Energy and the environment

There is little doubt that the large-scale utilization of fossil fuels is putting significant stress on the environment. The effects of combustion products on air quality and the climate are both local and global in nature. The local effects, primarily in the form of air pollution and smog formation in large urban areas, have been known for many decades, and in recent years government regulations to reduce the effects of air pollution have been significantly strengthened. These include both exhaust emission standards for vehicles as well as emissions regulations for large fixed installations, such as fossil-fueled power stations. These regulations have been pioneered in the USA by agencies such as the California Air Resources Board (CARB), and the US Environmental Protection Agency (EPA), but similar measures have now been adopted in most of the developed world. On a global scale, there is increasing evidence, and concern, about the role of CO2 and other so-called "greenhouse gases'' on global climate change. In this chapter we will examine both the localized and global effects of these air emissions, and describe current mitigation techniques.

3.1 localized environmental concerns

Localized air pollution, prevalent in the heavily populated areas of large cities, results from direct chemical reaction with the products of combustion and from the formation of ground-level ozone. Combustion products include carbon monoxide (CO), sulfur dioxide (SO2), nitrogen oxides (NOx), unburned hydrocarbons, and finally carbon dioxide (CO2), which is primarily of global concern. Carbon monoxide is a toxic gas which is usually formed in small concentrations from well-adjusted burners or internal combustion engines, but can be formed at higher levels if there is insufficient air present for complete combustion. In urban areas this is mainly a product of vehicle engine exhaust, although it has been greatly reduced by the widespread use of catalytic converters in car exhaust systems. As such, it is today rarely a threat to human health on its own. Sulfur dioxide is formed in the combustion process when fuels containing sulfur are burned, and this is now limited primarily to high-sulfur coal or in some cases to low-quality gasoline and diesel fuel containing high levels of sulfur. When SO2 is released to the atmosphere from power station chimneys or vehicle exhausts it can react with water vapor to form sulfuric acid, an important component of "acid rain.'' In sufficient concentrations this can be very damaging to human lung tissue, as well as to buildings, vegetation, and the environment in general. The emission of SO2 from coal-fired power stations, and subsequent acid rain formation, has been greatly reduced in recent years, however, by burning low-sulfur coal and by the installation of flue gas desulfurization (FGD) equipment. Emissions from vehicle exhausts have also been reduced by the on-going installation of sulfur removal equipment in oil refineries in order to remove sulfur from both gasoline and diesel fuel during the refining process.

Nitrogen oxides, NO and NO2, collectively described as "NOx," together with unburned hydrocarbons, are primarily a concern because of the potential to form ground-level ozone (O3). Nitric oxide (NO) is formed during the combustion of fossil fuels in the presence of nitrogen in the air, whether in motor vehicles, thermal power stations, or in furnaces and boilers used to heat homes and commercial buildings. The NO formed during the combustion process is normally converted rapidly to NO2 due to the presence of excess oxygen when it is discharged into the atmosphere. In the presence of sunlight, however, the NO2 may subsequently be dissociated, resulting in the free oxygen atoms reacting with O2 molecules to form high levels of "ground-level" ozone. Ozone is a very reactive oxidant and can cause irritation to the eyes and lungs, and can also destroy vegetation as well as man-made materials such as synthetic rubber and plastic. In high concentrations, found mainly in large urban centers with high levels of solar insolation and unburned hydrocarbons, it becomes "smog" with its characteristic brown color and odor. Smog contains a high concentration of highly reactive hydrocarbon free radicals, and not only causes visibility problems, but can result in severe health problems, particularly for people with asthma or other lung ailments. In response to environmental legislation in many parts of the world, techniques have been developed to significantly reduce the NOx emissions from stationary combustion equipment such as boilers and large furnaces. The production of NOx is directly related to the combustion temperature, and many companies have concentrated on reducing combustion temperatures, thereby reducing NOx formation. This has resulted in the development of so-called "Low-NOx" burners, which incorporate multi-staged combustion, or lean-burn technology in which excess air is used to reduce combustion temperatures. Where regulations are particularly stringent, a greater reduction in NOx emission levels can be achieved by selective catalytic reduction, in which the reducing agent ammonia reacts with NO to produce nitrogen and water. For motor vehicles, the development of the three-way catalytic converter, which has the ability to both oxidize unburned hydrocarbons and CO, and reduce NOx emissions, has been particularly effective in making modern vehicles much less polluting than has previously been possible. The introduction of the catalytic converter on gasoline vehicles has reduced the emission of NOx by over 90% compared with a vehicle without the device.

