Review on thermal-science fundamental research of pressurized oxy-fuel combustion technology

Abstract

As the next-generation oxy-fuel combustion technology for controlling CO₂ emissions, pressurized oxy-fuel combustion (POC) technology can further reduce system energy consumption and improve system efficiency compared with atmospheric oxy-fuel combustion. The oxy-fuel combustion causes high CO₂ concentration, which has a series of effects on the combustion reaction process, making the radiation and reaction characteristics different from air-fuel conditions. Under the pressurized oxy-fuel condition, the combustion reaction characteristics are affected by the coupling effect of pressure and atmosphere. The radiation and heat transfer characteristics of the combustion medium are also affected by pressure. In recent years, there have been many studies on POC. This review pays attention to the thermal-science fundamental research. It summarizes several typical POC systems in the world from the perspective of system thermodynamic construction. Moreover, it reviews, in detail, the current research results of POC in terms of heat transfer characteristics (radiant heat transfer and convective heat transfer), combustion characteristics, and pollutant emissions, among which the radiation heat transfer and thermal radiation model are the focus of this paper. Furthermore, it discusses the development and research direction of POC technology. It aims to provide references for scientific research and industrial application of POC technology.

Graphical abstract

Keywords

pressurized oxy-fuel combustion (POC) / CO₂ control / system efficiency / radiation heat transfer

1 Introduction

In response to escalating global electricity demand, the CO₂ emissions of the power industry are surging, contributing significantly to global warming. The effective reduction of carbon emissions has, therefore, become a worldwide hot topic. Developed countries/regions such as USA, European Union, and Canada, have successively put forward the goal of carbon neutrality. China, the present developing country, has also proposed the strategy of achieving carbon neutrality by 2060 [1,2]. Among various carbon capture and storage (CCS) technologies [3], oxy-fuel combustion holds considerable promise, which is considered one of the ways to reduce CO₂ emissions of coal-fired power plants on a large scale in the future. It uses pure oxygen and recirculated flue gas into the combustion chamber to obtain a high concentration of CO₂ flue gas, and CO₂ can be recovered directly through condensation and compression, showing many advantages [4]. However, oxygen production from air and CO₂ recovery is performed at a high pressure, while the oxy-fuel combustion boiler is operated at an atmospheric pressure. The pressure difference causes the auxiliary devices to generate significant energy consumption and lead to low system efficiency. Therefore, pressurized oxy-fuel combustion (POC) is proposed aiming to improve the system efficiency by increasing the combustion pressure and recovering the latent heat of flue gas. The high-pressure and low-temperature water leaving the feed pump enters the flue gas condenser for heat exchange with the flue gas containing latent heat. The feed water is preheated and injected into the boiler. At the same time, the flue gas is cooled, then, purified, and compressed to recover CO₂ [5]. On the gas side, the oxygen flow from the air separation unit (ASU) is mixed with the recirculated flue gas and injected into the pressurized boiler. The CO₂ recovered and stored through the POC system can regenerate synthetic fuel through thermal catalysis, electrocatalysis, and other methods, achieving net zero carbon emissions [6]. Fig.1 shows the schematic of a POC systeme turbine.

POC can improve the efficiency of the boiler and the steam power cycle while reducing energy consumption during CO₂ compression. The efficiency of POC is higher than that of the integrated gasification combined cycle (IGCC) system with a CO₂ capture system [5]. According to the research on POC power cycle by Haryanto & Hong [7], the overall net system efficiency increases by 3% when the pressure of the combustion chamber increases from 1 to 10 bar. Dobó et al. [8] measured the radiative heat flux in POC at 14 bar, which is 25 kW/m² greater than that in atmospheric pressure. Fig.2(a) is a comparison of the net output power and power consumption of each equipment in the pressurized and the atmospheric oxy-fuel combustion system. Fig.2(b) shows the impact of each equipment on the net output efficiency in the work process [9]. Under high-pressure conditions, the burner can recover more thermal energy from the flue gas, which is enough to replace the thermal load of the steam turbine feedwater heating system and reduce the steam emission. After balancing the heat recovery, ASU compression consumption, CO₂ compression consumption, and fan consumption, POC can increase the net output power and improve the boiler efficiency compared with atmospheric oxy-fuel condition, which is a very promising CO₂ control technology.

As a power production and CO₂ control technology, POC is based on the construction of a thermodynamic system. At present, there are many system designs, some of which have been in the pilot scale test. These systems are the basis for the study of POC technology. Moreover, the oxy-fuel combustion process produces a high concentration of CO₂, which has a series of effects on the combustion reaction process. At the same time, the high concentration of triatomic gases represented by CO₂ and H₂O greatly enhances the radiation heat transfer of the combustion medium [10]. Relevant studies have discussed the radiation heat transfer of oxy-fuel combustion [11,12]. The chemical reaction characteristics are affected by the coupling effect of pressure and atmosphere in pressurized oxy-fuel conditions. The radiation and heat transfer characteristics of the combustion medium are also affected by pressure. This is very important for the design and optimization of the POC system. In addition, it is noticed that there are many review papers on oxy-fuel combustion at present. Recently, Rahman et al. [13] reviewed the characteristics of nitrogen (N) migration and NO_x release of POC in detail. However, there are few reviews on POC. Compared with professional review papers in a certain field, there is a lack of review papers with broad topics in the field. On this basis, this paper reviews the typical work in the field of thermal science basic research of POC, including the typical POC system, the heat transfer characteristics (radiation and convection), and the combustion reaction characteristics, on which, the combustion radiation characteristic is focused. This paper aims to provide guidance and reference for researchers in this field.

2 POC system

The current pressurized combustion system can be classified into direct heating and indirect heating systems, depending on whether the working fluid and fuel contact each other. In a direct heating system, the working fluid and fuel come into direct contact and are heated directly. This system is based on the gas-steam combined cycle, and the general process is to gasify solid or liquid fuels to produce gas fuels which are then mixed with oxygen and circulating flue gas, and oxy-fuel combustion is performed. The flue gas generated is directly used as the working fluid of the gas turbine. The new system structure and mechanical design of the turbine should be adapted to the working fluid characteristics of oxy-fuel combustion [14–16]. Direct heating systems are highly efficient. However, they require the redesign of the gas turbine for oxy-fuel conditions. The indirect heating system uses a heat exchanger to transfer the heat of fuel combustion to the working fluid, following the Rankine cycle of a conventional power plant. Pressurization can increase the dew point of the flue gas, allowing the latent heat of the flue gas to be used in the Rankine cycle to improve the cycle efficiency. This is the focus of the current research on POC systems.

2.1 POC system using coal water slurry

Coal water slurry is widely utilized as a fuel in current POC systems due to its ability to be easily transported under pressure via pumps. The pressurization conditions facilitate the recovery of steam latent heat from flue gas. Typical systems include the ThermoEnergy Integrated Power System (TIPS) [17] and the ISOTHERM^® [18]. The TIPS system uses a condenser to recover the latent heat of the steam from the flue gas of the pressurized boiler, as shown in Fig.3(a). The feed water of the boiler is heated by the condensed steam, and the remaining flue gas is cooled to produce liquid CO₂. After purification, other polluting gases or non-condensable gases can be removed, which can realize the control of CO₂ and pollutants. The TIPS system developed by CANMET (Canada Centre for Mineral and Energy Technology) maintains a condenser temperature difference of over 100 °C and has a flue gas outlet temperature of 123–262 °C. The large temperature difference enhances heat exchange and reduces the size of the heat exchanger. The amount of recovered latent heat is proportional to the flue gas pressure [19]. The system efficiency increases gradually with pressure below 80 bar. Deng & Hynes [20] performed a process simulation analysis on the TIPS system using a 350 MW subcritical reheat boiler with a combustion pressure of 80 bar and found that high calorific value coal can achieve an efficiency of 27.54%, which is 3% higher than atmospheric oxy-fuel combustion. Using the condensate circulating water heated by the flue gas in the condenser, followed by the two-stage extraction steam from the IP and HP turbines, can increase system efficiency by 1.36% compared to only heating the feed water of the boiler by a flue gas condenser.

