Modelling of combustion characteristics of high ash coal char particles at high pressure: Shrinking reactive core model
Introduction
Large particles of coal and coal char are being increasingly used over the last few decades for utilization of coal. High-pressure fixed-bed gasifiers are employed commercially with coal particles in the size range of 3–5 mm. Most of the theoretical and experimental works on coal/char combustion available in literature [1], [2] are based on the coals with low ash content, typically around 5–8% by weight. In contrast, India and China have large reserves of coal with higher ash content due to its drift origin, typically 12–25% after treatment in coal washery, with the washery rejects containing as high as 30–50% ash.
Clean coal technologies like pressurized fluidized bed combustion (PFBC) and pressurized entrained flow gasification (PEFG) have attracted considerable theoretical and experimental research due to their greater operational efficiencies and low environmental impact. Higher operating pressure during combustion increases the coal throughput due to increased overall reaction rate [3], [4]. The combustion of medium sized coal particles (3–5 mm) in fluidized bed has obvious advantages in coal preparation and handling, but requires knowledge on structural change and diffusion effects [2], [4], [5]. During combustion the coal particles are immediately pyrolyzed to devolatilized char and evolved volatiles. The volatiles burn in the gas phase and help the process of combustion of devolatilized char. The combustion of volatiles occurs very fast and hence the overall combustion rate is controlled by relatively slow char combustion process, which involves interaction of homogeneous and heterogeneous chemical reactions with transport limitations.
Reddy and Mohapatra [6] presented an unreacted shrinking core char combustion model considering a single chemical reaction, while the present authors [7] proposed a more comprehensive model with multiple chemical reactions. In shrinking core model which is applicable to coal/char with very low porosities, the reaction occurs at the interface between fully converted porous ash and unreacted core. Fragile and low ash coal may call for the use of shrinking sphere model [8]. On the other hand oxygen can diffuse freely into the interior of char particles having high porosity and the reaction occurs throughout the entire volume of the particle leading to volume reaction model. Gupta and Saha [9], [10] presented a detailed analysis of volume reaction model for general gas–solid non-catalytic reactions. Another version of volume reaction model, called shrinking reactive core model was proposed by Ishida and Wen [11]. Since the reaction rate is faster near the particle surface than in the interior of the particle, the carbon will be depleted first at the surface forming an ash layer. Then the thickness of ash layer increases with a decrease in radius of the porous partially reacted core. Wen [12] compared the continuum model with shrinking core model. According to Wang and Bhatia [13] the char particle comprises microporous grains surrounded by mesopores and macropores. The heterogeneous gas solid reactions take place on the surface of micropores while macropores serve as channel for transportation of gaseous reactants and products. However, Biggs and Agarwal [14] proposed that it is the accessible porosity of the char particle that is responsible for intra-particle transportation of gaseous reactants and products. The structural parameters like micro-porosity, internal pore surface area and the accessible porosity of char gradually change during the course of combustion, which needs to be incorporated into the model. Among various structural models the random pore model developed by Bhatia and Perlmutter [15] is most widely accepted to predict the development of pore surface area during combustion and gasification of porous coal char. Lu and Do [16] incorporated the structural effects of minerals present in high ash coal char during combustion and gasification. A modified random pore model was employed by Sadhukhan et al. [17] in order to separate active carbon surface area from the inert ash surface area. Structural development of coal of Indian origin in fluidized bed combustion was studied experimentally.
Analytical solution is possible for volume reaction model involving chemical reaction and diffusion with constant diffusivity and first order reactions. Weiss and Goodwin [18] successfully used the approach to predict the combustion behaviour for the regeneration of carbonized catalyst particles. They observed that at low temperature (450 °C), when chemical reaction is controlling, the carbon burn-off is uniform throughout the particle and that at high temperatures (625 °C) the burn-off is diffusion controlled with negligible penetration of oxygen in core, thus characterized by shrinking unreacted core model. At intermediate-temperature volume reaction model with diffusion and chemical reaction is applicable. The effective diffusivity of oxygen within the particle depends on structural properties and changes during the course of combustion process.
