Environmental issues: emissions, pollution control, assessment and management

2 Environmental issues: emissions, pollution control, assessment and management P. F. NELSON, Macquarie University, Australia

DOI: 10.1533/9781782421177.1.21 Abstract: Coal utilisation in industry has a long and rich history as a provider of reliable energy for electricity production and industrial processing. The management of environmental issues arising from this use has become an increasingly central concern, and raises challenges for industry as it faces potential impacts on air and water quality, and climate, from the emissions associated with coal combustion and other industrial processes. The concerns include greenhouse gas emissions, acid gases, fine particles, and the leaching of toxic trace elements from ash and slag. In this chapter the nature of the emissions and their impacts, legislative approaches, and emissions and controls are described. Finally some observations on the future of coal use are made, and the impacts of advanced technologies on environmental issues associated with coal use are briefly discussed. Key words: coal, sulfur oxides, oxides of nitrogen, fine particles, emissions control, emissions regulations.

2.1

Introduction

Coal has a long and rich history (Freese, 2006) of use in providing a source of light, transport, and electricity for industry. However, utilisation of coal in power production is increasingly under challenge due to real or potential environmental impacts, such as greenhouse warming, acid deposition, urban smog production, trace toxic emissions and leaching of heavy metals to surface and groundwater. In fact, environmental regulations and agreements, enacted at both the national and international level, present a significant challenge to the future viability and operations of the coal and utility industries. Coal utilisation in power production and in metallurgical processes does have the potential, without careful management and application of appropriate technology, to lead to significant environmental problems. Coal’s abundance, security of supply and relative price stability are positive attributes, but contrast with its environmental performance compared to competitor fuels such as gas and some renewable sources of energy. 21 © Woodhead Publishing Limited, 2013

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Current discussions of coal’s impacts are focussed on greenhouse gas emissions. However in some ways, other trace emissions and waste disposal issues associated with coal mining, preparation, and combustion present significant, and more immediate, challenges to social and environmental acceptability. Increasingly, governments and the broader community are taking a more comprehensive approach to assessing the impacts of energy production on the environment. Assessments of costs of power production are now beginning to include costs associated with health and other impacts. The results of a 10-year EU study, for example, recently claimed that the cost of producing electricity from coal or oil would double and the cost of electricity production from gas would increase by 30% if external costs such as damage to the environment and to health were taken into account.1 These estimates of environmental and health impacts are presently at best rudimentary and therefore potentially misleading, but they do suggest that the future will increasingly see attempts to include these costs in assessments of power sources and other industrial processes using coal. The relationship between environment and human health is increasingly becoming a focal point for community concern, a reflection of better information and understanding, but also of concern over environmental influences where we have no direct control. The coal and power industries need to be well positioned to respond to these challenges, particularly in a more competitive environment where other fuels, such as gas and biomass, are being seriously considered as alternatives to coal. Community demand for activities to report against a triple bottom line (economic, social and environmental) will increase. Environmental considerations have been central to the effective utilisation of coal in industry for many years. Potential impacts on human and ecosystem health of coal combustion in power stations, and its use in other industrial processes such as steel making, have required the development of detailed environmental assessment, regulation, management, monitoring and modelling to reduce the risk of deleterious effects. Environmental concerns associated with the use of coal include: •

gaseous and particulate emissions produced in the combustion process, notably oxides of nitrogen (NOx), sulfur dioxide (SO2) and toxic trace elements which can result in atmospheric concentrations which exceed human health guidelines; • conversion of NOx and SO2 to acidic gases and particles which can contribute to atmospheric fine particle concentrations, and can be deposited to sensitive ecosystems through wet or dry deposition processes; 1

http://www.externe.info/ Accessed 18 November 2011.

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•

impacts of NOx emissions on regional air quality through photochemical smog formation; and • emissions to air, water and land from operations associated with the mining of coal and the subsequent disposal of ash and spoil. It is likely that environmental regulations and agreements, enacted at both the national and international level, will continue to present a significant challenge to the future viability and operations of the industries which currently rely on coal as an energy source. Technological and political initiatives to mitigate or adapt to greenhouse gas emissions are also certain to have a large and increasing impact. In this chapter the nature of the emissions and their impacts are first described (Section 2.3). A discussion of legislative approaches (Section 2.6.1) and emissions and controls follows (Section 2.6.2). Finally some observations on the future, and the impact of advanced technologies will be made (Section 2.7).

2.2

Emissions of acid gases

There have been long-standing concerns about emissions of acid gases from the combustion of coal. Emissions of sulfur oxides (largely sulfur dioxide (SO2)) and oxides of nitrogen (largely NO but also some NO2, which collectively are known as NOx), arise not only from S and N present in the coal but also from the combustion process itself. Both sulfur and nitrogen oxides contribute to acid deposition (colloquially known as ‘acid rain’) in which these gases, or the acids they are converted to, impinge on land, water or vegetation as a result of wet and dry deposition processes (Finlayson-Pitts and Pitts, 1986; Seinfeld and Pandis, 1998; Brasseur et al., 1999; FinlaysonPitts and Pitts, 2000, Jacobson, 2002, Brasseur et al., 2003).

2.2.1

Sulfur oxides

Figure 2.1 shows estimates of global emissions of SO2 in the period 1850– 2005 and illustrates the large increases in emissions of this pollutant in the twentieth century and the dominant contribution from coal combustion. The initial increase over this period was driven by industrialisation, largely in the United States and Europe, and decreases from about the 1970s were due to the enactment of clean air legislation. These decreases are now being offset by increasing emissions from developing countries, particularly China and India, and from the growth in international shipping. Figure 2.2 shows sulfur dioxide emissions for the US and Australia, and both illustrate the importance of fuel combustion for electricity production as the major source. In both cases coal is the major contributor to these

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Emissions (Gg SO2)

250 000

200 000

150 000

Total Petroleum combustion Coal combustion Other process Metal smelting International shipping Biomass fuel Grassland and forest fires Ag waste burning Waste

100 000

50 000

18 50 18 60 18 70 18 80 18 90 19 00 19 10 19 20 19 30 19 40 19 50 19 60 19 70 19 80 19 90 20 00

0

Year

35 000

Emissions (Gg SO2)

30 000 25 000 20 000

China Germany Japan UK USA Intl shipping India

15 000 10 000 5 000

18 50 18 60 18 70 18 80 18 90 19 00 19 10 19 20 19 30 19 40 19 50 19 60 19 70 19 80 19 90 20 00

0

Year

2.1 Global sulfur dioxide emissions from 1850 to 2005. Top: Emissions by source sectors. Bottom: Emissions from select countries and international shipping. (Source: Data from Smith et al. (2011) and references therein.)

emissions. In the Australian context there are also large contributions from non-ferrous metal smelting. Emissions of SO2 are primarily a function of the sulfur content of the coal, and usually can be reduced only by:

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Solvent Mobile Miscellaneous Industrial processes Fuel combustion Fires Dust 0

2

4

6

8

10

SO2 emissions (millions of short tons) Motor vehicles Basic ferrous metals Fertiliser and pesticide Commercial shipping Petroleum and coal products Metal ore mining Basic non-ferrous metal Electricity generation 0.00

0.10

0.50 0.20 0.30 0.40 SO2 emissions (mtonnes)

0.60

0.70

2.2 Top: US sulfur dioxide emissions as a function of sector for 2008. (Source: From USEPA National Emissions Database, www.epa.gov). Bottom: Australian sulfur dioxide emissions as a function of sector for 2009/2010. (Source: From Australian National Pollutant Inventory, http:// www.npi.gov.au)

• • •

fuel substitution (i.e., choosing a coal of lower S content); fuel treatment to reduce the S content; flue gas desulfurisation (FGD) in which scrubbing agents (usually based on Ca) react with SO2 to produce a disposable waste product; in some cases the product can be recovered for use as a building material.

Hence coal composition has a direct relationship to SO2 emissions, and it is often assumed that all the S in the coal is converted to SO2. In practice, this

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appears to be a reasonable assumption. Spero (1998) reports conversions of 70–95% of the theoretical emission for a range of Queensland coals. The reduction in the emissions is ascribed to retention of some of the sulfur in the ash via reaction with calcium which occurs naturally in the coals as calcite. However, it has also been claimed (Flagan and Seinfeld, 1988) that the high temperatures typical of pulverised coal flames promote the conversion of any CaSO4 that may be formed to CaO and SO2. While S capture is favoured thermodynamically as the products of coal combustion cool, the reactivity of lime is reduced dramatically by sintering, hence reducing the surface area on which the S capture can occur.