Fig.3(b) shows the flowchart of the patented ISOTHERM^® system. Massachusetts Institute of Technology (MIT) has conducted extensive research on this system [21–23]. The circulating flue gas is divided into two paths leading to the combustion chamber and the exit of the combustion chamber respectively. This mixing of the flue gas from the pressurized combustion chamber to about 800 °C eliminates the need for a radiation heat exchanger and reduces the initial investment costs. Italy currently has a 5 MW demo unit operating at a pressure of 4 bar. The feed water of the boiler is heated by the latent heat of flue gas. ITEA has also designed a 48 MW pilot plant with a pressure of 10 bar. The latent heat of flue gas is recovered to replace the extraction steam from low-pressure turbine while the extraction steam from high-pressure and intermediate-pressure turbines is retained. The flue gas condenser has a temperature difference of about 20 °C, with an outlet temperature difference of 50 °C. At a pressure of 11 bar, the maximum efficiency can be achieved by recovering a large amount of latent heat [24].

2.2 Flameless POC

Flameless POC (FPOC) [25] technology is a new type of combustion technology. Conventional combustion relies on a narrow flame front to facilitate the mixing of fuel and oxidant under high-temperature conditions, driving rapid chemical reactions and physical processes visible as a flame. In contrast, FPOC controls combustion conditions and airflow dynamics based on POC. This allows full premixing of fuel and the oxidant at a high pressure, realizing uniform combustion across the entire reaction zone. The large reaction volume and minimal temperature and concentration gradients prevent the formation of a narrow flame front structure. The combustion chemical reaction processes homogenously without a visible flame. This technology can be applied to various types of fuels, and achieve flameless combustion at a high pressure and at high temperatures (1400–1700 °C) [26]. Fig.4 shows the FPOC system diagram. The high-temperature and high-pressure CO₂ at the outlet of the combustor is mixed with the steam generated by a novel once-through steam generator (OTSG), and then passes through the heat exchanger and the condenser. It is then divided into two parts: one part is returned to the combustion chamber as a diluent, and the other part is sent to the compressor for compression and drying, followed by CO₂ capture and utilization. The technology reduces emissions of pollud utilization. The technology reduces emissions of pollutants such as nitrogen oxides, sulfur oxides, and particulate matter. The development of FPOC technology began in 2008 and is currently in the pilot stage, with a 5 MW small-scale test facility already established. The design, construction, and operation of a 25 MW large-scale experimental device [27] is being conducted by the US Department of Energy, ITEA, Southwest Research Institute, and other institutions. FPOC technology is expected to become a commercialized clean energy technology in the future [28,29].

2.3 Circulating fluidized bed POC system

The technology of fluidized bed boilers is widely adopted due to its high combustion efficiency. The use of a circulating fluidized bed boiler and the direct use of coal particles as fuel is an important technical route of POC. Fluidized bed boiler operates at a pressure range of 60–80 bar, and a temperature of 800–900 °C, ensuring efficient burning of coal. Fig.5 shows the schematic of a circulating fluidized bed POC (CFB-POC) system, including fuel supply, gas–solid separation, CFB boiler, exhaust gas treatment, heat recovery, and solid waste treatment. In this system, coal particles are fluidized along with inert solid particles, and achieve POC. The waste gas is separated with solid particles by a cyclone separator, while the released heat is effectively recycled. Compared with conventional POC systems, the utilization of circulating fluidized bed technology ensures superior gas–solid mixing and circulation, thereby enhancing combustion performance. Furthermore, the implementation of cyclone separators enables efficient separation and recovery of solid waste. These advancements contribute to improved energy efficiency. Shi et al. [30] analyzed the energy efficiency changes of the POC system using CFB technology. The results revealed that higher operating pressure is beneficial to the combustion in CFB under the oxy-coal condition. The net efficiency increased from 27.2% at atmospheric pressure to 30.5% at an optimum pressure of 11 bar.

2.4 Staged POC system

Gopan et al. [31] from the University of Washington (UW) proposed a staged POC (SPOC) system, as shown in Fig.6. This system employs a multi-stage combustion chamber to achieve different levels of oxy-fuel combustion at different stages to ensure sufficient combustion, improve combustion efficiency, and reduce pollutant emissions. In the SPOC system, fuel is distributed equally to each combustor. The initial combustor maintains an excess of O₂, effectively regulating the temperature and heat transfer of combustion products. Additionally, heat is introduced into the Rankine cycle during the initial stages. As the flue gas temperature decreases significantly, the first-stage products, including remaining O₂, enter the next combustor where excess fuel is injected, and more O₂ is consumed. This process continues until the oxygen is completely exhausted. This system does not require flue gas recirculation through a fan, achieving true zero flue gas recirculation and eliminating the load.

In addition, the system also recovers the heat dissipation of compressors at all levels in the air separation system by using condensate water, which further enhances the efficiency of the POC system and reduces investment costs [32]. The process simulation shows that the system efficiency is significantly improved by 6% compared with the atmospheric oxy-fuel system. The efficiency of burning high calorific value coal can reach 35.74%. Further study [33] indicates that the increase of pressure enhances system efficiency, but if the pressure exceeds 16 bar, the effect is weakened. After considering various parameters such as environment and economy, the optimal pressure is regarded as 16 bar. The water content in fuel markedly affects efficiency, with higher water content leading to reduced efficiency.

2.5 POC system coupled with waste heat recovery technology

The University of Hull in the UK has proposed a new POC power generation system [34] that couples liquid air energy power generation and organic Rankine cycle waste heat recovery technologies. The system liquefies the nitrogen flow obtained from air separation and compression, and then uses the liquefied nitrogen flow in the liquid air power generation (LAPG) system. This allows for the rational utilization of the nitrogen flow, which is normally directly used as the exhaust gas. The organic Rankine cycle is used to recover the waste heat in the air compression process, flue gas purification process, CO₂ purification, and compression unit, which can offset 65% of the energy consumption in these processes. The integration of these two technologies can improve the efficiency of the whole system by 12%–15% compared to a single POC system. Therefore, it is a novel proposal to use new waste heat recovery technologies to improve system efficiency in the field of POC technology.

In general, the indirect heating system based on the Rankine cycle is more widely used due to its low technical difficulty. It uses the latent heat in the high-pressure flue gas to heat the condensate water in the cycle, reducing steam extraction and improving system efficiency. For system design, flue gas condensers can achieve a higher system efficiency by replacing only the reheater for low-pressure steam extraction instead of all the reheaters. The combustion system can use coal water slurry as fuel. However, when the fuel moisture is very high, the excess latent heat cannot be recovered through the flue gas regenerator, which will significantly reduce the efficiency. Therefore, when utilizing coal water slurry, especially low-rank coal water slurry, its concentration should be monitored adequately to guarantee full recovery of the latent heat. In addition, pressurized fluidized bed (PFB) boilers can directly use pulverized coal as fuel, and the research and development of new high-efficiency POC systems is also an important research direction in recent years. To understand the research progress, more reported POC systems are summarized, and relevant information is listed in Tab.1.

Existing studies indicate that POC technology can improve the whole system efficiency, but its efficiency and optimal pressure vary with different system designs. It is worth noting that POC has the following advantages:

1) The dew point of flue gas rises under pressurization. The circulating condensate can be used to recover the latent heat of flue gas, which reduces the steam extraction and increases the output of the steam turbine. Pressurization also reduces the power consumption of flue gas circulation, which helps improve system efficiency.

