Catalytic processing of oxygenated organic compounds accomplishes deoxygenation through simultaneous dehydration, decarboxylation, and decarbonylation reactions occurring in the presence of zeolite catalysts. In the late 1970s, synthetic zeolites such as ZSM-5 were successfully used to convert oxygenated organic compounds into hydrocarbons [5, 6]. ZSM-5 proved to be particularly effective for the conversion of methanol to gasoline range hydrocarbons [7], which led to the commercialization of the methanol-to-gasoline process by Mobil. This discovery also stimulated research focused on the production of hydrocarbons from biomass-derived pyrolysis oil and from biomass pyrolysis vapors. To enhance understanding of the reaction mechanisms and identify the most favorable process conditions research was conducted using model compounds representing different chemical classes of bio-oil components. Extensive research in this area has been performed in several centers, especially at Laval University [8, 9], University of Saskatchewan [10-13], University of Thessaloniki [14], University of The Basque Country [15,16], University of Valencia [17], and at University ofMassachusetts [18]. In general those studies showed that in the temperature range of 350-450°C oxygenated organic compounds in contact with zeolite catalysts undergo a suite of chemical reactions including dehydration, decarboxylation, cracking, aromatization, alkylation, condensation, and polymer­ization. The product was always a two-phase liquid (aqueous and organic) and gas, while coke deposits formed on the catalyst surface. The conversion and the product
composition varied depending on the class of compounds tested. Using a fixed bed of ZSM-5 catalyst, high conversions (>90%) were obtained for alcohols, aldehydes, ketones, acids, and esters while phenols and ethers remained mostly unchanged. Alcohols and ketones reacted to produce high yields of aromatic hydrocarbons while acids and esters were mostly converted to gas, water, and coke with low yield of hydrocarbons. For example, initially complete acetic acid conversion declined to 60% after 3 h on stream with the total hydrocarbon yield below 10%. The high pro­duction of coke resulting in a rapid catalyst deactivation was observed especially for the compounds having low (<1) effective hydrogen index defined in [19] as:

‘ H

V C

where H, C, and O represent the number of moles of hydrogen, carbon, and oxygen in the feedstock. This coke is mostly produced by dehydration of oxygenated organic compounds containing high amount of oxygen; dehydration of low-oxygen-content compounds produces mostly hydrocarbons. Significant improvement in the produc­tion of hydrocarbons from low effective-hydrogen-index compounds such as acetic acid can be obtained by coprocessing with methanol, which has a hydrogen index equal 2 [19] .

In parallel to model compound studies, zeolites were also applied for deoxygen­ation of biomass pyrolysis oils and its fractions [8, 10, 20]. The reported hydrocar­bon yields were in the range of 12-15% but also high coke production and rapid catalyst deactivation were observed due to hydrogen deficiency (effective hydrogen index <0.3) of biomass pyrolysis oils.

Lignocellulosic biomass consists of a variety of materials with distinctive physical and chemical characteristics. Typically it is categorized into either woody, herba­ceous, or crop residues. Most of these biomass materials are already used, without preliminary conversion, as a fuel for heating purpose and also to produce steam for generating electricity. Direct combustion is best suited to biomass having low contents of moisture and ash. In fact, until the start of twentieth century, biomass and coal were the major sources of fuels and chemicals. The recent energy crisis, fluctuating oil prices, and political factors associated with the import of fossil fuels

focus again on the utilization of abundantly available biomass resources for producing easy-to-handle forms of energy such as gases, liquids, and charcoal [56,134]. Biomass may be converted to energy by many different processes such as biochemical, thermochemical, and hydrothermal pathways depending on the raw characteristics of the material and the type of energy desired (Fig. 6).

Thermochemical processes depend on the relationship between heat and chemical action as a means of extracting and creating products and energy. Pyrolysis, gasification, and liquefaction which are conducted at a temperature of several hundred degrees Celsius are categorized in thermochemical processes. Pyrolysis is defined as the thermal degradation of biomass in the absence of oxygen to produce condensable vapors, gases, and charcoal; in some instances, a small amount of air may be admitted to promote this endothermic process. The products of pyrolysis can be gas, liquid, and/or solid. In flash pyrolysis, biomass is rapidly heated (e. g., at rates of 100-10,000°C/s) to 400-600°C, while limiting the vapor residence time to less than 2 s [4] . The oil production is maximized at the expense of char and gas. Pyrolysis processes typically use dry and finely ground biomass. Pyrolysis and direct liquefaction processes are sometimes confused with each other, and a simplified comparison of the two follows. Both are thermochemical processes in which feedstock organic compounds are converted into liquid products. In the case
of liquefaction, feedstock macromolecules are decomposed into fragments of light molecules in the presence of a suitable catalyst. At the same time, these fragments, which are unstable and reactive, repolymerize into oily compounds having appropriate molecular weights [19]. With pyrolysis, on the other hand, a catalyst is usually unnecessary, and the light decomposed fragments are converted to oily compounds through homogeneous reactions in the gas phase.

