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.     

Effect of reactivity loss on apparent reaction order of burning char particles

Considerable debate still exists in the char combustion community over the expected and observed reaction orders of carbon reacting with oxygen. In particular, very low values of the reaction order (approaching zero) are commonly observed in char combustion experiments. These observations appear to conflict with porous catalyst theory as first expressed by Thiele, which suggests that the apparent reaction order must be greater than 0.5. In this work, we propose that this conflict may be resolved by considering the decrease in char reactivity with burnout due to ash effects, thermal annealing, or other phenomena. Specifically, the influence of ash dilution of the available surface area on the apparent reaction order is explored. Equations describing the ash dilution effect are combined with a model for particle burnout based on single-film nth-order Arrhenius char combustion and yield an analytical expression for the effective reaction order. When this expression is applied for experimental conditions reflecting combustion of individual pulverized coal particles in an entrained flow reactor, the apparent reaction order is shown to be lower than the inherent char matrix reaction order, even for negligible extents of char conversion. As char conversion proceeds and approaches completion, the apparent reaction order drops precipitously past zero to negative values. Conversely, the inclusion of the ash dilution model has little effect on the char conversion profile or char particle temperature until significant burnout has occurred. Taken together, these results suggest that the common experimental observation of low apparent reaction orders during char combustion is a consequence of the lack of explicit modeling of the decrease in char reactivity with burnout.     

Experimental investigation of NOx emissions during pulverized char combustion in oxygen-enriched air preheated with a circulating fluidized bed

An experimental investigation on NO_x emissions of pulverized char combustion in oxygen-enriched air was carried out in a new test system consisting of a circulating fluidized bed (CFB) and a down-fired combustor (DFC). In this paper, the pulverized char combustion characteristics and NO_x emission characteristics in the air conditions, the local oxygen-enriched air conditions in the CFB and the overall oxygen-enriched air conditions were studied. The results show that when the primary air oxygen concentration increased from 21.0% to 24.6% and to 28.2%, the ratio of fuel-nitrogen converted to nitrogen in the CFB increased from 39.7% to 45.0% and to 50.8%, respectively. This finding indicates that the preheating process in the CFB was one of the important reasons why the preheating combustion technology could significantly reduce NO_x emissions. Compared with the air combustion conditions, the NO emissions decreased to almost half of the original emissions when only the primary air oxygen concentration increased. On this basis, the NO emissions increased slightly when the air oxygen concentration was also increased in the DFC. Under these conditions, the variations in the char combustion efficiency were consistent with the variations in the temperature distribution. The feasibility and superiority of integrating the preheating combustion technology and oxygen-enriched air combustion technology are verified.     

煤粉高温富氧无油点火实验和数值模拟研究

提出了利用高温氧气与高浓度煤粉气流直接混合来实现煤粉气流点火的无油点火方式,采用煤粉高温氧气无油点火实验装置对煤粉气流的高温氧气无油点火过程进行了研究,利用数值模拟方法对该点火装置的流场特性进行了分析.结果表明：当氧气加热温度超过750℃后,设计工况条件下利用高温氧气可以顺利安全地实现煤粉气流的点火;提高一次风温度、增大一次风煤粉浓度及高温氧气风量或减小一次风速度有利于煤粉气流的着火和燃烧;煤粉气流在点火装置中心管喷口正前方首先开始着火,流场平均温度可达2 000 K以上,最高温度超过3 000 K,温度最高的区域位于中心管轴线的两侧.     

