The burning of coal for electricity generation not only results in hazard- ous emissions but also produces residues called coal combustion products (CCPs). Traditionally, the CCPs have been dumped in large piles and/or ash-ponds mostly around the power stations. The CCPs are generally ash materials, made of fine particles and they are also generated as coarse parti- cles. The generation of these products poses serious threats to air, water, and soil, and consequently to living organisms. The extent of the environmental effects caused by CCPs depends on
1. the coal source,
2. the combustion technology used and
3. the collection and segregation of the residues.
The two main residues that emerge out of coal combustion in power stations are fly ash (FA) and bottom ash (BA). In the past four decades, the power stations have been targeting on technologies that minimizes the effects of coal combustion in terms of limiting atmospheric pollution and reducing residues generated (Beer, 2000). The anticipation of restriction on the CO2 emissions has forced the coal-dependent power stations in Organi- zation for Economic Co-operation and Development (OECD) countries to improve the thermodynamic efficiency of the combustion technology used (Beer, 2000). Since carbon is the main coal constituent, the emissions related to carbon were highly significant and accentuated by most of the environ- mental agencies globally, for their contribution to greenhouse gas (GHG)
Table 6.1 Environmental Impact of Coal Production and Consumption Environmental
Impacts Coal Production Coal Consumption
Air Dust and noise Greenhouse gases
Land Soil erosion and acid mine
drainage Accumulation of hazardous
residues (ash materials) Water Acid mine drainage and move-
ment of nutrients and heavy metals to ground water.
Leaching and surface runoff of heavy metals from ash-ponds.
Biological
health Impairs local biodiversity Public health and safety concerns, especially respiratory issues.
emissions (Table 6.2). Hence, there have been a number of technologies aimed at reducing carbon-related atmospheric emission, such as postcom- bustion capture, oxyfuel combustion, and precombustion capture. The first two technologies are carried out by amine scrubbing, which separates CO2
and the precombustion capture that removes 85% of CO2, where hydrogen gas alone serves as a combustion fuel after the process (Gupta et al., 2003;
IEA, 2002). Apart from the discharge of carbon-associated release of pol- lutants, the coal-fired power industries also emit other harmful gases such as sulfur oxides (SOx) and nitrogen oxides (NOx). All these technologies come under clean coal technologies (CCTs) and are mainly practiced to mitigate air pollution (Beer, 2007), although there are changes in the nature of the CCPs produced as a result of the adaptation of CCTs (Table 6.3).
For example, the CCPs emerging from some technologies pertaining to NOx reductions contain large ammonia content (Butalia and Wolfe, 2001);
similarly technologies designed to remove SOx during coal combustion can lead to S-rich byproducts (Wang et al., 2006). These byproducts can be used as a major source of plant nutrients (Wang et al., 1995, 2006).
In the global perspective, CCTs reduce emission of several pollutants, decrease waste generation and increase the amount of energy gained per unit amount of coal combustion. These technologies include various chem- ical and physical treatments applied pre- or postcombustion (Table 6.3).
They may be broadly grouped into processes relating either to combus- tion efficiency, pollution control (especially, control and reduction of NOx, SO2, mercury and particulate matter) or carbon sequestration (Beer, 2007;
Franco and Diaz, 2009; Longwell et al., 1995). There are two major cat- egories in NOx reduction technologies: (1) combustion modifications and
Table 6.2 Emission of Greenhouse Gases (GHGs) based on Various Sectors
Sector Mt CO2e % Emission
Electricity and heat 10,318.6 26.9
Manufacturing and construction 4467.5 11.6
Transportation 4836.0 12.6
Other fossil fuel combustion 3527.4 9.2
Fugitive emissions 1579.0 4.1
Industrial processes 1369.4 3.6
Agriculture 5729.3 14.9
Land use change and forestry 5165.9 13.5
Waste 1360.5 3.5
Total 38,353.6
World Resources Institute (WRI), 2011
oalTechnologyCombustionProducts315 Low-NOx burners
(LNB)
Provides internal staged com- bustion, thus reducing peak flame temperatures and oxygen availability
• Low operating cost
• Compatible with other technolo- gies
• Moderate NOx removal (30–50%)
Ammonia- and carbon-rich FA
Melick et al., 2005
Overfire air (OFA) Staged combustion, creating
fuel-rich and fuel-lean zones • Low operating cost
