Catalytic effect of K and Na with different anions on lignocellulosic biomass pyrolysis

Abstract

Alkali metals (AMs) play an important role in biomass pyrolysis, and it is important to explore their catalytic effects so to better utilize biomass pyrolysis. This study analyzed the catalytic influence of K and Na with different anions (Cl^–, SO₄^2–, and CO₃^2–) on biomass pyrolysis, and explored the influence on the pyrolytic mechanism. AM chlorides (NaCl and KCl), sulfates (Na₂SO₄ and K₂SO₄) and carbonates (Na₂CO₃ and K₂CO₃) were mixed with cellulose and bamboo feedstocks at a mass ratio of 20 wt %, in order to maximize their potential on in situ upgrading of the pyrolysis products. AM chlorides had little effect on the pyrolysis products, whereas sulfates slightly promoted the yields of char and gas, and had a positive effect on the composition of the gaseous and liquid products. Carbonates noticeably increased the yields of the char and gases, and improved the C content of the char. Besides, AM salt catalysis is an effective method for co-production of bio-oil and porous char.

Keywords

alkali metal salt / cellulose / biomass / catalytic pyrolysis

1 Introduction

Biomass is the only carbon-containing renewable energy source with abundant reserves, and its effective utilization can play a vital role in the realization of global carbon-neutral materials to combat climate change. Biomass pyrolysis can yield gas, liquid and solid energy products [1,2], which is a promising biomass utilization technology. However, the pyrolytic process of biomass is very complicated because of its complex composition and inherent inorganic elements. Woody biomass is mainly composed of cellulose, hemicellulose and lignin. And cellulose is typically used as a model compound to study the pyrolytic characteristics of biomass [3,4]. Besides, it is worth noting that alkalis and alkali metals are essential elements in the growth of biomass, and affect the biomass pyrolysis process and the distribution of products [5,6]. Most biomass has a higher content of K and Na (alkali metals, AMs) than that of alkaline earth metals [7,8], and AMs exhibit a regulatory effect on the composition of bio-oil and the pore structure of char [9,10]. Therefore, in order to improve the efficiency of biomass utilization, it is necessary to understand the effects of AMs on the pyrolytic process of woody biomass.

Some researchers have studied the effects of K and Na on the pyrolysis characteristics of biomass [11−13]. Leng et al. [14] found that a low KCl content could obviously promote gas formation at the expense of bio-oil. Giudicianni et al. [7] reported that AMs catalyze the bond cleavage of macromolecules and increase char yield. Li et al. [15] investigated the catalytic effect of AMs during rice husk pyrolysis, and found that AMs could promote the yields of aldehydes, ketones, acids, and phenolics by enhancing the ring-fission reaction in the sugar units and ether bond fracturing in lignin. Zhu et al. [16] found that K was beneficial for increasing pyrolysis reactivity and promoting the formation of CO₂ and aldehydes. Sun et al. [17] reported that K promotes the dehydrogenation of tar during reforming. However, previous studies generally evaluated the effect of low AM content occurring naturally in biomass (< 2 wt %). Considering the non-negligible effect of inorganic ash on pyrolytic products, it would be interesting to try doping AMs at a high ratio, which might be an efficient and cost-effective way to improve biomass pyrolysis. Hence, it is necessary to study the catalytic effect of high AM content on pyrolysis products.

In addition, it was reported that the anions of metal salts play a significant role in the pyrolysis characteristics of biomass [18]. Chen et al. [19] found that alkaline additives could react with highly active O-containing species and carbon fragments to generate many vacancies, which can quickly be occupied by the anions of alkaline additives to form new O-containing groups in biochar. Fu et al. [20] reported that the synergistic effect of K⁺ and HCO₃^– was more effective for enhancing bio-oil yield and decreasing levoglucosan yield compared to CO₃^2–, OH^– and Cl^–. Shen et al. [21] found that K salts (KOH, K₂CO₃, and K₂C₂O₄), especially KOH, could decrease the decomposition temperature. However, most studies focused on understanding the effects of AM chlorides and carbonates on pyrolysis. The effects of AMs with different anions on the catalytic pyrolysis and product distribution of biomass require further study.

