Oxy-fuel Combustion Conditions Research Articles

Advanced orthogonal analysis of multi-factorial interactions influencing co-combustion performance and pollution reduction potential of coal gangue and biomass in oxy-fuel combustion conditions

In the current scenario of increasing energy scarcity and stricter environmental protection requirements, the utilization of coal gangue and biomass energy is gaining attention worldwide. Biomass and coal gangue are complementary in nature, and their co-combustion can improve the burnout efficiency of coal gangue and reduce pollutant emissions. In this study, coal gangue and wheat straw are selected as research subjects, the optimal process conditions for their co-combustion under the coupling of multiple parameters in oxy-fuel atmosphere through orthogonal experiments are investigated. The results indicate that the most significant factor influencing combustion characteristics is the heating rate. When the biomass content is 45 %, O2 concentration is 50 %, heating rate is 30℃/min and particle size is 180–250μm, the flammability and comprehensive combustion indexes of the samples are maximized, which indicates the optimum combustion characteristics. The primary factor affecting pollutant release is biomass content. When the biomass content is 45 %, O2 concentration is 75 %, combustion temperature is 700℃, and particle size is 180–250μm, the pollutant release concentration is minimized.

Review on Mercury Control during Co-Firing Coal and Biomass under O2/CO2 Atmosphere

Combining biomass co-firing with oxy-fuel combustion is a promising Bioenergy with Carbon Capture and Storage (BECCS) technology. It has the potential to achieve a large-scale reduction in carbon emissions from traditional power plants, making it a powerful tool for addressing global climate change. However, mercury in the fuel can be released into the flue gas during combustion, posing a significant threat to the environment and human health. More importantly, mercury can also cause the fracture of metal equipment via amalgamation, which is a major risk for the system. Therefore, compared to conventional coal-fired power plants, the requirements for the mercury concentration in BECCS systems are much stricter. This article reviews the latest progress in mercury control under oxy-fuel biomass co-firing conditions, clarifies the impact of biomass co-firing on mercury species transformation, reveals the influence mechanisms of various flue gas components on elemental mercury oxidation under oxy-fuel combustion conditions, evaluates the advantages and disadvantages of various mercury removal methods, and finally provides an outlook for mercury control in BECCS systems. Research shows that after biomass co-firing, the concentrations of chlorine and alkali metals in the flue gas increase, which is beneficial for homogeneous and heterogeneous mercury oxidation. The changes in the particulate matter content could affect the transformation of gaseous mercury to particulate mercury. The high concentrations of CO2 and H2O in oxy-fuel flue gas inhibit mercury oxidation, while the effects of NOx and SO2 are dual-sided. Higher concentrations of fly ash in oxy-fuel flue gas are conducive to the removal of Hg0. Additionally, under oxy-fuel conditions, CO2 and metal ions such as Fe2+ can inhibit the re-emission of mercury in WFGD systems. The development of efficient adsorbents and catalysts is the key to achieving deep mercury removal. Fully utilizing the advantages of chlorine, alkali metals, and CO2 in oxy-fuel biomass co-firing flue gas will be the future focus of deep mercury removal from BECCS systems.

Co-combustion characteristics and kinetics of sludge/coal blends in 21%–50% oxygen-enriched O2/CO2 atmospheres

This research investigated the thermogravimetric analysis (TGA) and combustion kinetics of sewage sludge and Australian sub-bituminous coal and their blends with different mixing ratios (sludge/coal = 100/0, 80/20, 50/50, 20/80, 0/100 by weight) under air atmosphere (O2/N2 = 21/79 by volume), oxy-fuel combustion (O2/CO2 = 21/79, 25/75, 35/65, and 50/50 by volume), and pyrolysis (pure N2 atmospheres). The gaseous products that evolved from the TGA experiments were also analyzed using a Fourier transform infrared (FTIR) spectrometer under pyrolysis conditions. Results showed that the mixed fuel with higher coal blending ratio had higher ignition temperature, lower burnout temperature and larger combustion characteristic index. Furthermore, TGA–FTIR experiments of the 20 % sludge + 80 % coal blended fuel under nitrogen atmospheres showed more gasification components in pure coal than in sludge. The combustion kinetics analytical results via the Flynn–Wall–Ozawa method indicated that the larger the conversion degree, the smaller the obtained activation energy value. Under the oxy-fuel combustion conditions, when the oxygen concentration increased from 21 % to 50 % for the 20 % sludge + 80 % coal blended fuel, the ignition and burnout temperatures decreased, and the combustion characteristic index increased nearly three times. Oxygen enrichment can clearly improve combustion characteristics and flammability.

