Abstract

An increasing emphasis on mitigating global climate change (global warming) over the last few decades has created interest in a broad range of sustainable or alternative energy systems to replace fossil fuel combustion. Biomass, when harvested responsibly, is a renewable fuel with many uses in replacing fossil fuels. Cofiring biomass with coal in traditional large-scale coal power plants represents one of the lowest risk, least costly, near-term methods of CO₂ mitigation. Simultaneously, it is one of the most efficient and inexpensive uses of biomass. Alternatively, biomass can be transformed into useful products through gasification to produce clean syngas for highly efficient gas turbines, or feedstock to produce light gases, fuels, chemicals or other products. A large portion of this investigation focused on the effect of cofiring biomass on the near burner region of a commercial coal flame. This research included first-of-their-kind field measurements of flame structure and particle properties in front of a full-scale burner fired with biomass and coal, including measurements of particle size and composition, gas velocity, composition, and temperature in the near-burner region of multiple cofired flames in a 350 MWe full-scale power plant in Studstrup, Denmark. A novel sampling and analysis technique was developed enabling the estimation of the fraction of biomass in the flow as a function of position and the burnout of biomass and coal particles separately. These data show that biomass particles do not follow gas stream lines to the same extent that coal particles do. This is consistent with the larger sizes, slower heating and reaction rates, and higher momentum of biomass particles. This research also includes first-of-their-kind single particle continuous measurements of particle mass, surface and internal temperature, size, shape, during biomass pyrolysis and gasification. The single particle measurements provided among the most highly resolved and repeatable biomass gasification results reported to date for wood, switchgrass and corn stover. All three samples showed greater gasification reactivity to H₂O than to CO₂. The experiments included results in both reactants individually and combined. One of the most important findings of this work was the experimental confirmation that as the char particles gasify, their ash fractions increase and reaction rates decrease on both an intrinsic and external surface area basis. The analyses in this work show that this decrease in burnout quantitatively corresponds to the change in the predicted fraction of the surface that is ash and does not reflect any change in organic reactivity. Reaction rate parameters suitable for relatively simple power-law models based on external surface area describe all the data reasonably well.

Degree

PhD

College and Department

Ira A. Fulton College of Engineering and Technology; Chemical Engineering

Rights

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