富氧燃烧烟气净化工艺研究进展

刘敦禹1,2,蔡雨阳1,2,金 晶1,2,许开龙1,2

(1.上海理工大学 能源与动力工程学院,上海 200093;2.上海市动力工程多相流动与传热重点实验室,上海 200093)

摘 要:随着工业的快速发展,化石燃料消耗与日俱增,造成了大量温室气体CO2的排放,全球气候变化形势不容乐观。为了减少CO2排放,需要对高浓度CO2进行捕获、利用与封存,而富氧燃烧技术能有效实现碳捕集,是目前最具潜力的碳减排技术之一。富氧燃烧过程中,SOx、NOx、Hg等污染物以及惰性气体的存在不利于碳的捕集与封存。烟气中各成分浓度会对管道运输、地质储存和提高采收率(EOR)产生影响,介绍了烟气中CO2及各种杂质浓度的不同标准,系统综述了国内外脱硫、脱硝、脱汞和惰性气体脱除以及联合脱除技术的研究进展。脱硫部分除介绍传统脱硫技术外,重点描述了富氧燃烧烟气中CO2气氛对SOx脱除的影响以及加压条件下SO2的转化与去除。发现CO2气氛下SO2的吸收速率相比N2气氛有所降低,且SO2吸收过程中临界pH发生变化。脱硝部分重点描述了氧化吸收法脱硝技术以及加压条件下NO的氧化机理,并对高压下NO的氧化动力学进行阐述。随着压力的增加,NO氧化速率常数呈先下降后上升的趋势,且证明了反应器压力对液体夹带率的影响比较显著。总结了Hg脱除技术中不同烟气成分对Hg氧化的影响,HCl与Cl2起到了明显的促进作用。对活性炭进行改性,增加孔结构比表面积以及吸附剂表面的活性位点,提高Hg的脱除效率。介绍了惰性气体的净化技术,主要采用变压吸附方法来吸附和解吸附,降低了CO2气流中惰性气体去除的成本,实现一部分惰性气体再次循环回到锅炉中,提高CO2的捕获。重点讨论了在烟气压缩液化系统中的联合脱除技术,有效利用压缩过程条件将SOx、NOx、Hg分别以硫酸、硝酸、Hg(NO3)2形式协同去除,随着压力的增加,SOx与NOx去除效率提高,有利于和HAMS(N-S化合物)的生成,同时也导致了N2O生成量增多。证明了Hg与NO2是气相反应,提出了高压下NO2与Hg反应产物的不确定性。简单介绍了低温碳捕集技术,有潜力取代洗涤器和其他烟气处理方法,但目前还缺乏可行性的研究。未来需对不同压力下NO氧化速率常数的变化趋势进行解释,高压下NOx与SOx联合脱除的产物以及NO2与Hg的反应产物进行分析。

关键词:富氧燃烧;碳捕集;烟气净化;联合脱除

中图分类号:X51

文献标志码:A

文章编号:1006-6772(2021)02-0079-13

收稿日期:2020-09-14;责任编辑:白娅娜

DOI:10.13226/j.issn.1006-6772.CCUS20091403

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基金项目:国家自然科学基金资助项目(51706143,51976129)

作者简介:刘敦禹(1984―)男,黑龙江尚志人,副教授,博士,研究方向为清洁燃烧。E-mail: liudunyu@usst.edu.cn。

通讯作者:金晶,教授,博士,研究方向为清洁燃烧。E-mail: alicejin001@163.com

引用格式:刘敦禹,蔡雨阳,金晶,等.富氧燃烧烟气净化工艺研究进展[J].洁净煤技术,2021,27(2):79-91.LIU Dunyu,CAI Yuyang,JIN Jing,et al.Research and development on the purification technology for oxy-fuel combustion flue gas[J].Clean Coal Technology,2021,27(2):79-91.

