Characteristics of Chlorine Releasing from Coal-Fired Power Plant

Abstract

Chlorine (Cl) released from coal-fired power plants can be harmful to power equipment, the ecological environment and human health. Here, we investigated the Cl releasing characteristics from four coal-fired power plants (CFPPs) in China’s Anhui province based on an ion chromatography analysis of the combustion by-production samples collected from different locations of the power plants. The results showed that Cl content in coals was low (198–367 μg·g⁻¹), which positively correlated with the contents of lead, mercury and total sulfur, but was weakly correlated with the moisture and ash yield in coal. The releasing rate of Cl during coal combustion was highly correlated with temperature and volatile matter, and most Cl is transferred into the flue gas. Dust collector and wet flue gas devices equipped in the CFPPs were robust for removing Cl in the particulate phase, and the fabric filter showed a higher removal efficiency than the electrostatic precipitator. This study can provide theoretical support for Cl pollution control in coal-fired power plants.

1. Introduction

In 2020, the global coal output reached 7.74 billion tons, of which China accounted for 51% [1]. Coal is still the main energy source in China, accounting for about 60% of the total energy consumption. Coal is the main source of power generation in China [2]. China takes up 53% of the coal-fired power generation in the world [3]. Chlorine (Cl) is one of the common elements in coal. It produces HCl, Cl₂ or toxic organic chlorides during coal combustion. It will cause serious harm to the atmospheric environment and human health [4]. In Europe, 75% of HCI emissions come from coal [5]. Studies have shown that the content of Cl in coal burning will also affect the form control of Pb and Hg in flue gas [6]. When the content of chlorine in coal exceeds 0.3%, it will seriously corrode the boiler pipes and the wall of the carbonization chamber [7]. It also causes crust and block, so as to shorten the life of the equipment. Therefore, it is of great significance to study the release and migration characteristics of Cl in coal-fired power plants for controlling pollutants from coal burning.

The release characteristics of Cl in the process of coal combustion from coal-fired power plants has attracted much attention. Deng [8] selected six pulverized coal boilers from four power plants in China for research, which showed that more than 96.99% of Cl in coal-fired power plants was released into the flue gas, and the dust collector and limestone-gypsum wet desulfurization device have a synergistic effect on the removal of Cl in the flue gas. Meanwhile, researchers have focused on the relationship between the Cl and other factors (e.g., ash content, concentration of other released components, combustion temperature) [9,10,11,12,13,14,15]. The types of the coal, as well as the combustion characterization of each region, could directly affect the Cl release characterization, while related studies in Anhui Province, the 8th most populous province in China, are still lacking.

At present, there are few studies on the release and migration characteristics of Cl in coal-fired power plants at home and abroad [16]. It is still difficult to determine the occurrence state of Cl in coal combustion, and there is a lack of research on the release characteristics of Cl in coal [17]. Relevant studies in China mainly focus on the characteristics of Cl during coal combustion, and only stay in the laboratory simulation research stage. Understanding the occurrence mode of chlorine can effectively reduce the harm to environment and equipment in the release of chlorine in coal. In this study, we collected samples of coal, bottom slag, desulfurization wastewater, flue gas and desulfurization gypsum from four coal-fired power plants in Anhui Province. The characteristics of Cl release and migration during coal combustion and its influencing factors were studied.

2. Materials and Methods

2.1. The Basic Information of Power Plant and Coal Quality

Anhui province is the main coal energy supply base in eastern China, which borders Jiangsu province, Zhejiang province and Shanghai [18]. There are many large coal-fired power plants in Anhui. The samples were collected from six pulverized coal boilers of four representative power plants in different cities of Anhui Province. The sampling location is shown in Figure 1. The electric boilers of the four coal-fired power plants were named HPA-1, HPB-2, HPB-3, HPC-4, HPC-5, HPD-6 and HPD-7. Detailed information on the power plants are shown in the

Table 1. The basic information for the Cupsful gas and APCD conditions and flue gas.

Plant	Age	Coal Area	Boiler No.	Boiler Type	Boiler Capacity/MW	Air Pollution Control Facilities	Flue Gas Velocity/(m·s−1)	Flue Gas Tem./°C
Suzhou	1995	Indonesia	HPA-1	subcritical	300	SCR + ESP + WFGD	8.8	70.4
Xuancheng	2003	Shandong	HPB-2	subcritical	300	SCR + FF + WFGD	9.7	69.2
Xuancheng	2003	Huainan	HPB-3	subcritical	600	SCR + FF + WFGD	10.1	71.3
Huainan	2005	Huainan	HPC-4	subcritical	600	SCR + ESP+ WFGD	9.8	65.5
Huainan	2005	Mongolia	HPC-5	subcritical	300	SCR + FF + WFGD	9.6	70.8
Huaibei	1969	Shanxi	HPD-6	subcritical	600	SCR + FF + WFGD	8.9	78.9
Huaibei	1969	Shanxi	HPD-7	subcritical	300	SCR + ESP+ WFGD	9.0	76.5

Note: ESP is electrostatic precipitator; SCR is selective catalytic reduction denitrification; WFGD is limestone gypsum wet desulfurization; FF is bag filter.

