Microalgae can be used to produce a number of valuable products (Belarbi et al., 2000; Del Campo et al., 2000; Li et al., 2001; Banerjee et al., 2002), animal food (Knauer and Southgate, 1999), human health food (Becker, 2007) and as a wastewater treatment (Travieso et al., 2002; Kebede-Westhead et al., 2006). In addition to these options, microalgae have been proposed as systems for the sequestration of CO2

(Sawayama et al., 1995; Zeiler et al., 1995; de Morais and Costa, 2007a, b) and the production of biofuels (Chisti, 2007). The biofuels include biogas (CH4) by anaerobic digestion of the biomass (Spolaore et al., 2006), biodiesel from microalgal oils (Nagle and Lemke, 1990; Sawayama et al., 1995; Minowa et al., 1995; Miao and Wu, 2006; Xu et al., 2006; Chisti, 2007), hydrogen (Fedorov et al., 2005) and the direct use of algae in emulsion fuels (Scragg et al., 2003).

Biodiesel is one of the sustainable fuels which can replace diesel as a transport fuel and is usually made by the transesterification of plant-derived oils, waste cooking oils and animal fats. However, microalgae should be considered as another source of biodiesel because of the following:

• They have higher photosynthetic efficiency than terrestrial plants.

• They have rapid growth rate, with doubling times of 8-24 h.

• They have high lipid content of 20-70%.

• They facilitate direct capture of CO2, 100 t algae fix ~183 t CO2.

• They can be grown on a large scale.

• They will not compete with terrestrial plants in food production.

• They produce valuable products.

• They include freshwater and marine species.

• They have a much better yield of oil per hectare, oil palm 5000 t/ha, algae 58,700 t/ ha (Table 7.5; Chisti, 2007).

The use of microalgal oil to produce biodiesel is very much in the developmental stage, and so it should be regarded as a third-generation biofuel.

To use microalgae for the production of biodiesel a number of processes must be carried out and these are outlined in Fig. 7.7 and consist of strain selection, large — scale cultivation, harvesting, extraction of the oil, production of biodiesel from the oil, and the economics of the process.

Any improvement in fuel crop yield in terms of quantity and quality will reduce the amount of land required and processing. It may be possible to improve the quantity of biofuels and change the quality using either conventional plant breeding or genetic manipulation. There is an embargo on genetically manipulated plants within the EU at present, but globally there are a number of countries that have grown large quanti­ties of genetically manipulated crops without any obvious problems. It would appear that the non-food crop would be most suitable for genetic manipulation if regulations were relaxed. Some suggest that genetic modifications are essential if biofuels are to be fully exploited. Some of the areas that genetic manipulation may be helpful in the development of biofuels are as follows:

• Improvement in photosynthesis.

• Improvement in salinity and drought resistance.

• Herbicide and pesticide resistance in fuel crops.

• Improvements in the quantity and quality of plant oils.

• Reduction in lignin in lignocellulose to make cellulose more available for digestion.

• Improvement in hydrogen production by microalgae.

• Improvement in conversion of lignocellulose, cellulose, and other sugars to ethanol.

Perennial grasses have in the past been used as fodder crops but now they are consid­ered suitable as energy crops because of their high content of cellulose and lignin. This gives the plant biomass a high heating value. Some examples of the heating values of biomass types are compared with fossil fuels in Fig. 4.2. The figure shows that the energy density of all the biomass types is lower than coal and especially gas and oil. As a consequence more biomass will be required to produce an equivalent amount of energy and thus more biomass will need to be transported.

Trials with a large number of perennial grasses have been carried out for energy in both the USA and Europe (Table 4.3). The criteria that were used in the selection as an energy crop were as follows:

1. Suitable for the climate in the region.

2. Easily propagated.

3. A consistent and high yield of biomass per hectare, probably the most important.

4. Positive balance of energy input versus output.

5. The crop can be cultivated in a sustainable manner.

6. Resistance to pests and diseases.

Fig. 4.2. Energy content of fossil fuels, a: SRC and perennial grasses.

7. Broad genetic diversity to enable species to be adapted to prevailing conditions.

8. Harvesting possible with existing technology.

9. Perennial.

10. Competitive on cost with food crops.

The four that have been chosen for further study are Miscanthus, switchgrass, reed canary grass and giant reed. Some of the properties of these grasses are compared with short rotation coppice of willow and poplar in Table 4.4.

