On the influence of the char gasification reactions on NO formation in flameless coal combustion

Flameless combustion is a well known measure to reduce NO_x emissions in gas combustion but has not yet been fully adapted to pulverised coal combustion. Numerical predictions can provide detailed information on the combustion process thus playing a significant role in understanding the basic mechanisms for pollutant formation. In simulations of conventional pulverised coal combustion the gasification by CO₂ or H₂O is usually omitted since its overall contribution to char oxidation is negligible compared to the oxidation with O₂. In flameless combustion, however, due to the strong recirculation of hot combustion products, primarily CO₂ and H₂O, and the thereby reduced concentration of O₂ in the reaction zone the local partial pressures of CO₂ and H₂O become significantly higher than that for O₂. Therefore, the char reaction with CO₂ and H₂O is being reconsidered. This paper presents a numerical study on the importance of these reactions on pollutant formation in flameless combustion. The numerical models used have been validated against experimental data. By varying the wall temperature and the burner excess air ratio, different cases have been investigated and the impact of considering gasification on the prediction of NO formation has been assessed. It was found that within the investigated ranges of these parameters the fraction of char being gasified increases up to 35%. This leads to changes in the local gas composition, primarily CO distribution, which in turn influences NO formation predictions. Considering gasification the prediction of NO emission is up to 40% lower than the predicted emissions without gasification reactions being taken into account.     

Oxidation mechanism of ammonia-N/coal-N during ammonia-coal co-combustion

Ammonia-coal co-combustion is a feasible approach to reduce CO₂ emissions during thermal power generation, it is necessary to study NO formation mechanism in ammonia-coal co-firing to realize low-carbon and low-nitrogen combustion. The experimental results showed that temperature and ammonia ratio have a significant effect on the NO formation. Under the same ammonia blending amount, the NO production increased first and then decreased with temperature increasing. Theoretical calculations revealed that the formation of NH in NH₃→NH→NO is one of the factors restricting ammonia combustion. NH oxidation on the char surface first occurred in the NH/coal/O₂ combustion system, and realized the conversion of N to NO, HNO and NO₂ through different reaction paths. Combined with the experimental and theoretical calculation results, it was concluded that the reduction of NO by ammonia/char is enhanced at high temperature (>1300 °C), which reduces the conversion of ammonia-N/coal-N to NO.     

Rapid pyrolysis and CO2 gasification of anthracite at high temperature

Anthracite could be burnt efficiently at high temperature utilizing oxy-coal technology. To clarify the effects of temperature and atmosphere on char porosity characteristics, char morphology, fuel-N conversion, and reducing products release, rapid pyrolysis and CO₂ gasification of anthracite was carried out in a high temperature entrained-flow reactor to simulate the condition in a pulverized coal furnace. Developed pore structure was formed in the gasification chars, which could be contributed to charCO₂ reaction at high temperatures. More mesopores were formed in internal carbon skeleton and retained against collapse and coalescent for gasification chars than pyrolysis chars. Compared with pyrolysis char, smoother and denser surface was observed in gasification char with the irregular bulges disappeared due to the destruction of external carbon skeleton. Char-N could be oxidized to NO in CO₂ atmosphere and then reduced to N₂ by (CN) on the char surface. Char-N release was greatly promoted due to gasification reaction along with poly-condensation at high temperature; and the preact release of char-N would result in a larger portion of NO_x reduction in the following reduction zone with the oxygen-staging combustion technology compared with that in air-staging combustion. Complementally, homogeneous reduction in NO_x emission would play a minor effect for anthracite in oxy-coal combustion because of the deficiency of CH₄ and HCN, especially at high temperature.     

Impacts of char structure evolution and inherent alkali and alkaline earth metallic species catalysis on reactivity during the coal char gasification with CO2/H2O

In this work, we studied the effects of char structural evolution and alkali and alkaline earth metallic species (AAEMs) catalysis on the reactivity during the char gasification with CO₂, H₂O, and their mixture. The gasified chars with different carbon conversion levels were prepared, and their physicochemical structures were characterized via nitrogen adsorption and FT‐Raman techniques. The concentrations of AAEMs in different modes were obtained by the sequential chemical extraction method. The reactivities of the raw and gasified chars were analyzed by the thermogravimetric analysis. The gasification atmospheres had varied effects on the physicochemical structure of coal char. The gasified char obtained in the CO₂ atmosphere had a lower aromatic condensation degree compared with that obtained in the H₂O atmosphere, irrespective of the temperature. The impact of the atmospheres on the specific surface area of the char varied with the temperature because H₂O and CO₂ have different routes of development of pore structure with coal char. A large specific surface area facilitates the exposure and dispersion of more AAEMs on the surface of the channel, which is conducive to their contact with the gasification agent to play the catalytic role. Thus, the reactivity of the gasified char is well correlated with its specific surface area at different gasification temperatures. In the absence of AAEMs, the chemical structure of coal char becomes the dominant factor affecting the reactivity.     

