A generalized mathematical model has been devised for use in analyzing the pyrolysis of arbitrarily specified (but typical) biomass feedstocks at atmospheric pressure. The model represents both the microparticle (kinetically controlled) and macroparticle (diffusion-limited) types of pyrolysis.

The microparticle portion of the model is based on a superposition of the multistep chemical-reaction kinetics of the primary constituents of biomass; namely, cellulose, hemicellulose, and lignin. The submodel for each primary constituent accounts for decomposition into tar, char, and gas, with secondary decomposition of tar (see Figure 1). The formation of char is represented as taking place via competitive primary reactions of the active feedstock. The macroparticle portion of the model is constructed by coupling the foregoing kinetics, along with appropriate heats of reaction and physical properties of constituents, with the porous-particle model described in the preceding article, "Mathematical Model of Pyrolysis of Biomass Particles" (NPO-20070).

Figure 1
Figure 1. This Generic Reaction Scheme represents the pyrolysis of each primary constituent (cellulose, hemicellulose, or lignin). As in the model of the preceding article, each reaction is assumed to be irreversible and of first order, with a rate Ki given by the Arrhenius equation.

The chemical-kinetics parameters in the model were obtained from a combination of previous mathematical-modeling studies and experimental data on the pyrolysis of representative feedstocks (cellulose, lignin, and beech and maple wood). Then the model with the exact same parameters was used to predict selected aspects of the pyrolysis of different feedstocks: The predictions agreed well with data from thermogravimetric-analysis (TGA) and isothermal experiments on the pyrolysis of untreated microparticle bagasse (see Figure 2) and cherry, oak, and pine wood. Considering that the proportions of the three primary constituents vary widely in these feedstocks, the results can be interpreted as signifying that the chemical-kinetics part of the model is unexpectedly robust.

Figure 1
Figure 2. The Normalized Mall-Loss Rate of untreated bagasse as a function of temperature under microparticle TGA conditions at a heating rate of 10 K/min as computed by the model is compared here with data from a previous TGA experiment.

Results obtained with the macroparticle model generally agreed with the experimental data. The small deviations were attributed to differences in particle geometry between simulations and experiment, catalytic effects of mineral matter not considered in the model, and differences in properties of various feedstocks.

This work was done by Josette Bellan and Richard S. Miller of Caltech forNASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.comunder the Physical Sciences category, or circle no. 166on the TSP Order card in this issue to receive a copy by mail ($5 charge).


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Generalized mathematical model of pyrolysis of biomass

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