Synthesis and characterization of biocoal from Cymbopogon citrates residue using microwave-induced torrefaction

Accepted Manuscript Synthesis and characterization of biocoal from Cymbopogon citrates residue using microwave-induced torrefaction I.A.W. Tan, N.M. Shafee, M.O. Abdullah, L.L.P. Lim

PII: DOI: Reference:

S2352-1864(17)30140-2 https://doi.org/10.1016/j.eti.2017.09.006 ETI 157

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Environmental Technology & Innovation

Received date : 26 April 2017 Revised date : 4 August 2017 Accepted date : 20 September 2017 Please cite this article as: Tan I.A.W., Shafee N.M., Abdullah M.O., Lim L.L.P., Synthesis and characterization of biocoal from Cymbopogon citrates residue using microwave-induced torrefaction. Environmental Technology & Innovation (2017), https://doi.org/10.1016/j.eti.2017.09.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis and Characterization of Biocoal from Cymbopogon citrates Residue using Microwave-Induced Torrefaction

I. A. W. Tana*, N. M. Shafeea, M. O. Abdullaha, L. L. P. Limb a

Department of Chemical Engineering and Energy Sustainability, Faculty of Engineering, Universiti Malaysia Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia

b

Department of Civil Engineering, Faculty of Engineering, Universiti Malaysia Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia * Corresponding author. Tel: +6082 583312; Fax: +6082 583410. E-mail address: [email protected] (I. A. W. Tan)

Abstract

This study investigates the effects of microwave-induced torrefaction on the characteristics of lemongrass (Cymbopogon citrates) residue. The effects of microwave power level and reaction time on the proximate and elemental contents, surface morphology, surface chemistry, textural properties, thermal stability, higher heating value (HHV), hydrophobicity, and mass and energy yield of the torrefied lemongrass residue were determined. The development of tubular structure on the torrefied lemongrass residue improved its grindability performance. HHV of 19.37 MJ/kg was achieved by lemongrass residue torrefied at 300 °C. The reduction of H/C and O/C ratios were 14.3% and 60.0%, respectively. Mass and energy yield of the torrefied lemongrass residue was 61.20-81.50% and 66.11-83.85%, respectively. The biocoal showed lower moisture absorbing capacity and was less easily to be segregated into fine particles.

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The enhancement of physicochemical and thermal characteristics of the lemongrass residue proved the feasibility of converting this biomass into biocoal via microwave-induced torrefaction followed by pelletisation.

Keywords: Biomass; Cymbopogon citrates; Torrefaction; Pelletisation; Biocoal.

1. Introduction

Biocoal is a product derived from biomass which has been treated to improve its physical and chemical properties. The biomass is normally heated to certain temperature to make it hydrophobic in nature and proceed with pelletisation process for easy storage and handling (Asadullah et al., 2014; Anupam et al., 2016). Biocoal has become the worldwide attraction due to its ability of increasing the biomass co-firing rates as well as reducing the carbon dioxide emission in pulverized-coal power plants. Various technologies have been used to produce biocoal from agricultural wastes such as hydrothermal carbonization, torrefaction, pyrolysis and gasification (Smith and Ross, 2016; Ghiasi et al., 2014). Torrefaction is a relatively mild thermochemical treatment of biomass carried out at low temperature range of 200–300 oC at atmospheric pressure under inert atmospheric conditions. The heating rate is usually kept below 50 oC/min, which is crucial in producing higher amount of solid product (Satpathy et al., 2014). Torrefaction helps to increase the energy density of biomass and eliminates the problems dealing with raw biomass such as high moisture content, hygroscopic behavior and low calorific value (Matali et al., 2016). Generally, torrefaction temperature range depends on the type of biomass studied. In a study conducted by Bergman et al. (2005), the torrefaction process resembled the