Experimental investigation on the NO formation of pulverized coal combustion under high-temperature and low-oxygen environments simulating MILD oxy-fuel combustion conditions

The NO formation experiments simulating moderate and intense low-oxygen dilution (MILD) oxy-coal combustion conditions were conducted on a laminar diffusion flame burner with the coflow temperatures of 1473–1873 K and the oxygen volume fractions of 5 %–20 % in O2/CO2, O2/Ar and O2/N2 atmospheres. The flame images of pulverized coal combustion were captured to obtain the ignition delay distances, and the axial species concentrations were measured to obtain the variation of NO formation and reduction. The NO yield in O2/Ar atmosphere decreased by nearly 0.2 when the oxygen volume fraction decreased from 20 % to 5 % and by about 0.05 when the coflow temperature decreased from 1873 K to 1473 K. The NO yield in O2/CO2 atmosphere was 0.1–0.15 lower than that in O2/Ar atmosphere. The optimal kinetic parameters of thermal NO and fuel NO formation rate were obtained by a nonlinear fit of nth-order Arrhenius expression. Finally, the relative contribution rates of thermal NO to total NO (Rth) and NO reduction to fuel NO (Rre) were quantitatively separated. Rth decreases with the increase of oxygen volume fraction, below 6 % at 1800 K, 25 % at 2000 K. Rre is almost unaffected by the coflow temperature and affected by the oxygen volume fraction, reaching 30 % at 5 % O2.

Comparison of the flame stabilities during oxy-methane and air-methane combustion in a two-layer porous burner

The development of the oxy-fuel porous media combustion technique not only takes advantage of the characteristics of porous media combustion, such as a relatively low lean-burn limit and small volume, but also dramatically facilitates carbon capture and storage. In this paper, a two-layer porous burner model is established, in which a two-temperature equation model is adopted for calculation purposes. Differences in combustion behaviour between oxy-fuel and air-fuel conditions are compared. The results show that the difference in physical properties between CO2 and air is the main reason for the significant variation of combustion behaviours under the conditions of oxy-fuel compared to air-fuel. The specific performance is lower combustion temperature and flame propagation speed. The temperature under oxy-fuel combustion conditions is 300 K lower than that under air-fuel combustion conditions at an equivalence ratio of 0.6, and the stable range of oxy-fuel combustion is only approximately 50% of air-fuel combustion. The impact of porous media material parameters on combustion behavior has been investigated. These variables include the equivalence ratio, material thermal conductivity, volume heat transfer coefficient, and extinction coefficient, while the ratio of O2/CO2 remains fixed at 0.21/0.79. The influence of these variables on the stable velocity range and temperature field is consistent, but a large difference in values occurs. Reducing the thermal resistance of the burner by adjusting the properties of the porous matrix can increase the velocity limit of stable combustion. However, even if the thermal conductivity and extinction coefficient are increased by 5 times, there will be no significant impact on the stable range. Reducing the pore size of the combustion zone by 50% will increase the stable range by over 100%.

