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).
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 mech- anisms, including the Zeldovitch mechanism:
O2+ N2« NO + N (2.3)
N + O2« NO + O (2.4)
N + OH « NO + H (2.5)
N2+ O + M « N2O + M (M is an ion) (2.6) 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 bio- mass burning. It is clear that a major contribution is from combustion of fuels includ- ing 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.
Sulfur dioxide
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 irri- tant, which can affect human health and damage plants. There are a number of nat ural 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 diox- ide 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:
Table 2.6. Sources of nitrogen oxide.
Source Nitrogen oxides (NOx) Mt/year Natural
Soil 18.1 Ammonia oxidation 10.2 Lightning 16.4 Anthropogenic
Fuel combustion 65.1
Aircraft 3.0 Industry 4.0
Biomass burning 36.8
Oil products contain between 0.1 and 3% sulfides.
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Natural gas can contain hydrogen sulfide which is often removed before use.
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Coal contains between 0.1 and –4% sulfur as inorganic iron pyrites and organic
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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, return- ing 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 per- haps 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
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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 tur- bine. This gives an efficiency of 53% compared with 35% for the conventional
Table 2.7. Typical emissions from a coal-fired power station prior to flue gas treatment. (Adapted from Roberts et al., 1990.) Chemical Concentration Air (oxygen depleted) ~80%
Water (H2O) ~4.5%
Carbon dioxide (CO2) ~12%
Carbon monoxide (CO) 40 ppm
Sulfur dioxide (SO2) 1000–1700 ppm Sulfur trioxide (SO3) 1–5 ppm Nitric oxide (NO) 400–600 ppm
Nitrogen dioxide (NO2) ~20ppm Nitrous oxide (N2O) ~40ppm Hydrochloric acid (HCl) 250 ppm Hydrofluoric acid (HF) <20ppm
Particulates <115 mg/m3
Mercury (Hg) 3 ppb
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,
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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
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limestone/gypsum method, where the flue gas is treated with calcium carbonate slurry (limestone). The calcium carbonate reacts with sulfur dioxide to yield insol- uble calcium sulfate (gypsum) which precipitates and can be removed.
Photochemical smog
The burning of fossil fuels also produces particulates, soot and black smoke from vehicles and power stations. Power stations now control these types of emissions and there are regulations on the particulate emissions from vehicles. Before the Clean Air Act in the UK coal fires were responsible for the creation of smog, a mixture of fog and smoke, in large cities but these do not occur now. However, photochemical smogs do occur at the present time where NOx, mainly NO, and unburnt hydrocarbons build up due to high traffic density in cities such as Mexico City, Bangkok and Los Angeles.
Nitric oxide (NO) is converted into nitrogen dioxide, and nitrogen dioxide, which catalyses photochemically the production of ozone, produces a corrosive smog.
Conclusions
It is clear from reports such as the IPCC (2007) and Wuebbles et al. (1999) that glo- bal warming is occurring and mankind’s activity is responsible for this. The causes of Table 2.8. Annual emissions (in tonnes) from typical conventional UK 2000 MW power stationsd compared with more efficient alternatives. (Adapted from IEE, 2002.)
Oil-fired Gas-fired combined- Emissions Coal-fired conventional conventional cycle gas turbine
Particulates 7,000 3,000 Nil
Sulfur dioxide 150,000 (15,000a & 75,000b) 170,000 Nil Nitrogen oxides 45,000 (30,000c) 32,000 10,000
Carbon monoxide 2,500 3,600 270
Hydrocarbons 750 260 180
Carbon dioxide 11,000,000 9,000,000 6,000,000
Hydrochloric acid 5,000 Nil Nil
Ash 840,000 Nil Nil
aFlue gas desulfuration.
bLow sulfur coal.
cLow NOx burners.
dA power station of this size will produce 12 TWh/year.
the warming are the greenhouse gases, largely carbon dioxide, which stop long-wave radiation from leaving the earth. These gases are responsible for maintaining the world’s temperature at an average of 14°C, whereas without these gases the tempera- ture would be at −19°C. However, mankind’s burning of fossil fuels releases greenhouse gases, which causes an increase in their levels in the atmosphere and which in turn increases the global temperature. Some still argue that global warming is nothing more than natural variation, but data gathered from a large number of scientific fields indicate that a change in temperature is occurring and the change is a consequence of mankind’s activity. The link between changes observed and the Industrial Revolution is difficult to refute. The consensus now is that the climate is being affected and the consequences may be severe.
3 Mitigation of Global Warming
Introduction
Greenhouse gases in the atmosphere are increasing, in particular carbon dioxide, from the steady values found before 1850. The increase in greenhouse gases appears to be due to the burning of fossil fuels, which has fuelled industrialization. In add- ition, the demands for energy are increasing as more countries become industrialized.
If this increase in greenhouse gases continues unchecked, the consensus is that the world’s climate may be adversely affected (Stern, 2006; IPCC, 2007). The major sources of global energy are the fossil fuels, the supply of which is finite. Therefore, given these factors, considerable efforts have been made to develop non-fossil energy sources. The sources should be both sustainable and renewable, and reduce or elim- inate greenhouse gas emissions to the atmosphere.
Renewable energy means an energy source that can be continually replaced, such as solar energy and plant materials, where the energy is obtained from the sun or stored as a consequence of photosynthesis. However, there are some restrictions to renewable energy sources as these should not be depleted faster than the source can renew itself.
Sustainable development focuses on the long-term development to allow a switch from the use of finite resources to those which can be renewed. Sustainability has also become a political movement involving groups working to save the environment.
Another term used for non-fossil energy sources is ‘carbon-neutral’ which means that either the energy production yields no carbon dioxide, such as solar and nuclear power, or the process only releases carbon dioxide previously fixed through photo- synthesis. In determining the carbon dioxide reduction for renewable energy sources, life-cycle analysis will determine the fossil fuel input into the production of the fuel and carbon dioxide produced. These must be taken into consideration when the car- bon dioxide savings are determined for some biofuels, which may be less than 100%
carbon-neutral.
Urgent action is needed if the atmospheric greenhouse gases are to be stabilized at levels that would avoid damaging climate changes. This chapter covers the possible options available for the stabilization of greenhouse gases. In 2005, the world’s emis- sions of greenhouse gases were 33.7 Gt which included 26.5 Gt of carbon dioxide (Quadrelli and Peterson, 2007). Table 3.1 lists the top 25 carbon dioxide-producing countries.
The USA is the largest producer of carbon dioxide, but China is predicted to overtake the USA by 2025 (World Resources Institute, 2006). Neither country has signed the Kyoto Protocol. Both India and China have had rapid economic growth in the past few years and will account for 45% of new energy demand by 2030 (IEA, 2007). Coal is being used to generate electricity in both China and India which will increase greenhouse gas emissions more rapidly than other fossil fuels.