Front. Environ. Sci. Eng. DOI 10.1007/s11783-014-0636-2

RESEARCH ARTICLE

Activity and characteristics of “Oxygen-enriched” highly reactive absorbent for simultaneous flue gas desulfurization and denitrification Yi ZHAO (✉), Tianxiang GUO, Zili ZANG School of Environmental Science & Engineering, North China Electric Power University, Baoding 071003, China

© Higher Education Press and Springer-Verlag Berlin Heidelberg 2014

Abstract An “Oxygen-enriched” highly reactive absorbent was prepared by mixing fly ash, lime and a small quantity of KMnO4 for simultaneous desulfurization and denitrification. Removal of SO2 and NO simultaneously was carried out using this absorbent in a flue gas circulating fluidized bed (CFB). The highest simultaneous removal efficiency, 94.5% of SO2 and 64.2% of NO, was achieved under the optimal experiment conditions. Scanning Electron Microscope (SEM) and Accessory X-ray Energy Spectrometer (EDX) were used to observe the surface characteristics of fly ash, lime, “Oxygen-enriched” highly reactive absorbent and the spent absorbent. An ion chromatograph (IC) and chemical analysis methods were used to determine the contents of sulfate, sulfite, nitrate and nitrite in the spent absorbents, the results showed that sulfate and nitrite were the main products for desulfurization and denitrification respectively. The mechanism of removing SO2 and NO simultaneously was proposed based on the analysis results of SEM, EDX, IC and the chemical analysis methods. Keywords “Oxygen-enriched” highly reactive absorbent, Surface characteristics, Flue gas circulating fluidized bed, Simultaneous desulfurization and denitrification

1

Introduction

A wet scrubbing process [1] of flue gas desulfurization using slurry of fly ash and alkaline materials as absorbent was reported in the 1970s. Since then, many reactive absorbents based on fly ash were proposed, the first class of them is slurry of fly ash which has low desulfurization Received August 30, 2012; accepted December 10, 2013 E-mail: [email protected]

efficiency [2–4]; the second kind of them is fly ash and lime /slaked lime complex absorbent [5–17]. To enhance the reactivity of absorbents, the preparation conditions such as digesting temperature, reaction time, pressure, weight ratio of fly ash to calcium hydroxide, ratio of liquid to solid and many other factors [6–9,11,15] were extensively investigated, from which reactive species such as calcium silicate and calcium aluminate were formed by the reaction of calcium hydroxide with silicon and aluminum compounds in fly ash; the third class of them is to improve the desulfurization efficiency of the fly ash / lime by adding some additives such as CaSO4, CaCl2 and other compounds [18–24]. Consecutive examinations concerning simultaneous flue gas desulfurization and denitrification have been widely reported [25–32] since 1990, but experiments were mostly conducted in small-scale reactors in the laboratory. Experiments performed in a flue gas circulating fluidized bed (CFB) have not been further developed. In this study, a new technology that uses an “Oxygen-enriched” highly reactive absorbent containing KMnO4 to remove SO2 and NO simultaneously in a flue gas CFB is reported. The mechanism of removing SO2 and NO in the use of this absorbent was proposed according to the analysis results of Scanning Electron Microscope (SEM), an X-ray Energy Spectrometer (EDX), an ion chromatograph (IC) and the chemical analysis.

2

Experiment

2.1 Preparation of and characterization of “Oxygen-enriched” highly reactive absorbent

According to our previous experimental results [25,26] and others' works [13,17,30], the “Oxygen-enriched” highly absorbent was prepared from fly ash, industrial lime and a

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small quantity of KMnO4. Fly ash was supplied by Baoding Power Plant of China Datang Electric power company (Baoding, China). The lime used was of industrial grade (Tianjin chemical regent company, China). KMnO4 was of regent grade (Tianjin chemical regent company, China). During the preparation, 65.5% (wt) of fly ash, 33%(wt) of lime, 1.5% (wt) of KMnO4 and water were added into a reactor, in which the ratio of water to solids was 20 to 1 in weight and then the slurry was heated at 363 K for 6 h while stirring. After hydration, the absorbent slurry was filtered and dried in air. The specific areas of fly ash, lime and “Oxygen-enriched” highly reactive absorbent were determined using a Specific surface tester (ASAP-2000, Micromeritics company, USA), following the standard BET method. The results are shown in Table 1 and it can be seen that “Oxygenenriched” highly reactive absorbent has higher specific surface area. 2.2 Experiments of simultaneous desulfurization and denitrification

