Recent advances in co-processing biomass feedstock with petroleum feedstock: A review

Abstract

Co-processing of biomass feedstock with petroleum feedstock in existing refineries is a promising technology that enables the production of low-carbon fuels, reduces dependence on petroleum feedstock, and utilizes the existing infrastructure in refinery. Much effort has been dedicated to advancing co-processing technologies. Though significant progress has been made, the development of co-processing is still hindered by numerous challenges. Therefore, it is important to systematically summarize up-to-date research activities on co-processing process for the further development of co-processing technologies. This paper provides a review of the latest research activities on co-processing biomass feedstock with petroleum feedstock utilizing fluid catalytic cracking (FCC) or hydrotreating (HDT) processes. In addition, it extensively discusses the influence of different types and diverse physicochemical properties of biomass feedstock on the processing of petroleum feedstock, catalysts employed in co-processing studies, and relevant projects. Moreover, it summarizes and discusses co-processing projects in pilot or larger scale. Furthermore, it briefly prospects the research trend of co-processing in the end.

Graphical abstract

Keywords

co-processing / biomass / bio-oil / petroleum feedstock / fluid catalytic cracking / hydrotreating

1 Introduction

The development of sustainable and lower-carbon intensive energy has attracted a growing interest due to the world-wide depletion of crude oil and environmental concerns [1–3]. Biomass feedstock, such as lipids and lignocellulose-derived bio-oil, has received considerable attention as biomass is the most abundant material and the only renewable carbon source [4]. Biomass feedstock with a high oxygen content, high viscosity, and high acidity, is incompatible with current petroleum infrastructures and cannot be directly used as drop-in biofuel, which is functionally equivalent to petroleum fuels and is fully compatible with existing petroleum infrastructure [5]. Drop-in biofuels are being considered as the most beneficial alternative to petroleum fuels for reducing carbon emissions from the transportation sector [5–8]. There has been increased attention in drop-in biofuel production over the last few years [9–11]. However, the dispersed and bulky characteristics of biomass make it difficult to be efficiently collected and transported. The resulting high logistics costs are an important factor that makes a stand-alone biorefinery not cost-competitive. Co-processing biomass feedstock in the petroleum refinery enables the use of the existing refining, transport, and distribution infrastructure, preserving the nominal operating conditions and avoiding the construction of parallel facilities, which significantly reduces operating expenses (OPEXs) and capital expenditures (CAPEXs) [12–14]. Co-processing biomass feedstock in an existing refinery can reduce costs by about 20% compared to a stand-alone biorefinery [15–17], demonstrating that co-processing is a more economically attractive way to incorporate renewable carbon into transportation fuels.

Drop-in biofuels derived from lipid and lignocellulose are defined as conventional drop-in biofuel and advanced drop-in biofuel, respectively [9]. Currently, conventional drop-in biofuel is the only drop-in biofuel that is produced at a commercial scale [18]. The feedstock for drop-in biofuels is gradually expanding from lipid to lignocellulose, considering the numerous advantages of lignocellulose, such as low cost, sustainability, and availability [18–21]. Conversion of solid biomass to liquid product is a prerequisite for integration with petroleum refinery. Hydrothermal liquefaction (HTL) and pyrolysis are two main thermochemical processes of lignocellulose to produce liquid bio-oil or bio-crude [22–24]. Previous studies have indicated that direct upgrading lignocellulose-derived bio-oil by conventional refinery processes, such as fluid catalytic cracking (FCC) or hydrotreating (HDT), led to severe char and coke formation due to chemical instability and the hydrogen-deficit nature of bio-oil [25–28]. Co-processing biomass feedstock with hydrogen-rich petroleum feedstock is a potential solution, with advantages of inhibiting coke formation as well as enhancement of hydrocarbon formation [29].

Various types of biomass feedstock have been explored in previous studies of co-processing. Physicochemical properties of some typical petroleum feedstock and biomass feedstock, such as straight run gas oil (SRGO), vacuum gas oil (VGO), lipid, HTL bio-oil, fast pyrolysis (FP) bio-oil, hydrodeoxygenation (HDO) bio-oil, and catalytic fast pyrolysis (CFP) bio-oil, are summarized in Tab.1, which indicate significant differences between biomass feedstock and petroleum feedstock. Petroleum feedstock contains minimal amounts of oxygenates and has a very low oxygen content (0.05%–1.5%) [30], whereas biomass feedstock, such as FP bio-oil, contains various types of oxygenates and has a high oxygen content (up to 50%) [31]. Due to the high oxygen content of biomass feedstock, the higher heating value (HHV) of biomass feedstock, such as HHV of FP bio-oil (19–22 MJ/kg), is much lower than that of petroleum feedstock, such as HHV of VGO (47.41 MJ/kg) [32–34]. HDO bio-oil and CFP bio-oil, which are upgraded from FP bio-oil through the catalytic hydrotreated process or catalytic process, have a lower oxygen content, lower acidity, and higher HHV compared to FP bio-oil. Those advantages of HDO bio-oil and CFP bio-oil make them more suitable to be integrated with current petroleum refineries.

The cracking of large molecules into small molecules and the removal of oxygen from biomass feedstock are two key procedures for converting biomass feedstock into liquid fuels. The FCC process, responsible for 20%–30% carbon emission from petroleum refineries [81], is typically used to convert high-molecular weight fractions of feedstock into light products, such as propylene, high-octane gasoline, and light cycle oil (LCO) [39]. Moreover, since the catalyst of the FCC process is regenerated in situ, the FCC process can be more flexible in handling different types of feedstocks. The HDT process (nondestructive hydrogenation) [82] not only removes heteroatoms (sulfur, nitrogen, and oxygen) and metals (vanadium (V) and nickel (Ni)) in the feedstock through simultaneous hydrodesulfurization (HDS), hydrodenitrogenation (HDN), HDO, and hydrodemetalization (HDM) reaction, but also enhances the calorific value of the products [30,83]. Therefore, the FCC and HDT processes, which are found in virtually any conventional refinery, are the two main processes that integrate biomass feedstock with current petroleum refineries and have been extensively studied in recent years [9,12]. The different properties of biomass feedstock (viscosity, oxygen content, and stability) lead to distinct results when they are co-processed with petroleum feedstock either through the FCC or HDT process. The schematic view of the typical co-processing routes is presented in Fig.1.

Although the introduction of biomass feedstock into FCC and HDT processes reduces dependence on petroleum and reduces carbon emission, this strategy faces some technical challenges. This paper focuses on presenting a state-of-the-art review on co-processing biomass feedstock with petroleum feedstock in existing refineries using FCC or HDT processes. Moreover, it gives specific attention to reaction chemistry, catalysts, projects related to these co-processing processes. Furthermore, it also summarizes and discusses co-processing projects in pilot or larger scale, and briefly prospects the research trends in the future.

2 Co-processing via catalytic cracking

The catalytic cracking reaction follows the carbenium ion mechanism. Both the Brønsted and Lewis acid sites on the FCC catalyst generate carbenium ions. These ions can then participate in various subsequent reactions [84]. While petroleum feedstock has a negligible amount of oxygenates [30,85–87], biomass feedstock has an oxygen content up to 50% [31,54]. The reaction of co-processing biomass feedstock with petroleum feedstock via catalytic cracking mainly includes cracking, isomerization, and hydrogen transfer reactions (Eqs. (1)–(3)) [84]. Oxygen atoms are removed through simultaneous dehydration, decarbonylation, and decarboxylation reaction (Eq. (4)) [54,88].

(1){\mathrm{R}\mathrm{C}}^{+}{\mathrm{H}\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_3\to {\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{H}={\mathrm{C}\mathrm{H}}_2+{\mathrm{C}}^{+}{\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2\mathrm{R}

(2){\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{H}}_2{\mathrm{C}}^{+}{\mathrm{H}\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2\mathrm{R}\to {\mathrm{C}\mathrm{H}}_3{\mathrm{C}}^{+}\mathrm{H}\mathrm{C}\mathrm{H}\left({\mathrm{C}\mathrm{H}}_3\right){\mathrm{C}\mathrm{H}}_2\mathrm{R}\mathrm{o}\mathrm{r}{\mathrm{C}}^{+}{\mathrm{H}}_2\mathrm{C}\mathrm{H}\left({\mathrm{C}\mathrm{H}}_3\right){\mathrm{C}\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{R}

(3){3\mathrm{C}}_n{\mathrm{H}}_{2n}\left(\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{f}\mathrm{i}\mathrm{n}\mathrm{s}\right)+{\mathrm{C}}_m{\mathrm{H}}_{2m}\left(\mathrm{n}\mathrm{a}\mathrm{p}\mathrm{h}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{n}\mathrm{e}\right)\to {3\mathrm{C}}_n{\mathrm{H}}_{2n+2}\left(\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{a}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{n}\mathrm{s}\right)+{\mathrm{C}}_m{\mathrm{H}}_{2m-6}\left(\mathrm{a}\mathrm{r}\mathrm{o}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{c}\right)

(4){\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\to \mathrm{a}{\mathrm{C}}_x{\mathrm{H}}_{2x}+2{\mathrm{O}}_y+\mathrm{b}{\mathrm{C}\mathrm{O}}_2+\mathrm{c}{\mathrm{H}}_2\mathrm{O}+\mathrm{d}\mathrm{C}\mathrm{O}+\mathrm{e}\mathrm{C}

2.1 Co-processing of lipids

Lipids, which mainly consist of vegetable oils, animal fats, and waste oils, are the majority source of production conventional drop-in biofuel nowadays [5,12,19]. Due to its lower oxygen content and relative chemical simplicity, mainly composed of straightforward esters, lipids are completely miscible with petroleum feedstock and can be easily cracked [12,89,90].

Soybean oils have been utilized as the feedstock in a commercial FCC process (Petrobras, BRA) in the 1980s [91]. Compared to the gasoline produced from petroleum feedstock, the resulting gasoline exhibited a higher octane number and significantly lower levels of impurities such as sulfur and nitrogen. Only a trace amount of oxygen was detected from the product. The stability of the product was basically the same as any other gasoline produced from FCC processes. When processing soybean oils in a pilot-scale FCC reactor, a lower gasoline yield and a higher LCO yield were produced compared to VGO [92]. It was found that soybean oil requires less heat to crack than VGO because the temperature change from the bottom to the top of the riser is less. This is probably due to the fact that the majority of the oxygen in soybean oil is removed as water, which is an exothermic reaction [93]. Several studies have pointed out that water is the main deoxygenated product when co-processing lipids with petroleum feedstock [38,94–96]. For example, it was reported that the majority of oxygen in rapeseed oil, soybean oil, and palm oil was transformed to water during catalytic cracking in a pilot-scale FCC reactor [38]. Gasoline yields tended to decrease as the proportion of vegetable oil increased. Coke yields were almost constant for all experiments [38]. Since there is no external hydrogen source added to the FCC process, the phenomenon that oxygen is removed mainly in the form of water is a noteworthy point. It was hypothesized that the hydrogen atoms in the water originated from the dehydrogenation of the carbon chains [95]. This speculation was confirmed by the presence of more aromatics in the reaction products compared to the reactants. When co-processing lipids and petroleum feedstock, it should be noted that different saturation levels of lipids will cause different product distribution since olefinic bonds are more reactive. More aromatic compounds were formed when less saturated lipids (soybean oil or waste cooking oil (WCO)) were co-processed with VGO compared to more saturated lipids (palm oil and animal fats), which led to a higher liquid product and a lower gas yield [97].

2.2 Co-processing of bio-oils

Compared to lipids, bio-oils are more economically viable. Simultaneously, processing of bio-oils is much more complicated than processing of lipids, since bio-oils are extremely complex mixtures of polar oxygenated compounds while lipids are straightforward esters [91].

