Oxy-fuel Combustion Environments Research Articles

Effect of Ash and Sulphate on Corrosion of Ni-Based Alloys in a Simulated Oxyfuel Combustion Environment

AbstractAlloys of Ni–25Cr–(2Mn–1Si) under mixed deposits of ash + (0, 10, 50 and 90) wt% sulphate were exposed to an Ar–60CO2–20H2O gas at 650 and 750 °C for up to 300 h, forming both protective chromia and regions of Ni-rich oxide. The presence of ash + sulphate mixtures improved Ni–25Cr alloy protection, increasing surface coverage by thin, protective chromia compared with the deposit-free condition. Increasing sulphate proportions in these mixtures led to an accelerated chromia scale growth and reduced internal oxidation zone (IOZ). These beneficial effects were more significant at 750 °C, where surface coverage by the protective scale was increased, and a chromia band was formed beneath nonprotective regions at the IOZ-substrate interface. Alloy additions of Mn and Si generally slowed the growth of outer NiO and IOZ but did not lead to exclusive chromia scale formation.

NOx and N2O emissions from Ca-rich fuel conversion in oxyfuel circulating fluidized bed combustion

A part of Estonia’s power generation is based on oil shale combustion in thermal power plants, resulting in high CO2 and pollutant emissions. The presented paper is a part of long-term experimental work to investigate the combustion of Ca-rich oil shale for air and oxyfuel combustion environments. The experiments were performed in a 60 kWth circulating fluidized bed (CFB) test facility under air, O2/CO2, and O2 with the recycled flue gas (RFG) modes of inlet O2 as vol.% ranging from 21% to 52%, and with RFG of 50% and 87%. The influence of different combustion atmospheres, excess oxygen volumetric ratios in the primary oxidizer (λPr), and dense bed temperatures (TDB), on the total NOx and N2O formations, have been studied in detail.The results show that specific concentrations of NOx (per MJ) were reduced by 14% in O2/CO2 mode, and 22% in O2/RFG compared to air combustion. N2O emissions were increased in OXY21 mode and reduced significantly from 20 mg/MJ to 4 mg/MJ with increasing inlet O2% to 52% under O2/CO2. NOx and N2O emissions were the lowest of all combustion experiments at high inlet O2% with RFG application (OXY87 + RFG). Under all tested atmospheres, NOx emissions were increasing with increasing excess oxygen in the primary oxidizer. N2O enhanced with excess oxygen, accounted for the homogenous gas-phase reaction of volatile-N and the heterogenous reaction of char-N. NOx emissions at low operating TDB were decreasing with increasing bed temperatures and increased at higher TDB > 770 °C in air combustion and TDB > 850 °C in OXY30. N2O emissions were slightly decreased at higher bed temperatures under air and O2/CO2 modes. Combustion efficiency enhanced at higher inlet O2% resulting in lower CO concentrations with reduced fuel burnout, and increased NOx and N2O emissions.Overall, NOx and N2O formations were more affected by the condition of operating parameters under both air and oxyfuel combustion experiments, and the obtained results are giving confidence that oxyfuel technology does not influence the release of nitrogen emissions from oil shale combustion. To lower the cost of CO2 impurities removal, the optimal operating conditions during oil shale CFB combustion can be confirmed after considering the discussed parameters. However, further CO2 purification resulting from oil shale oxyfuel flue gas stream is possibly required.