In gasification, oxygen-deficient thermal decomposition of organic matter primarily produces synthesis gas. Gasification can be thought of as a combination of pyrolysis and combustion. Gasification has a good potential for near-term commercial applica­tion due to the benefits over combustion including more flexibility in terms of energy applications, higher economical and thermodynamic efficiency at smaller scales, and potentially lower environmental impact when combined with gas cleaning and refining technologies. An efficient gasifier decomposes high molecular weight organic compounds released during pyrolysis into low molecular weight noncon­densable compounds in a process referred to as tar cracking. Undesirable char that is produced during gasification participates in a series of endothermic reactions at tem­peratures above 800°C which converts carbon into a gaseous fuel. Typically gaseous products include: CO, H2, and CH4. Fisher-Tropsch synthesis can be used to convert the gaseous products into liquid fuels through the use of catalysts. Gasification and pyrolysis both requires feedstock that contains less than 10% moisture [49, 85].

Biochemical processes takes place at ambient to slightly higher temperature levels using a biological catalyst to bring out the desired chemical transformation. Bioethanol from lignocellulosic biomass is produced mainly via biochemical routes. The biomass is first pretreated by different pretreatment methods (discussed later) for the improving the accessibility of enzymes. After the pretreatment, biomass goes through the enzymatic hydrolysis for conversion of polysaccharides into monomeric sugars such as glucose, xylose, etc. Subsequently, sugars are fermented to ethanol by the use of different microorganisms using the process called simulta­neous saccharification and co-fermentation (SSCF) [33].

FT reactor can be either a fixed bed reactor or a slurry bed reactor. In earlier studies and commercialization, fixed bed reactor technology was extensively used. Since FT process is exothermic, a careful control of heat and mass transfer is very impor­tant for CO conversion and catalyst selectivity and stability. In the fixed bed reactor, special efforts have been made to design the reactor internals to remove the heat and control the reactor temperature. More recently, slurry bed reactor is preferred because it offers distinct advantages for the control of both mass and heat transfer problems that may affect the reactor performance. Slurry bed reactor also allows more flexibility in the use of the catalyst particle size. For coal and petroleum derived syngas, commercial FT reactors are being operated by Shell, Exxon, Sasol, Syntroleum among others all over the world. Shell GTL PEARL project in Qatar produces 70,000 bbld (barrels per day). Shell also has a smaller commercial plant in Bintulu, Malaysia which produces 14,700 bbld. Syntroleum operates FT process in Australia. Sasol-QP GTL ORYX-1 project in South Africa produces 34,000 Bbld. These and many other commercial technologies can be readily used for the FT pro­cess that uses biosyngas. The size of FT process will depend on the size of the gasifier for an integrated process. For example, a BTL (biomass to liquid) plant producing 2,100 Bbld will require a gasifier producing 250 MWth [38].

While existing commercial technology for GTL and CTL can be used for BTL, the scale of BTL plant is going to be important. Unlike, coal and natural gas, biomass is difficult to transport and store, and the cost of feed preparation of biomass can become an important factor in the scale of BTL process. This issue has been briefly discussed later in the economy of scale of BTL operation. Since FT process is oblivi­ous to how syngas is produced (as long as its composition is not significantly varied), gasification technology is the key to the integration of GTL, CTL, or BTL process. In order to take advantage of the economy of scale, significant efforts are being made to examine CBTL (mixture of coal and biomass to liquid) process. As discussed later, this process offers some distinct advantages over CTL or BTL process.

Jun et al. [9] reported the influence of H2 content in feedgas on the conversion of CO and CO2 to hydrocarbons. The reaction with H2-deficient feed (CO/CO2 = 0.33, H2/ (2CO + 3CO2) = 0.44) showed that only CO was converted to hydrocarbons, while in H2-enriched feed (CO/CO2 = 0.33, H2/(2CO + 3CO2) = 1), CO2 was converted to hydrocarbons as well as CO. The high concentration of H2 was thought to promote the conversion of CO2 to CO by reverse WGS reaction, followed by FT reaction in which CO was further hydrogenated to hydrocarbons.

