资源描述
Experimental study on the absorption behaviors
of gas phase bivalent mercury in Ca-based wet
flue gas desulfurization slurry system
AbstractiGas phase oxidation and catalytic oxidation of element mercury (IIg°) to bivalent mercury (Hg2')were proposed to improve the mercury removal efficiency in the wet flue gas desulfurization (WFGD) system. However, the re-emission of Hg°, generated by the reduction of absorbed Hg2", would lead to a damping of the total mercury removal efficiency. In this paper, the absorption and reduction behaviors of bivalent mercury in the Ca-based WFGD slurry were evaluated in our purpose-built device. According to our experimental results, the slurry chemistry (such as CaS〇3 content, SO42 , Cl and pH value) had a strong influence on the reduction of absorbed bivalent mercury. And the inlet concentrations of SO2 and 〇2 contribute little to the mercury absorption. Within the typical pH value range of 4.5-5.5, about 70% of inlet bivalent mercury was converted to Hg°. The re-emission of Hg would be greatly retarded with the increase of [SO42 ] due to the formation of HgS〇4 or Hg3〇2S〇4. Moreover, it was found that Cl would also inhibit the reduction of bivalent mercury through the ligands reactions between Cl and Hgz .
1. Introduction
Mercury, due to its persistence, bio-accumulation and neurological toxicity, had received considerable attention from environmental engineers and environmental protecting institutions [1]. According to the conservative estimation of Environmental Protection Agency (EPA), the exposure reference dose of 0.1 fig (mercury)/kg (body weight)/day was justified to protect against harmful neurological effects during fetal development and early childhood [2].
Coal combustion was considered as the largest source of anthropogenic mercury emission. Mercury existed in the flue gas mainly in three forms: element mercury (HgO), oxidized mercury (Hg2一）and particle-bound mercury (Hg (P)) [3]. In the combustion zone, the mercury in the coal was first evaporated and turned to the elemental form (Hg°), which was then partly oxidized to oxidized mercury (Hg2 ) by some flue gas components such as IIC1, S〇2, NOx and fly ash as passing through the down stream of the combustion zone. Some mercury was associated to fly ash or particulates and turned to particle-bound mercury (Hg (P)), which was easily removed by dust collection [4, 5]. Since the mercury in the flue gas was mainly in element form (Hg°) and was hardly captured by flue gas cleaning equipment, the majority of mercury was emitted.
Active carbon injection method on mercury capture from coal derived flue gases had been widely investigated [6, 7]. However, the major drawback of its application was the rather high operating cost of using activated carbon. Many efforts had been done to improve the economy of mercury removal process such as the use of inexpensive powdered carbon sorbents [8-10]. Another way was to remove mercury and S〇2 simultaneously in the WFGD scrubber, which had been received many research interests nowadays [11-13]. Since mercury in flue gas mainly existed in element form, its solubility in the aqueous solution was very low. Tn general, the mercury removal efficiency by wet scrubbing varied from 40 to 80% depending on the coal type and combustion conditions. Thus, most of researches were focused on the oxidation technology of Hg° to Hg2+ by catalytic oxidation [14-18], photochemical oxidation [19-21] and gas-phase oxidation [22-25] to enhance its solubility, thereby improved the removal efficiency. However, little work had been conducted to identify the absorption of bivalency mercury and its reduction by the scrubber slurry. Some former
researchers had found the Ilg2' reduction and re-emitted to the flue gas during their researches [26-28]. However, detail information was not given due to the lack of information about the slurry’s chemical and physical properties as well as the complexity of the reactions. This drawback made it difficult to predict possible consequent re-emission and removal efficiency of mercury in FGD system.
The experimental work in this paper was carried on by assuming there was 100% mercury oxidation by the oxidation technology in gas phase. The main objective of the investigation was to examine the effects of slurry conditions (CaS〇3 content, pll value, SO42 , Cl , and slurry temperature) of the typical Ca-based FGD system on the bivalent mercury removal and its reduction behaviors. Further more, the contributions of main gas phase components (SO2 and O2) to bivalent mercury adsorption were evaluated as well.
2. Experimental and test methods
2.1. Experimental apparatus and materials
2.1.1. Experimental apparatus
In order to investigate the Hg2+ absorption and its reduction by the Ca-based WFGD slurry, a purpose-built device was designed, as shown in Fig.
Sampling
*l'cmpcralurc control device FIr. 1. The lab-device for Hg2* absorbing experiment.
The system is composed of three parts: the Hg2+ generation oven, the flue gas mixing oven and the Hg2f bubbling absorbing reactor. The bubbling reactor was of a volume of 400 mL. A ceramic prilling spray was attached to the end of the stem for generating tine bubbles and increasing the mixing of gas, liquid and solid phases.
The IIg2f generation oven was made of glass and located in a silicon oil bath to maintain a desired temperature of 60。(1 A Teflon stir-bar was immersed inside the bath and the silicon oil was stirred to keep a uniform temperature.
