Catalytic effects of metals on peat combustion

Abstract

A comparison was made of the effects of Fe, Mn, Cr, Ni, Co, Zn, Al, Mg and Ca on slow and rapid peat combustion. The concentrations used were 100–200 mmol kg −1 and lower and higher concentrations were also used for Fe. The order of the catalytic effects of the metals on the combustion of peat particles (100–125 μm diameter) in an entrained flow reactor (particle heating rate 15000 ± 5000 °Cs −1) was: Cr > Mn, Fe > Co, Ni > Ca > Zn, Mg > Al. In the presence of 100 mmol kg −1 Cr, the combustion time decreased by 26% compared with acid-washed peat having a low content of inorganic material. The decrease was only 4% with Al. Thermogravimetric experiments predicted the order: Fe, Cr > Mn, Ni > Co > Ca > Mg > Zn > Al (heating rate 0.17 °Cs −1). Experiments with Fe in the entrained flow reactor showed an increase in concentration from 105 to 330 mmol kg −1 to have little effect, but a decrease from 105 to 42mmol kg −1 weakened the catalytic effect markedly. The thermogravimetric experiments predicted that concentrations between 105 mmol kg −1 and 330 mmol kg −1 would strengthen the catalytic effect. Catalytic effects of metals on combustion can be predicted on the basis of atomic structure. Transition metals are good catalysts and the best of them have five or six electrons in d orbitals, whereas metals with only one stable oxidation state and completely occupied or empty s, p and d orbitals are poor catalysts. These effects probably prevail in the combustion of coals. In the case of dried Finnish bog peat, the catalytic effects of cations are mainly due to Fe, because the contents of other strong catalysts (Cr and Mn) are very low relative to Fe; Ca causes some additional effects.