The experiments of simultaneous desulfurization and denitrification were carried out using “Oxygen-enriched” highly reactive absorbent in a self designed flue gas CFB experimental system, as shown in Fig. 1. In this experiment, hot air containing SO2 and NO was used to simulate the actual flue gas and the absorbent reacted with SO2 and NO in the reactor. Reacted and unreacted solid particles of the absorbent were collected by a pneumatic cyclone and fabric filters and then most of them were recirculated into the flue gas CFB reactor, that is to say that the incompletely reacted adsorbent was fed back to the reactor to react with SO2 and NO continuously and the residual absorbent was thrown away. At the same time, the fresh absorbent was fed by a worm distributor. According to Eq. (1), an adding rate of fresh absorbent of 2.74 g$s–1, namely purging rate of spent absorbent was obtained under the given conditions: 3087 mg$m–1of the inlet SO2 concentration, 1032 mg$m–3 of the inlet NO concentration, 1.2 of Ca/(S + N), 4.7 mg$m–3 of the solids concentration and 4.1 kg$(m2$s–1) of the recirculating rate. When the experimental system was operated with the negative pressure using an induced draft fan, a pressure drop of 800 Pa was observed. 2.3

Analytical methods and instruments

A SEM (KYKY-2800B, Zhongyi company, China) and an accessory EDX(Vantage DIS type, Thermo NORAN

Fig. 1

The experimental apparatus of flue gas CFB system

Company, USA) were used to observe the surface properties and to determine relative surface contents of significant elements in the highly reactive absorbent, “Oxygen-enriched” absorbent and the spent absorbent. The components of removal products were analyzed by an ion chromatograph (792 Basic, Metrohm A G, Swiss). The concentrations of sulfur species (sulfate and sulfite) and nitrogen species (nitrate and nitrite) in the spent absorbent were determined using barium chromate photometry and N-(1-naphthyl)-ethylenediamine photometry, respectively. The concentrations of SO2 and NO at inlet and outlet were determined by a hand-running flue gas analyzer (MRU95/3, JNPT Navi Mumbai, Germany) and the removal efficiencies were also calculated from the measured concentrations of SO2 and NO. 2.4 Definitions of Ca/(S + N) and the efficiencies of desulfurization and denitrification

An important index, Ca/S, is usually used to evaluate the utilization of the absorbent and serves as a critical technological parameter to determine the amount of the absorbent added in flue gas desulfurization system. For the flue gas simultaneous desulfurization and denitrification process mentioned in this study, Ca/(S + N) was used instead of Ca/S and it can be expressed as Ca=ðS þ NÞ ¼ n1 =n2 ,

where n1 and n2 stands for the molar weight of calcium oxide in absorbents and the mixtures (SO2 + 1/2NO) in flue gas, respectively. The definition of stoichiometric relationship is based on the feed rate of calcium oxide, SO2

Table 1 Surface properties of fly ash, industrial lime and “oxygen-enriched” highly reactive absorbent substances –1

specific surface area/(m $g ) 2

(1)

fly ash

industrial lime

“oxygen-enriched” highly reactive absorbent

5.6

12.1

55.1

Yi ZHAO et al. Activity and characteristics of “O2-enriched” absorbent for desulfurization and denitrification

and NO to the reactor and the expected removal products mainly including CaSO3/CaSO4 and Ca(NO3)2/Ca (NO2)2 in the spent absorbent.

