Binder addition

Many biomass feedstocks possessed natural binding agents [Shaw, 2008]. However, additional binders are often added for better binding in densification. Various binders have been employed to improve the binding characteristics, compressive strength, and general quality of fuel briquettes. They can also reduce the energy cost of producing such briquettes by reducing the amount of compaction pressure or temperature required for conditioning. The use of binders during biomass densification also reduces the wear on production equipment and production costs [e.g., Kaliyan & Morey, 2010b; Tumuluru et al, 2011], for example, by reducing the compaction pressure, energy, and time required to densify a specific quantity of biomass material.

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Binders commonly used in briquetting include starch, molasses, lignosulphonates (in animal feed processing) or sulfonate salts made from lignin in pulp [Thomas et al, 1998; Williams & Nugrand, 2000; Tabil & Sokhansanj, 1996], or biomass wastes that are rich in natural binders, e.g., rice bran and sawdust [Chou et al, 2009]. Recent research has focused on developing new, cheaper and more sustainable binders, as well as optimising the ratio of binder to feed biomass. A variety of effects of binders on briquette quality have been reported:

Chin & Siddiqui [2000] reported a decrease in the relaxed density of briquettes with an increase in binder ratio for sawdust and coconut fiber, yet an increase in relaxed density of briquettes with an increase of binder ratio for peanut shell and palm fiber. Singh & Singh [1982] reported an increase in briquette strength with increased addition of a molasses and sodium silicate binder in briquettes from rice straw. Kaliyan & Morey [2009] discovered that solid bridges were made by natural binders such as lignin and protein, in binderless briquetting with corn stover and switch grass. They also found that temperatures in the range of glass transition (75 - 100o_{C) is important for efficient particle bonding.}

Oladeji & Enwerenmadu [2012] also showed a reduction of corn cob briquette density with increased addition of a starch binder.

Emerhi [2011] used three different organic binders including cow dung, wood ash and starch in briquetting of sawdust, to assess the effect on calorific value of the produced briquettes. Results showed that starch-bound briquettes produced the highest calorific value while ash bound briquettes had the least calorific value. Sivakumar et al [2012] showed that briquetting sawdust with a cow dung binder could be optimized to increase the thermal efficiency and methane content of the product gas in a downdraft gasifier.

Despite the advantages of using binders in biomass briquetting, problems have been encountered with some types of binders when fuel briquettes are converted to energy, including air emissions from pollutants in untreated materials, deposit formation and corrosion of equipment [Obernbergera & Theka, 2004]. Other binders may have resource problems, e.g., starch, which is also a food product. There is therefore a need to explore better and more environmentally friendly binders for biomass briquetting, and evaluate these with existing binders. The main binders investigated in this research include, starch, biosolids and microalgae.

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Figure 5a to c shows examples of starch, biosolids and microalgae as binders for briquetting. Table 5 compares the physical and chemical properties of starch, biosolids and micro-algae binders used in this study as gathered from sources in literature [Paine & Vadas, 1969; Merrill &Watt, 1973; Logan & Harrison, 1994: Stain, 1998; Andreoli et al, 2001; Dweck et al, 2006; Xiong et al, 2008; Barz, 2009;Phuphuakrat et al, 2010; Silva et al, 2012; Vardon et al, 2012; Egun and Abah, 2013; Bi & He, 2013; Jiang et al, 2014; Sudjito et al, 2014].

Figure 5: Binders for briquetting (a) starch, (b) biosolids, and (c) harvested concentrated microalgae [UFP, 2013; OSE, 2015; PJC, 2013]

Starch

Starch in its pure form is a tasteless and odourless white powder which can be sourced from various kinds of crops such as rice, wheat, cassava, yam, and potato. It has two major components: amylose and amylopectin [Satin, 1998]. These polymers are very different structurally, amylose being linear and amylopectin highly branched. The ratio of these two components influences its viscosity, shear resistance, gelatinization, textures, solubility, tackiness, gel stability, cold swelling and retrogradation of the starch [Satin, 1998; Oladeji & Enweremadu, 2012]. These components of starch are also regarded as one of the natural binding compounds present among protein and lignin part of various types of biomass.