Research and development on the purification technology for oxy-fuel combustion flue gas

LIU Dunyu1,2,CAI Yuyang1,2,JIN Jing1,2,XU Kailong1,2

(1.School of Energy and Power Engineering,University of Shanghai for Science and Technology,Shanghai 200093,China;2.Shanghai Key Laboratory of Multiphase Flow and Heat Transfer in Power Engineering,Shanghai 200093,China)

Abstract:With the rapid development of industry,the consumption of fossil fuels is increasing with each passing day,resulting in a large amount of greenhouse gas CO2 emissions. The situation for global climate change is not optimistic. In order to reduce the emission of CO2,it is necessary to capture,utilize and store the high concentration of CO2. Oxy-fuel combustion technology can effectively achieve carbon capture,which is considered as one of the most potential carbon emission reduction technologies. In the process of oxy-fuel combustion,the existence of pollutants(including SOx,NOx and Hg) and inert gas is not conducive to carbon capture and storage. Since the concentration of various components in flue gas will affect pipeline transportation,geological storage,and enhanced oil recovery(EOR),the different standards of CO2 and various impurities concentration in flue gas were introduced firstly,and the research progress of desulfurization,denitrification,mercury removal,inert gas removal,and combined removal technologies at home and abroad was systematically reviewed. In addition to the introduction of traditional desulfurization technology,the influence of CO2 atmosphere in oxy-fuel combustion flue gas on SOx removal and the conversion and removal of SO2 under pressure were mainly described. It is found that the absorption rate of SO2 in CO2 atmosphere is lower than that in N2 atmosphere,and the critical pH changes during the absorption process. In the denitration section,the denitration technology by oxidation absorption method and the oxidation mechanism of NO under high pressure were mainly described,and the oxidation kinetics of NO under high pressure was described. With the increase of pressure,the NO oxidation rate constant first decreases and then increases,which is proved that the effect of reactor pressure on liquid entrainment rate is significant. The effects of different flue gas components on Hg oxidation in Hg removal technology were summarized. HCl and Cl2 play a significant role in promoting Hg oxidation. The activated carbon is modified to increase the specific surface area of pore structure and the active sites on the surface of adsorbent,so as to improve the removal efficiency of Hg. In this paper,the purification technology of inert gas was introduced. Pressure swing adsorption method is mainly used to adsorb and desorb,which reduces the cost of inert gas removal in CO2 gas flow,realizes a part of inert gas recycling back to the boiler,and improves CO2 capture. The combined removal technology in the flue gas compression liquefaction system is mainly discussed,which effectively uses the compression process conditions to remove SOx,NOx and Hg in the form of sulfuric acid,nitric acid and Hg(NO3)2. With the increase of pressure,the removal efficiency of SOx and NOx increases,which is conducive to the formation of and HAMS(N-S compounds),and also leads to the increase of N2O production. It is proved that the reaction of Hg with NO2 is a gas phase reaction,and the uncertainty of the product of the reaction of NO2 with Hg under high pressure is proposed. The low temperature carbon capture technology is briefly introduced,which has the potential to replace the scrubber and other flue gas treatment methods,but there is still a lack of feasibility research. In the future,it is necessary to explain the change trend of NO oxidation rate constant under different pressures,analyze the products of NOx and SOx combined removal and the reaction products of NO2 and Hg under high pressure.

Key words:oxy-fuel combustion;carbon capture;flue gas purification;simultaneous removal

