Effect of the Chlorine Type, Concentration and the Adding Mode on the Mercury Transformation in. Coal-Fired Flue Gas

Transcription

1 AASCIT Journal of Environment 2017; 2(6): ISSN: (Print); ISSN: X (Online) Effect of the Chlorine Type, Concentration and the Adding Mode on the Mercury Transformation in Coal-Fired Flue Gas Zhenya Duan 1, 2, Shaopu Li 1, Enle Xu 1, *, Wenbo Huang 1, 2, Shuxiao Wang 2 Keywords Mercury Transformation, Chlorine, Solid Impregnation, Gas Adding, Coal-Fired Flue Gas Received: September 8, 2017 Accepted: November 16, 2017 Published: December 7, College of Electromechanical Engineering, Qingdao University of Science and Technology, Qingdao, China 2 State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Beijing, China address xel360@126.com (Enle Xu) * Corresponding author Citation Zhenya Duan, Shaopu Li, Enle Xu, Wenbo Huang, Shuxiao Wang. Effect of the Chlorine Type, Concentration and the Adding Mode on the Mercury Transformation in Coal-Fired Flue Gas. AASCIT Journal of Environment. Vol. 2, No. 6, 2017, pp Abstract Impacts of chloride type, adding concentration and adding mode on the mercury transformation of coal-fired flue gas were experimentally investigated in a one-dimensional drop tube furnace with continuous feeding unit. The results indicated that the mercury transformation rate of NaCl addition is higher than that of CaCl 2 for lignite, whereas that of CaCl 2 addition is higher than adding NaCl for sub-bituminous. For solid impregnation, the mercury transformation rate of lignite and sub-bituminous are the same which is 85% when the chlorine concentration of NaCl and CaCl 2 is both ppm, respectively. For HCl gas addition, the mercury transformation rate of pre-combustion is higher than post-combustion for lignite and sub-bituminous. The mercury transformation rate of lignite is about 82% when the chlorine concentration is 400.0ppm and that of sub-bituminous is about 78% when the chlorine concentration is 250.0ppm. In addition, considering the equipment corrosion and the operation difficulties, the solid immersion is the better way to add chloride. 1. Introduction Mercury emission has been recognized as a global air pollution problem owing to its toxicity, persistence, and long-range transportability which attracts more and more worldwide attentions [1-3]. Coal combustion is the largest source of mercury emissions to the atmosphere in the world [4]. China, with its more than 2000 coal-fired power plants [5], is the largest single emitter of atmospheric mercury in the world [6-8]. Mercury in the flue gas has three major chemical forms: particulate bound mercury (Hg p ), gaseous oxidized mercury (Hg 2+ ) and elemental mercury (Hg 0 ) [9-11]. The Hg p can be removed from flue gas by conventional air pollutant control devices in power plants, such as an electrostatic precipitator (ESP), a fabric filter or other particulate matter control equipment [12, 13]. The Hg 2+ is easier to capture as it is water soluble and can be removed by wet scrubber technologies [14]. However, the Hg 0 in the vapor phase, which has high volatility and low water soluble, is difficult to control and is likely to be emitted into the atmosphere [15, 16]. Thus, the removal of Hg 0 is the most important and difficult work in flue gas mercury control.

