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Mechanism of flue gas simultaneous desulfurization and denitrification using the highly reactive absorbent ZHAO Yi, SUN Xiaojun, XU Peiyao, MA Shuangchen, WANG Lidong & LIU Feng School of Environmental Science & Engineering, North China Electric Power University, Baoding 071003, China Correspondence should be addressed to Zhao Yi (email:
[email protected]) Received April 29, 2005
Abstract
Fly ash, industry-grade lime and a few ox idizing man ganese compound additive were used to prepare the “Ox ygen-riched” highly rea ctive absorbent for simultaneous desulfurizat ion and denitrification. Experiments of simultaneous desulfurization and denitrification w ere carried out using the highly reactive absorbent in the flue gas circulating fluidized bed (CFB) system. Removal efficiencies of 94.5% for SO2 and 64.2% for NO were obtained respectively. The s canning electron microscope (SEM) and accessory X-ray energ y spectrometer were used to observe micro-properties of the samples, including fly ash, common hig hly reactive a bsorbent, “Oxygen-riched” highly reactive absor bent and spe nt absorbent. The w hite fla ke layers were observed in the SEM images about surfaces of the common highly reactive absorbent and “Ox ygenriched” one, and the particle surfaces of the spent absorbent were porous. The content of calcium on su rface was higher than that of the average in the highly reactive abso rbent. The manganese compound additive dispe rsed uniformly on the surfaces of the “Ox ygenriched” highly reactive absorbent. There was a sulfur peak in the energy spectra pic tures of the spent absorbent. The component of the spent absorbent was analyzed with chemical ana lysis methods , and the results indicate d that more nitrogen sp ecies appeared in the absorbent ex cept sulfur species, and SO2 and NO were remove d by chemical absorption according to the e xperimental results of X-ray energy spectrometer and the chemical analysis. Sulfate being the main desulfurization products, nitrite was the main denitrification ones during the process, in which NO was oxidized rapidly to NO2 and absorbed by the chemical reaction. Keywords: highly r eactive absorbent, fly ash, sim ultaneous desulfur ization and de nitrification, micro-property, scanning electron microscope, X-ray energy spectra. DOI: 10.1360/03ye0041
Using slurry m ixed by fly ash an d alkaline m aterial, rem oval of SO2 in industrial waste gas had been reported in the 1970s, which was named the wet s crubbing[1]. Since Copyright by Science in China Press 2005
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then, there have been many reports about flue gas desulfurization using the reactive absorbent prepared with flying ash. There are four kinds as follows: (1) Using slurry of fly ̣ ash as absorbent [2 4], which m ainly utilize the propert y of fly ash for des ulphurization, ̣ but the rem oval ef ficiency is not high. (2) Using fly ash and lim e/slaked lim e[5 17] as absorbent. There have been a lot of re ports about the preparation conditions of this kind of absorbent, such as digesting temperature, time, pressure ratio by weight of fly ash and ̣ calcium hydroxide, a nd ratio of water a nd solid, e tc.[6 9,11,15]. (3) Using highly reactive ̣ absorbent. The additives, s uch as CaSO 4[18 24], CaCl 2 and ot her c ompounds[19], a re added to the fly ash/lime to improve the desulfurization activity. Unfortunately, the mentioned absorbents are only used for desulphurization, while simultaneous denitrification cannot be realized. The “Oxygen-riched” absorbent for simultaneous desulfurization and denitrification was prepared by adding oxidative rea gent to fly ash and lime. With the duct injection system and CFB as the wor king platform, the experiments about simultaneous removal of SO 2 and NO in flue gas have been done by using the proposed absoṛ bent, which are reported in our papers[25 27]. The “Oxygen-riched” absorbent is na med the fourth kind. The pres ent inves tigations of the f ormer three kinds of absor bents showed that the surface characteristic of the absorbents has a great effect on desulphurization. However, the surface characteristics of the fourth ki nd of absorbent and its reactions with SO 2 and NO in the CFB system have not been reported. SEM and the accessory X-ray energy spectrometer were used to analyze micro-properties and contents of signific ant e lements on the surface of the fly ash, the fly ash/lim e a bsorbent, “ Oxygen-riched” absorbent and reaction p roducts in this paper . T he chem ical analysis m ethods were adopted to determine the compositions of the products of desulfurization and denitrification. Thus the detailed mechanism of desulfurization and denitrification in the CFB system was investigated. 1