2.2.2

Nitrogen oxides

The formation of oxides of nitrogen (NOx) from the combustion of coal has been extensively studied. This is due to the importance of emissions of NOx and the role they play in a range of environmental problems: • • • • •

winter time urban pollution episodes; acidic deposition and acidification of remote soil and freshwater ecosystems; photochemical ozone formation; fertilisation of sensitive soil and plant ecosystems leading to changes and reductions in biodiversity; stimulation of plankton blooms in marine waters.

In addition, NOx controls the oxidising capacity of the troposphere, and hence the lifetimes of some greenhouse gases. The mechanism for formation is extremely complex; the complexity arises since NO can be produced by several pathways. These pathways are reasonably well understood, and for coal combustion the two most important are: the thermal (or Zeldovich) route in which atmospheric N2 in the combustion air is converted to NOx; and the fuel NOx route in which oxidation of nitrogen contained in the organic structures of the coal produces NOx. The relative contributions of thermal and fuel NOx to total NOx varies with nitrogen content of the fuel and combustion conditions; however, in modern plants burning coal where techniques are used to lower flame temperatures to reduce thermal NOx, fuel NOx can contribute > 80% to the total emissions. In contrast to the situation with coal sulfur, however, where essentially all of the sulfur is converted to SO2, the relationship between coal nitrogen and NO formation is complex and no simple relationship exists. Significant amounts of the coal nitrogen can be converted to N2 in the combustion process, and the efficiency of this conversion is a complex function

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of boiler design parameters, combustion conditions and coal characteristics. A summary of these factors is given below: Boiler design factors • firing mode (front wall, opposed wall, tangential) • capacity or maximum continuous rating (MCR, Mwe or steam flow rate) • burner type (e.g., low-NOx) • number and capacity of burners • burner zone heat release rate (plan, volume and basket) Boiler operating factors • • • •

load excess air or oxygen burner tilt burner swirl vane settings

Coal property factors • • • •

volatile matter content fuel ratio (FC/VM) coal carbon-to-hydrogen ratio (C/H) nitrogen content.

As discussed above, control of emissions of sulfur oxides is often achieved by the use of a coal of lower S content. In the case of N, however, there is much less justification for choosing coals on the basis of N content. Careful control of combustion conditions and boiler design can result in significant reductions in NOx formation. These factors are more important in achieving lower emission standards than coal nitrogen content. There are, however, a number of factors which suggest that the search for a relationship between coal quality and emissions of NOx will continue: •

in spite of the lack of a direct relationship, limitations on the allowable levels of coal nitrogen (usually less than 2% N, dry ash free basis) have been imposed by pollution control agencies (in Japan, for example); • there is considerable downward pressure on emission limits worldwide; • some recent results of large international research programmes show that even under staged combustion conditions (low overall NOx emissions) there are demonstrable effects of coal quality on NO formation.

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In developed countries coal combustion makes substantial contributions to total NOx emissions but motor vehicle emissions are also substantial, as illustrated in Fig. 2.3 which shows the major sources of NOx emissions in the US and Australia.

Solvent Mobile Miscellaneous Industrial processes Fuel combustion Fires Dust 0

2

4 6 8 NOx emissions (millions of short tons)

10

12

Basic non-ferrous metal Oil and gas extraction Coal mining Metal ore mining Biogenics Burning/wildfires Motor vehicles Electricity generation 0.00

0.10

0.20

0.30

0.40

0.50

NOx emissions (mtonnes)

2.3 Top: US nitrogen oxides emissions as a function of sector for 2008. (Source: From USEPA National Emissions Database, www.epa.gov). Bottom: Australian nitrogen oxides emissions as a function of sector for 2009/2010. (Source: From Australian National Pollutant Inventory, http:// www.npi.gov.au).

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Fine particles

Atmospheric particles (also known as particulate matter, PM) are receiving increased attention as a consequence of their effects on human health, visibility, acid deposition and global climate. Much of the recent attention has been directed at the inhalable fine particle fraction (less than about 1–2 μm in diameter) due to the potential impacts on human health. Statistical analyses of urban air pollution in the United States, Europe and elsewhere have revealed a strong correlation between fine particle concentrations and short-term impacts on health, such as mortality (Wilson and Spengler, 1996). Recent results (Pope et al., 2002) have extended these findings to long-term impacts. Fine particles arise from both natural and anthropogenic, and primary and secondary, sources. Here, primary refers to particles directly emitted by sources such as diesel vehicles, industrial processes and bushfires, and secondary refers to atmospheric gas-to-particle conversion processes. Coal combustion is a significant source and contributes to both primary and secondary fine particles.

2.3.1

Human health impacts

The relationship between exposure to air pollutants and potential health impacts has been recognised for many years, at least since increasing industrial development in Europe resulted in large increases in emissions of black smoke and acid gases. The quantitative relationship between extreme air pollution events and excess mortality has also been established for around 50 years, since the famous ‘London Smog’ of 1952. In that event, a strong rise in air pollution levels, particularly particles and SO2, was followed by sharp increases in mortality and morbidity. Effects of long-term exposure to much lower levels of pollutants have been more difficult to establish, in part because of the difficulties in separating the impacts of confounding factors on health outcomes. Recent epidemiological research, however, based on long-term observations in cities in the developed world, has consistently revealed an association between air pollution, particularly fine particles and human health impacts. These statistically based analyses of urban air pollution worlwide have revealed a clear correlation between PM concentrations and short-term impacts on health (Dockery et al., 1993; Wilson and Spengler, 1996; HEI, 2002). Further results (Pope et al., 2002) have extended these findings to long-term impacts. For example, Pope et al. (2002) found that each 10 mg m−3 increase in the concentration of fine particles (PM2.5) was associated with an

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Table 2.1 Dose-response relationships between PM exposure and health effects (derived from HEI (2002) which includes details of the references listed) Region

Reference

PM size fraction

3 Australian cities

Simpson et al., 2005

29 European cities (APHEA-2 project)

Pope and Dockery, 2006/Analitis

PM2.5 PM10 PM10

Increased risk in mortality per 10 μg/m3 of PM (95% confidence interval)

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29 studies from outside Western Europe and North America 90 US Cities (NMMAPS Project) Four Asian cities

Short term All-cause 0.9% (−0.7–2.5%) 0.2% (−0.8–1.2%)

Katsouyanni et al., 2001 Cohen et al., 2004

PM10

0.5% (0.4–0.6%)

Samet et al., 2000

PM10

0.5%

Health Effects Institute, 2010

PM10

0.6% (0.3–0.9%)

Chen et al., 2008

PM2.5

All-cause 6% (3–10%)

Laden et al., 2006

PM2.5

16% (7%–26%)

Jerrett et al., 2005 Pope et al., 2002

PM2.5 PM2.5

17% (5–30%) 6% (2–11%)

Cardiovascular

Respiratory

0.76% (0.47–1.05%)

0.58% (0.21–0.95%)

0.6% (0.2–1.1%)

0.3% (0.1–0.5%)

Cardiopulmonary

Lung Cancer

12% (−3–30%) 9% (3–16%)

44% (−2–211%) 14% (4–23%)

0.6% (0.4–0.8%)

Long term Meta-analysis of six cohort studies (five North America, one Europe) Harvard Six Cities – extended analysis Los Angeles (ACS study) ACS Study