0 引 言

化石燃料燃烧会产生大量CO2,过多CO2排放会导致温室效应,使全球气候变暖,减少CO2排放量已成为国际社会关注的焦点。当前全球气候变化形势不容乐观,1901—2012年,全球平均气温上升了0.89 ℃[1]。温度升高会导致冰川融化、海平面上升、珊瑚礁灭绝、全球粮食短缺等问题[2]。我国将近34.2%的CO2排放都来自燃煤发电,而燃煤发电在未来几十年依然是电力的主要来源[3]。2015年《巴黎协定》后中国就根据国情向全球作出“承诺”,宣布争取尽早实现2030年CO2达峰且单位GDP CO2排放比2005年下降60%~65%的目标[4]。碳捕集、封存和利用技术(Carbon capture,utilization and storage,CCUS)能有效应用于CO2减排,是应对气候变暖最有效的方法之一。常见的CO2捕集技术有燃烧前捕集、燃烧后捕集和富氧燃烧技术。燃烧前捕集用于煤气化联合循环发电(IGCC),燃烧后捕集指在煤粉或天然气联合循环发电厂的尾气侧捕获CO2。富氧燃烧技术又称空气分离/烟气再循环燃烧技术,具有相对成本低、易规模化、可改造存量机组、减排CO2力度大等诸多优势[5],被国内外认为是最具有发展前景和优势的直接CCUS技术之一[6]

1 富氧燃烧碳捕集纯度标准

富氧燃烧技术系统示意如图1所示。该系统先由空气分离装置进行高纯度O2提取,按一定比例与循环回来的锅炉尾部烟气混合,进入锅炉燃烧后产生含有高浓度CO2的烟气,随后经过烟气净化系统,再进入烟气压缩纯化系统,最后得到高浓度的液态CO2产品用以运输、利用和封存。

与传统燃烧相比,在富氧燃烧下,燃烧产生的SOx、NOx总量减少,但由于大量烟气再循环,烟气排放量减少,导致酸性污染物浓度升高[7-9]。除SOx、NOx等常见污染物,捕集后CO2产品中还含有多种杂质,如O2、H2O、CO以及惰性气体等,O2浓度相比传统燃烧略有上升,而H2O浓度上升约10%,这些杂质可能会对管道运输、地质储存和提高采收率(EOR)产生负面影响[10]。表1列出了捕集后CO2纯度以及杂质的标准,由于管道运输、地质储存和提高采收率等影响,需CO2体积分数>95%;H2O体积分数<500×10-6,也有建议应<50 ×10-6;O2体积分数应在100×10-6~1 000×10-6,石油运营商认为应<10×10-6,但这些限制需要试验验证;惰性气体、CO、SOx、NOx浓度分别需<4%、<2 000×10-6、<50×10-6、<100×10-6。根据氧气含量要求,可采用各种工艺生产纯度95%~99.999%的CO2。纯度越高,CO2捕集率就越低,更高的纯度意味着更高的资本和运营支出。为了达到CO2纯度标准,如何廉价、高效捕集和资源化利用富氧燃烧烟气中的污染物杂质已成为研究热点,本文针对富氧燃烧CO2捕集过程中SOx、NOx、Hg污染物以及惰性气体脱除进行了探讨。

图1 富氧燃烧技术系统示意[5]

Fig.1 Schematic diagram of oxy-fuel combustion system[5]

表1 CO2纯度标准[10]

Table 1 CO2 purity standards[10]

成分要求原因CO2>95%管道输送:由于存在O2、N2、CO等不凝组分,增加操作压力EOR:由于O2、N2和CO等不凝组分,提高最低混相压力(MMP)H2O<50×10-6管道输送:自由水的存在导致腐蚀和水合物形成[11]<500×10-6基于设计和操作考虑(即水在纯CO2中的溶解水平,以及H2S和CH4的影响)[12]O2<10×10-6[13-14]管道输送:腐蚀和两相流[15]地质储存:减少储存容量,降低注入和溶解度补集EOR:与油反应EOR:提高最低混相压力(MMP)[12]100×10-6~1 000×10-6(EOR)4%(蓄水层)EOR:与油反应,导致喷射点过热这个范围是因为缺乏实际的试验N2H2Ar<4%(所有非可压缩的)管道输送:由于两相流增加压力,导致管道直径增加[16]地质储存:与O2相同EOR:增加MMP[12,17]CO<2 000×10-6管道运输:健康和安全(H&S)考虑[12,17]地质储存:与O2相同EOR:增加MMP[12]NOx<100×10-6管道运输:健康与安全(H&S)[17]地质储存:可能与盖层和井组分发生反应,影响储层的完整性,但影响不大[18]SOx<50×10-6管道运输:健康与安全(H&S)[17]地质储存:可通过降低水的pH值,增加岩石矿物的溶解,并通过形成硫酸盐沉淀而使注入量发生改变,但影响比较小[18]

