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. 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 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.com under the Physical Sciences category, or circle no. 166on the TSP Order card in this issue to receive a copy by mail ($5 charge).

NPO-20068

Topics:
Mathematical models Physical Sciences Simulation and modeling
This Brief includes a Technical Support Package (TSP).
Document cover
Generalized mathematical model of pyrolysis of biomass

(reference NPO20068) is currently available for download from the TSP library.

Don't have an account?

Magazine cover
NASA Tech Briefs Magazine

This article first appeared in the February, 1998 issue of NASA Tech Briefs Magazine (Vol. 22 No. 2).

Read more articles from the archives here.

Overview

The document presents a generalized mathematical model for the pyrolysis of plant biomass, developed by researchers Josette Bellan and Richard S. Miller at NASA. Pyrolysis is a thermal decomposition process that occurs in the absence of oxygen, aimed at converting biomass or organic waste into valuable products such as biofuels, gases, and char. The model addresses the complexities of biomass pyrolysis, focusing on both micro-particle and macro-particle sizes, which are crucial for predicting commercially relevant pyrolysis processes.

The introduction highlights the shift in research focus from low-temperature pyrolysis, which maximizes char yields for charcoal production, to high-temperature pyrolysis that aims to increase tar and gas yields while minimizing char formation. This transition is driven by the demand for cleaner fuels and other applications, such as adhesives and resins. The document emphasizes the need for accurate mathematical models to aid in the design of scalable and efficient biomass conversion reactors.

Key aspects of the model include the kinetics of micro-particles, which must accurately predict temperature evolutions, bulk product groups, quantitative yields, and variations in yield based on temperature and feedstock type. The model incorporates a superimposed approach to cellulose, hemicellulose, and lignin, allowing for a more robust prediction of yield variations with temperature. This is essential for understanding the mechanisms of char production during pyrolysis.

The document is structured into several sections. Section 2 discusses the motivations behind the current work, while Section 3 presents the micro-particle kinetics and compares them with past experimental data on sub-millimeter biomass particle pyrolysis. Section 4 introduces the macro-particle model and its experimental comparisons, highlighting the differences in behavior between micro and macro particles. Finally, Section 5 concludes with discussions on the implications of the findings and potential future research directions.

Overall, this document serves as a comprehensive resource for understanding the mathematical modeling of biomass pyrolysis, providing insights into the kinetics involved and the factors influencing product yields. It underscores the importance of developing accurate models to optimize biomass conversion processes for sustainable energy production.