Category Archives: BIOFUELS

The Benefits and Deficiencies of Biofuels

Introduction

Biofuels are energy sources derived from biological materials and are therefore renewable and sustainable, and can go some distance in replacing fossil fuels and reducing carbon dioxide emissions. Their biological nature separates them from other renewable energy sources such as wind, wave and solar power. Biofuels can be solid, liquid and gaseous and can be used to generate electricity and as transport fuels. No matter how biofuels are used, they have both benefits and shortcomings, and in this chapter these are explored.

The benefits of biofuels whether globally or to a single country are as follows:

0. Reduction in crude oil use. Liquid biofuels can supplement or replace petrol and diesel, and at low levels of blending, little engine modification is required. Biodiesel can be used up to 100% in a conventional diesel engine but higher blends of ethanol (85%) require either modifications or a flexible fuel engine. Biomass and biogas can reduce fossil fuel use for electricity generation.

1. Improvements in engine performance. Ethanol has a very high octane number and has been used to improve the octane levels of petrol. It is also a possible replacement for methyl tertiary butyl ether (MTBE) which is being phased out as an octane enhancer. Biodiesel addition will enhance diesel lubricity and raise the cetane number.

2. Air quality. Biofuels can improve air quality by reducing the emission of carbon monoxide (CO) from engines, sulfur dioxide and particulates (PM) when used pure or in blends.

3. Reduction in the emission of the greenhouse gases (GHGs) carbon dioxide and methane. The replacement of fossil fuels with biofuels can reduce significantly the production of carbon dioxide, and the use of biogas reduces methane emissions.

4. Toxicity. Biofuels are less toxic than conventional fuels, sulfur-free, and are easily biodegradable.

5. Production from waste. Some biofuels can also be made from wastes, for example, used cooking oil can be used to make biodiesel.

6. Agricultural benefits. Biofuel crops of all types will provide the rural economy with an alternative non-food crop and product market.

7. Reduction of fuel imports. By producing fuels in the country, imports will be reduced and the security of energy supply will be increased.

8. Infrastructure. No new infrastructure is required for the first — and second-generation liquid biofuels and some of the solid and gaseous biofuels.

9. Sustainability and renewability. Biofuels are sustainable and renewable, as they are produced from plants and animals.

However, there are shortcomings to the use of biological materials to replace fossil fuels which are as follows:

Подпись: 167© A. H. Scragg 2009. Biofuels: Production, Application and Development (A. H. Scragg)

1. Biological material may not be able to produce enough fuel to replace fossil fuels completely, and extensive cultivation of biofuel crops will compete with food crops, perhaps driving up prices.

2. Large amounts of energy are required to produce some biofuels, giving them a low net energy gain.

3. Some of the second — and third-generation biofuels will require the introduction of a completely new infrastructure, for example, hydrogen.

Carbon capture and storage

In order to sequester carbon dioxide it first has to be extracted from the flue gases of power stations, cement works and refineries in a process known as ‘carbon capture and storage’. Carbon dioxide capture requires a relatively pure carbon dioxide stream for transport and storage. In some cases the carbon dioxide needs to be compressed, as mineral carbonization requires high pressures. There are three main methods of capturing carbon dioxide.

The first is post-combustion or process removal using absorption, adsorption, cryo­genic and membrane technologies. Adsorption uses amine-based solvents or cold metha­nol. Carbon dioxide can also be removed by passing over activated charcoal or through special membranes (Feron and Jansen, 1995; Grimston et al., 2001; Gronkvist et al, 2006). Advanced methods are adsorption on to zeolites or activated carbon fibres.

The second is pre-combustion gasification of coal in a power station which pro­duces a mixture of carbon monoxide and hydrogen. If this mixture is treated with steam, the carbon monoxide is converted to carbon dioxide and hydrogen. The hydrogen on combustion in the power station forms water and the carbon dioxide and water can easily be separated:

Gasification = CO + H2 (3.3)

Water shift reaction CO + H2 + H2O = H2 + CO2 (3.4)

The third method is to use oxygen in the combustion process, which produces a flue gas of carbon dioxide and water. An alternative to oxygen addition is to use a metal oxide as an oxygen source, known as chemical looping.

