1. Helsinki Univ. of Technology, Kemistintie 1, P.O.Box 6100, FIN-02015 TKK, Finland.
2. Corresponding author. Phone: 358-9-451 2555; Fax 358-9-462 373. E-mail: Anu.Kaukovirta-Norja@hut.fi.
3. Oy Lahden Polttimo AB, P.O. Box 22, FIN-15141 Lahti, Finland.