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roasting of coffee beans which were carried out at lower temperature and with the presence of air. It was mostly known as a pre-treatment process to upgrade the biomass properties so that it could be used as a feedstock in bio-renewable energy production. About 70% of the feed biomass was retained as solid product and the remaining 30% was converted into torrefaction gases. Mitchell and Elder (2010) stated that the suitable torrefaction temperature range was 220 to 300 ºC. Peng et al. (2013) in their study on torrefaction and densification of different species of softwood residues used the torrefaction temperature which ranged from 240 to 340 ºC. Torrefaction temperature ranging from 200 to 300 ºC has been applied in most studies dealing with lignocellulosic biomass (Chen et al., 2017; Wang et al., 2017; Martín-Lara et al., 2017; Kambo and Dutta, 2014; Kishor et al., 2014; Nhuchhen et al., 2014; Wang et al., 2012). Most of the biomass torrefaction studies and applications were performed by using conventional heating methods (Zhang et al., 2017, Chen et al., 2017; Wang et al., 2017; Yue et al., 2017; Martín-Lara et al., 2017; Xu et al., 2017). Conventional heating methods involve the transferring of energy to a material via three modes of conditions, namely conduction, convection and radiation from outside to the inside of a material (Kishor et al., 2014), and therefore are less preferable due to long processing time is required. The use of microwave radiation as a heat source can be a better alternative because of the advantages such as higher heating efficiency, excellent heat transfer, improved uniformity of heat distribution, better control over the heating process, fast internal heating, higher power densities, the ability to reach high temperatures at higher heating rates, and less processing time (Huang et al., 2017). This method of energy conversion is through reflective (conductors), transparent (insulators) and absorptive (dielectrics) reactions (Wang et al., 2012). Microwave heating, also known as dielectric heating, occurs where only specific materials that have dielectric properties can undergo the heating process. Basically, it is the conversion of

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electromagnetic energy to thermal energy through molecular interaction within the electromagnetic field, which provides better heat transfer, good penetration depth and fast torrefaction process. Wang et al. (2012) in their study on microwave-induced torrefaction of rice husk and sugarcane residues reported that the torrefaction process using microwave heating was an efficient and promising technology with a great potential in producing high quality fuel. Satpathy et al. (2014) also reported that microwave irradiation could be used effectively for torrefaction of wheat and barley straw investigated at moderate power and short process time. Microwave torrefaction of sewage sludge and leucaena was found to produce biochar with high HHV and fuel ratio as well as low atomic H/C and O/C ratios (Huang et al., 2017). Pelletisation, also called densification, has been reported to greatly enhance the bulk density of wood residues (Tumuluru et al., 2011). Torrefied biomass is easier to be pelletised because more fatty structures are developed during torrefaction that may act as binder (Pinto et al., 2017). The formation of pellets with torrefied biomass leads to less fines, higher uniformity, energy and bulk densities which improve the energetic conversion efficiency and reduce transportation and storage costs. Therefore, pellets using torrefied biomass are a better option to improve heating value, grindability, combustion performance, storage, transport and handling (Stelt et al., 2011). So far, most researches have been focused on the development of torrefaction kinetics and reactors, with very few studies being conducted on pelletisation of torrefied biomass (Peng et al., 2013). Lemongrass (Cymbopogon citrates) residue consists of high hemicellulose content of 58% from its original composition (Hussin et al., 2013). Approximately, there is 100 hectares of lemongrass farm available in Malaysia and the production of dry bagasse is about 200 tonne per year. Generally, the lemongrass residue such as leaves and stalks are left dried, burnt and

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naturally degraded in the fields without proper handling or disposal. Up to date, no study has been reported in the literature on the potential of converting lemongrass residue into biocoal. Therefore, this study aims to investigate the feasibility of producing biocoal from lemongrass residue by using microwave-induced torrefaction method followed by pelletisation. The torrefaction process was carried out within a narrow temperature range of 200 to 300 °C in an anoxic atmosphere. The torrefied lemongrass residue was then densified into pellets. The physicochemical and thermal characteristics of the biocoal produced, such as proximate contents, elemental contents, surface morphology, surface chemistry, textural properties, thermal stability, higher heating value (HHV), hydrophobicity, mass and energy yield with relate to the effects of microwave power level and reaction time were investigated.

2. Materials and Methods

2.1

Preparation of lemongrass residue

The raw lemongrass residue with initial moisture content of 50% (wet weight basis) was used as the raw material to prepare the biocoal. The sample was collected from local farm in Kuching, Sarawak, Malaysia. Immediately after collection, the sample was sealed in a plastic bag and subsequently stored in a refrigerator at 4 °C to prevent any loss of moisture, minimize biodegradation and off gassing. The lemongrass residue was grinded and sieved to 700 mm by using a sieve shaker. For initial moisture content determination, 5 g of triplicate samples were kept in an oven for 2 hours at 105±2 ºC. The average reading for initial moisture content was recorded and the samples were stored in a desiccator (Kishor et al., 2014).

