High-pressure and high-temperature steam can be used to treat the lignocellulose material where hemicellulose is hydrolysed by acids released during steam treatment. Acid addition increases the sugar yields but sulfuric acid can yield sulfur dioxide which can be inhibitory to further treatment. Steam treatment is less energy intensive than mechanical disruption.

The present and possible future replacements for fossil fuels have been described in Chapters 4, 5, 6 and 7. The biofuels can be solid, gaseous and liquid as follows:

Solid:

• Biomass.

• Waste materials.

Gaseous:

• Methane.

• Hydrogen.

• Dimethyl ether (DME).

Liquid:

• Methanol.

• Ethanol.

• Biobutanol.

• Synthetic Fischer-Tropsch (FT) petrol.

• Biodiesel.

• Bio-oil.

• Synthetic FT diesel.

• Microalgal biodiesel.

Solid fuel replacements

As described in Chapter 4, biomass in the form of wood, specific energy crops, crop residues and organic wastes can be used to replace coal and natural gas. Biomass can be burnt, co-fired with coal and gasified to generate electricity and heat. Small- to medium-size heating systems have been developed to use pelleted biomass. The pyrolysis and gasification of biomass can be used to produce a liquid fuel, which can be used for transport, electricity generation and to provide heat.

All plants fix carbon dioxide during photosynthesis, which is released again when the plant dies and the plant material is degraded by microorganisms. The carbon dioxide fixed is used to synthesize storage compounds such as starch and oils, and cellular struc­tural components such as cellulose and lignin. It is the structural components that are the slowest to degrade when the plant dies and these are the highest in woody plants.

It has been estimated that the amount of carbon taken up by vegetation was 3.2 Gt C/year, and 1.7 Gt C/year is lost mainly through deforestation, which gives a net increase of 1.5 Gt C/year. The carbon emissions from fossil fuels are 6.4 Gt C/year so that without any mitigation measures, some 23% of the carbon dioxide is removed. However, afforestation does not remove all the carbon dioxide produced and the atmospheric levels are still rising, but it does show the potential of biomass to seques­ter carbon dioxide. Table 3.7 indicates the potential of carbon dioxide sequestration by planting new forests, managing existing forests, managing crops, etc.

Woody plants have a carbon content of 0.54 kg carbon per kilogram of dry wood (Cook and Beyea, 2000). If tree growth is linear in the early years, the carbon dioxide removed can be calculated. The estimates for carbon dioxide sequestered by maize, switchgrass, short-rotation coppice willow and standing forest wood are shown in Table 3.8. Some of the carbon dioxide is only sequestered on a temporary basis as the switchgrass and short-rotation coppice will be burnt as a fuel, and other parts of the crops will be returned to the soil where they will be degraded. However, if the wood is used as construction material, the carbon dioxide will be locked up for considerably

longer. The carbon dioxide sequestered in forest material is less because growth is slower and forest regeneration, once harvested, is not always certain. The world is losing areas of dense forests either to building or agriculture, and reversal of this trend would help to reduce carbon dioxide levels in the short term.

If biomass is gasified at temperatures above 700°C, a mixture of gases and charcoal is produced. The gas produced contains mainly hydrogen, carbon monoxide, meth­ane, carbon dioxide and nitrogen. More hydrogen can be produced by a water-shift reaction where carbon monoxide is reacted with water to form carbon dioxide and hydrogen:

Biomass + heat + steam ^ H2 + CO + CO2 + CH4 (5.5)

+ hydrocarbons + charcoal

Water-shift reaction

CO + H2O = CO2 + H2 (5.6)

Using a fluidized bed gasifier with a catalyst it has been possible to obtain 60% hydrogen production. The main problem with gasification is the formation of tar, which can be minimized by gasifier design, control and additives (Ni et al., 2006). The hydrogen can be separated from the other gases by pressure swing adsorption.

Figure 7.3 gives an overall view of the methods that can be used to produce biofuels from coal and biomass, and Table 7.2 gives the maximum concentration of impurities that syngas should have in order to be suitable for FT synthesis. Too high a concen­tration of impurities will poison the cobalt catalyst in the process.

The exothermic FT synthesis combines H2 and CO when passed over a cobalt catalyst at a temperature of around 260°C producing a mixture of hydrocarbons including petrol (C8—Cn) with an average of C8H18 and diesel (Cn—C21) with an overall hydrocarbon average of C16H34.

