Ind. Eng. Chem. Res. 2004, 43, 4363-4368
4363
SEPARATIONS Mercury Removal from Flue Gas with Particles Generated by SO3-NH3 Reactions Joo-Youp Lee,† Soon-Jai Khang,*,† and Tim C. Keener‡ Departments of Chemical Engineering and Civil and Environmental Engineering, University of Cincinnati, Cincinnati, Ohio 45221
A lab-scale experimental apparatus was used to investigate mercury removal using particles generated from a simulated flue gas containing 10-40 ppb of gaseous elemental mercury at 178 °C. This operating temperature was chosen to represent the flue gas temperature after the air preheater in a typical power plant. The particles, mostly ammonium sulfates as confirmed by X-ray diffraction, were generated by the gas-phase reaction of SO3 and NH3 by maintaining 20-50 ppm SO3 with NH3/SO3 molar ratio of 2. The generated particles were captured by the glass-fiber filter which was also maintained at 178 °C. The results showed that as high as 49% of elemental mercury was captured by the particles. Introduction Among the air toxic metals addressed in the 1990 Clean Air Act Amendments, mercury from coal-fired utility plants is the hazardous air pollutant (HAP) of greatest concern because of its persistence and bioaccumulation as methyl mercury in the environment and its high neurodevelopment toxicity. The annual global mercury emission rate is estimated at 5000 tons per year and 80% of it is reported to result from anthropogenic activities.1 In the United States, about 3% of the global emissions is released, which amounts to about 150 tons per year. Of this, coal-fired combustors contribute approximately 46% (33% from utility boilers and 13% from commercial and industrial boilers). Municipal waste combustors and medical waste incinerators emit about 20% and 10%, respectively. Mercury emissions from medical waste incinerators and from municipal and hazardous waste combustors are already regulated in the United States. However, mercury emissions from coal combustion, which is estimated to be the largest source of anthropogenic mercury release in the U.S., are not currently regulated.1 It has been reported that a low concentration of gaseous mercury, on the order of 1 ppbv, exists in the flue gas when coal is burned. It can vary considerably over the range 3 to 70 µg/m3,2 even if more typically, emissions are reported to be about 10 µg/m3.3 Some of this variation is of course accounted for by mercury level in different coals, but the gaseous mercury concentration does not always correspond to the mercury content of the coal being burned, indicating a degree of mercury retention in the dust. However, the factors governing this degree of retention have not been fully investigated. It is important to understand the fate of mercury during combustion and in the flue gas to effectively * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Chemical Engineering. ‡ Department of Civil and Environmental Engineering.
control its emissions. It is generally known that mercury vapor is released in the form of elemental mercury, Hg0(g), during combustion, and that the elemental mercury may remain as a monatomic species or be transformed into oxidized forms (either Hg22+ or Hg2+) in the postcombustion environment. The oxidized forms of mercury are reported to be less volatile and more soluble in water than elemental mercury, and they readily react with other species and adsorb on particles, which potentially enables them to be captured in wet scrubbers and particulate control devices. However, the elemental mercury vapor is not effectively captured in conventional air pollution control devices because of its unique characteristics such as high volatility, insolubility in water, and nonreactivity with other species. From this point of view, dry sorbents have the potential to remove both elemental and oxidized forms of mercury. Therefore, sorbent injection method upstream of either an electrostatic precipitator (ESP) or baghouse would be one of the most promising methods for controlling mercury emissions from coal combustion, as virtually all coal-fired boilers are equipped with one of these two particulate control devices. ESPs are used in over 90% of the coal-fired power plants in the U. S. and the preferential escape of particles is in the 0.1-1.0 µm size. Fabric filters are used on the remaining units. Sorbent injection has advantages such as relatively easy retrofit and high applicability to both industrial and utility boilers. The conventional sorption method using bulk solid sorbents such as activated carbon relies on the mass transfer driven by the concentration gradient of mercury. Because of a low mercury concentration level in the flue gas, the mass transfer rate is very small, thus requiring a large number of very small sorbent particles. This limiting effect of diffusion is minimized by a very large number of very small particles in the flue gas and by allowing sufficient time for diffusion to take place from the bulk gas to particles. The most widely used and tested sorbents are activated carbons including
