Available online at www.sciencedirect.com ScienceDirect

Energy Procedia 75 ( 2015 ) 196 – 201

The 7th International Conference on Applied Energy – ICAE2015 Effect of on the Cracking of from the of a Pine Sawdust in a Fixed Bed Reactor Z.Z. Zhang, M.M. Zhu*, P.F. Liu, W.C. Wan, W.X. Zhou, Y.L. Chan and D.K. Zhang

Centre for Energy (M473), The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia

Abstract

The effect of biochar on the cracking of the tar from pyrolysis of a pine sawdust was experimentally studied. The pine sawdust was pyrolyzed in a fixed bed reactor at 1123K with and without a pine sawdust biochar placed downstream of the pyrolysis process to crack the pyrolysis tar. The cracking temperatures tested were 923, 1023 and 1123K and the vapour residence times of the tar passing through the biochar studied were 0, 1.2, 2.3 and 3.8s. The pyrolysis liquid yield decreased and pyrolysis gas yield increased with increasing biochar cracking temperature and the vapour residence time. The main pyrolysis gases were H2, CO, CH4 and CO2. As the cracking temperature increased, the H2 yield increased while CH4 yield decreased. The CO concentration decreased when the cracking temperature increased from 973K to 1023K but then increased when the temperature continued to increase. The trend of the change of CO2 concentration as a function of the cracking temperature was opposite to that of CO. As the vapour residence time increased, the yields of H2 and CO2 increased while CH4 and CO yields decreased. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (©http://creativecommons.org/licenses/by-nc-nd/4.0/ 2015 The Authors. Published by Elsevier ).Ltd. Peer-reviewSelection and/or under responsibility peer-review of under Applied responsibility Energy Innovation of ICAE Institute

Keywords: Biochar; Pine sawdust; Pyrolysis; Tar cracking

1. Introduction

The combined heat and power generation using gas engines fuelled with pyrolysis gas is one of the most promising technologies in decentralised area [1]. In order to enhance the quality and quantity of the pyrolysis gas for the gas engine, gas cleaning and upgrading using cracking method are often adopted [1-4]. A number of catalysts have been developed, which can be classified into two groups, namely, synthetic catalysts (e.g., transition-metals-based catalysts, activated alumina, alkali metal carbonates, FCC catalysts, and ) and minerals (e.g., ferrous metal oxides, clay minerals, olivine, and

* Corresponding author. Tel.: +61-8-6488 5528; fax: +61-8-6488 7622. E-mail address: [email protected]

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Applied Energy Innovation Institute doi: 10.1016/j.egypro.2015.07.299 Z.Z. Zhang et al. / Energy Procedia 75 ( 2015 ) 196 – 201 197 calcined rocks) [5]. Among these, biochar is one of the most promising catalysts because of its low cost and high catalytic activity for gas cleaning and pyrolysis oil cracking, and most importantly, it is a by- product of pyrolysis [5, 6]. Ekström et al. found that almost 94% tar was converted into gaseous products after passing the tar over black at 1023K [7].Likewise, Chembukulam et al. reported that the cracking of tar from the carbonisation of sawdust over a bed of resulted in almost complete decomposition of tar into gaseous products at 1223K [8]. However, with no carrier gas to swipe the volatiles away from the sawdust bed and towards the cracking charcoal bed, the secondary reactions of the tar in the gas phase with the already pyrolysed sawdust biochar layer may play a significant role in this step up. Gilbert et al. studied the tar decomposition behaviour over a bed of biochar in the temperature range of 873 to 1073K [9]. It was found that the overall pyrolysis gas yield increased with increasing cracking zone temperature. However, the enormous amount of the raw used in their experiments may affect the heat and mass transfers during the pyrolysis process. The present work investigated the cracking of the tar from pyrolysis of a pine sawdust over biochars produced from the same feedstock. The effect of vapour residence time and cracking temperature on the pyrolysis liquid and gas yields and the gas composition were studied.

Figure 1 A schematic diagram of pyrolysis – cracking experimental setup

2. Experimental

2.1. Materials

The pine sawdust from a local furniture manufacturer was first sieved into the size fraction of 2-2.8 mm for experimentation. The cracking biochar used in the present study was produced by pyrolysing the raw pine sawdust at 1123K in nitrogen and stored in a drying oven at 373K prior to use. The compositions of the raw pine sawdust and the cracking biochar were analysed and the results are listed in Table 1.

