外文翻譯石灰石脫硫?qū)ρh(huán)流化床中NOX排放的影響（可編輯）

1、外文翻譯-石灰石脫硫?qū)ρh(huán)流化床中nox排放的影響 英文原文 limestone effects on nox formation in cfb combustors abstract circulating fluidized bed cfb combustion technology has been widely used in power generation with the considerations of its advantages in economically controlling so2 and nox emissions. however, it is found

2、that nox emission increased up to 30% when limestone is added into the combustor for desulphurization, especially with rather high ca/s ratio. the phenomenon of nox augment was discussed based on associated mechanisms and chemical kinetics. the catalytic oxidization effect, preferential conversion e

3、ffect from hcn to nh3, and reduction effect on nox were analyzed. it is found limestone can act as a catalyst, increasing the reaction rates for the reaction associated with nox formation. furthermore, cao decomposed caco3 favors the conversion of hcn to nh3 when they are release in de-volatilizatio

4、n process. due to preferential oxidization of nh3 to no rather than the unstable n2o, nox emission increases with limestone injection. when high ca/s is used, the reduction effect becomes dominated and reduces the overall nox emissionkeywordscfb, limestone, desulphurization, nox formation, chemical

5、kinetics 1. introduction circulating fluidized bed cfb combustion technology has been widely used in power generation, because of its advantages in such as the high fuel flexibility for burning various kinds of coals and the high feasibility in economical emission control. given that the combustion

6、temperature in cfb combustors, e.g., boilers, is usually between 10201120k and secondary air can be injected at different ports along the axial direction, the nox concentration in the flue gas can be controlled to be much lower than those using other combustion technologies. shown in fig. 1, nox emi

7、ssion from a cfb combustors are lowest, in the range of 100-220 ppm, compared with other kinds of combustors 1fig. 1 nox emissionlevel for different combustion systems 3 when coals with high surface content are burned, limestone is added into cfb boilers for desulphurization. within the normal tempe

8、ratures, the efficiency of limestone desulphurization can be up to 90% 1. in addition to the desulphurization, limestone may play several other positive roles in improving the boiler performance of such as combustion, heat transfer, material balance and ash separation in the separator. however, obse

9、rved in practical operation and experiments, limestone addition in cfb combustors might cause a negative effect on the nox emissions. figure 2 depicts the nox emission of a commercial cfb boiler with limestone desulphurization, operating with bed temperature between 1150k and 1200k with ca/s ratio o

10、f 2.2. it can be seen, when limestone is injected, while n2o emission was little affected, the no remarkably increased by 50 ppm or about 30%2. figure 3 further depicts some experimental results of the influence ca/s ratio on the nox emission 3. when ca/s ratio changes is smaller than 2, the nox con

11、centration increases with the ca/s ratio. when ca/s ratio is higher than 2, the nox concentration decreases with the ca/s ratio though the mechanisms for nox formation in homogeneous reactions are rather clear, the studies on the mechanisms for nox formation in heterogeneous atmosphere, especially w

12、ith the presence of limestone, are limitedfig.2 nox emission with/without fig.3 nox concentration with differentlimestone desulphurizationca/s ratios tb1165k, o26% 2. nox formation mechanisms and chemical kinetics in a cfb combustor there are mainly three well-known mechanisms counting for the nox f

13、ormation in coal combustion 4: 1extended zeldovich or thermal mechanism in which o, oh, and n2 species are in equilibrium values and n atoms are in steady state. 2 prompt mechanisms where no is formed more rapidly than predicted by the thermal mechanism above, either by i fenimore cn and hcn pathway

14、s, or ii the n2o-intermediate route, or iii as a result of non-equilibrium concentrations of o and oh radicals in conjunction with the extended zeldovich scheme. 3fuel nitrogen mechanism, in which fuel-bound nitrogen is converted to no apparently, for a cfb combustor, neither the extended zeldovich

