Performance Simulations for Co-gasification of Coal and Methane

Abstract

In the process under development, coal suspended in mixtures of CH4, H2, and steam is rapidly heated to temperatures above 1,400°C under 5–7 MPa for at least 1 s. The coal first decomposes into volatiles and char while CH4 is converted into CO/H2 mixtures. Then the char is converted into CO/H2 mixtures via steam gasification on longer time scales, and into CH4 via hydrogasification. Throughout all stages, homogeneous chemistry reforms all intermediate fuel components into the syngas feedstock for methanol synthesis. Fully validated reaction mechanisms for each chemical process were used to quantitatively interpret a co-gasification test series in SRI’s lab-scale gasification facility. Homogeneous reforming chemistry generates equilibrium gas compositions at 1,500°C in the available transit time of 1.4 s, but not at any of the lower temperatures. Methane conversion in the gas phase increases for progressively hotter temperatures, in accord with the data. But the strong predicted dependence on steam concentration was not evident in the measured CH4 conversions, even when steam concentration was the subject test variable. Char hydrogasification adds CH4 to the product gas stream, but this process probably converts no more than 15–20% of the char in the lab-scale tests and the bulk of the char is converted by steam gasification. The correlation coefficient between predicted and measured char conversions exceeded 0.8 and the std. dev. was 3.4%, which is comparable to the measurement uncertainties. The evaluation of the predicted CH4 conversions gave a std. dev. greater than 20%. Simulations of commercial conditions with realistic suspension loadings and no diluents in the feed gave slightly lower conversions of both CH4 and coal, because hydrogasification accounts for more of the char conversion, and occurs at rates slower than for steam gasification.