Release of Sulfur and Chlorine Gas Species during Combustion and Pyrolysis of Walnut Shells in an Entrained Flow Reactor

The release behavior of sulfur and chlorine compounds into the gas phase of walnut shell particles (WNS) is studied with an entrained flow reactor. Experiments are carried out in nitrogen (N2), carbon dioxide (CO2) atmosphere and under air and oxy-fuel conditions at different temperatures (T = 1000–1300 °C) and stoichiometries (λ = 0.8–1.1). A total of 98.7% of fuel-bound sulfur volatilizes as sulfur dioxide (SO2), carbonyl sulfide (COS) and hydrogen sulfide (H2S) in the gas phase in N2 atmosphere at 1000 °C. As hydrogen chloride (HCl), 37.0% of the chlorine is released at this temperature. In CO2 atmosphere, a similar total release of sulfur and chlorine is observed (1000 °C). With each temperature increment, the release of SO2, H2S and HCl in the gas phase decreases (N2 and CO2 atmosphere). SO2 forms the major sulfur component in both atmospheres. In CO2 atmosphere, higher concentrations of COS were detected than in N2 atmosphere. Air and oxy-fuel combustion conditions show significantly lower SO2, COS and HCl concentrations as in N2 and CO2 atmosphere. No H2S is detected in the gas phase during any of the combustion trials.

Predicting the Equilibrium Product Formation in Oxy-fuel Combustion of Octane (C8H18) using Numerical Modeling

Preserving the environment is a major challenge for modern society, and reducing the greenhouse effect caused by combustion processes is a primary concern. It is known that nitrogen compounds (NOx) negatively impact air quality and public health. Internal combustion engines, responsible for nearly half of the atmosphere's pollutants, have prompted public policymakers to phase out gasoline and diesel-powered vehicles in the near future. Carbon capture and storage technologies, including oxy-fuel combustion, are being developed to address these environmental challenges. To predict exhaust pollutants such as NO and CO, assuming that the exhaust gases resulting from fuel and air combustion are in chemical equilibrium is a useful approximation. In the current case study, a numerical investigation on the impact of modifying the oxygen content of the reaction mixture from 21% to 100% on the equilibrium composition of C8H18 air-enriched combustion was conducted. Specifically, the equilibrium-constant approach routines were tailored for 10 species reported in the literature to the conditions of oxy-fuel combustion. Additionally, a comprehensive analysis was performed by varying the temperature and equivalence ratio alongside the oxygen level. The results emphasize the intricate interplay between various factors in oxy-combustion and provide valuable insight into the equilibrium product formation in oxy-fuel combustion. Notably, the non-uniform reduction of N2 as a function of O2 is highlighted, with an overall reduction rate of 0.93 observed across the range of O2 percentages from 21% to 99%.

Experimental study on NOx emission characteristics under oxy-fuel combustion

Abstract This study focuses on the emission characteristics of NOx under oxy-fuel combustion conditions. A comparative analysis with air combustion was performed. NOx emission, control measures and influence factors under different working conditions were studied. Experiments were carried out on a 3-MWth test platform and a laboratory platform. The ‘π’-type furnace was adopted, with the furnace width of 2.6 m, depth of 2.0 m and height of 10.5 m for the 3-MWth coal-fired boiler. NOx emissions at different oxygen concentrations and different air distribution were investigated; the effects of H2O and CO2 concentration on denitrification efficiency and SO2/SO3 conversion rate were explored. Experiment results suggest that, compared with air combustion, NO concentration (volume basis) at the furnace outlet under oxy-fuel combustion is higher than that of air combustion, but the amount of NOx emissions in the discharged gas significantly decrease compared to the air combustion conditions. In addition, the formation of NOx can be effectively controlled through staged combustion. Furthermore, the selective catalytic reduction (SCR) denitrification efficiency and the conversion rate of SO2 to SO3 decreases when the CO2 concentration and the H2O content increase, indicating that CO2 and H2O have an adverse effect on the performance of the catalyst. Additionally, compared with CO2 concentration, H2O content has a greater effect on catalyst performance.

