I n d . Eng. Chem. Res. 1988,27, 1775-1783
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Received for review November 9, 1987 Accepted May 23, 1988
Transport Model with Radiative Heat Transfer for Rapid Cellulose Pyrolysis Lee J. Curtis and Dennis J. Miller* Department of Chemical Engineering, Michigan State University, East Lansing, Michigan 48824-1226
A mathematical model is presented which describes mass and energy transport during rapid pyrolysis of fibrous cellulose particles. Radiative heat transfer within porous cellulose is modeled by using the method of zones. The kinetic model for pyrolysis developed by Bradbury e t al. is extended to include secondary decomposition of condensible liquids (tars) formed. Solution of the governing equations shows that both mass- and heat-transfer resistances influence product composition from pyrolysis even for cellulose particles as small as 0.5 mm in diameter. Heating rate has little influence on product composition, but increasing the total pressure results in a decreased condensible product yield. Radiative heat transfer plays a minor role within the solid for the conditions simulated. The model is useful for identifying critical parameters and conditions in pyrolysis and for predicting trends in product yields.
I. Introduction Since biomass materials are abundant, inexpensive, and renewable resources, their conversion to synthetic fuels and chemical feedstocks appears attractive. For these end uses, rapid pyrolysis is a potential route since it requires less energy input than steam gasification, requires no additional reagents, and produces very little char. Pyrolysis does have one important drawback, however: it is difficult to selectively produce high yields of valuable products. Therefore, the focus of much present pyrolysis research is on understanding pyrolysis chemistry and physics, both via experimentation and modeling, so that desired product yields might be enhanced. The need for this work is clearly outlined in the Kona workshop on thermochemical biomass conversion (Antal, 1984). Major products from rapid pyrolysis of cellulose are gases and a condensible fraction containing anhydrosugars and organics. A number of reaction models have been developed from experimental data to describe product formation pathways (Shafizadeh, 1968; Madorsky, 1964; Mok and Antal, 1983a); kinetic constants have been determined for several reaction steps (Bradbury et al., 1979; Hajaligol et al., 1982; Broido, 1976). The influences of heat- and mass-transfer resistances have been experimentally investigated, but unfortunately most studies only give qualitative information. The primary effect of mass-transfer resistance appears to be a decrease in condensible tar yield, as secondary decomposition to gases becomes more important with increasing residence time in the reacting region. Mok and Antal (1983b) and Shafizadeh and Fu (1973) both report a decrease in tar yield as pressure is increased, and Hajaligol et al. (1982) report a similar decrease as sample thickness is increased. Others have shown similar trends (Scott and Piskorz, 1982; Shafizadeh et al., 1979). Heat-transfer resistances result both from endothermicity of the primary reactions and the low thermal conductivity of cellulose. The thermal resistance becomes *To whom all correspondence should be addressed.
