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Impacts of Sulfur Oxides on Mercury Speciation and Capture by Fly Ash during Oxy-fuel PC Combustion Lawrence Phoa Belo, Liza Kay Elliott, Rohan J. Stanger, and Terry F. Wall Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01078 • Publication Date (Web): 30 Aug 2016 Downloaded from http://pubs.acs.org on September 8, 2016

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Manuscript

Impacts of Sulfur Oxides on Mercury Speciation and Capture by Fly Ash during Oxy-fuel PC Combustion Lawrence P. Belo1,2, Liza K. Elliott2, Rohan J. Stanger2 and Terry F. Wall2,* 1

Chemical Engineering, De La Salle University, 2401 Taft Avenue Manila 1004 Philippines 2

Chemical Engineering, The University of Newcastle, New South Wales 2308 Australia

KEYWORDS: Mercury, SO2, SO3, competition, fly ash, oxy-fuel combustion

ABSTRACT: Coal fired utility boilers is the single largest anthropogenic source of mercury emissions. Mercury is a naturally occurring trace element in coal and when combusted may exist in three different forms; Hg0, Hg2+ or Hg particulate.

During oxy-fuel combustion, impurity

concentrations, such as SOx, NOx and Hg, can be up to 4 times higher than concentrations in air combustion. Increased mercury concentration is of concern because mercury is known to attack aluminium heat exchangers required in the compression of CO2. Due to the elevated concentrations during oxy-fuel conditions, interactions of Hg and SOx was investigated in this study in order to verify if there is any competition between SOx and Hg. The effect of Hg, SOx, H2O and temperature on the native capture of Hg by fly ash was assessed using a quartz flow 1

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Manuscript reactor packed with fly ash to simulate a bag filter. Doubling Hg in the system from 5 µg/Nm3 to 10 µg/Nm3 doubled the amount of Hg captured in the fly ash from 1.6% to 2.8% and increases the amount of Hg unaccounted from 5.8% to 18.1%. Increased SO2 decreased the proportion of Hg0 in the flue gas. Temperature in the bag filter was found to have a large impact on the mercury capture by fly ash. As temperature was increased from 90°C to 200°C, Hg0 in the flue gas was found to increase from 77.9% to 98.3%, indicating better capture of Hg at lower temperatures.

1. INTRODUCTION Mercury is a naturally occurring element found in air, water and soil. It is released by natural and anthropogenic sources.

The US Environmental Protection Agency (EPA) considered

mercury to be a hazardous pollutant for both humans and the environment.

1

Apart from its

toxicity, it has received much attention due to its bio-accumulation tendency, difficulty to control and persistence in the atmosphere (0.5 to 2 years residence time).

2

The single largest

anthropogenic source of mercury is coal-fired utility boilers. 1,3,4 Coal contains trace amounts of mercury as contaminants depending on its rank, and is commonly associated with sulfur compounds in coals.

5,6

Due to the volatility of mercury and its compounds, when coal is

combusted, Hg in the fuel completely evaporates and is converted to elemental mercury vapour with typical concentrations of 0.3 – 35 µg/Nm3. Some reported values have reached 70 µg/Nm3. 2,4,6,7

Along with mercury emissions, coal-fired utility boilers also emit significant quantities of CO2. In 2010 alone, out of approximately 30 Gigatonnes CO2 emitted, 41% came from the energy sector of which 71% came from burning coal and peat.

8

In order to address the problems with 2


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Manuscript CO2 several technologies are being developed to capture, sequester or utilise CO2. Among these oxy-fuel combustion is seen as the most promising, however it is relatively new and many problems still remain unsolved.

9

During oxy-fuel combustion, N2 is eliminated from the flue

gas by combusting the fuel in oxygen mixed with recirculated flue gas to lower the temperature of the boiler and make up for the airborne N2 during conventional air-fired combustion. However due to this recycling stream, concentrations of the oxy-fuel impurities (e.g. CO2, SO2 and H2O) are increased by a factor of 4.

