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Renewable and Sustainable Energy Reviews
journal homepage: www.elsevier.com/locate/rser
A review on hydrothermal liquefaction of biomass
A.R.K. Gollakotaa, Nanda Kishoreb,⁎, Sai Gua,⁎
a Department of Chemical and Processing Engineering, University of Surrey, Guildford GU2 7XH, United Kingdomb Department of Chemical Engineering, Indian Institute of Technology Guwahati, Assam 781039, India
A R T I C L E I N F O
Keywords:Wet biomassDry biomassHydrothermal liquefactionHTL mechanismValue-added chemicalsEnergy efficiency
A B S T R A C T
The rapid depletion of conventional fossil fuels and day-by-day growth of environmental pollution due toextensive use of fossil fuels have raised concerns over the use of the fossil fuels; and thus search for alternaterenewable and sustainable sources for fuels has started in the last few decades. In this context biomass derivedfuels seems to be the promising path; and various routes are available for the biomass processing such aspyrolysis, transesterification, hydrothermal liquefaction, steam reforming, etc.; and the hydrothermal liquefac-tion (HTL) of wet biomass seems to be the promising route. Therefore, this article briefly enlightened a fewconcepts of HTL such as the elemental composition of bio-crude obtained by HTL, different types of feedstockadopted for HTL, mechanism of HTL processes, possible process flow diagrams for HTL of both wet and drybiomass and energy efficiency of the process. In addition, this article also enlisted possible future research scopefor concerned researchers and a few of them are setting up HTL plant suitable for both wet and dry biomassfeedstock; analyzing influence of parameters such as temperature, pressure, residence time, catalytic effects,etc.; deriving optimized pathways for better conversion; and development of theoretical models representing theprocess to the best possible accuracy depending on nature of feedstock.
1. Introduction
The rapid increase in energy demand and the lack in the supply tocater the needs, enables to find the alternatives to meet the currentdemands of the society which is of primary concern in terms of socioeconomics of the globe. The rapid growth rate of urbanization in thepast decades, water quality degradation became a public health threat.Eutrophication associated with nutrient content of wastewater andtreatment plant effluents have become a serious concern to waterenvironment. Combining these two critical environmental and socialcauses an alternate source for the conventional fossil fuels which has tobe sustainable, energy secured, and reduce global warming is highlydesirable. This lead several researchers to focus in search of alternatesource for commercialization from several decades and came up withone source which is nothing other than the biomass. Biomass sourcesinclude wood wastes, energy crops, aquatic plants, agricultural crops,and their waste products and municipal and animal wastes; and areconsidered as potential sources of fuels and chemical feedstock's. Thechoice of biomass and its derivatives generate high expectations for theproduction of fuels, raw materials for synthesis in the petrochemicalindustry and fine chemicals. Biomass, in addition to the production ofthe first generation biofuels (bioethanol and biodiesel), is also ofinterest for the production of bio oil due to its perspectives for
production by pyrolysis of lignocellulose biomass. Several statisticsrelated to production and consumption of biomass as an alternatedsource of energy is listed by several authors. As of 2010, world biofuelproduction has been largely focused on first generation bio fuelsproducing ethanol and biodiesel from starch, sugars, and vegetableoils. Advanced bio fuels or biofuels produced from lignocellulosematerials such as wood waste and straw made up to only 0.2% of totalbiofuel production. In recent years, research on using biomass forliquid fuels has been robust, ranging from studies of pyrolysis andhydrothermal liquefaction (HTL) of lignocellulose material, gasificationand biomass to liquid technologies to the upgrading processes.
Secondly, converting biomass from its natural solid form to liquidfuels is not a spontaneous process. The liquid fuels that humans haveharnessed on a large scale as fossil fuel took thousands of years ofgeochemical processing to convert biomass to crude oil and gas. Inrecent years, several conversion technologies have been developed toobtain liquid products from different kinds of biomass for use as fuelsand chemicals. Biomass conversion technologies are broadly classifiedinto two categories namely, biochemical and thermo- chemical con-version. In general, the unprocessed bio-oil derived from the biomasshas low energy density, high moisture content, and its physical form isnot free flowing that creates a problem as a feedstock for reciprocatingengines. Thermochemical conversions compared to biochemical con-
http://dx.doi.org/10.1016/j.rser.2017.05.178Received 14 August 2016; Received in revised form 26 April 2017; Accepted 20 May 2017
⁎ Corresponding author.E-mail addresses: [email protected] (N. Kishore), [email protected] (S. Gu).
Renewable and Sustainable Energy Reviews 81 (2018) 1378–1392
Available online 08 June 20171364-0321/ © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
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versions are processed at several higher degrees of temperature in thepresence of appropriate catalysts to obtain liquid products fromdifferent sources. In general, thermochemical conversions are muchrapid than the biochemical conversions. Thermochemical conversionconversions generally implied to upgrade biomass by heating underpressurized and oxygen deprived enclosure. It can be further classifiedinto combustion, gasification, pyrolysis, and direct liquefaction. Amongwhich direct liquefaction or hydrothermal liquefaction is one of thepromising routes that drawn attention in the recent past. This processis synonym of hydrous pyrolysis, but compared to pyrolysis HTL iscarried at lower temperatures and heating rates. In other words,hydrothermal liquefaction of biomass is the thermochemical conver-sion of biomass into liquid fuels by processing in a hot, pressurizedwater environment for sufficient time to break down the solid biopolymeric structure to mainly liquid components. Typical hydrother-mal processing conditions are 523–647 K of temperature and operat-ing pressures from 4 to 22 MPa. The low operating temperature, highenergy efficiency and low tar yield compared to pyrolysis is the keyparameter that drives the attention of researchers on the liquefactionprocess.
Till date all the paradigm research on the production of biofuel isrested on the solid dry wood but liquid bio-waste has a better edge overthe solid manure which is the critical processing parameter for thehydrothermal liquefaction process. Among several possibilities of wetbiomass, microalgae is considered to be an excellent source of biofuelproduction because of their advantages such as high photosyntheticefficiency, maximum biomass production, fast growth rate and lack ofarable soil requirements. A very little attention on the HTL of wetbiomass process is available as of now and very few attempts of pilotplants globally, for instance, Pittsburgh Energy Research centre of U.SBureau of Mines took place in 1990s but stopped later on due toeconomic crisis. Hence, an attempt has been made to bridge the gap inthe literature and create a possibility to take the research on HTL ofbiomass to next level by selecting the best available feedstock and theoperating conditions. Further research on tuning of the criticaloperating parameters like temperature and pressure for better yieldis still under progress. Thus, this article tries to enlighten the existingknowledge and research gaps available pertained to biomass andhydrothermal liquefaction. However, before presenting thorough lit-erature review along with HTL mechanisms, energy efficiency andadvantages/disadvantages of this process, a few details of elementalcomposition; and wet and dry biomass feedstock suitable for HTL arepresented below.
1.1. Elemental composition of biomass
Any biomass consists of elemental carbon (C), hydrogen (H),nitrogen (N), sulphur (S) and oxygen (O); and a few details of themare presented below.
1.1.1. Carbon (C)It is obviously the most important constituent of the biomass. It
comes from the atmospheric CO2 that become a part of the plantmatter during photosynthesis. Carbon represents the major contribu-tion to the overall heating value. During combustion, it is principallyremodelled into CO2 that is discharged within the atmosphere [1]. Inany combustion application an area of carbon is not combusted utterlyand ends up in emissions of change state gases, usually carbon-monoxide gas or PAH's [2]. Generally, the carbon content of the fuelis estimated via the composition of lignin, hemicellulose and cellulose.Similarly, the char content is majorly depended on the amount of thelignin i.e., for herbaceous biomass yields lower carbon content ascompared to wood biomass. Fuels rich in lignin, such as olive kernel,have carbon contents higher than 50% (by wt) on dry basis [3].
1.1.2. Hydrogen (H)It is another major constituent of biomass, as can be expected from
the chemical structure of the carbohydrate and phenolic polymers.During combustion, hydrogen is converted to H2O, significantlycontributing to the overall heating value [4]. The weight content ofhydrogen, on a dry basis, is usually slightly lower in herbaceousbiomass (5.5–6%) than in woody biomass (6–8%) [3].
1.1.3. Nitrogen (N)It constitutes the vital nutrient form for plants. For instance, the
nitrogen is applied as a fertilizer to soil that enhances the growth andyield of the plants. By the continual application of the nitrogen in themodern day agricultural era, the nitrogen content of herbaceousbiomass species acquires a greater amount of nitrogen ranging between(0.4–1.0% wt dry basis) compared to woody biomass [5]. Further, thepresence of nitrogen significantly contributes the degradation inbiochemical processes like digestion or fermentation. On the otherhand, nitrogen during combustion process doesn’t oxidize and there-fore its contribution to the overall heating values [3].