Therefore, in this study, we investigated the catalytic effects of common AM salts on the pyrolytic characteristics of woody biomass. As the most widely distributed and abundant organic component in nature, cellulose has been chosen as a model compound to study the pyrolysis characteristics of biomass. Considering the network of interlinked bonds between cellulose, hemicellulose and lignin, which may affect obviously the pyrolytic characteristics of biomass [7,22], low-ash bamboo was also chosen as a test material. Compared to the main trends of pyrolysis products from bamboo and cellulose, the effect of component connections in the pyrolysis products was studied, and the catalytic mechanisms of K and Na in biomass pyrolysis were further explored.

2 Experimental

2.1 Materials

Bamboo samples were collected from Chibi City, Hubei Province, and pure cellulose was purchased from Sigma-Aldrich. The ultimate and proximate analyses of the samples were conducted using a Vario EL II Elemental Analyzer (Germany) and a Navas SDTGA-2000 Analyzer (Spain), respectively. The detailed information of cellulose and bamboo was provided in a previous study [23]. Bamboo is mainly composed of C (49.4 wt %) and O (43.8 wt %), and has high volatile matter (85.4 wt %) and little ash (0.7 wt %). And cellulose has a higher O content (50.1 wt %) and H content (6.2 wt %), but lower C content (43.7 wt %), along with higher volatile matter (95.5 wt %) and negligible ash. All results are reported on a dry mass basis.

The AM salts chosen in the study included KCl, NaCl, K₂SO₄, Na₂SO₄, K₂CO₃ and Na₂CO₃ (purity > 99.7%), which were purchased from Sinopharm Chemical Reagent Co. Ltd., China.

2.2 Pyrolysis procedure

As described in our previous studies, pyrolytic experiments were performed in a fixed-bed reactor [24]. The system consists of a quartz reactor (diameter: 30 mm; length: 400 mm) with the front end connected to the carrier gas inlet. The pyrolysis vapor outlet on the back end is connected to liquid condensation and gas collection devices. Heating was provided by a Carbolite electric furnace (UK) with precise temperature control. To explore the maximum potential of the AM salts for biomass pyrolysis, the mass ratio of the salts in the mixture was set to 20 wt %. Before the test, 1.6 g of biomass sample and 0.4 g of salts were blended and ground in a ceramic mortar until the samples were thoroughly mixed. For each run, 2 g of prepared mixed sample were placed in a crucible (diameter: 28 mm; length: 50 mm), held at the top of the quartz reactor. Before the experiment, a flow of 500 mL·min^–1 of N₂ (> 99.999%) was introduced into the reactor to displace any air present. When the reactor reached a temperature of 550 °C, the purged N₂ flow was reduced to 100 mL·min^–1. The crucible was quickly inserted into a constant-temperature zone and maintained for 30 min for pyrolysis. The released volatiles were sent to a downstream condenser using ice water to collect the pyrolysis oil, and the gaseous products were collected and stored in a gasbag. After each run, acetone was used to clean the condenser and collect the bio-oil. The liquid yield was calculated using the mass difference before and after cleaning the condenser. Each run was repeated three times, and the yields of the three products were calculated as the average of the results of the three experiments. To avoid moisture absorption during sample storage, pyrolysis experiments were performed immediately after preparation of the mixed samples. A “blank” sample with mixed biomass and SiO₂ was employed to evaluate the experimental errors.

2.3 Product characterization

The analytical procedure for gas, bio-oil and char production was consistent with that of our previous work. The details have been described in a previous publication [23]. Briefly, the gas composition was analyzed using a Panna A91 GC (China) equipped with a thermal conductivity detector and a hydrogen flame ionization detector. The bio-oil composition was analyzed using an Agilent HP7890 GCMS coupled with an HP5975 MS detector and an HP-5MD capillary column. The water content of bio-oil was measured by a TitroLine KF-10 Karl-Fischer titrator (Germany) in accordance with the ASTM D 1744 standard. Char characterization and analysis included ultimate analysis (Vario MICRO cube, Elementar, Germany), X-ray diffraction (XRD, PANalytical B.V, X’Pert PRO, Netherlands) to determine the crystal structure, and Fourier-transform infrared spectroscopy (FTIR, VERTEX 70, Bruker, Germany) to analyze the surface functional groups. Further details on the char analysis, including the procedure and parameters, can be found in other publications [23].

