A numerical study on parametric sensitivity of the flow characteristics on pulverized coal gasification

In order to analyse the sensitivity on pulverized coal flames of variables such as initial turbulent intensity, steam addition, primary/secondary momentum ratio, and radiation heat transfer, a numerical study was conducted at the gasification process. Eulerian approach is used for the gas phase, whereas Lagrangian approach is used for the solid phase. Turbulence is modeled using the standard k–ϵ model. The turbulent combustion model incorporates the eddy dissipation model. The radiation heat transfer is solved using a Monte‐Carlo method. One‐step two‐reaction model is employed for the devolatilization of a Kideco coal. In pulverized flame of long liftoff height, the initial turbulent intensity is an important factor to predict the accurate flame front position. The radiation heat transfer and wall heat loss ratio distort the temperature distributions along the reactor wall, but do not affect the reactor performance such as coal burnout, residence time and flame front position. The primary/secondary momentum ratio only affects the position of flame front, but the coal burnout is slightly influenced. It is confirmed that the momentum ratio is a variable only associated with the flame stabilization. The addition of steam in the reactor has a detrimental effect on all the aspects, particularly in reactor temperature and coal burnout. The increase of liftoff height and dropped gas temperature mean the harm of flame stabilization in the reactor, and so the gasification reaction may be deactivated. Copyright © 2000 John Wiley & Sons, Ltd.     

Theory of reverse combustion linking

The present work suggests a theory of reverse combustion linking (RCL). RCL is a central unit operation of underground coal gasification (UCG) technology. The theory is based on analyzing the stability of different branches of the propagation speed curves and determining the regime that is responsible for propagation of the flame during RCL. The theory is in good qualitative and quantitative agreement with the data obtained in practical use of RCL in UCG operations.     

Understanding ignition processes in spray-guided gasoline engines using high-speed imaging and the extended spark-ignition model SparkCIMM: Part B: Importance of molecular fuel properties in early flame front propagation

Recent high-speed imaging of ignition processes in spray-guided gasoline engines has motivated the development of the physically-based spark channel ignition monitoring model SparkCIMM, which bridges the gap between a detailed spray/vaporization model and a model for fully developed turbulent flame front propagation. Previously, both SparkCIMM and high-speed optical imaging data have shown that, in spray-guided engines, the spark plasma channel is stretched and wrinkled by the local turbulence, excessive stretching results in spark re-strikes, large variations occur in turbulence intensity and local equivalence ratio along the spark channel, and ignition occurs in localized regions along the spark channel (based upon a Karlovitz-number criteria).In this paper, SparkCIMM is enhanced by: (1) an extended flamelet model to predict localized ignition spots along the spark plasma channel, (2) a detailed chemical mechanism for gasoline surrogate oxidation, and (3) a formulation of early flame kernel propagation based on the G-equation theory that includes detailed chemistry and a local enthalpy flamelet model to consider turbulent enthalpy fluctuations. In agreement with new experimental data from broadband spark and hot soot luminosity imaging, the model establishes that ignition prefers to occur in fuel-rich regions along the spark channel. In this highly-turbulent highly-stratified environment, these ignition spots burn as quasi-laminar flame kernels. In this paper, the laminar burning velocities and flame thicknesses of these kernels are calculated along the mean turbulent flame front, using tabulated detailed chemistry flamelets over a wide range of stoichiometry and exhaust gas dilution. The criteria for flame propagation include chemical (cross-over temperature based) and turbulence (Karlovitz-number based) effects. Numerical simulations using ignition models of different physical complexity demonstrate the significance of turbulent mixture fraction and enthalpy fluctuations in the prediction of early flame front propagation. A third paper on SparkCIMM (companion paper to this one) focuses on the importance of molecular fuel properties and flame curvature on early flame propagation and compares computed flame propagation with high speed combustion imaging and computed heat release rates with cylinder pressure analysis.The goals of SparkCIMM development are to (a) enhance our fundamental understanding of ignition and combustion processes in highly-turbulent highly-stratified engine conditions, (b) incorporate that understanding into a physically-based submodel for RANS engine calculations that can be reliably used without modification for a wide range of conditions (i.e., homogeneous or stratified, low or high turbulence, low or high dilution), and (c) provide a submodel that can be incorporated into a future LES model for physically-based modeling of cycle-to-cycle variability in engines.     