Table 1 Proximate and ultimate analysis of the pine sawdust and the pine biochar

Proximate Analysis (wt% d.b.) Ultimate Analysis (wt% d.a.f.) Sample Volatiles Fixed carbon Ash C H S O Pine sawdust 87.32 12.10 0.58 35.79 6.38 0.02 57.81 Pine biochar 3.57 94.24 2.19 78.77 1.8 0 19.43

2.2. Pyrolysis - Cracking Experiment

Figure 1 shows the experimental setup for the pyrolysis - cracking experiment. Furnace 1 was used as a dryer at a temperature of 373K to remove any remaining moisture from the raw sample. Furnace 2 was kept at 1123K, at which the pyrolysis of the pine sawdust took place. The catalytic cracking of the tar 198 Z.Z. Zhang et al. / Energy Procedia 75 ( 2015 ) 196 – 201

produced from the pyrolysis over biochar occurred in furnace 3 at different cracking temperatures (923, 1023 and 1123K) with different amounts of cracking biochar (0, 1.5, 3 and 5g). Knowing the amount of cracking biochar and the gas flow rate, the vapour residence time of the volatiles passing through the cracking char bed was calculated. In the current experiments, the vapour residence times were 0, 1.2, 2.3 and 3.8s corresponding to 0, 1.5, 3 and 5g cracking biochar, respectively. In a typical experiment, ca. 1 g of the pine sawdust was transferred to a stainless steel mesh basket and kept at the centre of furnace 1 prior to the start of the experiment, the desired amount of cracking biochar was placed in the isothermal zone of the furnace 3 and held with two mesh supports. The system was then purged with nitrogen at 500mL min-1, after which the three furnaces were heated to the required temperatures. The mesh basket loaded with the pine sawdust was inserted into the isothermal zone of furnace 2 where the pyrolysis starts. The pyrolysis gas and tar passed through the cracking biochar bed placed in furnace 3. The condensable volatiles exiting furnace 3, known as pyrolysis liquid, were then collected using a two-stage cold trap system, which consisted of a U tube and 4-meter long perfluoroalkoxy alkane (PFA) cooling coil immersed in a salt-ice cooling bath (253K) and ethanol-dry ice cooling bath (195K), respectively. The non-condensable volatiles, known as pyrolysis gas, were collected in a 2L pre-vacuumed gas bottle. Each experiment lasted 4 minutes as it was found that the pyrolysis completed within 4 minutes according to our preliminary studies. The biochar yield from the pyrolysis in Furnace 2 was determined by weighting the solid residues collected from the mesh basket after the pyrolysis experiment and divided by the weight of the original pine sawdust. The liquid yield was calculated from the differences in the masses of the two-stage cold trap system before and after the experiments. The composition of the pyrolysis liquids was analysed using a GC-MS (Agilent 7890-5975c). The GC-MS detectable compounds were normalized against the internal standard (Methyl Oleate), which was added into all liquid samples at a constant concentration. A GC (Agilent 7890) with two thermal conductivity detectors (TCD1 for H2 detection with N2 as carrier gas and TCD2 for CO, CO2 and N2 detection with He as carrier gas) and one flame ionization detector (FID for detection with He as carrier gas) was configured to analyse the pyrolysis gas. Knowing the concentration of each gaseous species and the total volume, the gas yield was calculated from the GC results.

Figure 2 Variations of the (a) pyrolysis liquid yield and (b) pyrolysis gas yield as a function of vapour residence time at cracking temperatures of 923, 1023 and 1123K

3. Results and discussion

3.1. Effect of vapour residence time and cracking temperature on the pyrolysis products yields

The average yield of biochar produced from the pyrolysis of the pine sawdust was approximately 14.3wt% – this was constant for all the experiments. The effects of vapour residence time and cracking Z.Z. Zhang et al. / Energy Procedia 75 ( 2015 ) 196 – 201 199 temperature on the pyrolysis liquid yield were illustrated in Figure 2 (a). In general, the yields of the pyrolysis liquids decreased with increasing vapour residence time at all three cracking temperatures tested. Such decreasing trend was more profound at higher cracking temperatures, which suggested that the catalytic activity of the biochar were higher at elevated temperatures. Figure 2 (b) shows the variations of the pyrolysis gas yield as functions of vapour residence time and cracking temperature. As can be seen, the pyrolysis gas production was enhanced by increasing the vapour residence time for all three cracking temperatures tested. However, no significant enhancements in the gas production can be achieved when further increasing the vapour residence time from 2.3 to 3.8s at cracking temperatures of 1023 and 1123K. At vapour residence times of 0 and 1.2s, it was found that the pyrolysis gas yield increased as the cracking temperature increased from 923K to 1123K. At longer vapour residence times studied, the existence of an optimal cracking temperature, which occurs at 1023K, was observed.

3.2. Effect of vapour residence time and cracking temperature on the GC-MS detectable tar compounds

The GC-MS detectable tar compounds are classified into four categories, heavy PAHs (e.g. 4-7 ring PAHs), light PAHs (e.g. 2-3 ring PAHs), light aromatics (e.g. single ring aromatics) and heterocyclic aromatics (e.g. oxygenated compounds) [10]. The effect of the vapour residence time and cracking temperature on the GC-MS detectable tar compounds was shown in Figure 3. Similar to the trend of liquid yield, the relative intensity of GC-MS detectable tar compounds decreased with increasing vapour residence time at all three cracking temperatures and was more profound at higher cracking temperatures. However, it is interesting that without biochar addition, the relative intensity of GC-MS detectable and the pyrolysis gas yield increased with increasing cracking temperature (Figure 3 and Figure 2b), while the pyrolysis liquid yield remains almost unchanged (Fig 2a). This may due to the thermal cracking of the GC-MS undetectable fractions, which resulted in the increases of gas yield and lighter fractions with increasing cracking temperature [11].