Krishnamoorthy et al. [34] analyzed the 13C content in CO, CO2, hydrocarbons and oxygenates after 13CO2 was added to H2/12CO reactants (508 K, 0.8 MPa, H2/ CO = 2). No 13C is detected in CO, suggesting that dilution of the CO reactant by 13CO molecules formed from 1 3CO2 via reverse WGS reaction is negligible at the reaction conditions. The hydrocarbon products have negligible 13C content, indicat­ing that CO2 is much less reactive than CO towards chain initiation and growth. Similarly, the addition of 14CO2 (1.4 mol%) to H2/12CO (1:1) did not lead to detect­able 14C contents in CO and hydrocarbons on Fe catalysts (513 K, 0.1 MPa, H2/CO= 1) [35]. However, Xu et al. [36] detected almost identical radioactivity per mole in hydrocarbons to that of the added, 4CO2 on a Fe-Si-K catalyst (543 K,

1.2 MPa, H2/CO/CO2 = 60/10/30), suggesting that each hydrocarbon molecule con­tained one 1 4C from 1 4CO2. The reaction temperature may decide the differences among the above studies.

Besides the studies on the possibility of CO2 participating in FT synthesis [9, 34­37], it is reported that CO2 added to syngas can affect selectivity of FT products such as C, + selectivity [34], olefin/paraffin ratio [16, 34]. Krishnamoorthy et al. found these phenomena resulted from conversion of CO2 to CO via reverse WGS reaction with CO2 addition to syngas [34], but the effects of CO2 addition on the selectivity are much smaller at 508 K than at 543 K. Calculation [38] indicates that a CO2/CO ratio of 16/1 is required to avoid any CO2 formation in the WGS reaction even under 543 K. The required ratio of CO2/CO is 4.4 under the experimental conditions of 508 K and 2.14 MPa (H2/CO = 2) [34]. These values also mean that CO2 is not reactive to influence WGS reaction, too.

In the case of CO2 cofed with CO, it can be converted to hydrocarbons at high temperature rather than low temperature. The factor is probably the thermodynamics of WGS reaction [39]. The extent of WGS reaction is thermodynamically favored at low temperatures. Correspondingly, reverse WGS reaction is evident at high temperature, and it promotes the CO2 conversion to hydrocarbons.

At low temperature, to use the CO2 contained in syngas relies mainly on the catalyst to activate CO2 . Therefore, our study was done at low temperature in order to develop Fe catalyst active for FT synthesis with CO2-containing syngas.

The purpose of process integration is to combine more than one unit operations into a single unit to reduce both capital and operational cost. In butanol fermentation, early reports on process integration were published in the late 1980s and the early 1990s [13, 52] where butanol fermentation was integrated with product separation. In the case of butanol production by fermentation, following process integrations can occur depending upon the feedstock used:

1. Fermentation and recovery of butanol.

2. Hydrolysis of feedstock and fermentation to butanol.

3. Hydrolysis of feedstock, fermentation, and separation of butanol.

In the case of fermentation and recovery, feedstock does not require hydrolysis such as glucose or if it requires hydrolysis, the later can be performed by the butanol producing culture while fermentation occurs. An example of this is the production of butanol from whey permeate (a byproduct of cheese making industry) and simul­taneous recovery by gas stripping. Whey permeate contains lactose (a disaccharide of glucose and galactose) which can be hydrolyzed by the culture into monomeric sugar units. Then both of these sugars can be converted to butanol. Butanol that is produced by the culture can be recovered simultaneously by one of the product recovery techniques mentioned in Sect. 4.

The second group is where a feedstock requires hydrolysis using exogenous enzymes. In this case, the butanol producing culture is not capable of hydrolyzing the substrate and hence either enzymes are added to the reactor or hydrolytic enzyme producing culture is propagated with the butanol producing culture. The conditions under which enzymes perform hydrolysis need to be close to the cultivation condi­tions of butanol producing culture. In this system, hydrolysis and fermentation are all combined. This group is called SSF (Simultaneous Saccharification and Fermentation). The third group is where hydrolysis, fermentation, and recovery are combined. In this case, enzymes or a hydrolytic enzyme producing culture hydroly­ses the feedstock such as cellulosic biomass, the butanol producing culture performs fermentation, and application of product (butanol) recovery technique recovers the product simultaneously from the reactor. The third process can be abbreviated as SSFR (simultaneous saccharification, fermentation, and recovery).

