Numerical modeling of a steam-assisted tubular coal gasifier

Understanding the heat and mass transfer phenomena in a coal gasifier is very useful for the assessment of gasifier performance and optimization of the design and operating parameters. In this paper, performance of an entrained flow air blown laboratory scale gasifier is numerically simulated with Fluent software. In the model, the continuous phase conservation equations are solved in an Eulerian frame, while those of particle phase are solved in a Lagrangian frame, with coupling between the two phases carried out through interactive source terms. The dispersion of the particles due to turbulence is predicted using a stochastic tracking model, in conjunction with the k–ε equations for the gas phase. The coal gasification model adopted includes devolatilization, combustion of volatiles, char combustion and gasification. The gasification performance inside the gasifier has been predicted for different air ratios as well as for different air and steam inlet temperatures. The overall temperature inside the gasifier is found to increase when the degree of air/steam pre-heating is increased, resulting in acceleration of the different reaction steps in the gasifier. The overall gasification performance indices such as carbon conversion, heating value of the exit gas and cold gas efficiency have been predicted. The predicted results show good agreement with available experimental data in literature.     

Coal conversion submodels for design applications at elevated pressures. Part I. devolatilization and char oxidation

Numerous process concepts are under development worldwide that convert coal at elevated pressure. These developments rely heavily on CFD and other advanced calculation schemes that require submodels for several stages of coal chemistry, including devolatilization, volatiles combustion and reforming, char oxidation and char gasification. This paper surveys the databases of laboratory testing on devolatilization and char oxidation at elevated pressure, first, to identify the tendencies that are essential to rational design of coal utilization technology and, second, to validate two well-known reaction mechanisms for quantitative design calculations.Devolatilization at elevated pressure generates less volatile matter, especially tar. Low-rank coals are no less sensitive to pressure variations than bituminous coals; in fact, coal quality is just as important at elevated pressure as it is at atmospheric pressure. Faster heating rates do not enhance volatiles yields at the highest operating pressures. The FLASHCHAIN^® predictions for the devolatilization database depict the distinctive devolatilization behavior of individual samples, even among samples with the same nominal rank. The only sample-specific input requirements are the proximate and ultimate analyses of the coal. There were no systematic discrepancies in the predicted total and tar yields across the entire pressure range. Char oxidation rates increase for progressively higher O₂ partial pressures and gas temperatures, but are insensitive to total pressure at constant O₂ mole fraction. Char burning rates become faster with coals of progressively lower rank, although the reactivity is somewhat less sensitive to coal quality at elevated pressure than at atmospheric pressure. An expanded version of the carbon burnout kinetics model was able to represent all datasets except one within useful quantitative tolerances, provided that the initial intrinsic pre-exponential factor was adjusted for each coal sample.     

Mathematical modeling of gasification of high‐ash Indian coals in moving bed gasification system

Simulation of gasification of high‐ash Indian coal in an updraft moving bed gasification system is presented in this paper. A steady one‐dimensional numerical model, which takes into account of drying, devolatilization, combustion and gasification processes, is used to solve the mass and energy balances in the gasification system. The results from the model have been validated against the experimental data available in literature for various types of coals. The predicted product gas composition, its calorific value and the exit temperature are in agreement with the reported results. The validated model is used to study the effect of input parameters such as oxygen content in air stream, steam flow rates and the pressure of the gasification system. Results indicate that the value of oxygen mole fraction around 0.42 in the oxidizer stream can provide optimum performance in oxygen‐based gasification systems. There is a range of steam‐to‐coal ratio that is dependent on the oxygen content in oxidizer stream. For air‐based systems, this value is around 0.4 and for oxygen‐based systems it is 1.5. The gasification performance improves with operating pressure significantly. An operating pressure of around 8 bar and higher, based on the application, can be used for achieving the required performance with high‐ash coals. The model is useful for predicting the performance of high‐ash Indian coals in a moving bed gasification system under different operating parameters. Copyright © 2013 John Wiley & Sons, Ltd.     