10.1021/ie0308386 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/19/2004
4364 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004
unpromoted and chemically promoted ones. This method showed a fairly good performance for municipal waste incinerators where mercury concentration is 1 or 2 orders of magnitude higher than that of coal-fired combustors. However, there are still problems associated with the use of activated carbons for mercury removal from flue gas: (1) activated carbons are general adsorbents, and cannot selectively capture mercury; (2) activated carbon has a poor capacity for mercury (carbon to mercury weight ratio of 3000 to 100 000:1); (3) good performance is obtained over a limited temperature range (typically below 150 °C) even for sulfur-impregnated carbons; and (4) they can be regenerated only a few times. In this work, the feasibility of an effective and economical method of removing mercury from flue gas utilizing the existing constituents of flue gas with a small amount of added SO3 or NH3 is experimentally investigated. It has been reported that a decrease in mercury concentration in the flue gas was observed around SCR unit by some field tests. Experiments were carried out for low and high concentrations of mercury, SO3, and NH3 gases at a fixed furnace temperature of 178 °C to examine the effect of particles on the mercury removal. The potential application of this method is also discussed. Acid Dew Point Estimation. Sulfur trioxide and water have a tremendous affinity for each other; when temperatures are lowered to the dew point, these two combine rapidly (less than one second) to form sulfuric acid.4 Despite the importance of the formation of sulfuric acid to the power generation industry, theoretical consideration of equilibrium data on low SO3 concentration has not been successful to allow good predictions of acid dew points. The currently available equilibrium data are empirically correlated and can be found in the literature4,5 as given below:
1000 ) 1.7842 + 0.0269 log PH2O TDP 0.1029 log PSO3 + 0.0329 log PH2O log PSO3 (1) where TDP is the acid dew point in K and Pi is the gas partial pressure in atmosphere. Reaction of NH3 with SO3. It has been reported that ammonia reacts directly with SO3 in the presence of water vapor to produce aerosol products below 235 °C6 or between 200 and 260 °C7. The NH3-SO3 reactions are fast and practically completed within the first 0.1 s. These reactions are as follows
2NH3 + H2O + SO3 f (NH4)2SO4(s)
(2)
NH3 + H2O + SO3 f NH4HSO4(s)
(3)
Because the products of the above reactions, ammonium sulfates and bisulfates, decompose above 235 °C, SO3 first reacts with H2O to produce sulfuric acid mist if SO3 is introduced into flue gas above the decomposition temperature. As mentioned earlier, SO3 has a tremendous affinity to react with H2O in less than one second to form sulfuric acid mist when temperatures are lowered to the dew point, depending on the concentrations of H2O and SO3.
H2O + SO3 f H2SO4(l)
(4)
The equilibrium calculation indicates that 97% of SO3 exists in the liquid form of H2SO4 with 100 ppmv SO3 and 14%(v) H2O at 235 °C. The liquid form of sulfuric acid (acid mists) reacts with ammonia only when the gas is cooled below sulfate decomposition temperature, 235 °C. Because the sulfuric acid mists grow through coalescence,
2NH3 + H2SO4(l) f (NH4)2SO4
(5)
NH3 + H2SO4(l) f NH4HSO4
(6)
it is important to produce fine aerosol particles as indicated by reactions 2 and 3, and avoid the reaction pathways of reactions 4-6. Therefore, it is desirable that SO3 and a stoichiometric amount of NH3 should be introduced into the gas stream whose temperature window is between acid dew points and the decomposition temperature of sulfates. Kiyoura and Urano8 have reported on the mechanism, kinetics, and equilibrium of the thermal decomposition of ammonium sulfate. They found that at temperatures greater than 170 °C ammonium sulfate releases ammonia and forms ammonium bisulfate according to the following reaction:
(NH4)2SO4(s) T NH4HSO4(s) + NH3v
(7)
The decomposition of ammonium sulfate between 100 and 170 °C was expressed by the reaction
2(NH4)2SO4(s) T (NH4)3H(SO4)2 + NH3v
(8)
where sulfate decomposed into bisulfate, and both sulfate and bisulfate combined to form triammonium hydrogen sulfate, (NH4)3H(SO4)2. They also found that ammonium bisulfate could release water vapor above 150 °C by forming ammonium pyrosulfate.
