As ethanol builds up in the fermenter it will begin to inhibit the growth of many microorganisms, and some microorganisms are very sensitive to ethanol (Cardona and Sanchez, 2007). To ameliorate this inhibition four methods for the removal of ethanol during fermentation have been used: vacuum, gas stripping, membranes and liquid extraction (Figs 6.11 and 6.12).

Ethanol has a boiling point of 78°C so that applying a vacuum to the fermenter will remove ethanol from the medium. This technique has not been widely used but in

Concentrated ethanol

►

Vacuum chamber

Feed

Cells

(b) Fermentation with gas stripping

Ethanol + carbon dioxide

Condenser

Fig. 6.11. Ethanol removal from fermentation by (a) vacuum and (b) gas stripping. (Redrawn from Cardona and Sanchez, 2007.)

one case a 12-fold increase in ethanol production was obtained. Gas stripping involves passing a gas, in many cases carbon dioxide, through the culture in a separate column to sweep out the ethanol (Fig. 6.11b). More complex arrangements have been designed and pilot plant units have been operated for some time. The use of ceramic membranes for the separation of ethanol from the growth medium has been investigated using a separate unit from the fermenter known as a pervaporation unit. The use of membranes to immobilize cells can also be combined with ethanol removal. It has been shown that pervaporation can reduce the cost of ethanol by 75% as much of the cost is associated with distillation.

Ethanol can be extracted from the fermentation medium by using a bio — compatable solvent such as aliphatic alcohols я-dodecanol, oleyl alcohol, and dibutyl

126

Ґ

v______

Decanter

To distillation

Aqueous

phase

Fig. 6.12. The removal of ethanol by (a) membrane (pervaporation) and (b) liquid extraction. (From Cardona and Sanchez, 2007.)

phthalate. The solvent can be added to the fermentation and the aqueous and water phases can be separated in a decanter. In a version of this technique a two-phase fermentation can be run using two incompatible polymers and the ethanol can be partitioned into one phase and the biomass in the other.

The gaseous fuels DME and hydrogen can be produced using a number of routes including those from biological materials. The carbon dioxide produced per unit of energy (g CO2/MJ) is shown in Fig. 8.21 for hydrogen and DME. The production of DME from syngas, hydrogen from electrolysis, natural gas and coal is compared with petrol, CNG, LPG, coal and gas in terms of carbon dioxide produced per unit of energy. The amount of carbon dioxide produced by DME is similar to petrol and LPG. Hydrogen production by all three routes produces more carbon dioxide than petrol especially when coal is used. This indicates one of the problems of producing hydrogen from fossil fuels.

In a study by the Joint Research Centre EU, the cost of reducing carbon dioxide emissions for a number of biofuels was calculated and some of the data is shown in Fig. 8.22. The GHGs avoided in a life-cycle analysis, well-to-wheel (WTW), are related to the cost (€/t) of carbon dioxide equivalents avoided. A life-cycle analysis systemati­cally identifies and evaluates opportunities for minimizing the overall environmental consequences of using resources and releases into the environment. In terms of GHGs avoided, biodiesel is slightly less expensive than ethanol whether produced from either sugarbeet or wheat. The fuels DME, ethanol and FT diesel produced from wood bio­mass avoid the most GHGs because of using a sustainable source and are inexpensive at around €100-500/t CO2 avoided. Unfortunately the production of the biofuels from wood has yet to be commercialized. Electrolysis of water to produce hydrogen using sustainable electricity from nuclear and wind power avoids large quantities of GHGs.

183 I

700

600

^ 500 CD

О 400

сл

300 200 100 0

(c)

Fig. 8.20. (a) The energy input (MJ/t) for the processes used to produce biodiesel from rapeseed under normal and low nitrogen cultivation. (b) The carbon dioxide (kg/t) produced from the various stages of biodiesel production from rapeseed under normal and low nitrogen conditions. (c) Greenhouse gas (kg/t) for biodiesel production from rapeseed using a conventional and low nitrogen process. (Redrawn from Mortimer et al, 2003.)

CNG, hydrogen from natural gas and ethanol from wheat only avoid moderate amounts of GHGs. Hydrogen for use in fuel cells produced by on-board reforming of petrol is the most expensive option.