Reactor performance with primary/secondary swirl intensity and direction in coal gasification process

In order to evaluate the effect of swirl direction and intensity of primary/secondary stream on pulverized coal gasification performance, a numerical study was conducted. Eulerian and Lagrangian approaches are used for the gas and solid phase, respectively. The computation code was formulated with PSI‐cell method, k–? model for turbulence flow, Monte‐Carlo method for radiative heat transfer, and eddy dissipation model for gas‐phase reaction rate. A one‐step two‐reaction model is employed for the devolatilization of Kideco coal. Flow and reactor performance are varied by primary/secondary swirl intensity and direction. For weak primary swirl, the WSF region is minimized at the secondary vane angle beginning generation of internal recirculation zone and having peak coal burnout. The flame stability is improved at counterswirl rather than coswirl due to its intense shear. Meanwhile, for strong primary swirl, flow distribution and coal burnout are the reverse trend with those of weak swirl and the flame stability is somewhat enhanced at coswirl rather than counterswirl. To improve coal burnout and flame stability, it is confirmed that the swirl condition be proposed for moving the flame front position toward upstream. Copyright © 2001 John Wiley & Sons, Ltd.     

Design and experimental study of pilot scale throat-less downdraft gasifier fed by rice husk and wood sawdust

In this work, pilot scale throat-less downdraft gasifier is fabricated and tested on rice husk and blend of rice husk-sawdust. The test aims to investigate effect of equivalence ratio on temperature profile, propagation front (flame propagation rate, bed movement rate, and effective propagation rate), and performance of the gasifier (composition of producer gas, heating value of producer gas, and thermal efficiency of the gasifier). Equivalence ratio investigated are 0.15, 0.20, and 0.25, while the blend ratio is 1:1 by mass. The results show that axial temperatures in the reactor surge faster with increasing equivalence ratio during the rice husk gasification and the blend gasification. Typically, flame propagation rate, bed movement rate, and effective propagation rate improve with rising equivalence ratio from 0.15 to 0.25. The best higher heating values and thermal efficiencies are obtained at equivalence ratio of 0.2 and 0.15 for the rice husk gasification and the blend gasification, respectively.     

Experimental simulate on hydrogen production of different coals in underground coal gasification

Research on hydrogen production from coal gasification is mainly focused on the formation of CO and H₂ from coal and water vapor in high-temperature environments. However, in the process of underground coal gasification, the water gas shift reaction of low-temperature steam will absorb a lot of heat, which makes it difficult to maintain the combustion of coal seams in the process of underground coal gasification. In order to obtain high-quality hydrogen, a pure oxygen-steam gasification process is used to improve the gasification efficiency. And as the gasification surface continues to recede, the drying, pyrolysis, gasification and combustion reactions of underground coal seams gradually occur. Direct coal gasification can't truly reflect the process of underground coal gasification. In order to simulate the hydrogen production laws of different coal types in the underground gasification process realistically, a two-step gasification process (pyrolysis of coal followed by gasification of the char) was proposed to process coal to produce hydrogen-rich gas. First, the effects of temperature and coal rank on product distribution were studied in the pyrolysis process. Then, the coal char at the final pyrolysis temperature of 900 °C was gasified with pure oxygen-steam. The results showed that, the hydrogen production of the three coal chars increased with the increase of temperature during the pyrolysis process, the hydrogen release from Inner Mongolia lignite and Xinjiang long flame coal have the same trend, and the bimodality is obvious. The hydrogen release in the first stage mainly comes from the dehydrogenation of the fat side chain, and the hydrogen release in the second stage mainly comes from the polycondensation reaction in the later stage of pyrolysis, and the pyrolysis process of coal contributes 15.81%–43.33% of hydrogen, as the coal rank increases, the hydrogen production rate gradually decreases. In the gasification process, the release of hydrogen mainly comes from the water gas shift reaction, the hydrogen output is mainly affected by the quality and carbon content of coal char. With the increase of coal rank, the hydrogen output gradually increases, mainly due to the increasing of coal coke yield and carbon content, The gasification process of coal char contributes 56.67–84.19% of hydrogen, in contrast, coal char gasification provides more hydrogen. The total effective gas output of the three coal chars is 0.53–0.81 m³/kg, the hydrogen output is 0.3–0.43 m³/kg, and the percentage of hydrogen is 53.08–56.60%. This study shows that two-step gasification under the condition of pure oxygen-steam gasification agent is an efficient energy process for hydrogen production from underground coal gasification.     