Ash characterisation formed under different oxy-fuel circulating fluidized bed conditions

The CO2-intensive oil shale power industry accounts for more than 70% of the Estonian energy sector, producing large amounts of ash that are mainly landfilled. Adopting oxy-fuel technology in the Estonian energy sector, in conjunction with CO2 storage, can be a critical tool for reducing carbon footprint. Nevertheless, several differences in ash formation under oxy-fuel conditions can be expected, which can lead to additional ash handling/utilization and related environmental concerns. In this context, the composition of oil shale ash was obtained from a series of combustion experiments in a 60 kWth circulating fluidized bed (CFB) test unit under both air and oxy-fuel combustion regimes studied by means of physical, chemical analysis as quantitative X-ray diffraction and element analysis for ashes from different separation ports. During the experimental work, special attention was given to understanding the impacts of the boiler temperature, recycled flue gas (RFG), and elevated oxygen concentration. As a result, the primary goal of this research is to characterise the ash produced by oxy-fuel combustion of oil shale in order to gain a better understanding of its chemical and mineral compositions, including Ca-Mg silicates and clay minerals, as well as to investigate the behaviour of the formation of free lime and anhydrite, which are the major compounds influencing potential ash utilisation routes. The results show that although the decomposition of calcium carbonates was limited during oxy-fuel combustion because of the high CO2 partial pressure, the contents of secondary silicates and anhydrite were slightly higher. The increased particle temperature and the higher SO2 partial pressure (with RFG) can explain this. The particle size distributions (PSDs) were wider under oxy combustion, and the specific surface area (SSA) in Filter ashes were much higher under elevated inlet O2% atmosphere. Oxy-fuel combustion had no significant impact on the concentration of elements in the ash. Overall, the mineralogy of the ashes formed in air and oxy-fuel combustion environments is similar, yet slight differences exist in the formation of clay minerals in the case of increased oxygen concentrations and with the application of RFG as a result of the increased residence time of the particles.

Comparison between pilot and lab scale testing of aluminide coated and uncoated ferritic steels under oxy-fuel and coal/thistle co-firing conditions

The present study compares the biomass corrosion behavior of two diffusion aluminide coating obtained by slurry application, which were deposited on two low-chromium content steels, ferritic-martensitic P92 (8.7 wt% Cr) and ferritic T22 (2 wt% Cr). Their performance degradation was conducted under an oxy-fuel combustion environment for both coated and uncoated materials both under laboratory conditions and in a pilot plant burning thistle for 500 h. Exposures were carried out in the laboratory at two different temperatures, 600 °C and 650 °C, under a model atmosphere consisting of 60 % CO2, 30 % H2O, 8 % O2, bal.% N2 (in vol%), 500 vppm HCl and 2 vppm SO2. The pilot plant used a mixed fuel of 60 wt% coal and 40 wt% thistle that was burnt and the samples were exposed to a temperature range of 600–620 °C. After testing, the results revealed that the aluminide-coated materials exhibited a very high resistance under both extreme scenarios, with a variable protective character related to their Al content. On the contrary, uncoated material exhibited severe degradation, in particular T22. Microstructural and morphological studies showed up similar corrosion patterns and products on coated and uncoated materials for both testing environments.

Quantifying the effect of CO2 gasification on pulverized coal char oxy-fuel combustion

Previous research has provided strong evidence that CO2 and H2O gasification reactions can provide non-negligible contributions to the consumption rates of pulverized coal (pc) char during combustion, particularly in oxy-fuel environments. Fully quantifying the contribution of these gasification reactions has proven to be difficult, due to the dearth of knowledge of gasification rates at the elevated particle temperatures associated with typical pc char combustion processes, as well as the complex interaction of oxidation and gasification reactions. Gasification reactions tend to become more important at higher char particle temperatures (because of their high activation energy) and they tend to reduce pc oxidation due to their endothermicity (i.e. cooling effect). The work reported here attempts to quantify the influence of the gasification reaction of CO2 in a rigorous manner by combining experimental measurements of the particle temperatures and consumption rates of size-classified pc char particles in tailored oxy-fuel environments with simulations from a detailed reacting porous particle model. The results demonstrate that a specific gasification reaction rate relative to the oxidation rate (within an accuracy of approximately +/- 20% of the pre-exponential value), is consistent with the experimentally measured char particle temperatures and burnout rates in oxy-fuel combustion environments. Conversely, the results also show, in agreement with past calculations, that it is extremely difficult to construct a set of kinetics that does not substantially overpredict particle temperature increase in strongly oxygen-enriched N2 environments. This latter result is believed to result from deficiencies in standard oxidation mechanisms that fail to account for falloff in char oxidation rates at high temperatures.

