An advantage of water as a reaction medium is that it is ecologically safe, cheap, and readily available but it also has some unique properties when heated under pressure. The hydrogen bonds are weakened, resulting in a change in dielectric constant, acidity, and polarity, each of which can increase opportunities for water to take part in chemical reactions. This change in properties allows water to act as a catalyst, lowering activation energies and allowing reaction pathways which would not occur at ambient conditions. Water is consumed by hydrolysis reactions and formed by dehydration reactions. It is also consumed in the water gas shift reaction which is active at higher temperatures. The critical point of water is at 374°C and 22.1 MPa, below this point the vapor pressure curve separates the liquid from the gaseous phase and the conditions are described as subcritical. Approaching the critical point, the densities of the two phases become more and more alike, and finally become identical at the critical point (Peterson et al., 2008). Above this point, the density of supercritical water is interchangeable without any phase transitions over a wide range of conditions. Depending where in the phase diagram the process conditions take place determines whether HTC, HTL, or HTG reaction conditions are favored, as can be seen in Fig. 17.1.

At ambient conditions, the miscibility of water for hydrocarbons and gases is poor but it is a good solvent for salts due to its high dielectric constant of 78.5 (Kruse and Dinjus, 2007). Just below the critical point, the miscibility of hydrocarbons is improved as the dielectric constant reduces to levels equivalent to solvents such as dichloromethane, decreasing further in the supercritical region. The change in dielectric constant with temperature can be seen in Fig. 17.2 which shows the effect of temperature at constant pressure (30 MPa on the properties of water). Even under HTC conditions (250°C), the dielectric constant has more than halved, increasing the solubility of organics and opening new reaction pathways. The reaction rates in hydrothermal media can be finely tuned by controlling the temperature and pressure influencing the activation energy of reactions. Above the supercritical point of water, the miscibility of hydrocarbons and gases is very high, increasing the rate of reactions. Below the supercritical point, miscibility of these compounds is not complete but is still increased compared to ambient conditions. When the process products cool back down to ambient conditions, the water and the organic fraction will separate once more; this makes distillation or other costly separation techniques unnecessary.

At high temperature and pressure below the supercritical point, the ionic product (pH) is up to three orders of magnitude higher than under ambient conditions and is plotted in Fig. 17.2. The high ionic product supports acid or base catalyzed reactions and can act as an acid/base catalyst precursor because of the relatively high concentrations of H₃O⁺ and OH⁻ ions from the self-dissociation of water (Kruse and Dinjus, 2007). The advantage of this is that the addition of acid or base catalysts can be avoided. The concentration of ions is at its maximum at 275°C, representing an optimum temperature for acid/base catalyzed reactions. Above 350°C, pH increases rapidly by five orders of magnitude or more (Kruse and Dinjus, 2007). The change in density of water at 30 MPa with temperature is also plotted in Fig. 17.2 and indicates that the most dramatic change takes place in the region of the critical point (375°C). Between 300 and 450°C, the density at 30 MPa changes from a liquid-like 750 kg/m³ to a gas-like 150 kg/m³, despite this dramatic change, there is no phase change taking place. This change in density directly correlates with properties such as solvation power, degree of hydrogen bonding, polarity, dielectric strength, diffusivity, and viscosity (Peterson et al., 2008).

The water provides a medium for a complex series of reactions which involve removal of hydroxyl groups through dehydration, removal of carboxyl and carbonyl groups though decarboxylation, and cleavage of many ester and ether bonds through hydrolysis (Funke and Ziegler, 2010). The different steps in HTC are summarized in Fig. 17.3.

The initial step is thought to be the hydrolysis of carbohydrates and proteins which consumes water. Hemicellulose appears to degrade first, at temperatures generally below 200°C, cellulose degrades at 200–230°C and lignin degrades between 220 and 260°C (Pastor-Villegas et al., 2006; Libra et al., 2011; Reza et al., 2013). The lignin is thought to remain relatively intact at lower temperatures but begins to degrade further above 250°C. The soluble hydrolysis products undergo further dehydration reactions yielding water and decarboxylation reactions yielding CO₂. These compounds undergo condensation and polymerization to larger molecules which undergo further aromatization (Funke and Ziegler, 2010; Kruse et al., 2013).

