Article pubs.acs.org/EF
A Study of Particle Size Effect on Biomass Torrefaction and Densification J. H. Peng,† H. T. Bi,†,* S. Sokhansanj,†,‡ and J. C. Lim† †
Clean Energy Research Centre and Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia, Canada ‡ Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States ABSTRACT: The particle size effect on torrefaction of pine particles and formation of torrefied pellets has been studied in a thermogravimetric analyzer (TGA) and a tubular fixed bed reactor. The fixed bed reactor was also used to produce torrefied samples for a single-die press unit for the densification test. Both the TGA and fixed bed reactor torrefaction test results showed that the torrefaction rate was affected by the particle size, especially at high temperatures. Although the temperature gradient inside particles smaller than 1 mm is very small during torrefaction, the internal diffusion of generated vapors inside particles imposes an impact on the global torrefaction reaction rate. The hard core or nonshrinkage particle model with a first order torrefaction reaction can predict the reaction data reasonably well, with the data-fitted effective vapor diffusivity coefficient. The densification tests showed that more energy was required to make pellets from larger torrefied particles, while the water uptake and Meyer hardness tests of pellets revealed that good quality torrefied pellets could be obtained from fine torrefied sawdust particles.
1. INTRODUCTION Wood torrefaction is a thermal treatment of wood under atmospheric conditions without air or oxygen in the temperature range 473−573 K.1 The target of torrefaction is to increase the energy density of wood on a mass basis. A number of papers has been published on the torrefaction for the different final uses, such as co-firing in existing coal-fired power stations,2 gasification,3,4 syn-gas production,5 and as barbeque fuel.6 The recent rapid growth of wood pellet industry is mainly driven by the incentive programs in Europe to reduce greenhouse gas emissions and the rising oil and natural gas prices. Bergman proposed a combined torrefaction and pelletization process for the production of the high energy density torrefied wood pellets.1 Torrefied pellets are suitable for long-distance transportation, especially for pellets produced in British Columbia (BC) of Canada and exported to Europe or Asia.7 We previously developed a two-component one-step kinetic model for the torrefaction of BC softwoods to establish a relationship among weight loss, torrefaction temperature, and residence time,8 then reported the effect of temperature and residence time on torrefaction and densification of different wood species.9 Table 1 summarizes some bench-scale test results of torrefaction of wood with different operating conditions reported in the open literature. Most previous torrefaction studies were conducted in fixed bed reactors. Besides the reaction temperature, reaction residence time, and wood species, particle size is also an important design parameter for torrefaction and densification, which was not reported in some studies. Also, Table 1 shows that there are inconsistent results from different research groups when one compares the data obtained at same operating conditions, such as 523 K for 30 and 60 min, respectively. This can be partly attributed to the difference in the reactor configurations and heating rate, but © 2012 American Chemical Society
particle size may also play important role. As Prins et al. (2006) postulated, for pyrolysis of wood below 573 K, the reactions were the rate-limiting step for particles smaller than 2 mm where the impact of intraparticle heat and mass transfer becomes insignificant.10 In this study, both a TG and a fixed bed reactor have been used to study particle size effect on torrefaction. A press machine was used to make torrefied pellets for the evaluation of the quality of the torrefied pellet based on density, energy yield, durability, and equilibrium water uptake rate.
2. EXPERIMENTAL SECTION 2.1. Samples. Pine chips from Fiberco and FPInnovations have been used as the test material in this study. For the preliminary study on particle size effect in a TGA, air-dried pine chips from Fiberco were crushed in a Model GP-140 disk grinder (Modern Process Equipment Inc., Chicago, IL) and then separated to three size groups in a Gilson Test-Master sieving device (Gilson Company Inc., Lewis Center, Ohio). Figure 1 shows the three pine sawdust samples prepared from Fiberco pine chips. A THELCO laboratory PRECISION oven (Thermo Electron Corporation) was used to measure the moisture content of the sawdust samples made from the FPInnovations pine chips. A 25 mL glass cylinder was used to determine the bulk density. A multipycnometer (Quantachrome Instruments, U.S.A.) was used to measure the particle density. Table 2 shows the properties of Fiberco pine samples. Pine chips from FPInnovations were prepared by drying in the PRECISION oven at 378 K for 24 h and crushing in a hammer mill (Glenmills Inc., U.S.A.; Model 10HMBL) Received: March 5, 2012 Revised: April 26, 2012 Published: April 27, 2012 3826
dx.doi.org/10.1021/ef3004027 | Energy Fuels 2012, 26, 3826−3839
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pine logging residue chips (20.9−70.6 mm long, 1.9−4.9 mm thick, 15.1−39.7 mm wide)
Bridgeman et al., 200813
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