6,8,10-13

Higher Hg concentrations are also expected. In

oxy-fuel combustion, mercury still represents an environmental problem but the higher Hg concentrations pose an additional operational / technological problem; Hg0 vapour in the flue gas can accumulate in the CO2 compression and processing units and corrode the brazed aluminium heat exchangers used in cryogenic liquefaction. 6,9 Understanding mercury speciation and factors affecting its distribution in the flue gas is important to be able to properly address and appropriate the cleaning options suitable for its removal. Mercury exists in three states; elemental mercury vapour (Hg0), oxidised mercury species (Hg2+) and particulate-bound mercury (HgP)

14

. The speciation is said to be dependent on the

concentration of sulfur and halogens in the coal,

1,6,7,15

flue gas temperature and

composition 1,7,8,16. Hg0 is released upon volatilisation at about 200°C and continues throughout coal combustion.

It can then be oxidised to Hg2+ via homogeneous (gas phase) and/or

heterogeneous (gas-solid) oxidation reactions during post-combustion and as the flue gas cools. 5,6,8,17

The oxidised mercury may either be gaseous (g) or solid (s) inorganic mercuric

compounds, Hg2+X (where X are anions, e.g. Cl2 (g), SO4 (s), O (g,s), S (s)). Zygarlicke, 2 and Monterroso, et al.

17

2,5

Galbreath and

detailed the possible transformations of mercury in a

combustion system, which are represented in Figure 1. Table 1 presents the appearance of 3


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Manuscript different Hg species typically resulting from the combustion of coal as summarised by Monterroso, et al.

17

In fly ash, the order of mercury appearance as per thermodynamic

equilibrium calculations is HgCl2 < HgS < HgO.

17

Highlighted in the table are the Mercury-

Sulfur compounds.

[Figure 1]

The gaseous Hg2+ species may either stay in the gas phase or be captured by the fly ash together with any solid Hg2+ as HgP. The gaseous Hg2+ is soluble and has a tendency to associate with particulate matter

18

and get captured as HgP. Hence, Hg2+ and HgP can easily be captured

through conventional air pollution control devices (APCDs) like electrostatic precipitators (ESP), fabric filter (FF) or bag houses, scrubbers, flue gas desulfurisers (FGDs) and selective catalytic reducers (SCR). 14,17,18 Amongst the three mercury species, Hg0 has been found to be the most difficult to handle due to its abundance,

19

chemical inertness,

8,19

insolubility in water,

8

high volatility

8

and lower

bonding energy on sorbent surfaces. 8,20 Sorbent injection using activated carbon (AC) has been viewed as one of the most effective methods for Hg0 removal. 18 The presence of unburnt carbon (UBC), i.e. coal chars or carbon in combustion fly ash (FA), is favourable in mercury capture. Injecting fly ash in bag filters has been shown to remove 13% ~ 80% of mercury at temperatures of 135°C ~ 160°C. 21 In studies by Ren, et al. 21 relating Ca-based sorbents and mercury capture revealed that the presence of SO2 positively influenced the active sites increasing the capture by 15% ~ 20%.

4


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Manuscript [Table 1] A study by Wang, et al.

18

presented evidence of increasing mercury removal with increasing

sulfur content in the coal. The presence of sulfur and chlorine could oxidise Hg0 (g) into Hg2+ by the following reactions. 18

SO2 ( g ) + O2 ( g ) + Hg 0 ( g ) → HgSO4 ( g , s)

(1)

HgCl2 ( g ) + SO2 ( g ) + O2 ( g ) → HgSO4 ( s ) + Cl2 ( g )

(2)

SO2 in the flue gas has been shown by several researchers6,18,21-25 to be an important factor in the oxidation of Hg0 and can overshadow the effects of Cl in the oxidation. At temperatures > 700 K, the major oxidised species is HgCl2 but at temperatures < 590 K, the major species is HgSO4 (s) 18 and as SO2 in the flue gas increased, Hg2+ increased and Hg0 18 decreased. Until 2015, the Callide Oxy-fuel Project (COP) was the largest operating oxy-fuel plant in the world and represented a significant milestone in the scale-up of oxy-fuel technology; combining electricity generation, retrofit potential and CO2 purification.

20,22,23

The COP boiler block

circuit contained no dedicated SOx, NOx or Hg control, but some Hg capture was expected in the fabric filter along with fly ash removal.

