Effect of Co-Firing Torrefied Woody Biomass with Coal in a 30 kWt Downfired Burner
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Mesquite and juniper can be beneficially utilized for gasification and combustion applications. Torrefaction has been considered to be one of the thermal pretreatment options to improve the chemical (e.g. heat content) and physical (e.g. grindability) properties of raw biomass. A simple three component parallel reaction model (TCM) was formulated to study the effect of heating rate, temperature, residence time and type of biomass on torrefaction process. Typically inert environment (e.g. N_(2), He, Ar) is maintained to prevent oxidation of biomass during torrefaction. A novel method for utilization of carbon dioxide as the pretreatment medium for woody biomass has been investigated in the current study. Both raw and the torrefied biomass (TB) were pyrolyzed using TGA under N_(2). The TB fuels were also fired with coal in a 30 kWt downfired burner to study the NOx emission. In addition, tests were also done using raw biomass (RB) (mesquite and juniper) blended with coal and compared with results obtained from cofiring TB with coal. A zero dimensional model has been developed to predict the combustion performance of cofired fuels. The results are as follows. TGA studies yielded global kinetics based on maximum volatile release (MVR) method. TCM predicts higher loss of hemicellulose upon torrefaction when compared to the other components, cellulose and lignin resulting in improved heat values of TB. Comparable mass loss at lower temperatures, improved grindability, and improved fuel properties were observed upon using CO_(2) as the torrefaction medium. Co-firing 10% by mass of raw mesquite with coal reduced the NOx emission from 420 ppm to 280 ppm for an Equivalence ratio (ER) of 0.9. Further cofiring TB with coal reduced the NOx emission by 10% when compared to base case NOx emission from combustion of pure PRB coal. NOx emission decreased with increase in equivalence ratio. In addition, a term used in the biological literature, respiratory quotient (RQ), is applied to fossil and biomass fuels to rank the potential of fuels to produce carbon dioxide during oxidation process. Lesser the value of ‘RQ’ of a fuel, lower the global warming potential.
Thanapal, Siva S (2014). Effect of Co-Firing Torrefied Woody Biomass with Coal in a 30 kWt Downfired Burner. Doctoral dissertation, Texas A & M University. Available electronically from