Methods to improve properties of fuel pellets obtained from different biomass sources: Effect of biomass blends and binders

Fuel Processing Technology 199 (2020) 106255

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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc

Research article

Methods to improve properties of fuel pellets obtained from different biomass sources: Effect of biomass blends and binders

T

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Shailendrasingh P. Rajput , Sachin V. Jadhav, Bhaskar N. Thorat Department of Chemical Engineering, Institute of Chemical Technology, N. Parikh Road, Matunga (E), Mumbai 400019, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Biomass Pellets Strength Calorific value Durability Binder

This study evaluates the effect biomass blending and binder addition on the strength, calorific value (CV), and durability by biomass pellets. Blended biomass materials (0 to 60 wt% additive) and three binders (2 to 6 wt%) (i.e.) recovered polyvinyl alcohol (rPVA); waste cooking oil (WCO) and waste lubricating oil (WLO) were used to make biomass pellets. Ground nut shell (GNS) pellets exhibited the highest CV but lowest strength which was improved by the addition of saw dust (SWD) and Leaf litter waste (LLW) additives. Contrarily, LLW pellets exhibited good strength but the minimum CV which was improved by GNS audition. The LLW biomass contributed as strength improver and GNS served as CV enhancer. The SWD biomass provided both strength and CV to the pellets. In case of binders, a significant increase in the strength of GNS pellets was observed on rPVA addition in comparison to LLW pellets and SWD pellets, whereas WCO and WLO showed a marginal reduction in strength of all the biomass pellets. The rPVA material worked as strength improving binder whereas WCO and WLO were responsible to increase the CV of the pellets.

1. Introduction Fuel wood is one of the primary energy sources for cooking and heating in rural areas across India. It is also a source of commercial energy such as brick kilns, hotels, and restaurants in semi-urban areas. The major consumers of fuel wood (i.e.) 83.5% are rural households and 23% are urban users [1,2]. The annual fuel wood consumption by 854 million people in India (i.e.) 216.4 MMTPA (million metric tons per Annum) with 27% collected from publicly owned forests [3]. To reduce the exploitation of forests and preserve flora and fauna, alternative sustainable biomass sources can be made available by using agro-residues, leaf litters and other forms of biomasses. Agro-waste mainly from rural parts of India and garden waste from urban areas is a potential raw material for biomass pellet production [4]. Around ~900 TPD of leaf litter waste (LLW) (i.e.) garden waste is generated in large cities like Mumbai [5,6]. Simillarly, India is the largest groundnut producer in the world accounting for an average of 7.6 MMTPA in 2016 and 6.2 MMTPA in 2017 [7,8] which results in generation of 1.5 MMTPA GNS biomasss that can potentially be used in fossil fuel replacement [9]. Like most agro-processing byproducts GNS and LLW have a low bulk density (< 150 kg/m3) and therefore cannot be efficiently used as a fuel and economically transported over long distances to areas where they can be effectively utilized [10]. These biomass have

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to be processed for conversion into usable form. As a result of high moisture content, irregular shape and sizes, and low bulk density, these biomasses are very difficult to handle, transport, storage, and utilize in its original form [11,12]. Solid fossil fuels such as coal have advantages when compare with biomass. The handling of raw biomass negatively influence the economies of biomass [13]. One of the solutions to this problem is densification of biomass materials into pellets, briquettes, or cubes and used as an energy source [6]. Use of briquettes for large and medium scale industrial thermal applications have been widely investigated [14]. On the other hand, pellets can also be used in small scale industrial thermal applications (i.e.) boilers and gasifiers. Biomass is abundantly available throughout India but in scattered form, as a result, decentralized pelletization would be an ideal option for locally producing fuel pellets. Fuel pellets prepared using locally available biomass (i.e.) sawdust (SWD), groundnut shells (GNS) and leaf litter waste (LLW) by adding low-cost binders like recovered polyvinyl alcohol (rPVA); waste cooking oil (WCO) and waste lubricating oil (WLO) would be one of the promising options for producing low cost pellets. These pellets containing complex compounds like rPVA, WCO, and WLO binders would be burnt in boilers safely at higher operating temperatures to ensure complete combustions. The binder addition could improve important properties of pellets like CV, strength, and durability.

Corresponding author. E-mail address: [email protected] (S.P. Rajput).

https://doi.org/10.1016/j.fuproc.2019.106255 Received 18 July 2019; Received in revised form 20 October 2019; Accepted 29 October 2019 0378-3820/ © 2019 Elsevier B.V. All rights reserved.

Fuel Processing Technology 199 (2020) 106255

S.P. Rajput, et al.

This paper evaluates preparation and characterization of fuel pellets from SWD, GNS and LLW by adding rPVA, WCO, and WLO binders. The focus of the study was to develop a better perspective on the properties of the biomass pellets, by preparing pellets having higher strength and CV.

