Renewable and Sustainable Energy Reviews 73 (2017) 1289–1299
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An overview of effect of process parameters on hydrothermal carbonization of biomass
MARK
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Sabzoi Nizamuddina, , Humair Ahmed Balochb, G.J. Griffina, N.M. Mubarakc,⁎⁎, Abdul Waheed Bhuttob, Rashid Abrod, Shaukat Ali Mazarib, Brahim Si Alie a
School of Engineering, RMIT University, Melbourne, 3001 Australia Department of Chemical Engineering, Dawood University of Engineering and Technology, MA Jinnah Road, 74800, Karachi, Pakistan c Department of Chemical Engineering, School of Engineering and Science, Curtin University, 98009 Sarawak, Malaysia d Engineering Services Center, Karachi Laboratories Complex, Pakistan Council of Scientific and Industrial Research, Karachi, Pakistan e Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia b
A R T I C L E I N F O
A BS T RAC T
Keywords: Process parameters Hydrothermal carbonization Catalyst High-energy solid fuel
The preceding decades witnessed hydrothermal processes being actively utilized all over the world, specifically in the developed zones. Their optimum usage is primarily sought for in terms of conversion of biomass into valuable solid, liquid and gaseous fuels. Indeed, Hydrothermal carbonization (HTC) is an effective and environment friendly technique; it possesses extensive potential towards producing high-energy density solid fuels. However, the production and quality of solid fuels from HTC depends upon several parameters; temperature, feed type, residence time, pressure and catalyst being the eminent ones. This study investigates the influence of operating parameters on solid fuel production during HTC. The biomass quality has also been analyzed in HTC by extending existing literature work through experiments that have been performed. Data including chemical composition, heating value, proximate analysis and ultimate analysis for different types of biomass was consequently collected and analyzed. It was found that reaction temperature, residence time and type of feed material are the primary factors that influence the HTC process. At higher temperatures, lower solid product is obtained; the carbon content increases, whilst the hydrogen and oxygen content decrease. Further, it has been found that higher lignin content in biomass leads to an increased solid fuel production.
1. Introduction Before the emergence of a fossil fuel based economy in the 19th century; biomass was the primary source serving as an integral contributor towards energy production. However, the realm of reduced energy output from biomass readily replaced the latter with fossil fuels along with increased acceptance. Further, fossil fuels also committed to serve as a cost effective and a feasible alternate for the world at large. The passage of time to date witnessed a rise in demand of energy due to an escalated growth in population and industrialization worldwide. The persistence of rapidly emerging new economies in the forthcoming decades is speculated to add apprehension to the existing gap between demand and supply. It is henceforth a consequence that concerns have apparently reached a pivotal point whereby it is anticipated that the fossil fuel energy will not be able to fulfil the demand. Further, environmental problems imposed by the liberation of
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greenhouse gases (GHG) pre-eminently carbon dioxide (CO2), nitrogen oxide (NOx) and particulate matter by utilization of fossil fuels continue to remain factors of concern [1]. As stated by the International Maritime Organization (IMO), there exists a need to reduce more than 80% of the engine-out NOx. The latter is supported by Fig. 1 that shows the emission of CO2 from fossil fuels and highlights a drastic increase of CO2 in the atmosphere. To further jeopardize the situation unbalanced market prices, and limited fuel availability [2–6] add to the drawbacks that deviate from the support of utilization of fossil fuels. It is therefore foreseen that a shift backwards towards a biomass-based economy is more than just envisioned. The world at large is being observed to be ushering towards alternates as an obligatory option. In terms of the subsets of latter biomass promises itself as being one of the few existing viable means of renewable energy resources that is capable of transformation into solid, liquid and gaseous fuels [7]. At present biomass is a rich source of energy
Corresponding author. Corresponding author at: Department of Chemical Engineering, Faculty of Engineering and Science, Curtin University, 98009 Sarawak, Malaysia. E-mail addresses:
[email protected] (S. Nizamuddin),
[email protected],
[email protected] (N.M. Mubarak).
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http://dx.doi.org/10.1016/j.rser.2016.12.122 Received 21 December 2015; Received in revised form 13 December 2016; Accepted 26 December 2016 1364-0321/ © 2016 Elsevier Ltd. All rights reserved.
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different types of biomass. HHV is calculated by using the following equations.
