Pretreatment of straw for power production by pyrolysis and char wash

Co-firing of straw and coal in existing pulverised coal-fired boilers is an option for biomass based power generation. However, the high chlorine and potassium content of straw may cause problems. Experiments with co-combusti on of straw and coal in power plants have shown that when a moderate amount of straw is applied (up to 20% on a thermal basis), the most serious probl ems are deactivation of SCR catalysts applied for NO reduction and deterior ation of the fly ash quality caused by potassium. To prevent these problems a pretreatment process is required in which the heating value of the straw is supplied to the boiler without introducing potassium into the furnace c hamber. A pretreatment process based on pyrolysis and char wash was investi gated. Straw is pyrolysed at moderate temperatures at which the potassium i s retained in the char. Potassium and residual chlorine are then extracted from the char by water, and char and pyrolysis gases may be co-fired with c oal. Fundamental laboratory studies acid technical investigations were cond ucted to evaluate the pretreatment concept. The investigations indicate, th at the low temperature pyrolysis of straw can be performed in a circulating fluid bed reactor applying only straw and straw char as bed material. With a bed temperature of approximately 550 degreesC no significant amount of p otassium is released to the gas phase. Applying a counter current moving be d to the char potassium extraction with water will probably be advantageous . (C) 2001 Elsevier Science Ltd. All rights reserved.