In addition to the chemical effects of ozone and smog formation, there is increasing interest in the health effects of particulate emissions, which are primarily a feature of coal combustion and diesel engine exhaust. The particles are formed through a complex process involving unburned hydrocarbons, sulfur dioxide, and NOx, primarily in fuel-rich flames such as those inherent in diesel engines and the pulverized coal combustion systems used in power stations. The particles formed have a wide size range, but the ones that have come under the most scrutiny for health reasons, and have been the subject of environmental legislation to limit their production, are those under 10 microns (1 micron = 10~3 mm) in diameter. This so-called PM10 matter can enter deep into the lungs and there is growing scientific consensus that these can then cause serious heart and lung complaints, including asthma, bronchitis, and even lung cancer and premature death. Recently there has been increasing concern about the very smallest particles, PM25, the material under 2.5 microns in characteristic diameter. There is some evidence that these may be of equal, or even greater, concern than the larger particles in that they have the ability to penetrate even deeper into the lungs. Particulate emissions from coal-fired power plants, which normally also include a significant fly ash content, have long been controlled by electrostatic precipitators, which use fine, electrically charged wires to attract the particulate matter, which is then periodically removed, usually by vibrating the wires. This technique tends to work well for large particle sizes, and in order to remove smaller size fractions the precipitator may be followed by a "bag-house," which is essentially a very large fabric filter. These techniques, however, are not sufficient for removing the very smallest particles, such as those produced by diesel engines. The removal of these very fine particles from diesel engines is particularly important in urban areas, where the population density is high, and people are in close proximity to diesel exhaust. In response to increasingly stringent regulations to limit the mass of particulate matter emitted by diesel engines, manufacturers have worked hard to reduce this by increasing fuel injection pressures. Ironically, some researchers have now expressed concern that this actually may have made matters worse, as the increased injection pressures result in much smaller particle sizes on average. The total mass of particulate matter emitted has been significantly reduced, but this has been achieved at the expense of producing many more of the very smallest particles. In recent years diesel engine manufacturers have been working to perfect a "particulate trap," to filter out the very fine particles contained in the exhaust gases. This is usually a very fine, porous, ceramic matrix which traps the particles but allows the gaseous exhaust products to pass through. After some hours of running the trap needs to be "regenerated," by burning off the entrapped particulate material. These devices have not yet been developed to the point where they are reliable enough, or inexpensive enough, to be routinely fitted to commercial vehicles.

3.2 global environmental concerns

On a global scale, it is the "greenhouse effect" and the prospect of global warming which has drawn the most attention. A simple diagram illustrating this effect is shown in Figure 3.1. Solar radiation produced as a result of the very high temperature of the sun is composed primarily of short wavelength visible or near-visible radiation, for which the atmosphere is largely "transparent." In other words, although a small fraction of this radiation is reflected by the earth's atmosphere back out into space, most of it passes straight through (as if the atmosphere is window glass) and warms the earth's surface. The warm earth then re-radiates some of this energy back out into space, but since it is produced at relatively low temperatures it is primarily long wavelength, or infra-red radiation. Some of the gases in the earth's atmosphere, just like window glass, are particularly opaque (or have a low "transmissivity") to this long-wavelength radiation, and are therefore referred to as "greenhouse gases" (GHGs). Much of the long wavelength radiation is therefore reflected back to the earth's surface and

Transmissivity The Atmosphere

there is then a net imbalance in the energy absorbed by the earth and that re-radiated back out, with the result being a warming of the earth's surface and the surrounding atmosphere, just as in a greenhouse.

