Numerical analysis of flue gas recirculation effects in a large-scale biomass grate furnace

This study investigates the effects of FGR on combustion performance and solid fuel bed dynamics in a 4 MW grate-fired biomass furnace using detailed CFD simulations. An embedded Eulerian bed model is employed to assess the influence of FGR on bed morphology and thermal conversion, while capturing the coupled interactions between the solid fuel bed and the gas phase. FGR is applied independently to the primary, secondary, and tertiary gas streams, with recirculation ratios of up to 50%. The results indicate that primary FGR has the most pronounced impact, causing a progressive reduction in fuel bed volume of up to 15% at 50% FGR, primarily due to accelerated char consumption. However, this is accompanied by a substantial decrease in bed temperature and thermal power output, as well as increased carbon monoxide emissions. FGR applied to the secondary air stream has a comparatively moderate effect, resulting in limited changes to fuel bed characteristics and modest increases in emissions. In contrast, tertiary FGR yields the most favorable outcomes by enhancing turbulence and mixing in the upper combustion chamber. This leads to improved combustion efficiency, a reduction in CO emissions of up to 98%, and an increase in thermal power output exceeding 1%.