1.1.4. Sulphur (S)This is considered to be the other important nutrient hindered in
structures of amino acids, proteins and enzymes for substantial plantgrowth similar to nitrogen. The high growth rate of most herbaceouscrops means that the sulphur concentration in plant biomass istypically higher than those in woody biomass. The concentration ofsulphur in wood can be as low as 0% (which means below the detectionlimit of most laboratory devices) and reaches up to 0.1% on a dry basisin exceptional cases, herbaceous biomasses have a sulphur contentranging from 0% to 0.2% or even higher [6]. In waste fractions, valuesup to approximately 1% have been reported. However, it's mostimportant impact relates to gaseous emissions, syngas cleaning ingasification processes and corrosion [3].
1.1.5. Oxygen (O)Oxygen constitutes the vital element in the chemical composition of
the biomass which is evident from the nature through photosyntheticprocess. The composition of the oxygen content controls the heatingvalue of the bio crude that is obtained via any processing technique [7];and majority of the bio crudes are limited their utilization just becauseof the excess content of the oxygen. It is evident that the oxygen contentin the phenolic compound structure are very complex to break (or toremove) it in the form of water so that to enhance the heating values. Itshould be noted that oxygen is not measured directly – its weightconcentration is estimated by subtracting from 100 the concentrationsof all other elements (C, H, N, S) and of the ash content in the dry fuel[3]. In the context of HTL of biomass, the elemental composition ofbio-crude obtained by the hydrothermal liquefaction of both wet anddry biomass is given in Table 1.
1.2. Biomass Feedstock
1.2.1. Dry feedstockThe tangled biomass properties lead to the braid chemical reactions
during the hydrothermal liquefaction procedure. Woody biomass/lignocellulose built is embedded with hemicellulose, cellulose and thelignin compounds; and hence the rigid crystallinity of the structuremakes it rigid and tougher to process. Further, accommodating acontinuous flow framework again involves the complexity of theprocedure that leads to the solubility criteria in the solvents due tothe hindered resistance to the structure of biomass. Thus the entireframework of the process is penalized economically. Despite, theintrinsic source of hydrogen and carbon, the processing of thelignocellulose biomass is limited due to the substantial amount of theoxygen. The detailed explanation of the individual fractions of ligno-cellulose is presented below.
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Table 1Elemental composition of the bio crude obtained through hydrothermal liquefaction of biomass (both wet and dry).
S.No Feedstock Elemental composition of product species after HTL References
C (%) H (%) N (%) O (%) S (%) Ash (%) Moisture (%) HHV % Yield
1 Spirulina algae 68.9 8.9 6.5 14.9 0.86 – – 33.2 32.6 Vardon et al. [8]2 Swine manure 71.2 9.5 3.7 15.6 0.12 – – 34.7 30.23 Anaerobic sludge 66.6 9.2 4.3 18.9 0.97 – – 32.0 9.44 Arthrospira platensis 74.5 10.2 6.8 7.5 1.0 – – 38.65 30 Lavanya et al. [9]5 Tetraselmis 71.4 9.5 5.7 12.3 1.1 – – 35.58 296 Nannochloropsis gaditana 76.1 10.3 4.5 8.8 0.4 – – 38.00 Barriero et al. [10]7 Scenedesmus almeriensis 74.9 9.1 5.9 9.6 0.7 – – 36.208 Nannochloropsis Sp 77.2 9.9 4.7 8.2 0.5 – – 399 Almeriansis 73.2 9.3 5.1 0.8 11.7 35.8 20.0 – 42.610 Gaditana 74.2 9.3 4.0 0.6 11.8 36.2 12.4 – 50.811 Nannochloropsis Oceana 77.6 4.9 3.4 – 0.3 – – 37.70 54.2 Caporgno et al. [11]12 Derbesia 73 7.5 6.5 10.6 0.7 – – 33.2 Neveux et al. [12]13 Ulva 72.6 8.2 5.8 11.0 0.4 – – 33.814 Chaetomorpha 70.9 7.7 6.8 11.4 0.1 – – 32.515 Cladophora 71.6 8.0 7.1 10.6 0.9 – – 33.316 Oedogonium 72.1 8.1 6.3 10.4 1.3 – – 33.717 Cladophora FW 71.1 8.3 6.8 10.6 1.3 – – 33.518 Aspen wood 75.2 8.2 0.5 15.8 0.3 0.48 3.8 34.3 Pedersen et al. [13]19 Scenedesmus 72.6 9.0 6.5 10.5 1.35 – – 35.5 Vardon et al. [14]20 Defatted scene 72.2 8.9 7.8 10.5 0.90 – – 35.321 Spirulina 72.2 9.1 8.1 9.2 1.41 – – 35.822 Nutrient depleted Oedogonium 65.8 8.5 1.5 18.3 – – – He et al. [15]23 Spent coffee grounds 71.2 7.1 3.0 18.7 – – – 31.00 Yang et al. [16]24 Pinus 60.1 6.0 1.2 32.7 0.1 – – 23.1 Cao et al. [17]25 Cupressus funebris 62.0 6.6 1.6 29.8 0.1 – – 25.126 Platanus 70.0 6.3 0.7 23.0 0.01 – – 28.627 Cinnamomum Camphora 74.0 8.4 1.5 16.1 0.01 – – 34.228 Pittosporum tobira 71.0 8.5 1.5 19.0 0.01 – – 32.929 Distylium racemosum 64.1 6.8 0.7 28.4 0.01 – – 26.330 Viburnum odoratissimum 71.7 8.1 1.2 19.0 0.01 – – 32.531 Salix alba 73.7 9.2 3.1 14.1 0.01 – – 35.632 Algal waste of Indiana polis 71.4 8.36 4.92 15.4 – – – 33.3 Chen et al. [18]33 Swine manure 71 8.9 4.1 0.21 14.2 35 – – 61 Toor et al. [19]34 Garbage 73.6 9.1 4.6 12.7 36 – – 2135 Indonesian biomass residue 67–80 6–8 0–2 11–23 30 – – 21–3636 Sawdust, rice husk, lignin – – – 837 Beech wood 76.7 7.1 0.1 16.1 34.9 – – 2838 Phytomass 76.6 7.6 2.1 0.1 13.6 – – –
39 Algae Dunaliella tertiolecta 63.55 7.66 3.71 25.8 30.7 – – 25.840 Porphyridium 66–83 5–11 0–12 0–1 8–27 22.8–36.9 – – 5–2541 Nannochloropsis 51 7 9 0.6 28.8 3 – 46 Valdez et al. [20]42 Acid mine drainage 68.8 7.9 7.1 – – 29.70 Raikova et al. [21]43 Cyanobacteria sp. 76.02 9.10 6.29 7.44 1.15 – – 36.51 Huang et al. [22]44 Bacillariophyta sp. 76.09 9.11 5.60 8.28 0.92 – – 36.4545 Seaweed 75.54 9.16 3.65 11.66 0.62 – – 35.97 Bach et al. [23]46 L.digitata 70.5 7.8 4.0 17 0.7 – – 32 17.6 Anastasakis and Ross [24]47 L.hyperbore 72.8 7.7 3.7 14.9 0.8 – – 33 9.848 L.saccharina 74.5 7.9 3.0 14.0 0.6 – – 33.9 1349 A.esculenta 73.8 8 3.8 14 0.8 – – 33.8 17.850 L.Saccharina 31.3 3.7 2.4 26.3 0.7 24.2 9.2 1251 Taihu cyanophyta 77.30 12.08 1.10 9.01 – – 41.73 Guo et al. [25]52 Coffee Husk 43.13 5.02 1.55 32.78 0.67 7.4 8.44 16.79 Braz et al. [26]53 Tucuma Seed 48.83 6.12 0.88 32.20 – 5.97 6.08 20.7754 Sugarcane 45.05 5.57 0.25 37.91 – 3.21 6.95 20.7755 Peanut shell 41.52 7.43 2.12 27.96 0.60 12.80 7.98 16.5256 Rice husk 31.47 6.67 1.04 23.03 0.50 29.53 8.19 15.3957 Pine sawdust 45.95 7.47 0.32 34.32 0.57 4.71 6.90 17.0358 Pond water algae biomass 46.09 6.22 9.70 37.35 0.64 Nautiyal et al. [27]59 Spirulina 48.10 6.97 10.14 34.13 0.6660 Chlorella 51.33 7.90 9.80 30.38 0.5961 Pond water algae oil 59.94 11.57 0.11 28.37 0.3162 Spirulina oil 66.73 12.40 0.50 20.21 0.1663 Cladophora glomerata 26.8 3.53 2.14 20.48 0.22 36.5 9.1 10.29 Plis et al. [28]64 Posidonia Oceanica 35.5 3.60 0.27 28.85 0.18 26.1 5.5 12.8265 Nannochloropsis gaditana 40.3 5.97 6.30 14.49 0.37 28.3 4.1 18.5366 Microcystis 42.26 6.27 7.88 43.07 0.52 6.14 9.59 16.2 Hu et al. [29]67 Almeriensis 41.78 6.81 7.94 42.93 0.55 14.5 17.6 Diaz-Rey et al. [30]68 S.japonica 34.7 5.5 1 41.6 0.3 18.3 7.4 12.1 Choi et al. [31]69 S.pallidum 22.64 4.90 2.96 21.34 1.42 36.44 10.30 9.63 Li et al. [32]70 P.elongata 35.81 5.93 6.86 51.40 27.45 11.55 12.54 Ceylan et al. [33]71 Sargassum sp 26.70 4.23 1.35 67.53 36.82 9.34 10.1072 U.fasciata 25.64 5.75 3.13 5.52 16 19.86 Trivedi et al. [34]
(continued on next page)
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1.2.1.1 CelluloseThe chemical formula of the cellulose is denoted by (C6H10O5)n
which is a long chain polysaccharide, high molecular weight, and withhigh degree of polymerization. Cellulose is a non-polar compound atambient temperature, but tends to soluble with the increase in thetemperature [54]. Monomers of glucose are stiffly bonded possessingthe formation of strong intramolecular and intermolecular interactionsamongst hydrogen bonds that originates the glucose monomers [55].
