Simultaneous Removal of SO2 and NO from Flue Gas with Calcium

The optimum condition for simultaneous SO2/NO removal was studied, including reaction temperature, O2 concentration, and relative humidity in the flue...
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Ind. Eng. Chem. Res. 2006, 45, 6099-6103

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Simultaneous Removal of SO2 and NO from Flue Gas with Calcium-Based Sorbent at Low Temperature Hu Zhang,† Huiling Tong,* Sujuan Wang, Yuqun Zhuo, Changhe Chen, and Xuchang Xu Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Tsinghua UniVersity, Beijing 100084, China

The calcium-based sorbent for simultaneous removal of SO2/NO was prepared with KMnO4 as additive. The activity of sorbent was studied individually in a fixed bed at low temperature. The experimental results showed that KMnO4 could highly enhance the sorbent ability for NO capture. It was found that temperature rise could improve SO2 capture, but could not influence NO removal so distinctively. The presence of water vapor in the gas could prominently improve the sorbent’s ability to capture SO2 and NO, and an optimal relative humidity existed for NO removal. O2 and KMnO4 were found to play an important role in NO removal. The optimum condition for simultaneous SO2/NO removal was studied, including reaction temperature, O2 concentration, and relative humidity in the flue gas. XRD and IC analysis indicated that SO2 was absorbed as sulfate with KMnO4 present and as calcium sulfite with KMnO4 absent. It was further deduced from the experimental results that NO was first oxidized into NO2 and then was removed by reaction with calcium hydroxide and calcium sulfite into nitrate and nitrite. 1. Introduction Sulfur dioxide and nitrogen oxides are the main emissions released from a coal-fired power plant. The air pollution legislation has become stricter in recent years, which requires the electric power industry to remove these emissions to a large extent. Current typical flue gas desulfurization (FGD) processes are wet scrubbing, spray drying, and semi-dry CFB-FGD (circulating fluidized bed flue gas desulfurization). And SCR (selective catalytic reduction) is the dominating technology for NOx control. Those technologies have been commercially put into practice because of high reliability and high removal efficiency. However, applying those technologies separately to remove SO2 and NOx in one plant will require a very high capital cost. Hence, it is necessary to find alternative technologies that could reduce SO2 and NOx at acceptable costs.1 The last 20 years was a crucial period for the development of combined removal SO2/NOx technology; nearly 70 processes appeared.2 Those technologies can be divided into two main groups according to their mechanism, one is that of combined SO2/NOx removal and the other one is that of simultaneous SO2/ NOx removal. The former includes those combining the separated FGD process and denitrification process into one system, such as NFT, DESONOX, and activated coke process. The latter means those technologies which can reduce SO2/NOx in one process and with the same sorbent, such as calciumbased sorbent process, SNRB (SOx-NOx-ROx BOX), NOXSO, electron beam process, and corona discharge process. Among them, the calcium-based sorbent process, removing SO2/NOx with the same sorbent at temperature between 60 and 120 °C, is regarded as a competitive technology for its lower equipment costs and cheaper sorbent, which can be installed after the precipitator of the coal-fired boilers. The study on simultaneous SO2/NOx removal with calciumbased sorbent started in the 1980s. In the traditional dry and semi-dry FGD process, researchers found the sorbent could * Corresponding author. Tel.: +86-106-278-8668. Fax: +86-106277-0209. E-mail: [email protected]. † E-mail: [email protected].

