Article pubs.acs.org/EF
Experimental and Modeling Study of the Effect of Torrefaction on the Rapid Devolatilization of Biomass Tian Li,*,† Liang Wang,‡ Xiaoke Ku,† Berta Matas Güell,‡ Terese Løvås,† and Christopher R. Shaddix§ †
Department of Energy and Process Engineering, Norwegian University of Science and Technology, Kolbjørn Hejes vei 1b, 7491 Trondheim, Norway ‡ SINTEF Energy Research, P.O. Box 4761 Sluppen, 7465 Trondheim, Norway § Combustion Research Facility, Sandia National Laboratories, Livermore, California 94550, United States ABSTRACT: In the present work, experimental and computational fluid dynamics (CFD) approaches were proposed and applied to assess rapid devolatilization behaviors of four types of biomass (forest residue, torrefied forest residue, Norwegian spruce, and torrefied Norwegian spruce). Biomass particles were subjected to devolatilization experiments at 1073 and 1473 K in a drop-tube reactor. Torrefaction was found to have consistent effects on the size reduction of studied biomass. In addition, similar behaviors of char fragmentation were observed for tested torrefied biomass after rapid devolatilization at 1473 K. Mass loss during devolatilization of biomass was highly dependent on heating condition. Both rates and extents of devolatilization of biomass were increased at elevated temperatures and heating rates. In comparison with raw feedstock, high char yields were realized with the torrefied biomass after devolatilization experiments. Evolution of elemental composition of studied biomass was found to be insensitive to tested conditions. However, organic composition of char was strongly affected by elemental composition of fuel, thus also influenced by torrefaction. CFD simulation showed that sizes of fuel particles had decisive effects on residence time of them in the reactor, especially particles with diameter larger than 355 μm. Particle temperature, in contrast, depended on both particle diameter and particle density. A modified two-competing-rates devolatilization model was also presented in the present work. On the basis of experimental data, one optimal set of kinetic parameters was obtained following a proposed procedure. The model predicted well the mass loss of all tested fuel and the evolution of each organic element in char at all operation conditions.
1. INTRODUCTION Gasification of biomass in high-temperature entrained flow reactors is a promising thermochemical conversion technology for utilizing biomass for chemical and fuel production. Hightemperature entrained flow gasification provides high carbon conversion with a high-quality tar-free syngas. However, biomass pretreatment is usually required because of some unwanted inherent properties of biomass, such as low energy density, heterogeneous nature, and hydrophilic property. Torrefaction is typically a mild pyrolysis in the temperature range of 473−573 K at atmospheric pressure in the absence of oxygen. Torrefaction upgrades biomass materials into high-quality solid fuels and has become widely used in recent years.1 Torrefied biomass has reduced moisture content, better grindability and storability, and improved hydrophobic behavior.2 However, owing to the changes of physicochemical properties of biomass during torrefaction, the behavior of torrefied biomass is different from its parent fuel in thermochemical conversion processes; thus, special attention needs to be given to understand its behavior in these environments. Devolatilization is normally the first step in the biomass gasification process. The devolatilization process is complicated and has decisive effects on overall fuel conversion efficiency and the gas/solid product distribution. Therefore, a better understanding of the biomass devolatilization process is crucial to achieving a more efficient gasification process.3 Devolatilization of biomass has been intensively investigated using thermogravimetric analyzers (TGAs) under well-controlled conditions.4,5 © XXXX American Chemical Society
Normally, milligrams of sample in a crucible or pan is heated up in the TGA according to a predetermined temperature program. The mass loss of tested sample is recorded, which is used for evaluation of the kinetic parameters of biomass devolatilization. The applicability of biomass devolatilization kinetics obtained by TGA at various heating rates has been discussed by Mehrabian et al.6 In addition, the single-particle/pellet reactor equipped with a balance has also been used to study the devolatilization of biomass at heating rate higher than that of the traditional TGA.7 The single-particle/pellet reactor and TGA are particularly useful for understanding thermal conversion of biomass in grates or in fluidized beds. However, intense devolatilization of biomass particles under high heating rates and short residence times (entrained flow gasification condition) cannot be achieved in a TGA or a single-particle/pellet reactor. An alternative approach to realize such conditions is to use a drop-tube reactor (DTR) or an entrained flow reactor (EFR). Nevertheless, the particle residence time and temperature are difficult to measure directly in such reactors. A lack of knowledge of the detailed thermal history of fuel particles makes a precise determination of devolatilization kinetics challenging. Li et al. studied rapid devolatilization of raw and torrefied palm kernel shell in a DTR operating between 773 and 1473 K.8 It was found that the devolatilization rate of palm kernel shell decreased as a result of torrefaction. Received: February 12, 2015 Revised: June 1, 2015