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8% increased risk of lung cancer mortality. A similar magnitude of impacts has been observed worldwide. Table 2.1 summarises the results of some of these studies. In summary, these studies suggest that atmospheric particles have substantial impacts on human health with more recent data indicating PM2.5 has more significant impacts than PM10. A feature of the many studies which have now been conducted in many different countries is the convergence of the results obtained. Research efforts (HEI, 2002) have begun to identify characteristics of particles that may induce health effects, suggested plausible biological mechanisms and identified groups of the population who appear to be at increased risk. However, there is still considerable debate about the characteristics of particles important in determining health impacts. Particle characteristics of potential importance include size, morphology and composition. It is clear that health effects are more significant for fine (less than 2.5 μm, PM2.5) and ultrafine (less than about 0.1–1 μm) particles than for larger material (HEI, 2002), but there is still considerable debate about the importance of particle number, mass and morphology. Particle composition is also likely to be important, and the US Health Effects Institute (HEI, 2002) has described the health effects of PM components, including metals and organic components. Table 2.2 provides an overview of the current understanding of the biological effects of the various components of the PM. Inter-related effects of particle size, composition and other characteristics and health effects are not yet completely understood, which in part may be attributable to the significant temporal and spatial variation observed in PM characteristics. The fine (PM2.5) fraction of ambient PM largely consists of carbon (elemental and organic), metals, sulfate, and nitrate. The relative contributions of these components varies spatially and temporally and will be determined, inter alia, by proximity to sources, time of day and year, and other factors. Critical reviews (Lighty et al., 2000; Jacobson et al., 2000; Monn, 2001; Valavanidis et al., 2008; Pelucchi et al., 2009; Burgan et al., 2010; Charlesworth et al., 2011) have summarised the current understanding in the area of PM composition, health effects and what is currently known about the size and composition of combustion aerosols and the organic fraction, and also of the spatial variability in composition. There remains intense activity in the area of PM and health effects, and particularly in investigating causal relations between fine particle composition and health effects. Okeson et al. (2003) consider the key issues particularly for combustion generated fine particles to be:

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Table 2.2 Chemical components of PM10 and their biological effects Component

Major subcomponents

Described biological effects

Metals

Iron, vanadium, nickel, copper, platinum and others

Organic compounds

Many are adsorbed onto particles; some volatile or semi volatile organic species form particles themselves.

Biological origin

Viruses, bacteria and their endotoxins, animal and plant debris (such as pollen fragments) and fungal spores.

Ions

Sulfate (usually as ammonium sulfate) Nitrate (usually as ammonium/sodium nitrate) Acidity (H+)

Reactive gases

Ozone, peroxides, aldehydes

Particle core

Carbonaceous material

Can trigger inflammation, cause DNA damage, and alter cell permeability by inducing production of reactive oxygen species (particularly hydroxyl free radicals) in tissues. Some may cause mutations, some may cause cancer, and others can act as irritants and induce allergic reactions. Plant pollens can trigger allergic responses in the airways of sensitive individuals; viruses and bacteria can provoke immune defence responses in the airways. Sulfuric acid at relatively high concentrations can impair muccociliary clearance and increase airway resistance in people with asthma; acidity may change the solubility (and availability of metals and other compounds adsorbed onto particles). May adsorb onto particles and be transported into lower airways, causing injury. Carbon induces lung irritation, epithelial cell proliferation, and fibrosis after long-term exposure.

Source: From HEI, 2002.

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• The magnitude of the impact of PM on human health depends on PM mass, size distribution, composition (polyaromatic hydrocarbons and metals such as Fe, V and Zn), the presence of biogenic components (endotoxins, pollens, bacteria, viruses) and other factors. • Particle size distribution ‘appears to have a modulating effect on the degree of toxicity beyond that anticipated due merely to deposition issues’ – as discussed above, there is evidence of increasing toxicty with decreasing particle size. • Relative roles for soluble and insoluble components of PM require further elucidation.

2.3.2

Climate effects

Fine particles (or aerosols) will also have major impacts on climate processes as they alter the energy balance of the atmosphere, and affect the absorption, scattering and emission of radiation within the atmosphere and at the Earth’s surface. Anthropogenic contributions to aerosols (primarily sulfate, organic carbon, black carbon, nitrate and dust) together produce both cooling and warming effects, and an indirect cloud albedo forcing. They also influence rainfall. However, aerosol impacts on the magnitude of the temperature response, on clouds and on precipitation remain subject to high uncertainty, as does the extent of the contribution from coal-derived particles. The net impacts of coal-fired power stations on the radiative balance (now and in the future) due to aerosols will be a complex function of the balance between primary and secondary aerosol formation, future emissions growth projections, and other issues. Quantifying these impacts is possible only with comprehensive global models of emissions growth, atmospheric composition, transport and transformations, and radiative forcing. Recent work by Shindell and co-workers at NASA Goddard Institute for Space Studies and Columbia University provide (Shindell et al., 2009; Shindell and Faluvegi, 2010) detailed insight into these complex interactions. Using a climate model, Shindell and Faluvegi (2010) undertake the first study of the spatial and temporal pattern of radiative forcing specifically for coal-fired power stations. They find that without substantial pollution controls (to reduce emissions of primary PM and secondary PM precursors such as SO2 and NOx), climate forcing in the near term (to the 2040s) is negative. Their results clearly show general cooling in the Northern Hemisphere, and more intense localised cooling near regions of high growth in emissions (i.e., China and India). If pollution controls on secondary PM precursors are enforced, however, the full positive forcing from CO2 is rapidly realised. In the longer term, global mean forcing from stable emissions is positive regardless of pollution controls. The authors conclude that their ‘results indicate that due to spatial

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and temporal inhomogeneities in forcing, climate impacts of multi-pollutant emissions can vary strongly from region to region and can include substantial effects on maximum rate-of-change, neither of which are captured by commonly used global metrics’.

2.3.3 The coal industry’s contribution to fine particles The coal industry makes significant contributions to fine particles in a number of ways: • Generation of dust and fine particles during open cut coal mining. • Direct emissions of fine particles from coal-fired power stations and other industrial processes which use coal. • Indirect contributions to fine particle loadings through conversion of gas phase emissions of SO2 and NOx to sulfate and nitrate. The indirect route to formation is very important in fine particle formation and requires additional discussion. The processes by which the gas to particle conversions occur are complex, with both homogeneous (gas phase) and heterogeneous (aqueous phase) processes contributing. Gas phase oxidation The following gas phase reactions have been identified as being most important (Seinfeld and Pandis, 1998; Brasseur et al., 1999, 2003; Finlayson-Pitts and Pitts, 2000): SO2: SO2 + OH → HOSO2 HOSO2 + O2 → SO3 + HO2 SO3 + H2O → H2SO4 net: OH + SO2 + O2 + H2O → H2SO4 + HO2 NOx: NO + O3 → NO2 + O2 NO + HO2 (RO2) → NO2 + OH (RO) NO2 + OH → HNO3

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The rate at which oxidation occurs is a function of hydroxyl radical (OH) concentration, which in turn is strongly influenced by temperature and radiation, by the concentrations of NOx, ozone (O3), water vapour and volatile organic compounds (VOCs), the latter being emitted from many anthropogenic and natural sources. It can be concluded that the rate of gas phase production of secondary particles within a power station plume is a strong function of the atmospheric environment into which the plume is emitted and transported. This is borne out by the literature review of Hewitt (2001) who reviewed conversion of SO2 and NOx to sulfates and nitrates in power station plumes and documented observed gas phase oxidation rates for SO2 in the range <0.1–30% h−1. As these oxidation processes are primarily dependent on the production of OH radicals under sunlight, the formation of gas-phase sulfate effectively ceases at night time and the formation of gas-phase nitrate slows to much lower rates. Reactions between excess ammonia, the major neutralising species in the atmosphere, and the acid aerosols formed by the above gas-phase processes result in the formation of ammonium nitrate and ammonium sulfate. The sulfate is almost exclusively ultra fine material (<1μm), but the nitrate can partition between PM2.5 and larger material. Aqueous phase oxidation Sulfur dioxide and nitrogen oxides can also undergo aqueous-phase oxidation to sulfate (as S(VI)) and nitrate. When ozone or hydrogen peroxide (H2O2) is present in cloud water, aqueous-phase oxidations rates of SO2 to SO42− can be 100s of per cent per hour. As a consequence, in-cloud aqueousphase SO2 oxidation is an efficient source of sulfate, which is then available to be deposited to the ground through wet deposition, or to condense and become sulfate particles if the cloud water evaporates. The relative contribution of gas-phase and aqueous-phase sulfate and nitrate production to the overall loading of particle mass in an airshed will depend strongly on the availability and characteristics of clouds, the likelihood that the clouds and the SO2 and NOx plumes will interact and the probability that rainout occurs. The key reactions are: SO2: SO2 + H2O → HSO3– + H+ → SO32– + 2H+ HSO3– + H2O2 → SO42– + H+ + H2O SO32– + O3 → SO42– + O2

