Introduction

One of the economic and energetic drawbacks in the processing of microalgae is the dewatering stage as microalgae typically only grow to a solids concentration of 1-5 g/l [8]. This makes concentration and drying challenging and energy intensive. Microalgae biofuel is most commonly produced by the extraction of lipids and subsequent transesterification to bio-diesel. Most common lipid extraction techniques require a dry biomass feedstock before transesterification as does conversion to thermal energy or syngas by combustion or gasification. This can account for as much as 25 % of the energy contained in the algae [29]. Hydrothermal processing avoids this step as the algae is processed as a slurry in hot compressed water and does not require drying. Operating conditions vary depending on the desired product. At low temperatures <200°C, the process is referred to as hydrothermal carbonisation (HTC) and predominantly produces a char. At intermediate temperatures ~200-375°C the process is referred to as hydrothermal liquefaction (HTL) and predominantly produces an oil. At high temperatures >375°C hydrothermal gasification (HTG) reactions occur, predominantly producing a syngas. The aim of these hydrothermal processing routes is to generate a product with higher energy density than the feedstock by removal of oxygen.

The char produced from HTC can be co-fired with coal or used as a biochar for soil amendment [30], the bio-crude from HTL can be upgraded to a variety of fuels and chemicals while the syngas from HTG can be used for combustion or converted to hydrocarbons by either biological or catalytic processing e.g. Fisher Tropsch synthesis.

11 Hydrothermal processing essentially simulates the natural processes which have taken place in nature in the production of fossil fuel reserves. All fossil fuel reserves have been created by the transformation of organic matter under pressure and heat over long periods of time. Coal is mainly formed from terrestrial plants while oil and gas is mainly the product of decaying phytoplankton and zooplankton. The process of applying high pressures and temperatures to organic matter in modern hydrothermal processing is therefore a way of speeding up nature‟s natural pathways to form a renewable fossil fuel. Just as in nature, the state and quality of the resulting fossil reserve is dependent on the severity of the environmental conditions. The products of hydrothermal processing are consequently often referred to as green coal, bio-coal, bio-crude and syngas.

Research has been carried out in all of these synthetic hydrothermal pathways and is discussed in the following sections.

Apart from the above mentioned hydrothermal processes, there are some additional wet processing methods which have been used for algal biomass as it is realized that wet extraction techniques offer a distinct energy requirement advantage. Levine et al. for example proposed the in situ lipid hydrolysis of wet algal biomass followed by supercritical transesterification with ethanol [31].

Alternatively Patil et al. have suggested the wet transesterification to fatty acid methyl esters in supercritical methanol [32]. There have also been limited studies on the co-liquefaction of algal biomass with coal or organic solvents to improve the yields and quality of bio-crude [33-34].

However this review will focus on the hydrothermal routes.

Water as a reaction medium has several advantages over chemicals as it is ecologically safe, cheap and readily available. When water is heated and compressed, the hydrogen bonds are weakened resulting in a change in dielectric content, acidity and polarity, each of which can increase opportunities for water to take part in reactions. This leads to water acting as a catalyst, lowering activation energies and allowing reaction pathways which would not occur at ambient conditions.

The critical point of water is at 374°C and 22.1 MPa, below this point the vapour pressure curve separates the liquid from the gaseous phase. Approaching the critical point, the density of the two phases become more and more alike and finally identical at the critical point [35]. 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 are placed determines if HTC, HTL or HTG reaction conditions are met as can be seen in Figure 1.1.

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Figure 1.1: Hydrothermal processing conditions in the water phase diagram; data from Perry's Chemical Engineers' Handbook [1]

During the carbonisation stage, the carbon concentration of the biomass is increased and the oxygen and mineral matter content are reduced, the gaseous product is low and a biochar is produced by carbonisation reactions. During liquefaction, biomass feedstocks are decomposed to smaller molecules which are reactive and can re-polymerize into oily compounds [36]. The main reaction steps during liquefaction have been summarised by Garcia Alba et al. as follows [37];

1. Hydrolysis of macromolecules (lipids, proteins and carbohydrates) into smaller fragments;

2. Conversion of these fragments by, for example, dehydration into smaller compounds;

3. Rearrangement via condensation, cyclisation, and polymerization producing new larger, hydrophobic macro-molecules.

The products from hydrothermal liquefaction consist of a bio-crude fraction, a water fraction containing some polar organic compounds, a gaseous fraction and a solid residue fraction. At the

13 conditions, consists of varying amounts of H2, CO, CO2, CH4 and light hydrocarbons. The initial reaction steps are the same as during liquefaction but the more severe conditions lead to the small fragments decomposing even further to low molecular weight gaseous compounds. At high temperatures >500°C H2 production is favoured while CH4 production is favoured at 350 to 500°C although all these conversion pathways can be influenced with the use of catalysts and pressure [38].