2 SOx脱除技术

煤燃烧排放的烟气中存在酸性气体污染物SOx,主要以SO2形式存在,是酸雨形成的主要原因,因此需在排入大气前脱除。富氧燃烧系统中SOx一般在压缩前脱除,传统烟气脱硫可分为干法(DFGD)、半干法(SDFGD)以及湿法脱硫技术(WFGD),其优缺点见表2。阿尔斯通公司[19]在不同典型脱硫方法下,对进入压缩系统前的SOx浓度进行分析,结果如图2所示,发现用湿法脱硫技术和湿式静电除尘器有较好的脱除效果。

目前,石灰石/石膏湿法烟气脱硫是国内外应用最广泛的一种脱硫工艺,该工艺采用石灰或石灰石作为脱硫剂对燃煤烟气进行脱硫[21],工艺流程为:从塔底部进入的烟气与自脱硫塔顶部淋下的含CaCO3的浆液充分混合接触后,烟气中SO2被吸收生成Ca(HSO3)2,Ca(HSO3)2落入反应槽中,然后通过鼓入的空气使Ca(HSO3)2被氧气氧化为CaSO4,CaSO4进一步发生结晶反应生成石膏(CaSO4·2H2O)。相关的反应方程式如下:

表2 干法、湿法、半干法脱硫技术优缺点[20]

Table 2 Advantages and disadvantages of dry,wet and semi-dry desulfurization technologies[20]

种类条件优点缺点干法烟气脱硫技术(DFGD)无液相参与,脱硫过程和处理脱硫产物均在干状态下进行设备腐蚀小、无废水排出、烟气无明显温降、有利于烟囱排气扩散设备庞大、反应速度慢、脱硫效率低于85%湿法烟气脱硫技术(WFGD)脱硫和处理脱硫产物均在湿状态下进行脱硫反应速度快、脱硫效率高达90%以上投资和运行维护费高、产生难处理脱硫产物、积垢和堵塞严重、设备腐蚀、造成二次污染、工艺流程复杂半干法烟气脱硫技术(SDFGD)具有干法与湿法两者的优点,脱硫和处理脱硫产物分别在干或湿状态下进行在湿状态下脱硫的半干法最优,脱硫效率高、反应速度快、无废水排出、脱硫后产物易于处理脱硫效率较高,设备磨损严重,原料成本比湿法和干法高

图2 脱硫技术的典型选择[19]

Fig.2 Typical selection of desulfurization technologies[19]

CaCO3+2SO2+H2OCa(HSO3)2+CO2

(1)

Ca(HSO3)2+O2+CaCO3+H2OCaSO4·2H2O+CO2

(2)

除了石灰石/石膏湿法脱硫技术外,氧化镁脱硫技术与氨法脱硫技术应用也比较广泛。氧化镁脱硫技术具有原料储备充足、脱硫效率高、不发生结垢、系统简单、能耗低、氧化镁可再生等优点[22]。反应机理为

MgO+H2OMg(OH)2

(3)

Mg(OH)2+SO2+xH2OMgSO3·xH2O,

(4)

MgSO3+1/2O2MgSO4

(5)

氨法脱硫技术中,SO2是气-液相或气-气相反应,传质阻力较小,因此反应速率快,脱硫效率最高可达98%,该过程相对简单,设备占地面积小,脱硫后副产品(NH4)2SO4是化肥的主要成分,回收后能降低运行成本[23-24]。相关反应如下:

2NH3+H2O+SO2(NH4)2SO3

(6)

SO2+(NH4)2SO3+H2O2NH4HSO3

(7)

NH3+NH4HSO3(NH4)2SO3

(8)

O2+2(NH4)2SO32(NH4)2SO4

(9)

除上述常见脱硫剂外,NaOH溶液对于酸性气体的吸收也十分有效[25]。Liu等[26]在CO2压缩前对钠基水溶液吸收SO2进行动态研究,考察了CO2与N2气氛下吸收的区别,提出了SO2随pH变化的吸收特性曲线,并发现CO2气氛下SO2吸收速率相比N2气氛有所降低,如图3所示。Hansen等[27]在湿法烟气脱硫条件下进行了同时吸收SO2和CO2试验验研究,发现CO2的存在会改变石灰水的pH使脱硫效率发生改变。Wappel等[28]研究了气液平衡状态下SO2和CO2的相互作用,发现在SO2/CO2/K2CO3体系中,SO2优先与K2CO3反应,存在的主要物质是亚硫酸盐,且SO2吸收到K2CO3溶液中是一个不可逆反应,硫会在溶剂中积聚,导致CO2吸收能力下降。有/无CO2条件下SO2在NaOH溶液中的吸收机理如图4所示。无CO2条件下,在pH=12.22~9.65时,SO2与OH-反应生成随着SO2逐渐吸收,pH降至4.82且生成低于4.82后,SO2与H2O反应生成H+有CO2存在条件下,SO2吸收反应的相应临界pH值发生变化,在pH=12.18~10.81,CO2被碱液吸收生成随后继续吸收CO2形成直到pH到达8.32,SO2开始与反应生成和CO2。Liu等[29]确定了一个pH窗口来提高SO2吸收率并能有效降低钠损失。Liu等[30]提出了一种基于双模理论的洗涤器用于富氧燃烧烟气中SO2的脱除,并对模型进行参数优化。

图3 不同溶液中SO2吸收速率与SO2分压的关系,并与计算得到的气相控制线进行比较[26]

Fig.3 Relationship between SO2 absorption rate and partial pressures of SO2 at different solutions and comparison with gas phase controlled lines[26]

加压条件下,烟气中部分SO2极易转化为SO3。空气气氛下SO3/SO2转化率为0.3%~1.1%,富氧燃烧方式下SO3/SO2转化率提高至1.3%~2.5%[31]。SO3的形成可以在FGD中进行控制,与SO2脱除技术相似,可以通过石灰石浆液、镁基吸收剂、钠基吸收剂进行脱除。采用湿式静电除尘器可显著去除烟气中的SO3

3 NOx脱除技术

酸性气体NOx也是煤燃烧烟气排放污染物的一种。传统脱硝技术分为干法脱硝技术与湿法脱硝技术。选择性催化还原技术(SCR)以及选择性非催化还原技术(SNCR)是目前最常用的干法脱硝技术。两者都是利用氨作为还原剂选择性地将NOx转化为N2和H2O,只是SNCR不需要添加催化剂。其他方法还有分子筛、活性炭吸附、等离子体法等[32-33]。陈松涛等[34]对富氧燃烧条件下SCR催化剂脱硝活性进行研究,发现CO2的存在会降低催化剂的脱硝效率,且随着CO2浓度的增加而增强,但不会导致催化剂失活。