2) Pressurization can minimize air leakage, increase CO₂ concentration, and partially compress CO₂ compared to atmospheric oxy-fuel combustion. Thus, the initial investment and operating costs of the CO₂ compression and purification system can be reduced.

3) In POC, high-pressure oxygen separated by ASU can be directly sent to the pressurized furnace without further decompression compared to atmospheric pressure systems. The pressurized CO₂ flue gas from POC can also directly enter CO₂ compression and storage without additional pressurization. Meanwhile, POC can also intensify heat transfer and reduce heat exchange area due to the increased flue gas density. The heat transfer coefficient of the heat recovery boiler for oxy-fuel combustion at an enhanced pressure of 10 bar is 3.6 times higher than that of the conventional air-fuel condition and 2.8 times higher than that of the atmospheric oxy-fuel condition. Compared with atmospheric pressure oxy-fuel combustion systems, the improvement in system efficiency and heat transfer capacity reduces the long-term investment cost of POC.

4) POC can use coal water slurry as fuel and adjust the temperature of various combustion equipment through flue gas circulation, enabling the use of low-grade coal instead of expensive fuel. Research on the ITEA system indicates that a 1% reduction in fuel costs increases overall system efficiency by 0.5%.

3 Heat transfer characteristics

The temperature in the oxy-fuel combustion furnace can reach over 1500 °C. Heat transfer in the furnace mainly relies on radiation heat transfer, and the heating surface mainly relies on convection heat transfer. The high-concentration CO₂ and H₂O atmosphere generated by oxy-fuel combustion, coupled with the high-pressure atmosphere environment, causes significant changes in the heat transfer characteristics of POC. Therefore, this section primarily focuses on the heat transfer characteristics.

3.1 Radiation heat transfer

During the combustion process of boiler and other combustion equipment, the radiation heat transfer is mainly dependent on CO₂, H₂O, and other triatomic gases, as well as soot particles and other radiation media. As pressure increases, the radiation characteristics of these materials change, impacting the overall heat transfer in the furnace. To calculate this radiation transfer process, the radiation transfer equation (RTE) is used, which can be expressed as [45]

(1)\begin{array}{rl}\frac{\mathrm{\partial }I\left(s,\overrightarrow{s}\right)}{\mathrm{\partial }s}=& -\left({\kappa }_{\mathrm{g}}+{\kappa }_{\mathrm{p}}+{\sigma }_{\mathrm{p}}\right)I\left(s,\overrightarrow{s}\right)+{\kappa }_{\mathrm{g}}{I}_{\mathrm{b},\mathrm{g}}\left(s\right)\\ & +{\kappa }_{\mathrm{p}}{I}_{\mathrm{b},\mathrm{p}}\left(s\right)+\frac{{\sigma }_{\mathrm{p}}}{4\text{π}}{\int }_{{\mathrm{\Omega }}_i=4\text{π}}^{}I\left(s,\overrightarrow{s}\right)\mathrm{\Phi }\left(s,\overrightarrow{s}\right)\mathrm{d}{\mathrm{\Omega }}_i,\end{array}

where I (s,\overrightarrow{s}) is the radiation intensity at s position in \overrightarrow{s} direction, {\kappa }_{\mathrm{g}} is the absorption coefficient of the gas mixture, {\kappa }_{\mathrm{p}} is the absorption coefficient of particles, and {\sigma }_{\mathrm{p}} is the scattering coefficient of particles. I_{\mathrm{b},\mathrm{g}}\left(s\right) is the blackbody radiation intensity of the gas at s position. I_{\mathrm{b},\mathrm{p}}\left(s\right) is the blackbody radiation intensity of particles at s position. \mathrm{\Phi } (s,\overrightarrow{s}) is the scattering phase function, representing the energy fraction scattered from the incident direction to the exit direction, and {\mathrm{\Omega }}_i is the solid angle. According to RTE, the radiation heat transfer in combustion media can be calculated by obtaining the radiation characteristics of gas and particle media. The typical numerical methods can be the Monte Carlo method, finite volume method (FVM), finite element method (FEM), and discrete ordinates method (DOM).

The Monte Carlo method [45] relies on probabilistic statistics, comprehensively considers the attenuation, wall reflection properties, and medium scattering interference during radiation transfer within combustion chambers, and transforms complex radiation detection problems into random sampling processes of emission, absorption, and scattering. Niu et al. [46] used the backward Monte Carlo ray-tracing approach based on radiation distribution factors to obtain high-resolution directional radiation information, and effectively calculated the radiation characteristics related to the transfer process and the energy-related temperature. However, this method requires a large amount of random sampling and has a low computational efficiency and reliability.

FVM is a numerical calculation method for solving partial differential equations. In the RTE of flame, FVM divides the solution area into a limited number of control volumes, and then applies the law of mass conservation and energy conservation to each control volume to obtain a discretized system of equations. Mishra et al. [47] used FVM to solve the radiation energy equation and proposed a hybrid strategy to study the radiation heat transfer problem.

FEM is widely popular among methods for obtaining the radiation characteristics of media because of its remarkable characteristics of grid flexibility and simple implementation. Razzaque et al. [48] applied FEM to the coupling problem of radiation heat transfer and conductive heat transfer in a rectangular box. Traditional FEM can only perform global solutions. To obtain local adaptive solutions, Wang et al. [49] used the discontinuous finite element method (DFEM) to calculate the radiative heat transfer of gray and isotropic scattering media questions.

DOM operates on the principle of dividing the solution domain into a limited number of discrete directions, and then applies the RTE in each discrete direction to obtain a system of discretized equations. Stamnes et al. [50] developed a Fortran-based DOM program for radiative transfer analysis. Sakami et al. [51] proposed an improved DOM that discretizes the radiative heat transfer in the participating medium into structured and unstructured grids. Paul [52] used DOM in a three-dimensional (3D) numerical simulation study of radiative heat transfer in gas turbine combustion chambers. They compared the differences between high-order and low-order predictions of radiation transfer performance and found that high-order predictions were more accurate.

3.1.1 Radiative heat transfer of gas medium

For the triatomic gas radiation medium represented by H₂O and CO₂, according to Kirchhoff”s laws, the spectral emissivity of the gas can be expressed as [45]

(2){\epsilon }_{\eta } =1-\mathrm{e}\mathrm{x}\mathrm{p}(-k_{\eta }PS),

where P is the pressure, and S is the path length. As shown in Eq. (2), the gas emissivity can be improved by increasing the pressure. From the perspective of spectroscopy, under the industrial combustion temperature, the impact of pressure on the gas spectral absorption coefficient {\kappa }_{\eta } at wavenumber \eta can be expressed by collision broadening [53]

(3){\kappa }_{\eta } =N{\sum }_i{S}_i{F}_i\left(\eta \right),

(4)F_i\left(\eta \right)= \frac{{\gamma }_i}{\pi \left[\right(\eta -{\eta }_i{)}^2+{\gamma }_i^2]},

(5)N=7.339\times 10^{21} \frac{P}{P_{0}T},

(6){\gamma }_i={\gamma }_{i0}\left(\frac{P}{P_0}\right)\sqrt{\frac{T_0}{T}},

where F_i(η) is the Lorentz line profile, η_i is the center wavenumber of the line, {\gamma }_i is the half-width of gas collision broadening, {\gamma }_{i0} is the half-width of the reference status, and N is the molecule number density of the radiating species.

