Production, Application and Development

A. H. Scragg

Biofuels are energy sources derived from biological materials, which distinguishes them from other non-fossil fuel energy sources such as wind and wave energy. Biofu­els can be solid, liquid or gaseous, and all three forms of energy are sustainable and renewable because they are produced from plants and animals, and therefore can be replaced in a short time span. In contrast, fossil fuels have taken from 10 to 100 million years to produce and what we are burning is ancient solar energy. In addition, the energy derived from plant material should be intrinsically carbon-neutral, as the carbon accumulated in the plants by the fixation of carbon dioxide in photosynthesis is released when the material is burnt.

At present it is clear that the supply of fossil fuels is finite and a time can be envis­aged when the supplies of fossil fuels become scarce or even run out. Also the burning of fossil fuels releases additional carbon dioxide into the atmosphere over and above that released in the normal carbon cycle. The accumulation of carbon dioxide in the atmosphere appears to be the major cause of global warming. The consensus suggests that the long-term effects of global warming will be severe, with drastic changes in climate and sea levels. At the same time, modern society requires increasing amounts of energy, most of which is obtained from fossil fuels. Thus, mankind is almost totally reliant on fossil fuels to provide electricity, heating/cooling and transport fuels. This reliance can be seen in the effects on countries when oil supplies are interrupted by wars, embargos and strikes. In 2008, the world suffered from rapid rises in oil prices which affected the price of many commodities.

Alternative energy supplies are needed, therefore, to provide both power and fuel for transport. The possible energy sources available are very diverse and include hydroelectric, nuclear, wind, biological materials and many others. Whatever energy source is used, it should be sustainable and as carbon-neutral as possible. Biofuels encompass the contribution that biological materials may make to energy supply and in particular liquid fuels for transport. Solid biofuels, principally biomass, have been used for thousands of years to provide heat and for cooking, and are used at present to generate electricity and in combined heat and power systems. The gaseous biofuel methane is produced by the anaerobic digestion of sewage, in landfills and is also used for electricity generation and for heat and power systems. It is the liquid biofuels that will be used to replace the fossil fuels petrol and diesel, and they have attracted much attention. At present liquid biofuels can be divided into first-, second — and third-generation biofuels. The first-generation biofuels consist of ethanol which is used to either supplement or replace petrol and biodiesel, as a replacement for diesel. Ethanol is produced from either sugar or starch which is extracted from crops such as sugarcane, sugarbeet, wheat and maize and can save 30-80% of greenhouse gas emissions when compared with petrol. Biodiesel is produced from plant oils and animal fats and can save between 44 and 70% greenhouse gases compared with diesel. At first glance this situation appears ideal but there are problems with first — generation biofuel production and supply. One problem is the amount of energy that is used to produce and convert the crops into biofuels. Another problem is the amount of biofuels needed to replace fossil transport fuels. In the UK, in 2006 19,918 million t of petrol and 23,989 million t of diesel were used, that is a total of 43,907 million t. To supply this tonnage is a formidable task. For example, to supply the diesel required in the UK using the oil-seed crop rapeseed, 113% of the agricultural land would be needed. This is clearly not possible and even at modest levels of diesel replacement the biofuel crops would compete with food crops. It is this feature that has brought forward many objections to biofuels, and they have been blamed for some food shortages; however, in reality food prices are influenced by a number of factors. Converting sensitive lands such as rainforests to grow biofuel crops has also engendered justifiable resistance. In addition, crops such as wheat and other starch — containing crops require considerable processing and energy input, and when inves­tigated by life-cycle analysis show only marginal gains in energy.