15、mechanism which depends strongly on temperature and only becomes significant when temperature is above 1750k, nor the prompt mechanism which becomes significant only with abundant chi radical and low o2 concentration is important. the dominate mechanism for nox formation in cfb combustor is fuel nit

16、rogen mechanism fuel nitrogen mechanism is rather complicated. part of nitrogen in coal is usually released in the form of hcn and nh3 as volatile nitrogen, and the rest remains as fixed nitrogen during the de-volatilization process. the ratio of volatile nitrogen to fixed nitrogen, as a result of d

17、e-volatilization, depends on coal type, temperature and heating rate of coal particles 5. normally, the oxidization of volatile nitrogen occurs in homogeneous gas-phase reactions immediately after de-volatilization, while the oxidization of fixd nitrogen in heterogeneous gas-solid reactions along wi

18、th fixed carbon combustion6, 7.3. limestone effects on no formation a cfb combustor3.1 catalytic effect for fuel-n oxidization given the residence time of flue gas in the combustor is short, it is impossible for every reaction to reach equilibrium. once the flue gas exits the combustor, it is cooled

19、 and the associated reactions stop. the reaction rates with no taking part in can be expressed as:where, kc is expressed by arrhenius law: therefore, small activate energies and large collision frequencies favor high reaction rate. only reactions with high reaction rates are significant to influence

20、 nox formation. with cao presence in a cfb combustor, the important reactions associated to nox formation are listed in tab.16.table 1. important no related reactions in a cfb combustor reaction molcmskg/moln2+ono+nn+o2no+on+hono+hno+ho2no2+hono+mno+mno+o+mno2+mn2+o+mn2o+m1.3010146.401094.0010131.00

21、10136.4010169.4010141.4010130.01.00.00.00.50.00.0315.8926.150.000.000.008.07586.609 it can be seen that most of the reactions for nox formation with is faster than those for nox decompositiontherefore in the limited residence time, the presence of limestone favors more nox.3.2 p referential conversi

22、on effect from hcn to nh3 the gaseous nitrogen matter exists in forms of aromatic or amino hydrocarbons in the beginning of the de-volatilization process, depending on the coal type, heating rate and temperature etc. most nitrogen in aromatic hydrocarbons is then converted to hcn while most nitrogen

23、 in amino hydrocarbons is decomposed to nh3. later, hcn and nh3 are oxidized following different pathways in a cfb combustor, hcn is preferentially oxidized to n2o in following pathways 7,8: hcn+onco+h (r1) nco+non2o+co (r2)and, nh3 is preferentially oxidized to nox in following three steps: step 1:

24、 nh3 nh2 nh3+ohnh2+h2o (r3) nh3+onh2+oh (r4) nh3+onh2+oh (r5) step 2: nh2 nh nh2+ohnh+h2o (r6) nh2+onh+oh (r7)nh2+hnh+h2 (r8) step 3: nh no nh+o2no+oh(r9) nh+ono+h (r10) nh+ohno+h2 (r11) it can be seen from above chemical schemes, two major forms of nitrogen compounds exist in a cfb combustor: n2o a

25、nd no and they are preferent-ially oxidized from hcn and nh3 respectively. when limestone is added for desulphurization, it produces cao during the pyrolysis process. then cao can react with hcn, converting hcn into nh3. cao+2hcncacn2+co+h2 r12 cacn2+3h2ocao+co2+2nh3 r13 cacn2+h2o+2h2+co2cao+2nh3+2c

26、o r14based on the preferential oxidization schemes of r3 to r1 1, nh3 is prone to form no,resulting in enhancement of nox formatio-n. however, even though n2o is the main pollutant in a cfb combustor, it is little affected by limestone injection. on one hand, n2o formation is enhanced in with homoge

27、neous reaction r2 and the heterogeneous reactions on carbon surface such as r15 and r16; one the other hand, the disassociation of n2o is also enhanced in reaction r17 and r18. cno+non2o+co r15 cn+non2o+c r16 2n2o2n2+o2r17 n2o+ cn2+co r18the reaction r17 is sensitive to temperature. when temperature

