Combined Heat and Power from Cotton Gin Trash through Fluidized Bed Gasification: Evaluation of a Heat Exchanger for Waste Heat Capture

The continued regulation of greenhouse gases (GHG) to reduce carbon emissions from coal fired power plants will likely reduce the availability of electricity and cause an increase in costs in the future. Agriculture facilities, such as cotton gins, in rural areas will be faced with challenges with regards for energy. A byproduct at a gin is cotton gin trash (CGT), a waste biomass with an energy content of 7000 Btu/lb. The energy in the CGT can be converted to useful energy, such as electricity and heat, through the process of fluidized bed gasification (FBG). FBG is a thermo-chemical process that converts a biomass to a combustible synthesis gas (syngas) that can be utilized in an engine to fuel a generator. The syngas produced from gasification typically exits the reaction chamber at a temperature of about 1300°F to 1400°F. Before the syngas is combusted in an engine / generator, the gas must be cleaned and cooled. A two tubed, counter-flow finned heat exchanger was designed and implemented with a pilot-scale fluidized bed gasification system located at Texas A&M University. The cooling agent through the heat exchanger was ambient air, in which the exiting heated air has the potential for drying cotton in a larger, commercial sized gasification system. Approximately 6.2 kW of thermal energy was available to be captured and transferred from the syngas to the air. Results from the experiments aided in determining the heat exchanger efficiency and effectiveness, along with overall heat transfer rate to provide thermal energy to a gin. Conclusions from this study were used for the design of a 250 kW fluidized bed gasification system that will be implemented at a cotton gin for combined heat and power.