Numerical modeling of NOx reduction using pyrolysis products from biomass-based materials

Pyrolysis products of biomass (bio-oils) have been shown to cause a reduction in NOx emissions when used as reburn fuel in combustion systems. When these bio-oils are processed with lime, calcium is ion-exchanged and the product is called BioLime™. BioLime™, when introduced into a combustion chamber, causes oils to pyrolyze and reduce NOx emissions through reburn mechanisms while simultaneously causing Ca to react with SO2. Thus NOx and SO2 emissions are reduced at the same time. The devolatilization rates of two biomass-based materials were studied using TGA and were related to the yield of pyrolysis gases and char during flash pyrolysis. Numerical simulations using CHEMKIN to model NO reduction through homogeneous gas phase reactions were reported. The numerical predictions were then compared to NOx emission levels from experiments in a down-fired combustor (DFC) to validate the model. A difference in NO reduction was observed by use of different bio-oils under similar operating conditions. This is believed to be due to the difference in yield of flash pyrolysis products of bio-oils. Also, different pyrolysis gases have different NOx reduction potentials. Knowledge of the relative contribution of pyrolysis gases in NO reduction will help choose a feedstock of biomass that will aid in increasing the yield of the desired species. A parametric analysis was done using the model to study the effect of varying concentrations of hydrocarbons, CO2, CO, and H2, and the results were then verified using a flow reactor. The analysis showed that hydrocarbons were mainly responsible for causing reduction in emissions of NO, whereas CO2, CO, and H2 have very little effect on NO reduction.