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 [39]. Just below the critical point, the miscibility for hydrocarbons is improved as the dielectric constant is in the range of 10, which would be equivalent to that of dichloromethane, decreasing further in the supercritical region. The change in dielectric constant can be seen in Figure 1.2 over a range of temperatures at 30 MPa pressure. It can be seen that even at HTC conditions of around 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 adjusted by means of temperature and pressure as this affects the dielectric constant which influences the activation energy of reactions. Above the supercritical point of water, the miscibility for hydrocarbons and gases is very high making it a good reaction medium for organics and gases. Below the supercritical point, miscibility of these compounds is not complete but increased compared to ambient conditions. When the process products cool back down to ambient conditions, the water and the organic compounds will separate as they are not soluble anymore; this makes distillation or other costly separation techniques unnecessary.

Figure 1.2: Density [40], static dielectric constant [41] at 30 MPa and ionic product [42] of water at 25 MPa

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At high temperature and pressure below the supercritical point, the ionic product is up to three orders of magnitude higher than under ambient conditions and is plotted on a logarithmic scale in Figure 1.2. The high ionic product supports acid or base catalysed reactions and can act as an acid/base catalyst precursor because of the relative high concentrations of H3O⁺ and OH^- ions from the self-dissociation of water [39]. 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 which is therefore the optimum temperature for acid/base catalysed reactions. Above 350°C the ionic product decreases rapidly by 5 orders of magnitude or more above 500°C [43]. The density of water at 30 Mpa over the hydrothermal temperature is plotted in Figure 1.2 and it can be seen 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; however 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 [35].

Some of the earliest work on hydrothermal processing was carried out at the Pittsburgh Energy Research Centre in the 1970s-1980s. Their process of biomass liquefaction was based on technology used for lignite coal liquefaction. Some early research into HTL of biomass was performed at the Royal Institute of Technology, Stockholm, the University of Arizona and the University of Toronto. All the early research investigated terrestrial biomass only and algal biomass was first investigated in the „90s. Hydrothermal gasification of biomass is first reported in the USA, at the Massachusetts Institute of Technology. More information about the history of the process development can be found in D.C. Elliott‟s recent chapter of Robert C. Brown‟s book

“Thermochemical Processing of Biomass” [44].

Hydrothermal processing of lignocellulosic biomass has received much more attention than algal biomass and has been extensively reviewed by Peterson et al. and Toor et al. [35, 45]. Terrestrial biomass consists mainly of cellulose, hemi-cellulose and lignin while algae consist of varying amounts of protein, starch like carbohydrates and lipids. If the algae is from a marine origin it typically contains large amounts of ash as salts and other mineral matter, some strains can have ash contents as high as 60 wt.% on a dry basis. Macroalgae are typically higher in carbohydrate and much lower in lipid content compared to microalgae, and if from a marine environment can be very high in ash content [46]. This can affect hydrothermal reactions and presents a significant difference to terrestrial biomass which is typically low in ash content. The different biochemical components undergo different reaction pathways in hydrothermal media and have previously been reviewed [35, 45].

15 Figure 1.3 describes an idealized closed loop HT concept with integrated nutrient recycling for algae cultivation. Figure 1.3 depicts a photo-bio reactor for microalgae cultivation where nutrients, water, light and CO2 are the only required inputs. A similar concept could be described for open pond cultivation or for macroalgae where the cultivation layout could include growth in marine environments. It is important to note that whilst the algal biomass is processed wet in hydrothermal processing, some dewatering is still required. Low cost dewatering is more challenging for microalgae than macroalgae but many processes are available such as flocculation. Algae is grown, harvested and dewatered to produce a slurry with a higher solid content. Subsequently the slurry is processed in hot compressed water to produce the desired primary energy product. The product phases include a gaseous fraction, process water, solid residue and a bio-crude. The amounts of each fraction depend on both the temperature and pressure of operation and the amount of biomass in the slurry. The solids:water ratio of the feedstock can be altered to the specific requirements and capabilities of the processing facility. When the solids content of the slurry is higher, the amount of water per mass of algae to be heated is less; reducing the energy requirements but at the same time more energy is spent in dewatering. One advantage of hydrothermal processing of algal biomass is that nutrients such as nitrogen, which are concentrated in the process water, can potentially be recycled [47-48]. The amount of nutrients in the process water varies with hydrothermal conditions and feed composition. In HTC and HTL, the gaseousfraction contains predominantly CO2 and can potentially be recycled to the cultivation step. In HTG, the syngas could be converted to a liquid fuel by Fischer-Tropsch after separation of CO2 and clean up or burnt in a gas turbine directly. The bio-crude produced by HTL can be further upgraded to produce fuels and chemicals. The solid residue, which still contains some nitrogen and minerals depending upon processing conditions, may be used as a fertilizer, as a fuel or as a biochar. Most of the current research and development into hydrothermal processing of algal biomass fits somewhere into this idealized concept. The technology is still very much in the research stages, most of which are still based on a laboratory scale and there are still many uncertainties and challenges associated with each of these process options. The following sections summarise the current state of research on the hydrothermal processing of micro and macro algae. Process conditions, feedstock, products, energy balance considerations and nutrient recycling possibilities are all discussed.

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Figure 1.3: Schematics of an integrated HT process with nutrient and CO2 recycle