Center for Sustainable Environmental Technologies

The goal of this project is to optimize performance of an indirectly heated gasification system that converts switch grass into hydrogen-rich gas suitable for powering fuel cells. Iowa State University has developed a thermally ballasted gasifier that uses a single reactor for both combustion and pyrolysis. Instead of spatially separating these processes, they are temporarily isolated. The producer gas is neither diluted with nitrogen or the produces of combustion. The heat released during combustion at 850 C is stored as latent heat in the form of molten salt sealed in tubes immersed in the fluidized bed. During the pyrolysis phase, which occurs at temperatures between 600 and 850 C, the reactor is fluidized with steam rather than air. Heat stored in the phase change material is released during this phase of the cycle to support the endothermic reactions of the pyrolysis stage. Because air is not used during the gas-producing phase of the cycle, nitrogen does not dilute the product gas, resulting in relatively high concentrations of hydrogen and carbon monoxide in the producer gas compared to conventional gasifiers. Iowa State has measured hydrogen content between 20 and 36 vol-%, which is a factor of 4 to 6 higher than found in gas from air-blown gasifiers.

The thermal ballasting system also more than doubles the carbon monoxide concentration. The carbon monoxide, along with the steam used to fluidize the reactor, can be shifted to additional hydrogen by the water-gas shift reaction, resulting in hydrogen concentrations exceeding 60 vol-%. The project employs a pilot-scale (5 ton per day) gasifier to evaluate the thermally ballasted system. Gasification at the pilot scale is important for obtaining realistic process data, especially for calculating energy flows through the system and assessing the practicality of feeding switch grass into the gasifier. A slipstream from the gasifier is used to evaluate gas cleaning and upgrading options. This slipstream includes: a guard bed designed to remove hydrogen sulfide and hydrogen chloride; a steam reformer designed to crack tar and decompose ammonia and high temperature and low temperature water-gas shift reactors to increase hydrogen content.