Experimental investigations of biomass gasification with carbon-dioxide
A sustainable energy cycle may include enhanced utilization of solar energy and atmospheric CO2 to produce biomass and enhanced utilization of exhaust CO2 from power plants for synthetic gas production. The reaction of carbon with CO2 is potentially one of the important processes in a future sustainable carbon cycle. Reactions involving carbon and CO2 are also relevant to the chemical process and metal industries. Biomass char has been recognized as a present and future alternative to fossil-fuels for energy production and fuel synthesis. Therefore, biomass char gasification with CO2 recycling is proposed as a sustainable and carbon-neutral energy technology. Biomass char is a complex porous solid and its gasification involves heat and mass transfer processes within pores of multiple sizes from nanometer to millimeter scales. These processes are coupled with heterogeneous chemistry at the internal and external surfaces. Rates for the heterogeneous carbon gasification reactions are affected by inorganic content of the char. Furthermore, pore structure of the char develops with conversion and influences apparent gasification rates. Effective modeling of the gasification reactions has relied on the best available understanding of diffusion processes and kinetic rate property constants from state of the art experiments. Improvement of the influences of inorganic composition, and process parameters, such as pressure and temperature on the gasification reaction rates has been a continuous process. Economic viability of gasification relies on use of optimum catalysts. These aspects of the current status of gasification technologies have motivated the work reported in this dissertation. The reactions between biomass chars and CO2 are investigated to determine the effects of temperature and pressure on the reaction rates for large char particles of relevance to practical gasification technologies. An experimental apparatus consisting of a high-pressure fixed-bed reactor with product gas sampling for tracking the reaction progress, supported by independent gravimetric measurements of mass loss, is described. The effects of pressure and temperature on the char-CO2 reaction are investigated at elevated pressures up to 10 atm. Measurements of reaction rates at multiple temperatures and pressures for a low-ash pinewood char are presented. Kinetic rate parameters for the char-CO2 reaction are reported with detailed uncertainty calculations and discussed in the context of the structural changes of the char with mass loss. The effects of pressure and temperature on the internal mass transfer processes and the intrinsic reaction rates are assessed using Thiele analysis for non-isothermal particles with the nth order and the Langmuir-Hinshelwood kinetic rate models. The effects of potassium, calcium and iron catalysts on the CO2 gasification rates of an activated coconut char are investigated. A catalyst treatment method for obtaining high catalyst loadings (~12 wt. %) is described. The effects of the catalysts on the surface reaction rates and the activation energies are reported. The results of this study are encouraging in the context of potential future discovery of a viable low-temperature catalytic gasification process for sustainable use of biomass as a renewable energy resource. Utilization of plant based substances such as citric acid to provide higher catalytic activity and the potential for utilizing the high initial activity of iron by using rust proofing compounds for maintaining high reactivity are recommended for further development.
Gore, Purdue University.
Chemical engineering|Mechanical engineering|Materials science
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