• No capital equipment required • Moderate NOx removal
(30–60%)
Ammonia-rich
FA Beer, 2000
Selective catalytic
reduction (SCR) Catalyst located in flue gas stream promotes reaction of NH3 with NOx
• High NOx removal (70–90%) Ammonia-rich
FA Franco and
Diaz, 2009 Selective noncatalytic
reduction (SNCR)—
urea injection
Injection of urea into furnace to react with NOx to form nitrogen and water
• Low capital coat • Relatively simple system • Moderate NOx removal:
Nontoxic chemical (25–50%)
Ammoniarich
FA Franco and
Diaz, 2009
Selective non catalytic reduction (SNCR)—
ammonia injection
Injection of ammonia into furnace to react with NOx to form nitrogen and water
• Low operating cost • Moderate NOx removal • (25–50%)
Ammonia-rich
FA Franco and
Diaz, 2009 Fluidized bed combus-
tion (FBC)
Injection of air into a bed of inert ash and crushed lime- stone
• Low cost for scrubber installation FBC ash Wang et al., 2006 Flue gas desulphuriza-
tion (FGD) A wet scrubbing process which uses limestone slurry as an absorbent
• High SOx removal (90–95%) FGD gypsum Srivastava, 2000
(2) postcombustion processes. The most common combustion modification processes are as follows:
1. Low-NOx burners (LNBs) are designed to burn in a lower maxi- mum flame temperature and a reduced oxygen concentration by con- trolled mixing of fuel and air, resulting in staged combustion (Magel et al., 1996), ensuring lower thermal and fuel NOx production. The main disadvantage of the LNBs is generation of high carbon combus- tion wastes (ash) as a result of the lower flame temperature employed during combustion, which makes the marketing of the ash undesirable (Melick et al., 2005).
2. Overfire air (OFA) technology involves the injection of air into the furnace above the normal combustion zone and this approach is gener- ally used in conjunction with operating the burners at a lower than nor- mal air-to-fuel ratio, which reduces NOx formation. The OFA process is frequently used in association with LNBs. The use of oxygen in the OFA reduces loss of ignition (LOI), thereby ensuring low carbon ash (Bool and Kobayashi, 2003).
3. Reburning process reduces NOx by injecting 10–25% of the coal in a separate reburn zone, where the fuel-rich conditions lead to the reduction of NOx formed in the normal combustion zone. The OFA technology is also employed above the reburn zone to complete the combustion. Hence, there are three zones in the furnace for this reburn- ing technology: (1) a combustion zone with an approximately normal air- to-fuel ratio; (2) a reburn zone, where added fuel results in a fuel-rich condition; and (3) a burnout zone, where OFA leads to completion of combustion (Moyeda, 2004).
4. Flue gas recirculation (FGR) is used to modify the conditions in the combustion zone (lowering the temperature and reducing the oxygen con- centration) which consequently decreases the formation of NOx. In FGR, the flue gas is recirculated to the furnace in the combustion zone and also used as a carrier to inject fuel into a reburn zone to increase penetration and mixing. Baltasar et al. (1997) observed marked decrease of NOx emissions in FGR technology without significant effects on the flame stability.
The postcombustion processes include addition of reactive substances or catalysts to convert the already formed NOx into molecular nitrogen or nitrates and the most prominent treatment processes are the following:
1. Selective catalytic reduction (SCR) reduces NOx in the presence of a catalyst and an injection reagent (vaporized ammonia—NH3).
The coal is burned along with the catalyst and NH3 in a catalyst vessel
installed downstream of the furnace at 570–750 °F, to produce nitrogen and water vapor. The catalyst plays an important role in chemisorption of NH3 onto its active sites. In this process, the N is released as an inert dinitrogen gas and the reactions involved in SCR process are as follows (US EPA, 1997):
4NO+4NH3+O2→4N2+6H2O (1) 2NO2+4NH3+O2→3N2+6H2O (2) 2. Selective noncatalytic reduction (SNCR) uses a reducing agent
(typically NH3 or urea), which is injected into the furnace above the combustion zone, where it reacts with NOx similar to SCR. The even distribution of the reducing agent in the furnace and sufficient residence time in the appropriate temperature range are the critical factors for SNCR technology (Wu, 2002).
Both SCR and SNCR processes can be used in conjunction with each other as a hybrid process and also with LNBs for synergistic benefits. The disadvantage of these methods is the production of NH3-rich ash, due to high levels of NH3 slip as part of the operational impacts, resulting in low commercial value of the ash (Wu, 2002).