3

Results and discussion

3.1 Optimum experimental conditions, desulfurization and denitrification efficiencies

The main experimental conditions, such as the content of KMnO4 in absorbent, Ca/(S + N), flue gas residence time, inlet flue gas temperature and humidity, will affect the efficiencies of desulfurization and denitrification. Hence, they were investigated experimentally in a flue gas CFB. Efficiencies of simultaneous desulfurization and denitrification versus the content of KMnO4 are shown in Fig. 2. It was found that although the mass percent of KMnO4 had a weak effect on desulfurization, it had a significant effect on denitrification. In the range of 0%(wt) to 1.5%(wt), the removal efficiencies enhanced rapidly with the mass percent of KMnO4. However, the change occurred at 1.5% (wt). Thereafter, the removal efficiencies remained constant, which might be due to saturation of the “oxidizing-points” on the surface of absorbent with an increase of the mass percent of KMnO4. The experiments of desulfurization and denitrification were carried out by varying Ca/(S + N) from 0.8 to 2.0. It was found that the removal efficiencies increased sig-

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nificantly when the Ca/(S + N) ranged from 0.8 and 1.2, In the range of 1.2 to 2.0 of the Ca/(S + N), the removal efficiencies almost stayed constantly at 94.8% for SO2 and 64.6% for NO. Therefore, the optimal Ca/(S + N) was set to 1.2. The variation of the efficiencies of desulfurization and denitrification as a function of the residence time of flue gas were studied experimentally. The efficiencies almost stayed constantly at 94.7% for SO2 and 64.7% for NO respectively when the residence time of flue gas laid between 2.4s and 2.7s. The optimal residence time was then determined to be 2.4s. Since the inlet flue gas temperature will greatly influence the efficiencies of desulfurization and denitrification, the variation of the efficiencies for removing SO2 and NO as a function of the inlet flue gas temperature ranging from 110°C to 170°C was investigated. The efficiencies increased with an increase in the inlet flue gas temperature up to 130°C. However, the efficiencies decreased sharply with the inlet flue gas temperature ranging from 130°C to 170°C. Thus the optimal inlet flue gas temperature was determined to be 130°C. Flue gas humidity is one of the important factors affecting removal efficiencies. The variation of the efficiencies for removing SO2 and NO as a function of flue gas humidity ranging from 1% (v/v) to 8% (v/v) was measured. The results indicated that overall the removal efficiencies increased with flue gas humidity. At lower humidity, the increasing trend was sharp with the humidity, while the trend become much smoother as the humidity increased further. Flue gas humidity was adjusted by spraying water into the flue gas CFB and was chosen as 5% (v/v). According to the experimental results mentioned above, the optimum experimental conditions were established as follows: the content of KMnO4, Ca/(S + N), the residence time of flue gas, the optimal inlet flue gas temperature and flue gas humidity were 5% (wt), 1.2, 2.4s, 130°C and 5% (v/v), respectively. Five parallel experiments were done under the optimum experimental conditions, the results are shown in Table 2. It showed that stable removal efficiencies of SO2 and NO were achieved. 3.2 SEM examinations of fly ash, industrial lime, absorbents and spent absorbents

Fig. 2 The effect of contents of KMnO4 on the efficiencies of desulfurization and denitrification

SEM images of fly ash, industrial lime, absorbents and the spent absorbents are shown in Fig. 3 to 6. It was clear that the surface of fly ash was relatively smooth, while those of

Table 2 Parallel experimental results of desulphurization and denitrification 1

2

3

4

5

average

sample variance/.S2

SO2 %

94.1

95.4

94.8

93.5

94.7

94.5

0.42

NO %

64.9

64.5

63.1

63.8

64.7

64.2

0.44

items efficiencies

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Fig. 3

Surface of fly ash particle (3000)

Fig. 6 Surface of the spent absorbent (980)

appeared on the surface of “Oxygen-enriched” highly reactive absorbent. With the spent absorbents, the surface was covered by porous substances, evidently different from that of the fresh one, which might be resulting from the removal products, such as calcium sulfate, sulfite, nitrate, nitrite and so on. 3.3

EDX study of fly ash, absorbent and spent absorbent

3.3.1 Surfaces of fly ash and “Oxygen-enriched” highly reactive absorbent

Fig. 4 Surface of Industrial lime particles (3000)

Fig. 5 Surface of “Oxygen-enriched” highly reactive absorbent (700) (A,B,C are for the part of erosive traces)

industrial lime, absorbents and the spent absorbents were much coarser. The erosive traces and the white flake layer