capture a certain NOx while removing SO2 from the flue gas.3-5 Therefore, NOx removal technologies at low temperature were studied.3,6-8 Some research focused on removing NO that were 90% of the NOx in the flue gas. But it was difficult to get high removal efficiency for restriction of NO’s inertial chemical characteristics. The others focused on removing nitrogen dioxide (NO2)7,8 since NO2 is more soluble in water than NO and it can easily react with alkali sorbent, such as Ca(OH)2. But an additional process is needed to oxidize NO into NO2 before applying the technology. Some research has been conducted to improve the NO removal efficiency by modifying calcium-based sorbents characteristics while the SO2 removal is not apparently affected.9,10 In this study, calcium-based sorbent was prepared by adding KMnO4 or other additives. The experiments were undertaken in a fixed-bed reactor to assess its ability to capture SO2 and NO in the flue gas at low temperature. The influence of parameters such as temperature, relative humidity (RH), and O2 concentration in the flue gas on NO/SO2 capture efficiency was studied. Based on experimental results, the mechanism for NO removal with a calcium-based sorbent was proposed. (Details of the RH control method are given in the Supporting Information.) 2. Experimental Section 2.1. Preparation of the Sorbent. The basic sorbent materials were reagent-grade calcium hydroxide and coal fly ash (weight composition: SiO2-48.45%, Al2O3-36.64%, Fe2O3-5.12%, CaO2.86%, MgO-1.02%, and another minor compound-5.91%). The additives used were reagent-grade NaOH, Na2SO3, and KMnO4 individually. To prepare the basic sorbent, Ca(OH)2 was mixed with coal fly ash at a weight ratio of 1:3. Then a 200.0 g dry mixture was added into the beaker with 1000 mL of deionized water. The slurry was heated to 90 °C and maintained for 9 h with continuous stirring. And then the sample was filtered and dried at 100 °C in the atmosphere until the weight no longer changed. The modified sorbent was prepared by reslurrying 5.0 g of basic sorbent with 0.25 g of additive at ambient temperature in 100 mL of deionized water, stirred continuously for 20 min, and then dried at 100 °C until no weight change.

10.1021/ie060340e CCC: $33.50 © 2006 American Chemical Society Published on Web 07/28/2006

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Table 1. Main Properties of Solids

Table 2. Experimental Conditions

material

BET surface area (m2 g-1)

mean particle diameter (µm)

silica sand fly ash Ca(OH)2 basic absorbent KMnO4 modified absorbent

0.34 1.47 6.2 15.14 19.04

340.3 73.1 10.4 111.2 95.7

In the experiment, the following sorbents were used: basic sorbent, modified sorbent by 10% KMnO4, 10% NaOH, and 10% Na2SO3 (weight ratio). To analyze the product composition with ion chromatography (IC), pure Ca(OH)2 modified by 10% KMnO4 was also used in the experiment. Quartz sand was used to mix with the solid reagents to prevent channeling and agglomeration in the reactor. The main properties of the solids are listed in Table 1. 2.2. Apparatus and Experimental Procedure. The experiments to remove the SO2 and NO simultaneously were conducted in a fixed-bed reactor under atmospheric pressure (see Figure 1). The cylindrical reactor (30 mm in diameter, 160 mm in height) was made of stainless steel with a sample holder. The reactor was submerged in the water bath, the temperature of which was regulated by a controller. The simulated flue gas was made of SO2, NO, O2, and N2, which was supplied by highpressure cylinders. Water vapor was supplied by N2 flow passing through a water humidifier (shown in Figure 1), so the desired relative humidity (RH) could be achieved by controlling the temperature of the humidifier and gas flow through it. Mass flow meters were used to control each gas flow. A bypass line across the reactor allowed the synthetic flue gas to stabilize before flowing into the reactor. All the tubes for water vapor transport were wrapped with heating tapes (temperature 110 °C) to prevent condensation. Water in the flue gas out of the reactor was removed by a cold trap before going into the gas analyzer (shown in Figure 1). The gas analyzing systems consisted of a Thermal Electron chemiluminescent NO-NO2-NOx analyzer (Model 42c) and a Thermal Electron pulsed fluorescent SO2 analyzer (Model 43c). The experimental errors mainly came from gas flow measurement, gas analysis, and temperature control. Under the experimental conditions, their errors were less than 2%, 3%, and 0.1 °C, respectively. The Brunauer-Emmett-Teller (BET) method was used to measure the surface area of the sorbents and products, the X-ray diffraction (XRD) method to analyze the chemical composition of products, and the IC method to detect the amount of sulfate, nitrite, and nitrate in the spent sorbent and then mass balance analysis was performed.

