外文翻譯--石灰石脫硫?qū)ρh(huán)流化床中nox排放的影響

1、<p><b>　　中文3132字</b></p><p><b>　　英文原文</b></p><p>　　Limestone Effects on NOx Formation in CFB Combustors </p><p><b>　　Abstract </b></p>

2、<p>　　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 foun

3、d 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

4、and chemical kinetics. The catalytic oxidization effect, preferential con</p><p>　　Keywords CFB, limestone, desulphurization, NOX formation, chemical kinetics </p><p>　　1. Introduction </p&

5、gt;<p>　　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 hi

6、gh feasibility in economical emission control. Given that the combustion temperature in CFB combustors, e.g., boilers, is usually between 1020~1120K and secondary air can be injected at different ports along the axial di

7、rection, the NOX concentration in the flue gas can be controlled to be much </p><p>　　Fig. 1 NOX emissionlevel for different combustion systems [3] </p><p>　　When coals with high surface content

8、 are burned, limestone is added into CFB boilers for desulphurization. Within the normal temperatures, the efficiency of limestone desulphurization can be up to 90% [1]. In addition to the desulphurization, limestone may

9、 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, observed in practical operation and experiments,

10、 limestone addition in C</p><p>　　Though the mechanisms for NOx formation in homogeneous reactions are rather clear, the studies on the mechanisms for NOx formation in heterogeneous atmosphere, especially wi

11、th the presence of limestone, are limited. </p><p>　　Fig.2 NOX emission with/without Fig.3 NOX concentration with differentlimestone desulphurization Ca/S ratios (Tb=1165K, [O2]=6% ) </p>

12、<p>　　2. NOx formation mechanisms and chemical kinetics in a CFB combustor </p><p>　　There are mainly three well-known mechanisms counting for the NOX formation in coal combustion [4]: (1)Extended Zeldo

13、vich ( 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,

14、 either by (i) Fenimore CN and HCN pathways, or (ii) the N2O-intermediate route, or (iii) as a result of non-equilibrium concentrations of O and OH radicals in con</p><p>　　Apparently, for a CFB combustor, n

15、either the Extended Zeldovich 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 radica

16、l and low O2 concentration is important. The dominate mechanism for NOX formation in CFB combustor is fuel nitrogen mechanism. </p><p>　　Fuel Nitrogen mechanism is rather complicated. Part of nitrogen in coa

17、l 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 de-vola

18、tilization, 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</p>

19、<p>　　3. Limestone effects on NO formation a CFB combustor</p><p>　　3.1 Catalytic effect for fuel-N oxidization </p><p>　　Given the residence time of flue gas in the combustor is short, i

20、t is impossible for every reaction to reach equilibrium. Once the flue gas exits the combustor, it is cooled and the associated reactions stop. The reaction rates with NO taking part in can be expressed as:</p>&l

21、t;p>　　Where, Kc is expressed by Arrhenius’ law:</p><p>　　Therefore, small activate energies and large collision frequencies favor high reaction rate. Only reactions with high reaction rates are significan

22、t to influence NOX formation. With CaO presence in a CFB combustor, the important reactions associated to NOx formation are listed in Tab.1[6].</p><p>　　Table 1. Important NO related reactions in a CFB combu

23、stor</p><p>　　It can be seen that most of the reactions for NOX formation with is faster than those for NOX decomposition. Therefore in the limited residence time, the presence of limestone favor

24、s more NOX.</p><p>　　3.2 P referential conversion effect from HCN to NH3 </p><p>　　The gaseous nitrogen matter exists in forms of aromatic or amino hydrocarbons in the beginning of the de-volat

25、ilization process, depending on the coal type, heating rate and temperature etc. Most nitrogen in aromatic hydrocarbons is then converted to HCN while most nitrogen in amino hydrocarbons is decomposed to NH3. Later, HCN

26、and NH3 are oxidized following different pathways. </p><p>　　In a CFB combustor, HCN is preferentially oxidized to N2O in following pathways [7,8]: </p><p>　　HCN＋O→NCO＋H

27、 （R1） </p><p>　　NCO＋NO→N2O＋CO （R2）</p><p>　　And, NH3 is preferentially oxidized to NOX in following three steps:</p><p&