Coal char particle secondary fragmentation in an entrained-flow coal-water slurry gasifier

Coal char particle size in the gasifier has an influence on the carbon conversion, gasifier slagging as well as the particle matter content in raw syngas. The particle size distribution in a bench-scale opposed multi-burner (OMB) gasifier illustrated that the secondary fragmentation behavior exists in the entrained-flow gasifier after the particles moving from high temperature impinging flame region to the gasification chamber outlet region. Particles larger than 200 μm in the impinging flame region have porosity structures with fragile shapes. Particle size distributions under different oxygen to carbon ratios (O/C) also indicate that there is an obvious fragmentation while the particle size is larger than 200 μm. As long as the coal char particles move from the impinging flame region towards the gasification chamber outlet region, the secondary fragmentation is probably taken place as a result of percolative fragmentation, undergoing gasification reactions and thermal stress. However, thermal stress fragmentation only has an influence on the particle sizes larger than 350 μm according to the calculation by a simplified mathematical model.     

The effect of a perforated plate on the propagation of laminar hydrogen flames in a channel – A numerical study

Laminar hydrogen flame propagation in a channel with a perforated plate is investigated using 2D reactive Navies-Stokes simulations. The effect of the perforated plate on flame propagation is treated with a porous media model. A one step chemistry model is used for the combustion of the stoichiometric H₂–air mixture. Numerical simulations show that the perforated plate has considerable effect on the flame propagation in the region downstream from the perforated plate and marginal effect on the upstream region. It is found to squeeze the flame front and result in a ring of unburned gas pocket around the flame neck. The resulting abrupt change in flow directions leads to the formation of some vortices. Downstream of the perforated plate, a wrinkled “M”-shape flame is observed with “W” shape flame speed evolution, which lastly turns back to a convex curved flame front. Parametric studies have also been carried out on the inertial resistance factor, porosity, perforated plate length and its location to investigate their effects on flame evolution. Overall, for parameter range studied, the perforated plate has an effect of reducing the flame speed downstream of it.     

The stabilization of a methane–air edge flame within a mixing layer in a narrow channel

The flame stabilization mechanism of a methane–air edge flame formulated in a narrow channel was experimentally investigated and compared with a simple analytical model. Non-premixed flames were classified into premixed flame modes and edge flame modes. The correlation between the propagation velocity and the fuel concentration gradient in a narrow channel was investigated and the applicability of ordinary edge-flame theory was appraised.     

Hydrogen flame propagation inside an obstructed chamber

Hydrogen is recognized as a most dangerous gas due to high ignition rate and flame speed. In this work, numerical simulations have been performed to simulate the flow pattern and flame deflagration of hydrogen gas inside a confined chamber with different obstacles. This study tries to disclose the transient progress of flame to define the key actual factors affecting flow feature and flame propagation. In order to simulate hydrogen flame deflagration the limited obstacle channel, a three-dimensional model is established and large eddy simulation (LES) method is applied for the simulation of the model. The impacts of obstacle geometry on the pressure distribution are carefully examined. Obtained results show that overpressure inside the confined channel significantly increase within 0.3–0.6 ms. In this work, comprehensive reliable correlation for prediction of the pressure inside the confined channel is present. Our findings clearly demonstrates that velocity-density coefficient plays significant role the pressure distribution inside the model.     