Preparation of the “Oxygen-riched” highly reactive absorbent
According to our previous experim ental results [25,26] and other research conclusions[13,17,28], the “Oxygen-riched” highly reactive a bsorbent was prepared from fly ash, industrial lim e a nd oxidiz ing manganese com pound additive. The prepara tion process was as follows. The mixture including fly ash and industrial lime with the ratio of 31 in weight, m anganese compound a nd water, was stirred and digested i n 363 K and dried after six hours. Thus, the “Oxygen-riched” highly reactive absorbent was achieved. The f ly ash used in the experiments cam e fro m Baoding Thermoelectricity Plant, whose compositions are s hown in Table 1. The c ontent of the ef fective calcia in the industrial lime was 90.77%, measured by the method of cane sugar. Table 1 Contents of components in fly ash Components SiO Content (%)
2
46.6
Al2O3 25.2
Fe2O3 11.1 3.3
CaO MgO 2.4
Combustion loss
Others
10.0
1.4
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2
Desulfurization and denitrification experiments in the flue gas CFB system
Using the simulative flue gas containing SO2 and NO, five parallel experiments of the simultaneous desulfuriz ation and de nitrification were carried ou t by using “ Oxygenriched” highly reactive absorbent in the flue gas FCB experimental system (shown in Fig. 1). The optimum experimental conditions are listed in Table 2, and the results are in Table 3.
Fig. 1. The experimental apparatus of the flue gas CFB system. 1, Air inlet; 2, steel bottle of SO2; 3, steel bottle of NO; 4, glass-rotor flow meter; 5, buffer bottle; 6, e lectric heater; 7, fluidized bed reactor; 8, w ater tank; 9, high-pressure pu mp; 10, spray nozzle; 11, screw m aterial-fed m achine; 12, vortex d ust re mover; 13, material-circling leg; 14, gas analysis instrument; 15, induced-draft fan. Table 2 CSO2,in (mg/m3) 3087 1032
CNO,in (mg /m3)
Table 3
Optimum experimental conditions of CFB
Gas settling time (s) 2.4
Inlet gas temperature (ć) 130
Ratio of Ca/(S+N) 1.2
Wet of flue gas (%) 5
Parallel experimental results of desulfurization and denitrification Items
Efficiencies SO2 (%) NO (%)
12 94.1 64.9
3 95.4 94. 64.5
8 93. 63.1
4
5
5 94.7 63.8 64.7
Average 94.5 64.2
Sample variance (S2) 0.42 0.44
Table 3 shows that the removal efficiencies of 94.5% and 64.2%, respectively for SO 2 and NO co uld be obtained by using the “Oxygen-riched” highly reactive absorbent under th e op timized ex perimental co nditions. Variance of the S 2 in five parallel experiments was relati vely sm all, which s howed that t he data was highly reproducible and precise. The capacity of the “Oxygen-riched” highly reactive absorbent was stable in the simultaneous denitrification and desulphurization. 3 Analytical studies on the micro-properties on surface of fly ash, industrial lime, absorbents and the spent absorbent The profile about micro-properties on surface of the fly ash, industrial lime, common Copyright by Science in China Press 2005
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highly reactive absorbent, “Oxygen-riched” highly reactive absorbent and the products were obser ved and analyzed by us ing S EM (KYKY-2800B type). The preparation of samples was as follows. The conductive two-sided gum paper was agglutinated on sample flat. Then the sample powder was uniformly sprinkled on the paper and the residual powder was blown out. After the conductive film layer was plated on the sample powder with the ion sputter method, it was observed with the electron microscope. The scanned images of samples are shown in Fig. 2̣Fig. 7.