Table 2 Pellet preparation using binders. Biomass

Case 4

Case 5

Case 6

rPVA

WCO

WLO

2 4 6 2 4 6 2 4 6

2 4 6 2 4 6 2 4 6

2 4 6 2 4 6 2 4 6

Binder amount (wt%)

2. Materials and methods SWD

2.1. Biomass and binders GNS

A mixture of sawdust (SWD) from various woods was obtained from the local sawmill in Mumbai, India. Groundnut shells (GNS) were procured from groundnut oil-seeds producing area (Dhulia district of Maharashtra, India) after the dehulling operation in feed preparation unit. The leaf litter waste was collected from the campus of Institute of Chemical Technology, Mumbai, India, comprising of leaves, twigs, and garden trimmings of mixed grasses and trees. Three binders namely rPVA, WCO and WLO were used in the investigation. The rPVA was an aqueous binder recovered from the textile industry effluent using the procedure described by Rajput and Thorat [15]. The second binder, WCO was procured from local eateries using palm oil for frying. Indian eateries usually use palm oil for frying purposes because of the low cost and ease of availability. Likewise, WLO was procured from a local garage. Five-liter cans of both types of oils were procured, and filtered to remove the contaminants and stored as a stock for further experimentation. These filtered oils from stock were used in all the experiments to maintain the uniformity.

LLW

(Model: SABC – 01) supplied by Rico scientific instruments, New Delhi, India. Ash content of pellets was measured using muffle furnace equipped with a thermostat, supplied by Meta-lab scientific industries, Mumbai, Maharashtra. India (Model: MSI-50A). The ash content was calculated as the percentage of residue remaining after dry oxidation at 600 °C using the following equation.

Ash (%) = [(Weightcrucible + ash –Weightcrucible)/(Weightcrucible + sample–Weightcrucible) ] × 100

(1)

The chemical composition analysis of the biomass samples was carried out to determine the amount of extractives, lignin, cellulose, hemicellulose and ash content. The amount of extractives in the biomass samples was determined using Soxhlet extractor setup of 250 ml at varying time intervals of 6, 18 and 24 h [16,17]. The % (w/w) of the extractives content was evaluated as the difference in weight between the raw extractive-laden biomass and extractive-free biomass. The total lignin content of the extractive free biomass was calculated by determining acid soluble and insoluble lignin. The determination of lignin from the biomass samples was carried out using the Klason method [18]. The soluble lignin fraction was determined according to TAPPI method UM 250 (TAPPI standards 1985) [19], at absorbance 280 nm in UV spectrometer using the formula below

2.2. Biomass pellets preparation Biomass size reduction was achieved using a shredder (Model no. CS 50) equipped with cyclone dust collector. The whole assembly was supplied by Bhide and Sons Pvt. Ltd., Sangli, Maharashtra, India. In preparation of biomass blended pellets, various proportions of biomass were added as an additive to a single biomass. In the description of mixed biomass pellets, first biomass indicates as an additive in the second biomass. For example, 20 wt% SWD-GNS pellets meaning 20 wt % of SWD was added as an additive in 80 wt% GNS biomass and this mixture was used to form pellets. Combinations of biomass blends with additives used in the study are shown in the Table 1. Similarly, pellets of each biomass, (i.e.) SWD, GNS and LLW were prepared with varying binder amounts (2, 4 and 6 wt%) of rPVA (Case 4), WCO (Case 5) and WLO (Case 6). Table 2 shows the various cases formulated to investigate the effect of binder on biomass pellets. The pellets were prepared using roller pelleting press (output 80–100 kg/h) machine supplied by Ecosense (EcoPellet maker -Model: PM 100). The pelleting machine consisted of a die with a tapered aperture and two rotating rollers push the feed material through this aperture in the stationary die to make a pellet.

Soluble Lignin = (A/100) × (dilution factor/original sample weight) × 100%

(2)

The filtrate from the H2SO4 treatment was thoroughly stirred and homogenized for analysis of cellulose and hemicellulose. Glucose (C1) and reducing sugar (C2) concentrations in the filtrate were determined according to a glucose oxidase-peroxidase assay kit and DNS method [20]. These concentrations were used for the analysis of cellulose and hemicellulose of the extractive free biomass using the following equations.

2.3. Chemical analysis

Cellulose content (%w/w) The CV of the pellets was measured using Bomb Calorimeter

= [(0.9/0.96) × (C1 × V)/original sample weight] × 100

Table 1 Pellet preparation using biomass as an additive. Case

Biomass

(3)

Hemicellulose content (%) = [(0.88/0.93) × ((C2 − C1) × V × a)/original sample weight)] × 100

Biomass additive amount (wt%)

(4) 1

SWD

2

GNS

3

LLW

20%

40%

60%

GNS LLW SWD LLW GNS SWD

GNS LLW SWD LLW GNS SWD

GNS LLW SWD LLW GNS SWD

In the Eq. (3), 0.9 is the coefficient obtained from the molecular weight of polymer and monomer of hexose. The saccharification yield was taken as 0.96. In the Eq. (4), 0.88 is the coefficient that results from the molecular ratio of the polymer and monomer pentose, 0.93 is the saccharification yield of xylane and xylose, C1 is the concentration of glucose (g/L) in the filtrate obtained from H2SO4 treatment, C2 is the concentration of reducing sugars in the (g/L) from the same filtrate. The 2