Some examples of carbon dioxide sources suitable for carbon capture and storage are given in Fig. 3.11. Cement production is one example where carbon dioxide can be sequestered, and should be as cement production contributes 6-7% of the total global carbon dioxide released into the atmosphere. Cement factories are stationary and hence suitable for retrofitting of carbon dioxide sequestration. In cement produc­tion, calcium carbonate (limestone) is converted to lime (CaO) in a rotary kiln, releas­ing carbon dioxide. The lime is then heated at around 1450°C to form clinker, which when mixed with 5% gypsum forms cement (Gronkvist et al., 2006). One tonne of cement releases about 500 kg carbon dioxide in its production. In addition, carbon dioxide is released from the fuel used to heat the kiln and has been estimated to be 275 kg carbon dioxide per tonne of lime, giving a total of 775 kg carbon dioxide produced per tonne of cement.

Whatever the process producing carbon dioxide, once the carbon dioxide has been separated it needs to be stored to keep it from reaching the atmosphere. The carbon dioxide can be stored using one of the following systems (Fig. 3.11):

1. Store underground in oil and gas reservoirs, deep saline aquifers, coal beds, active and uneconomical oil and gas reservoirs.

2. Hold in deep non-mineable coal formations and coal bed methane formations.

3. Store in deep oceans.

The storage underground could be part of enhanced oil recovery (EOR). At present 80% of recovered carbon dioxide is used in EOR and about 70 oil fields use this worldwide, sequestering some 31 million m3 of carbon dioxide per day. The retention times and capacities for carbon sequestration are given in Table 3.6.

The storage in the deep oceans has a number of possibilities, as the oceans con­tain 40,000 Gt carbon compared with 780 Gt in the atmosphere. Thus, the oceans are an immense carbon sink where captured carbon dioxide could be stored, and the options to put it in the oceans are as follows (Grimston et al., 2001):

• Dry ice released into the sea from ships.

• Liquid carbon dioxide injected at depth of 1000 m from a ship or ocean-bottom manifold forming a rising droplet plume.

• A dense carbon dioxide-seawater mixture formed at a depth of 500-1000 m forming a sinking current.

• Liquid carbon dioxide introduced into a sea bed depression forming a stable lake at a depth of below 4000 m.

Table 3.6. Global capacity and residence time for the various carbon sinks. (Adapted from Grimston et al, 2001.)

Sink

Capacity (GtC)

Retention time (years)

Oceans

1,000-10,000

Up to 1,000

Forestry

60-90

50

Agriculture

45-120

50-100

Enhanced oil recovery

20-65

10-50

Coal beds

80-260

>100,000

Oil and gas reservoirs

130-500

>100,000

Deep aquifers

30-650

>100,000

Hydrogen

Hydrogen is the first element in the periodic table, a colourless, odourless gas which is the most plentiful element in the universe. Hydrogen has been used extensively in the chemical industry in the manufacture of ammonia, methanol, petrol, heating oil, fertilizers, vitamins, cosmetics, lubricants, cleaners, margarine and as a rocket fuel. Hydrogen has been put forward as a new energy carrier in a system known as the ‘hydrogen economy’, which was first mentioned in 1972. In the hydrogen economy, hydrogen would be used as a fuel and to transport and store energy in the way that electricity is used (Winter, 2005; Clark and Rifkin, 2006). Hydrogen as an energy source has many advantages as it is non-toxic, high in energy, on combus­tion yields only water and it can be used both in fuel cells and internal combustion engines. Hydrogen has three times the energy content of petrol and methane at

141.9 MJ/kg but because of its low density it has very low energy content per unit volume (Table 5.4). Table 5.4 compares the energy per unit mass and unit volume for hydrogen, petrol and other gaseous fuels. It is clear that both methane and hydrogen in the gaseous state have low energy per unit volume but in hydrogen in liquid form the energy per unit volume was still low. However, there are disadvan­tages in the production, storage and flammability of hydrogen which have been used to question the adoption of hydrogen as an energy carrier (Hammerschlag and Mazza, 2005).

There are a number of chemical and biological routes for the production of hydrogen, but only some processes are renewable and sustainable. These methods are listed below where only the first three methods are operated at an industrial scale.

Non-renewable:

• Steam reformation of methane (natural gas).

• Coal gasification.

• Partial oxidation of heavy oil.

• Thermocatalytic treatment of water.

