SIMULATION OF SINGLE CHAR PARTICLE GASIFICATION IN CONDITIONS OF A COAL GASIFIER

Abstract

Coals from different regions of the world have different characteristics regarding mineral content and coal matrix. In high ash coals, beneficiation process is difficult for removal of the mineral matter that leads to the lower efficiency and greater environmental pollution by coal combustion. The carbon dioxide emissions from the coal combustion leads to global warming which has led to the international agreement that sets the target for controlling CO2 emissions. To achieve these goals, we require increasing energy efficiency by making changes in the combustion practices of the coal. Coal gasification has the potential to provide a solution to the increasing demand for energy through fuel synthesis. The compositions of H2 and CO in the product gases mostly depend on the reactant gas used the coal type, operating condition, and gasification process. The char gasification with steam enhances the production of H2 while with CO2, gasification improves the CO formation in the product gas. For the char gasification, a number of kinetic models have been proposed. The simplest of these models are homogeneous and shrinking-core model. In the homogeneous model, it is assumed that the particle size of char remains same but the density decreases during its gasification, whereas, in the shrinking-core model, the gaseous reactant are assumed to diffuse through the gas film and ash layer to reach the unreacted core surfaces. As reaction proceeds, the unreacted core of a char particle continues to shrinking. Based on the kinetic approach, many other researchers have also studied the modeling and simulation of coal gasification. The gasification of a single coal char particle is carried out by the active penetration of the reactant gases through the porous structure of the char particle leading to change in the porous structure which cause the changing in the effective diffusivity of the gaseous components and hence change in the reaction rates. Therefore, modeling of a single coal char particle is important for the analysis of the effects of different parameters on the coal char gasification which is useful for the design of a gasfier. The model for the gasification and combustion of a single coal char particle are developed by a few researchers. ii The rate of char gasification is affected by many parameters such as concentration of gasifying agent, temperature, and the physic-chemical properties of the char particle like effective diffusivity, carbon content, ash content, and porosity. Mathematical modeling and simulation of gasification of a single char particle for production of synthesis gas is challenging due to complex chemical reactions coupled with the changing physico-chemical properties of the char particle during gasification. In the present work we have developed a mathematical model for a single char particle. The resulting nonlinear partial differential equation representing unsteady state diffusion controlled reaction of a gasification agent (steam and mixture of steam and CO2) inside the non-catalytic porous solid (char particle) was solved numerically using staggered grid finite volume method with appropriate initial and boundary condition. The rate of steam gasification of char particle was found to be proportional to the steam concentration. Increasing the steam concentration from 0.5 to 2 mol/m3, the rate of steam gasification reaction increases rapidly which produces more hydrogen and carbon monoxide. The carbon monoxide is further reacted with steam through water gas shift reaction to form more hydrogen and carbon dioxide causing several fold increase of hydrogen fluxes at 2 mol/m3 compared to that at 0.5 mol/m3 steam concentration. At a lower reaction temperature (1100 K), less increments in the carbon monoxide flux is observed. Hence for obtaining high yield of hydrogen the steam concentration should be high. The reaction temperature is expected to be one of the most promising variables that directly affect the performance of a gasifier during gasification because the main gasification reactions are strongly endothermic. On increasing the reaction temperature beyond 1100K; the rate of reaction increases which shifts the reaction mechanism from the progressive reaction model to the shrinking core model. When the gasification reaction rates are slow (for example, at temperature 1000K), sufficient amount of gasifying agent diffuses deep into the particle and reaction continues throughout the char particle similar to the progressive reaction model. At higher temperature (>1100 K), however, the rate of reactions become fast which leads to maximum conversion of char near the outer surface resembling the shrinking core model. Thus at higher temperatures an un-reacted core is formed that shrinks with time. This study reveals that the highest concentration of CO and H2 in the synthesis gas is obtained iii when the reaction temperature is greater than 1200 K which also minimizes the fraction of CO2 in the product gas. Also, removal of the ash layer from the particle surface at time interval of 100 second for less ash content char (<1%) and 10 second for high ash content char (>25%) may give higher CO concentration. For obtaining high H2 concentration, the steam partial pressure should be kept high. Carbon concentration directly affects the rate of gasification of the char particle. Apparently, a higher carbon concentration should lead to higher conversion, however, the simulation results reveal that if the parameters like size, density, and porosity of the particle remain unchanged, the rate of percent conversion is high for low char concentration but low at high char concentration in the central core of the particle. Since the consumption near the particle surface is more for charcoal with higher carbon concentration. But in this case too, near the center, carbon consumption is high when its concentration is low. This is because of the fact that keeping density constant, reduction in carbon concentration ultimately leads to increase in the diffusivity of gasifying agent. Therefore availability of gasifying agent in the core region is more when carbon concentration is less. The effects of particle sizes were analyzed using particle sizes 5, 7.5, and 10 mm at the reaction temperature of 1100 K. It was observed that the overall rate of conversion increases with a decrease in the particle size. In a smaller particle, the steam diffuses deep in the char particle and reactions continue throughout the particle following the progressive model. In the case of large particle, reactions take place at the outer surface of the particle and steam does not reach deep into the particle follow the shrinking core model. Hence the rate of the surface reaction of large char particle is high compared to the small particle. The porosity of char particle affects the total conversion only in the initial phase of gasification, in the other words, the time required to reaches complete conversion is almost the same for all initial porosity values. The variation of initial porosity is more prominent towards the low porosity range. For higher initial porosity, curves between total conversion and time get closer to each other. Due to nearly same time requirement for complete conversion of char particle of different initial porosity, the flux ratios of H2 to CO and CO to CO2 remains the same for all initial porosity of char particle after a certain period of time (≈2500 s). iv Since the reactivity data of coal char particle with high ash content is limited, the study on the effect of ash content in the coal char particle on the gasification is important for the design of a gasifier to use high ash content in the coal char. Because of this, in the modeling of a single char particle is also included the effect of ash content on the conversion rate and overall rate of generation of gaseous components from the outer surface of the coal char particle. The study reveals that although product yield decline but the overall carbon conversion of high ash content coal char particle is higher than lower one. The variation of carbon conversion near the surface of coal char particle for the fraction of different ash content in the char particle are nearly same whereas towards the centre of the char particle, because of the available specific surface area for the gasification reactions, conversion is significantly high. In this work, unsteady state modeling and simulation for coal char gasification with varying gasifying agent composition of steam and CO2 is also developed. This study revealed that the highest fraction of H2 and CO with lower CO2 concentration (dry basis) is obtained when the gasifying agent composition is 70 % steam and 30 % CO2 in the gasifying medium. The synthesis gas obtained at this composition of reactant gases having higher heating value in comparison with the other cases. The work done in this study is organized into five chapters. The introduction of the gasification process is presented in the first chapter, a literature review related to gasification of char particles in conditions of a coal gasifier follows in the second chapter. Chapter 3 deals with the model development for single char particle gasification along with the porosity and diffusivity and kinetic model for gasification. Results of the simulation of single char particle gasification are presented and discussed in chapter 4. The numerical model is validated by comparing the experimental data. Then the parametric analysis is done with the change in each parameter’s value (keeping other parameters same). Chapter 5 summarizes the conclusions and recommendations drawn from this study.