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Table 3 Chemical composition of raw materials. Biomass type

Cellulose (wt%)

Hemicellulose (wt%)

Lignin (wt%)

Extractives (wt%)

Ash (wt%)

SWD GNS LLW

38.5 32.2 35.2

23 27.7 25.5

28.8 22.4 24.6

6 14.4 8

3.7 3.3 6.7

Ehsanollah Oveisi-Fordiie [25]. It is measured by the percentage of broken pellets when dropped from a certain height. Drop tests of the pellets were conducted with a sample mass of 100 g enclosed in 16 cm × 16 cm bag. To prevent the cushioning effect to the pellets during the drop test, the bags were made of thin georgette fabric. Several preliminary tests from various dropping heights on the concrete floor showed that the georgette fabric bags did not break open and hence were suitable for the drop test. In order to maintain uniformity in results, dropping height of 5.5 m was kept constant for all the test sets of various samples. After each test, the content of the drop test bag was sieved using 3.2 mm sieve and remains weighed. Durability (D) of pellets was calculated as the percentage of mass retained in the sieve [6].

total volume V of sugar solution, M the dry weight of biomass and the dilution factor is represented by A in the Eq. (2) [20]. The chemical composition of the three biomass types utilized in the study is presented in Table 3 and the values were similar to those reported in the literature [16,21]. GNS contains a high amount of extractives (14.4 wt%) compared to SWD (6 wt%) and LLW (8 wt%). The chemical composition of all the three biomasses was nearly similar except for ash content. Ash content of LLW (6.7 wt%) was higher as compared with SWD (3.7 wt%) and GNS (3.3 wt%). 2.4. Mechanical testing The strength of pellets is a very important parameter to determine the shelf life, burning characteristics, durability, and finally economics of the pellets. It is affected by various factors such as moisture content, particle size, biomass composition and presence of additives or binders [12]. The use of biomass type also significantly affects the strength of the pellets [22]. The lignin present in the biomass acts as a binder on melting at a high temperature of about 140 °C, imparting strength to the pellets. [23]. These high temperatures result from friction between die and roller rotating at high speed in the pelletizer. The conditions in the pelletizer create combined effect of heat, shear, and resistance to the feed material and push it through the tapered cavity of the die resulting in the densification of biomass. Presence of extractives plays a discouraging role in the compaction by acting as a lubricant in the cavity of the die and minimizing the resistance for compaction resulting in low strength of pellets [5]. The flexural strength of the pellet was measured using Brookfield texture analyzer supplied by AMETEK Instruments India Pvt. Ltd., Bangalore, India. The load cell of 500 N and a three-point bend fixture assembly (TA-TPB) was used for all the analysis. The pellets were placed horizontally on the support and the blade (TA7) was released at a crosshead speed of 30 mm min−1. A trigger load of 0.1 N was fed into the instrument with a target load of 200 N. The failure load at which pellets broke were recorded and logged into the computer. Pellets of 30 mm length and 8 mm diameter were selected from each batch for flexural strength analysis. Flexural strength tests were repeated 5 times for each sample to determine the deviations in the pellet forming process. This deviation was shown in the graphs using error bars. The flexural strength of the pellet was obtained by using the following equation [24].

σ = [FL/πR3]

D = [Mf is the mass (g) remaining on the sieve/Mi is initial mass (g)] × 100

(6)

3. Results and discussion To evaluate the performance of the pellets as a firewood substitute and sustainable fuel, it is important to gauge three main characteristics of the pellets (i.e.) strength, durability, and CV. Correspondingly the results are discussed in a similar structure with the strength of pellet being at the center of the discussion followed by durability and CV. 3.1. Strength of pellets This study evaluates the effect of biomass blending on the strength of mixed biomass pellets. Likewise, another method to improve the strength of pellets is to incorporate binders in the pelletization process [26]. The use of binders has shown to enhance the strength of pellets formed from biomass exhibiting lower binding abilities. This section discusses the effect of the two methodologies on the strength of the biomass pellets. 3.1.1. Effect of biomass blending on the strength of pellets Fig. 1 shows the effect of SWD addition as an additive on the strength of pellets obtained from LLW and GNS biomass pellets (Case 1). The strength of pure SWD pellets was inherently high as a result this biomass was added as an additive to LLW and GNS to prepare mixed biomass pellets. The composition of the additive (i.e.) SWD was varied from 0 to 60 wt% in the mixed biomass pellets. It can be seen that the strength of GNS and LLW pellets increased with the addition of SWD. In comparison to GNS, the increase in strength of LLW pellets with SWD addition was insignificant. The strength of pure SWD pellet was marginally greater than LLW pellets, as evidenced with the insignificant rise in strength of the pellet formed by the addition of SWD as an additive to LLW. The strength of LLW pellet with zero addition of SWD was 4304 kPa which increased to 4399 kPa by 20 wt% addition of SWD. The further increase in the SWD additive amounts to 40 and 60 wt% resulted in a minuscule increase in the strength of the pellets (~5 and 7.5% respectively). In comparison, the additive had a profound effect on pellets prepared from GNS. The strength of GNS without additive was 1382 kPa which increased by ~57% with the addition of 20 wt% SWD. The higher amounts of SWD (i.e.) 40 wt% addition showed 81% rise in strength of the pellets. Finally, a 165% rise was exhibited on the addition of 60 wt% SWD to GNS pellets. The variation in the strength of