The variation of pressure mainly affects the absorption coefficient of gas by affecting N. Therefore, the radiation characteristics of gas will be enhanced with the furnace pressure. Here, the standard calculation method of boiler in the Soviet Union [54], as expressed in Eqs. (7) and (8), can be referred to.

(7)a_{\mathrm{f}}= m(1- {\mathrm{e}}^{-\left(k_{\mathrm{q}}{r}_{\mathrm{q}}+k_{\mathrm{t}\mathrm{h}}\right)PS} )+(1-m)(1 -{\mathrm{e}}^{-k_{\mathrm{q}}{r}_{\mathrm{q}}PS}),

(8)a_1= \frac{a_{\mathrm{f}}}{a_{\mathrm{f}}+(1-a_{\mathrm{f}})\phi },

where m is the flame luminescence coefficient, k_q is the radiation attenuation coefficient of the triatomic gas in the flame, r_q is the volume fraction of triatomic gas, and k_th is the radiation attenuation coefficient of particles in the flame. φ is the insulation coefficient. The qualitative calculation based on Eqs. (7) and (8) indicates that the furnace flame emissivity a_f is a monotone-increasing function of pressure term P. This results in an increase in furnace emissivity, which enhances the radiation heat exchange.

Further studies indicate that the radiation intensity and emissivity increase with the pressure of gas mixture, especially when the pressure range is below 5 bar. However, variation in gas radiation characteristics is less sensitive if the pressure is greater than 5 bar. This is more obvious when the temperature is greater than 1500 K [55]. This is the temperature range of most interest in boilers, which, therefore, implies that as pressure increases in the boiler, the radiation heat transfer intensity does not increase significantly beyond a certain point. Further research [56] shows that the radiation heat transfer coefficient of flue gas in oxy-fuel combustion increases with the pressure. Under the condition of a O₂/CO₂ ratio of 21:79, the radiation heat transfer coefficient of the high temperature reheater in the boiler increases by 6.08% as the pressure rises from 1 to 5 bar, and increases by 3.21% as pressure increases from 5 to 10 bar. However, there is no significant increase in the coefficient when the pressure exceeds 10 bar.

At present, the gas radiation absorption coefficient k_ga in Eq. (1) is mainly calculated by using various models. Modest [57] reviews and summarizes various gas radiation models, which provides a reference for the complexity and accuracy requirements of different engineering problems, as shown in Fig.7. For the line by line (LBL) model, it mainly considers that the gas can emit and absorb photons within a certain wavelength range, resulting in an energy variation and spectral emission. This model establishes a connection between the spectrum and radiation parameters of gases using the data from HITRAN and HITEMP databases [58]. LBL is the most precise method for calculating gas radiation characteristics but requires detailed parameter checking for each spectral line, making it unsuitable for engineering applications. Its outputs are usually used as a benchmark to evaluate the accuracy and effectiveness of other methods.

The statistical narrow band (SNB) model is one of the waveband models, which is the closest to the LBL calculation results in all gas radiation models. Therefore, it is also often used as a reference model [59,60]. The SNB model of the EM2C laboratory in France selects narrow bands with an interval of 25 cm⁻¹, which is verified to be still applicable under pressurized conditions [53]. The mean transmissivity of an isothermal, homogeneous gas in the spectral waveband is expressed as

(9){\overline{\tau }}_{\mathrm{v}}= \mathrm{e}\mathrm{x}\mathrm{p}\left[-\frac{2\overline{\gamma }}{\overline{\delta }}\left(\sqrt{1+\frac{XPS\overline{k\delta }}{\overline{\gamma }}}-1\right)\right].

The parameters \overline{k} and \overline{\delta } are given by the EM2C database. The half-width of the mean spectral line of H₂O and CO₂ is expressed as [39]

(10)\begin{array}{rl}\overline{{\gamma }_{{\mathrm{H}}_2\mathrm{O}}}=& \frac{P}{P_{\mathrm{r}\mathrm{e}\mathrm{f}}}\{0.462\frac{T_{\mathrm{r}\mathrm{e}\mathrm{f}}}{T}{X}_{{\mathrm{H}}_2\mathrm{O}}+{\left(\frac{T_{\mathrm{r}\mathrm{e}\mathrm{f}}}{T}\right)}^{0.5}[0.0792(1-X_{\mathrm{C}{\mathrm{O}}_2}\\ & -X_{{\mathrm{O}}_2})+0.106X_{\mathrm{C}{\mathrm{O}}_2}+0.036X_{{\mathrm{O}}_2}]\},\end{array}

(11)\begin{array}{rl}\overline{{\gamma }_{\mathrm{C}{\mathrm{O}}_2}}=& \frac{P}{P_{\mathrm{r}\mathrm{e}\mathrm{f}}}{\left(\frac{T_{\mathrm{r}\mathrm{e}\mathrm{f}}}{T}\right)}^{0.7}[0.07X_{\mathrm{C}{\mathrm{O}}_2}\\ & +0.058(1-X_{\mathrm{C}{\mathrm{O}}_2}-X_{{\mathrm{H}}_2\mathrm{O}})+0.1X_{{\mathrm{H}}_2\mathrm{O}}],\end{array}

where P_ref = 1 bar, and T_ref = 296 K. X_{{\mathrm{H}}_2\mathrm{O}} and X_{{\mathrm{C}\mathrm{O}}_2} represent the mole fractions of H₂O and CO₂ respectively. The pressure term P mainly affects the half-width of the mean spectral line, and thus affects the radiation characteristics. The half-width of the mean spectral line increases with the pressure, while the transmittance of the gas decreases.

The total model mainly focuses on the overall radiative heat flux, other than the spectral distribution. After simplification, the total model can greatly reduce the amount of calculation while still ensuring accurate results for engineering purposes. Typical total models include the full spectrum k-distribution (FSK) model and the weighted sum of gray gases (WSGG) model. For POC, the WSGG model is particularly valuable. The core idea is to simulate the actual non-gray gases through several ideal gray gas. The total absorption or emissivity of the gas is calculated by the weighting factors and absorption coefficients of different gray gases.

(12)\alpha (T,X)= \epsilon (T,X)= \sum _{i=1}^{N_{\mathrm{g}}}{a}_i\left(T\right)\left(1-e^{-k_{i}XPS}\right),

(13){\sum }_{i=1}^{N_{\mathrm{g}}}{a}_i\left(T\right) =1.

where N_g is the number of gray gas, a_i is the weighting factor for each gray gas, k_i is the absorption coefficient, and X represents the sum of the mole fraction of H₂O and CO₂ in the mixed gas, expressed as

(14)X=X_{{\mathrm{H}}_2\mathrm{O}}+X_{\mathrm{C}{\mathrm{O}}_2}.

The emissivity formula of the WSGG model contains the pressure term P, which can theoretically calculate the gas radiation characteristics by adjusting this pressure term. The WSGG model has a higher computational efficiency and a lower accuracy, but it can also meet engineering requirements. Therefore, it is currently the most widely used radiation model in CFD software such as ANSYS Fluent. Xia et al. [61] used CFD simulation to design a unique burner based on the SPOC system. During the simulation process, the WSGG model was used to calculate the gas absorption. Fig.8 shows the temperature distribution of all the four stages in SPOC. In addition, the SLW model [62] is developed based on the WSGG model and gas spectrum database, which can further enhance the calculation accuracy. The full spectrum correlated-k distribution (FSCK) model [63] introduces the correlation absorption coefficient and modifies the FSK model under non-uniform conditions. The new correlation k values proposed by Wang et al. [64] can improve the operation efficiency of the FSCK model.

