Numerical study on biomass co-firing with coal in a pilot-scale pressurized oxy-fuel combustor

Abstract

As the next evolution in oxy-fuel combustion technology, pressurized oxy-combustion (POC) can potentially improve the process efficiency and economics. Biomass combustion is considered to be nearly CO2 neutral, and thus, a negative CO2 balance can be achieved by co-firing biomass in an oxy-fuel combustion process. However, this concept has not yet been demonstrated for the POC system. To address this need, and validate the feasibility of co-firing under these conditions, simulation investigations are conducted based on a pressurized pilot-scale facility. The influences of biomass co-firing ratio on the combustion characteristics and pollutant emissions are estimated. It is evident from the results that the diffusion flame formed by the fuel-oxidant reaction has a good fullness within the furnace, with the maximum flame temperature under each condition not exceeding 1950 K. Biomass typically contains higher moisture and volatile mass fractions compared to coal, leading to a lower temperature around the fuel inlet. In the POC scenario, char gasification plays a prominent role in the process of char consumption. In 100 % coal and 100 % biomass cases, the char consumption caused by the gasification reaches 80 % and 69 % respectively. The predicted NOx emissions in 0 %Bio, 50 %Bio, and 100 %Bio cases are no more than 10 ppm. Furthermore, NOx reduction by volatile and char is quantitatively estimated. The employed low equivalence ratio, coupled with the enhanced NOx reduction by volatile and char can be applied to account for the low NOx emissions in the POC system. This investigation is to provide a comprehensive assessment regarding the combustion and pollutant emissions within the pilot-scale pressurized oxy-combustor during co-firing biomass with coal.