Reaction kinetics of coal char in oxy-fuel combustion environment and characterization of reacted char

ABSTRACT The work primarily focuses on combustion kinetics of devolatilized coal char fines (75–120 µm), including the estimation of kinetic parameters and char morphological transformation in oxy-fuel environment (O2-CO2 and O2-N2). Experimental work has been carried out to study the effect of higher CO2 concentration and elevated O2 concentrations on the ignition, burnout behavior of pulverized coal char both non-isothermally and isothermally. The coal chars obtained from pyrolyzing three different types of coal are used in experiments; a high ash (HA) coal, a medium ash (MA) coal, and a low ash (LA) coal. Three different kinetic models, namely VM, GM and RPM, have been employed to determine the kinetic parameters. The kinetic parameters are observed to be a strong function of oxygen concentration for all the three models. The estimated activation energy employing non-isothermal analysis for LA coal char using VM lies in the range of 203–212 kJ/mol. In comparison, the activation energy predicted by GM for HA coal char lies in the range 137–144 kJ/mol. Moreover, the activation energy estimated for MA coal char by RPM varies in the range of 134–142 kJ/mol. It is also explored during the non-isothermal analysis that an increase in char particle size from 75–120 µm to 250–300 µm shows a small increase in the values of apparent activation energy and pre-exponential factor. The activation energy values calculated employing the isothermal method are 106–126 kJ mol−1 (using RPM) for coal char MA and 96–135 kJ mol−1 (using GM) for coal char HA. Both the isothermal and non-isothermal kinetic analysis reveals that the combustion of coal char LA follows VM while coal char MA and HA follows RPM and GM in order. SEM images and BET surface area analysis for partially converted coal chars are further analyzed to justify the suitability of different reaction mechanism followed by various coal chars.

A Computational Modeling of the Influence of Different Oxy-fuel Combustion Environment on Combustion Characteristics and Particles Temporal History

ABSTRACT In the present article, CFD simulation of various oxy-fuel combustion cases has been performed to find out the influences of oxy-fuel combustion environment on temperature distribution, char consumption, radiative properties and temporal variation of particle phase variables such as particle temperature, volatile and char mass fraction. The oxy-fuel combustion cases are obtained by varying O2 and CO2 concentrations while keeping the total volume flow rate constant. Four oxy-fuel cases are considered in the present work having oxygen concentration 21%, 25%, 30%, and 35%. The present article also investigates the influence of char gasification reactions on temperature distribution, species concentration and temporal profile of particle char mass fraction. The predicted temperature distribution and radiative properties under oxy-fuel cases are compared with the corresponding values obtained under air-fired combustion as for retrofit existing power plants comparable values are desirable. The oxy-30 case having 30% oxygen has almost identical temperature and radiative profiles. With an increase in oxygen concentration in oxy-fuel cases leads to an increased peak temperature and surface incident radiation due to the rise in local stoichiometry, which results in enhanced reaction kinetics and burning rate. The peak temperature obtained in oxy-25, oxy-30 and oxy-35 cases is around 10%, 18%, and 24% higher than the oxy-21 case. Reduced char burnout time is obtained due to the presence of gasification reactions.

Effects of Chlorine in Ash on the Corrosion Performance of Ni-Based Alloys in a Simulated Oxy-Fuel Environment

Alloy materials can be severely degraded by corrosive gases and coal ash in service of coal power plants at high temperature. To better understand alloy corrosion in coal power plants, it is necessary to study the effect of each constituent from coal combustion on alloy corrosion. Chlorine-containing compounds in coal are corrosion-accelerating agents. The concentration of chlorine increases a lot in the gas environment of oxy-fuel combustion due to the absence of airborne nitrogen gas acting as a diluent. The role of chlorine in ash corrosion is investigated in simulated oxy-fuel environments. Long time tests over 3000 h were performed on various alloys at 750 °C. The degradation depth, weight change, and microstructural characteristics of oxide scales on Ni-based alloys are reported after exposure at 750 °C in an oxy-fuel combustion environment for over 3000 h. Synchrotron nanobeam X-ray analysis was performed to evaluate the phase and chemical composition of the oxide layers on the alloy surface. Nanob...