The degree of this energy imbalance depends very much on the transmissivity of the atmosphere, in other words the degree to which the gases in the atmosphere either transmit or block the infra-red radiation from the earth. Climatologists refer to the effects of changes in the amount of solar radiation reaching the earth's surface as changes in the "radiative forcing'' of the atmosphere. Some gases are much more opaque to the long wave-length radiation leaving the earth's surface than others, and their relative effect is measured by their "global warming potential'' (GWP). Probably the most important of these gases is water vapor, and its concentration in the atmosphere can vary significantly, both spatially and temporally. However, the amount of water vapor in the atmosphere is primarily a function of natural processes, and it is therefore not usually considered to be an anthropogenic (man-made) GHG. The atmospheric gases which are anthropogenic in nature, and which have increased in concentration over time, include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and a variety of gases, such as the chlorofluorocarbons (CFCs), which exist in small quantities, but have a strong global warming potential. Since CO2 exists in the atmosphere in much greater quantity than the other anthropogenic GHGs, it is usually assigned a GWP rating of 1.0. The two next most important GHGs are CH4, with a GWP of 23, and N2O, with a GWP of296 (see Houghton, 2004). Even though CO2 has the lowest GWP of the three gases, it is by far the most important

Global carbon cycle gigatonnes carbon accumulating at 3.2± 0.2 atm°sphere 760

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Figure 3.2 The global carbon cycle. Source: Royal Commission on Environmental Pollution's 22nd Report: Energy - The Changing Climate.

because it is emitted in much greater quantity. Houghton estimated that CO2 has accounted for some 70% of the enhanced greenhouse effect resulting from the anthropogenic release of GHGs, while methane accounts for 24%, and N2O for 6%. For this reason CO2 has received the most attention from scientists and policymakers, although it is not the only GHG of importance. If over time the long-term average concentration of CO2 in the atmosphere increases, there will be a decrease in the long wavelength transmissivity of the atmosphere, resulting in more of the infra-red radiation being trapped. This will lead to an increase in the net energy being absorbed by the earth's surface and the atmosphere, with the result being an increase in the global average temperature. There is, therefore, increasing scrutiny of the "global carbon cycle'' and a concern with increasing concentration levels of CO2 in the atmosphere.

The "global carbon cycle,'' illustrated in Figure 3.2, taken from the report of the UK Royal Commission on Environmental Pollution, Energy - The Changing Climate (2000), shows the quite complex processes at work exchanging carbon between different parts of the earth and its atmosphere. The bold figures in each "reservoir" represent the amount of carbon stored, in units of gigatonnes (Gt - or billions of tonnes). The gray arrows represent natural exchanges between reservoirs, which are nearly in balance, while the bold arrows represent the net flux in each case. The figures in italics adjacent to each of the arrows show the

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Figure 3.3 Emissions of CO2 in the USA by sector, 1995. Source: Based on figures from the Energy Information Agency Emissions of Greenhouse Gases in the United States 1995.

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Figure 3.3 Emissions of CO2 in the USA by sector, 1995. Source: Based on figures from the Energy Information Agency Emissions of Greenhouse Gases in the United States 1995.

CO2 fluxes, in units of Gt/year of carbon, between the different reservoirs. It is clear that the natural fluxes are much greater than the anthropogenic flux resulting from the combustion of fossil fuels and industrial processes such as the production of cement. The net result of all of the net carbon fluxes shown is an accumulation of approximately 3.2 Gt/year of carbon in the atmosphere. In addition to carbon stored as CO2, there is approximately 4000 Gt of carbon stored as fossil fuels; coal, oil, and natural gas, in the earth's crust, as shown in Figure 3.2. It is the consumption of these resources that is the main source of the anthropogenic release of some 6.2 Gt/year of CO2 into the atmosphere. The fossil fuel reserves are relatively modest compared with the amount of carbon stored in the oceans, or in the earth as carbonate minerals, but are also much greater than the total carbon in the earth's atmosphere. They do, therefore, represent a substantial potential source of carbon which would be added to the atmosphere if they were all to be eventually consumed to provide mankind's energy needs without capturing and storing the CO2 released.