The rise and fall of water level due to tides can be harnessed to generate electricity; like hydropower it is a clean, reliable and long-lasting renewable and does not pro­duce carbon dioxide. Sites with a sufficient tidal range and area are limited and rep­resent only 10% of the energy that is available from hydroelectricity. One of the best-known tidal power stations was built at the River Rance, Brittany, in 1966 and has been working for over 40 years producing 550 MW (Charlier, 2007).

Wave power

Schemes for the harnessing of the rise and fall of waves are under investigation in a number of countries. Devices for the conversion of wave energy to shaft power or compression have been proposed and a number have been tested.

Hydrogen can be used to store energy, as a fuel for the internal combustion engine, gas turbine and the fuel cell (see section ‘Fuel cells’, Chapter 3).

Energy storage

There are a few examples of hydrogen being used to store energy in situations where excess electricity is being generated from sustainable sources such as wind and solar

power. These energy sources are intermittent and to balance out the dips in electricity production the stored hydrogen is used to fuel a generation system. One example is the island of Utsira off Norway where electricity is provided by wind power. Surplus electricity is used to electrolyse water and the hydrogen formed stored compressed in a large tank. When the wind does not blow the stored hydrogen is used in a modified internal combustion engine to drive a generator.

Another example was a combined photovoltaic and wind electricity generation system in Cooma, Australia (Shakya et al., 2005). Here again the surplus electricity was used to electrolyse water and the hydrogen stored compressed in a cylinder with a capacity of 5.5 m3 at 24.5 MPa (3552 psi). It was found that the electricity generated was more expensive than grid-connected electricity, as expected, but costs could be reduced as over 50% of the capital costs were the electrolyser and purification com­ponents. A reduction in the cost of these components would reduce costs consider­ably. The system still has great merit in situations where no grid supply is available.

There is a considerable body of information on the large-scale cultivation of micro­algae in bioreactors of various designs (Molina Grima et al., 2001; Scragg et al., 2002; Acien Fernandez et al., 2003; Chisti, 2007; de Morais and Costa, 2007a, b). The designs of photobioreactors can be divided into two types — open and closed — and the advantages and disadvantages of these types are outlined below:

146

Open bioreactors:

• Natural water, raceway ponds, inclined surfaces. These can suffer from: water and CO2 loss, contamination and pollution, requirement of large area, limitation on the number of species that can be grown, no process control, dependency on weather, poor mixing and low biomass (0.1-0.2 g/l).

Closed bioreactors:

• Stirred vessels, tubular bioreactor, laminar bioreactor, plastic bag vessels. Best for high-value products, process control, continuous culture possible, all types of algae grown, flexible production, not affected by weather, high biomass (2-8 g/l).

It would appear that the two best bioreactor designs are the raceway and tubular designs. The raceway is considerably simpler but mixing is limited, temperatures vary and the biomass concentration is low. The tubular bioreactors are enclosed and with good mixing and circulation a high biomass concentration can be achieved without contamination. They are, however, more expensive to operate and require cooling during daylight. A wide range of bioreactor designs have been used to culture micro­algae and some examples of the various designs are given in Table 7.6.

Fig. 7.8. Outline of the parameters which affect the growth of algae in bioreactors.

Whatever design is used, microalgal growth in bioreactors is influenced by four parameters: the supply of light, the supply of CO2, mixing (turbulence) and the build-up of oxygen (Grobbelaar, 1994) (Fig. 7.8). The maximum amount of light is about 30% saturation, and values above this can cause photobleaching, the loss of chlorophyll. The CO2 levels of 0.03% are below the optimum for growth, concentra­tion of 0.1% is more suitable, and there have been cases where 10% CO2 did not inhibit growth. In the light, microalgae can produce oxygen rapidly and a build-up of oxygen can inhibit growth. Mixing in the form of turbulence is essential to keep the cells in suspension and for gaseous exchange.

Commercial production of microalgae has been used to produce pigments, food supplements and shellfish food. The designs that have been used to produce micro­algae commercially are given in Table 7.7.

