RESUMO
Two experiments were performed with an atmospheric circulating fluidized bed gasifier (ACFBG), the first with pelletized straw and the second with loose straw, to investigate the occurrence of polycyclic aromatic hydrocarbons (PAHs), chlorophenols (CIPhs), polychlorinated biphenyls (PCBs), polychlorinated benzenes (ClBzs), polychlorinated dibenzo-p-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs) in the bottom ash and fly ash formed during gasification. Only PAHs were present in large amounts, and only in the fly ash, ranging from 300 to 555 mg/kg ash in the tests with pelletized straw and from 73 to 118 mg/kg ash in those with loose straw. These amounts are so high that environmentally safe disposal or reuse of the ash would be difficult, so the development of a technique to handle the problem was included in the project. The method investigated was to burn the fly ash in a circulating fluidized bed (CFB) boiler in order to destroy the PAHs. This worked surprisingly well, eliminating 99% of the PAHs, without any further formation of the other harmful organic compounds analyzed. Thus, this method could actually be useful in practice. Especially the fact that the formation of PCDD/Fs was minimal during gasification and further treatment of the ash in the CFB boiler makes the gasification technique highly competitive relative to conventional combustion methods.