Effect of H2O on NO at oxy-fuel combustion condition of ethanol/NH3 and ethylene glycol/ NH3

ABSTRACT In this paper, the formation and reduction of NO during the oxidation of C2H5OH/NH3 and (CH2OH)2/NH3 under oxy-fuel conditions were experimentally investigated to understand the effect of high H2O content on NO emission. We investigated the effects of high H2O concentration, binding of H2O to CO2, and OH/C ratio on NO formation and reduction. The results showed that the NO concentrations of C2H5OH/NH3 and (CH2OH)2/NH3 in different atmospheres increased with the increase in excess oxygen ratio (α) and showed the following order: O2/N2>O2/Ar>O2/CO2 and O2/Ar/H2O>O2/CO2/H2O, with significant differences. When combusted under a dry atmosphere (without H2O), (CH2OH)2/NH3 produces much less NO than C2H5OH/NH3. Taking the O2/N2 atmosphere as an example, the maximum NO conversion rate of (CH2OH)2/NH3 is 0.43, much lower than the maximum NO conversion rate of 0.60 for C2H5OH/NH3. This is because (CH2OH)2 has a larger OH/C ratio than C2H5OH, which will reduce the generation of CHi radicals and thus the chance that nitrogen will be oxidized to NO by CHi. The NO concentrations of both C2H5OH/NH3 and (CH2OH)2/NH3 will increase with H2O under wet atmosphere oxygen-lean and oxygen-rich conditions. Of these, the 40% H2O concentration condition has the most significant effect on NO emission. When the maximum NO emission was at α = 1.4in the O2/Ar atmosphere with 40% H2O, the NO emission of C2H5OH/NH3 was 437 PPM with a conversion rate of 0.63, and the NO emission of (CH2OH)2/NH3 was 449 PPM with a conversion rate of 0.42. This phenomenon may be due to the fact that decomposition of ethylene glycol consumes more OH radicals, thus leading to a much lower effect of H2O on NO production during (CH2OH)2/NH3 combustion than during C2H5OH/NH3.

A strategy to extend load operation map range in oxy-fuel compression ignition engines with oxygen separation membranes

The possibility of applying oxy-fuel combustion to a compression ignition engine (CIE) at different operating points regarding engine speed and load is studied in this work. To do so, a strategy is proposed to extend the load operation map range of a 2.2L turbocharged and direct-injection CIE under oxy-fuel combustion conditions using mixed ionic-electronic conducting membranes (MIEC) to acquire oxygen (O2) from the air. As nitrogen is not present in the intake flow, nitrogen oxide (NOx) emissions are eliminated, and carbon capture is allowed. The strategy consists of modifying exhaust gas temperature and oxygen-fuel ratio , generating temperature-lambda maps to decide the optimal combination in terms of engine performance. Thus, the system behavior is analyzed at three engine speeds (1250rpm, 2500rpm and 3500rpm) with different load levels and compression ratio (CR) of 20. The baseline engine model is calibrated with experimental data within part-load operation ranges, and an apparent combustion time model calibrated with computational fluid dynamics data is applied to simulate the combustion process under oxy-fuel combustion conditions. Finally, specific parameters are researched to verify whether the system produces enough energy to heat up the MIEC, generating the necessary oxygen. Indeed, a few of them have been found very useful as design parameters for finding out oxy-fuel engine operation limits and estimating MIEC design features.

In-situ desulfurization using porous Ca-based materials for the oxy-CFB process: A computational study

