Experimental investigation of intraparticle secondary reactions of tar during wood pyrolysis
Highlights
► This study can clarify intraparticle secondary reactions of tar. ► Intraparticle reactions occur during the pyrolysis of large wood particle. ► Polymerization of tar (intraparticle reactions) progresses at approximately 380 °C. ► Intraparticle tar decomposition to secondary gas progresses at approximately 400 °C. ► Char works as a catalyst and lowers the reaction temperature of tar cracking.
Introduction
Biomass, especially wood, is expected to be developed as a renewable energy resource to deal with global warming and the depletion of fossil fuels. Biomass is a carbon neutral fuel and hence it differs from fossil fuels [1], [2]; moreover, it may offer additional advantages of low sulfur and nitrogen content [2], [3]. Biomass can be converted to energy forms through various processes, such as thermochemical conversion and biochemical conversion. Gasification and pyrolysis are the most promising thermochemical conversion processes [4]. The energy efficiency of gasification is higher than that of combustion [2], [5]. The primary reaction of gasification is pyrolysis, which produces three main products: gas, char, and tar. However, tar is an undesirable product that causes many operational difficulties in biomass gasifiers. Tar formation and condensation in a gasification system lead to blockage and foul of process equipment such as engines and turbines [2], [6], [7]. Therefore, many researchers have been developing technologies for controlling or suppressing the tar formation during biomass pyrolysis [8], [9], [10].
When biomass is pyrolyzed, volatile matter, which includes several tar and gas species, is released by thermal scission of chemical bonds; char is also formed. These reactions are considered as the primary reactions of pyrolysis. Next, a portion of the tar undergoes secondary reactions consisting of its decomposition to secondary gas and secondary tar, and polymerization to secondary char. Secondary tar decomposition may occur homogeneously in the vapor phase and heterogeneously on the surfaces of the pyrolyzing solid or other beds. Moreover, secondary tar decomposition can also occur within (intraparticle) or outside (extraparticle) the biomass particles [11], [12]. Extraparticle tar decomposition reactions include homogeneous vapor phase cracking and heterogeneous conversion over the surface of the char or other beds. Boroson et al. [13] studied the homogeneous vapor phase cracking of wood pyrolysis tar using a two-stage reactor and reported 5–88% tar conversions at temperatures of 773–1073 K with residence times of 0.9–2.2 s.
The biomass feedstock used in commercial gasifiers has a relatively large particle size such as that of woodchips and wood pellets, because biomass materials unlike pulverized coal cannot be economically processed into fine sizes. Wood pellets measure 6–8 mm in diameter according to Swedish standards [14]. During the pyrolysis of large biomass particles, it was frequently mentioned that intraparticle secondary reactions, including intraparticle tar decomposition, are significant and affect product yields [11], [15]. Large biomass particles may provide sufficient residence time for tar cracking reactions inside the particles. Thus, tar may be cracked during transport in the pores of large biomass particles or the char layer. Many researchers have reported positive effects of biomass char on the tar cracking reaction in a two-stage reactor [1], [3], [16], [17], [18], [19]. However, there is little information and evidence regarding intraparticle secondary reactions of tar. We recently investigated intraparticle tar decomposition experimentally and numerically [20]. A calculation in our previous research indicated the possibility of intraparticle tar decomposition; however, this calculation did not consider heterogeneous reactions because their kinetic parameters were not available. In addition, the gas species produced through intraparticle tar decomposition have not been clarified. Further experimental studies should be conducted to investigate the intraparticle secondary reactions of tar.
In the pyrolysis of fine biomass particles, these reactions are negligible because of the short residence time of the traveling tar inside the particle. A simulation conducted by Di Blasi [11] showed that in small particles (less than 6 mm), intraparticle degradation of tar is negligible. Bryden and Hagge [21] reported that char does not provide additional holding time within the particles for secondary pyrolysis reactions in thermally thin particles (Biot number < 0.2). The characteristics of intraparticle secondary reactions of tar can be clarified according to the differences in such reactions between fine and large biomass particles by comparing the yields of pyrolysis products between fine and large particles.
The purpose of this study is to clarify intraparticle secondary reactions of tar experimentally by comparing the yields of pyrolysis products between the sawdust and wood cylinder, which represent fine and large biomass particles, respectively. Pyrolysis characteristics of both the samples under our experimental conditions were investigated by examining the temperature distribution of biomass particles, ultimate analysis of chars, and the weight fraction histories. Next, we discussed the effects of intraparticle secondary reactions of tar on the product yields and the reaction temperature.
Section snippets
Sample preparation
The samples used in this study were Japanese cypress sawdust and Japanese cypress wood cylinders. The properties of Japanese cypress wood (trunk) are shown in Table 1. The wood cylinder was milled and then sieved to produce sawdust with particles less than 1 mm in size. The initial weight of the sawdust for each experiment was approximately 50 mg. The wood cylinder had a diameter of 8 mm, length of 9 mm, and initial weight of approximately 180 mg.
Pyrolysis experiments
Fig. 1 shows a schematic diagram of the experimental
Pyrolysis characteristics
Fig. 4 shows the temperature histories measured at three locations within the wood cylinder during pyrolysis at a heating rate of 0.5 K/s. The temperatures at these three locations in the radial direction did not differ during pyrolysis. The thermal conductivity of wood across the grain (radial direction) is approximately one-third of that along the grain (axial direction) [23]. Thus, the temperatures along the axial direction should also be uniform, which implies that the temperature of
Conclusion
Intraparticle secondary reactions of tar during the pyrolysis of woody biomass were investigated experimentally. Pyrolysis products of the sawdust and wood cylinder were compared at low heating rate of 0.5 K/s. There was uniform temperature distribution within the wood cylinder. And, the ultimate analysis of chars did not vary along the radial direction. Therefore, a comparison of the sawdust and wood cylinder revealed intraparticle secondary reactions. The weight fraction histories of both the
Acknowledgment
This research was partially supported by the JFE 21st Century Foundation.
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