For lignocellulosic biomass, the net result is an increase in aromaticity with phenolic structures derived from the dehydration of lignin combined with aromatization resulting from carbonization of carbohydrates. Condensation and polymerization of the fragments can also form “humic-acid-like” material and “bitumen-like” material which tend to readhere to the hydrochar (Funke and Ziegler, 2010). The reaction pathways involved and composition and yield of products are strongly dependent on the type and composition of the feed (Funke and Ziegler, 2010) and are discussed in more detail in Section 17.3. The proportions of the biochemical components also influence the degradation pathway and operating temperature. In order to produce a coal-like product using hydrothermal carbonization, a reaction temperature of between 200 and 250°C is typically used (Funke and Ziegler, 2010).

When the temperature is increased to 250–375°C, hydrothermal liquefaction reactions start to dominate. During liquefaction, the biomass is decomposed to smaller molecules which are reactive and can repolymerize into hydrophobic, oily compounds (Zhang et al., 2010). The main reaction steps during liquefaction have been summarized by Garcia Alba et al. (2011) as follows:

The products from hydrothermal liquefaction consist of a biocrude fraction, a water fraction, a gaseous fraction, and a solid residue fraction. The majority of the inorganic material is concentrated in the solid residue and process water. The product distribution is largely affected by the biochemical composition of the feedstock. Lipids, for example, are almost entirely fractionated to the biocrude as fatty acids and alkanes. Carbohydrates on the other hand tend to form char. This can be avoided by keeping the water alkali to increase the biocrude yields. The processing of lignocellulosics generally requires an alkali catalyst to achieve satisfactory conversion to biocrude and to avoid excessive char formation. In HTC, the water is acidic and so favors the formation of char. Proteins are converted readily to biocrude via a stepwise reaction into amino acids and finally ammonium, increasing the pH further and promoting biocrude formation. This is why catalysts are not required in HTL when processing feedstock high in protein. A series of interactions between different biochemical components in the biomass can occur. For example, feedstocks containing protein and lipids can result in formation of fatty acid amides. Carbohydrates and protein fragments can react via the Maillard reaction resulting in a range of N compounds. The different steps in HTL are summarized in Fig. 17.3.

Hydrothermal gasification occurs in the higher-temperature region above the supercritical point where water is in the supercritical state. The primary product now becomes a syngas high in combustible gases such as H₂, CH₄, CO, and light hydrocarbons (C₂–C₃); however CO₂ is also produced. The energy requirements to maintain the water in its supercritical state are higher than the subcritical due to the higher operating temperatures required.

An overall simplified net reaction of biomass under supercritical gasification can be summarized as:

HTG has many promising attributes for the formation of H₂ from biomass as it can use a variety of feedstocks and presents a sustainable alternative to steam reforming of natural gas. Fig. 17.4 shows the equilibrium gas yields from a slurry of 5 wt% (weight percentage) wood sawdust adapted from Guo et al. (2007). When a high H₂ yield is required, reaction [17.iv] is undesired and should be suppressed to avoid the formation of methane from H₂. The yield of methane drops steadily from 400 to 800°C while the fraction of H₂ increases. Above 800°C, gasification is complete and reaction [17.i] can be used to predict the molar ratio of CO₂ and H₂. Therefore from a thermodynamic point of view, higher temperatures are not required during HTG. At lower temperatures, CH₄ production is favored. Although the amount of water to biomass is also significant in this context, a higher water content will move the equilibrium toward hydrogen and away from methane production by reaction [17.ii].

