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Development of Upgraded Bio-Oil Via Liquefaction and Pyrolysis
C H A P T E R
12 Development of Upgraded Bio-Oil Via Liquefaction and Pyrolysis Aisha Matayeva*,†, Francesco Basile*, Fabrizio Cavani*, Daniele Bianchi‡, Stefano Chiaberge‡ *Department of Industrial Chemistry “Toso Montanari”, University of Bologna, Bologna, Italy †Institut f€ ur Technische und Makromolekulare Chemie, RWTH Aachen University, Aachen, Germany ‡ Renewable Energy & Environmental R&D - Istituto Eni Donegani Eni spa, Novara, Italy
O U T L I N E 1 Current Processes for Conversion of Biomass into Bio-Oil 2 From Biomass to Bio-Oil Via Pyrolysis 2.1 Fundamentals of Pyrolysis 2.2 Pyrolysis Oil Properties 2.3 Catalytic Pyrolysis 2.4 Upgrading of Pyrolysis Oils 2.5 Pyrolysis Technologies 3 Bio-Oil Production by Hydrothermal Liquefaction 3.1 Fundamentals of HTL
Studies in Surface Science and Catalysis, Volume 178 https://doi.org/10.1016/B978-0-444-64127-4.00012-4
232 233 233 235 237 240 241 243 243
3.2 HTL Oil Properties 3.3 Catalytic HTL 3.4 Hydrothermal Upgrading 3.5 HTL Technologies 4 Comparative Studies on HTL and Pyrolysis Processes 5 Conclusions and Recommendations for Future Work References Further Reading
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# 2019 Elsevier B.V. All rights reserved.
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1 CURRENT PROCESSES FOR CONVERSION OF BIOMASS INTO BIO-OIL Environmental concerns and possible future shortages have boosted research into alternatives for fossil-derived products [1]. Biomass is one potential source of renewable energy and the conversion of plant material into a suitable form of energy, usually as electricity or fuel for an internal combustion engine, can be achieved using a number of different routes, each with specific pros and cons [2]. Depending on the biomass used in the biorefinery process, first-, second-, and thirdgeneration biofuels can be distinguished. Three main types of first-generation biofuels used commercially are biodiesel, bioethanol, and biogas, whose production process is considered an “established technology.” First-generation biofuels obtained through a biorefinery process of edible biomasses such as crop, sugar cane, and vegetable oils, are in competition with the food industry [3]. This creates several economical and ethical issues related to the food and
FIG. 12.1
agricultural land and water they need to grow. Development of the so-called second-generation biofuels are aimed to overcome the above issues. The feedstock is generally lignocellulosic-based biomass (wood waste, agricultural and forestry residues, sorted domestic organic wastes, etc.). Fuels from aquatic organisms (micro- and macroalgae, etc.) are often considered as thirdgeneration biofuels, since they avoid the competition with both food and soil use. These organisms have gained a growing interest in recent years as some of them contain a higher concentration of lipids, which are desirable components in biofuel production. The roadmap for biomass conversion into platform chemicals, which is able to substitute petroleum-derived equivalents, is complex and ranges from biological to severe catalytic thermochemical processes (Fig. 12.1). Bio-chemical processes, such as fermentation and anaerobic digestion, occur at lower temperatures in a diluted water phase, and most common types of biochemical processes are fermentation and anaerobic digestion. The fermentation uses microorganisms or enzymes to convert a
Biomass conversion processes.
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fermentable substrate into recoverable products, while an anaerobic digestion involves the bacterial breakdown of biodegradable organic material in the absence of oxygen over a temperature range from about 30°C to 65°C to produce biogas [4]. In comparison to the biochemical processes, thermochemical processes occur faster in the range of few seconds, minutes, or hours [5]. Biomass can be converted into bio-oil by two main thermochemical routes: pyrolysis and hydrothermal liquefaction (HTL). The characteristic and technique feasibility of the two thermo-chemical processes for bio-oil production are compared in Table 12.1. Pyrolysis is the thermochemical decomposition of dry organic matter (moisture content below 10% mass fraction) in the absence of oxygen at moderate temperatures (350–550°C) and atmospheric pressure [6], while HTL is performed in the presence of water at lower temperatures (200–350°C) and higher pressures (2–20 MPa), and can convert biomass with high moisture content (above 50% mass fraction) [7]. The resulting bio-oil product then can be upgraded TABLE 12.1 Comparison of Two Typical Thermochemical Processes for Bio-Oil Production Pyrolysis
HTL
Temperature (°C)
450–500
300–400
Pressure (MPa)
atmosphere
5–20
Pretreatment
Drying is necessary
Drying is not necessary
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by conventional hydrotreatment in order to produce advanced biofuels. Both pyrolysis and HTL technologies are promising approaches to solve the environmental problems, since both processes do not only produce useful energy in the form of liquid fuel, but also contribute to the disposal of household, agriculture, industrial, and municipal residues/ wastes. In other words, the bio-oil production from second- and third-generation biomass (domestic organic waste, municipal waste, sewage sludge, etc.) by HTL and pyrolysis has been a sustainable “waste-to-fuel” approach serving multiple purposes: waste/residue management, energy production, reduction of greenhouse gas emissions, and economic benefits. Therefore, the fundamentals of both pyrolysis and HTL, including their latest developments, are of great importance in evaluating the applicability of these processes within the waste management sector. In this regard, the main goal of this chapter is to bring together an overview of ongoing efforts and advances as well as the environmental and economic aspects of both pyrolysis and HTL technologies. Additionally, the influence of catalysts during the pyrolysis and HTL on the product yields and composition is also being considered.
Treatment conditions
Bio-oil characteristics HHV (MJ/kg)1
35
22
C
70
55
H
8
6
O
12
34
N
10
5
Elemental analysis (%)
2 FROM BIOMASS TO BIO-OIL VIA PYROLYSIS 2.1 Fundamentals of Pyrolysis Many reviews on pyrolysis can be found in the literature [8,9]. These reviews cover pyrolysis process design, pyrolysis reactors as well as its commercialization. According to the literature reviewed, pyrolysis is known to be conventional, fast, or flash, depending on the operating conditions employed. Conventional pyrolysis may also be termed slow pyrolysis [10], defined as the thermal cracking of organic-based materials in the absence of oxygen at slow heating
2. BIO-BASED PROCESSES AND BEYOND
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rates (0.1–10°C/s), while fast pyrolysis is a high-temperature process (400–550°C) in which biomass is rapidly heated (10–200°C/s) and decomposed to form vapors, aerosols, gases, and char. Bio-oil is collected by rapid quenching and condensing the vapors and by coalescence of aerosols. Flash pyrolysis is a thermal-cracking process at a very high heating rate (>1000°C/s) with a very short vapor residence time to minimize the secondary cracking to keep the liquid yields high [11,12]. The reactor is the core and generally the most researched aspect of pyrolysis, though control and improvement of liquid quality are receiving increased attention. Reactors described in literature, used in the pyrolysis of different wastes, include fixed bed reactors, batch or semibatch reactors, rotary kilns, fluidized bed reactors, microwave-assisted reactors, and some innovative solutions like plasma or solar reactors [13–15]. As it can be seen from Table 12.2, the reactor design used in pyrolysis units depends on the type of process. In slow pyrolysis, the most common reactors are drum, rotatory kilns, and screw/auger. In fast pyrolysis systems, reactors can be fluidized bed, rotating cones, entrained flow, vacuum, and ablative [16]. Typically, a pyrolyzer consists of a reactor, cyclone, and condenser. The cyclone separates solid products from liquid and gases. Once the
TABLE 12.2
solid products are separated, the vapor products are rapidly quenched in the condenser and then bio-oil is separated from other gases. Noncondensable gases are usually recycled into the pyrolyzer for fluidization and heating. In general, fluidized-bed reactors are used to study the behavior of fast pyrolysis and to investigate the secondary cracking of oil at longer residence times [17]. Fluidized-bed reactors are characterized by a high heating rate and a good blending of the feedstock. In these reactors, the heat transfer medium is the bed of particles (sand, catalyst, etc.) which is circulated, using high flow rates of gas from the reactor vessel into a burner. In the burner, the particles are exposed to oxygen with the gas, or solid reaction co-products, which are burned to heat the particles and then circulated back into the reactor vessel. As a result, the solid residence time is approximately the same as the vapor residence time, and the reactor operates at high superficial gas velocities. Flash vacuum pyrolysis had its beginnings in the 1940s and 1950s, mainly through mass spectrometric detection of pyrolytically formed free radicals. Meanwhile, many different types of apparatus and techniques have been developed, and the most important methods as well as a survey of typical reactions and observations, are well described [18].
Characteristics of Various Types of Pyrolysis Conventional
Fast
Flash
- Heating rate (C/s)
0.1–10
10–200
<1000
- Particle size (mm)
5–50
<1
<0.2
- Vapor residence time (s)
450–550
0.5–10
<0.5
Reactor configurations
Fixed bed, vacuum reactors
Ablative, auger, fluidized bed, circulating fluidized bed reactors
Fluidized bed, circulating fluidized bed reactors, downer reactors
Operating conditions
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In the rotating cone reactor, biomass particles and heat carrier particles are introduced near the bottom of the cone where the solids are mixed and transported upward by the rotating action of the cone. In this pyrolysis reactor rapid heating and a short residence time of the solids can be realized [19]. Biomass materials like wood, rice husks, or even olive stones can be pulverized and fed into the rotating cone reactor. Due to the a number of reactor types being used for the pyrolysis of waste, Scott and others [20] analyzed several reactors for fast biomass pyrolysis and concluded the general broadly desirable criteria including: simplicity in design, ease of scale-up, ease of control and operation, improved thermal efficiency, and possibility of compact, small-scale designs suitable for rural environments. Currently, fluid beds, circulating fluid bed and transport reactors, and auger pyrolysis reactors are reported to have a strong technology basis and high-market attractiveness, and more detailed information on various fast pyrolysis reactors can be found in previous chapters [13]. Within the reactor types the feedstock composition plays a big role to produce higher yield of bio-oil. Commonly, the land biomass residues were most suitable feedstock to produce pyrolysis oil. Most work has been performed on wood because of its consistency and comparability between tests [10]. However, other types of biomass have been used. For instance, a number of studies on oil production from algae as a nonpolluting, renewable source of energy can be found in the literature. For instance, authors [21] have shown that liquid fuel in the form of a mixture of hydrocarbons could be produced from the microalgae Dunaliella. Similarly, another work [22] came to the same conclusion that the properties of bio-oil of microalgae were more suitable for fuel use than pyrolysis oils from lignocellulosic materials. Representative values of properties for pyrolysis oil products derived from different sources are collected and listed in Table 12.3.
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2.2 Pyrolysis Oil Properties Pyrolysis oil is a liquid, typically dark redbrown to almost black [10] depending on the chemical composition of the biomass. It has a distinctive acid, smoky smell, and can irritate the eyes [3]. It is important to note that oil properties depend on feedstock and operating conditions but may change during storage due to a process indicated as “aging,” which is usually characterized by an increased viscosity within time and a phase separation of the oil in a watery phase and a viscous organic phase. Typically, the quality of pyrolysis oil from lignocellulose is too poor for a direct use as transportation fuel or even for direct upgrading to fuel precursors at existing oil refineries. It is very viscous, acidic (pH < 3), unstable and has a relatively lowenergy density (19 MJ/kg), compared with conventional fossil oil (30 MJ/kg) [32]. In general, the oil has a polar nature and does not mix readily with hydrocarbons due to the presence of large amounts of water and oxygenated components [9]. The presence of large amounts of oxygen in plant carbohydrate polymers means the pyrolytic chemistry differs sharply from these other fossil feeds. Condensed species in bio-oil are derived mainly from lignin, but also cellulose fragments. Bio-oil components’ molecular weights from several hundred or more can be obtained. They form as part of the aerosols. Free water in the biomass explosively vaporizes upon fast pyrolysis. It shreds the feed particles and aids heat transfer. Cellulose and hemicellulose also lose water, which contributes to the process. Chemically, bio-oil is a complex mixture of water, guaiacols, catecols, syringols, vanillins, furancarboxaldehydes, isoeugenol, pyrones, acetic acid, formic acid, and other carboxylic acids. It also contains other major groups of compounds, including hydroxyaldehydes, hydroxyketones, sugars, carboxylic acids, and phenolics. Phenolic compounds are often present in high
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236 TABLE 12.3
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Overview of Literature on Pyrolysis of Common Types of Biomass
Biomass Type
Reactor Type
T (°C)
Time (min)
A bench-scale fixed bed
600
–
Heating Rate (°C/min)
Oil Yield (wt.%)
HHV (MJ/kg)
Ref.
–
51.2
–
[23]
–
[24]
Second generation biomass Maize Stalk
45.5
Rice straw
50.2
Cotton straw
42.8
Corn cob
Fixed bed reactor
450
60
20
47.3
Wheat straw
36.7
Rice straw
28.4
Rice husk
38.1
Sugarcane bagasse
A Hastelloy-type horizontal cylindrical device of steel
380–780
20
120–127
47–38
—
[25]
Tea residue
CDS Pyroprobe 5200 pyrolyzer
450–700
0.3
20 C/ms
59
—
[26]
Waste cereals
Laboratory pyrolysis unit
800
10
46
27
[27]
62
14
36.5
–
[28]
24.29
38
[29]
Waste peanuts crisps Sewage sludge
A stainless-steel tube
500
20
Food waste
36.3
Wood
50.9
Jatropha (in form of seeds, cake, hulls)
A bench scale reactor
500°C
0.25
Algal waste
A stainless steel fixed-bed reactor
400–600
20
10
24.10 and 44.01
19.91
[30]
Chlorella protothecoides
A fluid bed reactor
500
0.05
600 C/s
18
29
[22]
23–31
[31]
Third generation biomass
Microcystis aeruginosa Laminaria digitata, Fucus serratus
24 A fluid bed reactor
500
concentrations (up to 50 wt.%), consisting of relatively small amounts of phenol, eugenol, cresols, and xylenols [17]. The bio-oils have a water contents of typically 20–25 wt.% [33], and
0.03
11–17
water cannot be removed by conventional methods like distillation. Upgrading via hydrodeoxygenation produces high-grade stable oil that can be burnt at
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a higher efficiency or refined into transportation fuels that are compatible with current infrastructure and vehicle technologies. Several works have been carried out on research and development of bio-oil for heat and power generation, including the design and study of specialized bio-oil [34]. Bio-oil can be used in the following applications: it can be combusted in industrial or residential boilers for heat and power generation, co-feed in natural gas-fired/coal-fired power plants, or converted into fuels (gasoline and diesel) and chemicals through hydrocracking or hydroprocessing. However, as discussed earlier, several bio-oil properties, including the high oxygen content, water content, oil instability and aging make the conversion into heat and power significantly more challenging and ultimately limit the range of its applications. Therefore, further research is needed to overcome remaining challenges in using these products because the environmental and economic benefits and success of pyrolysis technologies depend on addressing these challenges.
2.3 Catalytic Pyrolysis The complexity of bio-oil makes it challenging for further utilization as intermediate to produce transportation fuels. The chemistry of bio-oils can be manipulated by changing the thermal conditions of the process or by conducting pyrolysis in the presence of catalysts. It has been recognized that the application of catalysis could be of major importance in controlling the oil quality and its chemical composition [35]. Catalysts could be applied for a number of reasons, such as lower pyrolysis temperatures, a higher chemical and physical stability, high yields of target components, and an improved miscibility with refinery streams. Catalysts might be used at different stages in the fast pyrolysis process; they could be impregnated in the biomass feed, mixed with the biomass in the pyrolysis reactor (in situ operation), built in the process after the pyrolysis reactor for the reforming of primary pyrolysis
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vapors (ex situ operation) [36]. The major chemical reaction pathways involved in the biomass catalytic pyrolysis include deoxygenation, cracking, aromatization, ketonization, aldol condensation, hydro-treating and reforming (Fig. 12.2). By using an appropriate catalyst, a specific reaction can be selectively enhanced, leading to the increased production of desirable products, such as hydrocarbons and anhydrous sugars [37]. Catalytic deoxygenation is an efficient route to reduce the oxygen content in the bio-oil. Dehydration, decarboxylation, and decarbonylation are typical deoxygenation reactions to remove oxygen in the form of H2O, CO, and CO2, respectively. Catalytic cracking can convert large molecules and heavy organic chemicals into small molecular products. Meanwhile, the catalytic cracking of oxygenates also gives rise to the formation of aromatics and olefins. Aromatization is another main route to low-molecular weight oxygenates and olefins into aromatics. Under HZSM-5 catalysis, acids, alcohols, aldehydes, esters, ethers, and furans can be largely converted to aromatic hydrocarbons as the major products [38–40]. In addition, ketonization/aldol condensation is a promising route for the conversion of the carboxylic and carbonyl components into longer-chain intermediates that can be further upgraded into gasoline/diesel fuels. Ketonization is a reaction that converts two carboxylic acid molecules into a ketone, CO2, and H2O. This reaction can simultaneously remove carboxyl groups and achieve CdC coupling without consuming external hydrogen, thereby increasing the heating value and stability of the products [41]. However, the ketonization reaction mechanism is complex, and ketene, betaketoacids, carboxylates, and acyl carbonium ions are suggested to be the key intermediates. The surface acid-base properties of metal oxides potentially favor the catalytic pyrolysis of biomass to form desirable products and the different acid and base sites of the metal oxides alter the distribution of bio-oil components, promoting or inhibiting the formation of certain products.
