Lignocellulosic Biomass Solid Fuel Properties Enhancement Via Torrefaction

Available online at www.sciencedirect.com

ScienceDirect Procedia Engineering 148 (2016) 671 – 678

4th International Conference on Process Engineering and Advanced Materials

Lignocellulosic Biomass solid Fuel Properties Enhancement via Torrefaction S. Matali*, N.A. Rahman, S.S. Idris, N. Yaacob, A.B. Alias Faculty of Chemical Enginnering, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia

Abstract Torrefaction, also known as mild pyrolysis, is often carried out between temperature ranges of 200 and 300oC in anoxic conditions. It is a type of thermo-chemical pre-treatment process applicable to biomass in order to convert it into compatible energy fuels. Torrefaction has favorable effects on biomass, which includes increasing its energy density and eliminating problems commonly associated with raw biomass such as high moisture content, hygroscopic behavior and low calorific value. In this study, torrefaction of agricultural residue, oil palm frond (non-woody biomass) and short rotation energy crop, Leucaena Leucocephala (woody biomass) were conducted in a horizontal tube furnace at five temperatures and holding time of 60 min. High heating values, elemental and proximate analyses results, thermal degradation profiles of torrefied fibrous products were compared to its raw forms. It was concluded that as torrefaction conditions became more severe, this led to a more qualified and energy-dense solid fuel with higher fixed carbon content, increased calorific values and reduced hydrogen and oxygen contents. The results gained from this study may provide basic information for torrefied products application in combustor and/or gasifier design. © Published by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license ©2016 2016The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ICPEAM Peer-review under responsibility of the organizing committee of ICPEAM 2016 2016. Keywords: Biomass; Torrefaction; High heating value; Energy density

1. Introduction Over dependence on fossil fuels for primary source of energy supply has led to global energy crisis, increased greenhouse gases (GHGs) emissions and decline in fossil fuels reserves [1]. Thus, alternative energy from

*

Corresponding author. Tel.: +6-035-543-6328; fax: +6-035-543-6300. E-mail address: [email protected]

1877-7058 © 2016 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 the organizing committee of ICPEAM 2016

doi:10.1016/j.proeng.2016.06.550

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renewable sources is in focus to replace fossil fuels for greener energy options. Among renewable resources, biomass is considered as a feasible option due to its carbon neutrality, sustainable and vastly available. However, biomass has its limitations to be utilized particularly as a direct feedstock for power generation. These include low combustion efficiency attributable to its high moisture content, low energy density, hydrophilic behavior, high oxygen content which makes it susceptible to biological attack and biodegradation [2–5]. Furthermore, due to the tenacious and fibrous nature of ligno-cellulosic biomass, grindability is a challenge which requires high energy consumption during preparation of fuel [6]. To overcome these undesirable properties, torrefaction as a pretreatment process is introduced in order to improve biomass properties. Torrefaction is a thermo-chemical conversion method where biomass is subjected to thermal heating in the absence of air, typically in the temperature range of 200 to 300oC at atmospheric pressure [7]. Torrefaction differs from pyrolysis process where the former’s purpose is to retain solid mass yield while enabling its energy content to be conserved and with incomplete removal of volatile matters. It was also reported elsewhere that torrefaction enables the reduction of hydrogen and oxygen contents, which consequently will release less water vapor and smoke during combustion [8,9]. Torrefaction studies in previous years mainly focused on woody biomass [10–17], agricultural byproducts such as oil palm mill waste [3,18–22], corn stover/stalk [7,23–25] and various straw and grass species [26–28]. However, less emphasis on torrefaction study on agricultural field residue and potential energy crop is reported in published literature. In this study, an agricultural residue from oil palm plantation i.e. oil palm frond (OPF) and woody biomass from fast growing species, Leucaena Leucocephala (LL) will be subjected to torrefaction in order to investigate their potential usage as bioenergy fuel. OPF, a major residue from oil palm plantation has great potential due to its abundant availability and has high volatile content indicating high reactivity [29]. As aforementioned, LL is a fast growing tree and considered to be a potential energy crop [17]. Hence, effects of torrefaction temperatures at fixed holding time of 60 minutes on solid mass and energy yields, energy density and thermal degradation behavior via thermogravimetric analysis (TGA) will be presented. Results obtained from this study may provide basic knowledge related to influence of torrefaction process parameters on physico-chemical properties of the two biomass materials. 2. Experimental 2.1. Materials Oil palm frond (OPF) was obtained from local oil palm plantation while Leucaena Leucocephala (LL) was collected from open areas in Selangor, Malaysia. Bulky biomass samples were cut, chipped and grinded into small particle sizes of approximately 3 to 5 mm. Next, large biomass fibre was pulverized and sieved using a sieve shaker in order to obtain desired particle size of less than 212 µm. The fine fibrous samples were then oven-dried at 80oC for 24 hours to remove surface moisture, then stored in an airtight container and placed inside desiccators. 2.2. Torrefaction experiments Prior to the start of torrefaction experiments, nitrogen gas was purged into a horizontal furnace (MTI Corp., USA) equipped with a 80 mm-ID quartz tube reactor, attached to a precision temperature controller. Purging was done for 10-15 minutes at a rate of 1 L/min in order to provide inert condition within the reactor. Torrefaction experiments were carried out for sample weight of 10g at continuous nitrogen flow rate of 100 mL/min. Heating rate was set to 10oC/min for temperatures 200oC, 225oC, 250oC, 275oC and 300oC. Heating rate of 10oC/min is considered as slow in order to maintain the homogeneity of products [30]. After reaching desired temperature, biomass samples were held for continuous torrefaction holding time of 60 minutes. Chew and Doshi reported in their review paper that torrefaction time less than or equal to 1 hour is sufficient enough to produce solid fuels with higher energy density as compared to untreated biomass [31]. After torrefaction experiments were completed, furnace was turned off, left to cool down to ambient temperature where samples were taken out, weighed and stored in airtight containers.

