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(390a) Understanding the Role of Partial Oxidation Reactions during Autothermal Pyrolysis

Authors: 
Peterson, C., Iowa State University
Brown, R. C., Iowa State University
Gable, P. A., Iowa State University
Lindstrom, J. K., Iowa State University
Polin, J., Iowa State University
The goal of this project is to understand the partial oxidation reactions responsible for energy release during biomass autothermal pyrolysis. The enthalpy for pyrolysis is provided by exothermic oxidation reactions within the reactor instead of external energy supplied by heat transfer. This is achieved by reacting a small amount of oxygen (equivalence ratios of 0.06-0.10) with biomass in a fluidized bed reactor, which partially combusts the products of pyrolysis to overcome the endothermic enthalpy of pyrolysis. Experiments performed at Iowa State have shown that autothermal pyrolysis does not produce significant loss in bio-oil yield or quality as compared to traditional non-oxidative pyrolysis. Consequently, we are developing a chemical kinetic model of autothermal pyrolysis to better understand the source of energy for the process.

Based on product differences between autothermal pyrolysis and inert pyrolysis (bio-oil, biochar, and gases), it appears the enthalpy for pyrolysis is being provided primarily by the oxidation of char, light oxygenates, and phenolic compounds. However, to confirm these products are in fact oxidizing at pyrolysis temperatures (400-600 ˚C) further experiments are needed. To accomplish this, model compounds representative of pyrolysis products such as acetic acid, levoglucosan, and phenolic monomers are partially oxidized at pyrolysis temperatures in a modified isothermal fluidized bed reactor. The condensed liquid product and non-condensable gases are analyzed via gas chromatography (GC) to determine products of partial oxidation and estimate kinetic data. Preliminary results show that these products begin to oxidize at temperatures below 400 °C, much lower than current published reaction mechanisms predict. Likewise, the role of incomplete combustion that does not generate CO2 and H2O appears to occur at these low temperatures and equivalence ratios. In particular, the lignin-derived phenolic components appear to incorporate oxygen into their structures during partial oxidation rather than generate non-condensable gases. Char oxidation studies were also performed using both thermogravimetric analysis (TGA) and fluidized bed experiments. Both methods indicate that the amount of alkali and alkaline earth metals (AAEMs) present in the biochar influence rates of reaction and product composition (CO vs CO2). Determining the relationship between product oxidation at pyrolysis temperatures can identify not only the source of energy in biomass autothermal pyrolysis, but potentially expands autothermal processing to other applications as well.