1.2.1.2 HemicelluloseIt is a branched structure hetero polymer with amorphous structure
embracing pentose and hexose as a polymer. The composition of thehemicelluloses varies significantly for woody biomass and the grassybiomass which comprises xylan in the earlier and mannan, glucan andgalactan in the later [56]. Further, hemicellulose in comparison to thecellulose possess the weaker structure and less resistant to intramole-cular hydrogen bonding allows the easier disintegration of the mole-cules [57]. Hence, the conducive nature to hydrolysis of hemicelluloseis severe and prone to easy miscibility characteristic [58].
1.2.1.3 LigninThe other fraction of the lignocellulose biomass is lignin which is
treated as a natural polymer can be referred as an aromatic compoundin which phenyl-propane with hydroxyl and ethoxy groups are linkedvia ether bonds [59]. Lignin possess similar morphological character-istic of amorphous form as hemicellulose, less solubility similar tocellulose and the peculiar behaviour of hydrophobic nature. Also itpossesses the binding capacity of stem cells or fibrous contents ofplants fibrous contents of the plant to strengthen cell wall structure,improves the sublimation capacity of micro-organisms and to store theenergy [60]. In contrast to other bioorganic compounds the decay rateof lignin is very low, in other words, it is resistant to biologicaldegradation. The significant feature of lignin encompasses the higherenergy content compared to the other two compounds that leads tohigher heating value to the product and considered to be the mostvaluable phenol compound [61].
1.2.2 Wet biomass feedstock
1.2.2.1 Lipids/FatsThese are majorly non-polar compounds otherwise immiscible/
hydrophobic in nature that resembles the similarities with the aliphaticcompounds and particularly referred to triglycerides (TAG's) nothingother than triesters of fatty acids and glycerol [62]. At normaltemperatures, the fats are generally insoluble in solvents and graduallytends to be polar with the change in temperature. The similarphenomenon on the effect of the temperature on the solubility of thefats can be seen in the case of cellulose. The dielectric constant of thesolvent in particular water at sub-critical conditions allows the greatermiscibility [57]. This leads to the stability in the structure of the TGA'sfurther leading to the formation of the glycerol a major by-product ofthe bio-diesel production which is a combination of fatty acids,methanol and salts. During HTL, glycerol is not converted to any oilyphase rather it turns to be a water-soluble compound [63]. Furtherdegradation of glycerol leads to the product stream of acetaldehydes,propionaldehyde, acrolein, allyl alcohol, ethanol, formaldehyde, CO,CO2, and H2 [62].
1.2.2.2 Proteins/Amino acidsOne of the major constituent of microbial or algal biomass is
protein consisting several chains of peptide reduces to polymers ofamino acids. They are the building blocks of proteins and highlyheterogeneous and hence the complexity in degrading the amino acidsis challenging. The strong peptide chain of proteins undergoes dec-arboxylation, and deamination reaction upon hydrothermal liquefac-tion leads to the formation of hydrocarbons, amines, aldehydes andacids [19]. Further degradation resulting carboxylic acids, acetic acid,propionic acid, n-butyric acid, and iso-butyric acids.
2. Evolution of HTL process over decades
Environmental issues, sustainability, declining fossil fuels, andenergy security concerns have led the world to explore alternate, clean,
Table 1 (continued)
S.No Feedstock Elemental composition of product species after HTL References
C (%) H (%) N (%) O (%) S (%) Ash (%) Moisture (%) HHV % Yield
73 Xuzhou SS 47.4 7.9 8.2 35.2 1.3 56.8 5.6 Huang et al. [35]74 S.patens C.Agardh 17.7 14.38 15.47 Li et al. [36]75 E.grandis 47.19 5.77 0.21 46.59 0.24 0.09 18.07 Louw et al. [37]76 T.suesica 43.32 7.27 5.75 40.55 3.11 12.20 7.88 Zainan et al. [38]77 Dunaliella tertiolecta 53.3 5.2 9.8 31.7 – 19.8 Minowa et al. [39]78 Chlorella vulgaris 52.6 7.1 8.2 32.2 0.5 23.2 Biller and Ross [40]79 Nannochloropsis oculata 57.8 8.0 8.6 25.7 – 17.980 Botryococcus braunii 77.04 12.40 1.23 9.86 0.18 35.6 Liu et al. [41]81 Fucus vesiculosus 32.88 4.77 2.53 35.63 2.44 11.8 15.0 Ross et al. [42]82 Chorda filum 39.14 4.69 1.42 37.23 1.62 9.9 15.5583 Laminaria digitata 31.59 4.85 0.90 34.16 2.44 10 17.6084 Fucus Serratus 33.5 4.78 2.39 34.44 1.31 18.6 16.6685 Laminaria hyperborea 34.97 5.31 1.12 35.09 2.06 11.2 16.5486 Macrocystis pyrifera 27.3 4.08 2.03 34.8 1.89 18.5 16.087 Miscanthus 46.32 5.58 0.56 41.79 0.2 2.1 19.0888 Nannochloropsis Salina 55.16 6.87 2.73 33.97 1.27 2.48 4.95 25.40 Toor et al. [43]89 Mougeotia 41.51 5.59 5.40 27.03 0.51 19.96 5.14 16.63 Sharara et al. [44]90 Cladophora 33.79 4.73 6.35 21.27 1.57 32.29 5.09 14.5391 Seaweed meal 43.99 5.95 5.21 36.13 1.02 7.7 7.92 18.35 Lorenzo et al. [45]92 Synechococcus/Anabaena 42.78 7.74 7.91 38.1 7.4 9.61 Wagner et al. [46]93 Synechocystis 46.12 7.98 3.52 11.2 4.5 15.3794 A.azuera 40.82 5.56 0.63 52.99 10.61 6.03 52.99 Aysu et al. [47]95 Chlorella.sp 47.54 7.1 6.73 38.63 5.93 6.8 18.59 Phukan et al. [48]96 E.prolifera 35.20 5.20 2.10 32.98 15.91 8.61 13.4 Yang et al. [49]97 Datura Stramonium L 43.55 5.98 0.77 49.70 6.38 3.73 14.39 Aysu and Durak [50]98 E.Spectabilis 39.27 6.54 1.28 52.91 4.6 5.6 13.17 Aysu et al. [51]99 Brewers spent grain 46.6 6.85 3.54 42.26 0.74 4.5 8 20.39 Mahmood et al. [52]100 Laurel algae 48.97 6.38 3.02 41.63 10.53 9.95 19.77 Ertas and Alma [53]
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cheap and renewable energy sources. Though biomass is one of theprominent option for renewable and sustainable energy, choosing thereliable and efficient conversion methodology is a big challenge. Overthe decades of research lightened different thermo-chemical conversiontechnologies and each of them has their own advantages and dis-advantages. In this regard, HTL is found to be the suitable processavailable to tackle the wet feed stock conversion to fuel energy.Research on HTL can date back to the 1930s and there is considerableamount of literature available worldwide pertaining the wet biomassconversion. The pioneering work on the HTL process on a commercialscale of cellulose biomass to heavy oil was initiated at PittsburghEnergy Research Centre in 1970. This process of HTL was takenfurther by Schaleger et al. [64] at Lawrence Berkeley Laboratory,Thigpen [65] at Albany Biomass Liquefaction Experimental Facility.Further details of studies of the HTL process by several researchers arebriefly described below.
