Oxy-fuel Fluidized Bed Research Articles

Gaseous emissions from advanced CLC and oxyfuel fluidized bed combustion of coal and biomass in a complex geometry facility:A comprehensive model

Since the formation of gaseous pollutants during solid fuel combustion in circulating fluidized bed (CFB) units, especially under oxyfuel combustion and chemical looping combustion conditions, is an extremely complex process, the development of a simple predictive model of gas emissions is of practical significance. This paper introduces a novel approach based on fuzzy logic (FL), one of the main artificial intelligence (AI) methods, for the prediction of CO2, CO, NOx (i.e., NO + NO2), N2O, and SOx (i.e., SO2 + SO3) concentrations in flue gases from coal and biomass combustion in various operational sceneries. The simulations are carried out for different combustion environments, i.e., air-firing, oxyfuel, iG-CLC (in-situ gasification chemical looping combustion), and CLOU (chemical looping with oxygen uncoupling), under a wide range of operating parameters. A mixture of oxygen with CO2 was used for oxyfuel combustion. Three different oxygen carriers (OC) were examined in the study for iG-CLC and CLOU, i.e., ilmenite, copper oxide (60% wt.) with the support of carbonate waste from ore flotation, and copper oxide (60% wt.) with the support of ilmenite (20% wt.) and fly ash. The validation of the model was successfully performed on a complex geometry laboratory–scale 5 kWth Dual-Fluidized-Bed Chemical-Looping-Combustion of Solid-Fuels (DFB-CLC-SF) facility. The gaseous pollutant emissions predicted using the developed model were in good agreement with experimental results. The maximum relative error between measured and predicted by the model emissions was lower than 8%. The proposed method constitutes a quick and easy-to-run technique and a complementary tool compared to the experimental procedures and the programmed computing approach. The developed fuzzy logic-based model of gaseous emissions from advanced combustion (GasAdComb) of solid fuels can be easily applied by scientists and engineers to simulate and optimize coal and biomass combustion processes.

Effects of gas composition and operating pressure on the heat transfer in an oxy-fuel fluidized bed: A CFD–DEM study

CFD-DEM modeling was conducted to study the particle heat transfer in an oxy-fuel fluidized bed under different gas compositions and operating pressures. In particular, to determine the appropriate operating fluidization velocity for the pressurized oxy-fuel fluidized bed, three fluidizing parameters, the excess velocity Uex, fluidization number FN, and Froude particle number Frp, were evaluated by examining the similarities of the flow patterns and the particle heat transfer. The results revealed that the particle heat transfer is positively correlated with the gas density and specific heat capacity, as well as the fluidization velocity. Increasing the operating pressure augments the particle heat transfer, with the rate of increase being the highest at a constant Uex while being less significant at a constant Frp. Moreover, using the constant Frp under pressurized conditions makes possible an almost constant bed expansion and a more desirable particle temperature distribution, as well as a similar thermal input.

A Comprehensive Review of Ash Issues in Oxyfuel Combustion of Coal and Biomass: Mineral Matter Transformation, Ash Formation, and Deposition

To alleviate the global warming issue, various CO2 abatement technologies have been proposed. As one of the leading options for CO2 capture in power plants mostly firing coal and biomass, the oxyfuel combustion technology has been actively developed for decades. Progress at different stages of the development has been continuously updated in several reviews. However, they usually cover a wide range of topics, which vary significantly in detail. The ash issues are very critical to the design and operation of oxyfuel boilers, but may be one of the most controversial topics. The knowledge is widely scattered in different research articles, and a relatively comprehensive description is not available in the published reviews. Therefore, it is difficult for one to have a clearer picture of this important topic. The present review attempts to provide a more complete understanding. It pays particular attention to experimental data, because they are the basis for model development and full-scale simulation. Different from previous reviews that presented limited information on mineral matter transformation under oxyfuel conditions, this Review summarizes most of the related results reported in the open literature. Data on ash formation and deposition in oxyfuel combustion are also updated. They are organized according to different combustion technologies, i.e., oxyfuel pulverized fuel combustion (oxy-PFC) and oxyfuel fluidized bed combustion (oxy-FBC). By correlating between the three aspects, an in-depth understanding of ash issues in oxyfuel combustion may be achieved. At the end of this Review, key messages regarding mineral matter transformation, ash formation and deposition in oxyfuel combustion, and future research needs are presented.