Forages and specially cultivated grasses are considered some of the prominent potential second-generation bioenergy crops and are the subject of intense research and discussion. The chief perennial feedstocks include switchgrass (Panicum virga — tum L.), Miscanthus giganteus (Miscanthus sinensis x Miscanthus sacchariflorus), and alfalfa (Medicago sativa L.). Perennials require fewer agricultural resources and easily adapt to a range of environmental conditions when compared to first — generation grain crops [104]. The greatest advantages of switchgrass lies in its suit­ability to marginal and erosive lands and its responsiveness to nitrogen fertilization which can dramatically increase crop yields [83]. The cell wall composition of some of the perennial crops is presented in Table 2.

Miscanthus has the highest cellulose level when compared to other perennials and may result in higher ethanol yields, as ethanol production has been described to linearly increase with cellulose and hemicelluloses composition [62] . Furthermore, it has higher yields when compared to switchgrass [10, 57]. As with straws and stovers, perennials present the challenges of dealing with high lignin and ash con­tent which lead to increased crop recalcitrance for saccharification. Conversion of switchgrass to fuel ethanol has been reported as less efficient than corn stover, wheat straw, and even wood residues [134] . Thus, the utilization of specially cultivated perennials may be advantageous only in marginal lands.

Direct detection of low-molecular weight cellulose degradation products can be carried out by HPLC (high pressure liquid chromatography). Oligosaccharides of different length can be resolved and quantified by size exclusion chromatography [18]. Monosaccharides of different covalent structure (xylan, mannan, arabinose, etc.) can be resolved by ligand exchange chromatography, wherein the hydroxyl groups from the sugars interact specifically with resin-immobilized cations (Pb2+, Ca2+). Specificity is imparted by differences in the orientation of hydroxyl groups from sugar to sugar [ 36] . Although carbohydrates tend to have low extinction coefficients, they can readily be detected with an in-line refractometer, which pro­vides high signal to noise. Spectrophotometric detection of labeled sugars has also been used, and depending on the site of labeling, can provide information relating to the specificity and mechanism of degradation. Thin layer chromatography (TLC) also allows cellulose degradation products (particularly, short oligosaccharides) to be resolved. Sugars can be detected by reacting reducing sugars with a colorimetric reagent directly on the TLC plate [20, 85].

According to one of the various embodiments, a designer Calvin-cycle-channeled pathway is created that takes the Calvin-cycle intermediate product, 3-phospho — glycerate, and converts it into 1-butanol by using, for example, a set of enzymes consisting of (as shown with the numerical labels 34, 35, 03-05, 36-43 in Fig. 4): NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase 34, NAD — dependent glyceraldehyde-3-phosphate dehydrogenase 35, phosphoglycerate mutase 03, enolase 04, pyruvate kinase 05, citramalate synthase 36, 2-methylmalate dehydratase 37 , 3-isopropylmalate dehydratase 38 , 3-isopropylmalate dehydroge­nase 39, 2-isopropylmalate synthase 40, isopropylmalate isomerase 41, 2-keto acid decarboxylase 42, and alcohol dehydrogenase (NAD dependent) 43. In this pathway design, as mentioned earlier, the NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase 34 and NAD-dependent glyceraldehyde-3-phosphate dehydroge­nase 35 serve as a NADPH/NADH conversion mechanism that can convert certain amount of photosynthetically generated NADPH to NADH which can be used by the NADH-requiring alcohol dehydrogenase 43 (examples of its encoding gene with the following GenBank accession numbers: BAB59540, CAA89136, NP_148480) for production of 1-butanol by reduction of butyraldehyde.

According to one of the various embodiments, it is a preferred practice to also use an NADPH-dependent alcohol dehydrogenase 44 that can use NADPH as the source of reductant so that it can help alleviate the requirement of NADH supply for enhanced photobiological production of butanol and other alcohols. As listed in Table 1, examples of NADPH-dependent alcohol dehydrogenase 44 include (but not limited to) the enzyme with any of the following GenBank accession numbers: YP_001211038, ZP_04573952, XP_002494014, CAY71835, NP_417484,

EFC99049, and ZP_02948287.