In HTL, heterogeneous and homogeneous catalysts have been applied “in situ” during hydrothermal processing in an attempt to improve biocrude quality. Reductions in oxygen, increased yields, and higher energy recoveries have been obtained. However for feedstock high in protein, which results in high levels of nitrogen in the biocrude, the use of “in situ” catalysts has been less successful. In situ catalysis is often used during liquefaction of lignocellulosic feedstocks where an alkali is added such as NaCO₃, KOH, or K₂CO₃. For algal and other high N feedstocks, homogeneous catalysts such as formic and acetic acids have applied as well as heterogeneous catalysts; Pt/Al, NiMo, Ni-Ra, CoMo, Pt-C and many more (Ross et al., 2010; Biller and Ross, 2011; Duan and Savage, 2010; Luo et al., 2015). The effects of alkali catalysts on lignocellulosic feedstocks are reported to be quite beneficial in terms of yield improvements and biocrude quality. The alkali catalyst reduces acid catalyzed decomposition of carbohydrates, phenol aldehyde cross linking reactions and favors more desirable aldol condensation reactions. During HTL of lignocellulosics without alkali, the high concentration of produced acids results in a low pH (<4) which favors dehydration, polymer and char formation (Zhu et al., 2014). For microalgae feedstock, where the majority of HTL work has taken place, the use of catalysts only increases the yields slightly and reduces the oxygen content of biocrudes. Due to the high nitrogen content of algae, the biocrudes generally contain high levels of N (5–7%), which is an issue during combustion and hydrotreating. None of the “in situ” catalytic work to date has been able to address the issue of N in biocrude satisfactorily. Attention has therefore shifted toward the upgrading of the biocrude using either catalytic hydrotreating or catalytic hydrothermal treatment (Bai et al., 2014; Duan and Savage, 2011a,b; Elliott et al., 2013b). Hydrotreating involves processing the biocrude with hydrogen over a catalyst. Hydrogenation reactions convert oxygen, nitrogen, and sulfur into H₂O, NH₃, and H₂S, respectively. A more detailed description of the catalytic upgrading of the biocrude generated from HTL is described by Elliott in chapter “Production of biofuels via bio-oil upgrading and refining.”

Catalysis appears to have been most successful in HTG where good conversion rates of biomass to syngas can be achieved at temperatures below the supercritical point, hence performing subcritical catalytic hydrothermal gasification (CHG) (Elliott, 2015). Early work in HTG suggested that supercritical conditions were necessary to achieve reasonable gas formation rates from biomass but more recent work has shown that with the right catalysts, HTG can operate successfully at lower temperatures. The use of catalysis can also affect the selectivity toward H₂ or CH₄; NaOH for example shifts the reaction equilibrium toward the production of H₂, while the presence of Ni increases the production of CH₄. Catalysts also allow operation at much lower temperatures resulting in so-called catalytic HTG (or CHG). Temperatures can be lowered as far as 350–400°C if the primary aim is to produce methane rather than hydrogen. Ru and Ni catalysts have been shown to be most effective during HTG (Elliott, 2008). One of the main issues in catalytic HTG is the poisoning of active sites by sulfur. Researchers at the Paul Scherrer Institute have developed catalyst and reactor set-ups to avoid this problem, by precipitating salts such as sulfates prior to the catalyst bed (Luterbacher et al., 2009).

The process water is considered a waste product but advancements have recently been made in treating this stream by anaerobic digestion resulting in enhanced biogas yields (Fettig, 2010; Danso-Boateng et al., 2015; Reza et al., 2014a,b). The process water from HTC contains high levels of TOC and therefore linking to AD can produce significant levels of biogas. Levels of biological methane potential range from 0.5 to 1 L/g TOC (Wirth and Mumme, 2014; Oliveira et al., 2013). Some challenges remain, such as reducing inhibition to methanogens but integration has clear advantages and the integration of HTC with AD shows particular promise.