HHVI = 196/1000 (FC) + 14119/1000
(1)
HHVII = 312/1000 (FC) + 1534 /10000 (VM)
(2)
Values for fixed carbon (FC) and volatile matter (VM) are taken from the Table 2. Biomass has proven itself as not only being an efficient source of energy but also as a key component towards enhancing the workability of biofuels in comparison to non-renewable petroleum products. Simultaneously, biomass reduces deterioration of the environment by equating the CO2 amount released to the amount that is absorbed via photosynthesis. Annual growth of biomass (dry basis) is 118x109 tons on the continent basis [19]. International energy agency (IEA) states that the entire oil consumption over the world in 2007 amounted to be 3.53x109 tons that is equivalent to 148.26x1018 J of energy, which however is 10% lesser than the worldwide per annum growth of biomass in relation to the energy content [20]. On the other hand, one of the investigations by United Nations Conference on environment and development (UNCED) shows that biomass is speculated to supply energy globally to approximately 50% of the existing principal energy requirement by 2050 [21]. Production of biofuels and chemicals from biomass can play a vital balanced befitting addition to the world in terms of provision of cheap transportation, electricity, and chemicals such as bio-fertilizers, bio-plastics, and bio-polyesters [22]. It is therefore speculated that an adequate optimum utilization of such methodologies of relevance promises a dimension that extends beyond the sets of research, social and economic activities in terms of minimizing the interdependence on fossil fuels. Biomass offers itself as an attractive feed for biofuels for three reasons at large. First and foremost, it is a renewable source. Hence, in comparison to fossil fuels, it offers the options for developments in near future and serves as an everlasting solution. Secondly, it releases CO2 in fewer amounts coupled with lesser sulfur content. Hence, it is clearly an environmentally friendly source of energy and fuel. Third, it possesses economic potential under the scenario whereby prices of petroleum fuels rise in near future [23]. Furthermore, similar to fossil fuels, biomass promises a position as a feedstock for the production of valuable chemicals in industries. Jacob and Igor [24] consider that the energy consumption in the chemical sector is around 10% of the global consumption of energy. There exist a range of fuels that can be produced from biomass sources. These extend from gaseous fuels including hydrogen and methane to liquid fuels such as biodiesel, fischer tropsch diesel, vegetable oil, bio-alcohols (ethanol and methanol), biosynthetic oil [25] and solid fuel such as biochar and hydrochar. Biofuels can be classified as either primary or secondary fuels. Primary fuels are natural and unprocessed fuels. Primary fuels are made to undergo combustion directly for both heating and cooking purposes. Examples of primary fuels include firewood, wood chips, and pallets. Secondary fuels are derived from primary fuels via a wide variety of processes. These fuels have various applications in transport and industrial process [26]. Additionally, due to environmental benefits depicted by biofuels, the share of biofuels is committed to increase in the transportation sector with time. Biofuels beehive more advantages than fossil fuels in multiple aspects namely the availability of sources and lower emission of CO2. Hence, biofuels are environment-friendly fuels; they promise energy security, foreign exchange savings, rural development, sustainability and biodegradability [27,28]. However, the main underlying difference between biofuels and petroleum fuels lies in the amount of oxygen content present in either which is also accountable for making their properties widely differ. Biofuels contain 10-45% oxygen, whereas petroleum products have none. However, in order to support biofuels numerous studies have been recently conducted to justify their social and environmental effects and benefits
Fig. 1. Emission of carbon dioxide (CO2) [141].
Fig. 2. Biomass resources present for utilization [9].
available from agriculture, forests and energy crops [8]. Fig. 2 shows biomass resources that are theoretically existent; it reveals that most of the biomass comes from agricultural land [9]. Biomass is a biological matter that incorporates all living matter on the earth. Biomass mainly consists of cellulose (C6H10O5)x, hemicelluloses (C5H8O4)m, lignin [C9H10O3(COH3)0.9-1.7]n, small extractives [10,11], fats, proteins [12], sugars, arrowroots, water, ash, besides additional mixtures. The largest fraction of biomass is cellulose, which is 35–50% of biomass by weight. Hemicelluloses contains 20-35% of biomass by weight, lignin shares about 15-20% of biomass weight whilst the remaining 15-20% includes proteins, fats, extractives and ash content as shown in Fig. 3 [13]. The general classification of biomass with sub-groups is shown in Table 1. Biomass is characterized by its physical properties, proximate analysis and ultimate analysis [14]. These properties tend to change in every substance [15] with the growth environment [16], time and age [17,18]. Table 2 gives the ultimate analysis, proximate analysis and heating value (HHV) of
Fig. 3. Typical composition of biomass.
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Table 1 Classification of biomass [111]. Biomass group
Biomass sub groups, varieties and species
Wood and woody biomass
Coniferous or deciduous; angiospermous or gymnouspermous; soft or hard; stems, branches, foliage, bark, chips, lumps, pallets, sawdust, briquits, sawmill and various wood species Annual and perennial and field based or process based. Such as grasses and flowers, straws and other residues Marine or fresh water algae; macroalgae or microalgae, seaweed, kelp, lake weed etc Bones, meat-bone meal, chicken liter and manures etc Municipal solid waste, demolition wood, refuse-derived fuel, sewage sludge, hospital waste, paper-pulp sludge, chipboard, fiber, plywood, wood pallets and boxes, railway sleepers etc Blends from above varieties
Herbaceous and agricultural biomass Aquatic biomass Animal and human biomass waste Contaminated biomass and industrial biomass waste (semi-biomass) Biomass mixtures
In pyrolysis process, the water content present in biomass has negative effect on the process because it requires higher heat of vaporization [22]. HTC therefore increases the options of biomass usage as a feedstock together with the overall process economy. It is also capacitive of the overall energy used for it caters the moisture content problem as it uses drying. At hydrothermal water conditions, nitrogen atoms present in biomass are converted to N2O; while phosphorous, chlorine and sulfur are oxidized to their respective inorganic acids which are simultaneously neutralized with the addition of a suitable base [68]. Biomass normally contains 40–60% oxygen. This is the reason why the exclusion of oxygen from biomass serves as the most important objective to increase energy density. Oxygen may be removed from feed biomass either by dehydration (oxygen is eliminated from the biomass feed in the form of water) or by decarboxylation (oxygen is removed in the form of carbon dioxide). Biomass experiences hydration reactions at higher temperatures and pressures, even in excess of water [19]. It undergoes depolymerization reactions via hydrolysis at elevated temperatures because of high ionization of water at those temperatures [69]. At the same time, both acidic and basic nature [66] of water justify the capability of hydrolysis to change biomass into biofuels directly. This is elaborated further by adding the credibility of HTC delivering quantities greater than those in comparison to the pyrolysis process that does not involve water [70]. A complete overview of the aforementioned is detailed and discussed later. The basic nature of water weakens certain bonds in the organic material. As a result, new molecular fragments are generated which amend the reaction environment. At the same time, water acts as an acid catalyst, speeding up different reactions. A review of previous studies on a different type of biomass using HTC process at different parameters is given in Table 3.
