The Effect of Atmosphere on Elemental Mercury Release During Thermal Treatment of Two Bituminous Coals

The dynamic release behavior of elemental Hg (Hg) during thermal treatment of two bituminous coals was studied under N2, CO2 and air atmospheres. The results show that the profiles of Hg released present several peaks during thermal treatment of two bituminous coals. The Hg peak profiles for the same coal are different with the different atmosphere used. The amount of Hg released from the coals is about 92-94, 73-74 and 31-33% under N2, CO2 and air atmosphere, respectively. This indicates that the Hg is the dominant form during thermal treatment of the coals under N2 atmosphere whereas part of Hg is converted into the oxidized Hg during thermal treatment of the coals under CO2 and air atmosphere. The total Hg released is promoted by the volatile matter release to some extent while the Hg released is mainly affected by the atmospheres used.


Introduction
Mercury (Hg) is a toxic trace element in coal with a high volatility. 1 Because of the tremendous amount of coal used each year, its utilization has been one of the main sources of anthropogenic discharge of Hg. 2 Hg has the persistence and bioaccumulation character, which can strongly affect the environment as well as the human health. 3,46][7][8][9] Also, a series of strict policies for controlling Hg emissions have been established, including the emission standard of air pollutants for coal-fired power plant in China.To satisfy the emission standard of Hg, effective Hg control technology should be introduced to reduce the Hg emission.Consequently, it is important to develop effective Hg control technologies.
Generally, Hg released presents mainly in two forms (Hg 2+ and Hg 0 ) during coal combustion.The two forms of Hg show different characteristics and have different migration abilities in environment.The Hg 2+ is a local pollutant, which is water-soluble and can fall from the atmosphere quickly. 10,113][14] Therefore, the emission control of Hg 0 is harder than that of Hg 2+ and it becomes the main concern for Hg pollution control.
To develop an effective Hg controlling technology, it is necessary to understand the release behavior of Hg during coal thermal treatment because it happens in most coal conversion processes.5][16][17][18][19][20][21][22][23][24] However, the information about the effect of atmosphere on Hg 0 release during thermal treatment of bituminous coal is still limited.In the present paper, the effect of atmosphere on Hg 0 release from two bituminous coals during thermal treatment has been studied.

Coal samples
Two Chinese bituminous coals were used, which were labeled as 1 and 2. The coals were crushed and sieved to 0.16-0.27mm and dried before use.Proximate and ultimate analyses of the two coals and the concentration of Hg contents in the coals are shown in

Thermal treatment experiments
The thermal treatment process was carried out under N 2 , CO 2 and air atmosphere with a flow rate of 300 cm 3 min -1 in a fixed bed quartz tube reactor from room temperature to 1200 ºC at a heating rate of 20 ºC min -1 . 1 g (the precision is 0.0002 g) of coal sample was charged into a quartz boat.Then, the quartz boat with the coal sample was pushed into the constant temperature zone of the reactor.A thermocouple was placed in the center of the coal sample to measure the temperature.Before thermal treatment of coal, 10 min purge time for the reactor was used to assure the atmosphere (N 2 , CO 2 or air) purity.The release of Hg 0 from the thermal treatment of the coals was analyzed dynamically by coupling a temperature-programmed decomposition (TPD) unit with an on-line atomic fluorescence spectrometer (AFS) detector.The volatile products from the thermal treatment of the coals were swept into the AFS detector by purging gas continuously and the Hg 0 intensity is recorded by a computer.In this way, a dynamic Hg 0 release profile during coal thermal treatment can be obtained. 25The detection limit of Hg in this system is 100 ng N m -3 .The on-line mass spectrometry (MS) (Balzers QMS422) was used to monitor the volatile matter release during the thermal treatment of the coals.At the temperature of 1200 ºC, the boat with sample was moved quickly to the cold end of the reactor and cooled down under N 2 flow.The weights of the sample were weighed and recorded.The Hg contents in chars were analyzed.
Release ratio of elemental mercury is used to quantify the amount of Hg 0 released from coal, which is abbreviated as RRE and defined as: (1)   Release ratio of total Hg (RRT) is used to quantify the amount of total mercury released from coal, defined as: ( Volatile yield (VY) is used to evaluate the quantity of volatile matters released during the coal thermal treatment, defined as: (3)

Determination of mercury
The contents of Hg in the two coals and the chars were determined following the Chinese national standard (GB/T 16659-2008).Firstly, mercury in the coal sample was dissolved in HNO 3 -H 2 SO 4 solution and converted into Hg 2+ in the presence of V 2 O 5 .Then, the Hg 2+ is reduced to Hg 0 with the solution of KBH 4 .Finally, Hg 0 was detected by the atomic fluorescence spectrometer with the detection limit of 0.05 ng L -1 .The Hg 0 release profiles during the thermal treatment of the coals were obtained by the on-line AFS.