2NH4HSO4(s) T (NH4)2S2O7 + H2Ov
(9)
There is always ammonia slip (usually 99%) into the catalyst block. The catalyst bed was operated by injecting a small amount of diluted SO2 with O2 overnight at 380 °C. The conversion of SO2 into SO3 approached up to 70-80% after about 24 h with a continuous SO2 injection. The conversion reached almost 99% in about 48 h. The activation step seems to be a crucial factor to achieving
a satisfactory catalytic performance. The outlet line of the catalytic reactor was heated to 90 °C with a heating tape (Barnstead International) to prevent the condensation of SO3. EPA Method 820 was used to measure SO2 and SO3 concentrations at the inlet and outlet of the catalytic reactor. The sampling train employed impingers to separate and measure H2SO4 mist including SO3, SO2, and H2O separately by the barium-thorin titration method from emission sources. Sulfuric acid was captured by the first impinger and separated in 80% (v/v) 2-propanol solution, which inhibits the oxidation of SO2. Sulfur dioxide was captured in the second and third impingers containing 3% (v/v) H2O2 solution. In these two impingers, the collected SO2 was oxidized to H2SO4. It was found that almost all SO2 (upper limit of SO2 concentration ) 3%) couldbe collected efficiently in two midgets, each containing 15 mL of 3% H2O2 at a rate of 1 L/min for 20 min sampling time. Sulfate ion was titrated with barium perchlorate to produce insoluble barium sulfate.19 A metallochromic indicator, thorin, was added to the sample. The titration of sulfate was conducted in 80% 2-propanol to reduce the solubility of BaSO4. As the titrant is added to the sample, the barium preferentially reacts with sulfate. When the entire sulfate is reacted, barium reacts with the free yellow indicator to form the pink barium indicator complex (the sodium salt of 1-(o-arseno-phenyl-azo)-2naphthol-3: disulfonic acid), signifying the end-point of the titration. In situ generated aerosols turned out to be very soluble in water, and those particles would dissolve into water if they were exposed to even a very small amount of water. Therefore, extra care was taken to prevent water condensation throughout the system by heating the feed lines leading to the aerosol reactor. Ammonia gas was directly added to the aerosol reactor. A mercury permeation tube was used to maintain a constant rate of mercury release. A desired release rate of mercury was achieved with nitrogen carrier gas by
4366 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 Table 1. Summary of Mercury Capture Efficiencies at 40 ppbv Inlet Hg0 Concentrationa run number and experimental conditions
sampling time (min)
Aerosols generated outside the aerosol reactor (50 ppmv SO3, 140 ppmv NH3, 40 ppmv Hg0) 1. With glass woll packing + glass fiber filter 20 2. With glass wool packing 20 3. With glass wool packing 60 Aerosols generated inside the aerosol reactor 4. With glass fiber filter (50 ppmv SO3, 140 ppmv NH3, 40 ppm Hg0) 5. Measurements taken after 2 h of aerosol generation; With glass fiber filter (50 ppmv SO3, 140 ppmv NH3, 40 ppm Hg0) (1) 2000 ppmv SO3, 4000 ppmv NH3, 40 ppb Hg0 (2) 50 ppmv SO3, 140 ppmv NH3, 40 ppb Hg0 (3) Only Hg0 was injected (No SO3 and NH3 injection) a
capture efficiency (%) 14 11 24
20 20
19 49
20 20 20
23 43 35
Furnace temp. 178 °C.