Table 4.4. Properties of biomass crops. (From Powlson et al., 2005; Lewandowski etal., 2003.) Crop Poplar (SRC) Willow (SRC) Miscanthus sp. Switchgrass Reed canary grass Giant reed Yield t/ha/year 7 7 (15-30) 12 (5-44) (1 0-25) 10 (5-23) (15-35) 8 (7-13) 5-23 Establishment time 3 years+ 3 years+ 3 years+ 2-3 years+ 1 -2 years 1 -2 Photosynthetic pathway C3 C3 C4 C4 C3 C3 Fertilizer Low/medium Low/medium Low Very low Medium Moderate Water supply Wet Wet Not tolerant to stagnant water Drought tolerant Drought tolerant Drought tolerant Pesticide Low Low Low Very low Low Low Establishment costs High High Very high Very low Very low High Pest/disease — Beetle rust None None Some insect problems Few Day/length Long Long Long Short Long Long Plantation longevity 20 years 20 years 20 years 20 years 10 years n /a Energy content GJ /t 15 15 17.6-17.7 1 7.4 16.5-17.4 17.3-18.8 Output GJ/ha/year 105 105 260-530 (262-525) 174-435 (1 75-437) 240-600 (262-613) 88-403

In the case of sugarcane the sugar can be pressed from the cane and used directly in fermentation to produce ethanol. Starch on the other hand cannot be used by yeasts in fermentation and so has to be converted to glucose before it can be used. This is the main difference between the processes used in the USA and Brazil.

110

Figure 6.3 outlines the process that is used to obtain glucose from maize. The process was developed initially to produce starch from maize where the starch was used in food formulation, for the production of high fructose maize syrup, a low- calorie sweetener and a wide range of starch products. The harvested maize is soaked in water to loosen the kernels which are passed through a mill which removes the germ or embryo. The wash water is known as maize steep liquor and is one of the components of the medium used to grow penicillin. The separated germ is used for germ meal as a high protein supplement or pressed to extract maize oil (Mazola) used widely in cooking. The kernels, which now contain mainly starch, are ground, washed and centrifuged to remove fibre and gluten which are used in the food indus­try. What remains is starch which can be used in food and other products. Some of the starch is converted into glucose so that it can be processed into a mixture of glu­cose and fructose known as high fructose maize sugar (HFCS) for use as a low-calorie sweetener. The conversion of starch into glucose is carried out by starch-degrading enzymes, the amylases. Starch is synthesized in the chloroplast where glucose molecules are linked together with a-D-1,4 glycosidic linkages to form long chains (Fig. 6.4). At some stages, a branching by enzyme adds a side chain with a a-D-1, 6 linkage which gives starch a more rigid structure. The starch molecules can be bro­ken down either by hydrolysis with acid or by enzymatic breakdown. The enzymatic

process is normally used as it produces fewer by-products. The starch forms a stiff paste with water, which is first liquefied by heating to 60-80°C and the enzyme a-amylase added. The enzyme breaks the long glucose chains in the starch into shorter sections, known as long chain dextrins. In some cases, a high temperature a-amylase is used and the starch heated to 100°C. After a short period of time, the liquefied starch is cooled to 50-60°C and another enzyme, amyloglucosidase, added. This enzyme converts the dextrins into glucose.

Once glucose has been produced, it can be used as a substrate for ethanol fermen­tation by yeast. Figure 6.5 shows a typical process for the production of ethanol from glucose. The glucose from the starch is mixed with salts, nitrogen and phosphorus

and the mixture sterilized by heating to 120°C for 2-5 min in a continuous sterilizer. Once sterilized, the medium is run into a large bioreactor (200,000 l and above), yeast added from a seed bioreactor (1-10% volume of the main reactor) and the culture incubated at 30°C for a few days. Carbon dioxide produced during the fermentation can be adsorbed and used to make solid carbon dioxide, ‘dry ice’. Once fermentation has finished the yeast cells are removed by centrifugation and the medium, sometimes known as ‘beer’, is warmed by passing through a heat exchanger and then distilled. Distillation is needed to concentrate the ethanol and is the major energy-consuming stage. The fermentation yields about 10% ethanol and it needs to be more than 95% to be used as E95 or 100% if used as a blend. Heating a 10% ethanol solution will yield a vapour containing more ethanol than water and the remaining solution will contain more water so that a limited amount of enriched ethanol can be obtained. But with a series of distillations a concentration of 95.6% ethanol can be obtained. At 95.6% the liquid and vapour have the same concentration so no further concen­tration is possible. The mixture is known as an azeotrope. However, by using a distil­lation column separated by plates a series of separate distillations can be produced and this will give the azeotrope in one distillation. To produce anhydrous ethanol a second distillation is required where benzene is added and this on distillation gives pure ethanol and the benzene can be recovered and used again.