2.2.1 HTL bio-oil/FP bio-oil

HTL bio-oil: In general, HTL bio-oil has a lower oxygen content, a lower aromatics content, and a higher HHV compared to FP bio-oil [32,57,98]. Previous study reported that the conversion of catalytic cracking reaction decreased at all catalyst-to-oil (CTO) ratios when algae-derived HTL bio-oil was added to heavy vacuum gas oil (HVGO) [98]. This phenomenon could be explained by the fact that the large quantity of nitrogen-containing compounds present in HTL bio-oil reduced catalyst reactivity by competitively adsorbing on the acid sites responsible for cracking [54]. Although the HTL bio-oil contained abundant nitrogen-containing compounds, almost complete heteroatom was removed from the blend. The liquid products account for the majority of the products irrespective of different conversions [98]. In another study, black liquor-derived HTL bio-oil underwent an additional process of acid washing and HDO before being co-processed with VGO [99]. The addition of 10 wt.% (wt, mass fraction) of HTL bio-oil to VGO resulted in a minor decrease in gasoline yield and a slight increase in coke yield. However, with the addition of 30 wt.% of HTL bio-oil to VGO, a significant increase of coke yield was observed. Zhang et al. [100] conducted co-processing experiments with 5, 10, and 15 wt.% of HTL bio-oil. At a given conversion, the 5 wt.% of HTL bio-oil blend produced more gasoline and nearly the identical coke yield compared with pure VGO. Meanwhile, the 10 and 15 wt.% of HTL bio-oil blend produced a significantly higher coke yield, as shown in Fig.2. The conversion decreased markedly when the blending ratio of HTL bio-oil was higher than 10%, which was attributed to the deactivation of catalyst caused by the nitrogen-containing and oxygen-containing species in the HTL bio-oil [100].

FP bio-oil: FP bio-oil is the liquid product from the FP of biomass, which is composed of various polar oxygenates that are derived from the thermal decomposition of cellulose, hemicellulose, and lignin [29]. When the FP bio-oil was co-processed with atmospheric residue in a micro-activity testing (MAT) unit, the FP bio-oil derived from pine wood obtained a similar product distribution compared to pure atmospheric residue, while the FP bio-oil derived from wheat straw obtained a higher coke yield and a lower LPG yield [76]. This result indicated that different types of FP bio-oil would cause a significant shift in co-processing product distribution. When FP bio-oil was co-processed with VGO in a pilot-scale FCC process [92,101,102], it was observed that even a small addition of FP bio-oil would cause operational issues and have a significant influence on product distribution. It was necessary to modify the feed delivery system to avoid feed nozzle plug. The addition of 3 wt.% FP bio-oil to the VGO led to a lower gasoline yield, a lower LCO yield, and a higher coke yield [92]. Meanwhile, most of the FP bio-oil is converted into undesirable products, such as CO_x and water [92].

Commercial FCC processes mainly consist of four components, i.e., a riser, a stripper, standpipes, and a regenerator. A schematic of the typical FCC process is displayed in Fig.3 [103]. Catalytic cracking occurs in the riser where the catalyst contacts with the feedstock. The catalyst deactivation would occur along the riser as the coke formed as a byproduct gradually deposits on catalyst. After leaving the riser, the product and deactivated catalyst are separated by the stripper. Subsequently, the catalyst is sent to the regenerator through standpipe, where the coke deposited on catalyst will be burnt, causing the regenerated catalyst to have a high temperature. Finally, the high temperature regenerated catalyst is sent to the riser through the standpipe to initiate the next cycle of the FCC process [91].

FP bio-oil and VGO are immiscible due to the high polarity of FP bio-oil. In an FCC demonstration-scale process, FP bio-oil and VGO were injected into the riser reactor through two feed lines at different axial positions [39], which has a distinct difference from the FCC laboratory-scale processes in which a single feed line was used to inject the feedstock [41]. This configuration allowed both FP bio-oil and VGO to be heated to their respective optimal temperature. The biomass feedstock should not be heated over 50 °C in order to avoid plugging in the feed tank or feed line, while petroleum feedstock is generally heated at temperature between 180 and 320 °C to realize optimal dispersion [104]. Dispersant agent is unnecessary since VGO and bio-oil feed is separated and injected at different heights into the riser reactor. Different from most of FCC laboratory-scale processes which are operating isothermally, the temperature in FCC pilot-scale or demonstration-scale processes is gradually reducing from the bottom to the top of the riser reactor due to the endothermic cracking reaction, and the contact between the hot catalyst and the feedstock [39]. The temperature difference between the bottom and the top of the riser reactor is at least 100 °C, which causes a thermal shock at the bottom of the riser reactor between bio-oil and hot catalyst. The thermal shock would significantly facilitate the vaporizing and thermal cracking of heavy distillation fractions. The injecting point of pyrolysis oil is situated at the upstream of VGO, realizing a higher cracking temperature and a slightly longer residence time in the riser reactor [16]. The local CTO ratio at the bottom of the riser reactor is far more than the average CTO ratio, which will also promote the cracking reaction [39].

While no CO_x and water was produced during VGO cracking, some CO_x were observed in the regenerator [39]. The oxygen removal of FP bio-oil was mainly in means of water, followed by CO_x, which could be explained by the fact that the acidity of FCC catalyst caused dehydration reactions to play a dominant role. More CO was produced than CO₂, which indicated the leading role of the decarbonylation reaction compared to the decarboxylation reaction [91,104]. Compared to VGO cracking, an obvious decrease of coke formation was observed for the 5% FP bio-oil and almost identical coke yield was observed for the 10% FP bio-oil. The reason for this might be that for the 5% FP bio-oil, the dilution effect derived from the high water content (31.9 wt.%) of FP bio-oil was more obvious than its coke formation tendency, while for the 10% FP bio-oil, the dilution effect was not obvious [39]. In another study, up to 20 wt.% of FP bio-oil was co-processed with VGO [91,104]. Compared to VGO cracking, the same coke yield was observed for the 10% FP bio-oil and a higher coke yield was observed for the 20% FP bio-oil. The incremental yield of coke was lower than that observed in FCC laboratory-scale processes with the same or even a lower proportion of bio-oil blended [41]. The lower coke yields obtained at larger scale processes are attributed to its feeding system, which could better introduce bio-oil into the reactor and atomize bio-oil into small droplets [29]. It is clear that a co-processing study on a larger scale FCC process would more accurately mimic the FCC process in a refinery. Recently, Preem, the largest fuel company in Sweden, has successfully conducted a co-processing experiment at the Preemraff Lysekil refinery. A few hundred tons of FP bio-oil was co-processed at a ratio of 2 wt.%. The experiment lasted a couple of days and showed that the quality of the produced gasoline and diesel was not affected by the FP bio-oil. The next step will be to conduct a long-term test in which 50 thousand tons of FP bio-oil will be processed over a two-year period [105].

2.2.2 HDO bio-oil/CFP bio-oil

To reduce the oxygen content, water content, and total acid number (TAN) of HTL bio-oil or FP bio-oil, HTL bio-oil or FP bio-oil can be upgraded by catalytic process or catalytic hydrotreated process before being used for co-processing with petroleum feedstock. As shown in Tab.1, the HDO bio-oil and CFP bio-oil, with a lower oxygen content and acidity than FP bio-oil, are more suitable for insertion to FCC or HDT processes. The pretreatment scheme of low-grade bio-oil before co-processed in refinery is shown in Fig.4. The catalytic treatment of crude bio-oil will lead to a large amount of carbon loss [106]. For example, the carbon yields of CFP bio-oil produced from pine wood were 14%–23%, much lower than the 35%–42% carbon yields from pine HDO bio-oil and the 76.9% carbon yield from pine FP bio-oil [107–109]. Although the catalytic HDT of crude bio-oil can moderately retain carbon, the higher water content cannot be avoided [109,110].

HDO bio-oil: HDO bio-oil is produced from hydrotreatment of bio-oil via HDO reaction, which has a higher H/C (hydrogen to carbon) ratio and a lower oxygen content compared to FP bio-oil. However, the degree of HDO should be reasonable since excessive hydrotreatment would cause a high hydrogen consumption and a poor process economic feasibility [111,112].

HDO bio-oil was co-processed with petroleum feedstock to clarify the effect of different HDO degree of bio-oil on co-processing, and find out the optimal HDO degree for co-processing [102,113–115]. The gasoline yield increased when the bio-oil obtained from a higher degree of HDO was co-processed [114]. However, another study reported that there was no obvious relationship between gasoline yield and HDO degree, while the coke yield was inversely proportional to HDO degree [113]. The optimal HDO degree which obtained the highest octane number in naphtha fraction was a mild HDO, corresponding to a hydrogen consumption of 202 NL per kilogram of bio-oil [113]. In another study, although a high oxygen content (16.9 to 28 wt.%) was contained in HDO bio-oils, all experiments were successfully conducted, which yielded almost identical product distribution, indicating that mild HDO could be enough to generate the required quality of bio-oil for successful co-processing [115]. The most reactive components which tended to prevent successful co-processing could already be converted under mild HDO [115–117]. However, another study found that a mild HDO of the bio-oil (20 to 22 wt.% oxygen content) even made the bio-oil more difficult to be co-processed, resulting in an increased coke and a reduced gasoline. The blending ratio of the bio-oil cannot exceed 3 wt.%. A medium and severe HDO of bio-oil (1 to 12 wt.% oxygen content) was required to generate HDO bio-oil which was easier to handle. A 10 wt.% bio-oil can be successfully co-processed, whose yields of coke and gasoline were similar to VGO [102].

A successful cracking of pure HDO bio-oils was conducted [115]. The coke and dry gas yield from pure HDO bio-oils was significantly higher than that obtained from pure long residue or co-processing HDO bio-oil with a long residue, and was found to be inversely proportional to the HDO degree [115]. The enhanced coke formation from pure HDO bio-oils was attributed to the abundant polar oxygenates present in HDO bio-oils. These oxygenates would occupy the acid sites of catalyst, thereby impeding the cracking of HDO bio-oils [112,118,119]. To achieve the same conversion with co-processing, a higher CTO ratio was required for pure HDO bio-oil [115]. By comparing the experimental product profile with the putative product profile of pure HDO bio-oils obtained from linear extrapolation via experimental product profile of pure long residue and co-processing, it was found that the coke and dry gas yield from putative pure HDO bio-oil was much lower. The beneficial synergistic effect could be explained by the fact that there might exist a hydrogen transfer from the hydrocarbons in the long residue to the oxygenates in the HDO bio-oil. The coke formation rate of HDO bio-oil possibly declined due to the lower concentration of coke precursor caused by dilution of long residue [115,116].

Different ratios of HDO bio-oil was co-catalytic cracked with atmospheric residue at a CTO ratio of 2.5 or 3 [85]. At a CTO ratio of 2.5, the conversion was gradually decreased from 80% to 65% with the increase of the ratio of HDO bio-oil in the feed, while the conversion was almost identical for all feed except for the 20/80 HDO bio-oil/atmospheric residue blend with a little decrease at a CTO ratio of 3. This phenomenon indicated that some refractory compounds were existing in HDO bio-oil. A higher CTO ratio was required for co-processing to get equivalent conversion with pure petroleum feedstock since it was more difficult for HDO bio-oil [115,120] and CFP bio-oil [76] to be cracked than VGO. The lower crackability of CFP bio-oil was possibly caused by the high content of basic nitrogen in the CFP bio-oil, which acted as a catalyst poison [76]. However, in some researches, HDO bio-oil [113] and CFP bio-oil [86,121] had a higher crackability compared to VGO, because the conversion was higher when bio-oil was present. This distinct conclusion might be explained by different biomass feedstocks and setups used in their experiments.