23,26

Fabric filters are known to provide enhanced Hg

removal in comparison to Electrostatic Precipitators (ESPs) and are a feature of Australian power stations where coal-S (and SO3) is low. In the COP combustion system, the SOx and mercuryladen flue gas passes through the bag filter and is expected to be partially captured by the fly ash. 27

The purpose of this paper is to determine the extents of competition between Hg2+/HgP and SOx capture by the inorganic portion of fly ash during pulverised coal combustion in oxy-fuel and air combustion conditions. In particular, this paper has focussed on interactions occurring 5


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Manuscript specifically around the Fabric Filter, rather than in the post-combustion conditions which have received significantly more attention.

20

The fly ash used in this study were generated in a

German-Australian collaboration presented in previous publications.

6,10,12

This study

specifically focuses on the roles of temperature, gaseous Hg concentration, SOx and water vapour concentrations in the speciation and capture of Hg on fly ash.

2. EXPERIMENTAL A schematic diagram of the experimental setup is shown in Figure 2 and is divided into five major parts: (1) The SO3 generator where the gas mixture SO2, O2, H2O vapour and N2/CO2 are fed using Brooks mass flow controllers. A 2 L/min (25°C, 1 atm) gas mixture is preheated at 80°C (±10 K) using heating tape and then fed into a 1” I.D. quartz tube flow reactor controlled by a 45 cm electrically heated furnace. The reactor contains a ~2 g packed bed of fly ash acting as a solid catalyst supported by quartz wool and operated at a wall temperature of 700°C (±10 K) for SO3 production based on a preliminary experiments for SO2 to SO3 conversion; (2) A gas stream containing mercury (as Hg0) is produced by passing 1 L/min (25°C, 1 atm) N2 across a mercury permeation tube containing liquid mercury held at a constant temperature in a Labec NBCT7S water bath. The permeation rate of Hg0 was regulated by controlling the vapour pressure of Hg0 in the tube (supplied by Valco Instruments Co., Inc.) which was achieved by controlling the temperature of the water bath. The Hg0-laden gas is then fed after the 700°C SO3 generator to avoid any reaction/conversion of Hg0 with the generator ash and mixed with the gas before entering the simulated Bag Filter; (3) A Simulated Bag Filter consisting of a 1” I.D. quartz tube with a second packed bed of fly ash (~2g) supported by quartz wool. The temperature was maintained at 90°C to 200°C by a HTS/Amptek Heavy Amox Insulated Duo heating tape; 6


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Manuscript (4) Gas exiting the simulated bag filter passes through an insulated PFA line, kept as short as possible to limit heat loss and possible condensation before the gas entered a series of impingers in an ice bath. For measurement of all the mercury present in the gas stream, referred in this study as the total Hg (Hgtot), an impinger series of SnCl2–NaOH–Coil–Blank was used, shown by the symbol ① in Figure 2. The first impinger was a freshly prepared aqueous 0.05M SnCl2/0.1M HCl reduction solution; a modified concentration of the solution used by Spörl, et al.,6 which converts any Hg2+ present in the gas stream to Hg0. The gas which still contains high concentrations of SO2 is then scrubbed using a NaOH solution, protecting the mercury analysis from contamination by sulphur28. The coil and blank impingers were used to eliminate condensing moisture from the gas, again required to protect the mercury analyser. If measurement of Hg0 (rather than Hgtot, Hg0+Hg2+) in the gas stream was required, the SnCl2 was removed from the impinger series using 3-way valves (labelled as ② in the Figure). Any Hg2+ in the gas stream was collected in the NaOH impinger as Hg2+ is soluble. The Hg2+ was determined by difference between Hgtot and Hg0 experimental results. (5) The gas mixture then passes through an Ohio Lumex RA-915+ Mercury analyser that measures online Hg0 based on coldvapour atomic adsorption with mercury lamp as the radiation source at a wavelength (λ) 254 nm 29

. The signal is then converted to Hg0 concentration. The exhaust gases then pass through an

activated carbon bed before venting.

[Figure 2]

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Manuscript All tubing used in the experiments was PFA, selected for its limited interaction with mercury. Exposure of the gas to metallic surfaces was minimised where possible but the mercury injection into the gas stream and union connections on impingers were stainless steel. Experiments were conducted using fly ash obtained from experiments completed at the 20 kWth pilot plant located at the Institute of Combustion and Power Plant Technology (IFK), University of Stuttgart in Germany through a German-Australian collaboration. Details of the conditions at which the fly ash were obtained are discussed elsewhere, however the fly ash was produced under air and oxy-fuel firing similar to COP conditions and importantly contain almost no unburned carbon.

