ISSN 1068-364X, Coke and Chemistry, 2017, Vol. 60, No. 4, pp. 171–176. © Allerton Press, Inc., 2017. Original Russian Text © G.S. Nyashina, N.E. Shlegel, P.A. Strizhak, 2017, published in Koks i Khimiya, 2017, No. 4, pp. 40–46.
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Emissions in the Combustion of Coal and Coal-Processing Wastes G. S. Nyashina*, N. E. Shlegel**, and P. A. Strizhak*** Tomsk Polytechnic University, Tomsk, 634050 Russia *e-mail: gsn1@tpu .ru **e-mail:
[email protected] ***e-mail:
[email protected] Received January 19, 2017
Abstract⎯Power stations based on traditional hydrocarbon fuels—in particular, coal—are characterized by considerable atmospheric emissions. In the light of the annual increase in coal consumption and the ongoing environmental deterioration, significant decrease in atmospheric emissions per unit mass of coal consumed must be regarded as a high priority. In the present work, SO2, CO2, CO, NO, and NO2 emissions from power plants around the world are analyzed. The emissions formed in the combustion of traditional types of Russian coal and their processing wastes (filter cakes) are compared. The benefits of using coal–water slurry containing coal-processing wastes at power plants are outlined. Keywords: coal, filter cake, coal–water slurry, combustion, atmospheric emissions, energy sources DOI: 10.3103/S1068364X17040056
Power plants today are responsible for accelerating environmental pollution. Plants based on traditional fuels (coal, peat, fuel oil, gas) are of particular concern [1–3]. Up to 40–45% of all power generated worldwide derives from such sources. Five main pollutants account for 90–95% of the gross emissions from such plants [4]: ash particles, sulfur oxides (SOx), nitrogen oxides (NOx), carbon oxides (COx), and water vapor (H2O). Each of these has a negative environmental impact [5–8]. In particular, ash particles in the smokestack gases may contain carbon, silicon dioxide, aluminum and iron oxides, sulfur, some organic compounds, heavy metals, and other chemicals [5]. Oxides of sulfur and nitrogen are oxidized by atmospheric moisture to produce weak solutions of sulfuric and nitric acids, which are associated with acid rain [6, 7]. Increase in the content of nitrogen oxides damages the atmospheric ozone layer, which protects the Earth from UV radiation. Globally, power plants emit carbon dioxide (CO2) and water vapor (H2O), which intensify the greenhouse effect (increasing temperatures in the lower atmosphere) and give rise to more violent weather phenomena [8]. In the light of environmental problems, global warming, and the constant increase in the total power of power plants and energy consumption, the limitation of environmental pollutant emissions is of increasing concern. In many countries (especially China, India, the United States, and Russia), plans are underway for the use of power-plant wastes as raw
materials. The environmental impact of such plans must be assessed. In the present work, we analyze the emissions from the global power industry due to the combustion of traditional fuels and coal-processing wastes and seek means of reducing those emissions. GLOBAL FUEL CONSUMPTION AND POLLUTANT EMISSIONS Efforts to mitigate human impacts on climate have been most vigorous in the past 30–50 years [9–12]. A focus has been the reduction of atmospheric pollutant emissions from the power, chemical, and petrochemical industries. This impulse is reflected in the Kyoto Protocol (1997), an international document calling for reduction in greenhouse-gas emissions between 2008 and 2012 [14]. International documents between 2012 and 2016 similarly constrain global industrial activity. As we see in Fig. 1, the leading emitters of greenhouse gases are China, the United States, and India [13, 14]. In particular, coal-fuel power plants are the main power sources in China. In 2014, China accounted for 50.6% of world coal consumption [16]. Along with the growing coal consumption (Fig. 2), the emissions of oxides of sulfur, nitrogen, and carbon and solid particulates are rising, as well as emissions of harmful elements such as mercury (Hg), antimony (As), and selenium (Se) [3]. For example, according to data from the National Oceanic and Atmospheric Administration (United States), the carbon -dioxide concentration is increasing significantly each year
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Fig. 1. Global greenhouse-gas emissions (2014) [14].
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Global greenhouse-gas emissions: >37.92 Gt CO2 25.4
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Fig. 2. Coal consumption for power production in absolute terms and relative to the total coal consumption [3].
