Numerical modeling of biomass combustion in a stoker boiler

Biomass fuel is considered a promising substitute for traditional fossil fuels. Amid a great variety of methods for converting the energy in biomass fuel into usable energy, direct combustion is still the dominant technology employed by industry. Because biomass fuel possess a much wider range of physical and chemical properties than fossil fuel, its combustion behavior is similarly diverse (and typically differs from fossil fuel), with a similar range seen in emissions characteristics. To address the variability the fuel stream imposes on the system, this work endeavors to use numerical modeling to investigate biomass combustion in a stoker boiler to provide physically insightful details while requiring minimal time and effort relative to the traditional experimental approach.

In the first part of this work, a comprehensive model was developed to investigate the co-firing of different kinds of biomass including oat hulls, wood chips and natural gas with coal in a stoker boiler located in the Power Plant of the University of Iowa. Later, this model is employed in the optimization of the air supply system and plans for efficiently injecting light weight biomass, such as oat hulls, into the stoker boiler. The other key problem is in NOx prediction and reduction for the stoker boiler. This was by combining a standard CFD model describing the turbulent dynamics and combustion with several sub-models specifically developed for this study to model the fuel bed, fuel particle movement, and fuel gasification. To verify and baseline these sub-models, a series of experiments are performed, including a temperature measurement campaign for coal combustion in the boiler, a chemical lab analysis of oat hull chemical characteristics, an experiment measuring oat hull particles' physical properties, and a high-heating-rate gasification test of oat hulls. In particular, the stoker boiler temperature measurements are unique in the number of points measured and the range of firing conditions. The simulation showed that for the co-firing of oat hulls with coal, the flame temperature decreased with increasing oat hull fraction. The oat hull particles follow the flow and burn in suspension due to their light weight. The simulation showed that increasing injection velocity could slightly reduce the peak temperature and thereby reduce NOx levels. It was also observed that there is a critical velocity above which the trend of decreasing CO2 is reversed. The co-firing of other types of biomass such as wood chips and natural gas in the stoker boiler were also studied. The result of co-firing wood chips shows that adding wood chips decreases the flame. The flame zone is also shortened when compared to pure coal, primarily resulting from a higher oxygen environment above the coal bed due to the high oxygen content of the wood chips. Co-firing natural gas with coal resulted in the high temperature zone shifting from the back wall closer to the front wall, significantly reducing the overall flame length. The level of predicted NOx agreed very well with the experimental data. The simulations showed that injecting Urea with the secondary air system on the front wall can greatly reduce the NOx level inside the boiler for minimal cost and effort.