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2.2

Microwave-induced torrefaction procedure

2 g of the dried and sieved lemongrass residue was placed in a stainless steel sample holder. The sample holder was then placed in a modified bench top microwave oven (model R374AST, Sharp, Malaysia). Nitrogen gas was purged for 20 minutes before the torrefaction process started and continuously purged throughout the process in order to maintain the inert environment as well as to prevent combustion (Kambo and Dutta, 2014; Kishor et al., 2014). The microwave power level (100, 300, 500, 800 and 1000 W) and reaction time (30 and 40 minutes) were varied to obtain the desired torrefaction temperature. The microwave power was shut down once the desired values were achieved. The torrefied lemongrass residue was cooled to 80 °C under nitrogen gas flow and then placed in a desiccator for further cooling to room temperature. The mass of the torrefied lemongrass residue was recorded. The torrefied lemongrass residue samples were stored in air-tight containers for further characterization.

2.3

Pelletisation procedure

Starch was used as the binding agent. The samples were placed in a cylinder with diameter of 2 to 4 inches. Next, the piston was released and the compression process took place against a temporary stop. The instantaneous pressure of the pelletizer might exceed 448000 kPa which then produced a pellet with density of 1040 kg/m3. To discharge the pellet, the pressure was released and the stopper was removed (Kitani and Hall, 1989).

2.4

Characterization methods

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The lemongrass residue and the biocoal produced were characterized for their physicochemical and thermal properties to determine the effects of the torrefaction temperature and reaction time on its characteristics.

2.4.1 Proximate analysis

Proximate analysis was conducted to determine the percentage of moisture, volatile matter, fixed carbon and ash contents of the raw and torrefied lemongrass residue. The proximate analysis was carried out using the ASTM conventional method (Kambo and Dutta, 2014). The samples were placed in the muffle furnace at 103 ± 2 ºC for 16 hours. In accordance to ASTME871, the samples were then re-weighed and the changes in weight before and after the process were expressed as the moisture percentage (ASTM, 2015). For the determination of ash residue, the dried samples were ignited in the muffle furnace at 575 °C for 5 hours. Volatile matter of the samples was determined by firing the samples at 950 ºC for 7 minutes. The fixed carbon content was determined by subtracting the percentage of moisture, volatile matter and ash from 100 percent.

2.4.2 Elemental analysis

2.6 ± 0.2 mg of the samples and 2.6 ± 0.2 mg of vanadium oxide were taken in small aluminum pockets, which made the total weight more than 5.2 mg. The aluminum pocket was then placed inside an elemental analyser (model EA 1112 Series, Thermoflash, USA) for CHN determination. The temperature of the elemental analyser was set to 900°C and the carrier gases

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used were oxygen and helium. The running time was 7 minutes. The tests were conducted for triplicate and the average reading was recorded.

2.4.3 Higher heating value (HHV)

Higher heating value was determined using a bomb calorimeter (model 6400, Parr, USA) after the samples were dried in the oven at 105±2 ºC for 24 hours. The raw and torrefied lemongrass residue were pelletised prior to analysis. The pelletised samples were then combusted in the bomb calorimeter (Ashton, 2013).

2.4.4 Surface chemistry

The changes in the surface functional groups of the raw and torrefied lemongrass residue were determined using Fourier transform infrared spectrophotometer (model IRAFFINITY-1 CE, Shimadzu, Japan). Each spectrum was recorded in the wavenumber ranging from 4000 to 400 cm-1 with a resolution of 4 cm-1.

2.4.5 Textural analysis

The Brunauer-Emmett-Teller (BET) surface area, pore size distribution and pore volume of the raw and torrefied samples were determined by measuring the amount of nitrogen adsorbed on the solid surface at 77K by using Quantachrome Instruments. Prior to analysis, the samples were degassed at 150 °C for 10 hours under vacuum condition (pressure rise of 50 micron/min).

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2.4.6 Surface morphology

The surface morphology of the raw and torrefied lemongrass residue were analysed using a scanning electron microscope (model TM3030, Hitachi, Japan) at accelerating voltage of 10 kV. Prior to analysis, the samples were coated with pure gold (20 nm) using an auto fine coater. The scanning images were amplified by factors of 100, 1000 and 5000.