图4 有/无CO2存在下SO2吸收机理与临界pH值

Fig.4 Absorption mechanism and critical pH value of SO2 with or without CO2

富氧燃烧产生的NOx主要由NO组成,因为NO氧化为NO2的过程随着温度的升高而迅速下降,在高于700 ℃下(在锅炉内),平衡混合物中几乎不含有NO2[35],这使得NOx脱除具有挑战性。富氧燃烧控制NOx排放的技术手段有低氮燃烧、氧分级以及氧化吸收等。氧化吸收法较为常见,由于NO不溶于水,去除前需要氧化为NO2,然后再进行吸收脱除。NOx在水中氧化吸收机理如图5所示。湿法脱硝技术建立在此基础上且种类繁多,主要用一些强氧化剂来氧化NO生成NO2,然后被碱性溶液或水吸收,多应用于常压环境,但强氧化剂的使用成本较高。Yan等[36]采用4种氧化剂(NaClO2、NaClO、H2O2和KMnO4)和4种吸收剂(Ca(OH)2、CaCO3、NaOH和Na2CO3)研究NO氧化吸收过程的反应机理,发现NaClO2溶液为最佳氧化剂,在浓度为1.0%时,氧化率达100%,Ca(OH)2浆液的吸收性能最好。Hao等[37]研究了蒸发的H2O2与紫外光作用下NO的氧化机理,得出紫外能量密度和紫外波长对NO氧化有显著影响,降低pH值和增加O2浓度均可增强NO氧化的结论。美国BOC公司开发的LoTOx技术是将氧气/臭氧混合气体通入烟道中,利用臭氧的强氧化性将NO氧化为易溶于水的高价态氮氧化物,然后通过洗涤形成HNO3[38]

图5 NOx在水中氧化吸收机理[39]

Fig.5 Mechanism of NOx oxidation absorption in water[39]

加压条件下对氮氧化物的吸收已经应用于硝酸的生产,说明在一定压力下能促进NO的氧化,且进一步提高NOx脱除率。罗哲林[40]对加压条件下NO单独脱除过程进行研究,发现增加压力、延长停留时间、提高O2浓度和初始NO2浓度有利于NO吸收以及硝酸转化,而温度升高则有负面影响。黄强等[41]在试验基础上应用Aspen Plus建立了加压单独脱硝模拟流程,也得出相同结论,提出烟气中N2的存在不利于加压脱硝过程。

阎维平等[42]提出了一种富氧燃烧捕集CO2时加压降温的特殊工艺来回收NO,证明高压低温工况有利于NO氧化为NO2并转化为稀硝酸产品,得出压力为3 MPa、温度为30 ℃时NO的氧化率达到90%。陈曦等[43]对加压条件下氮氧化物的水吸收进行了研究,发现在高压(0.4~0.6 MPa)下氮氧化物的吸收率较高,且回收的硝酸的价值可以弥补气体压缩的成本。

一些学者进一步研究了高压下NO氧化动力学进。Cheng等[44]在压力0.1~3.0 MPa下对NO氧化进行研究,在已有试验结果和理论模型的基础上实现NO氧化动力学参数的回归。Liu等[45]研究了加压条件下的NO氧化并吸收到水中的动态过程,如图6(a)所示,NO2的传质主要由气相控制;图6(b)说明NO的吸收主要由液相控制;图6(c)发现在0.5~1.5 MPa,氧化速率常数随着压力的增大而减小,该现象可以用NO3或NO二聚体作为中间体的预平衡机制来解释,但随着反应器压力从1.5 MPa增加到2 MPa,NO氧化速率常数的增加暂时还无法解释,可能涉及到一些未知的链式反应;图6(d)表明随着反应器压力的增加,液体的最佳表面积减小,而液体体积增大。内表面上的液体可以通过3个过程产生:液体预加载、液体蒸汽冷凝和气体鼓泡夹带。随后的结果证明了反应器压力对液体夹带率的影响比较显著。

图6 氮氧化物吸收模型中最佳参数随压力的变化[45]

Fig.6 Optimum parameters change with reactor pressure based on absorption modeling of nitrogen oxides[45]