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FIG. 12.2 Schematic of the chemical reactions for biomass catalytic pyrolysis. Reproduced with the permission from S. Wang, G. Dai, H. Yang, Z. Luo. Lignocellulosic biomass pyrolysis mechanism: a state-of-the-art review, Prog. Energy Combust. Sci. (2017).
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Magnesium oxide (MgO) and calcium oxide (CaO), the typical base metal oxides catalyst used for biomass catalytic pyrolysis, are effective for deoxygenation via ketonization and aldol condensation of acid and ketone compounds. For instance, authors [42] revealed that with the addition of CaO in the pyrolysis of a cotton stalk the concentration of ketones increased, while that of acidic compounds decreased in the resulting product. The direct deoxygenating effect of CaO on bio-oil during pyrolysis of white pine in a fluidized-bed reactor was studied at 520°C [43]. According to the results obtained, it was shown that the oxygen content of the bio-oil decreased from 39% to 31% in the presence of CaO. Moreover, furfural, furfuryl alcohol, hydroxymethyl furfural, and 3-methyl-2-cyclopentanone are recognized as the products from dehydration reactions that improved with increasing the CaO loading. In addition to acidic and basic metal oxides, transition metal oxides with unique acid–base properties have been widely investigated for biomass catalytic pyrolysis, with various effects on the product distribution. Authors [44] screened the effect of 31 catalysts on the carbon yield of solid residue, gas and bio-oil from the pyrolysis at 500°C of pine sawdust with the ratio 1/1 (w/w). Tested catalysts included mesoporous silica-supported metal oxides, bulk metal oxides, clays, and zeolites. With all the tested catalysts the yields of bio-oil ranged from 14% to 58%, with large differences in the concentration of the heavy fraction (0%–70%). However, for most of the catalysts a reduction of heavy matter was accompanied by a significant diminution of bio-oil yields with respect to uncatalyzed pyrolysis of biomass. A significant decrease in nonvolatile fraction was obtained with ZnO, CuO, Fe2O3, and mixed-oxide catalysts. Zeolites, crystalline materials, are the most investigated solid acid catalysts for biomass catalytic pyrolysis. Mesoporous and macroporous zeolites may not favor reactions for specific products due to the high exposure of the active
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site to the substrate [45], while the microporous materials have been extensively studied in the catalysis of fast pyrolysis of lignocellulosic biomass due to their good deoxygenation performance and shapes selectivity for value-added hydrocarbons. In particular, ZSM-5 zeolite, especially the protonated form, HZSM-5, has been demonstrated to be the most effective catalyst for aromatic hydrocarbon production. During the biomass catalytic pyrolysis using HZSM-5, the primary pyrolysis vapor undergoes a series of reactions on both the surface and pores of the catalysts, including cracking, deoxygenation, decarboxylation, cyclization, aromatization, isomerization, alkylation, disproportionation, oligomerization, and polymerization [46]. Doping certain metal cations or oxides into zeolite has been explored as a method to increase the yield of desired compounds and to suppress coke formation by tuning the density and activity of the active sites. Authors [47] evaluated a set of commercial and laboratory-synthesized catalysts for their hydrocarbon production performance via the pyrolysis/catalytic cracking route from three types of biomass feedstocks: cellulose, lignin, and wood at temperatures ranging from 400°C to 600°C. Batch mode tests used three biomass feedstocks to identify the most promising catalysts from a set of 40 selected ones: 10 commercial zeolites (including ZSM-5, Y, and SAPO types), 22 laboratory-prepared ZSM-5 catalysts modified by substituting Al with different metals (cobalt, iron, nickel, cerium, copper, gallium, etc.), four laboratory zeolites, and four different silica and alumina materials. The highest yield of hydrocarbons (approximately 16 wt.%) was achieved using nickel, cobalt, iron, and gallium-substituted ZSM-5. According to the previous studies on the catalytic pyrolysis it is shown that the application of the proper catalyst is a fundamental step in developing catalytic pyrolysis as an effective process to convert biomass into a valuable liquid biofuel, and further research is still needed in this area.
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2.4 Upgrading of Pyrolysis Oils The high content of oxygenated compounds and the water content, and a low heating value of pyrolysis oil, make the direct co-processing of oil itself in standard refinery units problematic. Several pyrolysis process modifications are currently being studied to obtain the oil product with the better properties. Various upgrading routes have been studied: hydrodeoxygenation (HDO) to remove the oxygen under high pressures of hydrogen and in the presence of a catalyst; catalytic cracking using zeolites; and high-pressure thermal treatment, in which pyrolysis oil is thermally treated to obtain the oil with the higher energy density. Integrated co-processing would provide a relatively straightforward route to the production of liquid transport fuels via fast pyrolysis since oil refining infrastructure is already available. Combined hydrotreating and catalytic cracking appears to possess significant potential for the production of commodity chemicals. Additionally, a two-step hydroprocessing scheme comprising a mild stabilization step and a more intensive upgrading step are envisaged. Alternatively, the hydroprocessing step coupled with catalytic cracking is also being investigated. For instance, in the study [48], the applicability of HDO was investigated as the pyrolysis oil upgrading step to allow fluid catalytic cracking (FCC) co-processing. Different HDO reaction temperatures (230–340°C) were evaluated in a 5 L autoclave, keeping the other process conditions constant (total 290 bar, 5 wt.% Ru/C catalyst). After HDO, the upgraded pyrolysis oil underwent phase separation resulting in an aqueous phase, some gases, and the oil phase that was further processed in a micro-activity test (MAT) reactor (simulated FCC reactor). When the HDO reaction temperature was increased, a net transfer of organic components from the aqueous phase to the oil phase was observed, increasing the carbon recovery in the oil product (up to 70 wt.% of the carbon in
pyrolysis oil). Authors have demonstrated on a laboratory scale that it is possible to produce hydrocarbons from lignocellulosic biomass via a pyrolysis oil upgrading route. Correspondingly, it is also reported by other authors [13] that a combined HDO step and co-processing in an FCC unit produced transport grade fuel. As mentioned earlier, catalytic hydropyrolysis also represents an alternative approach. Cellulosic and woody biomass can be directly converted to hydrocarbon gasoline and diesel blending components through the use of integrated hydropyrolysis plus hydroconversion (IH2) [49]. The IH2 process consists of a pressurized fluidized-bed first-stage reactor for hydropyrolysis, followed by the hydroconversion step, which further removes oxygen from the biomass and fully converts it to gasoline and diesel products. Light gas from the hydroconversion step is separated and sent to a steam reformer which produces the hydrogen used in the process. The hydrocarbon liquid yield is 24–28 wt.%, which is comparable with fast pyrolysis coupled with FCC or HDO. The produced oil is highly deoxygenated (less than 1% oxygen) and contains no polynuclear aromatics, olefins, or reactive free radicals because high partial pressures of hydrogen and the catalyst are available during conversion. With this integration, and using the proper operating conditions, the process is self-sufficient as it requires no external source of methane or hydrogen. This technology might utilize domestic renewable biomass resources to produce transportation fuels in sufficient quantity and quality to substantially reduce the reliance on crude oil [50]. A number of interesting insights into pyrolysis oil hydroprocessing have been gained in recent years. Integrated upgrading approaches, that is, hydroprocessing followed by fluid catalytic cracking, appears to possess synergistic benefits. The production of commodity chemicals via hydroprocessing and catalytic cracking routes within the biorefinery infrastructure may enhance the economic viability of pyrolysis
2. BIO-BASED PROCESSES AND BEYOND
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and pyrolysis-related processes [13]. An important task is that upgrading should selectively target problematic oxygen functionalities and the future research work will need to address the search for cheap catalysts.
2.5 Pyrolysis Technologies Early investigations into the pyrolytic conversion of biomass to liquids were carried out in the 1970s by various workers, for example, the work by Knight and co-workers, and by Garrett Corp. (continued by Occidental Petroleum) [9]. Additional impetus for further process studies was given by the development in the early 1980s of the Waterloo Fast Pyrolysis Process (WFPP) at the University of Waterloo, Canada, and by the development of an “ablative” pyrolysis method at the National Renewable Energy Laboratory in the United States [51]. Up to 1989, the only European plant was a conventional pyrolysis demonstration plant of 500 kg/h operating in Italy for liquid and char production with approximately 25% yield for each. The pyrolysis plant was in operation from 1985 to 1990 and was the largest pyrolysis plant for liquids built in Europe until 1996. The design capacity of the plant was 1 t/h dry biomass, but only up to 500 kg/h was achieved on a continuous basis. The nature of the process was that relatively slow pyrolysis occurred in the stirred or fluid bed reactor producing secondary oil that had a low water tolerance and high viscosity. Although the product quality was poor, the availability of large quantities of pyrolysis liquids for testing and evaluation caused serious attention to be focused on direct liquefaction for the first time in Europe [9]. Since the late 1990s, the process realization emerged, resulting in the construction of pilot plants in Spain (Union Fenosa), Italy (Enel), United Kingdom (Wellman), Canada (Pyrovac, Dynamotive), Finland (Fortum), and the Netherlands (BTG). Meanwhile, many pilot plant projects stopped eventually after the initial testing.
For instance, the plants of Union Fenosa, Enel, Wellman and Pyrovac’s large-scale installation in Jonquiere, Canada, are no longer in operation. This may be because of a lack of confidence in economic prospects and markets at the time, or by legislative limitations [9]. Recent technological improvements to address logistical and technological challenges are making it more likely that fast pyrolysis will become a commercially viable technology. Oil companies and food/feed industries are building biofuel departments and are looking for existing knowledge matching their strategies and targets regarding renewable resources. A number of industrial-sized, fast pyrolysis oil plants have been constructed and are listed in Table 12.4. In 1998 Ensyn adapted the 45t/day unit to upgrade heavy oil. From 2005 onward, focus was again on renewable fuels, and a 75 t/day unit was commissioned in Renfrew Ontario in 2007 [50]. In 2012 KiOR began operating a 500 t/day facility in Columbus, processing woody wastes, in particular, pine waste [52,53]. During the KiOR process, wood chips are first seized and dried, then fed into a reactor, where they interact with a regenerated catalyst in a process similar to “cracking” at the traditional refinery. The vapors are separated from the solid catalyst and char in the separator by a series of cyclones. It is cooled yielding the crude that condenses into liquid TABLE 12.4 Fast Pyrolysis Oil Plants Pyrolysis Oil Plant
Technology
Capacity (kg feed/h)
Ensyn (Canada)
Fluid bed/riser
4500
Kior (United States)
Catalytic fast pyrolysis
21,000
BTG (Netherlands)
Rotating cone
5000
Fortum Valmet (Finland)
Fluid bed/riser
10,000
Gentling (Malaysia)
Rotating cone
2000
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and water. The liquid undergoes hydrotreating to produce gasoline and diesel fuel. In this unit, hydrogen is used to remove residual oxygen and render a hydrocarbon liquid product that can be fractionated into naphtha, kerosene, and distillate ranges by distillation. However, in 2014, KiOR shut down its Columbus, Ohio production facility since it was having difficulty with its production. Fortum commercialized the fast pyrolysis technology by building a pyrolysis-oil plant connected to the Joensuu power plant in Finland [54–56]. The integrated bio-oil plant, based on fast pyrolysis technology, is one of the first in the world on an industrial scale. The pyrolysis plant commenced in 2013. It is integrated with the combined heat and power production plant in Joensuu, which produces electricity and district heat and 50,000 t of bio-oil per year. The plant is located in Joensuu (Finland) with a capacity to produce 50 kt/year pyrolysis liquids. The feeding raw materials include forest residues and other wood-based biomasses. The product is used as a substitute for heavy fuel oil, and as raw material in the chemical industry or for biodiesel production in the future. BTG Biomass Technology Group, one of the pioneers in pyrolysis, started the fast pyrolysis developments at the beginning of the 1990s with a new reactor concept in which no inert gases were required to enable rapid mixing of biomass and hot bed material. This concept was the result of research done at the University of Twente where rapid mixing was achieved by mechanical mixing inside a rotating cone reactor [57]. Over the years BTG’s modified rotating cone technology has resulted in several patents. BTG has successfully tested over 45 different kinds of feedstock over the years, ranging from relatively simple feedstocks, like wood, to difficult feedstocks, like sludge or empty fruit bunch. In 2005 BTG Biomass Technology Group (The Netherlands) started the demonstration of the technology on empty fruit bunch in Malaysia on a scale of 2 t/h. Prior to feeding it to the
pyrolysis plant the empty fruit bunches is further sized and dried in a drier, where the moisture content is reduced down to about 5%–10%. Recently Fraunhofer Umsicht propose an in situ process that used the char produced by the pyrolysis collected in a reformer tube working at a temperature ranging from 550°C to 750°C as catalytic process for oil upgrading and char stabilization. The products depend on the biomass composition. Nevertheless, the main results are the increase of hydrogen concentration in the gas phase, an oil more easily separable from the water fraction and a more stabilized char. TCR process have shown to be effective in the pyrolysis of dried and pelletized biomass residues such as sludge, digestate, and other waste biomasses. Agri-Therm Inc., a spin-off company from Western University Institute for Chemicals and Fuels from Alternative Resources, has developed the first mobile pyrolysis system (MPS) for rapidly converting low value bio-residue at the source into higher-value bio-oil and bio-char for use in meeting renewable fuel content requirements and making special chemicals, pharmaceuticals, and materials. Mobile pyrolysis takes the pyrolysis plant directly into the agricultural and forestry operation, reducing transportation costs and converting low value stalks, leaves, straws, bagasse, chips, sawdust and branches, into higher value and high-energy density biooil and bio-char. The MPS100, built by Agri-Therm, was the first demonstration unit and it was designed to be an efficient, 10 dry tons per day. More importantly, the MPS generates its own self-sustaining thermal power (by combustion of the pyrolysis noncondensable gas in the central furnace), mitigating the need for access to the power grid and/or fuel depots. The second-generation MPS unit (MPS200) has been partially constructed and is currently undergoing cold and hot testing from 2013. The MPS200 has been professionally designed and engineered. The unit has design improvements that will increase reliability and
2. BIO-BASED PROCESSES AND BEYOND
3 BIO-OIL PRODUCTION BY HYDROTHERMAL LIQUEFACTION
output and will put Agri-Therm one step closer to a commercially proven mobile pyrolysis solution [16]. A further list of other global companies and research groups, with descriptions of their respective technologies for fast pyrolysis and upgrading, can be found in “Review of Fast Pyrolysis of Biomass and Product Upgrading.” [58]. Pyrolysis definitely proved to be a promising technology because its greatest strength is the flexibility of the technology to use a range of biomass feedstocks to produce renewable biofuels. Ongoing efforts should be focused on improving efficiencies, increasing product yields, and co-product utilization and valorization. That is why the challenges are related to improving: the operational reliability of demonstrationscale pyrolysis processes, the feedstock edibility, and the process integration and its control. In the meantime, the main focus should be on improvement of the quality and stability of the oil. In addition, the components of the oil, causing its specific characteristics (pH, aging, viscosity, etc.) have not been fully identified nor are the reactions taking place understood. One particular issue is the exact role of the various oxygen functionalities in the oil. It is important to establish which functionalities are desired and
243
which ones are undesirable, and to understand how to steer the pyrolysis process itself in this respect; by catalysis, for instance.