In the study of gas radiation characteristics under the POC condition, it is found that the half-width, spectral line intensity, and other parameters in the LBL model based on a high-resolution accurate database are directly associated with the pressure term. As a result, the LBL model can still be used as the most accurate calculation method as benchmark for scientific research on POC technology. Chu et al. [65,66] used the LBL model to study the gas radiation heat transfer of two-dimensional (2D) axisymmetric jet diffusion flame at high pressures. Due to the huge calculation time, this model cannot be applied to large-scale and 3D CFD calculation [67,68]. The SNB model and its correlated-k model (SNBCK) have a high accuracy in POC radiation calculation, while the WSGG model and FSCK model have a relatively lower accuracy [65]. Moreover, Zhao et al. [69] used the Edwards exponential broadband model to calculate the radiation characteristics of syngas in a radiation waste heat boiler under high-temperature and high-pressure conditions. The calculated flue gas temperature at the outlet is in good agreement with the experimental results, indicating that the broadband model is feasible in radiation calculation of pressurized engineering application. Furthermore, the broadband correlated-k model specifically for the POC condition was given by Li et al. [70].

Many proposed WSGG models are designed for atmospheric pressure conditions, but their applicability under pressurized conditions needs to be verified. In practical engineering applications, Xia et al. [61] used the WSGG model when simulating their POC system, mainly due to its broad application and low computational requirements. However, spectral characteristics change with pressure. As pressure increases, spectral lines broaden and overlap more [71], which leads to significant deviations when using atmospheric WSGG models at high pressures. In recent years, scholars have studied improvements to the WSGG model under pressurized conditions. Shan et al. [72] first proposed that the parameters of the WSGG model at atmospheric pressure are unsuitable for high-pressure conditions although the pressure term can be adjusted; thus, new parameters for the WSGG model must be developed. They established a new POC WSGG model suitable for H₂O/CO₂ mixture within the pressure range of 1 to 30 bar based on the SNB model [73]. Fig.9(a) displays a comparison between the new model and the SNB model in calculating the emissivity of the gas medium. It appears that the new model works well at different pressures, with an error of approximately 1% in predicting radiation heat flux at certain combustion temperatures. The new WSGG model exhibits a good applicability under all conditions (isothermal or non-isothermal, and homogeneous or heterogeneous). Coelho & França [74] proposed a method to calculate the WSGG coefficient at high pressure when the fixed molar ratio of the H₂O and CO₂ mixture is 2. Their method produced results with a relative deviation of less than 4% when compared to the reference LBL across all path lengths and temperatures ranging from 1 to 40 bar. Fig.9(b) shows that the radiative heat flux results, calculated by the model at high pressures of 10–40 bar, are the same as the benchmark results. In addition, the study suggests that the model is highly effective in inhomogeneous media when compared with homogeneous media. These findings imply that the model can be readily applied to the combustion processes. Guo et al. [75] also extended and improved the WSGG model under pressurized conditions. The pressure application range is extended to 30 bar. The updated parameters of the WSGG model (PWSGG-SK) use the correlated-k-distribution to deal with the non-isothermal conditions and perform data fitting by MATLAB software. It is found that the accuracy of the model is significantly improved under the condition of a small path length. In the high-pressure, non-isothermal and homogeneous gas mixture, the maximum error of the radiation source term is less than 8%. Fig.9(c) clearly shows that the results of the modified PWSGG-SK model at the pressure of 30 bar are in good agreement with the LBL results.

The WSGG model has been improved under pressurized conditions, but its predictive ability still has room for improvement. This paper summarizes some representative research results, and Tab.2 lists the key information of these studies, such as pressurization conditions, improvement methods for the model, and application prospects. These studies show that the development of the parameters of the WSGG model under pressurized oxy-fuel conditions is a feasible technical route. A wider range of parameters of the model should be established according to different application conditions, and corresponding mathematical optimization should be introduced to improve the performance of the model.

3.1.2 Particle radiation heat transfer

In oxy-fuel combustion of solid fuel, thermal radiation is influenced not only by triatomic gas but also by various particles in the participating medium. These particles include unburned pulverized coal particles, fly ash, and soot. In the presence of particles, the gas radiation is suppressed, and particles play an important role in the radiation heat transfer in the furnace [87]. The experimental measurement methods of spectral emittance of micro/nanostructured particles are introduced in detail in Shan et al. [88], which can provide reference. Particle radiation also depends on the parameters such as particle composition, density, temperature, concentration, and size distribution. The radiation characteristics of particles have been confirmed to change with their composition [89,90]. For the particles in a furnace, the Mie theory is the most accurate method to describe the characteristics of radiation. Under the condition of dilute phase particles, for incident radiation, the particles do not shade each other, and each particle can be regarded as scattering independently. The extinction, scattering, and absorption coefficients of a particle system can be obtained by linearly superposing the parameters of a single particle. When the particle concentration is large, the interaction between particles cannot be ignored. The conditions for judging dependent scattering can be expressed as [91]

(15)

where f_{\mathrm{v}} is the volume fraction of particles. Under low particle concentration conditions, the absorption coefficients {\kappa }_{\mathrm{p}} and scattering coefficients {\sigma }_{\mathrm{p}} of particle clouds with discrete particle size distribution are expressed as [45]

(16){\kappa }_{\mathrm{p}}= \frac{3}{2}{\sum }_{i=1}^{Nt}\frac{Q_{\mathrm{a}\mathrm{b}\mathrm{s}}(d_i,m,\lambda )\cdot \rho \cdot w_i}{\rho \cdot d_i},

(17){\sigma }_{\mathrm{p}}= \frac{3}{2}{\sum }_{i=1}^{Nt}\frac{Q_{\mathrm{s}\mathrm{c}\mathrm{a}}(d_i,m,\lambda )\cdot \rho \cdot w_i}{\rho \cdot d_i},

where\rho is the mean concentration of particle, Nt is the total number of particles, w_i is normalized weight, and d_i is the size of particle; Q_{\mathrm{a}\mathrm{b}\mathrm{s}} and Q_{\mathrm{s}\mathrm{c}\mathrm{a}} are the absorption and the scattering factor, respectively, which can be solved by the Mie theory. The parameter of the particle size x and complex refractive index m are the most important parameters in the Mie theory, expressed as

(18)x=\text{π}{d}_i/\lambda ,

(19)m=n-ki.

Bahador & Sundén [92] and Ates et al. [93] used the Mie theory to obtain the absorption coefficient, scattering coefficient, and the phase function of particle radiation in a biomass boiler and a circulating fluidized bed boiler, respectively. As shown in Eqs. (16) and (17), pressure has no obvious effect on the radiation characteristics of a single particle. However, Zeng et al. [94] used the Mie theory to study the particle radiation of the dilute phase flue gas under the condition of oxy-fuel combustion in a PFB and found that the mean absorption coefficient of the particle cloud increases as pressure is raised due to the increased particle concentration and radiation saturation. Their experimental results showed that the heat flux only increased by 0.47% when the particle concentration increased from 0.01 to 0.02 kg/m³. Therefore, the radiation model produced under atmospheric pressure may still be employed at high pressures in some cases.