Comparative study on combustion and oxy-fuel combustion environments using mixtures of coal with sugarcane bagasse and biomass sorghum bagasse by the thermogravimetric analysis

Biomass and coal have different physicochemical properties and thermal behavior. During the co-combustion of coal-biomass mixtures, their thermal behavior varies according to the percentage of each fuel in the mixture. Thereby, this research aims to characterize the thermal behavior of mixtures of coal, sugarcane bagasse, and biomass sorghum bagasse as biomass in simulated combustion (O2/N2) and oxy-fuel combustion (O2/CO2) environments. Experiments have been performed in duplicate on a thermogravimetric analyzer at heating rate of 10 °C/min. A uniform granulometry was considered for all materials (63 μm) in order to ensure a homogeneous mixture. Four biomass percentages in the mixture (10, 25, 50 and 75%) have been studied. Based on thermogravimetric (TG) and thermogravimetric (DTG) analyses, parameters such as combustion index, synergism, and activation energy have been determined, as well as the combustion environment influence on these parameters. The results indicate that, although sugarcane bagasse has the lowest activation energy, the thermal behavior of both types of biomass is similar. Thus, biomass sorghum bagasse can be used as an alternative biomass to supply the power required during sugarcane off-season. For both mixtures, optimal results were obtained at 25% of biomass. By analyzing the environment influence on combustion behavior, the results indicate that when N2 is replaced with CO2, it is observed an increase in reaction reactivity, a higher oxidation rate of materials and an improvement in evaluated parameters.

Oscillating coal and biomass flames: A spectral and digital imaging approach for air and oxyfuel conditions

The transient nature of a flame can be quantified by performing spectral and oscillatory analysis of its parameters, such as the flame's luminance and temperature. This paper presents an assessment of the effect of an oxyfuel environment on the combustion of two different solid fuels, a high volatile bituminous coal and a white wood biomass, in a 250 kWth pilot-scale combustion test facility. A digital flame monitoring system was fitted to the experimental furnace, and was used to record high speed videos of the flame. Transient signals for both digital luminance and temperature were obtained after the instantaneous frames were extracted from the original videos. Spectral analysis was performed over the transient signal in order to analyse the temporal coherence of the flame through a weighted oscillation frequency value. An additional parameter, the oscillation index, which accounts for the amplitude of the oscillation of the flame, was computed to complement the information recovered from the flame. The oscillation trends obtained from these experiments assess the dynamic response of the flame to different combustion environments within the furnace. In general, it was found that oxyfuel flames showed a discernible temporal repeatability and a lower magnitude of the oscillation of their flame parameters, and therefore are registered as being more stable than their counterpart under air combustion conditions. In addition, the biomass flames exhibit less sensitivity to the oxyfuel combustion environment than what was found with coal, which may allow future oxy-biomass regimes to operate under a much wider envelop of firing conditions.

SO3 formation and the effect of fly ash in a bubbling fluidised bed under oxy-fuel combustion conditions

This study investigates SO3 formation under air-fired and oxy-fuel combustion environments in a bench-scale bubbling fluidised bed. Flue gas compositions typical of air-fired and oxy-fuel environments were simulated with synthetic gas mixtures. Parameters such as bed temperature and SO2 and H2O concentrations were varied to determine their effect on the formation of SO3. Tests were also performed using three fly ashes produced from three different coals and combustion systems. Oxy-fuel combustion led to increased SO3 concentration as compared to typical air-fired environments. SO3 concentrations increased with combustion temperature and O2 concentrations. SO3 formation also depends on the chemical composition of the fly ash.

