Design and performance of a bench-scale steam fluidized gasifier for biomass analysis
Gasification has become an attractive technology that is capable of producing a rich syngas from a range of biomasses and other fuels. This syngas can then be further processed into liquid fuels through Fischer-Tropsch processes to provide an alternative to current fossil fuels or to provide local markets with a competitive fuel. The purpose of this study was to construct a four-inch bench scale fluidized bed gasifier to operate at a variety of stable conditions and to analyze the resulting syngas stream. A sampling system was constructed from a Perma Pure dryer that consists of a single Nafion® tube capable of removing water vapor from the syngas stream while preserving the other gas compounds for analysis. A heated filter held at 250°F was installed before the dryer to remove particulate matter and condense tars, thus preserving the integrity of the dryer. Using this sampling system provided a higher quality gas stream for analysis than conventional methods. Carbon monoxide, hydrogen, carbon dioxide, methane, ethane, acetylene, ethylene, pentane, butane, propane, propylene, methyl mercaptan and five unknown compounds were identified by the Varian CP-4900 micro gas chromatograph. However, the gas stream did not contain any hydrogen sulfide, meaning there likely is still a problem with condensation and dissolution. The reactor and sampling system have undergone a series of revisions to increase efficiency and accuracy from which a series of operating conditions could be tested. Reactor temperature, biomass feed rate and types of fuel were selected as the operating variables. Findings were compared to theoretical calculations for concentration and trends. Results show that increasing temperature increases the production of hydrogen, volumetric flow rate and carbon conversion and decreases the production of hydrocarbons. Increasing the feed rate of biomass did not result in an increase in carbon monoxide or hydrogen, but did increase the volumetric flow rate and hydrocarbon production. Increased biomass feed rate also resulted in lower carbon conversion. Pine chips and switch grass were tested in the reactor and showed similar gas composition. However, further testing is needed on switch grass for a more complete analysis. Tests show that the reactor is capable of producing a stable environment for a range of operating conditions and that the sampling system is capable of removing particulate matter and condensing tars. Further work on the reactor and sampling system can likely improve results.