Thermal–mechanical modelling around the cavities of underground coal gasification

Underground coal gasification (UCG) is an efficient method for the conversion of the deep coal resources into energy. This paper is concerned with a feasibility study of the potential of deeply lying coal seams (>1200 m) for the application of UCG combined with subsequent storage of CO₂ for a site located in Bulgaria. A thermal–mechanical coupled model was developed using the ABAQUS software package to predict the heat transfer, the stress distributions around the UCG and the consequent surface subsidence. Material properties of rocks and coal were obtained from existing literature and geomechanical tests which were carried out on samples derived from the demonstration site in Bulgaria. Three days of gasification has been simulated by assigning a moving heat flux on a cell of 2 m × 2 m × 2 m at a velocity of 2 m/day. Results of temperature and stress distribution showed that the developed numerical model was able to simulate the heat propagation and the stress distribution around cavities under a thermal–mechanical coupled loading during the UCG process. Also, the surface subsidence was found to be 0.08 mm after three days of gasification for the case studied. It is anticipated that the results of this paper can be used for the prediction and optimization of the UCG process in deep coal seams.     

Effect of obstacle arrangement on premixed hydrogen flame: Eddy-dissipation concept model based numerical simulation

In this paper, computational fluid dynamics (CFD) numerical simulation is used to analyze and discuss the horizontal propagation process of premixed hydrogen flame with obstacles. A total of three different obstacle channel arrangements at the blocking ratio of 0.5, which will affect the explosion flame and pressure development. The results show that the premixed flame is affected by flow instabilities and vortices when propagating through the obstacle channel, thereby distorting the flame. The vortices outside the flame boundary are more conducive to the acceleration of the flame. The continuous acceleration and synergistic promotion of the flame is more prominent due to the existence of the channel in the central axis of flame propagation, and the maximum velocity even achieved 307.91  m/s. The degree of the wrinkle of flame increases with the number of obstacle channels. The flame propagation process is always accompanied by pressure variations, and the dynamic pressure builds up at the flame front and intensifies periodically. But the downstream pressure gradually increases as the number of obstacle channels increases. CFD simulation of the explosion process clearly reveals the changing trends and interactions of explosion characteristic factors.     

煤气化过程的神经网络直接辨识

煤在水蒸汽环境中气化时,煤的热解和焦的气化同时发生,两个过程有一些相同的产物,因此无法通过测量确定两个过程各自的进行情况。如果用煤的热解模型和焦的气化模型共同描述煤在水蒸汽中的气化过程,由于缺乏必要的数据,难以保证模型参数的合理性。要直接描述煤热解和焦气化形成的综合过程,传统的数学回归方法又难以胜任。本文用神经网络方法对煤在水蒸汽中气化的综合过程进行了直接辨识,取得了较好的模拟效果,并对煤在水蒸汽中的气化过程有了更深入的了解,为煤气化过程的“活性焦比”模型提供了支持。     

Three-Dimensional Unstable Non-linear Numerical Analysis of the Underground Coal Gasification with Free Channel

The three-dimensional unstable non-linear coupled mathematical model on the underground coal gasification with free channel is established in this article. The determination methods for the major model parameters are explained. Adopting the SIMPLE method, the numerical solution of the mathematical model is found. Additionally, the patterns of variation and development for the temperature field, concentration field and pressure field in the gasification panel are studied. On the basis of the model test, the calculation results are analyzed. Research shows that calculated values of the temperature field are a little higher than the experimental ones, and the relative error for every measuring point is virtually within 16%. Also, a good conformity takes on between the experimental values for the concentration field of the gas compositions and the calculated ones. The simulated results indicate that the relative calculation error of the pressure field is 10.20%–19.44%. Through the analysis of measured data, the change mechanisms of the gas compositions, heating value and pressure field in the underground gasification with free channel are pointed out.     