The influence of different HDO bio-oil blending ratios on gasoline quality was investigated [85]. With the increase of the HDO bio-oil blending ratio, the yield of aromatics increased, the yield of iso-paraffins and olefins decreased, and the yield of n-paraffins and naphthenes remained unchanged. The identical changes of gasoline composition were also observed by Lappas et al. when HDO bio-oil was added to the VGO in an FCC pilot-scale process [120]. This phenomenon was attributed to the lower hydrogen content in aromatics, which made it more difficult to be cracked than paraffins. The aromatics accounted for about 40% in the gasoline fraction obtained from co-processing, which was attributed to initially high aromatic compound content in HDO bio-oil [85].

CFP bio-oil: CFP bio-oil is produced in a single step and external hydrogen source is not needed. The high pressure (200–300 bar) needed for HDO of bio-oil is avoided, creating a moderate reaction condition for CFP [86]. The use of CFP bio-oil can reduce the number of operation unit and improve economic feasibility of co-processing.

Previous study reported the organic yield of co-processing CFP bio-oil/VGO and HDO bio-oil/VGO were 30 and 24 wt.%, respectively [86]. The higher organic yield of the former demonstrated a promising future of co-processing CFP bio-oil/VGO in refineries. On the other hand, it was found that the co-processing of HDO bio-oil with petroleum feedstock resulted in a higher bio-carbon content in liquid product compared to CFP bio-oil [29]. A lower yield of hydrogen and coke was observed when CFP bio-oil was co-processed with SRGO, which indicated that the hydrogen from SRGO was transferred to CFP bio-oil to remove the oxygen in CFP bio-oil [122]. Meanwhile, coke formation was moderated due to the hydrogen transfer. The hydrogen yield for the blend should have been lower if all the oxygen was rejected as a form of water. The relative higher yield of hydrogen for the blend demonstrated that partial oxygen was removed as CO_x, which could be further confirmed from the fact that negligible amount of oxygen was detected from co-cracked products [122]. Contrary to the conventional idea that oxygen content of bio-oil should not be higher than 10 wt.% before entrance into FCC processes [123], Agblevor et al. [122] concluded that the criterion for the crackability of bio-oils should be the stability rather than oxygen content. This conclusion was verified by another study, which reported that HDO bio-oil containing 28 wt.% of oxygen could be successfully co-processed with petroleum feedstock [115]. In a pilot-scale FCC process, no operational problem was observed when the blending ratio of CFP bio-oil was below 10 wt.% [121]. The blending ratio of CFP bio-oil can be increased during co-processing in larger scale FCC processes, since they have a larger riser diameter and therefore lower the risk of riser blocking [121].

FP bio-oil, CFP bio-oil, and HDO bio-oil were co-processed with VGO under the FCC condition [41]. The highest coke yield was observed from co-processing FP bio-oil/VGO, which was caused by the highest amount of sugar like material in FP bio-oil. The product distribution obtained from CFP bio-oil and HDO bio-oil was very similar. Compared to HDO bio-oil, a slightly higher coke and gasoline yield was obtained from CFP bio-oil. The higher gasoline yield obtained from CFP bio-oil was attributed to the higher aromatics content of CFP bio-oil compared to HDO bio-oil. The aromatics could be directly transferred into gasoline fraction. The higher coke yield obtained from CFP bio-oil could be attributed to its lower H/C ratio, higher micro carbon residue value, heavier chemical composition [86], and higher content of high molecular mass lignin, which acted as a precursor for coke formation. Distinct coke yields from CFP bio-oil and HDO bio-oil indicated that oxygen content could not be used individually to judge the crackability of bio-oils when they were co-processed with petroleum feedstock [122]. Chemical composition of liquid product from pure VGO and co-processing FP bio-oil, CFP bio-oil, and HDO bio-oil was almost identical, which were benzenes, polycyclic aromatic hydrocarbons (PAHs), and straight chain hydrocarbons [41].

The detail information related to feedstock, operation parameters for co-processing, and product yield of research activities on co-processing biomass feedstock with petroleum feedstock in FCC process are summarized in Tab.2. Many co-processing researches reported a lower gasoline yield and a higher coke yield. The lower gasoline yield may be caused by the large amount of water contained in FP bio-oil, which dilutes the reactant stream. Coke yields were usually lower at larger scale reactors, which are attributed to its feeding system. Biomass feedstock and petroleum feedstock can be introduced into the riser at different feed nozzles, which allow them to be injected at their respective optimum temperatures. In most cases, it was found that upgraded bio-oil (HDO bio-oil and CFP bio-oil) was more suitable to be co-processed with petroleum feedstock than FP bio-oil. Most of the research on co-processing upgraded bio-oil with petroleum feedstock has reported higher coke yields compared to pure VGO cracking, which is consistent between CFP bio-oil/VGO and HDO bio-oil/VGO. However, the gasoline and LCO yield between CFP bio-oil/VGO and HDO bio-oil/VGO is inconsistent. The gasoline obtained from CFP bio-oil/VGO is usually slightly higher than that obtained in pure VGO. This phenomenon can be partly attributed to the higher aromatics content in CFP bio-oil compared to HDO bio-oil, since the aromatics might be transferred into gasoline fraction directly. On the other hand, there are no universal results from the gasoline and LCO yield obtained from HDO bio-oil/VGO, which was highly dependent on the CTO ratio and the ratio of HDO bio-oil in the feed.

2.3 Catalysts for co-catalytic cracking

FCC processes provide an easier-and-less-risky insertion point for biomass feedstock considering its in situ regenerated catalyst [5]. FCC catalysts can remove oxygen atoms from biomass feedstock effectively. As shown in Tab.2, FCC processes took place in the presence of E-CAT in most cases. E-CATs are fine spherical particles with an average particle size of 75 μm and a bulk density of 0.80–0.96 g/cm³ [91,124,125]. A representative E-CAT contains 10–30 wt.% Y zeolite dispersed in a matrix composed of alumina, clay (usually kaolin), and binder (Fig.5) [12,126]. Zeolite, also called a molecular sieve, is the main active components of FCC catalysts [125] and Y zeolite is the most commonly used zeolite in FCC processes [125,127]. Zeolite catalysts require little investment for greater profits. In other words, zeolite catalysts have always been the biggest bargain for refineries. This is why E-CAT is the dominant catalyst in FCC processes [84]. The large molecules contained in petroleum feedstock are deemed to be pre-cracked on the FCC catalyst matrix due to its acidity and macro/meso porosity [128]. HZSM-5 zeolite can be added to increase gasoline octane and light olefin yields [129,130].

Three zeolite-based catalysts, i.e., E-CAT, HY, and HZSM-5, were used for cracking VGO and HDO bio-oil/VGO to investigate the effect of catalyst structure on product quality [134]. The catalytic cracking reactions for both pure VGO and HDO bio-oil/VGO are affected by the pore size and Si/Al ratio of the zeolites, leading to changes in the conversion level and the product distribution. The formation rates of products are reduced when co-processing HDO bio-oil/VGO, except for the formation of coke and aromatics [134]. The results can be attributed to the limited access of oxygenates into zeolite pores. As shown in Fig.6, most of the oxygenated compounds contained in bio-oil cannot enter the HZSM-5/HY channels [134]. Excessive coke on the external surface of zeolite would be formed by pre-cracking and deoxygenating of oxygenates, resulting in pore blockage and lower reaction rates. Furthermore, competition for zeolite acid sites between hydrocarbon cracking and oxygenates deoxygenation (DO) on the surface accounts for the observed impacts of co-processing on product quality [135]. Fogassy et al. [136] found that bio-carbon in reactant (8.5%) was not evenly distributed among co-processing products using the ¹⁴C isotopic analysis method. Gasoline contained only 7.2% of bio-carbon while the majority of bio-carbon flowed to coke (15.8%) and gases (10.6%). As discussed above, bio-molecules were essentially cracked in the mesopores of the FCC catalyst, occupying acid sites (essentially Lewis type, typical of extra-framework alumina deposits) which would otherwise be used for VGO cracking reactions that produce gases [136]. On the other hand, gasoline was mainly derived from intra-framework cracking of fossil hydrocarbon molecules [136]. Therefore, the fraction of bio-carbon in the gases is higher and the one in the gasoline is lower. The higher fraction of bio-carbon in coke indicates that the increased coke formation during co-processing can be attributed to the bio-molecules condensation reaction on the external surface of zeolite, which could easily form polyaromatic coke precursors. Moreover, VGO cracking was significantly influenced during co-processing by the hydrogen deficit, caused by the reduction of oxygenates through the dehydration reaction [134]. More polyaromatics would be generated due to hydrogen deficit and thus more coke would be formed from VGO cracking [29,136].

When co-processing biomass feedstock with petroleum feedstock, polar oxygenates in biomass feedstock are easily adsorbed and trapped in the pores of zeolite, resulting in deactivation of the zeolite [137]. However, the adverse effects of oxygenates on E-CAT are not as apparent as those on pure zeolite catalyst [138]. The impact of different FCC catalysts, i.e., a 100% E-CAT and a mixture of 90% E-CAT and a 10% ZSM-5 additive, on the conversion of co-processing was investigated [138]. Three types of oxygenates, i.e., acetic acid, hydroxy-acetone, and phenol, were mixed with gasoil to simulate the co-processing of bio-oils. Compared to pure gasoil cracking, the overall conversion was higher when oxygenates were co-processed with both catalysts. However, when co-processing oxygenates, the gasoil conversion over pure E-CAT was not changed significantly, while that over a mixture of 90% E-CAT and 10% ZSM-5 additive was decreased. This phenomenon could lead to the conclusion that oxygenates would more easily contact with ZSM-5 sites compared to fossil molecules, and thereby the interaction between gasoil and ZSM-5 was weakened [29]. Moreover, Ni and V present in crude oil were reported to be beneficial for successful co-processing [139]. Although the presence of Ni and V may have negative impacts on FCC catalysts, such as collapse of the structure and reduction in active sites [140], the adverse effects can be circumvented by the addition of metal passivators, such as antimony, tin, and bismuth [84,140,141]. In addition, the alumina in FCC catalysts can also trap the metal contaminants, thus mitigating the adverse effects of metal contaminants on the active zeolite acid site [141]. Gerards et al. [139] found that the addition of Ni and V greatly increased the Lewis acid sites of zeolite catalyst. The increased Lewis acid sites are beneficial for the conversion of oxygenates and adsorption of unconverted oxygenates, thereby providing more Brønsted acid sites for cracking reactions, and further reducing the adverse effects of oxygenates on zeolite [139].

When co-processing biomass feedstock with petroleum feedstock, it is important to consider the presence of alkali and alkaline earth metals (AAEMs), such as Ca, K, Mg, and Na, in the biomass feedstock. AAEMs not only neutralize the acid sites, but also cause collapse of the catalyst structure through reactions between alkali metal oxides and silica or alumina, which can lead to catalyst deactivation [62]. Pretreatment of biomass feedstock, such as acid washing and filtration, is essential prior to insertion into the refinery to achieve efficient removal of AAEMs [19,142]. Acid washing has been used to remove AAEMs from HTL bio-oil [99]. AAEMs would not leach into the surrounding bio-oil and are concentrated in biochar. Therefore, most of the AAEMs and other inorganic substances can be eliminated from the biomass feedstock by removing the biochar through filtration, such as hot gas filtration and microfiltration [142–146]. Bio-oil with almost zero alkali metal content was generated after separating biochar from pyrolysis before condensation through hot gas filtration [147].

3 Co-processing via HDT

Currently, the technology for producing drop-in biofuel through the hydrotreatment of lipids has reached a commercial level [5]. Neste has already achieved commercial production of renewable diesel from lipids with a capacity of 4000 barrels per day in 2007 [148]. In the case of oleic acid, oxygen atoms in lipids are removed through simultaneous HDO, decarbonylation, and decarboxylation reaction (Eqs. (5)–(7)) [149]. Due to the expensive investment costs associated with stand-alone biomass hydrotreatment, it is economically viable only for high throughput plants. However, considering the dispersed, bulky and seasonal characteristics, and availability of biomass, it is challenging to establish stand-alone biomass hydrotreatment plants with a high throughput. The co-HDT technology is regarded as a promising approach as it can utilize existing refinery facilities and make full use of dispersed biomass resources. More efforts should be directed toward investigating the co-HDT technology that allows for partial substitution of petroleum feedstock with biomass feedstock [12].

