Crops specifically grown for energy biomass include short rotation coppice with willow (Salix sp.) and poplar (Populus sp.), perennial grasses miscanthus, switchgrass (Panicum virgatum), reed canary grass (Phalaris arundinacea) and giant reed (Arundo donax).

Coppice has been used in Europe for centuries with long rotations of 10-30 years using hazel (Corylus avellana L), black alder (Alnus blutinosa L), and to a lesser extent oak (Quercus robur L), ash (Fraxinus excelsior L), elm (Ulmus sp.) and horn­beam (Carpinus betulus L). Latterly non-indigenous species have been used. In the coppice system the stems are cut back at close to soil level after 3-10 years. This encourages the growth of a large number of stems which can be harvested after 3-4 years in the case of willow and poplar. The other species have a slower growth rate and will be harvested after 10-30 years. Table 4.2 gives the yield and energy content of the short rotation coppice species. Willow and poplar yield between 6 and 17 t/ha/ year with a calorific value of 15 GJ/t. Plantations of 15,000 cuttings/ha for willow and 10-12,000 cuttings/ha for poplar are planted in winter or spring. After 1 year the single stem is cut back to soil level to produce a multi-stemmed stool which can be harvested every 3-4 years. Harvest is mechanical and the stems chipped and dried for combustion. Willow is the preferred species in the UK, where the plantations can last for 25-30 years. Much of the biomass is co-fired with coal in power stations.

The large-scale production of ethanol as a fuel started in Brazil in 1975, followed by the USA in 1978. The amount of ethanol produced in the world in 2001 was 22,540 billion l (Fig. 6.1); global production was dominated by Brazil and the USA. Brazilian production is based on the fermentation of sugar from sugarcane, whereas the USA used starch extracted from maize. By 2006, global production had increased to 51 billion l (13.5 billion gallons), which represents 4.6% of global petrol consumption. Bioethanol has the potential to replace 353 billion l of petrol, which is 32% of the global petrol consumption (1103 billion l) when used as 85% addition (E85) (Balat et al., 2008). In 2006, the USA produced 4.85 billion gallons (18.3 x 109 l) of bioethanol which has overtaken the production by Brazil of 4.49 billion gallons (16.97 x 109 l) (Fig. 6.2).

The plants installed in Brazil are small, in the region of 100,000 l in capacity, compared with the large units installed in the USA at around 0.8 billion l per year.

Fig. 6.1. Distribution of production of ethanol worldwide in 2001, from a total of 22,540 billion l. (From Licht, 2006.)

Fig. 6.2. Distribution of ethanol production worldwide in 2006, from a total of 51,000 billion l. (From Balat et al., 2007.)

Table 6.5 lists some of the largest ethanol producers in the USA, with the largest vol­ume produced at 4.1 billion l (1.08 billion gallons). This difference is in part due to the USA plants being attached to very large maize-processing plants, whereas in Brazil locally grown sugarcane is processed in smaller units avoiding extensive transport.

Cetane numbers rate the ignition properties of diesel fuel as a measure of the fuel’s ignition on compression as measured by ignition delay. Cetane affects smoke produc­tion on start-up, drivability before warm-up and diesel knock at idle. Cetane number is measured in a single-cylinder engine compared with reference blends of n-cetane and heptane. Cetane numbers for biodiesel are influenced by the fatty acid ester pro­file making up the biodiesel (Table 7.15). In general, as the number of carbon atoms in the fatty acid esters increases, so does the cetane number (Fig. 7.15). However, as the number of double bonds increases the cetane number decreases (Fig. 7.16), so for a high cetane number long-chain saturated fatty acids are needed (Graboski and McCormick, 1998; Knothe et al., 2003). The formation of pollutants is dependant on ignition delay and cetane number and a high cetane number gives less NOx.

One of the perceived problems with all biofuels is that if they require too much energy for their production then this makes them uneconomic and unsustainable. The energy input is measured by the net energy value (NEV), the ratio of the energy obtained versus the energy required to generate the fuel. If this is less than one it indicates an unsustainable energy source.

Energy balances for biomass

The net energy balance or NEV for biomass combustion or use in electricity generation is shown in Fig. 8.33. Electricity generated from fossil fuel had an NEV of 0.32 and electri­city generation with all forms of biomass had much higher values. This indicates that these are more sustainable sources. Electricity generated from SRC and Miscanthus, either directly or after gasification, had NEV values of around 5. Combustion of willow and the perennial grasses had NEV values of 15 and above. The differences between combustion and electricity generation reflect the losses that occur in electricity generation.