(5)

In the above equation, σ = Flexural strength (kPa), F = Load at break (N), L = Support Span (m) and R = Radius of pellet (m). 2.5. Durability The durability of pellets is an another important property to determine effectiveness and usability of the pellets. Pellets with low durability may indicate problems in maintaining structural integrity during handling, transport, and operations. Pellets disintegrate easily either due to moisture absorption or falling or friction. The durability indications breakage tendancy of the pellets. Lower durability exhibiting pellets have a higher chance of breakage. Durability is used for quantifying the quality of pellet's integrity via drop test. The drop test was performed as per the procedure used by 3

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Fig. 1. Effect of addition of SWD as an additive on the strength of pellets obtained from LLW and GNS biomass.

The strength of LLW pellets without any addition of GNS was 4303 kPa which decreased to 4097 kPa which represents a decrease of 5% on the addition of 20 wt% GNS. The addition of 40 and 60 wt% GNS in LLW decreased the strength of the GNS-LLW mixed biomass pellets by 20 and 36% respectively, in comparison to pure LLW pellets. The strength variations in case of GNS added pellets increased with GNS amount. The variations increased from 2 to 6% when GNS addition was increased 20 to 60 wt% in GNS-SWD mixed biomass pellet. The variations in strength were negligible when GNS was added to LLW, resulting in a minor variation of 1 to 2% in all the compositions of 20 to 60 wt% GNS. Fig. 3 shows the effect of LLW as an additive to prepare GNS and SWD mixed biomass pellets (Case 3). The addition of LLW as an additive helped to increase the strength of LLW-GNS pellets, while it decreased the strength of LLW-SWD pellets. At 20 wt% LLW addition, LLW-GNS mixed pellets strength was 1779 kPa as compared to 4653 kPa for LLW-SWD pellets, which represents a rise of ~29% as compared to 5% with respect to pure GNS and SWD pellets. Higher amounts of additive (i.e.) 40 wt% of LLW represented a rise 94% for LLW-GNS pellets in comparison to 10% rise in LLW-SWD pellets. Finally, at 60 wt% LLW additive, the rise in strength was ~149% for LLWGNS pellets as compared to 2% in LLW-SWD mixed biomass pellets. Contra positive effect on strength of LLW addition to SWD was observed. The strength of mixed biomass (i.e.) SWD-LLW pellets decreased inappreciably in comparison to pure SWD pellet. The drop in strength was 0.5, 1 and 3.5% when 20, 40 and 60 wt% LLW biomass was added to SWD, forming pellets of relative strengths. The deviations in the strength, when LLW was added as an additive were hardly noticeable in both the biomass at all the compositions. The effects of adding LLW additive to GNS based mixed pellets are governed by similar phenomenon in case of SWD-GNS pellets as explained above in Fig. 1. The addition of LLW having marginally lower lignin content (Table 3) resulted in reduction of the lignin amount during the LLWSWD pellets preparation, resulting in slight decrease in the strength of the LLW-SWD pellet with rise in LLW amount.

pellets when SWD was added to LLW was nominal, 2–5%, in all the additive amounts (i.e.) 0 to 60 wt%. On the other hand, the variations in strength were lower when SWD was added to GNS, the strength of the pellet varied from 2 to 4%. The reason for marginal rise in strength of SWD-LLW pellets could be because of similar lignin and extractive content of both biomasses (Table 3), making very small difference in the natural binder (i.e.) lignin amount in the pelletization process. But in case of SWD-GNS pellets, the addition of SWD had profound effect on strength of the pellets. This can be attributed to increase in the amount of lignin (Table 3) in pelletization process with higher amounts of SWD addition, increasing the binding ability of SWD-GNS pellets. The presence extractives during pelletization process would be reduced by addition of SWD in the SWD-GNS pellets. The presences of higher extractives in pure GNS (Table 3) are well known to reduce the strength of the pellets by lubricating the aperture of the pelleting machine [5]. It is suggested that extractives also affect pellet durability by blocking the binding sites for hydrogen bonding to occur between particles [27] The hydrophobic nature of the extractives were most likely responsible for lowering the compression strength, due to presence of a chemical weak boundary layer, limiting the adhesion mechanism to van der Waals forces [28]. A counter effect was observed when GNS was used as an additive to prepare mixed pellets of LLW and SWD. Fig. 2 shows the effect of GNS as an additive to prepare LLW and SWD pellets (Case 2). The addition of the GNS as an additive to LLW and SWD pelletization had a detrimental effect on the strength of the mixed biomass pellets. The percent rise in GNS amount decreased the strength of the mixed biomass pellets. The strength of SWD pellets decreased from 4414 to 4290 kPa which represents a drop of 3% in the strength of the mixed pellet at 20 wt% GNS addition. Further increase in GNS amounts of 40 and 60 wt%, resulted in a sharp drop of 17 and 43% in the strength of the pellets respectively. Primarily, one of the reasons for the lower strength of mixed pellets could be diminished the strength of GNS (i.e.) 1382 kPa in comparison to 4414 kPa of SWD. The binding ability of high-density material like SWD would be curtailed by addition of low-bulk-density (i.e.) GNS, in turn affecting the densification process. It can be argued that when pellets are prepared of mixed biomass GNS and SWD, the low-density material GNS will cushion the higher density material SWD during the pelletization process, resulting in lower strength pellets. This observation can also be seen in the case of GNS and LLW mixed biomass pellets.