2 AASCIT Journal of Environment 2017; 2(6): Some researchers suggested that the Hg 0 can be oxidized to form the Hg 2+, most likely by chlorine species [17, 18], in order to increase the removal efficiency of mercury. Zhuang et al. [19] investigated mercury transformations in coal flue gas when firing sub-bituminous coal with a CaCl 2 additive based on the pilot-scale experiments. The CaCl 2 additive with the sub-bituminous coal resulted in approximately 50% oxidized mercury compared to the dominance of Hg 0 in the baseline flue gas as a result of reactive chlorine species formed in coal flue gas. Agarwal et al.[20] found that hydrogen chloride (HCl) is the primary chlorine species responsible for Hg 0 oxidation in the temperature range from approximately 320 C to 538 C, while Cl 2 is the dominant species in the temperature range from approximately 121 C to 232 C. Xu et al. [21] investigated the effects of physicochemical properties on the mercury adsorption performance of three fly ash samples. The results indicated that the samples modified with CuBr 2, CuCl 2 and FeCl 3 showed excellent performance for Hg removal, because the chlorine in metal chlorides acts as an oxidant that promotes the conversion of Hg 0 into its oxidized form Hg 2+. Cu 2+ and Fe 3+ can also promote Hg 0 oxidation as catalysts. Zhang et al. [22] added CaCl 2 to a commercial selective catalytic reduction (SCR) catalyst in a fixed bed to evaluate the effect on elemental mercury oxidation. By adding 1.0wt.% CaCl 2 to the catalyst, the Hg 0 oxidation efficiency rose from 7.7% to 78.8%. Liu et al. [23] experimentally investigated mercury oxidation in the presence of HCl and O 2 in a fixed-bed reactor. Mercury oxidation was improved significantly in the presence of HCl and O 2, and the Hg 0 oxidation efficiencies decreased slowly as the temperature increased from 200 C to 400 C. All previous research demonstrated the chlorine species can effectively oxidize Hg 0 to Hg 2+, however, they did not study on the best adding concentration of the appropriate additives for different kinds of coals. The paper aims to investigate the impacts of chloride type, concentration, and adding mode on mercury transformation in the one-dimensional drop tube furnace with continuous feeding lignite and sub-bituminous through the experimental method. 2. Experiments 2.1. Experimental Device The configuration of the one-dimensional drop tube furnace system is given in Figure 1. The system is composed of an air distribution unit, a feeding unit, a combustion unit, a flue unit, and a monitoring unit. The air distribution unit consists of an air compressor, an air storage tank, a mass flow controller and a rotor flow meter. The atmospheric air is fed into the air storage tank after being pressed by the air compressor. Part of the air is supplied to the furnace combustion with the flow rate of 0.6m 3 h -1 and the other part feeds into the Thermo on line monitoring system as the diluent gas. The Thermo on line monitoring system can continuously test the mercury concentration in flue gas and upload the data to receiver which can save and display the test results. The feeding unit includes a micro-feeder and a feeder controller. The micro-feeder can mix the coal sample with the air supplied by the air storage tank, and then the mixed gas will flow into the one-dimensional drop tube furnace through the pipeline. The combustion unit is composed of one-dimensional drop tube furnace controlled by the setting program. The furnace body is divided into the upper, middle and lower sections. Each section can be heated independently and has a built-in temperature probe to measure the temperature and control the heating current. The experimental temperature is about 1100 C. Figure 1. Configuration of one-dimensional drop tube furnace system. 1. Air compressor; 2. Air storage tank; 3. Mass flow controller; 4. Rotor flow meter; 5. Micro-feeder; 6. Feeder controller; 7. Tube furnace; 8. Sectional temperature control instrument; 9. Hopper; 10. Quartz glass tube; 11. Tail gas treatment device; 12. Flue gas mercury analyzer system