(5){\mathrm{C}}_{17}{\mathrm{H}}_{33}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}+4{\mathrm{H}}_2\to \mathrm{n}-{\mathrm{C}}_{18}{\mathrm{H}}_{38}+2{\mathrm{H}}_2\mathrm{O}

(6){\mathrm{C}}_{17}{\mathrm{H}}_{33}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}+2{\mathrm{H}}_2\to \mathrm{n}-{\mathrm{C}}_{17}{\mathrm{H}}_{36}+\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}

(7){\mathrm{C}}_{17}{\mathrm{H}}_{33}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\to \mathrm{n}-{\mathrm{C}}_{17}{\mathrm{H}}_{36}+\mathrm{C}{\mathrm{O}}_2

3.1 Co-processing of lipids

When co-processing lipids with petroleum feedstock in HDT processes, the level of unsaturation and free fatty acid content present in lipids should be considered. Lipids with a lower unsaturation degree have worse fluidity, which requires preheating of the pipelines in the supply section. The preheating process not only leads to additional energy consumption, but may also exacerbates the polycondensation reaction of biomass feedstock, which is unfavorable for continuous operation [150]. The higher the free fatty acid content, the stronger the overall acidity of feedstock, which can lead to corrosion of equipment [12]. The above phenomenon can be avoided by reasonably selecting the type of lipids and controlling the blending ratio of lipids. Tóth et al. [151] conducted co-HDT of SRGO–LCO–WCO three component blends and concluded that the WCO content should not be higher than 15%, as the desulfurization reaction is suppressed by the high oxygen content of WCO. From the perspective of hydrogen consumption, previous study showed that the optimal blending ratio of WCO should be lower than 10% [152]. Compared with pure heavy atmospheric gas oil HDT, hydrogen consumption increased only about 6.5% when the 5%–10% WCO was co-hydrotreated.

Although some challenges remain, co-HDT of lipids with petroleum feedstock is now commercially available as demonstrated by Topsoe and Petrobras [5,12,153]. For example, three undisclosed refineries in Australia, Europe, and the USA utilized DO catalyst from Topsoe to co-hydrotreat lipids in the range of 0–5%, 2%–5%, and 10%–20%, respectively [153]. However, lignocellulose has numerous advantages compared to lipid, such as low cost, sustainability, and availability. The annual production of lignocellulose was estimated to be 10–50 billion tons around the world, which accounts for 50% of world biomass [154]. However, the annual production of lipid used for biofuels is unstable, since the availability of edible lipids will change along with market competition [155]. The biomass feedstock for co-HDT should gradually expand from lipid to lignocellulose [148].

3.2 Co-processing of bio-oils

The co-HDT technology primarily removes heteroatoms (O, N, S, metals) through HDO, HDN, HDS, and HDM reactions [1,156,157]. The most important reaction for hydrotreatment of petroleum feedstock is HDS reaction while for biomass feedstock, HDO is the most important reaction [158]. There may be competition between different reactions during co-HDT. Therefore, before investigating the co-HDT of bio-oils with petroleum feedstock, it is important to clarify the impact of heteroatom-containing species in biomass feedstock on the hydrotreatment of petroleum feedstock and the interaction among different reactions using model compounds.

3.2.1 Model compounds

The impact of different oxygen-containing species in biomass feedstock on the hydrotreatment of petroleum feedstock was studied by Bui et al. [159] and Pinheiro et al. [160]. The addition of guaiacol resulted in the emergence of new products such as phenol, and cresol compared with the conversion of SRGO alone [159]. It was found that the introduction of guaiacol inhibited the progress of the HDS reaction at low temperatures. This inhibitory effect gradually disappeared with the increase of reaction temperature, as shown in Fig.7. The reason for this is that intermediate phenols derived from guaiacol compete with the most active sulfur-containing species through a competitive adsorption mechanism at the initial stage of the reaction and inhibit the HDS reaction. With the increase of temperature, phenolic species were converted into hydrocarbons by HDO reaction, and the inhibition effect on HDS reaction disappeared. Pinheiro et al. [160] investigated the effect of different bio-oil model compounds on the hydrotreatment of SRGO. The result showed that those oxygenated compounds whose HDO reaction produces water (alcohols, ketones, and ethers) showed no inhibition on HDS and HDN reactions. On the other hand, propionic acid and ethyl caprate significantly inhibits the HDS and HDN reactions. CO_x is produced when propionic acid and ethyl caprate are used as additives. The water gas shift reaction and methanation reaction associated with CO_x may occupy the catalyst active sites that should be used for HDS and HDN reaction.

During co-HDT, the removal of sulfur-containing species in petroleum feedstock will produce H₂S through the HDS reaction, which will further affect the HDN and HDO reaction. Zhu et al. [161] used N,N-diethyldodecanamide (DEDAD) as model compounds of nitrogen and oxygen-containing components in the HTL bio-oil derived from wet waste, and studied the effect of H₂S on the HDN and HDO reaction of DEDAD. The results showed that the introduction of H₂S decreased the yield of amines and increased the yield of C₁₂ compounds. It is inferred that the presence of H₂S strongly inhibits the HDO reaction of DEDAD (the rate constant decreases from 1.21 to 0.15) and promotes the HDN reaction of amines (the rate constant increases from 0.086 to 0.68), as shown in the blue box in Fig.8.

The reaction pathways and kinetic parameters of representative heteroatom-containing model compounds of biomass feedstock (dodecanoic acid and 4-propylphenol) and petroleum feedstock (dibenzothiophene and quinoline) during co-HDT are presented in Fig.9 [161]. As shown in Fig.9(a), dodecanoic acid either undergoes the decarboxylation and decarbonylation reaction to generate CO₂, CO and C₁₁ hydrocarbons through DO pathway, or undergoes the hydrogenation (HYD) pathway to generate H₂O and C₁₂ hydrocarbons. The rate constant of the HYD pathway (1.57) is much larger than that of the DO pathway (0.094). Therefore, it proceeds as the dominant pathway. For 4-propylphenol, there are two competing reactions with similar rate constants, as shown in Fig.9(b). 4-propylphenol can be first hydrogenated to form oxygen-containing intermediates by the HYD pathway, and then deoxygenated to form cycloalkanes. 4-propylphenol can also be directly converted to aromatics by direct deoxygenation (DDO), and cycloalkanes can be generated by further HYD reaction. As shown in Fig.9(c), direct desulfurization (DDS) of dibenzothiophene to the corresponding hydrocarbon products is the dominant reaction (rate constant 0.48). Dibenzothiophene can also react through the HYD reaction to generate sulfur-containing partially hydrogenated intermediates, which can be desulfurized by further HYD reaction. As shown in Fig.9(d), for pyridines, the HYD reaction of the aromatic ring occurs first. Subsequent partially or fully hydrogenated nitrogen-containing heterocycles undergo the denitrogenation (DN) reaction to produce hydrocarbon products. It is obviously observed that the rate constant of pyridines (0.01) is the lowest among these model compounds, which indicates that the removal of nitrogen-containing species is a big challenge during co-HDT [161].

3.2.2 Bio-oil

To the best of the authors’ knowledge, there is limited research on the co-HDT of bio-oil and petroleum feedstock. The experimental results of co-HDT HTL bio-oil with SRGO showed that the diesel obtained had a lower boiling point range and a higher density [162]. From the perspective of density, the diesel complied with the specifications for road diesel (0.82–0.845 g/cm³). The variation in density of the diesel can be attributed to changes in its chemical composition: it contained more aromatic compounds. Due to the higher density of cyclic structures compared to linear structures with the same carbon atoms, the diesel exhibited a higher density [162]. Additionally, the hydrotreatment experiments on pure HTL bio-oil demonstrated that almost all oxygen was removed (≤ 0.2 wt.%), while other studies required more severe reaction conditions to achieve the same effect [163]. It was concluded that this phenomenon might be attributed to the use of distilled HTL bio-oil in this study, which removed the heaviest fraction that was difficult to process, and therefore achieved complete DO of the bio-oil under milder conditions [162].

In another study, HTL bio-oil was also distilled prior to co-HDT with VGO [164]. The aim of this study was to provide hydrotreated VGO streams containing bio-content for subsequent refining processes, such as FCC and hydrocracking. Through check-back experiments using pure VGO every 200 h, it was found that when the percentage of bio-oil was higher than 10%, the efficiencies of HDS and HDN reactions decreased by 20.2% and 7.2%, respectively, compared to the initial values, implying a significant catalyst deactivation. In addition, the HDS efficiency of the blends with 5 and 10 vol.% (vol.%, volume fraction) ratios was almost the same as that of pure VGO when the reaction temperature was higher than 370 °C. Therefore, it was concluded that the reaction conditions with an HTL bio-oil blending ratio below 10% and a temperature above 370 °C are more favorable for successful co-HDT. In addition to reasonable control of the blending ratio of biomass feedstock and reaction conditions, pretreatment of biomass feedstock is also very critical for successful co-HDT. It has been reported that HDS and HDN efficiencies are higher in co-HDT of hydrotreated HTL bio-oil with VGO compared to that of co-HDT of raw HTL bio-oil with VGO [165], which indicated that appropriate hydrotreatment of biomass feedstock is helpful for diminishing the negative impact of biomass feedstock on the petroleum feedstock.

It has been reported that the presence of CO_x inhibits HDS and HDN reactions during co-processing [164,166,167]. Pinheiro et al. [168] investigated the influence of CO_x by injecting the same amount of CO_x, as produced in the co-HDT experiment, in the SRGO HDT experiment. With the increasing of injected CO_x flow rate from 0 to 14 mmol/h, the HDS efficiency was decreasing from 98% to 92%, and the HDN efficiency was decreasing from 95% to 76% [168]. The decrease of the HDS and HDN efficiencies with the increase of CO_x indicated that the decrease in the efficiencies of HDS and HDN reactions was indeed caused by the CO_x generated during the experiment [168]. It has been reported that the presence of CO will greatly inhibit the HDS reaction by directly affecting the rupture of the C–S bond [169]. With the increase of CO₂ content, the water gas shift reaction and methane steam reforming reaction will be promoted, which may result in considerable hydrogen consumption and inhibition of HDS and HDN reactions [170]. An additional HDT experiment with pure SRGO was performed after the co-HDT experiment. The results were almost identical with the pure SRGO HDT experiment performed with fresh catalyst, indicating that the oxygen-containing compounds themselves do not deactivate the catalyst [166]. This conclusion is consistent with the studies of others [116,171]. Although the HDS reaction was inhibited by the addition of HDO bio-oil to the LCO, the HDS efficiency returned to the initial level again when pure LCO was used as feedstock after the removal of HDO bio-oil [171]. The decrease of the HDS efficiency due to the introduction of bio-oil can be solved by increasing the reaction temperature. The pressure of the reactor remained stable during the 37-day-long tests, indicating that there was no permanent deactivation of catalyst and no clogging of the catalyst bed. Similarly, a check-back experiment using pure SRGO after co-HDT showed that the HDS efficiency returned to the initial value, indicating that no permanent deactivation of the catalyst occurred during the co-HDT [116].

The distribution of the molecular weight of the co-hydrotreated products was found to be lower than that of the bio-oil, indicating that the high molecular weight compounds in the bio-oil were also broken to some extent during the co-HDT [116]. This result is consistent with another study [172], in which the percentage of compounds in a low boiling point range was increased after co-HDT HTL bio-oil with heavy gas oil.