3 BIO-OIL PRODUCTION BY HYDROTHERMAL LIQUEFACTION 3.1 Fundamentals of HTL HTL involves the thermochemical conversion of a broad range of biomass types in the presence of hot compressed water at subcritical conditions into a liquid product known as bio-oil [59]. HTL requires an operating temperature of 300–350°C at 5–20 MPa for 5–60 min, wherein water is in the liquid form [60]. Near and supercritical water have been successfully used for carbonization, gasification, liquefaction, and upgrading of hydrocarbon resources including crude oils, microalgae, lignocellulosic biomasses, and wastes. Fig. 12.3 shows temperature/pressure ranges for hydrothermal water processes over the phase diagram of water. Water’s triple point and critical point are shown in the phase diagram; the hydrothermal water processes are typically performed in a narrow window just greater than the vapor-liquid co-existence curve
FIG. 12.3 Phase diagram of water superimposed with different HTW technologies. c-SCWG, catalytic supercritical water gasification; HTC, hydrothermal carbonization; HTL, hydrothermal liquefaction; nc-SCWG, noncatalytic supercritical water gasification; and SCWU, supercritical water upgrading. Reproduced with the permission from M.T. Timko, A.F. Ghoniem, W.H. Green. Upgrading and desulfurization of heavy oils by supercritical water, J. Supercrit. Fluids (2015).
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or at pressures slightly in excess of water’s critical pressure (221 bar). Increasingly reactive conditions are encountered as temperature is increased while holding pressures greater than water vapor pressure. On the time scale of hours and in the 150–200°C range, hydrothermal carbonization (HTC) converts water-soluble and water insoluble carbon fractions into insoluble carbonaceous solids (biochar), while HTL converts feedstocks into the energy-rich bio-oil. Supercritical upgrading converts heavy oils into lighter oils on the time scale of minutes and at temperatures ranging from 300°C to 500°C. Under these conditions, reaction rates are appropriate for a large-volume product (upgraded crude oil), while minimizing gas and solid byproducts. Accordingly, at the higher temperature, conditions used for SCW-gasification (SCWG), hydrocarbons break down into a mixture of H2, CO, and methane [61]. Subcritical water (<150–374°C, 0.4–22.1 MPa) and supercritical water (>374°C, >22.1 MPa) are nontoxic and environmentally friendly media with good mass transfer and heat transfer characteristics. The properties of water, such as the density and dielectric constant, can be continuously controlled between gas-like and liquid-like values by varying the temperature and pressure. For example, at a pressure of 25 MPa, the dielectric constant decreases from approximately 78 at 25°C to 27 at 250°C, and to 2 at 400°C [62]. This decrease in dielectric constant increases the solubility of small organic compounds. Thus, the polarity of water and hence its ability to dissolve various solids, liquids, and gases that are otherwise insoluble or sparingly soluble, can be significantly enhanced by transforming ordinary water into supercritical water. In addition, water under subcritical conditions can act as an acid or base catalyst, whereas supercritical water offers the unique possibility of shifting the dominant reaction mechanisms from free radical to ionic through modification of the water density [63]. Therefore, the role of water in hydrothermal processing cannot be underestimated.
The HTL process begins with solvolysis of biomass in micellar forms, the disintegration of biomass fractions (cellulose, hemicellulose, and lignin), and thermal depolymerization into smaller fragments. HTL, which mimics the processing of fossil fuels buried deep inside the earth, occurs in minutes or hours. HTL produces oil with low oxygen content as opposed to other processes like fast pyrolysis. HTL proves to be very energy efficient as it entails temperatures lower than those reached during pyrolysis. Conditions such as temperature, pressure, and reaction times influence the conversion of biomass into bio-oil [64]. Hydrothermal processing was initially developed for turning coal into liquid fuels, but recently, the technique has been applied to a number of feedstocks, including woody biomass, agricultural residues, and organic wastes (e.g., animal wastes, municipal solid wastes [MSW], and sewage sludge). The biomass type also affects the nature of the bio-oil (Table 12.5). Loosely packed biomass liquefaction results in bio-oil with high oxygen and moisture content that is undesirable as it lowers the quality and HHV of the fuel efficiency. Table 12.5 summarizes representative data of hydrothermal liquefaction of common types of biomass. The major components of waste biomass can be roughly classified into proteins, lipids, carbohydrates, lignin, and ash. Unlike biodiesel production, which only extracts lipids from algae, HTL could utilize the whole algae biomass including carbohydrates and proteins, which consequently allows the use of fast-growing species with low lipid content hence avoids the demand for promoting lipid accumulation [74]. The envisioned pathway is for the macromolecules of biomass (lipids, proteins, and carbohydrates) to be hydrolyzed into small fragments which subsequently can be converted to smaller compounds, for instance the amino acids transform into hydrocarbons, amines, aldehydes, and acids through decarboxylation and deamination, etc. In hot compressed water,
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3 BIO-OIL PRODUCTION BY HYDROTHERMAL LIQUEFACTION
TABLE 12.5
Overview of Literature on HTL of Common Types of Biomass Reactor Type
T (°C)
Time (min)
Oil Yield (%)
HHV (MJ/kg)
Ref.
Pulp/paper sludge powder
A bomb reactor system
250–280
15–120
20–45
35
[65]
Blackcurrant pomace (Ribes nigrum L.)
A 0.6 L stainless steel stirred batch reactor
583
240
30
35.9
[66]
Pinewood
A Parr reactor of 300 mL capacity
250
120
28
–
[67]
Digested sludge
A Parr (4500) 2-L reactor
300
30
9.4
32
[68]
Dairy manure
A bench top 300 mL stainless steel reaction pressure vessel
350
15
19
32.16
[69]
Brown type onion (Allium cepa L.)
A batch reactor
100–300
5
Negligible (0.05)
A self-designed batch reactor system
325
45–60 min
21.10
Nannochloropsis sp.
A 100 mL batch reactor (4593)
260
60
Algae Dianchi Lake in China
A 100 mL batch reactor (4593)
300
60
Biomass Type Second generation biomass
[70]
Third generation biomass Cyanobacteria sp. Bacillariophyta sp.
triacylglycerides are readily hydrolyzed to glycerol and fatty acid [75]. During the HTL process, the conversion of glycerol is inclined to produce water-soluble compounds such as methanol, acetaldehyde, propionaldehyde, acrolein, allyl alcohol, ethanol, and formaldehyde, with some gas products, mainly CO, CO2, and H2 [76]. The lipid content is proved to contribute directly to the HTL bio-oil yield, and high-lipid algae can produce higher bio-oil yields in comparison to low-lipid algae. The carbohydrate content of the biomass varies depending on the biomass type. The most abundant carbohydrates in aquatic biomass are cellulose, hemicelluloses, and starch. In addition, depending on the type of species, hemicellulose, as well as various other heteropolysaccharides, may also be present [76]. Under hydrothermal conditions, carbohydrates undergo rapid hydrolysis to form glucose and other monosaccharides which are then further degraded.
33.87–36.51
[71]
55
37
[72]
18.4
35.5
[73]
18.21
The main liquefaction mechanism is believed to be the carbohydrates breaking down to polar water-soluble organics, such as organic acids (i.e., formic, acetic, lactic), aldehydes, benzenes and alcohols, all carrying a substantial amount of oxygen [65,75,77]. The aldehyde- and benzenetype structures may further produce larger hydrocarbons, which are then part of the biocrude fraction [77]. Higher protein content is a primary difference between algae biomass and terrestrial biomass [78], which causes the high content of nitrogen in the resulting bio-oil produced from HTL. Proteins are polymers of amino acids linked by peptide bonds. They consist of a carbon-nitrogen bond between the carboxyl and amine groups forming long chains, which can be hydrolyzed in subcritical waterforming amino acids [79]. The amino acids can be further converted to carbonic acids and amines by decarboxylation and to ammonia and organic
2. BIO-BASED PROCESSES AND BEYOND
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acids by deamination [80]. The products from these reactions may then repolymerize and react with the sugars present from other components of the feedstock through Maillard-type reactions to produce long-chain hydrocarbons and aromatic ring-type structures like phenols, or nitrogen heterocyclics such as indole or pyrrole, which are commonly found in the bio-crude derived from HTL [76]. Some species, such as Spirulina algae, containing as high as about 65–70wt.% protein have been reported and the protein content in the aquatic biomass is highly dependent on the cultivation conditions [68,81,82]. Sorted by contributions and tendencies to contribute to the oil yield in HTL, the components of biomass are in the following order: lipid > protein > carbohydrate [83]. Although the lipid fraction is typically the main target of oil production, it was reported that 0%–15% of the produced bio-oil is converted from carbohydrates and proteins. This indicates that hydrothermal liquefaction is a promising process because of its ability to convert the protein and carbohydrate fractions to bio-oil. Biller and Ross [83] studied HTL of different model components representative of the lipid, protein, and carbohydrate fraction of microalgae, and even created an equation to estimate the biocrude oil yield as an relation to the mass fraction of algae fraction multiplied by their respective individual yields, obtained from the liquefaction of model compounds. However, the equation fit well only for the strains tested in their study (Nannochloropsis occulata and Chlorella vulgaris), and big deviations can be found for other strains under the same reaction conditions. Accordingly, each microalgae fraction does not behave independently during the HTL process and its contribution to the overall yield depends on an interaction among components of algae; for example, Maillard reactions between carbohydrates and proteins, or condensations between degradation products from lipids and proteins. Generally, the HTL products consist of four phases: a gas-phase of mainly CO2, a top-phase
of bio-oil, an aqueous byproduct, and a bottom phase with mainly solid residue. Depending on the physical properties (e.g., density and hydrophobicity), the oil is either just gravimetrically separated from the aqueous phase or extracted with an organic solvent [84]. The complex chemical reaction pathways of biological molecules in hydrothermal processes include depolymerization, fragmentation, dehydration, and decarboxylation. A better understanding of the reaction pathways and kinetics operative during HTL could provide opportunities for innovation that improve the process. The literature review [85] provides a broad information about the reactions of the most abundant types of biomolecules (e.g., proteins, lipids, and carbohydrates) in hydrothermal water conditions. This review collects information pertinent to the behavior of microalgae biomolecules (e.g., proteins, polysaccharides, lipids, chlorophyll) and their hydrothermal decomposition products (e.g., amino acids, sugars, fatty acids) in high temperature water. Representation of reactions of different biomacromolecules in microalgae, depicting products, which can be formed during hydrothermal liquefaction, is presented in Fig. 12.4.
3.2 HTL Oil Properties Bio-crude oil is much like a petroleumderived product but with a higher viscosity and oxygen content, typically 10–20 wt.% compared with 1% in conventional petroleum [86]. These oils could be upgraded catalytically to yield an organic distillate product which is rich in hydrocarbons and useful chemicals. Compared with bio-oil obtained from the fast pyrolysis method, their yield from the HTL process is much lower and it features higher heating value. Bio-oils from HTL have viscosities varying from liquid to tar-like, and compose a very complex matrix containing small molecules of diverse polarity and boiling points, as well as large oligomeric components [84].
2. BIO-BASED PROCESSES AND BEYOND
O
Hydrolysis
R
O HO
R
R
OH R Fatty acids
H2N
Amino acids
Protein
R
NH2 Amine
R
O R
R
N NH
NH
O
O
N
R
Pyrazine
R
OH Alcohols
OH Organic acid
R N
N
O
O
Pyrrolidinedione
Maillard reaction
R
H N
H N
H N
R¢
Pyrrolidine
N Indole derivatives
Indole
OH
OH O
O HN
R¢
N
H H Ammonia
HO
Amide
OR¢
R
R
H
OH
R¢
Esters
O
OH R
R
O C O Carbon dioxide
Deamination
Hydrolysis
C C N C C N R
Decarboxylation
O
N R¢
O
Lipid
O H H O H H
Pyrrolidine derivatives of fatty acid
O
OH + OH Glycerol
N
Alkenes
Decarboxylation
O O O
+
Alkanes
R O
O
R
Decarboxylation
O
OH O
OH HO
O OH OH
Carbohydrate
OH OH O Hydrolysis
OH
HO H
+ HO
OH OH Reducing sugar
R¢ N
R¢
OH O
R
OH
Pyrrolidinone
N
O N
Sugar Degradation
R
O
R¢
Cyclic oxygenates
R
Quinoline
Pyridinol
O HN
NH
O
R
R
OH
Phenol derivatives Temperature (°C)
0
100
200
250
300
320
FIG. 12.4 General reaction network of HTL of low-lipid microalgae. Reproduced with the permission from C. Gai, Y. Zhang, W.T. Chen, P. Zhang, Y. Dong. An investigation of reaction pathways of hydrothermal liquefaction using Chlorella pyrenoidosa and Spirulina platensis. Energy Convers. Manag. (2015).
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12. PRODUCTION AND UPGRADING OF BIO-OIL VIA LIQUEFACTION AND PYROLYSIS
3.3 Catalytic HTL The use of catalysts is a challenging factor in biomass liquefaction as it can reduce the required reaction temperature, enhance reaction kinetics, and improve the yield of desired products. The use of catalysts in hydrothermal liquefaction processes is intended to improve process efficiency by reducing char and tar formation. Two types of catalysts, homogeneous and heterogeneous, are reported in the literature and are summarized here. The literature analysis shows that the alkali hydroxides, carbonates, bicarbonates, and alkali formates as homogeneous catalysts, were intensively studied in the catalytic liquefaction. Based on the previous study, the activity and selectivity of alkali solutions are placed in the following order: K2CO3 > KOH > Na2CO3 > NaOH [87]. Authors showed that alkali carbonates catalytically increased the conversion rate and the generation of bio-oil more than alkali hydroxides due to its secondary reaction with water, forming bicarbonates and hydroxides. Homogeneous catalysts offer some advantages: decreased solids production, increased bio-crude yield, and improved bio-crude properties. Specifically, alkaline catalysts are able to enhance the yield of heavy oils and decrease the formation of residues, while acid catalysts (e.g., sulfonic acids and sulfuric acid) promote the condensation of lignin materials, consequently increasing the amount of insoluble residue [88], even though they are capable of decreasing the temperature and time required for the liquefaction of lignocellulosic biomass by enhancing the hydrolysis of cellulosic components. Other catalysts were reported to be used in the liquefaction of lignocellulosic biomass, such as FeS, Rb2CO3, Ba(OH)2, and ZnCl2, CuO [89]. Although the use of homogeneous catalysts seems to be effective, the separation of these catalysts may become problematic and require additional input energy. The application of heterogeneous catalysts in HTL have also discussed to improve bio-crude quality [90,91].