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NOx: Gas phase followed by aqueous phase (in droplets, cloud particles): NO2 + O3 → NO3 + O2 NO3 + NO2 → N2O5 N2O5 + H2O (l) → 2H+ + 2NO3– net: 2NO2 + O3 + H2O (l) → 2H+ + 2NO3– + O2 A determining parameter for fine particle impacts is particle size and hence the aerosol size fractions in which the sulfate and nitrate PM resides. There is substantial evidence (Seinfeld and Pandis, 1998) that secondary sulfate resides in the fine fraction (i.e., less than 1 μm) as a result of gas-phase condensation or from heterogeneous aqueous-phase reactions. Nitrate PM has been observed to reside in both the fine and coarse particle fractions. Fine fraction (<1 μm) nitrate aerosol is usually in the form ammonium nitrate and is a result of gas-phase condensation. Coarse fraction nitrate is formed from the reaction of nitric acid with sodium chloride or aerosol crustal material (i.e., sea salt and dust (Seinfeld and Pandis, 1998). PM2.5/PM10

Fine ash SO3 SO2

H2SO4 /Sulfate (NH4)2SO4

PM2.5

Sulfate (NH4)2SO4

NO/NO2

PM2.5

Nitrate NH4NO3

PM2.5/PM10

HNO3(g)

FF/ESP

2.4 Schematic of fine particle formation mechanisms from the gas and particulate phase emissions from coal-fired power stations.

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Based on the discussion in this section, coal-fired power stations can contribute to atmospheric particle loadings in three ways. These are summarised schematically in Fig. 2.4. An assessment of power station impacts on atmospheric PM is therefore complex and not simply dependent on direct emissions of the fine components of the ash which escape capture in the air pollution control devices.

2.4

Trace elements

2.4.1 Trace element concentrations and modes of occurrence Trace toxic metals and elements are also present in coals in varying concentrations (Swaine, 1990, 1995; Davidson and Clarke, 1996; Dale et al., 1999; Dale, 2003), and some will be emitted with the fine particulate material, or perhaps in the gas phase for some very volatile compounds. Release of trace elements2 into the environment is an area of increasing concern given the relatively high toxicity of some of these species. As a consequence, pollution control agencies in many countries are evaluating the environmental impacts of emissions of toxic trace elements, and in some cases instituting regulations to limit these emissions. Utilisation of coal in combustion and other industrial processes results in potential releases of trace elements in a number of ways. The more volatile may be emitted in the gas phase or enriched on the fine (sub-micron) particulate fraction, and hence escape capture by electrostatic precipitators (ESPs), fabric filters (FFs), or other air pollution control devices (APCDs). Alternatively, trace elements may reside in the fly ash collected by gas cleaning devices or in the bottom ashes or slags. Their ultimate fate in the latter case will depend on the utilisation and/or disposal options chosen for the ash or slag, and in many cases will be determined by the leachability of the trace elements. The nature of the fine particles released from coal-fired power stations has been studied for many years. The early work is excellently summarised and reviewed by Damle and co-workers (Damle et al., 1982). More recent work at full scale (Kauppinen and Pakkanen, 1990) and at pilot scale (Galbreath et al., 2000) has extended our understanding of the size and composition as a function of size. Current understanding of combustion aerosols or fine particles has been reviewed by Lighty and co-workers (2000). 2

Trace elements of concern include those regulated under the 1990 US Clean Air Act amendments (antimony, arsenic, beryllium, cadmium, chromium, cobalt, lead, manganese, mercury, nickel and selenium, as well as the radionuclides uranium and thorium).

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2.4.2 Vaporisation, condensation and fine particle formation Inorganic material, including the trace elements, can occur in coal in different mineralogical environments, and this can have a significant effect on their behaviour or deportment during combustion. Three broad classes of inorganic material in coals have been described: • inorganic material present in the original sediment layers which formed the organic structure of the coal. This is usually organically bound and described as an inherently-bound metal; • some inorganic material may not be incorporated into the organic structure; instead the structure forms around it; this is described as a mineral inclusion; • some material enters the sediment layers after they have decomposed, or mix into the coal during geological events after the coal seam has formed and during mining operations; these are described as mineral exclusions. Trace elements can reside in any of these structures. These residences, or modes of occurrence, have been studied with a range of analytical techniques and reviewed by Davidson (2000). Davidson concludes that completely reliable techniques have yet to be developed for the analysis of trace element modes of occurrence. The importance of trace element modes of occurrence may be illustrated by considering the results of Teng (Doctoral thesis in Mechanical Engineering, MIT as quoted by Senior et al. (2000)). Teng measured the fractional vaporisation of As, Se, Cr and Co under conditions approximating those found in full scale boilers. It was found that the fractional vaporisation was similar for all four elements at ~ 0.2–0.4. This is surprising since: •

The oxides of As and Se have high vapour pressures (As2O3 boils at 465°C and SeO2 and SeO3 at 317°C and 180°C respectively) and might be predicted to completely vaporise under these conditions, and • The oxides of Cr and Co are refractory with very high boiling points, and hence might be predicted to not vaporise to any significant extent. These differences have been rationalised (Senior et al., 2000) on the basis of modes of occurrence. Chromium and cobalt are often organically bound in northern hemisphere coals, although comparative studies of modes of occurrence have shown poor agreement for these elements. Elements associated with the organics would be expected to be released with the vola-

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tiles regardless of their vapour pressure, and are often lost during ASTM devolatilisation tests. By contrast, As and Se are often associated with the pyrite in northern hemisphere coals. Their release has been postulated to be retarded since they must diffuse out of the pyrhoittite particles formed in the early stages of combustion. A refinement to the model of trace element vaporisation has been developed to account for this diffusional resistance (Senior et al., 2000). These results have clear implications for coals from different sources, such as those with low pyrite contents. In that case trace element associations may be rather different, leading to different fractional vaporisations. Particle size distributions of ash produced during coal combustion have long been known to be multi-modal in character. The majority of the ash is greater than 1 μm in size, and results from pulverised fuel burnout leading to an ash residue, and particle fragmentation processes. A much smaller proportion of fine material is also produced. The nature and composition of this material is of great importance, since: •

Collection efficiency in electrostatic precipitators is lowest for particles in the 0.1–1 μm size range (Helble, 2000). • In the context of trace element deportment, the nature of the fine ash is very important since trace elements have been observed to be enriched in the fine fraction. • The major health effects observed recently for air pollutants are associated most strongly with fine particulate material. • Light scattering is greatest for particulate material in the submicron range; hence visibility impacts are highest for this range. Submicron particulate material produced in combustion processes largely arises from vaporisation and condensation processes, although there is evidence for a contribution, probably from fragmentation processes, to particles in the 0.7–3.0 μm size range (Linak et al., 2003). The mechanism for the formation of submicron-size particles is widely understood to result from homogeneous condensation of flame-volatilised species, namely the refractory oxides SiO2, CaO, MgO and Fe2O3. Flagan and co-workers (Flagan and Friedlander, 1978; Taylor and Flagan, 1982) described the vaporisation/condensation processes. The main features of this mechanism are as follows: • During combustion highly reducing conditions can exist inside coal particles. • Under these conditions refractory oxides can be reduced to more volatile suboxides or elements; for example, in the case of Si:

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The coal handbook SiO2(s) + CO(g) → SiO(g) + CO2(g)

• The volatile species is transported away from the particle into the bulk gas where O2 concentrations are significantly higher, and the suboxide or element is re-oxidised: SiO(g) + O2(g) → SiO2(g) + O(g) •

Provided the vapour pressure of the oxide exceeds the saturated vapour pressure, spontaneous condensation will occur, and nuclei of the submicron fume will be formed.