However, the resistance to biofuels need not be the case as the first-generation biofuels bioethanol and biodiesel were only intended to be used as a 5% addition to fossil fuels to comply with the EU directives, and to show that fossil fuels could be replaced. It was clear that any more would compromise food crops. It is the second — and third-generation biofuels that should replace the bulk of the transport fossil fuels. The second-generation biofuels are ethanol, produced directly from lignocellulose, and the gasification of lignocellulose and waste organic materials producing petrol, diesel, methanol and dimethyl ether. Lignocellulose and organic wastes are available in large quantities and their use does not compromise food crops. Lignocellulose is often the discarded portion of food crops such as straw. The third-generation biofuels are hydrogen, produced either by the gasification of lignocellulose or directly by microalgae, and biodiesel produced from oil accumulated by microalgae. These second — and third-generation biofuels should not compromise food crops, but to bring these fuels into production will require both research and investment. To stop the use of land areas, such as the rainforests, for first-generation biofuels will prob­ably require legislation.

Nitrous oxide (N2O)

Nitrous oxide is an active greenhouse gas found at a very low concentration of 310 ppbv (parts per billion by volume) in the atmosphere. On a molecule-to-molecule basis nitrous oxide is 200 times more effective than carbon dioxide in absorbing infrared radiation

and is also involved in the degradation of ozone. The gas is produced naturally by the denitrification of nitrate by microbial activity in soil and sea. Nitrous oxide is produced in a sequence of reactions leading from nitrate to nitrogen gas and is shown below:

NO3 > NO2 > NO > N2O > N2 (2.1)

The sources of nitrous oxide are given in Table 2.4. The addition of nitrogen-based fertilizer to soils increases the rate of denitrification. Nitrous oxide is mainly lost in the stratosphere by photodegradation:

2N2O + hv(light) = 2N2 + O2 (2.2)

Introduction

At present we are living in a situation where the world’s demand for energy continues to increase at a predicted annual rate of 1.8%, especially as countries develop, while at the same time the supply of energy appears limited. The reason for this is that 75-85% of the world’s energy is supplied by the fossil fuels — coal, gas and oil (IEA, 2002; Quadrelli and Peterson, 2007) — and the supply of these is finite. In addition, the burn­ing of fossil fuels has increased the atmospheric concentration of some greenhouse gases that are responsible for global warming. Other consequences of burning fossil fuels include the production of acid rain, smog and an increase in atmospheric particles. In addition, the world’s population is expected to expand at about 1% per year, which will mean that global energy requirements will continue to rise. It is predicted that fossil fuels will continue to dominate the energy market for some time and oil will be the most heavily traded fuel. The Middle East contains the bulk of the oil reserves and, therefore, much of the global oil supply will increasingly be obtained from this area. This will increase the world’s vulnerability to price shocks caused by oil supply disruption from this somewhat unstable area. Against this background, all countries’ (including the UK’s) access to adequate energy supplies will become increasingly important at a time when oil supplies are declining, such as the North Sea’s oil and gas. Alternative sources of energy, which are renewable and with sustainable supplies, are required. Renewable energy sources can provide a constant supply of energy, and examples are hydroelectri­city, wind and wave power, and geothermal — and biological-based fuels. It would be foolish to think that any one of these renewable energy sources could completely replace fossil fuels. However, if each of the renewable sources can make a contribution, when combined they may be able to replace fossil fuels, although this would probably need to be in conjunction with a reduction in energy use, and an increase in its efficiency. The challenge for all countries is therefore to move to a more secure, low-carbon energy production, without undermining their economic and social development.

In this book, the problems associated with fossil fuel use are outlined, and how the adoption of alternative fuels can mitigate global warming. Other chapters cover biologically produced, solid, gaseous and liquid fuel production with their advantages and disadvantages.

over 200 times lower than carbon dioxide, it is 21 times more effective at adsorbing infrared radiation than carbon dioxide. Methane levels are over twice what they were in pre-industrial times and have been increased by human activities such as rice cultivation, coal mining, waste disposal, biomass burning, landfills and cattle farms (Fig. 2.7). Ruminants can produce up to 40 l of methane per day. Methane is mainly removed from the atmosphere through reaction with hydroxyl radicals where it is a significant source of stratospheric water vapour. The remainder is removed through reactions with the soil and loss into the stratosphere.