(Fig. 3) [14]. The sources of CO2 emissions are as follows: power generation 38%; transportation 31%; industry 14%; residential and commercial sectors 10%; biomass combustion 7%. Another major coal consumer is India, which occupies fifth place globally in terms of power generation (210 GW) [1]. In India, as in China, coal is the primary fuel (55% of power generation). With coal consumption of 503 million t, the corresponding emissions are as follows: 580 kt ash particles; 2100 kt sulfur oxides (SO2); 2000 kt nitrogen oxides (NO), and 665 million t carbon oxides (CO2) [1]. The NO and NO2 emissions of coal-fired power plants in China using desulfurization of the smokestack gases and selective catalytic reduction were compared in [15].This comparison, taking account of the type of coal burned (bituminous coal, anthracite, lig-
nite) and the power of the plants (<600 MW, 600– 800 MW, >800 MW), shows that the range of emissions is 11.1–479 ppm (13.87–598.75 mg/m3) for NO and 0.19–4.26 ppm (0.36–8.14 mg/m3) for NO2. That is considerably less than for traditional coal combustion at power plants. The conclusions were that, with increase in power of the plant, the concentration of nitrogen oxides (NOx) in the emissions decreases and that the NOx emissions are greater when burning anthracite than when burning bituminous coal [15]. Copious emissions not only pollute the environment but impair human health. In India, for the period 2010–2011, heavy ash particles resulted in (8.0–11.5) × 105 premature deaths and 20 million cases of asthma, according to the estimates in [1]. In many countries, power plants are improving their pollution-control measures (for example, by means of catalytic selective reduction) [15, 16]; and developing so-called clean-coal technologies based on efficient coal combustion and state-of-the-art power generation [3]. For example, 63–82% reduction in greenhouse-gas emissions has been possible by carbon capture and storage [17, 18] and carbon capture and utilization [10, 13] (Fig. 4). Emissions are considerably reduced by the combustion of coal dust in oxygen [24, 25] and by integrated gasification in a closed cycle. The reduction in emissions is greatest in the combustion of captured CO2 from the smokestack gases in a gas-turbine cycle [18]. A promising method of reducing atmospheric gas emissions, without any additional expenditures, is to switch from the direct use of coal to the preparation of water–coal suspensions that are subsequently burned in furnaces. The transition to such coal–water slurries improves power-plant economics by permitting the COKE AND CHEMISTRY
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Potential global warming, CO2/(kW h)
use of low-grade fuel or processing wastes. The complete combustion of coal in the slurries significantly reduces the waste of fuel resources and minimizes atmospheric emissions. USING COAL–WATER SLURRIES AT THERMOELECTRIC PLANTS The replacement of traditional coal by coal–water slurry is under active development in China, Japan, Sweden, Germany, the United States, and elsewhere [26, 27]. In China, in particular, three research centers are working on that topic. Seven enterprises produce coal–water slurry, and power plants produce up to 2 million kW of energy on the basis of that fuel [26, 27]. By 2020, the production of coal–water slurry in China is expected to reach a record: 100 million t/yr [26, 27]. For lack of experimental data on the combustion of coal–water slurry and its environmental impact, this fuel is not widely used as a fuel in Russia. It is of interest to compare the environmental impact of coal and coal–water slurry. The ignition and subsequent combustion of a droplet of composite fuels was studied in [28–30]. The basic principles of stable coal–water slurries were outlined in [28–30]. To assess the volume of gaseous emissions from the combustion of coal–water slurries, we modify the apparatus used in [30]. Specifically, we add a Testo-340 gas analyzer with four sensors: for O2 (0–21%, error 0.1%); CO2 (0–20%, error 0.1%); CO (0–10000 ppm, error 100 ppm); SO2 (0–2000 ppm, error 10 ppm); and NOx (0–2000 ppm, error 10 ppm). A built-in modular probe takes samples of the smokestack gases in fuel combustion. The temperature of the gases in the combustion region is 500–1000°C. This range is selected because considerable emission of sulfur oxides (SOx) and nitrogen oxides (NOx) is observed above 500°C. The mass of composite fuel used in each experiment is 1 g. The main fuel components are ground K, G, and SS coal and filter cake of similar rank. The Table 1 summarizes the characteristics of the fuel components.
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[19] [20] [21] [22] [23]
600 500 400 300 200 100 0
Traditional process
Carbon capture
Combustion in oxygen
Fig. 4. Potential global warming associated with the gasturbine cycle, for various technologies [19–23].