This model was extended by Sarofim and co-workers (Haynes et al., 1982; Quann and Sarofim, 1982; Quann et al., 1982) to account for observed distributions of inorganic species in the fine ash products from coal combustion. Submicron particle formation is important in trace element deportment, since although trace toxic species such as As and Se can also vaporise due to the formation of volatile combustion products, these species are often not present in sufficient concentrations to homogeneously condense. Condensation of these species on existing particles is, in that case, more likely. The fine particles contribute relatively more to the available surface area, so enrichments in the fine particle fraction are often observed as detailed above. Helble (2000) has developed a model, which includes trace element concentrations as a function of particle size, and size dependent particulate capture efficiencies. Using this model he is able to show that the predictions of emitted concentrations of volatile trace elements such as arsenic and selenium can be improved. Table 2.3 Collection efficiencies (CE) for total particles and for each element Element

Total particle CE

Element CE

Hg Se As Cd Pb Ni Cr Co Be Mn Sb

99.1 99.0 99.2 99.2 99.2 99.1 99.2 98.9 99.0 99.2 98.9

28.9 49.1 96.1 96.1 96.8 97.6 98.0 98.2 98.3 98.5 98.5

Source: Adapted from Helble, 2000.

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It had been known since the work of Davison et al. (1974)) that the fine particle fraction of fly ash could be enriched in trace elements compared with the fraction of trace elements in the parent coal. This is due to the volatilisation of some elements in the boiler and their subsequent condensation in the cooler sections of the flue gas stream. There has been considerable work investigating these observations for a variety of ESP stations burning different coals (Helble, 2000). These studies have found different behaviour for different elements and their transport through the ESP. While there are exceptions, most studies find the same elements enriched in the fine particle fraction. For instance, most studies examined by Helble (2000) found enrichment for the elements As, Cd, Pb, Sb and Se while most found depletion in Mn. This is in general accord with the findings of Meij (1994) who carried out studies on ESPs at coal-fired power stations in the Netherlands, and proposed that the observed results could be explained in terms of the element volatility. The overall capture efficiencies for individual trace elements for ESPs were estimated by Helble (2000) for a number of elements, and Table 2.3 illustrates typical collection efficiencies observed. Table 2.3 shows that for most elements except Hg and Se, the two most volatile elements present, the element capture efficiency is almost as high as the total particle collection efficiency. However, for the reasons stated earlier, small variations between the overall collection efficiency and the individual collection efficiency may have resulted from enrichment in the fine fraction.

2.4.3

Leaching of trace elements from coal waste products

The issue of use and disposal of the coal combustion and processing byproducts inevitably formed when coal is utilised will remain a major challenge. These by-products or waste material consist primarily of fly ash, along with boiler bottom ash, slags, scrubber sludge, and various liquid wastes. These materials contain potentially toxic trace metals (arsenic, mercury, chromium, lead, selenium, cadmium, and boron). Although successful management strategies have been developed in industrialised countries to avoid major environmental problems with the disposal and/or use of these materials, toxic contaminants can leach from the waste into groundwater and surface water when the waste is not properly disposed of. Waste generation is an unavoidable consequence of coal processing. If an ongoing use for the ash cannot be established it is normal for the material to be disposed of to land-based waste repositories. These may be purpose-built, such as ash and slag dams or the infilling of geographic features and mine voids. Notwithstanding which disposal method is used the principle of producer responsibility would suggest that the need for ongoing

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maintenance and monitoring of such repositories remains for many years into the future. There are indications worldwide that these responsibilities are increasingly being recognised. For example, the Minerals Council of Australia (MCA, 2007) has explicitly recognised this responsibly in its key indicators of sustainable development: •

•

from merely after the fact environmental remediation of mine sites and emissions to land, air and water, [otherwise known as end of pipe remedies], to before the fact risk based prevention; management; from the rehabilitation and reclamation of used mine sites to a beyond life of mine, longer term management for sustainable ecosystems.

Clearly the MCA believes that responsibility extends beyond the waste pipe and to the legacy that is left for the community. It is equally clear that an ongoing commitment by coal waste producers will also be required to ensure that the disposal strategies adopted today continue to be effective into the future. It should be in both the community and the generator’s interest to minimise the amount of ash that goes to waste repositories and to maximise the amount marketed for productive purposes (Aynsley et al., 2003). However, recent research has identified some of the many constraints which may prevent this from occurring: • Cementious product use is the major market for ash, because the value to the end user compensates the significant costs of extracting and transporting the ash, to be attractive to other users, either costs need to be lowered or price maintained (Heeley, 2003). • Potential high volume applications need to have a higher value to the user than the cost of supply and delivery; if these applications cannot be identified then subsidies from ash producers would be required to encourage increased use (Heeley, 2003). • Constraints to increased use may be exacerbated by the lack of extended responsibility by coal producers for their product, competition between ash producers and vertical integration of the cement, concrete and quarrying industries (Heeley, 2003). • Fly ash penetration into the high volume structural fills market requires not only approval by state environmental regulators but also acceptance by the generators that they will not receive premium prices for incremental increases in ash sales (Innes and Davis, 2000). • Uncertainty by regulators on the environmental effects following the use of ash as backfill where a potential for contamination of water resources exists (Ward et al., 2006b).

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Table 2.4 Chronology of USEPA activities and regulations with respect to coal waste regulation Year

Action

1980

Solid Waste Disposal Act Congress: Amendments to the temporarily exempts fly ash, bottom Resource Conservation and ash, slag and FGD wastes from Recovery Act (RCRA) of regulation as hazardous wastes 1976 directs EPA to study adverse effects on human health and the environment Report to Congress, Wastes Tentative determination that these from the Combustion of materials do not warrant regulation Coal in Electric Utility as hazardous wastes Power Plants Consent decree Divided wastes into two categories: those generated by electricity generation (Category 1) all remaining wastes (Category 2) Regulatory determination Category 1 wastes do not warrant regulation when managed alone (i.e., not co-managed with lowvolume wastes) Report to Congress Tentative determination that Category 2 materials should also be exempt Regulatory determination Confirmed that Category 2 materials should also be exempt Determined that ‘national regulations under subtitle D of RCRA are warranted for (coal waste materials) when they are disposed of in landfills or surface impoundments’ Also suggested that regulations are also warranted when these wastes are used to fill surface or underground mines Still awaiting regulations EPA plans to develop national regulations for coal waste disposal in surface impoundments, landfills, and as mine filling but has yet to do so

1988

1992

1993

1999 2000

2008

•

Effect

Imposition of ‘land fill levies’ by regulators on large tonnage ash usage projects resulting from the classification of ash as an industrial waste resulting in its replacement by virgin materials (Ward et al., 2006a).

Given these barriers to beneficial use of coal waste materials it is more than likely that this issue will be managed through a risk assessment process.

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Some examples of the examination of the impacts of ash disposal on human health and ecological risk are revealing. The US EPA performed a very comprehensive assessment of human health and ecological risk associated with waste disposal in the 1990s. A chronology of the US EPA activities is provided in Table 2.4. The fact that the EPA did not release national guidelines for coal waste disposal has been highly controversial, and has led to a range of activities and responses (NRC, 2002, 2006). The National Research Council (NRC) through its Board on Earth Sciences and Resources/Committee on Mine Placement of Coal Combustion Wastes (NRC, 2006) found that: Coal mine disposal is a viable management option as long as (1) combustion residue placement is properly planned and is carried out in a manner that avoids significant adverse environmental and health impacts and (2) the regulatory process for issuing permits includes clear provisions for public involvement. This can be achieved by identifying the characteristics of the residues, considering options for productive reuse of them, assessing the potential disposal sites, identifying management and engineering design of placement activities, and monitoring the site for potential contamination.

The provisions identified by NRC require a highly developed system of environmental management, governance and compliance. They also imply a long-term approach to the management of sites where this type of disposal is practised. The response of the public and NGOs to this lack of action by EPA has been highly critical. Recent reports claiming that: ‘new EPA risk assessment finds extraordinary cancer risk from coal ash; lack of federal regulations endanger U.S. water supplies’3 and ‘ash from power plants used to fill mines is poisoning water throughout PA; Local group petitions EPA for assistance’4 demonstrate that this is a current issue of high importance for some who live and work in the vicinity of coal disposal sites. Many of these potential problems arise from old and poorly managed disposal sites, but they do illustrate the importance of: • long-term ownership and management of disposal sites, • recognition of the need to manage the potential liabilities associated with these sites, • proper governance of this issue in a way that involves regulatory bodies, community groups, and the industry. 3

4

Cancer: Coal’s Hidden Cost, available at:http://www.earthjustice.org/news/press/007/ cancer-coals-hidden-cost.html, accessed 6 March 2007. New Study Reports Pennsylvania Groundwater Contamination from Coal Ash, available at: http://www.earthjustice.org/news/press/007/new-study-reports-pennsylvaniagroundwater-contamination-from-coal-ash.html, accessed September 2007.