The major crops for the first-generation biofuels ethanol and biodiesel are rapeseed, soybean, sunflower, oil palm, sugarcane, sugarbeet and maize. The ability to grow these crops in low moisture and saline conditions would relieve the pressure that biofuels have on prime agricultural land.

Fig. 8.30. The effect of light intensity on growth and carbon dioxide assimilation by plants.

Reed canary grass is a species indigenous to Europe belonging to the Gramineae family. It is adapted to low temperatures and short growing times. Reed canary grass is a C3 grass which grows to 3 m in height, is propagated by seed and is harvested once a year.

Giant reed

Giant reed is also an indigenous species belonging to the Gramineae family. It is a tall perennial C3 grass which can reach heights of up to 8-9 m. The giant reed tolerates a variety of conditions but prefers well-drained soils. It is propagated by rhizomes rather than seed and the yield can reach 100 t/ha/year under optimum conditions.

All perennial grasses are regarded as drought tolerant, require few inputs and grow on poor land. However, establishment of these crops is not easy and can vary greatly. Yields of biomass can also vary and appear to be related to nitrogen and water availability. Switchgrass requires as much water as traditional crops and is responsive to nitrogen fertilizer but too much usage will give a problem of lodging.

One of the reasons for substituting ethanol for petrol is that ethanol can be produced from biological material and hence is both renewable and sustainable, and reduces carbon emissions.

The ability of microorganisms to produce alcohol from sugars has been known since Egyptian times and could be regarded one of the first uses of biotechnology. The best-known and most widely used microorganism involved with the production of ethanol has been the yeast Saccharomyces cerevisiae, but it is not the only one. In the absence of oxygen, the yeast will switch its metabolism to fermentation producing ethanol and carbon dioxide. The lack of oxygen inhibits the citric acid cycle so pyru­vate would be expected to accumulate (Fig. 6.7). However, under these conditions

Fig. 6.6. Typical process for the production of ethanol from sugarcane in Brazil. (Modified from Poole and Towler, 1989.)

pyruvate is converted to acetaldehyde by the enzyme pyruvate decarboxylase with the release of carbon dioxide. Acetaldehyde is then converted to ethanol by the enzyme alcohol dehydrogenase. The overall equation is given below:

C6H12O6 = 2C2H5OH + 2CO2

theoretical yield of ethanol from this equation is 51% of the substrate added but some energy is required to maintain the cells so that the yield is about 95% of the theoretical yield with pure substrates. However, with industrial systems the best yields are around 91%. The concentration of ethanol obtained by fermentation is normally from 5 to 10% as ethanol begins to inhibit growth above 5%. Concentrations of 10% ethanol can be obtained with pure substrates. The reason for the loss of viability as the ethanol concentration increases is that ethanol is a solvent and dis­rupts the cells’ lipid-protein membrane making it increasingly leaky. Yeast strains with a higher tolerance to ethanol have membranes containing a higher proportion of longer-chain unsaturated fatty acids. Higher concentrations of ethanol can be obtained using high concentrations of substrates, and ethanol tolerant (10-18%) strains but the process is much slower. The limitation of S. cerevisiae is that it cannot utilize pentoses such as xylose and arabinose and more complex carbohydrates like starch and cellulose.

In contrast, unsaturated fatty acids give better lubricity and cold flow characteris­tics than saturated fatty acids (Knothe, 2005). Desulfurization of conventional diesel leads to a considerable loss in lubricity, which is required for the functioning of the engines’ pumps and injectors. A number of studies have shown that the addi­tion of small quantities (5%) of biodiesel can increase the lubricity of conventional diesel (Hu et al., 2005; Knothe, 2005). The polar elements in biodiesel help to form a layer on the metal parts of the engine, reducing engine wear. Lubricity is measured by the ball-on-cylinder lubricity evaluator (BOCLE) test or the high frequency reciprocating rig (HFRR) test and those fuels with good lubricity give values of 4500-5000 g. Figure 7.19 shows the effect on lubricity of adding biodiesel to con­ventional diesel.