Beckman and Elliott [66] performed comparison studies on yieldsand properties of oil obtained from direct thermochemical biomassliquefaction process. The authors developed correlations for the keyparameters such as temperature, pressure, time, catalyst and con-cluded that the changes in the governing parameters has a significantimpact on the product output. White and Wolf [67] developed a uniquemethod of pumping concentrated viscous biomass slurries throughliquefaction system. The authors introduced the extruder – feedersystem to pump the high viscous liquids for the liquefaction process.One of the pioneering works of the HTL of microalgae with high lipidcontent was done by Dote et al. [68]. In the years that followed severalresearchers had begun to explore this technology and verified theproperties of algae bio-oil in comparison with the petroleum Minowaet al. [39]. Itoh et al. [69] developed a scale up of HTL of sewage sludgeoperating at temperature of 300 °C and pressure of 10 MPa. Sawayamaet al. [70] studied energy consumption for the HTL process of variousalgae and presented the energy consumption ratio for the HTL couldproduce net energy with low energy consumption. Also HTL opens upnew possibilities for combined fuel production and waste watertreatment in densely populated areas where large amounts of wetwaste materials are available. Behrendt et al. [71] reviewed the directliquefaction especially for lignocellulose biomass through sub criticaland super critical solvents. Since 2009, the study gradually grabbed theattention of some reputed international research laboratories exploringthe liquefaction reaction parameters that includes residence time,reaction temperature, catalyst selection and loading. Peterson et al.[57] provided a comprehensive overview on the hydrothermal liquefac-tion of bio-oil from biomass using sub and super critical water orsolvent extraction technique. Further the products of HTL processcomprises a wide range of value added chemical compounds whichinclude branched aliphatic compounds, aromatics and phenolic deri-vatives, carboxylic acids, esters, and nitrogenous ring structures Xiuet al. [72]. Savage [73] published a brief but insightful overview on thehydrothermal liquefaction of microalgae. This was supported by theseveral other researches of Alba et al. [74], Minowa et al. [39], Rosset al. [75], Vardon et al. [14], Zou et al. [76]. Researchers have recentlyposted a renewable algal biorefinery based on HTL of algae Jena et al.[77], Alba et al. [74,78]. Recently the prominence of algae as thefeedstock for HTL is addressed by Barreiro et al. [10] and Chow et al.[79]. Latest review of Elliott et al. [80] and Jindal and Jha [81]enlightened some of the previous and current research works onhydrothermal liquefaction of biomass (both solid and liquid) whichincluded studies of Appell et al. [82], Kaufman et al. [83], Inoue et al.[84], Chornet and Overend [85], Ocfemia et al. [86], Heilmann et al.[87], Elliott [7], Akhtar and Amin [88], Xu et al. [89] and Tekin et al.[90].
Further literature pertaining to the reaction mechanisms of HTL iscritical in understanding the process so as to better design the reactorsand control the process. At present, understanding of HTL mechanismsis largely qualitative and indicative although efforts have been made
through years by many researchers Balat [91], Demirbas [92,93],Minowa et al. [94], Yuan et al. [95]. Generally, biomass is first brokenup into fragments by hydrolysis, and then degraded into smallercompounds by dehydration, dehydrogenation, deoxygenation anddecarboxylation. At the end, some complex chemicals may be synthe-sized by repolymerization. Biocrude product generated in the repoly-merization process usually contains acids, alcohols, aldehydes, esters,ketones, phenols and other aromatic compounds Chornet and Overend[85]. Hietala et al. [95] introduced new dimensions of kinetic model forfast hydrothermal liquefaction of biomass with good yield of 41% at400 0C and short residence time. However, the exact mechanisms orpathways of the HTL still remain unclear mainly due to the complexityof the feedstock and HTL products, as well as the seemingly-infinitepossible intermediate reactions. Hydrothermal liquefaction now is inthe transient state from lab-pilot scale to pilot-industrial scale, whichmakes the mechanisms study more important and urgent. With thesupport of mechanisms work, development of large-scale units could bemore reasonable and high-grade products may be produced, adding asolution to current environmental and energy problems.
Recently, immense amount of the work pertaining to hydrothermalliquefaction of wet and dry biomass through different techniques hasbeen carried out by many researchers. For instance, Alhassan et al. [96]presented glimpses of HTL process on dry biomass and obtained bio-oil with properties equivalent to pyrolysis oil. Costanzo et al. [97]obtained bio-oil derived from wet biomass and reported that thecomposition of the bio-oil is equivalent to conventional gasoline.Further, to obtain higher percentages of yield Lavanya et al. [9]introduced the blending of algal crude to petro crude that is turnedto diesel fraction. Pedersen et al. [13] obtained aspen wood derivedbio-oil characteristics equivalent to gasoline which supports theresearch work of Costanzo et al. [97]. Zhu et al. [98] reported thathydrothermal liquefaction of wet and dry biomass results in the higheryields of phenolics at the conventional operating temperatures ratherthan desired aromatics. Maddi et al. [99] made an attempt to obtainvalue added chemicals through hydrothermal liquefaction of bio-oiland succeeded in obtaining the same. Another important aspect of thisHTL process is the introduction of continuous flow reactor which yieldsa significant amount of heating values which was reported by Pedersenet al. [13]. Recent review of Elliot et al. [80] also stressed theimportance of introducing the continuous flow reactors rather thanconventional batch process for the higher yields of HTL products.
Several other explorations on the critical parameters such as energyconsumption economics by Sawayama et al. [70]; reaction mechan-isms/pathways [91–94,100]; algal biomass as feedstock [97,74,78];generation of value added chemicals by Xiu et al. [72]; temperature,pressure, reaction residence time and catalyst loading effects byAnastasakis and Ross [101] and more insights by several researchers[36,40,75,76,82,84–89,102–104] are praiseworthy. Finally, Table 2presents the comprehensive details of aforementioned and some moreliterature in tabular form pertained to various aspects of hydrothermalliquefaction over the years.
In the past decade, there were also several review papers on variousaspects of HTL. For instance, Peterson et al. [57] presented sub- andsupercritical water technologies available for thermochemical biofuelproduction from various types of biomass. Toor et al. [134] reviewedthe subcritical water technologies for production of biocrude frombiomass; and they summarized that the chemical properties of bio-crude are highly dependent of the biomass composition. Akhtar andAmin [88] reviewed the literature pertaining to HTL of biomass withthe aim to report process conditions such as final liquefactiontemperature, residence times, rate of biomass heating, size of biomassparticles, types of solvents, etc. for optimum bio-oil yield. They havealso included brief discussion on decomposition mechanisms duringhydrothermal liquefaction. Tekin et al. [90] explained the physico-chemical properties of water under subcritical and supercritical condi-tions on HTL and interactions of water with biomass during HTL
A.R.K. Gollakota et al. Renewable and Sustainable Energy Reviews 81 (2018) 1378–1392
1382
Table
2Su
mmaryof
literature
pertained
tohyd
rothermal
liqu
efaction
ofbiom
ass(bothwet
anddry).
S.No
Auth
or
Desc
ription
Feed
Pro
cess
conditions
Pro
ducts
1Alhassanet
al.[96]
Hyd
rothermal
liqu
efaction
ofde-oiledJa
trop
hacu
rcas
cake
usingdeep
eutectic
solven
ts(D
ESs)as
catalyst
andco
solven
tsDe-oiledJa
trop
hacu
rcas
cake
Tem
p:25
0°C
HHV
values
oftheHTLbiooilis
25.01MJ/kg
.Pressure:4.5MPa
Reactor:Batch
2Anastasakisan
dRoss
[24]
Hyd
rothermal
liqu
efaction
offourbrow
nmacro
alga
ecommon
lyfound
ontheUK
coasts:Anen
ergetican
alysis
oftheprocess
andcomparison
withbio-ch
emical
conversionmethod
s
Digitata,
hyp
erbo
rea,
saccharina,
escu
lenta
Tem
p:35
0°C
Highnitrogencontentis
observed
from
thebiooilob
tained
indicatingthenecessity
forupgrad
ation.
Pressure:
Reactor:Batch
3Bachet
al.[23]
Fasthyd
rothermal
liqu
efaction
ofNorwegianalga
-Screeningtests
Lam
inaria
saccharina
Tem
p:35
0°C
Highestyieldof
79%
bio-oilis
obtained
with35
.97MJ/kg
HHV.
Pressure:16
.5ba
rReactor:Muffle
furn
ace
4Barrieroet
al.[10]
Hyd
rothermal
liqu
efaction
ofmicroalga
e:Effecton
theproduct
yieldsof
thead
ditionof
anorga
nic
solven
tto
separatetheaq
ueo
usphasean
dthe
biocrudeoil
Nan
nochloropsisga
ditan
a,Sc
ened
esmusalmeriensis
Tem
p:35
0°C
Organ
icsolven
tdichloromethan
eincreasestheyieldof
biocrudeat
higher
oxyg
enan
dnitrogencontents.
Pressure:2MPa
Reactor:Micro
autoclav
e5
Cap
orgn
oet
al.[11]
Hyd
rothermal
liqu
efaction
ofNan
nochloropsisOcean
icain
different
solven
tsNan
nochloropsisOcean
ica
Tem
p:24
0–30
0°C
Alcoh
olco
solven
tsyieldhighbiooil~60
%.