even more pronounced as the high porosity char layer is formed during pyrolysis. Several models have been developed based on the assumption that heat transfer limits pyrolysis rate, and the models consider only heat-transfer resistances (Kung, 1972; Kansa et al., 1977). In extreme cases, a shrinking core mode of pyrolysis is assumed (Chan et al., 1985; Kanury, 1972). Models including both mass- and heat-transport effects have been developed primarily for predicting the rate of wood pyrolysis and combustion. Fan et al. (1978) have developed a general pyrolysis model which includes generation, reaction, and diffusion of the gas-phase components within the solid. However, convective flow and solid structure changes are not included in their model. Antal (1985) has set forth “general” transport equations for the pyrolysis of cellulose, and Kothari and Antal (1985) have presented numerical solutions to some simplified forms of the equations. In these model equations, however, two important phenomena were not explicitly included: the effects of secondary decomposition reactions on overall product yield, and the contribution of radiation to overall heat transfer within the cellulose particle. These phenomena may play an important role in determining overall pyrolysis behavior, especially at the high temperatures and heat fluxes encountered in flash pyrolysis (Kansa et al., 1977). Chan et al. (1985) included both secondary reactions and a diffusive radiation term as a correction to the thermal conductivity in their model for slow pyrolysis, but to date, no detailed treatment of radiation has been conducted for cellulose. This paper presents a model for flash pyrolysis which includes a semirigorous treatment of radiation within a “gray” cellulose solid and the secondary decomposition of pyrolysis tars. The objectives of developing the model are to investigate the importance of radiative energy transport within the porous solid and the effects of transport resistances on overall product yield. 11. Model Development Cellulose is modeled as a one-dimensional porous slab (half-thickness = L ) of randomly oriented fibers located
0888-5885/88/2627-17~5~0~.50/0 0 1988 American Chemical Society
1776 Ind. Eng. Chem. Res., Vol. 27, No. 10, 1988 Table I. Product Gas Composition component mole fraction
co
0.454 0.096 0.094
COZ
H2 H20 CH4
0.100
0.170 0.058 0.020 1.0
C2H4
CZH, total
between two identical radiating surfaces. During pyrolysis, the volatile products formed within the solid flow outward to the surfaces, while residual char remains as the solid skeleton. The spatial dimension of the solid is taken to be constant during pyrolysis; this is based on observations in our laboratory (Kim et al., 1987) and by Mok and Antal (1983b), in which it is reported that the char has structural integrity and retains the shape of the original sample. Thus, the solid merely becomes more porous as cellulose is consumed. Also, it is assumed that the vapor and solid phases are in thermal equilibrium at each point within the solid matrix. Volatile pyrolysis products are grouped into two components: (1)noncondensible gases of a fixed composition of CO, CO,, H20,HP,and small amounts of hydrocarbons, and (2) a condensible "tar" fraction. The gas composition is given in Table I; the tar fraction is assigned the properties of levoglucosan. This binary vapor phase was chosen to simplify calculations and to allow for secondary reaction of tars during transport out of the solid. 11.1. Radiation in Cellulose. 11.1.1. Diffuse Radiation. Radiation within porous solids has traditionally been treated as an addition to the conduction term, where the radiative flux is given in dimensionless form as
This treatment of radiation as a diffusive process is an accurate approximation (Hottel and Sarofim, 1967) only when the mean free photon path is much smaller than the particle dimension (Le., when radiation penetrates only a short distance in the solid). The optical thickness, 7 , which is a measure of the opaqueness of the solid, is defined as
Figure 1. Radiation to zone i from other zones j and from surfaces SI and Sp.
The method of zones, which is a discretized version of the rigorous equations describing radiation within a gray gas, is thus used to describe the radiative flux within cellulose. This technique, developed by Hottel and Sarofim (1967), treats the absorbing medium as a number of isothermal zones, each of which is interacting with all other zones. Although an approximation, the method is accurate when 7 C 0.4 for each zone, a condition which arises in the present simulation if each zone is designated as the region between adjacent difference points. Figure 1illustrates the basic idea of the method of zones. It is seen that any zone i interacts with other zones j and surfaces S1and Sz by both direct and reflected radiation. The direct view factors, defined as the fraction of one zone or surface directly seen by another zone or surface, are functions of absorption coefficient, position, and the overall sample dimension 2L: (1)zone i-zone j
g,gj = 2[53(70y) - E3(7,y) - M 7 g J (2) zone i-surface 1; zone i-surface 2
where K is the absorption coefficient (inverse centimeters). In this study, it is assumed that cellulose is a gray material composed of fibers of diameter df and having porosity 6 . If it is assumed that the fibers are opaque and the gas is nonabsorbing, then the fraction of radiation which passes through a thickness of one fiber diameter is equal to the porosity, t. The local absorption coefficient for cellulose is therefore estimated as (3)
In general, the diffusive approximation of radiative heat transfer is valid only when 7 > 3 for any solid region of interest (Hottel and Sarofim, 1967). 11.1.2. Method of Zones. In the cellulose simulated in this study, overall sample optical thickness ranges from 1.2 to 12. To solve the differential equations describing heat and mass transfer within the solid, 20-40 individual difference points must be used, resulting in 7
