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Ind. Eng. Chem. Res. 1996, 35, 1711-1721
1711
Thermal Lag, Fusion, and the Compensation Effect during Biomass Pyrolysis† Ravi Narayan and Michael Jerry Antal, Jr.* Hawaii Natural Energy Institute and Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, Hawaii 96822
Results from a numerical model for endothermic biomass pyrolysis, which includes both high activation energy kinetics and heat transfer across a boundary layer to the reacting solid particle, are presented. The model accounts for conventional thermocouple thermal lag and unconventional thermal lag due to heat demand by the chemical reaction (which is governed by Arrhenius kinetics). Biomass fusion, first identified quantitatively by Le´de´ and Villermaux, is shown to be a manifestation of severe thermal lag that results from the chemical reaction heat demand. Over the wide range of conditions studied, the true substrate temperature remains almost constant during pyrolysis, as is the case with compounds undergoing fusion or sublimation at constant pressure. A simple algebraic model, whose derivation presupposes the idea that biomass pyrolysis mimics the melting of a block of ice, accurately predicts the maximum value of thermal lag during pyrolysis. Unidentified thermal lag in TGA experiments lowers the values of the apparent activation energy and frequency factor associated with the experimental data but approximately retains the true value of their ratio. Thus, the widely varying values of kinetic parameters for cellulose pyrolysis reported in the literature may be a result of differing thermal lag characteristics of the experiments. Introduction Attendees of the 1980 Specialists Workshop on Fast Pyrolysis of Biomass in the scenic resort town of Copper Mountain, CO, witnessed an astounding demonstration. Jim Diebold and Tom Reed of the Solar Energy Research Institute replaced the blade of a coping saw with an electrical resistance wire that glowed red when they turned the power on. Diebold proceeded to use the refurbished saw to rapidly cut intricate patterns in strips of wood, as though he were using a hot knife to slice a cold stick of butter (Diebold, 1980). Many of the attendees were intimately familiar with the earlier work of Berkowitz-Mattuck and Noguchi (1963), Martin (1964), Lincoln (1965), and Lewellen et al. (1976), who first predicted and then demonstrated the complete pyrolytic vaporization of cellulose by intense heat fluxes; nevertheless, none present at the demonstration imagined that wood could be sliced by a hot wire so easily. Diebold went on to design a vortex reactor for the flash pyrolysis of biomass, which became a centerpiece of the U.S. Department of Energy program to convert biomass into useful liquid fuels (Diebold and Scahill, 1985). Another attendee of the meeting, Dr. Jacques Le´de´, returned to France, where he, Professor Jacques Villermaux, and their colleagues initiated an elegant study of the melting rates of rods of ice, paraffin, and polyamide 11 pressed against a spinning hot disk. They compared these data with measurements of the rate of pyrolysis of a rod of wood pressed against the same hot spinning disk. As described in a series of brilliant papers (Villermaux and Antoine, 1980; Le´de´ et al., 1985; Martin et al., 1986; Villermaux et al., 1986; Le´de´ et al., 1987, 1988), Le´de´ and his co-workers found that the wood rod behaved both experimentally and theoretically as though it melted at a temperature of 466 °C. In spite of the many advances (see Antal and Varhegyi, 1995) which have occurred in the field of biomass pyrolysis † We dedicate this paper to our colleague Dr. Jacques Le ´ de´ of the Laboratoire des Sciences du Ge´nie Chimique, CNRSENSIC, Nancy, France.
