Journal of Combustion

Volume 2018, Article ID 6931483, 12 pages

https://doi.org/10.1155/2018/6931483

## Simulation of Coal and Biomass Cofiring with Different Particle Density and Diameter in Bubbling Fluidized Bed under O_{2}/CO_{2} Atmospheres

^{1}School of Energy & Environment, Southeast University, Nanjing, Jiangsu 210096, China^{2}Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Nanjing, Jiangsu 210096, China

Correspondence should be addressed to Lingling Zhao; nc.ude.ues@gnilgnil_oahz

Received 3 May 2018; Revised 26 July 2018; Accepted 18 August 2018; Published 12 September 2018

Academic Editor: Hiroaki Watanabe

Copyright © 2018 Chao Chen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

A 2D dynamic model for a bubbling fluidized bed (BFB) combustor has been developed for simulating the coal and biomass cofiring process under 21% O_{2}/79% CO_{2} atmosphere in a 6 kWth bubbling fluidized bed, coupled with the Euler-Euler two-phase flow model. The kinetic theory of binary granular mixtures is employed for the solid phase in order to map the effect of particle size and density. The distribution of temperature, volume fraction, velocity, gas species concentration, and reaction rates are studied with numerical calculations. The simulated temperature distribution along the height of the combustor and outlet gas concentrations show good agreement with experimental data, validating the accuracy and reliability of the developed cofiring simulation model. As indicated in the results, there are two high temperature zones in the combustor, which separately exist at the fuel inlet and dilute phase. The reaction rates are related to the species concentration and temperature. The higher concentration and temperature lead to the larger reaction rates. It can be seen that all of the homogeneous reaction rates are larger at the fuel inlet region because of rich O_{2} and volatiles. High mass fraction of volatile gas is found at the fuel inlet, and the main reburning gas at the dilute phase is CH_{4}. The mass fraction distribution of CO is related to the volume fraction of fuel which is due to the fact that the source of CO is not only from the devolatilization but also from the gasification. On the basis of this theoretical study, a better understanding of flow and combustion characteristics in biomass and coal cofiring under oxy-fuel atmospheres could be achieved.

#### 1. Introduction

Biomass is a carbon-neutral fuel with a large reserve available, producing no net CO_{2} emissions in its life cycle, and can reduce net CO_{2} emissions effectively in coal-based power plants [1]. Biomass cofiring with coal provides an alternative way to utilize biomass fuel effectively due to its’ many advantages such as low risk, good emission properties, and better fuel economy [2, 3]. Fluidized bed (FB) combustion technology is suitable for biomass and coal cocombustion due to its high combustion efficiency for various types of fuel and better emission characteristics [4–6]. What is more, in comparison with pulverized-fuel (PF) boilers, fluidized bed combustors could be designed with a relatively small investment for conversion from coal combustion to biomass and coal cofiring.

Oxy-fuel combustion technology is a type of carbon-capture technology first proposed by Abraham et al. in 1982 [7]. Several experimental investigations demonstrated the potential and economic value of its application in fluidized bed combustors [8–10]. In recent years, researchers have paid more attention to biomass and coal cofiring in oxy-fuel fluidized bed combustors [11]. Tan et al. [12] conducted a series of tests on combustion and emission characteristics of wood cofiring with coal in a pilot-scale oxy-fuel circulating fluidized bed (CFB) combustor and concluded that it is feasible for achieving negative emissions of CO_{2}. Duan et al. [13] studied NO emission from biomass and coal cofiring in a 10 kWth CFB under oxy-fuel conditions. It was found that the emission of NO in an O_{2}/CO_{2} atmosphere is lower than that in an air atmosphere. Experiments were carried out by Kumar and Singh [14, 15] to investigate the temperature profile, gas emission, particle size distribution, and combustion efficiency of four kinds of biomass cofiring with coal under an oxy-fuel atmosphere in a 20 kWth CFB.

Computational fluid dynamics (CFD) has been widely used to study the combustion and flow characteristics of biomass cofiring with coal [16–21]. Gungor [22] simulated the biomass and coal cofiring in CFB using a developed model which investigated the effect of biomass fraction on CO, N, and SO_{2} emissions. Zhang et al. [23] simulated the combustion of low density biomass (oat hulls) cofiring with coal in a fluidized bed combustor by using an Euler-Lagrange model and found that the mass fraction of biomass can affect the peak temperature of the furnace, while the adjustment of the secondary air can strongly affect the combustion of oat hulls. However, there have been only a few studies in which biomass cofiring in FB was simulated under an oxy-fuel atmosphere. Bhuiyan et al. [24, 25] simulated biomass cofiring in PF under an O_{2}/CO_{2} atmosphere and studied the effects of oxygen concentration and biomass ratio on combustion characteristics.