It is generally considered that for continuous operation, the process waters will need to be recycled to minimize water usage (Uddin et al., 2014). During HTC, the soluble organics in the water phase are both being produced and adsorbed although there is evidence that the production dominates (Uddin et al., 2014), which means it will continue to increase on multiple cycling. The level of inorganics will also increase leading to a build-up of inorganics in the reactor (Uddin et al., 2014). This may cause problems with salt corrosion and may require additional measures to precipitate salts with possible recovery of nutrients. Recycling of process water can be considered for each of the hydrothermal process routes and in HTC it has been shown to increase the HTC yields of the hydrochar (Uddin et al., 2014).

The advantage of continuous flow systems include higher throughput and improved heat recovery. Continuous operation generally has a lower residence time than batch operation although there are benefits of higher residence times for product distribution as described further in Section 17.6. As residence times decrease due to increasing flow rate, the reactor output increases and energy requirements decrease. Careful consideration should be given to flow rates, heating rates, and residence times in terms of biocrude yield, chemical energy recovery, capital reactor cost, and energy requirements. The optimum conditions for continuous HTL however need to be carefully adjusted as, for instance, operation at lower residence times also reduces the chemical energy recovery. The effect of this trade-off should become more apparent with the transition from lab-scale to pilot- and full-scale HTL systems.

The general steps anticipated in a modern hydrothermal liquefaction facility are summarized in Fig. 17.6(a) for lignocellulosic biomass. The processing and pumping of feed slurries of low particle size may require pretreatment for certain feedstock. For woody biomass this typically includes size reduction and alkaline treatment to obtain a pumpable and stable slurry.

The feedstock is then processed via HTL at temperatures of around 350°C and pressures of 180 bar for approximately 15 min. Phase separation occurs spontaneously after the reaction resulting in a gaseous phase rich in CO₂, solid residue, the biocrude, and an aqueous phase. The process water can be recycled reducing water requirements which can enhance biocrude yields. Waste process water can be treated anaerobically or treated via catalytic HTG or CHG to produce either a methane-rich or hydrogen-rich syngas (Elliott et al., 2013b; Cherad et al., 2015). Anaerobic digestion of HTL process waters has not been demonstrated experimentally and could prove difficult due to compounds in the water phase such as furfurals and phenols which are inhibitory to AD. There is potential for the recovery of N and P following treatment of the water by either AD or hydrothermal gasification. The produced biocrude requires upgrading via hydrotreating to produce final fuels or a refinery feedstock.

A conceptual scheme for the production of biofuel from microalgae by hydrothermal liquefaction is described in Fig. 17.6(b). A benefit of processing algae by HTL is the potential for recycling of nutrients back to cultivation.

Microalgae require no pretreatment due to their small particle size and are pumped more easily as a slurry. The processing of other biomass may require grinding or maceration, adding additional cost. Algae still require dewatering during harvesting to produce a slurry containing approximately 20% solids. Feedstock such as sewage sludge would operate similarly to algae but still require dewatering and thickening before processing. Some research has shown it is possible to recycle the process water after dilution to algal cultivation and the algae can utilize the organic carbon by heterotrophic growth however this has not been demonstrated at scale (Biller et al., 2012).

The general steps anticipated in a modern hydrothermal gasification facility are summarized in Fig. 17.7.

Careful control of the process conditions and feedstock can result in highly functionalized carbon materials (Libra et al., 2011). Higher temperatures or longer residence times result in higher energy densification although yields of hydrochar are typically lower. A higher biomass to water loading will result in higher yields of hydrochar and lower yields of soluble hydrocarbons in the water (Stemann et al., 2013). Water can be recycled resulting in improved energy densification and increase yields of hydrochar (Stemann et al., 2013). The acidic nature of the process water due to high levels of organic acids can increase the reaction rates of HTC and is an added benefit of water recirculation (Berge et al., 2011). Typical yields of hydrochar are listed in Table 17.2 for a variety of typical feedstocks.