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Table 5: Comparison of basic properties of starch, biosolid and micro-algae

Properties Starch Biosolid Micro-algae Reference

Calorific value (MJ/kg dry mass) 18 6-19 15-23

[Silva et al, 2012; Andreoli et al, 2001; Dweck et al, 2006; Merrill and Watt, 1973]

Ash content (% dry mass) 0.08 31 10

[Jiang et al, 2014; Xiong et al, 2008; Phuphuakrat et al, 2010; Vardon et al, 2011]

Moisture content (% undried

mass) 4-11 5-11 7

[Jiang et al; 2014; Egun and Abah, 2013; Xiong et al, 2008]

Volatile matter ((% dry mass) - 39-57 67 [Jiang et al, 2014; Phuphuakrat, 2010; Sudjito et al, 2014] Bulk density (kg/m3 _{dry mass) 617 400-800 370-435} [ Egun and Abah, 2013; Logan and Harrison, 1994]

Amylopectin (%) 0-70* N/A N/A [Satin, 1998]

Cellulose - 1 7.1 [Ververis et al, 2006; Hattori and Mukai, 1986]

Lignin (% dry mass) - 10-10.3 2 [Ververis et al, 2006; Vardon et al, 2011; Hattori and Mukai, 1986] Hemicellulose (%dry mass) - - 16.3 [Ververis et al, 2006; Hattori and Mukai, 1986]

Protein (% dry mass) 0.23 15-35 64** [Xiong et al, 2008; Vardon et al, 2006; Sudjito et al, 2014]

Fat (% dry mass) 0.075 13 2-10

[Silva et al, 2012; Andreoli et al, 2001; Dweck et al, 2006; Merrill and Watt, 1973]

Nitrogen (% dry mass) NA 3.3 -3.7 1.6 -6.8*** [Bi & He, 2013; Barz, 2009]

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Properties Starch Biosolid Micro-algae Reference

Chlorine (% dry mass) NA 0.02 1.97 [Sudjito et al, 2014; Barz, 2009]

Calorific value (MJ/kg) 17.5 10.1 -16.2 18.59 [Silva et al, 2012; Bi & He, 2013; Barz, 2009]

Lipid (%) NA NA 21.3 -30.8 [Bi & He, 2013]

NA = not available

* The remainder of the starch is assumed to be amylose **Value obtained from different strains of microalgae

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Starch has various applications as a binder in non-food industries such as textiles, cosmetics and pharmaceuticals, explosives, paper, construction, etc. Its high energy content, and chemical and structural properties make it a promising binding agent for fuel briquetting. Addition of water and heat to starch granules causes swelling, which results in the formation of intermolecular hydrogen bonds between the amylose and amylopectin components of starch, followed by loss of the individual crystalline structure of the two components [Tako & Hizikuri, 2002]. This leads to formation of a viscous solution that undergoes retrogradation, i.e., gelling, during cooling or storage. The viscosity of hydrated starch increases its shear and tensile strengths. The fluidity and viscoelasticity of the produced solution [Tako & Hizikuri, 2002] gives it the ability to occupy the void spaces present within and between biomass particles, forming solid bridges that become stronger upon air-drying.

Biosolids

Biosolids are the residue from anaerobic digestion of waste activated sludge from municipal wastewater treatment. Biosolids contain valuable organic matter and high content of natural binding compounds such as lignin and protein (Table 5), which are useful in solid compaction processes [Silva et al, 2012] (2.4.6.5).

In its untreated state, biosolids contains pathogenic organisms present in municipal wastewater [EC, 2012]. Therefore, it has become a requirement to treat biosolids before disposal, application on farm land or other applications [ADAS, 2001]. Conventional treatment destroys at least 99% of the pathogens; this has been superseded by enhanced treatment which ensures that 99.99% of pathogens are destroyed [ADAS, 2001]. The binding ability of a particular biosolid is highly influenced by the type of waste and treatment method it undergoes.

Microalgae

Algae consist of large group photosynthetic, heterotrophic organisms from different phylogenetic groups, representing many taxonomic divisions. They are distributed worldwide, inhabiting pre-dominantly fresh and sea water ecosystem [Guschina & Harwood, 2013].

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The use of microalgae as a source of renewable oils for biofuel production has gained significant attention in the recent years; this is attributed to the potential benefits presented by microalgae biomass, for example, easy to cultivate, ability to capture carbon during growth, waste management potential (waste water), high lipid content etc. Under the right conditions, some algae strains can produce 50% of their dry weight in the form of lipids suitable for fuel production. However, the remaining 50% contains large amount of fixed carbon and energy. The efficient recovery of the energy and carbon entrained in this residue is important for improved environmental and economic sustainability of algal biofuels [Jarvis, 2011].

Algal residue has a potential application in material binding due to its high quantity of protein and other biomass tissue including cellulose, hermicellulose and lignin. In the presence of moisture, algae residue releases a binding substance that act as glue between loose material particles, this facilitates the formation of solid bridges and closing of void spaces between biomass particles [Ververis et al, 2006]. For example, fresh water micro algal biomass was found to increase the mechanical strength of paper pulp [Ververis et al, 2006].