Pre-treatment of Biomass
Most of the challenges that co-firing poses to boiler operation originate from fuel properties (the differences in characteristics of coal and biomass) and can be summarized as follows:
- Pyrolysis starts earlier for biomass than for coal
- The volatile matter content of biomass is higher than in coal
- The fractional heat contribution by volatile substances in biomass is approximately 70% compared with 30-40% in coal
- The specific heating value [kJ/kg] of volatiles is lower for biomass compared with coal
- Biomass char has more oxygen compared with coal and it is more porous and reactive
- Biomass ash is more alkaline in nature, which may aggravate the fouling problems
- Biomass can have high chlorine content.
Biomass has usually high moisture content resulting in a relatively low calorific value of the fuel, which might negatively affect its combustion properties. Additionally biomass feedstocks have low bulk density and potentially high chlorine content as well as hydrophilic character. Biomass properties which set demanding requirements for power plant include total ash content, its melting behavior and chemical composition - typically biomass contains fewer ashes than coal but alkaline metals that are usually responsible for fouling of heat transfer surfaces are high in biomass ashes.
Raw biomass has much lower bulk density than coal resulting in expensive storage, transport and handling, and has non-friable character resulting in problems with biomass grindability. Most of the challenges that co-firing poses to boiler operation originate from biomass properties and therefore improving the properties e.g. by pre-treatment can be applied as one of the measures to avoid or reduce these challenges.
Different methods of Biomass Pre-treatment