Relative contents of the main elements on the surface of fly ash and “Oxygen-enriched” highly reactive absorbent were analyzed by EDX, the results are shown in Table 3. The average energy spectrum on the surface of “Oxygenenriched” highly reactive absorbent is shown in Fig. 7. It can be seen from Table 3 that the relative contents of Si, Al and Fe elements were much higher than that of Ca, Ti and K elements contained in fly ash. With “Oxygen-enriched” highly reactive absorbent, the relative content of Ca was much higher than that of fly ash, according to Fig. 7 and Table 3. It can be speculated that these Ca species might be calcium hydroxide, which could be resulting from the hydration of industrial lime during the preparation of the absorbent. At that time, plentiful heat was discharged accompanying the dissolution of industrial lime, the calcium oxide particle was subsequently transformed into micro particle of calcium hydroxide. With the weight ratio of 3:1 of industrial lime to fly ash, the relative content of calcium should be about 15%. However, the measured value in Fig. 7 and Table 3 were higher than the predicted one, the possible reasons accounting for that may be the adsorption of excessive Ca (OH)2 on the surface of fly ash during drying of absorbent, leading to a higher Ca content. According to previous works [9,13,19,20], the surface of fly ash can be corroded by Ca(OH)2 due to the reaction among Ca (OH)2, aluminum and silicon compounds in fly

Yi ZHAO et al. Activity and characteristics of “O2-enriched” absorbent for desulfurization and denitrification

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Table 3 Relative contents of the main elements on the surface of fly ash and “Oxygen-enriched” highly reactive absorbent relative contents of the main elements

samples

Al

Si

K

Ca

Ti

Fe

fly ash

18.15

27.91

0.82

0.70

0.54

2.19

“oxygen-enriched” highly reactive absorbent

18.71

30.48

2.16

40.24

0.79

4.44

Mn

3.19

Fig. 7 Average energy spectrum on the surface of the “Oxygen-enriched ” highly reactive absorbent

Fig. 8 Average energy spectrum on the surface of the spent absorbent

ash during the digesting process, therefore a rough surface of the absorbent was created. Figure 7 and Table 3 showed that the manganese element existed in “Oxygen-enriched” highly reactive absorbent, indicating that KMnO4 dispersed on the surface during the preparation of absorbent. 3.3.2 Surface of spent “Oxygen-enriched” highly reactive absorbent

Figure 8 showed the energy spectrum of the spent absorbent. Compared with Fig. 7 and Table 3, the sulfur species appears evidently on the surface, which proves

that the absorption occurred in sulfur species and the absorbent. 3.4 Ion chromatography and chemical analysis of the removal products

The contents of sulfur and nitrogen species in the unreacted absorbent and the spent absorbent were determined by chemical analysis, and the resulted are listed in Table 4, it was clear that the molar ratio of sulfate to sulfite in the spent absorbent was 1.85 and that of nitrate and nitrite was 2.98. With regard to the fresh absorbent, a small quantity of sulfate was detected, which might come from fly ash. Sulfite, nitrate, and nitrite were not detected, which

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Table 4 Contents of sulfur and nitrogen species in unreacted absorbent and spent absorbent (mmol$g–1) the contents of sulfur and nitrogen species in unreacted absorbent [SO42–] 0.002

the contents of sulfur and nitrogen species in spent absorbent

[SO32–]

[NO–]

[NO3–]

[SO42–]

[SO32–]

[NO2–]

[NO3–]

0

0

0

0.999

0.538

0.555

0.186

Fig. 9

Ion chromatogram of the spent absorbent

indicated that sulfate was the main desulfurization product and nitrate was the main denitrification one. The ion chromatogram of the spent absorbent is shown in Fig. 9 and basically the results coincide with that of chemical analysis.

