Gasification: Eliminating Risks Associated With Co-Firing Biomass

Abstract

Under certain greenhouse gas (GHG) regulation scenarios, older coal-fired units may be faced with the prospect of shutdown before reaching the end of their useful life. Repurposing this existing asset for 100% biomass fuel is a more efficient use of capital than compared to building a new stand-alone unit. Biomass co-firing is an alternative for an owner to consider to address GHG regulation impacts on older coal-fired power boilers and the growing demands of pending legislation. “Direct” co-firing is a baseline approach where finely divided biomass is injected directly into the boiler furnace. Direct co-firing experience is typically less than 5% heat rate, and technical upper limits have been described in EPRI literature (1) as approximately 10% of boiler heat. Direct co-firing also does not enhance the opportunity to co-fire biomass with natural gas. Direct biomass co-firing may require extensive renovations and emissions/particulate control devices. “Indirect” co-firing is an alternative process that mitigates process risk by first converting the biomass into a fuel gas and then cleaning this gas to remove alkali and chloride contaminants prior to combustion in the power boiler furnace. Indirect co-firing may be a superior approach from an operations perspective because it protects against forced outages and repair costs expected with direct co-firing (2). Gas cleaning to remove alkali metals from the fuel gas prior to combustion reduces process risk by reducing fouling and slagging potential. Removing chloride from the fuel gas dramatically reduces the corrosion potential. Beyond reducing process risk, separating biomass ash before combustion retains the value in separate co-product ash streams, as it prevents intermingling with the coal ash. This paper describes technical and economic considerations for indirect co-firing, contrasted with direct co-firing approaches. The renewable energy ratio of a co-fired unit could be significantly increased by employing biomass gasification of the solid fuel with gas cleanup, in contrast to process risks, added emissions control costs, and technical limitations of direct co-firing of the solid biofuel.