Combustion interactions in oxy-fuel firing of coal blends: An experimental and numerical study

Abstract

Coal blends are commonly used in thermal power plants and oxy-fuel combustion also attracts great concerns recently. However, little has been done on oxy-fuel combustion of coal blends. In this paper, combustion tests are performed in a drop tube furnace for various coal blends under O₂/CO₂ mixtures, which are also reproduced numerically. Strong combustion interactions between the parent coals in a blend are observed. The ignition of the low-volatile coal is promoted due to the rapid combustion of the volatiles from the high-volatile coal. However, the char burnout of the low-volatile coal is compromised due to the rapid O₂ consumption by the large amount of volatiles. The interactions under oxy-fuel conditions are more sensitive to the excess O₂, inlet oxidizer temperature and coal particle size. 1) The increase in the excess O₂ tends to weaken the combustion interactions, in which both the ignition promotion and burnout inhibition effects on the low-volatile coal become less apparent. 2) The increase in the inlet oxidizer temperature further promotes the ignition of the low-volatile coal whereas weakens the burnout inhibition of the low-volatile coal. 3) The effects of reducing particle size on the combustion interactions are similar to those of increasing inlet oxidizer temperature. Since biomass often contains a high volatile content, the findings may shed light on the synergistic effects in oxy-fuel co-combustion of biomass and coal.

Introduction

Driven by economics, coal sulphur content to meet emission regulations, and/or the effects of different coals on boiler performance, coal blends have been commonly used in pulverized-fuel power plants [1,2]. Su et al. [3] review the combustion studies of coal blends in laboratory-, pilot-, and full-scale facilities and conclude that most coals appear to interact with each other in a blend, especially a blend of low- and high-volatile coals. A high-volatile coal usually improves ignition, flame stability and carbon burnout of a blend with a low-volatile coal or anthracite [[4], [5], [6]]. The combustion behaviour of a coal blend deviates from the linear summation of the component coals due to their interactions [[7], [8], [9]]. The relative ignition and flame stability of a coal blend is more sensitive at lower furnace temperature and lower thermal input. The carbon burnout of a coal blend is roughly a linear weighted average if the effect of Hardgrove grindability index of the component coals is controlled. Biswas et al. [10] investigate the combustion behaviour of blends of two coals of the same rank but with wide variation in mineral matter content and observe a nonlinear effect on the burnout in the drop tube furnace. Coal blends with less than 50% of the high-ash coal show better burnout than the individual coals. Faúndez et al. [11] carry out ignition tests on blends of three coals of different ranks and observe that the low-rank coals (subbituminous, high-volatile bituminous) enhance the ignition of the higher-rank coal (low-volatile bituminous) in all the blends. Chi et al. [12] conclude from their experimental studies in a drop tube furnace that the ignition behaviour of a coal blend is similar to that of the high-volatile coal in the blend and the similarity degree depends on the mass fraction of the high-volatile coal. The experimental study of different blending methods for a high-volatile sub-bituminous coal and a low-volatile bituminous coal under air-fuel conditions shows that the in-furnace blending combined with below medium stoichiometric conditions is an attractive method to improve carbon burnout and reduce NO_x emissions [13]. Sarkar et al. [14] experimentally study the combustion characteristics of blends of a low-ash coal and a high-ash coal and found that the burnout temperature of a blend is non-additive and close to that of the component coal with higher burnout temperature.

The combination of oxy-fuel and coal blend combustion may not only reduce the CO₂ emissions but also improve both the energy utilization and conversion efficiency [6]. Compared to coal blend combustion in air-fuel conditions, oxy-fuel combustion of coal blends has been rarely investigated. The use of CO₂ or the mixture of CO₂ and H₂O vapour as the diluent in oxy-fuel combustion, instead of N₂ in air-fuel combustion, largely changes both combustion physics and combustion chemistry, due to the great differences in the physical properties and chemical effects of the different diluents, as reviewed in Ref. [15]. There is no consistent conclusion on the effects of the combustion atmosphere on fuel particle ignition and char burnout. From combustion chemistry point of view, CO₂ and H₂O steam gasification reactions tend to increase the overall char consumption rate, but both the gasification reactions are strongly endothermic which tends to cool the char particle and reduces the char oxidation rate. From combustion physics point of view, the higher heat capacity of CO₂ (in comparison to N₂) tends to reduce flame temperatures and the lower O₂ binary diffusivity in CO₂ (compared to O₂ in N₂) reduces the O₂ flux in the particle boundary layer, both of which reduce char conversion rate. As a result, the combustion behaviours and interactions in oxy-fuel combustion of a coal blend may be different from those under conventional air-fuel conditions. It is worthy of investigation in order to design and/or optimize coal blend combustion under oxy-fuel conditions. A couple of studies on fuel blends combustion under oxy-fuel conditions are done very recently. Yao et al. investigate oxy-fuel combustion of the blends of semi-coke and bituminous coal and find interesting results in the ignition temperature, burnout behaviour and activation energies [16]. Issac et al. experimentally and numerically study the combustion performance of the blends of coals of different grades with woodchips and wood char under air and oxy-fuel conditions and the findings are beneficial for enhancing fuel flexibility and combustion efficiency in power plants by properly blending different fuels [17].

In our previous study of coal blend combustion, the effects of atmosphere, char-CO₂ reaction and reactor temperatures in a drop tube furnace on the interactions between the component coals in the blend are investigated experimentally and numerically [18]. It is observed that the high-volatile coal promotes the devolatilization but compromises the char combustion of the low-volatile coal. Compared to the interactions under air-fuel condition, the devolatilization promotion is weaker and the char burnout inhibition is stronger in 21% O₂/79% CO₂ atmosphere. When O₂ concentration in the O₂/CO₂ mixture increases to around 30–35%, the combustion interactions between the two coals are similar to those in air-fuel condition.

This paper extends our previous study by examining the effects of excess oxygen coefficient, inlet gas temperature and particle size on the combustion interactions of different coals in a blend under O₂/CO₂ atmospheres. First, combustion tests of coal blends under excess oxygen coefficients of λ = 1.0, 1.2 and 1.4 are performed, respectively. Then, numerical simulations are carried out to reveal more details of the observed combustion interactions under these excess oxygen conditions, and to examine the impacts of the inlet oxidizer temperature and particle size on the combustion interactions.