The study of co-combustion characteristics of coal and microalgae by single particle combustion and TGA methods

The co-combustion characteristics of coal and microalgae with different blending ratios and under different atmospheres are studied by single particle combustion and thermogravimetric analysis methods. The combustion processes of coal, microalgae and their blends in the single particle combustion experiment have two stages, while the combustion process of coal in the thermogravimetric analysis experiment only has one stage. With the increasing blending ratio of microalgae, flames of volatiles and char of fuels become dimmer and smaller, and the average flame temperature decreases from about 1400 °C to about 1200 °C. The ignition delay time decreases from 200 ms to 140 ms, and the experimental ignition delay time of blended fuels is lower than the theoretical ignition delay time, which demonstrates that the synthetic effect between coal and microalgae exists. To analyze the influence of oxy-fuel atmosphere on the combustion characteristics, the air is replaced by the O₂/CO₂ atmosphere. The replacement decreases the luminosity, size and average temperature of flames. The average flame temperature of volatiles decreases from 1449.4 °C to 1151.2 °C, and that of char decreases from 1240.0 °C to 1213.4 °C. The replacement increases the ignition delay time of fuel from 80 ms to 100 ms. Increasing mole fraction of O₂ in O₂/CO₂ atmosphere can offset these influences. With the increasing mole fraction of O₂, flames of volatiles and char of fuels become brighter and larger, the average flame temperature increases from about 1100 °C to about 1300 °C, while the ignition delay time decreases from 100 ms to 77 ms.     

Thermogravimetric analysis and process simulation of oxy‐fuel combustion of blended fuels including oil shale,semicoke, and biomass

Oxy‐fuel (OF) combustion is considered as one of the promising carbon capture and storage technologies for reducing CO₂ emissions from power plants. In the current work, the thermal behaviour of Estonian oil shale (EOS) and its semicoke (SC), pine saw dust, and their blends were studied comparatively under model air (21%O₂/79%Ar) and OF (30%O₂/70%CO₂) conditions using thermogravimetric analysis. Mass spectrometry analysis was applied to monitor the evolved gases. The effect of SC and pine saw dust addition on different combustion stages was analysed using kinetic analysis methods. In addition, different co‐firing cases were simulated using the ASPEN PLUS V8.6 (APV86) software tool to evaluate the effects of blending EOS with different biomass fuels of low and high moisture contents. The specific boiler temperatures of each simulated case with the same adjusted thermal fuel input were calculated while applying the operation conditions of air and OF combustion. According to the experiment and process simulation results, the low heating value and high carbonate content of SC brings along endothermic decomposition of carbonates, which negatively affects the heat balance during the conventional co‐combustion of EOS with SC. Instead, firing of EOS with SC and biomass in OF process can be an effective solution to reduce the environmental impact in terms of the reduction of CO₂ emissions and ash. Furthermore, the sensible heat from SC can positively affect the energy balance of the system as the endothermic effect of decomposition of CaCO₃ (for both EOS and SC) can be avoided in OF combustion.     

A review of recent developments in carbon capture utilizing oxy‐fuel combustion in conventional and ion transport membrane systems

Fossil fuels provide a significant fraction of the global energy resources, and this is likely to remain so for several decades. Carbon dioxide (CO₂) emissions have been correlated with climate change, and carbon capture is essential to enable the continuing use of fossil fuels while reducing the emissions of CO₂ into the atmosphere thereby mitigating global climate changes. Among the proposed methods of CO₂ capture, oxyfuel combustion technology provides a promising option, which is applicable to power generation systems. This technology is based on combustion with pure oxygen (O₂) instead of air, resulting in flue gas that consists mainly of CO₂ and water (H₂O), that latter can be separated easily via condensation, while removing other contaminants leaving pure CO₂ for storage. However, fuel combustion in pure O₂ results in intolerably high combustion temperatures. In order to provide the dilution effect of the absent nitrogen (N₂) and to moderate the furnace/combustor temperatures, part of the flue gas is recycled back into the combustion chamber. An efficient source of O₂ is required to make oxy‐combustion a competitive CO₂ capture technology. Conventional O₂ production utilizing the cryogenic distillation process is energetically expensive. Ceramic membranes made from mixed ion‐electronic conducting oxides have received increasing attention because of their potential to mitigate the cost of O₂ production, thus helping to promote these clean energy technologies. Some effort has also been expended in using these membranes to improve the performance of the O₂ separation processes by combining air separation and high‐temperature oxidation into a single chamber. This paper provides a review of the performance of combustors utilizing oxy‐fuel combustion process, materials utilized in ion‐transport membranes and the integration of such reactors in power cycles. The review is focused on carbon capture potential, developments of oxyfuel applications and O₂ separation and combustion in membrane reactors. The recent developments in oxyfuel power cycles are discussed focusing on the main concepts of manipulating exergy flows within each cycle and the reported thermal efficiencies. Copyright © 2010 John Wiley & Sons, Ltd.     

