外文翻译--石灰石脱硫对循环流化床中NOX排放的影响 (1)

英文原文Limestone Effects on NOx Formation in CFBCombustorsAbstractCirculating Fluidized Bed (CFB) combustion technology has been widely used inpower generation with the considerations of its advantages in economically controlling SO2and NOx emissions. However, it is found that NOXemission 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 effect fromHCN 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 NH3when they arerelease in de-volatilization process. Due to preferential oxidization of NH3to NO rather thanthe unstable N2O, NOXemission increases with limestone injection. When high Ca/S is used,the reduction effect becomes dominated and reduces the overall NOXemission.Keywords CFB, limestone, desulphurization, NOXformation, chemical kinetics1. IntroductionCirculating 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 temperature in CFB combustors, e.g., boilers, is usually between 1020~1120K and secondary air can be injected at different ports along the axial direction,the NOXconcentration in the flue gas can be controlled to be much lower than those using other combustion technologies. Shown in Fig. 1, NOx emission from a CFB combustors are lowest, in the range of 100-220 ppm, compared with other kinds of combustors [1].Fig. 1 NO X 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 temperatures, 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, observed in practical operation and experiments, limestone addition in CFB combustors might cause aemissions. Figure 2 depicts the NOx emission of a commercial negative effect on the NOXCFB boiler with limestone desulphurization, operating with bed temperature between 1150K and 1200K with Ca/s ratio of 2.2. It can be seen, when limestone is injected, while O emission was little affected, the NO remarkably increased by 50 ppm or about 30%[2].N2Figure 3 further depicts some experimental results of the influence Ca/S ratio on the NOxconcentration increases emission [3]. When Ca/S ratio changes is smaller than 2, the NOXconcentration decreases with with the Ca/S ratio. When Ca/S ratio is higher than 2, the NOXthe 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 with the presence of limestone, are limited.Fig.2 NOX emission with/without Fig.3 NOX concentration with different limestone desulphurization Ca/S ratios(Tb=1165K, [O2]=6% ) 2. NOx formation mechanisms and chemical kinetics in a CFB combustorThere are mainly three well-known mechanisms counting for the NO X formation in coal combustion [4]: (1)Extended 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 pathways, 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. (3)Fuel Nitrogen Mechanism, in which fuel-bound nitrogen is converted to NO.Apparently, for a CFB combustor, neither the Extended Zeldovich mechanism which depends strongly on temperature and only becomes significant when temperature is above 1750K, nor thPrompt mechanism which becomes significant only with abundant CH ieradical and low O2 concentration is important. The dominate mechanism for NO X formation in CFB combustor is fuel nitrogen 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 de-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 with fixed carbon combustion[6, 7].3. Limestone effects on NO formation a CFB combustor3.1 Catalytic effect for fuel-N oxidizationGiven the residence time of flue gas in the combustor is short, it is impossible forevery 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:βατb ac NO C C k C =∂∂- Where, K c is expressed by Arrhenius’ law:)exp(RTE AT k B c -= Therefore, small activate energies and large collision frequencies f a vor high reactionrate. Only reactions with high reaction rates are significant to influence NO Xformation. With CaO presence in a CFB combustor, the important reactionsassociated to NOx formation are listed in Tab.1[6].Table 1. Important NO related reactions in a CFB combustorReaction A (molcms)B E (kg/mol) N 2+O=NO+N N+O 2=NO+O N+HO=NO+H NO+HO 2=NO 2+HO NO+M=NO+M NO+O+M=NO 2+M N 2+O+M=N 2O+M 1.301014 6.40109 4.0010131.001013 6.401016 9.401014 1.401013 0.0 1.0 0.0 0.0 0.5 0.0 0.0 315.89 26.15 0.00 0.00 0.00 8.075 86.609It can be seen that most of the reactions for NO X formation with is faster than thosefor NO X decomposition. Therefore in the limited residence time, the presenceof limestone favors more NO X .3.2 P referential conversion