During HTC the biomass undergoes energy densification to a hydrochar which resembles a “lignite-like coal” with an energy density of 28 MJ/kg (Libra et al., 2011; Hoekman et al., 2013). As the temperature increases, yields of hydrochar will decrease, resulting in higher energy densification (Berge et al., 2011; Hoekman et al., 2013). Typically, 60–90% of the HHV of the biomass or waste is maintained within the hydrochar, representing approximately 75–80% of the original carbon (Ramke et al., 2009). Generally with lignocellulosic biomass, a biomass with a heating value of 16–18 MJ/kg can produce a biocoal with an energy density between 28 and 30 MJ/kg. Higher ash feedstock such as sewage sludge, MSW, and digestate produce more moderate energy densification due to the higher levels of inorganic material. The change in O/C ratio can be seen in the Van Krevelen diagram shown in Fig. 17.9. This indicates that the hydrochar produced has an energy density similar to that of a lignite coal. The morphology of the hydrochar particles is dependent on the feedstock composition and contains interesting carbon nanostructures (Titirici and Antonietti, 2010). The outer shell of this particle is thought to be more hydrophilic whereas the inner core is thought to be hydrophobic, this hydrophobicity increases with reaction (He et al., 2013). The hydrophobicity of the hydrochar reduces its water content and is likely to improve its handling and storage properties.

In addition to the increased energy density, the hydrochar also has different ash behavior when heated. Ash high in alkali metals, chloride, and sulfur can result in problems with slagging and fouling during combustion. An additional benefit of HTC is the improved combustion behavior of the resulting hydrochar due to removal of many of the problematic inorganic materials. Fig. 17.10 shows typical levels of reductions in inorganic species in biomass. Alkali metals such as potassium and sodium are significantly reduced together with high levels of chloride, sulfur, and a portion of the nitrogen and phosphorus. The impact of this is to increase ash melting temperatures and reduce corrosion. This change in ash behavior during heating is measured using an ash fusion test. An example for miscanthus is shown in Fig. 17.11, HTC is shown to increase the ash transition temperatures resulting in reduced problems with slagging.

The HTC gaseous products are largely CO₂ although there are also small amounts of CO, CH₄, and H₂ present. Increasing reaction severity will result in higher levels of decarboxylation and a corresponding increase in CO₂.

Biocrude is more viscous that pyrolysis oil; however it is less dense. It contains a large number of different molecular compounds with a range of molecular weights (average MW ∼1000). The molecular composition is largely influenced by feedstock composition and operating conditions.

Several groups have investigated the composition of HTL biocrude from different feedstocks. Torri et al. described in detail the composition of biocrude derived from microalgae (Torri et al., 2011). Identified compounds from microalgae biocrude are attributed to their origin of either carbohydrate, protein, lipid, or algaenan. Lipids generally result in free fatty acids and alkanes, while carbohydrates form oxygenated compounds, phenols, alcohols, and cyclic ketones. Degradation products from protein include indoles, pyrroles, and pyrazines. The characterization of biocrudes is an area of ongoing investigation due to its complex composition and high molecular weight complicating its characterization. Although named biocrude, it is not a petroleum analogue due to the higher oxygen and nitrogen content in the oils. Particularly when processing feedstocks with a large protein content the nitrogen content is found to be undesirably high. This has been shown to be an issue in the liquefaction of microalgae due to their high protein content of up to 70%. This issue is not present for the liquefaction of lignocelluloses; however their liquefaction results in lower yields and higher viscosities. There have been several feedstocks under investigation as shown in Table 17.4 for the production of biocrude. Microalgae have been shown to result in the highest yields and HHV particularly when high-lipid-containing algae were utilized. The liquefaction of lignocelluloses generally results in lower yields but carbon efficiencies of >50% are still achieved.

Manures and sludges appear to be a promising feedstock for HTL with good energy recoveries but their investigation has been limited to date and not employed on continuous reaction systems. Continuous HTL has been performed on feedstocks such as corn stover, DDGS, forest residue, macroalgae, and microalgae. PNNL published a carbon balance on the liquefaction of lignocelluloses on their continuous bench scale systems. Overall carbon yield, including hydrotreatment of the biocrude product, was nearly 50% (Elliott et al., 2015).