Particulate matter formation mechanism during pressurized air-and oxy-coal combustion in a 10kWth fluidized bed

Pressurized fluidized bed oxy-fuel combustion is believed to be a promising clean coal technology for power plants to achieve economically efficient CO2 capture. However, studies on the particulate matter (PM) formation mechanism under pressurized conditions were very limited. In this work, a 10 kWth pressurized oxy-fuel fluidized bed combustor (POFBC) was used, and 0.1/0.3/0.5 MPa were conducted for both air- and oxy-coal combustion. The effects of combustion atmosphere and pressure on particle size distribution (PSD) and size segregated composition of PM were explored. The results indicated that the concentration of PM1 generated in oxy-fuel combustion was higher than that in air combustion regardless of pressure, but this difference decreased with the increase of pressure, and the influence of combustion atmosphere on the concentration of PM1-10 was not obvious. In oxy-fuel combustion, the elevated pressure reduced the concentration of PM1 but increased the concentration of PM1-10. Moreover, elevated pressure can decease the yield of most elements in submicron particles. Specifically, it appeared that the effect of pressure on the yields of volatile species (Na, K, and S) in PM1 were not significant, but the yields of refractory species (Fe and Si) in PM1 were obviously smaller at elevated pressures.

Modelling the co-firing of coal and biomass in a 10 kWth oxy-fuel fluidized bed

The oxy-fuel co-firing of solid fuels (such as coal, biomass, and solid waste) in fluidized beds is one of the most promising technologies for the industrial application of CO2 capture and waste disposal. However, both the practical experimentation and numerical simulations for elucidating co-firing in an oxy-fuel fluidized bed are still limited. In this study, a multiphase particle-in-cell scheme based on a 3D Eulerian–Lagrangian model was further developed following our previous research on oxy-fuel co-firing in a micro fluidized bed (Powder Technol. 2020, 373, 522–534). The refined JL 4-step mechanism for the CO-CO2 homogeneous reactions, the heterogeneous reactions of char oxidation and gasification, the heterogeneous reactions of NO and N2O formation from char-N, and the self-desulfurization effect were comprehensively considered. The improvement of the models was verified through the continuous operation of 10 kWth oxy-fuel fluidized bed tests (Fuel 2021,286,119,312; Energy Fuels, 2020, 34, 7373–7387), and the effects of the biomass blending ratio (Mb) on co-firing characteristics were discussed. It was found that the improvements could enhance the adaptability of the models to the oxy-fuel atmosphere, and the accurate prediction of NO, N2O, SO2. With an increase in Mb, the main reaction zone expanded or moved up along the riser height, and the volume of the high-temperature area increased, which promoted the burnout of particles and CO2 emission when Mb is 50%. The high volatility of biomass increased O2 consumption and CO concentration at the upper part of the riser, reduced N2O formation, and had a significant impact on NO reduction. The low sulfur content and high Ca/S ratio of the biomass considerably reduced the SO2 concentration. The simulation results also provided helpful information for the design and operation control of oxy-fuel co-firing of coal and biomass in a fluidized bed, such as the oxidant supply in different areas and grades, appropriate increase in the riser height, and reasonable adoption of Mb.

Power-to-Green Methanol via CO2 Hydrogenation—A Concept Study including Oxyfuel Fluidized Bed Combustion of Biomass

