Combustion of millimeter sized coal particles in convective flow

Continuous mass versus time data for single coal particles from 5.3 to 9.9 mm with gas temperatures of 900–1200K, Reynolds numbers of 63–126, and oxygen concentrations of 4.5–21 % are presented. Devolatilization and char burn models for millimeter sized particles are formulated and compared with the experimental results. The devolatilization rate is most sensitive to particle size and gas temperature. The char reactivity depends on initial size, Reynolds number, and oxygen concentration. The devolatilization rate agrees with the model of Anthony and Howard when volatile yields are provided from experimental data. The char burning rate follows a diffusion controlled shrinking sphere model when a diffusion screening factor is included.     

A comparison of various models in predicting ignition delay in single-particle coal combustion

In this paper, individual coal particle combustion under laminar conditions is simulated using models with various levels of complexity for the particle and gas phase chemical kinetics. The mass, momentum and energy governing equations are fully coupled between the particle and the gas phase. In the gas phase, detailed chemical kinetics based on GRI3.0 and infinitely-fast chemistry are considered and compared. For the particle phase, models for vaporization, devolatilization and char oxidation/gasification are considered, and the Kobayashi–Sarofim devolatilization model is compared to the Chemical Percolation Devolatilization (CPD) model. Ignition delay is used as a quantitative metric to compare the simulation prediction with experimental data, with careful attention given to the definition of ignition delay in the simulations. The effects of particle size, coal type and gas-phase temperature on the ignition delay are studied and compared with experimental data.     

Auto-gasification of a biofuel

A novel mechanism for gasifying a char is described. For thermally large particles (i.e., Bi > 0.1) the temperature distribution is non-uniform. Because different temperature regimes exist in the particle, the stages of drying, devolatilization (or pyrolysis), and reaction of the char may overlap. At some point the particle’s surface is fully devolatilized, while the particle’s interior is still undergoing drying and devolatilization. As H₂O and CO₂ flow out from the particle they pass through the hot surface layers of char. If the temperature is high enough, the char may be gasified. Black liquor was used here as a sample fuel. It has desirable properties for such auto-gasification; thermally large particles, a high initial water-content and a very porous and highly reactive char. Detailed numerical simulations suggest that 30 to 40% of the char may be converted simultaneously with devolatilization by auto-gasification. The larger the particle and the higher the temperature, the larger is the fraction of char gasified. For coal and peat, a typical particle size is too small for this mechanism to play any role when fired in pulverized fuel or fluidized bed furnaces. For burning wooden logs, the particle size is large and the pyrolysis time is ≈10 min., so then auto-gasification might be important.     

燃煤链条炉排工业锅炉燃烧数值模型研究

针对燃煤链条炉排工业锅炉的燃烧过程中床层内部存在复杂的传热、传质及物理化学反应过程等特点开发了三维床层和炉膛耦合的燃烧数值计算模型,模型包含了煤燃烧过程中水分蒸发、挥发析出、气相反应、焦炭燃烧和传热传质等基本的化学物理过程,同时考虑了大粒径煤颗粒内部的非等温传热特性,并通过实验测试与数值模拟对数值模型进行校核,实验结果与模型计算吻合得较好,从而验证了计算模型的准确性。燃煤链条炉排工业锅炉燃烧数值模型的建立为实现燃煤工业锅炉的优化设计和运行指导提供了先进的设计手段和理论支持。     

Fuel conversion characteristics of black liquor and pyrolysis oil mixtures: Efficient gasification with inherent catalyst

Alkali metals inherent in black liquor (BL) have strong catalytic activity during gasification. A catalytic co-gasification process based on BL with pyrolysis oil (PO) has the potential to be a part of efficient and fuel-flexible biofuel production systems. The objective of the paper is to investigate how adding PO into BL alters fuel conversion under gasification conditions. First, the conversion times of single fuel droplet were observed in a flat flame burner under different conditions. Fuel conversion times of PO/BL mixtures were significantly lower than PO and comparable to BL. Initial droplet size (300–1500 μm) was the main variable affecting devolatilization, indicating control by external heat transfer. Char oxidation was affected by droplet size and the surrounding gas composition. Then, the intrinsic reactivity of char gasification was measured in an isothermal thermogravimetric analyser at T = 993–1133 K under the flow of CO₂–N₂ mixtures. All the BL-based samples (100% BL, 20% PO/80% BL, and 30% PO/70% BL on mass basis) showed very high char conversion. Conversion rate of char gasification for PO/BL mixtures was comparable to that of pure BL although the fraction of alkali metal in char decreased because of mixing. The reactivities of BL and BL/PO chars were higher than the literature values for solid biomass and coal chars by several orders of magnitude. The combined results suggest that fuel mixtures containing up to 30% of PO on mass basis may be feasible in existing BL gasification technology.     