Pressure:32
–
43ba
rReactor:Batch
6Chan
etal.[105
]Effectof
process
param
eterson
hyd
rothermal
liqu
efaction
ofoilpalm
biom
assforbio-oilproductionan
ditslife
cycleassessmen
tRaw
fruitbu
nch
,palm
Mesocarp
fibrean
dpalm
kern
elsh
ell
Tem
p:33
0–39
0°C
Optimum
biooilyieldis
obtained
at39
0°C
and25
MPa
Pressure:25
–
35MPa
Reactor:Batch
7Chen
etal.[18]
Hyd
rothermal
liqu
efaction
ofmixed
culture
alga
lbiom
assfrom
waste
water
treatm
entsystem
into
bio-crudeoil
Feedstockfrom
Onewater
INC.
Indianap
olis
Tem
p:26
0–32
0°C
HHVva
lues
of33
.3MJ/Kgisviab
lewithhyd
rocarbon
s,fattyacids,
cyclic
amines.
Pressure:1MPa
Reactor:Batch
8Costanzo
etal.[97]
Effectof
low
temperature
hyd
rothermal
liqu
efaction
oncatalytic
hyd
roden
itrogenationof
alga
ebiocrudean
dmod
elmacromolecules
Sorokiniana,
minutissim
a,bijuga
Tem
p:22
5°C
Bio
oilwithsimilar
compositionto
gasoilob
tained
viaHTL/H
DO
process
usingRu/C
catalyst.
Pressure:20
.7ba
rReactor:Batch
9Dav
idet
al.[106
]Review
andexperim
entalstudyon
pyrolysis
andhyd
rothermal
liqu
efaction
ofmicroalga
eforbiofuel
production.
Nan
nochloropsis
Tem
p:25
0°C
Algal
HTLis
very
han
dywhen
compared
topyrolysis
interm
sof
yield.
Pressure:10
–
25MPa
Reactor:
Con
tinuou
s10
Ebo
ibiet
al.[107
]Hyd
rothermal
liqu
efaction
ofmicroalga
eforbiocrudeproduction:
improvingthebiocrudeproperties
withva
cuum
distillation
Tetraselm
is,Sp
irulina
Tem
p:30
0–35
0°C
ThehighestHHVof
40MJ/kg
was
obtained
aftertheprocess
for
thesp
ecified
feed
stock.
Pressure:30
bar
Reactor:Batch
11Guoet
al.[25]
Areview
ofbio-oilproductionfrom
hyd
rothermal
liqu
efaction
ofalga
eAlgal
feed
stock
Tem
p:43
0–53
0°C
Freefattyacids,
phen
olsan
darom
atic
compou
nds.
Pressure:24
–
34MPa
Reactor:Batch
12Guoet
al.[108
]Hyd
rothermal
liqu
efaction
ofcyan
ophyta:
Evo
lution
ofpoten
tial
bio-
crudeoilproductionan
dcompon
entan
alysis
Cyanop
hyta
Tem
p:37
0°C
Hightemperaturesfavo
uredphen
ols,
benzenes,alka
nes
andfatty
acidsan
dlow
temperaturesfavo
uredalka
nes
andheterocyclic
compou
nds.
Pressure:1.5P
siReactor:Batch
13Haa
rlem
mer
etal.
[109
]Analysis
andcomparison
ofbiooils
obtained
from
hyd
rothermal
liqu
efaction
andfast
pyrolysis
ofbe
achwoo
dBeech
woo
dTem
p:30
0°C
Highiodineva
luebiooilis
obtained
Pressure:1MPa
Reactor:Batch
14Had
hou
met
al.[110
]Hyd
rothermal
liqu
efaction
ofmillwaste
water
forbio-oilproductionin
subc
riticalconditions
Oilmillwaste
water
Tem
p:20
0–30
0°C
Amax
imum
yieldof
58wt%
was
achievedhav
ingmajoritycontent
ofphen
olics.
Pressure:19
0ba
rReactor:Batch
15Heet
al.[15]
Con
tinuou
shyd
rothermal
liqu
efaction
ofmacro
alga
ein
thepresence
oforga
nic
co-solvents
Oed
ogon
ium
Tem
p:30
0–35
0°C
Presence
ofco
solven
thas
minor
effect
ontheproduct
yield.
Pressure:10
bar
Reactor:Batch
16Hietala
etal.[95]
Aqu
antitative
kinetic
mod
elforthefast
andisothermal
hyd
rothermal
liqu
efaction
ofNan
nochlorosisSp
Nan
nochlorosisSp
Tem
p:10
0–40
0°C
Bio
crudeyieldas
highas
46%
isachieva
bleat
400°C
and1min
residen
cetime.
Pressure:40
0ba
r(con
tinued
onnextpage)
A.R.K. Gollakota et al. Renewable and Sustainable Energy Reviews 81 (2018) 1378–1392
1383
Table
2(con
tinued
)
S.No
Auth
or
Desc
ription
Feed
Pro
cess
conditions
Pro
ducts
Reactor:Batch
17Hog
non
etal.[111
]Com
parison
ofpyrolysis
andhyd
rothermal
liqu
efaction
ofChlamyd
omon
asreinhardtii.Growth
studieson
therecovered
hyd
rothermal
aqueo
usphases
c.reinhardtii
Tem
p:22
0–31
0°C
Yield
ofbio-oilishigher
inpyrolysisan
den
ergy
ratios
arehigher
inHTL.
Pressure:10
MPa
Reactor:Batch
18Gao
etal.[17]
Bio-oilproductionfrom
eigh
tselected
greenlandscap
ingwastesthrough
hyd
rothermal
liqu
efaction
Lan
dscap
ewaste
leav
esTem
p:30
0°C
Thepresence
ofva
lue-ad
ded
chem
icals,
phen
olics,
ketones,esters,
acids,
andalcohols,
inbio-oils
areconfirm
edPressure:2MPa
Reactor:Batch
19Lav
anya
etal.[9]
Hyd
rothermal
liqu
efaction
offreshwater
andmarinealga
lbiom
ass.
Arthrosp
iraPlatensis,
tetraselmis
spTem
p:20
0–35
0°C
Introd
ucedaprocedure
toblen
dalga
lcrudeto
petrocrudean
dyielding18
%dieselfraction
.Pressure:18
0ba
rReactor:Batch
20Lyu
etal.[112
]Twostag
enan
ofiltrationprocess
forhigh-valuech
emical
production
from
hyd
rolysatesof
lign
ocellulose
biom
assthrough
hyd
rothermal
liqu
efaction
Ricestraw
Tem
p:30
0°C
TNSF
hyd
rolysatesinto
threeparts:su
garconcentrates,mon
ophen
ols,
cyclop
entanon
es,acid
acids.
Pressure:12
MPa
Reactor:Batch
21Mad
diet
al.[99]
Quan
titative
characterization
oftheaq
ueo
usfraction
from
hyd
rothermal
liqu
efaction
ofalga
eChlorella,
Scen
edesmusob
liqu
es,
Tetraselm
isTem
p:35
0°C
Aceticacid,propan
icacid,pyridine,
aceton
e,ethan
olan
dother
orga
nic
chem
ical
compou
ndsarequ
antified
.Pressure:20
.7MPa
Reactor:
Con
tinuou
s22
Meryemog
luet
al.
[113
]Biofuel
productionby
liqu
efaction
ofKen
afbiom
ass
Ken
afan
dwheatstraw
biom
ass
Tem
p:20
0–35
0°C
Higher
yieldsareseen
at30
0°C
andreducedthereafter.
Pressure:5MPa
Reactor:Batch
23Nazariet
al.[114
]Hyd
rothermal
liqu
efaction
ofwoo
dybiom
assin
hot
compressed
water:
catalyst
screen
ingan
dcomprehen
sive
characterization
ofbio-crudeoils
Birch
woo
dsaw
dust
Tem
p:30
0°C
Occurence
ofphen
olic
derivatives
andaliphatic
compou
nds
increasedsign
ifican
tlyin
thepresence
ofcatalyst
than
withou
tcatalyst.
Pressure:90
bar
Reactor:Stirred
reactor
24Neveu
xet
al.[12]
Bio
crudeyieldan
dproductivityfrom
thehyd
rothermal
liqu
efaction
ofmarinean
dfreshwater
greenmicroalga
eOed
ogon
ium,Derbe
sia,
Ulva
Tem
p:35
0°C
Derbe
sia
>Ulva
>Oed
ogon
ium.This
isthesequ
ence
ofthe
higher
biocrudeproduced
Pressure:25
0ba
rReactor:Batch
25Patilet
al.[115
]Hyd
rothermal
liqu
efaction
ofwheatstraw
inhot
compressed
water
and
subc
riticalwater-alcoh
olmixtures
Wheatstraw
Tem
p:35
0°C
Amax
imum
of28
MJ/kg
HHV
isob
tained
.Pressure:20
0ba
rReactor:Tubu
lar
26Ped
ersenan
dRosen
dah
l[116
]Productionof
fuel
range
oxyg
enates
bysu
percritical
hyd
rothermal
liqu
efaction
oflign
ocellulosicsystem
sAsp
enwoo
dTem
p:40
0°C
Themajoritygrou
psob
tained
arecyclop
entanon
esan
dox
ygen
ated
arom
atics.