3 Results and discussion

3.1 Effect of varying K₂CO₃ content

Fig.1(a) shows the product yields obtained from the pyrolysis of cellulose using various amounts of K₂CO₃. Compared to the calculated value, with a small amount of K₂CO₃ addition (2% K₂CO₃), there is a significant change in the product yields. It is found that K₂CO₃ addition results in a decrease in the liquid product yield with increasing gaseous and solid product yields, particularly for gaseous products. It is possibly because K₂CO₃ addition mainly promotes the secondary cracking and polycondensation of volatile. Besides, with the increase of K₂CO₃ addition, the inhibiting effect on liquid yield shows no significant change, but the promoting effect on gas yield increases slightly, while the promoting effect on char is slightly weakened.

Fig.1 (a) Product distribution, (b) gas composition, and (c) the compositions of liquid products for cellulose pyrolysis with K2CO3 addition.

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Fig.1(b) shows the composition of the gaseous products obtained from the pyrolysis of cellulose with various amounts of K₂CO₃. Compared with the calculated value, it is found that at a low level of K₂CO₃ addition (2% K₂CO₃), the CO₂ yield significantly increases, by 1.98 mmol·g^–1, while the yield of CO also increases, by 0.56 mmol·g^–1. It might be due to that K₂CO₃ addition promotes the breaking of glucoside bond and the breaking of pyran ring to release CO₂, as well as the decarbonylation of intermediate products to generate CO. With an increase in the K₂CO₃ content, the promoting effect on CO₂ and H₂ gradually increases, but the promoting effect on CO and CH₄ has no significant change, which indicates that a high addition of K₂CO₃ is conducive to the deoxidation of biomass and the release of H₂.

Fig.1(c) shows the compositions of the liquid products from pyrolysis of cellulose with different contents of K₂CO₃. The results show that with a low content of K₂CO₃ addition, there is significant inhibition on the generation of anhydrosugar and pyran. However, there is an obvious increase in the formation of linear aliphatic compounds and cyclopentanone; as well as a weak synergistic effect on furans and phenyls. It might be attributed to that K₂CO₃ addition effectively intensifies the ring-opening reaction of pyran ring to generate linear intermediates, some of which undergo cyclisation to form cyclopentanones and furans. Besides, with the increase of K₂CO₃ addition, the inhibitory effects on anhydrosugar and pyran are enhanced, and there is almost no anhydrosugar and pyran generation at 5% K₂CO₃. With a further increase in K₂CO₃ content, promotion of the generation of linear aliphatic compounds reaches a maximum and then gradually weakens. Moreover, there is a significant synergistic effect on cyclopentanone production with an increase in K₂CO₃ addition, which reaches a maximum at 20% K₂CO₃, while the promotion of furan production gradually weakens and there is a slight inhibitory effect at high K₂CO₃ content (20%). Meanwhile, the promoting effect on phenols is strengthened at 20% K₂CO₃. It might be due to that the breakage of C–O bonds of furan and pyran rings and ketol-isomerase intensify with an increase of K₂CO₃ addition to generate cyclopentanone, whereas the addition of a high dosage of K₂CO₃ might accelerate the aromatisation of light intermediates to form phenyl.

