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Supplier Information Pack Algal processing systems School of Engineering 15 Mar 2012 Contents EXTRACTION PROCESSES.......................................................................................... 4 SOLVENT (HEXANE) EXTRACTION ......................................................................... 4 SELECTIVE EXTRACTION .......................................................................................... 5 SUPERCRITICAL FLUID EXTRACTION .................................................................... 5 CONVERSION PROCESSES ......................................................................................... 5 THERMOCHEMICAL CONVERSIONS ....................................................................... 6 BIOCHEMICAL CONVERSIONS ................................................................................. 6 GASIFICATION .............................................................................................................. 6 PYROLYSIS .................................................................................................................... 8 THERMOCHEMICAL LIQUEFACTION ...................................................................... 9 DIRECT COMBUSTION .............................................................................................. 10 BIOCHEMICAL CONVERSIONS ............................................................................... 11 CHEMICAL TRANS-ESTERIFICATION .................................................................... 11 ANAEROBIC DIGESTION (AD) ................................................................................. 13 Note This document is intended for the use of organisations that are interested in supplying components and services for the algal industry. It is presented for information and discussion purposes only and represents a snapshot of the industry at the time of production and has been gathered from a number of publicly available information sources. Cranfield accepts no liability for any activity arising out of any inaccuracy in this publicly available material. EXTRACTION PROCESSES Summary: Basically, extraction follows cell disruption to actualise effective fractionation by recovering all targeted components of the microalgae at the maximum scale. Solvent extraction is an established method used to extract metabolites such as astaxanthin, βeta-carotene and fatty acids (Molina Grima et al., 2003). Types: Hexane extractions, selective extraction, subcritical water extractions, supercritical fluid (carbon dioxide) extraction, and free fatty acid extraction. Solvent (Hexane) extraction Summary Hexane extraction, for example, is an oil-specific extraction process without in the first place having the services of cell disruption (Greenwell et al., 2010). However, an ideal lipid extraction process for microalgae fuels production needs to be selective towards desirable products (neutral lipids) to minimize co-extraction of non-lipid contaminants (Halim et al., 2011). Crude extracts are purified using different types of chromatographic techniques to isolate the desired metabolites. Supercritical fluid chromatography and high pressure liquid and gas chromatographic techniques are employed to recover astaxanthin, carotene and poly-unsaturated fatty acids. Reverse phase chromatography, silica gel adsorption chromatography, argentated silica gel chromatography and ion-exchange chromatography, have all been used for recovering pure fatty acids and proteins respectively (http://www.algaeindustrymagazine.com; Molina Grima et al., 2003) Valensa International, a biofuel company based in Eustis, Florida, USA has a high quality organic extraction facility that extracts astaxanthin without chemicals. The entire extraction process is a computer controlled process (temperature, pressure and flows) employing high pressure liquid chromatography and gas chromatography to extract astaxanthin and other products http://www.algae industrymagazine.com/astaxanthin- extraction. Materials: Hexane, ethanol, chloroform, diethyl ether, Throughput: Microalgal lipid extraction with solvents has not been tried on a large scale despite routine analysis on a laboratory scale. This may stem from the fact that variables affecting lipid extractions are not well understood and most lipid composition assessment processes have been focussing on other products such as nutraceuticals (Frank et al., 2011; Halim et al., 2011). However, Stephenson et al. (2010), simulated a solvent recovery using HYSYS software which determined that a 1:2 solvent:biomass volumetric ratio would recover 99% of the algal triacylglycerides. Other authors modelled extractions by solar drying and considered a regional facility that would operate on a larger scale to take care of the operating cost (Lundquist et al., 2009). Soxhlet hexane extraction was found by Halim et al. (2010) to be less