Gasification of Coal Chars with H₂O and H₂O-O₂ Mixtures —Effects of Surface Area of Char and and Ciexustubg CH₄—

Abstract

To inquire into the properties responsible for the reactivity of coal gasification and the effect of added CH₄ on the rate of steam gasification, char(C) and porous char (PC) prepared from the same parent coal were gasified with the mixtures composed of H₂O(0, 24%)-O₂(0, O.5, 3, 6%)-N₂ under atmospheric pressure at 600-950°C. The fuel ratio of six kinds of coals used here were in the range of 1.4 to 7.9. The gasification rate and the composition of product gas were investigated by an apparatus combined with a thermobalance and two gaschromatographs, using the 100 mg sample (average diameter: 0.5-2.0 mm, mainly 1.0 mm). The reactivity was evaluated quantitatively by applying the rate equation, f=1-exp(-aθ^b), (a, b =constants) to the data of fractional gasification (f) vs. time(θ).
Main results obtained were as follows: ( 1 ) Either N₂ or CO₂ were used as adsorbates to measure specific surface area(Sgo); in the former case the ratio of Sgo of PC to that of C were in the range of about 50 to 260, and in the latter case, of about 7 to 130. But the difference in the reactivity between PC and C showed only a few times at most, and there were almost no difference in the composition of product gas by steam gasification of PC and C.
(2) Activation energies of steam gasification of chars prepared from four parent coals were very close to that of pure porous carbon containing neither mineral matters nor ash.
(3) Low level CH₄ and/or H₂ mixed with steam showed a pronounced retarding effect on the rate of steam gasification. This was interpreted as the decreasing of surface complex, (OH)C, formed by dissociation of H₂O.
(4) The rate of gasification with H₂O (24%)-O₂(1%)-N, mixed with low level CH₄(3-6%) was almost close to that with O₂(1%)-N₂, and product gas was rich in CO and H₂ although methane was not consumed at all. This was interpreted well by the homogeneous radical chain oxidation scheme of CO and H₂.
(5) Steam gasification of char was chemically controlled at temperatures lower than about 1000°C. The pore size distribution and/or pore surface area gave almost no influence on gasification rate. The chemical properties of char surface and the chemical interaction between carbonaceous surface and gasifying H₂O molecule were seemed to be essentially predominant factors governing the reactivity of gasification of char.