Figure 1. Schematic chart of respective system for SO2/NO simultaneous removal.

SO2 inlet concentration (ppmv) NO inlet concentration (ppmv) O2 (%) relative humidity (%) reaction temperature (°C) sorbent (g) quartz sand (g) gas flow rate (N mL min-1)

500 200 0-10 0-80 70-90 0.5 10.0 1300

The experiment conditions are listed in Table 2. The removal efficiency of SO2 and NO was calculated as follows:

ηSO2 ) (CinSO2 - CoutSO2)/CinSO2 ηNO ) (CinNO - CoutNO)/CinNO where CinSO2, CoutSO2, CinNO, and CoutNO represent respectively the SO2 and NO concentration in the flue gas at the inlet and outlet of the reactor. 3. Results and Discussion 3.1. Effect of Additives on NO Removal. The ability to remove NO is the key factor in evaluating a certain additive in simultaneous SO2/NOx removal process. Figure 2 shows the effect of three additives on NO removal. It was found that NaOH or Na2SO3 additive had little effect on NO removal, but KMnO4 could apparently enhance the sorbent’s capture ability for NO in the flue gas. Our experimental results indicate that NaOH and Na2SO3 cannot capture NO directly. However, previous researchers reported that NaOH and Na2SO3 were effective in improving the calcium-based sorbent’s capture ability for NO in the flue gas.3 Different from that in our paper, their additives were added into the sorbent at the beginning of hydration. It meant that additives can improve fly ash dissolution and act with fly ash by pozzolanic reaction to produce certain active species in the sorbent preparing period,3 which may be effective in NO removal. 3.2. Effect of Temperature on SO2/NO Removal. Temperature is an important operating factor in desulfurization and denitrification process. To find an optimal temperature window for SO2 and NO simultaneous removal, it is necessary to explore the temperature influence on SO2 and NO capture, respectively. Figure 3 shows the experimental results of SO2 and NO removal efficiency under different reaction temperature; the sorbent used in the experiment was basic sorbent modified by 10% KMnO4.

Figure 2. Effect of additives on NO removal. Reactor conditions: 0.5 g of sorbent, 5% additive, 80 °C temperature, 50% RH, 500 ppm SO2, 200 ppm NO, 5% O2, 1300 mL/min gas flow.

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Figure 3. Effect of temperature on (a) SO2 and (b) NO removal. Reactor conditions: 0.5 g of 10% KMnO4-modified sorbent, 50% RH, 500 ppm SO2, 200 ppm NO, 5% O2, 1300 mL/min gas flow. Table 3. Mass Comparison of Total S and N between Gas and IC Analysis of Spent 10% KMnO4-Modified Ca(OH)2 Sorbenta reaction temp (°C)

IC-S (×10-5 mol)

gas-S (×10-5 mol)

IC-N fixed (×10-5 mol)

gas-N removed (×10-5 mol)

60 70 80

60.91 19.04 21.40

64.09 20.42 22.13

9.60 3.90 6.13

10.29 3.97 6.59

a Reaction conditions: 0.5 g of sorbent, 15.4% H O, 500 ppm SO , 200 2 2 ppm NO, 5%O2, 1300 mL/min gas flow.