Nitrogen oxide formation from australian coals

The evolution of fuel nitrogen during devolatilization and the formation of NO_x during combustion were studied for two Australian coals in crucible, thermobalance, and rapid heating (drop-tube furnace) experiments. The evolution of coal nitrogen during devolatilization was dependent on both temperature and mode of heating. Under near stoichiometric combustion, 20–30% of coal nitrogen was converted to NO_x, Conversion increased markedly with increased fuel-lean conditions. The NO_x formed from volatiles was proportional to the fraction of coal nitrogen evolved as HCN and NH₃. The combustion of char at various temperatures and stoichiometries showed that the conversion of char nitrogen to NO_x depended primarily on char burnout. The contribution of char nitrogen to NO_x formation was greater than that of volatile nitrogen under fuel-rich conditions.     

An Eulerian model for the simulation of an entrained flow coal gasifier

A numerical model is used to simulate the gasification of coal inside an entrained flow gasifier. The model is based on the Eulerian–Eulerian concept. Both gas and particulate phases conservation Eulerian equations are solved. The model used includes the coal particle processes, such as drying, volatilization, heterogeneous reactions of combustion and gasification, particle drag and turbulent dispersion, as well as heat-up.The model is applied to the investigation of the gasification of coal in a commercial entrained flow gasifier, from which experimental data is available. The results obtained show good agreement for both the main and minor species, and temperature.     

Formation of the structure of chars during devolatilization of pulverized coal and its thermoproperties: A review

The paper provides an overview of current studies on the behaviour of coal during devolatilization, especially the experimental studies and modelling efforts on the formation of char structure of bituminous coals in the open literature. Coal is the most abundant fossil fuel in the world. It dominates the energy supply in the future and plays an increasing role particularly in the developing countries. Coal utilization processes such as combustion or gasification generally involve several steps: i.e., the devolatilization of organic materials, homogeneous reactions of volatile matter with the reactant gases and heterogeneous reactions of chars with the reactant gases. The devolatilization process exerts its influence throughout the life of the solid particles from the injection to the burnout, therefore is the most important step which needs to be understood. When volatile matter is generated, the physical structure of a char changes significantly during the devolatilization, some accompanying a particle's swelling. The complexity of a char's structure lies in the facts that the structure of a char itself is highly heterogenous inside an individual particle and between different particles and the chemistry of a char is strongly dependent on the raw coal properties. A char's structure is strongly dependent on the heating conditions such as temperature, heating rate and pressure. Understanding the swelling of coal and the formation of char's pore structure during the devolatilization of pulverized coal is essential to the development of advanced coal utilization technologies. During combustion and gasification of pulverized coal, the behaviour of individual particles differs markedly due to the variation of their maceral composition. Particles with different maceral constituents generate different types of char structure. The structure of a char has a significant impact on its subsequent heterogeneous reactions and ash formation. The review also covers the most recent studies carried out by the authors, including the experimental observations of the thermoplastic behaviour of individual coal particles from the density fractions using a single-particle reactor, the experimental analysis on chars prepared in a drop tube furnace using the density-separated coal samples, the development of a mathematical model for the formation of char's pore structure based on a simplified multi-bubble mechanism and the investigation on the effect of pressure on char formation in a pressurized entrained-flow reactor.     

Combustion of particles in a large pulverized brown coal flame

A model of the ignition of a polydisperse cloud of brown coal particles, in a known gas environment, is presented and used to predict the behavior of the particles in a burner jet of a utility boiler. The model allows for drying, devolatilization, and char combustion of the particles. It is assumed that the volatiles burn in the free stream so that char combustion can occur during volatiles evolution, the diffusion of oxygen to the particle surface being inhibited due to the net outflow of volatiles. The model is used to calculate the behavior of a cloud of p.f. size particles along the centerline of a brown coal burner jet in which the gas temperature and composition have been measured. Rates of volatile release and char combustion are calculated and shown to be in agreement with measurements of volatile material in the flame. It is found that particles smaller than about 80 μm contribute most to the ignition of the jet and that they closely follow the local gas temperature. The unique character of brown coal of combustion, its high volatile evolution on rapid heating, the high activity of its char at low temperature, and the demonstrated ignition of its char without a jump in temperature make the overlap of devolatilization and char combustion more likely than with other coals. The mathematical formulation that allows this overlap gives oxygen consumption levels consistent with measurement. An analysis is made of the relative importance of radiation from the flame front to the particle, and entrainment of hot combustion gases into the jet. It is found that the radiation is of secondary importance compared to the effect of entrainment which is the controlling mechanism in the initial heating of the particles. Also, the significance of the assumption that the volatiles burn in the free stream is discussed.     