Fig. 2. Surface of fly ash particle (3000×).
Fig. 3. Industrial lime particles (3000×).
Fig. 4. Surface of common highly reactive absorbent particle (700×).
Fig. 5. Surface of “ absorbent (700×).
Fig. 6. Surface of particle (980×).
Fig. 7. Surface of another absorbent particle (980×).
an absorbent product product
Oxygen-riched” highly reactive
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It was shown that particles of fly ash w ere relatively smooth, although there wer e some abnorm al protuberances on the surface. T he s urface of industry li me particles mostly in 2 m diameter was obviously coarse than that of fly ash, which was usually in tens of m icrons diam eter. The com mon highly reactive absorbent particles were very coarse. T here were obviously erosive tra ces on t he s urface, which w as attached to the white flake layer. The scanned s urface image of the “Oxygen-riched” highly reactive absorbent particles was sim ilar to that of the common highly reactive ones. Seeing the micro-surface, the spent absorbent particles were evidently dif ferent fro m fresh ones. The surface of the spent absorbent no longer had the white flake layer, while the porous characteristic was observed, which would be caused by complicated physics and chemical processes on the absorbent surface. During the process of sim ultaneous desulfurization and denitrification in the flue ga s CFB syste m, the s ubstances on the a bsorbent s urface may transform as follows: dissolved, reacted, dried, and circulated in material. In the above process, the surface of abs orbent particles was covere d with products, such as calcium sulfate, sulfite, nitrate and nitrite, etc. These products were porous under the condition of the flue gas CFB. Accompanying by collision and abrasion between the particles, the products flaked off continuously, which led to exposure of fresh absorbent. It was one of the key reasons for high removal efficiencies of SO2 and NO. 4
Energy spectra analyzing of fly ash, industrial lime, absorbent and products
A scanning electron microscope (SEM, KYKY-2800B type) equipped with the X-ray energy spectrometer (EDS, Vantage DIS type, Thermo NORAN Co mpany in USA) was used to analyze the surfac e compositions of the s amples. The ordinate of energy spectrum figure was photon counts and the abscissa was ener gy. Elements from 11Na to 92U could be analyzed by the X-ray energy spectrometer. 4.1
Average energy spectra pictures on surface of fly ash
Average energy spectra pictures on surface of fly ash are shown in Fig. 8. The r elative content of the main elements on the surface of fly ash is shown in Table 4.
Fig. 8. Average energy spectra pict ure of the s urface of fly ash. (Notes: The Auru m peak came from the Au film used as conductive medium in the preparation of samples and the same as follows.)
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Table 4 Relative contents of the main elements on the surface of fly ash Al Si 18.23
K 26.22
0.62 1.
Ca 07 1.
Ti Fe 01 3.
87
From Fig. 8 and Table 4, the compounds of Si, Al, Fe and Ca were the main composition in fly ash, in which there was also a small quantity of Ti and K elements. 4.2
Energy spectra pictures of the common highly reactive absorbent
(i) Energy spectra pictures of point a of the common highly reactive absorbent in Fig. 4. From Fig. 9, relative content of Ca at point a in Fig. 4 was high, while that of Si, Al and K was low with others in trace. Hence, the main substance in point a m ight be calcium hydroxide, which wa s hydrated from the industrial lim e in the preparation of absorbent. Abundant heat was di scharged with dissolution the industrial lim es, which transformed calci um oxide particles into micro partic les of calcium hydroxide. A little calcium hydroxide dissolved into wa ter. As micro-particles of calcium hydroxide coexisted with fly ash, part of them and relative in solution would cover the surface of fly ash in the process of drying, which appeared as the white flake layer. (ii) Energy spectra picture of common highly reactive absorbent at point b in Fig. 4. Compared Fig. 10 with Fig. 8, the relative co ntent of Ca at point a was higher than that
Fig. 9. Scanned energy spectra pictures at point a in Fig. 4.
Fig. 10. Scanned energy spectra pictures at point b in Fig. 4.
