The oxidation of calcium sulphide at the temperatures of fluidized-bed combustors

Book · Sat Dec 31 04:00:00 EST 1994

OSTI ID:93165

Davies, N H; Laughlin, K M ^[1]; Hayhurst, A N ^[2]

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1. Coal Research Establishment, Cheltenham (United Kingdom)
2. Univ. of Cambridge (United Kingdom). Dept. of Chemical Engineering

Calcium sulphide is a major product from coal gasification processes that use limestone as a desulphurizing agent; also, it may be formed in fuel-rich regions of fluidized-bed combustors. Complete conversion of CaS to CaSO{sub 4} via the gas-solid reaction CaS + 2O{sub 2} {r_arrow} CaSO{sub 4} is desirable to allow safe disposal of the bed ash material. This reaction has been investigated at temperatures from 650 C to 1,050 C using thermogravimetric analysis. The results revealed, inter alia, a discontinuity in product formation with increasing temperature: Above 850 C, CaO is also formed with the evolution of SO{sub 2}. This has been shown to be a result of the apparent solid-solid-reaction CaS + 3CaSO{sub 4} {r_arrow} 4CaO + 4SO{sub 2}, which occurs above about 900 C, when a liquid melt is probably produced. The observation of CaO production at 850 C during CaS oxidation, however, is a consequence, of the exothermicity of the reaction forming CaSO{sub 4} raising the particle temperature above 900 C, thereby initiating the solid-solid reaction between product CaSO{sub 4} and reactant CaS. This rise in temperature, coupled to the change in porosity caused by both CaSO{sub 4}, and CaO formation, gives rise to slightly sigmoidal mass-time plots from the thermogravimetric experiments. A kinetic study of the oxidation of CaS to CaSO{sub 4} has shown it to be first order in O{sub 2}, with a rate constant of 4.5 {+-} 2.0 {times} 0{sup {minus}4} exp[{minus}(5.7 {+-} 1.5) 10{sup 3}/T] m/s. The overall conclusion is that, although CaS is converted to CaSO{sub 4} at the operating temperatures of fluidized-bed combustors, reaction between these two solids starts for particle temperatures between 850 C and 900 C, probably via a melt, and results in the undesirable evolution of SO{sub 2}.

• OSTI ID: 93165
• Report Number(s): CONF-940711--
• Country of Publication: United States
• Language: English

Related Subjects

01 COAL, LIGNITE, AND PEAT
ACTIVATION ENERGY
CALCIUM SULFATES
CALCIUM SULFIDES
CHEMICAL REACTION KINETICS
COAL
COMBUSTION KINETICS
EXPERIMENTAL DATA
FLUIDIZED-BED COMBUSTION
OXIDATION
SORBENT INJECTION PROCESSES
SULFUR DIOXIDE
SYNTHESIS
TEMPERATURE DEPENDENCE
THERMAL GRAVIMETRIC ANALYSIS