6,10,12

Table 2 present the physical characteristics of the fly ash used in the

Hg adsorption and competition experiments.

[Table 2]

The experimental design for the study to test the competition between SOx and Hg in fly ash is presented in Table 3. Hg0 injections at 5 and 10 µg/m3 were selected to simulate typical concentrations found in practical oxy-fuel combustion. 6 The Hg0 permeation rate was calculated using the Antoine equation with coefficients and temperature correction factors provided by the supplier and the concentration produced by the mercury permeation tube was measured by the mercury analyser prior to starting the experiments. Each experimental run lasted 30 minutes. After completion of the experiment, the ash in the simulated bag filter was removed and analysed for Hg and S. The Hg adsorbed in the ash was determined by placing the ash in the Ohio Lumex RA-915+ with RP-M-324M mercury analyser. A quartz ladle, serving as a sorbent trap, is inserted into the analyser’s thermo-catalytic 8


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Manuscript conversion chamber heated to 680°C where bound mercury (Hg0, Hg2+, HgP) is thermally decomposed and converted to Hg0 to be measured by the analyser.

29

This was converted to

Particulate Hg (HgP) as given in equation (3) where Hgin ash represents the measured amount of Hg in the filter ash in ng. Hginput represents the total amount of mercury introduced into the gas stream prior to the simulated bag filter over the entire experimental time (ng/Nm3 Hg).

% Hg P =

Hgin ash ×100% Hginput

(3)

Any Hg that is neither measured (i.e. Hg0, HgP) nor calculated (i.e. Hg2+) is collectively termed in this study as unaccounted Hg and is calculated by difference and given by equation (4).

(

Unaccounted Hg = 100% − % Hg 0 + % Hg 2 + + % Hg P

)

(4)

[Table 3]

3. RESULTS AND DISCUSSION 3.1 Effect of Hg concentration The effects of increasing Hg0 vapour concentration was evaluated (Figure 3). It can be seen from the plot that the HgP captured by the fly ash associated with doubling the Hg0 input increases the amount of Hg0 which may have been converted to Hg2+ and captured by ash from 1.6% to 2.8%, roughly double the amount. Similarly, increasing the partial pressure of Hg0 increased the proportion of unaccounted Hg from 5.8% to 18.1%.This mercury is not retained in the ash but may be adsorbed on surfaces throughout the experimental rig, expected to be the

9


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Manuscript section between the heated bag filter simulator and the impinger train where the gas undergoes cooling.

[Figure 3]

3.2 Effect of SOx as SO2/SO3/H2SO4 on Hg capture Studies

6,7,9,20

have shown that Hg competes with SOx during capture in fly ash. Wilcox et

al. 7 noted that sulfur can have either a positive or a negative impact in oxidising and capturing Hg0 on activated carbon depending on its form, species and presence. In order to investigate the effects of different forms of SOx in the flue gas on the Hg capture of inorganic fly ash, this paper divided the effects of SOx as follows: (1) purely SO2, no SO3 present, where the gas mixture (SO2, O2, H2O, N2) does not pass through the SO3 generator bed of fly ash; (2) SO2, SO3 and H2SO4 wherein the gas mixture (SO2, O2, SO3, H2SO4) passes through a fixed bed of fly ash to generate SO3 (and H2SO4 upon coming in contact with water vapour, based on results by Belo, et al.

30

) and (3) SO2 and SO3 only, where water was absent to test the effect of the absence of

H2SO4. For all these experiments the simulated bag filter (BF) temperature was set at 150°C. Figure 4(a) presents the effect of SO2 input on the Hg capture of fly ash when no SO3 is present, i.e. no generator ash was used. With the input Hg0 fixed at ~5 µg/Nm3, increasing the concentration of input SO2 from 0 ppm to 1000 ppm shows that Hg0 measured by the analyser decreased. The amount of HgP did not vary greatly ranging from 1.2% to 2.1% of the input Hg, while the amount of unaccounted Hg increased significantly. Kellie, et al.