2H2 + CO = CH2 + H2O (7.3)

The FT synthesis unit operations are given in Fig. 7.4 when using dried biomass. The dried biomass is gasified in an entrained flow gasifier at 900-1300°C in the presence

139 I

of steam and oxygen. In some cases, the biomass may be pretreated by pyrolysis or torrefaction (Fig. 7.3) or even taken from a fluidized bed gasifier. The ash is removed and the gas is cleaned of sulfur-containing compounds, and then the CO/H2 ratio is adjusted by the water shift reaction. The cleaned gas is then passed over a cobalt cata­lyst in the FT reactor producing a range of hydrocarbons from CH4 to waxes. The alpha factor shown in Fig. 7.5 describes the proportion of the various products formed, and this is affected by the catalyst used and process conditions. Maximum diesel production is around 30% of the total products at an alpha value of 0.85-0.9. The lower-temperature conditions which favour diesel production are 260°C, with cobalt-based catalyst at a pressure of 15-40 bar.

The process of gasification, gas cleaning and FT synthesis is a complex chemical process where the larger the scale, the more economic the process (Fig. 7.6). As the size increases, the conversion costs reduce, levelling out at around 1800 MWth (mega­watts thermal) while the other costs remain static.

Thus, the production plant, using biomass to produce syngas and FT products, will be much larger compared to other biomass processes because of the increased efficiency

Fig. 7.5. The effect on the products formed in the Fischer-Tropsch process of the alpha factor, the probability of chain growth.

Fig. 7.6. The effect of scale on the economics of the Fischer-Tropsch process. (Redrawn from van der Drift and Boerrigter, 2006.)

of the FT process at the larger scales. The fossil fuel-based FT plants are huge, above 1000 MWth. With biomass, there may be a problem in supplying such a large process without extensive transport of biomass from distant sources. This may mean that any biomass-based plant is likely to be smaller at 100 MWth. However, there are ways to treat biomass to reduce its volume, so that it can be transported easily to the large central FT plant. The first is torrefaction, where biomass is heated at 250-300°C, which turns it into a brittle, solid mass that can be treated like coal. The second option is pyrolysis at 500°C that converts the biomass into oil/char slurry (Fig. 7.3).

At present syngas is mainly used by the chemical industry (Fig. 7.2), but some 8% (500 PJ per year) is used to produce fuels called GTL. FT processes are operated by Sasol in South Africa, and Shell in Bintulu, Malaysia. These are large plants of 1000 MWth due to the economies of scale and in one case use natural gas (CH4).

To supply the EU-25, ten large plants of 1000 MWth would be required. At present, small — to medium-scale gasification systems of biomass are used for distrib­uted heat and power (CHP) production. The larger scale of the GTL production also allows for the possibility for CO2 capture and storage.

Both diesel and petrol replacements can be produced from biomass and waste organic materials by gasification followed by FT synthesis. This process yields a mixture of

Fig. 8.26. The percentage of agricultural land required to produce 5, 20 and 100% of UK diesel using biodiesel, FT diesel and microalgae. The UK’s agricultural land is 18,016,981 ha (18 Mha) and the diesel required is 23,989,000 t. Yield of biodiesel is 1t/ha, FT diesel 1.35 t/ha and microalgal biodiesel 22.93 t/ha.

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Fig. 8.28. The percentage of agricultural land required to produce 5, 20 and 100% ethanol replacement for petrol from sugar and FT petrol. The petrol use in the UK is 26,916,216,000 l (19,918,000 t) and the agricultural land is 18,016,981 ha. Ethanol yields of sugarbeet are 2500-3000 l/ha; wheat 500-2750 l/ha; lignocellulose 1840-3580 l/ha. FT petrol and FT kerosene yields are 1.35 t/ha.

hydrocarbon which includes diesel and petrol (naphtha) fractions. It has been estimated that FT diesel production is about 45% efficient and therefore the 7,854,422 t of woody biomass available in the UK would yield 3,534,489 t FT diesel if all the products from the FT process were diesel. Other authors give different efficiencies for the pro­duction of liquid fuels from biomass. Van der Drift and Boerrigter (2006) quote that the biomass to syngas efficiency was 80% and syngas to liquid fuels 71%, giving an overall efficiency of 57%, and suggest a value of 63% for the production of liquid fuels if all the products are regarded as diesel.