Effect of turbulence and devolatilization models on coal gasification simulation in an entrained-flow gasifier

Numerical simulations of the oxygen-blown coal gasification process inside a generic entrained-flow gasifier are carried out. The Eulerian–Lagrangian approach is applied to solve the Navier–Stokes equations and the particle dynamics. Seven species transport equations are solved with three heterogeneous global reactions and two homogeneous reactions. Finite rates are used for the heterogeneous solid-to-gas reactions. Both finite rate and eddy-dissipation combustion models are calculated for each homogeneous gas-to-gas reaction, and the smaller of the two rates is used. Four different devolatilization models are employed and compared. The Kobayashi model produces slower devolatilization rate than the other models. The constant rate model produces the fastest devolatilization rate. The single rate model and the chemical percolation model produce moderate and consistent devolatilization rate. Slower devolatilization rate produces higher exit gas temperature and higher CO and CO₂ mass fractions, but lower H₂ and heating value, and hence, achieves lower gasification efficiency. Combustion of volatiles is modeled with two-stage global reactions with an intermediate stage via benzene.Turbulence models significantly affect the simulated results. Among five turbulence models employed, the standard k–ε and the RSM models give consistent results. The time scale for employing stochastic time tracking of particles also affects simulated result. Caution has to be exerted to select the appropriate time constant value. Smaller particles have a higher surface/volume ratio and react faster than larger particles. However, large particles possessing higher inertia could impinge on the opposing jet and change the thermal-flow filed and the reaction rates.     

Modeling of laminar pulverized coal flames with speciated devolatilization and comparisons with experiments

In an earlier mathematical model of laminar pulverized coal–air combustion, supported by added CH₄, it was assumed that the volatiles from the coal consisted solely of CH₄ and HCN. A revised model is introduced with speciated devolatilization rate constants for tar, CH₄, CO, CO₂, H₂O, H₂, and HCN. It is assumed that these rate constants can also be applied to the devolatilization of the tar. In addition, it is assumed that the soot is predominantly carbon and is oxidized by the attack of O, H, OH, and O₂, in the same way as the coal char. Because the devolatilization rate is strongly dependent on particle temperature, the latter has to be determined accurately from the momentum and energy equations of the particle. The model is one-dimensional, with axial radiative transfer. The introduction of soot formation and speciation of the volatiles results in much improved accuracy in the prediction of species and temperature profiles in subatmospheric combustion on a flat flame matrix burner. It is possible to derive an overall global devolatilization rate constant that agrees reasonably with the measurements. These computations suggest that the effective area of the assumed spherical coal char particles is four times greater than that of the assumed sphere. Modeling of atmospheric pressure flames suggests that in this case, the value of 4 should be reduced, probably because, as pressure increases, the diffusion flux of reactant is reduced. Subatmospheric pressure laminar burning velocities are predicted with satisfactory accuracy over the full range of overall equivalence ratios. Previous measurements of laminar burning velocity at atmospheric pressure are reviewed. However, the various means of supporting a stable coal flame and the associated uncertain geometries make it impossible to apply the present model to the different conditions. It is suggested that burning velocities measured on a flat flame burner, with a controlled amount of methane to support the combustion of a pulverized coal/air mixture, would provide a good test of the reactivities of different coals.     