3.2 Effect of K and Na on product yields

Fig.2 shows the product yields obtained from the pyrolysis of cellulose and bamboo with the addition of different AM salts. Compared to the blank sample, the chloride salts (KCl and NaCl) have little effect on the product yields. KCl slightly reduces the gas yields from cellulose and bamboo by approximately 0.6 and 1.0 wt %, respectively. NaCl increases the amount of gas released from cellulose by approximately 1.4 wt %, but it has no pronounced effect on bamboo. When AM sulfates (K₂SO₄ and Na₂SO₄) are added, little effect (less than 1.0 wt %) is observed on char yield. However, the liquid yield decreases by 1.7–3.4 wt %, and the gas yield increases by 1.5–2.3 wt %. This indicates that AM sulfates might promote the secondary cracking of volatile components to form lighter molecular compounds. Compared with chlorides and sulfates, AM carbonates (K₂CO₃ and Na₂CO₃) significantly promote char yields by 1.7–2.1 wt % for cellulose and 3.2–3.5 wt % for bamboo. Furthermore, when K₂CO₃ and Na₂CO₃ are added, the gas yield from cellulose increases from 10.3 to 20.2–22.8 wt %, while the liquid products decrease from 48.1 to 32.9–34.9 wt %. There is a similar but smaller effect on gas and liquid yields from bamboo. It indicates that K₂CO₃ and Na₂CO₃ might also promote the cracking of volatile components, resulting in less liquid product and more gaseous product.

Fig.2 Product distribution for (a) cellulose and (b) bamboo pyrolysis with the addition of various AM salts.

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3.3 Effect of K and Na on the gaseous products

Fig.3 shows the composition of gaseous products obtained from the pyrolysis of cellulose and bamboo with various AM salts. When KCl is added, there is little effect on the gas composition distribution, whereas K₂SO₄ and K₂CO₃ have a pronounced effect. K₂SO₄ increases the CO₂ yield of cellulose from 1.2 to 1.9 mmol·g^–1, whereas that of bamboo increases by 0.2 mmol·g^–1. This indicates that K₂SO₄ might promote the decarboxylation and decarbonylation reactions during pyrolysis. For cellulose, K₂SO₄ might mainly accelerate the formation of polycarboxylic intermediates via dehydration of hydroxyl group and keto-enol tautomerism, resulting in higher CO₂ production when compared with bamboo. In addition, K₂SO₄ addition causes a slight increase in CH₄ and H₂ generation (~0.1 mmol·g^–1), which is mainly derived from the reconstruction of molecular fragments and demethylation and dehydrogenation reactions during the process of char cyclization [25]. Therefore, the addition of K₂SO₄ might accelerate the dehydrogenation and demethylation reaction of volatiles. Compared with the relatively weak effects of KCl and K₂SO₄, K₂CO₃ has a noticeable effect on the gas composition. The CO₂ content of cellulose and bamboo increase more than twice and the CH₄ content increases by 20–100 wt %. Meanwhile, H₂ generation is greatly increased by more than ten times from 0.1 to 1.0–1.2 mmol·g^–1. CO generation is noticeably different between cellulose and bamboo pyrolysis with K₂CO₃ addition. The CO gas yield from cellulose increases from 1.6 to 2.3 mmol·g^–1, but that from bamboo decreases from 1.6 to 1.4 mmol·g^–1. It has been reported that K₂CO₃ has the capability to promote remodelling reaction of the pyran ring (rich in cellulose) to form a large amount of molecular fragments [20]. Then, these fragments might undergo a series of dehydrogenation and deoxygenation reactions to form permanent gases, such as CO, CO₂, and H₂. Bamboo mainly composes of holocellulose and lignin. Therefore, the increase in CO₂ and H₂ production might be attributed to the interaction between holocellulose and K₂CO₃, while the decrease in CO might be related to the weakening of decarbonylation and deoxygenation during lignin pyrolysis under the promotion of K₂CO₃.

Fig.3 Gas product characteristics for (a) cellulose and (b) bamboo pyrolysis with the addition of various AM salts.

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Several main differences can be observed in the effects of the K and Na salts. First, there is nearly no change in CO and H₂ generation during cellulose pyrolysis when NaCl and Na₂SO₄ are added, although the CO₂ yield is increased by 1.2–1.6 mmol·g^–1 for both Na salts. In addition, NaCl has little effect on the gas composition from bamboo pyrolysis, whereas Na₂SO₄ increases the CO and CO₂ yield by 0.2 mmol·g^–1. This implies that NaCl and Na₂SO₄ might have negative effects on the heating values of the gaseous products. Considering the effects of KCl and K₂SO₄, AM sulfates might promote the pyrolysis of volatile components during pyrolysis, resulting in a decrease in liquid production but a significant increase in CO₂ and CO yields. AM chlorides have a negligible effect on the transformation from volatile to gases. The effect of Na₂CO₃ addition is similar to that of K₂CO₃. The production of CO₂ and H₂ from cellulose and bamboo doubles; however, the production of CO declines. Nevertheless, Na₂CO₃ is less effective than K₂CO₃, which might be due to the difference in the strengths of K and Na metallicity [26].