It can be seen that when the relative humidity of flue gas is unchanged, the removal of SO2 was enhanced with the temperature rise, while the removal of NO was not so distinct to temperature change as that of SO2. This probably could be explained by the double effect of reaction temperature on SO2 and NO removal. On one hand, temperature rise could enhance the chemical reaction rate and the ionic diffusion rate; on the other hand, high temperature would reduce water accumulating and gas dissolving on the sorbent surface, and baffle SO2 and NO oxidation, because they are exothermic reactions. Thus, for SO2 removal, the positive effect of temperature, rather than the negative effect, may play a major role, and for NO removal, both sides may be in equilibrium. It is interesting that many previous reports on the temperature effect are inconsistent. In the same temperature range, Paul Chu3 reported that the temperature increase could enhance the SO2 removal but decrease the NO removal, Zhao Yi10 reported the SO2/NO simultaneous removal efficiency was cut down with temperature increase. However, Sakai’s6 result was opposite to Zhao Yi’s report. It should be mentioned that the relative humidity of flue gas was changed with temperature (the volume ratio of H2O fixed in flue gas) in the above research, which were different from that in our experiment. To compare with other researchers’ results, similar experiments were undertaken, where the reaction temperature was changed while keeping the 15.4% volume ratio of H2O in the flue gas unchanged. It was found that the SO2 removal efficiency decreased with temperature rise; meanwhile, the NO removal exhibited irregular characteristics with temperature rise. (Experimental results are listed in Table 3.). Maybe overlapping effects of temperature and relative humidity could explain such an inconsistency. 3.3. Effect of Relative Humidity on SO2/NO Removal. The presence of water vapor in the flue gas could prominently improve the sorbent to capture SO2 and NO. Figure 4 shows the experimental results of RH on SO2/NO removal at 80 °C. It can be seen that the water absorbed on the sorbent surface plays a major role in SO2 removal and the SO2 removal efficiency increased monotonically with the rising of RH. And

it seems with the existence of an optimal RH on NO removal, the removal efficiency went up while the RH increased in the range of 0-20% and decreased while the RH kept rising over 20%. Water accumulating on the surface and interior pore of sorbent forms a liquid film, which could make the reaction gas be more easily absorbed by the sorbent rather than gas-solid reaction.7,11 When RH in the gas is high, water accumulation on the sorbent is greater. Thus, the effects of RH on SO2 capture can be easily understood. But it is difficult to explain the decline of NO removal when the RH increased over 20%, the possible reason of which is that the reaction was surface-catalyzed. When more water than necessary was on the sorbent surface, the water seemed to foul the surface reaction. 3.4. Effect of O2 Content on SO2/NO Simultaneous Removal. Figure 5 shows the effect of O2 concentration on SO2/ NO removal, which was consistent with Paul Chu’s3 data that the O2 presence in flue gas could enhance the removal of both SO2 and NO. Actually, higher O2 concentration in the flue gas can remove more NO and the effect of O2 on NO removal will be discussed in detail later. The SO2 removal efficiency increased when the O2 was changed from 1% to 5%, whereas it did not increase further, even dropped a little, while O2 content changed from 5% to 10%, probably due to the competition between the SO2 and NO on the sorbent. Generally, O2 can improve the removal of SO2 and NO because it can accelerate the oxidation of SO2 to SO3 and NO to NO2 by KMnO4. It can be assumed that when the NO removal reaches a specific level, it will consume the surplus sorbent which can be assigned to react with SO2 initially. 3.5. Solid Products Analysis. XRD analysis was applied to detect the products of SO2/NO removal qualitatively. The results showed that SO2 was fixed as SO42- for the KMnO4-modified sorbent and was absorbed as SO32- while KMnO4 was absent in the sorbent. Nitrate or nitrite was not found by XRD, the probable reason of which is that the amount in the spent sorbent was too little or their crystal quality was not good enough for XRD detection. An unexpected result was that MnS and Mn3N2 were found in the used KMnO4-modified sorbent. It may imply that KMnO4 took part in the reaction with SO2 and NO to some extent. The IC method was used to detect the composition and amount of sulfate and nitrate in the used sorbent quantitatively. NO3- and NO2-, not found by XRD method, were detected by the IC method in the spent 10% KMnO4-modified Ca(OH)2 sorbent,. The results implied that NO was absorbed as nitrate or nitrite by the calcium sorbent. SO42- was also detected in the product. The total sulfur and nitrogen from IC analysis on

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Figure 4. Effect of RH on (a) SO2 and (b) NO removal. Reaction conditions: 0.5 g of 10% KMnO4-modified absorbent, 80 °C reaction temperature, 500 ppm SO2, 200 ppm NO, 5% O2, 1300 mL/min gas flue.