Single particle ignition and combustion of anthracite,semi-anthracite and bituminous coals in air and simulated oxy-fuel conditions

A fundamental investigation has been conducted on the combustion behavior of single particles (75–150 μm) of four coals of different ranks: anthracite, semi-anthracite, medium-volatile bituminous and high-volatile bituminous. A laboratory-scale transparent laminar-flow drop-tube furnace, electrically-heated to 1400 K, was used to burn the coals. The experiments were performed in different combustion atmospheres: air (21%O₂/79%N₂) and four simulated dry oxy-fuel conditions: 21%O₂/79%CO₂, 30%O₂/70%CO₂, 35%O₂/65%CO₂ and 50%O₂/50%CO₂. The ignition and combustion of single particles was observed by means of three-color pyrometry and high-speed high-resolution cinematography to obtain temperature–time histories and record combustion behaviors. On the basis of the observations made with these techniques, a comprehensive examination of the ignition and combustion behaviors of these fuels was achieved. Higher rank coals (anthracite and semi-anthracite) ignited heterogeneously on the particle surface, whereas the bituminous coal particles ignited homogeneously in the gas phase. Moreover, deduced ignition temperatures increased with increasing coal rank and decreased with increasing oxygen concentrations. Strikingly disparate combustion behaviors were observed depending on the coal rank. The combustion of bituminous coal particles took place in two phases. First, volatiles evolved, ignited and burned in luminous enveloping flames. Upon extinction of these flames, the char residues ignited and burned. In contrast, the higher rank coal particles ignited and burned heterogeneously. The replacement of the background N₂ gas of air with CO₂ (i.e., changing from air to an oxy-fuel atmosphere) at the same oxygen mole fraction impaired the intensity of combustion. It reduced the combustion temperatures and lengthened the burnout times of the particles. Increasing the oxygen mole fraction in CO₂ to 30–35% restored the intensity of combustion to that of air for all the coals studied. Volatile flame burnout times increased linearly with the volatile matter content in the coal in both air and all oxygen mole fractions in CO₂. On the other hand, char burnout times increased linearly or quadratically versus carbon content in the coal, depending on the oxygen mole fraction in the background gas.     

A study on municipal solid waste (MSW) combustion in N2/O2 and CO2/O2 atmosphere from the perspective of TGA

In this paper, the combustion behavior of municipal solid waste (MSW) is carried out in a thermogravimetric analyzer under different N₂/O₂ and CO₂/O₂ atmospheres with temperature ranging from 100 °C to 1000 °C. TG (thermogravimetric) and DTG (derivative thermogravimetric) curves are analyzed. The nth order reaction fitting model is used to yield the activation energy of reduction process according to the degree of conversion. The results indicate that all samples lose most their weight between 200 °C and 540 °C. As the oxygen concentration increased, conversion rate curves and DTG curves shift to lower temperature without significant change in its shape. At the same oxygen concentration, the peak values in CO₂/O₂ atmosphere are smaller than those in N₂/O₂ atmosphere, indicating that CO₂ has a higher inhibitory effect than N₂ on MSW combustion. After 600 °C, the weight loss peak appears much later in CO₂/O₂ atmosphere than it does in N₂/O₂ atmosphere. With the increase of heating rate, the maximum weight loss rates of samples increase obviously. The three-step reaction of nth order reaction model fits the weight loss very well.     