Methane hydrates have been proposed as a potential source of energy but the exploitation of these deposits has its problems. It has been estimated that methane hydrates represent 21 x 1015 m3 of methane (11,000 Gt carbon) (Kvenvolden, 1999). The carbon content of these hydrates is greater than that contained in all the fossil fuels (Fig. 2.8) (Lee and Holder, 2001).

Fig. 2.8. The carbon content (Gt) of methane hydrates compared with other sources. (Redrawn from Lee and Holder, 2001.)

As a result, any controlled release of methane from the hydrate deposits may have a significant effect on global warming. There is increasing evidence that major releases of methane from hydrates have occurred in the past and have been associated with warming events, although insufficient methane may have been released to be responsible for the full rise in temperature (Glasby, 2003). One consequence of global warming may be the dissociation of some of the shallow hydrate deposit, further increasing global warming. Slow release of methane in the sea would result in its oxidation before reaching the surface but large-scale sediment slumping, such as the Storegga slide off Norway displacing 3900 km3 of sediment, may release huge quanti­ties of methane.

In the past, the world’s energy supply was based on wood, a renewable resource, which was used for cooking, heating and smelting. Later, water and wind power were harnessed and used especially throughout Europe. The Industrial Revolution was initiated using water power but this was soon replaced by coal, which had high energy content and was freely available. Oil and gas use developed after coal and in the face of what appeared to be unlimited supplies of fossil fuels, water and wind power were abandoned. The worldwide economic growth since the mid-20th century has been sustained by an increasing supply of fossil fuel. Huge infrastructures have been organized to supply these fuels, and whole communities have grown to extract fossil fuels, particularly coal.

The global use of energy, and therefore fossil fuels, has increased steadily ever since the Industrial Revolution in the 1800s, and Fig. 1.1 shows the world’s total energy consumption in gigatonnes of oil equivalent (Gtoe, 109 t of oil equivalent) up to the present and predictions up to 2030. At present the annual consumption is around 10 Gtoe and the world’s energy needs are projected to grow by 55% from 2005 to 2030 at an average rate of 1.8% (IEA, 2005a, 2007). Estimates of the current world energy demand in exajoules (10 x 1018 J) are given in Table 1.1 with a consensus value of 410 EJ. The increases predicted in the world’s primary energy demand up to 2030 can be seen in Fig. 1.1 (IEA, 2005a). The predicted values may be underesti­mated because of the recent increases in the Indian and Chinese economies. These emerging economies are growing rapidly and China is increasing coal extraction and continues to build coal-fired power stations. The increases in oil consumption by Asia compared to North America and Europe can be seen in Fig. 1.2.

In the use of energy there is a correlation between average income and energy consumed. Figure 1.3 shows the relationship between average income and oil con­sumption. It is clear that as countries become more developed, the demand for energy will increase considerably, and this is happening in particular with China and India.

Fig. 1.2. Daily oil consumption (in 1000 barrels of 159 l each). ▲ North America; ■ Europe; ♦ Asia-Pacific; • Middle East. (Redrawn from Guseo et a/., 2007.)

Fig. 1.3. The relationship between average income and oil consumption (1 barrel =

159 l). (Redrawn from Alklett, 2005.)

As well as an increase in the combustion of fossil fuel, the pattern of consumption has changed dramatically. Coal fuelled the Industrial Revolution in the 1800s, and even by the 1930s, over 70% of the world’s energy was still derived from coal. Since the mid-2000s, oil and gas have replaced coal as the main world energy sources, with smaller contributions made by nuclear, biomass and hydroelectric resources. Figure 1.4 shows the sources of world primary energy supply represented as percent­ages. It is clear that the supply is now dominated by gas and oil.