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There are many good practice examples of ash disposal and use where the environmental impacts are shown to be acceptable and the risk is often managed by engineering approaches (ICCT, 1999; UKQAA, 2003). However, the issues listed above become increasingly important for coal companies selling into developing countries if they have a commitment to moving beyond simple compliance issues in the sustainable use of coal.

2.5 2.5.1

Environmental issues in iron making and advanced coal processing technologies Iron making

Large quantities of coal are also used in iron making, and although the environmental issues created are similar to those for coal combustion for electricity production, some differences are worth noting. Currently there are two major iron-making technologies: reduction of iron in the blast furnace, and secondary production of iron from recycled scrap in electric arc furnaces. The blast furnace traditionally relies on the use of coke as a reductant and to provide mechanical strength and porosity in the blast furnace charge. Coke is produced by the slow pyrolysis of coals with specific properties resulting in the release of oils, tars, nitrogen compounds such as ammonia and nitrogen oxides, and sulfur dioxide, and a solid ash product. The coke is a relatively pure carbon but its production clearly has the potential to create significant air and water pollution issues: • The initial pulverisation of the feed coal can create a particulate emission source. • The volatile material released during pyrolysis includes some of the major air pollutants (CO, NO2, SO2) and highly toxic materials (benzene, naphthalene, phenol, polycyclic aromatic compounds (PACs), hydrogen cyanide, and hydrogen sulfide). • The coke is pushed out of the oven in batches creating additional opportunities for particulate emissions. • The coke is usually quenched with water which creates a wastewater disposal problem. Historically, coke making has been both the largest source of emissions from iron-making, and because of the design of the coke ovens, the most problematic to control using air pollution control devices. Nonetheless, every stage of the iron-making process has the potential to release significant emissions of air pollutants. Particulates are ubiquitous at virtually every stage of the

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process. The blast furnace generates large quantities of sulfur and nitrogen oxides, and carbon monoxide. Casting and rolling contribute to sulfur oxide emissions as sulfur is released from the hot metal surfaces. Wastewater is also generated throughout the process (e.g., as cooling water, and from wet scrubbers) but in more recent times has been more carefully managed through increased rates of reuse and recycling. Large volumes of solid wastes (ash, slags) are also a significant by-product but extensive research over many years has demonstrated that much of this material is relatively benign, has low leaching rates of toxic compounds and can be gainfully utilised in products such as aggregates for use in road making and concrete. Well-developed protocols have been created for these uses in many jurisdictions.5 Finally iron-making has a very high energy demand and consequently makes significant contributions to greenhouse gas emissions. The secondary production of iron from scrap materials in electric arc furnaces (EAFs) does not involve the use of coal and has been claimed to offer significant environmental advantages compared to the blast furnace. However, the source of the energy used to produce electricity in the EAF has an impact on greenhouse gas emissions, and the presence of chlorine in the scrap feed to EAFs results in potential emissions of dioxins (Lee et al., 2005; Chang et al., 2006; Kao et al., 2007; Wang et al., 2009; Wang et al., 2010a; Wang et al., 2010b; Chiu et al., 2011; Lin et al., 2011).

2.5.2

Advanced coal processing technologies

Cleaner or advanced techniques for extracting the energy in coals have been developed. In the first instance, these were designed to produce inherently lower emissions of acid gases and particles through coal gasification to produce a fuel gas, and redesign of the gas cleaning system so that it was an inherent part of the overall process rather than a retrofit to existing pulverised fuel/combustion systems as much of the De-NOx and De-SOx control systems have been. Perhaps the most comprehensive assessment of the potential environmental impacts of coal gasification systems is that undertaken by the US DoE (NETL, 2002). This concludes that gasification has fundamental environmental advantages over direct coal combustion. Commercial-scale plants for both integrated gasification combined cycle (IGCC) electric power generation and chemicals applications have already successfully demonstrated these advantages. The superior environmental capabilities of coal gasification apply to all three areas of concern: air emissions, water discharges, and solid wastes. 5

See, for example, http://www.environment-agency.gov.uk/business/topics/waste/32154. aspx

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In many ways it is unfortunate that these advantages have not resulted in more widespread application of gasification and other advanced coal utilisation technologies, but high cost and uncertainties about greenhouse gas mitigation agreements have not been conducive to such developments.

2.6 2.6.1

Control of emissions Legislation and guidelines

Internationally there is significant concern over the adverse environmental impacts of air and water pollutants. In response, environmental legislation is enacted that aims to reduce the exposure of the general population to these harmful substances. For example, the level of what is deemed to be acceptable ambient air concentrations usually takes into account both exposurerelated health risks as well as what is realistically attainable in a given time frame. The latter is variable between countries, mainly depending on their level of economic development. Attaining these air quality goals usually requires other legislation and guidelines to reduce emissions at their source. The approach taken may be based on environmental quality, such as imposing emission limit values on individual emitters, or by controlling emission inventories through economic incentives or national ceilings. Alternatively, it may be technology-driven. In most places, a combination of one or more approaches is used. The variation in these approaches makes only a brief summary possible here, and concentrates as an example on emission limits for coal-fired power plants. An emission limit is simply the maximum level a particular pollutant can be emitted from a particular emission source or source type. It is usually expressed as a concentration. Regulated pollutants include primary particulate emissions as well as SO2 and NOx that can form secondary PM. Table 2.5 summarises emission limits placed on coal-fired power stations in several parts of the world. The table shows that limits are usually conditional on factors such as the age of the plant, with new plants having more stringent requirements; and the size of the plant, with lower limits for larger plants. Legislation for some countries, particularly China, includes variable emission limits based on factors such as plant size, coal type and location conditions. In Europe, more relaxed NOx standards (200 mg/m3) apply in older plants, but they are limited to 1500 hours of operation per year. Some of these details have been excluded from Table 2.5 to focus more on the limits most applicable to higher generation-capacity stations, operating most of the year, using high quality coal rather than small boilers using low quality coal. Under these constraints, emission limits are at the lower end of any variable range. Emission regulations for Chinese coal-fired power stations built from 1997 onwards are in-line with those from much more developed countries (Table 2.5). However,

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Table 2.5 Coal-fired power plant emission limits for several countries Date plant commenced/ licensed

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Australia/New South Wales (Protection of the Environment Operations (Clean Air) Regulation 2010) Europe (Directive 2001/80/EC)

(Directive 2010/75/EU, applies fully in 2020; linear decrease from ‘2001/80/EC’ limits between 2016 and 2020)

USA (New Source Performance Standards: Federal Register/Vol. 71, No. 38 /27 Feb 2006)

Emission limits (plant size) Solid particles

NOx

SO2

Before 1972 1972–Aug 1987 Aug 1987–Sep 2005 After Sep 2005 Before 27 Nov 2002

400 mg/m3 250 mg/m3 100 mg/m3 50 mg/m3 100 mg/m3 (<500 MWth) 50 mg/m3 (≥500 MWth)

2500 mg/m3

No limits

After 27 Nov 2002 Before 7 Jan 2013

30 mg/m3 25 mg/m3 (<300 MWth) 20 mg/m3 (≥300 MWth) 20 mg/m3 (<300 MWth) 10 mg/m3 (≥300 MWth) 0.05 g/kWh 0.023 g/kWh

After 7 Jan 2013

Before 28 Feb 2005 After 28 Feb 2005

800 mg/m3 500 mg/m3 600 mg/m3 (≤500 MWth) 500 mg/m3 (>500 MWth) (currently) 200 mg/m3 (>500 MWth) (from 1 Jan 2016) 200 mg/m3 200 mg/m3

200 mg/m3 (<300 MWth) 150 mg/m3 (≥300 MWth) 0.73 g/kWh 0.47 g/kWh

2000–400 mg/ m3 (sliding scale inversely proportional to plant size below 500 MWth and above 100 MWth) 200 mg/m3 250 mg/m3 (<300 MWth) 200 mg/m3 (≥300 MWth) 200 mg/m3 (<300 MWth) 150 mg/m3 (≥300 MWth) 0.93 g/kWh 0.65 g/kWh

South Korea (Sloss, 2009)

Before 1995

250 mg/m3

1995–1998

150 mg/m3 (<30 000 m3/h) 100 mg/m3 (>30000 m3/h) 50 mg/m3

after 1998

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China (Steinfeld et al., 2009)

Before 1997 1997–2003 After 2003

200 mg/m3 50a mg/m3

719 mg/m3 (350 ppmv)

1100 mg/m3 650 mg/m3 450 or 650b mg/m3

3429 mg/m3 (1200 ppmv) 1429 mg/m3 (500 ppmv)

772 mg/m3 (270 ppmv) (<500 MWe) 429 mg/m3 (150 ppmv) (>500 MWe) 1200 mg/m3 400a mg/m3 400a mg/m3

a Higher limits apply in mine-mouth stations in the western region, as well as for plants with FGD installed before regulation commenced, as well as for those using low-quality waste coal. b 450 for >20% volatile matter (dry ash free).