Ethanol

Fig. 8.31. The introduction of xylose-metabolizing enzymes into Saccharomyces cerevisiae. The shaded area indicates the introduced enzymes. (From Prasad et al, 2007.)

Different measures have been proposed to reduce global warming caused by the burn­ing of fossil fuels. However, the various bodies involved differ in defining the steps that need to be taken. Seven steps have been suggested by Mathews (2007), which require no further technological advances to be implemented, and should reduce emissions by 70% by 2050.

These steps are as follows and involve both carbon taxes and permits:

1. A global carbon pricing regime based on carbon taxes and permits.

2. Global satellite monitoring of greenhouse gas emissions.

3. Compensating developing countries for preserving rainforests.

4. Creation of a global market for responsible biofuels.

Each region can certainly pursue biofuels adapted to conditions found there, such as rapeseed for biodiesel in Europe (Ryan et al., 2006), but it is unrealistic to see temper­ate regions becoming self-sufficient in biofuels. It is far more expedient to open up the

world market and to encourage trade in biofuels, both to accelerate the utilization of biofuels as a defence against global warming, and to encourage industrial development of tropical countries as the world’s supplier of biofuels:

5. Creation and furtherance of markets for renewable electricity.

6. A global moratorium on building new coal-fired power stations.

7. Creation of global incentives for developing countries that are moving to adopt non-fossil-fuel industrial pathways.

Hoffert et al. (2002) stated: ‘[Stabilizing climate is not easy. At the very least, it requires political will, targeted research and development and international cooperation. Most of all it requires the recognition that although regulation can play a role, the fossil fuel greenhouse effect is an energy problem that cannot be simply regulated away.’

All the above measures are a mechanism to implement the following measures which can be used to reduce global warming. These are simply:

1. Burn less fuel.

2. Sequester carbon dioxide produced.

3. Use renewable alternative fuels.

Introduction

In contrast to the solid biofuels, described in Chapter 4, gaseous biofuels can not only be used for both electricity generation and heating, but also most importantly as a transport fuel. A list of gaseous biofuels is given below:

Gaseous fuels:

• Methane (biogas).

• Hydrogen.

• Dimethyl ether (DME).

Methane or biogas can be used to replace natural gas (methane) which is a fossil fuel for electricity generation and for cooking and heating. For land transport, there are a small number of modified internal combustion engines using gases derived from fossil fuels such as liquid natural gas (LNG), liquid petroleum gas (LPG) and compressed natural gas (CNG). Biogas, hydrogen and dimethyl ether have been proposed as replacements for these transport fuels. Hydrogen has also been proposed as a fuel for gas turbines.

Gaseous fuels have problems of storage and supply not encountered with either solid or liquid fuels. Storage of gas at atmospheric pressure is not practical so the gas has to be compressed to high pressure or liquefied at low temperatures to reduce its volume. Compression to pressures of 200 bar and liquefaction, which for hydrogen needs a temperature of -253°C, expends a considerable amount of energy and subse­quent storage has to be in strong pressure vessels or in well-insulated tanks. The lower energy density of the gaseous fuels compared with liquid fuels means that larger fuel tanks are required in vehicles. One advantage is that transport of gaseous fuels can be carried out using pipelines which are used at present for natural gas although in the case of hydrogen its low density may encourage leaks. All gaseous fuels are inflammable, especially hydrogen, which introduces safety problems when these gases are stored in vehicles. The dangers of hydrogen fires are often illustrated by the crash of the airship Hindenberg, but as hydrogen diffuses so rapidly any spill in an open space may disperse before anything can happen.

In this case the hydrolysis of cellulose is carried out separately from the fermentation but if the products of hemicellulose are to be included a second bioreactor is used to

123 I

ferment pentose sugars such as xylose. The use of a separate bioreactor gives a higher yield of ethanol and less energy is required. Considerable efforts have been made to improve the yield of ethanol using the normal yeast fermentation. A number of process changes have been investigated in order to improve the economics of etha­nol production. The traditional method of fermentation has been batch culture in a non-stirred cylindro-conical vessel where the sugar and salts are inoculated with yeast and the fermentation allowed to proceed until the sugar is exhausted. Other forms of bioreactor operations and designs have been investigated in order to improve ethanol productivity, as this affects the cost of the final product. The fer­menter can be operated in a batch-fed mode where batches of fresh medium are added at times during the fermentation. This avoids substrate inhibition where a high substrate concentration at the beginning of the fermentation may inhibit growth. In the continuous mode medium is added continuously throughout the fer­mentation and cells and medium removed at the same rate to keep the volume in the fermenter the same. This allows the operator to run the fermenter for a long period without having to waste time cleaning, refilling and sterilizing the fermenter. Fermenters of various designs where cell recycling has been used have considerably higher productivity which is due to maintaining a high cell density throughout the process.