Renewable:

• Electrolysis of water using electricity, only renewable if sustainable electricity sup­ply used such as wind or solar power.

• Photocatalytic splitting of water using TiO2.

Table 5.4. Comparison of the energy content of liquid and gaseous fuels. (Adapted from Midilli et a/., 2005.)

Fuel

Energy content mass (MJ/kg)

Energy content volume (MJ/l)

Petrol (liquid)

47.4

34.8

LPG (liquid)

48.8

24.4

LNG (liquid)

50.0

23.0

Hydrogen (liquid)

141.9

11.9

Hydrogen (gas)

141.9

0.012

Methane (liquid)

50.2

36.4

Methane (gas)

50.2

0.039

• Gasification of food waste, sewage sludge, biomass.

• Pyrolysis of biomass.

• Biological processes:

1. anaerobic metabolism (dark fermentation);

2. photosynthetic hydrogen production (direct biophotolysis);

3. indirect hydrogen production;

4. photo-fermentation;

5. carbon monoxide metabolism, water-gas-shift reaction.

The most widely used process for producing hydrogen is steam reforming of natural gas which is the least expensive process and produces 97% of the hydrogen made. The other two commercial processes are the gasification of coal (Chapter 4) and the partial oxidation of heavy oils. Clearly none of these processes are renewable and sustainable.

There are a number of renewable methods of producing hydrogen but to date these are all at the experimental stage (Kapdan and Kargi, 2006). The theme of the book is biofuels and the production of energy from biological materials and there­fore this section will concentrate on the biological production of hydrogen. As can be seen from the list there are a number of biological processes which result in hydrogen.

Synthetic Diesel, FT Synthesis

The Fischer-Tropsch (FT) synthesis was developed in the 1930s, by which a gas con­taining carbon monoxide (CO) and hydrogen (H2) can be converted into long-chain hydrocarbons which have properties similar to crude oil products. A gas containing as its main components H2 and CO can be produced by the high-temperature gasification of coal, biomass and waste, and is known as syngas. The gasification process produces a mixture of CO, H2, methane (CH4), carbon dioxide (CO2), nitrogen (N2) and water (H2O). Natural gas can also be used in the FT process. The FT synthesis was used to produce diesel and petrol in World War II using coal as the starting material (Prins et al., 2004). At present, syngas is mainly used by the chemical industry (Fig. 7.2) for ammonia production and only 8% is being used to produce hydrocarbon-based fuels called ‘gas to liquid’ (GTL) fuels where natural gas is used. In order to make the pro­cess sustainable, coal and gas should be replaced with biomass and waste materials. However, the process is costly and so this fuel is still under development.

Syngas production

At present, there are two industrial methods of producing syngas from biomass: a fluidized bed gasifier and entrained flow gasifier. The fluidized bed gasifier converts

Подпись: 23% Ammonia □ H2 refineries □ Methanol Elec □ GTL □ Other

%

Fig. 7.2. Present industrial uses of syngas: ammonia production, hydrogen for refineries, methanol production, electricity generation and GTL which is the conversion of gas to a liquid fuel. (From van der Drift and Boerringter, 2006.)

Pre-treatment Syngas

production

 

Syngas

conditioning

 

Fuels

produced

 

Fig. 7.3. The processes that can be used to prepare syngas made from biomass for the Fischer-Tropsch synthesis of fuels. (From van der Drift and Boerrigter, 2006.)

 

image123

biomass using an air-blown circulating fluidized bed operating at 900°C, but as the gas formed is not clean the system requires a catalytic reformer to remove many of the contaminants (Fig. 7.3). The gas from the fluidized bed gasifier contains H2, CO, CO2, H2O and considerable amounts of hydrocarbons such as CH4 benzene and tars (Table 7.1). The second option is entrained flow gasification where higher tempera­tures (1300°C) are used. This system requires a supply of very small particles to burn

138

Main constituents

Vol % dry wt

Lower heating value (LHV%)

Carbon monoxide (CO)

18

27.8

Hydrogen (H2)

16

21.1

Carbon dioxide (CO2)

16

Water (H2O) 2

13

Nitrogen (N2)

42

Methane (CH4)

5.5

24.1

Acetylene (C2H2)

0.05

0.4

Ethylene (C2H4)

1.7

12.4

Ethane (C2H6)

0.1

0.8

BTX

0.53

10.5

Tars (total)

0.12

2.8

BTX, benzene, toluene, xylenes.

correctly so that any material used has to be milled, which is energy-intensive and makes handling difficult.