3 70 Zhenya Duan et al.: Effect of the Chlorine Type, Concentration and the Adding Mode on the Mercury Transformation in Coal-Fired Flue Gas Figure 2 provides the schematic diagram of flue unit and sampling points. The exhaust comes from the furnace to the hopper through a connecting piece which connected with a long quartz glass tube with six sampling points. In the experiments, considering the residence time of the flue gas in coal-fired power plant is about 5-7s, so the first sampling point regards as gas injection point with residence time of 5.08s whereas the second sampling point with residence time of 6.78s regards as gas sampling point. Mercury FreedomTM flue gas mercury analyzer system which is produced by Thermo Fisher Company in America and can measure the content of the total mercury and the Hg 0, is used as the monitoring unit with the detection limit of 0.01 µg m Experimental Materials Lignite and sub-bituminous, which are the main combustion coals in China and contains more elemental mercury than others in the combustion process, were chosen as the experimental coals. Table 1 gives the characteristic parameters of the coal samples which were obtained from the average of the two samples. The coals test methods obey the Standards of GB/T and GB/T for industry Figure 2. Schematic diagram of flue unit and sampling points. Table 1. Results from proximate and ultimate analysis for tested coal samples. analysis and elemental analysis, respectively. The device of DMA-80 direct mercury analyzer was used to analyze the mercury content of the coal, and the standard test method is ASTM D of the direct pyrolysis-cold atomic absorption spectrophotometry method. The standard test method of GB/T , which is the high temperature combustion hydrolysis-potentiometric titration, was adopted to determine of chlorine in coal. Type of coal Industry analysis (%) elemental analysis v (%) Hg Cl Mad Vad Aad FCad C H O N S (ppm) (ppm) lignite sub-bituminous Note: Mad: Moisture; Vad: Volatile; Aad: Ash; FCad: Fixed Carbon The impact of chlorine adding mode on mercury transformation was studied by two methods of solid impregnation and gas adding in this paper. In the solid impregnation method, the 45.0g raw coal sample was covered by different concentrations of NaCl or CaCl 2 solutions. The mixture is dried in drying box at the temperature of 39 C for 72 hours before being mixed evenly with the solution. The next procedure is to ground the coal sample inside the mill Table 2. Information of solid impregnation. tank and to mix under high speed movement by ball mill with maximum particle size less than 80 mesh produced by Changsha Tianchuang Powder Technology Company. Finally, the sample was stored in the zip-lock bags. The detailed information of solid impregnation by adding NaCl and CaCl 2 is provided in table 2. The chlorine concentration in coal flue gas can be calculated based on the burning 15.0g coal and in 66min in each experiment. NaCl CaCl 2 Chlorine Chlorine Add chlorine solution solution solution solution concentration of concentration in quality (mg) concentration (g L -1 ) volume (ml) concentration (g L -1 ) volume (ml) coal (ppm) coal flue gas (ppm)

4 AASCIT Journal of Environment 2017; 2(6): Table 3. Information of gas adding by adding HCl. Serial number HCl concentration (ppm) HCl flow (L min -1 ) Chlorine concentration in coal flue gas (ppm) The gas adding method, on the other hand, divides into pre-combustion addition and post-combustion addition. The coal sample is ground by ball mill until the maximum particle size is less than 80 mesh and then it is stored in the zip-lock bags after being dried in constant temperature drying box. The pre-combustion addition is to add HCl gas at the coal sample inlet to make sure fully mixing with the coal sample. The post-combustion addition is to injecte HCl gas from the first hole of the quartz glass tube. Table 3 gives the detailed information of the HCl gas adding Quality Control of Mercury Concentration In order to ensure the stability of the system and the accuracy of the experimental data, the measured mercury concentration in the flue gas of raw coal sample was compared to the theoretical mercury concentration. Equation 1 was given to calculate the theoretical mercury concentration based on the mass balance of mercury in this system. D π v ρ 2 cg c1 = Q where c 1 is the theoretical mercury concentration of the flue gas, µg m -3 ; D is the diameter of feeding pipe, 0.015m; v represents the feeding speed, 0.1m h -1 ; c g represents mercury content in coal which has been provided in Table 1, ppm; Q 1 represents the flow rate of air, 0.6m 3 h -1 ; ρ is the stacking density calculated by the quality and the volume of coal powder in feeding pipe, which means the density of coal powder, kg m -3. Table 4. Theoretical concentration and measured concentration of mercury from different coals. Type of coal lignite sub-bituminous Stacking density (kg m -3 ) Theoretical mercury concentration (µg m -3 ) Measured mercury concentration (µg m -3 ) Relative error (%) Table 4 provide the comparison between the measured and the theoretical mercury concentration for the lignite and sub-bituminous. It is observed the maximal relative error is less than 1%, indicating the experimental method and the equipment are reliable. In addition, for the purpose of reducing the measurement error and increasing the representativeness of experimental 2 1 (1) data, blank samples for two kinds of coal were set, and each experimental data was obtained from the average of three samples. The influence of environmental parameters can be reasonably neglected with the standard deviation of parallel samples being not more than 10% and the comparison of the experimental samples and the blank samples Computing Method The E oxi is the mercury transformation rate, which can be calculated by the following equation E oxi Ct Co (%) = 100% (2) C where C t and C o represent the total mercury concentration and the content of Hg 0 in flue gas, respectively, with unit of, µg m -3. The E oxi of the lignite and sub-bituminous blank sample is about 30% and 53%, respectively. 3. Results and Discussion 3.1. Solid Impregnation Figure 3(a) shows the relation between the E oxi and the chlorine concentration by adding NaCl and CaCl 2 respectively for lignite. For the additive of NaCl, the E oxi increases significantly from 30% to 85% when the chlorine concentration increases from 0 to ppm, but the E oxi appears to decrease slowly when the concentration continues to increase. The reasons of this trend may be that with the increment of chloride concentration, the active site of fly ash was saturated and can t adsorb more Hg 0 [14]. For the additive of CaCl 2, the E oxi increases dramatically at the beginning and then tends to keep stable with the increment of the chloride concentration. When the concentration is more than ppm, the E oxi is about 55%. In summary, the chlorine concentration plays a key role in promoting the mercury transformation when the concentration is less than ppm, and the chlorine concentration shows no more effect on the mercury transformation when the concentration exceeds ppm. According to the Figure 3(a), the additive of NaCl shows no obvious difference with the CaCl 2 on promoting the mercury transformation at a low concentration, while its impact on mercury transformation becomes significant when the chlorine concentration is higher than ppm. Therefore, for the lignite, the NaCl is more effective than the CaCl 2 on t

5 72 Zhenya Duan et al.: Effect of the Chlorine Type, Concentration and the Adding Mode on the Mercury Transformation in Coal-Fired Flue Gas promoting the mercury transformation, and the best chlorine concentration is about ppm. Figure 3. Impact of the chlorine concentration on the E oxi in lignite (a) and sub-bituminous (b). Figure 3(b) gives the impact of the chlorine concentration on the E oxi for sub-bituminous. For the additive of NaCl, the E oxi increases significantly from 53% to 72% when the chlorine concentration increases from 0 to ppm, but the E oxi has little change when the concentration continues to increase. For the additive of CaCl 2, the E oxi increases dramatically at the beginning and then tends to decrease with the increment of the chloride concentration. When the concentration is about ppm, the maximum E oxi is about 85%. According to the Figure 3(b), the additive of CaCl 2 doesn t show much difference with NaCl on promoting the mercury transformation at low concentrations, while its impact on mercury transformation is more obvious than NaCl when the chlorine concentration is more than ppm. Therefore, it is concluded the CaCl 2 is more effective than the NaCl on promoting the mercury transformation for the sub-bituminous, and the best chlorine concentration is about ppm Gas Adding Figure 4(a) shows the relation between the E oxi and the chlorine concentration of lignite flue gas at two different ways of pre-combustion addition and post-combustion addition. In the case of pre-combustion addition, the E oxi increases significantly from 30% to 82% when the chlorine concentration increases from 0 to 400.0ppm, but the E oxi maintains between 82% and 85% when the concentration continues to increase. For the post-combustion addition, on the other hand, the E oxi increases significantly from 30% to 71% when the chlorine concentration increases from 0 to 250.0ppm, but the E oxi decreases slowly when the concentration continues to increase. Figure 4. Impact of the chlorine concentration on the E ox in lignite (a) and sub-bituminous (b).