In terms of gaseous product utilization, the content of combustible components in the gases and their heat values are key parameters. As shown in Tab.1, without the addition of AM salts, the CO₂ contents are 36.96 vol % for cellulose and 43.37 vol % for bamboo, respectively. The lower heating values (LHVs) are reasonable, at 9.45 and 9.96 MJ·Nm^–3 for cellulose and bamboo, respectively. However, when K₂CO₃ is added, the production of CO₂ noticeably increases, resulting in the lowest LHV of 7.17 MJ·Nm^–3. For cellulose, the addition of AM salts reduces the LHVs of all the gaseous products. However, for bamboo, AM chlorides and sulfates do not reduce the LHV of the gases and the rate of reduction caused by carbonate is lower than that of cellulose.

Tab.1 Characteristics of gas and liquid products

Item	Cellulose								Bamboo
	Blank	KCl	NaCl	K2SO4	Na2SO4	K2CO3	Na2CO3		Blank	KCl	NaCl	K2SO4	Na2SO4	K2CO3	Na2CO3
Gas composition distribution and heat value/vol %
CO2	37.0	43.5	45.7	50.8	44.5	49.9	52.9		43.4	46.8	44.2	43.4	43.4	51.7	54.1
CO	52.8	47.1	45.1	38.3	45.0	32.2	32.4		40.8	38.8	40.2	40.8	40.5	21.2	22.0
CH4	6.7	6.8	6.7	6.7	7.2	4.8	6.6		12.4	11.3	12.6	12.5	13.0	9.3	11.0
H2	3.5	2.7	2.5	4.2	3.3	13.2	8.1		3.5	3.1	3.0	3.3	3.1	17.8	12.9
LHV/(MJ·Nm–3)	9.5	8.7	8.4	7.7	8.6	7.2	7.3		10.0	9.3	9.9	10.0	10.1	7.9	8.1
Properties of liquid products
Water content/wt %	46.8	47.2	47.7	46.5	44.9	48.7	49.5		39.8	40.7	39.6	42.8	37.7	44.9	42.7

3.4 Effect of K and Na on the liquid products

Fig.4 shows the composition of the liquid products obtained from the pyrolysis of cellulose with various AM salts. With no salt added, cellulose exhibits a very high selective yield of pyran products (approximately 74.2%). When AM salts are added to cellulose, the content of pyran compounds in the liquid product is reduced, which might be attributed to promotion of the ring-opening reaction of the pyran ring and an increase in short-chain molecules, such as furans and cyclopentanones.

Fig.4 Liquid product characteristics from cellulose pyrolysis with the addition of various AM salts.