2.4.7 Thermogravimetric analysis

The thermal stability of the raw and torrefied lemongrass residue were examined in an inert atmosphere by using a thermogravimetric analyser (model Pyris-1, Perkin Elmer, USA). At a heating rate of 20 °C/min, the heating program was set in a range of 35 to 600 °C under the flow of nitrogen at 20 mL/min (ASTM, 2015).

2.4.8 Hydrophobicity test

a) Equilibrium moisture content (EMC) Both raw and torrefied lemongrass residue pellets were exposed to a control environment for 24 hours, where the temperature was ranging from 22 to 25 °C with relative humidity of 40% to 50%. The samples were then dried at 100°C in an oven for 16 hours. The changes in weight of the samples before and after the drying process were determined and expressed as the EMC (Kambo and Dutta, 2014).

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b) Water resistance Both raw and torrefied lemongrass residue pellets were immersed in distilled water for 2 hours at room temperature. Then, the samples were placed on an aluminum sheet and exposed to a control environment for 4 hours, where the temperature was ranging from 22 to 25 °C with relative humidity of 40 to 50%. Then, the weights of the samples were determined and any occurrence of weight changes was expressed as the moisture uptake and the average values were recorded (Kambo and Dutta, 2014).

2.4.9 Mass and energy yield

The percentages of mass and energy yield of the raw and torrefied lemongrass residue were calculated as below (Kishor et al., 2014): Percentage of mass yield,



100%

(1)

Percentage of energy yield, ∗



∗

100%

(2)

Energy density ratio, (3) where Mtb

: Mass of torrefied biomass (g)

Mrb

: Mass of raw biomass (g)

HHVtb

: Higher heating value of torrefied biomass (MJ/kg)

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HHVrb

: Higher heating value of raw biomass (MJ/kg)

3. Results and discussion

3.1 Effect of microwave power level on torrefaction temperature

The temperature profile of lemongrass residue at different microwave power level is shown in Fig. 1. Initially, the microwave power level of 100 W symbolized by P10 in Fig. 1. was tested. The average heating rate obtained was relatively low at 4 °C/min which caused the torrefaction temperature of 200 °C could not be achieved. As the microwave power levels were increased to 300, 500 and 800 W, these resulted in average heating rate of 20 °C/min. However, the final temperatures achieved after 40 minutes were all lower than 200 °C. The desired torrefaction temperature of 200 °C was achieved at microwave power level of 1000 W after 30 minutes of reaction time. The results agreed with the study conducted by Wang et al. (2012) on rice husk and sugarcane residues which found that higher microwave power level led to higher heating rate and final temperature. Similar trend on the temperature profile was observed by Satpathy et al. (2014) for torrefaction of wheat and barley straw. Figure 1

3.2 Effect of torrefaction temperature on physical appearance of lemongrass residue

The changes in color of the raw lemongrass residue were found to be greatly influenced by the torrefaction temperature. The torrefaction temperature of 200 and 250 °C did not affect

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much on the physical appearance of the samples as the color only changed from light brown to medium brown. However, the torrefaction temperature of 300 °C changed the color of the sample from medium brown to dark brown. The results obtained were consistent with the studies carried out by Stelte et al. (2014) and Nhuchhen et al. (2014) which reported that the biomass changed color when the torrefaction temperature was varied. As the torrefaction process underwent an exothermic reaction, the losses of moisture content, carbon dioxide and large amounts of acetic acid and phenols had given a big impact on the color changes of the samples. The color changes were greatly influenced by the formation of chromophoric groups in the lignin materials during the heating process. The chromophoric groups consisted of carbonyls, hydroxyls, methoxyls and phenolic compounds, which had the ability to absorb incident light due to the increment of wavelengths after the heat treatment (Tumuluru et al., 2010).

3.3 Proximate analysis

From the proximate analysis shown in Table 1, the fixed carbon content of the lemongrass residue was found to increase significantly after the torrefaction process. This suggested that none of the non-combustible material was driven away in the torrefaction process. The increment in the fraction of fixed carbon was expected once the raw lemongrass residue underwent pretreatment process as it would result in the loss of moisture content even in a very small volume once the torrefaction temperature was raised. Dhungana (2011) suggested that the increment in the amount of fixed carbon might also be due to charring of biomass, cracking of volatile matters and decomposition of hemicelluloses into more stable compounds. Acharya (2013) reported that the performance of combustion for biomass was influenced by concentration

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of cellulose, hemicelluloses, lignin, lipids, proteins, simple sugars and starches as different species exhibited different nature of plant tissues, development phases and planting environments. Table 1