One of the major problems with biodiesel is its poor low-temperature flow proper­ties shown by its high cloud and pour points. At the cloud point, long-chain fatty

Fig. 7.15. The effect of ester chain length on cetane number. (Redrawn from Graboski and McCormick, 1998.)

Fig. 7.16. The effect of double bonds on cetane number. (From (▲) Graboski and McCormick, 1998; (■) Knothe et al., 2003.)

acid esters begin to form small wax crystals and when these reach 5 pm in size, the fuel begins to look cloudy. As the temperature decreases, the crystals grow and aggregate in a form which can plug filters and eventually it ceases to pour. Often low-temperature filterability (LTFT) is used as a low-temperature characteristic which is generally halfway between cloud and pour point. Several methods have been used to improve low-temperature characteristics including additives, branched chain esters and winterization.

Treatment with chemical additives, pour depressants, is one way that conven­tional diesel is modified to improve its low-temperature characteristics.

The molecular structure of pour depressants is polymeric hydrocarbon chains with polar groups which function by adsorption, co-crystallization, nucleation and improved wax solubility. Vegetable oils lack polar groups but ozonation can reduce the pour point (Soriano et al., 2005). Other cold flow improvers have been tested, and these reduced the pour point but had little effect on cloud points (Chiu et al., 2004).

Using branched alcohols, such as iso-propanol or iso-butyl alcohol in place of methanol and ethanol in the transesterification process, yields esters which have lower pour points. However, this appeared to increase the viscosity of the biodiesel and also resulted in incomplete esterification.

Winterization of biodiesel has been carried out which increases the unsaturated fatty acid esters. The saturated fatty acids are removed as they have higher melting points. The process reduces biodiesel yields and the high concentration of saturated fatty acids means a lower cetane value.

A life-cycle analysis is often used to demonstrate that one product is environmentally superior to another and to identify stages where a reduction in use of resources and emissions can be achieved. This type of analysis is ideal to determine the impact of biofuels on global warming using a WTW analysis. A WTW analysis combines the fuel production system, well-to-tank (WTT), the energy and GHGs associated with delivering the fuel to the vehicle’s tank. The second part is tank-to-wheel (TTW), the energy expended and GHGs produced by the vehicle power train. These are com­bined to produce the WTW values.

In a recent study, the Joint Research Centre has given the energy use and GHG emissions in a WTW for a number of fuels including ethanol and biodiesel. The study was not a complete life-cycle analysis as it does not include the energy used and GHG emissions produced in building the production facilities and vehicles. Nevertheless the data illustrate the problems with some biofuel sources.

The reduction in fossil fuel use for electricity generation, heating/cooling and trans­port may involve a large number of measures, some of which are as follows:

• Increased engine efficiency.

• Increased power generation efficiency.

• Local electricity generation and distribution.

• Better home insulation.

• Fewer car and lorry journeys.

• Greater use of public transport.

• Greater use of biofuels.

• Alternative power systems.

• Changes in house design.

• Reduction in long-distance transport of material which can be sourced locally, and reduction in air miles.

Energy efficiency measures such as insulation, building design, light bulbs, stand-by default on televisions and other consumer electronics have been estimated to make a major contribution to reduction in energy use. The EU Emissions Trading Scheme and the Climate Change Levy should also encourage cost-effective energy saving, estimated at reducing carbon emissions by 6-9 Mt. Transport uses over 30% of the total energy, therefore continued increases in engine efficiency should give significant savings.

Methane (CH4) is a natural gas produced by the breakdown of organic material in the absence of oxygen in wetlands, termite mounds, and by some animals. In addition, methane is a greenhouse gas which is 23 times as effective as carbon dioxide, but because of its low concentration is only responsible for 15% of global warming. Mankind is also responsible for the release of methane through biomass burning, agri­culture (rice paddies), cattle, and release from gas exploration (Chapter 2, Table 2.5).

The reasons for considering methane (biogas) as a possible biofuel are as follows:

• Increases in the costs of waste disposal due to regulation and taxes have encour­aged the investigation of alternative methods of disposing of waste.

• The EU directive on use of renewable fuels and the Renewable Transport Fuel Obligation (RTFO) in the UK, and methane is a renewable fuel.