Within circulating fluidized bed (CFB) processes, gas and solid behaviors are mutually affected by operating conditions. Therefore, understanding the behaviors of gas and solid materials inside CFB processes is required for designing and operating those processes. In addition, in order to minimize the environmental impact, modeling to reduce pollutants such as SOx emitted from those processes is essential, and simulation reproduction is necessary for optimization, but little is known. In this study, the gas and solid behaviors in a pilot-scale circulating fluidized bed combustor were investigated by using computational particle fluid dynamics (CPFD) numerical simulation based on the multiphase particle-in-cell (MP-PIC) method under oxy-fuel combustion conditions. In particular, the combustion and in-situ desulfurization reactions simultaneously were considered in this CPFD model. Effect of fluidization number (ULS/Umf) was investigated through the comparison of particle circulation rates with regards to the loop seal flux plane and bed height in the standpipe. In addition, the effects of parameters (temperature, Ca/S molar ratio, and particle size distribution), sensitive indicators for the desulfurization efficiency of limestone, were confirmed. Based on the cycle of the thermodynamic equilibrium curve of limestone, it is suggested that direct and indirect desulfurization occur simultaneously under different operating conditions in CFB, creating an environment in which various reactions other than desulfurization can occur. Addition of the reaction equations (i.e., porosity, diffusion) to the established simple model minimizes uncertainty in the results. Furthermore, the model can be utilized to optimize in-situ desulfurization under oxy-CFB operating conditions.

Desulfurization in co-firing of sewage sludge and wooden biomass in a bubbling fluidized bed combustor under air and oxy-fuel conditions

This work presents results of an experimental investigation of a direct addition of Ca-based sorbent for SO2 capture in a bubbling fluidized bed combustion. The fuel of interest was secondary biomass represented by a dried sewage sludge, which was mixed with primary biomass represented by wooden pellets in a weight ratio of 30:70. The performance of this SO2 capture approach is compared in air and oxy-fuel combustion conditions, since the different gas environment in the bed significantly affects the SO2 capture process. The experiments were carried out in an experimental bubbling fluidized bed combustor with a power load of about 30 kW.Sewage sludge is typical for its high ash content and has sufficient dimensions and attrition resistance to create the fluidized bed without the support of external bed materials. However, it typically consists of S-containing organic substances, resulting in the presence of SOX in the off-gas. A reference case was initially conducted without any sorbent addition in order to investigate the effect of sulfur self-retention in the fuel ash. The sulfur retention in the ash reached almost 45 % in air- and up to 55 % under oxy-combustion.In the next phase, the performance of Ca-based sorbents in sulfur capture was evaluated. The sorbents were represented by two types of limestone with diverse CaCO3 content produced at different locations. The experiments were carried out with three Ca/S mole ratios (1.5, 3, and 4.5). The effect of the fluidized bed temperature was also investigated as the most important parameter.The SO2 capture ratio (used as a performance indicator) was calculated based on emission factors, due to the different specific volumes of flue gas in the air and oxy-fuel modes. The results show in general a significantly higher SO2 capture ratio under oxy-fuel conditions. The differences are more significant in lower Ca/S ratios, where the SO2 capture ratio increased from 46 % under air conditions to 75 % under oxy-fuel for Ca/S = 1.5. In the case of the highest Ca/S ratio examined (Ca/S = 4.5), the increase in SO2 capture was from 90 % to 95 %.

Particle Size Distribution and Enrichment of Alkali and Heavy Metals in Fly Ash on Air and Oxy-Fuel Conditions from Sludge Combustion

Comparative tests in air and oxy-fuel combustion were conducted in a 30 kWth circulating fluidized bed (CFB) pilot plant for waste sludge combustion. General combustion characteristics of the CFB, such as pressure profiles, temperatures along the bed, and flue gas composition, were different under the air and oxy-fuel conditions. At the bottom and in the fly ash, alkali and heavy metals had different distributions under the air and oxy-fuel combustion conditions. The particle size distribution in fly ash from air combustion was dominated by coarse particles, over 2.5 μm in size, whereas with oxy-fuel combustion, most particles were submicron in size, approximately 0.1 μm, and a smaller quantity of coarse particles, over 2.5 μm in size, formed than with air combustion. Mass fractions of Al, Ca, and K, below 2.5 μm in size, were found in the ashes from oxy-fuel combustion and in higher quantity than those found in air combustion. Submicron particle formation from Cr, Ni, Cu, and Zn in the fly ash occurred more during oxy-fuel combustion than it did in air combustion.

