Simulation of the sulfur retention in a oxy-fuel 20 MW CFB combustor

Abstract

Oxy-fuel combustion is a CO2 capture technology which consists of burning the fuel with a mix of pure oxygen and recycled flue gas, mainly composed of H2O(v) and CO2. Therefore, after steam condensation, the CO2 concentration in the flue gas may be enriched up to 95% (dry basis). Many of current researches are related to oxy-fuel pulverized coal combustion. However, circulating fluidized bed combustors can be a promising technology because in addition to other advantages, they have the possibility of carrying out the in situ desulfurization process via Ca-based sorbent added into the combustor which is highly dependent on the temperature and concentration of CO2. Because under oxy-fuel combustion the sorbent can be surrounded by CO2 concentrations ranging from 60 to 90%, the sulfation of the Ca-based sorbent can be performed at calcining (CaO solid reactant) or non-calcining conditions (CaCO3 solid reactant).

A particle sulfation model has been developed to describe the limestone sulfation reaction which is carried out in two steps. The simplicity of the kinetic model permitted it to be incorporated easily in a 1.5D computational simulation model of an oxy-fuel CFB to predict the sulfur retention reached inside the boiler. The model is capable of giving important information about the oxy-coal combustion process such as longitudinal profiles of the gases (O2, CO2, H2O, SO2, etc.), char concentration distribution, combustion efficiency and sulfur retention at different operating conditions. A simulation considering the main operating variables such as temperature, sorbent reactivity, O2 concentration fed, type of coal and the use of a desulfurization unit in the recycled flue gas was carried out to determine the most influential ones. The model is considered a useful tool to optimize the oxy-fuel combustion process in circulating fluidized bed combustors.