Oxy-fuel Fluidized Bed Combustion Research Articles

Particle-scale investigation of oxy-fuel combustion in a pressurized fluidized bed

A CFD-DEM model that includes chemical reactions such as fuel pyrolysis, combustion, gasification, volatile combustion, and the formation and reduction of gaseous pollutants was developed in the open-source MFIX. The extended model was validated at particle and reactor scales by comparison with experimental data. Subsequently, the model was applied to study the oxy-fuel combustion in a pressurized fluidized bed operating at varied pressures between 0.2 MPa and 0.8 MPa and O2 concentrations between 30 % and 50 %, yielding abundant particle-scale information. The results revealed that fuel combustion is positively impacted by volatile combustion. Raising the pressure or the O2 concentration, especially the pressure, enhances the combustion rate and temperature of fuel particles. The heat transfer of fuel particles during stable combustion is dominated by the reaction heat, followed by the convection heat, radiation heat, and particle–wall conduction heat, while heat conduction between particles can be safely ignored due to its small contribution. The heat and mass transfer of fuel particles is greatly improved under higher operating pressures but insignificantly affected by the O2 concentration of the combustion medium.

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.

Coal combustion emissions and ash formation characteristics during oxy-fuel combustion in a 100 kWth pressurized circulating fluidized bed

Pressurised circulating fluidized bed oxy-fuel combustion (PCFB-OFC) is a promising technology for CO2 capture because of its high carbon capture efficiency and net efficiency. However, there is a lack of comprehensive experimental studies on PCFB thermal state experimental devices due to the complexity of their design, construction, and operation. In this study, a 100 kWth PCFB-OFC experimental device was developed, and a series of coal combustion experiments were conducted under 0.1 to 0.6 MPa at an average oxygen concentration of 30% with a fixed peroxygen coefficient β (1.1–1.3). The effect of combustion pressure was investigated to determine the influence of pressure on coal combustion efficiency, furnace temperature distribution, gaseous pollutant emissions, and fly ash chemical composition. The experimental results showed that the increase in pressure increased the particle concentration in the dilute phase zone and increased the temperature at the top of the furnace, with a more uniform temperature distribution. The carbon content in the fly ash and the CO emissions in the flue gas were gradually reduced, and the combustion efficiency increased to 97.3%. The emissions of NO and N2O gradually decreased with the increase in load, and the NO emissions reduced by approximately 70% at 0.6 MPa. The emission of SO2 showed a downward trend as the pressure increases. Affected by the increase in pressure, the self-desulfurization capability of fly ash was improved. In addition, lower K2O and Na2O contents may reduce the deposition of fly ash in the furnace.

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.

Feature Selection and Uncertainty Analysis for Bubbling Fluidized Bed Oxy-Fuel Combustion Data

This paper presents a novel feature extraction and validation technique for data-driven prediction of oxy-fuel combustion emissions in a bubbling fluidized bed experimental facility. The experimental data were analyzed and preprocessed to minimize the size of the data set while preserving patterns and variance and to find an optimal configuration of the feature vector. The Boruta Feature Selection Algorithm (BFSA) finds feature vector’s configuration and the Multiscale False Neighbours Analysis (MSFNA) is newly extended and proposed to validate the BFSA’s design for emission prediction to assure minimal uncertainty in mapping between feature vectors and corresponding outputs. The finding is that the standalone BFSA does not reflect various sampling period setups that appeared significantly influencing the false neighborhood in the design of feature vectors for possible emission prediction, and MSFNA resolves that.

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.

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.

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.