No matter which method is used to produce the gas, extensive syngas cleaning and conditioning are required before the FT process can be used to produce liquid fuels as the contaminates inhibit the catalyst. The syngas also needs to have a H2/CO ratio of 2:1. The concentration of CO and H2 can be adjusted in the water shift reactor which converts CO to H2 and CO2. The reverse can also be carried out as the syngas com­position varies depending on the feedstock.

Forward (<250°C)

CO + H2O = CO2 + H2

(7.1)

Backward (>500°C)

H2 + CO2 = CO + H2O

(7.2)

European biofuel supplies

The International Energy Agency predicts that the world’s energy consumption will increase by 1.7% per annum (IEA, 2005a). If this is applied to the UK’s transport fuel, the UK would require 63,000,000 t of liquid fuels by 2010 and 67,402,000 t by 2020. However, a recent report forecast that the total use of fuel in the EU 25 (JRC, 2007) will increase up to 2010 and then slowly decline (Fig. 8.24).

Diesel for cars will steadily increase due to their better fuel consumption and emis­sions while petrol use will decline. The only group which appears to increase over the time period is the diesel demand by heavy goods vehicle (HGV) transport which is a feature of economic growth within the EU. The increase in fuel use from 2005 to 2020

Fig. 8.24. Present and future use of road fuels in the EU 25. (Redrawn from JRC, 2007.)

image187

Fig. 8.25. Biodiesel production in the EU 25 for the years 2002-2007 in 1000 t. (From European Biodiesel Board, 2007.)

represents an increase of 0.69%, much less than predicted, from global energy use but much of the global growth is probably from developing nations.

The major biofuel produced in Europe is biodiesel and in 2005 Europe consumed

178.178.0 t of fossil fuel diesel. The European production of biodiesel is given in Fig. 8.25 where Germany is by far the largest producer, although the UK’s production has recently increased significantly. The total biodiesel produced in Europe was 6,069,000 t which was considerable, but still only represents 2.45% of the total diesel used.

UK biofuels supply

The UK needs to provide a secure supply of fuel at a time when the UK is increasingly dependent on imports for energy production. At present the UK imports 5% of its gas and 34% of its oil. However, this is changing as the UK continental shelf oil and gas production will reduce over the next 25 years.

The current figure for the use of liquid fossil fuels in the UK (2005 figures) is

58.818.0 t (Table 6.1) principally made up of petrol 19,918,000 t, diesel 23,989,000 t and kerosene for aviation 10,765,000 t.

In a recent report the Biofuels Research Advisory Council (Biofuels in the Euro­pean Union, a vision for 2030 and beyond, 2006), the average annual growth for primary energy was predicted to be 0.6% for the UK, which matches the EU 25 pre­diction. This would include an increase in energy imports from 47.1% in 2000 to 67.5% in 2030. The largest increase will be in fuel for heavy transport.

It is not an impossible task for the UK, to provide 5% of the total diesel used as biodiesel as this would require 6.6% (1.1 Mha) of the total agricultural land set down to rapeseed (Fig. 8.26). In addition, there are 50-90 million l of used cooking oil which could provide 0.24-0.44% of the total diesel. Other estimates indicate that 1.15 Mha of agricultural land would be required to meet the 5% obligation (Rowe et al., 2009).

In the UK, it is rapeseed that dominates the oilseed production, and Fig. 8.26 shows the land required to produce 5, 20 and 100% of the UK’s diesel from rapeseed. It is clear that beyond a 5% substitution, biodiesel from rapeseed will require a sig­nificant amount of agricultural land which will impact on food crops. Therefore, other sources of biodiesel will be needed and Fig. 8.26 also shows the land required for two alternative sources of diesel, FT diesel and microalgae.