[29–39]. In this study, an overview on the effect of operating parameters on yield as well as on the quality of the hydrochar produced from hydrothermal carbonization (HTC) process is highlighted. Further, there is a critical discussion on the effect of conversion of biomass into solid fuels with their compositions being consequently evaluated. Furthermore, composition and characterization of different biomass are discussed. Conclusions are drawn in the light of literature; with recommendations being henceforth provided. 2. Hydrothermal carbonization (HTC) process The HTC process is an environment friendly technology. It thermally converts a variety of biomass feedstocks into high carbon content containing, smokeless solid fuels [40,41]. HTC of biomass is studied along with water by application of higher temperatures (180–250 °C) at elevated pressure (2-10MPa) for several hours [38]. It was Bergius and Specht [42] who introduced HTC process for the first time. They converted cellulosic material into high carbon material. The mixture of cellulose and water was heated in a closed vessel at 250–310 °C. As a result, the solid with lower O/C ratio was produced suggesting HTC as a successful conversion process. This was followed by further modifications and improvements in HTC processes that were carried out by Berl and Schmidt. They studied various saccharides at 150–35 0 °C [42]. Through relevant studies conducted since the last decade, HTC has emerged as a promising technology due to its inherent benefits [43–47] such as conversion of biomass into numerous products. Studies have also shown a consideration of biomass as a solid fuel that is comparable to brown coal [32,43–49], a source of bio-oil [22,50–53], soil amendment to improve fertility of soil and crop yields [48,54–56], as an activated carbon material that can be used as an adsorbent for water purification systems or CO2 sorption [54], and as a low-cost adsorbent or permeable reactive barrier for Uranium (VI), Copper and cadmium contaminated waters [57,58], carbon material with nanostructure [54,59,60], carbon catalyst which can be used in the production of fine chemicals [38], and lastly a carbon material that can enhance fuel cells efficiency [54]. Fig. 4 shows the schematic diagram of HTC with some of the possible applications of hydrochar [61]. The HTC process may be classified as either Direct HTC or Catalytic HTC process. In Direct HTC process, only water and feed are heated in a reactor at different temperature ranges in the absence of a catalyst, whereas in catalytic HTC process, a catalyst is used. HTC yields solid, liquid, and gaseous products [32]. The product distribution is primarily dependent on the feedstock and process temperature. However, there does exist an influence of the reaction time and the biomass to water ratio [52,53,62,63]. Presence of water accelerates the carbonization process of biomass and simultaneously affects the product distribution [32,64,65]. The reason for the latter is self-evident; water in biomass serves as both a reacting medium and a reactant [66,67]. Thermodynamically, water is a completely oxidized composite that is not capacitive of a high heating value. Hence, water serves as a perfect reaction agent with the capacity to conduct such types of reactions in contrast to the pyrolysis reactions.
3. Process parameters HTC process requires knowledge of the effect of process parameters; reaction temperature, feedstock, reaction time, catalyst and pressure. The effects of each process parameter are further discussed in the following section. 3.1. Effect of temperature Temperature is suggested as the key parameter for HTC process. The elementary role of temperature is to offer heat of disintegration for fragmentation of the biomass bond. Variations in the yields of the processes describe and measure the competency of temperature to crumble the biomass. Effectiveness of biomass conversion increases with an increase in temperature. This is generally because of additional energy being delivered by the temperature to break the biomass bonds [71]. The main products of HTC process are gas, liquid and solid products. Usually, moderate temperatures favour oil yield, whereas higher temperatures generate gas and the char products dominantly. This can be achieved both by secondary decomposition and bourdard gas reaction in which gas formation is more favourable [72,73] or even by the recombination of free radical reactions where a greater amount 1291
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Table 2 Characterization of various types of biomass. Biomass type