Figure 5. Effect of O2 concentration on (a) SO2 and (b) NO removal. Reactor conditions: 0.5 g of 10% KMnO4-modified absorbent, 80 °C reaction temperature, 50% RH, 500 ppm SO2, 200 ppm NO, 1300 mL/min gas flue.

the product was compared with the experimental results from gas analysis (see Table 3). Good mass balance showed that the gas analysis method used in the paper was credible. 3.6. Mechanism of NO Removal. The mechanism of SO2 removal by calcium sorbent has been fully studied.12 The common view was that SO2 was first dissolved into H2O film and transformed into SO32- and then reacted with Ca2+ dissociated, and the final product was CaSO3, which could be oxidized into CaSO4 while the reaction occurred in the oxidizing atmosphere. However, the mechanism for NO removal has not been clearly proposed so far. Nelli7 summarized the mechanism for NO2 removal with calcium-based sorbent: NO2 can be removed by reaction with surface water on the sorbent or with SO32- together with H2O present. The reaction steps are as follows:

3NO2(g) + H2O(l) T 2HNO3(l) + NO(g)

(1)

2NO2 + SO32- + H2O f 2NO2- + SO32- + 2H+ (2) In the experiments, O2 was found to play an important role in NO removal. Further experiments were undertaken to study the effect of O2 on NO removal. Figures 6 and 7 show the results of NO removal with sorbents under the condition of O2 present or absent. It can be seen that when O2 is not present in the flue gas, the basic sorbent has no activity on NO capture; meanwhile, the 10% KMnO4-modified sorbent showed a certain activity on the reaction, but its activity decreased fast after the first 5 min.

Figure 6. Effect of O2 on NO removal with basic sorbent. Reactor conditions: 0.5 g of sorbent, 80 °C reaction temperature, 50% RH, 500 ppm SO2, 200 ppm NO, 1300 mL/min gas flue.

Such activity could probably be the result of oxygen molecule present in KMnO4. It could be suggested that oxygen is indispensable for NO removal. According to the above results and IC analysis of the product, the mechanism of NO removal can be surmised as oxygen first oxidizing NO into NO2, and then NO2 was removed by the reaction steps of (1) and (2). It means that NO cannot be directly removed with a calciumbased sorbent before being oxidized into NO2. To remove NO in the flue gas means to remove NO2 in the experiment. A

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temperature and O2 concentration with moderate RH were the optimum conditions for simultaneous SO2 and NO capture. SO2 was absorbed as sulfate with KMnO4 present and as sulfite with KMnO4 absent. NO could not be removed with a calcium-based sorbent before being oxidized into NO2, and its product was nitrate and nitrite. Acknowledgment This research was funded by the National Basic Research Program of China (No. 2006CB200301). Supporting Information Available: Details of the RH control method (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. Figure 7. Effect of O2 on NO removal with 10% KMnO4-modified sorbent. Reaction conditions: 0.5 g of sorbent, 80 °C reaction temperature, 50% RH, 500 ppm SO2, 200 ppm NO, 1300 mL/min gas flue.

Figure 8. Reaction routes of simultaneous removal of SO2 and NO with calcium-based sorbent.

reaction scheme for simultaneous removal of SO2 and NO was proposed as shown in Figure 8. According to this deduction, the experimental results can be well explained for NO removal influenced by O2 concentration in the flue gas and presence of NO3- and NO2- ions in the product. 3.7. Effect of KMnO4 on SO2/NO Removal. The experimental results showed that KMnO4 as additive could highly improve the ability of calcium-based sorbent to capture NO. Apparently, the strong oxidization characteristics of KMnO4 promoted the reaction of NO with O2. As a result, more NO2 was produced and then removed by calcium hydroxide. However, the Mn-S and Mn-N species found in the spent sorbent by XRD indicated that the role of KMnO4 was not simply as a promoter but it was involved in the removal of SO2 and NO as a reactant. The role of KMnO4 in NO removal needs further study. 4. Conclusion KMnO4 as additive could prominently enhance the calciumbased sorbent’s reactivity toward NO removal. Higher reaction

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ReceiVed for reView March 21, 2006 ReVised manuscript receiVed June 9, 2006 Accepted June 28, 2006 IE060340E