effect from HCN to NH 3The gaseous nitrogen matter exists in forms of aromatic or amino hydrocarbons in thebeginning of the de-volatilization process, depending on the coal type, heating rate andtemperature etc. Most nitrogen in aromatic hydrocarbons is then converted to HCN whilemost nitrogen 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＋O→NCO＋H （R1）NCO＋NO→N2O＋CO （R2）And, NH3 is preferentially oxidized to NO X in following three steps:Step 1: NH3→NH2NH3＋OH→NH2＋H2O （R3）NH3＋O→NH2＋OH（R4）NH3＋O→NH2＋OH（R5）Step 2: NH2→NHNH2＋OH→NH＋H2O （R6）NH2＋O→NH＋OH（R7）NH2＋H→NH＋H2 （R8）Step 3: NH→NONH＋O2→NO＋OH （R9）NH＋O→NO＋H （R10）NH＋OH→NO＋H2 （R11）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 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+2HCN→CaCN2+CO+H2 (R12)CaCN2+3H2O→CaO+CO2+2NH3 (R13) CaCN2+H2O+2H2+CO2→CaO+2NH3+2CO (R14) Based on the preferential oxidization schemes of (R3) to (R1 1), NH3 is prone to form NO,resulting in enhancement of NOxformatio-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 homogeneous reaction (R2) and the heterogeneous reactions on carbon surface suchas (R15) and (R16); one the other hand, the disassociation of N2O is also enhanced in reaction (R17) and (R18).CNO＋NO→N2O＋CO (R15) CN＋NO→N2O＋C (R16) 2N2O→2N2＋O2 (R17) N2O＋C→N2＋CO (R18) The reaction (R17) is sensitive to temperature. When temperature is above 1250K, more than90% of N2O is decomposed into N2 and O2. Experimental study [9] showed that the temperature threshold 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 eduction effects of CaO on NOx formationposed from CaCO3 facilitate NO X formation As shown in Fig. 3, although CaO decomwhenCa/S is rather small, it can also suppress NO X formation when Ca/S is large. The possible reasons are:1. SO2 favors to convert HCN to NH3, resulting in more NO product according to the 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.2. In the hot combustor, CaO absorbs SO2 to form CaSO3, which acts as a reduction agent for NO in reaction [9]:2NO＋CaSO3→N2O＋CaSO4 (R19) 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 reaction is as following:4NH3＋6NO→5N2＋6H2O (R20)4. NO adheringon the surface of CaO particles is easier to be reduced by CO or other reduction agents.4. ConclusionsWhile 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,e associated reaction rates. The chemical kinetics shows that HCN is prone to influencing thform unstable N2O while NH3 form rather stable NO X, and part of HCN released fromdecomposed from CaCO3. The de-volatilization process is converted into NH3 by CaOcatalytic effect and preferential conversion of HCN to NH3 increases the NO emission. With high Ca/S, the reduction effect becomes dominated and reduces the overall NO X emission.It is important to optimzing Ca/S ratio in CFB boiler design and operation for controlling both SO2 and NO X emissions.References:[1] Feng Junkai, Y ue Guangxi, Lu Junfu. Circulating Fluidized Combustion Boiler [M].Chinese ElectricPower Press. 2003.[2] Zhou Haosheng, Lu Jidong, Zhou Hu. Nitrogen Conversion in Fluidized Bed Combustionof Coal WithLimestone Addition. Journal of Engineering Thermophysics, 2000,9 V ol.21 No.5:647~651[3] Feng Junkai, Shen Y outing. Boiler’s principle & calculation (II). Science Press. 1998, 206~207[4] Bowman C T. control of combustion generated Nitrogen Oxide Emissions: T echnology DrivenRegulation.Proc. 24th Combustion Inst. 1992:859~878[5] Han Caiyua, Xu Mingho. Coal dust combustion., Science Press, 2001: 449~450[6] Moria Hori. Combustion Science and T echnology. 1980:23~131[7] Ren W ei, Zhang Jiansheng, Jiang Xiaoguo, Lu Junfu, Y ao Jiheng, Y ue Guangxi. Experimental Studyon Nitrous Oxide for Mation During Char at Combustion at Fluidized Bed condition. Acta scientiaeCircumstantiae,2003.5 V ol.23 No.3:408~410[8] Ren W ei, Xiao Xianbin Lu Junfu, Y ue Guangxi, Research on Conversion ofNitrogen in Char DuringCombustion Under Fluidized Bed Condition. Journal of China University of Mining & T echnology, 2003.5V ol.32 No.3:259~266[9] Zhou Lixing, Lu Jidong, Zhou Hu. Nireogen Conversion in Fluidized BedCombustor of Coal WithLimestone Addition. Journal of Engineering Thermophysics. V ol.21, No.5:647~651[10] Mike Braford, Rajiv Grover, Pieter Paul. Controlling NO X Emission: Part 2.Chemical Engineering Progress, 2002, 98(4): 38~42中文翻译石灰石脱硫对循环流化床中NOX排放的影响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。