controlling the temperature of the permeation tube immersed in a water bath, the temperature of which was controlled by a heater controller. Mercury mass balances were taken around the system (at the outlets of the mercury permeation tube and the filter system) and at least 90% mercury mass balance was achievable. EPA Method 101A21 was employed to analyze elemental mercury captured in in situ generated particles on the filter and in the sampling impingers. The elemental mercury captured in particles on the filter was extracted by the following acid digestion procedure. The filter, including particulate matters, was placed in 20-40 mL of absorbing solution containing 4% (w/v) KMnO4 in 10% (v/v) H2SO4 solution. The beaker was heated on a steam bath until most of the absorbing solution was evaporated. Then, 20 mL of concentrated nitric acid was added to the beaker and heated at 70 °C for 2 h with a watch glass covering. The solution was filtered through Whatman No. 40 filter, and the filtrate collected was then analyzed by a cold vapor atomic absorption (CVAA) system (model MAS50B, Bacharach, Inc.). A set of three impingers containing the same absorbing solution used to trap elemental mercury on the filter was used to capture elemental mercury in the outlet gas stream. The solutions collected from impingers and used to wash residual mercury from the filter holder were filtered through Whatman No. 40 filter paper to remove brown manganese dioxide precipitate. The filter was allowed to digest the brown precipitate with 25 mL of 8 N HCl for at least 24 h at room temperature. A small amount of sodium chloride-hydroxylamine solution in 1 mL increments was added to the impinger solutions until the purple color of KMnO4 solution disappeared. The impinger solutions were then analyzed to determine elemental mercury concentration by CVAA after adding 5 mL of SnCl2 solution to reduce mercury oxidized by KMnO4 solution back to an elemental form. It was confirmed that, in most cases, more than 90% mercury was captured in the first impinger. A borosilicate glass tube (ACE Glass Inc.) of 5-cm i.d. and 60-cm length was used for generating in situ aerosols by SO3-NH3 reaction (in most cases, stoichiometric ratio of SO3/NH3 ) 2). A mixing cup was placed inside the aerosol reactor to provide a better mixing of all inlet constituents. The reactor temperature was maintained at 178 °C and monitored with a thermocouple installed in the middle of the reactor, well above an acid dew point, which is estimated as about 131 °C using eq 1 for a flue gas with 20 ppmv SO3 and 2.5% water concentrations throughout all the experiments. The typical compositions of the gas mixture at the reactor inlet were 20 ppmv SO3, 40 ppmv NH3, 10 ppbv Hg, 12% CO2, 3% O2, 2.5% H2O, and nitrogen (balance). A PTFE in-line filter with 47-mm filter diameter
(Savillex Corp.) was installed to capture aerosols after the aerosol reactor in the same furnace. Glass fiber filters without organic binders with nominal pore size diameters of 0.7 µm (Millipore Corp.) were used. Total flow rate of 2.246 L/min (measured at 21 °C) passed through sampling impingers placed at the exit of the furnace and provided a residence time of about 9 s through the aerosol reactor at 178 °C. The process line from the filter outlet to the inlet of sampling impingers for mercury measurement was also heated using a heating tape to prevent the condensation of water and mercury vapor. The gas flow rate was measured at the outlet of impinger system using a bubble flow meter. The aerosol reactor and filter system were cleaned with water before every experiment, and blank measurements at the outlets of a mercury permeation tube and the filter system were performed to ensure that there was no residual mercury in the system. No ammonia was injected until a reliable mercury mass balance was obtained. Results and Discussion Several experiments were carried out to examine the effectiveness of mercury capture by in situ generated aerosols as products of NH3-SO3 reactions and to understand the capture mechanism. The configuration of aerosol injection, filter, and the concentrations of SO3, NH3, and Hg were varied to study the mercury capture efficiency. The particles collected on the filter were analyzed by X-ray diffraction to identify the reaction products as shown in the following section. Experimental Results with 40 ppbv Inlet