Gasification of Coffee Grounds in Dual Fluidized Bed: Performance Evaluation and Parametric Investigation

Abstract

Coffee grounds refer to a kind of high-moisture biomass refuses generated in drink works. With a national technical program, we recently worked on converting this biomass waste into middle-caloric product gas. The conversion proceeded in two consecutive steps, via first fuel drying/upgrading and in turn pyrolytic gasification of the dried fuel with the so-called dual fluidized bed gasification (DFBG) technology. The present paper investigated the gasification of dried coffee grounds in a 5.0 kg/h pilot DFBG facility, with the aim of demonstrating the adaptability of the technique to the tested fuel and clarifying its chemically possible efficiency. It was shown that gasifying dried coffee grounds with moisture of about 10 wt % through DFBG at about 1073 K is easy to convert more than 70% of fuel's C into product gas, and the gas can have a higher heating value (HHV) over 3500 kcal/m3n. Nonetheless, the tar load in the product gas was sometimes up to 50 g/m3n. Increasing the steam/fuel mass ratio and decreasing the fuel particle size reduced the tar yield, but the available reduction degree was limited. Inclusion of a small amount of air into steam (gasification reagent) appeared efficient to lower the tar content of the product gas, but only the O2/C molar ratios below 0.1 are applicable in the view of preventing the HHV of the product gas from becoming lower than 3000 kcal/m3n. As a consequence, other tar elimination techniques were suggested to be necessary for the gasification of coffee grounds via DFBG. Furthermore, the paper demonstrated that for DFBG using steam as the gasification reagent its attainable C conversion likely determines the parameters, such as H conversion, cold gas efficiency, tar content in the product gas, and product gas molar composition.