3.1.2. Effect of binders on strength of biomass pellets The investigation of binder addition for improving the performance of pelletization is widely addressed [29,30]. Binders form bridge or matrix to make strong inter-particle bonding with biomass components [14 ], improving the strength and CV of the pellets. The selection of 4

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S.P. Rajput, et al.

Fig. 2. Effect of addition of GNS as an additive on the strength of pellets obtained from LLW and SWD biomass.

and 6 wt% binder addition. As the rPVA binder contains water and dissolved polymer, the existence of water caused wettability of the bulky material allowing the polymer to spread over and improving adhesion between the biomass particles and improving the strength of the pellet. The moisture contains in the binder evaporated during the pelleting process. Further rPVA would have melted at pelleting temperatures acting as an adhesive imparting the strength to the pellets. The variation in the strength of pellets prepared by GNS ranged from 3 to 8% for 2 to 6 wt% binder amount. Bulky nature of GNS material has large void space resulting in entrapment of binder into the void causing the interlocking and capillary attraction between the particles imparting strength to the pellet [31,32]. The LLW and SWD are densified forms of biomass having low voidage and less chance for the polymer material to bind with the biomass particles and improve the strength of the pellet. On addition of 2 wt%

binders is defined by cost and environmental friendliness. The use of textile effluent recovered material as binder has not been investigated for biomass pelletization; as a result rPVA was chosen for pelletization process and compared with the other two binders. One of the most important factors in binder addition is the cost involved; materials like rPVA end up in the effluent of textile industry effluents, creating environmental concerns. The value addition by using rPVA as binder would incentivize the textile industry effluent treatment process and provide economic benefits to biomass pelletization. Fig. 4 represents the effect of rPVA binder on the strength of pellets obtained from SWD, GNS, and LLW (Case 4). The predominant effect of binder addition was seen on GNS pellets in comparison to LLW and SWD pellets. It was observed that the strength increased considerably on adding rPVA binder. At lower amounts of 2 wt% addition resulted in a sharp increase of 60% in strength, followed by 107 and 163% with 4

Fig. 3. Effect of addition of LLW as an additive on the strength of pellets obtained from GNS and SWD biomass. 5

Fuel Processing Technology 199 (2020) 106255

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Fig. 4. Effect of rPVA binder on the strength of pellets obtained from SWD, GNS and LLW biomass.

decrease in GNS pellet strengths were 24, 27 and 31% respectively when 2, 4 and 6 wt% binder was added to the pellet making process. In LLW and SWD, the pellet strength reduced by 16 and 10% respectively on 2 wt% WLO binder addition. A higher amount accounted for 18 and 14% reduction in strengh when 4 wt% WLO binder was added to LLW and SWD pellets respectively. At the highest WLO binder content of 6 wt%, the reduction in strength of both the biomass pellets was identical; a 23% reduction was noted in LLW and SWD. The strength of a pellet can be attributed to the following phenomenon [32]:

rPVA binder, the LLW and SWD showed an increase of 17 and 10% in the pellet strength respectively, in compassion to the pure biomass pellets. Higher binder amounts increased the strength of LLW and SWD pellets to a lesser extent as compared to GNS pellets. The 4 wt% rPVA binder addition showed an increase of 22 and 20% in LLW and SWD pellets respectively. The maximum increase of 33% was exhibited when 6 wt% binder was added to LLW as compared to 26% in SWD at the same binder amount. The variation in the strength of LLW and SWD pellets was only 1–4% which was expected considering the nature of biomass. Likewise, other binder materials WCO and WLO have little possibility of further utilizing and ending up in domestic and urban waste. Oil addition can improve wood fuel properties, but densification of wood with added oil can be more challenging. Availability of WCO is not a limitation since large amounts are available from restaurants, food processing industry and households, prompting its use as a binder. Vehicle serving centers generate large quantities of WLO since all vehicle need to replace engine oil periodically. Fig. 5 shows the effect of using WCO as a binder in the pelleting of GNS, LLW, and SWD (Case 5). The use of WCO was investigated as an alternative binder due to its availability and low cost. The objective was to improve the strength of the pellets and impart higher CV. It was observed that the strength of GNS pellet decreased with the addition of WCO. In comparison to 0% binder, 2 wt% addition of WCO reduced the strength of GNS pellet by 15%. This reduction rose to 43% when the binder amount was increased to 4 wt%. The further reduction in the strength of pellet was 49% on 6 wt% WCO addition. In comparison to GNS, the strength of LLW and SWD pellets was not convincingly decreased on the addition of WCO. At lower binder amounts (i.e.) 2 wt% the strength of LLW and SWD decreased by 15 and 7% respectively, while higher amounts showed similar reductions. LLW pellets strength decreased by 21 and 24% on addition of 4 and 6 wt% binders. On the other hand, the reduction in the case of SWD was 24 and 26% respectively, on the addition of 4 and 6 wt% binders. Equivalent effect on the strength of pellet was observed when the binder changed from WCO to WLO. The effect of WLO as a binder on the strength of the SWD, GNS, and LLW biomass pellets is represented in Fig. 6 (Case 6). This indicated that the type of oil used did not affect the strength of the pellet. A nominal reduction in strength of pellets was seen in all the biomass pellets prepared using oils as a binder. The

i. ii. iii. iv.

Solid bridges, adhesion and cohesion forces. Surface tension and capillary pressure. Attraction forces between solids. Interlocking bonds.

The oil layer on the sawdust particles led to the surface deactivation, which resulted in the deactivation of all the mechanisms that include contact sites and attraction forces between surfaces lowering the strength of the pellet. According to Misljenovic et al. [32] only interlocking and capillary action and attraction between the particles due to entrapped oil gave the strength to pellets. The interlocking was poor because of the oil's lubricating effect which caused sliding between the particles reducing the strength of the pellets. In addition, oil would lubricate the dye aperture of the pelleting press reducing the resistance for compaction.

3.2. Durability of pellets A direct relationship between the strength and durability of the pellet was observed. The higher strength resulted in greater durability of the pellet. The highest durability of 97% was shown by SWD pellets, moderate durability of 94.3% observed for LLW pellets, whereas GNS pellets exhibited 82.6% durability which was lowest among all the three biomass. Table 4 shows the effect of biomass blending on the durability of pellets. The addition of SWD as blending material in other biomass (i.e.) GNS and LLW increased the durability of the pellets. The rise in the durability of GNS pellets was prominent in comparison to LLW. The GNS pellet durability rose by 6.4% when 60 wt% SWD was added to form SWD-GNS mixed biomass pellet. In comparison, 6

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Fig. 5. Effect of WCO binder on the strength of pellets obtained from SWD, GNS and LLW biomass.

durability incresed insignificantly (1.7%) when 60 wt% SWD was added in LLW to produce SWD-LLW mixed biomass pellets. The addition of GNS to other biomass had a negative effect on the durability of the mixed biomass pellets, which was also seen in case of strength of the pellet. The durability was decreased by 8.3 and 10% when 60 wt% GNS was added to LLW and SWD based mixed biomass (GNS-LLW and GNS-SWD) pellets, respectively. The addition of LLW also reduced the durability of mixed biomass (i.e.) LLW-SWD pellets, but it increased the durability of LLW-GNS mixed biomass pellets. Marginal reduction of 1.5% in durability was recorded on the addition of 60 wt% LLW to SWD to produce LLW-SWD mixed biomass pellets. Moreover, the durability of GNS increased by 5.4% on addition of 60 wt % LLW to the mixed biomass LLW-GNS pellets. Addition of such biomass like SWD improved the durability of weaker pellets (i.e.) GNS, while it inappreciably affected the durability of relatively higher

Table 4 Effect of biomass blending on the durability of pellets. Binder amount wt. (%)

Durability (%) Biomass GNS

LLW

SWD

Binders

0 2 4 6

rPVA

WCO

WLO

rPVA

WCO

WLO

rPVA

WCO

WLO

82.6 83.8 85.4 88

82.6 82 81.5 81

82.6 81.6 81 80.5

94.3 95 95.5 96

94.3 94 93.7 93

94.3 94 93 92

97 97.3 97.6 98

97 96.5 95.8 95

97 96.5 95 94

Fig. 6. Effect of WLO binder on the strength of pellets obtained from SWD, GNS and LLW biomass. 7

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behavior was consistent with the strength results. A prominent effect on durability was seen in GNS pellets as compared to LLW and SWD pellets. Increasing the rPVA binder amount increased the durability of the pellets, while it decreased with increase in WCO and WLO binder amounts. On addition of 6 wt% rPVA, a 5.4% increase in durability was seen in GNS pellets as compared to 1.7 and 1% in LLW and SWD pellets. The reduction in durability with the use of WCO and WLO addition was very similar. Both these binders very marginally decreased the durability of all the three biomass pellets. It can be concluded that biomass exhibiting higher strength contributed to the better durability of the mixed biomass pellets. The addition of such biomass improved the durability of the pellets exhibiting lower durability. Further, the addition of aqueous-based binders had a positive impact on the durability of the pellets, as compared to oil-based binders.