Pressure:30
0ba
rReactor:Batch
micro
27Ped
ersenet
al.[13]
Con
tinuou
shyd
rothermal
co-liquefaction
ofaspen
woo
dan
dglycerol
withwater
phaserecirculation
Asp
enwoo
dan
dglycerol
mixture
Tem
p:40
0°C
Bio
crudewithhighqu
alityof
34.3
MJ/kg
HHVva
lue,an
dva
luab
lech
emicalsin
therange
ofC6-C
12similar
toga
soline.
Pressure:30
0ba
rReactor:
Con
tinuou
s28
Raiko
vaet
al.[21]
Assessinghyd
rothermal
liqu
efaction
fortheproductionof
bio-oilan
den
han
cedmetal
recovery
from
microalga
ecu
ltivated
onacid
mine
drainag
e
Arthrosp
iraplatensis,
Chlamyd
omon
assp
Tem
p:31
0–35
0°C
HTLproducedbio-oilforupgrad
ingprocess
from
theacid
mine
drainag
ealso
theprevention
ofAMD.
Pressure:12
0ba
rReactor:Batch
29Red
dyet
al.[117
]Tem
perature
effect
onhyd
rothermal
liqu
efaction
ofNan
nochloropsis
gaditan
aan
dChlorellasp
Nan
nochloropsisga
ditan
aan
dChlorellasp
Tem
p:30
0°C
Higher
yieldsof
HHVof
39MJ/kg
ispossiblewiththefeed
stock.
Pressure:20
0Psi
Reactor:Batch
30Sh
akya
etal.[118
]Effectof
temperature
andNa 2CO3catalyst
onhyd
rothermal
liqu
efaction
ofalga
eNan
nochloropsis,
Pav
lova
,Isochrysis
Tem
p:25
0–35
0°C
Bio
oilwithheatingva
lueof
37MJ/kg
isob
tained
whichis
equivalen
tto
heavy
crudeoil.
Pressure:35
Psi
Reactor:Batch
31Singh
etal.[119
]Catalytic
hyd
rothermal
liqu
efaction
ofwater
hyacinth
Water
hyacinth
Tem
p:25
0–30
0°C
Max
imum
bio-oilyieldof
23%
isob
tained
usingcatalyst
KOH
and
K2CO3withless
oxyg
en.
Pressure:13
00Psi
Reactor:Batch
32Singh
etal.[120
]Hyd
rothermal
liqu
efaction
ofrice
straw:Effectof
reaction
environmen
tRicestraw
Tem
p:28
0–32
0°C
Bio
oilyieldof
17%
and78
%conversionwithether
solublebio-oil
under
O2en
vironmen
t.Pressure:6–9MPa
(con
tinued
onnextpage)
A.R.K. Gollakota et al. Renewable and Sustainable Energy Reviews 81 (2018) 1378–1392
1384
Table
2(con
tinued
)
S.No
Auth
or
Desc
ription
Feed
Pro
cess
conditions
Pro
ducts
Reactor:Autoclav
e33
Sudasingh
eet
al.
[121
]Hyd
rothermal
liqu
efaction
oilan
dhyd
rotreatedproduct
from
pine
feed
stockch
aracterizedby
heteronucleartw
odim
ension
alNMR
spectroscopyan
dFT-ICR
masssp
ectrom
etry
Pine
Tem
p:40
0°C
Fusion
ofFT-ICR
further
simplified
iniden
tification
ofthe
compou
ndspresentin
thebio-oilob
tain
through
HTLprocess.
Pressure:30
00Psi
Reactor:
Con
tinuou
s34
Tek
inet
al.[122
]Hyd
rothermal
liqu
efaction
ofbe
achwoo
dusinganaturalcalcium
borate
mineral
Beach
woo
dTem
p:25
0–35
0°C
Mostof
theproduct
stream
isof
phen
ols,withan
optimum
HHVof
23.81M
J/kg
Pressure:4–
16.5
MPa
Reactor:Batch
35Tianet
al.[123
]Hyd
rothermal
liqu
efaction
ofharvested
high–ashlowlipid
alga
lbiomass
from
Dianch
ilake
:Effects
ofop
erational
param
etersan
drelation
sof
products
Algal
bloo
msof
Dianch
ilake
Tem
p:26
0–34
0°C
18.4%
yieldof
biocrudewithhighestcontentof
ash41
.6%
isob
tained
through
thefeed
stock.
Pressure:3MPa
Reactor:Batch
36Tran[124
]Fasthyd
rothermal
liqu
efaction
forproductionof
chem
icalsan
dbiofuels
from
wet
biom
assusingaplugflow
reactor
Wet
slurryfeed
Tem
p:37
4°C
Possiblech
allengesan
dcu
rren
tstatusof
possiblesolution
sto
the
challenges,
focu
singon
theneedof
develop
ingasp
ecialplug-flow
reactorforscalingupof
theHTLprocess
Pressure:22
.1MPa
Reactor:Plugflow
reactor
37Valdez
etal.[20]
Hyd
rothermal
liqu
efaction
ofnan
nochlorpsissp
:Sy
stem
atic
studyof
process
variab
lesan
dan
alysis
oftheproduct
fraction
sNan
nochlorpsissp
:Tem
p:25
0–40
0°C
Pressure:30
000P
siReactor:Batch
38Vardon
etal.[14]
Thermochem
ical
conversionof
raw
anddefattedalga
lbiom
assvia
hyd
rothermal
liqu
efaction
andslow
pyrolysis
Scen
edesmus,
Spirulina
Tem
p:30
0–45
0°C
Wet
alga
lbiomassfavo
urs
theHTLprocess
that
yieldshigher
HHV
values
of35
–37
MJ/kg
Pressure:10
–
12MPa
Reactor:ba
tch
39Wan
get
al.[125
]Hyd
rothermal
liqu
efaction
ofLitseaCube
baseed
toproduce
bio-oil
LitseaCube
baTem
p:29
0°C
Theliqu
efiedbio-oilwas
characterizedin
higher
C,H
,contentan
dmuch
lower
Ncontent.
Pressure:–
Reactor:Batch
40Wan
get
al.[126
]Hyd
rothermal
liqu
efaction
oflign
ite,
wheatstraw
andplastic
waste
insu
bcritical
water
foroil:product
distribution
Lignite,
wheatstraw
Tem
p:26
0–32
0°C
Addingtourm
alineduringHTLincreasestheoilyield,higher
conversionrates.
Pressure:2–5MPa
Reactor:Batch
41Xuan
dEtcheverry
[127
]Hyd
roliqu
efaction
ofwoo
dybiom
assin
suban
dsu
per
critical
ethan
olwithiron
basedcatalysts
Jack
pinewoo
dTem
p:47
3–62
3K
Highyieldbio-oilof
63%
withFeS
O4catalyst
withdom
inan
tphen
olic
compou
nds.
Pressure:2–
10MPa
Reactor:Batch
42Xuet
al.[89]
Hyd
rothermal
liqu
efaction
ofChlorellapyren
oidosaforbio-oil
productionov
erCe/HZSM
-5Chlorellapyren
oidosa
Tem
p:30
0°C
Zeo
lite
Ce/HZSM
-5exhibitsgo
odpoten
tial
forpreparationof
bio-
oilfrom
microalga
ewithhighefficien
cyPressure:
Reactor:Autoclav
e43
Yan
etal.[128
]Com
positionof
thebio-oilfrom
thehyd
rothermal
liqu
efaction
ofduck
weedan
dtheinfluen
ceof
theextraction
solven
tsDuckweed
Tem
p:35
0°C
Extractionsolven
tswithhighpolarityyieldshigher
%of
biooilof
26%.
Pressure:5kP
aReactor:Batch
44Yan
get
al.[16]
Hyd
rothermal
liqu
efaction
ofsp
entcoffee
grou
ndsin
water
med
ium
for
bio-oilproduction
Spen
tcoffee
grou
nd
Tem
p:20
0–30
0°C
Amax
imum
of31
MJ/kg
HHV
valueof
biooilis
form
edPressure:0.5–
2MPa
Reactor:Autoclav
e45
Yim
etal.[129
]Metal
oxidecatalysedhyd
rothermal
liqu
efaction
ofMalaysian
oilpalm
biom
assto
bio-oilunder
super
critical
conditions.