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in spite of this, China is currently estimated to contribute, and will continue to contribute, approximately one third of the world’s SO2 emissions, one quarter of the world’s PM emissions and one quarter of the world’s NOx emissions under most projected CO2-abatement scenarios (Cofala et al., 2010). It also has significantly higher problems with PM pollution than most other countries. Coal-fired power is a dominant source of China’s SO2 emissions, though large contributions are also made by other industrial sources such as iron and steel production. Transport is a small but growing source while household contributions continue to fall as electricity and gas replace the use of coal for heating within homes (Cao et al., 2009). The shift from coal to gas and electricity for household heating and cooking has also been very beneficial with regards to PM pollution since residential coal-fired boilers and stoves, unlike industrial sources, are largely unregulated. Winter concentrations of PM2.5 in Beijing declined by large amounts in the early 2000s (Wang et al., 2008) though in rural areas household sources are still a major source of PM and have a highly negative impact on indoor air quality (Florig et al., 2002). China’s rapid economic growth and the high proportion of coal-fired power in its energy mix are important factors in China’s problems with primary and secondary PM emissions. However, the limited abatement, particularly of SO2, is another very important factor (Cao et al., 2009; Steinfeld et al., 2009). One reason for low SO2 abatement is that emission limit exemptions apply to a large number of smaller power stations, depending on where they are and what type of coal they burn. This can mean SO2 emission limits of anywhere up to 1200 mg/m3, even for new plants (Steinfeld et al., 2009). Another cause is the low level of compliance in meeting emission limits. There are several reasons for this. The first is that SO2 scrubbers are much less effective when damaged by coal with higher sulfur than design specifications allow (Xu, 2011), and power producers, both knowingly and unknowingly, run a wide variety of coal types through their boilers (Steinfeld et al., 2009). A second reason is that fines for noncompliance are not high enough to deter offenders. Many power stations have found it more profitable to pay the fines than to install costly abatement technologies. Non-compliance is also made more difficult to detect because of activities such as tampering with continuous emission measurement devices, turning on SO2 scrubbers only for inspections, and bribing regulatory officials (Xu, 2011). Table 2.5 shows the trend towards lower emission limits over time. The most common development is to apply stricter regulations to successive generations of new plants. However, some regulations, such as NOx from 2016 in European Directive 2001/80/EC, apply to existing plants, requiring those plants to retrofit required control technologies. The future Directive 2010/75/EU, which comes into force from 2016, will apply relatively strict

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Table 2.6 Primary measures for pollutant reduction and emission control Combustion Capacity modifications derating

Air and fuel modifications

Solid fuels PM control

Lower volume flow and higher oxygen surplus reduces temperature and slagging

Solid fuels SO2 control

Reduced temperature reduces sulfur volatilisation

Pre-drying, Liquid ash gasification, removal, pyrolysis of cyclone fuel, burner in fuel additives, slag tap i.e. low furnace melting additives for slag tap furnace with liquid ash removal (tested for pressurised coal combustion for gas turbines) Use of low Burner with sulfur fuel separate and sorbent additive fuel injection additives, i.e. lime and limestone for fluidised bed combustion Mixing and finer Low NOX burners grinding of fuel, fluegas recycling reduces NOX production

Reduced Solid fuels temperature NOX Control (reduction of NOX production)

Burner In-furnace modifications combustion modifications Liquid ash removal, slag tap furnace; circulating fluidised bed combustion, coarse ash control

Over fire air injection of absorbents, i.e. limestone

Staged combustion and reburning

Source: Eurelectric, 2001.

limits for existing plants (that is, those built before 2013) including 20 mg/ m3 PM concentrations for large (>300 MW) power stations.

2.6.2

Process impacts on emissions

In some cases plant design and process operations may be used to control emissions of the major pollutants produced in coal utilisation. Table 2.6 summarises these so-called primary measures for PM, SO2, and NOx control in large combustion plants.The measures include combustion and burner/furnace

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modifications. In many cases they rely on careful control of temperatures to, for example, reduce thermal NOx production, and are based on detailed scientific understanding of trace species chemistry at high temperatures.

2.6.3

Sulfur oxides

Reduction and/or control of SO2 emissions can be achieved by the use of lower sulfur fuels, by coal cleaning to remove sulfur or by installation of post-processing gas cleaning systems. The move towards low sulfur coals can be encouraged by the use of economic instruments such as cap and trade schemes, as successfully implemented in the US Acid Rain Program (USEPA, 2011). It is possible to use coal cleaning to reduce the sulfur content of coal to the levels needed to comply with most environmental regulations provided most of the sulfur is present in inorganic forms. However, coal cleaning adds significant additional costs and may result in the loss of some combustible material. A number of gas cleaning systems and techniques are used to reduce SO2 emissions from coal processing: wet scrubbers, spray dry scrubber and sorbent injection. The choice of SO2 removal technology depends on the required emission reductions, plant size and operating conditions, sulfur content in the coal and the capital and operating costs of the various technology options. Wet scrubbers are the most commonly used flue gas desulfurisation (FGD) technology due to their high SO2 removal efficiency, reliability and competitive cost. Wet scrubbers based on limestone are the most common type. Lime can also be used as a scrubbing reagent but it is generally more expensive. The overall chemical reactions relevant to limestone scrubbing of SO2 are: SO2 + H2O → H2SO3 CaCO3 + H2SO3 → CaSO3 + CO2 + H2O CaSO3 + ½O2 + H2O → CaSO4.H2O A major advantage of limestone scrubbing is that gypsum (CaSO4.H2O) is produced in the process, and this can be sold for wall board or gyprock manufacture. The sorbent used in spray dry scrubbers is typically lime or calcium oxide. Unlike a wet FGD, a lime spray dryer is located before the PM control device. Atomised or aqueous lime slurry is sprayed into an absorption tower, SO2 is absorbed by the droplet and reacts with lime. The resulting dried and therefore solid particles are entrained in the flue gas, along with fly ash, and are collected in a PM control device.

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Lime spray dryers are sometimes used as an alternative to wet scrubbing since they provide simpler waste disposal and can be installed with lower capital costs, particularly in smaller plants. Sulfur dioxide emissions can also be reduced by adding a dry alkaline powder into the combustion zone or the flue gas duct. Sulfur dioxide adsorbs on the surface of the alkaline particles and reacts to form solid sulfite and sulfate which can be collected in PM control devices. Sorbents include limestone (CaCO3), hydrated lime and dolomite (CaCO3. MgCO3).