The particle radiation model has received relatively little attention compared to gas radiation. The particle media produced by combustion are dispersed in the gas medium. When studying the radiation characteristics of gas-particle mixtures, particles can be treated as participating media which is similar to gases. Based on the Mie theory and optical parameters such as complex refractive index, the influence of particles on radiation is expressed as a function of emission and absorption coefficients. This method separately calculates the radiation of particles and gases, which is relatively simple. Another method is to develop a coupled model of gas radiation and particle radiation, considering the interaction between gas and particles. This multi-component radiation transfer model can describe the complex radiation phenomena more accurately. Researchers have tried to use the WSGG model to explain the absorption and radiation characteristics of particles in the medium, but its accuracy is still not ideal [95]. Laubscher & Rousseau [96], considering the influence of different particle sizes on ash emissivity and scattering efficiency, developed a new gray particle radiation model using the empirical data of ash absorption and scattering efficiency, in which, particles are directly simplified as gray bodies for radiation heat transfer. The model improves the calculation efficiency but reduces the calculation accuracy. Modest & Riazzi [97] proposed the idea of using the FSK model to study gas-particle mixtures. Guo et al. [98] improved it by combining a non-gray radiation characteristic model with an FSK model, and obtained accurate and efficient results. These particle radiation models are based on the independent scattering of particles and the radiation characteristics of particles are also different with wavelength. With the increase in particle concentration, the interaction between particles cannot be ignored. Therefore, the above model is not suitable for application in fluidized bed boilers and other equipment with high particle concentrations. To deal with this problem, Cao et al. [99] developed the WSCK model, combining the Mie theory and experimental empirical formulations, while drawing on the WSGG and FSK models. Fig.10 shows the effect of particle concentration distribution on the incident flux on the wall. This model improves the particle scattering coefficient according to particle-dependent scattering, leading to a greater accuracy [100]. More detailed information on the establishment and application of the multi-component radiation model is summarized in Tab.3.

Section 3.1 provides a detailed summary of gas and particle radiation models under POC. These theoretical models can accurately describe the radiation characteristics of media, providing a basis for accurate calculation of the radiation heat transfer process in the POC system. CFD simulation is based on these theoretical models to obtain heat transfer characteristics under large-scale industrial dimensions, providing a reference for the design and operation of industrial systems. The self-developed gas radiation model can be coupled to the CFD software through the user-defined functions (UDF) module to solve the absorption coefficient, thereby adapting to different working conditions. The radiation characteristic parameters of particles calculated using the Mie theory also provide a reference for CFD. In summary, these theoretical models provide guidance for simulation and industrial applications.

For the research on radiation heat transfer in POC systems, in addition to the establishment of theoretical models and simulation work, many scholars have made contributions in experimentally studying the impact of pressure on radiation heat transfer. Kim et al. [102] used a laboratory-scale 15 kW_th fluidized bed boiler to analyze radiation characteristics in POC. The results showed that pressurization increases the flame temperature and enhances the heat recovery rate. The total heat recovery rate of the system at 10 bar increased by 3.3% compared with atmospheric pressure. Zhou et al. [103] conducted experiments and simulations of a 100 kW_th CFB-POC system. The FSCK model combined with the Mie theory was selected as the theoretical model. They found that the radiation in the lower area of boilers was greater than that in the upper area. Specifically, at a pressure of 6 bar, the radiation at a height of 3.5 m was 17 W/m² higher than that at a height of 5 m. Pressurization causes particle deposition, which greatly increases the particle concentration in the dense phase, causing the total radiation to increase. Combining theory and experiment, it is found that pressure has a promoting effect on radiation heat transfer.

3.2 Convection heat transfer

The convective heat transfer of flue gas will be improved under pressurized conditions. From the perspective of molecular dynamics, the increase of pressure enhances the collision frequency between molecules of flue gas and reduces the mean free path. Ultimately, this enhances the energy exchange process. The heat release coefficient of the convection heating surface of the boiler tube bundle can be expressed as

(20)Nu=h\frac{d}{\lambda }=C ({\mathit{R}\mathit{e})}^m({Pr)}^n.

The Prandtl number Pr is only slightly related to the mean temperature of the gas and has little effect on heat transfer. Different pressure boilers are designed with varying flow channel areas to ensure uniform flue gas flow rate. An increase in pressure can increase the number of molecules and density of the flue gas per unit volume, resulting in a higher Reynolds number Re. As a result, the convective heat transfer coefficient of POC flue gas is greater than that of atmospheric flue gas, even at the same flow rate. Through CFD simulation, Lian et al. [104] found that increasing pressure enhanced the heat transfer ability of particles. Ma et al. [105] used theoretical calculations to verify that the convective heat transfer coefficient and pressure drop on the flue gas side of POC system with a fixed structure increased compared with atmospheric conditions. The relevant data are summarized in Tab.4 to compare the flue gas side parameters under atmospheric and POC. Pressurization reduces flue gas volume, decreasing flow rates by 98.25%. However, the flue gas characteristics also change during pressurization, resulting in a 21.70% increase in the flue gas side heat transfer coefficient.

There also exists radiation heat transfer in the convection areas, represented by a high-temperature reheater and superheater. In the flue gas of air-fuel combustion, N₂, which lacks radiation ability, constitutes a significant proportion. In contrast, the flue gas of oxy-fuel combustion primarily consists of CO₂ and H₂O, significantly enhancing the radiation capacity. Pressurization promotes the generation of such triatomic gases, further enhancing radiation heat transfer in the convection areas. Yin et al. [106] simulated the convection area under different combustion conditions, and the variation of heat transfer coefficient is shown in Fig.11. With fixed combustion equipment, the transition from air-fuel combustion to oxy-fuel combustion and then to POC results in an increased heat transfer coefficient in the convection area. Specifically, the radiation heat transfer coefficient exhibits a significant increase, while the convective heat transfer coefficient shows a relatively smaller increase. Additionally, due to the low flue gas temperature at the high-temperature superheater, which is located behind the reheater, the radiation heat transfer coefficient of the high-temperature superheater is relatively small compared to that of the reheater, while the convective heat transfer coefficient is larger.

Calculation of flue gas enthalpy is an effective means to examine convection heat transfer conditions. For POC, the thermodynamic properties of real gases such as high-concentration CO₂ and H₂O in the flue gas change with the pressurization process, and the phase state may change. Therefore, the conventional method for calculating the enthalpy of an ideal gas under atmospheric pressure is no longer applicable. Dong et al. [107] treated the flue gas as ideal mixtures of several actual gases, other than directly as ideal gases. Based on the virial state equation, thermal properties were modified by using of deviation function, and then the calculation equation of the enthalpy value of the flue gas in the POC was deduced, as shown in Eq. (21). Analysis of the equation shows that enthalpy initially rises with pressure. However, once the critical pressure is exceeded, real gases liquefy, causing the enthalpy to drop. This method provides an effective way to study the physical properties of flue gas under pressurized oxy-fuel conditions. It can also be combined with convective heat transfer coefficient research to gain a more comprehensive understanding of energy transfer during combustion [108].

(21)h_{P,T}=h_{P_0,T_0}+{\int }_{T_0}^T c_P^0\mathrm{d} T-{\int }_{P_0\to 0}^P[R{T}^2(\frac{\mathrm{\partial }Z}{\mathrm{\partial }T}{)}_P{]}_T\mathrm{d} P,

where h_{P_0,T_0} is the reference enthalpy, when P₀ = 0, T₀ = 273 K, defined h_{P_0,T_0} = 0, Z is the compress factor.

Based on the above analyses of radiation and convection heat transfer, it can be concluded that POC increases the overall heat transfer capacity of the system. Regarding radiation heat transfer, temperature is the main influencing factor. Therefore, the radiative heat transfer of pressurized combustion is primarily concentrated in the higher-temperature furnace flame area. This can be further improved by utilizing high-temperature radiation materials or water-cooled wall coatings to elevate the flame area temperature. Concerning convection heat transfer, it mainly occurs within the flue gas region. Pressurization can increase the flue gas flow rate, thereby enhancing the convective heat transfer capacity. System design optimizations could include modifying the flue geometry and dimensions to increase the heat transfer surface area and improve gas flow conditions. Process control employing an optimized air-fuel ratio could also be considered. When designing POC systems, a compact furnace coupled with an enlarged flue can decrease the overall system size and burner investment costs while maintaining heat exchange performance. A balanced approach considering both radiation and convection heat transfer can lead to more efficient system designs.