In conclusion, appropriate pretreatment of biomass feedstock, such as distillation and hydrotreatment, reasonable control of the blending ratio of biomass feedstock and reaction conditions, is helpful for diminishing the negative impact of biomass feedstock on the petroleum feedstock. The decrease in the efficiencies of HDS and HDN reactions during the co-HDT was due to the presence of CO_x rather than the biomass feedstock itself. The detailed information related to feedstock, operation parameters for co-HDT, and results of research activities on co-processing biomass feedstock with petroleum feedstock in HDT processes are summarized in Tab.3.

3.3 Catalysts for co-HDT

Ni (Co)-Mo based catalysts have been commercially applied in HDT processes for co-processing biomass feedstock with petroleum feedstock due to their excellent activity for heteroatoms removal reactions [172–174]. Typically, these catalysts undergo a two-stage sulfidation process to improve the catalytic activity before being used in HDT processes. In the first stage, molybdenum oxide is converted to molybdenum oxysulfide, while in the second stage, molybdenum oxysulfide is converted to molybdenum sulfide (active phase) [172]. Ni and Co can not only activate the C-heteroatom bond but also donate electrons to Mo, which weakens the bond between Mo and S, resulting in sulfur vacancies [175]. The increase of sulfur vacancies resulted in an enhanced reactivity of HDS and HDO [175].

Catalyst deactivation has been a persistent issue during co-HDT of biomass feedstock with petroleum feedstock. The high oxygen content in biomass feedstock is typically removed through HDO reaction, decarbonylation reaction, decarboxylation reaction, and dehydration reaction under the catalytic action of Ni (Co)-Mo based catalysts [176,177]. The water generated from these reactions and the nature of high-water content in biomass feedstock, combined with high-temperature reaction environment can promote the structure collapse of \gamma-Al₂O₃ support [164]. The nature of high TAN value in biomass feedstock will cause the corrosion of HDT equipment and the pipelines. It is worth noting that the HDO reaction is an exothermic reaction accompanied by the consumption of hydrogen. Elevated temperature may exacerbate catalyst coking, and even lead to catalyst melting, resulting in structural changes of catalyst [5]. The excessive progress of the HDO reaction may reduce the reaction pressure. This phenomenon can be effectively prevented by improving the existing cooling system and/or controlling the blending ratio of biomass feedstock. A typical experimental unit used for HDT experiments is shown in Fig.10. The catalyst is usually placed on a catalyst bed located in the middle of the reactor. To avoid possible overheating during the co-HDT process, Preem designed a hydrotreater with four catalyst beds. The distance between the catalyst beds effectively prevents overheating. The raw tall diesel is injected directly into the first and second catalyst beds without heating, providing a quenching effect [178].

In addition, the presence of biomass feedstock will lead to a sharp increase in CO_x generated through the decarbonylation and decarboxylation reaction, respectively, which will also affect the activity of Ni (Co)-Mo based catalysts. As discussed in Section 3.2.2, the water gas shift reaction and the methane steam reforming reaction will be facilitated with the increase of CO₂ content, which may lead to the reduction of H₂ partial pressure and the inhibition of HDS and HDN reactions [170].

In conclusion, the addition of biomass feedstock with high oxygen content will have an impact on the removal of heteroatoms and the lifetime of the catalyst. The development of HDT catalysts suitable for the co-processing of biomass feedstock with petroleum feedstock is still the focus of the issue. To cope with the sulfur leaching from the catalyst surface due to the high oxygen content present in biomass feedstock, the inclusion of hydrogen sulfide in the carrier gas can be considered to promote the regeneration of sulfide active sites, thus maintaining the activity of the catalyst. To prevent the formation of coke precursors, TiO₂, CeO₂, and ZrO₂, which have moderate acidity and the ability to activate oxygen-containing compounds, are probably suitable as a substitute for the strongly acidic \gamma-Al₂O₃ [175]. To tackle the issue of the increased hydrogen consumption caused by DO, appropriately increasing the hydrogen pressure and reducing the blending ratio of the biomass feedstock can be considered. Additional hydrogen donors such as methanol can also be introduced. Suitable hydrogen donors can not only provide hydrogen, but also serve as a solvent to dissolve biomass feedstock and prevent their polycondensation reactions at high temperatures.

4 Perspectives and conclusions

Production of low-carbon fuels through co-processing using existing infrastructure in petroleum industry offers a potential route for commercialization of biomass utilization. Up-to-date research activities of co-processing biomass feedstock with petroleum feedstock via FCC or HDT processes are reviewed in this paper. The influence of various types and diverse physicochemical properties of biomass feedstock on the processing of petroleum feedstock, catalysts employed, and relevant projects associated with previous studies are also discussed.

As discussed above, much effort has been dedicated to advancing co-processing technologies. Though significant progress has been made, numerous challenges continue to impede the development of co-processing technologies. These challenges include immiscibility between biomass feedstock and petroleum feedstock, reactor plugging caused by premature catalyst deactivation, and significant differences between the micro-scale reactors and the industrial ones. To further promote the development of co-processing technologies, the following aspects need to be considered in future research:

1) Selection of appropriate biomass feedstock and petroleum feedstock is important for desirable generation of products and stable operation of co-processing reactors. Different combinations of biomass feedstock and petroleum feedstock can have a significant influence on the co-processing process, such as feedstock supply, conversion rates, catalyst deactivation, and product distribution. The bio-liquid produced from pyrolysis or solvent liquefaction of lignocellulose with a high oxygen content is not well compatible with current oil refineries. Though fractionation has been explored to improve the miscibility by removing out heavy fractions in the bio-liquid, performing a certain level of hydrotreatment is a more common means to improve the quality of biomass feedstock. Upgraded bio-oil, which have a lower oxygen content and a lower acidity compared to crude bio-oil, are generally more suitable for co-processing. A relatively higher blending ratio and a better product quality can be achieved when co-processing upgraded bio-oil. However, HDO bio-oil with a higher quality means more hydrogen consumption during bio-oil production, while CFP bio-oil with a higher quality means a lower bio-oil yields for the biomass catalytic pyrolysis process. It is important to find the optimal trade-off between the economics of the co-processing and the degree of bio-oil pretreatment. When co-HDT biomass feedstock, the CO_x released from biomass feedstock has a significant effect on desulfurization and DN. Bio-liquids which are rich in alcohols, ketones, and ethers in their chemical composition will have less impact on desulfurization and DN of petroleum feedstock than bio-liquids which are rich in acids and esters in their chemical composition, since the oxygen in alcohols, ketones, and ethers is removed as water while the oxygen in acids and esters is removed as CO_x. Moreover, the AAEMs contained in the biomass feedstock may deactivate HDT or FCC catalysts. Acid washing and filtration of bio-oil are efficient methods to remove AAEMs present in biomass feedstock, which will deactivate catalysts.

2) As discussed above, most co-processing catalysts used in previous studies, such as E-CAT for FCC processes and Co (Ni)-Mo/γ-Al₂O₃ for HDT processes, are typically developed for upgrading conventional petroleum feedstock. The design of petroleum catalysts does not adequately take into account the characteristics of biomass feedstock. Future catalyst designs should take into consideration the unique physiochemical properties of biomass feedstock to further optimize the co-processing process. For co-processing biomass feedstock with petroleum feedstock, the AAEMs present in the biomass feedstock may cause catalyst deactivation through reactions between alkali metal oxides and silica or alumina. The development of Anti-AAEMs catalysts to prevent premature catalyst deactivation is a possible future direction for catalysts. However, since the greatest advantage of zeolite catalysts is their cheapness, their cost should always be taken into account when developing new catalysts. The production of high-quality biomass feedstock which has a lower AAEMs content by optimizing the means of pretreatment of biomass feedstock and adopting the optimal reaction conditions is also the direction of development. Moreover, the CO_x generated from the biomass feedstock has an inhibitory effect on desulfurization and denitrification capabilities of the HDT catalyst. Considering the fact that the sulfur content of diesel is restricted to be below 10 mg/L in most countries [180,181], the reduced HDS efficiency due to the blending of biomass feedstock must be minimized or eliminated to produce fuels that comply with the regulation. It is recommended to develop catalysts that can selectively deoxygenate biomass feedstock through HDO instead of decarboxylation or decarbonylation, to reduce the production of CO_x. Alternatively, it is also suggested to develop catalysts that are insensitive to CO_x in co-processing.

3) Micro-reactors, such as Parr reactors and micro-fixed bed reactors have been extensively used in previous studies. However, there are significant differences between those micro-reactors and the reactors used in commercial petroleum refineries, such as the feed delivery system, temperature profile, riser diameter, and CTO ratio. In detail, for FCC micro-reactors, the feedstock is introduced to the riser through a single line, the reactor interior is isothermal, reactions are at non-steady-state, and reaction times are typically 10–100 s. For FCC industrial reactors, the feedstock is introduced to the riser through separate pipelines, the temperature of the riser decreases gradually from the bottom to the top, reactions are at steady-state, the reaction time is usually 1 s, and the CTO ratio at the bottom of the riser is much higher than the average value. Moreover, FCC industrial reactors has a diameter of 0.8 m, much larger than the diameter of MAT reactor (14 mm) [41,182]. The knowledge gap between the micro-scale reactors and the industrial ones should be considered. When introducing biomass feedstock in FCC or HDT processes, the higher cost derived from premature catalyst deactivation, deterioration of product quality, and higher hydrogen consumption deserves thoughtful consideration.

References

[1] Sousa-AguiarE FXimenesV LAlmeidaJ M A R D, . Catalysts for co-processing biomass in oil refining industry. In: Frusteri F, Aranda D, Bonura G, eds. Sustainable Catalysis for Biorefineries. RSC Green Chemistry, 2018, 1–24

[2] Guilhot L. An analysis of China’s energy policy from 1981 to 2020: Transitioning towards to a diversified and low-carbon energy system. Energy Policy, 2022, 162: 112806

[3] Olabi A, Abdelkareem M A. Renewable energy and climate change. Renewable & Sustainable Energy Reviews, 2022, 158: 112111

[4] Liu Y, Yang X, Zhang J. . Process simulation of preparing biochar by biomass pyrolysis via Aspen plus and its economic evaluation. Waste and Biomass Valorization, 2022, 13(5): 2609–2622

[5] van Dyk S, Su J, Mcmillan J D. . Potential synergies of drop-in biofuel production with further co-processing at oil refineries. Biofuels, Bioproducts & Biorefining, 2019, 13(3): 760–775

[6] Jeswani H K, Chilvers A, Azapagic A. Environmental sustainability of biofuels: A review. Proceedings of the Royal Society of London. Series A, 2020, 476(2243): 20200351

[7] WoodwardS. Biofuels: A Solution for Climate Change. National Renewable Energy Laboratory, Golden, CO, US, 1999

[8] Paratzos S, Mcmillan J D, Saddler J N. The Potential and Challenges of Drop-in Biofuels. IEA Bioenergy Task 39 Report, 2014

[9] Yazdanparast R, Jolai F, Pishvaee M. . A resilient drop-in biofuel supply chain integrated with existing petroleum infrastructure: Toward more sustainable transport fuel solutions. Renewable Energy, 2022, 184: 799–819

[10] van Dyk S, Su J, McMillan J D, et al. Drop-in Biofuels: The Key Role that Co-processing Will Play in Its Production. IEA Bioenergy Task 39 Report, 2019

[11] Moon M, Park W K, Lee S Y. . Utilization of whole microalgal biomass for advanced biofuel and biorefinery applications. Renewable & Sustainable Energy Reviews, 2022, 160: 112269

[12] DasP. Pyrolytic bio-oil—Production and applications. In: Tuli D, Kasture S, Kuila A, eds. Advanced Biofuel Technologies. Elsevier, 2022, 243–304