The combustion of fossil fuels is the primary source of CO2 emissions, and as such can be traced back to the major energy end-use sectors, including residential and commercial buildings, industrial processes, and transportation. Figure 3.3 shows the distribution of

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Figure 3.4 Atmospheric CO2 concentrations. Source: IPCC Climate

Change 2001: The Scientific Basis.

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Figure 3.4 Atmospheric CO2 concentrations. Source: IPCC Climate

Change 2001: The Scientific Basis.

CO2 emissions by end-use sector in the USA for the year 1995. Contributions from each end-use sector naturally vary from one country to another, depending on the state of industrial development, and particularly on the number of motor vehicles in operation. In the highly industrialized countries, for example, transportation, industrial processes, and electric power generation tend to be the dominant users of fossil fuels, and therefore also the dominant sources of CO2 emissions. Nearly 35% of the total emissions shown in Figure 3.3, for example, originate from electrical powerplants. In less-developed nations, fossil fuel use, and therefore CO2 emissions, may be heavily weighted towards domestic heating and cooking, rather than to the use of motor vehicles. In some sectors the use of fossil fuels, and therefore CO2 emissions, can be reduced by switching from a high-carbon content fuel like coal, to a lower carbon content fuel, such as natural gas. This has been done in parts of Europe, for example, where coal-fired power stations have been replaced by natural gas-fueled combined cycle gas turbines (CCGTs). Also, increasing the end-use efficiency in any sector can be effective in reducing energy consumption, thereby reducing CO2 emissions. This increase in efficiency may be easier to achieve in some sectors, for example domestic home heating, than in others, such as transportation. However, the introduction of fuel efficiency standards for motor vehicles in the USA, as well as increased fuel costs and switching from gasoline to more efficient diesel engines in some markets, has led to significant gains in the efficiency of automobiles over the past three decades.

Figure 3.4, from the Intergovernmental Panel on Climate Change, or IPCC (2005), shows the concentration of CO2 in the

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Figure 3.5 Earth's surface temperature change. Source: IPCC Climate Change 2001: The Scientific Basis.

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Figure 3.5 Earth's surface temperature change. Source: IPCC Climate Change 2001: The Scientific Basis.

atmosphere over the last 1000 years. It can be seen that the CO2 concentration prior to the industrial revolution, beginning in the late eighteenth century, was nearly constant at a level of 280 parts per million (ppm). During the nineteenth and twentieth centuries the level has increased rapidly, reaching approximately 370 ppm today. This concentration represents the total carbon content of some 760 Gt currently in the atmosphere, as shown in Figure 3.2.

The effect of this large increase in CO2 concentration on the earth's surface temperature can be seen in Figure 3.5, with data from various sources, including thermometer measurements over the past two centuries, and temperatures inferred from tree rings, ice cores, and other historical records for earlier times. It can be seen that there is a very good correlation between the increase in global CO2 concentration (as seen in Figure 3.4) and the increase in the earth's temperature.

Scientists working with the Intergovernmental Panel on Climate Change (IPCC) have also done extensive computer modeling of the greenhouse gas effect to try to predict the effect of further increases in CO2 concentration levels on global average temperatures. The computer models have used a number of different emissions and economic activity scenarios in order to better estimate the likely range of CO2 concentration and average global temperature rise. The results of these calculations show that CO2 concentration will likely reach a value

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Figure 3.6 Predicted temperature change, natural forcing only.

Source: IPCC Climate Change 2001: The Scientific Basis.

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Figure 3.6 Predicted temperature change, natural forcing only.