2.6.4

Nitrogen oxides

Modifications to the combustion process and flue gas cleaning techniques based on ammonia injection into the flue have both been used commercially to control NOx emissions. The most common combustion modification is air staging, often in the form of so-called low NOx burners. By far the most common flue gas cleaning technique is selective catalytic reduction (SCR) with ammonia. Application of the most appropriate of these technologies is dependent on the degree of control to be achieved, and the cost. SCR can achieve the greatest reductions (it is required, for example, for attainment of some stringent control limits) but the costs of the catalyst are high. Coal quality will have some influence on this technology, particularly sulfur content and the contents of trace elements which may act as poisons for the catalyst (alkali ions, particularly in sulfated form, arsenic and selenium). Air staging can be used to reduce NOx emissions substantially, and in some locations will be the preferred technique. Extensive experimental and computational studies have established the conditions and reactions responsible for NOx reductions under air staged conditions. In order to appreciate its implications it is necessary to first review briefly the current understanding of coal nitrogen conversion to NOx, and of the mechanisms of NOx reduction in air-staged combustors. The success of air staging in reducing NOx relies on rapid release and conversion of volatile nitrogen to HCN and NH3 which burn under fuel rich conditions; encouraging formation of N2, and hence minimising NOx formation and char burnout on addition of second stage oxygen (over fire air (OFA)). Clearly the timescales for volatile nitrogen release and for conversion of tar nitrogen to HCN and NH3 play an important role in determining the optimum conditions (e.g., size or duration of the fuel rich stage) for air staging. A wealth of experimental evidence shows that the N released with the volatiles is very efficiently converted to N2 if the combustion of volatiles is carried out under fuel rich conditions. It follows that a simple correlation between the coal N content and NO formation is not to be expected. Rather

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Table 2.7 Comparison of particulate matter air pollution control devices

Technology

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Electrostatic Precipitator (ESP)

Removal efficiency %

Other performance parameters

<1 μm

2 μm

5 μm

>10 μm

Parameter

Value

>96.5

>98.3

>99.95

>99.95

Operating temperature

80–220°C (cold ESP) 300–450°C (hot eSP) 0.1–1.8%

Energy consumption (% of electric capacity) Pressure drop Fabric filter

Remarks

>99.6

>99.6

>99.9

>99.95

Operating temperature Energy consumption (% of electric capacity)

Pressure drop

0.15–0.3 kPa

Can handle very large gas volumes with low pressure drops Low operating costs except at very high removal rates May not work very efficiently with ash having very high electrical resistivity

150°C (polyester) Bag life decreases as coal 260°C sulfur content increases (fibreglass) and as the filtering velocity increases 0.2–3% Individual bags fail at an average annual rate of about 1% of installed bags The pressure drop increases 0.5–2 kPa as the particle size decreases for a given flue gas throughput

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the behaviour of the N retained in the char and burnt out in the second stage with the over fire air becomes of crucial importance. Experimental studies (Jones et al., 1995) support this overall framework, and provide a way in which coal quality considerations may be used to discriminate between coals in terms of their NOx formation under staged conditions of low NOx formation. Coals from the US, UK, Colombia, South Africa, and Australia were included in this study. The coals ranged in N content from 1.62% to 2.01% and in volatile matter (VM) content from 29.0% to 40.1%. As expected, emissions did not correlate simply with nitrogen content or VM, since under staged conditions what is important is the conversion of the N which remains with the char. Using a function based on both coal volatile matter release at the high heating rates (VMHHR) which apply in real furnaces and on nitrogen content, a good correlation is observed. These results suggest that a future evolving coal quality indicator for NOx could be based on both the coal N content and on the volatiles release. The differences between coals are rather small but in the context of a control strategy for NOx these differences, and the flexibility in coal selection they imply, may become crucial.

2.6.5

Particulate matter

In modern coal-based processors, particulate emissions are usually controlled by the use of electrostatic precipitators (ESP) or fabric filters (FF). The selection of PM control technology depends on coal type, plant size, boiler type and configuration, and the level of control required (i.e., efficiency). Both FF and ESP technologies are highly efficient and capable of removing particulates to a level well below the emission limits, although FFs are more efficient in removing fine particles in ultra fine particle range (less than 1 μm). A comparison of the performance and operating characteristics of FFs and ESPs is given in Table 2.7.

2.7

Future trends

Technological and political initiatives to mitigate or adapt to greenhouse gas emissions are likely to have a significant impact on energy intensive industries, and it is also likely that pressures to reduce emissions of carbon dioxide and pollutant gases and particles will only intensify. An interesting source for potential future developments in the energy sector is provided by the International Energy Agency World Energy Outlooks (IEA, 2010, 2011).

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Coal

Total non-coal

Nuclear

Renewables

Oil

Gas 0

200

400

600

800

1000

1200

1400

1600

Incremental energy demand (mtoe)

2.5 Incremental world primary energy demand by fuel, 2000–2010. (Source: From IEA, 2011.)

These outlooks depend on critical assumptions about political decisions and energy subsidies, projections of world economic activity and energy demand; so they are subject to significant uncertainty in any quantitative sense. Nonetheless, the thrust of the conclusions in recent Outlooks (IEA, 2010, 2011) is compelling, and the implications for coal use in industry significant. The 2010 Outlook (IEA, 2010) observes that ‘emerging economies, led by China and India, will drive global demand higher’, but that global demand for each fuel source will increase, and that fossil fuels will continue to account for over one-half of the increase in total primary energy demand to 2035. Non-OECD countries are likely to account for more than 90% of the projected increase (IEA, 2010), and world electricity demand is expected to grow more strongly than any other final form of energy. Electricity generation is entering a period of transformation as investment shifts to low carbon technologies – the result of higher fossil fuel prices and government policies to enhance energy security and to curb emissions of CO2. In the New Policies Scenario, fossil fuels – mainly coal and natural gas – remain dominant, but their share of total generation drops from 68% in 2008 to 55% in 2035, as nuclear and renewable sources expand. The shift to low carbon technologies is particularly marked in the OECD. Globally, coal remains the leading source of electricity generation in 2035, although its share of electricity generation declines from 41% now to 32% (IEA, 2010). In fact, there are already indications that coal will remain a key player in meeting energy needs into the future. Coal has met almost half of the increase in global energy demand over the last decade (IEA, 2011), as

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shown in Fig. 2.5. Maintaining current policies would result in an additional 65% increase in coal use by 2035, but there is serious uncertainty about such projections. China’s consumption is a critical determining factor, and current indications from its planning processes are that it will seek to reduce the energy and carbon intensity of its economy with major impacts on coal markets. Future investments in coal generating infrastructure are increasingly likely to be concentrated in more efficient coal-fired power plants and carbon capture and storage (CCS) technology (IEA, 2011), but there are still significant barriers to overcome particularly for CCS. The Outlook concludes (IEA, 2011) that ‘if CCS is not widely deployed by the 2020s, an extraordinary burden would rest on other low-carbon technologies to deliver lower emissions in line with global climate projections’. The implications for the traditional environmental issues associated with coal use in industry (air pollutants, acid deposition, ash management) of these changes in primary energy supply will be profound. In OECD and developed economies ever-more stringent controls on emissions of gas and particulate pollutants is already evident. The new EU Directive 2010/75/ EU on Industrial Emissions (EU, 2010) will impose significantly more stringent standards for emissions of these traditional pollutants and will reportedly result in plant closures without costly retrofitting of more effective air pollutant control devices. Development of a legally-binding instrument for mercury control and management is also likely to result in specific legislation for emissions from coal-fired plant, as coal use is the largest anthropogenic source of this pollutant. Indeed, the US and Canada have already developed specific rules for mercury emissions. It is worth emphasising that new technologies for coal use in electricity production aimed at CCS have inherently lower emissions of traditional gas and particulate emissions, so that efforts to reduce greenhouse gas emissions are likely to have co-benefits for traditional emission concerns. There are also clear indications of a switch from coal to gas in OECD countries for new build electricity plant (IEA, 2010, 2011).

2.8

Sources of further information and advice

The references provide significant details, and the following websites are good sources of additional information: 1. IEA Clean Coal Centre Coal on Line Website: http://www.coalonline. info/site/coalonline/content/home 2. European Union, Integrated Pollution Prevention and Control, Reference Document on Best Available Techniques for Large Combustion Plants, July 2006; ftp://ftp.jrc.es/pub/eippcb/doc/lcp_bref_0706.pdf

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3. USEPA Advanced Technologies for the Control of Sulfur Dioxide Emissions from Coal-Fired Boilers, Topical Report Number 12, June 1999; http://www.netl.doe.gov/technologies/coalpower/cctc/topicalreports/pdfs/topical12.pdf 4. American Coal Foundation, at: http://www.teachcoal.org/aboutcoal/articles/coalenv.html 5. Co-operative Research Centre for Coal in Sustainable Development Power Station Handbook, www.ccsd.biz 6. World Coal Organisation, http://www.worldcoal.org/coal-the-environment/

2.9

References

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