The fatty acid profile of biodiesel will reflect the fatty acid profile of the oil used in its production. Table 7.14 gives the fatty acid profiles of biodiesel produced from a variety of oils. The most striking differences are perhaps the biodiesel produced from palm, coconut and linseed oil. Both palm and coconut oil contain high levels of lauric acid and linseed a high concentration of linolenic acid. This will have a profound effect on the characteristics of the biodiesel. The high level of saturated fatty acids in palm biodiesel will mean poor cold temperature characteristics with a cloud point of 8°C and a pour point of 6°C. Linseed biodiesel has a high proportion of unsaturated fatty acid esters and should exhibit rapid oxidation and polymer formation. The properties of biodiesel is affected by the proportion of long — and short-chain fatty acids and the presence of one or more double bonds (saturated and unsaturated) (Fig. 7.14).

Fig. 7.14. The effect of chain length and degree of saturation on cetane number, oxidative stability, lubricity and cold flow.

Biodiesel has a similar energy content and viscosity as diesel in addition to a number of positive attributes such as increased flash point, non-toxic, rapidly bio­degradable, lower emissions and increased lubricity (Table 7.15). Negative aspects are the poor low-temperature characteristics and oxidative stability. Many of these properties are affected by the fatty acid ester content depending on chain length and the presence of double bonds.

Other parameters which are included in the table are acid value and oxidative stability. Acid value is the measure of the unsaturation of the fatty acids in the mixture

Table 7.15. Properties of various biodiesel. (From Graboski and McCormick, 1998; Sheehan et al., 1998; Williamson and Badr, 1998; Srinvastava and Prasad, 2000; Mittlebach and Gangl, 2001; Al-Widyan et al., 2002; Antolin et al., 2002; Haas, 2005; Sarin et al., 2007.) Density Cetane Energy Viscosity Cloud Pour Flash Acid value Oxidation Oil source (mg/ml) number content (kJ/kg) (cPs) point (°C) point (°C) point (°C) (gig/100 g) stability (h) Soybean 0.88 52.5 40.0 4.2 2 -1 169 0.15 3.8 Rapeseed 0.88 52.2 40.5 5.8 -1 -6 162 0.16 5.6 0.32 Sunflower 0.88 48.0 40.0 4.22 0 -4 1 83 0.179 0.2 1.73 Cotton 0.88 51.2 40.2 — — 3 110 — — Palm oil 0.87 53.0 39.5 4.5 8 6 174 0.24 13.37 Tallow 0.88 58.8 40.0 4.45 12 9 106 — Linseed 0.88 — 40 3.32 0 — — 0.335 — Jatropha — 57.1 — 4.4 4 — 163 0.48 3.23 Pongamia — 55.1 — 4.16 4 — 141 0.1 2.35 Cooking oil 0.88 — 39.3 15.1 0 0 109 Soybean 0.88 51.3 — 4.3 6 — 169 soapstock

measured as gI2/100 g sample. Oxidative stability is the time required to induce the production of volatile breakdown products when incubated at elevated temperatures, in hours.

The sugars produced by the hydrolysis of lignocellulose are glucose from cellulose, and xylose, arabinose, glucose, mannose and galactose from hemicellulose and galac — turonic acid from pectin. Saccharomyces cerevisiae can ferment glucose, mannose and galactose but not the other sugars. There are some yeasts which can metabolize xylose, but only a few can ferment xylose to ethanol. One approach was to introduce xylose catabolism into ethanol-producing yeast. Xylose-utilizing yeasts Pichia stipitis, Pachysolen tannophilus and Candida shehatae have very fastidious growth condi­tions. In S. cerevisiae, xylose is reduced to xylitol and xylitol is reduced to xylulose which is slowly metabolized. In the xylose-metabolizing yeasts, xylose is converted into xylulose by xylose reductase and xylitol dehydrogenase. In the reaction NADH and NADP are produced which need to be regenerated and can normally be carried out aerobically, but under anaerobic conditions no electron acceptors are available. A recombinant S. cerevisiae has been produced with the complete xylose pathway from P. stipitis, but the cells only metabolize xylose aerobically (Fig. 8.31) (Prasad et al., 2007), and under anaerobic conditions xylitol was produced.

Arabinose is also present in lignocellulose hydrolysates which S. cerevisiae cannot ferment. Overexpression of all structural genes of a fungal arabinose pathway pro­duced a S. cerevisiae strain capable of fermenting arabinose. However, the production was too low to be commercial.

