Biomass co-firing and coal-to-biomass conversion on existing coal-fired boilers have seen increased interest, especially in Europe, due to regulatory drivers of CO_2 reduction. Understanding in-furnace combustion and heat transfer of biomass co-firing or 100% conversion becomes critical in designing systems to meet expected performance of the boilers. In this paper, the in-furnace combustion of coal-to-biomass conversion for a 130MW wall-fired utility boiler was studied using ANSYS FLUENT Computational Fluid Dynamics (CFD) code. The application of Rotating Opposed Fired Air (ROFA) was modeled and evaluated for combustion improvement and NOx reduction. The modeling results found that biomass combustion and heat transfer patterns are different from coal due to the difference of fuel physical and chemical properties, particle size and ash concentration in the flue gas. Near-burner flames of the biomass case are stable under modeled conditions using the proposed biomass replacement burner. Overall gas temperature and peak temperature in the combustion zone are lower than coal case. The predicted model outlet gas temperature for biomass case increases by 43°C from coal baseline due to reduced radiant heat transfer to waterwall and radiation dominated superheater. The predicted un-burned carbon (UBC) loss of the biomass case is lower than that of the coal baseline case because of the improved combustion by ROFA. NOx emission from the biomass case is significantly reduced in comparison with coal baseline case, predominantly due to low nitrogen biomass fuel along with combustion staging afforded by the ROFA system. The predicted NOx emission of the 100% biomass case is below 200 mg/Nm~3 Industrial Emission Directive (IED) compliance limit with primary method only. Ash deposition tends to increase due to high alkali mineral content, but is likely manageable with proper sootblowing for this woody biomass. Corrosion on waterwalls tends to be less for biomass, but corrosion on leading-edge superheater surfaces needs to be addressed for this particular biomass fuel by applying in-furnace additives.
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