Figure 3 Variation of the GC-MS detectable tar compounds as a function of residence time and cracking temperature

3.3. Effect of vapour residence time and cracking temperature on the major gas compounds

The gaseous products generated from pyrolysis of the pine sawdust is mainly composed of hydrogen, carbon monoxide, carbon dioxide, methane and a small amount of other light (HCs), such as ethane, ethylene, propane etc. Figure 4 shows the yields of the major gases produced from pyrolysis as a function of vapour residence time identified by GC at different cracking temperatures. As can be seen from Figure 4 (a), H2 production increased as the vapour residence time increased. In comparison with that without the cracking biochar addition, the concentration of H2 in the pyrolysis gas increased by ca. 80% at 3.8s vapour residence time at the cracking temperature 1123K. It is also noted that the effect of cracking biochar on the yield of H2 was more profound at higher cracking temperatures 200 Z.Z. Zhang et al. / Energy Procedia 75 ( 2015 ) 196 – 201

than that at lower cracking temperatures. Figure 4 (b) and (c) show that the concentration of carbon monoxide decreased while the concentration of CO2 increased as the vapour residence time increased from 0 to 2.3s, and then the concentrations of both CO and CO2 flattened as the vapour residence time continued to increase from 2.3s to 3.8s. For the experiments without the use of cracking biochar, increasing the cracking temperature resulted in an increasing trend in CO concentration and a decreasing trend in CO2 concentration. However, the use of cracking biochar changed these trends. At all three vapour residence times studied, increasing the cracking temperature from 973K to 1023K decreased the CO concentration and increased CO2 but further increasing the cracking temperature led to the change of CO and CO2 to the opposite direction. As illustrated in Figure 4 (d) and (e), both methane and other light hydrocarbons decreased with increasing vapour residence time at the cracking temperatures of 1023 and 1123K. Such trend was not observed at cracking temperature of 923K. The reduction of methane and other light hydrocarbons with the existence of cracking biochar was more significant at higher cracking temperatures than that at lower cracking temperatures. Without the cracking biochar, the slight decrease in the H2 and CO2 and increase in the CO and CH4 concentrations suggest that the reverse water-gas shift reaction and the methanation reactions may play an important role in the process. While for the experiments with the use of the cracking biochar, the water- shift reaction, the gas reforming reactions and the methane cracking reaction enhanced the H2 production and lowered the concentration of methane and other light hydrocarbons in the final gas products. It is also obvious that these reactions were more prevalent at longer vapour residence times and higher cracking temperatures. The CO produced together with the steam in the gas phase further promoted the water–gas shift reaction, which increased the amount of hydrogen and carbon dioxide while decreased CO amount in the final gaseous product. However, when the cracking temperature was high enough, the steam and carbon dioxide reactions may promote the CO production. This explains why CO concentration in the pyrolysis gas decreased first as the temperature increased from 973K to 1023K but then increased as the cracking temperature further increased.

Figure 4 Variation of the yields of major pyrolysis gases as a function of residence time and cracking temperature

4. Conclusions

The effect of a pine sawdust biochar in the cracking of tar from pyrolysis of the pine sawdust was studied Z.Z. Zhang et al. / Energy Procedia 75 ( 2015 ) 196 – 201 201 in a fixed bed reactor. Pyrolysis liquid yield decreased while the pyrolysis gas yield increased with increasing vapour residence time and cracking temperature. As a result of a series of gas reforming and cracking reactions, increasing the cracking temperature reduced the concentrations of methane and other light hydrocarbon but increased the concentration of H2 in the pyrolysis gas. The concentration of carbon dioxide increased while the carbon monoxide decreased with the increase of vapour residence time. The concentration of CO decreased when the cracking temperature increased from 973K to 1023K but increased again when the temperature was further increased, while the trend of CO2 concentration as a function of the cracking temperature was opposite to that of CO. As the vapour residence time increased, the yield of H2 and CO2 increased while the CO and CH4 yields decreased.

Acknowledgements

Financial and other support have been received from the Australian Research Council under the ARC Linkage Projects Scheme (ARC Linkage Project LP100200135), BHP Billiton Iron Ore Pty Ltd, and Ansac Pty Ltd.

References

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Dr Mingming Zhu is a research fellow at the Centre for Energy at the University of Western Australia. His research interests include in diesel engines, pyrolysis of biomass and wastes, combustion of biogas in gas engines, biochar slurry preparation and combustion, anaerobic digestion, and characterisation and utilisation of heavy oil.