Syngas Production via CO2 Enhanced Gasification of Biomass Fuels

Abstract

This paper presents experimental results of decomposition tests for biomass to syngas conversion. The gasification process was found to yield improved char conversion and higher levels of H2 and CO for various CO2 recycle ratios. Carbon monoxide production from steam gasification was enhanced by increasing the CO2 input flow rates. The evolution of H2 gas only became significant at high gasification temperatures above 650°C for the wood and 500°C for the herbaceous and nonwood biomass samples studied. Using thermogravimetric analysis (TGA), gas chromatography (GC), atomic absorption spectroscopy (AAS), calorimetry, and the scanning electron microscope with energy dispersive X-ray analysis (SEM/EDX) the nature of the biomass composition and ash residue, and the mass decay of biomass sources including various woods and grasses were studied. These were poplar, red oak, sugar maple, white pine, spruce, Douglas fir, pine needles, maple bark, alfalfa, cordgrass, and American beachgrass. Hydrogen, carbon monoxide and methane gas evolution as a function of temperature was also quantified. The woods and grasses had similar TGA curves with a third level mass step during high temperature steam gasification showing completed mass loss by 900–1,000°C. Two distinct regimes of mass decay, representing pyrolysis and gasification and char burnout, were found to correlate well with the two corresponding gas evolution regimes for CO and H2. An SEM/EDX analysis also showed high levels of potassium (K), magnesium (Mg), and phosphorus (P) in the ash residue. The mineral content of the biomass sources, and particularly the high alkaline content of the grassy feedstocks used in the present study, were held responsible for the corrosion of the quartz TGA furnace. This composition necessitates the careful selection and possible need for preprocessing of biomass fuels to minimize corrosion of the operating equipment. Gasification prior to high-temperature combustion enables the removal of the corrosive ash elements such as potassium and chlorine that would otherwise be problematic.