[13] Bezergianni S, Dimitriadis A, Kikhtyanin O. . Refinery co-processing of renewable feeds. Progress in Energy and Combustion Science, 2018, 68: 29–64

[14] Bhatt A H, Zhang Y, Heath G. Bio-oil co-processing can substantially contribute to renewable fuel production potential and meet air quality standards. Applied Energy, 2020, 268: 114937

[15] Shahriar M F, Khanal A. The current techno-economic, environmental, policy status and perspectives of sustainable aviation fuel (SAF). Fuel, 2022, 325: 124905

[16] TalmadgeMJiangY JAskanderJ, . Strategies for Co-processing in Refineries: Techno-economic & Refinery Impact Analysis. National Renewable Energy Laboratory, Golden, CO, USA, 2020

[17] Han X, Wang H, Zeng Y. . Advancing the application of bio-oils by co-processing with petroleum intermediates: A review. Energy Conversion and Management: X, 2021, 10: 100069

[18] Andrade M C, Gorgulho Silva C D O, De Souza Moreira L R. . Crop residues: Applications of lignocellulosic biomass in the context of a biorefinery. Frontiers in Energy, 2021, 16(2): 224–245

[19] Karatzos S, Van Dyk J S, Mcmillan J D. . Drop-in biofuel production via conventional (lipid/fatty acid) and advanced (biomass) routes. Part I. Biofuels, Bioproducts & Biorefining, 2017, 11(2): 344–362

[20] van Dyk S, Su J, Ebadian M. . Production of lower carbon-intensity fuels by co-processing biogenic feedstocks: Potential and challenges for refineries. Fuel, 2022, 324: 124636

[21] Saravanan A, Senthil Kumar P S, Jeevanantham S. . Recent advances and sustainable development of biofuels production from lignocellulosic biomass. Bioresource Technology, 2022, 344: 126203

[22] Rodionova M V, Bozieva A M, Zharmukhamedov S K. . A comprehensive review on lignocellulosic biomass biorefinery for sustainable biofuel production. International Journal of Hydrogen Energy, 2022, 47(3): 1481–1498

[23] Jena U, Das K. Comparative evaluation of thermochemical liquefaction and pyrolysis for bio-oil production from microalgae. Energy & Fuels, 2011, 25(11): 5472–5482

[24] Seo M W, Lee S H, Nam H. . Recent advances of thermochemical conversion processes for biorefinery. Bioresource Technology, 2022, 343: 126109

[25] StöckerM. Perspectives for thermochemical conversions of lignocellulosic biomass. Small, 2023, early access, doi:10.1002/smll.202302495

[26] ChenGZhangRMaW, . Catalytic cracking of model compounds of bio-oil over HZSM-5 and the catalyst deactivation. Science of the Total Environment, 2018, 631–632: 1611–1622

[27] Liao H T, Ye X N, Lu Q. . Overview of bio-oil upgrading via catalytic cracking. Advanced Materials Research, 2013, 827: 25–29

[28] Lahijani P, Mohammadi M, Mohamed A R. . Upgrading biomass-derived pyrolysis bio-oil to bio-jet fuel through catalytic cracking and hydrodeoxygenation: A review of recent progress. Energy Conversion and Management, 2022, 268: 115956

[29] Zhang M, Hu Y, Wang H. . A review of bio-oil upgrading by catalytic hydrotreatment: Advances, challenges, and prospects. Molecular Catalysis, 2021, 504: 111438

[30] Stefanidis S D, Kalogiannis K G, Lappas A A. Co-processing bio-oil in the refinery for drop-in biofuels via fluid catalytic cracking. Wiley Interdisciplinary Reviews. Energy and Environment, 2018, 7(3): e281

[31] SpeightJ G. Handbook of Petroleum Refining. Boca Raton: CRC Press, 2016

[32] Zhang R, You Z, Ji J. . A review of characteristics of bio-oils and their utilization as additives of asphalts. Molecules, 2021, 26(16): 5049

[33] Jacobson K, Maheria K C, Kumar Dalai A K. Bio-oil valorization: A review. Renewable & Sustainable Energy Reviews, 2013, 23: 91–106

[34] Liu D, Li Z, Wu C. . Exploration of in-situ formed MoS_x catalyst for co-hydrodeoxygenation of sawdust and vacuum gas oil in pilot-scale plant. Applied Catalysis B: Environmental, 2021, 297: 120499

[35] Wu N, Niu Q, Pieters J. . Influence of torrefaction as pretreatment on the fast pyrolysis of sugarcane trash. Energy Conversion and Management, 2023, 291: 117291

[36] Kumar S, Chandra Srivastava V C, Nanoti S M. . Removal of refractory sulfur and aromatic compounds from straight run gas oil using solvent extraction. RSC Advances, 2014, 4(73): 38830–38838

[37] Mikulec J, Kleinová A, Cvengroš J. . Catalytic transformation of tall oil into biocomponent of diesel fuel. International Journal of Chemical Engineering, 2012, 2012: 215258

[38] Sano Y, Choi K H, Korai Y. . Adsorptive removal of sulfur and nitrogen species from a straight run gas oil over activated carbons for its deep hydrodesulfurization. Applied Catalysis B: Environmental, 2004, 49(4): 219–225

[39] Bielansky P, Weinert A, Schönberger C. . Catalytic conversion of vegetable oils in a continuous FCC pilot plant. Fuel Processing Technology, 2011, 92(12): 2305–2311

[40] Pinho A, de Almeida M B, Mendes F L. . Fast pyrolysis oil from pinewood chips co-processing with vacuum gas oil in an FCC unit for second generation fuel production. Fuel, 2017, 188: 462–473

[41] Naik D V, Kumar V, Prasad B. . Catalytic cracking of pyrolysis oil oxygenates (aliphatic and aromatic) with vacuum gas oil and their characterization. Chemical Engineering Research & Design, 2014, 92(8): 1579–1590

[42] Lindfors C, Paasikallio V, Kuoppala E. . Co-processing of dry bio-oil, catalytic pyrolysis oil, and hydrotreated bio-oil in a micro activity test unit. Energy & Fuels, 2015, 29(6): 3707–3714

[43] de Paz Carmona H, Vráblík A, Hidalgo Herrador J M. . Animal fats as a suitable feedstock for co-processing with atmospheric gas oil. Sustainable Energy & Fuels, 2021, 5(19): 4955–4964

[44] Shi Z, Zhao B, Tang S. . Hydrotreating lipids for aviation biofuels derived from extraction of wet and dry algae. Journal of Cleaner Production, 2018, 204: 906–915

[45] Udayan A, Pandey A K, Sirohi R. . Production of microalgae with high lipid content and their potential as sources of nutraceuticals. Phytochemistry Reviews, 2022, 22: 833–860

[46] Awogbemi O, Onuh E I, Inambao F L. Comparative study of properties and fatty acid composition of some neat vegetable oils and waste cooking oils. International Journal of Low Carbon Technologies, 2019, 14(3): 417–425

[47] Aniza R, Chen W H, Lin Y Y. . Independent parallel pyrolysis kinetics of extracted proteins and lipids as well as model carbohydrates in microalgae. Applied Energy, 2021, 300: 117372

[48] De Paz Carmona H, Horáček J, Tišler Z. . Sulfur free supported MoC_x and MoN_x catalysts for the hydrotreatment of atmospheric gasoil and its blends with rapeseed oil. Fuel, 2019, 254: 115582

[49] de Paz Carmona H, de la Torre Alfaro O, Brito Alayon A. . Co-processing of straight run gas oil with used cooking oil and animal fats. Fuel, 2019, 254: 115583

[50] Hidalgo J M, Horaček J, Matoušek L. . Catalytic hydrocracking of vacuum residue and waste cooking oil mixtures. Monatshefte für Chemie, 2018, 149(6): 1167–1177

[51] RashidUHazmiB. Advances in production of biodiesel from vegetable oils and animal fats. In: Lalthazuala Rokhum S, Halder G, Assabumrungrat S, eds. Biodiesel Production: Feedstocks, Catalysts, and Technologies. John Wiley & Sons Ltd., 2022

[52] Andari F, Kittel J, Fernandes J. . High temperature corrosion in various grades of vegetable and waste oils used for bio-fuel production. Corrosion Science, 2022, 206: 110501

[53] ZhangFYiJPanW, . A new process for the production of second-generation biodiesel from waste oils and fats. Biomass Conversion and Biorefinery, 2022, early access, doi:10.1007/s13399-022-03149-x

[54] Tian C, Li B, Liu Z. . Hydrothermal liquefaction for algal biorefinery: A critical review. Renewable & Sustainable Energy Reviews, 2014, 38: 933–950

[55] Cerqueira H, Caeiro G, Costa L. . Deactivation of FCC catalysts. Journal of Molecular Catalysis A Chemical, 2008, 292(1–2): 1–13

[56] Sharma N, Jaiswal K K, Kumar V. . Effect of catalyst and temperature on the quality and productivity of HTL bio-oil from microalgae: A review. Renewable Energy, 2021, 174: 810–822

[57] Zou S, Wu Y, Yang g M. . Production and characterization of bio-oil from hydrothermal liquefaction of microalgae Dunaliella tertiolecta cake. Energy, 2010, 35(12): 5406–5411

[58] Xiu S, Shahbazi A. Bio-oil production and upgrading research: A review. Renewable & Sustainable Energy Reviews, 2012, 16(7): 4406–4414

[59] ChengFBrewerC E. Best practices for bio-crude oil production at pilot scale using continuous flow reactors. In: Jacob-Lopes E, Aguiar Severo I, Queiroz Zepka L, eds. 3rd Generation Biofuels. Elsevier, 2022, 1061–119

[60] Park H, Cruz D, Tiller P. . Effect of ash in paper sludge on enzymatic hydrolysis. Biomass and Bioenergy, 2022, 165: 106567

[61] Zhang Q, Chang J, Wang T. . Review of biomass pyrolysis oil properties and upgrading research. Energy Conversion and Management, 2007, 48(1): 87–92

[62] Chang S H. Bio-oil derived from palm empty fruit bunches: Fast pyrolysis, liquefaction and future prospects. Biomass and Bioenergy, 2018, 119: 263–276

[63] Lange J P. Renewable feedstocks: The problem of catalyst deactivation and its mitigation. Angewandte Chemie International Edition, 2015, 54(45): 13186–13197

[64] Dabros T M, Stummann M Z, Høj M. . Transportation fuels from biomass fast pyrolysis, catalytic hydrodeoxygenation, and catalytic fast hydropyrolysis. Progress in Energy and Combustion Science, 2018, 68: 268–309

[65] Alvarez-Chavez B J, Godbout S, Palacios-Rios J H. . Physical, chemical, thermal and biological pre-treatment technologies in fast pyrolysis to maximize bio-oil quality: A critical review. Biomass and Bioenergy, 2019, 128: 105333

[66] Zhang L, Liu R, Yin R. . Upgrading of bio-oil from biomass fast pyrolysis in China: A review. Renewable & Sustainable Energy Reviews, 2013, 24: 66–72

[67] Pietraccini M, Badu P, Tait T. . Study of flash pyrolysis and combustion of biomass powders using the Godbert-Greenwald furnace: An essential step to better understand organic dust explosions. Process Safety and Environmental Protection, 2023, 169: 458–471

[68] Sekar M, Mathimani T, Alagumalai A. . A review on the pyrolysis of algal biomass for biochar and bio-oil — Bottlenecks and scope. Fuel, 2021, 283: 119190

[69] Attia M, Farag S, Chaouki J. Upgrading of oils from biomass and waste: Catalytic hydrodeoxygenation. Catalysts, 2020, 10(12): 1381

[70] Zacher A H, Olarte M V, Santosa D M. . A review and perspective of recent bio-oil hydrotreating research. Green Chemistry, 2014, 16(2): 491–515