Several metal oxide catalysts including MnO, MgO, NiO, ZnO, CeO2, La2O3, etc., were employed in supercritical hydrothermal liquefaction of empty fruit bunch derived from oil palm residues at 390°C and at reaction time of 1 h. Among the tested catalysts, four most active metal oxides with lower electronegativity such as CaO, MnO, La2O3 and CeO2 were selected, and gave maximum relative yield of bio-oil at about 1.40 times than the one obtained without the catalyst [92]. In one study [93], rice straw was treated under hydrothermal conditions at different temperatures (200–300°C) for the reaction time of 120 min with and without NiO nanocatalysts. The tested nanocatalyst increased the bio-oil yield up to 13.2% and 17.2% for light and heavy oils, respectively, at 300°C and the carbon recovery was significantly improved. Authors [94] showed that bio-oil yields were higher with the following the order of nano-Ni/SiO2 > zeolite > Na2CO3 in hydrothermal liquefaction of microalgae Nannochloropsis sp. The highest bio-oil yield (30.0%) was obtained at 250°C by using nano-Ni/SiO2. Moreover, applying a catalyst resulted in a decrease in the oxygen and the nitrogen contents of the bio-oil and consequently an increase in its heating value. Crude bio-oils were produced from the microalga Nannochloropsis sp. via reactions in liquid water at 350°C in the presence of six different heterogeneous catalysts (Pd/C, Pt/C, Ru/C, Ni/SiO2-Al2O3, CoMo/γ-Al2O3 (sulfided), and zeolite) under inert (helium) and highpressure-reducing (hydrogen) conditions [95]. In the absence of added H2, all of the catalysts tested produced higher yields of crude bio-oil from the liquefaction of Nannochloropsis sp., but the elemental compositions and heating values of the product (about 38MJ/kg) were largely insensitive to the catalyst used. The crude bio-oil yields range from a low of 35% from uncatalyzed liquefaction without H2 up to 57% from Pd/C catalyzed liquefaction without H2. In both the presence and absence of H2, the
2. BIO-BASED PROCESSES AND BEYOND
3 BIO-OIL PRODUCTION BY HYDROTHERMAL LIQUEFACTION
supported Ni catalyst produced a crude bio-oil with a sulfur content below the detection limits. Ru/C is also shown to increase the oil yield from 48.6% to 77.2% produced from kenaf biomass at 300°C [96]. Apart from catalysts, there have been some studies devoted to investigating the effect of co-solvents on the performance of the HTL process of waste biomass [97], since it has been reported that at the critical point of water the liquefaction process was operated under challenging conditions. The organic solvents such as ethanol, methanol, propanol, butanol, tetralin and ethyl acetate, etc., have been studied as alternative reaction solvents to replace water [98]. Solvent selection is very important to obtain high liquefaction oil yields and improve the oil’s properties [99]. The presence of solvent dilutes the concentration of the products, thus tending to minimize cross-linked reactions and reverse reactions. In the liquefaction experiments [100] carried out in a 1000 mL autoclave at different temperatures from 573 to 653 K for Spirulina microalgae, the maximum oil yield increased gradually with increasing the temperature. In the resulting oil, the nitriles (22.7%) were dominated when the 1,4-dioxane was used, whereas the esters were dominant in case of methanol (35.5%) and ethanol (26.3%). After HTL the nitrogen content in the bio-oil changed from the initial content with 8.9% to 7.5%, 7.9%, and 9.5% in the presence of methanol, ethanol and 1,4-dioxane, respectively. However, authors [101] performed the HTL for the same highprotein microalgae feedstock (Spirulina) in the presence of sub- and supercritical ethanol in the presence of FeS and Na2CO3 catalysts. The bio oil yield ranged from 35.4% to 45.3%, depending on reaction temperature and the solvent filling rate. There have been numerous studies [102] on the beneficial effect of methanol and ethanol on the bio oil yield. From a commercial point of view, the solvent should either be cheap and easily recoverable, be produced within the process, and/or be co-processed with the bio-oil to the end products [103].
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3.4 Hydrothermal Upgrading One implementation of liquefaction is the hydrothermal upgrading (HTU) process for the conversion of wet biomass at 573–623 K and 15–18 MPa into biocrude oil. The HTU process may at first glance seem quite similar to the catalytic liquefaction process; however, there are a number of differences. Mainly, the HTU consists of an initial hydrothermal treatment followed by catalytic treatment, converting the biocrude from the initial step into other hydrocarbons. The research and development of the HTU (Hydrothermal Upgrading) process started in 1982 in the Shell Research Laboratory in Amsterdam. In Ref. [104] six HTU units, wood was converted into 3600 t/day of biocrude containing 10% oxygen, then it was upgraded by catalytic hydrodeoxygenation in a central facility. Because bio-oil is produced in an aqueous environment, it may be beneficial from a process engineering point of view to carry out hydrotreating in the same environment. For instance, authors performed [94] hydrodenitrogenation of crude oil produced from HTL of algae in the presence of high-temperature water, a formic acid mixture and H + ZSM-5 nanocatalyst. The effect of the reaction temperature (250–500°C), formic acid as a hydrogen source and nanocatalyst on nitrogen elimination was investigated comprehensively. Maximum nitrogen removal (75%) was obtained in the presence of the nanocatalysts performed at 400°C. Similar results were seen in Ref. [63] catalytic hydrothermal upgrading of algal bio-oil at 400°C for 240 min with the addition of 6 MPa and 10% nine zeolite catalysts in supercritical water. The results showed that compared with noncatalytic upgrading reactions, all of the zeolites promoted denitrogenation, deoxygenation, and desulfurization of the pretreated bio-oil due to the presence of the acid sites. In one study [105], a duckweed biocrude produced from the hydrothermal liquefaction of Lemna minor was treated in subcritical water at
2. BIO-BASED PROCESSES AND BEYOND
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12. PRODUCTION AND UPGRADING OF BIO-OIL VIA LIQUEFACTION AND PYROLYSIS
350°C with added H2 in the presence of several different commercially available materials such as Ru/C, Pd/C, Pt/C, Pt/c-Al2O3, Pt/C-sulfide, Rh/c-Al2O3, activated carbon, MoS2, Mo2C, Co-Mo/c-Al2O3, and zeolites. The effect of the catalysts on the yields, product distributions, the deoxygenation, denitrogenation, and desulfurization of biocrude were examined. All the materials showed catalytic activity for deoxygenation and desulfurization of the biocrude but only Ru/C showed activity for denitrogenation. Among the examined catalysts, Pt/C showed the best performance for deoxygenation. The oil produced with Ru/C shows the lowest sulfur, the highest hydrocarbon content (25.6%), the highest energy recovery (85.5%), and the highest higher heating value (42.6 MJ/kg). The study [106] was focused on the impact of low temperature HTL (LT-HTL), followed by a higher temperature HTL on the minimization of heteroatoms which produce a refinery quality bio-oil. Authors performed consecutive low(225°C for 15 min) and high- (350°C for 60 min) temperature subcritical liquefaction and subsequent hydrodeoxygenation in the presence of Ru/C and CoMo-S catalysts for Chlorella sorokiniana, Chlorella minutissima, and Scenedesmus bijuga algae feedstocks. This pretreatment step reduced nitrogen heteroatom and generated a biocrude with free fatty acids and unsaturated hydrocarbons. After HDO using Ru/C (5%) the final yields of the highest quality oil (the lowest nitrogen level with 3.2%–3.9%) ranged from 15% to 22%, indicating the two-stage strategy might be promising for HTL of a high-protein algae biomass due to perceivable fulfillment in heteroatom reduction. Correspondingly, the results of authors [107] designated a two-step treatment approach (noncatalytic treatment followed by catalytic upgrading) with different heterogeneous catalysts showing captivating approaches for the hydrothermal catalytic upgrading of algal biocrude. Among different catalysts tested, Raney-Ni
(1.6%) and HZSM-5 (1.8%) were the most suitable ones for denitrogenation (1.6% and 1.8% N in resulting bio oil), however using HZSM-5 produced the upgraded oil in the lowest yield (53.1%). Because Ru/C and Raney-Ni catalysts showed the best performance for deoxygenation and denitrogenation, authors performed one more experiment with a combination of these two catalysts and obtained the highest upgraded oil yield—77.2% with the 2% nitrogen content. Similarly, authors [108] reported the activities of several different couple of catalysts for hydrothermal hydrodeoxygenation and hydrodenitrogenation of pretreated algal oil at 400°C for 4 h in supercritical water. The yield in upgraded oil ranging from a low of 63.2 wt.% in the Ru/C + Rh/γ-Al2O3-catalyzed reaction, to a high of 77.2 wt.% in the Ru/C + Mo2Ccatalyzed reaction. All the couple of catalysts exhibited excellent performance with respect to deoxygenation, denitrogenation, and, in particular, desulfurization. Ru/C + Mo2C, Ru/C + Pt/γ-Al2O3, and Ru/C + Pt/C performed best with respect to deoxygenation, denitrogenation, and desulfurization and produced upgraded oils with O, N, and S contents of 0.1, 1.8, and 0.065 wt.%, respectively. The upgraded oil produced with Ru/C + Mo2C retained 89.7% of the heating value of the original pretreated oil and contained 98.5% of the components boiling below 450°C. According to the results obtained, authors selected two component catalyst mixtures as a promising strategy for upgrading bio-oil derived from in supercritical water.
3.5 HTL Technologies As early as the 1920s, first experimental data from Berl supported the concept of oil production from plant biomass [109]. Pioneering liquefaction work was done by Appell and coworkers at the Pittsburgh Energy Research Center (PERC) in the 1970s, which was later demonstrated in a pilot plant in Albany, Oregon.
2. BIO-BASED PROCESSES AND BEYOND
4 COMPARATIVE STUDIES ON HTL AND PYROLYSIS PROCESSES
The PERC process consisted of converting dried-wood in an anthracene oil at 300–370°C in the presence of a catalyst Na2CO3 and reducing gas (CO/H2, operating pressure 20 MPa). Anthracene oil was progressively replaced by recycling the oil produced. However, serious technical problems, caused by undissolved solids and an increase of medium viscosity, prevented the pilot unit from operating after 1981. However, while in operation, the unit produced 5000 kg of oil on a continuous basis [110]. After reconsidering the problems of the PERC process, the Lawrence Berkeley Laboratory introduced a pretreatment of wood prior to its liquefaction, in a smaller unit with a reactor volume of 1 L. The pretreatment was a mild acid hydrolysis (pH ¼ 10–8) at 180°C for 45 min and assisted to weaken the lignocellulosic material. A water/wood slurry was then prepared, and pH was adjusted to 8 using Na2CO3. This step— wood liquefaction in an aqueous medium— lasted 10–60 min. These two trials, undertaken to develop a single-step liquefaction process, showed the need for pretreatment [111]. The CatLiq biomass conversion process has been developed over the past 10 years and has been developed in scales from 1 to 100 kg/h. Both anaerobic digested sewage sludge and corn silage has been demonstrated as working feedstocks, resulting in bio-oil as the main product in regards to energy, yield of which is highly tunable by process parameters [112]. Other direct liquefaction of organic substances, developed by Hoch-schule f€ ur Angewandte Wissenschaften Hamburg, Germany (HAW), EPA’s Water Engineering Research Laboratory, Cincinnati, Ohio, United States, developed a prototype sludge-to-oil reactor system (STORS), the thermal depolymerization technology, designed by Changing World Technologies, Inc. (CWT), are described in detail in the review [74]. Although the interest in HTL-based processes remains a key driver for the production of fuels
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and chemicals toward an HTL bio-refinery concept, there have been many challenges facing the commercialization of these technologies, including expensive and complex reactors that require high-capacity water handling equipment. Overall poor understanding of mass balance make it difficult to accurately measure product yields during the hydrothermal run [60]. Compared with flash pyrolysis, HTL is at a nearly developmental stage, and challenges for the coming years are: improvement of oil production rate and energy efficiency of the process, understanding the reaction mechanisms and kinetics, and development of pilot-scale plant. It still remains an interesting topic to investigate the interaction of operation parameters on the oil yield and quality during biomass conversion and optimize the process conditions as well as to learn more about the mechanisms involved in the HTL process in order to achieve a desirable oil yield and quality. Because there is a wide range of feedstock that could be used, a systemic approach in determining the effects of various components to oil yield and quality is needed. For the upscale, a detailed economic feasibility study is needed in order to assess the profitability of the process.
4 COMPARATIVE STUDIES ON HTL AND PYROLYSIS PROCESSES The bio-oil produced from the both direct liquefaction and pyrolysis is a complex mixture containing significant quantities of nitrogen and oxygen with high TAN (total acid number). Thus, further treatment, such as denitrogenation and deoxygenation, is required to meet the transportation fuel. To date, only a few studies have reported a parallel comparative evaluation of the yields and fuel properties of bio-oil obtained by pyrolysis and liquefaction. For instance, authors [113] provide a comparative assessment of bio-oil yield and qualities from a
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microalgal feedstock using slow pyrolysis and HTL. Based on the results obtained, HTL resulted in higher bio-oil yields (41%) compared with pyrolysis (23%–29%). Moreover, bio-oil produced from HTL was found to have higher energy density and superior fuel properties, such as thermal and storage stabilities, compared with pyrolysis bio-oil. Another study [114] comparatively evaluated the bio-oil yield and quality from a microalgal feedstock using HTL, alcoholysis, pyrolysis, and hydropyrolysis. It was shown that HTL resulted in a bio-oil with higher energy density and superior fuel properties, such as thermal and storage stabilities, compared with three other conversion routes. In Ref. [6] HTL and pyrolysis of Chlamydomonas reinhardtii was compared. The bio-oils were produced with similar HHV and elemental composition; however, GC-MS analysis revealed different composition. Pyrolysis bio-oils were mainly composed of proteins derived compounds whereas HTL bio-oils were composed of proteins, lipids and carbohydrates derived compounds. Energy ratios (energy produced in the form of bio-oil divided by the energy content of the initial microalgae) between 66% at 220°C and 90% at 310°C in HTL were obtained, whereas it was in the range 73%–83% at 400–550°C for pyrolysis. The higher heating value of the HTL bio-oil was increasing with the temperature while it remained constant for pyrolysis. Based on the results obtained from the comparative evaluations on the yields and properties of the bio-oils, it should be concluded that HTL oil has a higher yield and higher energy density. However, it is also worth to mentioning that all comparatives studies considered employed third-generation biomass, such as algae feedstock that preferably produces the higher oil yield by HTL than pyrolysis, while the further research is required on the studies on the oil production from the second-generation biomass such as wood wastes, organic food residues, etc.
5 CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK Bio-oil produced from thermochemical conversion technologies, such as pyrolysis and HTL, is a very challenging mixture of biochemicals and water. Finding pathways to maximize its commercial value has been, and continues to be, a priority. The nature and yield of bio-oil from hydrothermal technologies depends on several factors, such as catalyst, feedstock type, the nature of the solvent, and process conditions employed in the processes. Thus choice and selection of the thermochemical process for a particular biomass type is important in view of its major influence on the yield and desired properties of the final product. The major differences between HTL and pyrolysis are as follows: – HTL occurs at 5–20 MPa pressure and low temperature (250–350°C), whereas pyrolysis proceeds at near atmospheric pressure (0.1–0.5 MPa) and 400–600°C temperature. – HTL can convert high moisture biomass without water evaporation and using the excess water in biomass as a highly reactant medium. This eliminates the need for drying, whereas pyrolysis requires complete drying. – HTL results in a higher yield of bio-oil whereas pyrolysis, in most cases (except fast pyrolysis), leads to the production of higher quantities of solid char than other products. Moreover, HTL has not yet been widely commercialized as the pyrolysis process due to a number of technological gaps as well as the lack of a deep knowledge about the HTL chemical pathway. For both thermochemical processes, along with technological constraints, there are economic bottlenecks because technologies uses high pressure or temperature equipment; the processes have high-capital investments. Considerable challenges remain in the area of catalyst recycling and regeneration in order to improve the lifetime and efficiency of the
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hydrothermal processes. In view of this, hydrothermal technologies, both HTL and pyrolysis, hold significant promise, but research and development are still needed to the commercial deployment.
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microalgae in sub- and supercritical ethanol, Fuel Process. Technol. 92 (1) (2011) 147–153. C. Capello, U. Fischer, K. Hungerb€ uhler, What is a green solvent? A comprehensive framework for the environmental assessment of solvents, Green Chem. 9 (9) (2007) 927–934. F. Lemoine, I. Maupin, L. Lemee, J.M. Lavoie, J. L. Lemberton, Y. Pouilloux, et al., Alternative fuel production by catalytic hydroliquefaction of solid municipal wastes, primary sludges and microalgae, Bioresour. Technol. 142 (2013) 1–8. F. Goudriaan, D.G.R. Peferoen, Liquid fuels from biomass via a hydrothermal process, Chem. Eng. Sci. 45 (9) (1990) 2729–2734. C. Zhang, P. Duan, Y. Xu, B. Wang, F. Wang, L. Zhang, Catalytic upgrading of duckweed biocrude in subcritical water, Bioresour. Technol. 166 (2014) 37–44. W. Costanzo, R. Hilten, U. Jena, K.C. Das, J.R. Kastner, Effect of low temperature hydrothermal liquefaction on catalytic hydrodenitrogenation of algae biocrude and model macromolecules, Algal Res. 13 (2016) 53–68. X. Bai, P. Duan, Y. Xu, A. Zhang, P.E. Savage, Hydrothermal catalytic processing of pretreated algal oil: a catalyst screening study, Fuel 120 (2014) 141–149. Y. Xu, P. Duan, B. Wang, Catalytic upgrading of pretreated algal oil with a two-component catalyst mixture in supercritical water, Algal Res. 9 (2015) 186–193. E. Berl, Science oil production from plants, Science 99 (2573) (1944) 309–312. D.C. Elliott, Historical developments in hydroprocessing bio-oils, Energy Fuel 21 (3) (2007) 1792–1815. J.M. Bouvier, M. Gelus, S. Maugendre, Wood liquefaction an overview, Appl. Energy 30 (2) (1988) 85–98. R.P. Nielsen, G. Olofsson, E.G. Søgaard, CatLiq - high pressure and temperature catalytic conversion of biomass: the CatLiq technology in relation to other thermochemical conversion technologies, Biomass Bioenergy 39 (2012) 399–402. U. Jena, K.C. Das, J.R. Kastner, Effect of operating conditions of thermochemical liquefaction on biocrude production from Spirulina platensis, Bioresour. Technol. 102 (10) (2011) 6221–6229. P. Duan, B. Wang, Xu Y. catalytic hydrothermal upgrading of crude bio-oils produced from different thermo-chemical conversion routes of microalgae, Bioresour. Technol. 186 (2015) 58–66.