28、 is above 1250k, more than90% of n2o is decomposed into n2 and o2. experimental study 9 showed that the temperaturethreshold for initialing n2o decomposition is lowered by limestone to be from 1100k to 950kwith limestone, and more than 70% of the n2o is decomposed at temperature of 1100k.3.3 r educt

29、ion effects of cao on nox formation as shown in fig. 3, although cao decomposed from caco3 facilitate nox formation whenca/s is rather small, it can also suppress nox formation when ca/s is large. the possible reasons are: 1so2 favors to convert hcn to nh3, resulting in more no product according to

30、thekinetics discussed in previous section. from the other view, the depletion of so2 at large ca/s ratios obstacles hcn conversion, resulting in less no product2in the hot combustor, cao absorbs so2 to form caso3, which acts as a reduction agent for no in reaction 9: 2no+caso3n2o+caso4 r19 3cao and

31、some other materials in a cfb combustor will promote the conversionreaction of no to n2 10,11. the materials include the al2o3 and mgo in ash and fe2o3 on the combustor wall. it reaction is as following: 4nh3+6no5n2+6h2or20 4 no adheringon the surface of cao particles is easier to be reduced by co o

32、r other reduction agents.4. conclusions while limestone favors desulphurization in a coal-fired cfb combustor, but it might increase pollutant no emission. limestone can act as a catalyst to enhance nox formation, influencing the associated reaction rates. the chemical kinetics shows that hcn is pro

33、ne to form unstable n2o while nh3 form rather stable nox, and part of hcn released from de-volatilization process is converted into nh3 by cao decomposed from caco3. the catalytic effect and preferential conversion of hcn to nh3 increases the no emission. with high ca/s, the reduction effect becomes

34、 dominated and reduces the overall nox emission.it is important to optimzing ca/s ratio in cfb boiler design and operation for controlling both so2 and nox emissions.references:1feng junkai, yue guangxi, lu junfucirculating fluidized combustion boiler m chinese electricpower press. 2003.2zhou haoshe

35、ng, lu jidong, zhou hunitrogen conversion in fluidized bed combustion of coal withlimestone addition. journal of engineering thermophysics, 2000,9 vol.21 no.5:6476513feng junkai, shen youting. boilers principle & calculation ii. science press. 1998, 2062074bowman c t. control of combustion generated

36、 nitrogen oxide emissions: technology driven regulation.proc. 24th combustion inst. 1992:8598785han caiyua, xu mingho. coal dust combustion., science press, 2001: 4494506moria hori. combustion science and technology. 1980:231317ren wei, zhang jiansheng, jiang xiaoguo, lu junfu, yao jiheng, yue guang

37、xi. experimental study on nitrous oxide for mation during char at combustion at fluidized bed conditionacta scientiaecircumstantiae,2003.5 vol.23 no.3:4084108 ren wei, xiao xianbin lu junfu, yue guangxi, research on conversion of nitrogen in char duringcombustion under fluidized bed condition. journ

38、al of china university of mining & technology, 2003.5vol.32 no.3:2592669 zhou lixing, lu jidong, zhou hunireogen conversion in fluidized bed combustor of coal withlimestone addition. journal of engineering thermophysics. vol.21, no.5:64765110mike braford, rajiv grover, pieter paulcontrolling noxemis

39、sion: part 2chemical engineeringprogress, 2002, 984: 384211 zhongzhaoping,lanjixiang,hanyongsheng reducingdesulfurizationandammoniainjectiondenitrification in a coal-fired fluidized bed combustion with fly-ash recyclejournal of combustionscience and technology,1997, 31: 4753中文翻譯 石灰石脫硫?qū)ρh(huán)流化床中nox排放的影響