[71] Zhang L, Gong K, Lai J. . Chemical composition and stability of renewable hydrocarbon products generated from a hydropyrolysis vapor upgrading process. Green Chemistry, 2017, 19(15): 3628–3641

[72] Stummann M, Høj M, Schandel C B. . Hydrogen assisted catalytic biomass pyrolysis. Effect of temperature and pressure. Biomass and Bioenergy, 2018, 115: 97–107

[73] Zhang X, Wang T, Ma L. . Hydrotreatment of bio-oil over Ni-based catalyst. Bioresource Technology, 2013, 127: 306–311

[74] Mortensen P M, Grunwaldt J D, Jensen P A. . A review of catalytic upgrading of bio-oil to engine fuels. Applied Catalysis A, General, 2011, 407(1–2): 1–19

[75] Ali H, Kansal S K, Saravanamurugan S. Upgradation of bio-oil derived from various biomass feedstocks via hydrodeoxygenation. In: Li H, Saravanamurugan S, Pandey A, eds. Biomass, Biofuels, Biochemicals. Elsevier, 2022, 287−308

[76] Zhang C, Zhang Z C. Essential quality attributes of tangible bio-oils from catalytic pyrolysis of lignocellulosic biomass. Chemical Record, 2019, 19(9): 2044–2057

[77] Eschenbacher A, Myrstad T, Bech N. . Co-processing of wood and wheat straw derived pyrolysis oils with FCC feed—Product distribution and effect of deoxygenation. Fuel, 2020, 260: 116312

[78] Chen X, Chen Y, Yang H. . Catalytic fast pyrolysis of biomass: selective deoxygenation to balance the quality and yield of bio-oil. Bioresource Technology, 2019, 273: 153–158

[79] Zhang C, Zhang Z C. Essential quality attributes of tangible bio-oils from catalytic pyrolysis of lignocellulosic biomass. Chemical Record, 2019, 19(9): 2044–2057

[80] Santosa D M, Kutnyakov I, Flake M. . Coprocessing biomass fast pyrolysis and catalytic fast pyrolysis oils with vacuum gas oil in refinery hydroprocessing. Energy & Fuels, 2022, 36(20): 12641–12650

[81] Dabros T M H, Stummann M Z, Høj M. . Transportation fuels from biomass fast pyrolysis, catalytic hydrodeoxygenation, and catalytic fast hydropyrolysis. Progress in Energy and Combustion Science, 2018, 68: 268–309

[82] de Mello L F, Gobbo R, Moure G T. . Oxy-combustion technology development for Fluid Catalytic Crackers (FCC)–large pilot scale demonstration. Energy Procedia, 2013, 37: 7815–7824

[83] SpeightJ G. The Refinery of the Future. Massachusetts: Gulf Professional Publishing, 2020

[84] Jones D S. Pujadó P P. Handbook of Petroleum Processing. Dordrecht: Springer Science & Business Media, 2006

[85] SadeghbeigiR. Fluid Catalytic Cracking Handbook: An Expert Guide to the Practical Operation, Design, and Optimization of FCC Units. Oxford: Butterworth-Heinemann, 2020

[86] Huynh T M, Armbruster U, Atia H. . Upgrading of bio-oil and subsequent co-processing under FCC conditions for fuel production. Reaction Chemistry & Engineering, 2016, 1(2): 239–251

[87] Thegarid N, Fogassy G, Schuurman Y. . Second-generation biofuels by co-processing catalytic pyrolysis oil in FCC units. Applied Catalysis B: Environmental, 2014, 145: 161–166

[88] Seiser R, Olstad J L, Magrini K A. . Co-processing catalytic fast pyrolysis oil in an FCC reactor. Biomass and Bioenergy, 2022, 163: 106484

[89] Corma A, Huber G W, Sauvanaud L. . Processing biomass-derived oxygenates in the oil refinery: Catalytic cracking (FCC) reaction pathways and role of catalyst. Journal of Catalysis, 2007, 247(2): 307–327

[90] Huber G W, Chheda J N, Barrett C J. . Production of liquid alkanes by aqueous-phase processing of biomass-derived carbohydrates. Science, 2005, 308(5727): 1446–1450

[91] Ragauskas A J, Williams C K, Davison B H. . The path forward for biofuels and biomaterials. Science, 2006, 311(5760): 484–489

[92] de Rezende Pinho A, De Almeida M B B, Mendes F L. . Production of lignocellulosic gasoline using fast pyrolysis of biomass and a conventional refining scheme. Pure and Applied Chemistry, 2014, 86(5): 859–865

[93] BrydenKWeatherbeeGHabibE T Jr. FCC pilot plant results with vegetable oil and pyrolysis oil feeds. In: Biomass 2013, Washington, DC. Washington, DC: U.S. Department of Energy, 2013, 1--17

[94] Su J, Cao L, Lee G. . Determining the amount of ‘green’coke generated when co-processing lipids commercially by fluid catalytic cracking. Biofuels, Bioproducts & Biorefining, 2022, 16(2): 325–334

[95] Bielansky P, Reichhold A, Schönberger C. Catalytic cracking of rapeseed oil to high octane gasoline and olefins. Chemical Engineering and Processing, 2010, 49(8): 873–880

[96] Dupain X, Costa D J, Schaverien C J. . Cracking of a rapeseed vegetable oil under realistic FCC conditions. Applied Catalysis B: Environmental, 2007, 72(1–2): 44–61

[97] Yarlagadda P S, Hu Y, Bakhshi N N. Effect of hydrothermal treatment of HZSM-5 catalyst on its performance for the conversion of canola and mustard oils to hydrocarbons. Industrial & Engineering Chemistry Product Research and Development, 1986, 25(2): 251–257

[98] Melero J A, Clavero M M, Calleja G. . Production of biofuels via the catalytic cracking of mixtures of crude vegetable oils and nonedible animal fats with vacuum gas oil. Energy & Fuels, 2010, 24(1): 707–717

[99] Santillan-Jimenez E, Pace R, Morgan T. . Co-processing of hydrothermal liquefaction algal bio-oil and petroleum feedstock to fuel-like hydrocarbons via fluid catalytic cracking. Fuel Processing Technology, 2019, 188: 164–171

[100] Melin K, Strüven J O, Eidam P. . Hybrid gasoline production from black liquor through coprocessing. Energy & Fuels, 2022, 36(19): 12004–12009

[101] Zhang Y, Alvarez-Majmutov A. Production of renewable liquid fuels by coprocessing HTL biocrude using hydrotreating and fluid catalytic cracking. Energy & Fuels, 2021, 35(23): 19535–19542

[102] BrydenKWeatherbeeGHabibE T. Flexible pilot plant technology for evaluation of unconventional feedstocks and processes. 2023–11-29, available at website of Grace

[103] ZacherA. Optimizing Co-processing of Bio-oil in Refinery Unit Operations Using a Davison Circulating Riser (DCR). Pacific Northwest National Laboratory Report. 2015

[104] Vogt E T, Weckhuysen B M. Fluid catalytic cracking: Recent developments on the grand old lady of zeolite catalysis. Chemical Society Reviews, 2015, 44(20): 7342–7370

[105] Pinho A, de Almeida M B, Mendes F L. . Co-processing raw bio-oil and gasoil in an FCC unit. Fuel Processing Technology, 2015, 131: 159–166

[106] Lindfors C, Elliott D C, Prins W. . Co-processing of biocrudes in oil refineries. Energy & Fuels, 2022, 37(2): 799–804

[107] Chen X, Che Q, Li S. . Recent developments in lignocellulosic biomass catalytic fast pyrolysis: Strategies for the optimization of bio-oil quality and yield. Fuel Processing Technology, 2019, 196: 106180

[108] Iisa K, French R J, Orton K A. . Production of low-oxygen bio-oil via ex situ catalytic fast pyrolysis and hydrotreating. Fuel, 2017, 207: 413–422

[109] Tran Q K, Le M L, Ly H V. . Fast pyrolysis of pitch pine biomass in a bubbling fluidized-bed reactor for bio-oil production. Journal of Industrial and Engineering Chemistry, 2021, 98: 168–179

[110] Dayton D C, Hlebak J, Carpenter J R. . Biomass hydropyrolysis in a fluidized bed reactor. Energy & Fuels, 2016, 30(6): 4879–4887

[111] Wang K, Dayton D C, Peters J E. . Reactive catalytic fast pyrolysis of biomass to produce high-quality bio-crude. Green Chemistry, 2017, 19(14): 3243–3251

[112] Arbogast S, Bellman D, Paynter D, et al. Commercialization of pyrolysis oil in existing refineries—Part 1. 2017-1-11, available at website of Hydrocarbon Processing

[113] Samolada M, Baldauf W, Vasalos I. Production of a bio-gasoline by upgrading biomass flash pyrolysis liquids via hydrogen processing and catalytic cracking. Fuel, 1998, 77(14): 1667–1675

[114] Gueudré L, Chapon F, Mirodatos C. . Optimizing the bio-gasoline quantity and quality in fluid catalytic cracking co-refining. Fuel, 2017, 192: 60–70

[115] Lutz H, Büchele M, Knaus F. . Wood derived fast pyrolysis bio-liquids as co-feed in a fluid catalytic cracking pilot plant: Effect of hydrotreatment on process performance and gasoline quality. Energy & Fuels, 2022, 36(17): 10243–10250

[116] de Miguel Mercader F, Groeneveld M, Kersten S. . Production of advanced biofuels: Co-processing of upgraded pyrolysis oil in standard refinery units. Applied Catalysis B: Environmental, 2010, 96(1–2): 57–66

[117] de Miguel Mercader F, Groeneveld M, Kersten S R A. . Hydrodeoxygenation of pyrolysis oil fractions: process understanding and quality assessment through co-processing in refinery units. Energy & Environmental Science, 2011, 4(3): 985 − 997

[118] Baldauf W, Balfanz U, Rupp M. Upgrading of flash pyrolysis oil and utilization in refineries. Biomass and Bioenergy, 1994, 7(1–6): 237–244

[119] Fogassy G, Thegarid N, Toussaint G. . Biomass derived feedstock co-processing with vacuum gas oil for second-generation fuel production in FCC units. Applied Catalysis B: Environmental, 2010, 96(3–4): 476–485

[120] Adjaye J D, Bakhshi N N. Production of hydrocarbons by catalytic upgrading of a fast pyrolysis bio-oil. Part I: Conversion over various catalysts. Fuel Processing Technology, 1995, 45(3): 161–183

[121] Lappas A, Bezergianni S, Vasalos I. Production of biofuels via co-processing in conventional refining processes. Catalysis Today, 2009, 145(1–2): 55–62

[122] Wang C, Li M, Fang Y. Coprocessing of catalytic-pyrolysis-derived bio-oil with VGO in a pilot-scale FCC riser. Industrial & Engineering Chemistry Research, 2016, 55(12): 3525–3534

[123] Agblevor F A, Mante O, McClung R. . Co-processing of standard gas oil and biocrude oil to hydrocarbon fuels. Biomass and Bioenergy, 2012, 45: 130–137

[124] Talmadge M S, Baldwin R M, Biddy M J. . A perspective on oxygenated species in the refinery integration of pyrolysis oil. Green Chemistry, 2014, 16(2): 407–453

[125] YangW C. Handbook of Fluidization and Fluid-particle Systems. New York: CRC Press, 2003

[126] Komvokis V, Tan L X L, Clough M, et al. Zeolites in fluid catalytic cracking (FCC). In: Xiao F, Meng X, eds. Zeolites in Sustainable Chemistry: Synthesis, Characterization and Catalytic Applications. Springer, 2016, 271–297

[127] Schuurman Y, Fogassy G, Mirodatos C. Tomorrow’s biofuels: Hybrid biogasoline by co-processing in FCC units. In: Triantafyllidis K S, Lappas A A, Stöcker M, eds. The Role of Catalysis for the Sustainable Production of Bio-fuels and Bio-chemicals. Elsevier, 2013, 321–349