A revolution of the global energy industry is without an alternative to solving the climate crisis. However, renewable energy sources typically show significant seasonal and daily fluctuations. This paper provides a system concept model of a decentralized power-to-green methanol plant consisting of a biomass heating plant with a thermal input of 20 MWth. (oxyfuel or air mode), a CO2 processing unit (DeOxo reactor or MEA absorption), an alkaline electrolyzer, a methanol synthesis unit, an air separation unit and a wind park. Applying oxyfuel combustion has the potential to directly utilize O2 generated by the electrolyzer, which was analyzed by varying critical model parameters. A major objective was to determine whether applying oxyfuel combustion has a positive impact on the plant’s power-to-liquid (PtL) efficiency rate. For cases utilizing more than 70% of CO2 generated by the combustion, the oxyfuel’s O2 demand is fully covered by the electrolyzer, making oxyfuel a viable option for large scale applications. Conventional air combustion is recommended for small wind parks and scenarios using surplus electricity. Maximum PtL efficiencies of ηPtL,Oxy = 51.91% and ηPtL,Air = 54.21% can be realized. Additionally, a case study for one year of operation has been conducted yielding an annual output of about 17,000 t/a methanol and 100 GWhth./a thermal energy for an input of 50,500 t/a woodchips and a wind park size of 36 MWp.

Movement and combustion characteristics of densified rice hull pellets in a fluidized bed combustor at elevated pressures

Pressurized oxy-fuel fluidized bed combustion is regarded as a promising carbon capture technology, which can realize negative CO2 emission through burning biomass. However, given its high volatile content and low density, biomass has been frequently reported to demonstrate poor mixing in industrial FB combustors. Especially in the case of pressurized oxy-fuel combustion, the gas-solid flow in the reactor, fuel particle motion and combustion process will all change and influence each other, which is very important for the particle conversion process and the safety of boiler operation. To correlate the location information with the combustion process in pressurized oxy-fuel combustion, densified rice hull pellets, serving as a typical biomass, were selected as the fuel, and entire whole combustion process was recorded by a color video camera to facilitate the temperature measurement of the volatile flame and char particle by two-color pyrometry. The distribution of a densified rice hull pellet at any given location was obtained from statistics derived from the images. Results showed that the pressure and atmosphere change does not strongly affect the probability of densified rice hull pellets appearing at a given location during the devolatilization process itself. However, during the char combustion process, the probability of finding a particle at the bed surface increased with pressure whereas particle concentrations in the splash zone showed little change, despite the enhanced bubble collapse at elevated pressures. However, the temperature of the volatile flame and char particles increases with the pressure whereas the burnout time of biomass particle decreases due to better oxygen transport.

The Gas Interchange between Bubble and Emulsion Phases in a Pressurized Fluidized Bed by Computational Fluid Dynamics Simulations

This study characterizes the effects of pressure, superficial velocity, and particle size on gas interchange between bubble and emulsion phases, in an oxy-fuel fluidized bed under high temperature....

Three-dimensional simulation of the co-firing of coal and biomass in an oxy-fuel fluidized bed

The co-firing of coal and biomass in fluidized beds under oxy-fuel conditions is an important approach with significant industrial prospects, for both carbon dioxide capture and disposal of biomass waste produced by agriculture and forestry. Numerical simulation is essential in the design of structural and operational parameters for oxy-fuel fluidized beds, as is experimentation. Following our previous research on the oxy-fuel combustion of coal in a fluidized bed (Powder Technol. 2019, 349, 40–51), this paper develops 3D Eulerian-Lagrangian simulations for the co-firing of coal and biomass in an oxy-fuel fluidized bed based on the multiphase particle-in-cell (MP-PIC) scheme. The particle field is described by the discrete particle method (DPM) and the gas field is calculated by large eddy simulation (LES). Different models are used for the reaction of coal and biomass, including devolatilization, combustion of char and volatiles, and the production of pollutants. The numerical simulation was verified by combustion experiments carried out in a micro-fluidized bed reactor with an online mass spectrometer. Based on the simulations, the effects of biomass mass ratio, combustion atmosphere, oxygen concentration, and combustion temperature on the co-firing of coal and biomass in an oxy-fuel fluidized bed are discussed.