An experimental study of propane combustion in a fluidized-bed gasifier

This paper reports an experimental study of the combustion of volatiles from coal, simulated by propane, and its interaction with char gasification reactions in a fluidized-bed coal gasifier (FBG). Experiments were performed under conditions of propane pyrolysis (in a bed fluidized by nitrogen and steam), propane reacting with oxygen and steam (in a bed fluidized by air and steam), char gasification only (in a bed fluidized by air and steam without propane) and in char gasification (in a bed fluidized by air and steam with propane). Axial concentration profiles of various species were obtained at 750, 850 and 950 °C. It was observed that the combustion of propane in an FBG, but without char, behaves as reported in the literature for fluidized-bed combustion (FBC). However, upon introducing char into the bed to simulate the reducing atmosphere in an FBG, oxygen was rapidly consumed within a short distance of the distributor, by significant partial oxidation of both propane and its decomposition products to carbon monoxide. The char was found to aid the pyrolysis of propane, limiting the amount of hydrocarbons surviving into the freeboard. The experimental results reported here are believed to be the first observations on the combustion of volatiles under conditions in an FBG.     

Transient gasification and syngas formation from coal particles in a fixed‐bed reactor

An experimental investigation on gasification and syngas formation from coal particles in a fixed‐bed reactor is conducted; particular attention is paid to the transient reaction dynamics. Three different coals, including two high‐volatile coals and a low‐volatile coal, are taken into consideration. In the initial reaction period, a two‐stage reaction is clearly observed; specifically, an exothermic reaction followed by an endothermic reaction is exhibited. Meanwhile, seeing that the devolatilization and pyrolysis reactions are pronounced, the initial concentrations of H₂ and CH₄ are relatively high, especially for the former. With increasing time, the interaction between coal and char particles is dominated by the latter, the concentrations of CO and CO₂ thus become higher. From the observation of syngas combustion, the entire gasification intensity proceeds from intensified growth, rapid decay, and then to progressive decay with increasing reaction time. For the two high‐volatile coals, the mass depletion is enhanced markedly once the reaction temperature is as high as 1000°C, whereas it is insensitive to the temperature for the low‐volatile coal. Nevertheless, it is found that, based on the weights of moisture and volatile matter, their relative release ratio from the low‐volatile coal is better than that from the high‐volatile coals. This implies that the final devolatilization and pyrolysis extent is not determined by coal grade. Copyright © 2006 John Wiley & Sons, Ltd.     

Study of devolatilization during chemical looping combustion of large coal and biomass particles

Chemical Looping Combustion (CLC) is one of the emerging technologies for carbon capture, with less energy penalty. The present way of using pulverized coals in a fluidized bed (FB)-CLC have limitations like loss of unconverted char and gaseous combustibles, which could be mitigated by use of coarser fuel particles. Devolatilization time is a critical input for the effective design of FB-CLC systems, primarily when large fuel particles are used. The present study investigates the devolatilization time and the char yield of three coals of two shapes, namely, two high ash Indian coals and a low ash Indonesian coal and a wood (Casuarina equisetifolia) in the size range of +8–25 mm, at different fuel reactor temperatures (800–950 °C) of a hematite based CLC unit. The devolatilization times of single fuel particles during CLC are determined using a visual method called ‘Color Indistinction Method’. Indonesian coal has the longest devolatilization time among the fuels, and biomass has the least. Increasing the bed temperature enhances the rate of volatile release, whereas this effect is less pronounced in larger particles. Devolatilization of Indonesian coal is found to be strongly influenced by the changes in operating conditions. With the decrease in sphericity, a maximum of 56% reduction in devolatilization time is observed for the +20–25 mm slender particles of Indonesian coals when compared to the near-round particles. The maximum average char yields at the end of the devolatilization phase for coal and biomass are about 55–76% and 16% respectively. Char yield in coal particles increases with an increase in particle size, whereas biomass particles show relatively consistent yield across all experimental conditions. Increase in bed temperature reduces the char yields of coal up to 12% and in biomass up to 30%. High volatile Indian coal is the most influenced fuel by the changes in fuels shape. A correlation for determining devolatilization time under CLC environment is presented, and it successfully fits most of the experimental values within ±20% deviation for coals (R² = 0.95) and within ±15% deviation for biomass (R² = 0.97).     