40、 摘 要 循環(huán)流化床燃燒技術(shù)已經(jīng)在能源動力領(lǐng)域得到了廣泛地運用,因為它具有能夠十分經(jīng)濟地控制燃燒過程中so2 和 nox 的排放。但是運行實踐表明,加入石灰石對循環(huán)流化床燃燒過程中nox的排放有一定的負(fù)面影響,煙氣中的nox濃度增大到30%,為了提高脫硫效率采用較高的鈣硫比時,nox的排放濃度也會增大。本文綜合分析了石灰石脫硫?qū)ρh(huán)流化床中nox排放的影響機理,并從化學(xué)動力學(xué)的角度對該結(jié)果進行了理論分析。石灰石反應(yīng)生成的氧化鈣對揮發(fā)分中的nh3 氧化生成no 的反應(yīng)有較大的催化作用,促進了no 的生成。氧化鈣還能促進揮發(fā)分中的hcn向nh3 轉(zhuǎn)化,由于hcn氧化傾向于生成熱穩(wěn)定性較差的n2o而

41、nh3 氧化傾向于生成no, 故hcn 向nh3 的轉(zhuǎn)化也使流化床排煙煙氣中的nox 濃度有所增大。氧化鈣對nox 生成的促進作用都占主導(dǎo)地位,過高的鈣硫比使nox 排放濃度增大。 關(guān)鍵詞 循環(huán)流化床, 石灰石, 脫硫, nox, 催化化學(xué)動力學(xué) 1. 介紹 循環(huán)流化床燃燒技術(shù)已經(jīng)在能源動力領(lǐng)域得到了廣泛地運用,由于其煤種適應(yīng)性和低成本污染物排放控制等優(yōu)點,已成為很有競爭力的一種潔凈煤技術(shù)。由于循環(huán)流化床中燃燒溫度一般處于10201120k的溫度范圍內(nèi),并且可以采用更為自由的二次風(fēng)布風(fēng)方式,可以抑制熱力型nox 的生成,使nox 的排放濃度大大低于采用其他燃燒方式。從圖1可以看出,與其他燃燒方

42、式相比,循環(huán)流化床的nox 的排放濃度最小,在100-220ppm1。 圖1 不同燃燒方式nox 排放水平3 當(dāng)煤粒表面充分燃燒時,在循環(huán)流化床中加入石灰石脫硫。在正常溫度范圍內(nèi),由于石灰石脫硫的效率相對較高通常能達到90%以上1。但是,研究表明石灰石作為脫硫劑加入后,對流化床的運行產(chǎn)生的影響是多方面的,例如石灰石的加入在有效的脫去so2 等硫化物的同時,會影響爐膛中的燃燒情況,傳熱情況,改變循環(huán)流化床的物料平衡,使分離器和除塵器的負(fù)擔(dān)增大等。但是,運行和試驗數(shù)據(jù)也表明,石灰石的加入對循環(huán)流化床nox 的排放有一定的負(fù)面影響,圖2描繪了一臺商業(yè)循環(huán)流化床鍋爐在運行溫度為1150k和1200k,

43、ca/s比為2.2,脫硫工況時nox的排放情況。為了提高脫硫效率采用較高的鈣硫比時nox 的排放濃度通常也會有所增大,增大了污染物的排放,應(yīng)該引起重視。從圖中可以看出,當(dāng)石灰石加入后,對n2o的排放影響較小,而no卻增加了50ppm,或者為30%2。圖3進一步描繪了一些關(guān)于ca/s比對nox排放影響的實驗結(jié)果3。當(dāng)改變ca/s比稍小于2時,nox濃度隨著ca/s比的增加而增加;當(dāng)ca/s比超過2時,nox濃度隨著ca/s比的增加而減小。即使nox在類似反應(yīng)中的生成機理已很明了,對在另類空間特別是在石灰石加入的情況下,nox的生成機理的研究還相當(dāng)有限。 圖2 加入石灰石前后nox排放 圖3 鈣硫