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2.3. Characterization experiments Proximate analysis and thermal decomposition profiles on both fibrous torrefied samples were accomplished using a thermogravimetric analyser Mettler Toledo/TGA/ SDRA51e under inert nitrogen gas at ambient pressure with constant flow-rate of 50 ml/min, and at heating rate of 10°C/min for temperature range of 25-900°C. Each sample of weight 20 mg was placed directly into an alumina crucible where prior to analysis, temperature was kept isothermal for one minute until steady condition was obtained before ramping to the desired temperature. Weight loss was recorded throughout the test with respect to temperature and time and the resulting thermogravimetric (TG) profile was then further processed to obtain differential thermogravimetric (DTG) profile. High heating values (HHVs) and elemental analyses were achieved using IKA-works C5000 calorimeter and Thermo-Finnigan Flashed 1112 analyser, respectively. The experiments were replicated at least twice to obtain reproducibility and in accordance to the required standard procedure of the American Society for Testing and Materials (ASTM). The relations that define mass yield, energy yield and energy density of torrefied samples are: Mass Yield, MY(wt%) = (mtorr/mraw) x 100

(1)

Energy Yield, EY (wt%) = MY x (HHVtorr/HHVraw)

(2)

Energy Density, ED = EY/MY

(3)

where mtorr is mass of torrefied sample, mraw is mass of raw sample, both in mg, HHVtorr and HHVraw are high heating values of torrefied and raw samples, respectively, in MJ/kg. 3. Results and Discussions 3.1. Elemental and proximate analyses The results of elemental and proximate analyses of raw and torrefied fibrous samples at 60 minutes holding time are listed in Table 1. For comparison purpose, two coals of different ranks i.e. sub-bituminous and bituminous coals were also displayed. In general, results gained indicated that oxygen and hydrogen contents decreased with the increase of torrefaction temperature, whereas, elemental carbon content increased. Elemental oxygen and hydrogen reductions were up to 28% and 34% for torrefied OPF and LL, respectively, while elemental carbon increased approximately to 37% for both torrefied samples at the highest torrefaction temperature (300oC). Decrease in hydrogen and oxygen contents is generally attributable to destroyed hydroxyl group (-OH) in biomass samples during torrefaction, which consequently produced solid hydrophobic fuel [15,32]. Table 1. Elemental/Proximate analysis of raw and torrefied oil palm frond (OPF) and Leucaena Leucocephala (LL) at holding time 60 min.

Raw OPF Torr. OPF-200ooC Torr. OPF-225oC Torr. OPF-250oC Torr. OPF-275oC Torr. OPF-300 C Raw LL Torr. LL-200ooC Torr. LL-225oC Torr. LL-250oC Torr. LL-275oC Torr. LL-300 C Sub-bituminous fcoale Bituminous coal

Elemental Analysis (dafa, %) C H N O* 41.75 5.51 1.39 51.36 42.82 6.09 1.68 49.41 47.32 5.76 1.38 45.54 48.04 4.84 2.01 45.11 51.55 3.92 2.47 42.06 56.68 3.76 2.73 36.83 45.82 6.75 1.43 46.00 50.76 8.21 0.26 40.77 52.21 6.03 1.66 40.10 51.24 5.76 1.21 41.78 54.61 5.84 1.15 38.40 62.59 5.53 1.47 30.41 50.28 4.78 1.83 43.10 68.73 5.54 1.95 23.56

Atomic H/C ratio 1.57 1.69 1.45 1.20 0.91 0.79 1.76 1.93 1.38 1.34 1.27 1.05 1.13 0.96