Gasification inhibition in chemical-looping combustion with solid fuels

Chemical-looping combustion (CLC) is a novel technology that can be used to meet growing demands on energy production without CO₂ emissions. The CLC process includes two reactors, an air and a fuel reactor. Between these two reactors oxygen is transported by an oxygen carrier, which most often is a metal oxide. This arrangement prevents mixing of N₂ from the air with CO₂ from the combustion giving combustion gases that consist almost entirely of CO₂ and H₂O. The technique reduces the energy penalty that normally arises from the separation of CO₂ from other flue gases, hence, CLC could make capture of CO₂ cheaper. For the application of CLC to solid fuels, the char remaining after devolatilization will react indirectly with the oxygen carrier via steam gasification. It has been suggested that H₂, and possibly CO, has an inhibiting effect on steam gasification in CLC. In this work experiments were conducted to investigate this effect. The experiments were conducted in a laboratory fluidized-bed reactor that was operating cyclically with alternating oxidation and reduction periods. Two different oxygen carriers were used as well as an inert sand bed. During the reducing period varying concentrations of CO or H₂ were used together with steam while the oxidation was conducted with 10% O₂ in N₂. The temperature was constant at 970 °C for all experiments. The results show that CO does not directly inhibit the gasification whereas the partial pressure of H₂ had a significant influence on fuel conversion. The results also suggest that dissociative hydrogen adsorption is the predominant hydrogen inhibition mechanism under the laboratory conditions, thus explaining why char conversion is much faster in a bed of oxygen carrying material, compared to an inert sand bed.     

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 100 μm 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.     

Pressurized chemical-looping combustion of coal with an iron ore-based oxygen carrier

Chemical-looping combustion (CLC) is a new combustion technology with inherent separation of CO₂. Most of the previous investigations on CLC of solid fuels were conducted under atmospheric pressure. A pressurized CLC combined cycle (PCLC-CC) system is proposed as a promising coal combustion technology with potential higher system efficiency, higher fuel conversion, and lower cost for CO₂ sequestration. In this study pressurized CLC of coal with Companhia Valedo Rio Doce (CVRD) iron ore was investigated in a laboratory fixed bed reactor. CVRD iron ore particles were exposed alternately to reduction by 0.4 g of Chinese Xuzhou bituminous coal gasified with 87.2% steam/N₂ mixture and oxidation with 5% O₂ in N₂ at 970 °C. The operating pressure was varied between 0.1 MPa and 0.6 MPa. First, control experiments of steam coal gasification over quartz sand were performed. H₂ and CO₂ are the major components of the gasification products, and the operating pressure influences the gas composition. Higher concentrations of CO₂ and lower fractions of CO, CH₄, and H₂ during the reduction process with CVRD iron ore was achieved under higher pressures. The effects of pressure on the coal gasification rate in the presence of the oxygen carrier were different for pyrolysis and char gasification. The pressurized condition suppresses the initial coal pyrolysis process while it also enhances coal char gasification and reduction with iron ore in steam, and thus improves the overall reaction rate of CLC. The oxidation rates and variation of oxygen carrier conversion are higher at elevated pressures reflecting higher reduction level in the previous reduction period. Scanning electron microscope and energy-dispersive X-ray spectroscopy (SEM-EDX) analyses show that particles become porous after experiments but maintain structure and size after several cycles. Agglomeration was not observed in this study. An EDX analysis demonstrates that there is very little coal ash deposited on the oxygen carrier particles but no appreciable crystalline phases change as verified by X-ray diffraction (XRD) analysis. Overall, the limited pressurized CLC experiments carried out in the present work suggest that PCLC of coal is promising and further investigations are necessary.     