However, the FT process produces a mixture of HC and the yield of diesel is generally around 30% of the total HC produced. This would represent 1,060,346 t of diesel from 7,854,422 t of biomass. The demand for diesel is 23,989,000 t and therefore FT diesel could provide 5.6% of this demand. Clearly any improvement in efficiency would improve the yield, but at present to increase the amount of FT diesel more biomass crops will need to be planted. In some cases with the FT processes, if the naphtha fraction is recycled it gives an overall yield of diesel of above 50%. The FT production of petrol has the same problems as diesel and petrol constitutes 30-40% of the total product.

The land required to produce FT diesel is shown in Fig. 8.26. The supply of FT diesel assumes a yield of biomass of 10 t/ha, 45% conversion, and 30% of the products are diesel giving a yield of 1.35 t/ha of FT diesel. A 5% addition would require

888,0 ha, 4.9% of the agricultural land, and if 100% replacement was attempted it would use 98% of the present agricultural land. Even if 100% of the FT products were diesel, this would still represent 23-29% agricultural land, a figure which is too high as it would affect food production. The land required providing similar quantities of FT petrol and FT kerosene is given in Fig. 8.28. A 5% addition of FT petrol would need 4.1% of the agricultural land and 100% would require 82% of the land. The amount of kerosene required is less than diesel and petrol and therefore less land is needed with 5% requiring 2.2% of the agricultural land and 100% requiring 44.3% of the land. If all the biomass available in the UK was converted to FT petrol, it would yield 3.5 million t if solely used for petrol production, which is 13.0% of the total use. Thus, FT synthesis of petrol and diesel uses non-food crops but it still requires large land areas to give 100% replacement, which may compromise food crops. Another feature is that the woody biomass available in the UK is widely distributed so that not all of it will be available and it may need transporting some distance.

In addition, another problem with the conversion of biomass to fuels is that to be efficient FT production plants have to be large and it is envisaged that the UK would only need three to four of these units. These large units would involve the transportation of large quantities of biomass and subsequent use of fuel. The develop­ment of small, efficient, regional FT plants would be a considerable advance in the provision of FT diesel and petrol.

An alternative to the FT synthesis has been developed where the synthesis gas is converted into ethanol by a bacterial culture (section ‘Commercial Lignocellulose Processes’, Chapter 6). A company, Bioengineering Resources Inc. (BRI), has devel­oped this system and one unit uses 100,000 t of waste, generates 5-6 MW of power and yields 6-8 million gallons of ethanol (22.68-30.24 million l).

If the UK woody biomass of 7,854,422 t was available, it would supply 78 BRI units yielding 1769-2358 million l representing between 6.6 and 8.7% of the ethanol required for 100% replacement of petrol.

The Nature of Biofuels: First-, Second — and Third-generation Biofuels

The alternative energy sources are derived from biological material and it is these sources that are the main focus of the book. Recently the use of biological materials to provide a source of energy that is renewable and can mitigate carbon dioxide accu­mulation has attracted considerable attention (Chum and Overend, 2001; Hamelinck et al., 2004; Cockroft and Kelly, 2006).

The range of biofuels that can be produced is listed below and includes biofuels that are being used at present and others which are still at the development stage. Biological-based fuels can be solid, liquid and gaseous, and the physical state of the fuel greatly influences the way it is used. It is the developmental stage that has been used to divide biofuels into first-, second- and third-generation biofuels (Fig. 4.1). Those biofuels currently used and produced in large quantities are the first-generation biofuels. The biofuels that have been produced but technical difficulties and high costs have delayed their application on a large scale are the second generation. The third-generation biofuels are those which are still at the research and development stage.

Solid fuels:

• Biomass.

• Wastes.

Gaseous fuels:

• Methane (biogas).

• Hydrogen.

• Dimethyl ether (DME).

Liquid fuels:

• Methanol (FT origin).

• Ethanol.

• Biobutanol.

• Synthetic petrol (FT origin).

• Synthetic diesel (FT origin).

• Biodiesel (esters).

• Biodiesel (bio-oil).

• Biodiesel (plant and microalgal hydrocarbons).

• Biodiesel (microalgal oils).

The first-generation biofuels are represented by biomass, biogas, biodiesel and etha­nol. Biomass is not included in Fig. 4.1 as it is mainly combusted or co-fired with coal to produce electricity. However, biomass in the form of wastes and lignocellulose can

also be converted into biogas, diesel, petrol, methanol and dimethyl ether using gas­ification (Fig. 4.1).