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in Fig. 8. It w as shown that external Ca entered the s urface of fly as h after digestion. It was also shown in Fig. 8 that the relative content of Si was close to that of Al in fly a sh, which was different from that of C a. However, in Fig.10 the relative content of S i was much larger than that of Al. At the same time, there was less difference between Si and Ca. It was assumed that the silicon com pound in fly ash reacted w ith Ca(OH)2 in to CaSiO3 on the surface during the diges ting process, which was cons istent with refs. [9, 13, 18, 20]. During the reaction process, the surface of fly ash was corroded by Ca(OH)2, which led to the rough surface of prepared absorbent. (iii) Energy spectra pictures of the common highly reactive absorbent at point c. Fig. 11 shows the energy spectra pictures of the beading surface at point c in the highly reactive absorbent, which is similar to that in Fig. 10. Because SiO2 was the main substance in the beading, there was more Si in Fig. 11, which was more than that in fly ash. The relative contents of the main elements at point c of the samples are shown in Table 5.
Fig. 11. Scanned energy spectra pictures at point c in Fig. 4. Table 5 Relative contents of the main elements on surface of beading in the common highly reactive absorbent Al Si 9.83 31.
K Ca 07 1.
26
6.28
Ti
Fe
0.00
3.24
(iv) Average energy spectra pictures on surface of the common highly reactive absorbent. The average energy spectra pictures on surface of the samples are shown i n Fig. 12. The relative content of the main elements are shown in Table 6.
Fig. 12. Average energy spectra on surface of common highly reactive absorbent.
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Table 6 Relative contents of the main elements on surface of common highly reactive absorbent Al Si 13.61 18.
73
K
Ca
0.51
21.36
Ti Fe 0.62
1.88
Fig. 12 and Table 6 indicate that the relative content of Ca was highe r than that of Si and Al, because part of Ca(OH)2 reacted with the substance on surface of the fly ash and some other Ca(OH) 2 absorbed into the surface of fly ash. However, from the amounts of the adding industrial lime and fly ash with the weight ratio of 3 1 during preparation of common highly reactive a bsorbent, the r elative content of calcium should be about 15 percent, which was lower than that in Fig. 12. It m ight be that Ca(OH)2 could not completely spread into the inside of fly ash during digestion. Thus the relative content of Ca on surface of absorbent was more than the average content, which enhanced the availability of Ca in the common highly reactive absorbent. Because SO2 and NO were easily ̣ reacting with Ca on the surface[10 12], the preparation method of a bsorbent was be neficial to promote the efficiency of desulfurization and denitrification. 4.3
Energy spectra picture of “Oxygen-riched” highly reactive absorbent
(i) Energy spectra pictures of “Oxygen-riched” highly reactive absorbent at point A in Fig. 5. The energy spectr a pictures of point A on s urface of sam ples in Fig. 5 are shown in Fig. 13. The relative contents of the main elements are shown in Table 7.
Fig. 13. Energy spectra pictures at point A in Fig. 5. Table 7 Relative contents of the main elements (wt.%) of point A on surface of samples in Fig. 5 Al Si 15.90
K 34.78 1.
46
Ca 40.18 0.
Ti 69
Fe
Mn
4.92
2.07
According to the relative contents of elements in Fig.13 and Table 7, it was ass umed that the main substance at point A in Fig. 5 was Ca(OH)2, which came from hydration of industry-grade lim e in preparati on of the hi ghly reac tive absorbent and a dhered to t he surface of fly ash in the drying process. From the rather high content of silicon and aluminum, it was also assum ed that there were com pounds containing s ilicon and alum inum, such as hydration calcium silicate, calcium aluminate and calcium aluminosilicate, etc. Dif ferent from the above ener gy spe ctra pictures, there was a peak of m anganese www.scichina.com
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element, which was that the “Oxygen-riched” highly reactive absorbent made by adding oxidizing manganese additive to the common highly reactive absorbent. (ii) Energy spectra pictures of “Oxygen-riched” highly reactive absorbent at point B in Fig. 5. The energy s pectra pictures of point B on surface of sam ples in Fig. 5 are shown in Fig. 14. The relative contents of the main elements are shown in Table 8.