Further Reading [115] C. Gai, Y. Zhang, W.T. Chen, P. Zhang, Y. Dong, An investigation of reaction pathways of hydrothermal liquefaction using Chlorella pyrenoidosa and Spirulina platensis, Energy Convers. Manag. 96 (2015) 330–339.
2. BIO-BASED PROCESSES AND BEYOND
12 Development of Upgraded Bio-Oil Via Liquefaction and Pyrolysis Aisha Matayeva*,†, Francesco Basile*, Fabrizio Cavani*, Daniele Bianchi‡, Stefano Chiaberge‡ *Department of Industrial Chemistry “Toso Montanari”, University of Bologna, Bologna, Italy †Institut f€ ur Technische und Makromolekulare Chemie, RWTH Aachen University, Aachen, Germany ‡ Renewable Energy & Environmental R&D - Istituto Eni Donegani Eni spa, Novara, Italy
O U T L I N E 1 Current Processes for Conversion of Biomass into Bio-Oil 2 From Biomass to Bio-Oil Via Pyrolysis 2.1 Fundamentals of Pyrolysis 2.2 Pyrolysis Oil Properties 2.3 Catalytic Pyrolysis 2.4 Upgrading of Pyrolysis Oils 2.5 Pyrolysis Technologies 3 Bio-Oil Production by Hydrothermal Liquefaction 3.1 Fundamentals of HTL
Studies in Surface Science and Catalysis, Volume 178 https://doi.org/10.1016/B978-0-444-64127-4.00012-4
232 233 233 235 237 240 241 243 243
3.2 HTL Oil Properties 3.3 Catalytic HTL 3.4 Hydrothermal Upgrading 3.5 HTL Technologies 4 Comparative Studies on HTL and Pyrolysis Processes 5 Conclusions and Recommendations for Future Work References Further Reading
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246 248 249 250 251 252 253 256
# 2019 Elsevier B.V. All rights reserved.
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12. PRODUCTION AND UPGRADING OF BIO-OIL VIA LIQUEFACTION AND PYROLYSIS
1 CURRENT PROCESSES FOR CONVERSION OF BIOMASS INTO BIO-OIL Environmental concerns and possible future shortages have boosted research into alternatives for fossil-derived products [1]. Biomass is one potential source of renewable energy and the conversion of plant material into a suitable form of energy, usually as electricity or fuel for an internal combustion engine, can be achieved using a number of different routes, each with specific pros and cons [2]. Depending on the biomass used in the biorefinery process, first-, second-, and thirdgeneration biofuels can be distinguished. Three main types of first-generation biofuels used commercially are biodiesel, bioethanol, and biogas, whose production process is considered an “established technology.” First-generation biofuels obtained through a biorefinery process of edible biomasses such as crop, sugar cane, and vegetable oils, are in competition with the food industry [3]. This creates several economical and ethical issues related to the food and
FIG. 12.1
agricultural land and water they need to grow. Development of the so-called second-generation biofuels are aimed to overcome the above issues. The feedstock is generally lignocellulosic-based biomass (wood waste, agricultural and forestry residues, sorted domestic organic wastes, etc.). Fuels from aquatic organisms (micro- and macroalgae, etc.) are often considered as thirdgeneration biofuels, since they avoid the competition with both food and soil use. These organisms have gained a growing interest in recent years as some of them contain a higher concentration of lipids, which are desirable components in biofuel production. The roadmap for biomass conversion into platform chemicals, which is able to substitute petroleum-derived equivalents, is complex and ranges from biological to severe catalytic thermochemical processes (Fig. 12.1). Bio-chemical processes, such as fermentation and anaerobic digestion, occur at lower temperatures in a diluted water phase, and most common types of biochemical processes are fermentation and anaerobic digestion. The fermentation uses microorganisms or enzymes to convert a
Biomass conversion processes.
2. BIO-BASED PROCESSES AND BEYOND
2 FROM BIOMASS TO BIO-OIL VIA PYROLYSIS
fermentable substrate into recoverable products, while an anaerobic digestion involves the bacterial breakdown of biodegradable organic material in the absence of oxygen over a temperature range from about 30°C to 65°C to produce biogas [4]. In comparison to the biochemical processes, thermochemical processes occur faster in the range of few seconds, minutes, or hours [5]. Biomass can be converted into bio-oil by two main thermochemical routes: pyrolysis and hydrothermal liquefaction (HTL). The characteristic and technique feasibility of the two thermo-chemical processes for bio-oil production are compared in Table 12.1. Pyrolysis is the thermochemical decomposition of dry organic matter (moisture content below 10% mass fraction) in the absence of oxygen at moderate temperatures (350–550°C) and atmospheric pressure [6], while HTL is performed in the presence of water at lower temperatures (200–350°C) and higher pressures (2–20 MPa), and can convert biomass with high moisture content (above 50% mass fraction) [7]. The resulting bio-oil product then can be upgraded TABLE 12.1 Comparison of Two Typical Thermochemical Processes for Bio-Oil Production Pyrolysis
HTL
Temperature (°C)
450–500
300–400
Pressure (MPa)
atmosphere
5–20
Pretreatment
Drying is necessary
Drying is not necessary
233
by conventional hydrotreatment in order to produce advanced biofuels. Both pyrolysis and HTL technologies are promising approaches to solve the environmental problems, since both processes do not only produce useful energy in the form of liquid fuel, but also contribute to the disposal of household, agriculture, industrial, and municipal residues/ wastes. In other words, the bio-oil production from second- and third-generation biomass (domestic organic waste, municipal waste, sewage sludge, etc.) by HTL and pyrolysis has been a sustainable “waste-to-fuel” approach serving multiple purposes: waste/residue management, energy production, reduction of greenhouse gas emissions, and economic benefits. Therefore, the fundamentals of both pyrolysis and HTL, including their latest developments, are of great importance in evaluating the applicability of these processes within the waste management sector. In this regard, the main goal of this chapter is to bring together an overview of ongoing efforts and advances as well as the environmental and economic aspects of both pyrolysis and HTL technologies. Additionally, the influence of catalysts during the pyrolysis and HTL on the product yields and composition is also being considered.
Treatment conditions
Bio-oil characteristics HHV (MJ/kg)1
35
22
C
70
55
H
8
6
O
12
34
N
10
5
Elemental analysis (%)
2 FROM BIOMASS TO BIO-OIL VIA PYROLYSIS 2.1 Fundamentals of Pyrolysis Many reviews on pyrolysis can be found in the literature [8,9]. These reviews cover pyrolysis process design, pyrolysis reactors as well as its commercialization. According to the literature reviewed, pyrolysis is known to be conventional, fast, or flash, depending on the operating conditions employed. Conventional pyrolysis may also be termed slow pyrolysis [10], defined as the thermal cracking of organic-based materials in the absence of oxygen at slow heating
2. BIO-BASED PROCESSES AND BEYOND
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rates (0.1–10°C/s), while fast pyrolysis is a high-temperature process (400–550°C) in which biomass is rapidly heated (10–200°C/s) and decomposed to form vapors, aerosols, gases, and char. Bio-oil is collected by rapid quenching and condensing the vapors and by coalescence of aerosols. Flash pyrolysis is a thermal-cracking process at a very high heating rate (>1000°C/s) with a very short vapor residence time to minimize the secondary cracking to keep the liquid yields high [11,12]. The reactor is the core and generally the most researched aspect of pyrolysis, though control and improvement of liquid quality are receiving increased attention. Reactors described in literature, used in the pyrolysis of different wastes, include fixed bed reactors, batch or semibatch reactors, rotary kilns, fluidized bed reactors, microwave-assisted reactors, and some innovative solutions like plasma or solar reactors [13–15]. As it can be seen from Table 12.2, the reactor design used in pyrolysis units depends on the type of process. In slow pyrolysis, the most common reactors are drum, rotatory kilns, and screw/auger. In fast pyrolysis systems, reactors can be fluidized bed, rotating cones, entrained flow, vacuum, and ablative [16]. Typically, a pyrolyzer consists of a reactor, cyclone, and condenser. The cyclone separates solid products from liquid and gases. Once the
TABLE 12.2
solid products are separated, the vapor products are rapidly quenched in the condenser and then bio-oil is separated from other gases. Noncondensable gases are usually recycled into the pyrolyzer for fluidization and heating. In general, fluidized-bed reactors are used to study the behavior of fast pyrolysis and to investigate the secondary cracking of oil at longer residence times [17]. Fluidized-bed reactors are characterized by a high heating rate and a good blending of the feedstock. In these reactors, the heat transfer medium is the bed of particles (sand, catalyst, etc.) which is circulated, using high flow rates of gas from the reactor vessel into a burner. In the burner, the particles are exposed to oxygen with the gas, or solid reaction co-products, which are burned to heat the particles and then circulated back into the reactor vessel. As a result, the solid residence time is approximately the same as the vapor residence time, and the reactor operates at high superficial gas velocities. Flash vacuum pyrolysis had its beginnings in the 1940s and 1950s, mainly through mass spectrometric detection of pyrolytically formed free radicals. Meanwhile, many different types of apparatus and techniques have been developed, and the most important methods as well as a survey of typical reactions and observations, are well described [18].
Characteristics of Various Types of Pyrolysis Conventional
Fast
Flash
- Heating rate (C/s)
0.1–10
10–200
<1000
- Particle size (mm)
5–50
<1
<0.2
- Vapor residence time (s)
450–550
0.5–10
<0.5
Reactor configurations
Fixed bed, vacuum reactors
Ablative, auger, fluidized bed, circulating fluidized bed reactors
Fluidized bed, circulating fluidized bed reactors, downer reactors
Operating conditions
2. BIO-BASED PROCESSES AND BEYOND
2 FROM BIOMASS TO BIO-OIL VIA PYROLYSIS
In the rotating cone reactor, biomass particles and heat carrier particles are introduced near the bottom of the cone where the solids are mixed and transported upward by the rotating action of the cone. In this pyrolysis reactor rapid heating and a short residence time of the solids can be realized [19]. Biomass materials like wood, rice husks, or even olive stones can be pulverized and fed into the rotating cone reactor. Due to the a number of reactor types being used for the pyrolysis of waste, Scott and others [20] analyzed several reactors for fast biomass pyrolysis and concluded the general broadly desirable criteria including: simplicity in design, ease of scale-up, ease of control and operation, improved thermal efficiency, and possibility of compact, small-scale designs suitable for rural environments. Currently, fluid beds, circulating fluid bed and transport reactors, and auger pyrolysis reactors are reported to have a strong technology basis and high-market attractiveness, and more detailed information on various fast pyrolysis reactors can be found in previous chapters [13]. Within the reactor types the feedstock composition plays a big role to produce higher yield of bio-oil. Commonly, the land biomass residues were most suitable feedstock to produce pyrolysis oil. Most work has been performed on wood because of its consistency and comparability between tests [10]. However, other types of biomass have been used. For instance, a number of studies on oil production from algae as a nonpolluting, renewable source of energy can be found in the literature. For instance, authors [21] have shown that liquid fuel in the form of a mixture of hydrocarbons could be produced from the microalgae Dunaliella. Similarly, another work [22] came to the same conclusion that the properties of bio-oil of microalgae were more suitable for fuel use than pyrolysis oils from lignocellulosic materials. Representative values of properties for pyrolysis oil products derived from different sources are collected and listed in Table 12.3.
235
2.2 Pyrolysis Oil Properties Pyrolysis oil is a liquid, typically dark redbrown to almost black [10] depending on the chemical composition of the biomass. It has a distinctive acid, smoky smell, and can irritate the eyes [3]. It is important to note that oil properties depend on feedstock and operating conditions but may change during storage due to a process indicated as “aging,” which is usually characterized by an increased viscosity within time and a phase separation of the oil in a watery phase and a viscous organic phase. Typically, the quality of pyrolysis oil from lignocellulose is too poor for a direct use as transportation fuel or even for direct upgrading to fuel precursors at existing oil refineries. It is very viscous, acidic (pH < 3), unstable and has a relatively lowenergy density (19 MJ/kg), compared with conventional fossil oil (30 MJ/kg) [32]. In general, the oil has a polar nature and does not mix readily with hydrocarbons due to the presence of large amounts of water and oxygenated components [9]. The presence of large amounts of oxygen in plant carbohydrate polymers means the pyrolytic chemistry differs sharply from these other fossil feeds. Condensed species in bio-oil are derived mainly from lignin, but also cellulose fragments. Bio-oil components’ molecular weights from several hundred or more can be obtained. They form as part of the aerosols. Free water in the biomass explosively vaporizes upon fast pyrolysis. It shreds the feed particles and aids heat transfer. Cellulose and hemicellulose also lose water, which contributes to the process. Chemically, bio-oil is a complex mixture of water, guaiacols, catecols, syringols, vanillins, furancarboxaldehydes, isoeugenol, pyrones, acetic acid, formic acid, and other carboxylic acids. It also contains other major groups of compounds, including hydroxyaldehydes, hydroxyketones, sugars, carboxylic acids, and phenolics. Phenolic compounds are often present in high
2. BIO-BASED PROCESSES AND BEYOND
236 TABLE 12.3
12. PRODUCTION AND UPGRADING OF BIO-OIL VIA LIQUEFACTION AND PYROLYSIS
Overview of Literature on Pyrolysis of Common Types of Biomass
Biomass Type
Reactor Type
T (°C)
Time (min)
A bench-scale fixed bed
600
–
Heating Rate (°C/min)
Oil Yield (wt.%)
HHV (MJ/kg)
Ref.
–
51.2
–
[23]
–
[24]
Second generation biomass Maize Stalk
45.5
Rice straw
50.2
Cotton straw
42.8
Corn cob
Fixed bed reactor
450
60
20
47.3
Wheat straw
36.7
Rice straw
28.4
Rice husk
38.1
Sugarcane bagasse
A Hastelloy-type horizontal cylindrical device of steel
380–780
20
120–127
47–38
—
[25]
Tea residue
CDS Pyroprobe 5200 pyrolyzer
450–700
0.3
20 C/ms
59
—
[26]
Waste cereals
Laboratory pyrolysis unit
800
10
46
27
[27]
62
14
36.5
–
[28]
24.29
38
[29]
Waste peanuts crisps Sewage sludge
A stainless-steel tube
500
20
Food waste
36.3
Wood
50.9
Jatropha (in form of seeds, cake, hulls)
A bench scale reactor
500°C
0.25
Algal waste
A stainless steel fixed-bed reactor
400–600
20
10
24.10 and 44.01
19.91
[30]
Chlorella protothecoides
A fluid bed reactor
500
0.05
600 C/s
18
29
[22]
23–31
[31]
Third generation biomass
Microcystis aeruginosa Laminaria digitata, Fucus serratus
24 A fluid bed reactor
500
concentrations (up to 50 wt.%), consisting of relatively small amounts of phenol, eugenol, cresols, and xylenols [17]. The bio-oils have a water contents of typically 20–25 wt.% [33], and
0.03
11–17
water cannot be removed by conventional methods like distillation. Upgrading via hydrodeoxygenation produces high-grade stable oil that can be burnt at
2. BIO-BASED PROCESSES AND BEYOND
2 FROM BIOMASS TO BIO-OIL VIA PYROLYSIS
a higher efficiency or refined into transportation fuels that are compatible with current infrastructure and vehicle technologies. Several works have been carried out on research and development of bio-oil for heat and power generation, including the design and study of specialized bio-oil [34]. Bio-oil can be used in the following applications: it can be combusted in industrial or residential boilers for heat and power generation, co-feed in natural gas-fired/coal-fired power plants, or converted into fuels (gasoline and diesel) and chemicals through hydrocracking or hydroprocessing. However, as discussed earlier, several bio-oil properties, including the high oxygen content, water content, oil instability and aging make the conversion into heat and power significantly more challenging and ultimately limit the range of its applications. Therefore, further research is needed to overcome remaining challenges in using these products because the environmental and economic benefits and success of pyrolysis technologies depend on addressing these challenges.