4 Combustion reaction characteristics

Solid fuels are mainly used in large power stations, such as coal and biomass. However, the combustion of these fuels under pressurized oxy-fuel conditions can result in complex chemical reactions. This review focuses on the study of combustion chemical reaction characteristics of solid fuel.

4.1 Combustion characteristics

The combustion characteristics of coal under pressurized oxy-fuel conditions are different from those of atmospheric oxy-fuel combustion and air-fuel combustion, involving ignition characteristics, combustion characteristics, burnout characteristics, and other aspects. The increase in pressure promotes the combustion reaction process, but the promotion effect of pressure weakens after 5 bar [109].

Pressure affects the ignition mechanism of pulverized coal for oxy-fuel combustion, making the combustion temperature different and affecting the ash formation behaviors of mineral matters. The transformation of the ignition mechanism was explored by thermogravimetric experiments. Pulverized coal ignites heterogeneously at atmospheric pressure. Currently, the combustion temperature of oxy-fuel combustion of pulverized coal is relatively high. After a slight pressurization, the ignition state transitions to homogeneous. At this point, the combustion temperature starts to decrease. At 10 bar, the temperature is the lowest, and the combustion characteristics are optimal [110]. However, as the pressure continues to increase, volatile diffusion coefficient decreases and the ignition state begins transitioning from homogeneous to heterogeneous at 40 bar. It is not until 60 bar that the ignition mechanism fully transitions to heterogeneous ignition [111]. Therefore, during the gradual pressurization process from atmospheric pressure, the actual ignition mechanism undergoes a transition from heterogeneous to homogeneous and back to heterogeneous.

Pressurization has a positive effect on coal pyrolysis and combustion reaction rate [112]. The increase in pressure promotes dehydrogenation reaction and coal structure decomposition, improves the combustion performance of coal, promotes the decomposition of coal molecules into smaller fragments, contributes to the release of CO₂ and small particles, and accelerates the combustion reaction process. At the same time, the pressure increases the volume molar concentration of O₂, which is directly proportional to the oxy-fuel combustion reaction rate and greatly increases the combustion rate [113].

The combustion of pulverized coal volatile has a great impact on the whole combustion process. In the O₂/CO₂ atmosphere, because of gasification, a “flame layer” will be formed on the particle surface, making the volatile flame in O₂/CO₂ atmosphere darker than that in O₂/N₂ atmosphere at the same oxygen ratio. Furthermore, the ignition delay of particles in the O₂/CO₂ atmosphere is longer than that in O₂/N₂ atmosphere at the same pressure [114]. In the case of oxy-fuel combustion, the volatile from coal pyrolysis becomes more difficult to release with the increase of pressure, which reduces the release rate of volatile at the ignition stage, leading to an extended-release time and combustion duration of volatile. Therefore, ignition delay is extended, which brings adverse effects [115]. However, the effect of pressure on volatile combustion characteristics is also related to the coal type. As shown in Fig.12, the volatile burning time and ignition delay time of Powder River Basin (PRB) sub-bituminous coal both increase with pressure, while the ignition delay time of Shenhua (SH) bituminous coal first increases and then decreases with pressure [116]. Geng et al. [117] conducted lignite devolatilization experiments under different atmospheres in a horizontal tube furnace, and studied the oxy-fuel combustion reactivities of char through a thermogravimetry. The results show that pressurization has a positive impact on the increase in volatile component yield from lignite pyrolysis at a high CO₂ concentration, and adding water can further increase the volatile component yield.

In the POC process, coal char combustion is essential as it accounts for over 90% of coal particle combustion time. Moreover, the temperature of char plays a decisive role in the emission and safety of pollutants during boiler operation. The pressurized condition makes the char particles more broken, increases the effective reaction area, and advances the ignition time [118]. Pang et al. [119] conducted an oxy-fuel combustion test on char after high-temperature carbonization of anthracite coal in a PFB reactor, and studied the effects of combustion pressure and oxygen concentration on the burnout time and reactivity of char particles. Fig.13(a) shows that the burnout time decreases significantly with the increase of combustion pressure and oxygen concentration. The burnout time is shortened by nearly half with the pressure increasing from 1 to 5 bar. If the pressure continues to increase, char combustion will transition from the dynamic combustion zone to the diffusion combustion zone, and the promotion effect will weaken. Li et al. [114,120] studied the combustion mechanism of lignite coal char particles in a POC fluidized bed boiler through experiments and simulations. The results are shown in Fig.13(b). The change pattern of the burnout time of lignite particles with pressure is similar to that of char. With the increase of pressure, the O₂ diffusion mass flux increases. So do the flame temperature and char temperature. However, the increment gradually decreases, and the flame size narrows and lengthens. The burnout time of particles is significantly shortened.

Biomass is a carbon-neutral fuel derived from solar energy. Therefore, combining carbon capture technology with biomass fuel can help achieve negative carbon emissions. Some scholars have studied the POC characteristics of biomass fuel. Yang et al. [121] used the PFB to study the oxy-fuel combustion characteristics of high-density rice husk particles. As shown in Fig.14, compared with the air-fuel condition, the mean temperature of volatile flame is lower under the oxy-fuel condition. With the increase of pressure, the mean- and peak- temperatures of volatile flame and char surface rise. Besides, the particle burnout time is significantly shortened. However, due to the high content of alkali metal, the combustion of biomass fuel is very easy to be fouling and slagging, which can be unsafe for boiler operations. Therefore, coal and biomass are often blended for co-combustion to prevent slagging and to improve combustion performance. Liu et al. [122] conducted blending and co-firing of coal and biomass in a 10 kW_th pilot stage PFB and found that increasing pressure and biomass blending ratio can increase the fuel burnout rate and combustion efficiency. However, the increase in pressure will cause the ignition delay of the mixed fuel. Considering the promotion effect of pressure on combustion and the inhibition effect on oxygen diffusion, the optimal mixed combustion pressure should be 10 bar [123]. Studies on POC technology of biomass fuel are significant for achieving the carbon neutrality goal.

In the process of summarizing the above combustion characteristics, it is found that the test equipment widely involves thermogravimetric analyzers [111], tube furnaces [117], and laboratory-scale fluidization bed boilers [119]. The thermogravimetric test can provide key kinetic parameters for numerical simulations, and the tube furnace experimental results provide guidance for obtaining the correction factors of pyrolysis or combustion model in numerical simulations. Numerical simulation provides an efficient research method for industrial applications such as the design and optimization of industrial systems. The research results of the laboratory-scale burner provide a preliminary understanding of the combustion characteristics, and can be used as part of technical verification before industrial scale-up to ensure that the operation on a larger scale is feasible and reliable. By deeply understanding the basic characteristics of the combustion process through numerical simulation, industrial systems can be better designed and adjusted to improve efficiency and reduce emissions.

4.2 Pollutants emission characteristics

Like air-fuel combustion condition, the POC process also produces pollutants such as NO_x, SO_x, and trace metals. The process of nitrogen migration in POC is affected by pressure. With the increase of pressure, more fuel-N is converted to HCN, while the conversion rate of NH₃ is gradually reduced. The emission process of NO_x is roughly consistent with the combustion process. It is found that the generation characteristics of NO from coal combustion are affected by coal pyrolysis and the reduction of NO by CO and char [124,125]. When the oxygen ratio is low, the reduction effect of CO and char on NO is stronger under pressurized conditions, and the conversion rate of NO is gradually reduced. Conversely, when the oxygen ratio is higher (more than 25%), increasing pressure will accelerate the combustion rate of volatile matter, which enhances the effect of accelerating pyrolysis, and increases the NO conversion rate.