Hg0 Concentration. A series of experiments was conducted by varying the position of aerosol generation, mercury sampling time, filter type, and inlet concentrations of SO3 and NH3 to examine the factors affecting the capture of elemental mercury. The results are summarized in Table 1. The first three data (runs 1-3) shows mercury capture efficiencies when aerosols are injected after being generated outside the aerosol reactor. Comparing run 1 with run 2, a glass wool prefilter packed in the filter holder in front of the regular filter showed a slightly higher capture efficiency of mercury. In addition, as mercury sampling time increased from 20 to 60 min (runs 2 and 3), the capture efficiency also increased more than 200%. Particles were generated outside (runs 1-3) and inside (runs 4-6) the aerosol reactor to explore the effect of in situ generated particles on the efficiency of mercury capture. Compared to run 1, higher capture efficiency was obtained in run 4 using in situ generated particles although only a glass fiber filter was used. Preformed particles were not as efficient as the in situ generated
Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4367 Table 2. Summary of Mercury Capture Efficiencies at 10 ppbv Inlet Hg0 Concentrationa capture efficiency (%) measured by run number and experimental conditions 7. Injection of 100 ppmv SO3 and 200 ppmv NH3 for 5 h followed by 20 ppmv SO3 and 40 ppmv NH3 (1) SO3 and NH3 injection (2) only Hg was injected (no SO3 and NH3 injection) average 8. Injection of 100 ppmv SO3 and 200 ppmv NH3 for 4 h followed by 20 ppmv SO3 and 40 ppmv NH3 (1) SO3 and NH3 injection (2) only Hg was injected (no SO3 and NH3 injection) average 9. Injection of 400 ppmv SO3 and 800 ppmv NH3 for 3 h followed by 20 ppmv SO3 and 40 ppmv NH3 10. Injection of 400 ppmv SO3 and 800 ppmv NH3 for 1 h followed by 20 ppmv SO3 and 40 ppmv NH3 11. Injection of 400 ppm SO3 and 800 ppm NH3 12. Injection of 20 ppm SO3 and 40 ppm NH3 a
sampling time (min)
gas impingers
digestion of particles/filter
20 50
27 35 31
23
30
25 32 29 30
33 18
60
42
20
60 60
14 40
13 21
20 20
Furnace temp. 178 °C.
particles. A significant improvement was achieved in run 5 after populating aerosol particles in the reactor by injecting 50 ppm SO3 and 140 ppm NH3 into the reactor for 2 h prior to mercury addition. Additional particles in the system appear to contribute to this improvement. A series of experiments was performed with a glass wool packing in run 6 to study the effect of filter cake and in situ generated particles. Mercury capture efficiency at each step was obtained by analyzing the impinger solution taken every 20 min. High concentrations of SO3 and NH3 gases were injected with 40 ppb Hg for the first 20 min in run 6-1 and then 50 ppm SO3 and 140 ppm NH3 were injected with 40 ppb Hg for another 20 min in run 6-2. A significant increase in the capture efficiency of elemental mercury was obtained by combining the in situ particle generation and filter cake (particles accumulated in run 6-1 on the glass wools). The last test was done with no particle generation in the system for an additional 20 min, which resulted in 35% capture efficiency. It seems that mercury capture efficiency was not sensitive to the concentration levels of SO3 and NH3 if enough particles were generated, and the particles already collected on the filter partially retained the mercury adsorption capabilities. Experimental Results with 10 ppb Inlet Hg Concentration. In this set of experiments, the inlet concentration of elemental mercury was reduced from 40 to 10 ppb to simulate a more realistic mercury level in a flue gas while varying the degree of preformation of aerosol particles in the reactor prior to mercury addition. Mercury capture efficiencies for the injection of 20 ppm SO3, 40 ppm NH3, and 10 ppb Hg are shown in Table 2. At the beginning of experiments for runs 7-10, high concentrations of SO3 and NH3 (a stoichiometric ratio of 2) were injected into the system for a period of time (1-5 h) to examine the effect of mercury adsorption on particles on the filter as in the previous run 5. After high concentrations of SO3 and NH3 were injected into the system for a period of time, low concentrations of SO3 and NH3 were injected for about 30 min to stabilize the system before elemental mercury vapor was injected. Then, mercury vapor was added to the system and mercury vapor measurement through sampling impingers was also started. The mercury capture efficiency was measured by both the gas sampling impingers and the digestion of particles on the filter. The difference between these two measurements could be attributed to mercury adsorption on particles