2.3 Catalytic Pyrolysis The complexity of bio-oil makes it challenging for further utilization as intermediate to produce transportation fuels. The chemistry of bio-oils can be manipulated by changing the thermal conditions of the process or by conducting pyrolysis in the presence of catalysts. It has been recognized that the application of catalysis could be of major importance in controlling the oil quality and its chemical composition [35]. Catalysts could be applied for a number of reasons, such as lower pyrolysis temperatures, a higher chemical and physical stability, high yields of target components, and an improved miscibility with refinery streams. Catalysts might be used at different stages in the fast pyrolysis process; they could be impregnated in the biomass feed, mixed with the biomass in the pyrolysis reactor (in situ operation), built in the process after the pyrolysis reactor for the reforming of primary pyrolysis
237
vapors (ex situ operation) [36]. The major chemical reaction pathways involved in the biomass catalytic pyrolysis include deoxygenation, cracking, aromatization, ketonization, aldol condensation, hydro-treating and reforming (Fig. 12.2). By using an appropriate catalyst, a specific reaction can be selectively enhanced, leading to the increased production of desirable products, such as hydrocarbons and anhydrous sugars [37]. Catalytic deoxygenation is an efficient route to reduce the oxygen content in the bio-oil. Dehydration, decarboxylation, and decarbonylation are typical deoxygenation reactions to remove oxygen in the form of H2O, CO, and CO2, respectively. Catalytic cracking can convert large molecules and heavy organic chemicals into small molecular products. Meanwhile, the catalytic cracking of oxygenates also gives rise to the formation of aromatics and olefins. Aromatization is another main route to low-molecular weight oxygenates and olefins into aromatics. Under HZSM-5 catalysis, acids, alcohols, aldehydes, esters, ethers, and furans can be largely converted to aromatic hydrocarbons as the major products [38–40]. In addition, ketonization/aldol condensation is a promising route for the conversion of the carboxylic and carbonyl components into longer-chain intermediates that can be further upgraded into gasoline/diesel fuels. Ketonization is a reaction that converts two carboxylic acid molecules into a ketone, CO2, and H2O. This reaction can simultaneously remove carboxyl groups and achieve CdC coupling without consuming external hydrogen, thereby increasing the heating value and stability of the products [41]. However, the ketonization reaction mechanism is complex, and ketene, betaketoacids, carboxylates, and acyl carbonium ions are suggested to be the key intermediates. The surface acid-base properties of metal oxides potentially favor the catalytic pyrolysis of biomass to form desirable products and the different acid and base sites of the metal oxides alter the distribution of bio-oil components, promoting or inhibiting the formation of certain products.
2. BIO-BASED PROCESSES AND BEYOND
FIG. 12.2 Schematic of the chemical reactions for biomass catalytic pyrolysis. Reproduced with the permission from S. Wang, G. Dai, H. Yang, Z. Luo. Lignocellulosic biomass pyrolysis mechanism: a state-of-the-art review, Prog. Energy Combust. Sci. (2017).
2 FROM BIOMASS TO BIO-OIL VIA PYROLYSIS
Magnesium oxide (MgO) and calcium oxide (CaO), the typical base metal oxides catalyst used for biomass catalytic pyrolysis, are effective for deoxygenation via ketonization and aldol condensation of acid and ketone compounds. For instance, authors [42] revealed that with the addition of CaO in the pyrolysis of a cotton stalk the concentration of ketones increased, while that of acidic compounds decreased in the resulting product. The direct deoxygenating effect of CaO on bio-oil during pyrolysis of white pine in a fluidized-bed reactor was studied at 520°C [43]. According to the results obtained, it was shown that the oxygen content of the bio-oil decreased from 39% to 31% in the presence of CaO. Moreover, furfural, furfuryl alcohol, hydroxymethyl furfural, and 3-methyl-2-cyclopentanone are recognized as the products from dehydration reactions that improved with increasing the CaO loading. In addition to acidic and basic metal oxides, transition metal oxides with unique acid–base properties have been widely investigated for biomass catalytic pyrolysis, with various effects on the product distribution. Authors [44] screened the effect of 31 catalysts on the carbon yield of solid residue, gas and bio-oil from the pyrolysis at 500°C of pine sawdust with the ratio 1/1 (w/w). Tested catalysts included mesoporous silica-supported metal oxides, bulk metal oxides, clays, and zeolites. With all the tested catalysts the yields of bio-oil ranged from 14% to 58%, with large differences in the concentration of the heavy fraction (0%–70%). However, for most of the catalysts a reduction of heavy matter was accompanied by a significant diminution of bio-oil yields with respect to uncatalyzed pyrolysis of biomass. A significant decrease in nonvolatile fraction was obtained with ZnO, CuO, Fe2O3, and mixed-oxide catalysts. Zeolites, crystalline materials, are the most investigated solid acid catalysts for biomass catalytic pyrolysis. Mesoporous and macroporous zeolites may not favor reactions for specific products due to the high exposure of the active
239
site to the substrate [45], while the microporous materials have been extensively studied in the catalysis of fast pyrolysis of lignocellulosic biomass due to their good deoxygenation performance and shapes selectivity for value-added hydrocarbons. In particular, ZSM-5 zeolite, especially the protonated form, HZSM-5, has been demonstrated to be the most effective catalyst for aromatic hydrocarbon production. During the biomass catalytic pyrolysis using HZSM-5, the primary pyrolysis vapor undergoes a series of reactions on both the surface and pores of the catalysts, including cracking, deoxygenation, decarboxylation, cyclization, aromatization, isomerization, alkylation, disproportionation, oligomerization, and polymerization [46]. Doping certain metal cations or oxides into zeolite has been explored as a method to increase the yield of desired compounds and to suppress coke formation by tuning the density and activity of the active sites. Authors [47] evaluated a set of commercial and laboratory-synthesized catalysts for their hydrocarbon production performance via the pyrolysis/catalytic cracking route from three types of biomass feedstocks: cellulose, lignin, and wood at temperatures ranging from 400°C to 600°C. Batch mode tests used three biomass feedstocks to identify the most promising catalysts from a set of 40 selected ones: 10 commercial zeolites (including ZSM-5, Y, and SAPO types), 22 laboratory-prepared ZSM-5 catalysts modified by substituting Al with different metals (cobalt, iron, nickel, cerium, copper, gallium, etc.), four laboratory zeolites, and four different silica and alumina materials. The highest yield of hydrocarbons (approximately 16 wt.%) was achieved using nickel, cobalt, iron, and gallium-substituted ZSM-5. According to the previous studies on the catalytic pyrolysis it is shown that the application of the proper catalyst is a fundamental step in developing catalytic pyrolysis as an effective process to convert biomass into a valuable liquid biofuel, and further research is still needed in this area.
2. BIO-BASED PROCESSES AND BEYOND
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2.4 Upgrading of Pyrolysis Oils The high content of oxygenated compounds and the water content, and a low heating value of pyrolysis oil, make the direct co-processing of oil itself in standard refinery units problematic. Several pyrolysis process modifications are currently being studied to obtain the oil product with the better properties. Various upgrading routes have been studied: hydrodeoxygenation (HDO) to remove the oxygen under high pressures of hydrogen and in the presence of a catalyst; catalytic cracking using zeolites; and high-pressure thermal treatment, in which pyrolysis oil is thermally treated to obtain the oil with the higher energy density. Integrated co-processing would provide a relatively straightforward route to the production of liquid transport fuels via fast pyrolysis since oil refining infrastructure is already available. Combined hydrotreating and catalytic cracking appears to possess significant potential for the production of commodity chemicals. Additionally, a two-step hydroprocessing scheme comprising a mild stabilization step and a more intensive upgrading step are envisaged. Alternatively, the hydroprocessing step coupled with catalytic cracking is also being investigated. For instance, in the study [48], the applicability of HDO was investigated as the pyrolysis oil upgrading step to allow fluid catalytic cracking (FCC) co-processing. Different HDO reaction temperatures (230–340°C) were evaluated in a 5 L autoclave, keeping the other process conditions constant (total 290 bar, 5 wt.% Ru/C catalyst). After HDO, the upgraded pyrolysis oil underwent phase separation resulting in an aqueous phase, some gases, and the oil phase that was further processed in a micro-activity test (MAT) reactor (simulated FCC reactor). When the HDO reaction temperature was increased, a net transfer of organic components from the aqueous phase to the oil phase was observed, increasing the carbon recovery in the oil product (up to 70 wt.% of the carbon in
pyrolysis oil). Authors have demonstrated on a laboratory scale that it is possible to produce hydrocarbons from lignocellulosic biomass via a pyrolysis oil upgrading route. Correspondingly, it is also reported by other authors [13] that a combined HDO step and co-processing in an FCC unit produced transport grade fuel. As mentioned earlier, catalytic hydropyrolysis also represents an alternative approach. Cellulosic and woody biomass can be directly converted to hydrocarbon gasoline and diesel blending components through the use of integrated hydropyrolysis plus hydroconversion (IH2) [49]. The IH2 process consists of a pressurized fluidized-bed first-stage reactor for hydropyrolysis, followed by the hydroconversion step, which further removes oxygen from the biomass and fully converts it to gasoline and diesel products. Light gas from the hydroconversion step is separated and sent to a steam reformer which produces the hydrogen used in the process. The hydrocarbon liquid yield is 24–28 wt.%, which is comparable with fast pyrolysis coupled with FCC or HDO. The produced oil is highly deoxygenated (less than 1% oxygen) and contains no polynuclear aromatics, olefins, or reactive free radicals because high partial pressures of hydrogen and the catalyst are available during conversion. With this integration, and using the proper operating conditions, the process is self-sufficient as it requires no external source of methane or hydrogen. This technology might utilize domestic renewable biomass resources to produce transportation fuels in sufficient quantity and quality to substantially reduce the reliance on crude oil [50]. A number of interesting insights into pyrolysis oil hydroprocessing have been gained in recent years. Integrated upgrading approaches, that is, hydroprocessing followed by fluid catalytic cracking, appears to possess synergistic benefits. The production of commodity chemicals via hydroprocessing and catalytic cracking routes within the biorefinery infrastructure may enhance the economic viability of pyrolysis
2. BIO-BASED PROCESSES AND BEYOND
241
2 FROM BIOMASS TO BIO-OIL VIA PYROLYSIS
and pyrolysis-related processes [13]. An important task is that upgrading should selectively target problematic oxygen functionalities and the future research work will need to address the search for cheap catalysts.
2.5 Pyrolysis Technologies Early investigations into the pyrolytic conversion of biomass to liquids were carried out in the 1970s by various workers, for example, the work by Knight and co-workers, and by Garrett Corp. (continued by Occidental Petroleum) [9]. Additional impetus for further process studies was given by the development in the early 1980s of the Waterloo Fast Pyrolysis Process (WFPP) at the University of Waterloo, Canada, and by the development of an “ablative” pyrolysis method at the National Renewable Energy Laboratory in the United States [51]. Up to 1989, the only European plant was a conventional pyrolysis demonstration plant of 500 kg/h operating in Italy for liquid and char production with approximately 25% yield for each. The pyrolysis plant was in operation from 1985 to 1990 and was the largest pyrolysis plant for liquids built in Europe until 1996. The design capacity of the plant was 1 t/h dry biomass, but only up to 500 kg/h was achieved on a continuous basis. The nature of the process was that relatively slow pyrolysis occurred in the stirred or fluid bed reactor producing secondary oil that had a low water tolerance and high viscosity. Although the product quality was poor, the availability of large quantities of pyrolysis liquids for testing and evaluation caused serious attention to be focused on direct liquefaction for the first time in Europe [9]. Since the late 1990s, the process realization emerged, resulting in the construction of pilot plants in Spain (Union Fenosa), Italy (Enel), United Kingdom (Wellman), Canada (Pyrovac, Dynamotive), Finland (Fortum), and the Netherlands (BTG). Meanwhile, many pilot plant projects stopped eventually after the initial testing.
For instance, the plants of Union Fenosa, Enel, Wellman and Pyrovac’s large-scale installation in Jonquiere, Canada, are no longer in operation. This may be because of a lack of confidence in economic prospects and markets at the time, or by legislative limitations [9]. Recent technological improvements to address logistical and technological challenges are making it more likely that fast pyrolysis will become a commercially viable technology. Oil companies and food/feed industries are building biofuel departments and are looking for existing knowledge matching their strategies and targets regarding renewable resources. A number of industrial-sized, fast pyrolysis oil plants have been constructed and are listed in Table 12.4. In 1998 Ensyn adapted the 45t/day unit to upgrade heavy oil. From 2005 onward, focus was again on renewable fuels, and a 75 t/day unit was commissioned in Renfrew Ontario in 2007 [50]. In 2012 KiOR began operating a 500 t/day facility in Columbus, processing woody wastes, in particular, pine waste [52,53]. During the KiOR process, wood chips are first seized and dried, then fed into a reactor, where they interact with a regenerated catalyst in a process similar to “cracking” at the traditional refinery. The vapors are separated from the solid catalyst and char in the separator by a series of cyclones. It is cooled yielding the crude that condenses into liquid TABLE 12.4 Fast Pyrolysis Oil Plants Pyrolysis Oil Plant
Technology
Capacity (kg feed/h)
Ensyn (Canada)
Fluid bed/riser
4500
Kior (United States)
Catalytic fast pyrolysis
21,000
BTG (Netherlands)
Rotating cone
5000
Fortum Valmet (Finland)
Fluid bed/riser
10,000
Gentling (Malaysia)
Rotating cone
2000
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and water. The liquid undergoes hydrotreating to produce gasoline and diesel fuel. In this unit, hydrogen is used to remove residual oxygen and render a hydrocarbon liquid product that can be fractionated into naphtha, kerosene, and distillate ranges by distillation. However, in 2014, KiOR shut down its Columbus, Ohio production facility since it was having difficulty with its production. Fortum commercialized the fast pyrolysis technology by building a pyrolysis-oil plant connected to the Joensuu power plant in Finland [54–56]. The integrated bio-oil plant, based on fast pyrolysis technology, is one of the first in the world on an industrial scale. The pyrolysis plant commenced in 2013. It is integrated with the combined heat and power production plant in Joensuu, which produces electricity and district heat and 50,000 t of bio-oil per year. The plant is located in Joensuu (Finland) with a capacity to produce 50 kt/year pyrolysis liquids. The feeding raw materials include forest residues and other wood-based biomasses. The product is used as a substitute for heavy fuel oil, and as raw material in the chemical industry or for biodiesel production in the future. BTG Biomass Technology Group, one of the pioneers in pyrolysis, started the fast pyrolysis developments at the beginning of the 1990s with a new reactor concept in which no inert gases were required to enable rapid mixing of biomass and hot bed material. This concept was the result of research done at the University of Twente where rapid mixing was achieved by mechanical mixing inside a rotating cone reactor [57]. Over the years BTG’s modified rotating cone technology has resulted in several patents. BTG has successfully tested over 45 different kinds of feedstock over the years, ranging from relatively simple feedstocks, like wood, to difficult feedstocks, like sludge or empty fruit bunch. In 2005 BTG Biomass Technology Group (The Netherlands) started the demonstration of the technology on empty fruit bunch in Malaysia on a scale of 2 t/h. Prior to feeding it to the
pyrolysis plant the empty fruit bunches is further sized and dried in a drier, where the moisture content is reduced down to about 5%–10%. Recently Fraunhofer Umsicht propose an in situ process that used the char produced by the pyrolysis collected in a reformer tube working at a temperature ranging from 550°C to 750°C as catalytic process for oil upgrading and char stabilization. The products depend on the biomass composition. Nevertheless, the main results are the increase of hydrogen concentration in the gas phase, an oil more easily separable from the water fraction and a more stabilized char. TCR process have shown to be effective in the pyrolysis of dried and pelletized biomass residues such as sludge, digestate, and other waste biomasses. Agri-Therm Inc., a spin-off company from Western University Institute for Chemicals and Fuels from Alternative Resources, has developed the first mobile pyrolysis system (MPS) for rapidly converting low value bio-residue at the source into higher-value bio-oil and bio-char for use in meeting renewable fuel content requirements and making special chemicals, pharmaceuticals, and materials. Mobile pyrolysis takes the pyrolysis plant directly into the agricultural and forestry operation, reducing transportation costs and converting low value stalks, leaves, straws, bagasse, chips, sawdust and branches, into higher value and high-energy density biooil and bio-char. The MPS100, built by Agri-Therm, was the first demonstration unit and it was designed to be an efficient, 10 dry tons per day. More importantly, the MPS generates its own self-sustaining thermal power (by combustion of the pyrolysis noncondensable gas in the central furnace), mitigating the need for access to the power grid and/or fuel depots. The second-generation MPS unit (MPS200) has been partially constructed and is currently undergoing cold and hot testing from 2013. The MPS200 has been professionally designed and engineered. The unit has design improvements that will increase reliability and
2. BIO-BASED PROCESSES AND BEYOND
3 BIO-OIL PRODUCTION BY HYDROTHERMAL LIQUEFACTION
output and will put Agri-Therm one step closer to a commercially proven mobile pyrolysis solution [16]. A further list of other global companies and research groups, with descriptions of their respective technologies for fast pyrolysis and upgrading, can be found in “Review of Fast Pyrolysis of Biomass and Product Upgrading.” [58]. Pyrolysis definitely proved to be a promising technology because its greatest strength is the flexibility of the technology to use a range of biomass feedstocks to produce renewable biofuels. Ongoing efforts should be focused on improving efficiencies, increasing product yields, and co-product utilization and valorization. That is why the challenges are related to improving: the operational reliability of demonstrationscale pyrolysis processes, the feedstock edibility, and the process integration and its control. In the meantime, the main focus should be on improvement of the quality and stability of the oil. In addition, the components of the oil, causing its specific characteristics (pH, aging, viscosity, etc.) have not been fully identified nor are the reactions taking place understood. One particular issue is the exact role of the various oxygen functionalities in the oil. It is important to establish which functionalities are desired and
243
which ones are undesirable, and to understand how to steer the pyrolysis process itself in this respect; by catalysis, for instance.