Note, the 2-keto acid decarboxylase 42 (e. g., AAS49166, AD A65057, C AG34226, AAA35267, CAA59953, A0QBE6, A0PL16) and alcohol dehydrogenase 43 (and/ or 44) have quite broad substrate specificity. Consequently, their use can result in production of not only 1-butanol but also other alcohols such as propanol depending
on the genetic and metabolic background of the host photosynthetic organisms. This is because all 2-keto acids can be converted to alcohols by the 2-keto acid decarboxylase 42 and alcohol dehydrogenase 43 (and/or 44) owning to their broad substrate specificity. Therefore, according to another embodiment, it is a preferred practice to use a substrate-specific enzyme such as butanol dehydrogenase 12 when/ if production of 1-butanol is desirable. As listed in Table 1, examples of butanol dehydrogenase 12 are NADH-dependent butanol dehydrogenase (e. g., GenBank: YP_148778, NP_561774, AAG23613, ZP_05082669, ADO12118) and/or NAD(P) H-dependent butanol dehydrogenase (e. g., NP_562172, AAA83520, EFB77036, EFF67629, ZP_06597730, EFE12215, EFC98086, ZP_05979561).

In one of the various embodiments, another designer Calvin-cycle-channeled 1-butanol production pathway is created that takes the Calvin-cycle intermediate product, 3-phosphoglycerate, and converts it into 1-butanol by using, for example, a set of enzymes consisting of (as shown with the numerical labels 34, 35, 03, 04, 45-52, and 40-43 (44/12) in Fig. 4): NADPH-dependent glyceraldehyde-3-phos — phate dehydrogenase 34, NAD-dependent glyceraldehyde-3-phosphate dehydroge­nase 35, phosphoglycerate mutase 03, enolase 04, phosphoenolpyruvate carboxylase 45, aspartate aminotransferase 46, aspartokinase 47, aspartate-semialdehyde dehy­drogenase 48, homoserine dehydrogenase 49, homoserine kinase 50, threonine syn­thase 51, threonine ammonia-lyase 52, 2-isopropylmalate synthase 40, isopropylmalate isomerase 41 , 3-isopropylmalate dehydrogenase 39 , 2-keto acid decarboxylase 42, and NAD-dependent alcohol dehydrogenase 43 (and/or NADPH — dependent alcohol dehydrogenase 44, or butanol dehydrogenase 12).

According to another embodiment, the amino-acids-metabolism-related 1-butanol production pathways (numerical labels 03-05,36-43 and/or 03,04,45-52, and 39-43 (44/12)) can operate in combination and/or in parallel with other photo­biological butanol production pathways. For example, as shown also in Fig. 4, the Fructose-6-phosphate-branched 1-butanol production pathway (numerical labels 13-32 and 44/12, can operate with the parts of amino-acids-metabolism-related pathways (numerical labels 36-42, and/or 45-52 and 40-42) with pyruvate and/or phosphoenolpyruvate as their joining points.

Examples of designer Calvin-cycle-channeled 1-butanol production pathway genes (DNA constructs) are shown in the DNA sequence listings (SEQ ID NOS: 58-81 of the US Patent Application Publication No. 2011/0177571 A1). The net results of the designer photosynthetic NADPH-enhanced pathways in working with the Calvin cycle (Fig. 4) are photobiological production of 1-butanol (CH3CH2 CH2CH2OH) from carbon dioxide (CO2 ) and water (H2 O) using photo­synthetically generated ATP (adenosine triphosphate) and NADPH (reduced nico­tinamide adenine dinucleotide phosphate) according to the following process reaction:

4CO2 + 5H2O ^ CH3CH2CH2CH2OH + 6O2

According to Su et al. [82], extraction and lipase-catalyzed transesterification with methyl acetate and ethyl acetate can be done under the same conditions. They can be simply combined to a two-step-onepot in situ reactive extraction. First, the alkyl acetates were performed as the extraction solvent and afterwards as the transesterification agent. Then, by removing the catalyst, defatted plant material (by filtration), and the solvent (by evaporation), the methyl/ethyl esters were obtained.

The negative effects of glycerol and alcohol on lipase can be reduced by substi­tuting short-chained alkyl acetates for short-chained alcohols as acyl acceptors for fatty acid esters production. Short-chained alkyl acetates are also appropriate solvents for seed oil extraction. Therefore, Su et al. [82] adopted methyl acetate and ethyl acetate as extraction solvents and transesterification reagents at the same time for in situ reactive extraction of J. curcas L seed. Fatty acid methyl esters and ethyl esters were respectively obtained with higher yields than those resulted in by conventional two-step extraction/transesterification. The improvement varied from

1.3 to 22%. The key parameters such as solvent/seed ratio and water content were further examined to find their effects on the in situ reactive extraction. The highest J. curcas ethyl/ethyl esters could achieve 86.1 and 87.2%, respectively, under the optimized conditions [82] .