Table 1.2 gives the current and predicted sources of primary energy in the world in terms of Gtoe. Nuclear power, once thought to be a limitless source of energy, has increased slowly since the late 1980s due to problems of radioactive waste disposal

Fig. 1.4. Present sources of world primary energy supply for 2005 (shown as percentages of a total of 11.43 Gtoe). (Redrawn from IEA, 2007.)

and decommissioning of old power stations. The nuclear contribution has remained stable, whereas biomass and renewables have shown a steady increase. In 2004, fossil fuels were used to produce 72% of the world’s electricity with coal producing 39%, oil 8% and gas 25% (Quadrelli and Peterson, 2007).

The use of gas is predicted to continue to replace coal for electricity generation as it is a cleaner fuel producing fewer greenhouse gases. Coal is predicted to increase by 50%, whereas gas is expected to increase by 88%. The reduction in carbon diox­ide when switching from coal to gas follows the formulae given below:

(coal) 2(CH) + 2%O2 = 2CO2 + H2O (1.1)

(natural gas) CH4 + 2O2 = CO2 + 2H2O (1.2)

The pattern of change in energy use in the UK has mirrored the global pattern, where coal has been superseded by gas for electricity generation (Fig. 1.5), and the nuclear and the renewables sectors have also increased their contribution. In Fig. 1.5, nuclear power is combined with renewable energy sources but as the supply of uranium is finite, some regard nuclear power as non-renewable.

The overall fuel consumption by the various domestic and industrial sectors is given in Table 1.3, which shows that transport uses 37.1% of the energy. In the case of electricity generation, large losses of energy occur during generation (55.2 Mtoe) and distribution (19.1 Mtoe) from a total energy consumption of 232.1 Mtoe. Combined, these are 74.7 million t of oil equivalents (Mtoe) or 31.9% of the total energy produced. This is a consequence of large centralized electricity generation, where waste heat cannot be used, and the transportation of electricity over long distance, which involves losses.

An outline of the major flows of energy within the UK is shown in Fig. 1.6. The complete pattern of flow is more complex than that shown in the figure, which only shows the major routes, but it is clear that oil is used exclusively to produce transport fuels. Gas is used both for heat and electricity generation, whereas coal is mainly used for electricity generation.

Fig. 1.5. UK energy consumption changes since 1980. (From BERR, 2007.)

The fuels used for electricity generation in the world and the UK are given in Table 1.4. It is clear that worldwide coal is still the major fuel in electricity generation, with gas, nuclear and hydroelectric resources making similar contributions. The renewable electricity generation such as wind, solar and biomass only contributed 2.2% of the total worldwide. However, in the EU and UK, gas is used to produce a large proportion of the area’s electricity. The move from coal to gas for electricity generation in the UK can be seen in Fig. 1.7, and this has altered the overall fuel usage (Fig. 1.5).

The contribution of renewable energy sources to electricity generation is very small for the UK, so it does not show in Fig. 1.7, but a more detailed list of renewable sources of energy in the UK in 1990 and 2006 is given in Table 1.5. The total renew­able energy was 4.4 Mtoe in 2006 from a total energy use of 159.5 Mtoe.

The table excludes nuclear power, but the two sources that have increased are landfill gas (methane) and wind/wave energy. Landfill sites used to be disposal sites and no attempt was made to collect the methane gas formed in the anaerobic diges­tion of the organic components of the waste. This has changed now and collection systems are installed during the filling of the landfill site. Wind power technology is now fairly mature and wind farms have been constructed in areas of consistently high wind. The positioning of some wind farms has seen objections raised due to noise, effects on birds and interference with radar. Many of these objections may not be found with offshore wind farms. Wave power is still under development with a number of systems designed to extract energy from waves directly from ocean cur­rents. The row labelled ‘Other biofuels’ includes ethanol and biodiesel, which have seen a rapid increase in production in the last few years.

The carbon dioxide concentration in the atmosphere is low (368 ppmv; 0.03%) com­pared with oxygen and nitrogen but it is a greenhouse gas and is responsible for 55% contribution to global warming. There is a continual flow between the atmosphere and organic and inorganic carbon in the soils and oceans (Fig. 2.3). Plants on land and in sea fix carbon dioxide in photosynthesis and this is balanced by carbon dioxide pro­duced by respiration of animal and plants and microbial decomposition of biological materials. Carbon dioxide is also locked up in plant and animal debris in soils and the oceans act as a very large sink where carbonate rocks and reefs also store carbon.