Combustion of highly non-porous coal char with high ash has been mostly analyzed by shrinking unreacted core model which is quite restricted in its applicability. Combustion of porous coal char with high ash, which is the case in the present work, involves pore diffusion in char and will not follow shrinking unreacted core model. Shrinking reactive core model is a generalized model that can handle all type of coal char, porous or non-porous, under diverse operating conditions. It also predicts correctly the formation of two distinctly separate zones; ash layer and partially converted char core.
Everson et al. [19] employed a simplified pseudo steady state isothermal shrinking reactive core model of Ishida and Wen [11] without ash layer. Mass and heat transfer phenomena outside and within the particle play significant role during combustion of millimeter size char particles which was not adequately addressed by them. Change in diffusivity due to pore structural evolution was also not included. The effective diffusivity calculated by them showed decreasing trend with increasing temperature, which is contrary to the trend predicted by molecular theory.
In the present work a much more involved shrinking reactive core model was proposed incorporating all phenomena of mass and heat transfer outside the particle and within. Transport phenomena in both ash layer and partially reacted core were included. The physical properties were treated as function of composition and temperature and the effect of pore structure evolution is also incorporated. The fully transient model consists of three sub-models (1) multiple reaction kinetic model of char combustion under pressurized condition, (2) random pore model for intra-particle pore surface evolution and pore diffusion and, (3) convective and radiative heat transfer outside the particle and conductive heat transfer within the particle. The SRK equation of state was used to deal with high pressure.
This single-particle model is highly versatile and can handle combustion of coal char in fluidized bed, drop-tube furnace, flow reactor and pulverized fuel combustor at atmospheric as well as high pressure.
The coupled sub-models were solved simultaneously using implicit finite volume method by a FORTRAN code developed in-house. The model predicted the location of reactive core radius, radial temperature profile, total burnout time and mass-loss profile and was validated with the published experimental results of Everson et al. [19] for pressurized combustion of high ash coal char in a thermogravimetric analyzer at various temperatures with different oxygen/nitrogen mixtures.
Section snippets
Char combustion model
Arthur [20] proposed that during oxidation of carbon in coal above 1273 K, the gaseous product is entirely CO and over a temperature range of 673–1173 K the composition of the product may be given by
Biggs and Agarwal [14] used a similar expression to predict the flue gas composition for combustion of porous char particle in a fluidized bed over a wide range of particle size and operating temperature. For the purpose of oxygen balance for char combustion reaction the
Numerical method
mk,s and Ts are the principal variables. The governing differential equations (Eqs. (6), (11)) were solved simultaneously along with the initial and boundary conditions to find out the values of principal variables as function of time and radial position. Finite volume method using implicit formulation was adopted to solve the model equations. Finite volume method was successfully employed to solve the sets of coupled partial differential equations along with non-linear boundary conditions by
Estimation of parameters for the model
The kinetic and pore parameters depend on the type of char and structural development during combustion at a given condition. Reaction rate constant, order of reaction and the pore parameter may be estimated from the model for a particular char type and operating condition. Everson et al. [19] conducted combustion experiments at high pressure (487 kPa) with high ash (47%) coal char (3 mm) at different temperatures for varying oxygen mole fraction. They estimated kinetic parameters with a
Conclusion
A shrinking reactive core model has been employed in order to predict the combustion behaviour of a single char particle at high pressure. The fully transient model is suitable for high-pressure combustion of high ash coal/char and incorporates intrinsic reaction kinetics, mass and heat transfer limitations including variation in effective diffusivities and pore surface area. Though it was developed primarily for coal char, the approach is suited for different biomass as well. Parameters were
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2021, Chemical Engineering JournalCitation Excerpt :It was found that an increase in pressure decreased the yield of gas and tar products during the devolatilization process [28], and a significant influence of pressure on the particle swelling ratio was also observed [29]. Many studies found the elevated pressure accelerated the combustion rate and decreased the burnout time [30 31]. However, Monson et al. [32] and Lester et al. [33] showed the reaction rate first increased with pressure and then decreased when the pressure was up to 0.5 MPa.