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Energy & Fuels 2006, 20, 2202-2210
Diffusional Effects in CO2 Gasification Experiments with Single Biomass Char Particles. 1. Experimental Investigation A. Go´mez-Barea,* P. Ollero, and C. Ferna´ndez-Baco Chemical and EnVironmental Engineering Department, Escuela Superior de Ingenieros (UniVersity of SeVille), Camino de los Descubrimientos s/n, 41092 SeVille, Spain ReceiVed NoVember 8, 2005. ReVised Manuscript ReceiVed April 12, 2006
Kinetic studies were conducted to elucidate the effects of diffusional limitations at particle scale during thermogravimetry analysis (TGA) char reactivity tests. The main objective was to identify the limiting phenomena that may occur during biomass char gasification experiments with CO2. The char used was wood matter from pressed-oil stone (WPOS) (this is also called orujillo). The gasification rates of WPOS were measured at four different particle sizes (0.06, 0.9, 1.2, and 2.1 mm), three CO2 partial pressures (0.20, 0.35, and 0.50 bar), and four temperatures (800, 850, 900, and 950 °C). Literature on single-particle studies and diffusional effects that occur in char gasification kinetic experiments were critically surveyed. We found neither general criteria for a kinetic model to be selected nor a quantitative consensus about the impact of variables such as particle size on the diffusional interferences. Our own experiments indicate the extraordinary importance of the diffusional effects at high temperature and large particle size, i.e., a large Thiele modulus. The CO2 partial pressure was seen to have a secondary role, compared to temperature and particle size, within the range analyzed. The results presented in this study reveal that biomass-derived char particles inside industrial gasifiers may be limited by diffusional effects. Consequently, suitable kinetic-particle models that capture plausible physical limitations should be used when simulating such gasifiers. In the second part of this work, which is given in a companion paper [Energy Fuels 2006, 20, xxxx], a simple particle model is developed and validated with the experimental results reported in the present article.
1. Introduction Biomass gasification is a flexible process which produces a fuel gas that is suitable for feeding efficient gas engines and gas turbines, co-firing with coal in existing boilers, and the synthesis of biofuels. Large-scale applications are usually performed in fluidized systems, because fixed-bed design has strong up-scaling limitations over 1-2 MWth. As a biomass particle is fed into a fluidized-bed (FB) gasifier, the particle is heated and the moisture and volatile gases are driven off. The final stage is a heterogeneous gas-solid reaction between the char and steam and carbon dioxide, which are usually rate-limiting. This makes the gasification rate of char one of the most important pieces of information for evaluating the entire gasification process, because it determines the required volume of a gasifier. Therefore, knowledge of the char reactivity is a decisive factor in the design process and is a reason numerous studies have been performed on char gasification kinetics.1,2 If diffusional film and intraparticle mass- and heat-transfer processes in char particles are not rapid enough, the actual gasification rate differs from the intrinsic one evaluated in bulkgas conditions. Under usual operating conditions in biomass gasifiers (and often in coal gasifiers), the gasification rate usually lies in the transition between the chemically controlled and porediffusion-controlled regions. As a result, the overall gasification * Author to whom correspondence should be addressed. Tel.: +34 95 4487223. Fax: +34 95 4461775. E-mail address:
[email protected]. (1) Molina, A.; Mondrago´n, F. Reactivity of coal gasification with steam and CO2. Fuel 1998, 77 (15), 1831-1839. (2) Liliedahl, T.; Sjo¨stro¨m, K. Modelling of char-gas reaction kinetics. Fuel 1997, 76 (1), 29-37.