Evolution of pore structure and fractal characteristics of coal char during coal gasification

This study classifies the evolutionary properties of coal char pore structure which occur during coal gasification. CO₂ gasification of various coal samples was carried out in a fixed bed reactor. The resulting chars were analysed using N₂ isothermal adsorption/desorption and scanning electron microscopy (SEM), coupled with fractal theory. Analytical results indicate that the pore structure of coal char underwent micropore evolving, enlarging and overlapping, while more mesopores and macropores developed with continued gasification. The surface area of coal char increased to its maximum value when carbon conversion reached approximately 50%. Fractal calculation results showed that two types of fractal structures associated with the coal char surface and pore structure underwent stereome development and elapsing. However, the evolutionary properties were unique for different coal samples. High rank coal had a complex spatial structure with more micro-pores, whereas lower rank coal had a much flatter spatial structure.     

The fate of char-N at pulverized coal conditions

The fate of char-N (nitrogen removed from the coal matrix during char oxidation) has been widely studied at fluidized bed conditions. This work extends the study of char-N to pulverized coal conditions. Coal chars from five parent coals were prepared and burned in a laboratory-scale pulverized coal combustor in experiments designed to identify the parameters controlling the fate of char-N. The chars were burned with natural gas (to simulate volatiles combustion) in both air and in a nitrogen-free oxidant composed of Ar, CO₂, and O₂. In some experiments, the char flames were doped with various levels of NO or NH₃ to simulate formation of NO_x from volatile-N (nitrogen removed during coal devolatilization). The conversion of char-N to NO_x in chars burned in the nitrogen-free oxidant was 50-60% for lignites and 40-50% for bituminous coals. In char flames doped with NO_x, the apparent conversion of char-N to NO_x (computed using the NO_x measurements made before and after the addition of char to the system) decreased significantly as the level of NO_x doping increased. With 900 ppm NO_x present before the addition of char, apparent conversion of char-N to NO_x was close to 0% for most chars. While there is no clear correlation between nitrogen content of the char and char-N to NO_x conversion at any level of NO_x in the flame, the degree of char burnout within a given family of chars does play a role. Increasing the concentration of O₂ in the system in both air and nitrogen-free oxidant experiments increased the conversion of char-N to NO_x. The effects of temperature on NO_x emissions were different at low (0 ppm) and high (900 ppm) levels of NO_x present in the flame before char addition.     

Detailed kinetic analysis and modelling of the dry gasification reaction of olive kernel and lignite coal chars

The dry gasification process of solid fuels is a promising pathway to mitigate and utilize captured CO₂ emissions toward syngas generation with tailored composition for several downstream energy conversion and chemical production processes. In the present work, comprehensive kinetic analysis and reaction modelling studies were carried out for olive kernel and lignite coal chars gasification reaction using pure CO₂ as gasifying agent. Chars reactivity and kinetics of the gasification reactions were thoroughly examined by thermogravimetric analysis at three different heating rates and correlated with their physicochemical properties. The reactivity of olive kernel char, as determined by the mean gasification reactivity and the comprehensive gasification characteristic index, S, was almost three times higher compared to that of the lignite coal char. It was disclosed that the fixed carbon content and alkali index (AI) have a major impact on the reactivity of chars. The activation energy, E_a, estimated by three different model-free kinetic methods was ranged between 140 and 170 kJ/mol and 250–350 kJ/mol for the olive kernel and lignite coal chars, respectively. The activation energy values for the lignite coal char significantly varied with carbon conversion degree, whereas this was not the case for olive kernel char, where the activation energy remained essentially unmodified throughout the whole carbon conversion range. Finally, the combined Malek and Coats-Rendfrem method was applied to unravel the mechanism of chars-CO₂ gasification reaction. It was found that the olive kernel char-CO₂ gasification can be described with a 2D-diffusion mechanism function (D2) whereas the lignite coal char-CO₂ gasification follows a second order chemical reaction mechanism function (F2).     

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.     

Natural draft and forced primary air combustion properties of a top-lit up-draft research furnace

Worldwide, over four million people die each year due to emissions from cookstoves. To address this problem, advanced cookstoves are being developed, with one system, called a top-lit up-draft (TLUD) gasifier stove, showing particular potential in reducing the production of harmful emissions. A novel research furnace analogy of a TLUD gasifier stove has been designed to study the TLUD combustion process. A commissioning procedure was established under natural draft and forced primary air conditions. A visual assessment was performed and the temperature and emissions profiles were recorded to identify the combustion phases. The efficiency was evaluated through the nominal combustion efficiency (NCE = CO₂/(CO₂ + CO)), which is very high in the migrating pyrolysis phase, averaging 0.9965 for the natural draft case. Forced primary air flows yield similar efficiencies. In the lighting phase and char gasification phase the NCE falls to 0.8404 and 0.6572 respectively in the natural draft case. When providing forced primary air flows, higher NCE values are achieved with higher air flows in the lighting phase, while with lower air flows in the char gasification phase. In the natural draft case high H₂ emissions are also found in the lighting and char gasification phases, the latter indicating incomplete pyrolysis. From the comparison of the natural draft with the forced draft configurations, it is evident that high efficiency and low emissions of incomplete combustion can only be achieved with high controllability of the air flow in the different phases of combustion.     