31

has reported that

when SO2 concentration in the flue gas increased, the observed Hg0 concentration decreased.

10


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Manuscript An experiment with and without water vapour at the same SO2 input concentration was also investigated (3rd and 4th bars Figure 4(a)), it showed that purely addition of H2O vapour does not have much effect on the capture of Hg on fly ash, HgP. The HgP captured by the fly ash were 2.3% when water is not present and 1.2% with water in the system. However, the addition of 2%v H2O vapour to the system, increased the unaccounted Hg from 13.4% and 17.3%. Using the SO3 generator, 30 the effect of varying SO3 concentration has been investigated and the result is plotted in Figure 4(b). It can be seen from the plot that increasing SO3 concentrations from 0 to 15 ppm produced three notable points: (a) the HgP in all three levels of SO2 (SO3) are similar in level, ranging from 1.5% to 1.9%; (b) as the SOx concentration is increased, the unaccounted Hg increases from 4.7% to 16.1%; and (c) at 15 ppm SO3 a Hg2+ of 7.1% was measured. It could be deduced that enhanced Hg0 oxidation can be observed at higher SO3 input concentrations. The capture behaviour exhibited in this plot is similar to works completed by other investigators

9,21,32

where in the absence of SO2, the Hg0 adsorption effectiveness of the

sorbents is lower; whilst with SO2, the adsorption was enhanced.

[Figure 4]

Fernandez-Miranda, et al.

9

noted that in combustion atmospheres, Hg0 oxidation may have

occurred via the reaction in equation (5):

2Hg 0 ( g ) + 2SO2 ( g ) + O2 ( g ) → 2HgO ( g ) + 2SO3 ( g ) fast

(5)

However in this study where baseline Hg0 was kept constant at ~5 µg/Nm3, the presence of 10 ppm SO3 (Figure 5) did not change the oxidation of mercury compared to the equivalent system without SO3 (667 ppm SO2) (Figure 4), but when the SO3 concentration increased to 11


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Manuscript 15 ppm, 1000 ppm SO2, a significant oxidation amounting to 7.1% Hg2+/HgTot occurred which was not present when no SO3 was available. The unaccounted Hg increased as SOx input was increased in both Figures 4(a) and (b).

[Figure 5]

Producing SO3 and cooling the gas in the presence of water will produce H2SO4. Therefore it is not clear whether SO3 or H2SO4 is producing the enhanced oxidation of Hg0 observed. Figure 5 shows the effect of purely SO2/SO3 and SO2/SO3/H2SO4 on mercury. A practical oxy-fuel SO2 input (667 ppm) giving rise to SO3 of 10 ppm was used. Wilcox et al.

7

stated that Hg in its

atomic state acts as a base and can readily react with acidic sites on carbon surfaces. Similarly, introducing water vapour in the system to produce H2SO4 from SO3 provides acid sites on the fly ash surface by the reaction in equation (6). 9

SO3 ( aq, g ) + H 2O ( l , g ) → H 2 SO4 ( aq, g )

fast

(6)

Once Hg0 is oxidised and becomes acidic, it will compete with other acidic gases on the basic sites that are available on carbon surfaces. However, the fly ash used in this study is very low in UBC (< 0.1%w) and so the effect of fly ash as an adsorbent (with its inorganic mineral content) is observed. 7 It can be seen that with a similar SOx input, both systems have roughly the same Hg0 measured but the BF fly ash without water vapour in the system has captured roughly 4 times HgP (from 1.6% to 6.8%).

Fernandez-Miranda, et al.

9

compared the effect of different water 12


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Manuscript concentrations simulating air and oxy-fuel combustions and found that Hg0 oxidation was favoured by the presence of more H2O vapour. Hence, without H2O the Hg measured as HgP may have been Hg0 captured by the fly ash while with H2O there could have been oxidation but not fully captured by the fly ash. With the presence of H2O, it can also be seen that the amount of unaccounted Hg has increased. One more important point in this plot is that without H2O, there is no unaccounted mercury. In fact, the total is slightly higher than 100%, which is due to the errors associated in the online measurements and errors in analytical techniques.