3 BIO-OIL PRODUCTION BY HYDROTHERMAL LIQUEFACTION 3.1 Fundamentals of HTL HTL involves the thermochemical conversion of a broad range of biomass types in the presence of hot compressed water at subcritical conditions into a liquid product known as bio-oil [59]. HTL requires an operating temperature of 300–350°C at 5–20 MPa for 5–60 min, wherein water is in the liquid form [60]. Near and supercritical water have been successfully used for carbonization, gasification, liquefaction, and upgrading of hydrocarbon resources including crude oils, microalgae, lignocellulosic biomasses, and wastes. Fig. 12.3 shows temperature/pressure ranges for hydrothermal water processes over the phase diagram of water. Water’s triple point and critical point are shown in the phase diagram; the hydrothermal water processes are typically performed in a narrow window just greater than the vapor-liquid co-existence curve
FIG. 12.3 Phase diagram of water superimposed with different HTW technologies. c-SCWG, catalytic supercritical water gasification; HTC, hydrothermal carbonization; HTL, hydrothermal liquefaction; nc-SCWG, noncatalytic supercritical water gasification; and SCWU, supercritical water upgrading. Reproduced with the permission from M.T. Timko, A.F. Ghoniem, W.H. Green. Upgrading and desulfurization of heavy oils by supercritical water, J. Supercrit. Fluids (2015).
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or at pressures slightly in excess of water’s critical pressure (221 bar). Increasingly reactive conditions are encountered as temperature is increased while holding pressures greater than water vapor pressure. On the time scale of hours and in the 150–200°C range, hydrothermal carbonization (HTC) converts water-soluble and water insoluble carbon fractions into insoluble carbonaceous solids (biochar), while HTL converts feedstocks into the energy-rich bio-oil. Supercritical upgrading converts heavy oils into lighter oils on the time scale of minutes and at temperatures ranging from 300°C to 500°C. Under these conditions, reaction rates are appropriate for a large-volume product (upgraded crude oil), while minimizing gas and solid byproducts. Accordingly, at the higher temperature, conditions used for SCW-gasification (SCWG), hydrocarbons break down into a mixture of H2, CO, and methane [61]. Subcritical water (<150–374°C, 0.4–22.1 MPa) and supercritical water (>374°C, >22.1 MPa) are nontoxic and environmentally friendly media with good mass transfer and heat transfer characteristics. The properties of water, such as the density and dielectric constant, can be continuously controlled between gas-like and liquid-like values by varying the temperature and pressure. For example, at a pressure of 25 MPa, the dielectric constant decreases from approximately 78 at 25°C to 27 at 250°C, and to 2 at 400°C [62]. This decrease in dielectric constant increases the solubility of small organic compounds. Thus, the polarity of water and hence its ability to dissolve various solids, liquids, and gases that are otherwise insoluble or sparingly soluble, can be significantly enhanced by transforming ordinary water into supercritical water. In addition, water under subcritical conditions can act as an acid or base catalyst, whereas supercritical water offers the unique possibility of shifting the dominant reaction mechanisms from free radical to ionic through modification of the water density [63]. Therefore, the role of water in hydrothermal processing cannot be underestimated.
The HTL process begins with solvolysis of biomass in micellar forms, the disintegration of biomass fractions (cellulose, hemicellulose, and lignin), and thermal depolymerization into smaller fragments. HTL, which mimics the processing of fossil fuels buried deep inside the earth, occurs in minutes or hours. HTL produces oil with low oxygen content as opposed to other processes like fast pyrolysis. HTL proves to be very energy efficient as it entails temperatures lower than those reached during pyrolysis. Conditions such as temperature, pressure, and reaction times influence the conversion of biomass into bio-oil [64]. Hydrothermal processing was initially developed for turning coal into liquid fuels, but recently, the technique has been applied to a number of feedstocks, including woody biomass, agricultural residues, and organic wastes (e.g., animal wastes, municipal solid wastes [MSW], and sewage sludge). The biomass type also affects the nature of the bio-oil (Table 12.5). Loosely packed biomass liquefaction results in bio-oil with high oxygen and moisture content that is undesirable as it lowers the quality and HHV of the fuel efficiency. Table 12.5 summarizes representative data of hydrothermal liquefaction of common types of biomass. The major components of waste biomass can be roughly classified into proteins, lipids, carbohydrates, lignin, and ash. Unlike biodiesel production, which only extracts lipids from algae, HTL could utilize the whole algae biomass including carbohydrates and proteins, which consequently allows the use of fast-growing species with low lipid content hence avoids the demand for promoting lipid accumulation [74]. The envisioned pathway is for the macromolecules of biomass (lipids, proteins, and carbohydrates) to be hydrolyzed into small fragments which subsequently can be converted to smaller compounds, for instance the amino acids transform into hydrocarbons, amines, aldehydes, and acids through decarboxylation and deamination, etc. In hot compressed water,
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3 BIO-OIL PRODUCTION BY HYDROTHERMAL LIQUEFACTION
TABLE 12.5
Overview of Literature on HTL of Common Types of Biomass Reactor Type
T (°C)
Time (min)
Oil Yield (%)
HHV (MJ/kg)
Ref.
Pulp/paper sludge powder
A bomb reactor system
250–280
15–120
20–45
35
[65]
Blackcurrant pomace (Ribes nigrum L.)
A 0.6 L stainless steel stirred batch reactor
583
240
30
35.9
[66]
Pinewood
A Parr reactor of 300 mL capacity
250
120
28
–
[67]
Digested sludge
A Parr (4500) 2-L reactor
300
30
9.4
32
[68]
Dairy manure
A bench top 300 mL stainless steel reaction pressure vessel
350
15
19
32.16
[69]
Brown type onion (Allium cepa L.)
A batch reactor
100–300
5
Negligible (0.05)
A self-designed batch reactor system
325
45–60 min
21.10
Nannochloropsis sp.
A 100 mL batch reactor (4593)
260
60
Algae Dianchi Lake in China
A 100 mL batch reactor (4593)
300
60
Biomass Type Second generation biomass
[70]
Third generation biomass Cyanobacteria sp. Bacillariophyta sp.
triacylglycerides are readily hydrolyzed to glycerol and fatty acid [75]. During the HTL process, the conversion of glycerol is inclined to produce water-soluble compounds such as methanol, acetaldehyde, propionaldehyde, acrolein, allyl alcohol, ethanol, and formaldehyde, with some gas products, mainly CO, CO2, and H2 [76]. The lipid content is proved to contribute directly to the HTL bio-oil yield, and high-lipid algae can produce higher bio-oil yields in comparison to low-lipid algae. The carbohydrate content of the biomass varies depending on the biomass type. The most abundant carbohydrates in aquatic biomass are cellulose, hemicelluloses, and starch. In addition, depending on the type of species, hemicellulose, as well as various other heteropolysaccharides, may also be present [76]. Under hydrothermal conditions, carbohydrates undergo rapid hydrolysis to form glucose and other monosaccharides which are then further degraded.
33.87–36.51
[71]
55
37
[72]
18.4
35.5
[73]
18.21
The main liquefaction mechanism is believed to be the carbohydrates breaking down to polar water-soluble organics, such as organic acids (i.e., formic, acetic, lactic), aldehydes, benzenes and alcohols, all carrying a substantial amount of oxygen [65,75,77]. The aldehyde- and benzenetype structures may further produce larger hydrocarbons, which are then part of the biocrude fraction [77]. Higher protein content is a primary difference between algae biomass and terrestrial biomass [78], which causes the high content of nitrogen in the resulting bio-oil produced from HTL. Proteins are polymers of amino acids linked by peptide bonds. They consist of a carbon-nitrogen bond between the carboxyl and amine groups forming long chains, which can be hydrolyzed in subcritical waterforming amino acids [79]. The amino acids can be further converted to carbonic acids and amines by decarboxylation and to ammonia and organic
2. BIO-BASED PROCESSES AND BEYOND
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acids by deamination [80]. The products from these reactions may then repolymerize and react with the sugars present from other components of the feedstock through Maillard-type reactions to produce long-chain hydrocarbons and aromatic ring-type structures like phenols, or nitrogen heterocyclics such as indole or pyrrole, which are commonly found in the bio-crude derived from HTL [76]. Some species, such as Spirulina algae, containing as high as about 65–70wt.% protein have been reported and the protein content in the aquatic biomass is highly dependent on the cultivation conditions [68,81,82]. Sorted by contributions and tendencies to contribute to the oil yield in HTL, the components of biomass are in the following order: lipid > protein > carbohydrate [83]. Although the lipid fraction is typically the main target of oil production, it was reported that 0%–15% of the produced bio-oil is converted from carbohydrates and proteins. This indicates that hydrothermal liquefaction is a promising process because of its ability to convert the protein and carbohydrate fractions to bio-oil. Biller and Ross [83] studied HTL of different model components representative of the lipid, protein, and carbohydrate fraction of microalgae, and even created an equation to estimate the biocrude oil yield as an relation to the mass fraction of algae fraction multiplied by their respective individual yields, obtained from the liquefaction of model compounds. However, the equation fit well only for the strains tested in their study (Nannochloropsis occulata and Chlorella vulgaris), and big deviations can be found for other strains under the same reaction conditions. Accordingly, each microalgae fraction does not behave independently during the HTL process and its contribution to the overall yield depends on an interaction among components of algae; for example, Maillard reactions between carbohydrates and proteins, or condensations between degradation products from lipids and proteins. Generally, the HTL products consist of four phases: a gas-phase of mainly CO2, a top-phase
of bio-oil, an aqueous byproduct, and a bottom phase with mainly solid residue. Depending on the physical properties (e.g., density and hydrophobicity), the oil is either just gravimetrically separated from the aqueous phase or extracted with an organic solvent [84]. The complex chemical reaction pathways of biological molecules in hydrothermal processes include depolymerization, fragmentation, dehydration, and decarboxylation. A better understanding of the reaction pathways and kinetics operative during HTL could provide opportunities for innovation that improve the process. The literature review [85] provides a broad information about the reactions of the most abundant types of biomolecules (e.g., proteins, lipids, and carbohydrates) in hydrothermal water conditions. This review collects information pertinent to the behavior of microalgae biomolecules (e.g., proteins, polysaccharides, lipids, chlorophyll) and their hydrothermal decomposition products (e.g., amino acids, sugars, fatty acids) in high temperature water. Representation of reactions of different biomacromolecules in microalgae, depicting products, which can be formed during hydrothermal liquefaction, is presented in Fig. 12.4.
3.2 HTL Oil Properties Bio-crude oil is much like a petroleumderived product but with a higher viscosity and oxygen content, typically 10–20 wt.% compared with 1% in conventional petroleum [86]. These oils could be upgraded catalytically to yield an organic distillate product which is rich in hydrocarbons and useful chemicals. Compared with bio-oil obtained from the fast pyrolysis method, their yield from the HTL process is much lower and it features higher heating value. Bio-oils from HTL have viscosities varying from liquid to tar-like, and compose a very complex matrix containing small molecules of diverse polarity and boiling points, as well as large oligomeric components [84].
2. BIO-BASED PROCESSES AND BEYOND
O
Hydrolysis
R
O HO
R
R
OH R Fatty acids
H2N
Amino acids
Protein
R
NH2 Amine
R
O R
R
N NH
NH
O
O
N
R
Pyrazine
R
OH Alcohols
OH Organic acid
R N
N
O
O
Pyrrolidinedione
Maillard reaction
R
H N
H N
H N
R¢
Pyrrolidine
N Indole derivatives
Indole
OH
OH O
O HN
R¢
N
H H Ammonia
HO
Amide
OR¢
R
R
H
OH
R¢
Esters
O
OH R
R
O C O Carbon dioxide
Deamination
Hydrolysis
C C N C C N R
Decarboxylation
O
N R¢
O
Lipid
O H H O H H
Pyrrolidine derivatives of fatty acid
O
OH + OH Glycerol
N
Alkenes
Decarboxylation
O O O
+
Alkanes
R O
O
R
Decarboxylation
O
OH O
OH HO
O OH OH
Carbohydrate
OH OH O Hydrolysis
OH
HO H
+ HO
OH OH Reducing sugar
R¢ N
R¢
OH O
R
OH
Pyrrolidinone
N
O N
Sugar Degradation
R
O
R¢
Cyclic oxygenates
R
Quinoline
Pyridinol
O HN
NH
O
R
R
OH
Phenol derivatives Temperature (°C)
0
100
200
250
300
320
FIG. 12.4 General reaction network of HTL of low-lipid microalgae. Reproduced with the permission from C. Gai, Y. Zhang, W.T. Chen, P. Zhang, Y. Dong. An investigation of reaction pathways of hydrothermal liquefaction using Chlorella pyrenoidosa and Spirulina platensis. Energy Convers. Manag. (2015).
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3.3 Catalytic HTL The use of catalysts is a challenging factor in biomass liquefaction as it can reduce the required reaction temperature, enhance reaction kinetics, and improve the yield of desired products. The use of catalysts in hydrothermal liquefaction processes is intended to improve process efficiency by reducing char and tar formation. Two types of catalysts, homogeneous and heterogeneous, are reported in the literature and are summarized here. The literature analysis shows that the alkali hydroxides, carbonates, bicarbonates, and alkali formates as homogeneous catalysts, were intensively studied in the catalytic liquefaction. Based on the previous study, the activity and selectivity of alkali solutions are placed in the following order: K2CO3 > KOH > Na2CO3 > NaOH [87]. Authors showed that alkali carbonates catalytically increased the conversion rate and the generation of bio-oil more than alkali hydroxides due to its secondary reaction with water, forming bicarbonates and hydroxides. Homogeneous catalysts offer some advantages: decreased solids production, increased bio-crude yield, and improved bio-crude properties. Specifically, alkaline catalysts are able to enhance the yield of heavy oils and decrease the formation of residues, while acid catalysts (e.g., sulfonic acids and sulfuric acid) promote the condensation of lignin materials, consequently increasing the amount of insoluble residue [88], even though they are capable of decreasing the temperature and time required for the liquefaction of lignocellulosic biomass by enhancing the hydrolysis of cellulosic components. Other catalysts were reported to be used in the liquefaction of lignocellulosic biomass, such as FeS, Rb2CO3, Ba(OH)2, and ZnCl2, CuO [89]. Although the use of homogeneous catalysts seems to be effective, the separation of these catalysts may become problematic and require additional input energy. The application of heterogeneous catalysts in HTL have also discussed to improve bio-crude quality [90,91].