The traditional fermentation vessel is not stirred but stirred tanks can be used to give good mixing and a more rapid growth rate. Alternative fermenter designs have been tested in order to improve the rate of growth and ethanol production. The tower fermenter is just an elongated tank with a high aspect ratio. The fluidized bed fer­menter operates by mixing the cells by pumping the medium up through the base of the fermenter, thus fluidizing the cell mass at the bottom of the tank. A membrane fermenter keeps the cells separate from the medium with a semi-permeable membrane. This allows the fermenter to retain a high cell density and thus a higher rate of etha­nol production. Examples of the productivity of these various systems are given in Table 6.9.

Another method for maintaining a high cell density is to immobilize the cells on or in some form of support. This retains the cells within the bioreactor at a high density and allows for a continuous process. Examples of immobilized cells are given in Table 6.9. It is difficult to compare results as the glucose used differs in concentra­tion but it is clear that cell recycling results in increased productivity.

Simultaneous saccharification and fermentation (SSF)

One of the most important advances in ethanol production was the development of simultaneous saccharification and fermentation (SSF). In this system, yeast ferments the glucose produced by the cellulase enzymes in the same vessel and at the same time. The cellulase enzymes therefore do not suffer from feedback inhibition from their products glucose and cellobiose as the fermentation removes these inhibitors. This increases hydrolysis rates, reduces enzyme levels, shortens process time and requires smaller bio­reactor volumes. The drawbacks to the system are the differences in the optimal condi­tions for the enzymes and yeasts which reduces the process rate. The cellulase normally operates at 40-50°C, whereas yeast fermentation is carried out at 30°C. One way of avoiding this is to use thermotolerant yeasts like Kluyveromyces marxianus. The use of yeasts that can assimilate pentoses, for example Candida acidothermophilum, C. brassicae and Hansenula polymorpha, would also improve the process.

The hydrolysis can be carried out by an enzyme mixture or enzyme-producing microorganisms. The organisms used in SSF are often T. reesei which provides the enzymes and S. cerevisiae for the fermentation, run at a temperature of 38°C. The temperature is a compromise between yeast optimum of 30°C and the hydrolysis optimum of 45-50°C. The major advantages of SSF have been found to be increase in hydrolysis due to the reduction in feedback inhibition, lower enzyme or organism requirement, higher product yield, less contamination as sugar levels are kept low, shorter process time and smaller bioreactor volumes.

One of the advantages of biofuels is the possible reduction in engine emissions which contribute to global warming and atmospheric pollution. Here again most of the stud­ies have been carried out with the first-generation biofuels, ethanol and biodiesel.

Biodiesel

There have been a large number of studies on the exhaust emissions from engines using a variety of biodiesel types and concentrations (Graboski and McCormick, 1998; Willianson and Badr, 1998; EPA, 2002). However, it is difficult to compare results as different engines, conditions, and blends have been used. Figure 8.6 shows the mean of a number of studies on the effect of using 100% rapeseed biodiesel on the important engine emissions: hydrocarbons (HC), CO, nitrous oxides (NOx), and PM. Rapeseed biodiesel is the main biodiesel produced in the EU and the consensus shows a considerable reduction in the emission of HC and PM and a small increase in NO . The increase in NO was probably due to an increase in combustion tem­perature. The mean of the three studies on the effect of sunflower biodiesel on engine emissions is shown in Fig. 8.7. With sunflower biodiesel, the reduction in HC was

same as rapeseed biodiesel, and CO and PM were further reduced, but NOx emissions increased. The emissions from an engine fuelled with 100% waste olive oil biodiesel at different loads are shown in Fig. 8.8 (Dorado et al., 2003). As the load increases the reduction in CO, NOx and sulfur dioxide decreases to zero at the highest load. A different result was observed when a 50% sunflower biodiesel blend was used in a marine diesel engine (Fig. 8.9) (Kalligeros et al., 2003). With 50% sunflower biodiesel, the emissions decrease as for waste olive oil biodiesel but do not reach zero at the highest load. The advantages of using biodiesel to reduce emissions may therefore be eliminated when the engine is used at high loads. However, the reduction in emissions may depend on the test engine used.

Fig. 8.8. The effect of waste olive oil biodiesel (100%) on the percentage of changes in emissions from a diesel engine compared with diesel at various loads (Nm). (From Dorado et al., 2003.)