Ethanol can be produced in the UK from sugar extracted from sugarbeet, starch extracted from wheat and cellulose from lignocellulose material. The yields of ethanol per hectare from sugar, starch and lignocellulose crops vary depending on the crop as can be seen in Fig. 8.27. Crop yields also depend on the crop cultivation and climate conditions. The average values are between 2000-3000 l/ha for sugarbeet but the temperate crops cannot compete with sugarcane at 6000 l/ha and sorghum at 4000 l/ha. However, as these crops cannot be grown in the UK, wheat and sugarbeet have to be used. The amount of land for ethanol production at 5, 20 and 100% substitu­tion is shown in Fig. 8.28. Ethanol production from sugarbeet would require 50-60% of the agricultural land for 100% replacement, wheat 54-298% agricultural land and lignocellulose 42-81% agricultural land.

Conclusions

The reports by Stern (2006) and the IPCC (2007) outline the consequences of global warming, and it is clear that efforts should be made to reduce the emissions of green­house gases from fossil fuels globally.

The IPCC has come up with four scenarios predicting the global atmospheric carbon dioxide levels depending on what measures are taken towards their reduction. If the amount of carbon dioxide released per year was retained at present values, the carbon dioxide would reach 550 ppm by 2050. Carbon dioxide emissions are still increasing so that 550 ppm may be reached before 2050. A level of 550 ppm is pre­dicted to give a 2°C increase in global average temperature. At present there is hope to reduce carbon dioxide release, so that a value of 450 ppm is reached by 2100. Small increases in temperature seem insignificant, but these can have far-reaching effects such as the melting of sea ice. Some consider that even if we stopped carbon dioxide emissions now, the tipping point may have already been reached and a rapid and long-lasting increase in temperature is inevitable.

The chapter outlines the methods that are currently available or under develop­ment for the reduction of atmospheric carbon dioxide which include: burn less fuel, sequester the carbon dioxide and use non-carbon-dioxide-producing energy. Within these broad categories there are many options and no one option will provide a com­plete solution, but in concert they may well affect the outcome. The solution is not the science but rather the politics where countries have to reduce carbon dioxide emissions, while at the same time producing growth in their economies and increasing prosperity. It seems that if energy supply is to be sustainable and carbon-neutral it cannot be obtained at the same time as continued growth of the economy. However, developing countries will not stop their development in order to reduce carbon diox­ide emissions despite the warning that global warming will affect developing coun­tries the most. Considerable political effort and legislation will be needed if global warming is to be halted.

Methanol

Methanol (CH3OH) is a simple alcohol commonly known as ‘wood alcohol’. It is a toxic, colourless, tasteless liquid with a faint odour which can be used in a spark ignition engine. Its characteristics are given in Table 6.2, where it is compared with petrol. Methanol contains considerably less energy than petrol but the high octane rating gives more power and acceleration. It is less flammable than petrol but burns with a nearly invisible flame, making flame detection difficult. Methanol is toxic, corrosive and as it is miscible in water a spill can be an environmental hazard, but methanol offers impor­tant emissions improvements, reducing hydrocarbons and nitrogen oxide.

In the early days of motoring, methanol was used in internal combustion engines as a blend with petrol. It was used as a motor fuel in Germany in the Second World War because of the shortage of oil (Antoni et al., 2007). Because of its low energy content (19.9 MJ/kg), since the 1970s it has only been used in special cases such as Indianapolis car racing, and even the Indy cars will switch to ethanol in 2007 (Solomon et al., 2007). Another use for methanol is in the production of an anti­knock agent for use in petrol. Methanol is converted into methyl-tert-butyl-ether

© A. H. Scragg 2009. Biofuels: Production, Application and Development (A. H. Scragg) 105

Table 6.1. Liquid fuel use in the UK in 2006 and in the EU 25 and world in 2005 (tonnes x 1000). (From IEA, 2008b; Energy Statistics, BERR, 2007.)

Fuel

UK consumption in 2006

EU 25 consumption in 2005

World consumption in 2005

LPG (liquid

288

3,428

16,207

petroleum gas) Petrol

19,918 (33.9%)

107,752 (31.5%)

876,286 (44.3%)

Petrol (aviation)

46

142

2,252

Jet kerosene

10,765 (18.3%)

51,453 (15%)

229,026 (11.6%)

Kerosene

3,457

4,909

625

Diesel

23,989 (39.3%)

178,178 (52%)

687,935 (34.8%)

Other fuels

355

1,178

161,785

Total

58,818

342,131

1,974,116

Table 6.2. The characteristics of petrol and methanol.