The first-generation biofuels are produced from energy crops such sugarcane, sugarbeet, maize, wheat, rapeseed, soybean and sunflower. However, to completely replace the fossil fuels gas, petrol and diesel large areas of land will be required. Hence, there is not enough land to grow sufficient energy crops without competing with food crops for land. For these reasons second — and third-generation biofuels are under development (Fig. 4.1). The second-generation biofuels will be produced from ligno — cellulose biomass and wastes which have much better yields per hectare as the whole of the harvested plant will be used. The higher yields will mean that second-generation biofuel production will compete less with food crops. The direct production of hydro­gen and extraction of oil for biodiesel from microalgae are third-generation biofuels which will not compete with food crops. Microalgae can be grown on non-agricultural land or in marine conditions and because they are some 50-100 times more productive than biofuel crops much less land will be required (Chisti, 2007).

Therefore, it is essential that second — and third-generation biofuels are developed as first-generation biofuels can only realistically supply 5% of the fuels required.

Ethanol and ethanol-petrol blends are not new as fuels for the internal combustion engine, since these fuels were proposed in the late 1800s by early car manufacturers. Henry Ford once described ethanol as the ‘fuel for the future’. During the First and Second World Wars, ethanol was mixed with petrol in order to preserve oil stocks. After the First World War, petrol dominated the fuel market although ethanol still continued to be used as an octane enhancer (anti-knock) in the 1920s but this was superseded by tetra-ethyl lead. The use of ethanol as a fuel re-emerged in the 1930s in the USA, where ethanol produced from maize was sufficiently cheap to be used in blends. It was used in concentrations of 5-17.5% to produce a blend called ‘gasohol’ and marketed as ‘Agrol’. In the UK, gasohol was marketed by the Cleveland Oil Company under the name of ‘Discol’ in the 1930s, a blend which continued to be sold until the 1960s. In the USA, gasohol was dropped by 1945 due to the availability of cheaper petrol.

In 1975 Brazil introduced the ‘Proalcool’ Programme to produce ethanol from sugarcane as a fuel to replace petrol as a response to oil price rises from 1973. The rise in oil prices was due to an Organization of the Petroleum Exporting Countries (OPEC) oil embargo as a consequence of the Arab-Israeli War in 1973. The reasons for the development of ethanol as a fuel in Brazil were to reduce the imports of petrol as Brazil had few oil fields, to open up areas of the country for cultivation, to provide employment, to increase the industrial base, and to develop ethanol exports of plant and expertise. In addition, Brazil is one of the largest producers of sugar from sugar­cane so that a good substrate for ethanol production was readily available which did not require processing. The production of ethanol was encouraged by grants and subsidies to make ethanol cheaper than petrol. By the late 1980s about 50% of the cars used 95% (E95) ethanol as a fuel. However, price rises and a sugar shortage have reduced ethanol use to about 20% of vehicles, although 40% of the total fuel used is ethanol. One unforeseen outcome of the development of a large ethanol industry in Brazil producing 16.97 x 109 l in 2006 (4.49 billion gallons) is a flourishing export market for ethanol. In 2005 Brazil exported 100 million gallons to India, USA and Europe.

The USA initiated the production of fuel ethanol in 1978 with an Energy Tax Act where gasohol was defined as a blend of petrol containing more than 10% ethanol. The Act exempted ethanol from the US$0.40/gallon tax on petrol. Apart from the tax changes, support for the ethanol industry was in the form of agricultural subsidies and tax credits awarded to blenders. The driving factors for the development of an ethanol industry were similar to those in Brazil. In the case of the USA, ethanol was produced from maize starch rather than from sugarcane. In addition, the price of chemi cally produced ethanol in the USA increased which made biologically produced ethanol more economic. In the 1970s chemically produced ethanol was selling at US$0.145/l but in the 1980s the increase in the feedstock increased ethanol prices to US$0.53/l, which was the same price as biologically produced ethanol. The tax exemption rose to US$0.60/gallon in the mid-1980s but was reduced in 2005 to US$0.51/gallon. An additional reason for the production of alcohol as a fuel was the low prices that the farmers were getting for their maize. At present, fuel ethanol accounts for 7% of the maize crop, boosting farm incomes by US$4.5 billion and is responsible for 200,000 jobs. In the 1980s, lead in petrol was removed and ethanol was of interest to increase the octane value. The first replacement for lead in petrol was MTBE made from methanol but concerns over its toxicity has seen a change to ETBE. After considerable debate in the USA, the Renewable Fuel Standard (RFS) was signed in 2005 which required both biodiesel and bioethanol to be blended in petrol and diesel to the value of 7.5 billion gallons (US) a year by the year 2012. At the present the replacement of petrol with ethanol is also driven by the need to reduce carbon dioxide emissions, as ethanol is a carbon-dioxide-neutral sustainable product.