Ash deposition during pressurized oxy-fuel combustion of Zhundong coal in a lab-scale fluidized bed

The paper presents the ash deposition behavior during pressurized oxy-fuel fluidized bed combustion (POFBC) of Zhundong coal at 0.3 MPa, which has never been reported before. Ash deposits collected from different positions (windward, leeward and vertical sides) of the probe under different atmospheres were investigated in detail. The results showed that the inside deposits, i.e. the initial deposit layers, on the different positions shared the similar composition and morphology. However, the atmosphere had a significant impact on the ash deposition behavior. The inside deposits have more regular spheres and larger size under 30% O2/70% CO2 condition than those from 21% O2/79% N2 and 21% O2/79% CO2 combustion, probably due to the intensive submicron particle coagulation. Furthermore, the particles contained in the inside deposits had higher Fe and Cl in 30% O2/70% CO2 combustion compared with that in the other two combustion modes. However, the content of S in the deposits was the highest under 21% O2/79% CO2 condition, which may be attributed to the fact that high CO2 partial pressure strengthens self-desulfurization ability of ash. In addition, S and Si contributed significantly to the compositions of the inside deposits.

Oxy-pressurized fluidized bed combustion: Configuration and options analysis

Bio-energy carbon capture and storage is a promising option to mitigate or eliminate CO2 emissions from fossil fuel-based heat, steam, and power generation. Among the different carbon capture technologies, oxy-fuel fluidized bed combustion is an attractive option due to its fuel flexibility and moderate operating temperature, making it suitable for bio-energy carbon capture and storage. A recent breakthrough in the development of this technology is operation under pressurized conditions. In order to assess the benefits of oxy-pressurized fluidized bed combustion, a number of design configurations and options were analyzed via process simulation. Three coals, one coke, and one torrefied hardwood biomass were selected for this work. A series of parameters, including reactor and system configuration, were varied to determine their impact on key performance metrics such as oxygen requirement, power balance, availability of high temperature heat, and by-product generation. The shift from coal to torrefied biomass only marginally affected the total amount of recoverable heat for power production, but resulted in significant reductions in both power and oxygen requirements. It was observed that an increase in operating pressure led to a higher amount of available heat due to the recovery of the latent heat of H2O condensation. An optimum operating pressure, ranging from 7 to 15 bar(g), was obtained when considering the total power consumed within the power plant. Results showed that the total quantity of recoverable high temperature heat is slightly higher for the circulating bed configuration in comparison to that of the bubbling. The use of a flue gas wet sulphuric acid process for sulphur removal resulted in the production of a high purity sulphuric acid product (>98 wt%) and increases in useable heat were noted at elevated pressures where total sulphur capture could be pushed to as high as 99.97%. Additionally, changes in flue gas O2 concentration via the wet sulphuric acid process and sorbent injection into the combustor were found to have significant impacts on process performance and natural gas/O2 demands.

Numerical simulation of fluidization for application in oxyfuel combustion

<p style='text-indent:20px;'>This paper is concerned with the simulation of multiphase flow hydrodynamics in an experimental oxyfuel fluidized bed combustor designed for biomass fuels. The aim is to perform cross-validation between several models and solvers that differ in the description of some phenomena in question. We focus on the influence of turbulence modeling, inter-phase drag force models, the presence of biomass in the mixture. Also the possibility to simplify the full 3D description to a quasi-1D model is tested. However, the results indicate that such simplification is not suitable for chaotic phenomena in considered scenarios. The models were developed using ANSYS Fluent and OpenFOAM CFD software packages as well as our in-house CFD code CFBSim. The quantities relevant for comparison (the densities of the dispersed solid phases and the phase velocities) are presented in the form of cross-section averaged vertical profiles.

Release and migration characteristics of sodium and potassium in high alkali coal under oxy-fuel fluidized bed combustion condition

Zhundong coal is promising to replace inferior coal in fossil-fueled power plants, considering its high abundance, low contents of sulfur and nitrogen. However, the relative high contents of alkalis, especially sodium and potassium, damage the operational economy and safety badly since the release of alkali metals in flue gas can lead to fly ash-related problems such as slagging, ash accumulation and corrosion. Although, oxy-fuel circulating fluidized bed (CFB) combustion is beneficial to reduce the release of sodium and potassium by lowering the operation temperature, the ash formation characteristics of Zhundong coal under the oxy-fuel CFB is still scarce. Our results showed that the residual amounts of sodium and potassium in the ash samples, both tend to decrease with increasing temperature no matter under air combustion or oxy-fuel combustion. However, the sodium content in the cinder obtained under the oxy-fuel combustion atmosphere was higher than that under the air combustion atmosphere, and the insoluble sodium, mainly existed in the form of aluminosilicates, was the main form. In addition, the increase of steam content in both atmospheres reduced the sodium content in the cinder while the residual amount of potassium increased. Density functional theory calculations verified the transformation behavior between sodium and potassium and the inhibiting effect of steam. This study elucidates the release and migration characteristics of sodium and potassium in Zhundong coal both experimentally and theoretically and will benefit the popularity of Zhundong coal.