Fig. 14. Energy spectra pictures at point B in Fig. 5. Table 8 Relative contents of the main elements at point B on surface of samples in Fig. 5 Al Si 29.70
K 52.17 1.
77
Ca 12.06 0.
Ti 50
Fe
Mn
2.68
1.13
Seen from Fig. 14 and Table 8, the relati ve contents of silicon and alum inum were very high and that of calciu m was rather high. It was assumed that the m ain substances at point B in Fig. 5 were calcium silica te, ca lcium alu minate a nd other c ompounds, which contained s ilicon and aluminum, such as silicon dioxide, aluminate sesquioxide, etc. (iii) Energy spectra pictures of point C in “Oxygen-riched” highly reactive absorbent in Fig. 5. The energy s pectra pictures of point C on surface of sam ples in Fig. 5 are shown in Fig. 15. The relative contents of the main elements are shown in Table 9. The energy spectra pictures and the re lative contents of the main elements at point B were
Fig. 15. Energy spectra pictures at point C in Fig. 5.
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Table 9 Relative contents of the main elements at point C on surface of samples (wt.%) in Fig. 5 Al Si 31.44
K 54.36
1.96 9.
Ca 45
Ti 0.90
Fe
Mn
1.31 0.
58
similar to that of point C in Fig. 5. It was assumed from the very hi gh relative contents of silicon and aluminum and ra ther high of calcium that the main substances at point C were calcium silicate, calcium aluminate and other compounds, which contained silicon and aluminum, such as silicon dioxide, aluminate sesquioxide, etc. (iv) Average ener gy spectr a pictures on surface of “Oxygen-riched” highly reactive absorbent. The average energy spectra pictures are shown in Fig. 16. The relative contents of main element (wt. %) are shown in Table 10.
Fig. 16. Average energy spectra pictures on surface of “Oxygen-riched” highly reactive absorbent. Table 10 Relative contents of the main elements on the surface of “Oxygen-riched” highly reactive absorbent (wt. %) Al Si 18.71
K 30.48 2.
16
Ca 40.24 0.
Ti 79
Fe
Mn
4.44
3.19
By comparing Fig. 16 with Fig. 12, and Table 6 with Table 10, it is found that Fig. 16 is very sim ilar with Fi g. 12. However, there is a peak of manganese element in Fig. 16, and there is the rela tive content of m anganese element in Table 10. Either from the average value or the points A, B and C on the surface of the particle, the statistical tables of the m ain ele ments show the existence of m anganese el ement in t he “ Oxygen-riched” highly reactive absorbent. It is proved that the oxidizin g additive spr ead uniformly during the preparation of the “Oxygen-riched” highly reactive absorbent. 4.4 Average ener gy spectra pictures on surface of “ Oxygen-riched” highly rea ctive absorbent product The average energy spectra pictures on the surface of the “Oxygen-rich” highly reactive absorbent after the desulfuriza tion and denitrification in the CFB are shown in Fig. 17. The relative contents of main elements (wt. %) are shown in Table 11.
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Fig. 17. Average energy spectra pictures on the surface of the absorbent products. Table 11 Relative contents of main elements on the surface of spent absorbent (wt. %) Al Si 20.98
K 42.03 2.
66
Ca 22.19 1.
