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Title: Low-Order Modeling of Internal Heat Transfer in Biomass Particle Pyrolysis

Abstract

We present a computationally efficient, one-dimensional simulation methodology for biomass particle heating under conditions typical of fast pyrolysis. Our methodology is based on identifying the rate limiting geometric and structural factors for conductive heat transport in biomass particle models with realistic morphology to develop low-order approximations that behave appropriately. Comparisons of transient temperature trends predicted by our one-dimensional method with three-dimensional simulations of woody biomass particles reveal good agreement, if the appropriate equivalent spherical diameter and bulk thermal properties are used. We conclude that, for particle sizes and heating regimes typical of fast pyrolysis, it is possible to simulate biomass particle heating with reasonable accuracy and minimal computational overhead, even when variable size, aspherical shape, anisotropic conductivity, and complex, species-specific internal pore geometry are incorporated.

Authors:
; ;
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org.:
USDOE Office of Energy Efficiency and Renewable Energy (EERE), Bioenergy Technologies Office (EE-3B)
OSTI Identifier:
1270780
Report Number(s):
NREL/JA-2700-66823
Journal ID: ISSN 0887-0624
DOE Contract Number:
AC36-08GO28308
Resource Type:
Journal Article
Resource Relation:
Journal Name: Energy and Fuels; Journal Volume: 30; Journal Issue: 6
Country of Publication:
United States
Language:
English
Subject:
09 BIOMASS FUELS; fast pyrolysis; simulation; biomass