Effect of oxy-fuel combustion with steam addition on coal ignition and burnout in an entrained flow reactor

The ignition temperature and burnout of a semi-anthracite and a high-volatile bituminous coal were studied under oxy-fuel combustion conditions in an entrained flow reactor (EFR). The results obtained under oxy-fuel atmospheres (21%O₂-79%CO₂, 30%O₂-70% O₂ and 35%O₂-65%CO₂) were compared with those attained in air. The replacement of CO₂ by 5, 10 and 20% of steam in the oxy-fuel combustion atmospheres was also evaluated in order to study the wet recirculation of flue gas. For the 21%O₂-79%CO₂ atmosphere, the results indicated that the ignition temperature was higher and the coal burnout was lower than in air. However, when the O₂ concentration was increased to 30 and 35% in the oxy-fuel combustion atmosphere, the ignition temperature was lower and coal burnout was improved in comparison with air conditions. On the other hand, an increase in ignition temperature and a worsening of the coal burnout was observed when steam was added to the oxy-fuel combustion atmospheres though no relevant differences between the different steam concentrations were detected.     

Experimental study of the high-temperature slagging characteristics of coal ash-biomass ash blends under different atmospheres

The blends of coal ash and straw ash with the proportion of 50%, 30% and 10% were selected for high-temperature slagging experiments under different atmospheres (Air/N₂/CO₂). The main components of the slags were the mixtures of Si-Al-Ca-O. The straw ash in low proportion was first to melt and then bonded slags. The eutectic reaction of blend ashes were facilitated by increasing the proportion of straw ash, which enhanced the effect of slagging. Slagging in oxygen-containing atmospheres (Air, CO₂) was much more serious than that in N₂ atmosphere. The formation of iron glass was promoted under air atmosphere, and K, Na, Cl elements in straw ash volatilized significantly while few of them existed in the form of feldspar compounds or sulfates. Mullite in coal ash was consumed and then produced calcium feldspar because of the eutectic reaction with CaO. However, the slagging characteristics were not the same in reducing atmospheres. In CO₂ atmosphere, the surface was eroded into gully apertures because of the gasification, and Fe₂O₃ was reduced to FeO which enhanced slagging. But the effect of N₂ atmosphere on the blend ashes was weak. The mineral decomposition and reorganization made the slag present fragmented, while the element Fe was present in the form of Fe₂O₃ under N₂ atmosphere. In this case, the strong polarity of Fe³⁺ weakened the melting of the slag.     

Kinetics of gasification of coal,biomass and their blends in air (N2/O2) and different oxy-fuel (O2/CO2) atmospheres

Kinetics of bituminous coal and palm shells were evaluated using thermo-gravimetric analysis under different environments (N₂/CO₂/O₂). The observed percent mass loss of biomass was higher than mass loss percentage of coal because biomass had higher reactivity and volatile matter content. Ignition temperatures of pure coal, biomass and their blends were also investigated and it was observed that biomass blends had improved ignition properties in both air and oxy-fuel environments. However, the combustion mechanism wasn’t affected. Different mixtures of CO₂/O₂ were also used on 10% palm shell–90% coal samples and compared with air as the reference. At the same composition of oxygen in oxy-fuel as that of simulated air, ignition temperatures were slightly higher and mass loss percentages were marginally lower. However, this difference due to heat capacities of N₂ and CO₂ was meager and was considered negligible. Upon increasing O₂ content, lower ignition temperatures were observed. Kinetics of coal, palm shell and their blends were determined at different gas mixture compositions using Doyle’s and Coats-Redfern’s models. For both models, E was found to decrease with increasing palm shell composition in coal as well as increasing O₂ concentration in oxy-fuel. However, a reverse trend was observed for the pre-exponentional factor(A).     

Combustion behavior of single particles from three different coal ranks and from sugar cane bagasse in O2/N2 and O2/CO2 atmospheres