Malaysian
oilpalm
Tem
p:39
0°C
Thepresence
ofphen
ols,
ketones,arom
atic
carbox
ylic
acidsare
confirm
ed.
Pressure:25
MPa
Reactor:Batch
46Zhan
get
al.[130
]Hyd
rothermal
liqu
efaction
ofChlorellapyren
oidosain
Ethan
ol-w
ater
for
biocrudeproduction
Chlorellapyren
oidosa
Tem
p:28
0°C
Ethan
olwater
solution
has
best
yieldof
57.3
wt%
biocrude.
Pressure:1MPa
Reactor:Batch
47Zhan
get
al.[131
]Thermograv
imetrican
dkinetic
analysis
ofbiocrudefrom
hyd
rothermal
liqu
efaction
ofEnteromorphaprolifera
Enteromorphaprolifera
Tem
p:57
3K
Atw
ostep
mechan
ism
willb
eabe
tter
persp
ective
inop
timizingthe
kineticsof
HTLprocess.
Pressure:1MPa
Reactor:Batch
48Zhen
get
al.[132
]Alkalinehyd
rothermal
liqu
efaction
ofsw
inecarcassesto
bio-oil
Swinecarcasses
Tem
p:25
0°C
62.2
wt%
ofbio-oilan
d22
wt%
solidresidueis
obtained
Pressure:–
Reactor:Batch
(con
tinued
onnextpage)
A.R.K. Gollakota et al. Renewable and Sustainable Energy Reviews 81 (2018) 1378–1392
1385
process. Tian et al. [156] presented review on HTL from algalprospective for algal biorefinery including the factors such as feedstockpreparation, scale-up of algae HTL and process integration. Guo et al.[25] also presented the details of research works pertaining toproduction of bio-oil from HTL of algae and reported that the proper-ties of resulting bio-oil from HTL are affected by temperature, reactiontime, algae species, algae concentration, reaction atmosphere, catalystsand sub and supercritical water reaction conditions. Elliott et al. [80]presented the review of research papers of HTL with aim to describeevolution of HTL processes from batch to continuous state. Furtherthey described the mass and energy balances from the existing processmodels which are used for process costs calculations with the aim toprovide optimum way for commercial likelihood of technology. Jindaland Jha [81] reviewed effects of process variables such as biomasscomposition, temperature, residence time, pressure, biomass heatingrate, particle size, catalysts, etc. on the yield and quality of bio oil.Dimitriadis and Bezergianni [147] presented review on HTL of variousfeedstock such as woody biomass, wastes, plastics and microalgae forbio-crude production through HTL. On the other hand, this articlebriefly reviewed the elemental composition of bio-crude obtained byHTL, different types of feedstock adopted for HTL, mechanism of HTLprocesses, possible process flow diagrams for HTL of both wet and drybiomass and energy efficiency of the process. In addition, this articlealso enlisted a possible future research scope for concerned research-ers.
3. HTL process mechanism
The mechanism of the hydrothermal liquefaction has not beenelucidated much in the literature. The pathway of HTL comprises threemajor steps, depolymerisation followed by decomposition and recom-bination [19]. The details of these are explained in the further sections.Briefly, biomass is decomposed and depolymerized in to small com-pounds. These compounds may be highly reactive, thus polymerizingand forming bio crude, gas and solid compounds is essential. Furtherthe critical process parameters such as temperature, residence time,the process of repolymerization, condensation and decomposition ofthe components from the different phases may vary accordingly. Sincebiomass is a complex mixture of carbohydrates, lignin, proteins, andlipids, the reaction chemistry and mechanisms of biomass liquefactionare consequently also complex [135,136]. Fig. 1 presents the schematicoverview of the reaction pathway and the details of the steps areenlisted [137].
3.1. Depolymerisation of the biomass
Depolymerisation of biomass is a sequential dissolving of macro-molecules through utilization of their physical and chemical properties.The intrinsic hemicellulose and cellulose biopolymers contributespositively towards the thermal stability of the biofuel [138].Depolymerisation process overcomes the difficulties and obstinateproperties of lignocellulose biomass that mimics the natural geologicalprocesses of producing fossil fuels. The decisive parameters tempera-ture and pressure changes the structure of the long chain polymersconsisting of hydrogen, oxygen, and carbon to shorter chain hydro-carbons [139]. Further, the energy contents of the organic materials arerecycled in the presence of water.
3.2. Decomposition of the biomass monomers by cleavage,dehydration, decarboxylation and deamination
This step involves the loss of water molecule (dehydration), loss ofCO2 molecule (decarboxylation) and removal of amino acid content(deamination) [140]. The dehydration and decarboxylation facilitatethe removal of oxygen from the biomass in the form of H2O and CO2
respectively. Biomass comprising macromolecules are hydrolysed toTable
2(con
tinued
)
S.No
Auth
or
Desc
ription
Feed
Pro
cess
conditions
Pro
ducts
49Zhuet
al.[98]
Hyd
rothermal
liqu
efaction
ofba
rley
straw
tobio-crudeoil.Effects
ofreaction
temperature
andaq
ueo
usphaserecirculation
'sBarleystraw
Tem
p:28
0–40
0°C
Thecrudebio-oilcompositionis
adjusted
byreducingtheam
ount
ofphen
olsan
dacids
Pressure:35
0ba
rReactor:Batch
50Zhuet
al.[133
]Influen
ceof
alka
licatalyst
onproduct
yieldan
dproperties
via
hyd
rothermal
liqu
efaction
ofba
rley
straw
Barleystraw
Tem
p:30
0°C
Morephen
olic,less
carbox
ylic
compou
ndswithcatalyticactivity
andhighcarbon
anden
ergy
recovery
withou
tan
ycatalyst
isob
served
.Pressure:90
–
112ba
rReactor:Autoclav
e
A.R.K. Gollakota et al. Renewable and Sustainable Energy Reviews 81 (2018) 1378–1392
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form polar oligomers and monomers [141]. Water at high tempera-tures and pressure breaks down the hydrogen bonded structure ofcellulose and causes the formation of glucose monomers. Becausefructose is more reactive than glucose, it rapidly degrades in plethora ofproducts by the different types of reactions, including isomerization,hydrolysis, dehydration, reverse-aldol defragmentation, rearrangementand recombination reactions [142]. The majority of the degradationproducts such as polar organic molecules, furfurals, glycoaldehydes,phenols and organic acids are highly soluble in water.
3.3. Recombination and repolymerization of reactive fragments
This is the third step in which the reverse to the initial processingsteps are happening due to the unavailability of the hydrogen com-pound [19]. If hydrogen is freely available in the organic matrix for theliquefaction process the free radicals will be capped yielding the stablemolecular weight species. At conditions the unavailability of thehydrogen or the concentration of the free radicals are excessively large,the fragments are recombined or repolymerized to form high molecularweight char compounds otherwise noted as coke formation [143].
3.4. Hydrothermal liquefaction process
Hydrothermal liquefaction is a biomass to bio-liquid conversionroute carried out in water at moderate temperature of 280–370 °C and
high pressures (10–25 MPa) that has a liquid bio crude as mainproduct along with the gaseous, aqueous and solid phase byproducts[144]. The product stream of this process have high energy content andunique feature of enhanced heat recovery in comparison to otherprocesses [145]. Many complex reactions take place during thetransformation of biomass into crude oil like products [146]. In thisregard, the process mechanism is classified for two types of feedstocknamely lignocellulose biomass (dry feedstock) and algal biomass (wetfeedstock). The sub classification or the major fractions of the dry/lignocellulose biomass is hemicellulose, cellulose and lignin [147]. Onthe other hand, the wet feed stock composition comprises of lipids,proteins, carbohydrates and algaenans. The description of HTL of thedry and wet feedstock of biomass are presented herein.
3.4.1. HTL of lignocellulose biomassThe reactor design in the hydrothermal processing can be either
batch or continuous. Continuous reactors require feeding system thatoperate under pressure and include either slurry based pump or lockand hopper systems for large particles. The detailed pathway of thebiomass to bio-crude through hydrothermal liquefaction of lignocellu-lose biomass was shown in Fig. 2 adopted from Biller and Ross [148].The feedstock requires pre-treatment especially the woody biomass forthe reasons of reducing the particle size, removing the contaminants,and alkaline treatment to obtain a stable slurry for easy pumping. Thefeedstock is then processed via HTL at temperature of around 350 °C
Fig. 1. Reaction pathway of the hydrothermal liquefaction.
Biomass/Wet Waste Pre-treatment Hydrothermal
LiquefactionPhase
Separation Hydrotreating
Anaerobic Digestion/Hydrothermal gasification
Aqueous Phase
Aqueous Fertilizer Process Heat andPower Integration
H2
WaterRecirculation
Biochar/fertilizer
Hydrocarbon Fuels
Fig. 2. Process flow diagram of HTL process of lignocellulose biomass adopted from Biller and Ross [148].