Another approach is to expand the substrate range of ethanol-producing bacteria. One example was to insert the Z. mobilis ethanol pathway into microorganisms which can use xylose, such as Escherichia coli. This involves the insertion of pyruvate decarboxylase and alcohol dehydrogenase (Fig. 8.32).

Another group of enzymes of interest are the cellulases and genetic manipulation has been used to increase their efficiency and production. To date cellulases have been expressed in S. cerevisiae, Z. mobilis, E. coli and Klebsiella oxytoca but with limited success (Chang, 2007).

In 1990, the UK government published its first white paper on sustainability, fol­lowed by, in 1999, a paper, A Better Quality of Life. More recently the Department of Trade & Industry (DTI) produced a white paper, Our Energy Future — Creating a Low Carbon Economy in 2003 in which there was an aim to reduce greenhouse emis­sions by 60% by 2050 compared with the 1990 value.

As part of the measures to reduce carbon dioxide emissions, the UK has intro­duced a major policy, the Renewable Transport Fuel Obligation (RTFO), formerly Non-Fossil Fuels Obligation, in response to the EU Biofuels Directive (2003/30/EC). The government has enacted the Renewable Obligation Order, under which 10% of electricity generation should come from renewables by 2010. Most of the renewable — produced electricity is expected to come from wind and co-firing with biomass. In the UK, 6.7% of the electricity should be generated from biomass by 2006/2007 and 15.4% by 2015/2016. The current schemes under the Renewable Obligation Order include the following:

• Any biomass, including imported coconut and olive waste, can be co-fired until March 2009.

• Of the total produced, 25% of co-fired biomass must be from energy crops from April 2009 until March 2010.

• Also, 50% of co-fired biomass must be from energy crops from April 2010 until March 2011.

• In addition, 75% of co-fired biomass must be from energy crops from April 2011 to March 2016.

• Co-firing ceases to be eligible for renewable obligation certificates (ROCs) after March 2016.

The most recent figures given for the UK are shown in Table 3.3 and Fig. 3.5, where the greenhouse gases are quoted as millions of tonnes of carbon. Emissions of carbon

779.9

(%)

CO2 GHG

Fig. 3.5. UK greenhouse gas and carbon dioxide emissions. The greenhouse gases include carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride and allow for EU ETS in 2005 and 2006. (Redrawn from Defra, 2008.)

dioxide fell by 12% between 1990 and 2006, helped by emissions trading in 2005 and 2006 (EU ETS). The overall drop in greenhouse gases which include carbon dioxide, methane, nitrous oxide and the chlorofluorocarbons and sulfur hexafluoride was 20.6% (Table 3.3) in 2006, well below that required by the Kyoto Protocol. The factors contributing to this reduction are given as less coal and oil used and more gas and renewables used, particularly to generate electricity (Defra, 2008).

The RTFO also sets levels of biofuels, bioethanol and biodiesel to be added to overall fuels sales with a final value of 5% by volume. The levels of obligation are given in Fig. 3.6 in terms of volume. If the energy content of the fuel is used, the add­ition represents 5.75% because of the lower energy content of biofuels. However, the levels of biofuel use in the UK have suffered some difficulties and are still low (Table 3.4). The UK Report to the European Commission quoted these difficulties as ‘the level of duty differential in the UK and the length of certainty the mechanism offers has proved insufficient to stimulate the level of investment in production capacity and infrastructure required to meet the Directive’s objectives’.

The development of biofuels and the effects of policy can be seen in the contrast­ing positions in the UK and Germany. The markets for biofuels have been compared for the UK and Germany in terms of the directives, policies and standards (Bomb et al., 2007). In 2002, the German government exempted all biofuels from excise tax until 2009 to encourage their adoption. Germany is now the largest producer of biodiesel in the EU, and after 2004 the excise duty was exempted for biodiesel blends. This has made biofuels competitive with fossil fuels in terms of cost. However, some groups in Germany have been critical of biofuels, including Friends of the Earth (Bomb et al., 2007). At present the market for 100% biodiesel has shifted to trucks, and automobile manufacturers are making efforts to provide warranties for 100% biodiesel and blends for cars. In 2004 no ethanol was produced in Germany and any used was imported from Brazil, but a number of ethanol plants are now under

construction. Ethanol use in Germany has been encouraged with support of the farm sector, energy security and reduction in carbon dioxide emissions. Not only has there been an exemption of excise tax but additional factors such as set-aside premiums, partial state monopoly and price controls for agricultural products.

