435165 Comprehensive Study of Biomass Pyrolysis and Gasification Kinetics, Pore Stucture and Surface Area Modeling

Thursday, November 12, 2015: 9:55 AM
250B (Salt Palace Convention Center)
Ruochen Wu, Chemical Engineering, Brigham Young University, Provo, UT

Biomass gasification process is comprised of four stages, drying, devolatilization or pyrolysis, oxidation and gasification. Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures in the absence of oxygen (or any halogen). It involves the simultaneous change of chemical composition and physical phase. Pyrolysis is a set of complex reactions observed in organic materials exposed to high temperatures. The pyrolysis stage is a really rapid process, usually less than 1 minute. And GC-MS (Gas chromatography–mass spectrometry) is set up to collect continuous data for the production gases during pyrolysis stage. 

The experimental data in this investigation illustrates that the residence time of pyrolysis process is around 40 seconds.  Another important result from this model is the fraction of mass loss at the end of pyrolysis. Typically, the fraction of mass loss for poplar is the highest among the three types of biomass material, usually between 92% and 96%. And this fraction is about 88%-90% for switchgrass and corn stover.

The pyrolysis process involves a huge amount of complex reactions and product gases. In the experiments, CH4, C2H2, CO, NO, CO2, benzene and toluene are on the list of my interests. CO is a little tricky when analyzing the GC-MS data, because N2 and CO2 has the same molecular weight and mass-to-charge ratio (m/z). To overcome this difficulty, argon is acted as purge gas to create an atmosphere free of N2. A typical data for the pyrolysis gases analysis indicates that H2O appears first, followed rapidly by benzene and toluene, then by C2H2, CH4 and NO.

Char reactivity is demonstrated to be correlated with the char porous structure and surface area. The rate determining step in biomass gasification is the char-CO2 or Char-H2O reaction. In this report, the char conversion is introduced, to determine the reactivity of biomass chars. Char conversion is the ratio of the instant mass loss value and the mass loss value at the end of devolatilization. As biomass chars of different type and different size will have different total reaction time and reaction rate, it’s more obvious to compare the reaction rates at a same char conversion, instead of residence time or other parameters. Internal surface area data are collected by N2 adsorption, for chars produced at different reacting conditions (different concentrations of CO2 and H2O and different temperatures). Models have been developed to describe the surface area and pore distribution of biomass chars, which also illustrate the intrinsic rate of biomass gasification.

Some publications suggest that CO2 adsorption gives different results with N2 adsorption. Thus, CO2 adsorption tests have been performed to the same biomass chars, and the surface area are applied to develop this model.

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