Water &NutrientsWater &Nutrients
AlgaeCultivation Water Removal Hydrothermal
Liquefaction Hydrotreating
Catalytic HydrothermalGasification
Hydrocarbon Fuels
H2
Process Heatand PowerIntegration
Aqueous Phase
Solids, P recycle
Nutrients & CO2
CO2
Fig. 3. Process flow diagram of HTL process of algal biomass/wet biomass adopted from Biller and Ross [148].
A.R.K. Gollakota et al. Renewable and Sustainable Energy Reviews 81 (2018) 1378–1392
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and a pressure of 150 bar for 15 min approximately. Phase separationoccurs spontaneously at these operating conditions resulting thegaseous stream of CO2, solid residue, bio-crude and small traces ofaqueous phase [149]. The aqueous phase/water can be recirculated toHTL unit which reduces the water requirement and enhance the bio oilyield. The other stream of the water that is generated through HTLprocess can be treated anaerobically or via catalytic hydrothermalgasification technique to produce methane rich or hydrogen richsyngas. Anaerobic digestion of the water from the HTL process can’tbe treated to full extent due to the presence of phenols and furfuralsthat delimits the anaerobic digestion process.
During the phase separation process the obtained solid phasematerial can be directly used as a bio char/fertilizer. The producedbio-crude may not be directly used as an alternate which requiresfurther hydrotreatment for commercial utilization. However, compli-cated reaction mechanism is not required for the upgrading procedureas the bio-crude obtained through HTL is less in moisture, oxygencontent and hence the fine hydrotreatment will enhance the quality ofbio-crude. Finally, the amount of the heat generated through the entireprocess can be efficiently utilized to operate the hydrothermal gasifierthat refines the process economy.
3.4.2. HTL of wet/algal biomassThe major benefit of processing algae/wet biomass is the potential
for recycling of nutrients back to cultivation. The procedure of treatingwet biomass is similar to that of treating lignocellulose with a fewmodifications. The schematic of the procedure treating the wet/algalfeedstock is presented in Fig. 3 adopted from Biller and Ross [148].
The major difference lies in treating the wet feedstock compared to dryfeedstock is ignoring the pre-treatment step.
Generally, pre-treatment is essential as the particle size of the microalgae is quite small compared to lignocellulose that eliminates thecomplexities of pumping slurries to the reactor. Further, dewateringprocedure at harvesting step tends to produce the feedstock slurry with20% solids. Then the slurry is passed through the HTL reactor yieldsbio crude that has to be hydro-treated further to produce hydrocarbonfuels. During the HTL process the aqueous phase generated isrecirculated back to the algae cultivation procedure, CO2 that isreleased can be utilized in the process of photosynthesis in the growthof algae for next batch. During hydrotreatment the nutrients areseparated that are sent back to the algae cultivation procedure. Thisovercomes the shortfalls of the HTL process economy.
4. Energy efficiency of HTL process
The ultimate target of the hydrothermal liquefaction procedure is toconvert the biomass to bio crude that can be utilized as an alternate tothe commercial fossil fuels. However, the economy of the process ishighly questionable due to the utilization of higher pressure conditions.Fig. 4 describes the utilization of various product streams that canaffect the energy efficiency in terms of materials and feedstock. HTL ofwet biomass i.e., algal feedstock is a multifaceted processing techniquethat can generate the nutrients which can be recycled for the growth ofalgae, waste water can be used for the water requirements and CO2 forthe photosynthesis reaction.
It has the potential to bottleneck the current algal refiningprocedure including consumption of water, fertilizer and energy input.One of the cutting edge benefits of this HTL process is it consumes 10–15% of the energy in the feedstock biomass yielding energy efficiency of85–90%. HTL can recover more than 70% of the feedstock carboncontent that can be utilized for the carbon capture procedures. Theother technical edge of this process is the bio-crude that is obtainedfrom this process doesn’t require much treatment/upgrading proce-dures for the commercial utilization. From Fig. 4, it is seen that severalby-products are obtained through this HTL process like fertilizer in theform of a solid content, bio crude alternate to fossil fuels, the processedwater can be treated for the utilization and the nutrient cycle for the
Fig. 4. Schematic of Energy efficient usage HTL process using algal biomass. This figure was drawn from Refs. [150–156].
Table 3Comparison of bio-crude by HTL and pyrolysis adopted from Elliot et al. [157].
Elemental composition Hydrothermal liquefaction Pyrolysis
C (wt%) 73 58H (wt%) 8 6O (wt%) 16 36S (ppm) < 45 29Moisture 5.1 24.8HHV (MJ/kg) 35.7 22.6Viscosity (cPs) 15,000 59
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regeneration of the algal growth. A comparison on the bio-crudesobtained via pyrolysis and hydrothermal liquefaction procedures areshown in Table 3 whereas, the major product compounds obtained byHTL are presented in Table 4. From Table 3 it can be observed thatexcept the sulphur contents and viscosity, the performance of HTL issuperior to pyrolysis.
5. Conclusion
Because of the ordinant dictations for the fuels, the alarming levelsof CO2 emissions into atmosphere, and depletion of conventional fossilfuels the desideratum for the renewable fuels is evident. In this context,biomass derived fuels seems to be the promising path which isrenewable and redundant. Various techniques are available for thebiomass processing such as pyrolysis, transesterification, hydrothermalliquefaction, steam reforming, etc. However, bio-oil (or bio-crude)obtained from various process suffers from several demerits such ashigh oxygen content, rendering its acidic value, instability, miscibility,low heating values, etc. Hence utilization of bio-crude requires theoptimal processing technique to efficiently convert the biomass to bio-crude i.e., an alternate for future use. Amongst the aforementionedtechniques hydrothermal liquefaction of biomass seems to be thepromising route for the engenderment of bio-oil equipollent to
conventional crude oil. Briefly, this article enlightened the elementalcomposition of bio-crude obtained by HTL, different types of feedstockadopted for HTL, mechanism of HTL processes, possible process flowdiagrams for HTL of both wet and dry biomass; advantages/disadvan-tages and energy efficiency of the process. Finally, challenges stillsubsist with this field which could serve as future scope for concernedresearchers. A very few of these challenges are listed below:
1. Set up an HTL pilot plant by analyzing the effects of feedstock (wetbiomass) conversion efficiency.
2. Analyze the complexities of the critical parameters like temperature,pressure and the residence time for HTL of a given feedstock.
3. Examine possible pathways for optimized conversion via HTL (ofboth wet and dry biomass feedstock and their mixtures).
4. Explore the relationships between bio-crude oil yield and valueadded chemical species and study the effects of catalyst on produc-tion of individual specific chemical compounds.
5. Investigate the reaction pathways of HTL process of model com-pounds as well real biomass.
6. Develop theoretical, especially computational fluid dynamics based,models to analyze the complex inherent properties like thermody-namic properties of the bio-crude, viscosity effects etc. so that tooptimize the processing parameters.
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Table 4Major product compounds of bio-oil obtained by HTL process.
Product compounds Reference
MonoaromaticsPhenol Duan and Savage [158]Benzene/Toluene/Styrene Ross et al. [75]Cholesterol and Cholestene Jena and Das [77]Vitamin E Zou et al. [76]Fatty AcidsMyristic acid Brown et al. [159]Palmitic acid Shuping et al. [160]Steric acid/oleic acid Valdez et al. [161]Tetradecanoic and Octanoic acid Biller and Ross [40]Hexadecanoic acid Jena et al. [162]Arachidic acid Duan and Savage [158]Eicosapentaenoic acid Brown et al. [159]Alkane/AlkeneHexadecane/hexadecane Yang et al. [163]Heptadecane/Heptadecene Biller and Ross [40]Pentadecane/Pentadecene Zou et al. [76]Phytane/Phytene Shuping et al. [160]Docosane and Cholestane Valdez et al. [161]Cycloalkane Anastasakis and Ross [24]Eicosane, Coprostane, Triconate, Hentriaconate,
DotriaconateDuan and Savage [158]
Polyaromatic compoundsNaphthalene Yang et al. [163]Quinoline Jena et al. [162]Indene Brown et al. [159]Anthracene and Phenanthrene Yang et al. [163]Pyrene and carbozole Zhou et al. [164]Fluorene Duan and Savage [158]Nitrogen compoundsPiperidines Jena and Das [77]Pyrrols/Pyrrolidines Biller and Ross [40]Indoles Yuan et al. [100]Pyridines Valdez et al. [161]Pyrazines Zhou et al. [164]Pyrrolidinones Shuping et al. [160]Amides Valdez et al. [161]Amines and nitriles Zou et al. [76]Other Oxygenated compoundsEsters Jena et al. [151]Aldehydes Anastasakis and Ross [24]Ketones Ross et al. [75]Alcohols Yang et al. [165]Acetic acid Jena and Das [77]Furans Jena et al. [162]
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