In contrast, in the UK, the excise tax on biodiesel was reduced in 2002 by £0.20 and ethanol by £0.20 in 2005. The excise tax still remained at £0.275 on both bio­fuels, which means that only biodiesel produced from used cooking oil is competitive in price with fossil fuels. However, production of biodiesel in the UK has increased and in 2008, 404,000 t of biodiesel from a variety of oil sources and 55,000 t of bioethanol from sugarbeet were produced.

The main driver for the introduction of biofuels in Germany has been the exemp­tion of excise tax, which was not implemented in the UK, and explains the differences in production in both countries. The tax concession is expected to end in 2009 in Germany and it will be interesting to see how biofuels will compete after this time. The result of the policies or the lack of policies in the two countries explains the contrasting adoption of biofuels, and the conclusions of a study of the two countries are as follows (Bomb et al., 2007):

• Consumers will buy fuel on price, rather than for green credentials.

• National government commitment for the establishment of a biofuel industry is required.

• Low-level blending is the easiest method of introducing biofuels, but sufficient volumes for anything more than 5% addition are not available.

• Niche markets are available for biofuels in areas of environmental sensitivity.

• Oil companies are more supportive of biodiesel than bioethanol.

• Environmental impacts and carbon balances vary between biofuels, and there have been some questions about bioethanol.

• A fuel certification system is needed for sustainability and fuel composition.

• Support for biodiesel and bioethanol does not stop other technologies from being developed.

Pyrolysis is the heating of the biomass in the absence of air at temperatures of 300- 500°C. Under these conditions the products are gas, charcoal and an oil (bio-oil) which after treatment can be used in a diesel engine. The main treatment is to reduce the viscosity which is too high to be used in a diesel engine. The possible uses of pyrolysis products are given in Fig. 4.8. The crude bio-oil can be used in gas turbines and engines but for the standard diesel engine it requires upgrading. The bio-oil can also be used in boilers and co-fired in power stations and after gasification it can be

converted into transport fuels. It can be a source of chemicals. The charcoal can be used for industrial processes or as a source of heat. The pyrolysis liquid can have a number of names such as pyrolysis oil, bio-oil, bio-crude-oil and wood oil. Bio-oil is a dark brown acidic liquid consisting of a complex mixture of oxygenated hydrocar­bons and water which is not miscible with petroleum-based fuels. Some of the proper­ties of wood-derived bio-oil are given in Table 4.10.

Bio-oil can replace fuel oil in static operations such as boilers, furnaces, engines and turbines for the production of heat and electricity but to be used as a transport fuel the viscosity needs to be reduced.

Conclusions

Biomass is a solid biofuel which can be in all forms of wood — from trees, crop resi­dues, animal and municipal waste — in addition to crops specifically grown for energy.

Energy can be extracted from biomass by direct burning, co-firing with coal, gasifica­tion and pyrolysis. Gasification and pyrolysis can yield liquid fuels and can be used in electricity generation. The process for the generation of petrol and diesel in the Fischer-Tropsch (FT) process is discussed in Chapter 7. The main use of biomass has been in the generation of electricity and in combined heat and power systems.

The estimates of the energy available in biomass vary greatly from 93 to 1135 EJ against the global energy requirement of 425 EJ. It is clear at first glance that bio­mass can provide a significant amount of energy and Table 4.7 indicates that most regions do not use a high proportion of their potential. Biomass use is perhaps more efficient than biodiesel and bioethanol as the whole plant can be used rather than only a small proportion. However, other studies indicate that there may have been an overestimation of the biomass actually available. The surplus biomass may be much smaller after woodfuel has been taken into consideration. In addition, biomass is widely spread and seasonal and will require collection and transport if it is to be used and this will reduce the amount available. The other problem is that biomass can only be used once and there are insufficient amounts available for all uses espe­cially in the UK. It may be that the FT process for diesel and petrol production will be the most suitable for biomass. For biomass to be adopted as a sustainable source of energy, it requires government help in terms of tax concessions and provision of sites. The lack of government support is not the only impediment to the introduction of biofuels, as local opposition to renewable schemes such as wind farms has been strong in many cases. One example of local opposition triumphing over government initiative was the projected biomass electricity generation plant in Cricklade, Wiltshire, which was not granted planning permission (Upreti and van der Horst, 2004). A number of other similar schemes (27%) have been rejected for similar rea­sons. The plan was to build a 5.5 MW power station at Kingshill Farm, an estab­lished recycling site, to generate electricity under a Non-fossil Fuel Obligation (NFFO 3) contract capable of supplying electricity to more than 10,000 homes, at a time when Swindon was expanding rapidly. The site required 36,000 t of dry wood supplied from a 30-mile radius using forestry wastes and SRC. The rationale for the site was as follows:

1. Good access to forestry wastes.

2. The area was suitable for growing SRC.

3. Good road connections for fuel delivery.

4. Good access to electricity distribution.

5. It delivered electricity in a decentralized location.

6. Local employment, 15 permanent jobs and other jobs during construction.

7. Diversification in local agriculture.

However, despite these points individuals and organizations opposed the develop­ment and the main objections included:

• It would set a precedent for other local industrial developments.

• It would contradict local designations, the Area of Special Archaeological Significance and Rural Buffer Zone.

• It would lead to a large increase in the movement of heavy goods vehicles (HGVs).

• The six chimneys proposed were very tall and would affect the view.

• Approximately 117 million l of water would be lost into the atmosphere.

• The power station would produce smell, dust, noise and other emissions.

• Long-term general health impacts.

• Damage to Cricklade’s south-east meadows and water systems.

• Possible lack of compensation if anything should go wrong.

• There would be a negative effect on property prices.

These concerns were clearly very powerful and in September 2000 the application was rejected for the following reasons:

The Biomass Power Station is a major development proposal which would, if allowed, seriously undermine the openness of the rural landscape, resulting in a loss of coun­tryside creating an inappropriate form of major development in the Rural Buffer, contrary to the Wiltshire Plan Review and Policy DP 13 of the Wiltshire County Structure Plan 2011 Proposed Modifications.

(Upreti and van der Horst, 2004)

It was concluded that public relations strategies by developers, role of the media in amplifying risk, lack of proper information and lack of public understanding of bio­mass power plants were the main reasons for the lack of success. The UK government needs to generate a public awareness of the benefits and need for sustainable electric­ity generation or NIMBYism will prevail.

Once the hemicellulose and lignin have been removed ethanol production from cel­lulose and is often carried out in two stages, first the hydrolysis of cellulosic material to sugars by cellulase enzymes, and second the fermentation of the sugars. The rate of cellulose hydrolysis is about half the speed of the production of ethanol by yeast. A number of process formats have been developed.

There have been a number of studies on fuel economy and emissions concentrating on biodiesel and bioethanol. Biofuels have different energy content from the fuels that they replace, and therefore this will affect their fuel economy. Fuel economy can be meas­ured in terms of volumetric fuel consumption, brake-specific fuel consumption (BSFC, kg/kWh fuel flow/power) and brake thermal efficiency by measuring the torque and fuel consumption. In other studies, fuel consumption was determined from the carbon dioxide emissions and fuel carbon content but a more accurate value can be obtained when carbon dioxide emissions and fuel consumption are combined. The fuel con­sumption is normally measured at various power outputs (kW) and loads (Nm).

Biodiesel

The effect of the concentration of biodiesel used in blends on BSFC measured as MJ/ kWh is shown in Fig. 8.1. Although the histogram shows a drop in BSFC as the

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biodiesel concentration increases, the final value is only reduced by 4%, and with a standard deviation of 1.5%, it appears that fuel composition has little effect on fuel consumption. The type of oil used to produce biodiesel has also been investigated, and little difference can be seen in the fuel consumption using a variety of biodiesel (Fig. 8.2). This is perhaps not surprising as the calorific value of biodiesel made from various oils varies very little.

The change in engine load will also affect fuel consumption. A number of studies have been carried out and Fig. 8.3 shows a comparison of diesel with a 50% mixture of sunflower oil biodiesel blended with marine diesel tested in a single cylinder,

indirect injection diesel engine. In the case of the marine diesel when compared with a blend of 50% sunflower biodiesel, there was little difference in BSFC over the loads imposed apart from an increase in fuel consumption from around 400 g/h to over 900 g/h. In a similar study on the effect of load on BSFC using a biodiesel made from waste olive oil compared with diesel and oil, there was an increase of 7.5% in fuel consumption compared with diesel (Fig. 8.4).