Co-firing of coal and biomass in oxy-fuel fluidized bed for CO2 capture: A review of recent advances

Abstract The co-firing of coal and biomass in oxy-fuel fluidized beds is one of the most promising technologies for capturing CO2. This technology has attracted wide attention from academia and industry in recent years as a negative emission method to capture CO2 produced by carbon contained in biomass. In the past decades, many studies have been carried out regarding experiments and numerical simulations under oxy-fuel combustion conditions. This paper firstly briefly discusses the techno-economic viability of the biomass and coal co-firing with oxy-combustion and then presents a review of recent advancements involving experimental research and computational fluid dynamics (CFD) simulations in this field. Experimental studies on mechanism research, such as thermogravimetric analysis and tube furnace experiments, and fluidized bed experiments based on oxy-fuel fluidized beds with different sizes as well as the main findings, are summarized as a part of this review. It has been recognized that CFD is a useful approach for understanding the behaviors of the co-firing of coal and biomass in oxy-fuel fluidized beds. We summarize a recent survey of published CFD research on oxy-fuel fluidized bed combustion, which categorized into Eulerian and Lagrangian methods. Finally, we discuss the challenges and interests for future research.

Fundamental study on fuel-staged oxy-fuel fluidized bed combustion

The concept of a fuel-staged oxy-CFB (oxy-fuel circulating fluidized bed) combustion technology is proposed by this article to extend the operation range of an existing air-fired CFB boiler retrofitted for oxy-fuel operation. Experiments were designed to study the combustion behavior of a single coal particle at different locations of the furnace (dense bed and freeboard), which should differ largely from those in the conventional feeding to dense bed. Three different kinds of coal under various atmospheres were studied in a visualized fluidized bed combustor, by on-line monitoring the temperature of fuel particle and volatile flame simultaneously. As expected, shorter ignition delay time (td) was observed in the dense bed due to the stronger heat transfer than that in the freeboard. However, surprisingly, shorter devolatilization time (tv), higher peak temperature (Tpeak) and shorter burnout time (tb) were observed in the freeboard of the furnace, which were probably due to higher mass transfer rate of oxygen from gas phase to the reacting particle. When N2 was replaced by CO2, both the td and tb increased while the flame and particle temperature decreased significantly.

Numerical and Experimental Study on Oxy-fuel Coal and Biomass Co-firing in a Bubbling Fluidized Bed

As one of the most promising approaches to reduce CO2 emissions from power plants, oxy-fuel fluidized bed combustion technology still has a lot of basic uncertainties requiring in-depth studies, especially in application for coal and biomass co-firing. This work focuses on results of coal co-firing with biomass under O2/CO2 combustion conditions in a 6 kWth bubbling fluidized bed combustor. With the same thermal input maintained, computational fluid dynamics (CFD) was adopted as a predictive tool to examine the effects of firing coal and co-firing biomass with coal under O2/CO2 atmospheres. Hydrodynamics, heat transfer, species concentration, and char gasification in a fluidized bed were investigated. In comparison to coal combustion, the hydrodynamic and temperature profiles of biomass co-firing are not affected much when maintaining the same thermal input. The temperature of biomass co-firing at the bottom of the dense-phase zone, which is mainly the char combustion area, is about 10 K lower than that o...