This transesterification method reduces the risk of deactivation of enzyme by short-chained alcohol and glycerol because in the reaction short-chained alcohol is substituted with short-chained alkyl acetate and no glycerol is produced. Furthermore, such a route to fatty acid esters can decrease the expense associated with solvent extraction and oil cleanup. Due to its low cost production, in situ reactive extraction would be very promising for fatty acid esters production [82] .

5.1.1 Thermochemical Conversion

Thermochemical biomass conversion covers the thermal decomposition and chemical reformation of organic material in biomass to yield fuel and chemical products. Microalgae biomass can be converted into a fuel product by pyrolysis, thermochemical liquefaction and gasification [210].

Pyrolysis: Pyrolysis is the decomposition of biomass to bio-oil, syngas and charcoal at medium to high temperatures (350-750°C) under oxygen deficiency [81]. Microalgae biomass is suited to this process as their high lipid contents along with resolvable polysaccharides and proteins can be easily pyrolysed into bio-oils and syngas [92]. However, there are many technical challenges, notably, the high moisture content of the algal biomass and the bio-oil produced is generally high in nitrogen, and is acidic, unstable, viscous, and contains solids and dissolved water [35, 135]. Therefore, the derived bio-oils require further processing through hydrogenation and catalytic cracking to lower oxygen and remove alkalis [54].

Several studies have investigated the pyrolysis characteristics of microalgae biomass. Miao et al. [136] successfully carried out fast pyrolysis of Chlorella pro — thothecoides and Microcystis aeruginosa grown phototrophically. They recorded bio-oil yields of 18% (higher heating value (HHV) of 30 MJ kg-1) and 24% (HHV of 29 MJ kg_1), respectively. Miao et al. [136] recorded bio-oil yields of 57.9% (HHV of 41 MJ kg-1) with heterotrophically grown C. prothothecoides, which was

3.4 times higher compared to phototrophic production. Demirbas [53] also obtained a bio-oil yield of 55.3% (HHV of 39.7 MJ kg-1) at 502°C for C. prothothecoides. After extraction of bio-oil from Nannochloropsis sp. biomass, Pan et al. [159] pyrol- ysed the residual algal cake with and without a catalyst and recorded HHVs of 32.7 and 24.6 MJ kg-1, respectively. Notably, the catalysed pyrolysis produced oil with a lower oxygen content and fewer harmful compounds.

Significant research gaps exist in current knowledge relating to the specifications of converting algal biomass into bio-oil. Optimal residence time, temperature, and the effect of different feedstock species and growing conditions on the pyrolysis process are all major areas that need further investigation.

Thermochemical liquefaction: Thermochemical liquefaction is a low-temperature (300-350°C), high-pressure process aided by a catalyst in the presence of hydrogen to produce bio-oil [81]. The process decomposes organic materials down to smaller higher-density molecules, by utilising the high activity of water at sub-critical condi­tions [160]. The ability of thermochemical liquefaction to take in wet microalgal biomass (thereby avoiding drying) and convert it to a bio-oil makes it more attractive for commercial exploitation [44, 60, 138] . However, the process reactors and fuel — feed systems for thermochemical liquefaction are more complex and expensive.

Several studies on thermochemical liquefaction of microalgae achieved positive outcomes. For example, maximum bio-oil yields of 64% (HHV of 45.9 MJ kg-1) and 42% (HHV of 34.9 MJ kg-1) were recorded for thermochemical liquefaction of Botryococcus braunii and Dunaliella tertiolecta, respectively [60, 138]. Thermochemical liquefaction is seen as a promising method for energy production due to its acceptance of high moisture content biomass. However, major research gaps exist in current knowledge with organic solid concentration, optimal residence time, temperature and catalytic conditions all major areas that require significant research.

Gasification: This process refers to the partial oxidation ofbiomass at high tempera­tures (800-1,000°C) to yield syngas, a mixture of predominantly CO, H2 and CH4 [44], which can be burnt directly or used as a fuel. Several studies have investigated the viability of energy production by gasification of microalgae biomass. For exam­ple, Hirano et al. [88] gasified Spirulina at temperatures ranging from 850to1,000°C and determined the gas composition required to attain a theoretical yield of metha­nol. The highest methanol yield of 0.64 g methanol per 1 g of algal biomass was achieved at 1,000°C. Tsukahara and Sawayama [210] concluded from their analysis that low-temperature gasification of microalgae to produce fuel and co-products is a viable pathway for bioenergy production and GHG mitigation.