Performance characteristics of a pilot-scale biomass gasifier using oxygen-enriched air and steam

This study is to investigate the characteristics of a biomass gasification system using mixtures of “oxygen-enriched air” and steam as the gasifying agent for increasing the syngas heating value and combustible gas constituents. This study also aims to characterize the effects of oxygen-and-steam gasification on ammonia concentration that can lead to significant NOx emissions from syngas combustion. Experiments are conducted using a pilot-scale, pressurized bubbling fluidized bed gasifier with a capacity of five tons per day. Pure oxygen is added to air before mixing with steam for gasification. A significant amount of steam is required to control the reactivity of the system at high oxygen levels. The oxygen content in the enriched air varies from 21, 45, to 80vol.% on dry basis, corresponding to 21, 30, and 40vol.% on wet basis respectively. The bed temperature is maintained at 800°C for all tests. Three different biomass feedstocks with nitrogen contents varying from 0.05 to 1.4wt.% are used for study (i.e., pine, maple-oak mixture, and discarded seed corn). The syngas dry composition is measured using a microgas chromatograph while ammonia concentration and moisture content are measured using a modified IEA Tar Protocol and Karl Fischer Titration respectively. Results indicate that oxygen-enriched air and steam gasification favors the production of combustible gas components including hydrogen, carbon monoxide, methane, and lighter hydrocarbons. When 40% oxygen is used, hydrogen increases by 70%, 47%, and 32% for pine, maple-oak, and seed corn respectively, while CO increases by 34%, 18%, and 8.6% respectively. Overall, it is found that oxygen and steam gasification is most effective for feedstock with low nitrogen and moisture contents. Results also show that ammonia and NOx concentrations in syngas increase as oxygen enrichment increases. The lower heating value of syngas can increase by as much as 43% for the feedstock studied. When the oxygen level increases from 21% to 40%, the H₂/CO ratio also increases from 0.59 to 0.75, 0.67 to 0.84, and 0.36 to 0.43 for pine, maple-oak, and seed corn respectively. Despite the improvement, the H₂/CO ratio is still moderate. The moderate H₂/CO ratio is explained by the high water content in syngas at high oxygen and steam conditions, indicating a large amount of un-reacted steam at the current gasifier temperature at 800°C.