Numerical simulation of circulating fluidized bed oxy-fuel combustion with Dense Discrete Phase Model

Computational fluid dynamics (CFD) has become a valuable tool to study the circulating fluidized bed (CFB) combustion process. This work conducts the CFB oxy-fuel combustion process with different O2 concentrations by using the Dense Discrete Phase Model (DDPM) based on the Eulerian-Lagrangian concept, in which the gas is treated as a continuous phase whereas the solid is tracked in a discrete way. The effects of heat and mass transfer, gas-turbulence as well as the homogeneous and heterogeneous reactions are considered. Simulated results are analyzed both qualitatively and quantitatively in terms of particle flow structure, temperature, carbon conversion and gas composition. Results show that the physical and thermo-chemical properties of the coal particles with different diameters vary greatly, which illustrates the significant particle size distribution (PSD) effects on the simulated results. Besides, with the increase of the inlet O2 concentration, the coal particles exhibit a different distribution of the flow structure and combustion characteristics due to both a decrease of the superficial gas velocity and an increase of the chemical reaction rate. The simulated results are well validated by the available experimental data, indicating the feasibility of the established DDPM for the CFB combustion process.

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 assessment and optimization of a pressurized fluidized bed oxy-fuel combustion power plant with CO2 capture

In this paper, a 600 MW pressurized fluidized bed oxy-fuel combustion power plant integrated with an air separation unit (ASU) and a CO2 compression and purification unit (CPU) was presented. Pressurization increases the flue gas dew point, and more phase-change heat of moisture is thereby available in the flue gas. In the baseline case of 10 bar, the net power efficiency of the plant was 33.40%. The effect of pressure on the plant performance was important whereas the effect of fluidized bed temperature was marginal. The plant with wet mode flue gas recirculation offered higher net power efficiency than the dry mode. Heat integration of ASU, CPU and acid condenser with the steam cycle was conducted to maximize the power production. The use of heat from the acid condenser increased the net power efficiency by 0.81% point over the baseline case. The heat recovery from ASU boosted the net power efficiency by 2.22% points, while the heat recovery from CPU resulted in the net power efficiency improvement of 0.34% point. In the optimization combining ASU, CPU and acid condenser, the net power efficiency increased to 36.83% (LHV), which was competitive over other oxy-fuel combustion counterparts.

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...

Advances in reduction of NO and N2O1 emission formation in an oxy-fired fluidized bed boiler

Fossil fuel combustion is one of the major means to meet the mounting global energy demand. However, the increasing NOx and N2O emissions arising from fossil fuel combustion process have hazardous effects. Thus, mitigating these gases is vital to attain a sustainable environment. Interestingly, oxy-fuel combustion in fluidized bed for carbon capture and minimized NOx emissions is strongly sustainable compare to the other approaches. It was assessed that NOx formation and fuel-N conversion have significant limitation under oxy-fluidized bed compared to air mode and the mechanism of NOx formation is still deficient and requires further development. In addition, this review paper discussed the potential of primary measure as low emission process with others supplementary techniques for feasible NOx reduction. The influences of combustion mode, operating parameters, and reduction techniques such as flue gas recirculation, oxygen staging, biomass co-firing, catalyst, influence of fluidized bed design and structure, decoupling combustion and their merges are respectively evaluated. Findings show that significant minimization of NOx emission can be achieved through combination of primary and secondary reduction techniques.

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.

Combustion of Lignite Coal in a Bubbling Fluidized Bed Combustor under Oxyfuel Conditions

Oxyfuel combustion is one of the leading technologies considered for capturing CO2. The field of oxyfuel combustion in a bubbling fluidized bed is recently addressed only marginally, but it is more suitable for lower capacity loads. The paper presents some results from bubbling fluidized bed oxyfuel combustion of lignite coal from 25kW experimental device and 250kW pilot BFB boiler. The methodology of the results presentation is stated and confrontation of the experimental results with the mathematical model is presented. The experiments were focused on identification of the oxyfuel regime operation and determination of the different CO2/O2 ratio in fluidized medium. The CO2 flue gas concentration was for the majority of experiments kept in the range of 84 to 91% and the oxygen concentration in flue gas from 4 to 8% in case of smaller laboratory combustor. Pilot BFB combustor suffers more with air leakage but the results correspond to the reality of the real operation of these facilities in an industrial scale.