effective than supercritical carbon dioxide extraction, achieving a comparable lipid yield of ~ 0.058 g lipid extract/g dried microalgae in 5.6 x the required time. Cheng et al. (2011), moreover, compared lipid extractions from microalgae by organic solvent and supercritical CO2. The extraction yield obtained by using the Super-critical Fluid Extraction (SFE) method combined with bead-beating was 98.7%. The analysis concluded that SFE is effective and has a higher selectivity for triglycerides extraction. Selective extraction Selective extraction is explained by Hejazi et al. (2002) who used a two-phase system, i.e. aqueous and organic phases, for the selective extraction of carotenoids from micro algal biomass Dunaliella salina. The procedure takes advantage of two biocompatible solvents with low and high hydrophobicity. The method is envisaged for the selective extraction of specific lipids (triglycerides) suitable for biofuels (U.S. DOE, 2010). Supercritical fluid extraction Valensa International uses ultra-high CO2 pressure in the extracting process where ultra- high CO2 pressure (600 bars) is pumped through extraction vessels to extract astaxanthin. Use of CO2 is promoted because of its critical properties (i.e. critical temperature of 31.1°C and pressure 73.9 bars), low toxicity, and chemical inertness, and its preference to alternatives such as ethane, methanol, water etc. The most significant part in this kind of extraction is the product separation from the solvent (CO2). In the Valensa experience, after the reaction is completed, the solvent and the product is pumped into what is called the ‘blow down vessel’, where pressure is reduced in stages and then CO2 boils up as CO2 gas, the oil containing astaxanthin goes down where it is collected and the gas goes up, is recycled, collected, and compressed (http://www.algaeindustry.com). However, US. DOE., (2010), report that the process is energy intensive and scaling up the process is mainly for analytical purposes. Cost range: Generally, energy consumption and water recycling elevate cost. Benchmark: extraction process consumes not less than 10% of the total energy load (U.S. DOE, 2010). Energy balance: Xu et al. (2011) evaluate the energy balance of dry and wet routes of the micro algal biofuel concept by coupling a nearby power plant to the process. The analysis indicates that the energy can be improved for oil extraction in the wet route and indicating the possibility of producing higher value products. End product: TAG, biodiesel, hydrocarbons, higher value products. Issues: Choices of upstream and downstream operations; presence of water, increasing temperature and pressure, cell disruption; separation of desired extracts; energy consumption and water cycle (U.S. DOE, 2010). Areas for development: Increasing temperature and pressure assists extraction and cell disruptions regulate temperature and pressure requirements. CONVERSION PROCESSES Summary: Algal biomass can be converted to energy mainly by two main type of chemical and physical processes: thermochemical and biochemical conversions. Thermochemical Conversions: Gasification, pyrolysis, thermo-chemical liquefaction and direct combustion. Biochemical Conversions: Trans-esterification, anaerobic digestion, alcoholic fermentation, and photo biological hydrogen production. Thermochemical conversions: This entails the thermal decomposition of organic components in biomass to produce fuel products. When biomass is heated under oxygen-deficient conditions, it produces gas or syngas (hydrogen or carbon monoxide) that can be directly burned or further processed into other products (gaseous or liquid). Gasification Summary Gasification is the partial oxidation of algal biomass into combustible gas mixtures (CO, CO2, H2, CH4, C2H4, + impurities of N2, S, alkali compounds and tars) either known as producer gas or synthesis gas (syngas) under a controlled amount of air and high temperatures (800-1000°C and 600-1400°C) depending on the relative compositions of the component gases. Syngas can be produced from biomass via two routes; 1) Catalytic with temperature requirements of 900°C; and 2) Non-catalytic which requires a very high temperature of 1300°C. Syngas can be used for the generation of heat and power as well as for the production of chemicals and transportation fuels (Brennan and Owende, 2010; Damartzis and Zabaniotou, 2011; Douglas et al., 2011; Naik et al., 2010). Table 2 Typical combustion and gasification products. Liquid Char Gas Fast pyrolysis 75% 12% 13% Carbonisation 30% 35% 35% Gasification 5% 10% 85% Efficiency: Gasification can produce syngas from a wide variety of potential feed stocks. Products’ heating values depend on oxygen, air, or mixtures of these, as the gasifying agents. Air gasification produces a low heating value product (4-7) MJ/N m; gasification with oxygen or steam leads to a product with medium heating value (10-14 MJ/N m), while high