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Numerical Optimization on Char Conversion and NOx Emission under Various Operating Conditions in a Retrofit Biomass
Viet Thieu Trinh1,2, Byoung-Hwa Lee3, Seung-Mo Kim3
1School of Mechanical Engineering, Pusan National University, 2, Busandaehak-ro 63 beon-gil, Geumjeong-gu, Busan 46241, Republic of Korea.
ACS Omega
|June 5, 2023
Summary
Retrofitting fossil boilers with biomass improves electricity supply and lowers costs. Optimal combustion conditions, particularly a 40% secondary air ratio, significantly reduce unburned carbon and bottom ash while maximizing char conversion.
Area of Science:
- Energy Engineering
- Combustion Science
- Environmental Technology
Background:
- Retrofitting fossil fuel boilers with biomass is crucial for addressing electricity shortages and reducing power plant costs.
- Biomass combustion in retrofitted boilers faces challenges including high unburned carbon, significant bottom ash, and nitrogen oxide (NOx) emissions.
Purpose of the Study:
- To determine optimal combustion conditions for a 125 MWe pulverized biomass boiler retrofitted from an anthracite down-fired boiler.
- To evaluate the influence of secondary air ratio and burner standby location on combustion efficiency and emissions.
Main Methods:
- Utilized computational fluid dynamics (CFD) with commercial software (Fluent ANSYS) to simulate biomass combustion.
- Investigated air distribution, focusing on secondary air ratio and burner standby configurations.
- Calculated key parameters: biomass ash mass, char conversion, high zone temperature, and NOx formation/reduction.
Main Results:
- A secondary air ratio of 40% (Case R) significantly reduced bottom ash to 247 kg/h and increased char conversion to 97.33%.
- Case R exhibited superior air-biomass mixing, indicated by a lower standard deviation temperature (240 K).
- Optimal conditions (Burner A standby, 40% secondary air ratio) yielded 98.43% char conversion, 204 kg/h bottom ash, and 106 ppm NOx.
Conclusions:
- Optimizing the secondary air ratio to 40% is key for efficient biomass combustion in retrofitted boilers.
- The selected optimal case demonstrates a successful balance between high char conversion, low bottom ash, and reduced NOx emissions.
- CFD modeling provides a valuable tool for determining optimal operational parameters in biomass retrofitted power plants.
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