Desulfurization in co-firing of sewage sludge and wooden biomass in a bubbling fluidized bed combustor under air and oxy-fuel conditions

Abstract

This work presents results of an experimental investigation of a direct addition of Ca-based sorbent for SO2 capture in a bubbling fluidized bed combustion. The fuel of interest was secondary biomass represented by a dried sewage sludge, which was mixed with primary biomass represented by wooden pellets in a weight ratio of 30:70. The performance of this SO2 capture approach is compared in air and oxy-fuel combustion conditions, since the different gas environment in the bed significantly affects the SO2 capture process. The experiments were carried out in an experimental bubbling fluidized bed combustor with a power load of about 30 kW.Sewage sludge is typical for its high ash content and has sufficient dimensions and attrition resistance to create the fluidized bed without the support of external bed materials. However, it typically consists of S-containing organic substances, resulting in the presence of SOX in the off-gas. A reference case was initially conducted without any sorbent addition in order to investigate the effect of sulfur self-retention in the fuel ash. The sulfur retention in the ash reached almost 45 % in air- and up to 55 % under oxy-combustion.In the next phase, the performance of Ca-based sorbents in sulfur capture was evaluated. The sorbents were represented by two types of limestone with diverse CaCO3 content produced at different locations. The experiments were carried out with three Ca/S mole ratios (1.5, 3, and 4.5). The effect of the fluidized bed temperature was also investigated as the most important parameter.The SO2 capture ratio (used as a performance indicator) was calculated based on emission factors, due to the different specific volumes of flue gas in the air and oxy-fuel modes. The results show in general a significantly higher SO2 capture ratio under oxy-fuel conditions. The differences are more significant in lower Ca/S ratios, where the SO2 capture ratio increased from 46 % under air conditions to 75 % under oxy-fuel for Ca/S = 1.5. In the case of the highest Ca/S ratio examined (Ca/S = 4.5), the increase in SO2 capture was from 90 % to 95 %.