1. Bulgarian Academy of Sciences, Laboratory of Chemistry of Solid Fuels, Institute of Organic Chemistry, Academy G. Bonchev 9 Street, Sofia 1113, Bulgaria, Instituto Nacional del Carbon, Consejo Superior de Investigaciones Cientificas, Apartado de Correos 73, 33080 Oviedo, Spain, and Institute of Technology of Agricultural Producta, Nagref, 1 S. Venizelou, 14123 Lykovrissi, Greece