Simulation of char burnout characteristics of biomass/coal blend with a simplified single particle reaction model

A single particle reaction model has been developed to study the char burnout characteristics of co-firing of coal and biomass under air and oxy-fuel combustion conditions. Two types of coal/biomass blend, i.e., Brisbane bituminous coal blended with woodchip, and Loy Yang lignite coal blended with woodchip, have been studied. The model is validated with different sets of experiments and CFD simulation from the literature. The effects of biomass fraction, particle size, oxygen level and gas temperature have been investigated. It is found that increasing biomass blending ratio, oxygen concentration, [O2], or the gas temperature, tg, increases char reactivity and conversion rate and reduces char burnout time. The current numerical study demonstrates that comparing to traditional CFD simulation, the proposed single particle reaction model takes much less simulation time and efforts, while able to provide a deep insight into char burnout characteristics in co-combustion process.

Experimental study and modelling of pressurized oxy-coal combustion: Effect of devolatilization conditions on the physical structure and combustion characteristics of char particles

Pressurized oxy-coal combustion has been regarded as a promising carbon capture and storage (CCS) technology, and the combustion characteristics of char particles under pressurized oxy-fuel combustion conditions have been extensively investigated. However, most previous studies used the char samples prepared under the atmospheric N2 condition rather than under the pressurized CO2 condition, which may lead to the misunderstanding of the combustion characteristics. In this study, the effects of devolatilization atmosphere and pressure on the physical structure of char particles were experimentally investigated. The char particles produced under different devolatilization conditions were then used for the oxy-char combustion experiments conducted in a fluidized bed with the operating pressure between 0.1 and 0.5 MPa. In view of the limitations of the experiment, an experimental verified particle-scale char combustion model was further used for predicting the char particle’s combustion reactivity and temperature, and the influence of physical structure on the combustion characteristics was analyzed. The results obtained from the experiments demonstrated that higher devolatilization pressure led to larger mean pore sizes and specific surface areas of char particles. The char particles obtained with CO2 atmosphere had larger specific surface areas but smaller mean pore sizes than those obtained with N2 atmosphere. Besides, increasing devolatilization pressure resulted in shorter burnout times of char particles. The simulation results showed that using the char samples prepared under the atmospheric N2 condition for the pressurized oxy-fuel combustion experiments led to the underestimations of char combustion reactivity and peak temperature.

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.

Decoupling Investigation of Furnace Side and Evaporation System in a Pulverized-Coal Oxy-Fuel Combustion Boiler

A distributed parameter model was developed for an evaporation system in a 35 MW natural circulation pulverized-coal oxy-fuel combustion boiler, which was based on a computational fluid dynamic simulation and in situ operation monitoring. A mathematical model was used to consider the uneven distribution of working fluid properties and the heat load in a furnace to predict the heat flux of a water wall and the wall surface temperature corresponding to various working conditions. The results showed that the average heat flux near the burner area in the air-firing condition, the oxy-fuel combustion with dry flue gas recycling (FGR) condition, and the oxy-fuel combustion with wet flue-gas recycle condition were 168.18, 154.65, and 170.68 kW/m2 at a load of 80%. The temperature and the heat flux distributions in the air-firing and the oxy-fuel combustion with wet FGR were similar, but both were higher than those in the oxygen-enriched combustion conditions with the dry FGR under the same load. This study demonstrated that the average metal surface temperature in the front wall during the oxy-fuel combustion condition was 3.23 °C lower than that under the air-firing condition. The heat release rate from the furnace and the vaporization system should be coordinated at a low and middle load level. The superheating surfaces should be adjusted to match the rising temperature of the flue gas while shifting the operation from air to oxy-fuel combustion, where the distributed parameter analytical approach could then be applied to reveal the tendencies for these various combustion conditions. The research provided a type of guidance for the design and operation of the oxy-fuel combustion boiler.

Oxy-Combustion of Solid Recovered Fuel in a Semi-Industrial CFB Reactor: On the Implications of Gas Atmosphere and Combustion Temperature.