Rice husk Sal seed husk Olive husk Peanut hull Hazelnut shell Brazil nut shell Akhrot shell Coconut shell Pistachio shell Groundnut shell Chapparal wood Spruce wood Ailanthus wood Beech wood Bamboo wood Red wood Douglas fir wood Casurina wood Neem wood Subabul wood Wood Bark Douglas fir bark Wood chips Mulberry stick Peach pit Pine needles Sena leaves Coconut fibre Coconut coir Corncob Tea waste Walnut shell Sunflower shell Rice straw Wheat straw Corn straw Soya bean Corn cob Cotton stalk Cotton shuck Peanut stalk Peanut shuck Sesame stalk Broad bean stalk Rape stalk Willow tree Rubber plant Poplar Pine tree Phoenix tree Birch tree Met sequoia Coal Red oak wood Barley straw Flax straw Timothy grass Pinewood Pinuspinaster Cedrusatlantica Castaneasativa Fogussylvatica Quercusrobur Prunusaviurn Salix babilonica Bowdichianitida Hymenia courbaril Wood pallets (pine) Paper plant residue Green house residue Sunflower pellets Olive cake pellets
Proximate analysis
Ultimate analysis
Heating value
References
FC
VM
Ash
C
H
O
HHV I
HHV II
16.95 28.06 18.4 21.09 28.3 22.2 18.78 22.1 16.84 21.6 18.68 29.3 24.8 24.6 11.24 19.92 12.6 19.58 12.19 18.52 31.8 32.79 23.5 22.8 19.8 26.12 25.5 26.6 29.7 11.5 13 37.9 19.8 15.55 14.96 14.83 15.62 15.8 18.57 20.74 15.66 16.85 17.3 18.9 17.26 15.55 18.3 15.42 14.45 18.29 13.68 16.11 43.6 21.5 4.8 8.8 16 10.3 14.1 16.7 20.3 13.9 18 15 16.8 18.2 17.6 14.5 19.5 5.5 19.5 15.7
61.81 62.54 77.5 73.02 69.3 76.1 79.98 77.19 82.03 72.7 75.19 70.2 73.5 74 86.8 79.72 87.3 78.58 85.86 81.02 66.6 65.46 76.4 75.1 79.1 72.38 57.2 70.6 66.58 87.4 85.5 59.3 76.2 61.1 63.96 62.74 68.95 70.24 67.63 62.16 66.67 61.64 68.93 68.44 72.99 69.2 62.92 74.04 76.5 68.68 74.91 74.3 2.4 78.6 78.5 80.3 77.9 82.4 85.8 82.9 79.6 85.7 81.7 84.9 80.8 81.8 81.7 80.4 60.5 61 65.2 64.2
21.24 9.4 4.1 5.89 1.4 1.7 1.2 0.71 1.13 5.7 6.13 1.5 1.7 0.4 1.95 0.36 0.1 1.83 1.93 1.2 1.6 1.75 0.1 2.1 1.1 1.5 17.3 2.8 3.72 1.1 1.5 2.8 4 15.25 12.45 13.12 6.08 7.5 6.41 6.88 9.12 12.15 6.11 5.03 3.6 6.17 9.9 2.63 0.89 5.28 2.36 2.2 8.3 0.5 9.8 3 1.1 1.5 0.2 0.4 0.1 0.5 0.3 0.4 2.4 0.1 0.7 0.2 13.5 31 4.1 8.2
38.5 48.12 49.9 45.77 52.9 49.15 49.81 50.22 48.79 48.59 46.9 51.9 49.5 49.5 48.76 50.64 50.64 48.5 48.26 48.15 53.1 53.1 48.1 44.23 49.14 48.21 36.2 46.43 50.29 49 48 53.4 47.4 38.52 42.11 42.69 43.16 44.53 46.1 44.54 40.28 45.9 41.34 42.16 42.42 46.79 48.69 47.46 49.41 48.14 48.32 47.98 81.5 50 41.4 43.1 42.4 49 48.4 50.3 47.1 46.2 47.2 48.6 47.2 52.3 48.3 45.5 33.8 47.1 44.1 42.1
5.2 6.55 6.2 5.46 5.6 5.7 5.64 5.7 5.91 5.64 5.08 6.1 6.2 6.2 6.32 5.98 6.18 6.24 6.27 5.87 6.1 6.1 5.99 6.61 6.34 6.57 4.72 5.49 5.05 5.4 5.5 6.6 5.8 6.13 6.53 6.16 6.9 6.89 6.85 6.6 7.18 6.74 6.57 6.13 7.06 7.1 7.29 6.74 7.67 7.88 8.36 6.82 4 6 6.2 6.2 6 6.4 6 5.6 4.9 5.8 5.5 5.8 5.6 6.1 5.7 6.6 4 7.4 5.17 4.99
34.61 35.93 42.0 39.56 42.7 42.8 42.94 43.37 43.41 39.49 40.17 40.9 41 41.2 42.77 42.88 43 43.12 43.46 44.75 40.6 40.6 45.74 46.25 43.52 43.72 37.49 43.78 39.63 44.5 44 45.4 41.3 39.28 40.51 42.69 44.76 45.97 43.35 46.66 42.47 42.79 45.16 45.28 46.1 40.6 39.03 44.5 42.19 39.84 40.6 43.98 3.3 42.4 51.7 49.9 50.4 44.4 45.3 43.6 47.7 47.2 46.8 45.3 44.4 41.3 45.1 47.7 39.1 10.9 34.6 31
17.44 19.62 19.23 18.25 19.67 18.47 17.80 18.45 17.42 18.35 17.78 19.86 18.98 18.94 16.32 18.02 16.59 17.96 16.51 17.75 20.35 20.55 18.73 18.59 18.00 19.24 19.12 19.33 19.94 16.37 16.67 21.55 18.00 17.17 17.05 17.03 17.18 17.22 17.76 18.18 17.19 17.42 17.51 17.82 17.50 17.17 17.71 17.14 16.95 17.70 16.80 17.28 22.66 18.33 15.06 15.84 17.26 16.14 16.88 17.39 18.10 16.84 17.65 17.06 17.41 17.69 17.57 16.96 17.94 15.20 17.94 17.20
14.77 18.35 18.93 17.78 19.46 18.60 18.13 18.74 17.84 17.89 17.36 19.91 19.01 19.03 16.82 18.44 17.32 18.16 16.97 18.21 20.14 20.27 19.05 18.63 18.31 19.25 16.73 19.13 19.48 17.00 17.17 20.92 17.87 14.22 14.48 14.25 15.45 15.70 16.17 16.01 15.11 14.71 15.97 16.40 16.58 15.47 15.36 16.17 16.24 16.24 15.76 16.42 13.97 18.77 13.54 15.06 16.94 15.85 17.56 17.93 18.54 17.48 18.15 17.70 17.64 18.23 18.02 16.86 15.36 11.07 16.09 14.75
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[112] [112] [113] [114] [113,114] [115] [112,116] [112,116] [114] [112,114] [112] [117] [117] [117] [112] [91] [112] [116] [112] [116] [117] [118] [114] [119] [119] [117] [114] [117] [117] [113] [113] [113] [113] [120] [120] [120] [120] [120] [120] [120] [120] [120] [120] [120] [120] [120] [120] [120] [120] [120] [120] [120] [121] [121] [122] [122] [122] [122] [123] [123] [123] [123] [123] [123] [123] [123] [123] [124] [124] [124] [124] [124] (continued on next page)
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Table 2 (continued) Biomass type
Sewage sludge Oil palm shell Japanese apricot tree Alfalfa Stalk Urban wood waste Switch Grass Sawdust Sugarcane bagasse Almond Shells Almond Hulls Olive Pitts Willow wood Hybrid poplar
Proximate analysis
Ultimate analysis
Heating value
References
FC
VM
Ash
C
H
O
HHV I
HHV II