28、gt;　　Step 1: NH3→ NH2 </p><p>　　NH3＋OH→NH2＋H2O （R3） </p><p>　　NH3＋O→NH2＋OH

29、 （R4） </p><p>　　NH3＋O→NH2＋OH （R5） </p><p>　　Step 2: NH2→ NH

30、 </p><p>　　NH2＋OH→NH＋H2O （R6） </p><p>　　NH2＋O→NH＋OH （R7） <

31、/p><p>　　NH2＋H→NH＋H2 （R8） </p><p>　　Step 3: NH→ NO </p><p>　　NH＋O2→

32、NO＋OH （R9） </p><p>　　NH＋O→NO＋H （R10） </p><p>　　NH＋OH→NO＋H2

33、 （R11）</p><p>　　It can be seen from above chemical schemes, two major forms of nitrogen compounds exist in a CFB combustor: N2O and NO and they are preferent-ially oxidized fro

34、m 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.</p><p>　　CaO+2HCN→CaCN2+CO+H2

35、 (R12) </p><p>　　CaCN2+3H2O→CaO+CO2+2NH3 (R13) </p><p>　　CaCN2+H2O+2H2+CO2→CaO+2NH3

36、+2CO (R14) </p><p>　　Based on the preferential oxidization schemes of (R3) to (R1 1), NH3 is prone to form NO,resulting in enhancement of NOx formatio-n.</p><

37、p>　　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 homogeneous reaction (R2) and the heterogeneous react

38、ions on carbon surface such as (R15) and (R16); one the other hand, the disassociation of N2O is also enhanced in reaction (R17) and (R18).</p><p>　　CNO＋NO→N2O＋CO (

39、R15) </p><p>　　CN＋NO→N2O＋C (R16) </p><p>　　2N2O→2N2＋O2 (R17) </p>&

40、lt;p>　　N2O＋ C→N2＋CO (R18) </p><p>　　The reaction (R17) is sensitive to temperature. When temperature is above 1250K, more than90% of N2O is decomposed into N2

41、 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

42、temperature of 1100K.</p><p>　　3.3 R eduction effects of CaO on NOx formation </p><p>　　As shown in Fig. 3, although CaO decomposed from CaCO3 facilitate NOX formation whenCa/S is rather small,

43、it can also suppress NOX formation when Ca/S is large. The possible reasons are: </p><p>　　1. SO2 favors to convert HCN to NH3, resulting in more NO product according to the

44、 kinetics discussed in previous section. From the other view, the depletion of SO2 at large Ca/S ratios obstacles HCN conversion, resulting in less NO product. </p><p>　　2. In the hot

45、 combustor, CaO absorbs SO2 to form CaSO3, which acts as a reduction agent for NO in reaction [9]: </p><p>　　2NO＋CaSO3→N2O＋CaSO4 (R19) </p>

46、<p>　　3. CaO and some other materials in a CFB combustor will promote the conversion reaction of NO to N2 [ 10，11]. The materials include the Al2O3 and MgO in ash and Fe2O3 on the combustor wall. It

47、 reaction is as following: </p><p>　　4NH3＋6NO→5N2＋6H2O (R20) </p><p>　　4. NO adheringon the surface of CaO particles is easier to be reduced by CO or o

48、ther reduction agents.</p><p>　　4. Conclusions </p><p>　　While limestone favors desulphurization in a coal-fired CFB combustor, but it might increase pollutant NO emission. Limestone can act as

49、a catalyst to enhance NOx formation, influencing the associated reaction rates. The chemical kinetics shows that HCN is prone to form unstable N2O while NH3 form rather stable NOX, and part of HCN released from de-volati

50、lization process is converted into NH3 by CaO decomposed from CaCO3. The catalytic effect and preferential conversion of HCN to NH3 increases t</p><p>　　References:</p><p>　　[1]Feng Junkai, Y

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60、cience and Technology,1997, 3(1): 47~53</p><p><b>　　中文翻譯</b></p><p>　　石灰石脫硫?qū)ρh(huán)流化床中NOX排放的影響 </p><p><b>　　摘 要</b></p><p>　　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

61、泛地運(yùn)用，因為它具有能夠十分經(jīng)濟(jì)地控制燃燒過程中SO2 和 NOx 的排放。但是運(yùn)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(jī)理，并從化學(xué)動力學(xué)的角度對該結(jié)果進(jìn)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(jìn)了