rate of a single char particle is determined by combining the intrinsic chemical reaction rate with intraparticle and external diffusional rates. Any char-particle kinetic model to be included as a submodel in a gasifier should be capable of capturing diffusional effects. Experimental data for CO2 and H2O gasification have been reported using thermogravimetric analysis (TGA), fixed bed, laminar flow (drop tube), entrained flow, and FB reactors. To simulate the high heating rates needed to meet the requirements of commercial equipment, some researchers have used droptube reactors. With this method, however, it is usually the average gasification rate that is measured and it is difficult to give the rate variation as the reaction proceeds. There are also problems such as uneven temperature distribution along the length of reactor, uncertainty in reaction temperature, and reaction time. Using a FB reactor to determine the char reactivity seems to overcome many of drawbacks that are associated with the drop-tube reactor; however, kinetic research in an FB is difficult, because of its complex fluid dynamics. This makes it difficult to separate the kinetic information from mass transfer and/or fluid-dynamic influence. In addition, the combustion of volatiles changes the temperature conditions of devolatilization and, hence, the final composition (and, therefore, reactivity). Reactivity determined in TGA may deviate from that observed in commercial equipment, in which fuel particles often undergo rapid heating during devolatilization. However, the study of char gasification under controlled conditions such as TGA can provide invaluable information for estimating diffusional effects in a char particle inside a gasifier. The study of diffusional effects is precisely the main purpose of this study. Therefore,
10.1021/ef050365a CCC: $33.50 © 2006 American Chemical Society Published on Web 07/12/2006
Diffusional Effects in CO2 Gasification. 1
the experiments presented here are performed in a TGA system. One should keep in mind that the slow heating rate of TGA makes it likely that char reactivity will be slower than the resulting char produced from a particle fuel fed to a gasifier, especially in a fluidized bed. This makes the reactivity value obtained in TGA conservative. For prediction of the real gas composition and char-consumption rate in a real gasifier, it is necessary to consider the two heterogeneous char-gasification reactions (with CO2 and H2O in atmospheric gasifiers, and with H2 in pressurized gasification), as well as the water-gas shift reaction. Nonetheless, the limitations caused by physical effects in a char particle are similar for all these heterogeneous reactions. The clarification of diffusional effects in the gasification of char particles is achieved in this work by studying, experimentally, the combined effects of particle size at different CO2 partial pressures and temperatures in single char particles during TGA reactivity tests. This makes it possible to identify the limiting phenomena that may occur during WPOS-char gasification experiments with CO2 experimentally. In a companion article, a simple particle model capable of reproducing the experiments in a simple manner is developed.35 This sheds light on the physical-chemical phenomena involved in a typical gasification test and explains the different rate-limiting phenomena under different experimental conditions. 2. Literature Survey 2.1. Single-Particle Studies. The literature on gasification kinetics is extensive. However, kinetic studies that involve single macrosized char particles are not so abundant. Ergun3 showed that particle effects were of minor importance during the fluidized-bed gasification of active carbon and graphite 0.081.8 mm in size. DeGroot and Shafizadeh4 gasified 0.4-0.85 mm particles of Douglas fir and cottonwood charcoal in CO2. They correlated their experiments with an overall expression in terms of the initial particle size, and a reaction order of 0.7 was observed. Groeneveld and van Swaaij5 investigated carbon particle profiles in particles with dimensions of 40 mm × 20 mm × 20 mm. Neither a shrinking core model (SCM) nor a uniform conversion model (UCM) predicted their experiments satisfactorily. A local volumetric model enabled them to explain conversion profiles found experimentally. The overall reaction order observed was 0.7. Standish and Tanjung6 analyzed the effects of temperature, CO2 gas composition, and particle size in 10-34-mm-sized wood charcoal gasification. They correlated their observations with an apparent nth-order kinetic expression that has a reaction order of 0.71. The initial particle size effect was also included in the expression raised to a power of -0.81. Although the resulting kinetics was apparent (and not intrinsic), they claimed that the correlation that was developed served for comparison with other charcoal gasification data, as long as similar rate control conditions applied. However, this is a considerable restriction, because most kinetic studies do not provide a formal analysis of diffusional effects.7 (3) Ergun, S. Kinetics of the reaction of carbon dioxide with carbon. Phys. Chem. 1956, 60, 480-485. (4) DeGroot, W. F.; Shafizadeh, F. Kinetics of gasification of Douglas Fir and Cottonwood chars by carbon dioxide. Fuel 1984, 63, 210-216. (5) Groeneveld, M.; van Swaaij, W. Gasification of Char Particles with CO2 and H2O. Chem. Eng. Sci. 1980, 35, 127-313. (6) Standish, N.; Tanjung, A. F. Gasification of single wood charcoal particles in CO2. Fuel 1988, 67, 666-672. (7) Go´mez-Barea, A.; Ollero, P.; Arjona, R. Reaction-diffusion model of TGA gasification experiments for estimating diffusional effects. Fuel 2005, 84, 1695-1704.
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The work by Standish and Tanjung was further analyzed by Dasappa et al.,8 who developed a nonisothermal computational model to explain some “open” aspects of the Standish and Tanjung work. They succeeded in explaining some of Standish and Tanjung’s findings. In addition, they computationally verified conclusions already given by Ergun.3 Hawley et al.9 analyzed intraparticle mass resistance with a simple catalytic model of 1-2 mm wood char particles. They concluded that, up to 5 mm, the particle size did not alter the resulting kinetic expression. van den Aarsen10 analyzed heat- and mass-transfer effects in wood and rice-husk particles. For 1-mm particles, he estimated Thiele moduli of