Thermodynamic Analysis and Optimization of an Oxyfuel Fluidized Bed Combustion Power Plant for CO2 Capture

In this paper, a 600 MW supercritical oxyfuel fluidized bed combustion power plant is integrated with an air separation unit (ASU) and a flue gas compression and purification unit (CPU). The plant performance is investigated, and two flue gas recirculation modes (i.e., wet and dry modes) are compared under different operating conditions. In the initial analysis, the dry-mode plant has a net power efficiency of 31.6%, while the wet-mode plant has a net power efficiency of 31.5%. In the sensitivity analysis, the air ingress has the most significant influence on the net power efficiency of the plant. The furnace combustion temperature also affects the net power efficiency. The plant is optimized by recovering heat from the ASU, CPU, and acid condenser and utilizing the heat to preheat the feedwater. Although the dry-mode plant has a higher efficiency before optimization, the wet-mode plant has a higher efficiency after optimization; that is, the net power efficiency increases by 2.7% for the wet mode compare...

Flow characteristics in pressurized oxy-fuel fluidized bed under hot condition

Pressurized oxy-fuel fluidized bed (POFB) combustion is regarded as a promising technology for carbon capture from coal-fired power plants. High pressure and temperature conditions have important impacts on the flow characteristic of fluidized bed, and understanding them will help to optimize the design and operation of the POFB boiler. In this work, experiments were carried out in two pressurized fluidized bed (PFB) devices (a hot PFB and a “visual PFB”) both operated under high temperature (20–800°C) and high pressure conditions (0.1–1.0 MPa). Four parameters including the minimum fluidization velocity (umf), the minimum bubbling velocity (umb), bubble diameter (Db) and bubble frequency (f) were examined in this study. Results showed that the umf decreases with rising pressure and temperature. Based on our results a formula was fitted for calculating the minimum fluidization velocity in PFB, with a relative error less than 15%. With the increase of fluidization number (w), the bubble size and tail vortex increased gradually, the bubbles tended to merge, and the shape of bubbles became more irregular. The Db decreases with the increase of temperature and pressure at the same w. The f increases with increased w, while it decreased with the increase of temperature and pressure.

Conversions of fuel-N to NO and N2O during devolatilization and char combustion stages of a single coal particle under oxy-fuel fluidized bed conditions

The conversions of fuel-N to NO and N2O during devolatilization and char combustion stages of a single coal particle of 7 mm in diameter were investigated in a laboratory-scale flow tube reactor under oxy-fuel fluidized bed (FB) conditions. The method of isothermal thermo-gravimetric analysis (TGA) combing with the coal properties was proposed to distinguish the devolatilization and char combustion stages of coal combustion. The results show that the char combustion stage plays a dominant role in NO and N2O emissions in oxy-fuel FB combustion. Temperature changes the trade-off between NO and N2O during the two stages. With increasing temperature, the conversion ratios of fuel-N to NO during the two stages increase, and the opposite tendencies are observed for N2O. CO2 inhibits the fuel-N conversions to NO during the two stages but promotes those to N2O. Compared with air combustion, the conversion ratios of fuel-N to NO during the two stages are lower in 21%O2/79%CO2, and those to N2O are higher. At <O2> = 21–50% by volume, the conversion ratios of fuel-N to NO during the two stages reach the maximum values at <O2> = 30% by volume, and those to N2O decrease with increasing O2 concentration. H2O suppresses the fuel-N conversions to NO and N2O during the two stages. A higher coal rank has higher total conversion ratios of fuel-N to NO and N2O. Fuel-N, volatile matter, and fixed carbon contents are the important factors on fuel-N conversions to NO and N2O during the two stages. The results benefit the understanding of NO and N2O emission mechanisms during oxy-fuel FB combustion of coal.

Reduction of recycled NO over char during oxy-fuel fluidized bed combustion: Effects of operating parameters

To investigate the effects of operating parameters on the recycled NO reduction over char during oxy-fuel fluidized bed combustion, tests with two types of char were carried out in a bubbling fluidized bed system. The reduction ratios of recycled NO were calculated based on the test results. It was found that char combustion obviously affected the recycled NO reduction in the oxy-fuel fluidized bed combustor. The reduction of recycled NO was promoted by the increase of oxygen concentration in the feeding gas. The reduction ratio of recycled NO could be improved by the higher reaction temperature. Although the smaller sizes of char particles were conducive for char combustion, char particle sizes had little effects on the reduction of recycled NO. The prolonging of gas residence time favored the reduction of recycled NO over char. In addition, the effects of various char types on the recycled NO reduction were also evaluated in this study.