3.4 Elemental analysis

Table 2 lists the elemental analysis of raw and torrefied lemongrass residue. The changes of elemental composition were the most significant at the torrefaction temperature of 300 °C. The C content increased by 0.49, 2.29 and 3.96%, respectively at torrefaction temperatures of 200, 250 and 300 °C. The H content on the other hand decreased by 3.12, 8.77 and 8.92%, respectively at the three torrefaction temperatures studied. The O content also experienced reduction in amount by 2.08, 1.84 and 58.79%, respectively at three torrefaction temperatures studied. The results were consistent with Kishor et al. (2014) which stated that the removal of volatiles resulted in the increment of C content and reduction of both H and O contents. In a study on wood briquette torrefaction carried out by Felfli et al. (2005), the highest C content was achieved at torrefaction temperature of 270 °C. In this study, as the torrefaction temperature was ranged from 200 to 300 °C, the degradation of hemicelluloses was believed to occur completely at temperature near 300 °C. At the same time, cellulose started to degrade which then released oxygenated and aromatic compounds. At torrefaction temperature of 200 and 250 °C, the difference of H/C and O/C between the raw and torrefied samples was not significant. However, as the torrefaction temperature was increased to 300 °C, both H/C and O/C ratios showed a significant reduction. The reduction of H/C and O/C ratios at torrefaction temperature of 300°C

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were 14.3% and 60.0%, respectively. The results were consistent with findings reported in the literature which showed that at higher torrefaction temperature, the H/C and O/C ratios were decreased significantly (Kishor et al., 2014; Wang et al., 2012). It is very common for H/C and O/C to undergo reduction during the variation of torrefaction temperature. The increase in carbon content generally leads to the reduction of both H/C and O/C molecular ratios. Felfli et al. (2005) and Kishor et al. (2014) in their studies concluded that low H/C and O/C molecular ratios gave a good sight in combustion point of view where less water vapor and smoke were present. In addition, the energy losses during combustion and gasification were decreased. During the torrefaction process, the oxygen removal was found to be quicker compared to hydrogen, which was mainly due to extensive extraction of oxygenated compounds to carbon dioxide and carbon monoxide (Peng et al., 2013). Kambo and Dutta (2014) concluded that the achievement for both ratios depended on the type of pre-treatment used for the biomass samples. Table 2

3.5 Surface morphology analysis

From the SEM image of raw lemongrass residue as shown in Fig. 2(a), many inclusions in the thick-walled fibers of the sample could be observed. The rupture of pores could be seen on the samples torrefied at 200 and 250 °C, as shown in Figs. 2(b) and 2(c), respectively. When the torrefaction temperature was increased to 300 °C, the original biomass particles lost their original porous structure, but with formation of more compact and dense structure, as shown in Fig. 2(d). A tubular structure was clearly found on the surface of the lemongrass residue torrefied at 300 °C. With the aid of torrefaction, the inclusions started to deplete and completely disappeared at

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300 °C as the tubular structure started to develop. The upgrading of the surface structure of the lemongrass residue through torrefaction process had improved the fibrous nature of the biomass. The changes in the microstructure of the lemongrass made it less friable and improved its grindability performance. Chen et al. (2014) explained that the disappearance of those inclusions structure made the grindability of torrefied biomass greater compared to the raw ones. In a study conducted by Chen et al. (2014), the presence of tubular structure at higher torrefaction temperature was found to be mainly due to the large degradation of hemicelluloses structure and a very small amount of lignin. Funke and Ziegler (2010) explained that the thermal pre-treatment of biomass such as torrefaction and hydrothermal carbonization could cause the biomass to loss its structure due to the degradation and depolymerisation reactions of hemicelluloses. Figure 2

3.6 Surface chemistry analysis

FTIR spectra of the raw lemongrass residue illustrated major peaks at 3234.62, 2916.37, 2848.86, 1604.77, 1246.02, 1033.85, 516.92 and 503.42 cm-1. The peaks were assigned to functional groups such as alcohols, alkenes, ether and ester groups. The broad peak at 3234.62 cm-1 corresponded to the stretching of hydroxyl functional group (O-H bond). The peaks recorded at 2916.37 and 2848.86 cm-1 represented the stretching of asymmetric and symmetric of methyl functional group (H-C-H bond). A very slight symmetric stretching of alkenes functional group (C-C=C bond) was observed at 1604.77 cm-1. On the other hand, the two peaks observed at 1246.02 and 1033.85cm-1 defined the stretching of ether and ester functional group (C-O bond). The results were consistent with the study conducted by Benavente and Fullana (2015) on the