Results and Discussion
Dynamic release behavior of Hg 0 during thermal treatment of two coals under N 2 The dynamic Hg 0 release profiles versus the increase of temperature for these two coals were measured using TPD-AFS technique and the results are shown in Figure 1, which the intensity of all the profiles is normalized for easy comparison.
Figure 1 shows that the initial Hg 0 release from the two coals is around 150 ºC and most of the Hg 0 releases at temperatures below 600 ºC.6][27][28][29] Note that the profiles present the Hg peaks at similar temperature range for these two coals.For example, two typical peaks can be observed for the two coals.One peak was located in the range of 150 to 400 ºC and the other peak was located in the range of 500 to 600 ºC.It indicates the similar modes of occurrence of Hg in the coals, which possibly are organicbound and pyrite-bound Hg in coal. 30However, the minor peak at 800-900 ºC is only shown for coal 1, while it is not shown for 2. This result indicates the existence of different modes of occurrence of Hg in the coals.
Generally, the N 2 is an inert gas and cannot react with coal or other gas.Therefore, the Hg released profiles under N 2 shown in Figure 1 can be regarded as the thermal stability of the mercury in the coals.2][33] However, Figure 1 shows the Hg released peak at 800-900 ºC for coal 1.According to Guo et al. 30 the Hg released above 700 °C should be silicate-bound Hg in coal.In addition, the major mineral elements in the coals listed in Table 2 shows that 1 has higher content of SiO 2 and Al 2 O 3 than coal 2. Therefore, the Hg peak at 800-900 ºC for coal 1 is probably due to the influence of silicates or aluminosilicates. 34he amount of Hg released after TPD process for the two coals is listed in Table 3, in which the amount of Hg released as Hg 0 (RRE) was calculated based on AFS profile 25 and the amount of total Hg released (RRT) was calculated by comparing the mercury content in raw coal and char at 1200 ºC.The result shows that higher than 92% of total Hg in the coals releases in Hg 0 form under N 2 .Meanwhile, approximately 94-95% of total Hg in the coals releases out during thermal treatment under N 2 .Because the RRE is nearly equal to the RRT for the two coals used, it may be concluded that the Hg 0 is dominant form of Hg released from the coals during thermal treatment under N 2.