4 Hg脱除技术

汞及其化合物对环境和人体健康均有害,汞的去除一直是关注的焦点。元素汞的去除对富氧燃烧工艺过程有重要影响,其会与下游的液化材料(铜焊铝热交换器)发生反应导致设备腐蚀[46]。汞的形态对汞的去除十分重要,烟气中汞有3种形态:元素汞(Hg0)、氧化汞(Hg2+)和颗粒汞(Hgp)。煤燃烧过程中各种形态汞的转化过程如图7所示。氧化态Hg2+和颗粒结合态Hgp都可以被传统的烟气处理装置捕获,如湿法烟气脱硫和静电除尘器[47]。单质汞在水中溶解性差,稳定性好,难以去除。因此,有必要研究富氧燃烧条件下元素汞排放控制的方法。

图7 煤燃烧过程中汞的转化过程[48-49]

Fig.7 General mercury transformation process during coal combustion[48-49]

富氧燃烧烟气中的成分对汞的氧化脱除起一定作用。Wang等[50]研究了富氧燃烧中HCl、NO、水蒸气和SO2对汞排放及其形态的影响,发现上述物质都会导致Hg2+含量增加,且水蒸气浓度对汞的氧化有显著促进作用,HCl通过产生Cl自由基也能促进汞的氧化。Li等[51]对富氧燃烧CO2压缩过程中酸性气体脱除汞的机理进行研究,发现在高压下能促进NO氧化为NO2并与Hg0反应,可以有效去除烟气中的Hg0杂质,SO2不与Hg0反应,但影响Hg0和NO2的反应,抑制了汞的去除。Wu等[52]分别在O2/CO2和O2/N2气氛下进行了Cl2、HCl、NO、SO2均相氧化Hg0的试验研究,结果表明,Cl2能显著促进Hg0的氧化,NO对Hg0的氧化为5%~30%,取决于NO浓度并随温度升高而降低。

活性炭(AC)在汞控制中得到了广泛应用,被认为是一种成熟而有前途的脱汞技术。活性炭在高压和有氧气存在的条件下可能发生爆炸,因此在较低压力下产生作用[53]。影响活性炭吸附汞能力的因素很多,包括活性炭的表面积、颗粒大小、孔隙率以及烟气汞浓度、烟气温度、烟气成分等[54]。Min等[55]发现在基准气体条件下,汞的吸附效率随比表面积的增加呈线性增加。粒径增大会显著降低汞的吸附容量,而较高的初始汞浓度则导致汞吸收量增加[56]。Miller等[57]发现在107 ℃下,O2、CO2、N2和H2O气氛下褐煤基活性炭对汞的吸附效率仅为10%~20%,当加入HCl、NO、NO2其中一种时,汞的吸附效率达到了90%~100%。

Hg0在吸附剂上的吸附过程主要包括物理吸附和化学吸附,可以通过一些改性方法来增强吸附剂对Hg0的吸附过程,主要是改善吸附剂的表面孔结构来增加比表面积以及增加吸附剂表面的活性位点。活性炭的价格昂贵,为提高活性炭的利用效率,采用很多活性炭改性方法来提高汞的脱除效率,如图8所示,主要有卤化物和硫改性、酸和碱改性、金属和金属氧化物改性、微波和等离子体改性和复合改性[58]。一般情况下,汞脱除效果顺从AC < Cl浸渍AC < Br浸渍AC < I浸渍AC[59]。Sun等[60]用HNO3浸泡活性炭,以增强活性炭中的含氧官能团,试验发现改性后的吸附剂对Hg0的去除率显著提高。Li等[61]研究了Pd/活性炭吸附剂对烟气中Hg0和H2S吸附性能的影响,结果表明该吸附剂具有良好的除Hg0和H2S的性能。Xu等[62]制备并应用MnOx/石墨烯复合材料来提高烟气中元素汞(Hg0)的捕获能力,发现MnOx/石墨烯吸附Hg0的主要机理是催化氧化和吸附。Zhang等[63]研究了氧低温等离子体对活性炭(AC)表面性能和脱汞性能的影响,由于氧低温等离子体处理后增加了活性炭表面的酯基(C—O)、羰基(CO)和吸附活化位,提高了元素汞的吸附。