The flue gas mixing oven, which was also made of glass, was located in a different silicon oil bath. After passing through the flue gas mixing oven, the Hg2+-containted simulated flue gas and the diluent gas were well mixed and heated up to the desired temperature. By adjusting the flow rate of the Hg2f-containted gas and the diluent gas, the designed flowrate and mercury concentration were both obtained. All the connectors and lines were made of glass or Teflon.
At the beginning of each experiment, 300mL of absorption slurry was added to the Hg2+ bubbling reactor and then heated up to the predetermined temperature. 2 L/min of synthetic gas containing80|xg/m3 was introduced into
SI
1
iii
the Hg2+bubbling reactor. By determining the Hg24 and Hg° concentration of the outlet simulated flue gas, the effect of different parameters on absorption and its reduction by the slurry were evaluated. The exhaust gas was decontaminated by passing through two washing-bottles and one adsorbent column. The bottles contained 200mL 4% (w/v) KMn〇4 with 10% (v/v)H2S〇4 solution and the adsorbent column contained 200mL C\ impregnated AC. After that the gas was directly emitted to outdoor.
2.1.2. Materials
Calcium hydroxide, concentrated sulfuric acid, concentrated nitric acid, potassium chloride and potassium dichromate were supplied by Sinopharm Chemical Reagent Co.Xtd. Concentrated hydrochloric acid was bought from Hangzhou Chemical Reagent Co., Ltd. All the chemicals mentioned above were of analytical grade. Mercuric chloride and calcium sulfite were bought from Shanghai Shenbo Chemical Co., Ltd. and the purities were higher than 99.9%. The water used in our tests was ultrapure water and supplied by the Zhejiang University.
2.2. Test methods
2.2.1. Hg2+ and Hg° measurement
The Hg2+ and Hg° concentration were determined using the Ontario Hydro method. Flue gas passed though four impingers containing 20mL absorption solution. The first and second impingers contained Imol/L KC1 with 5% (v/v) IIC1 solution, which were used to absorb Ilg2 . Ilg°was absorbed by the 0.05% (w/v) KMn〇4 with 5% (v/v) H2S〇4 solution in the third and fourth impingers. After absorbing, several drops of 10% (w/v) were added to the
absorption solution until slightly purple color was stably obtained. All samples were then analyzed for mercury concentration using cold-atomic fluorescence spectroscopy (AFS-230E,Bei jing Kechuanghaiguang Instrument Co., Ltd.).
0.05% NaBH4 with 1% NaOH solution was selected as the reduction reagent in our experiment according to the instrument introduction. The maximum measurement error during our study was within 2%.
2.2.2. Calculation
In this article, the Hg2 removal efficiency ( ti Hg2' ), the Hg2+ reduction rate (T| Hg° ) and the total mercury removal efficiency ( r\ Hg(tot)) were calculated as follows:
"„#(*) - i〇〇
lOO
(2)
C3)
where *n IIg2 is the IIg2 removal efficiency, Tillg0 the Hg2 reduction
rate by the slurry, irillg(tot) the total mercury removal efficiency, cllg2 out the Hg2+concentration in the outlet gas, cllg2+in the Ilg2' concentration in the inlet gas, and cHg°out is the Hg° concentration in the outlet gas.
3. Results and discussion
3.1. Effect of slurry chemistry
3.1.1. Effect of CaS03 content
In the Ca-based WFGD slurry, CaS〇3 was used as the main S〇2 absorbent during its daily operation. Its effect on IIg2+ absorption and reduction was shown in Fig. 2.
100
90
80
70
60
50
AO
30
20
10
0
£
Fig. 2. EfFea of CaS〇3 content on Hg2+ absorption and reduction. Experimental conditions: gas flow ratc=2L/min: [Hg2*]=80p,g/m3; pH value=5.0; slurry volume-300 mL: gas temperature** 110 C: slurry temperature=40 C.
It could be seen that the CaS〇3 content in the slurry had neglected influence on Hg2 absorption, but had significant effect on the Ilg2' reduction. When the CaS〇3 content was low (<2%), increasing the CaS〇3 content would inhibit the IIg2+ reduction and the total mercury removal efficiency increased from 45% to 71%. While the CaS〇3 content was higher than 2%, the increase of CaS〇3 content would enhance the Hg2 r reduction. As a result, the total mercury removal efficiency decreased to 48% (8% CaS〇3).When the CaS〇3 content was very low(2%), as the low solubility of CaS〇3 in liquid, most CaS〇3 was suspended in the solution. The bivalent mercury in the slurry solution might be adsorbed to the Ca-particles (CaS〇3 particles) [29], which made the reduction more difficult. But when the CaS〇3 content was higher than 2%, the adsorption of bivalent mercury did not increase apparently. But S(IV) concentration increased and the effect of S(IV) on reduction dominated the reaction, which
enhanced the mercury reduction. The possible reduction mechanism of Hg2+ by S (IV) could be described as follows:
Hg(s)2^ ^ Hg(L)2^
⑷
Hg(L)2+ +SO32 HgS03
(5)
HgS〇3 — Hg‘十 S03
(6)
Hg — + S032- — Hg° + S03
(7)
+ Hg • 一 Hg° + Hs(l>2 +
(8)
3.1.2. Effect of CaS04 content
CaS〇4 was also the main component of Ca-based WFGD slurry. Thus, the effect of CaS〇4 content had been also investigated and the results were shown in Fig. 3.