Over many millennia some of the plant and animal debris have been converted by high pressure and temperature into fossil fuels, oil, gas and coal. It is the burning of fossil fuels that is altering the balance of the atmospheric carbon dioxide.

Annual carbon dioxide emissions from the use of coal, gas and oil were above 23Gt in 2000 having risen from 15.7 Gt in 1973 and 0 in pre-industrial times (IEA, 2002). Carbon dioxide emissions depend on energy and carbon content of the fuel, which ranges from 13.6 to 14.0 Mt C/EJ for natural gas, 19.0 to 20.3 for oil and 23.0 to 24.5 for coal (Wuebbles et al., 1999).

The human activities that are responsible for greenhouse gas emissions are given in Fig. 2.9, from which it is clear that the energy sector dominates production.

Fig. 2.9. Anthropogenic sources of greenhouse gases. (From Quadrelli and Peterson, 2007.)

Agriculture produces in the main the greenhouse gases methane (CH4) and nitrous oxide (N2O) from cultivation and livestock. When considering carbon dioxide, we see that the energy sector produces 95% carbon dioxide when Annex 1 countries are surveyed, with 4% methane and 1% nitrous oxide. The carbon dioxide emissions of sectors of the energy sector are given in Fig. 2.10. This use of fossil fuels appears to be responsible for the rapid increase in atmospheric carbon dioxide since the 1800s, and the IPCC predict that the carbon dioxide levels will continue to increase to values of 700 ppm by the year 2100 if nothing is done (Fig. 2.11). A number of scenarios have been developed by the IPCC based on various assumptions on the degree of reduction in greenhouse emissions. The carbon dioxide emitted from the electricity sector is the largest followed by that from transport. Although coal only represents 25% of the fossil fuels used for electricity generation, it generates more carbon dioxide as it con­tains a higher carbon content (Fig. 2.12). Figure 2.12 shows the fuels used in the global energy supply and the proportion of carbon dioxide that these produce. It is clear that coal use produces the greater proportion of carbon dioxide.

The carbon dioxide emissions in the 1990s were estimated to be 6.3 ± 0.4 PgC/year (6.3 Gt), which resulted in an increase in atmospheric carbon dioxide of 3.2 ± 0.1 Pg/ year while the remainder was adsorbed by the oceans and land (Glasby, 2006). This equates to 25.2 Gt of carbon dioxide per year. There have been a number of estimates and calculations on the levels of carbon dioxide that would be obtained if various reduction scenarios were implemented (Fig. 2.11) (IPCC, 2007). The present-day global reserves of oil, gas and coal (Tables 1.9 and 1.10) are about 1091-1268 Gt carbon. If these reserves were all used, the final atmospheric carbon dioxide level would be 2000-2200 ppm (IPCC, 2006; Glasby, 2006). One of the targets is to hold atmospheric

Fig. 2.10. A total of 26.5 Gt carbon dioxide emissions in the world by sectors. (From Quadrelli and Peterson, 2007.)

carbon dioxide at 450 ppm, which represents an increase of 70 ppm from present values equivalent to 13.2 PgC or 3.8% of fossil fuel reserves. If the target is 750 ppm, an increase of 350 ppm, this would be equal to 66 PgC (19%) (Glasby, 2006).