since Le´de´ and Villermaux reported the results of their studies, the mystery remains: “Why should the flash pyrolysis of a rod of wood resemble the melting of a block of ice?” The chief difference between the pyrolysis of biomass and the thermal degradation of familiar solid fossil fuels (e.g., coals and peat) is the dependence of the biomass char yield on experimental conditions. Virtually no char is formed when the pyrolytic vapors are rapidly removed from the vicinity of the unreacted substrate (as in the experiment conducted by Le´de´ and his colleagues); whereas char yields as high as 47.5% are observed when the vapors are held captive in the presence of the reacting solid at elevated pressures (Mok et al., 1992). This extraordinary range of biomass char yields is an outcome of the fact that heterogeneous, vapor-solid secondary reactions are effectively the only source of char born during biomass pyrolysis (Antal and Varhegyi, 1995). Because the char-forming reactions are exothermic and the vaporization reaction is endothermic, a linear relationship exists between the pyrolytic heat of reaction and the observed char yield (Mok and Antal, 1983; Mok et al., 1992). In the case of cellulose (the principal component of most biomass materials), the char-forming reactions liberate 600 J/g when a char yield of 40% is realized, whereas the vaporization reaction consumes from 240 J/g at atmospheric pressure to 335 J/g under vacuum while realizing a 10% yield of char (Mok and Antal, 1983). This variability in the heat of reaction confounded early workers in the fire and flammability community, who were concerned with the slow pyrolysis of macroscopic biomass samples. For example, Kung (1972) refers to “the rather elusive value of the endothermicity”. Bamford et al. (1945), Akita (1959), Roberts and Clough (1963), Tinney (1965), and Roberts (1970) all employed exothermic values of the heat of reaction, whereas Tang and Neill (1964), Kung (1972), Kung and Kalelkar (1973), Lee et al. (1976), Kansa et al. (1977), and Bennini et al. (1991) reported endotherms. More recent workers (Chan et al., 1985, 1988; Pyle and Zaror,
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1712 Ind. Eng. Chem. Res., Vol. 35, No. 5, 1996
1984; Capart et al., 1988; Alves and Figueiredo, 1989) utilized variable or multiple values for the heat of reaction according to the situation. Researchers concerned with the flash pyrolysis of microscopic biomass samples, where the char yield is low and the vapors quickly escape the solid matrix, evolved a somewhat different perspective on the pyrolytic heat of reaction. Following workers in the coal community (Russel et al.,, 1979; Sprouse, 1980; Misra and Essenhigh, 1988; Hajaligol et al., 1988), who assumed the heat of reaction term to be negligible, some biomass researchers (Van der Kaaden et al., 1986; Funazukuri et al., 1986) overlooked its contribution to heat demand at high heating rates. But careful experimentalists (Bradbury et al., 1979; Antal et al., 1980; Simmons and Gentry, 1986; Bilbao et al., 1987a,b, 1992; Radlein et al., 1991) repeatedly observed enhanced thermal lag during rapid pyrolysis, which is a signature of the endotherm. Those modelers (Kothari and Antal, 1985; Villermaux et al., 1986; Simmons and Gentry, 1986; Curtis and Miller, 1988; Bilbao et al., 1987b, 1989a,b, 1992) who included the endotherm in their flash pyrolysis simulations always noted its impact on temperature profiles. For example, Curtis and Miller (1988) observed “Because the overall pyrolysis is highly endothermic, the temperatures within cellulose remain relatively low ...”. Surprisingly, insight into the importance of the endotherm was not limited to modelers of flash pyrolysis. In his numerical study of the pyrolysis of wood slabs, Kung (1972) noted that the effects of the endothermic pyrolysis reaction were “evidently an important matter”. The underlying reason behind the important effects of the endotherm is intuitively obvious: when flash pyrolysis occurs, the reaction rate is high; consequently, the heat demand associated with the endothermic pyrolytic reaction is also large. As the pyrolysis temperature increases, the heat demand increases dramatically according to Arrhenius kinetics. For example, at a heating rate of 80 °C/min the rate of cellulose pyrolysis reaches a maximum value at about 388 °C (Antal and Varhegyi, 1995). Using the accepted value (Varhegyi et al., 1994; Antal and Varhegyi, 1995) for the activation energy of cellulose pyrolysis (238 kJ/mol), the rate of heat demand by the endotherm increases by a factor of 100 at the “fusion” temperature (466 °C) reported by Le´de´ and his colleagues. Clearly, heat transfer problems across the boundary layer into the reacting solid substrate become acute at high heating rates. Such problems were noted a decade ago by Kothari and Antal (1985), who studied cellulose pyrolysis using concentrated solar energy, and have been a focus of recent work in Le´de´’s laboratory (Le´de´ et al., 1992a,b; Le´de´ and Villermaux, 1993; Le´de´, 1994, 1995). One objective of this paper is to show that the fusion phenomenon observed by Le´de´ and his colleagues is an artifact of the severe thermal lag which occurs during the rapid, endothermic pyrolysis of biomass materials. To achieve this objective, we employ relatively simple numerical models to simulate weight loss and heat demand. We show that the maximum thermal lag predicted by these simulations can be well represented by an algebraic equation, whose derivation presumes pyrolysis to be a phase change phenomenon. The spirit of this inquiry is similar to the derivation of the Thiele modulus: no attempt is made to include all the intricacies of the problem in the numerical models. Our purpose is to provide a simple algebraic method for estimating the magnitude of the thermal lag and to show how severe
thermal lag assumes the guise of a phase change phenomenon. A second objective of this work is to show how thermal lag affects experimental measurements of the kinetic rate constants. The compensation effect is simply an outcome of undetected thermal lag. Thermal Lag Model Because thermogravimetry is usually employed to study biomass pyrolysis kinetics and modern instruments are designed to minimize thermal lag, this paper uses the conditions of thermogravimetry as a setting to illustrate the subtle effects of thermal lag on kinetic measurements. State-of-the-art thermogravimetry constantly monitors the weight of a very small (