Effect of potassium-doping and oxygen concentration on soot oxidation in O2/CO2 atmosphere: A kinetics study by thermogravimetric analysis

In oxy-fuel combustion, switching from nitrogen to carbon dioxide atmosphere with higher oxygen concentration will affect the oxidation rate of soot, and it was observed in our recent study on soot emission from biomass pyrolysis that potassium (K) crystals are embedded in soot and its precursor clusters. In this study, the effects of K-doping (KCl and K2SO4) and O2 concentration on soot oxidation in O2/CO2 atmosphere are studied using thermogravimetric analysis (TGA), and the extent of catalysis is compared with that in O2/N2 atmosphere. The delays on start, peak and end temperatures of soot oxidation are observed in O2/CO2 atmosphere. However, increase in O2 concentration which promotes oxidation significantly reduces the delay. All the K-doping cases results in accelerated soot oxidation rate, but catalytic role of the K-doping in O2/CO2 is significantly lower than that in O2/N2 because the CO2-enriched environment inhibits the performance of potassium as oxygen carrier. The accelerating degree from K-doping is also affected by the potassium type, doping mass and oxygen concentration. KCl acts as a better, more efficient doping agent than K2SO4 with the increase in doping mass. The catalytic effect of K2SO4 will not change and even decrease at 375μmol(K)/g(soot) for K2SO4 while the catalytic role of KCl keeps increasing even at 600μmol(K)/g(soot) for KCl. In O2 concentration range of 5–30%, the accelerating degree from K-doping presents the minimum value around 15%. This phenomenon strongly approves the hypothesis that potassium as the oxygen carrier and accelerating the oxygen transportation, because in the cases of without K-doping and at a high O2 concentration there is no additional active site for more O2 adsorption thus inducing the slow accelerating degree. The kinetic analysis indicates the first order reaction for soot oxidation and also a good compensation relation between apparent activation energy E and logarithmic frequency factor A. E is generally reduced with the atmosphere changing from O2/N2 to O2/CO2, with K-doping, and with O2 concentration decreasing. This study is beneficial to demonstrate the mechanism of how potassium doping and oxygen concentration affect soot oxidation rate in oxy-fuel combustion environment.

Air and oxy-fuel combustion kinetics of low rank lignites

The air and oxy-fuel combustion processes of two low-grade lignite coals were investigated by thermogravimetric analysis (TGA) method. Coals were provided from two different coal mines in the Aegean region of Turkey. Oxy-fuel combustion experiments were carried out with three different gas mixtures of 21% O2–79% CO2; 40% O2–60% CO2 and 50% O2–50% CO2 at 950 °C and heating rates of 10 °C/min, 20 °C/min and 40 °C/min. The kinetics of the oxy-fuel combustion of coals were studied by using four different methods namely, Coats-Redfern (model-fitting method), Friedman (FR), Flynn–Wall–Ozawa's (FWO) and Kissinger–Akahira–Sunose's (KAS) methods. The apparent activation energies of combustion process calculated by FWO method are slightly but systematically higher than that calculated by the KAS and FR methods for the oxy-fuel atmospheres. Combustion behavior of both coals in the oxy-fuel combustion environment could vary significantly, likely due to their characteristics such ash and volatile matter contents.