Ti 35
Fe
Mn
S
3.47
0.08
7.24
It is s hown in T able 10 there was no sulfur element on surface of the unrea cted absorbent, while the relative content of sulfur in the spent absorbent is given in Table 11. It indicated that sulfur species were absorb ed in the re move re action. Unfortunately, the Vantage DIS X-ray ener gy spectrometer can on ly analyze the elem ents between Na and U. As the nit rogen e lement coul d not be analyzed, the absorption of ni trogen spe cies could not be reflected directly by the energy spectra pictures. The absorption of nitrogen species was verified by the chemical analytical methods. 5
Results of chemical analysis
The contents of sulfate, sulfite, nitrate and nitrite in the unr eacted absorbent and the products were analyzed with che mical analysis. The contents of sulfate and sulfite were determined by barium chrom ate photom etry. The c ontent of nitrite w as det ermined by N-(1-naphthyl)-ethylenediamine p hotometry, and that of nitrate by reduction of zinc powder. The contents of sulfur and nitrogen species are listed in T able 12 f or the unreacted absorbent and the products. It was seen that the molar ratio of sulfate and sulfite in the spent absorbent was 1.85, and that of nitrite and nitrate was 2.98. With regard to the un reacted absorbent, a small quantity of sulfate was detected out, whic h might come from fly ash. Moreover, sulfite, nitrate and nitrite were not detected out in unreacted absorbent, which indicated that sulfate was the m ain desulfurization product, and nitrite was the Table 12 Contents of sulfur and nitrogen species in unreacted absorbent and products (mmol/g) The contents of sulfur and nitrogen species The contents of sulfur and nitrogen species in unreacted absorbent in absorbent products [SO2 4 ][ 0.002 0
SO2 3 ]
[NO] [
NO3 ]
[SO2 4 ][
SO2 3 ]
0
0
0.999
0.538
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[NO2 ] [ 0.555
NO3 ] 0.186
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main denitrification one. 6
Mechanism of desulfurization and denitrification in CFB system
Integrated with the res ults obtained from the X-ray ener gy spectr ometer and the chemical analysis, chem ical absorption of SO 2 and NO was confirmed in the CFB system. Being the prim ary com position of NO x in flue gas, dissolved NO in water was much lower than that of other species such as NO 2, HNO 2 and HNO 3. Although NO could be oxidized to NO 2 under natural conditions, it was not realized in practice because of the s hort settling ti me of a bout one to three s econds. Th e oxidizing additive contained in the “Oxygen-riched” highly reactive absor bent, NO might be rapidly oxidized to NO 2 and absorbed by the absorbent. The possible reaction process was inferred as follows: SO Ca(OH) CaSO NO+M
2+H2OėH2SO3
(1)
2+H2SO3ėCaSO3+2H2O 3+O2+NOė(reactive
(2)
complex compounds)ėCaSO4+NO2
(oxidant)ėNO2+M (reductive results)
(3) (4)
3NO
2 +H2Oė2HNO3 + NO
(5)
NO
2+NO+H2Oė2HNO2
(6)
Ca(OH)
2+2HNO3ėCa(NO3)2+
2H2O
(7)
Ca(OH)
2+2HNO2ėCa(NO2)2+2H2O
(8)
where (2) and (3) are the key steps of desulphurization, and (4), (6) and (8) are the main process of denitrification. 7 Conclusions During the preparation of highly reactive absorben t, particles of Ca( OH)2 coexisted with fly ash, which reacted in part with surface substances of fly ash. Thus the surface of fly ash was corroded, and that of absorbent appeared very coarse. Som e other Ca( OH)2 covered the surface of fly ash in the form of “white flake layer” during the hydration and drying. The fly ash being the carrier of Ca(OH)2 in the process, dispersion of Ca(OH)2 was enhanced and the effective reaction area was enlarged. The oxidizing additive was dispersed unif ormly in the “ Oxygen-riched” highly reactive absorbent. Thus NO could be oxidized and removed by the alkaline material in the absorbent. Using the highly reactive absorbent, removal efficiencies of 94.5% for SO2 and 64.2% for NO were obtained respectively in the CFB system. www.scichina.com
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The s ulfur ele ment pe ak in the e nergy s pectra pi ctures of the spent absorbent c onfirmed that the re moval reaction was chemical absorption between SO 2 in flue gas and the absorbent, which was further verified by the re sults of chemical analysis. I n the removal process of NO in the flue gas, it w as oxidized firstly and then absorbed chem ically, which was confirmed by the chemical analysis. Sulfate and nitrite were the m ain products of desulfurization and denitrification respectively. Acknowledgements This work was supported by th e Significant Pre-research Foundation of t he North China Electric Power University.
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