高压O2/CO2气氛下煤焦理论计算研究

研究表明在6MPa压力,富氧浓度为30%的条件下的增压富氧燃烧的经济性可以达到最佳。在此气氛下用一个简单的燃烧模型对碳球燃烧的情况进行理论计算,与空气气氛下的燃烧情况作对比,得到增压富氧燃烧在煤粉的燃烧时间和对烟气捕集回收二氧化碳上的优越性。从而为增压富氧燃烧的进一步发展提供理论基础。     

煤粉高温富氧无油点火的优化设计研究

设计了一种煤粉高温富氧无油点火装置,利用高温氧气与高浓度煤粉气流的混合直接点燃煤粉,进而取代油枪,实现煤粉的无油点火。通过冷态流场测定实验对点火装置进行了结构优化,并以此为基础,对煤粉高温富氧无油点火进行了数值模拟。结果表明:煤粉高温富氧无油点火装置采用三角形导流锥,中心管喷口扩口半角取15°,高温氧气通道喷口扩口半角取25°时组织的流场状况较好;点火装置形成的富氧燃烧高温火炬温度最高可在3 000 K上,烟气平均温度可在2 000 K以上,能有效保证主燃烧器的顺利点火。     

On the burning behavior of pulverized coal chars

A model that predicts the physical changes that pulverized coal char particles undergo during combustion has been developed. In the model, a burning particle is divided into a number of concentric annular volume elements. The mass loss rate, specific surface area, and apparent density in each volume element depend upon the local particle conditions, which vary as a consequence of the adsorbed oxygen and gas-phase oxygen concentration gradients inside the particle. The model predicts the particle's burning rate, temperature, diameter, apparent density, and specific surface area as combustion proceeds, given ambient conditions and initial char properties. A six-step heterogeneous reaction mechanism is used to describe carbon reactivity to oxygen. A distributed activation energy approach is used to account for the variation in desorption energies of adsorbed O-atoms on the carbonaceous surface. Model calculations support the three burning zones established for the oxidation of pulverized coal chars. The model indicates two types of zone II behavior, however. Under weak zone II burning conditions, constant-diameter burning occurs up to 30% to 50% conversion before burning commences with reductions in both size and apparent density. Under strong zone II conditions, particles burn with reductions in both size and apparent density after an initial short period (<2% conversion) of constant-diameter burning. Model predictions reveal that early in the oxidation process, there is mass loss at constant diameter under all zone II burning conditions. Such weak and strong burning behavior cannot be predicted with the commonly used power-law model for the mode of burning employing a single value for the burning mode parameter. Model calculations also reveal how specific surface area evolves when oxidation occurs in the zone II burning regime. Based on the calculated results, a surface area submodel that accounts for the effects of pore growth and coalescence during combustion under zone I conditions was modified to permit the characterization of the variations in specific surface area that occur during char conversion under zones II conditions. The modified surface area model is applicable to all burning regimes. Calculations also indicate that the particle's effectiveness factor varies during conversion under zone II burning conditions. With the adsorption/desorption mechanism employed, a near first-order Thiele modulus-effectiveness factor relationship is obeyed over the particle's lifetime.     

Kinetics of devolatilization and oxidation of a pulverized biomass in an entrained flow reactor under realistic combustion conditions

In this paper the results of a complete set of devolatilization and combustion experiments performed with pulverized (∼500 μm) biomass in an entrained flow reactor under realistic combustion conditions are presented. The data obtained are used to derive the kinetic parameters that best fit the observed behaviors, according to a simple model of particle combustion (one-step devolatilization, apparent oxidation kinetics, thermally thin particles). The model is found to adequately reproduce the experimental trends regarding both volatile release and char oxidation rates for the range of particle sizes and combustion conditions explored. The experimental and numerical procedures, similar to those recently proposed for the combustion of pulverized coal [J. Ballester, S. Jiménez, Combust. Flame 142 (2005) 210-222], have been designed to derive the parameters required for the analysis of biomass combustion in practical pulverized fuel configurations and allow a reliable characterization of any finely pulverized biomass. Additionally, the results of a limited study on the release rate of nitrogen from the biomass particle along combustion are shown.     

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.     