In Fig. 5, we show the experimental concentrations in the emissions on combustion of the composite fuels. (The maximum concentrations are given.) It follows from Fig. 5a that the CO2 emissions depend significantly on the temperature (regardless of the fuel considered). This is because the yield of gases increases with rise in the temperature. Thus, volatiles are liberated before the coke residue ignites and burn in the gas phase, heating the coke particles. The diffusing oxygen is consumed in the combustion of volatiles. The reaction of carbon and oxygen is determined by the reactions [31]
C coke + O 2 → CO 2; C coke + 1 2O 2 → CO. As we see in Fig. 5a, the maximum CO2 concentration is less in the combustion of the filter cake than in coal combustion. Given the cost of the coal and the filter cake, the use of filter cake is both economical and environmentally important. As we see in Fig. 5b, the CO concentration is lower with high temperatures in the combustion chamber. At high temperatures, the combustion of the coal particles is more complete. For filter cakes of different
Table 1. Characteristics of coal and filter-cake samples Technical analysis Sample K filter cake K coal G filter cake G coal SS filter cake SS coal
Elementary composition (daf), % a
Ad, %
V daf, %
Q s,V , MJ/kg
C
N
S
26.46 14.65 33.82 17.8 50.89 21.68
23.08 27.03 43.11 41.36 30.16 27.40
24.83 29.76 22.16 27.82 15.23 26.23
87.20 79.79 75.12 79.31 87.47 77.30
2.05 1.84 0.02 0.01 2.15 1.93
1.022 0.868 0.226 0.414 0.444 0.326
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Fig. 5. Temperature dependence of the maximum concentrations of CO2 (a), CO (b), SO2 (c), and NOx (d) emissions from SS (1), K (2), and G (3) coal and SS (4), K (5), and G (6) filter cake.
rank, the CO concentration falls within a relatively narrow range (250–400 ppm). For coal, the range is larger (Fig. 5b). In general, the quantity of CO emitted in coal combustion depends on the ash content [34]. In combustion, the ash envelopes the combustible components of the fuel and denies access to atmospheric oxygen. As a result, combustion is incomplete. The CO concentration is highest for SS coal, with 21.68% ash, as we see in the table: 150–600 ppm. For K coal, with 14.65% ash, the CO concentration is 50– 200 ppm. The use of coal–water slurries significantly lowers the emissions of SO2 (Fig. 5c) and NOx (Fig. 5d). The sulfur content in the coal greatly affects the SO2 emissions in the combustion of coal and filter cakes. The SO2 emissions are greatest for K coal and filter cake
and least for G coal and filter cake. This correlates with the sulfur content, as we see in the table. Likewise, analysis of the experimental data in Fig. 5d shows that the NOx emissions are proportional to the nitrogen content in the fuel, as we see in the table. The decrease in the NOx and SO2 emissions from the coal– water slurries is primarily due to the presence of water. The presence of water reduces the rate of adiabatic combustion, which depresses the formation of sulfur and nitrogen oxides. The evaporation of water from the coal–water slurry leads to the formation of superheated steam, which reacts with the coal to form carbon monoxide CO and free hydrogen H2. The gas formed reduces the sulfur and nitrogen oxides, with decrease in their content in the smokestack gases [33]. COKE AND CHEMISTRY
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Thus, in both economic and environmental terms, it is expedient to develop coal–water slurries for use at thermal power stations in countries which largely rely on coal and its processing wastes as fuels (China, India, Russia, the United States, etc.). It is obvious that the active use of filter cakes will have no harmful environmental impact. On the contrary, this will reduce the volume of stored wastes and eliminate the need to work new coal and oil fields. (The annual production and stored reserves of filter cakes are sufficient for many regions of the world.) CONCLUSIONS Analysis of the experimental results indicates the typical concentrations of industrial emissions for different regions of the world and what is involved in decreasing them. Attention focuses on the most harmful emissions formed in coal combustion: suspended particulates and oxides of sulfur, nitrogen, and carbon. Globally, the leading emitters of greenhouse gases are China, India, the United States, and Russia. A promising method of reducing atmospheric gas emissions, without any additional expenditures, is to switch from the direct use of coal to the preparation of water–coal suspensions (coal slurries) that are subsequently burned in furnaces. The transition to such coal–water slurries permits considerable decrease in SO2 and NOx emissions. As well as coal particles, such slurries may be based on coal-processing wastes (filter cakes). ACKNOWLEDGMENTS Financial support was provided by the Russian Scientific Foundation (project 15-19-10003).
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Translated by Bernard Gilbert
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