heating value product streams are obtained if steam is used (steam reforming) (Damartzis and Zabaniotou, 2011). Syngas is a low calorific gas (typical 4-6 MJ m-3) which can be used to produce a range of transportation fuels and chemical intermediates. For fuels, hydrogen by water-gas- shift (WGS) reaction is the main syngas derived route to fuels. The WGS reaction uses CO and H2O to give H2 and O2 and it can be used to upgrade producer gas to syngas by enriching the H2 content, or to produce hydrogen (Naik et al., 2010). For diesel range hydrocarbon productions, Fisher-Tropsch (F-T) syntheses with a CO or Fe catalyst, or dimethyl ether (DME), or gasoline range hydrocarbons via methanol synthesis over a Cu based catalyst have been used followed by sets of reactions (selective-hydro- treatment processes, CO hydrogenation and products separation) to produce hydrocarbons or oxygenated fuels, light fuels, Synthetic Paraffinic Kerosene (SPK) and diesel (Damartzis and Zabaniotou, 2011; Dmitri and Julian 2001; Naik et al., 2010). Process: Gasification processes operate at temperatures of 800-900°C or even higher, as cited in numerous literatures. The operation, notwithstanding the types and configurations, consists of four steps, viz: drying (stripping off moisture from the feedstock), pyrolyis/devolatization, reduction and combustion. In the pyrolysis, volatile substances are removed in the form of light hydrocarbons; CO and CO2 and also liquid- long chain hydrocarbons known as tar are obtained. The reduction zones underline the main process of gasification where the raw materials are transformed into syngas through a series of endothermic reactions using oxygen from the air and/or steam. The residual char matrix is further exposed to higher temperatures in the combustion zone, producing more volatile (gaseous) products and the necessary heat required in the reduction zone. Fluidized bed, fixed bed and entrained flow reactors are the three main types of reactor that are utilized for these transformations (Damartzis and Zabaniotou, 2011). Throughput: Hirano et al. (1998) gasified Spirulina at temperature ranges from 850- 1000°C to determine the gas composition required to generate a theoretical yield of biomass, and found that algal biomass gasification at 1000°C produced the highest theoretical yield of 0.64g methanol from 1g of biomass. Gasification has many possibilities to produce a wide range of fuels with acceptable properties. Due to its flexibility, it can be used to create various products and can also be integrated into existing thermochemical infrastructures. Through the same process, capital investment reduction is possible through feeding the algae into coal gasification plant. Energy balance: A positive energy balance ratio of 1:1 was obtained by Hirano et al. (1998) on gasification of Spirulina to produce methanol. They attributed a low value to the use of an energy intensive centrifuge process during biomass harvesting. Minowa and Sawayama (1999) partially oxidised the microalgae C. Vulgaris in a novel system for energy generation with the entire nitrogen component converted into fertilizer quality ammonia. Cost range: Capital investment reduction is very possible if algae are fed into existing coal gasification plant. Since Fischer Tropsch Synthesis (FTS) is an exothermic process, heat integration is also possible, especially for drying algae during harvesting/dewatering to significantly cut the cost of production. End-product: Bio-syngas, hydrogen, liquid hydrogen fuels (via FTS) and higher alcohol synthesis (methanol, ethanol etc.) Issues: Cost of clean-up and tar reforming – a remedy has been devised that includes entrained-flow gasification at higher temperatures. Areas for Development: Not enough data and very few life cycle assessments. However, the gasification process is very versatile and flexible, since bio-syngas can be used to produce a number of products. Secondly it can integrate algal feedstock into existing thermochemical infrastructure (coal gasification plant), coupled with the advantage of the excess heat integration of the FTS exothermic process for drying algae during the harvesting and dewatering operation (US. DOE., 2010). Pyrolysis Summary Pyrolysis is a thermal process that heats biomass at moderately high temperatures 350°C-700°C in the absence of air, to produce bio-oil (liquid), charcoal (solid) and fuel gaseous products. Using pyrolysis to convert biomass into liquid has received a wide range of coverage portraying promising outcomes for large scale production that could replace petroleum based fuels (Dermibas, 2006). Basically, the main objective of pyrolysis is the final recovery of a biofuel with medium-low calorific power. To produce different products using pyrolysis, different levels of heat are used to break down biomass. As such, and depending on the operating conditions, pyrolysis can be subdivided into three classes. Types: Flash pyrolysis, fast pyrolysis and slow pyrolysis. Flash pyrolysis: This is a type of pyrolysis that heats biomass at a moderate temperature (500°C) The process is likely to be successful for future replacement of fossil fuels with biomass derived liquid fuels (Brennan and Owende, 2010). This is because a lower temperature over a longer period of time maximizes the production of high-value solid char and high temperatures produce volatile gases. Within the process, bio-oil production from biomass pyrolysis is typically produced through flash pyrolysis. The produced oil can be mixed with the char to obtained a bio-slurry that can be converted (conversion efficiency: 70%) to syngas via gasification (gasifier condition: 26 bars, and 927-1227K).The produced bio-crude can be utilized in engines and turbines. Flash pyrolysis has a fast heating rate (>1,000K/s), short residence time (0.5-1s) and very fine particles (<0.2mm). The percentage yield of liquid products is generally favoured by short residence time, fast heating rates and moderate temperatures (Huber et al., 2006; Naik et al., 2010). Fast pyrolysis: Fast pyrolysis is a thermal process that rapidly heats biomass to a carefully controlled temperature (~ 500°C), then very quickly cools the volatile products (-2s) formed in the reactor. Fast pyrolysis has a high temperature range, with fast heating rate (10-200K/s), short residence time (0.5-10s) coupled with fine particles (<1 mm) (Naik et al., 2010). The process is quite promising for the production of liquid and/or gaseous products. Initial biomass decomposition produces vapours, aerosols and charc (from which upon condensation and cooling a dark brown liquid is formed. Past pyrolysis produces 60-75% of bio-oil, 15-25% solid char, and 10-20% non-condensed gases, depending upon feedstock (Brennan and Owende, 2010; Naik et al., 2010). Slow pyrolysis: This is a conventional type of pyrolysis that uses lower temperatures (550-950K) over a longer time to maximize the production of a high-carbon solid char while high temperatures produce mostly volatile gases. Conventional pyrolysis occurs under slow heating rate (0.1-1K/s) and residence time is 45-550s (Naik et al., 2010). Table 3 Comparison of operating parameters and expected yields for pyrolysis processes Mode Conditions Liquid% Char% Gas% Flash pyrolysis Moderate temperature (500°C) short hot 75 2 13 vapour residence time (about 1s) Fast pyrolysis Moderate temperature (500°C), moderate hot 50 20 30 vapour residence time (about 10-20s) Slow pyrolysis Low temperature (400°C), very long solids 30 35 35 residence time (Brennan and Owende, 2010) Characteristics Efficiency: The key to maximizing production of high-carbon solid char is heat transfer, as mentioned by Dr Peter Fransham, President of Advanced Biorefinery Inc. in Ottawa, Ontario, when he gave a tutorial on the basics of pyrolysis. Slow pyrolysis transfers heat at a rate of 1,000°C per minute, and fast pyrolysis increases the rate to 1,000°C per second (Renewable fuels R&D Biomass magazine September, 2008 page 52). Throughput: Pyrolysis can result in different kinds of liquids, depending on the reaction condition. The bio-oil produced tends to favour shorter residence time, fast heating values and moderate temperatures with an added advantage of compatibility with existing refinery streams, coupled with some upgrading and purification processes (hydro-treating and hydrocracking) for generating standard diesel fuel. Energy balance: Very little information on energy balance. Cost range: No detailed cost benefit analysis is found in the literature at present. Apparently, and due to inherent dehydration processes consequent from the stumbling blocks of moisture content, it will definitely not be cost-competitive in the short-term. The cost implication could be reduced if inexpensive and improved dewatering and extraction processes are also put in place. End product: Bio-oil from pyrolysis, solid char and non-condensed oil. Transportation fuels – liquid or gas and bio-chemicals. Issues: Moisture content is the stumbling block for pyrolysis; significant dehydration process must be performed upstream for process to work. There is significant knowledge gap about the specifications for converting algal bio-oil such as residence time, temperature to produce different algal bio-oils and the resulting characteristics of pyrolysis oil. (US. DOE., 2010). Areas for development: 1) Conversion specification and the specification for the resulting products; 2) Development of optimal residence time, and optimum temperature needed to transform different algal feedstock into bio-oil; 3) Transportation issues: stabilisation of some physical parameters (viscosity) of the produced bio-oil. Thermochemical liquefaction Summary: Thermochemical liquefaction is a technology employed to transform wet algal biomass into a range of liquid fuels. The process proceeds with a low temperature requirement (300-350°C) and high pressure (5-20MPa) aided by a catalyst in the presence of hydrogen to produce bio-oil (Brennan and Owende, 2010). Characteristics Process: The subcritical water (water held in a liquid state above 100°C by pressure application) breaks down the biomass material down to shorter and smaller molecules with a higher energy density. The process approximates the natural geological processes in the formation of petroleum-based fuels over some timescales (U.S. DOE, 2010). Efficiency: The efficiency of the process was claimed by Gourdiaan et al., (2000) to be as high as 75%. Throughput: Several studies have been conducted on thermochemical liquefaction on biomass and specifically on microalgal biomass (Brennan and Owende, 2010). Minowa et al. (2005) have reported the liquefaction of Dunaliella tertiolecta with a moisture content of 78.4wt% producing an oil yield of 37% organic basis at 300°C and 10MPa. The reaction temperature was 340°with holding time of 60min, oil viscosity of 150-330mPas coupled with calorific value of 36kJ/g-1 comparable to fuel oil. A maximum yield of 64% was reported by Minowa et al. (1995) using Botryococcus braunii by thermochemical liquefaction at 300°C catalysed by sodium carbonate. Now that liquefaction of algae is becoming popular, liquefaction techniques have been found to be more effective for the extraction of microalgal biodiesel than using the supercritical carbon dioxide (Aresta et al., 2005). Energy balance: Dote et al. (1994) achieved an energy balance for the process output/input of 6.67:1 by using thermochemical liquefaction at 300°C on B. braunii to maximize a yield of 64% dry wt. basis with higher heating value HHV of 45.9MJ/kg-1. Moreover, Minowa et al. (1995) obtained a positive energy balance of 2.94:1 on testing the viability of thermochemical liquefaction for the conversion of Dunaliella tertiolecta with an HHV of 34.9MJ/kg-1 and obtained an oil yield of 42% dry wt. Cost range: No detailed cost range was found in the literature. End product: Bio-crude, liquid fuels and heavy oil. Areas for development: Liquefaction of algae is becoming popular and is a promising technology but more information is needed about hydrothermal liquefaction of algae before it can mature to a commercially viable stage. DIRECT COMBUSTION Summary: Combustion is a chemical reaction between a fuel and oxygen in which biomass is burnt in the presence of air to convert the stored chemical energy in the biomass into hot gases, usually in a furnace, boiler or steam turbine at temperatures above 800°C (Goyal, 2008). Characteristics Efficiency: The combustion process can be feasible for biomass with <50% dry weight (McKendry, 2002). The conversion efficiency of large biomass-to-energy plants can be affected by the high moisture content of biomass. As such, heat integrated with power generation can thus improve the overall efficiency. Throughput: The products of combustion are carbon dioxide and water with release of heat. For the heat produced to be immediately used as storage is not a viable option. Direct combustion systems are often fixed bed or fluidized bed. The fluidized beds’ positive aspects in comparison to those of the fixed bed may include flexibility to changes in biomass fuel properties, sizes and shapes, fuel moisture content up to 60%, effective handling of agricultural residues and high-ash fuels coupled with low NOx emissions. Energy balance: Net energy conversion efficiencies for biomass combustion can be attained when co-combusted in coal fired plants or larger systems (>100 MW). But Kadam (2002) reports that a Life Cycle Assessment (LCA) of coal-algae co-firing could lead to lower greenhouse gas (GHG) emissions and less air pollution. Cost range: Cost implication varies widely depending on algal species and harvesting technology. End product: Hot gases, steam, heat and electricity. Issues: Air and sulphur pollution, and formation of particulate matter, and their consequent control. Areas for development: More research on the direct combustion of algal biomass is needed to determine its viability. BIOCHEMICAL CONVERSIONS CHEMICAL TRANS-ESTERIFICATION Summary: Chemical trans-esterification is a conversion technology that is employed to convert triacylglycerols extracted from algae biomass to fatty acid methyl esters (FAMEs), which is simply a displacement of an alcohol from an ester by another alcohol under the influences of super critical fluids, enzymatic, acid-catalysed or alkali- catalysed (U.S. DOE, 2010; Xuan et al., 2009). Following a trans-esterification reaction, the two phases (glycerine and esters) that are produced are separated by evaporation, decanting and centrifuging. Processing routes: The routes use different heating systems and catalysts such as acid-catalysed (H2SO4, HCl or H3PO4) and base-catalysed (methanol, ethanol and sodium ethanolate). Heating systems include microwaving, which increases the kinetics of