Effect of CO2 gasification reaction on char particle combustion in oxy-fuel conditions

CO2 gasification of coal char may play an important role in oxy-combustion environments with flue gas recirculation (FGR), but its effect on the overall reaction rate has not been clearly understood. To give clarity to the likely impact of CO2 gasification on the oxy-combustion of pulverized coal chars, burnout simulations of coal char particles were carried out, adopting apparent char reactivity and a single-film model that includes the Stefan flow effect on mass and energy transfer. Three oxygen concentrations (21%, 30%, and 5% O2), representing air, oxy-fuel, and oxygen-deficient combustion environments were simulated. A new experimental approach was used to directly measure the CO2 gasification rate of a subbituminous coal char at high temperatures and atmospheric pressure. The measured gasification rate is somewhat higher than previous measurements. The simulation results show that the endothermic gasification reaction reduces the char particle temperature and thereby reduces the oxidation rates. However, due to the contribution of the direct gasification reaction on carbon consumption, the char burnout time and the carbon consumption were improved. The gasification reaction has a greater influence on the char burnout time and the relative carbon consumption in an oxygen-deficient environment and on the drop of particle temperature in an oxygen-enriched environment (for a given gas temperature). In addition, the influence of the gasification reaction on char combustion increases as the gas temperature increases and as the particle size increases. Further, it was observed that the impact of the gasification reaction is dependent on the presumed kinetic rate, which highlights the importance of using reliable kinetic parameters in simulations. Based on the present results, it is important to include the gasification reaction by CO2 when simulating char combustion in oxy-fuel combustion environments.

Effect of coal ash on the performance of alloys in simulated oxy-fuel environments

Oxy-fuel combustion can increase efficiency and reduce pollution from coal power plants. To study material performance in an oxy-fuel environment, both Fe- and Ni-based alloys were exposed to simulated oxy-fuel environments at 750°C for long terms up to 6300h. The effects of ash and gas compositions (SO2, CO2, H2O and O2) on the performance of alloys were studied. Results from these tests are used to address the role of CO2, steam, and ash in the long-term corrosion performance of alloys. Detailed results are presented on weight change, scale thickness, internal penetration, microstructural characteristics of corrosion products, and cracking of scales for the alloys. Oxide scales were studied by synchrotron nano-beam X-ray diffraction.An incubation period and a propagation period were observed for ash corrosion. Before incubation time, alloys gain weight. After incubation time, localized corrosion damages the protective scales, alloys lose weight and corrosion rates dramatically increase. Since the incubation time can be over 4000h, long-term test results are more helpful in evaluating the alloy performance in an oxy-fuel combustion environment with ash than the results from a short-term test. The mechanism of ash corrosion is also discussed.

Soot Reactivity in Conventional Combustion and Oxy-fuel Combustion Environments

Soot Reactivity in Conventional Combustion and Oxy-fuel Combustion Environments

In-situ Measurement of Sodium and Potassium Release during Oxy-Fuel Combustion of Lignite using Laser-Induced Breakdown Spectroscopy: Effects of O2 and CO2 Concentration

Laser-induced breakdown spectroscopy (LIBS) was used in this study to measure quantitatively the sodium (Na) and potassium (K) release from burning coal particles under oxy-fuel combustion environments. A specially designed laminar premixed burner was employed to provide a postflame environment with different O-2 and CO2 concentrations, in which the effects of O-2 and CO2 on the release of Na and K during coal oxy-fuel combustion were studied systematically. For the devolatilization stage, neither O-2 nor CO2 had significant influence on the Na and K release. The release of Na and K during the char stage, however, changed significantly at different O-2 and CO2 concentrations. Under these experimental conditions, when the O-2 concentration increased from 3.9% to 10.6%, the peak concentration of Na at the char stage increased from 15.2 mg/m(3) to 33.7 mg/m(3), and the maximum concentration of K increased from 6.2 mg/m(3) to 11.7 mg/m(3). When the CO2 concentration increased from 35.8% to 69.4%, the release of Na and K was inhibited during the char stage, with the peak concentration decreasing from 8.9 mg/m(3) to 6.9 mg/m(3) for Na and from 3.7 mg/m(3) to 2.4 mg/m(3) for K. During the ash stage, the release of Na and K decreased with the O-2 concentration, whereas it increased with the CO2 concentration. (Less)

High-temperature-oxidation-induced ordered structure in Inconel 939 superalloy exposed to oxy-combustion environments