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DOI: 10.1021/acs.energyfuels.5b00348 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
2. EXPERIMENTAL SECTION
In addition, torrefied biomass displayed higher activation energies of devolatilization reactions. Xiu et al. investigated the devolatilization characteristics of several biomass materials using a plasma-heated laminar EFR operating between 750 and 900 K.9 Conversion of biomass fuel particles was determined by the ash tracer method and a single first-order reaction (SFOR) model was used to investigate the devolatilization process. For the studies briefly reviewed above, particle temperature was assumed to be constant during the devolatilization process, equal to the reactor wall temperature. In addition, residence time was assumed to be the same for all fuel particles fed into a reactor and was calculated on the basis of the velocity of the particle entrainment gas. These assumptions may lead to considerable errors when deriving biomass devolatilization kinetics. Computational fluid dynamics (CFD) analysis, taking account of fluid mechanics, heat mass transfer, solid−fluid interaction, etc., can provide temperature and residence time of fuel particle in a DTR with minimal inputs from experiments. Moreover, CFD simulation is relatively easy to scale up and therefore has the potential to be adapted to full-scale industrial devices with low cost and short turnaround time. As a result of limited experimental data, there are only few studies dedicated to the investigation of devolatilization of biomass in DTRs with CFD analysis. Sun et al. examined the flash pyrolysis of rice husk and sawdust over 973−1273 K via Ansys Fluent CFD software.10 The gas yield at the exit of the reactor was well-predicted by the CFD model (deviations less than 7%). However, because of the limitation of the simplified SFOR, it was concluded that the accurate predictions versus the residence time were impossible. Simone et al. systematically evaluated the biomass devolatilization kinetics with CFD-aided experiments.11 The Ansys CFX CFD software was used in this study, with an Eulerian/ Lagrangian approach. A subroutine was used to search optimal kinetic parameters of an SFOR model for biomass devolatilization. Some important observations were noted, such as the importance of the particle size distribution, the limited utility of complicated swelling or shrinking models, and that the SFOR model was insufficient for predictions of biomass devolatilization over an extended temperature range. It is worth noting that the DTR was operated at the laminar flow regime at relatively low temperatures of 673−1073 K, which may differ significantly from the typical industrial EFR. Clearly, more efforts are needed on time-resolved rapid devolatilization of biomass, especially torrefied biomass, with CFD-aided DTR study. The objective of this work is to investigate the devolatilization of raw and torrefied lignocellulosic biomass under hightemperature and high heating rate conditions with CFD-assisted experiment and, in combination with our previous study on conversion behavior of biomass char,12 to reveal the effect of torrefaction on thermochemical conversion of biomass under industrial related conditions. Both raw and torrefied biomass were devolatilized in an electrically heated DTR. The char residue at various residence times were extracted and characterized. Furthermore, a new procedure to determine biomass devolatilization kinetics was developed. Taking advantage of CFD simulation, the thermal histories of biomass particles during devolatilization were estimated and used to find kinetic parameters of a modified two-competing-rates devolatilization model. Furthermore, the modeled mass loss as well as the conversion of individual organic elements was compared with measured data.
2.1. Fuel and Characterization Method. Four types of biomass were tested in this study: forest residues (FR), Norwegian spruce (NS), torrefied forest residues (TFR), and torrefied Norwegian spruce (TNS). NS is a relatively clean and pure woody biomass in comparison to FR, which is a mixture of the tops and branches of trees (including needles). All four types of biomass were ground and compressed to 6 mm pellets without additional binders. The torrefied biomass pellets were produced by feeding the pellets into a torrefaction reactor consisting of four horizontal, electrically heated screw conveyors arranged in series. In the torrefaction process, the pellets were preheated at 498 K for 5 min and then torrefied at 548 K for 30 min.13 The mass yields for TNS and TFR are 83 and 94%, respectively. All raw and torrefied biomass pellets were ground and sieved to yield particles with sizes in the range of 212−300 μm. A summary of fuel properties is listed in Table 1.
Table 1. Properties of Biomass Fuel FR
TFR
NS
TNS
Proximate Analysis (as Received, wt %) moisture 6.3 4.2 5.0 3.8 volatiles 70.0 61.6 77.1 72.3 fixed carbon 21.5 31.5 17.5 23.4 ash 2.2 2.7 0.4 0.5 volatiles (dry, ash-free) 76.5 66.2 81.5 75.6 Ultimate Analysis (Dry, Ash-Free, wt %) C 52.1 59.5 46.7 52.8 H 6.1 5.6 6.2 5.8 N 0.5 0.6 0.1 0.1 S