Characteristics

Petrol

Methanol

Boiling point (°C)

35-200

65

Density (kg/L)

0.74

0.79

Energy (MJ/kg)

44.0

19.9

Flash point (°C)

13

65

Octane number

90-100

91

(MTBE), an anti-knocking agent, by acid catalysis with isobutene. MTBE has been added to petrol replacing the lead-based compounds used previously but now banned. However, there have been concerns about the carcinogenicity and groundwater con­tamination by MTBE and it is being replaced by ethyl-tert-butyl-ether (ETBE) which can be made from ethanol.

At present, most methanol is made from natural gas but renewable sources such as woody crops, agricultural residues, forestry waste and industrial and municipal waste can be used to produce methanol by either thermochemical conversion or gas­ification. The gasification of the biomass at high temperatures (above 700°C) in the presence of oxygen results in a mixture of gas, tar and charcoal due to partial oxida­tion. The gasification process needs biomass with moisture content of 10% or below. The gas formed is called ‘syngas’, and is a mixture of carbon monoxide and hydrogen which can be converted into methanol if passed over Cu/Zn/Al catalysts.

A more recently developed biological method for producing methanol is the de-esterification of the methylated carboxyl groups of galacturonic acid by a pectin methyl esterase to give methanol (Antoni et al., 2007). Pectin is a major component of plant cell walls and one suitable substrate for this process is sugarbeet pulp which contains 60% pectin on a dry weight basis.

Since 2000 in the USA, no cars have been run on 100% methanol, but some 15,000 M85 (85% methanol combined with 15% petrol) vehicles are in operation, mainly in California and New York. Perhaps the most promising use for methanol is in hydro­gen fuel cell vehicles where it is converted into hydrogen on board the vehicle.

Transesterification

Biodiesel is a replacement for diesel and is produced by reacting plant oils and animal fats with an alcohol to form a mixture of fatty acid esters in a reaction known as transesterification. Biodiesel is available commercially and should be regarded as a first-generation biofuel. The idea of splitting the triglycerides in fats and oils and using the resulting esters as a fuel has been around for a considerable time. Walton, in 1938, suggested the splitting of triglycerides (Graboski and McCormick, 1998), and there is a report of fatty acid esters being used as a fuel in the Congo in 1937 (Knothe, 2001). Subsequently, there have been a number of reports of using plant oil/diesel blends in engines where the problems of high viscosity of oil were encountered. One of the first reports of the use of esters was in 1980 using sunflower oil esters which appeared to remove many of the problems associated with untreated oils, in particular, viscosity. Since then, there has been a considerable number of reports on the production of fatty acid esters from a wide range of fats and oils. The European quality standards for fatty acid methyl esters, known as biodiesel, came into force in 2004 and are known as EN 14214 (biodiesel) and EN 14213 (heating fuel) (Schober et al., 2006).

Transesterification of plant oils is the conversion of the triglycerides which make up oils into fatty acid esters and glycerol. Triglycerides are the main component of fats and oils and consist of three long-chain fatty acids linked to a glycerol backbone. When the triglyceride reacts with an alcohol, the three fatty acids are released and combined with the alcohol to form alkyl esters. Transesterification of pure oils can be carried out rapidly with methanol and NaOH as the catalyst (Van Gerpen, 2005). Methanol is normally used as the alcohol, although ethanol, 2-propyl and 1-butyl will also suffice (Lang et al., 2001).

CH2

CH2OH

R1COOCH3

CH2 + 3 CH3OH

NaOH catalyst

= CHOH

+ R2COOCH3

(7.4)

CH2

CH2OH

R3COOCH3

triglyceride methanol

glycerol

methyl esters

The reaction can be catalysed by alkalis, acids, lipase enzymes and inorganic hetero­geneous catalysts (Fukuda et al., 2001; Vincente et al., 2004). The conditions for catalysis are a temperature near to the boiling point of methanol (60°C), although room temperature will suffice with pure oil, a molar ratio of alcohol/oil of between 3:1 and 6:1, and NaOH as the catalyst. The stoichiometric molar ratio of methanol/ oil is 3:1 but in order to drive the reaction towards ester formation the ratio is increased to ratios of up to 9:1. The effect of the molar ratio of methanol/oil on the process of transesterification is shown in Fig. 7.9.