62、NO 的生成。氧化鈣還能促進(jìn)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(jìn)作用都占主導(dǎo)地位，過高的鈣硫比使NOx 排放濃度增大。 </p><p>　　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) </p>&l

63、t;p><b>　　1. 介紹 </b></p><p>　　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(yùn)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1020～1120K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

64、出，與其他燃燒方式相比，循環(huán)流化床的NOx 的排放濃度最小，在100-220ppm[1]。</p><p>　　圖1 不同燃燒方式NOx 排放水平[3]</p><p>　　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(dá)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(yùn)行產(chǎn)生的影響是多方面的，例如石灰石的加入在

65、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(yùn)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(yùn)行溫度為1150K和1200K，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

66、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(jìn)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(jī)理已很明了，對在另類空間特別</p><p>　　圖2 加入石灰石前后Nox排放

67、 圖3 鈣硫摩爾比對NOx 濃度</p><p>　　濃度比較 的影響(Tb=1165K, [O2]=6%) </p><p>　　2. 循環(huán)流化床中NOx 生成途徑與化學(xué)動力機(jī)理</p><p

68、>　　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(jī)理虧愛很多，它由(i)燃燒時空氣中的氮和燃料中的碳?xì)潆x子團(tuán)如CH 等反應(yīng)生成HCN 和N， 再進(jìn)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(tuán)濃度不均勻與熱力機(jī)理共同作用導(dǎo)

69、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成NO。</p><p>　　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(tuán)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(jī)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。</p><p>　　燃料型NOx 的生成機(jī)理非常復(fù)

70、雜。煤被加熱時，煤中的揮發(fā)分便熱解析出，燃料中氮有機(jī)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

71、釋放[6,7]。</p><p>　　3. CaO 的加入對循環(huán)流化床中NOx 濃度的影響</p><p>　　3.1對燃料型N的催化作用 </p><p>　　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(dá)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(dá)：</p><p>　　其中

72、Kc 由Arrhenius 公式：</p><p>　　因此，小的活化能和大的碰撞機(jī)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1[6]：</p><p>　　表1. 循環(huán)流化床鍋爐種NO主要相關(guān)反應(yīng)</p><p>　　從上表可以看到，大多反應(yīng)NOX的生成速率要大于NOX的分

73、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。</p><p>　　3.2 對HCN轉(zhuǎn)化NH3的傾向的影響 </p><p>　　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

74、NH3通過不同的途徑被氧化。</p><p>　　HCN 傾向于通過下述反應(yīng)生成N2O[7,8]：</p><p>　　HCN＋O→NCO＋H （R1） </p><p>　　NCO＋NO→N2O＋CO

75、 （R2）</p><p>　　而NH3被氧化生成NOx 的轉(zhuǎn)化率較大，其反應(yīng)步驟有三：</p><p>　　Step 1: NH3→ NH2 </p><p>　　NH3＋OH→NH2＋H2O

76、 （R3） </p><p>　　NH3＋O→NH2＋OH （R4） </p><p>　　NH3＋O→NH2＋OH

77、 (R5） </p><p>　　Step 2: NH2→ NH </p><p>　　NH2＋OH→NH＋H2O （R6） </p><p>　　NH2＋O→NH＋OH

78、 （R7） </p><p>　　NH2＋H→NH＋H2 （R8） </p><p>　　Step 3: NH→ NO

79、 </p><p>　　NH＋O2→NO＋OH （R9） </p><p>　　NH＋O→NO＋H （R10） </

80、p><p>　　NH＋OH→NO＋H2 （R11）</p><p>　　從上面的化學(xué)方程式可以看到，在循環(huán)流化床燃燒中主要有兩種氮的化合物形式：N2O和NO，它們分別由HCN和NH3 氧化而來。當(dāng)石灰石加入爐膛脫硫時，石灰石在煅燒過程中產(chǎn)生了CaO。CaO 再與

81、HCN反應(yīng)，促使HCN 向NH3 轉(zhuǎn)化：</p><p>　　CaO+2HCN→CaCN2+CO+H2 (R12) </p><p>　　CaCN2+3H2O→CaO+CO2+2NH3