44、摩爾比對nox 濃度 濃度比較的影響tb1165k, o26%2. 循環(huán)流化床中nox 生成途徑與化學(xué)動力機理 通常,煤燃燒生成nox 的途徑主要有3 個4:1熱力型nox,由空氣中的氮氣在高溫下氧化而生成,此時o,oh和n2處于均衡狀態(tài),n原子處于穩(wěn)定狀態(tài);2快速型nox,no的生成速率要比上面講的熱力型機理虧愛很多,它由i燃燒時空氣中的氮和燃料中的碳氫離子團如ch 等反應(yīng)生成hcn 和n, 再進一步與氧作用,以極快的速率生成,或者ii由n2o快速分解,或者iii由o與oh離子團濃度不均勻與熱力機理共同作用導(dǎo)致產(chǎn)生;3 燃料型nox,由燃料中含有的氮化合物在燃燒過程中熱分解而又接著被氧化而生

45、成no。 燃燒溫度低于1750k時幾乎觀測不到高溫型nox 的生成反應(yīng),快速型nox 是產(chǎn)生于燃燒時chi 類離子團較多、o2濃度相對低的富燃料燃燒,一般多發(fā)生于內(nèi)燃機中。因此,循環(huán)流化床鍋爐燃燒中nox 的生成主要是燃料型nox。 燃料型nox 的生成機理非常復(fù)雜。煤被加熱時,煤中的揮發(fā)分便熱解析出,燃料中氮有機化合物首先被熱分解成hcn 和nh3 等中間產(chǎn)物,它們隨揮發(fā)分一起從燃料中析出,稱之為揮發(fā)分n。揮發(fā)分n 析出后,仍殘留在焦炭中的氮稱為焦炭n。燃料n 轉(zhuǎn)化為揮發(fā)分n 和焦炭n 的比例與煤種、熱解溫度及加熱速度等有關(guān)5。通常當(dāng)煤種的揮發(fā)分含量高,熱解溫度和加熱溫度提高時,揮發(fā)分n 增

46、加而焦炭n 相應(yīng)地減少。揮發(fā)分中氮最終以n2、nox 和n2o 的形式釋放,焦炭氮隨著焦炭的燃燒逐步釋放6,7。3. cao 的加入對循環(huán)流化床中nox 濃度的影響3.1對燃料型n的催化作用 通常情況下,煙氣在爐膛中停留時間有限,不可能使每種反應(yīng)都達到均衡。一旦爐膛中產(chǎn)生煙氣,就會被冷卻,連鎖反應(yīng)就會停止。no參與的反應(yīng)速率可以用下面的公式表達:其中kc 由arrhenius 公式: 因此,小的活化能和大的碰撞機率會導(dǎo)致高的反應(yīng)速度。只有反映具有高的反應(yīng)速度才會對nox生成有影響。在cao加入循環(huán)流化床鍋爐中后,聯(lián)合產(chǎn)生nox的主要反應(yīng)列于下表16: 表1. 循環(huán)流化床鍋爐種no主要相關(guān)反應(yīng)反

47、應(yīng)molcmskg/moln2+ono+nn+o2no+on+hono+hno+ho2no2+hono+mno+mno+o+mno2+mn2+o+mn2o+m1.3010146.401094.0010131.0010136.4010169.4010141.4010130.01.00.00.00.50.00.0315.8926.150.000.000.008.07586.609 從上表可以看到,大多反應(yīng)nox的生成速率要大于nox的分解速率。因此,在有限的煙氣停留時間內(nèi),石灰石的加入會導(dǎo)致更多nox的生成。3.2 對hcn轉(zhuǎn)化nh3的傾向的影響 在燃煤析出揮發(fā)分過程中,燃料n是以芳香環(huán)形式還是以炭氫化合物形式會發(fā)出來與煤種、加熱速度及熱解溫度等有關(guān)。以芳香環(huán)形式存在于煤中的燃料氮在揮發(fā)分燃燒過程中主要生成hcn,而以胺形態(tài)存在的燃料氮則主要以nh3 的形式析出,然后,hcn和nh3通過不同的途徑被氧化。 hcn 傾向于通過下述反