Effects of coal moisture content on carbon deposition in coke ovens

Abstract

The formation of carbon deposits from coal pyrolysis in a two-stage reactor was studied at a constant cracking temperature of 900°C for several moisture contents of the coal charge ranging from 0.8 to 15 wt%. Whatever the coal moisture content, the formation of the deposits, the mass of which was continuously monitored during a pyrolysis run by a gravimetric balance system, was characterized by two distinct stages. The average deposition rate decreased progressively with increasing 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 contents < 8 wt%, deposits were characterized by the superimposition of a spherulitic and a laminar texture, but the texture beyond 8% was only laminar. A model of the temperature field within the coal mass is proposed to explain the effect of moisture on carbon deposit formation. Results 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 effect on the gas phase in which deposits are formed but acts directly within the coal mass, modifying both the yield and the composition of the volatile matter and consequently its capacity to form deposits.