• The greater use of biomass in the UK.

• Improvements in air quality by the introduction of biofuels.

• Reduction in methane released into the atmosphere to comply with the Kyoto Protocol.

• Reduction in natural gas imports as much of the natural gas is supplied from unstable areas.

Methane is produced under anaerobic (no oxygen) conditions where organic material is broken down by a consortium of microorganisms. The three main sources of mate­rial for anaerobic digestion are given in Fig. 5.1.

In the UK, biogas is produced by anaerobic digestion of sewage sludge (190,000 t of oil equivalent) but landfill produces the bulk of the gas (1,320,000 t of oil equiva­lent) (Fig. 5.2). The bulk of the biogas is used to generate electricity. It has been estimated that the UK is capable of producing 6.3 million t of oil equivalent (Mtoe) as methane (NSCA, 2006).

Fig. 5.3. Uses of landfill biogas in the USA. (Redrawn from Themelis and Uloa, 2007.)

The generation of electricity also appears to be the main use in the USA (Fig. 5.3). In the USA, 3.7 billion Nm3 of methane is produced in landfill sites generating 1071 MW of power (Themelis and Ulloa, 2007).

In simultaneous saccharification and co-fermentation (SSCF) technology enzymatic hydrolysis of lignocellulose continuously releases hexose (glucose) and pentose (xylose) sugars and two organisms jointly assimilate the pentoses and hexoses. Examples of the organisms are Pichia stipitis and Brettanomyces clausennii and in other cases recombinant microorganisms have been used.

Consolidated bioprocessing

In this process, fermentation and enzyme production are carried out by a single microorganism growing in a single bioreactor. There are no microorganisms which can carry out this process at present but some are under development.

The carbon dioxide produced during the synthesis of diesel and biodiesel combined with carbon dioxide produced when the biodiesel is burnt is given in Fig. 8.18. Diesel produces around 80 g CO2/MJ compared with 43.7 g CO2/MJ for rapeseed biodiesel which is a reduction of 45%. In the case of diesel produced by the FT process by the

gasification of coal, natural gas and biomass, the carbon dioxide produced varies considerably. Both natural gas and coal FT diesel produce more carbon dioxide than diesel, 98 g CO2/MJ and 233 g CO2/MJ, respectively. The use of biomass in the syn­thesis of FT diesel yields only 5 g CO2/MJ which represents a 94% reduction in car­bon dioxide compared with mineral diesel.

The carbon dioxide fixed during growth and its distribution in products during the production of biodiesel from rapeseed is shown in Fig. 8.19. The rapeseed plant

Rapeseed + atmospheric
carbon dioxide

і

2.85 kg CO2 / kg biodiesel
2227 kg CO2

Fig. 8.19. The distribution of carbon dioxide in the production of biodiesel from rapeseed. (From Peterson and Hustrulid, 1998; Mortimer et a/., 2003.)

fixes a total of 15,736 kg CO2/ha, where 13,336 kg was retained in the plant and the remaining 2400 kg stayed in the soil sequestered by the soil microorganisms. The yield of the seed was 2240 kg/ha, 28.5% of the total plant material, containing 4800 kg carbon dioxide/ha. The rest of the plant, the straw, contains 8514 kg CO2/ha, 63.8% of the carbon dioxide fixed, which is often ploughed into the soil. The oil extracted from the seed contains 840 kg/ha which is an oil yield of 37.5% leaving the meal or cake containing 2389 kg CO2/ha to be used as feed or fuel. On transesterification the oil is converted into 840 kg biodiesel and 78 kg glycerol. The biodiesel represents some 9.2% of the whole plant and yields 2227 kg CO2/ha when used as a fuel. Figure 8.19 also includes the energy content of the biodiesel and the co-products where it can be seen that the meal and straw contain more energy than the biodiesel. For these reasons, it may be more efficient to convert the whole plant or biomass into a biofuel rather than just the oil extracted from the seed. This should be the nature of the second-generation biofuels.

The unit operations used in the production of biodiesel have been evaluated for their carbon dioxide and GHG production and energy input (Fig. 8.20a, b,c). It is clear that the major carbon dioxide-producing stages were esterification, use of nitro­gen fertilizer and to a lesser extent solvent extraction of the oil. A second process has been included where solvent extraction has been replaced by cold pressing and low nitrogen growing conditions. This results in more than 50% reduction in energy input, carbon dioxide and GHG emissions. Thus, the processes of biofuel production can be made more environmentally suitable by modifications to the key stages of the process.