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1、外文翻译-石灰石脱硫对循环流化床中NOX排放的影响 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中文翻译 石灰石脱硫对循环流化床中NOX排放的影响
40、 摘 要 循环流化床燃烧技术已经在能源动力领域得到了广泛地运用,因为它具有能够十分经济地控制燃烧过程中SO2 和 NOx 的排放。但是运行实践表明,加入石灰石对循环流化床燃烧过程中NOx的排放有一定的负面影响,烟气中的NOx浓度增大到30%,为了提高脱硫效率采用较高的钙硫比时,NOx的排放浓度也会增大。本文综合分析了石灰石脱硫对循环流化床中NOx排放的影响机理,并从化学动力学的角度对该结果进行了理论分析。石灰石反应生成的氧化钙对挥发分中的NH3 氧化生成NO 的反应有较大的催化作用,促进了NO 的生成。氧化钙还能促进挥发分中的HCN向NH3 转化,由于HCN氧化倾向于生成热稳定性较差的N2O而
41、NH3 氧化倾向于生成NO, 故HCN 向NH3 的转化也使流化床排烟烟气中的NOx 浓度有所增大。氧化钙对NOx 生成的促进作用都占主导地位,过高的钙硫比使NOx 排放浓度增大。 关键词 循环流化床, 石灰石, 脱硫, NOX, 催化化学动力学 1. 介绍 循环流化床燃烧技术已经在能源动力领域得到了广泛地运用,由于其煤种适应性和低成本污染物排放控制等优点,已成为很有竞争力的一种洁净煤技术。由于循环流化床中燃烧温度一般处于10201120K的温度范围内,并且可以采用更为自由的二次风布风方式,可以抑制热力型NOx 的生成,使NOx 的排放浓度大大低于采用其他燃烧方式。从图1可以看出,与其他燃烧方
42、式相比,循环流化床的NOx 的排放浓度最小,在100-220ppm1。 图1 不同燃烧方式NOx 排放水平3 当煤粒表面充分燃烧时,在循环流化床中加入石灰石脱硫。在正常温度范围内,由于石灰石脱硫的效率相对较高通常能达到90%以上1。但是,研究表明石灰石作为脱硫剂加入后,对流化床的运行产生的影响是多方面的,例如石灰石的加入在有效的脱去SO2 等硫化物的同时,会影响炉膛中的燃烧情况,传热情况,改变循环流化床的物料平衡,使分离器和除尘器的负担增大等。但是,运行和试验数据也表明,石灰石的加入对循环流化床NOx 的排放有一定的负面影响,图2描绘了一台商业循环流化床锅炉在运行温度为1150K和1200K,
43、Ca/S比为2.2,脱硫工况时NOx的排放情况。为了提高脱硫效率采用较高的钙硫比时NOx 的排放浓度通常也会有所增大,增大了污染物的排放,应该引起重视。从图中可以看出,当石灰石加入后,对N2O的排放影响较小,而NO却增加了50ppm,或者为30%2。图3进一步描绘了一些关于Ca/S比对Nox排放影响的实验结果3。当改变Ca/S比稍小于2时,Nox浓度随着Ca/S比的增加而增加;当Ca/S比超过2时,Nox浓度随着Ca/S比的增加而减小。即使NOx在类似反应中的生成机理已很明了,对在另类空间特别是在石灰石加入的情况下,Nox的生成机理的研究还相当有限。 图2 加入石灰石前后Nox排放 图3 钙硫
44、摩尔比对NOx 浓度 浓度比较的影响Tb1165K, O26%2. 循环流化床中NOx 生成途径与化学动力机理 通常,煤燃烧生成NOx 的途径主要有3 个4:1热力型NOx,由空气中的氮气在高温下氧化而生成,此时O,OH和N2处于均衡状态,N原子处于稳定状态;2快速型NOx,NO的生成速率要比上面讲的热力型机理亏爱很多,它由i燃烧时空气中的氮和燃料中的碳氢离子团如CH 等反应生成HCN 和N, 再进一步与氧作用,以极快的速率生成,或者ii由N2O快速分解,或者iii由O与OH离子团浓度不均匀与热力机理共同作用导致产生;3 燃料型NOx,由燃料中含有的氮化合物在燃烧过程中热分解而又接着被氧化而生
45、成NO。 燃烧温度低于1750K时几乎观测不到高温型NOx 的生成反应,快速型NOx 是产生于燃烧时CHi 类离子团较多、O2浓度相对低的富燃料燃烧,一般多发生于内燃机中。因此,循环流化床锅炉燃烧中NOx 的生成主要是燃料型NOx。 燃料型NOx 的生成机理非常复杂。煤被加热时,煤中的挥发分便热解析出,燃料中氮有机化合物首先被热分解成HCN 和NH3 等中间产物,它们随挥发分一起从燃料中析出,称之为挥发分N。挥发分N 析出后,仍残留在焦炭中的氮称为焦炭N。燃料N 转化为挥发分N 和焦炭N 的比例与煤种、热解温度及加热速度等有关5。通常当煤种的挥发分含量高,热解温度和加热温度提高时,挥发分N 增
46、加而焦炭N 相应地减少。挥发分中氮最终以N2、NOx 和N2O 的形式释放,焦炭氮随着焦炭的燃烧逐步释放6,7。3. CaO 的加入对循环流化床中NOx 浓度的影响3.1对燃料型N的催化作用 通常情况下,烟气在炉膛中停留时间有限,不可能使每种反应都达到均衡。一旦炉膛中产生烟气,就会被冷却,连锁反应就会停止。NO参与的反应速率可以用下面的公式表达:其中Kc 由Arrhenius 公式: 因此,小的活化能和大的碰撞机率会导致高的反应速度。只有反映具有高的反应速度才会对NOX生成有影响。在CaO加入循环流化床锅炉中后,联合产生NOX的主要反应列于下表16: 表1. 循环流化床锅炉种NO主要相关反应反
47、应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 从上表可以看到,大多反应NOX的生成速率要大于NOX的分解速率。因此,在有限的烟气停留时间内,石灰石的加入会导致更多NOX的生成。3.2 对HCN转化NH3的倾向的影响 在燃煤析出挥发分过程中,燃料N是以芳香环形式还是以炭氢化合物形式会发出来与煤种、加热速度及热解温度等有关。以芳香环形式存在于煤中的燃料氮在挥发分燃烧过程中主要生成HCN,而以胺形态存在的燃料氮则主要以NH3 的形式析出,然后,HCN和NH3通过不同的途径被氧化。 HCN 倾向于通过下述反应生成N2O7,8: HCN+ONCO+H (R1) NCO+NON2O+CO (R2)而NH3被氧化生成NOx 的转化率较大,其反应步骤有三: Step 1: NH3 NH2 NH3+OHNH2+