Citation Formats

Wiggins, Gavin M., Ciesielski, Peter N., and Daw, C. Stuart. Low-Order Modeling of Internal Heat Transfer in Biomass Particle Pyrolysis. United States: N. p., 2016. Web. doi:10.1021/acs.energyfuels.6b00554.
Wiggins, Gavin M., Ciesielski, Peter N., & Daw, C. Stuart. Low-Order Modeling of Internal Heat Transfer in Biomass Particle Pyrolysis. United States. doi:10.1021/acs.energyfuels.6b00554.
Wiggins, Gavin M., Ciesielski, Peter N., and Daw, C. Stuart. 2016. "Low-Order Modeling of Internal Heat Transfer in Biomass Particle Pyrolysis". United States. doi:10.1021/acs.energyfuels.6b00554.
@article{osti_1270780,
title = {Low-Order Modeling of Internal Heat Transfer in Biomass Particle Pyrolysis},
author = {Wiggins, Gavin M. and Ciesielski, Peter N. and Daw, C. Stuart},
abstractNote = {We present a computationally efficient, one-dimensional simulation methodology for biomass particle heating under conditions typical of fast pyrolysis. Our methodology is based on identifying the rate limiting geometric and structural factors for conductive heat transport in biomass particle models with realistic morphology to develop low-order approximations that behave appropriately. Comparisons of transient temperature trends predicted by our one-dimensional method with three-dimensional simulations of woody biomass particles reveal good agreement, if the appropriate equivalent spherical diameter and bulk thermal properties are used. We conclude that, for particle sizes and heating regimes typical of fast pyrolysis, it is possible to simulate biomass particle heating with reasonable accuracy and minimal computational overhead, even when variable size, aspherical shape, anisotropic conductivity, and complex, species-specific internal pore geometry are incorporated.},
doi = {10.1021/acs.energyfuels.6b00554},
journal = {Energy and Fuels},
number = 6,
volume = 30,
place = {United States},
year = 2016,
month = 6
}
  • We present a computationally efficient, one-dimensional simulation methodology for biomass particle heating under conditions typical of fast pyrolysis. Our methodology is based on identifying the rate limiting geometric and structural factors for conductive heat transport in biomass particle models with realistic morphology to develop low-order approximations that behave appropriately. Comparisons of transient temperature trends predicted by our one-dimensional method with three-dimensional simulations of woody biomass particles reveal good agreement, if the appropriate equivalent spherical diameter and bulk thermal properties are used. Here, we conclude that, for particle sizes and heating regimes typical of fast pyrolysis, it is possible to simulatemore » biomass particle heating with reasonable accuracy and minimal computational overhead, even when variable size, aspherical shape, anisotropic conductivity, and complex, species-specific internal pore geometry are incorporated.« less
  • Using various assumptions as described, it is seen that, for example, a particle dimension of 0.01 cm (thickness = 0.02 cm or 200 microns) might be investigated kinetically up to about 450-500/sup 0/C without taking transport limitations due to internal particle profiles into account. If external heat transfer to the particle surface is also slow, then even lower temperature limits would occur. In many cases, Equation 13 can be used to estimate the maximum particle size for various values of the dimensionless parameters. It should also be noted that the characteristic dimension for a bed of fine particles would bemore » bed depth rather than particle size.« less
  • Smoke particle emissions from the combustion of biomass fuels typical for the western and southeastern United States were studied and compared under high humidity and ambient conditions in the laboratory. The fuels used are Montana ponderosa pine (Pinus ponderosa), southern California chamise (Adenostoma fasciculatum), and Florida saw palmetto (Serenoa repens). Information on the non-refractory chemical composition of biomass burning aerosol from each fuel was obtained with an aerosol mass spectrometer and through estimation of the black carbon concentration from light absorption measurements at 870 nm. Changes in the optical and physical particle properties under high humidity conditions were observed formore » hygroscopic smoke particles containing substantial inorganic mass fractions that were emitted from combustion of chamise and palmetto fuels. Light scattering cross sections increased under high humidity for these particles, consistent with the hygroscopic growth measured for 100 nm particles in HTDMA measurements. Photoacoustic measurements of aerosol light absorption coefficients reveal a 20% reduction with increasing relative humidity, contrary to the expectation of light absorption enhancement by the liquid coating taken up by hygroscopic particles. This reduction is hypothesized to arise from two mechanisms: 1. Shielding of inner monomers after particle consolidation or collapse with water uptake; 2. The contribution of mass transfer through evaporation and condensation at high relative humidity to the usual heat transfer pathway for energy release by laser heated particles in the photoacoustic measurement of aerosol light absorption. The mass transfer contribution is used to evaluate the fraction of aerosol surface covered with liquid water solution as a function of RH.« less
  • Here, direct numerical simulation of convective heat transfer from hot gas to isolated biomass particle models with realistic morphology and explicit microstructure was performed over a range of conditions with laminar flow of hot gas (500 degrees C). Steady-state results demonstrated that convective interfacial heat transfer is dependent on the wood species. The computed heat transfer coefficients were shown to vary between the pine and aspen models by nearly 20%. These differences are attributed to the species-specific variations in the exterior surface morphology of the biomass particles. We also quantify variations in heat transfer experienced by the particle when positionedmore » in different orientations with respect to the direction of fluid flow. These results are compared to previously reported heat transfer coefficient correlations in the range of 0.1 < Pr < 1.5 and 10 < Re < 500. Comparison of these simulation results to correlations commonly used in the literature (Gunn, Ranz-Marshall, and Bird-Stewart-Lightfoot) shows that the Ranz-Marshall (sphere) correlation gave the closest h values to our steady-state simulations for both wood species, though no existing correlation was within 20% of both species at all conditions studied. In general, this work exemplifies the fact that all biomass feedstocks are not created equal, and that their species-specific characteristics must be appreciated in order to facilitate accurate simulations of conversion processes.« less
  • A simplified heat transfer model is analyzed in order to estimate an upper bound for biomass particle size in conducting experimental pyrolysis kinetics. In determining intrinsic kinetic rates, it is desirable that the entire particle be at reactor temperature for the duration of the chemical reaction. By comparing characteristic times for reaction rates versus heat up rates, an approximate boundary for particle size can be constructed as a function of temperature; above this boundary, the reaction rate is strongly heat transfer dominated, and below the boundary the reaction rate is kinetically controlled. Using parameters for cellulor pyrolysis, it is estimatedmore » that a 100 ..mu.. particle will be heat transfer limited due to internal heat transfer at temperatures above 500/sup 0/C.« less