In the 1950s, coal remained the main source of fuel for home heating, industry and electricity generation. Nuclear power started in the 1960s but supplied only 3.5% of western Europe’s electricity by 1970. As the economies of European countries increased, oil was increasingly imported and much of it from the Middle East. In 1971, the USA became for the first time an oil importer, which increased worldwide oil demand, and at the same time Kuwait and Libya reduced oil production. These developments were the first indication of the dependence of developed countries on imported fossil fuels, espe­cially oil, and the need for secure supplies of fuel. The first crisis in oil supply was caused by the Israel-Arab War in 1973 when the Organisation of Petroleum Exporting Countries (OPEC) imposed an oil embargo on the USA and the Netherlands and reduced production. This increased the oil prices rapidly (from US$3 to US$11 a barrel), which highlighted the instability of oil supplies in the UK and Europe. This encouraged non-OPEC countries to search for oil. In 1967, the UK started piping natural gas ashore from the North Sea and later on in the 1970s and 1980s, oil was discovered in the deeper parts of the North Sea. The supply of natural gas encouraged the switch from ‘town gas’ produced from coal, to natural gas for home heating and cooking. In 1981, the UK was self-sufficient in oil as the North Sea oil fields were exploited (Hammond, 1998). The Iran-Iraq war in 1979-1982 caused a second crisis in oil when the price rose to US$38 a barrel. In the UK, in 1979, the energy sector was privatized with the exception of the nuclear sector where there were concerns about the costs of decommissioning of nuclear plants. The importation of cheap coal was also permitted and this, in combination with a cheap supply of natural gas, meant that deep coal mining was drastically reduced. The increased supply of natural gas saw the introduction of combined cycle gas turbine (CCGT) for electricity generation, as this system released less carbon dioxide. This ‘dash for gas’ also reduced the demand for coal, which dropped from 60% in 1972 to 18.7% in 2006. Continued instability in the oil and gas markets and the depletion of the North Sea oil and gas supplies have strengthened the need for fuel security in the UK. In 2008, there was another oil crisis where oil peaked at around US$150 a barrel, which empha­sizes the dependence of developed countries on oil. The reasons for these rapid rises were unclear, as there was no war interrupting supplies. Oil supplies appear to be adequate, so it may be lack of refining capacity and speculation that caused the rises in price.

The burning of coal and oil also produces a number of harmful compounds other than carbon dioxide including carbon monoxide, sulfur dioxide and oxides of nitrogen.

Nitrogen oxides (NOx)

NOx is the term given to the two oxides of nitrogen, nitric oxide (NO) and nitrogen oxide (NO2) where nitric oxide predominates. These contribute towards acid rain as they are

converted into nitric acid in the atmosphere. The natural sources of nitric oxide (NO) and nitrogen oxide (NO2) are soils, ammonia oxidation and lightning (Table 2.6).

High temperatures such as those generated by lightning can produce nitrogen oxides. At high temperatures nitrogen reacts with oxygen by a number of mech­anisms, including the Zeldovitch mechanism:

The reaction yields a mixture of NO and N2O, with NO dominating at ~90%. The high temperature reaction occurs in vehicle engines, aircraft engines and during bio­mass burning. It is clear that a major contribution is from combustion of fuels includ­ing hydrogen in engines of various types.

Most oxides of nitrogen are oxidized to NO2 in the atmosphere which in the presence of water produces nitric acid. The nitric acid contributes to acid rain and the deposition of nitrogen into rivers and lakes can cause eutrophication. In humans NOx has been implicated in an increased susceptibility to asthma.

Nobody would dispute that fossil fuels supplies are finite, but what is disputed is the extent of the reserves remaining, and how long these will last. Over the years, there have been a large number of estimates based on present consumption, reserves and predicted new sources (Grubb, 2001; Bentley, 2002; Greene et al., 2006).

New oil fields have been found both on land and under the sea bed, but these new fields are being found in increasingly hostile environments. It has been concluded that the world is halfway through its recoverable oil, except for the Middle East (Bentley, 2002). Table 1.6 shows some of the estimates for the peak of production, known as ‘peak oil’ (Fig. 1.8), a time after which production declines, and the time when fossil fuels run out.

The International Energy Agency (IEA, 2005 a) has predicted that the supply of crude oil will peak around 2014 and then decline and coal will last until 2200 (Evans,

17

1999). The decline in available coal and crude oil should cause the prices of these fuels to rise, which would limit their use.