Modeling char oxidation behavior under Zone II burning conditions at elevated pressures

For accurate modeling of the coal combustion process at elevated pressures, account must be made for variations in char-particle structure. As pressure is increased, particle swelling increases during the devolatilization of certain bituminous coals, yielding a variety of char-particle structures, from uniform high-density particles to thin-walled non-uniform low-density particles having large internal void volumes. Since under Zone II burning conditions the char conversion rate depends upon the accessibility of the internal surfaces, the char structure plays a key role in determining particle burnout times. In our approach to characterize the impact of char structure on particle burning rates, effectiveness factors appropriate for thin-walled cenospherical particles and thick-walled particles having a few large cavities are defined and related to the effectiveness factor for uniform high-density particles that have no large voids, only a random distribution of pores having a mean pore size in the sub-micron range. For the uniform case, the Thiele modulus approach is used to account for Zone II type burning in which internal burning is limited by the combined effects of pore diffusion and the intrinsic chemical reactivity of the carbonaceous material. In the paper, the impact of having a variety of char structures in a mix of particles burning under Zone II burning conditions is demonstrated.     

Thermochemical conversion of brown coal and biomass in a pressurised fluidised bed gasifier with hot gas filtration using ceramic channel filters: measurements and gasifier modelling

Biomass (wood and Miscanthus pellets) and Rhenish brown-coal were gasified using a 1.5 MW_th (max.) pressurised fluidised bed gasification (PFBG) installation. NO_x precursor and tar emissions were studied by varying process parameters like the fuel type, pressure, temperature and air factor. Carbon conversions were well above 80%. Fuel-nitrogen conversion into NH₃ was mostly above ca. 50%. Fuel-nitrogen conversion into HCN was significantly lower. Results were in-line with comparable investigations with bottom feeding. Measurements of the gas composition for Miscanthus gasification were compared with a steady-state model based on elementary reaction chemistry and heterogeneous gas–char reactions related to the emission of nitrogen species. A flash pyrolysis model (FG-DVC-biomass version) was applied to determine the initial yields. Measurements and model simulation results were in reasonably good agreement.     

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.     

Combustion and gasification of coals in fixed-beds

Fixed-bed processes are commercially used for the combustion and conversion of coal for generation of power or production of gaseous or liquid products. Coal particle sizes in fixed-bed processes are typically in the mm to cm diameter range, being much larger than in most other coal processes. This review provides a broad treatment of the technology and the science related to fixed-bed systems. Commercialized and developmental fixed-bed combustion and gasification processes are explored, including countercurrent, cocurrent, and crosscurrent configurations. Ongoing demonstrations in the U.S. Clean Coal Technology program are included. Physical and chemical rate processes occurring in fixed-bed combustion are summarized, with emphasis on coal devolatilization and char oxidation. Mechanisms, rate data and models of these steps are considered with emphasis on large particles. Heat and mass transfer processes, solid flows, bed voidage, tar production and gas phase reactions were also considered. Modeling of fixed-bed processes is also reviewed. Features and assumptions of a large number of one- and two-dimensional fixed-bed combustion and gasification models are summarized while the details of a recent model from this laboratory are presented and compared with data. Research needs are also discussed.     

Numerical studies on the combustion properties of char particle clusters

Particle clustering is an important phenomenon in dense particle–gas two-phase flow. One of the key problems worth studying is the reacting properties of particle clusters in coal particle combustion process in the dense particle region. In this paper, a two-dimensional mathematical model for the char cluster combustion in airflow field is established. This char cluster consists of several individual particles. The comprehensive model includes mass, momentum, and energy conservation equations for both gas and particle phases. Detailed results regarding velocity vector, mass component, and temperature distributions inside and around the cluster are obtained. The micro-scale mass and heat transfer occurred inside and around the char cluster are revealed. By contrastively studying the stable combustion of char particle clusters consisting of different particles, the combustion properties of char clusters in various particle concentrations are presented and discussed.     

