Two-dimensional computational fluid dynamics simulation of coal combustion in a circulating fluidized bed combustor

Abstract

A computational fluid dynamics (CFD) approach was applied to simulate the air-coal two-phase flow and combustion characteristics in a 50 kW circulating fluidized bed (CFB) combustor. Eulerian–Granular multiphase model with a drag coefficient correction based on the extended energy-minimization multi-scale (EMMS/matrix) model was used to study the gas–solid hydrodynamics. One energy conservation equation was applied to the mixture of gases and solids, considering heat conduction, heat convection and heat sources from chemical reactions. Reactions during coal combustion included moisture evaporation, dry coal devolatilization, volatile combustion, char combustion and char gasification. The model predicted the main features of the complex gas–solid flow, including the cluster formation of the solid phase along the walls, the flow structure of upward flow in the core and downward flow in the annular region. The voidage and temperature profiles in the furnace and the concentrations of gas components from the riser were validated with experimental data. Distributions of reaction rates in the riser were also obtained. This indicates a promising way to simulate the coal combustion in CFB to help understanding the combustion mechanism and designing new combustion technologies.

Introduction

During the past 20 years, circulating fluidized bed combustion technology has gained worldwide significant attention. With its well-known benefits of fuel flexibility, high combustion efficiency and low emissions, as well as easier temperature control during oxy-fuel combustion [1], CFB combustion technology becomes one of the most important developing directions for coal-fired boilers [2].

Mathematical modeling and simulation is helpful to better understanding of the combustion process and will be significant for CFB scale-up. The modeling researches in the end of the 20th century were reviewed and the furnace approach models were divided into three levels of sophistication [2]. Several models were developed and improved in the last decade. Huilin et al. established a 1.5-D steady state mathematical model for CFB coal combustion boilers [3] and the group investigated particle clustering effects on combustion and emission in a CFB riser via a 2-D CFD approach [4], [5], [6], [7], [8]. Adenz et al. [9], [10] studied the carbon combustion efficiency and sulfur retention sub-models based on a previous model of fast bed CFB combustor. The model was applied for co-combustion of coal and biomass [10]. Gungor and Eskin established a 2-D CFB coal combustion model [11] with validation data from a pilot-scale 50 kW CFB combustor and an industrial-scale 160 MW CFB combustor. Sensitivity analysis of the operation parameters was carried out [12].

Macroscopic (semi-empirical) models for fluid dynamics of CFB units were presented with emphasis on applications for conditions relevant to industrial units such as fluidized-bed combustors [13]. A 3-D CFB model, initially developed in 1989, was modified to include the specific features under oxygen-enriched atmosphere [14], [15]. Another 3-D CFB model [16], [17], [18], improved with semi-empirical models for concentration and temperature distributions, can predict the local conditions inside combustion chamber with arbitrary shapes and co-combustion of fuels.

CFD modeling of fluid dynamics has already reached a high level with two different approaches, the Eulerian–Eulerian approach and Eulerian–Lagrange approach. It was found that the multi-scale CFD approach in terms of EMMS/matrix seems to reach a mesh-independent solution of the sub-grid structure, and succeeds in predicting the axial profile and flow regime transitions [19], [20]. Zhou et al. investigated the gas-particle turbulent flow in a CFB riser numerically by large eddy simulation (LES) coupled with Lagrangian approach [21]. While numerical modeling of reactive multiphase flows is still at an early stage [16]. CFD-based numerical model is used to simulate the gasification of coal [22] and biomass [23] inside an entrained flow gasifier. A 3-D numerical model was developed to simulate the coal gasification in a fluidized bed gasifier [24]. Jung and Gamwo [25] have developed the reaction kinetics model of the fuel reactor in a chemical looping combustion process and implemented the kinetic model into a multiphase hydrodynamic model, MFIX, developed earlier at the National Energy Technology Laboratory.

Our recent thorough literature review shows that Euler–Granular CFD-based reaction models have not been adapted to circulating fluidized bed coal combustion processes in the open literature. In this study, an Euler–Granular model using the Kinetic Theory of Granular Flow (KTGF) and EMMS/Matrix correction was employed to simulate the hydrodynamics of gas–solid flow in a CFB riser, coupled with heat transfer and chemical reaction sub-models. The simulated voidage and temperature distribution and gas composition from the furnace outlet were validated against the experimental data from a 50 kW pilot CFB combustor at the Southeast University, China. Gas–solid flow patterns, gas velocities, particle velocities, composition profiles of gas product and other important characteristics in a circulating fluidized bed coal combustor were predicted.