Wet algae slurries have been converted into an upgradeable biocrude by HTL in a bench-scale continuous-flow system at PNNL (Elliott et al., 2013b). Direct biocrude recovery could be achieved without the use of a solvent and biomass trace components could be removed. High yields of biocrude (up to 82 wt% on a carbon basis) could be obtained with high slurry concentrations of up to 34 wt% of dry solids (Elliott et al., 2013b). Researchers at Aarhus University have processed DDGS in a continuous system for 24 h and were able to recover 60% carbon in the biocrude and a yield of 39 wt% (Mørup et al., 2015).

Continuous-flow HTL work by Jazrawi et al. (2013), Biller et al. (2015), and the batch work by Faeth et al. (2013) suggests that higher heating rates and lower residence times favor the production of biocrude. Fast heating rate batch systems have resulted in biocrude yields exceeding 60 wt% and energy recoveries of around 90% from microalgae. Fast heating rates have not been employed on other feedstocks to date but it is possible that similar effects can be seen. A drawback of the very fast heating and low residence times is a reduction in deoxygenation of the biocrude. Operating with longer residence times results in a reduced oxygen content and higher carbon content corresponding to an increased HHV. For example, in one study the change in reaction times from 10 to 90 min resulted in a reduction in oxygen content from 16% to 8% (Faeth et al., 2013). The quality of the biocrude is superior at the longer residence times and despite its lower yield, is more suitable for upgrading due to lower H₂ demand for hydrodeoxygenation. The increased HHV at longer residence time results in a higher chemical energy recovery of the biocrude when processing algae.

The continuous HTL studies by Elliott et al. (2013b) and Morup et al. (2015) differ to those by Jazrawi et al. (2013) and Biller et al. (2015) as they did not employ the use of solvents to recover the biocrude. This has two advantages: firstly it avoids the use of toxic solvents which incur cost and can be harmful and secondly it adds undesired compounds to the biocrude and can therefore diminish its quality. Xu and Savage (2014) demonstrated in a batch HTL study the effect of using DCM for the recovery of biocrude (Xu and Savage, 2014). It was shown that additional biocrude is recovered from the aqueous phase compared to using no solvents. This accounted to about 8% of the total biocrude. There were remarkable differences between the biocrude recovered from the aqueous phase to the biocrude which was recovered directly without DCM. The O and N contents were found to be twice as high, adding additional N and O to the biocrude when recovered using DCM from all product phases, thereby diminishing its quality.

There have been several papers investigating the use of lignocellulosic biomass feedstocks for HTG, for example Waldner and Vogel (2005) report a methane yield of 0.33 g of CH₄ per g of wood. Schumacher et al. (2011) investigated the supercritical water gasification of macroalgae at 500°C and produced 12 and 13 g of H₂/kg seaweed. More recently, Onwudili et al. (2013) investigated the catalytic SCWG of S. latissima producing 30 g of H₂/kg seaweed in the presence of sodium hydroxide as a catalyst and noting a doubling of methane yield in the presence of nickel catalyst to 112 g CH₄/kg seaweed (Onwudili et al., 2013). The main challenges associated with HTG of biomass slurries are the selection of suitable catalysts which are not deactivated as well as gas separation and scaling of reactor systems.

The types of sugars and polar compounds produced from HTC of woody biomass and microalgae are discussed by Hoekman et al. (2013) (Broch et al., 2013; Reza et al., 2014c). Lignocellulosic feedstocks produce higher levels of 5-HMF and furfural than other biomass. Levels of galactose, xylose, mannose, and fructose are highest at the lowest temperatures. As the temperature increases the levels of glucose and levoglucosan increase. The compounds in the process water can be classified as sugar-derived or lignin-derived. The main sugar derived components include furfural, 2-ethyl-5-methyl furan, and 2-hydroxy-3-methyl-2-cyclopentanon (Xiao et al., 2012). Furfural is a degradation product of cellulose. 5-HMF and furfural dominate in the process water at higher temperatures (Hoekman et al., 2013). High levels of volatile fatty acids (VFA) are produced such as butanoic acid, propanoic acid, acetic acid, and lactic acid. The level of VFA in the water reduces with increasing reaction time (Danso-Boateng et al., 2015). Lignin can depolymerize and undergo repolymerization resulting in the formation of phenolic fragments such as 2,6-dimethoxyl phenols, dimethoxy benzaldehyde, and esters.