Several metal oxide catalysts including MnO, MgO, NiO, ZnO, CeO2, La2O3, etc., were employed in supercritical hydrothermal liquefaction of empty fruit bunch derived from oil palm residues at 390°C and at reaction time of 1 h. Among the tested catalysts, four most active metal oxides with lower electronegativity such as CaO, MnO, La2O3 and CeO2 were selected, and gave maximum relative yield of bio-oil at about 1.40 times than the one obtained without the catalyst [92]. In one study [93], rice straw was treated under hydrothermal conditions at different temperatures (200–300°C) for the reaction time of 120 min with and without NiO nanocatalysts. The tested nanocatalyst increased the bio-oil yield up to 13.2% and 17.2% for light and heavy oils, respectively, at 300°C and the carbon recovery was significantly improved. Authors [94] showed that bio-oil yields were higher with the following the order of nano-Ni/SiO2 > zeolite > Na2CO3 in hydrothermal liquefaction of microalgae Nannochloropsis sp. The highest bio-oil yield (30.0%) was obtained at 250°C by using nano-Ni/SiO2. Moreover, applying a catalyst resulted in a decrease in the oxygen and the nitrogen contents of the bio-oil and consequently an increase in its heating value. Crude bio-oils were produced from the microalga Nannochloropsis sp. via reactions in liquid water at 350°C in the presence of six different heterogeneous catalysts (Pd/C, Pt/C, Ru/C, Ni/SiO2-Al2O3, CoMo/γ-Al2O3 (sulfided), and zeolite) under inert (helium) and highpressure-reducing (hydrogen) conditions [95]. In the absence of added H2, all of the catalysts tested produced higher yields of crude bio-oil from the liquefaction of Nannochloropsis sp., but the elemental compositions and heating values of the product (about 38MJ/kg) were largely insensitive to the catalyst used. The crude bio-oil yields range from a low of 35% from uncatalyzed liquefaction without H2 up to 57% from Pd/C catalyzed liquefaction without H2. In both the presence and absence of H2, the
2. BIO-BASED PROCESSES AND BEYOND
3 BIO-OIL PRODUCTION BY HYDROTHERMAL LIQUEFACTION
supported Ni catalyst produced a crude bio-oil with a sulfur content below the detection limits. Ru/C is also shown to increase the oil yield from 48.6% to 77.2% produced from kenaf biomass at 300°C [96]. Apart from catalysts, there have been some studies devoted to investigating the effect of co-solvents on the performance of the HTL process of waste biomass [97], since it has been reported that at the critical point of water the liquefaction process was operated under challenging conditions. The organic solvents such as ethanol, methanol, propanol, butanol, tetralin and ethyl acetate, etc., have been studied as alternative reaction solvents to replace water [98]. Solvent selection is very important to obtain high liquefaction oil yields and improve the oil’s properties [99]. The presence of solvent dilutes the concentration of the products, thus tending to minimize cross-linked reactions and reverse reactions. In the liquefaction experiments [100] carried out in a 1000 mL autoclave at different temperatures from 573 to 653 K for Spirulina microalgae, the maximum oil yield increased gradually with increasing the temperature. In the resulting oil, the nitriles (22.7%) were dominated when the 1,4-dioxane was used, whereas the esters were dominant in case of methanol (35.5%) and ethanol (26.3%). After HTL the nitrogen content in the bio-oil changed from the initial content with 8.9% to 7.5%, 7.9%, and 9.5% in the presence of methanol, ethanol and 1,4-dioxane, respectively. However, authors [101] performed the HTL for the same highprotein microalgae feedstock (Spirulina) in the presence of sub- and supercritical ethanol in the presence of FeS and Na2CO3 catalysts. The bio oil yield ranged from 35.4% to 45.3%, depending on reaction temperature and the solvent filling rate. There have been numerous studies [102] on the beneficial effect of methanol and ethanol on the bio oil yield. From a commercial point of view, the solvent should either be cheap and easily recoverable, be produced within the process, and/or be co-processed with the bio-oil to the end products [103].
249
3.4 Hydrothermal Upgrading One implementation of liquefaction is the hydrothermal upgrading (HTU) process for the conversion of wet biomass at 573–623 K and 15–18 MPa into biocrude oil. The HTU process may at first glance seem quite similar to the catalytic liquefaction process; however, there are a number of differences. Mainly, the HTU consists of an initial hydrothermal treatment followed by catalytic treatment, converting the biocrude from the initial step into other hydrocarbons. The research and development of the HTU (Hydrothermal Upgrading) process started in 1982 in the Shell Research Laboratory in Amsterdam. In Ref. [104] six HTU units, wood was converted into 3600 t/day of biocrude containing 10% oxygen, then it was upgraded by catalytic hydrodeoxygenation in a central facility. Because bio-oil is produced in an aqueous environment, it may be beneficial from a process engineering point of view to carry out hydrotreating in the same environment. For instance, authors performed [94] hydrodenitrogenation of crude oil produced from HTL of algae in the presence of high-temperature water, a formic acid mixture and H + ZSM-5 nanocatalyst. The effect of the reaction temperature (250–500°C), formic acid as a hydrogen source and nanocatalyst on nitrogen elimination was investigated comprehensively. Maximum nitrogen removal (75%) was obtained in the presence of the nanocatalysts performed at 400°C. Similar results were seen in Ref. [63] catalytic hydrothermal upgrading of algal bio-oil at 400°C for 240 min with the addition of 6 MPa and 10% nine zeolite catalysts in supercritical water. The results showed that compared with noncatalytic upgrading reactions, all of the zeolites promoted denitrogenation, deoxygenation, and desulfurization of the pretreated bio-oil due to the presence of the acid sites. In one study [105], a duckweed biocrude produced from the hydrothermal liquefaction of Lemna minor was treated in subcritical water at
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350°C with added H2 in the presence of several different commercially available materials such as Ru/C, Pd/C, Pt/C, Pt/c-Al2O3, Pt/C-sulfide, Rh/c-Al2O3, activated carbon, MoS2, Mo2C, Co-Mo/c-Al2O3, and zeolites. The effect of the catalysts on the yields, product distributions, the deoxygenation, denitrogenation, and desulfurization of biocrude were examined. All the materials showed catalytic activity for deoxygenation and desulfurization of the biocrude but only Ru/C showed activity for denitrogenation. Among the examined catalysts, Pt/C showed the best performance for deoxygenation. The oil produced with Ru/C shows the lowest sulfur, the highest hydrocarbon content (25.6%), the highest energy recovery (85.5%), and the highest higher heating value (42.6 MJ/kg). The study [106] was focused on the impact of low temperature HTL (LT-HTL), followed by a higher temperature HTL on the minimization of heteroatoms which produce a refinery quality bio-oil. Authors performed consecutive low(225°C for 15 min) and high- (350°C for 60 min) temperature subcritical liquefaction and subsequent hydrodeoxygenation in the presence of Ru/C and CoMo-S catalysts for Chlorella sorokiniana, Chlorella minutissima, and Scenedesmus bijuga algae feedstocks. This pretreatment step reduced nitrogen heteroatom and generated a biocrude with free fatty acids and unsaturated hydrocarbons. After HDO using Ru/C (5%) the final yields of the highest quality oil (the lowest nitrogen level with 3.2%–3.9%) ranged from 15% to 22%, indicating the two-stage strategy might be promising for HTL of a high-protein algae biomass due to perceivable fulfillment in heteroatom reduction. Correspondingly, the results of authors [107] designated a two-step treatment approach (noncatalytic treatment followed by catalytic upgrading) with different heterogeneous catalysts showing captivating approaches for the hydrothermal catalytic upgrading of algal biocrude. Among different catalysts tested, Raney-Ni
(1.6%) and HZSM-5 (1.8%) were the most suitable ones for denitrogenation (1.6% and 1.8% N in resulting bio oil), however using HZSM-5 produced the upgraded oil in the lowest yield (53.1%). Because Ru/C and Raney-Ni catalysts showed the best performance for deoxygenation and denitrogenation, authors performed one more experiment with a combination of these two catalysts and obtained the highest upgraded oil yield—77.2% with the 2% nitrogen content. Similarly, authors [108] reported the activities of several different couple of catalysts for hydrothermal hydrodeoxygenation and hydrodenitrogenation of pretreated algal oil at 400°C for 4 h in supercritical water. The yield in upgraded oil ranging from a low of 63.2 wt.% in the Ru/C + Rh/γ-Al2O3-catalyzed reaction, to a high of 77.2 wt.% in the Ru/C + Mo2Ccatalyzed reaction. All the couple of catalysts exhibited excellent performance with respect to deoxygenation, denitrogenation, and, in particular, desulfurization. Ru/C + Mo2C, Ru/C + Pt/γ-Al2O3, and Ru/C + Pt/C performed best with respect to deoxygenation, denitrogenation, and desulfurization and produced upgraded oils with O, N, and S contents of 0.1, 1.8, and 0.065 wt.%, respectively. The upgraded oil produced with Ru/C + Mo2C retained 89.7% of the heating value of the original pretreated oil and contained 98.5% of the components boiling below 450°C. According to the results obtained, authors selected two component catalyst mixtures as a promising strategy for upgrading bio-oil derived from in supercritical water.
3.5 HTL Technologies As early as the 1920s, first experimental data from Berl supported the concept of oil production from plant biomass [109]. Pioneering liquefaction work was done by Appell and coworkers at the Pittsburgh Energy Research Center (PERC) in the 1970s, which was later demonstrated in a pilot plant in Albany, Oregon.
2. BIO-BASED PROCESSES AND BEYOND
4 COMPARATIVE STUDIES ON HTL AND PYROLYSIS PROCESSES
The PERC process consisted of converting dried-wood in an anthracene oil at 300–370°C in the presence of a catalyst Na2CO3 and reducing gas (CO/H2, operating pressure 20 MPa). Anthracene oil was progressively replaced by recycling the oil produced. However, serious technical problems, caused by undissolved solids and an increase of medium viscosity, prevented the pilot unit from operating after 1981. However, while in operation, the unit produced 5000 kg of oil on a continuous basis [110]. After reconsidering the problems of the PERC process, the Lawrence Berkeley Laboratory introduced a pretreatment of wood prior to its liquefaction, in a smaller unit with a reactor volume of 1 L. The pretreatment was a mild acid hydrolysis (pH ¼ 10–8) at 180°C for 45 min and assisted to weaken the lignocellulosic material. A water/wood slurry was then prepared, and pH was adjusted to 8 using Na2CO3. This step— wood liquefaction in an aqueous medium— lasted 10–60 min. These two trials, undertaken to develop a single-step liquefaction process, showed the need for pretreatment [111]. The CatLiq biomass conversion process has been developed over the past 10 years and has been developed in scales from 1 to 100 kg/h. Both anaerobic digested sewage sludge and corn silage has been demonstrated as working feedstocks, resulting in bio-oil as the main product in regards to energy, yield of which is highly tunable by process parameters [112]. Other direct liquefaction of organic substances, developed by Hoch-schule f€ ur Angewandte Wissenschaften Hamburg, Germany (HAW), EPA’s Water Engineering Research Laboratory, Cincinnati, Ohio, United States, developed a prototype sludge-to-oil reactor system (STORS), the thermal depolymerization technology, designed by Changing World Technologies, Inc. (CWT), are described in detail in the review [74]. Although the interest in HTL-based processes remains a key driver for the production of fuels
251
and chemicals toward an HTL bio-refinery concept, there have been many challenges facing the commercialization of these technologies, including expensive and complex reactors that require high-capacity water handling equipment. Overall poor understanding of mass balance make it difficult to accurately measure product yields during the hydrothermal run [60]. Compared with flash pyrolysis, HTL is at a nearly developmental stage, and challenges for the coming years are: improvement of oil production rate and energy efficiency of the process, understanding the reaction mechanisms and kinetics, and development of pilot-scale plant. It still remains an interesting topic to investigate the interaction of operation parameters on the oil yield and quality during biomass conversion and optimize the process conditions as well as to learn more about the mechanisms involved in the HTL process in order to achieve a desirable oil yield and quality. Because there is a wide range of feedstock that could be used, a systemic approach in determining the effects of various components to oil yield and quality is needed. For the upscale, a detailed economic feasibility study is needed in order to assess the profitability of the process.
4 COMPARATIVE STUDIES ON HTL AND PYROLYSIS PROCESSES The bio-oil produced from the both direct liquefaction and pyrolysis is a complex mixture containing significant quantities of nitrogen and oxygen with high TAN (total acid number). Thus, further treatment, such as denitrogenation and deoxygenation, is required to meet the transportation fuel. To date, only a few studies have reported a parallel comparative evaluation of the yields and fuel properties of bio-oil obtained by pyrolysis and liquefaction. For instance, authors [113] provide a comparative assessment of bio-oil yield and qualities from a
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microalgal feedstock using slow pyrolysis and HTL. Based on the results obtained, HTL resulted in higher bio-oil yields (41%) compared with pyrolysis (23%–29%). Moreover, bio-oil produced from HTL was found to have higher energy density and superior fuel properties, such as thermal and storage stabilities, compared with pyrolysis bio-oil. Another study [114] comparatively evaluated the bio-oil yield and quality from a microalgal feedstock using HTL, alcoholysis, pyrolysis, and hydropyrolysis. It was shown that HTL resulted in a bio-oil with higher energy density and superior fuel properties, such as thermal and storage stabilities, compared with three other conversion routes. In Ref. [6] HTL and pyrolysis of Chlamydomonas reinhardtii was compared. The bio-oils were produced with similar HHV and elemental composition; however, GC-MS analysis revealed different composition. Pyrolysis bio-oils were mainly composed of proteins derived compounds whereas HTL bio-oils were composed of proteins, lipids and carbohydrates derived compounds. Energy ratios (energy produced in the form of bio-oil divided by the energy content of the initial microalgae) between 66% at 220°C and 90% at 310°C in HTL were obtained, whereas it was in the range 73%–83% at 400–550°C for pyrolysis. The higher heating value of the HTL bio-oil was increasing with the temperature while it remained constant for pyrolysis. Based on the results obtained from the comparative evaluations on the yields and properties of the bio-oils, it should be concluded that HTL oil has a higher yield and higher energy density. However, it is also worth to mentioning that all comparatives studies considered employed third-generation biomass, such as algae feedstock that preferably produces the higher oil yield by HTL than pyrolysis, while the further research is required on the studies on the oil production from the second-generation biomass such as wood wastes, organic food residues, etc.
5 CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK Bio-oil produced from thermochemical conversion technologies, such as pyrolysis and HTL, is a very challenging mixture of biochemicals and water. Finding pathways to maximize its commercial value has been, and continues to be, a priority. The nature and yield of bio-oil from hydrothermal technologies depends on several factors, such as catalyst, feedstock type, the nature of the solvent, and process conditions employed in the processes. Thus choice and selection of the thermochemical process for a particular biomass type is important in view of its major influence on the yield and desired properties of the final product. The major differences between HTL and pyrolysis are as follows: – HTL occurs at 5–20 MPa pressure and low temperature (250–350°C), whereas pyrolysis proceeds at near atmospheric pressure (0.1–0.5 MPa) and 400–600°C temperature. – HTL can convert high moisture biomass without water evaporation and using the excess water in biomass as a highly reactant medium. This eliminates the need for drying, whereas pyrolysis requires complete drying. – HTL results in a higher yield of bio-oil whereas pyrolysis, in most cases (except fast pyrolysis), leads to the production of higher quantities of solid char than other products. Moreover, HTL has not yet been widely commercialized as the pyrolysis process due to a number of technological gaps as well as the lack of a deep knowledge about the HTL chemical pathway. For both thermochemical processes, along with technological constraints, there are economic bottlenecks because technologies uses high pressure or temperature equipment; the processes have high-capital investments. Considerable challenges remain in the area of catalyst recycling and regeneration in order to improve the lifetime and efficiency of the
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REFERENCES
hydrothermal processes. In view of this, hydrothermal technologies, both HTL and pyrolysis, hold significant promise, but research and development are still needed to the commercial deployment.
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2. BIO-BASED PROCESSES AND BEYOND