.

2.2. Sample Collection and Experimental Analysis

Samples of coal, fly ash, bottom slag and desulfurization gypsum were collected in January, May, September and December 2020 using the shrinkage method [19], and about 1 kg of each sample was collected. We put them in plastic bags and took them to the laboratory for analysis. The sampling points of flue gas are located in the front and rear of the dust collector and the desulfurization device. The sampling locations are shown in Figure 2. The collection used the M26A method recommended by USEPA [20]. The sampling instruments used are as follows: suction nozzle, sample tube, pitot tube, filter paper holder, six series conical flasks, differential pressure gauge and metering system, and so forth. The flue gas samples were collected by the automatic constant velocity sampling system. Granular Cl was captured by quartz fiber filter paper. HCl in the flue gas was absorbed by sulfuric acid solution (0.1 mol·L⁻¹) in the first three absorption flasks, and Cl in the flue gas was absorbed by NaOH solution (0.1 mol·L⁻¹) in the last three absorption flasks.

The solid sample was crushed, sieved, mixed, shrunk, dried and ground, and then we used the Eshka mixture fusion sample—potassium thiocyanate titration method to measure the content of Cl in each sample. The detailed experimental analysis process referred to GBT (3558-2014) [21]. The flue gas samples were treated with the digestion method, and were then analyzed by ion chromatography (ICS-2000). The mercury content was determined by the DMA-80 mercury meter, and the content of lead in the sample was determined by ICP-MS according to DZ/T0223-2015. The sulfur in coal is determined by the Eschka method [22]. The air drying method is used to measure moisture. That is, air drying coal samples are taken into the air blast drying oven and the loss of moisture is calculated. Ash content was determined by the slow ash method. The muffle furnace heated to 815 °C, and the residual mass is the ash content of the coal.

2.3. Experimental Reagents and Consumables

The experimental reagents used are as follows: ammonium ferric sulfate, potassium thiocyanate, sodium chloride, phenolphthalein, nitric acid, deionized water, sodium hydroxide, iodine, and so forth; and the consumables are as follows: muffle furnace, automatic temperature controller of a high temperature electric furnace, several porcelain crucible, analytical balance, electronic balance, alkali burette, pipette, filter paper, funnel, and so forth.

2.4. Quality Assurance and Control

In the field collection process, each flue gas was measured three times in parallel and the average value was taken. During the experimental analysis, each sample was also analyzed three times and the average value was taken. We make a group of laboratory blank for each two samples. No residue of Cl was detected in the blank test. The recovery rate is 79% to 108%. The relative standard deviation is 4.55%. The detection limit of Cl is 2–3 μg·g⁻¹, within the acceptable range.

3. Results

3.1. Residual Characteristics of Cl in Coal Combustion

Table 2. Contents of Cl and trace elements in coal.

Boiler Number Boiler No	Cl (μg·g−1)	Pb (μg·g−1)	Hg (μg·g−1)	Water Water Content/%	Ash Ash Content/%	Volatile Vdaf/%	Total Sulphur Total Sulfur/%
HPA-1	367	7.76	0.09	2.31	25.65	26.71	0.32
HPB-2	219	6.54	0.21	3.34	31.30	23.27	0.43
HPB-3	208	8.21	0.25	3.14	32.21	25.64	0.45
HPC-4	235	9.12	0.19	4.56	27.89	31.67	0.29
HPC-5	202	8.65	0.65	3.53	21.12	23.29	0.56
HPD-6	198	7.69	0.43	3.67	35.12	31.41	0.36
HPD-7	217	6.54	0.55	1.95	19.64	24.67	0.39

shows that the Cl content of coal collected in four coal-fired power plants is generally low. The maximum value of Cl content in coal is 367 μg·g⁻¹ with coal from Indonesia, and the Cl content in coal from China is around 200 μg·g⁻¹. The average value is 213 μg·g⁻¹. Indonesian coal used in the Suzhou power Plant has the highest Cl content, while local coal used in the Huaibei Power Plant has the lowest Cl content. It can be seen from

Table 3. Distribution of chlorine content in coals from different regions of the world.