esterification (Reffat and Sheltaway, 2008). But Yuan et al. (2008) report that microwave processing may be cost intensive. Other methods of heating that may reduce the reaction time, reaction temperature and energy input (Kalva et al., 2008) include ultrasonic and cavitation methods; even though achieving scalable production via ultrasonic devices requires significant research and innovations (U.S. DOE, 2010). Another processing route to convert biomass into green diesel fuels that are compatible with petroleum derived diesel fuels is also focused on the esterification of oils with methanol to produce FAMEs. The process uses hydro-treating of vegetable oil that is based on catalytic saturation, hydro-oxygenation, decarboxylation and hydro- isomerization. This technology is applicable for a variety of bio-oil feedstock to produce an isoparaffin-rich diesel (aromatic and sulphur free) substitute with a very high cetane blending value and good cold flow properties. The higher quality diesel is readily blended with diesel fuel and therefore does not depend on process configuration and/or feed origin in contrast to fatty methyl esters (Naik et al., 2010). Efficiency: Amin (2009) reports the achievable conversion of triglycerides to biodiesel (98%) and indicates that the produced biodiesel can be compatible with conventional petroleum derived diesel with a much lower cold filter plugging point of- 11 in comparison with that of diesel fuel. Throughput Microalgal species Chlorella and Botryococcus can reach up to 50% and 80% lipid productivity respectively and thus can be excellent sources of biodiesel. The microalgal C. protothecoides and Microcytic aeruginosa have the possibility for a large alkane chain distribution (10-30 carbons and 10-28 carbons respectively) (Miao et al., 2004). Biofuel from microalgal C. protothecoides and Microcytic aeruginosa has calorific values of 30MJ/kg-1 and 29MJ/kg-1 due to their high carbon and hydrogen content and low oxygen content, which indicates that the level of stability for biofuel obtained from microalgae is high compared to biofuels from plants (Miao et al., 2004), which is of high importance for aviation fuel specification Costa and Morais (2011) report a mean annual productivity of micro algal biomass in a tropical climate region as 1.53kg/m-3/day-1, with a mean 30.0% of lipids extracted from the biomass, and the concentration per hectare of total area is around 123.0m3 for 90.0% of the 365 days of a year. They thus calculated the yield of biodiesel from microalgae as 98.4m3ha-1. They suggest a possible scenario (even if the concentration of lipids in the microalgal biomass were to be 15.0% of dry weight) that for the production of 5.4 billion m3 of biodiesel, an area of approximately 5.4Mha must be calculated, which represents only 3.0% of the area currently used for the cultivation of plants for biodiesel. Table 4 Comparison of properties of biodiesel, diesel fuel and ASTM standard Properties Biodiesel from Diesel fuel ASTM microalgae standard Density kg l-1 0.864 0.838 0.86-0.90 Viscosity (mm2 s-1,cSat 40°C 5.2 1.9-4.1 3.5-5.0 Flash point (OC) 115 75 Min 100 Solidifying point °C -12 -50-10 - Cold filter plugging point °C -11 -3.0 (max. -6.70) Summer max 0 Winter max ,-15 Max 0.5 Acid value (mg KOH g-1) 0.374 Max 0.5 - Heating value MJ kg-1 1.81 1.81 - (Amin, 2009) Cost range: Using microwaves enhances the kinetics of trans-esterification but is cost-prohibitive. The process of trans-esterification is relatively mature. End-product: Biodiesel. Servicing and/or issues: Development of a homogeneous and robust catalyst with minimal sensitivity to tackle contaminants present in the algal extract. Lowering reaction temperatures and reaction rates is also needed for an efficient trans- esterification process. Areas for development: Application of industrial-scale ultrasonic devices can enhance the efficiency and thus produce thousands of barrels per day, but for scalable and commercial quantities an innovative technology is required for complete deployment of biofuel production. ANAEROBIC DIGESTION (AD) Summary: Micro algal anaerobic digestion is a process very similar to the intent of biogas generation. AD is the conversion of organic waste into a mixture of methane and carbon dioxide, commonly termed as biogas. Attention is paid to this conversion system due to the increasing cost of energy production. Biogas major components include methane and carbon dioxide plus atmospheric nitrogen, oxygen and traces of organic compounds which are collectively known as landfill gas (LFG). LFG is a low quality gas that requires purification from volatile organic contaminants and CO2. LFG is used for electricity generation, use in internal combustion engines, turbines, micro turbines, direct use in boilers, dryers, kilns, green houses and cogeneration (Naik et al., 2010). Process efficiency: Brennan and Owende (2010) describe the three sequential stages