Despite the general agreement in these dates, there is still considerable debate over the quantity of known and unknown oil reserves. These figures will clearly have a considerable influence on the lifetime of the oil production. The estimates given in Table 1.6 are based on conventional oil reserves. Data indicate that two-thirds of oil-producing countries are past their peak of conventional oil production, including the USA, Iran, Libya, Indonesia, the UK and Norway (Bentley et al., 2007). The esti­mates have taken into account the proved oil, probable reserves of oil and the rate of discovery. The rate of discovery of new oil fields controls oil production. When oil production and oil discovery are plotted for the UK, it can be seen that discoveries

Fig. 1.9. The discovery of proved and probable oil (2P) and oil production for the UK. Oil discovery measured as millions of barrels of oil per year and production thousands of barrels per day. (From Bentley et al., 2007.)

peaked in the 1970s, while production peaked in 1999 (Fig. 1.9). In mitigation the UK has a few potential sources of oil, including the deep Atlantic, on land, and small pockets of oil in existing fields. Other estimates suggest that 64 of the 98 oil — producing countries have passed their peak of oil production so that the same condi­tions apply to many other countries (see www. lastoilshock. com). Economic factors will also affect oil availability as higher prices will encourage exploration, expensive recovery, the use of marginal oil fields and depress oil demand, though these effects are unlikely to greatly affect estimates of oil reserves (Bentley et al., 2007).

However, there are technologies available that can be applied to extract more oil from existing oil fields and there are also unconventional oil sources. Improved oil recovery (IOR) involves techniques like horizontal drilling and improved manage­ment. Enhanced oil recovery (EOR) involves technologies to mobilize oil trapped in the well and includes gas injection, steam flooding, polymer addition and combustion in situ. Depending on the geology of the oil field, the oil enhancement can range from 10 to 100%. Recent studies have indicated that EOR can temporarily increase the rate of oil production, but the consequence is an increase in the rate of depletion (Gowdy and Julia, 2007). Figure 1.10 shows the oil production from the Forties oil­field where EOR (carbon dioxide flood) was applied in 1987.

Taken in the context of the history of mankind, the use of fossil fuels has been with us for only a short time, as can be seen in Fig. 1.11 (Aleklett, 2005). This means that the stocks have to be conserved, and alternatives introduced that are not reliant on plants and animals that died some 1-100 million years ago. A comparison between biomass currently grown and fossil fuel production in the industrial carbon cycle is shown in Fig. 1.12. It is clear that fossil fuels are not being replaced as conditions are different now and the timescales preclude any form of replacement.

П0

Fig. 1.10. Oil production in the North Sea Forties oilfield (i) through carbon dioxide flood. (From Gowdy and Julia, 2007.)

?

1850

Fig. 1.11. The short history of fossil fuels. (From Aleklett, 2005.)

Fig. 1.12. Petrochemical carbon cycle compared with the use of biomass. (Modified from Faaij, 2006.)

Three non-conventional sources of oil exist: heavy oil, oil shales and tar sands. Reserves of heavy oils are found in Venezuela and the oil is mobile under normal-well conditions but is extremely viscous on extraction. The bulk of oil shales are found in the USA with some in Estonia, Brazil and China. Shale oil is unfinished oil made up of kerogen oil because it has not been exposed to high temperatures. Shales deposits can contain between 4 and 40% kerogen, which can be released when the rock is heated to 300-400°C.

Tar sands contain 10-15% bitumen and are found close to the surface, so that it can be recovered using open cast techniques. The mined and in situ treatment involve heating to allow the oil to separate from the sand. Once extracted, the tar is heated to 500°C to yield kerosene and other distillates. Although tar sands do occur world­wide, 85% of the tar sands are found in Alberta, Canada. Both shale oil and tar sands require considerable energy to extract and process and therefore produce more car­bon dioxide. These sources have been of little economic interest until recently.