The combustion behavior of single fuel particles was assessed in O₂/N₂ and O₂/CO₂ background gases, with oxygen mole fractions in the range of 20–100%. Fuels included four pulverized coals from different ranks (a high-volatile bituminous, a sub-bituminous and two lignites) as well as pulverized sugarcane-bagasse, a biomass residue. Particles of 75–90 μm were injected under laminar flow in a bench-scale, transparent drop-tube furnace (DTF), electrically-heated to 1400 K where, upon experiencing high heating rates, they ignited and burned. The combustion of individual particles was observed with three-color optical pyrometry and high-speed high-resolution cinematography to obtain temperature and burnout time histories. Based on combined observations from these techniques, a comprehensive understanding of the behaviors of these fuels was developed under a variety of conditions, including simulated oxy-fuel combustion. The fuels exhibited distinct combustion behaviors. In air, the bituminous coal particles burned in two distinctive modes; the volatiles burned in bright envelope flames surrounding the devolatilizing char particles followed by heterogeneous char combustion. The volatile matter of sub-bituminous coal particles burned either in subdued envelope flames, surrounding devolatilizing and occasionally fragmenting chars, or heterogeneously at the char surface. Lignite particles typically burned with extensive fragmentation, and their volatiles burned simultaneously with the char fragments. The volatiles of bagasse particles burned in spherical and transparent envelope flames. Increasing the oxygen mole fraction in N₂, increased flame and char surface temperatures, and decreased burnout times; particles of all fuels burned more intensely with an increasing tendency of the volatiles to burn closer to the char surface. When the background gas N₂ was substituted with CO₂, the combustion of all fuels was distinctly less intense; at moderate O₂ mole fractions (<30%) most particles did not ignite under active flow conditions in the furnace (they did ignite under quiescent gas flow conditions in the DTF). Increasing the oxygen mole fraction in CO₂ increased the likelihood of combustion and its intensity. Combustion of volatiles in envelope flames was suppressed in the presence of CO₂, particularly under active gas flow in the DTF.     

Effect of different conditions on the combustion interactions of blended coals in O2/CO2 mixtures

The combination of oxy-fuel and blended-coal combustion may be one of these effective methods to both reduce CO₂ emissions and improve energy utilization efficiency in coal-fired power stations. The aim of this study is to investigate oxy-fuel combustion interactions of blended coals under different conditions using a thermo-gravimetric analyzer. The results show that compared with those in an O₂/N₂ mixture, the promotive and inhibitive effect and the comprehensive interactions are considerably weaker in an O₂/CO₂ mixture. In the O₂/CO₂ mixture, both increasing the O₂ concentration and decreasing the particle size result in decreasing the promotive effect but increasing the inhibitive effect and the comprehensive interactions, which increase the non-additive combustion characteristics. Enhancement of the heating rate increases the promotive effect but decreases the inhibitive effect and the comprehensive interactions, which weaken the non-additive combustion characteristics. Of these factors, the effects of the oxygen concentration and heating rate on comprehensive interactions are greater than that of particle size. This study provides useful information for the design and optimization of thermo-chemical conversion systems of coal blends in the O₂/CO₂ atmosphere.     

Investigation of Coal Fueled Chemical Looping Combustion Using Fe_3O_4 as Oxygen Carrier：Influence of Variables

Chemical-looping combustion (CLC) is a novel combustion technique with inherent CO2 separation.Magnetite (Fe3O4) was selected as the oxygen carrier.Shenhua coal (Inner Mongolia,China),straw coke and natural coke were used as fuels for this study.Influences of operation temperatures,coal to Fe3O4 mass ratios,and different kinds of fuels on the reduction characteristics of the oxygen carrier were investigated using an atmosphere thermogravimetric analyzer (TGA).Scanning electron microscopy (SEM) was used to analyse the characteristic of the solid residues.Experimental results shown that the reaction between the coal and the oxygen carrier become strong at a temperature of higher than 800℃.As the operation temperature rises,the reduction conversion rate increases.At the temperatures of 850oС,900℃,and 950℃,the reduction conversion rates were 37.1%,46.5%,and 54.1% respectively.However,SEM images show that at the temperature of higher than 950℃,the iron oxides become melted and sintered.The possible operation temperature should be kept around 900℃.When the mass ratios of coal to Fe3O4 were 5/95,10/90,15/85,and 20/80,the reduction conversion rates were 29.5%,40.8%,46.5%,and 46.6% respectively.With the increase of coal,the conversion rate goes up.But there exist an optimal ratio around 15/85.Comparisons based on different kinds of fuels show that the solid fuel with a higher volatile and a more developed pore structure is conducive to the reduction reactivity of the oxygen carrier.     