Numerical simulation of the devolatilization of a moving coal particle

The devolatilization of an isolated coal particle moving relative to the surrounding gas is numerically simulated using a competing reaction model of the pyrolysis and assuming that the released volatiles burn in an infinitely thin diffusion flame around the particle or not at all. The temperature of the particle is assumed to be uniform and the effects of the heat of pyrolysis, the intraparticle mass transfer resistance, and the variation of the particle radius are neglected. The effects of the size and velocity of the particle and of the temperature and oxygen mass fraction of the gas on the particle and flame temperature histories, the devolatilization time and the yield of light and heavy volatiles are investigated. The motion of the particle may have an important effect on the shape and position of the flame of volatiles, but it has only a mild effect on the devolatilization process for the particle sizes typical of pulverized coal combustion. This effect increases for large particles or in the absence of radiation. The relative motion enhances the heat transfer between the particle and the gas, causing the devolatilization time to decrease at high gas temperatures and to increase at low gas temperatures. The numerical results are compared with a blowing-corrected Nusselt number correlation often used in heat transfer models of the process.     

Numerical Studies on Combustion Characteristics of Interacting Pulverized Coal Particles at Various Oxygen Concentration

The two-dimensional laminar combustion characteristics of coal particles at various oxygen concentration levels of a surrounding gas have been numerically investigated. The numerical simulations, which use the two-step global reaction model to account for the surrounding gas effect, show the detailed interaction among the inter-spaced particles, undergoing devolatilization and subsequent char burning. Several parametric studies, which include the effects of gas temperature (1700 K), oxygen concentration, and variation in geometrical arrangement of the particles on the volatile release rate and the char burning rate, have been carried out. To address the change in the geometrical arrangement effect, multiple particles are located at various inter-spacings of 4–20 particle radii in both streamwise and spanwise directions. The results for the case of multiple particles are compared with those for the case of a single particle. The comparison indicates that the shift to the multiple particle arrangement resulted in the substantial change of the combustion characteristics and that the volatile release rate of the interacting coal particles exhibits a strong dependency on the particle spacing. The char combustion rate is controlled by the level of oxygen concentration and gas composition near particles during combustion. The char combustion rate is highly dependent on the particle spacing at all oxygen levels. The correlations of the volatile release rate and the change in total mass of particles are also found.     

Modeling of fixed bed downdraft biomass gasification: Application on lab‐scale and industrial reactors

This study aimed at presenting a model to simulate downdraft biomass gasification under steady‐state or unsteady‐state conditions. The model takes into account several processes that are relevant to the transformation of solid biomass into fuel gas, such as drying; devolatilization; oxidation; CO₂, H₂O, and H₂ reduction with char, pressure losses, solid and gas temperature, particle diameter, and bed void fraction evolution; and heat transfer by several mechanisms such as solid–gas convection, bed–wall convection, and radiation in the solid phase. Model validation is carried out by performing experiments in two lab‐scale downdraft fixed bed reactors (unsteady‐state conditions) and in a novel industrial pilot plant of 400 kW_th–100 kW_e (steady‐state conditions). The capability of the model to predict the effect of several factors (reactor diameter, air superficial velocity, and particle size and biomass moisture) on key response variables (temperature field, maximum temperature inside the bed, flame front velocity, biomass consumption rate, and composition and calorific value of the producer gas) is evaluated. For most response variables, a good agreement between experimental and estimated values is attained, and the model is able to reproduce the trend of variation of the experimental results. In general terms, the process performance improves with higher reactor diameter and lesser air superficial velocity, particle size, and moisture content of biomass. The steady‐state simulation appears to be a versatile tool for simulating different reactor configurations (preheating systems, variable geometry, and different materials). Copyright © 2013 John Wiley & Sons, Ltd.     

Numerical Simulation of Pressure Effects on the Gasification of Australian and Indian Coals in a Tubular Gasifier

This article presents a numerical study on the effect of pressure on the gasification performance of an entrained flow tubular gasifier for Australian and Indian coals. Gasification using a substoichiometric amount of air, with or without steam addition, is considered. The model takes into account phenomena such as devolatilization, combustion of volatiles, char combustion, and gasification. Continuous-phase conservation equations are solved in an Eulerian frame and those of the particle phase are solved in a Lagrangian frame, with coupling between the two phases carried out through interactive source terms. The numerical results obtained show that the gasification performance increases for both types of coal when the pressure is increased. Locations of devolatilization, combustion, and gasification zones inside the gasifier are analyzed using the temperature plots, devolatilization plots, and mass depletion histories of coal particles. With increase in pressure, the temperature inside the gasifier increases and also the position of maximum temperature shifts upstream. For the high-ash Indian coal, the combustion of volatiles and char and the gasification process are relatively slower than those for the low-ash Australian coal. The mole fractions of CO and H₂ are found to increase with increase in pressure, in all the cases considered. Further, the effects of pressure on overall gasification performance parameters such as carbon conversion, product gas heating value, and cold gas efficiency are also discussed for both types of coals.     