Sulfur dioxide (SO2) is another major pollutant emitted in large quanti- ties from the burning of coal with high sulfur content. More than 95% of sulfur is converted to SO2 from coal combustion (Franco and Diaz, 2009).
Along with NOx, SO2 is capable of producing acid rain. Widespread occur- rence of acid precipitation and dry deposition results in industrial emis- sions of SOx and NOx (Longhurst, 1991). These gases are transformed in the atmosphere as sulfuric and nitric acids (Table 6.4); transported over long distances and deposited on vegetation, soils, surface water, and build- ing materials. While the majority of NOx emissions are local/natural origin, SOx emissions are often transboundary in nature (Bolan et al., 2007). The average annual ratio of sulfuric acid to nitric acid is about 2:1 in North America but nitric acid is becoming progressively more important because of the installation of flue gas desulphurization (FGD) systems in coal-fired power stations (Dick et al., 2000).
Similar to NOx removal, SO2 can also be removed using both combus- tion modifications and postcombustion technologies. The most prominent combustion modification process for SO2 removal is the fluidized bed com- bustion (FBC), where the coal is burnt in a bed of inert ash and crushed limestone. The bed is held in suspension by injecting air through a perforated
floor. The limestone reacts with the released SO2 to form calcium sulfate (Eqns (8) and (9)). The use of lower (between 1449 and 1598 °F) FBC tech- nology compared to the conventional coal-fired furnaces (2552–2912 °F) results in optimum S capture, thereby minimizing the usage of limestone and also reducing NOx formation (Terman et al., 1978). Therefore, these boilers possess potential in coal-fired power plants for meeting air qual- ity standards without the usage of highly expensive SO2 scrubbers. The conversion of limestone into calcium sulfate is described by the following equations (Wang et al., 2006):
CaCO3→CaO+CO2 (7) CaO+SO2→CaSO3 (8) 2CaSO3+O2→2CaSO4 (9) The FBC ash is formed as a mixture of conventional coal combustion ash (either bed or fly ash), the SO2 reaction product (primarily anhydrite, CaSO4) and unspent sorbent (Stehouwer et al., 1999; Wang et al., 2006).
Hence, the FBC ash is highly alkaline and has the potential to be used as liming material and S fertilizer.
In United States, the SO2 reduction is largely carried out using the postcombustion technology, namely, FGD. The advantage of this system is that it can be installed to any existing conventional coal-fired power station and has the capability of removing up to 95% or more SO2 in
Table 6.4 Transformation of Sulphur Dioxide and Nitric Oxide in Atmosphere Process Transformation Equation H+ (molc mol−1) Oxidation of
sulfur dioxide 2SO2+O2→SO3 (3) 0
Hydrolysis of
sulfur trioxide SO3+H2O→H2SO4→SO4+2H+ (4) +2 Photochemical
oxidation of nitric oxide
O3+NO→N2O+O2 (5) 0 Hydrolysis of
nitrogen dioxide
2NO2+H2O→HNO3+HNO2
→NO3+H+ (6)
+1
flue gases (Franco and Diaz, 2009; Srivastava, 2000). The reagents that are used as sorbents to scrub flue gases include limestone (CaCO3), lime (CaO) or caustic soda (NaOH). Generally, FGD processes are classified as wet or dry scrubbers. A wet FGD process is the commonly used, which produces slurry waste, which is a saleable byproduct. However, the dry FGD technology produces solid waste which is easily disposable compared to the former process. The value-added CCPs emerging from FGD process are termed as FGD gypsum (Shahandeh and Sumner, 1991;
Sumner, 2007).
Although the development of combustion systems in electricity genera- tion had been discussed widely since the peak industrial revolution era (mid 1900s), the CCTs have started emerging only during the 1970s which is called as Environmental Era (Beer, 2000). After the initial attention on satis- fying the demands of CO2 mitigation, the focus shifted toward reducing the emission of nitrogen (NOx) and sulfur (SOx) oxides, which also contribute to acid rain and climate change. The CCPs generated from NOx technolo- gies are commercially undesirable because of NH3- and carbon-rich ashes.
However, they can be used as plastic fillers after chemical processing, which can act as an alternative for the usage of chemical fillers such as calcium carbonate or aluminosilicate (Huang et al., 2003). Hence, the products from SO2 reduction processes such as FGD and FBC will be broadly discussed in this review.