Coal District	Chlorine Content (μg·g−1)	Average (μg·g−1)
China	50~150 [24]	200
United States	50~870 [25]	628
Malaysia	10~209 [26]	139
Indonesia	320~550 [27]	/
Germany	140~250 [27]	/
Australia	10~1100 [28]	150
Britain	10~1110 [29]	440
South Africa	10~340 [27]	30

that the content of Cl in Chinese coal is far lower than that in other countries. The content of Cl in Chinese coal is generally 210–274 μg·g⁻¹, and the average content of Cl in American coal is 628 μg·g⁻¹ [23].

In order to study whether coal quality has an effect on coal Cl content, SPSS software was used to analyze the correlation between Cl and other trace elements and harmful substances (lead and mercury) in coal (

Table 4. Person Correlation Analysis between Cl and Coal Quality in Coal.

	Cl	Water Content/%	Ash Content/%	Vdaf/%	Total Sulfur/%	Pb	Hg
Cl	1
Water content/%	0.22 *	1
Ash content/%	−0.32	0.46	1
Vdaf/%	0.24	0.66 *	0.45	1
Total sulfur/%	0.56 *	0.79	0.54 **	0.56	1
Pb	0.68 *	0.001	−0.12	0.21	0.56 *	1
Hg	0.89 **	0.21	0.12	−0.43	0.21		1

Note: ** means significant correlation at the level of 0.01; * means significant correlation at the level of 0.05.

). The absolute value of correlation coefficient R is above 0.5, so A and B are considered to be strongly correlated. Between 0.3 and 0.5, the correlation can be considered weak. Below 0.3, no correlation is considered. The results showed that Cl in coal is significantly related to lead, mercury and total sulfur content in coal. Studies have reported that the content of CI in coal combustion affects the migration and morphological transformation of lead and mercury [30]. Hence, the removal of pollution of lead, mercury, and sulfur from coal can be considered in terms of cooperative control of Cl in coal. The correlation between Cl and ash content can be inferred as organic and inorganic affinities. Cl in coal is negatively correlated with ash content to a certain extent, indicating that Cl in coal chlorine may exist in the form of organic matter [31]. The correlation with moisture, ash and volatile matter in coal is weak.

3.2. Cl Migration Characteristics in Coal Combustion

Through combustion and the flue gas pollution control device, the Cl in coal is transferred to the net flue gas, desulphurization wastewater, desulphurization gypsum, fly ash and bottom slag. The distribution of chlorine in by-products of coal is shown in Figure 3 and

Table 5. Person Correlation Analysis between Cl and Coal Quality in Coal.

Boiler	Net Flue Gas (Cl) (mg·m−3)	Fly Ash (Cl) (μg·g−1)	Bottom Slag (Cl) (μg·g−1)	Desulphurization Wastewater (Cl) (mg·L−1)	Desulfated Gypsum (Cl) (μg·g−1)
HPA-1	1.19	89	99	3601	234
HPB-2	0.98	78	87	2890	178
HPB-3	0.67	81	101	3421	198
HPC-4	0.96	67	95	2689	167
HPC-5	0.84	76	105	3451	212
HPD-6	1.02	68	89	4321	176
HPD-7	0.89	89	96	3331	198

. Cl in fly ash and bottom slag is between 67.1 and 105 μg·g⁻¹. The Cl in fly ash and bottom slag is relatively low, which indicates that they have no enrichment effect on Cl. It also indicates that the Cl release rate of coal combustion is high, and most of the coal is transferred to flue gas. This result is consistent with Vassilev et al. [32] and Deng et al. [33]. Most of the Cl in the flue gas is dissolved in water and transferred to the desulfurization waste due to the efficient synergistic removal of Cl in the flue gas by WFGD. As can be seen from Table 5, the Cl in the desulfurization wastewater is 2689—4321 mg·L⁻¹, which is relatively high. It is reported that the high concentration of Cl ions in desulfurization wastewater will cause many problems, such as reducing desulfurization efficiency, accelerating equipment corrosion and affecting gypsum quality, which is also the technical bottleneck problem encountered by realizing zero discharge in coal-fired power plants at present. Cl is also transferred to desulphurized gypsum, which has a range of 167–234 μg·g⁻¹. The content is close to or above the secondary standard (≤200 μg·g⁻¹) for desulphurized gypsum. Desulfurization gypsum in coal-fired power plants is mainly used in the construction industry, so the pollution of chlorine in desulphurization gypsum is also a concern in coal-fired power plants.

3.3. Characteristics of Cl Releases from Coal Combustion

During high temperature combustion, some of Cl is released into flue gas, some is left in the bottom slag of coal. In order to study the release degree of Cl in coal, according to formula (1) [34]. Calculate the release rate of Cl in coal. The coal-fired release rates of seven boilers are shown in

Table 6. The release rate of Cl in coal.