There are two main approaches to describe the hydrodynamic behavior of particles in the fluidized bed: Lagrange (discrete element) and Eulerian (continuum). The Lagrange approach tracks every individual particle in the random flow field by solving its motion equation. The external force directly acting on each particle is taken into account. On the other hand, the Eulerian approach describes the carrier and the dispersed phase with a set of continuum equations representing conservation of mass, momentum, and energy of either phase within a fixed element volume. In the simulation of a dense fluidized bed, the discrete particles are approximated as continuous phase for describing the gas-particle and particle-particle interaction, which is based on the kinetic theory of granular flow (KTGF). The KTGF is developed from the kinetic theory of dense gases [26]. It is based on the Boltzmann equation, which represents the relative disordered motion of particles according to the collision and fraction through the granular temperature (* θ*). Goldschmidt et al. [27] demonstrated that simulations using KTGF agree well with the elastic particles model of a fluidized bed. Nevertheless, the original KTGF [28] cannot model a mixture with different size/density particles, due to the assumption that all particles have equal granular temperature [29, 30]. Hence, Lu et al. [30, 31] extended the KTGF model to a binary granular mixture with different granular temperatures by means of the Maxwell distribution. Wang et al. [32] simulated combustion and desulfurization processes in CFB with various particle size/density (coal/limestone) using the KTGF model with different granular temperatures. It is obvious that the flow and combustion characteristics of fuel are greatly affected by the particle size and density [33]. The assumption of average density and particle size limits the accuracy of simulations [21, 32]. Therefore, it is necessary to establish a model of coal and biomass cofiring in FB with different particle size and density.

In this study, the combustion and flow characteristics of biomass cofiring with coal in a 6 kWth oxy-fuel bubbling fluidized bed combustor have been investigated using CFD. The constitutive properties of the dispersed solid phases are predicted with the kinetic theory of binary granular mixtures. The unsteady processes of gas-solid two-phase flow, heat, and mass transfer incorporating the devolatilization, heterogeneous, and homogeneous reactions are considered in this simulation.

#### 2. Computational Modelling

Based on an Euler-Euler approach, a complicated model including gas-solid multiphase flow, interphase heat and mass transfer, devolatilization of coal and biomass, and homogeneous and heterogeneous reactions has been established. In order to ensure the good convergence and acceptable computational time, the established model was simplified; the main assumptions adopted in this study are listed as follows. (1) The mesh model is assumed as 2-dimensional with the furnace depth of 0.05 m. The widths of dilute and dense zones in the 2D case are determined based on the corresponding cross-section area in the 3D furnace. (2) The solid phase is composed of coal and biomass particles with different density and diameter. Both of them are assumed as inelastic spheres. (3) The gas is considered to be incompressible and ideal and there is no slippage near the wall. (4) The intensity of granular collision is independent with bed temperature. (5) The radiant heat transfer in the furnace is ignored, but the wall radiation is taken into account in the overall wall heat transfer coefficient. For good computational convergence, the heat transfer between coal and biomass particles is neglected. (6) Energy transfer induced by the effect of pressure, viscous dissipation, and compositional diffusion is ignored. (7) The reactions of denitration and desulfurization processes are neglected in the combustion submodel due to the less influence on temperature field.

##### 2.1. Hydrodynamic Model

###### 2.1.1. Gas Phase

The governing equations for gas phase are shown as (1)-(3), where the subscripts s and g represent solid phase and gas phase, respectively. , , ,* p, H,* and are the volume fraction, the density, the instantaneous velocity, the pressure, the enthalpy, and the temperature, respectively. is the mass source term for gas phase contributed by chemical reactions. is the stress-strain tensor. is the gas-solid drag coefficient which is defined as Gidaspow model [47]. means the mass transfer from solid phase to gas phase. in (3) is the energy source term, including the heat generation per unit volume and the heat exchange caused by convection between gas and solid. is the coefficient of heat transfer between gas and solid calculated by Gunn’s model [48]. The species conservation equation is expressed by (4), where subscript* i*=1-8 representing the 8 kinds of species (H_{2}O, H_{2}, CO, CO_{2}, CH_{4}, C_{2}H_{6}, tar, and O_{2}) in the gas phase. is the mass fraction for gas* i*, is the diffusion flux caused by concentration gradient of species , and is the substance reaction rates during chemical reactions.

###### 2.1.2. Particle Phase

The continuity, momentum, and energy equations of solid phase are shown in (5)-(7). The same characters share the same definitions with the gas phase. is the drag coefficient between different particle phases. Based on KTGF for binary granular mixture model, each particle phase has different size, density, and granular temperature. The kinetic energy equation for particle phases is shown in (8), where is the granular temperature for each particle phase. , , and are the diffusion coefficient for particle phase, the dissipation rate caused by particle inelastic collision, and kinetic energy exchange between gas and particles, respectively. The detailed derivation for KTGF of binary granular mixture model is given in [30, 32].

##### 2.2. Chemical Reaction Model

The cofiring process of coal and biomass is defined as 3 parts: water evaporation and devolatilization, homogeneous reactions (volatile combustion), and heterogeneous reactions (char oxidation and gasification). All of the reaction rates are shown in Table 1.