Kinetics of woodchips char gasification with steam and carbon dioxide

Kinetics of woodchips char gasification has been examined. Steam and CO₂ were used as the gasifying agents. Differences and similarities between kinetics of steam gasification and CO₂ gasification have been discussed. Comparison was conducted in terms of gasification duration, evolution of reaction rate with time and/or conversion, and effect of partial pressure on reaction rate. Reactor temperature was maintained at 900 °C. Partial pressure of gasifying agents varied from 1.5 bars to 0.6 bars in intervals of 0.3 bars. Steam and CO₂ flow rates were chosen so that both gasifying agents had equal amount of oxygen content. CO₂ gasification lasted for about 60 min while steam gasification lasted for about 22 min. The average reaction rate for steam gasification was almost twice that of CO₂. Both reaction rate curves showed a peak value at certain degree of conversion. For steam gasification, the reaction rate peak was found to be at a degree of conversion of about 0.3. However, for CO₂ gasification the reaction rate peak was found to be at a conversion degree of about 0.1. Reaction rates have been fitted using the random pore model (RPM). Average structural parameter, ψ for steam gasification and CO₂ gasification was determined to be 9 and 2.1, respectively. Average rate constant at 900 °C was 0.065 min⁻¹ for steam gasification and 0.031 min⁻¹ for CO₂ gasification. Change in partial pressure of gasifying agents did not affect the reaction rate for both steam and CO₂ gasification.     

Nitric oxide destruction during coal and char oxidation under pulverized-coal combustion conditions

A modified drop-tube reactor that allows particle distribution over the reactor cross-sectional area, and oxidation of chars produced in situ, was used to study the conversion efficiency of char nitrogen to nitric oxide (α_NO). The results confirm previous findings by other investigators that α_NO decreases as the weight of char burned increases. α_NO for coal was the same as (at 4% O₂) or lower than (at 20% O₂) that for an equal mass of char during oxidation. Since coal will yield approximately half its mass as fixed carbon, these results suggest that the local stoichiometry surrounding the particle is responsible for the observed reduction in α_NO as sample size increases. The analysis of the exhaust gases showed increases in HCN concentration and a decrease in CO₂/CO ratio as sample size increased, suggesting that local stoichiometry influences α_NO. Additional experiments showed that α_NO decreased as the background NO concentration was increased, at rates that diminished as the oxygen concentration increased, independent of particle size. The steep reduction in NO production as the background NO concentration increased was explained by the destruction of NO in the gas phase.     

Experimental study on the key factors affecting the gasification performance between different biomass: Compare citrus peel with pine sawdust

This work aims to reveal the advantages of citrus peel gasification and investigate the key factors affecting gasification performance. The gasification performance of citrus peel and pine sawdust are compared in a fixed bed reactor, and the reactivity and properties of biochar were investigated. The results showed that the H₂ yield and carbon conversion efficiency of citrus peel gasification were 34.35 mol/kg_biomass and 66.30%, respectively, which were higher than those of pine sawdust. Due to the high reactivity of citrus peel char, it only takes 100 min for the citrus peel to complete the gasification reaction, which is significantly faster than pine sawdust. Although the specific surface area of citrus peel char is lower than that of pine sawdust char, both the low degree of graphitization and the high catalytic index (2426.96) are favorable for the conversion of char, which ultimately lead to the high reactivity of citrus peel char.     

Gasification of a pulverized sub-bituminous coal in CO2 at atmospheric pressure in an entrained flow reactor

Char gasification by CO₂ may play an important role in oxy-fuel applications and affect particle temperature histories and overall reaction rates during combustion. This paper presents the results of a complete set of experiments of char gasification in CO₂ performed with a pulverized Indonesian sub-bituminous coal in an entrained flow reactor under realistic conditions; series of burnout curves at different reactor temperatures (1040–1300 °C) and CO₂ concentrations (0.7–100%) reveal consistent trends in the gasification rates. The study included also devolatilization and oxidation tests with this coal in the same experimental facility. The data are used to derive apparent kinetics for the three processes, in a manner similar to that followed in a previous work for the oxidation of a pulverized coal. The gasification kinetic parameters and reaction rates measured are then compared with values taken or derived from previous works by others, obtained by thermogravimetric analysis or experiments in entrained flow reactors. Finally, the relevance of char gasification in the overall reaction rate under conditions representative of those in an industrial boiler is explored, in particular for the case of oxy-coal combustion.     