Table 5 Effect of binder addition on the durability of pellets. Additive amount wt. (%)

Durability (%) Additive GNS

LLW

SWD

Biomass

0 20 40 60

LLW

SWD

GNS

SWD

GNS

LLW

94.3 92 88 86

97 94 89 87

82.6 84.4 86 88

97 96.6 96 95.5

82.6 85 87 89

94.3 95 95.5 96

3.3. Calorific value of pellets

strength biomass (i.e.) LLW. Table 5 shows the effect of binder addition on the durability of pellets. The addition of rPVA binder showed a positive impact on the durability of all the pellets, as compared to WCO and WLO binders. This

The most important criteria for the selection of fuel pellets is the CV. The energy density of biomass is lower than that of fossil fuel which makes it less preferable for use as fuel. Biomass characteristics such as

Fig. 7. Calorific values of pellets obtained from biomass blending. In the figure a) effect of GNS additive, b) effect of SWD additive, c) effect of LLW additive. 8

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Fig. 8. Calorific value of pellets obtained by the addition of binders. In the figure a) effect of WCO binder, b) effect of WLO binder, c) effect of rPVA binder.

GNS pellets decreased slightly when SWD was used as an additive. In comparison to GNS pellets, maximum 3% reduction was observed in SWD-GNS pellets on 60 wt% SWD addition. A 15% rise was noted on equivalent addition of SWD to LLW to form SWD-LLW mixed biomass pellets. The addition of GNS as an additive in mixed biomass pellets exhibited the best result in all the combinations. The CV of GNS-LLW mixed biomass pellets increased by 21% when 60 wt% of GNS was added to LLW. On the other hand, adding GNS to SWD for preparing GNS-SWD pellet provided a mere 3.6% rise in CV. The addition of LLW as an additive in mixed biomass pellets negatively altered the CV. The effect increased with the rise in LLW additive amount. At 60 wt% LLW addition, CV decreased to 15 and 13% for LLW-GNS and LLW-SWD pellets respectively. The addition of binder for pellet making also enhances the CV of the pellet. The type of binder used in the pelleting process alters the CV of the pellets. Fig. 8 represents the effect of binder addition on the CV of biomass pellets obtained from SWD, GNS, and LLW. Addition of aqueous binder (i.e.) rPVA had an inconsiderable effect on the CV of the pellets. In all the biomass pellets, the maximum increase in CV was 3% with 6 wt% rPVA addition, although the CV increased when the binder

high moisture content, low CV, friability, and others lead to the high combustion operating costs. The main concern relevant to the use of biomass energy is mainly related to the low CV of biomass. The CV of biomass pellets is also affected by the ash content. The higher the ash content of the raw material, the lower the CV. The ash in the pellets generates no energy and causes problems of slagging and results in deposit formation on the burners [33]. As a result, methods to improve the CV and reduce ash formation by the use of binders and combinations of biomass are being extensively pursued [34]. Fig. 7 represents the CV of mixed biomass pellets prepared from GNS, SWD and LLW biomass. Considering these three biomasses, GNS pellets showed the highest CV followed SWD and LLW. The three biomass pellets without additive addition exhibited CV of 4600, 4356 and 3458 kcal/kg respectively. These values were similar to those reported in the literature. The CV of 4143 kcal/kg for SWD was reported by Cheng et al. [31] and 4109 kcal/kg was reported by Zhu et al. [35]. Agricultural residues like rice straw also show similar CV 4469 kcal/kg [36]. The addition of additives had both positive and negative effects on the net CV of the pellets. On using SWD as an additive, the CV of LLW and GNS pellets had no cogent effect. The CV of the mixed SWD9

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Fig. 9. Effect of biomass additives on ash formation. In the figure a) effect of GNS additive, b) effect of SWD additive, c) effect of LLW additive.

[32]. The results indicate that the type of oil (i.e.) vegetable or petroleum inexorably alter the CV of the pellets.

amount was increased. As the rPVA binder is an aqueous mixture which contains only 7 wt% PVA which add to the CV of the pellet. The increase in binder amount increased the rPVA content in the pellets improving the CV. The GNS and LLW pellets showed a similar rise of 3% as compared to 2% shown by SWD at constant binder amount of 6 wt% rPVA. A similar observation is made by Deshannavar et al. [36] exhibiting a marginal rise in CV when starch was added as a binder to rice straw briquettes. The CV of rice straw briquettes increased from 4374 to 4436 kcal/kg when 6 wt% starch was added as a binder [36]. The addition of WCO and WLO had a positive impact on the CV of the pellets at all the binder amounts investigated. The most noticeable effect of oil-based binders was seen in the case of LLW pellets. At 6 wt% WLO and WCO binder amount, the CV of 3900 and 4083 kcal/kg was exhibited by LLW pellets which indicates a rise by 18 and 15% over binder-free pellets. The oil-based binder amount had similar results for both GNS and SWD pellets. The maximum rise in the CV of 12 and 13% was observed when 6 wt% WCO and WLO were added to GNS pellets. In the case of SWD, the increment in CV was 13 and 15% on the addition of 6 wt% of both WCO and WLO binders. The GNS and SWD pellets had a CV of 5167 and 4910 kcal/kg when WCO was used as a binder at 6 wt % addition. Adding WLO slightly increased theCV of GNS and SWD pellets to 5219 and 5002 kcal/kg. Misljenovic et al. reported the CV of 4680 kcal/kg when 2.2 wt% vegetable oil was added to SWD pellets