Therefore, the fatty acid content of biodiesel has to be a compromise between the best cetane and oxidative stability and cold flow characteristics. It has been suggested that the ideal fatty acid composition for biodiesel would be 10% myristic (C14:0), 50% palmitoleic (C16:1) and 40% oleic (C18:1) acids. This composition is perhaps the aim of all forms of plant breeding or some form of blend of oil.

Although not directly involving biofuels, the development of alternative drive systems that do not use fossil fuels for transport is important in the reduction in fossil fuel use and greenhouse gas emissions. There are a number of systems being tested including fuel cells, electric cars and hybrid systems.

Fuel cells

Fuel cells have had a long development, including use in the NASA Apollo pro­gramme in 1960, and since 1990 an experimental transportation system has been introduced. A fuel cell consists of two electrodes — the anode and cathode — divided by an electrolyte (Fig. 3.7). Hydrogen is run into the anode, where a platinum-coated

Fig. 3.7. Outline of a fuel cell.

proton-exchange membrane splits the hydrogen into hydrogen ions (protons) and electrons. The protons pass through the electrolyte to the cathode where they combine with oxygen, forming water. The electrons produce an external current which can be used to run an electric motor.

Fuel cells are classified by their operating temperature which is also determined by the electrolyte (Stambouli and Traversa, 2002). Table 3.5 gives some of the char­acteristics of fuel cells. Fuel cells can be combined in stacks, connected in series to produce the desired voltage. The number of fuel cells in a stack determines the voltage and the surface of each cell determines the current. Proton exchange and solid oxide fuel cells are the most advanced and have been fitted into experimental cars.

Two recent developments in fuel cell technology are the direct carbon fuel cell and the microbial fuel cell. In the direct carbon fuel cell, fine particles of carbon (10-1000 nm) are mixed with molten lithium, sodium, or potassium carbonate at 700-800°C (Cooper, 2006). The molten salt is introduced into the anode compart­ment and air to the cathode (Fig. 3.8). Electrons are carried from the carbonate to the cathode. Oxygen passes through a membrane which reacts with carbon, releasing electrons, forming carbon dioxide.

The microbial fuel cell derives energy from organic compounds metabolized by microorganisms. Figure 3.9 shows the layout of a microbial fuel cell. Microbes in the anode chamber oxidize substrates added to the chamber, generating electrons and protons as found in the chemical fuel cell. Carbon dioxide is formed but as organic substrates are used, the carbon dioxide released is only that fixed during photosynthesis.

Cathode

Proton

exchange

membrane

Fig. 3.8. Direct carbon fuel cell. (Redrawn from Cooper, 2006.)

The reactions are as follows when using acetate as a substrate:

CH3COOH + 2H2O ® 2CO2 + 7H+ + 8e — (3.1)

At the cathode the protons react with oxygen:

O2 + 4e — + 4H+ ® 2H2O (3.2)

To extract electrons to the anode, mediators have to be added to the anode chamber. These mediators move across the microbial cell membrane where they are reduced

and pass out of the cell, releasing the electrons to the anode. Mediators are dyes and metallorganics such as neutral red, methylene blue, thionine, Meldola’s blue and 2-hydroxy-1,4-naphthoquinone. However, the instability of the mediators limits their use, but recently a group of bacteria, the anodophiles, have been isolated. These bac­teria (including Shewanella putrefaciens, Geobacteraceae sulfurreducens, Geobacter metallireducens and Rhodoferax ferrireducens) attach themselves and transfer elec­trons directly to the anode (Du et al., 2007). Some microbial fuel cells have been inoculated with bacteria mixtures such as sewage sludge and sediments which have the advantage of a wider substrate range. The amount of electricity provided by the microbial fuel cells is still very low, but they can be stacked and used to produce hydrogen, for wastewater treatment and as biosensors.

Alternative biological fuel cells have the microbial cells replaced with enzymes. This has the advantage of having a higher volumetric catalytic capacity, and it avoids toxic oxidation products. One of the fuels tested in an enzyme-based fuel cell is gly­cerol, one of the by-products of biodiesel production. Glycerol is a non-toxic, non­volatile, high-energy density substrate (6.3 kWh/l) for a cell containing the enzymes alcohol dehydrogenase and aldehyde dehydrogenase (Arechederra et al., 2007).