7 15.75 12.2 15.62 12.5 12.19 9.34 15.0 20.71 20.07 21.2 16.07 12.49
44.6 77.91 87.72 68.02 52.56 65.19 55.03 76 73.8 75.6 82.22 84.81
41.5 6.34 0.08 5.84 4.08 7.63 0.69 11.3 3.29 6.13 3.2 1.71 2.70
52 51.1 49.6 40.60 33.22 39.68 32.06 44.8 49.3 47.53 48.81 49.90 50.18
6.3 6 5.9 5.15 3.84 4.95 3.86 5.4 5.97 5.97 5.79 5.90 6.06
32.1 42.9 44.5 36.02 27.04 31.77 27.04 39.6 40.63 39.16 43.48 41.80 40.43
15.49 17.21 16.51 17.22 17.30 16.93 17.27 16.57 18.18 18.05 17.31 16.78 17.54
9.03 16.87 17.26 17.04 14.8 16.24 17.63 16.91 18.12 17.58 17.66 14.37 18.04
[124] [125] [125] [126] [126] [126] [126] [113, 127] [113, 128, 129] [128] [114] [130] [130]
Fig. 4. Schematic of HTC process for possible applications [61]. Fig. 5. Yields of paper industry waste as a function of temperature [142].
of char will be formed [22]. Effect of temperature on distribution of the yield of liquefaction product from paper industry waste is well defined in Fig. 5, which shows that lower temperature favours maximum solid production. As the temperature rises the solid production decreases gradually, whilst the liquid and gaseous products increase. Generally, solid product is dominant in the temperature ranges of 150–200 °C; at moderate temperature ranges of 250–350 °C the liquid yield will be higher, whereas above 350 °C, gas formation yield leads to solid and liquid products. Fig. 6 provides information about the variation in solid yields produced from different types of biomass as a function of temperature in HTC process. Results of studies conducted by Liu et al. [74] and Sun et al. [75] demonstrated that the rates of solid products were very high at temperatures below 200 °C. An increase in temperature to intermediate range > 200–250 °C caused the solid product to decrease
alongwith another increase at temperatures > 280 °C. Carbonization at lower temperatures produces a higher amount of solid. At a higher temperatures carbonization is greater resulting in the formation of more liquid and gaseous products along with lower solid products. Extensive research work has been carried out to study the effect of temperature on product distribution during HTC process [76–79]. The biomass selected as a feed included enteromorphaprolifa, cattle manure, Spirulinaplatensis and Cunninghamialanceolata. It was observed that the solid product decreases as temperature increased. Chen et al. [80] reported similar results for sugar cane bagasse. Liu et al. [81] investigated the comparative effect of temperature from 150–375 °C on two types of biomass; coconut fiber and eucalyptus leaves. They found that the solid yield decreases rapidly with an increase in temperature to
Table 3 Review of hydrochar synthesis using HTC process. Biomass feed type
Temperature (oC)
Reaction time
Reactor type
Solid yield %
Reference
Mix wood Microalgae Olive mill wastewater Empty fruit bunch Eucalyptus sawdust Barley straw Poplar wood Olive residues Wheat straw Palm shell Switchgrass
215–295 190–210 180, 220 150–350 250
5–60 min 30–120 min 840 min 20 min 120 min
Parr stirrered pressure reactor Parr stirrered stainless steel reactor Lab-scale autoclave Parr stirrered pressure batch reactor Autoclave reactor
[53] [50] [131] [49] [132]
180–230
480 min
Stainless steel autoclave
180–260 300–400
30–120 min 60–180 min
Amar autoclave batch reactor Cylindrical metal container
50.1–69.1 25.3–45.7 ~30 49–76 40 37 51.9–89.9 49.0–75.4 53.7–80.1 38.7–63.0 35.2–82.0
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[134] [135]
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[87] and Karagoz et al. [88]. The HHV of HTC-char is strongly dependent on the initial composition of feed material. Berge et al. [62] reported that the HHV of biochar produced from the waste paper is 23.9 MJ/kg. HHV of the wood mixture was calculated by Hoakman et al. [53] and it was found to fall between the range of 22.5–29.5 MJ/ kg. The HHV of sargassumhorneri is similar to HHV value of wood waste and paper waste due to similar lignocellulosic composition of paper waste, wood waste and sargassumhorneri [89]. Table 4 gives the summary of lignocellulosic composition of different types of biomass. The mechanism of HTC process includes decarboxylation, dehydration, condensation polymerization, hydrolysis, and aromatization. Understanding the nature of these mechanisms depends primarily upon the type of biomass feed [32]. In general, the effect of feed composition plays as a crucial parameter in the formation of hydrochar. The precursor materials influence the pore size distribution in the final product. The composition of lignin and cellulose content in the precursor materials play a significant role in the porosity of hydrochar produced during HTC process. Fig. 6. Effect of temperature on solid yield during HTC of various biomass feed types [72–74,76–79,81,143].