Percolation theory for flame propagation in non- or less-volatile fuel spray: A conceptual analysis to group combustion excitation mechanism

A percolation theory for flame propagation in non- or less-volatile fuel spray is developed based on a cubic lattice model representing a local spray state. The interdroplet flame propagation characteristics found from microgravity experiments on flame spread along a linear droplet array are applicable to describing interdroplet flame propagation between neighboring droplets in any distribution of droplets because the effect of heat conduction from the flame front is shielded by the nearest unburned droplet, which acts as a heat sink. Thus, once the method by which the unburned droplet nearest to the flame front is ignited is identified and formulated into a simple algorithm rule, we can examine by computer simulation the statistical flame propagation behavior in a non- or less-volatile fuel spray in the framework of the percolation theory. In non- or less-volatile fuel, an unburned droplet swallowed by an envelope diffusion flame of other droplets is heated and becomes a new supplier of fuel vapor to the flame front, allowing the flame front to advance. For randomly distributed droplets, the flame front selects the path that minimizes its propagation time. These two phenomena occur when the grid spacing of the cubic lattice model is equal to the maximum flame radius of an isolated droplet immersed in the same air conditions as the local spray state. Furthermore, physical considerations reveal that the lattice size that leads to statistically meaningful information can be rather small, i.e., 20×20×20 vertices. Therefore, the proposed percolation theory is tractable and useful in finding the probability that a flame front propagates across a spray element and for exploring the mechanism of the excitation of group combustion for non- or less-volatile fuel sprays.     

Hydrogen-air deflagrations in open atmosphere: Large eddy simulation analysis of experimental data

The largest known experiment on hydrogen-air deflagration in the open atmosphere has been analysed by means of the large eddy simulation (LES). The combustion model is based on the progress variable equation to simulate a premixed flame front propagation and the gradient method to decouple the physical combustion rate from numerical peculiarities. The hydrodynamic instability has been partially resolved by LES and unresolved effects have been modelled by Yakhot's turbulent premixed combustion model. The main contributor to high flame propagation velocity is the additional turbulence generated by the flame front itself. It has been modelled based on the maximum flame wrinkling factor predicted by Karlovitz et al. theory and the transitional distance reported by Gostintsev with colleagues. Simulations are in a good agreement with experimental data on flame propagation dynamics, flame shape, and outgoing pressure wave peaks and structure. The model is built from the first principles and no adjustable parameters were applied to get agreement with the experiment.     

Mathematical model for coal conversion in supercritical water: reacting multiphase flow with conjugate heat transfer

Providing heat for supercritical water gasification (SCWG) of coal by coupling subsequent products oxidation in integrated supercritical water reactor (ISWR) provides an effective method for directional control of temperature field and avoids excessive hot spots caused by uniform heating. An exploratory numerical model incorporating particle-fluid flow dynamics, multispecies transport and thermal coupling between endothermic coal gasification and exothermic product oxidation was established to simulate the reacting multiphase flow process of coal conversion in a novel lab-scale ISWR. An eleven-lump kinetic model was proposed for the prediction of chemical reactions. And the thermal coupling relationship was described by conjugate heat transfer boundary conditions (BC). Detailed physical and chemical field distribution in ISWR were analyzed and influence factors were discussed. The results showed that oxidation of gas products as inner heat source could promote the gasification reaction with only slight or even little maximum temperature increase of the pressure-bearing wall. Coal feeding rate and oxygen supply method significantly affected the field distribution. The multi-injection compressed-air supply method provided a more uniform temperature field but would reduce heat transfer temperature difference. The carbon gasification efficiency (CGE) in the gasification zone could easily reach up to 97% under mild conditions (less than 650 °C).     

A comprehensive mathematical model of a serial composite process for biomass and coal Co-gasification

A comprehensive mathematical model to simulate a serial composite process for biomass and coal co-gasification has been built. The process is divided into combustion stage and gasification stage in the same gasifier, it is a new process for the co-gasification of biomass and coal. The model is based on reaction kinetic, hydrodynamics, mass and energy balances, it is a one-dimensional, K-L three-phase, unsteady state model. The model is divided into two sub-models, one is the combustion sub-model, the other is the coal-biomass serial gasification sub-model. Combustion sub-model includes coal pyrolysis, dense phase combustion, and dilute phase combustion model. Gasification sub-model includes biomass pyrolysis, dense phase coal gasification, dense phase biomass gasification, and dilute phase gasification model. The model studies the effects of key parameters on gasification properties, including gasification temperature, S/B, B/C, and predicts the composition of product gas and gas calorific value along the reactor's axis at different time. The model predictions agree well with experimental results and can be used to study and optimize the operation of the process.