Experimental investigation of combustion characteristics and NOx formation of coal particles using laser induced breakdown spectroscopy

To study the mechanism of coal combustion and NO_x formation, the combustion of coal particles in different atmospheres (O₂/N₂, O₂/CO₂) with different O₂ concentrations was investigated using the CO₂ laser as a heat source. The spatial distribution of atoms and groups (e.g., H at 656.2 nm, O at 777.3 nm, CN at 388.3 nm) relating to the combustion flame were measured simultaneously using laser induced breakdown spectroscopy (LIBS). The residual energy was measured during the collection of LIBS spectra in the combustion process, which could be characterized the temperature profiles of combustion flame due to the positive correlation with temperature. The combustion stage could be clearly discriminated by the emission of H and CN, along with the flame temperature. The residual energy obtained in different atmospheres indicated that the impact of combustion atmosphere on flame temperature is greater in the char combustion stage rather than volatile combustion stage. It was determined from the temporal and spatial distribution of residual energy and CN intensity that a higher flame temperature leads to a higher concentration of CN. The correlation between the generation of CN and the NO_x formation was also investigated to show that the formation approaches of NO_x are similar in the O₂/CO₂ and O₂/N₂ atmospheres, while the fuel-N conversion paths are different between volatile combustion and char combustion stages. The measurement of temporal and spatial distributions of LIBS spectra with varying flame temperatures is significant in revealing the mechanism of coal-particle combustion and NO_x formation.     

The intrinsic reactivity of coal char conversion compared under different conditions of O2/CO2, O2/H2O and air atmospheres

The reasons for the intrinsic reactivity differences in coal char conversion under an O₂/H₂O atmosphere compared with that under an O₂/CO₂ or O₂/N₂ atmosphere have been investigated in a thermogravimetric analyzer by a simple variable activation energy (SVAE) method combined with an adsorption/desorption reaction mechanism. The results show that only CO₂ or H₂O chemisorption occurred in the non-isothermal experiments, not gasification; however, the intrinsic reaction rate (IRR) of coal char conversion at the same O₂ concentration still increases in an orderly manner under O₂/CO₂, O₂/N₂ and O₂/H₂O atmospheres. This result is due to the different chemisorption mechanisms of CO₂ and H₂O, namely, the production of C(CO), C(OH) and C(H) from CO₂ and H₂O chemisorption. At the same O₂ concentration, the trends and magnitudes of variable activation energies for coal char combustion under O₂/CO₂ and O₂/N₂ atmospheres are similar, while they are very different from those under O₂/H₂O conditions. Therefore, CO₂ has little influence on the reactivity, while H₂O changes the reactivity. In addition, according to the developed reaction mechanism, it is concluded that the SVAE method contributes to the characteristic intrinsic reactivity of coal char conversion under different atmospheres.     

Research on kinetics of limestone desulphurization reactions under a reducing atmosphere

With the development of the second generation pressurized fluidized‐bed combustion combined cycle (PFBC‐CC), integrated gasification combined cycle (IGCC) and the technology of staged combustion for lower emission of NOx, limestone desulphurization under a reducing atmosphere is gradually becoming a very important problem. This paper presents static tests of limestone desulphurization reaction under a reducing atmosphere. The experiment was done by an atmospheric thermogravimetric analyzer (TG‐DTG8110, made in Japan). The sorbent is limestone from Nanjing, China. Its CaCO₃ percentage is 93·75% and particle mean diameter is 0·63mm. Mixed gas concentrations are as follows. (N₂+H₂S) mixed gas: [H₂S] is 1·366E4, 2·277E4 and 2·73E4mg(Nm³)⁻¹; (O₂+N₂) mixed gas: [O₂] is 3, 5, 7 and 100%. Test temperatures are 920–930°C and 835–845°C. The optimal reaction conditions were obtained from the experiment. Sulphidation reaction at about 925°C and oxidation reaction at about 848°C are carried out easily. Through numerical handling, the dynamic equations of sulphidation and oxidation reactions are also obtained. Copyright © 1999 John Wiley & Sons, Ltd.     

