Contaminant Estimates and Removal in Product Gas from Biomass Steam Gasification

Abstract

Permanent gas species, tar compounds, sulfur compounds, and ammonia produced from a bench-scale (∼1 kg/h) fluidized-bed biomass gasifier were analyzed. Two commercial Ni-based catalysts and one commercial ZnO sorbent were evaluated under varied conditions by quantifying contaminants from the reactor inlet and outlet with specific sampling and analysis methods. The Ni catalysts targeted tar destruction and ammonia reduction, and the ZnO sorbent was selected for sulfur compound removal. Tar components were identified by gas chromatography−mass spectrometry (GC−MS) and quantified by GC−flame ionization detector (FID). A total of 13 compounds (≥C6) were identified in raw product gas, principally “lighter tar” species with an average concentration of 15.5 g m−3 (dry gas basis). For tar species that were not detected by GC, a gravimetric method was used to quantify the portion of “heavier tar” (5.3 g m−3 dry gas basis). These data are raw gas tar concentrations for the gasifier-operating conditions used for the remainder of the tests. The performance of two commercial Ni catalysts were evaluated by comparing the concentrations of both “lighter tar” and “heavier tar” after the raw gas passed through the tar-reforming reactor. Concentrations of hydrogen sulfide (H2S), carbonyl sulfide (COS), and thiophene (C4H4S) in the raw, dry, product gas averaged 93, 1.7, and 2.2 ppmv, respectively. C4H4S and two additional sulfur compounds, benzothiophene and one unidentified compound (UN1), were found in the tar-trapping solution. Removal of sulfur compounds using the ZnO sorbent at varied temperatures and gas hourly space velocities (GHSVs) was investigated. The primary sulfur component, H2S, was reduced to less than 1 ppmv; COS was not reduced significantly; and C4H4S concentrations were not affected at all. The average NO and ammonia concentrations were determined to be 8.2 and 2662 ppmv in the dry gas, respectively. Both were successfully converted to permanent gas species by Ni catalysts.