The extent of the known reserves and those reserves to be discovered have been subject to a large number of estimates, and examples of two are given in Tables 1.7-1.9. There is reasonable agreement between the two estimates where it is clear that the bulk of the fossil fuel reserves are in coal. At present just below half of the total conventional oil reserves have been consumed, although there are considerable unconventional reserves. The conventional oil reserves and those predicted constitute

170 and 139 gigatonnes (Gt) of carbon, respectively. With oil containing 86% carbon, this represents 197.6 and 280.2 Gt of oil, respectively. These figures correlate well with the estimates of oil and gas reserves given on a regional basis in Table 1.9. The consumption rates have been given as 3.5 and 3.08 Gt per annum, respectively, which, if correct, means that the reserves will last between 40 and 53 years.

The concentration of sulfur dioxide is less than 1 ppb in clean air to 2 ppm in highly polluted areas, with levels typical at 0.1-0.5 ppm. Sulfur dioxide is a respiratory irri­tant, which can affect human health and damage plants. There are a number of natural and anthropogenic sources of sulfur dioxide, but the latter is by far the largest source. Marine phytoplankton produces dimethyl sulfide, which is converted into sulfur diox­ide in the atmosphere, hydrogen sulfide is formed by anaerobic decay and volcanoes emit sulfur dioxide. Most of the sulfur dioxide produced by human activity is from the burning of fossil fuels and the sources in fuels are listed on the following page:

• Oil products contain between 0.1 and 3% sulfides.

• Natural gas can contain hydrogen sulfide which is often removed before use.

• Coal contains between 0.1 and -4% sulfur as inorganic iron pyrites and organic thiophenes.

Burning fossil fuels in power stations has been the main source of sulfur dioxide, as can be seen from the typical emissions from a power station (Table 2.7). The main emissions include carbon dioxide, sulfur dioxide from the sulfur compounds in the coal, and nitrous oxides from the nitrogenous compounds. In the atmosphere sulfur dioxide is rapidly oxidized to sulfuric acid. The pH of clean rainwater is about 5.6 due to dissolved carbon dioxide. However, rainwater in the presence of pollutants sulfur dioxide and nitrous oxides forms sulfuric, sulfurous and nitric acid which can reduce the pH to 1. These acids have a short residence time in the atmosphere, return­ing to the surface as rain. The acid rain has an effect on water bodies, vegetation and buildings. The problem of changes in water and soil pH has been of concern since the late 1960s.

Acidification of waters causes an increase in the leaching of toxic metal ions into the water and also changes the flora and fauna. Acid rain has been blamed for the death of trees in a number of forests in Scandinavia and the USA. The effect is per­haps indirect as the change in pH of the soil may leach out toxic metals and change the uptake of ions by plants.

Acid rain has an effect on buildings, particularly those made from limestone. Concern about acid rain was sufficient to initiate legislation to reduce emissions.

At present, methods of reducing sulfur dioxide emissions are as follows, and examples are given in Table 2.8:

• Reduction in fuel use by improvements in combustion and reduction in energy loss. Combined cycle gas turbine is one such system where the hot gases from the turbine are used to generate steam, which is then used to run a conventional tur­bine. This gives an efficiency of 53% compared with 35% for the conventional

coal-fired stations with sulfur dioxide removal. Clean coal systems have also been introduced using fluidized bed combustion and gasification.

• The use of low sulfur fuels. Inorganic sulfur compounds in coal, such as pyrites, can be removed by catalytic hydrodesulfuration. Organic sulfur compounds, principally thiophenes, can be removed by microbial action (McEldowney et al., 1993). Replacing coal with natural gas also reduces sulfur emissions as natural gas contains little sulfur.

• Sulfur compounds can be removed from the flue gas and the most common is the limestone/gypsum method, where the flue gas is treated with calcium carbonate slurry (limestone). The calcium carbonate reacts with sulfur dioxide to yield insol­uble calcium sulfate (gypsum) which precipitates and can be removed.