The conversion rate of fuel-N to NO₂ initially increases and then decreases as pressure increases. However, the conversion rate of fuel-N to NO_x increases continuously as pressure rises [126], as shown in Fig.15(a). At the same time, temperature is also a crucial factor that impacts NO_x emissions. Increasing temperatures promote the oxidization of N to NO, which is beneficial for the reduction of higher-order N. However, as the temperature gradually increases, the rate of reducing N consumption exceeds the rate of reducing high-order N to NO, leading to increased NO emissions, while the emissions of NO₂ have the opposite trend [127]. In terms of NO_x control technology, NO can be oxidized to NO₂, which can then be absorbed by water or an alkaline liquid. Kameda et al. [128] developed a selective catalytic reduction (SCR) technology to absorb and treat NO with Mg–Al oxide slurry and water.

The emission law of SO₂ in POC process is like that of air-fuel combustion [129]. In addition to the influence of fuel characteristics, the SO₂ emission will decrease with the pressure [130,131]. The variation of SO₂ emission with pressure is shown in Fig.15(b). Under the condition of POC, the SO₂ emission initially decreases as the temperature rises, but it reaches a minimum at 1173 K before increasing again. In terms of desulfurization, limestone is still a common adsorbent in POC. Moreover, the increase of combustion pressure can promote the desulfurization process under the condition of fixed CO₂ concentration [132].

In terms of the chemical reaction mechanism of pollutant generation, Liang et al. [127,131] described the chain reaction of NO_x and SO_x generation process based on the reaction mechanism database, which is summarized in Fig.16. Ajdari et al. [133] proposed a non-equilibrium level rate model that can be used to illustrate the NO_x and SO_x co-absorption chemistry of POC flue gases, including 34 reactions of 39 compounds, and it is found that pH affects the reaction path and the products in the liquid phase. To better adapt to engineering calculations, two simplified mechanism models have been developed for accurate prediction of contaminant removal and absorption in the liquid phase components [134]. The total package simplified mechanism includes 12 reactions of 20 compounds while the pH characteristic mechanism has 7 or 8 reactions of 14–17 reactants. Based on Ajdari’s simplified mechanism, Tumsa et al. [135] studied the effect of different parameters on the desulfurization and denitrification efficiency using a direct contact tower. The results indicate that the increase in pressure promotes NO_x absorption by water, while the removal efficiency of SO_x depends largely on the pH value and gas–liquid ratio. Some studies have confirmed that the chemical reaction of H₂O and O₂ with SO₂ and NO_x at a high pressure can realize the desulfurization and denitrification under the POC condition [136]. Although there are few studies, some scholars use coal and biomass co-combustion to improve combustion and reduce pollutant emissions. Some studies have shown that compared with single coal combustion, co-combustion of coal and biomass can effectively reduce the content of NO_x and SO₂ in flue gas, and the emission reduction effect on NO_x and SO_x is more obvious with the increase of pressure and biomass mixing ratio [137]. The best sulfur fixation effect occurs when the biomass mixing ratio is 0.5, as the conversion rate of S into SO₂ in the fuel is the lowest and the conversion rate of CaSO₄ in the ash is the highest [138]. Sahu & Prabu [139] analyzed the sulfide control in the co-combustion of municipal waste and coal. Ultrasonic desulfurization technology can remove 95.9% of SO_x in the flue gas. To summarize the impact of pressure on pollutant emissions under the POC condition and the removal efficiency of different desulfurization and denitrification devices from different research results, this review summarizes the relevant research results. The data results are detailed in Tab.5 and Tab.6.

There are few studies on the emission of fine particle and trace elements under the POC condition. Italian energy company ENEL investigated the emission of particle from its 5 MW ITEA demonstration unit at a pressure of 4 bar [148]. It is found that the characteristics of the emission curve of particles from fossil fuels at the nanoscale level (3–100 nm) are similar. There are differences between different fuels, coal combustion releases particles with a bimodal distribution at 3–50 nm. The particle emissions from POC boiler are lower when compared to conventional power plants. Additionally, the increase in pressure reduces the emission of insoluble small particles such as Fe and Si, but has little effect on volatile matter like Na, K, and S [140]. In terms of trace elements, Dong [150] simulated the emission of trace elements Hg and As under the POC condition using FactSage software. It is found that these two elements exist in the form of Hg (g), HgCl₂ (g), HgO (g) and AsO (g), As₄O₆ (g), and As₂O₅ (s). As pressure increases, the temperature of the HgCl₂ (g) conversion zone increases, the mole fraction of HgO (g) increases, and the temperature of the As₂O₅ (s) conversion zone increases. Furthermore, Xue et al. [151] conducted a study to determine the effect of different fuels (100%; 70% coal mixed with 30% biomass fuel straw) on the emission of semi-volatile heavy metal elements (Pb, Zn, and Cd). It is found that co-combustion inhibited the release of Pb, Zn, and Cd in the flue gas, making it more deposited in the bottom ash and reducing the emission of heavy metal pollutants when compared with coal combustion alone.

In general, there are relatively few experimental studies on the emission of fine particles and trace elements under the POC condition. There is also few developments and design of corresponding removal technology. Further research in these areas could be considered in the future.

5 Conclusions

POC technology can solve the problems of low efficiency and high cost of atmospheric oxy-fuel combustion systems, and can further promote the industrialization of CO₂ control technology. Several institutions have proposed and designed their own POC systems, all of which have advantages and disadvantages. Scholars have conducted many studies and obtained many results on combustion reaction kinetics, radiation heat transfer, convective heat transfer, and pollutant emission. However, there are still many shortcomings, and it is far from industrial application. Future research should mainly focus on the following aspects:

1) The current studies mainly focus on the combustion characteristics of POC based on experimental results. There is less development of theoretical combustion models under pressurized oxy-fuel conditions. Establishing theoretical or statistical models suitable for numerical simulation based on the reaction kinetic parameters of different solid fuels under different pressurized conditions will promote CFD simulation research on POC, and promote the design and development of new burners and systems. In the context of carbon neutrality, research on alternative energy sources such as biomass fuels and coal-biomass blends for POC is also essential.

2) POC gas has enhanced radiation heat transfer capability. In its calculation, LBL models, narrow-band models, etc. are relatively accurate but computationally inefficient. Developing a WSGG model suitable for POC gas engineering calculations is of great significance. Based on this, the development of a particle radiation model that combines calculation efficiency and accuracy and a model that can couple gas and solid radiation characteristics is a further research focus. In addition, it is necessary to conduct necessary convection heat transfer experiments of flue gas under oxy-fuel conditions to improve the calculation method of radiation and convection heat transfer of boilers, and provide guidance for the design of boilers and burners.

3) Research on the generation and emission of pollutants in POC mainly focuses on N and S element compounds, especially the chemical reaction mechanism and coupling interaction rules of pollutant generation. However, there is a lack of extensive experimental research on particles and trace metals, which should be further studied. In addition, there is currently less research on the removal and coordinated removal technology of pollutants in POC systems, which needs to be considered.

4) Overall, the energy consumption of the oxy-fuel combustion system is relatively high. Consideration should be given to developing a new POC cycle system or modifying the existing system, or using the coupling of multiple technologies integration to improve system efficiency. Considering that the current system mostly uses coal water slurry and gas fuel, only PFB oxy-fuel combustion systems can directly use solid fuels such as coal. In the future, it is possible to develop and design POC boilers and systems that can directly use coal or biomass fuel, and actively couple with corresponding technologies to improve energy conversion efficiency, such as coupling solar-driven POC systems for overall coal/biomass gasification. In addition, current research on system efficiency is mostly limited to process simulation and theoretical analysis. Institutions with technical ability can actively promote pilot-scale experiments, accumulate data and experience, and promote the industrialization process.

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