Cofiring low-rank coal and biomass in a bubbling fluidized bed with varying excess air ratio and fluidization velocity

Abstract

Cofiring coal and biomass is one of the alternatives to reduce emissions from thermoelectric power plants. The influence of biomass fraction in the mixture is an important research topic to increase coal substitution and achieve lower GHG emissions. A combustion system based on laboratory-scale bubbling fluidized bed technology was used to analyze the burning of different ratios (5, 10 and 15%) of mixed biomass and low-rank mineral coal. Two operational parameters were varied, fluidization velocity (3umf and 6umf) and excess air ratio (50 and 70%). Previous studies focused mainly on gasification processes, where excess air is a negative parameter. For combustion processes, high values of excess air are mandatory. Experimental results showed that fuel mixtures containing up to 15% of biomass gives good results in terms of emissions and temperature stability control. Emissions were slightly increased as well as the operating temperature with the presence of biomass. The operating regimes with higher fluidization velocity and excess air ratio presented better performance, however, biomass volatiles release at the freeboard should be the main concern for this type of application. Therefore, it was concluded that the combined burning of coal with biomass is a feasible alternative to reduce emissions without impairing efficiency.