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With the addition of AM sulfates, the content of pyran products decreases from 74.2% to 66.0%–68.1%, among which the relative content of levoglucosan changes slightly, from 59.2% to 60.9% and 57.7% (Table S1, cf. Electronic Supplementary Material), but the yields of pyrone and dianhydro-α-d-glucopyranose decrease significantly. It indicates that Na₂SO₄ and K₂SO₄ mainly affect pyrone and dianhydro-α-d-glucopyranose, which have poor thermal stability, while have little effect on levoglucosan products with strong thermal stability in the volatiles from cellulose. When NaCl and KCl are added, the content of pyran products decreases from 74.2% to 51.7% and 61.6%, respectively. And NaCl has a more significant effect than that of KCl, which corresponds to the fact that NaCl gives a higher yield of CO and CO₂ as shown in Fig.3 in Section 3.3. Meanwhile, levoglucosan content in the pyran products decreases significantly (Table S1). It indicates that the AM chlorides might have a stronger ring-opening effect than AM sulfate salts, which is sufficient to promote the ring-opening reaction of the stable levoglucosan. Furthermore, AM chlorides greatly increase the relative content of furan compounds in the liquid product, from 6.8% to 17.3% for NaCl and to 13.0% for KCl. It indicates that the molecular fragments from pyranose cracking with AM chlorides might be prone to form furfural, 5-hydroxymethylfurfural and other furan compounds. Compared with sulfates and chlorides, AM carbonates have the opposite effect on the composition of the liquid products from cellulose, which correlates with the change in the char yield and gas composition. The most noticeable change is that all the dehydrated sugar products (such as levoglucosan and dianhydro-α-d-glucopyranose) disappear, but traces of pyrone remain (Table S1). Besides, the contents of cyclopentanone, benzene, and short-chain molecules increase significantly. Di Blasi et al. [27] reported a similar phenomenon when studying the effects of AM carbonates on biomass pyrolysis. It might be attributed to the fact that K₂CO₃ and Na₂CO₃ greatly enhance ring opening and continue to promote further cracking and condensation reactions of the pyrolytic intermediates, some of which undergo Grob cracking, dehydration, and keto-enol tautomerism to form light molecule compounds [28], which results in light gas products and light short-chain molecule increase significantly. Besides, the relative content of hydroxyacetone increases from 0.9% to 12.0%–14.6% (Table S1), which might mainly come from pyran ring cracking and reconstruction, and on the other hand, some other pyrolytic intermediates might occur dehydrogenation, deoxygenation and cyclization to form cyclopentanone and phenyl compounds [29]. Meanwhile it is worth noting that cyclopentanone and phenyl compounds are important intermediates in the formation of char [30]. Hence, the addition of K₂CO₃ and Na₂CO₃ significantly increase char yields during pyrolysis. Furthermore, the total content of cyclopentanone and phenyl compounds also increases from 3.3% to 56.0%–59.6%. Besides, AM carbonates slightly affect the total content of furan compounds (6.4%–7.0%), but the primary product changes from furfural to furfuralcohol (shown in Table S1). It might be due to that AM carbonates change the ring opening, remodelling, and cyclization reaction modes of cellulose pyrolysis, which results in the change in the side chain connections of the furan compounds [28].

Fig.5 shows the effect of the AM salts on the liquid products of bamboo pyrolysis. First, AM chlorides and sulfates cause only a slight compositional change in the liquid product from the pyrolysis of bamboo. For example, phenyl compounds show the greatest increase with KCl addition, only 4.4%, and short-chain compounds are slightly reduced by ~3.3%, and there is a minor decrease (~2.2%) in cyclopentanone, pyran, and furan products. It indicates that AM chlorides and sulfates might have a slight effect on pyrolytic process of bamboo, which corresponds to the weak change in pyrolysis product yields (Fig.2) and the composition of the gaseous products (Fig.3). Compared with AM chlorides and sulfates, the additions of K₂CO₃ and Na₂CO₃ have a significant promotion effect on the generation of phenyl compounds from bamboo pyrolysis, increasing from 44.3% to 78.2%. Meanwhile, the short-chain compounds decrease greatly, especially acetic acid, which is not detected. It might be attributed to that AM carbonates react with acetic acid to form acetate and CO₂, and then the unstable acetate might further decompose to become carbonate again. In summary, AM carbonates might promote the dehydrogenation, deoxygenation, and aromatization of light molecular fragments into phenyl compounds, resulting in the increase of char yield and production of more CO₂ and H₂.

Fig.5 (a) Liquid product characteristics and (b) the phenyl group composition from bamboo pyrolysis with the addition of various AM salts.

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Fig.5(b) shows the composition of the phenyl compounds from bamboo pyrolysis with the addition of AM carbonates. It is found that K₂CO₃ and Na₂CO₃ additions promote the formation of alkyl-phenyl compounds, which increase from 20.0% to 47.7% for K₂CO₃ and 47.9% for Na₂CO₃, while phenol and methoxy-phenyl compounds have little change. It indicates that light intermediates might be more prone to aromatize to form alkyl-phenyl compounds than methoxy-phenyl compounds with the effect of K₂CO₃ and Na₂CO₃, and methoxy group structure might not be formed during the molecular fragment recombination. Besides, a variety of dimethylphenol appear in the liquid products with the addition of K₂CO₃ and Na₂CO₃, which might be attributed to the non-directional recombination between unsaturated molecular fragments. And the contents of 4-ethylphenol (11.7%) and 4-methylphenol (6.4%) are much higher than those in the blank group, which might be because K₂CO₃ and Na₂CO₃ promote the depolymerization reaction of lignin.