3.3. Effect of reaction time HTC is recognized as a slow reaction. For HTC, the reported reaction time varies from several minutes to a few days. Reaction time however only influences the hydrolysis reactions up to a certain range of time beyond which it does not have any specific impact on the process. Generally, it is observed that a greater quantity of solid product is obtained at higher reaction times. Reaction duration defines the product composition as well as overall biomass conversion. Under supercritical conditions, the hydrolysis rate and degradation of biomass rate are relatively fast [90]. Hence, a comparatively smaller duration of time is required to decompose biomass effectively. Various studies and research have been conducted to examine the effect of reaction time for solid, liquid or gaseous product. Boocock et al. [91] conducted a study in which the results proved that longer reaction times are required for increased bio-oil production with the exception of those that possess a very high biomass to water ratio. Yan et al. [92] observed a slight increment in bio-oil production with longer run times. Longer reaction times support bio-oil yield and conversions of biomass to products at a lower temperature (150 °C). At higher temperatures (250–280 °C) however, the overall conversion and gaseous product rate increases [93]. Effect of variation in run times on conversions showed that an increase in time increases the conversion [94]. Zhang et al. [70] observed that longer residence time supports gaseous and solid product formation in contrast to liquid product formation. Further, residence time affects the product composition also. According to Karagöz, et al. [93] the products not only differ at shorter and longer run times; they also differ at lower and higher temperatures (180 °C or 250 °C). According to the results of experiments that have been previously conducted using a short run time; the solid fuels produced have higher heating values. This can be either due to the removal of oxygen present in biomass or by hydrolysis of hemicellulose. Some studies carried out at a lower run time (up to half of an hour) resulted in the production of HTC solid with a heating value equal or greater than lignin [95,96]. The reaction times influence the formation of hydrochar during HTC process. During HTC process the higher the reaction time the greater the formation of defined structure porosity, pore volume and high BET surface area and vice versa.
350 °C for both feed materials. However, from and beyond 350– 375 °C, solid yield reduces gradually. Lu et al. [82] further clarified that the carbon distribution is also affected by temperature. Temperature influences the characteristics of solid fuel produced by HTC process. Jamari et al. [42] investigated the effect of temperature on empty fruit bunch (EFB) during HTC. It was observed that the carbon value increased with increase in temperature in contrast to hydrogen and oxygen content. At higher temperatures, the oxygen and hydrogen contents decreased validating the removal of these substances with temperature. Additionally it is observed that higher temperatures during HTC process increase the magnitude of aromatization of carbon structure. Furthermore, the observation that materials undergo increased aromatization at high temperature is heavily attributed to the arrangement of the hydrochar and aromatic structures with fewer reactive sites, less abrupt decomposition and a well-organized hydrochar structure that was supportably obtained under optimum conditions. Hence, the effect of temperature is a vital role in the formation of hydrochar. 3.2. Effect of feedstock The structure and composition of biomass types vary from each other due to the difference of growing environment and growth time. Major components of biomass are cellulose, hemicelluloses and lignin. Every component behaves differently along with temperature variations. Generally, higher content of cellulose and hemicelluloses enhance the oil yield. Hindsight, it is also true that higher the content of lignin in biomass the greater the char produced. This is because lignin is hard to degrade due to its complex branching and thus remains as a residue [68]. Under hydrothermal conditions, cellulose hydrolyzes significantly above 200 °C [68], hemicelluloses at around 180 °C and lignin degrades at around 200 °C [32]. Zhong et al. [83] investigated the effect of lignin content present in four types of wood biomass including Cunninghamialanceolata, Fraxinusmandshurica, Pinusmassoniana, Lamb and PopulustomentosaCarr on the oil yield. The experimental observation proved that higher lignin content results in decreased bio-oil and increased biochar production. The oil yield was reduced by repolymerization and cyclization of lignin fragments of liquid oil [84]. Yang et al. [85] discussed the weight loss behavior of cellulose, hemicelluloses, and lignin of lignocellulosic biomass. They observed that 94.5% loss of cellulose, 80% of hemicellulose and 54.3% loss of lignin at 400° C, 268° C, and 900 °C, respectively. This clearly shows that the degradation of biomass with higher lignin content produces more char [86]. Similar results were reported by Gani et al.