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torrefaction of olive mill waste which reported that the bands appeared at 1162 to 1243 cm-1 were assigned to ether, alcohol and ester groups, which made up the chemical structure of hemicelluloses and cellulose. FTIR spectra of the lemongrass residue torrefied at 200 and 250 °C showed two peaks at characteristic bands of 1732.08 and 1724.36 cm-1. These bands represented the stretching of carboxylic acids (C=H bond), mainly came from hemicellulose composition. The breakdown of hemicelluloses was much easier compared to cellulose and lignin due to its amorphous structure (Kambo and Dutta, 2014). Commonly, hemicelluloses started to degrade at temperature ranging from 200°C to 300°C. The peak observed at 1514.12 cm-1 represented the vibrations of aromatic ring (C=C bond). This band disappeared when the torrefaction temperature was increased to 300°C. Stelte et al. (2014) suggested that the band was correlated with the lignin structure; however at higher temperature ranging from 275 to 300 °C, the band disappeared due to fully degradation of lignin.

3.7 Textural analysis

The BET surface area obtained for the raw lemongrass residue and the sample torrefied at 300°C were 15.16 and 81.39 m2/g, respectively. This indicated that the conditions of volatilization of hemicelluloses and cellulose had fully taken place in the torrefaction process. The small opening on the surface of raw lemongrass residue as illustrated in Fig. 2(a) had been destructed by the degradation of hemicelluloses and cellulose during the torrefaction process. In a study conducted by Kambo and Dutta (2014), the surface area of the raw miscanthus was increased by 26% after undergoing the torrefaction process at 260 °C. It was explained that the internal structure of the torrefied miscanthus had become turbostratically arranged layer-like

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structure, or to be exact, a formation of tubular structure could be clearly seen. Liu et al. (2014) also studied the specific surface area of torrefied corn stalk and cotton stalk, in which the results showed improvement of 6.61-70% and 9.29-89%, respectively after torrefaction process. It was suggested that the development of new pores was affected by the severity of the torrefaction process.

3.8 Thermogravimetric analysis

The TGA profile of the raw lemongrass residue showed that the first thermal decomposition was observed at temperature around 30 to 150 °C, where the percentage of weight reduction was 11.36%, which represented the loss of moisture content through evaporation of unbound water from the biomass. Once the temperature achieved 100 °C, the structure of lignin started to degrade at a very slow rate, however, hemicelluloses and cellulose only started to degrade at temperature ranging from 220 to 400 °C. Nhuchhen et al. (2014) stated that the decomposition of lignin took into account a broad temperature range due to its characteristics of having various oxygen functional groups from its original structure, which was also known as the most thermally stable structure compared to hemicelluloses and cellulose. Next, a very significant thermal decomposition was observed at torrefaction temperature ranging from 200 to 300 °C, where the percentage of weight reduction was 63.93%, which represented the occurrence of depolymerization and partial devolatilisation of the hemicelluloses (Acharya, 2013). At 315 °C, the weight of the sample was further reduced by 9.09%. The reduction of weight was continued up to 418 °C, where the sample was completely diminished. The thermal decomposition of cellulose took place at temperature above 300 °C, as well as lignin, which had

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been continually decomposed starting from 100 °C. It could be concluded that the devolatisation process was not fully complete within the torrefaction temperature range studied. Instead, it was further developed at higher operating temperature, thus producing more volatile gases and reducing the solid product yield (Nhuchhen et al., 2014). The TGA profile of the lemongrass residue torrefied at 300 °C showed that the sample lost most of its moisture content during the torrefaction process; however the carbon content had been increased. The first thermal decomposition was observed at temperature around 24 to 150 °C, giving weight reduction of 7.45% which was due to the loss of moisture content (Funke and Ziegler, 2010). A very significant thermal decomposition was observed at torrefaction temperature ranging from 200 to 300 °C, resulting in weight reduction of 55.02%. This result was expected because torrefied sample had undergone thermal treatment prior to the thermogravimetric analysis, in which the inner composition had been improved. The weight loss was drastic when the temperature was increased to 600 °C, giving weight reduction of 26.82%. The evolution of volatiles and greater conversion of lignin during this temperature range resulted in a high amount of weight loss (Wang et al., 2012; Benavente and Fullana, 2015; Eseltine et al., 2013). As compared to raw lemongrass residue, torrefied lemongrass residue was not fully diminished even though the temperature reached 600 °C. Nhuchhen et al. (2014) explained that the burnout rate of torrefied biomass decreased due to low volatile content and it took longer time for the torrefied biomass to be fully burnt. Hence, it could be concluded that the combustion characteristics of the torrefied lemongrass residue had been improved.