4 Mechanism of desulfurization and denitrification in flue gas CFB system Fly ash, the key material in the preparation of this absorbent, is chiefly composed of SiO2, Al2O3, Fe2O3, and CaO. During the preparation of absorbent, the noncrystal SiO2 and Al2O3 were gradually activated by alkali substance, in which calcium hydroxide diffused to the spherical glass body surface on fly ash to trigger chemical reaction and erosion, which dissolved the glass body, destroyed the grid of Si-O, Al-O and finally activated SiO2 and Al2O3 to react with calcium hydroxide. For the process, some gelling amorphous materials and plentiful pores were formed. From the SEM images shown in Figs. 3, 4 and 5, the surface microstructures of material particles changed significantly before and after digesting. In addition, although simultaneous desulfurization and denitrification were carried out at a flow reactor [20,33], the data of the removal products of NO such as nitrate and nitrite have not been found to verify the chemical reaction between the absorbent and NO, in which NO might be

adsorbed merely. In our viewpoint, adding KMnO4 into absorbent is necessary in order to remove NO and SO2 simultaneously. The predominant form of NOx in flue gas is presented as NO with much lower solubility in water than NO2, HNO2 or HNO3, which is the main reason why NOx cannot be removed together with SO2 from flue gas by the present desulfurization techniques. Therefore, the key for simultaneously removing NO and SO2 from flue gas was to oxidize NO into NO2 rapidly. With “Oxygen-enriched” highly reactive absorbent containing KMnO4, many oxidizing sites on the surface can oxidize NO to NO2 within the residence time 2.4s of flue gas CFB. NO2 could be first adsorbed physically together with SO2 on the surface of the absorbent and then SO2 and NO2 were dissolved into the water film of the absorbent and reacted rapidly with Ca(OH)2 in the absorbent when the atomizing water was sprayed into the flue gas CFB. According to the results obtained from the EDX, IC and chemical analysis, it can be estimated that SO2 and NO were removed by chemical absorption. The products of the desulfurization and denitrification were calcium sulfate, calcium sulfite, calcium nitrate and calcium nitrite. The chemical processes in the flue gas CFB can be described by the following global reaction schemes: SO2 þ H2 O↕ ↓H2 SO3

(2)

Yi ZHAO et al. Activity and characteristics of “O2-enriched” absorbent for desulfurization and denitrification

H2 SO3 þ KMnO4 ↕ ↓H2 SO4 þ Mðreductive  resultsÞ (3) CaðOHÞ2 þ H2 SO3 ↕ ↓CaSO3 þ 2H2 O CaSO3 þ O2 þ NO↕ ↓ ðreactive  complex  compoundsÞ↕ ↓CaSO4 þ NO2

(4) (5)

NO þ 1=2O2 ↕ ↓NO2

(6)

NO þ KMnO4 ↕ ↓NO2 þ Mðreductive  resultsÞ

(7)

3NO2 þ H2 O↕ ↓2HNO3 þ NO

(8)

NO2 þ NO þ H2 O↕ ↓2HNO2

(9)

CaðOHÞ2 þ 2HNO3 ↕ ↓CaðNO3 Þ2 þ 2H2 O

(10)

CaðOHÞ2 þ 2HNO2 ↕ ↓CaðNO2 Þ2 þ 2H2 O

(11)

where (3) and (4) are the key steps of desulfurization, and (7) and (9) are the main process of denitrification.

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Conclusions

During the preparation of “Oxygen-enriched” highly reactive absorbent, partial Ca(OH)2 reacted with aluminum and silicon compounds in fly ash to form coarse particles. Most of Ca(OH)2 covered the surface of absorbent in the form of “White flake layer” during the hydration and drying. KMnO4 was dispersed on the surface of the absorbent. The removal efficiencies of 94.5% for SO2 and 64.2% for NO were obtained respectively under the optimal experimental conditions. This process can meet the requirements of the Air Quality Standard and the Emission Standard of Air Pollutants for coal-fired industrial boilers. From the presence of sulfur and nitrogen species in Ion chromatogram of the spent absorbent in Ion chromatogram it can be inferred that the removal reaction was chemical absorption of SO2 and NO, and sulfate and nitrite were the main removal products of desulfurization and denitrification. Acknowledgements The work was supported by the National High Technology Research and Development Program of China (863 Program, No. 2013AA065403), Program for Changjiang Scholars and Innovative Research Team in University (IRT1127) and Zhejiang Provincial Engineering Research Center of Industrial Boiler & Furnace Flue Gas Pollution Control, Hangzhou, China (311202).

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