From this figure, it could be found that the increasing of the CaS〇4 had little influence on both bivalent mercury absorption and its reduction in the slurry. Though the solubility of CaS〇3 was lower than CaS〇4, the adding of CaS〇4 would slightly lower the SO32 concentration. It might inhibit the formation of HgS〇3 by reaction of the aqueous Hg2’ with SO32 in Eq. (5)， which was reported to be unstable and immediately decomposed to Ilg' and SO3' [30,31]. The S032' in the slurry was much more than the absorbed Hg2+ and the slight decrease of SO32" due to the adding of CaS〇4 could be ignored to the formation of HgS〇3 for its instant reaction. The reduction of Hgf to Hg°
6.5
The pH of the slurry
As shown in Fig. 4, within the pH value range (4.5-5.0) of the typical Ca-based FGD system, about 70% of total divalent mercury was converted to Hg° and re-emitted to gas phase. As a result, the total mercury removal efficiency was only about 20%. About 30% of the total bivalent mercury was converted to Hg°when the pH value was higher than 5.5 or lower than 4.0. The S(IV) in solution existed mainly in the form of SO2 II2O under the low pH value (<4.0), which might not react with Hg2 and enhance the reduction. When pH value >5.0, the S(IV) in the slurry was mainly in the form of SO32 . The re-emission of Hg° to the gas-phase mainly came from the decomposed of the HgS03 by the reaction of SO32" with Hg2 in the slurry according to the former
could be performed through Eq. (7) or Eq. (8). Though the lower of SO32 might inhibit the Eq. (7) reduction, the reduction could go through Eq. (8) and did not affect the Hg2" reduction process. So the later simulated Ca-based FGD slurry was prepared by CaS〇3，not the mixture of CaSO；? and CaS〇4, for the convenience of the experiment.
3.1.3. Effect of initial pH value and temperature
Fig. 4 showed the effect of the initial pH value on Hg2+ absorption and reduction in the slurry.
{o/JAOU-sealaAoluaa:
200000000 9^87654321
30 35 40 45 50 55 60
Temperature ("C )
As shown in Fig. 5, the temperature had no effect on Hg2+ absorption during low temperature range (30-40 °C), but the Hg2* reduction was enhanced when the temperature was higher than 40 °C. This result was consistent with the results of the previous study [32, 34]. At temperatures below 45 °C, the reaction Eq. (6) between S03 and HgSO was promoted, inhabiting the decomposition ofHgS03.
researchers [Eqs. (5)-(8)] [31,32]. But when the pH value was in the rang of 4.5-5.0, the S(IV) was mainly in the form of HS03 . Both HgS03 and HgS03H+ were formed by the reaction of Hg2+ with HSO3 in the solution. HgS〇3H+ would also decompose to form Hg° according to the previous work [33]. And the decomposition of HgS〇3H+was kinetically significant and its rate constant was much higher than that of IIgS〇3, thereby accelerating the reduction reaction.
The effect of slurry temperature on Ilg2' absorption and reduction by CaS〇3 was studied in our experiment and the result was shown in Fig. 5.
{sergpeoPAOUl^
K)gog<20goo
9<876I5<43<21
>QOOOQOOOO >91876514321
HgS03 + S〇32-^ Hg(S〇3)22-
3.1.4. Effect of SO/' concentration
(9)
0.000 0.005 0.010 0.015 0.020 0.025 0.030
The conccntraiion of SO.2' (mol/L)
, »>.i.
It was found that the S〇4~ concentration had significant effect on Hg reduction. When the concentration of SO42 increased from 0 to 0.01 mol/L, the Hg'T reduction ratio decreased from 45% to 10%. When the S〇4~~ concentration further increased, the value of Hg2' reduction ratio remained stable. As a result, 85% of total mercury was removed by the slurry.
The oxidized mercury was considered to be absorbed by the solid particles presenting in the slurry after the S〇2-CaO reaction [29]. And it might have two possible ways to prevent Hg2* reduction: (i) it was more difficult to reduce bivalent mercury to element mercury after Hg2 ' adsorbed and wrapped by slurry particles and (ii) the Hg*"^ reacted with SO42 to form a stable compound, HgSCU.The IIgS〇4 was greatly hydrolyzed in an acidic solution to form Hg302S〇4, which was readily in dilute mineral acids and existed in the solution, as un-dissociated molecules [35]. The mercury in the WFGD byproduct was reported to be stable [36], which would partially support this conclusion. The
1一 # |
In
展开阅读全文
浙ICP备2021020529号-1 | 浙B2-20240490 