Numerical prediction of the transport and pyrolysis in the interior and surrounding of dry and wet wood log

The numerical simulation of the pyrolysis process of a dry and wet birch wood log in a cylindrical heating chamber is preformed. The model includes the flow inside and outside the porous wood log and accounts for convective, conductive and radiative heat transfer. A two-step pyrolysis reaction scheme is used to model the conversion from wood to tar, gas and char. The results of the simulations compare well with the authors experimental data which are presented in terms of radial temperature distribution and mass reduction, for both dry and wet cases. Our transient simulations provide us with the detailed flow field inside and outside the wood log. It clearly shows not only the existence but also the structure of the pyrolysis gas plumes leaving the wood. These plumes have only been visualised experimentally by few authors [Brackmann C et al. Optical and mass spectroscopy study of the pyrolysis gas of wood particles. Appl Spectros 2003;57(2):216–22, [12]] without any quantitative measurements and the present investigation gives a realistic estimation that we presently use to evaluate its impact on the heat and mass transfer, and on the momentum balance and the particle dispersion in a near future work. The gas plumes have a maximum velocity magnitude ranging between 0.1 and 0.2 m s⁻¹ and vanish when all the wood gas is produced. It is shown that increasing the convective flow around the wood log do not significantly modify the pyrolysis gas plume structure and seems to have small effect on the overall heating and the pyrolysis process which are mainly controlled by the thermal radiation from the hot surrounding walls.     

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.     

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.     

Modeling of single coal particle combustion in O2/N2 and O2/CO2 atmospheres under fluidized bed condition

A one-dimensional transient single coal particle combustion model was proposed to investigate the characteristics of single coal particle combustion in both O₂/N₂ and O₂/CO₂ atmospheres under the fluidized bed combustion condition. The model accounted for the fuel devolatilization, moisture evaporation, heterogeneous reaction as well as homogeneous reactions integrated with the heat and mass transfer from the fluidized bed environment to the coal particle. This model was validated by comparing the model prediction with the experimental results in the literature, and a satisfactory agreement between modeling and experiments proved the reliability of the model. The modeling results demonstrated that the carbon conversion rate of a single coal particle (diameter 6 to 8 mm) under fluidized bed conditions (bed temperature 1088 K) in an O₂/CO₂ (30:70) atmosphere was promoted by the gasification reaction, which was considerably greater than that in the O₂/N₂ (30:70) atmosphere. In addition, the surface and center temperatures of the particle evolved similarly, no matter it is under the O₂/N₂ condition or the O₂/CO₂ condition. A further analysis indicated that similar trends of the temperature evolution under different atmospheres were caused by the fact that the strong heat transfer under the fluidized bed condition overwhelmingly dominated the temperature evolution rather than the heat release of the chemical reaction.     

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.     

Stoichiometry and coal-type effects on homogeneous vs. Heterogeneous combustion in pulverized-coal flames

Results of experiments to determine the extent of heterogeneous combustion in the rapid-devolatilization regime of fuel-rich bituminous and subbituminous coal-dust flames are reported. Major gas concentrations, gas and particle temperatures, and solids proximate and elemental compositions were measured as functions of residence time in pulverized-coal/O₂/Ar flames stabilized on a flat-flame burner. Proximate fixed carbon and volatile matter measurements are corrected to account for devolatilization exceeding that predicted by ASTM proximate analysis. The corrected proximate data are used to determine the fraction of volatile matter consumed heterogeneously in situ and the fraction released to the gas phase by pyrolysis. Seven to thirty-five percent of the initial corrected volatile matter is removed heterogeneously. A mass transfer analysis is applied to predict a time dependent critical particle size such that the volatiles flux emerging from larger particles is sufficient to prevent surface oxidation. Critical particle radii as large as 39 μm are calculated. Results of the volatile matter partitioning and critical radius calculations indicate that heterogeneous combustion is more important in the leaner flames and for the subbituminous coal.