3.3 Effect of Bag Filter Temperature In order to test the saturation of fly ash on Hg capture, different temperatures were tested to check whether temperature would have an effect on the capture capacity of the bag filter fly ash. Figure 6 shows the Hg captured in fly ash (HgP) at different temperatures. The mercury is measured as Hg0 because the analysis is completed by heating the ash above 680ºC where Hg is converted to Hg0 and released as a gas. The plot shows that, with similar Hg0 feed and atmosphere, lower temperatures correspond to better capture by fly ash. The lowest temperature, 90°C, is below the acid dew point and has resulted in a significant increase of mercury collection by the ash. Acid dew point was estimated for SO3 ~ 10 ppm and H2O = 2% vol using ZareNezhad’s correlation.

33

One run, completed at 150°C, was performed at an extended time

of 60 minutes, it was found that there is only 21% better capture (6.03 ng Hg/g FA compared to 4.98 ng Hg/g FA for exposure of 30 minutes) indicating the fly ash is approaching saturation at 60 minutes.

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Manuscript [Figure 6]

Figure 7 shows the effect of varying bag filter temperatures on the capture of Hg on fly ash. It can be noted that enhanced capture can be observed at lower temperatures. As temperatures increased from 90°C to 120°C capture by fly ash (HgP) was decreased from 9.1% to 2.8% and eventually to just 1.9% at 150°C and 1.2% at 200°C. Hg0 and HgP measurements also showed consistent trends in decreased capture as the temperature is increased.

[Figure 7]

Each of the figures shown in this article show increasing proportions of mercury that cannot be accounted for with increasing conversion to Hg2+ or collection of the Hg by particulate matter when water is present. This suggests the same mechanism that results in the collection of mercury by particles is also affecting the loss of mercury in the experiment and this mechanism could possibly be associated with H2SO4 formation or condensation.

It is likely that

condensation of mercury species on particles will also result in condensation on tubing. The PFA lines between the simulated bag filter and impinger setup in Figure 2 were washed with DDI waster and the resulting liquid was sent out to a third party laboratory be tested for possible sulfate and Hg condensation, test results detected no Hg (< 0.0001 mg/L) in the lines, nor did any of the impinge solutions SnCl2 and NaOH appear during the test.

However, the T-

connection where the Hg0 and oxidant gases passed through prior to the simulated bag filter 14


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Manuscript showed a measurable reading indicating that although the lines were all heated and kept above the acid dew point (> 200°C), some Hg was lost and may have formed oxidised Hg salt, e.g. HgSO4, during the runs in the Swagelok connection. 4. CONCLUSIONS

The SOx and mercury interaction and speciation in fly ash during coal combustion was investigated experimentally using Hg0 injection and selected flue gas impurities and a simulated bag filter using a low carbon fly ash. The following conclusions were drawn: Pilot scale tests indicated that switching from air to oxy increased the proportion of Hg2+/Hgtotal in the flue gas. Increasing Hg (double) in the flue gas prior to the bag filter increases the proportion of the HgP in the fly ash but increases the amount of unaccounted Hg as well. Increase in SO2 showed decrease in proportion of Hg0 in the flue gas and in the fly ash, but increases the amount of unaccounted Hg and is recommended for further studies. Additionally, an increase in SO3 (and H2SO4) resulted in significant conversion of Hg0 to Hg2+ and a better mercury capture by fly ash. The bag filter temperature was seen to have a negative correlation on Hg capture. An increase in BF temperature by 60°C (from 90 to 150°C) caused a decrease in HgP capture by 20%. Overall, sulfur oxides (SO2, SO3 and H2SO4) were found to enhance oxidation of elemental mercury to oxidised mercury. The presence of SO3 was found to lessen the Hg adsorption on fly ash. Temperature has a negative impact on Hg capture by fly ash.

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Manuscript ACKNOWLEDGMENTS

The authors wish to acknowledge financial assistance provided by Xstrata Coal Low Emissions R&D Corporation Pty Ltd; and the financial assistance provided through the Australian National Low Emissions Coal Research and Development (ANLEC R&D). ANLEC R&D is supported by Australian Coal Association Low Emissions Technology Limited and the Australian Government through the Clean Energy Initiative. The Institute of Combustion and Power Plant Technology (IFK), University of Stuttgart, Germany for the collaborative research initiative.

AUTHOR INFORMATION

Corresponding Author:

*Terry F. Wall: [email protected]

Notes:

The authors declare no competing financial interest.