煤质对水煤浆气化性能的影响研究

煤质与气流床气化炉的匹配性至关重要,其不但影响气化炉的运行条件,也影响气化性能。本文选择了10种来自新疆和陕西北部的煤样进行了工业分析、元素分析、灰组成分析、灰熔点分析以及成浆性测试,并筛选出适合水煤浆气化的煤样。同时借助Aspen Plus软件对适合水煤浆气化的煤样在相同的煤浆浓度、碳转化率及操作压力条件下开展煤质对水煤浆气化性能影响的模拟分析。结果表明煤中灰含量越高,冷煤气效率和有效气含量越低,比氧耗和比煤耗越高;煤中O/C质量比和H/C质量比的增加也会导致冷煤气效率和有效气含量降低,比氧耗和比煤耗增加。因此从水煤浆气化经济性考虑,建议水煤浆气化煤质灰含量小于9.0wt%,煤中O/C质量比小于0.173,H/C质量比小于0.065。     

Coal conversion submodels for design applications at elevated pressures. Part II. Char gasification

This paper surveys the database on char gasification at elevated pressures, first, to identify the tendencies that are essential to rational design of coal utilization technology, and second, to validate a gasification mechanism for quantitative design calculations. Four hundred and fifty-three independent tests with 28 different coals characterized pressures from 0.02 to 3.0 MPa, CO₂ and steam mole percentages from 0 to 100%, CO and H₂ levels to 50%, gas temperatures from 800 to 1500 °C, and most of coal rank spectrum. Only a handful of cases characterized inhibition by CO and H₂, and only a single dataset represented the complex mixtures of H₂O, CO₂, CO, and H₂ that arise in practical applications. With uniform gas composition, gasification rates increase for progressively higher pressures, especially at lower pressures. Whereas the pressure effect saturates at the higher pressures with bituminous chars, no saturation is evident with low-rank chars. With fixed partial pressures of the gasification agents, the pressure effect is much weaker. Gasification rates increase for progressively higher gas temperatures. In general, gasification rates diminish for coals of progressively higher rank, but the data exhibit this tendency only for ranks of hv bituminous and higher.

These tendencies are interpreted with CBK/G, a comprehensive gasification mechanism based on the Carbon Burnout Kinetics Model. CBK/G incorporates three surface reactions for char oxidation plus four reactions for gasification by CO₂, H₂O, CO and H₂. Based on a one-point calibration of rate parameters for each coal in the database, CBK/G predicted extents of char conversion within ±11.4 daf wt% and gasification rates within ±22.7%. The predicted pressure, temperature, and concentration dependencies and the predicted inhibiting effects of CO and H₂ were generally confirmed in the data evaluations. The combination of the annealing mechanism and the random pore model imparts the correct form to the predicted rate reductions with conversion. CBK/G in conjunction with equilibrated gas compositions accurately described the lone dataset on complex mixtures with all the most important gasification agents, but many more such datasets are needed for stringent model evaluations.

Practical implications are illustrated with single-particle simulations of various coals, and a 1D gasifier simulation for realistic O₂ and steam stoichiometries. The rank dependence of gasification rates is the determining factor for predicted extents of char conversion at the gasifier outlet. But soot gasification kinetics will determine the unburned carbon emissions for all but the highest rank fuels. Only gasification kinetics for gas mixtures with widely variable levels of H₂O, H₂, and CO are directly relevant to gasifier performance evaluations.     

The devolatilization of coal and a comparison of chars produced in oxidizing and inert atmospheres in fluidized beds