The effect of increasing concentrations of biodiesel on engine emissions is shown in Figs 8.10 and 8.11. As the concentration of commercial biodiesel in blends increased, the emission of CO was reduced and NOx increased (Fig. 8.10). When soybean biodiesel was tested in contrast to commercial biodiesel, CO was not reduced significantly but HC and PM were reduced and NOx increased.

In general, emissions from diesel engines running on blends or 100% biodiesel showed a reduction in CO, HC and PM, but an increase in nitrous oxide (NOJ levels. The reason for this change in emissions is thought to be the higher oxygen content of biodiesel, which gives a more complete combustion of the fuel and this reduces CO, HC and PM. The Environmental Protection Agency (EPA) has compiled the results of a number of studies on the effect of biodiesel content on emissions and the results were

similar to those observed in Figs 8.10 and 8.11. In a study using a MAN diesel bus engine, the fuel injection characteristics were different for diesel and rapeseed-derived biodiesel (Kegl, 2008). The biodiesel when injected into the engine forms a longer and narrower spray than mineral diesel, caused by a higher injection pressure, increased by low fuel vaporization and atomization due to higher surface tension and viscosity.

The reasons for the increased NOx production when using biodiesel may be the higher combustion temperature and injection characteristics. The increase in nitrous oxide (NOx) is probably due to the raised combustion temperature which is known to increase NOx formation. Advanced injection is caused by the higher bulk modulus of compressibility of biodiesel which allows the pressure wave from the pump to the nozzle to speed up, therefore advancing the timing. It has been observed that retard­ing the timing can in some way reduce NOx emissions. In order to reduce the emission of NOx with biodiesel, the injection timing was altered and the optimum setting was found to be 19° (°CA BTDC, degree of crankshaft angle before top dead centre) compared with 23° for diesel. The effect of altering the injection timing on emissions of CO and NO is shown in Fig. 8.12. The lowest NO emission was obtained at 21°, but the lowest CO emission was at 24°. In all the studies on emissions, no evidence has been given that the engines were optimized for biodiesel, and therefore modifica­tions such as altering the timing may reduce emissions of NOX.

Another way of reducing NOx production is to use exhaust gas recycling (EGR). Diesel engines fuelled with Jatropha oil biodiesel produce more NOX than diesel (Pradeep and Sharma, 2007). In this case exhaust gas recirculation was tested as a system to reduce NOX. The exhaust gases consisting of carbon dioxide and nitrogen are recirculated and injected into the engine inlet, reducing the oxygen concentration and combustion temperature which reduces NOX. The level of recirculation is critical because if the oxygen is reduced too far, incomplete combustion will produce higher levels of hydrocarbon, CO and smoke. In this case with a single cylinder diesel engine, the optimum recirculation was 15% as can be seen in Fig. 8.13.

Fig. 8.12. Effect of injection advance on emissions, the normal setting for diesel is 23°. (From Carraretto et al., 2004.)

100
80
60
0
Biodiesel + 15% EGR
Diesel	Biodiesel
□ Combustion time DNo*.

Fig. 8.13. Comparison of combustion duration (degrees) and NOx emissions for Jatropha sp.-derived biodiesel and diesel using exhaust gas recirculation. (From Pradeep and Sharma, 2007.)

Whatever biodiesel is used the scale of reduction in emissions will also be depend­ent on the engine characteristics such as combustion chamber design, injector nozzle, injection pressure, air-fuel mixture, load and other features. Therefore, the reduction in emission will vary from one diesel engine design to another.

Ethanol

Small quantities of ethanol (3-6%) have been added to petrol to increase the oxygen content to ensure complete combustion and reduce the emission of HC and PM. When high concentrations of ethanol are used such as the E85 fuel in a standard petrol engine, CO and NOx are reduced compared with petrol but there is an increase in HC (Fig. 8.14). In the flexible fuel engine where the conditions are optimized for E85 fuel, emissions of CO and NO were reduced but HC and methane were increased.

x

Fig. 8.14. The effect of E85 on the emis­sions from a standard engine and a flexible fuel engine. CO, carbon monoxide; NOx, nitrous oxides, CH4, methane. (From Wang et al., 1999.)

Dimethyl ether (DME)

A number of studies have been carried out on the emissions from a compression igni­tion engine (diesel) running on DME and DME blends. DME has been shown to pro­duce low noise, smoke-free combustion and reduced NO when used in an internal combustion engine (Huang et al., 2006). DME, because of its high cetane number and low boiling point, has been are developing truck and bus transport fuelled by DME. The emission levels from these development vehicles when run on DME show virtually no PM and low levels (0.5-2.0 g/kwh) of NOx.