Experimental studies of ash film fractions based on measurement of cenospheres geometry in pulverized coal combustion

In pulverized coal particle combustion, part of the ash forms the ash film and exerts an inhibitory influence on combustion by impeding the diffusion of oxygen to the encapsulated char core, while part of the ash diffuses toward the char core. Despite the considerable ash effects on combustion, the fraction of ash film still remains unclear. However, the research of the properties of cenospheres can be an appropriate choice for the fraction determination, being aware that the formation of cenospheres is based on the model of coal particles with the visco-plastic ash film and a solid core. The fraction of ash film X is the ratio of the measuring mass of ash film and the total ash in coal particle. In this paper, the Huangling bituminous coal with different sizes was burnt in a drop-tube furnace at 1273, 1473, and 1673 K with air as oxidizer. A scanning electron microscope (SEM) and cross-section analysis have been used to study the geometry of the collected cenospheres and the effects of combustion parameters on the fraction of ash film. The results show that the ash film fraction increases with increasing temperature and carbon conversion ratio but decreases with larger sizes of coal particles. The high fraction of ash film provides a reasonable explanation for the extinction event at the late burnout stage. The varied values of ash film fractions under different conditions during the dynamic combustion process are necessary for further development of kinetic models.     

Unburnt carbon and ashing behavior for slow burning of lignite under oxygen-enriched combustion conditions

Oxygen-enriched combustion of coal has so far been investigated using either entrained flow reactors or fluidized-bed combustors where burning takes place rapidly due to high heating rates. On the contrary, this paper presents the results of slow burning of coal under oxygen-enriched combustion conditions. Ashing behavior of mineral matter-rich low-rank coal from Turkish Tekirdag-Malkara lignite under oxygen-enriched conditions was investigated to determine the effect of this combustion technique on unburnt carbon, mineralogical characteristics and the burning performance. These experiments showed that the influence of O₂ concentration on ashing is much more evident than the temperature during oxygen-enriched combustion.     

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.     

富氧煤粉气流着火机理的实验研究

针对环境氧浓度发生变化时,煤粉气流的反应动力学级数和机理都会发生变化的情况,采用一维火焰炉对神木烟煤在富氧条件下的着火机理进行了实验研究.结果表明:采用着火时烟气中CO和CO2体积分数的变化规律来判断煤粉气流的着火是可行的;当环境中氧气体积分数从21%升高到40%时,神木烟煤的着火完成了从均相着火向多相着火的转变,同时随着着火温度的降低,着火时刻烟气中热解产物的含量迅速降低.     

煤粉炉燃烧效率工程预测模型

整理了前人有关煤焦燃烧反应的动力学数据，在此基础上，建立了适合于工程计算的煤粉炉燃烧效率预测模型。该模型能够定量分析煤种、煤粉细度、运行氧量、炉膛温度、燃尽高度等多种参数对燃烧效率的影响。采用该模型对实际运行锅炉的计算表明，它能够较为准确地预测飞灰未燃尽碳含量，可以用于煤粉锅炉燃烧效率预报分析，为选择合适的设计和运行参数提供指导。图6表3参3。     

Multivariable optimization of reaction order and kinetic parameters for high temperature oxidation of 10 bituminous coal chars

In industrial pulverized fuel combustion, char oxidation is generally limited by the combined effects of chemical reactions and pore diffusion. Under such conditions, char oxidation is frequently predicted by power law models, which despite their simplicity, are widely used in the comprehensive CFD modeling of pulverized coal boilers. However, there is no consensus on the apparent reaction order given by such models. This study developed a systematic approach which gives consistent values over a range of conditions. Apparent reaction orders for 10 bituminous coal chars were investigated with three different oxygen concentrations, ranging from 4 to 12 vol.%, and a gas temperature of 1223 K for each char. Experimental burnout profiles of the chars were obtained by means of an Isothermal Plug Flow Reactor operating at industrially realistic heating rates (10⁴ K/s). For various reaction orders between 0.05 and 2.00, kinetic parameters were independently determined, following numerical procedures recently suggested in the literature. The resulting values were incorporated into an empirical power law model and compared to experimental data for the 10 chars, over a burnout range of 0–75%. The best fit to the experiments occurs with apparent reaction orders of around one for all the chars.     