Source: IPCC Climate Change 2001: The Scientific Basis.

ranging between about 550 and 900 ppm by the end of the twenty-first century, depending on the particular scenario chosen. These models have also examined the relative effect on global temperature of "natural forcing'' of the atmosphere, due to variations in solar output, for example, and the so-called "anthropogenic forcing'' due to man-made emissions of greenhouse gases. Figures 3.6 to 3.8 show the results of the model predictions for a base-case scenario compared with measured values of the temperature change from 1850 to 2000. The model predictions have been conducted first with the assumption of only natural forcing, then with only anthropogenic forcing, and finally with both natural and anthropogenic forcing, as shown in Figures 3.6 to 3.8 respectively.

In Figure 3.6, it can be seen that there is quite a poor correlation between the predicted temperature rise, assuming only natural forcing, and the rise obtained from actual observations. This is particularly true for about the first 25 years when the industrial revolution was well under way, and for the last 25 years during which there has been strong economic activity in many countries, with a consequent substantial increase in CO2 emissions. With the assumption of only anthropogenic forcing in the model, as shown in Figure 3.7, the prediction is much better during the early and late years, but not very good

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Figure 3.7 Predicted temperature change with anthropogenic forcing only. Source: IPCC Climate Change 2001: The Scientific Basis.

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Figure 3.7 Predicted temperature change with anthropogenic forcing only. Source: IPCC Climate Change 2001: The Scientific Basis.

during the two decades between 1950 and 1970, when there was a noticeable decrease in solar intensity.

Finally, by including the effects of both natural and anthropogenic forcing in the model, the predicted temperature rise, as shown in Figure 3.8, matches very closely with the observed temperature records. The results from these three sets of predictions provides very strong evidence that the rapid increase in temperature observed over the last 50 years is very likely due to anthropogenic effects, and can be almost entirely attributed to the burning of fossil fuels.

Although the model results shown in Figures 3.6 to 3.8 were obtained for the base-case emissions and economic activity scenario, calculations were also conducted by researchers for a range of alternative scenarios, known as SRES (Special Report on Emissions Scenarios), as described by the IPCC. The results of predictions for the next 120 years for the complete range of these scenarios are shown in Figure 3.9. Results are shown both for a full range of several models using all of the SRES scenarios, and for a more restricted ensemble of models, again using the full range of SRES scenarios. These calculations predict an overall increase in global average temperature between 1990 and 2100 to range from a low of 1.4 °C to a high of 5.8 °C. At the higher end of this range, there would no doubt be significant

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Figure 3.8 Predicted temperature change from both natural and anthropogenic forcing. Source: IPCC Climate Change 2001: The Scientific Basis.

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Figure 3.8 Predicted temperature change from both natural and anthropogenic forcing. Source: IPCC Climate Change 2001: The Scientific Basis.

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Figure 3.9 Temperature change predicted for various emission and economic activity scenarios. Source: IPCC Climate Change 2001: The Scientific Basis.

changes to the global climate, including more frequent and more severe storms, melting of the polar ice caps, and more frequent occurrence of droughts. There would also be a significant rise in mean sea level, predicted to be up to one meter, leading to widespread erosion and flooding in coastal areas worldwide.

Given the real threat that such climate change would have on mankind's well-being, and on the global economy, scientists, engineers, and policymakers are now discussing long-term mitigation techniques to minimize, or at least reduce, the rapid increase in global CO2 concentration levels being predicted for the twenty-first century. At the present time these discussions are primarily focused on obtaining international agreement for limiting the production of greenhouse gases under the auspices of the United Nations Framework Convention on Climate Change (UNFCCC), which was formally adopted in 1992 in New York. Under this convention the most heavily industrialized countries, including the OECD members and 12 countries with "economies in transition," sought to return their greenhouse-gas emissions to 1990 levels by the year 2000. This was followed by the "Kyoto Protocol,'' committing signatories to specific action, which was proposed in Kyoto, Japan in 1997. Under this agreement, the industrialized countries listed in Annex 1 to the Kyoto accord agreed to reduce their emissions of a suite of six greenhouse gases below the levels produced in 1990 by "targets" of between 0% and 8%, averaged over the period from 2008 to 2012, as shown in Table 3.1. In one or two special cases

Table 3.1. Kyoto accord targets

''Annex 1'' countries Target %

European Union-15, Bulgaria, Czech Republic, Estonia, Latvia, -8

Lithuania, Romania, Slovakia, Slovenia, Switzerland

Canada, Hungary, Japan, Poland -6

Croatia -5

New Zealand, Russian Federation, Ukraine 0

Norway 1

Australiaa 8

Iceland 10

Note:

a The USA and Australia did not ratify the agreement. Source: Houghton, 2004.