Atomic O/C ratio 0.92 0.87 0.72 0.71 0.61 0.49 0.75 0.60 0.58 0.61 0.53 0.36 0.64 0.26

Proximate Analysis (dbb, %) VMc FCd Ash 79.37 20.63 25.60 79.18 20.82 15.86 75.70 24.30 38.87 70.94 29.06 29.10 60.73 39.27 16.44 46.37 53.63 30.00 72.70 15.25 12.04 65.70 13.63 20.67 56.76 12.18 31.06 76.56 21.53 1.91 57.10 21.01 21.89 37.78 30.69 31.52 44.28 45.01 10.71 33.42 58.63 7.95

*Oxygen calculated by difference; adry ash free basis; bdry basis; cVM – volatile matter; dFC – fixed carbon, eAdaro coal; f

Silantek coal

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A van Krevelen plot is also shown in Figure 1 to demonstrate the change in atomic ratio of H/C as a function of atomic ratio O/C. Calculations of these ratios are referred to the works of Bridgeman et al [26]. Reduction of these atomic ratios implies the measures of pyrolysis efficiency and oxidation degree of torrefied products [30]. Figure 1 shows that elementary composition of torrefied biomass moves towards coal with better results obtained from torrefied LL. In this case, torrefied products subjected to torrefaction temperature above 275oC even surpassed subbituminous coal. This is in agreement with many literatures that have reported torrefied biomass characteristics came close to that of lignite coal [6,7,12,33–35].

Increasing Ttorr

Fig. 1. Van Krevelen plot of raw, torrefied biomass samples at holding time 60 min and coals of different rank.

In view of torrefaction effects on proximate analysis, with the rise in torrefaction temperature, fixed carbon content increased while volatile matter reduced significantly as shown in Table 1. Increase of fixed carbon was more than doubled for both torrefied biomass samples with torrefied OPF having the highest value of 54 wt%. Influence of temperature was more apparent for temperature range above 250oC due to enhanced decomposition of hemicellulose. It was reported by Tumuluru that hemicellulose decomposes extensively into volatiles and char-like solid product during mild to severe torrefaction temperatures i.e. 235-275oC [7]. High fixed carbon in torrefied biomass is favoured as this will give substantial contribution to thermal energy release when it is burned, particularly when it is to be blended with coal [36]. Similar observations on the extensive volatile matter reduction was also reported in the study of wood chips/logging residues [14-15,35]. Lowering of volatile matter was consistent with increasing torrefaction temperature with 48 wt% and 42 wt% volatiles decrements for torrefied OPF and torrefied LL, respectively. The difference on volatile matter reduction between the two biomass types was due to variance in cellulosic content mainly carbohydrate fraction which are easily degraded during thermal treatment [35]. (a)

(b)

Fig. 2. (a) Fixed carbon (b) Volatile matter change as a function of temperature at holding time 60 min

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3.2. Solid mass yield, energy yield and energy analysis Table 2 shows the high heating values (HHVs), solid mass and energy yields as well as energy densities of raw and torrefied biomass samples at various temperatures. Solid mass yield is defined as the mass ratio of torrefied biomass over its raw form (refer equation 1) and typical solid yield ranges from 50 to 90 wt% [4]. Mass yields decreased to about half of its original weight when torrefied temperature reached 300oC for both samples. This is due to the effect of moisture removal and volatiles released i.e. hemicellulose and some short-chain lignin compounds during thermal treatment [37]. As for energy yield, the values generally reduced from its raw form by 29 wt% and 40 wt% for torrefied OPF and LL, respectively. Energy yield can be regarded as a significant indicator to the amount of energy retained after torrefaction [17]. However, with reference to Equation 2, energy yield is largely dependent on mass yield values and it was also reported that energy yield is directly linked to biomass type [3,38]. Table 2. High Heating Values (HHVs), mass yield, energy yield and energy density of raw and torrefied oil palm frond (OPF) and Leucaena Leucocephala (LL) at holding time 60 min

Raw OPF Torr. OPF-200oC Torr. OPF-225oC Torr. OPF-250oC Torr. OPF-275oC Torr. OPF-300oC Raw LL Torr. LL-200oC Torr. LL-225oC Torr. LL-250oC Torr. LL-275oC Torr. LL-300oC Sub-bituminous coal Bituminous coal

HHV (MJ/kg) 17.67 18.57 20.34 20.98 22.77 25.16 17.93 18.31 18.54 19.28 21.73 24.92 26.65 30.96 (a)

Mass Yield (wt%) 95.0 87.0 75.0 63.0 50.0 92.0 88.9 76.7 68.8 43.2 -

Energy Yield (wt %) 99.9 100.2 89.1 81.2 71.2 93.9 91.9 82.4 83.3 60.1 -

Energy Density 1.05 1.15 1.19 1.29 1.42 1.02 1.03 1.08 1.21 1.39 -

(b)