Processing of feedstocks high in nitrogen, such as manures or algae, leads to increased nitrogen in the process water. Values of total nitrogen have been reported as 3000–8000 mg/L, mainly in the form of ammonium and nitrates. Organic nitrogen such as pyrroles and pyrrolidinones cyclic dipeptides, aromatic nitrogen compounds, amines, amides, and amino acids have been identified in the process water (Garcia Alba et al., 2013). The pH of process waters from lignocellulosic biomass range from 3 to 4, whereas with high protein feedstocks, such as microalgae, have been shown to have a higher pH. This is due to the effect of the N-compounds in the process water (Broch et al., 2013).

Table 17.6 lists some of the main chemicals identified in the process waters from the different hydrothermal routes and indicates the relative amounts under different conditions.

Algal biomass produces significantly higher levels of arabitol and mannose than lignocellulosic and reflects the different carbohydrate components in algae. The main acids produced from microalgae include malonic acid, lactic acid, succinic acid, and glutaric acid. The maximum amounts of sugars from microalgae were shown to be at 230°C, above which they degrade further and increase the levels of organic acids. The process water can also contain heavy metals and alkali salts (Reza et al., 2013; Smith et al., 2016).

Several approaches have been used to tackle the problem of the wastewater byproduct. For example in the case of microalgae HTL, PNNL applied catalytic hydrothermal gasification (CHG) effectively for HTL process water clean-up and fuel gas production from water-soluble organics, allowing the water to be considered for recycling of nutrients to the algae growth ponds (Elliott et al., 2013a). In the case of the process water from macroalgae HTL, CHG conversion of 99.2% of the carbon in the aqueous phase was achieved (Elliott et al., 2013b).

Anaerobic digestion of the aqueous phase from hydrothermal processing has been suggested in various reports; however experimental work has only been successfully carried out on the process water from HTC. Further research is required to evaluate the potential for HTL process water by AD and the inhibitory effect on methanogenesis. Wirth and Mumme (2014) have demonstrated that the process water can be treated using AD. Typical values of TOC are in the range 10–20 g/L and Erlach and Wirth have shown the biological methane potential (BMP) to be in the region of 0.5–1 L methane per gram of TOC (Erlach et al., 2011). BMP has been estimated from the Buswell equation for a range of feedstock such as sewage sludge and digestate (Danso-Boateng et al., 2015).

The potential for recycling nutrients from the HTL and HTG process water for algae cultivation has been attempted on a laboratory scale (Biller et al., 2012; Garcia Alba et al., 2013; Jena et al., 2011). The recycling of nutrients is regarded to be essential in microalgae biofuel systems since the energy, cost, and carbon emissions associated with nutrient production are very high. In most cases, it was necessary to significantly dilute the process waters to avoid growth inhibition from components such as phenols and metals such as nickel (Biller et al., 2012). Another study investigated the use of the HTL aqueous byproduct as a substrate for microbial growth under heterotrophic rather than phototrophic conditions. The microbial strains were shown to have a higher tolerance to the aqueous phase, leading to significantly reduced dilution requirements compared to microalgae (Nelson et al., 2013).