of AD as hydrolysis, fermentation and methanogenesis. Breaking down of complex compounds into soluble sugars occurs in the hydrolysis step, followed by the conversion of sugars into alcohols, acetic acid, volatile fatty acids (VFAs) and gas containing H2 and CO2 by fermentative bacteria, which is metabolised into methane CH4 (60-70%) and CO2 (30-40%) by methanogens. The AD process has greater propensity for nutrient recycling and energy recovery (Sialve, 2009). Throughput: Depending on the substrate employed for digestion, optimal operating conditions for biogas production can be defined such as pH (6.8-7.5), temperature (30.0-35.0°C), C/N ratio (20-30), total solids (7.0-9.0%) and time (20-40 days) (Omer and Fadalla, 2003). Basically, the AD operating process is affected by organic loadings, temperature, pH, retention time in reactors, mesophilic conditions (35°C) and thermophilic conditions (55°C) (Harun et al., 2010). But Vergara-Fernandez et al. (2007) show that a biogas with methane concentration (50-65%) can be generated from marine algae which can be used as a fuel gas, can be converted to generate electricity (Harun et al., 2010) and can also be used onsite to assist some of the processes that require thermal energy, e.g., drying the algae (Vera-Morales and Schafer, 2009). Table 5 shows the methane yield produced from different algae species (Harun et al., 2010). Table 5 Methane yield from different algae species Biomass Methane yield (m3kg-1) Laminaria sp. 0.26-0.28 Gracilaria sp. 0.28-0.4 Macrocystis 0.39-0.41 L. Digitata 0.5 Ulva sp. 0.20 (Adopted from Harun et al., 2010) The solid residual biomass after digestion can be further utilized for bio-fertilizer production, incinerated or used in animal feed (Costa and de Morais 2011). AD process is particularly suitable for high moisture (80-90%) organic waste, which can be useful for optimization of wet algal biomass. AD process can be utilised to generate liquid fuel and bio-fertilizer for agricultural production (Brennan and Owende, 2010; Costa and de Morais, 2011; Naik et al., 2010). Currently, research attention has been diverted to the production of liquid fuels (methanol) than gaseous fuels (methane) from anaerobic digestion. This is because of the inherent cost connected to methane purification and collection. The advantages for methanol production includes: a) Production of low sulphur and low ash fuels in commercial quantities; and b) Liquid fuel is much easier to handle, store and transport than gaseous products (Naik et al., 2010). The bio-refinery concept of anaerobic digestion (AD) is shown in figure 2 below. Fig. 2. Anaerobic digestion (Naik et al., 2010). Bio-fertilizer Biomass Anaerobic digestion Biogas/landfill gas Methanol Catalytic CH4 Refrigeration or conversion CO2 wash process Liquid CO2 Cost range: Zamalloa et al. (2011), estimate the cost range of biomass to be £86-124 ton-1 dry matter (DM) available for bio-methanation under the premise of achievable up- concentrations of 0.2-0.6kg/m-3 to 20-60kg DM/m-3 plus effective bio-methanation of the concentrate at a loading rate of 20kg DM/m-3d-1. Energy balance: Algae biomass total energy module would be in the order of £0.170-0.087 kWh-1 considering a carbon credit of about £30 ton-1 CO2 (eq) (Zamalloa et al., 2011). End-product: Methane and liquid fuels (methanol) production for electricity generation, using internal combustion engines, turbines, micro turbines, direct use in boilers, dryers, kilns, greenhouses and cogeneration, and bio-fertilizer for agricultural production. Issues: Biogas cleans up. Various technologies employed for biogas clean up include: membrane separation, adsorption, absorption and cryogenic distillation. Energy consumption for biogas clean-up is another issue to put into consideration. Nutrient recovery and CH4 emissions from AD and biogas clean up. Areas for development: Conversions of high amounts of polysaccharides and other oligosaccharides available in the algal residues and remnants that are potential feedstock for fermentation processes into ethanol and other biofuels. Current research activities focus on the research on liquid fuels instead of gaseous fuels due to the inherent cost of methane purification, collection, handling, and transportation and storage difficulties associated with gaseous products. Important area for R&D is desalination of marine algae for the purpose of anaerobic digestion. Removal of hydrogen sulphide could be facilitated by oxygen produced by algae biomass growth, this area is still very immature in terms of technology readiness. Algae co-digestion can improve methane yields but still needs optimisation research, Glycerol which is produced as by product in the transesterification process of algae oil could be co- digested in anaerobic digestion as could improve methane yield however again optimised procedures have not been identified yet.
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