3.9 Higher heating value (HHV) measurements

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In correlation with the elemental analysis results, the amount of carbon content of the raw lemongrass residue was found to be higher after undergoing the torrefaction process. This resulted in the increment of heating value of the torrefied products (Wang et al., 2012). The HHV of the raw lemongrass residue and the samples torrefied at 200, 250 and 300 °C was found to be 17.93, 18.45, 18.76 and 19.37 MJ/kg, respectively. Commonly, the raw sample tends to have a very low heating value due to high moisture content and low percentage of fixed carbon (Batidzirai et al., 2013). In a study conducted by Tumuluru et al. (2010), the HHV of raw wood chips was found to be 50% lower than the torrefied wood pellets. Raw reed canary grass also showed almost similar characteristics where the HHV was found to be 10% less than the torrefied sample. The results obtained in this study were consistent with Keipi et al. (2014) which reported that the HHV increment after torrefaction of eight types of woody biomass was 9%. The elemental analysis showed that the oxygen content of the torrefied lemongrass residue was reduced from 37.93% to 15.63%, at torrefaction temperature of 300 °C. Almost 60% of oxygen content reduction was obtained after torrefaction process, which was desirable for producing higher-grade biocoal. The higher oxygen content might lower the heating value as well as the needs of having larger amount of plants and auxiliary equipment due to the large production of flue gas during combustion process (Nhuchhen et al., 2014).

3.10 Hydrophobicity

From the hydrophobicity test conducted, the structure of the raw lemongrass residue pellet was found to be fibrous and the moisture absorbing capacity was higher compared to the torrefied samples. The percentage of moisture absorbed by the raw and torrefied lemongrass

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residue pellets is tabulated in Table 3. When immersed in the water, the raw lemongrass residue pellet started to segregate into fine particles in less than 2 minutes. This indicated the hygroscopic nature of the raw lemongrass residue pellet, which initiated the growth of biological organism. In addition, it would also add more cost and difficulties for handling and storage purposes (Nhuchhen et al., 2014). Acharya (2013) stated that the level of hydrophobicity was greatly influenced by the severity of torrefaction. When the torrefaction temperature was increased from 200 to 300 °C, the moisture absorbing capacity of the torrefied lemongrass residue pellet became lower. The pellet of lemongrass residue torrefied at 300 °C showed the greatest hydrophobic characteristics as no crumbling of the pellet was found after 2 hours of immersion in water. The hydrophobicity results obtained in this study were consistent with Dhungana (2011) which reported that the raw biomass pellet was easily disintegrated into fine particles which indicated higher moisture absorbing capacity as compared to the torrefied biomass where only 7-20% of moisture was being absorbed. Table 3

3.11 Mass and energy yield

The degradation of mass yield was found to be significant within the torrefaction temperature range of 200 to 300°C. When the temperature dropped to below 300 °C, the mass yield became constant again. The mass yield obtained for the lemongrass residue torrefied at 200, 250 and 300 °C were 81.50, 74.30 and 61.20 %, respectively. The losses of hydroxyl group and hemicellulose composition during torrefaction process contributed to a mass loss of the raw lemongrass residue. The results obtained agreed with the study conducted by Acharya (2013) on