Contrary to HTC, where only limited reports on continuous processing are available, researchers have published increasing number of reports on the continuous operation of HTL. The first continuous HTL facilities were constructed in the 1970s and 1980s during the development of the HTU process, the PERC Albany Research Facility and the Lawrence Berkeley Laboratory Process with significant throughputs of 10–18 kg/h (dry solids) (Zhu et al., 2014). Table 17.8 presents a summary of continuous reactor systems in the recent literature; it can be seen that the capacities are still relatively low. The development of continuous HTL reactors is still in the early stages and further research needs to be performed to evaluate the best reactor designs, construction materials, and fluid dynamics. Elliott et al. (2015) describe the development of continuous HTL systems in their review in detail. To advance the technical maturity of HTL, PNNL identified specific challenges including reducing the risk of large-scale pumpability, reducing capital costs by moving away from a continuous stirred-tank reactor configuration to a scalable plug-flow reactor configuration, and understanding appropriate materials of construction for process design. A further study by PNNL on continuous HTL (Zhu et al., 2014) on forest residue and corn stover feedstocks demonstrated that nothing other than size reduction (formatting) was required. The process was configured in a PFR design which maintained a small CSTR as a preheater. Within the limits of the corrosion assessment testing, the study showed that stainless steels were suitable for HTL applications.

With funds from the Australian National Collaborative Research Infrastructure Strategy (NCRIS) a continuous flow pilot-scale hydrothermal processing unit became operational at the University of Sydney in 2012. The design of the unit is based on coiled stainless steel tubes submerged into a fluidized sand bath in a plug flow reactor type set-up.

Some noticeable outcomes of the study include: (1) more severe reaction conditions led to the highest yields, lowest oxygen but increased nitrogen contents in the biocrude; (2) higher solids loadings increase biocrude yields, as shown in PNNL studies, and reduce carbon losses within the system; (3) the “inverse scaling” effect of pumping and pressure control led to the conclusion that scaling up of the Sydney design should result in better controllability and reduced potential of formation of agglomerates and deposits, which can lead to blockages.

Researchers at Aarhus University investigated the commissioning of a PFR processing DDGS over 24 h using an induction heater with heating rates of 17°C/min. It was found that the mean residence time varied from the expected value and the system can therefore not be considered a plug flow type reactor but merely a tubular type reactor (Mørup et al., 2015).

Industrial installations of the HTL technology exist but currently limited information of capacities, reactor design, and operating conditions are available. Sapphire Energy Inc. (USA) and Muradel Ply Ltd. (Australia) are examples of companies working on liquid fuel production using HTL from microalgae and Steeper Energy (Demark) are investigating the use of lignocellulosic biomass. Muradel currently has a 3 t/day (30 wt% solids) continuous HTL facility in operation and is planning to increase the capacity to 6.5 t/day and a total output of 50 million L/annum by 2020, while Sapphire Energy is planning on constructing a plant in 2018 to produce ∼200 million L biocrude per annum (Energy, 2013).

There are only limited reports on hydrothermal gasification in larger than laboratory scale. At PNNL bench-scale systems were developed as early as 1989 (Elliott et al., 1989). This work was used to design a scaled-up mobile reactor system, designed for up to 0.5 t of wet feed per day (Elliott et al., 1994). PNNL demonstrated successfully the continuous gasification of biomass to methane-rich product gas. Initially problems occurred with plugging of the catalyst bed by biomass which was solved by a two-step process where biomass was liquefied in a CSTR before HTG. The design temperature of the mobile unit mounted on the back of a pick-up truck was 350°C (Duan and Savage, 2010c). The demonstration unit worked well, but some poisoning of the catalyst was observed.

A continuous HTG reactor has also been developed at the Paul Scherrer Institute (see Fig. 17.7 and references therein). This design aims to remove salts, particularly sulfates prior to the reactor bed. Sulfur has been shown to poison the active catalyst sites and this is one of the main issues of HTG.

At the Karlsruhe Institute of Technology the largest plant for hydrothermal biomass gasification was installed with a throughput of 100 kg/h slurry (∼20 wt% solids). After some “teething troubles” during the beginning of operation (plugging, problems concerning feeding in the biomass), various biomass feedstocks, eg, corn silage, were successfully converted to gas. The VERENA test facility is designed for a maximum of about 700°C and 35 MPa (Kruse, 2009).

Currently the development of HTG reactors is less advanced than for the HTC and HTL pathways and requires expanded process development to take the technology to a scale of industrial demonstration.