3.4. Ash formation from pellets Ash formation is one of the important parameter determining the acceptability of biomass-based renewable fuels. Biomass fuels generate a wide range of ash depending upon the inorganic materials present in the biomass or introduced during the processing steps. The formation of ash affects particulate emissions from biomass combustion units and also can cause problems due to slagging, deposit formation and corrosion [37,38]. The chemical composition of biomass fuels and especially the contents of ash-forming elements influence the choice of an appropriate combustion and process control technology [39,40]. It affects deposit formation, fly ash emissions and ash handling as well as ash utilization/disposal options. The formation of ash from various biomass blends is shown in Fig. 9. The addition of additives seems to influence the ash forming phenomenon whereas binder addition has no effect on ash content. As a result, ash formation was only reported in the mixed biomass pellets. In all the three biomass, LLW exhibited the highest ash content, followed by SWD and GNS. LLW exhibited ash content of 6.7% which was twice the value seen in GNS (3.3%) and SWD (3.7%). Fasina et al. [10] reported ash content of 2.91 wt% on combustion of GNS 10

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Funding

pellets. Zhu et al. [35] found the ash content of 3.91 wt% when sawdust pellets were combusted. The addition of GNS and SWD seems to decrease the ash formation in LLW pellets. Addition of 20 wt% SWD to LLW to form SWD-LLW pellets reduced ash content by 8.6%. The reduction in ash content increased to 26.6% when SWD content reached 60 wt% in SWD-LLW pellets. The 20 wt% GNS addition to LLW pellets was responsible for ash reduction by 10.44%, higher amounts of additive resulted in a ~30% reduction in ash formation from GNS-LLW pellets. In GNS mixed pellets, additive addition unremarkably affected ash formation. The basic ash content of GNS pellets was less, to begin with, which slightly increased on the addition of SWD in the SWD-GNS mixed biomass pellets. Addition of 60 wt% SWD insignificantly increased the ash content of the SWD-GNS mixed biomass pellets from 3.3 to 3.55%. On the other hand, higher ash containing biomass (i.e.) LLW addition to GNS (LLW-GNS pellet) was responsible for an indicative increase in ash formation from 3.3 to 5.33% which indicated an increase of 60% in the total ash formation. The addition of LLW to SWD to prepare mixed biomass pellets (LLW-SWD) exhibited congruent results to GNS based mixed biomass (i.e.) LLW-GNS pellets. The supplementation of LLW in SWD (LLW-SWD) pellets caused ash content to increase from 3.7 to 5.53 wt%, representing a rise of ~50%. The lower ash content of GNS slightly reduced the ash content of SWD mixed biomass (GNS-SWD) pellets by 0.23% at 60 wt% addition of GNS as an additive.

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4. Conclusion This work evaluated the use of additives and binders to improve quality of biomass pellets. GNS pellets showed highest CV but lowest strength among all the biomass studied, whereas SWD pellets showed the highest strength. It can be concluded that GNS can be used as an additive to improve CV, whereas SWD can be used as an additive to improve the strength of mixed biomass pellets. On the other hand, SWD pellets have a uniform and fine particle size distribution which resulted in the higher strength. At par strength of LLW biomass with SWD and abundant availability makes LLW as the desired additive to improve the strength of mixed biomass pellets for rural and urban applications. The rPVA binder improved the strength of all biomass pellets but slightly reduced CV due to its aqueous nature. Conversely, WCO and WLO increased the CV but insignificantly decreased the strength of the pellet. Among the three binders investigated, rPVA binder would be used to improve the strength of biomass pellets inherently having a higher CV like GNS, whereas WCO and WLO would be used to improve the CV of pellets of biomass like LLW. The availability of WCO and WLO would be an influential factor in its wide-scale applicability as compared to rPVA. The effect of these binders on combustion characteristics and emissions would have to be carried out before wide-scale applications of these binders in biomass pelletization can be explored. Nomenclature SWD GNS LLW rPVA WCO WLO CV TPD MMTPA

sawdust ground nut shell leaf litter waste recovered polyvinyl alcohol waste cooking oil waste lubricating oil calorific value tons per day million metric tons per annum

Acknowledgments The authors are thankful to M/s S. A. Pharmachem, Mumbai, and in particular Mr. V. D. Patil for his support for developing a membrane separation process for the production of rPVA binder. 11

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