Fig. 3. (a) High heating values of different biomass types and coal (b) Energy densities of torrefied biomass as a function of temperature at holding time 60 min

Increments in higher heating values and energy densities are presented in Figure 3(a) and 3(b). Similarly, as torrefaction temperatures intensified, HHV increased and correspondingly their energy densities. Energy density is defined as amount of chemical energy stored in fuel per unit volume and if energy density is more than unity, desired energy gain can be achieved [30]. HHVs of torrefied biomass samples in this study are comparable to that of subbituminous coal and energy densities increased by factors of 1.42 and 1.39 for torrefied OPF and LL, respectively, at

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the maximum torrefaction temperature of 300oC. Notably, energy density rises significantly at torrefaction temperature exceeding 250oC but with lower solid mass and energy yields. Nam et al. reported that torrefaction temperature of 250oC can be considered as ‘middle condition’ of average preferred energy density and yield, depending on the purpose of the torrefied products [27]. 3.3. Thermal decomposition behavior of torrefied biomass Figure 4(a) and 4(b) displays the thermal decomposition profiles i.e. DTG curves for raw and torrefied OPF and LL fibrous samples at 200oC, 250oC and 300oC, respectively, with comparison in different coal ranks. As shown from the figures, distinctive torrefaction temperature effect was observed. Initial small peaks below temperature 150oC were due to moisture loss during drying step. Raw samples showed sharp curves during temperature range of 200-250oC, which were attributable to hemicellulose thermal degradation. In contrast, for torrefied samples above 250oC, near absence of hemicellulose degradation was detected which is in agreement with findings by Arias et al. that hemicellulose’s thermal degradation temperature range is between 220 oC and 350oC [11]. DTG peaks were shifted to the right as more volatiles were released causing peak heights to be reduced, indicating lesser reactivity. However, it should be noted that, for pyrolysis done in wider temperature range, major decompositions were due to cellulose as evident in higher DTG peaks in the range of 305-375oC [39]. Generally, thermal degradation profiles emulate those of coals as torrefaction temperatures were increased to 300oC. These trends of DTG profiles were of similar findings by various researchers [14,17,27,38]. (a)

(b)

Fig. 4. DTG decomposition profiles of (a) OPF (b) LL at holding time 60 min

4. Conclusion Torrefaction effect on lignocellulosic biomass materials i.e. oil palm frond (non-woody biomass) and Leucaena Leucocephala (woody biomass) was investigated. Both torrefied biomass materials underwent physico-chemical changes, which include mass reduction, rise in energy content and chemical compositions. As agreed by various previous researches, these changes were more evident as torrefaction temperatures were increased. Solid mass yield decreased up to 43%, which consequently made energy yield reduced up to 60%. However, high heating values improved up to 25 MJ/kg as related to its raw form of 18 MJ/kg where these enhanced values are comparable to that of lignite and sub-bituminous coals. This was further validated by van Krevelen plot in which biomass materials torrefied above 275oC were observed to approach higher coal rank atomic ratios. It is recommended that temperature range of 250oC to 300oC with holding time of 1 hour to be applied during torrefaction process in order to produce potentially good quality biochar. Apart from utilization of agricultural field residue and recognizing potential energy crop from Leucaena Leucocephala, results obtained in this study demonstrated that torrefaction is a viable option to convert raw feedstock into a promising bioenergy fuel. If torrefied products are intended to be co-combusted with coal or utilized in gasification process, recommended further works on torrefaction should focus on parameter optimization based on specific biomass type, densification, grindability, kinetics behavior and most importantly, combustion behavior and its kinetics.

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Acknowledgement The authors gratefully acknowledge the financial support received from Ministry Higher Education of Malaysia through Fundamental Research Grant Scheme (FRGS) (600-RMI/FRGS 5/3 (98/2014)), Research Management Institute (RMI) and Faculty of Chemical Engineering, Universiti Teknologi MARA, Shah Alam, Selangor, Malaysia. The authors would also wish to thank Mr Syed Muhammad Azlan Sayed Idrus for providing samples of Leucaena Leucocephala for this research study. References [1] S. Ren, H. Lei, L. Wang, Q. Bu, S. Chen, and J. Wu, “Thermal behaviour and kinetic study for woody biomass torrefaction and torrefied biomass pyrolysis by TGA,” Biosyst. Eng., vol. 116, pp. 420–426, 2013. [2] G. Toscano, A. Pizzi, E. Foppa Pedretti, G. Rossini, G. Ciceri, G. Martignon, and D. Duca, “Torrefaction of tomato industry residues,” Fuel, vol. 143, pp. 89–97, 2015. [3] Y. Uemura, W. N. Omar, T. Tsutsui, and S. B. Yusup, “Torrefaction of oil palm wastes,” Fuel, vol. 90, pp. 2585–2591, 2011. 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