Reduction in Carbon Dioxide Emissions used at 100% or as an oxygenated addition to diesel. When DME was used in a diesel engine, it reduced NOx and SOx emissions and was sootless (Semelsberger et al., 2006). Large motor manufacturers when Using Biofuels

Biomass

The carbon dioxide fixed during photosynthesis is released when the biological material is burnt which means that there is no net gain in atmospheric carbon dioxide. This indi­cates that biological materials are ‘carbon neutral’ in nature and therefore ideal for the mitigation of global warming. However, one of the main arguments against biofuels of all types is that fossil fuels are used and carbon dioxide released during the production of biofuels. Therefore, biofuels are not 100% carbon-neutral. Clearly fossil fuel will pro­duce the greatest amounts of carbon dioxide as they release carbon dioxide fixed millions of years ago, whereas biomass has fixed its carbon dioxide in the last 10 years. Table 8.2 gives some values for the carbon dioxide generated per megajoule (MJ) of energy during the combustion of fossil fuels. Coal and coke have the highest carbon content and pro­duce the highest levels of carbon dioxide. Natural gas (methane) has the lowest GHG emissions of the fossil fuels and is one of the reasons why electricity generation was switched to gas in the UK in the 1990s. To be suitable for carbon dioxide mitigation, biofuels will have to have GHG emissions much lower than those for fossil fuels.

The bioenergy crops and waste biological materials described in Chapter 4 if used for energy will clearly reduce the amount of carbon dioxide accumulating in the atmosphere. The carbon dioxide emissions from biomass crops were compared with those emitted from the solid fossil fuels coal and coke (Fig. 8.15). The carbon dioxide produced per megajoule when various biomass sources are burnt or used to generate electricity are compared with two solid fossil fuels coal and coke in

Fig. 8.15. Carbon dioxide emissions from bioenergy crops when burnt or used to generate electricity. SRC, short rotation coppice. (From Gustavsson et al., 1995; Dubisson and Sintzoff, 1998; Matthews, 2001; Bullard and Elsayed et al., 2001; Heller et al., 2001; Keoleian and Volk, 2005.)

Fig. 8.16. Greenhouse gases saved when biomass is either gasified or combusted in g CO2 equivalents/MJ. (From Lettens et al. 2003.)

Fig. 8.15. The combustion of the biomass produces 20 times less carbon dioxide than coal. When short rotation coppice (SRC) and Miscanthus are used to generate elec­tricity more carbon dioxide is formed per unit of energy than simple combustion.

Biomass can also be gasified and the gas used as fuel for gas turbines to produce electricity. Combustion and gasification as a source of energy have been compared using three biomass sources in terms of the amounts of carbon dioxide saved (Fig. 8.16) (Lettens et al., 2003). The perennial grass Miscanthus sp. gives the least carbon dioxide and mixed coppice the greatest, and there appears to be little signifi­cant difference between combustion and gasification.

The fission process releases large amounts of energy, about 50 million times that of coal on a weight basis, which means that very little uranium fuel is required. No combustion is involved so that there are no emissions but fission generates radioac­tive materials, some of which have very long lives. There are also considerable prob­lems in the reprocessing and disposal of spent fuel, the possibility of leaks or accidents, and the decommissioning of the power stations at the end of their working life. The accidents at the nuclear-generating plants at Three Mile Island and Chernobyl have shown that, despite very stringent safety arrangements, accidents can occur. This has made the public wary of nuclear power and more likely to accept alternative sources of power. Nuclear power is also regarded as not sustainable as there is a limited supply of uranium and its production involves the production of greenhouse gases.

In this case, growing in the light photosynthesis is used for growth and to store car­bohydrates. When the organism is switched to aerobic dark conditions the stored carbohydrates or cell material is metabolized in the same way as in Fig. 5.12, yielding hydrogen. This type of two-stage process has been observed in Cyanobacteria sp.

Photo-fermentation

Some photoheterotrophic bacteria (purple non-sulfur bacteria) such as Rhodobacter sp. and Rhodospirillum sp. convert organic acids in the light into carbon dioxide and hydrogen (Fig. 5.13). The key enzyme in these organisms is nitrogenase which requires ATP to produce hydrogen. The nitrogenase is inhibited by oxygen, ammonia and high nitrogen to carbon ratios so that oxygen-free conditions are required.

Carbon monoxide metabolism (water-shift reaction)

Some photoheterotrophic bacteria, for example Rhodospirillum rubrum, can metab­olize carbon monoxide in the dark in a reaction similar to the water-shift reaction:

CO + H2O = CO2 + H2 (5.11)