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Manuscript REFERENCES (1)

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Tables Table 1.

Typical mercury species found in coal combustion showing initial and final

temperatures of appearance including the temperature at which maximum concentrations were obtained. 17,34

Mercury compound HgCl2 Hg2Cl2 HgBr2 HgS (black) (metacinnabar) HgS (red) (cinnabar) HgSO4 Hg2SO4 HgO

Temperature at maximum concentration (°C) 120 ± 10 80 ± 5 and 130 ± 10 110 ± 5 205 ± 5 and 245 ± 5

Initial temperature of appearance (°C) 70 60 60 170

Final temperature of appearance (°C) 220 220 220 290

310 ± 10

240

350

540 ± 20 280 ± 10 505 ± 5

500 120 430

600 480 560

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Tables Table 2. Characteristics of the Fly Ash Used in the Experiment.

Fe2O3 content (wt %, dry basis) Unburned Carbon (wt %, dry basis) Mean diameter, d10 (µm) Mean diameter, d50 (µm) Mean diameter. d90 (µm) Initial Hg0 content (ng/g fly ash) BET (N2) surface area (m2/g of fly ash)

6.55 < 0.1 2.33 7.45 20.70 5.34 0.9455

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Tables Table 3. Configurations for the study on SOx versus Hg in fly ash during combustion. Inlet gas Hg0 SO2 (SO3) H2O Bag Filter Temp 3 Experiment µg/m ppm %v °C 5, 10 667 (10) 2 150 1: Variable Hg0 2 150 5 0 (0), 667 (10), 1000 (15) 2: Variable SOx 5 667 (10) 0, 2 150 3: Variable H2O 5 667 (10) 2 90, 120, 150, 200 4: Variable BF Temp NOTE: The SO3 conversion and setup used in the experiments have been discussed in detail in previous work.30

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Figures

Figure 1. Routes of Hg transformation in a coal combustion system. 2,17,20,34

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Figures

Figure 2. Schematic of the experimental rig used for the SO3 and Hg competition.

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Figures

100% 5.8%

% Hg speciation by analyser

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Energy & Fuels

95% 1.6%

Unaccounted 18.1%

90%

Hg2+

85% 80%

Hg(P) Hg(0)

92.6%

2.8%

75%

79.1%

70% 5

10 Hg0 input, ug/m3

Figure 3. Effect of increasing Hg0 concentration on fly ash capture. Input: Hg0 = varied 510ug/Nm3, SO2 (SO3) = 667 (10) ppm, O2 = 3.33%, H2O = 2%v, balance N2. BF Temp = 150°C

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Figures

Figure 4. (a) Effect of SO2 input and (b) Effect of SO2/SO3 input on the Hg capture of fly ash. Input: Hg0 ~ 5 µg/Nm3, 0–1000 ppm SO2, 3.33% O2, with (w) and without (w/o) 2%v H2O, balance

N2.

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Figures 100%

% Hg speciation measured by analyser

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6.8%

5.8%

0.5%

1.6%

95%

Unaccounted Hg(P) Hg2+ Hg(0)

90% 95.8%

92.6%

85%

80%

0

2 H2O input, %v

Figure 5. Effect of H2O input in the fly ash capture. Input: Hg0 ~ 5 µg/Nm3, SO2 (SO3) = 667 (10) ppm, O2 = 3.33%, H2O = 0 and 2%v, balance N2. BF temp = 150°C.

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Figures

Figure 6. Mercury captured/retained in fly ash at different temperatures. Input: Hg0 = 5 µg/Nm3, O2 = 3.33%, H2O = 2%v balance N2.

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Figures 100% % Hg speciation measured by analyser

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8.3% 12.2%

5.4%

0.5% 1.2%

Unaccounted

1.9%

Hg(P)

2.8% 1.9%

90%

Hg2+ 98.3%

9.1%

Hg(0)

92.6%

80% 0.8%

86.9%

77.9%

70% 90

120

150

200

Temperature, °C

Figure 7. Effect of bag filter temperature in the fly ash capture. Input: Hg0 = 5 µg/Nm3, O2 = 3.33%, H2O = 2%v balance N2.

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Impacts of Sulfur Oxides on Mercury Speciation ... - ACS Publications

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