Author's School

School of Engineering & Applied Science

Author's Department/Program

Energy, Environmental and Chemical Engineering

Language

English (en)

Date of Award

Winter 1-1-2012

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Richard L Axelbaum

Abstract

Cofiring of biomass in pulverized coal boilers for large-scale power generation requires that current combustion standards of stability, reliability, emission and fuel conversion efficiency are maintained and/or improved. While the properties of biomass fuels are highly variable and can differ significantly from that of coal, in general biomass fuels have greater volatile matter content and larger particle size. The larger size is due to both fuel preparation methods as well as the physical properties of the biomass material. These two characteristics significantly impact the structure of the volatile flame, which is the zone dominated by the combustion of volatiles in the near burner region. The length and location of the volatile flame is important not only to flame stability, but also to the formation of pollutants such as NOx. Previous experiments have shown an increase in N conversion to NO when cofiring during both air-fired and oxyfuel combustion, despite the wood waste having less fuel-bound nitrogen. CFD simulations reveal that the impact of biomass fuels on the volatile flame length lead to increased NO formation.

Changes in volatile flame structure and NO formation are investigated using experimental and computational fluid dynamic: CFD) methods for cofired flames of pulverized coal and wood waste. Volatile flame length is measured experimentally using gaseous species measurements of CO and CO2 in a 35 kWth combustion facility. A numerical study aids in the interpretation of the impacts of wood waste on the volatile flame. Lastly, a simple model is developed to predict the effects of particle size on flame length. A dimensionless number, the Volatile Flame Number: VF#), assists in the comparison of flame and particle devolatilization processes.

The length of the fuel-rich volatile flame zone is found to be sensitive to the location of volatile matter release and the amount of volatiles available in the near burner region. Larger particles with high axial momentum and longer heating times breakthrough the volatile flame zone to release volatiles downstream into areas rich in oxygen. The delayed release of volatiles leads to less volatiles in the fuel-rich region and shorter volatile flames, which augments breakthrough of particles before complete release of volatiles. Increased volatile matter content, characteristic of biomass fuels, leads to an increased volatile flame length and reduced particle breakthrough when all volatiles are released in the near burner region. Particle breakthrough is shown to occur for all biomass cofired flames, but the critical size for breakthrough occurs for low cofiring ratios due to the significant particle size difference between coal and wood waste. As the cofiring ratio increases, so does the participation of wood particles in the volatile flame leading to longer flames and less breakthrough. This work will demonstrate that flame structure can be optimized for desired volatile flame lengths and minimum emissions by selecting biomass properties such as particle size and volatile matter content.

DOI

https://doi.org/10.7936/K77W698Z

Comments

Permanent URL: http://dx.doi.org/10.7936/K77W698Z

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