Boiler	Cl in the Coal (μg·g−1)	Cl in Bottom Slag (μg·g−1)	Release Rate of Cl (%)
HPA-1	367	99	97.12
HPB-2	219	87	98.35
HPB-3	208	101	99.16
HPC-4	235	95	98.69
HPC-5	202	105	99.56
HPD-6	198	89	98.15
HPD-7	217	96	99.67

. Cl release rate of the boilers is above 97.12%. The maximum is 99.67%. This indicates that most of Cl in coal is released into flue gas. Therefore, the Cl content in boiler outlet flue gas is mainly determined by coal Cl. In addition, whether the release of Cl in coal is affected by combustion temperature and other factors needing further study.

$$Y=\frac{F\times C-f\times C}{F\times C}\times 100\%.$$

(1)

In the formula, F is the daily coal consumption (t·d⁻¹), C is w (Cl) in coal (μg·g⁻¹), f is the amount of daily residue produced (t·d⁻¹), c is w (Cl) in slag (μg·g⁻¹).

To study whether the temperature of coal combustion affects the release rate of Cl in coal, the coal is burned at different temperatures and the release rate of Cl is determined. As shown in Figure 4a, it was found that the combustion temperature of coal has a strong correlation with the Cl release rate in coal (R² = 0.95). The higher the combustion temperature, the higher the coal release rate. When the temperature reaches 700 °C, the Cl in the coal has been released completely. Figure 4b shows the relationship between Cl release rate and volatile matter in coal combustion. It can be seen that there is also a good correlation between Cl release rate and volatile matter (R² = 0.96).

3.4. Effects of Pollution Control Devices on the Occurrence of Cl in Flue Gas

For analyzing the influence of the flue gas pollution control device on Cl form and distribution in flue gas, different forms of Cl before and after the dust collector were determined as shown in Figure 5. The concentration of particulate Cl in the flue gas of each boiler decreases significantly after the dust collector. Particle chlorine removal efficiency is 82.4–93.06%. That is probably because the dust collector has a good removal effect on the particles. HCl removal efficiency in flue gas after the dust collector is relatively low, between 2.19% and 11.26%. Among the seven boilers, the removal effects of particle chlorine from boilers (HPA-2,HPA-3,HPA-5 and HPA-6) installed with a bag dust collector are higher than that equipped with electrostatic precipitators. This may be due to the fact that the bag dust collector can capture smaller fly ash than the electrostatic precipitator [35]. The fine fly ash trapped on the cloth bag has a longer contact time with the gaseous chloride in the flue gas, thus enabling more adsorption of the gaseous chloride in the flue gas. As shown in Figure 5a, after the flue gas passes through the desulfurization device, the HCl in the flue gas has a good removal effect in the desulfurization device. The removal efficiency is above 91%, which may be due to the HCl being soluble in water. In addition, some of the granular Cl is removed synergistically in the desulfurization device, and the total Cl concentration of the outlet flue gas also decreases significantly. From Figure 5b, it can be seen that the outlet flue gas is dominated by HCl and the particle state is relatively small. The dust collector has a higher removal effect on the particle Cl in the flue gas, while the limestone wet desulfurization device has a higher removal effect on the HCl in the flue gas. The content range of Cl in clean flue gas is 1.11–1.70 mg·m⁻³ after the dust collector and the desulphurization unit. According to Integrated emission standard of air pollutants (Cl₂ < 85 μg·m⁻³), it fulfils the requirements [36]. Therefore, coal-fired power plants should pay more attention to the environmental hazards caused by chlorine in desulfurization wastewater and gypsum.

4. Conclusions

The coal combustion by-production samples collected from different locations of power plants were analyzed by ion chromatography to investigate the Cl releasing. The results showed that Cl content in coal was low. The maximum value was 367 μg·g⁻¹, the minimum value was 198 μg·g⁻¹, and the mean value was 213 μg·g⁻¹. It is positively correlated with the contents of lead, mercury, and total sulfur, and is weakly correlated with the moisture and ash yield. Meanwhile, the releasing rate of Cl during coal combustion was highly correlated with temperature and volatile matter, and most Cl was transferred into the flue gas. The content of Cl in the flue gas at the boiler outlet is mainly determined by the content of Cl in the coal. The dust collector has a good dust removal effect on particle Cl, and the removal effect of bag dust collectors on particle Cl is higher than that of the electrostatic precipitator. The limestone wet desulfurization device has a higher removal effect on flue gas HCl. This study can provide theoretical support for Cl pollution control in coal-fired power plants.