Kinetics of CO2 gasification of biomass char in granulated blast furnace slag

The CO₂ gasification reactions of biomass char in granulated BFS (blast furnace slag) were isothermally investigated using a thermogravimetric analyzer with the temperature ranging from 1173 K to 1323 K. The effects of temperature, biomass type and granulated BFS on the kinetic characterizations of CO₂ gasification of biomass char were illuminated. The kinetic mechanism models and parameters were obtained through a novel two-step calculation method. The results indicated that the CO₂ gasification reactivity of biomass char as conversion and gasification index increased with the increase of temperature and it could be promoted through granulated BFS. The CO₂ gasification reactivity of CS (cornstalk) char with lower alkali index was lower than that of PS (peanut shell) char. The A₄ model (Avrami-Erofeev (m = 4) model) and A₃ model (Avrami-Erofeev (m = 3) model) were demonstrated as the best appropriate models for CO₂ gasification of CS char and PS char, respectively. The gasification activation energy of CS char ranging from 155.08 to 160.48 kJ/mol was higher than that of PS char whether with or without granulated BFS. Granulated BFS could decrease the activation energy of CO₂ gasification of char at any biomass type.     

An investigation on the mechanism of the increased NO2 emissions from H2-diesel dual fuel engine

The addition of hydrogen (H₂) into the intake air of a diesel engine was found to significantly increase the emissions of nitrogen dioxide (NO₂). Previous research demonstrated a strong correlation between the emissions of NO₂ and unburned H₂ in exhaust gas. However, the mechanism whereby H₂ addition in increasing NO₂ formation in a H₂-diesel dual fuel engine. Previously has not been investigated.This research numerically verified the hypothesis that the increased NO₂ emissions observed with the addition of H₂ was formed through the conversion from NO to NO₂ during the post combustion oxidation process of the unburned H₂ when mixed with the hot NO-containing combustion products. A variable volume single zone model with detailed chemistry was applied to simulate post-combustion oxidation process of the unburned H₂ and its effect on NO₂ emissions. The mixing of the unburned H₂ with the NO-containing hot combustion products was found to convert NO to NO₂. Such a conversion is promoted by the hydroperoxyl (HO₂) radical formed during the oxidation process of the H₂. The factors affecting the NO₂ formation and its destruction include the concentration of NO, H₂, O₂, and the temperature of the bulk mixture. When H₂ and hot NO-containing combustion products mixed during the early stage of expansion stroke, the NO₂ formed during H₂ oxidation was later dissociated to NO after the complete consumption of H₂. The complete combustion of H₂ exhausted the source of HO₂ necessary for the conversion from NO to NO₂. The mixing of H₂ with combustion products during the last part of the expansion stroke was not able to convert NO to NO₂ since the temperature was too low for H₂ to oxidize and to provide the HO₂ needed. The bulk mixture temperature range suitable for meaningful conversion from NO to NO₂ aided by HO₂ produced during the oxidation of H₂ was examined and presented.     

Char nitrogen conversion: implications to emissions from coal-fired utility boilers

The contribution of nitrogen present in the char on the production of nitrogen oxides during char combustion was analyzed. A literature review summarizes the current understanding of the mechanisms that account for the formation of NO and N₂O from the nitrogen present in char. The review focused on: (1) the functionalities in which nitrogen is present in the coal and how they evolve during coal devolatilization; (2) the mechanism of nitrogen release from the char to the homogeneous phase and its further oxidation to NO; and (3) the reduction of NO on the surface of the char. The critical analysis of these three issues allowed identification of uncertainties and well-founded conclusions observed in the literature for this system.

The existing models for the production of nitrogen oxides from char-N were also reviewed. A critical analysis of the assumptions made in these models and how they affect the final predictions is presented. Finally, a simplified version of these models was used to perform a parametric analysis evaluating the impact of several parameters on the total conversion of char-N to NO. These parameters include: (1) the rate of NO reduction on the char surface; (2) the rate of carbon oxidation; and (3) early vs. late nitrogen release during the char oxidation process. The results underscore the importance of the reaction of NO reduction on the char surface to the final conversion of char-N to NO.