Tab.1 lists the water content of the liquid products. The water contents of the pyrolysis oils from cellulose and bamboo are 46.9 wt % and 39.8 wt %, respectively. Na₂SO₄ has a significant effect on water production, reducing the water content to 44.9 wt % and 37.7 wt % for cellulose and bamboo, respectively. This indicates that Na₂SO₄ can upgrade the pyrolysis oil to reduce the water yield. In contrast, AM carbonates increase the water content in the pyrolysis liquid from both raw materials. It is also found that AM carbonates simultaneously reduce the quality of both gas and liquid products.

3.5 Effect of K and Na on the char products

Because AM salts remain solid during pyrolysis, they can be recycled from the char product to improve process efficiency and reduce material costs. In this study, char samples without water washing were analyzed using XRD, and the results indicated that no AM salt reacted with the carbon substrate. All the AM salts were removable by washing with water. Fig.6 shows the XRD patterns of char samples obtained from cellulose and bamboo after washing. It is found that all the char samples present large peaks near 2θ = 23°, which corresponds to amorphous carbon structure. In addition, no other noticeable diffraction peaks appear in the spectra, indicating that all AM salts in the char were removed.

Fig.6 XRD analysis of char products from (a) cellulose and (b) bamboo pyrolysis, with variable alkali salts after water washing.

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Fig.7 shows FTIR spectra of the washed char samples. The spectra of the cellulose- and bamboo-derived char produced with AM chlorides are nearly identical to those of the blank samples, indicating that AM chlorides have almost no effect on the formation of functional groups on the char surface. In addition, a noticeable difference is observed when AM sulfates and carbonates are added. With the addition of AM sulfates, the peaks related to the aromatic ring C–H bending vibration intensifies, and the maximum peak changes from 875 to 810 cm^–1, indicating that the char structure has changed from complex and multi-substitution aryl structure to simple substituted aryl structure. Meanwhile, the aryl ether C–O–C absorption peak at 1260 cm^–1 is more obvious in contrast to that of the blank sample. It indicates that AM sulfates promote the remaining of a large number of aryl ether linkages between the carbon matrix structure. Reflecting on the discussion in the previous sections, the change of the liquid product composition with the addition of AM sulfates is less than that with AM chlorides, but the change in the gas product composition is significantly greater than that with AM chlorides, which might be due to the increase of decarboxylation and decarbonylation reaction. As the AM carbonates are added, the intensity of the aromatic ring C–H vibration peaks in the FTIR spectra became weaker, but the peaks of fused-ring structure in char intensify. It might be attributed to that K₂CO₃ and Na₂CO₃ promote the dehydrogenation reaction, which corresponds to the increase of H₂ production. And absorption peak at 1800–900 cm^–1 intensifies in the FTIR spectra, which proves that the AM carbonate has a significant influence on the char formation process.

Fig.7 FTIR analysis of char products from (a) cellulose and (b) bamboo pyrolysis, with the addition of variable alkali salts after water washing.

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Tab.2 shows the results of the ultimate analysis and porosity of the char samples with different AM salts. AM chlorides have almost no effect on the elemental composition of the char from cellulose and bamboo, which is consistent with the finding of the characterization of the gaseous and liquid products. Compared with chlorides, AM sulfates slightly increase the carbon content of the cellulose char but noticeably reduce the carbon content of bamboo char. This suggests that sulfates might lead to carbon loss from the lignin during pyrolysis. Interestingly, carbonates increase the carbon content of char from 84–85 wt % to 86–89 wt % (dry and ash-free basis), which indicates that carbonates might promote the production of high carbon content char at a lower pyrolysis temperature. Along with an increase in carbon content, the AM carbonates also improve the porosity of the char samples. For example, K₂CO₃ significantly increases the surface area of the char from 7 and 8 m²·g^–1 to 79 and 116 m²·g^–1 for cellulose and bamboo, respectively. This degree of increase is the highest among all the AM salts employed in this study.