82、 (R13) </p><p>　　CaCN2+H2O+2H2+CO2→CaO+2NH3+2CO (R14) </p><p>　　從上面的化學(xué)方程(R3)到(R11)表明，NH3更易于從NO轉(zhuǎn)化而來，導(dǎo)致NOx排放的增加。</p><p>　　但是，即使N2O是循環(huán)流化床中的主要污染物，石灰石的

83、加入對它的影響較小。一方面，反應(yīng)(R2)和發(fā)生在炭表面的的反應(yīng)如(R15)和(R16)加強(qiáng)了N2O的生成；另一方面，反應(yīng)(R17)和(R18)加強(qiáng)了N2O的分解。</p><p>　　CNO＋NO→N2O＋CO (R15) </p><p>　　CN＋NO→N2O＋C

84、 (R16) </p><p>　　2N2O→2N2＋O2 (R17) </p><p>　　N2O＋ C→N2＋CO

85、 (R18) </p><p>　　反應(yīng)(R17)對溫度十分敏感，在高溫下，這個反應(yīng)十分迅速。在1250K ℃時，N2O 轉(zhuǎn)化成N2和 O2的轉(zhuǎn)化率達(dá)到了90%以上。實驗[9]表明，加入石灰石脫硫以后，能降低N2O 的轉(zhuǎn)化溫度，N2O 的初始分解溫度從1100K降低到950K，1100K時N2O 的轉(zhuǎn)化率達(dá)到70%以上。</p><p>　　3.3 CaO對NOx排放的

86、負(fù)面影響</p><p>　　如圖3所示，雖然在Ca/S很低時，從CaCO3分解出的CaO氧化鈣對CFB 中NOx 的生成有促進(jìn)作用，但是，在Ca/S很大時，石灰石脫硫?qū)FB 煙氣中NOx 排放濃度有降低，可能的原因有：</p><p>　　1．實驗表明，SO2 對HCN 向NH3 轉(zhuǎn)化的反應(yīng)也有促進(jìn)作用，根據(jù)上面的討論會導(dǎo)致更多NO排放。從另一方面看，在大Ca/S比時，SO2的大量減少

87、，抑制了HCN的轉(zhuǎn)化，使NO的生成減少。</p><p>　　2．氧化鈣與SO2發(fā)應(yīng)生成的CaSO3 在高溫下有較強(qiáng)的還原性，反應(yīng)方程式為[9]：</p><p>　　2NO＋CaSO3→N2O＋CaSO4 (R19) </p><p>　　3．

88、CaO和流化床循環(huán)灰中的其他一些物質(zhì)對NH3還原NOx 生成N2 的反應(yīng)有催化促進(jìn)作用[10,11]，這些物質(zhì)包括煙灰中的Al2O3 和 MgO 以及爐墻上的Fe2O3。反應(yīng)式為：</p><p>　　4NH3＋6NO→5N2＋6H2O (R20) </p><p>　　4．NO 能與CaO表面吸附的CO 等

89、還原性物質(zhì)發(fā)生反應(yīng)，降低煙氣中NOx 濃度。</p><p><b>　　4. 結(jié)論</b></p><p>　　在燃煤的CFB 鍋爐中，加入石灰石脫硫能達(dá)到較高的脫硫效率，使SO2 排放濃度大大降低。但是，會增加污染物NO的排放。石灰石反應(yīng)生成的氧化鈣對NOx的生成起到催化作用，加快連鎖反應(yīng)的反應(yīng)速度?；瘜W(xué)動力學(xué)也表明，HCN氧化傾向于生成熱穩(wěn)定性較差的N2O而NH

90、3 氧化傾向于生成。CaCO3分解生成的CaO還能促進(jìn)揮發(fā)分中的部分HCN向NH3 轉(zhuǎn)化，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(jìn)作用都占主導(dǎo)地位，過高的鈣硫比使NOx 排放濃度增大，所以在CFB 中采用氧化鈣脫硫時，要綜合考慮石灰石對SO2 的脫除和NOx 的生成的促進(jìn)作用，以這兩類污染物對環(huán)境的綜合影響最低作為目標(biāo)來確定鈣硫比，使流化床鍋爐的環(huán)保性能發(fā)揮到最優(yōu)。</p&g