DIOXINS AND RECYCLING OF RESOURCES- III THERMODYNAMIC CALCULATION ON DIOXINS FORMATION IN COMBUSTION PROCESS

摘要

The thermodynamic calculations were carried out in order to investigate the conditions of dioxins/furans (Dioxins) formation at high temperatures in combustion processes. It is found that (1) Dioxins are never formed in case that surplus oxygen exists in the system, namely under the condition of complete combustion and (2) Dioxins are not formed when solid carbon can be deposited, because the solid carbon is thermodynamically more stable than the carbonaceous matters including Dioxins. On the other hand, in fact, Dioxins are formed even at 1073K under "the complete combustion conditions", in which surplus oxygen exists in the system, because it is almost impossible to burn out the fly carbonaceous matters perfectly in the actual combustion furnaces. Thermodynamically, under the assumption that the carbon deposition does not proceed, Dioxins are calculated to be formed at high temperature of 1073K in some range of high CO/CO_2. The condition is considered to hold at the vicinity of the fly carbonaceous matters in the combustion furnaces, in actual. Under the condition that the fly carbonaceous matters exist, even if the reaction of 2CO->C +CO_2 (carbon deposition) proceeds, the reaction of C + CO_2 ->2CO (CO formation; oxidation of the fly carbonaceous matter by CO_2) will be occurred at the same time. Then, the high CO/CO_2 ratio is kept around the fly carbonaceous matters, at which Dioxins can form. Dioxins can be considered to form by assuming the existence of the heterogeneous site with high CO/CO_2 ratio at the vicinity of the fly carbonaceous matters.