In order to investigate the factual air pollutant emissions from Henan’s power sector in 2010, SO
2, NO
x and PM emissions from 24 generating sets from 15 coal-fired power plants have been measured. It is shown that SO
2 emission values from 22 of 24 generating sets conform to the requirements, which is causing by the high performance of the flue gas desulfurization system. Much higher NO
x emissions indicate that the construction of flue gas denitrition systems is necessary. PM emissions varied from 2.3 kg to 299.9 kg per hour. Total sulfur, moisture, ash and volatile content, and net caloric value of coals were investigated to elucidate the relationship between coals and air pollutant emissions.
Air Pollutant Emissions; Flue Gas Desulfurization (FGD); Flue Gas Denitrition; Total Sulfur; Ash Content1. Introduction
In recent years, the energy demand has rapidly increased, like thermal power, iron & steel, building materials, etc., so the coal consumption has increased, so do air pollutant emissions (SO2, NOx and PM). Aside from the documented effects on human health [1], the high concentrations of acid gases emissions can lead to the acidic deposition, haze, stunted plant growth, species decline, and corrosion of the national heritage, in other words, damage to concrete and limestone buildings, statues, monuments, and other historic structures [2,3]. While particulate matters (PM10 and PM2.5), are inhalable and can penetrate deeply into the cardiovascular system, thus causing most types of respiratory illness, heart disease and strokes [4].
Coal-fired power plant has been considered as a very important source of regional air pollution and ecosystem acidification, due to its huge emissions of acidic pollutants. For power sector developed fast in the past 20 years in China, SO2, NOx and PM emissions of coal-fired power plants increased by 1.5, 1.7 and 1.2 times, respectively. The SO2 emission of coal fired power sector was estimated to be 11,801 kt in 2010. The NOx emission would increase from 6965 kt in 2005 to 9680 kt in 2010. The TSP, PM10, and PM2.5 emissions would be 2540, 1824 and 1090 kt in 2010 respectively [5]. With increasing environmental pressure, Chinese government has made the decision that coal-fired power sector would be the most important source of regional atmospheric emission abatement in the near future, and power plants are thus anticipated to face more stringent environmental regulations related to siting and operation.
Not only China government takes strict measure to control power plant’s air pollutants emissions and pay attention to the monitoring and management of thermal generator sets [6], so as to prevent air pollution, but also several mitigation technologies are adopted to reduce the values. The most pronounced technologies are filtering stack emissions using various industrial dust collectors, flue gas desulfurization (FGD) and denitrition systems [7-9]. Henan Province with one of the largest coal consumption is also one of the largest air pollutant emission area in China. To supply a clear emission picture of power sector, since 2005, Henan had shut down 9.78 million kW of small thermal power units, and by the end of 2008, all of its coal-fired power units had been equipped with desulfurization facilities, being able to reduce SO2 by 930,000 t. In order to investigate the factual air pollutant emissions from Henan’s power sector in 2010 and the efficiency of FGD systems, measurements have been carried out on 24 generating sets from 15 coal-fired power plants. Total sulfur, moisture, ash and volatile content, and net caloric value of coals were investigated to elucidate the relationship between coals and air pollutant emissions.
2. Materials and Methods2.1. Apparatus
A sulfur analyzer (LECO, S-144DR) was used for the measurement of total sulfur in the coal. An automatic calorimeter (SDACM-3000) was applied for the measurement of net calorific value at constant volume of the coal. A gas conditioning unit (Kane, KM 9008) was used, in case the build-up of water in the water trap for the long period continuous monitoring. A portable multi-gas analyzer (Kane, KM 9106) was used to monitor the gas [10]. The stainless steel sample probe of sufficient length to traverse the sample points, and teflon sample line linked the probe with analyzer were used for sampling. After last sampling, the analyzer pumps fresh air into the sensors to allow toxic sensors to be set to zero and the Oxygen sensor to be set to 20.9%. And before sampling, a calibration was performed with the standard gas at known concentrations. A smoke and dust (gas) automatic tester (LY30121H) was applied for moisture detection. Microstructures of PM were observed through a stereo microscope (OLYMPUS SXZ16).
2.2. Sampling Site and Sampling Points
Sampling site and sampling points are based on the Chinese Standard [11]. The concentration of SO2 was measured at the input and output of the flue gas desulfurization (FGD) system during the experimental period, as shown in Figure 1. Sampling is performed at a site located at least eight stack or duct diameters downstream and two diameters upstream from any flow disturbance such as a bend, expansion, or contraction in the stack, or from a visible flame. And the situation of traverse points being too close to the stack walls is not employed. As all the stacks in our experiment are rectangular stacks, to determine the number of traverse points shall first divide the stack cross-section into as many equal rectangular elemental areas as traverse points, and then locate a traverse point at the centroid of each equal area according to the example.
2.3. Sampling and Moisture Correction
Position the probe at the first sampling point. Purge the system for at least two times the response time before recording any data. Then, traverse all required sampling points, sampling at each point for an equal length of time and maintaining the appropriate sample flow rate. At least one valid data point per minute during the test run shall be recorded.
During the experiment, we shall determine the moisture content of the flue gas and correct the measured gas concentrations to a dry basis according to the Section 5.2.2.3 in the Chinese Standard GB/T 16157-1996. And the dry basis results can be deduced by following equation:
CD = CW/(1-BWS);
BWS = Moisture content of sample gas;
CD = Pollutant concentration adjusted to dry conditions, mg/m3;
3. Results and Discussion3.1. Characteristic of Coals
As well as known, annual emission of each unit can be calculated based on unit-specific fuel consumption and emission factor. Emissions of SO2, NOx and PM from power plants at province level were calculated using Equations (1)-(3) respectively [5,12].
where subscripts i, j, k, m, n, and y stand for province, power unit, boiler type, fuel type, emission control technology and particulate size; EF is the emission factor; A is the coal consumption; C is the application rate of emission control technology; h is the removal efficiency of emission control technology; Scont is the sulfur content of fuel; Sr is the sulfur retention in ash; AC is the ash content of fuel; ar is the ratio of bottom ash; and f is the particulate mass fraction by size.
During the experimental period, coals used for each generating set were systematically analyzed. Total sulfur [13], moisture, ash and volatile content [14], and net caloric value [15] are important coal quality parameters. These parameters are summarized in Table 1 and Figure 2. As shown in Equation (1), the sulfur content (Scont) of fuel is the key factor influencing SO2 emissions. In Table 1 and Figure 2, all the Scont values are below 2.0, except those of I plant. Too high Scont shall lead to high concentration of SO2 in raw gas, which may be beyond the capacity of FGD and causing the high SO2 emissions. It is corroborated by the high SO2 emission from I plant (vide infra). According to Equation (2), fuel types (m) influence the NOx emissions profoundly, but in the coalfired power plants, the coal quality parameters shall take the charge. As we all know that high moisture, low volatile contents and caloric values will have a negative
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