Kinetics,thermodynamics and synergistic effects analyses of petroleum coke and biomass wastes during H2O co-gasification

In this study, the H₂O co-gasification of petroleum coke (PC) with low (sulfur and V₂O₅ contents) and different five kinds of biomass wastes were conducted using a thermogravimetric analyzer (TGA). The biomass used were the agricultural wastes (rice husk (RH), rice stalk (RS), and cotton straw (CS)) and by-product wastes (sawdust (SD) and sugar cane bagasse (SCB)). Their reactivities, kinetics and thermodynamics parameters were investigated and compared in detail as well as a synergistic effect during co-gasification of the blends. The kinetics and thermodynamics parameters were estimated by using the homogeneous model (HM) or the first-order chemical reaction (O1) and shrinking core models (SCM) or Phase boundary controlled reactions (R2 and R3). It was found that the biomass wastes was significantly improved the blends gasification reactivity. The obvious significant synergistic effect was observed in the char gasification stage of the blends compared with the pyrolysis stage. Compared to other models the phase boundary controlled reaction (R2) was found to be the best model to predict the experimental data of the co-gasification process. For both reaction stages of single fuels, SD showed the lowest values of activation energy and thermodynamics parameters. The blends of PC: SD and PC: CS provided the lowest activation energy and thermodynamics parameters for the pyrolysis stage and the char gasification stage, respectively. The co-gasification of PC and biomass wastes are a promising technique for the efficient utilization of PC and biomass wastes.     

Pyrolysis characteristics of Turkish lignites in N2 and CO2 environments

Pyrolysis characteristics and kinetic parameters of two Turkish lignites having different ash contents (Orhaneli as low ash and Soma as high ash sample) were studied under N₂ and CO₂ atmospheres by means of thermogravimetric analysis. The isoconversional kinetic methods of Flynn?Wall??Ozawa, Kissinger??Akahira??Sunose, and Friedman were employed to estimate the activation energy and pre-exponential factors. The experiments were conducted at four different heating rates of 5, 10, 15, and 20°C/min within the temperature range of 50??950ºC. The obtained results indicated that changing the pyrolysis ambient had no significant effect on the devolatilization region up to 700°C. The char formation region in N₂ atmosphere was due to the CaCO₃ decomposition and was more significant for Soma lignite due to its high ash content. However, in CO₂ atmosphere, the gasification reaction took place at temperatures higher than 700°C. The decomposition process of CaCO₃ in CO₂ atmosphere was hampered up to temperatures higher than 900°C. The estimated activation energies were found to have approximately similar trends under different atmospheres. For Orhaneli lignite, the average activation energy values were higher in CO₂ environment. However, for Soma lignite due to decomposition of CaCO₃, the activation energy values were higher in N₂ atmosphere. The mean uncertainty values were assessed for the activation energy values obtained for all test cases.     

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.     

Experimental study on ignition and combustion of coal-rice husk blends pellets in air and oxy-fuel conditions

The ignition and combustion characteristics of anthracite-rice husk (AC-RH) and bituminous coal-rice husk (BC-RH) pellets were investigated in a vertical heating tube furnace under different experimental condition, for gas temperature (873 K–1073 K) and under air and different oxygen concentration (21–70%) in CO₂/O₂ atmosphere. The investigation of the ignition and combustion characteristics focused on ignition mechanism, ignition delay, ignition temperature and combustion process. AC-RH pellets had two ignition mechanism in CO2/O2 atmosphere: homogeneous ignition of volatile and heterogeneous ignition of char. Heterogeneous ignition region decreased while homogeneous ignition increased as rice husk blending ratio increased in oxygen concentration-gas temperature plane. Only homogeneous ignition was observed when rice husk blending ratio was 30%. As for BC-RH pellets, only homogeneous ignition occurred in all experimental conditions. The effect of the rice husk blending on the anthracite was more pronounced than the bituminous coal for ignition mechanism. As oxygen concentration increased, a significant reduction in ignition delay and ignition temperature was observed at low rice husk blending ratio and low gas temperature. but at 1073 K, high oxidizer temperature weakened the effect of biomass blending and oxygen concentration on ignition delay and ignition temperature. Meanwhile, at 20% and 30% rice husk blending ratio, it also weakened the effect of oxygen concentration and oxidizer temperature on ignition delay and ignition temperature. In contrast, blending ratio had a more significant effect on ignition behavior. The replacement of N₂ by CO₂ at the same oxygen concentration contributed to an increase in ignition delay time and internal ignition temperature, which suppressed the ignition behavior. Different ignition mechanisms corresponded to different combustion processes.     

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

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