This is a study of the devolatilization of coal in a laboratory-scale bed of silica sand, fluidized with either air or N₂ and electrically heated to 750 or 900°C. Coal particles (diameter 1.4–1.7 or 2.0–2.36 mm) were fed in batches to the surface of the bed and allowed to devolatilize in either an oxidizing atmosphere of air or inert N₂. In the first case, combustion of the volatiles occurred, but there was only thermal decomposition (pyrolysis) in the second situation. The resulting chars were recovered and analyzed for composition and structure, so that comparisons could be made between the effects of devolatilization with combustion and of pyrolysis in an inert atmosphere. It was found that the fractions of C and H in the char were only slightly sensitive to the type of fluidizing gas used. The amount of nitrogen in the recovered char and also the devolatilization time showed no dependence on the type of fluidizing gas, whereas BET areas were slightly larger after combustion in air. It is concluded that these effects are small relative to other errors, inherent in experiments on coal combustion, so that chars prepared in a bed fluidized by hot N₂ are very similar to those formed during coal combustion at nominally the same temperature. Equally, the overall composition of the volatile matter released during combustion in a fluidized bed is the same as in pyrolysis in nitrogen. The effects of other parameters, such as the temperature of the bed, the flow rate of the fluidizing gas and the size of the coal particles are also discussed in detail. It is concluded that most of the volatiles burn in a fluidized bed (at or less than 900°C) far away from the original coal particle. Also, NO_x is in effect a primary product of devolatilization, being produced in appreciable amounts when coal is heated in inert N₂. The ratio of C/N in the volatiles is found to be a constant during the latter stages of devolatilization, but beforehand at lower temperatures, carbon species are preferentially released. Overall, devolatilization of small particles (< 2.4 mm) in a fluidized bed at 900°C is kinetically controlled. The activation energy is small, being 15 ± 6 kJ/mol.     

NOx and H2S formation in the reductive zone of air-staged combustion of pulverized blended coals

Low NO_x combustion of blended coals is widely used in coal-fired boilers in China to control NO_x emission; thus, it is necessary to understand the formation mechanism of NO_x and H₂S during the combustion of blended coals. This paper focused on the investigation of reductive gases in the formation of NO_x and H₂S in the reductive zone of blended coals during combustion. Experiments with Zhundong (ZD) and Commercial (GE) coal and their blends with different mixing ratios were conducted in a drop tube furnace at 1200°C–1400°C with an excessive air ratio of 0.6–1.2. The coal conversion and formation characteristics of CO, H₂S, and NO_x in the fuel-rich zone were carefully studied under different experimental conditions for different blend ratios. Blending ZD into GE was found to increase not only the coal conversion but also the concentrations of CO and H₂S as NO reduction accelerated. Both the CO and H₂S concentrations inblended coal combustion increase with an increase in the combustion temperature and a decrease in the excessive air ratio. Based on accumulated experimental data, one interesting finding was that NO and H₂S from blended coal combustion were almost directly dependent on the CO concentration, and the CO concentration of the blended coal combustion depended on the single char gasification conversion.Thus, CO, NO_x, and H₂S formation characteristics from blended coal combustion can be well predicted by single char gasification kinetics.     

Effect of coal type on gasification in pressurized drop tube furnace

In order to understand the effect of coal type on coal gasification process at 15 atmospheric pressure of pressurized drop tube furnace (PDTF), a numerical study was conducted. Eulerian approach is used for the gas phase, whereas Lagrangian approach is used for the solid phase. Turbulence is modelled using the standard k–ε model. The turbulent gas‐phase combustion model incorporates the eddy dissipation model. One‐step two‐reaction model is employed for the devolatilization. Effect of coal type on carbon conversion at the same coal feed rate and gas flow rate cannot be verified due to the variation of equivalence ratio according to coal composition. Therefore, the same equivalence ratio is chosen to evaluate the effect of coal type on gasification. It is found that the volatile release based on experimental results should be taken in computations to predict accurate carbon conversion, especially in coal gasification due to the low gasification reaction. Even at over 1500 K and 15 atmospheric pressure, at which reactions are primarily diffusion‐controlled, the exit carbon conversion varies with the coal type. The temperature gradient in near‐burner region becomes gentle with increasing proximate volatile and moisture contents, but the volatiles released can make the temperature gradient steep by means of the fast reaction with oxygen. Copyright © 2001 John Wiley & Sons, Ltd.     

温度对中试规模的喷动流化床煤部分气化行为的影响

在构建的热输入2MW的中试规模加压喷动流化床部分气化试验装置上,对徐州烟煤的加压部分气化行为进行了研究.重点考察了气化温度对煤气成分、煤气热值、碳转化率和煤气产率等气化指标的影响.研究结果表明,煤气各组分的浓度,特别是甲烷浓度对气化温度非常敏感,碳转化率和煤气产率随温度的升高而增加,在试验的温度区间内,温度对煤气热值影响不大.部分气化炉所产生的煤气和半焦的热值均满足第二代增压流化床联合循环发电系统的要求.