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torrefaction of oats at temperature ranging from 210 to 300 °C, which reported that nearly 71% of mass yield was obtained at 30 minutes residence time of torrefaction process. Meanwhile, the percentage of energy yield was found to decrease with the increment of torrefaction temperature, which was 83.85, 77.72 and 66.11 %, respectively at 200, 250 and 300 °C. Asadullah et al. (2014) reported around 73% yield of biocoal derived from palm kernel shell was achieved at optimum temperature 300 °C with residence time of 20 minutes. Benavente and Fullana (2015) also observed the reduction of energy yield in torrefied olive mill waste in which the percentages of reduction ranged from 40 to 99 %, with increment of torrefaction temperature from 150 to 300 °C. This was greatly impacted by the degradation of hemicelluloses and the devolalitization of product gases such as carbon monoxide, carbon dioxide, water vapor and acetic acid (Benavente and Fullana, 2015). The results obtained in this study were supported by Kishor et al. (2014) which claimed that both microwave power level and reaction time affected the performance of mass yield and energy yield. The untreated biomass sources commonly have very low energy density which makes them not suitable in energy conversion system. Large volume of biomass feed is therefore needed to produce the amount of desired power. The lemongrass residue torrefied at 200, 250 and 300 °C showed an increment of energy density ratio of 1.03, 1.05 and 1.08, respectively. The results were consistent with the study conducted by Acharya (2013) in which the torrefaction of oats also resulted in reduction of mass and energy yield, but increased the energy density. Nhuchhen et al. (2014) explained that the presence of higher fraction of lignin inside the torrefied biomass allowed more extraction of energy. It was also suggested that for the insignificant increment of energy density, it could be improved by using wet torrefaction method such as hydrothermal carbonization.

21   

4. Conclusions

Biocoal was successfully produced from lemongrass residue using microwave-induced torrefaction followed by pelletisation. The torrefied lemongrass residue showed the highest HHV of 19.37 MJ/kg at torrefaction temperature of 300 °C. Mass and energy yield of the biocoal was 61.20-81.50% and 66.11-83.85%, respectively. The increment of HHV was due to the high percentage of fixed carbon (37.77%) and reduction of H/C and O/C ratios, which proved that the microwave-induced torrefaction enhanced the moisture and fixed carbon contents of the sample. The torrefied lemongrass residue was less friable with improved grindability performance, and showed better combustion properties with lower moisture absorbing capacity.

Acknowledgements

The authors acknowledge the research grants provided by Universiti Malaysia Sarawak under Special Grant Scheme F02/SpGS/1406/16/7 and Centre for Renewable Energy, Faculty of Engineering, UNIMAS under CoERE/Grant/2013/03.

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Figure Captions Fig. 1

Temperature profile of lemongrass residue at different microwave power level

Fig. 2

SEM images of lemongrass residue (5000x) (a) raw; (b) torrefied at 200°C; (c) torrefied at 250°C and (d) torrefied at 300°C

List of Tables Table 1

Proximate analysis of raw and torrefied lemongrass residue

Table 2

Elemental analysis of raw and torrefied lemongrass residue

Table 3

Percentage of moisture absorbed by raw and torrefied lemongrass residue pellets

Table 1 Proximate contents (%)

Raw

Torrefaction Temperature (°C) 250

300

Moisture

2.50

1.59

1.41

1.35

Volatile Matter

79.10

70.2

66.3

60.4

Fixed Carbon

16.84

26.73

30.88

37.77

Ash

1.56

1.48

1.41

0.48

27   

200

Table 2 Torrefaction

Elemental analysis (wt. %)

O/C ratio

condition

C

H

N

O

Raw

47.18

13.12

1.77

37.93

0.28

0.80

200 °C – 30 minutes

47.41

12.71

2.73

37.14

0.27

0.78

250 °C – 30 minutes

48.26

11.97

2.53

37.23

0.25

0.77

300 °C – 30 minutes

49.05

11.95

23.36

15.63

0.24

0.32

28   

H/C ratio

Table 3 Torrefaction

Weight reduction before

Weight reduction after

conditions

immersed in water (%)

immersed in water (%)

Raw

13.97

32.47

200 °C – 30 minutes

8.71

29.55

250 °C – 30 minutes

7.76

26.55

300 °C – 30 minutes

5.49

24.48

29   

Fig. 1

30   

(a)

(b)

(c)

(d) Fig. 2

31   

Graphiccal abstract

((a)

(b)

((c)

(d)

Scaanning electrron microsccopy imagess of lemongrrass residuee (5000x): (aa) raw; (b) 200°C 2 toorrefaction; (c) 250°C toorrefaction and (d) 3000°C torrefacttion

 

32   

Highlights



Biocoal was synthesized from lemongrass residue via microwave-induced torrefaction.



Lemongrass residue torrefied at 300 °C showed higher heating value of 19.37 MJ/kg.



Energy yield of the biocoal produced was 66.11-83.85%.



Mass yield of the biocoal produced was 61.20-81.50%.



Microwave-induced torrefaction improved the grindability of the biocoal.

33