sticking on the wall surfaces of aerosol reactor, connection tubes, and filter holder, or adsorption on the wall itself. The net mercury capture efficiencies based on the particle-digestion method were in the range of 13-33%. The experimental runs 11 and 12 were conducted to investigate the effect of inlet concentration of SO3 and NH3 on mercury capture efficiency by injecting Hg, SO3, and NH3 simultaneously (no preformation of particles before mercury measurement). The result shows that high concentration of SO3 and NH3 does not give much difference on mercury capture efficiency, indicating that the inlet concentration level of 20 ppm SO3 and 40 ppm NH3 is high enough to capture elemental mercury vapor. When the experimental results obtained here are compared with those conducted with 40 ppb inlet mercury concentration, inlet elemental mercury concentration seems to have a strong effect on the total elemental mercury captured. This effect was also observed in the experiments carried out by Krishnan et al.22 with 30 and 60 ppb inlet mercury concentrations for elemental sulfur-impregnated activated carbons, whereas it was found to be insignificant for thermally promoted activated carbons. The particles collected on the filter were also analyzed by X-ray diffraction, and the diffraction patterns for the collected particles with and without mercury injection are shown in Figure 2. The clearly identifiable reaction product by the SO3-NH3 reaction was ammonium sulfate with three strongest peaks at 2θ ) 20.5, 20.3, and 29.3. Considering the amount of NH3 at a stoichiometric ratio of 2 injected into the system, it is quite reasonable that the major final reaction product appears to be ammonium sulfate. When the X-ray diffraction patterns obtained from mercury injection are compared with those obtained from no mercury injection, elemental mercury does not seem to be discernible in the particle sample collected. However, CVAA analysis confirms that the sample contains mercury contents in the particles. Therefore, it seems that X-ray diffraction may not be an appropriate method to identify a small amount of elemental mercury captured in ammonium sulfate particles. The amount of elemental mercury captured in particles was approximately 13 µg Hg/g sulfate particles for run 10. The amount of particles generated was measured by taking a difference in mass between a blank filter installed before an experiment and collected particles with the filter after the experiment and estimated for run 10 by assuming that only ammonium sulfate was formed as reaction products of
4368 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004
Figure 2. X-ray diffraction results of collected particles for (a) SO3-NH3 reaction only and (b) Hg + SO3-NH3 reaction experiments at 178 °C.
SO3 and NH3 based on the X-ray diffraction results. The amount of particles measured was 20.7 mg and that estimated was 14.7 mg. Conclusions An effective and economical method to capture gaseous elemental mercury utilizing the existing constituents of flue gas with a small amount of added SO3 or NH3 was experimentally studied. The experimental results showed that in situ generated particles as reaction products of SO3 or NH3 had a sorption capacity to capture elemental mercury without further treatment on particles. With the inlet mercury concentration of 40 ppb, the mercury capture efficiency varied between 11 and 49%. With the inlet mercury concentration of 10 ppb, the capture efficiency varied between 13 and 33%. Increasing concentration levels of SO3 and NH3 beyond 20 and 40 ppm, respectively, did not improve the mercury capture efficiency. Further experiments are planned to study the capture of elemental mercury with respect to temperatures of the aerosol reactor and filter, sampling time, and added components in a simulated flue gas such as fly ash. Nomenclature TDP ) acid dew point, K Pi ) gas partial pressure, atm Subsripts v ) volume
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Received for review November 25, 2003 Revised manuscript received April 2, 2004 Accepted April 30, 2004 IE0308386