However, alkalis and acids are not the only catalysts which can be used in the trans­esterification reaction and these include enzymes and solid catalysts. Some of the solid catalysts are listed in Table 7.11.

Transesterification using heterogeneous catalysts has been investigated using basic zeolites and alkaline metal compounds. Metal oxides, hydroxides and alkoxides have been used to transesterify rapeseed oil (Gryglewicz, 1999) where calcium oxide was the most effective. Metal oxides and those loaded with Al2O3, SiO2 and MgO were also used to treat rapeseed oil (Peterson and Scarrach, 1984).

Oil extracted from Pongamia pinnata has been transesterified using a solid Li/ CaO catalyst even in the presence of 0.48-5.75% free fatty acids (Meher et al.,

154

2006b) and Jatropha curcas oil using CaO (Zhu et al., 2006). A number of modified zeolites have been used successfully to transesterify soybean oil (Suppes et al., 2004). Much of the research has been with solid base catalysts but solid acid catalysts have also been used. Tungstated zirconia, a solid super acid catalyst, has been used to transesterify soybean oil at 200-300°C, and has given a conversion of over 90% (Furuta et al., 2004). More recently, amorphous zirconia combined with titanium and aluminium has been shown to give over 95% conversion of soybean oil at 250°C (Furuta et al., 2006).

Microbial lipases have the ability to transesterify oils in the presence of metha­nol. These enzymes function in the presence of water and the catalyst and salts do not need removing at the end of the reaction (Table 7.12). However, the enzymes are more expensive than the simple inorganic catalysts. Some of the expense of using enzymes can be reduced by enzyme immobilization which allows a continu­ous process and increases the working life of the enzyme (Ban et al., 2001; Fukuda et al., 2001).

Transesterification has also been carried out using supercritical methanol, ethanol, propanol and butanol. The process does not require a catalyst but high tem­peratures (~300°C) and pressures (8 MPa) (Cao et al., 2005; Demirbas, 2006a, b).

In addition to improving the production of ethanol by modifications to the process, modern molecular biology offers chances to alter all aspects of ethanol production. In terms of bioethanol genetic modification can be applied both to the biomass sources and the fermenting organisms.

The biomass sources for ethanol are sugars, starches and lignocellulose. Increas­ing sugar content of sugarcane and sugarbeet plants had reached a limit but recently a sucrose isomerase enzyme has been targeted to sugarcane vacuoles, which converts the glycosidic linkage in sugar to a 1,6-fructoside. This allows the accumulation of 0.5M isomaltose (palatinose) in sugarcane stems in addition to sucrose which increases the overall sugar content (Gressel, 2008). However, the one enzyme capable of degrading isomaltose only degrades it slowly but it is hoped to increase the rate greatly by gene shuffling. A change in starch composition to make its conversion into sugar easier is possible and is under investigation.

The development of some second — and third-generation biofuels depends on the ability to process lignocellulose. If lignocellulose is to be used for ethanol produc­tion, it has to be broken down into sugars. The lignin content of wood, straw and grasses reduces the rate of cellulose hydrolysis due to steric hindrance of the cellulo­lytic enzymes. A reduction in lignin or an increase in cellulose would increase the production of sugars. Plant material with more cellulose and less lignin has been reported where partial silencing of the phenylpropanoid pathway enzymes leading to lignin reduces the lignin content (Morohoshi and Kajita, 2001; Gressel and Zilberstein, 2003). In this way an increase in the digestibility of maize, sorghum, pearl millet, poplar and pine has been achieved. A possible problem with the reduc­tion in lignin is loss of structural strength and subsequent lodging (blowing down in strong winds). The dwarf and semi-dwarf wheat and rice will probably not suffer from this and there appears to be no correlation between lodging and lignin content (Gressel, 2008).

The best and most widely used ethanol-producing microorganism Saccharo- myces cerevisiae has a rapid growth rate, and a tolerance to ethanol accumulating in the medium. However, S. cerevisiae has only a limited substrate range restricted to a few sugars and it is unable to metabolize starch and lignocellulose or the pen­tose sugar from lignocellulose. There are ethanol-producing bacteria, perhaps the best known is Zymomonas mobilis, but these also have a restricted range of sugars that they can ferment. One solution to this dilemma is to use genetic manipulation to introduce the ability to use alternative substrates like starch and xylose into these organisms.