微富氧条件下煤粉燃烧及NO生成特性的研究

利用热重分析技术对微富氧条件下煤粉的燃烧特性进行了研究,并与富氧条件下煤粉的燃烧特性进行了对比,利用固定床测定了燃煤NO的生成规律,分析了反应气氛和煤种的影响．结果表明：随着氧体积分数增加,微富氧条件下煤粉的燃烧向低温区移动,综合燃烧特性指数S逐渐增大;在相同的氧体积分数下,由于N2和CO2的物性差异,煤粉的微富氧燃烧特性优于富氧燃烧特性,但当氧体积分数升高到40％时,两种气氛的燃烧特性差别不大;反应气氛和煤种均对燃料氮的转化率影响显著;氧体积分数升高或N2的参与会使反应温度上升,影响燃料氮的转化率;煤的挥发分和元素氮的质量分数也会影响燃料氮的转化率．     

An investigation of the combustion of pulverized coal-air mixture in different combustor geometries

A two-dimensional theoretical study of the flow and combustion of pulverized coal by diffusion flames is presented. The model predicts gas flows, species concentrations, and temperatures in combustors having specific geometries. The conservation equations are solved utilizing the κ-ε turbulence model. Coal devolatilization is modeled by the two-competing-reactions scheme, which generates two sets of volatiles and char, each by a specific rate constant, described in Arrhenius form. Char combustion from devolatilization occurs by reaction with oxygen, carbon dioxide, water, particle dispersion, and radiative heat transfer between furnace wall and particles. The model is used to investigate the interaction between flow and combustion in flames produced by arranging the locations of the primary inlet and the secondary air inlets in a furnace. The predictions, which could be valuable for designing furnaces, indicate that a centered primary inlet and a minimum recirculation are some of the criteria that could favorable for combustion.     

The effect of wood biomass blending with pulverized coal on combustion characteristics under oxy-fuel condition

In this study, combustion from the co-firing of coal and wood biomass, and thermal characteristics such as ignition temperature, burn-out temperature, and activation energy were discussed using a thermogravimetric analyzer (TGA). We investigated the effects of biomass blending with two kinds of pulverized coal (bituminous Shenhua, and sub-bituminous Adaro) under air and oxy-fuel conditions. The coal fraction in the blended samples was set to 1, 0.8, and 0.5. The oxygen fraction in the oxidant was set to 0.21, 0.3, 0.5, and 0.8. The ignition temperature was governed by the fuel composition, particularly in the blended biomass which has a much higher content of volatile matter comparing to coal. However, the burnout temperature, which shows a strong relationship with char combustion, depended on the oxidant ingredients rather than on the fuel components. Thermal characteristics such as ignition, burnout temperature, reaction region, and heat flow were very similar between air and a 0.3 oxygen concentration under oxy-fuel conditions with Shenhua coal.     

旋风炉内气相燃烧及两相流动的数值模拟

在有反应两相流动及煤粉燃烧的全双流体模型（ＰＴＦ模型,ｐｕｒｅ ｔｗｏ－ｆｌｕｉｄ ｍｏｄｅｌ）基础上,采用修正的ｋ－ε－ｋｐ两相湍流模型,对旋风炉内的湍流气相燃烧（甲烷和一氧化碳的燃烧）及在气相燃烧条件下的两相流动进行了数值模拟研究,模拟结果表明,在有燃烧的情况下,在旋风炉的底部存在近壁回流区,该回流区有利于火焰稳定,气粒两相切向速度分布具有类似的Ｒａｎｋｉｎｅ涡结构,该研究为煤粉燃烧的数值模拟     

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.