In the integrated oxy-fuel combustion and the turbine power generation system, turbine alloys are exposed to high temperature and an atmosphere comprised of steam, CO2 and O2. While surface and internal oxidation of the alloy takes place, the microstructure in the subsurface region also changes due to oxidation that results in the loss of the strengthening precipitates. In an earlier study of the oxidation of Inconel 939 Ni-based superalloy exposed to oxy-fuel combustion environment for up to 1000h, a high-temperature-oxidation-induced phase transformation in the sub-surface region was noticed and a two-phase region formed at the expense of strengthening γ' phase. While one of the two phases was identified as the Ni-matrix γ solid solution, face-center-cubic) phase, the other product phase remained unidentified. In this study, the crystal structure of the unknown phase and its orientation relationship with the parent Ni-matrix phase was investigated through electron diffraction and high-resolution transmission electron microscopy. It was determined that the crystal structure of the unknown phase could be modeled as a ternary derivative of the ordered η-Ni3Ti phase (D024) structure with lattice parameters of a=.5092nm and c=.8336nm, α=90°, β=90° and γ=120°.

Soot Reactivity in Conventional Combustion and Oxy-fuel Combustion Environments

A study of the reactivity of soot produced from ethylene pyrolysis at different temperatures and CO2 atmospheres toward O2 and CO2 has been carried out using a thermogravimetric analyzer. The purpose was to quantify how soot reactivity is affected by the gas environment and temperature history of the carbon, as well as to compare the soot reactivity toward O2 and CO2. Soot samples were either oxidized in 5% O2 or gasified in 10, 50, and 90% CO2 atmospheres, during non-isothermal runs at 10 K/min. Soot oxidation was observed at temperatures of 400–500 K lower than soot gasification, showing higher reactivity toward oxygen than CO2. Independent of the environment history of the soot samples, the soot samples formed at lower temperatures have higher reactivity toward both O2 and CO2 than the soot samples obtained at higher temperatures. The presence of CO2 during the formation of the soot only affected the soot reactivity at the highest formation temperature (1475 K) and CO2 concentration (78.5%). Under these conditions, the soot reactivity was observed to increase by a factor of about 2.6 compared to soot formed in N2 at the same temperature. We attribute the increased reactivity to a higher micropore surface area, facilitated by the gasification reaction at a high temperature. The intrinsic kinetics for oxidation and gasification of soot were obtained by applying the volumetric reaction model.

Effect of CO2 and steam gasification reactions on the oxy-combustion of pulverized coal char

For oxy-combustion with flue gas recirculation, elevated levels of CO2 and steam affect the heat capacity of the gas, radiant transport, and other gas transport properties. A topic of widespread speculation has concerned the effect of gasification reactions of coal char on the char burning rate. To asses the impact of these reactions on the oxy-fuel combustion of pulverized coal char, we computed the char consumption characteristics for a range of CO2 and H2O reaction rate coefficients for a 100μm coal char particle reacting in environments of varying O2, H2O, and CO2 concentrations using the kinetics code SKIPPY (Surface Kinetics in Porous Particles). Results indicate that gasification reactions reduce the char particle temperature significantly (because of the reaction endothermicity) and thereby reduce the rate of char oxidation and the radiant emission from burning char particles. However, the overall effect of the combined steam and CO2 gasification reactions is to increase the carbon consumption rate by approximately 10% in typical oxy-fuel combustion environments. The gasification reactions have a greater influence on char combustion in oxygen-enriched environments, due to the higher char combustion temperature under these conditions. In addition, the gasification reactions have increasing influence as the gas temperature increases (for a given O2 concentration) and as the particle size increases. Gasification reactions account for roughly 20% of the carbon consumption in low oxygen conditions, and for about 30% under oxygen-enriched conditions. An increase in the carbon consumption rate and a decrease in particle temperature are also evident under conventional air-blown combustion conditions when the gasification reactions are included in the model.