(Australia, Iceland, and Norway) the agreed targets were actually an increase from the 1990 levels due to the difficulties for smaller economies in making the necessary changes to their energy supply system. The 15 European Union (EU) countries (prior to the admission of 10 new countries in 2004) agreed to obtain the 8% reduction in GHG emissions on average across the whole community. Subsequent negotiations between the EU countries has resulted in the UK target for the period 2008-2012 being set at 12.5% below the 1990 level, for example.

The Kyoto Protocol was subject to final ratification by the countries who were "parties" to the convention, and the protocol was to enter into force on the 90th day after the date on which not less than 55 parties to the convention, incorporating Annex 1 countries which accounted in total for at least 55% of the total GHG emissions for 1990 from that group, submitted their final ratification notice to the UN. The USA and Australia subsequently went on record as saying that they would not ratify the agreement. As of November 2,2004,127 states and regional economic integration organizations had submitted their notice of ratification, and the total percentage of emissions from 'Annex 1' countries ratifying the agreement was 44.2%. The protocol would therefore come fully into force ifeither the USA, which accounts for 36.1% of Annex 1 emissions, or the Russian Federation, which accounts for 17.4% of these emissions, were to submit their notice of ratification. Although the USA indicated that they would not ratify, the President of Russia signed a federal law to ratify the protocol on November 4, 2004. The Kyoto Protocol then came into force on February 16, 2005, 90 days after Russia's notice of ratification was received by the UN in New York.

Whether or not most countries will actually meet these targets is in considerable doubt, particularly given the fact that very few mitigation techniques have been established to date. Also, the fact that the world's largest economy has not signed on to the agreement raises the issue of industrial competitiveness among those countries which do undertake mitigation measures. This will be particularly important given the growing economic power (and greenhouse gas emissions) of the rapidly growing economies, such as China and India, which have not been a party to the Kyoto Protocol. Although some countries (such as the UK) have made considerable progress in meeting their Kyoto targets, this has largely been the result of widespread ''fuel switching'' from coal to natural gas for electrical power generation. The fuel share for gas- and coal-fired power generation in the UK are now just about equal, at nearly 40% each, while in 1990 there was essentially no

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Figure 3.10 CO2 emissions in the UK - 1990-2000. Source: UNFCCC.

gas-fired power generation. Figure 3.10 shows the annual CO2 emissions for the UK, over the period 1990 to 2000, as reported to the UNFCCC. Given that CO2 represents about 70% of the potential global warming effect of all GHG emissions worldwide, it is an important marker for achieving the Kyoto targets. The Kyoto target for the UK of 12.5% below 1990 levels for all GHGs is also shown on the left-hand side of Figure 3.10 in terms of a CO2 emissions target. It can be seen that with the exception of one or two years, there has been a steady decrease in CO2 emissions over the decade (colder than normal temperatures during the winter of 1995-96 resulted in the spike in emissions for 1996). The emission levels appear to have started to move up again in 2000, but it is too soon to know whether or not this is a long-term trend and whether the UK's Kyoto target will be achieved in the 2008-2012 time-frame of the accord.

The widespread fuel switching has been done primarily because of the lower capital cost of building natural gas-fired power stations compared with coal-fired ones, and also due to the relatively low cost of natural gas at the time these plants were constructed during the 1990s. The inherently lower carbon content of natural gas, coupled with the significantly increased efficiency of the combined cycle gas turbine power stations, has resulted in a large reduction in CO2 emissions. With natural gas prices rising rapidly in recent years, however, and with concerns about shortages of natural gas supplies, this

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Figure 3.10 CO2 emissions in the UK - 1990-2000. Source: UNFCCC.

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