Tab.2 Characteristics of char products

Item	Cellulose								Bamboo
	Blank	KCl	NaCl	K2SO4	Na2SO4	K2CO3	Na2CO3		Blank	KCl	NaCl	K2SO4	Na2SO4	K2CO3	Na2CO3
Ultimate analysis/wt %, dry and ash free basis
C	85.4	85.2	86.0	86.0	86.1	89.3	88.8		84.5	84.0	84.5	82.5	81.6	88.7	86.4
H	3.0	3.0	3.0	2.9	2.9	2.0	2.0		2.9	2.7	2.0	2.5	3.1	2.0	1.7
O*	11.6	11.8	11.0	11.1	10.9	8.7	9.3		12.6	13.3	13.5	15.0	15.3	9.3	11.9
Pore properties of washed chars
SBET/(m2·g–1)	7.5	8.2	27.8	16.7	21.9	79.6	9.5		8.6	6.9	7.9	11.6	6.2	116.8	52.5
Smicro/(m2·g–1)	0.0	0.0	7.9	1.9	0.0	57.6	0.0		0.0	0.0	0.0	0.0	0.0	87.2	23.5
Vtotal/(10−3cm3·g–1)	10.1	9.8	17.0	10.3	27.6	94.6	6.5		9.1	18.2	22.1	19.3	9.5	201.2	128.1
Pore size/nm	5.4	4.7	2.2	3.0	4.9	1.9	6.9		4.1	8.4	9.9	7.5	10.8	2.5	3.5

3.6 Mechanism of the influence of AM salt addition on biomass pyrolysis products

Combined with the changes in gas-, liquid-, and solid-phase products after AM salt addition, it is found that the effect of AM carbonates is stronger than that of AM sulfates, followed by that of AM chlorides. The main difference between these AM salts is the solid-phase contact reaction. Fig.8 illustrates the comparison of the effect of AM salts addition on biomass pyrolysis with consideration of all the three-phase products. AM chlorides and sulfates have little effect on the pyrolysis of the gaseous and liquid products from bamboo; however, both reduce the amount of pyran compounds in the liquid products from cellulose. AM chlorides might promote the cracking of volatiles and ring-opening reaction of pyran rings from cellulose to form light gaseous molecules, and accelerate further cyclization reactions to generate furans and phenyl compounds. And AM sulfates inhibit the cracking of cellulose through solid-solid phase contact reactions, resulting in the large aryl ether structure in char and the decrease of pyrone and dianhydro-α-d-glucopyranose content. AM carbonates greatly affect the pyrolysis of cellulose and bamboo, because of their strong alkalinity, and etch the surface of biomass as the temperature increases, and substantially increase pore expansions. Meanwhile, AM carbonates intensify the solid-solid phase contact reaction between salts and biomass, which might significantly accelerate ring-opening reaction of cellulose, and promote further cleavage and condensation of intermediate products after ring-opening reaction, and reacting with acetic acid to form CO₂. Hence, AM carbonates change the main composition of the gaseous and liquid products.

Fig.8 Effect paths map of AM salts during biomass pyrolysis.

Full size|PPT slide

4 Conclusions

AM salts play an important role in the yields and properties of biomass pyrolysis products. AM carbonates (K₂CO₃ and Na₂CO₃) are shown to be more effective than other salts in promoting the pyrolysis process, because they can significantly increase the char yield and enhance the vapor crack to gaseous products. They also have an important upgrading effect on product quality, including enriching the phenolics in the bio-oil and increasing the carbon content and surface area of the char. Although AM sulfates are less effective than AM carbonates, Na₂SO₄ is capable of increasing the heating value of gases and reducing the water content of the bio-oil.

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Competing interests

The authors declare that they have no competing interests.

Acknowledgements

The authors wish to express their great appreciation for the financial support received from the National Natural Science Foundation for Distinguished Young Scholars (Grant No. 52125601) and the National Natural Science Foundation of China (Grant No. 52306244). And the technical support was provided by the Analytical and Testing Centre at Huazhong University of Science & Technology.

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