35
The previous study reported that the modes of occurrence of Hg in the coals can be characterized based on the profiles in Figure 1 and the detailed discussions were given in the literature. 30The Hg in coal can be separated  into four modes of occurrence of Hg: the organic-bound Hg, the silicate-bound Hg, the pyrite-bound Hg and the HCl-soluble Hg. 30 For coal 1, it contains all the four modes of occurrence of Hg whereas coal 2 contains three modes of occurrence of Hg without the silicate-bound released in the temperature range > 750 °C. 30namic release behavior of Hg 0 during thermal treatment of two coals under CO 2 The dynamic Hg 0 release profiles versus the increase of temperature for two coals under CO 2 atmosphere are shown in Figure 2. Also, the intensity of the profiles is normalized.
Similar to the release behavior of Hg 0 under N 2 , the profiles also show several peaks and the Hg 0 starts to release at about 150 ºC.Note that the shape of the peaks < 600 ºC under CO 2 is similar to that under N 2 except that the intensity of the corresponding peaks under CO 2 is slightly lower than that under N 2 .However, the peaks at 800-900 ºC under N 2 for coal 1 almost disappear after the thermal treatment under CO 2 .
The RRE and RRT under CO 2 are listed in Table 3.It shows that the amount of Hg 0 released under CO 2 counts for 73-74% of Hg in the coals, which is lower than that under N 2 .However, the amount of total Hg released counts for 96-98% of Hg in the coals and higher than that under N 2 at temperature range studied.It indicates that CO 2 atmosphere restrains the Hg 0 release and promotes the total Hg release to some extent. 36n fact, the thermal treatment of the coals under CO 2 < 800 °C is the process of pyrolysis due to the gasification of the coals that cannot occur at lower temperature range.Therefore, the behavior of Hg 0 release is similar to that under N 2 at lower temperature range.Generally, CO 2 can be produced as part of volatile matter during coal pyrolysis. 37,38Also, the result of MS verifies the production of CO 2 during pyrolysis of the coals.The CO 2 release behavior during thermal treatment of two coals under N 2 is shown in Figure 3. Theoretically, coal pyrolysis is a complex chemical reaction and CO 2 gas is part of the reaction product. 37When the additional CO 2 is introduced into the reaction system, it inhibits the reaction to a certain degree according to the theory of chemical reaction balance, leading to the decrease of the volatile matter produced during coal pyrolysis.Consequently, Hg 0 as part of volatile matter or reaction product during coal pyrolysis is also inhibited, resulting in a lower Hg peaks than that under N 2 .
The thermal treatment of coal under CO 2 > 800 °C is a process of coal gasification because coal can react with CO 2 at high temperature. 39,40And the coal gasification promotes the volatile matter release, leading to a dramatic increase of VY for both coals (see Table 3).It seems that the Hg 0 release should be promoted by the coal gasification because the coal matrix is destroyed and the Hg in it can easy release out during the coal gasification.However, contrary to what we suspect, the Hg 0 release even decreases.It might be caused by CO 2 .For example, CO 2 can react with C (C + CO 2 = 2CO) during coal gasification. 40Similarly, CO 2 could react with Hg 0 during coal gasification, resulting in a decrease of Hg 0 release. 41t should be pointed out that the VY for the two coals at temperature < 800 °C under CO 2 is less than that under N 2 whereas the VY at temperature > 800 °C under CO 2 is higher than that under N 2 .For example, the VY for coal 1 at 700 and 1200 ºC under N 2 is 25.38 and 30.84%, respectively, while that under CO 2 is 24.27 and 77.97%, respectively.This implies that the CO 2 atmosphere restrains the volatile matter release < 800 °C and promotes the volatile matter release > 800 °C.Dynamic release behavior of Hg 0 during thermal treatment of two coals under air The dynamic Hg 0 release profiles versus the increase of temperature for two coals under air are shown in Figure 4.The intensity of the profiles is also normalized.
Figure 4 shows that the profile of the peaks is different with that under N 2 and CO 2 atmosphere.In addition, the peak intensity under air is distinctly lower than that under N 2 and CO 2 atmosphere, which indicates a lower Hg 0 release under air.Note that the peaks in Figure 4 show similar profiles at 200-500 °C for both coals, possibly attributing to the similar modes of occurrence of Hg in the coals.However, the peaks at temperature > 500 °C under N 2 and CO 2 for these two coals almost disappear after the thermal treatment under air.This result implies that the Hg released at temperature > 500 °C under air is mainly oxidized Hg.
The RRE and RRT under air (in Table 3) show that the amount of Hg 0 released under air counts for 31-33% of Hg in coal and is distinctly less than that under N 2 and CO 2 .However, the amount of total Hg released is higher than that under N 2 and CO 2 .This result indicates that a large part of Hg 0 has been converted into oxidation state of Hg by the oxygen in the air. 42Table 3 shows that the VY under air is higher than that under N 2 and CO 2 .However, the higher VY under air does not promote the Hg 0 release because of the formation of the oxidized Hg during thermal treatment of the two coals under air. 42

Conclusions
A study was made to understand the release behavior of Hg 0 from two bituminous coals under different atmospheres during thermal treatment.The profiles of Hg 0 released present several peaks during thermal treatment of these two coals.With the different atmosphere used, the profiles are also different for the same coal, which indicates that the release behavior of Hg 0 is affected by the atmosphere used.Generally, the RRE is in the order of N 2 > CO 2 > air.The RRE for both coals is higher than 92% under N 2 , which is nearly equal to the RRT under N 2 atmosphere.This result indicates that the Hg 0 released is the dominant form during thermal treatment of these two coals under N 2 .The RRE is approximately 73-74 and 31-33% under CO 2 and air atmosphere, respectively, which is less than the corresponding RRT.This indicates that a part of Hg 0 has been converted into the oxidized Hg during thermal treatment of these two coals under CO 2 and air atmosphere.

Table
. The major mineral elements of the two coals detected by X-ray fluorescence analysis are shown in Table2.Vol.27, No. 12, 2016

Table 1 .
Proximate and ultimate analyses of the coals a By difference; V: volatile; A: ash; M: moisture; as: as received; daf: dry and ash free.

Table 2 .
Major mineral elements in the coals a a Dry basis.The Effect of Atmosphere on Elemental Mercury Release During Thermal Treatment of Two Bituminous Coals

Table 3 .
The amount of Hg released and VY during thermal treatment of two coals a Based on the AFS profile; b according to the mercury content in raw coal and char; c according to the mass of raw coal and char; RRE: release ratio of elemental mercury; RRT: amount of total Hg released; VY: volatile yield.Vol.27, No. 12, 2016