Oxy-fuel combustion of refuse waste is gaining considerable attention as a viable CO2 negative technology that can enable the continued use of stationary combustion plants during the transition to renewable energy sources. Compared to fossil fuels, waste-derived fuels tend to be highly heterogeneous and to contain a greater amount of alkaline metals and chlorine. Therefore, experimental studies are mandatory to thoroughly elucidate refuse materials’ combustion and pollutant formation behavior. This paper presents an experimental investigation on the air and oxy-fuel combustion of solid recovered fuel at a 200 kWth circulating fluidized bed facility. In the course of two experimental campaigns, the effects of combustion atmosphere and temperature on pollutant formation (i.e., NOx, SO2, and HCl) and reactor hydrodynamics were systematically studied. In contrast to air-firing conditions, the experimental results showed that oxy-fuel combustion enhanced the volume concentration of NOx by about 50% while simultaneously decreasing the fuel-specific NOx emissions (by about 33%). The volume concentrations of SO2 and HCl were significantly influenced by the absorption capacity of calcium-containing ash particles, yielding corresponding values close to 10 and 200 ppmv at 871–880 °C under oxy-fuel combustion conditions. In addition, the analysis of hydrodynamic data revealed that smooth temperature profiles are indispensable to mitigate bed sintering and agglomeration risks during oxy-fuel operation. The results included in this study provide a valuable contribution to the database of experimental information on the oxy-fuel combustion of alternative fuels, which can be applied in future process model validations and scale-up studies.

Co-combustion of high and low ash lignites with raw and torrefied biomass under air and oxy-fuel combustion atmospheres

ABSTRACT Co-combustion characteristics of high and low ash lignites blended with raw and torrefied pine woodchips were studied by Thermogravimetric Analyzer (TGA) under air and oxy-fuel conditions. The lignites were blended with biomass samples at the mass fraction of 50/50 wt.%. Three heating rates of 10, 20, and 40°C/min were chosen, and the characteristic temperatures, including initial, ignition, and burnout temperatures, were obtained. In order to estimate the activation energies of the co-combustion of the blends, Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose, and Friedman kinetic methods were employed. Additionally, to assess the summative behavior of the fuel blends, the relative error as a degree of synergism was calculated based on the difference between theoretical and experimental DTG profiles. It was seen that co-combustion of torrefied biomass with the low ash Orhaneli lignite in air resulted in the average relative error of 21.41%, indicating the maximum synergism for the blend. This value was 9.59% under oxy-fuel combustion atmosphere. Blending torrefied biomass with the high ash Soma lignite resulted in average relative errors of 1.34% and 1.45% under air and oxy-fuel combustion atmospheres showing an insignificant synergetic effect. An improvement in combustion performance was noticed under oxy-fuel combustion conditions. The average activation energy values for the blend of torrefied biomass and Orhaneli lignite were 54.47 and 112.48 kJ/mol under air and oxy-fuel combustion atmospheres, which were lower than that of the parent fuels indicating higher reactivity of the blends. This trend was not seen for Soma lignite. The associated uncertainty values for the FWO method were in the range of 3.57% to 12.45% making it a proper tool for obtaining the kinetic parameters.

Effect of the microstructure of Haynes 282 nickel-based superalloys on oxidation behavior under oxy-fuel combustion conditions

The effect of the Haynes 282 microstructure on its oxidation behavior was investigated using X-ray diffraction, scanning electron microscopy, electron backscatter diffraction, transmission Kikuchi diffraction, and transmission electron microscopy. Specimens with different grain sizes were oxidized at 760 °C in O2 + 100 ppm SO2 to simulate oxy-fuel combustion conditions. Small-grained, deformed specimens showed a high oxidation rate at the initial stage. However, the small-grained specimen exhibited superior oxidation resistance at the later stage of oxidation, with Cr depletion being suppressed only in this specimen. These different oxidation behaviors are discussed in terms of Cr and Ti supplied from the matrix.