A number of studies using a variety of biodiesel blends and engines have shown that increases in BSFC can vary from 0 to 13.8% fuel consumption increase over diesel (Table 8.1). Biodiesel showed higher fuel consumption of 13.8% measured as BSFC using soy oil biodiesel in a four cylinder, four-stroke turbocharged John Deere engine (Monyem and Van Gerpen, 2001). The same result was found with biodiesel

from waste olive oil run in a three cylinder, four-stroke Perkins diesel engine where the BSFC was 7% greater (Dorado et al., 2003). The reduction in fuel consumption may be a consequence of the lower energy density of biodiesel (33-42 MJ/kg) com­pared with diesel (46 MJ/kg). The variation in the reduction in BSFC may also be due to the different combustion conditions found in the different engines.

The following are possible renewable, sustainable energy sources which will also reduce greenhouse emissions:

• Nuclear power.

• Hydroelectric.

• Tidal.

• Wave.

• Wind.

• Geothermal.

• Solar.

• Biological.

In photosynthesis, solar energy is used by photosystem II to split water and release oxygen, electrons and protons. Photosystem I uses solar energy to produce the reducing power required to fix carbon dioxide, and the electrons are passed along the electron transfer system, eventually generating ATP. In the direct use of solar energy to convert water into hydrogen the electrons are transferred along the electron transfer chai until the penultimate step catalysed by ferredoxin, where the electrons are transferred to a hydrogenase, converting protons into hydrogen (Fig. 5.12). These reactions are carried out by green microalgae and blue-green Cyanobacteria sp.

However, hydrogenases are inhibited by oxygen so that the concentration of oxygen needs to be kept below 0.1%. In the case of C. reinhardtii oxygen is removed

Glucose

I

I

Fig. 5.11. The pathway involved in the production of acetone and butanol by Clostridium acetobutylicum.

H2O

by respiration but because substrate is consumed the efficiency is low. In some cyano­bacteria such as Anabena cylindrica photosynthesis is split between two types of cells. Photosystem II functions in the vegetative cells whereas the heterocysts contain the carbon-fixing portion and a hydrogenase enzyme which is protected from the inhibi­tory effects of oxygen by a thick cell wall.

A number of herbaceous plants accumulate long-chain hydrocarbons (terpenes) par­ticularly those in the Euphorbiaceae such as the annuals Hevea brasiliensis, Euphorbia lathyris (3-10 t dry weight/ha/year) and Calotropis procera (10.8-21.9 t dry weight/ ha/year). The hydrocarbons are produced, as latex, which consists largely of long — chain C 30 triterpenoids which can be cracked (pyrolysis) to form petrol and diesel. These herbaceous plants can be grown in various parts of the world and give quite good yields in terms of dry weights per hectare. Trees like Eucalyptus globus, Pittosporum resiniferum and Copaifera multijuga also produce oils, often in the fruit as in P. resiniferum, which can also be converted into petrol and diesel. However, with the Brazilian tree C. multijuga, the trunk can be tapped and the oil used directly as a diesel replacement.

Microalgal Hydrocarbons

A small number of algae are capable of producing terpenoid oils, one of which is Botrycoccus braunii, which is reported to accumulate up to 86% dry weight as oil (Dote et al., 1993). Hydrocracking of the oil yielded 62% petroleum, 15% aviation fuel, 15% diesel and 3% heavy oil. The large-scale cultivation of this alga is under development.

In the UK it has been estimated that there are 7.8 million t of wood biomass available which could be used to produce electricity (Table 4.8, Woodfuel, 2007). The energy content of wood biomass is around 15 GJ/t and therefore 7.8 million t represents an energy content of 0.118 EJ. The UK electricity demand in 2005 was 387.3 TWh (1012) which is 1394 PJ (1015 J). Electricity generation is around 30% efficient and therefore

195 I

0.118 EJ of energy would yield 35.4 PJ electricity which represents 2.54% of the total requirement. To supply all the electricity would require 92 Gt of biomass which at 12 t/ha represents 7.6 Mha, which is 42.6% of the agricultural land. Another study by Powlson et al. (2005) estimated that wood biomass, waste straw and the conver­sion of some grassland to biomass could yield 12.2% of the UK’s electricity. The yields from forestry waste, wheat straw, sugarbeet, set-aside and specifically grown biomass are given in Table 4.9. The woody biomass can only be used once and per­haps it would be better to use the biomass for the FT synthesis of liquid fuels and generate electricity from other renewable sources.