3.4. Effect of pressure Pressure is another factor which significantly influences the degradation of biomass in hydrolysis process. Biomass decomposition and hydrolysis rate can be controlled by keeping pressure above the critical pressure of medium. This may significantly boost satisfactory reaction routes that thermodynamically favour conversion of biomass to valuable products. Pressure has a positive effect on the density of solvent or 1294
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Table 4 Summary of composition of different types of biomass. Fuel
Cellulose
Hemicellulose
Lignin
Extractive/ash
References
Almond shell Beech wood White poplar European birch White willow Corncob Monterey pine Douglas fir Hazelnut shell Olive husk Spruce wood Sunflower shell Tea waste Walnut shell Wheat straw Empty Fruit Bunch Fiber Shell Hazelnut seed coat Wood bark Corn Stover Tobacco stalk Tobacco leaf Ailanthus wood Softwood (av.) Hardwood (av.) Waste material Acacia Mangium Bagasse Bagasse pith Coconut husk Coconut shell Corn stalk Kenaf Oil palm EFB Oil palm husk Oil palm petioles Oil palm shell Pineapple leaf Rice husk Rice straw Rubber tree Bamboo Newspaper Chinquapin Japan cedar Banana stem Coastal Bermuda grass Switch grass
50.7 45.3 49.0 48.5 49.6 50.5 41.7 42.0 26.8 24 49.8 48.4 30.2 25.6 28.8 26 19 22 29.6 24.8 51.2 42.4 36.3 46.7 45.8 45.2 50.6 43.12 49.20 45.18 30.55 26.49 42.43 42.60 35.71 34.28 37.01 27.7 32.16 44.14 36.26 45.84 47 40-55 46 35 37.92 25 45
28.9 31.2 25.6 25.1 26.7 31 20.5 23.5 30.4 23.6 20.7 34.6 19.9 22.7 39.4 43 37 26 15.7 29.8 30.7 28.2 34.4 26.6 24.4 31.3 29.2 72.14 74.98 72.16 56.45 79.29 68.18 81.27 65.57 61.31 71.65 21.6 63.2 82.42 74.45 73.84 25 25-40 20 24 71.15 35.7 31.4
20.4 21.9 23.1 19.4 22.7 15 25.9 27.8 42.9 48.4 27 17 40 52.3 18.6 24 33 46 53 43.8 14.4 27 12.1 26.2 28 21.7 24.7 29.91 19.54 22.13 38.82 35.54 21.73 10.31 21.97 31.91 20.94 44 18.68 42.58 34.73 21.42 21 18-30 30 33 12.25 6.4 12
2.5 1.6 0.2 0.3 0.3 3.5 0.3 0.4 3.3 9.4 2.5 2.7 9.9 2.8 3.3 7 11 6 1.4 1.6 3.7 2.4 17.2 0.5 1.7 2.7 4.5 6.76 1.28 1.24 2.65 1.86 3.27 1.24 5.67 6.99 2.06 10.31 6.55 2.48 5.28 2.86 7 4 8 3.66 -
[113] [113] [1] [1] [1] [113] [1] [1] [113] [113] [113] [113] [113] [113] [113] [136] [136] [136] [117] [117] [117] [117] [117] [117] [117] [117] [117] [137] [137] [137] [137] [137] [137] [137] [137] [137] [137] [138] [137] [137] [137] [137] [139] [1] [139] [139] [137] [140] [140]
HTC process. Hence, the effect of increased pressure is the enhancement in the role in the formation of hydrochar in HTC process.
medium as it increases their density. The higher rate of extraction and disintegration of biomass is achieved by using high-density solvents [22]. This is because the high-density medium infiltrates sufficiently in the fragments of biomass. Pressure has minute or even negligible influence on both liquid oil and gas yield [97–100] at supercritical conditions. It is for this reason that the effect of pressure on the properties of water or on solvent is negligible in supercritical region. The pressure in the reactor can be raised; either by raising the temperature directly or by adding fluids like nitrogen. The reaction inside the reactor is affected by pressure according to Le Chatelier’s principle. The direction to which the equilibrium shifts at higher pressure; shifts from solid to liquid phase or vice versa are decided according to this principle. At the same time, it is a fact that the shift is towards the direction constituting a lesser number of moles. For example in both decarboxylation and dehydration reactions are depressed at higher pressure [32]. The effect of pressure in HTC greatly influences the formation of hydrochar. High pressures lead to high temperatures; the breaking of biomass composition occurs hastily and final product obtained with high quality of hydrochar is obtained in
3.5. Effect of catalyst The use of small amounts of hydrolytic agents or catalysts considerably enhances the hydrolysis level. Catalysts differ according to the hydrolysis process employed. In general, acid catalysts are the most effective for hydrolysis [101], whereas the basic catalysts hinder the formation of char and support the formation of liquid oil. The use of catalyst also causes reduction in NOx. This is because the catalytic chemical reactions quickly convert NOx into nitrogen and water. Karagoz et al. [102] studied the effect of RbOH and CsOH (basic catalysts) on pine wood and observed that liquid product increased as the solid was reduced. A similar effect was observed by using K2CO3 (base) on sawdust [103]. Standards for the selection of good catalyst demand that they should be thermally stable, effective, cost effective and have high selectivity towards the required yield [104]. A number of catalysts 1295
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Fig. 7. Different catalyst with their ability to perform reactions and their respective advantages and disadvantages [144]. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
high energy density obtained from organic matters such as dung, firewood, and agricultural wastes. It is considered to be an outstanding contender solid fuel because of its low fiber structure and high heating value which makes it equally comparable to coal. Further, the elemental composition of HTC hydrochar is approximately equal to that of lignite or sub-bituminous coal. It is extremely feasible for degradation during land applications and has numerous promising benefits related to land fertility [64]. There are two eminent processes that produce char from different biomass sources; pyrolysis and HTC. The difference between pyrolysis and HTC is that the pyrolysis deploys dry biomass and it is not environment friendly due to the volatile matters released whereas the HTC process on the contrary is environment-friendly. In HTC, water is used as a solvent. Further, HTC has a sum of further practical benefits also. In HTC, complex drying and expensive separation process are theoretically saved because in HTC the need for special drying of feed is eliminated. It is also easy to separate solid product through filtration from the mixture. Char has great mechanical strength, sufficient pore size distribution, and higher pore surface area making it useful to serve as a fuel or chemicals. It can be used as an absorbent for separation and purification, catalyst support, solvent recovery and automobile exhaust emission control [106]. Biomass with higher moisture content produces a greater amount of char [25]. Char has many applications: it may be used as a fuel in boilers in the form of briquettes, mixed with other biomass like sugar cane baggasse, for the production of activated carbon, carbon nanotubes and further, can be processed as well to form hydrogen-rich gas [107]. Furthermore, HTC produced carbonaceous materials has applications in the field of catalysis, adsorption, and electrochemistry [108–110]. Hydrochar has a lower specific area which may be improved either by thermal pretreatment or by removing extractable to make it comparable to conventional charcoal [95,96]. A coal similar to solid fuel with higher energy and higher carbon content can be produced from biomass with low heating value and high moisture content through HTC process. The effect of process parameters has
are considered by international scientists for higher conversion of biomass to biofuels [89,103,105]. Examples include sodium hydroxide (NaOH), potassium hydroxide (KOH), sodium carbonate (Na2CO3), potassium carbonate (K2CO3), propanol (C3H8O), ethanol (C2H6O), citric acid. Fig. 7 gives a variety of catalyst (blue boxes) with their ability to perform different reactions (yellow boxes) and main advantages and drawbacks (green boxes). The traits of a catalyst bear dire significance to the formation of hydrochar. Catalysts have influenced effects that serve as a seed in the production of hydrochar during HTC process although small amounts are deployed for the enhanced rate of formation of hydrochar. Therefore, catalyst particle diffusion into the biomass to helps to breakdown the lignin and cellulose compound to form hydrochar during HTC process.
