Steam/oxygen gasification system for the production of clean syngas from switchgrass

Abstract

A pilot-scale 25kg/h fluidized bed, oxygen/steam blown gasifier and syngas cleaning system was developed to convert switchgrass into clean syngas. The system is rated for operation at gage pressures up to 1bar. The reactor vessel incorporated a novel guard heating system to simulate near-adiabatic operation of large commercial-scale gasifiers, and was effective for gasification temperatures up to 900°C. After removing particulate from the gas stream via cyclones, a warm-gas cleaning operation based on oil scrubbing was used to remove tars. Sulfur compounds were removed via solid-phase adsorption. Ammonia was removed by water scrubbing. Baseline gasification tests with steam and oxygen were conducted at equivalence ratios (ER) between 0.21 and 0.38 using switchgrass as fuel. Measurements on the raw and cleaned syngas included permanent gas composition, C2 hydrocarbons, water, heavy and light tars, gasification residues (char and ash), hydrogen sulfide (H2S), carbonyl sulfide (COS), carbon disulfide (CS2), ammonia (NH3), and the first reported measurements of hydrogen cyanide (HCN) for oxygen/steam blown gasification. Heavy tars were removed with high efficiency by the method employed, although more difficult to remove light tars reduced overall tar removal efficiency to less than 80%. The sulfur scrubbing system demonstrated 99.9% removal efficiency, resulting in less than 200ppb of H2S in the cleaned gas. The NH3 scrubbing system also accomplished greater than 99.9% removal efficiency, resulting in final NH3 concentrations of less than 1ppm.