The transesterification reaction requires catalysis and apart from alkali catalysts others have been used including acids, enzymes and solid catalysts (Suppes et al., 2004; Vincente et al., 2004; Meher et al., 2006a). The alkali-catalysed transesterifica­tion is by far the fastest process (Fig. 7.10), but is sensitive to impurities in the raw materials.

The presence of water and free fatty acids in the oil consumes alkali, and forms soaps which in turn produce emulsions. Emulsions stop the separation of glycerol as the reaction proceeds, which reduces the yield of biodiesel (Fig. 7.11).

Подпись: Molar ratio (MeOH/oil) -A-TAG Fame Fig. 7.9. The effect of the methanol/oil ratio on methyl ester production. MeOH, methanol; TAG, triacylglycerols; FAME, fatty acid methyl esters. (Redrawn from Freedman et al, 1986.)

Fig. 7.10. The production of methyl esters during NaOH-catalysed transesterification. (Redrawn from Freedman et al., 1986.)

Подпись:Подпись: Free fatty acid (%) -A- 0 H20 0.9% H2O Fig. 7.11. Effect of the presence of free fatty acids and water on the NaOH- catalysed transesterification of beef tallow. (Redrawn from Ma et al., 1998.)

R-COOH + KOH = R-COO-K+ + H2O (7.5)

fatty acid potassium soap

In extreme cases, the treated oil will set into a gel formed from a combination of glycerol and soap. An ester yield of less than 5% was obtained in the presence of 0.6% free fatty acids (Canakci and van Gerpen, 1999; Usta, 2005). Therefore, oils containing no water and less than 0.5% free fatty acids are required for successful alkali catalysis. These properties can be obtained with most plant oils, but waste cooking oils, rendered fats and some plant oils contain between 0.7 and 24% water and 0.01-75% free fatty acids (Zhang et al., 2003; Meher et al., 2006a; Canakci, 2007). Unfortunately, there are large amounts of unrefined plant oils, waste cooking oils and soapstocks available for biodiesel production. Acid catalysts, mainly sulfuric, hydrochloric and phosphoric acids, have not been used widely as the reaction is very much slower than the alkali catalysts (Fig. 7.12), but acid catalysis is not affected by free fatty acids.

Therefore, a two-stage process has been developed where in the first stage acid catalysis is used to esterify the free fatty acids, and the alkali-catalysed system is used in the second stage to transesterify the triglycerides (Zullaikah et al., 2005; Wang et al., 2006) (Fig. 7.13).

image133

Fig. 7.13. The two-stage production of biodiesel from oil containing 50% free fatty acids. Stage one is catalysed by sulfuric acid and the second is alkali-catalysed. FFA, free fatty acids; FAME, fatty acid methyl esters; TAG, triacylglycerols. (From Zullaikah et a/., 2005.)

Hydrogen production in microalgae

One of the sustainable methods of producing hydrogen is to use anaerobic photosyn­thetic bacteria which use light and organic acids to produce hydrogen. However, yields of hydrogen are low and are only produced under stress conditions, so that genetic manipulation may be able to increase yields without using stress conditions.

Energy and Fuel Generation Using Biomass

There are four processes whereby biomass can be used to generate electricity or pro­duce fuels. These options are given in Fig. 4.5 and include direct combustion, co-firing, gasification and pyrolysis.

image059

Fig. 4.5. The direct use of biomass for the production of fuel and energy. Heat and electricity is produced through combined heat and power systems (CHP).

Combustion

Biomass has been used for many years to provide domestic heating using direct com­bustion in fires and stoves. These systems are normally not very efficient and produce some emissions of soot and dust. Small to medium-scale heating systems using pel­leted biomass have been developed for houses and larger buildings such as schools. These modern heating systems have improved efficiencies and reduced emissions. Large-scale combustion of biomass for the production of electricity is used in many countries. The technology involved in the combustion of wood and forest residues can be conventional pile burning, stationary, moving, vibrating, suspension and fluid­ized beds. Wastes can be incinerated to produce heat and power and are found widely distributed in Europe. Incineration is central to the treatment of domestic waste although there have been some concerns about emission from incinerators. The appli­cation of fluidized beds and advanced gas cleaning has given an efficiency of 30-40% for electricity production at a scale of 50-80 MW combined with flue gas cleaning.