4. Critical discussion Biomass is an attractive feed for biofuels production. A range of fuels can be produced from biomass sources. These include gaseous fuels including hydrogen, methane as well as liquid fuels such as biodiesel, fischer tropsch diesel, vegetable oil and bio-alcohols (ethanol and methanol), biosynthetic oil and biochar. Biofuels hold more advantages than fossil fuels in various aspects: availability of resources, reduced emission of GHGs that makes it an environmental-friendly fuel, energy security, foreign exchange saving, rural development, sustainability, and biodegradability. Till today, the process of solid biofuel production using HTC has not reached optimum conditions for high yield of biochar. Previous studies suggest that various operating parameters influence the production of hydrochar using HTC process. The amount of char produced during HTC process in separation process technology has become a major concern for development. In the treatment process of adsorption, char is used as a new absorbent to replace activated carbon (AC). This is because the cost of production of AC is higher in comparison to the hydrochar produced during HTC process. Hydrochar is a solid product with high carbon content and 1296
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great influence in the formation of hydrochar during HTC process. Tuning of each and every single process parameter to achieve high yield, high purity, well organized structure and well developed pore size and high BET surface area is achieved once the process parameters have been optimized. Hence, high yield of hydrochar produced using HTC process is expected to create a major breakthrough in the near future. 5. Conclusions This paper provides detailed information guiding in detail towards the synthesis of char using HTC process. Dehydration, decarboxylation, aromatization, hydrolysis and condensation polymerization reactions take place during HTC process. These reactions do not represent consecutive reaction steps but rather form a parallel network of different reaction paths. The presence of water accelerates the carbonization process of biomass and affects the product distribution as well. Simultaneously, water in biomass serves not only as a reacting medium but also as a reactant. Temperature is found to be the most influential factor in the HTC process followed by residence time with feed type material being positioned third. Higher temperatures produce a lower solid product, whereas lower temperatures produce solids at a higher rate. Temperature affects the physical and chemical characteristics of the solid fuel produced; at a higher temperature, the carbon content is higher whereas hydrogen and oxygen content is lower. This results in a greater HHV value of biochar. The effect of feed material on HTC is observed by the structure of biomass; biomass with higher content of cellulose or hemicellulose will produce less solid fuel, whereas the biomass with higher lignin content will produce more solid fuel. The reaction time defines the product composition as well as overall biomass conversion. For HTC reactions, the residence time is reported to vary from several minutes to a few days. It is reported that longer residence time supports gaseous and solid product formation, in contrast to liquid product formation. Pressure has a positive effect on the density of solvent or medium as an increase of pressure leads to an increase in density. This is why the higher rate of extraction and disintegration of biomass is achieved using high-density solvents. The use of small amounts of hydrolytic agents or catalysts considerably enhances the hydrolysis level. Standards set for selection of a good catalyst that gives optimum performance include thermal stability, effectiveness, cost effectiveness and high selectivity towards required yield. Acknowledgments This work was supported by university of Malaya for fully funding under HIR-MOHE Account no. D-000030-16001. References [1] Yu Y, Lou X, Wu H. Some recent advances in hydrolysis of biomass in hotcompressed water and its comparisons with other hydrolysis methods. Synthesis 2008;35:36. [2] Okkerse C, Van Bekkum H. From fossil to green. Green Chem 1999;1:107–14. [3] Fischer G, Schrattenholzer L. Global bioenergy potentials through 2050. Biomass Bioenergy 2001;20:151–9. [4] Huber GW, Iborra S, Corma A. Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering. Chem Rev 2006;106:4044–98. [5] Krewitt W, Simon S, Graus W, Teske S, Zervos A, Schäfer O. The 2 C scenario—a sustainable world energy perspective. Energy Policy 2007;35:4969–80. [6] Fayaz H, Saidur R, Razali N, Anuar F, Saleman A, Islam M. An overview of hydrogen as a vehicle fuel. Renew Sustain Energy Rev 2012;16:5511–28. [7] Demirbaş A. Biomass resource facilities and biomass conversion processing for fuels and chemicals. Energy Convers Manag 2001;42:1357–78. [8] Saxena R, Adhikari D, Goyal H. Biomass-based energy fuel through biochemical routes: a review. Renew Sustain Energy Rev 2009;13:167–78. [9] Jacob AM, Igor VB. The potential of biomass in the production of clean transportation fuels and base chemicals. Production and purification of ultraclean transportation fuels: American Chemical Society; 2011. p. 65–77.
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