[128] Ma Z, Wei L, Zhou W. . Overview of catalyst application in petroleum refinery for biomass catalytic pyrolysis and bio-oil upgrading. RSC Advances, 2015, 5(107): 88287–88297

[129] CormaAOrchillésA V. Current views on the mechanism of catalytic cracking. Microporous and Mesoporous Materials, 2000, 35–36: 21–30

[130] Vermeiren W, Gilson J P. Impact of zeolites on the petroleum and petrochemical industry. Topics in Catalysis, 2009, 52(9): 1131–1161

[131] Jae J, Tompsett G A, Lin Y C. . Depolymerization of lignocellulosic biomass to fuel precursors: Maximizing carbon efficiency by combining hydrolysis with pyrolysis. Energy & Environmental Science, 2010, 3(3): 358–365

[132] Ferella F, D’Adamo I, Leone S. . Spent FCC E-CAT: towards a circular approach in the oil refining industry. Sustainability, 2018, 11(1): 113

[133] Ferreira J M M, Sousa-Aguiar E F, Aranda D A G. FCC catalyst accessibility—A review. Catalysts, 2023, 13(4): 784

[134] Ihli J, Jacob R R, Holler M. . A three-dimensional view of structural changes caused by deactivation of fluid catalytic cracking catalysts. Nature Communications, 2017, 8(1): 809

[135] Fogassy G, Thegarid N, Schuurman Y. . From biomass to bio-gasoline by FCC co-processing: Effect of feed composition and catalyst structure on product quality. Energy & Environmental Science, 2011, 4(12): 5068–5076

[136] Huuska M, Rintala J. Effect of catalyst acidity on the hydrogenolysis of anisole. Journal of Catalysis, 1985, 94(1): 230–238

[137] Fogassy G, Thegarid N, Schuurman Y. . The fate of bio-carbon in FCC co-processing products. Green Chemistry, 2012, 14(5): 1367–1371

[138] Graça I, Comparot J D, Laforge S. . Effect of phenol addition on the performances of H–Y zeolite during methylcyclohexane transformation. Applied Catalysis A, General, 2009, 353(1): 123–129

[139] Graça I, Ribeiro F R, Cerqueira H. . Catalytic cracking of mixtures of model bio-oil compounds and gasoil. Applied Catalysis B: Environmental, 2009, 90(3–4): 556–563

[140] Gerards R, Fernandes A, Graça I. . Towards understanding of phenolic compounds impact on Ni- and V-USY zeolites during bio-oils co-processing in FCC units. Fuel, 2020, 260: 116372

[141] Akah A. Application of rare earths in fluid catalytic cracking: A review. Journal of Rare Earths, 2017, 35(10): 941–956

[142] Clough M, Pope J C, Lin L T X. . Nanoporous materials forge a path forward to enable sustainable growth: Technology advancements in fluid catalytic cracking. Microporous and Mesoporous Materials, 2017, 254: 45–58

[143] Zacher A H, Olarte M V, Santosa D M. . A review and perspective of recent bio-oil hydrotreating research. Green Chemistry, 2014, 16(2): 491–515

[144] Elliott D C. Water, alkali and char in flash pyrolysis oils. Biomass and Bioenergy, 1994, 7(1–6): 179–185

[145] Javaid A, Ryan T, Berg G. . Removal of char particles from fast pyrolysis bio-oil by microfiltration. Journal of Membrane Science, 2010, 363(1–2): 120–127

[146] Hoekstra E, Hogendoorn K J, Wang X. . Fast pyrolysis of biomass in a fluidized bed reactor: in situ filtering of the vapors. Industrial & Engineering Chemistry Research, 2009, 48(10): 4744–4756

[147] Zhong D, Chang Z, Zeng K. . Solar pyrolysis of biomass—Part II: The physicochemical structure evolution of char. Fuel, 2023, 333: 126474

[148] Baldwin R M, Feik C J. Bio-oil stabilization and upgrading by hot gas filtration. Energy & Fuels, 2013, 27(6): 3224–3238

[149] Su J, Van Dyk S, Saddler J. Repurposing oil refineries to “stand-alone units” that refine lipids/oleochemicals to produce low-carbon intensive, drop-in biofuels. Journal of Cleaner Production, 2022, 376: 134335

[150] Chen S. Green oil production by hydroprocessing. International Journal of Clean Coal Energy, 2012, 1(4): 43–55

[151] Xu J, Brodu N, Abdelouahed L. . Investigation of the combination of fractional condensation and water extraction for improving the storage stability of pyrolysis bio-oil. Fuel, 2022, 314: 123019

[152] Tóth C, Sági D, Hancsók J. Diesel fuel production by catalytic hydrogenation of light cycle oil and waste cooking oil containing gas oil. Topics in Catalysis, 2015, 58(14–17): 948–960

[153] Bezergianni S, Dimitriadis A, Karonis D. Diesel decarbonization via effective catalytic co-hydroprocessing of residual lipids with gas–oil. Fuel, 2014, 136: 366–373

[154] Kubička D, Tukač V. Hydrotreating of triglyceride-based feedstocks in refineries. In: Murzin D Y, ed. Advances in Chemical Engineering. Elsevier, 2013, 141–194

[155] Sánchez O J, Cardona C A. Trends in biotechnological production of fuel ethanol from different feedstocks. Bioresource Technology, 2008, 99(13): 5270–5295

[156] Wang J, Singer S D, Souto B A. . Current progress in lipid-based biofuels: Feedstocks and production technologies. Bioresource Technology, 2022, 351: 127020

[157] Guilhaume N, Schuurman Y, Geantet C. The role of catalysis in the valorization of woody biomass fast pyrolysis liquids: Overview and contribution of IRCELYON. Catalysis Today, 2021, 373: 5–23

[158] Chen S Y, Nishi M, Mochizuki T. . Co-processing of jatropha-derived bio-oil with petroleum distillates over mesoporous CoMo and NiMo sulfide catalysts. Catalysts, 2018, 8(2): 59

[159] de Santos Brandão J G, Manmohandas H V. A simplified kinetic model for continuous hydrotreating of HTL biocrude. Thesis for the Master’s Degree. Aalborg: Aalborg University, 2020 (in Denmark)

[160] Bui V N, Toussaint G, Laurenti D. . Co-processing of pyrolisis bio oils and gas oil for new generation of bio-fuels: Hydrodeoxygenation of guaïacol and SRGO mixed feed. Catalysis Today, 2009, 143(2): 172–178

[161] Pinheiro A, Hudebine D, Dupassieux N. . Impact of oxygenated compounds from lignocellulosic biomass pyrolysis oils on gas oil hydrotreatment. Energy & Fuels, 2009, 23(1–2): 1007–1014

[162] Zhu C, Gutiérrez O Y, Santosa D M. . Kinetics of nitrogen-, oxygen- and sulfur-containing compounds hydrotreating during co-processing of bio-crude with petroleum stream. Applied Catalysis B: Environmental, 2022, 307: 121197

[163] Sauvanaud L, Mathieu Y, Corma A. . Co-processing of lignocellulosic biocrude with petroleum gas oils. Applied Catalysis A, General, 2018, 551: 139–145

[164] Baker E G, Elliott D C. Catalytic hydrotreating of biomass-derived oils. In: Soltes Ed J, Milne T A, eds. Pyrolysis Oils from Biomass. ACS Publications. 1988

[165] Xing T, Alvarez-Majmutov A, Gieleciak R. . Co-hydroprocessing HTL biocrude from waste biomass with bitumen-derived vacuum gas oil. Energy & Fuels, 2019, 33(11): 11135–11144

[166] Badoga S, Alvarez-Majmutov A, Chen J. Mild hydrotreatment of biocrude derived from hydrothermal liquefaction of agriculture waste: Improving biocrude miscibility with vacuum gas oil to aid co-processing. Biofuels, Bioproducts & Biorefining, 2022, 16(3): 785–798

[167] Pinheiro A, Hudebine D, Dupassieux N. . Membrane fractionation of biomass fast pyrolysis oil and impact of its presence on a petroleum gas oil hydrotreatment. Oil & Gas Science and Technology–Revue d'IFP Energies nouvelles, 2013, 68(5): 815–828

[168] Chen W, Cao J, Fu W. . Molecular-level insights into the notorious CO poisoning of platinum catalyst. Angewandte Chemie International Edition, 2022, 61(16): e202200190

[169] Pinheiro A, Dupassieux N, Hudebine D. . Impact of the presence of carbon monoxide and carbon dioxide on gas oil hydrotreatment: Investigation on liquids from biomass cotreatment with petroleum cuts. Energy & Fuels, 2011, 25(2): 804–812

[170] PhilippeMRichardFHudebineD, . Transformation of dibenzothiophenes model molecules over CoMoP/Al₂O₃ catalyst in the presence of oxygenated compounds. Applied Catalysis B: Environmental, 2013, 132–133: 493–498

[171] Bezergianni S, Dagonikou V. Effect of CO₂ on catalytic hydrotreatment of gas−oil. Canadian Journal of Chemical Engineering, 2015, 93(6): 1017–1023

[172] Dimitriadis A, Meletidis G, Pfisterer U. . Integration of stabilized bio-oil in light cycle oil hydrotreatment unit targeting hybrid fuels. Fuel Processing Technology, 2022, 230: 107220

[173] Borugadda V B, Chand R, Dalai A K. Screening suitable refinery distillates for blending with HTL bio-crude and evaluating the co-processing potential at petroleum refineries. Energy Conversion and Management, 2020, 222: 113186

[174] Sánchez-Anaya O, Mederos-Nieto F S, Elizalde I. . Producing hybrid fuels by hydrotreating Jatropha curcas L. and gasoil mixtures in a batch reactor. Journal of the Taiwan Institute of Chemical Engineers, 2021, 128: 140–147

[175] Sági D, Baladincz P, Varga Z. . Co-processing of FCC light cycle oil and waste animal fats with straight run gas oil fraction. Journal of Cleaner Production, 2016, 111: 34–41

[176] Mortensen P M, Grunwaldt J D, Jensen P A. . A review of catalytic upgrading of bio-oil to engine fuels. Applied Catalysis A, General, 2011, 407(1–2): 1–19

[177] Nunes V O, Fraga A C, Silva R V S. . Chemical characterisation of sugarcane bagasse bio-oils from hydrothermal liquefaction: Effect of reaction conditions on products distribution and composition. Journal of Environmental Chemical Engineering, 2021, 9(6): 106513

[178] Li T, Su J, Wang H. . Catalytic hydropyrolysis of lignin using NiMo-doped catalysts: Catalyst evaluation and mechanism analysis. Applied Energy, 2022, 316: 119115

[179] Egeberg R, Knudsen K, Nyström S. . Industrial-scale production of renewable diesel. Petroleum Technology Quarterly, 2011, 16(4): 59–65

[180] de Paz Carmona H, Svobodova E, Tišler Z K. . Hydrotreating of atmospheric gas oil and co-processing with rapeseed oil using sulfur-free PMoC_x/Al₂O₃ catalysts. ACS Omega, 2021, 6(11): 7680–7692

[181] Pan L, Liu P, Li Z. A discussion on China’s vehicle fuel policy: Based on the development route optimization of refining industry. Energy Policy, 2018, 114: 403–412

[182] Williams M, Minjares R. A Technical Summary of Euro 6/VI Vehicle Emission Standards. The International Council on Clean Transportation (ICCT) Report, 2016

[183] Wu Y, Peng L, Qin L. . Validation and application of CPFD models in simulating hydrodynamics and reactions in riser reactor with Geldart A particles. Powder Technology, 2018, 323: 269–83

RIGHTS & PERMISSIONS

Higher Education Press