EFFECTS OF COAL MOISTURE-CONTENT ON CARBON DEPOSITION IN COKE OVENS

The formation of carbon deposits from coal pyrolysis in a two-stage re actor was studied at a constant cracking temperature of 900 degrees C for several moisture contents of the coal charge ranging from 0.8 to 1 5 wt%. Whatever the coal moisture content, the formation of the deposi ts, the mass of which was continuously monitored during a pyrolysis ru n by a gravimetric balance system, was characterized by two distinct s tages. The average deposition rate decreased progressively with increa sing moisture content up to 8 wt%, then increased sharply beyond 8 wt% . Deposits observed under the polarized light microscope presented two different structures depending on the coal moisture content. For cont ents < 8wt%, deposits were characterized by the superimposition of a s pherulitic and a laminar texture, but the texture beyond 8% was only l aminar. A model of the temperature field within the coal mass is propo sed to explain the effect of moisture on carbon deposit formation. Res ults from this model as well as analyses of primary and secondary tars indicate that in the reactor the water added to the coal has no effec t on the gas phase in which deposits are formed but acts directly with in the coal mass, modifying both the yield and the composition of the volatile matter and consequently its capacity to form deposits. (C) 19 96 Elsevier Science Ltd.