6 AASCIT Journal of Environment 2017; 2(6): According to the Figure 4(a), the post-combustion addition method is better than the pre-combustion addition method on promoting the mercury transformation when the chlorine concentration is less than ppm, while the pre-combustion addition is more effective than the post-combustion at higher concentrations. In the lignite case, the best choice of chlorine concentration is ppm in the pre-combustion addition method. Figure 4(b) shows the relation between the E oxi and the chlorine concentration at two ways of pre-combustion addition and post-combustion addition for sub-bituminous flue gas. In the case of pre-combustion addition, the E oxi increases dramatically at the beginning and then tends to keep stable with the increment of the chloride concentration. Particularly, the E oxi is between 78% and 82% when the chlorine concentration is more than ppm. For the post-combustion addition way, the maximal E oxi is about 70% when the chlorine concentration is 15.0 ppm. However, the E oxi decreases at the beginning and then tends to keep stable around 60% with the continuing increment of the chloride concentration. The reason may be that the HCl gas remains a very short residence time in the quartz glass tube, which leads to incompletely reaction between the HCl gas and flue gas. It can be found from Figure 4(b) that there is negligible difference of the E oxi between pre-combustion and post-combustion at low chlorine concentration, however, the E oxi of pre-combustion is higher than post-combustion when the chlorine concentration is more than 75.0ppm. So the best choice of chlorine concentration is 250.0ppm in the way of pre-combustion addition for the sub-bituminous Comprehensive Analysis Between Solid Impregnation and Gas Adding Table 5. Comparison of different chlorine adding modes. Coal Add category Add location Chlorine concentration in coal fired flue gas (ppm) E oxi (%) The operation level Cost NaCl solid impregnation easy low Lignite CaCl 2 solid impregnation easy low HCl pre-combustion difficult high HCl post-combustion difficult high NaCl solid impregnation easy low Sub-bituminous CaCl 2 solid impregnation easy low HCl pre-combustion difficult high HCl post-combustion difficult high The maximum E oxi at different chlorine adding modes was provided in Table 5, from which it can be found that the effect of NaCl addition on mercury oxidation for lignite is better than that of CaCl 2 addition, whereas the results are opposite to sub-bituminous. The reason, according to the data in Table 1, may be contributed to the higher sulfur content in sub-bituminous than lignite. The higher sulfur content will generate more SO 2 in the combustion gas to expand the mercury oxidation, especially with the existence of calcium. In addition, Table 5 also shows that the chlorine concentration in adding gas method is much higher than solid impregnation method whereas the maximum E oxi have little difference between the two methods. The high chlorine concentration will result in the corrosion of the equipment, and the adding gas method need to establish special gas generating device and reform the system which is expensive and hard to achieve. By comprehensive comparison, the solid immersion is the best way to add chloride and has a bright prospect. 4. Conclusions The E oxi was investigated under the solid impregnation and gas adding methods for the lignite and sub-bituminous. The experimental data indicated the solid impregnation method with NaCl additive is better than CaCl 2 in lignite, with the E oxi of about 85% at the best chlorine concentration of ppm. However, the additive of CaCl 2 is better than NaCl for sub-bituminous, with the E oxi of about 85% at the best chlorine concentration of ppm. In the gas adding method, the E oxi for pre-combustion is higher than post-combustion in lignite and sub-bituminous. The E oxi is about the 82% at the best choice of chlorine concentration of 400.0ppm in lignite and the E oxi is about the 78% at the best chlorine concentration of 250.0ppm in sub-bituminous. In addition, considering the equipment corrosion and the operation difficulties, the solid immersion is the best way to add chloride. It is indicated through this research the solid impregnation of chloride addition with existing flue gas pollution control equipment is an effective method to control mercury emission and has a bright prospect. And it is expected the research result could play an important role in deepening understanding of the law of mercury speciation transformation in flue gas by the addition of chlorine and could show practical significance in promoting the development of coal-fired mercury pollution control technology. Acknowledgements This work reported here was supported by the Science and Technology Planning Project of Shandong Provincial Education Department (J15LC16), State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex (No. SCAPC201405),and the National Key Basic Research Development Plan (2013CB430001). We express our grateful thanks to them for their financial support.

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