图8 活性炭改性方法[64]

Fig.8 Modification methods of activated carbon[64]

5 惰性气体净化技术

在CO2捕集中一定量的CO2会与惰性气体一起排出,这限制了CO2回收的效率与纯度,需要降低CO2气流中的惰性气体浓度。富氧燃烧烟气中的N2、O2、Ar等惰性气体的脱除方法理论上有:低温分离和变压吸附脱除。低温分离是利用CO2与N2、O2、Ar等杂质在低温下的不同沸点进行分离,从而得到高纯度的CO2产品。变压吸附方法(PSA)是通过改变压力来吸附和解吸附,不仅可以提高CO2回收效率,还降低了CO2气流中惰性气体去除成本,实现一部分惰性气体再次循环回锅炉中,提高CO2的捕获。Hack等[65]提出了一种零排放闭循环的系统,如图9所示。该系统中N2、O2、Ar等惰性气体采用变压吸附方法分离,剩下一部分排入空气分离装置进行下一次循环,实现了高纯度的CO2捕获。

图9 零排放闭环富氧燃烧系统[65]

Fig.9 Zero emission closed loop oxy-fuel combustion system[65]

6 联合脱除技术

近年来,在富氧燃烧CO2压缩过程中联合脱除SOx、NOx和Hg的技术得到了广泛研究。CO2压缩捕集时的加压降温过程为脱除SO2和NOx提供了有利条件,另外,由于SO2和NOx浓度高于空气燃烧,使得脱除效率高,并能够资源化回收硫酸和硝酸。Air Products公司提出了在CO2压缩过程中以硝酸和硫酸的形式除去NOx和SOx以及以Hg(NO3)2的形式去除Hg0的简化反应机理[66-67],如图10所示。该机理涉及4个步骤:NO与O2在高压下氧化为NO2;NO2与SO2反应生成SO3,在与H2O反应生成H2SO4;残余的NO2与水反应生成HNO3;Hg与HNO3反应生成Hg(NO3)2

德国Linde公司开发了一种湿法洗涤方法:在CO2进入压缩机前,烟气脱硫系统除去95%~99%的SO2,随后烟气被压缩至1.8 MPa,使NO转化为NO2并在氨溶液中化学吸收以形成硝酸铵[68]。法国Air Liquide公司开发了一种常压去除SO2和高压去除NO的洗涤系统,该方法使用苛性碱溶液吸收SOx到较低浓度,然后烟气通过四级压缩系统除去NO[69]。普莱克斯公司设计了2套SOx、NOx、Hg的脱除方法用于高硫煤和低硫煤燃烧后的烟气净化[70]。烟气首先进入压缩机,冷凝过程中使一部分NO和SO2生成H2SO4和HNO3,随后送到活性炭床进行氧化(SO2→SO3,NO→NO2),饱和后用水洗涤使其再生(SO3→H2SO4,NO2→HNO3),几乎所有的NO都作为相应的酸被吸收和除去。美国Air Products公司提出的高压联合脱除技术由于简单、成本低和副产物具有资源化利用价值等优点,具有较大商业应用价值[71],该技术系统示意如图11所示。烟气首先进入压缩机被压缩至1.5 MPa,促进NO氧化为NO2,得到的NO2将SO2氧化为SO3,NO2同时被还原为NO,烟气经冷却器冷却后进入第1级逆流气液接触反应器,在该反应器中SO3与H2O充分接触生成H2SO4。根据铅室反应机理,在SO2完全被氧化生成H2SO4后,NO2开始与H2O反应生成HNO2和HNO3,同时Hg与HNO3发生反应以Hg(NO3)2形式脱除。脱硫后的烟气经压缩机压缩至3 MPa,再经冷却器冷却后进入第2级逆流气液接触反应器,该反应器中未除尽的NO被氧化为NO2并与H2O反应生成HNO2和HNO3,该反应系统可以达到100%的SO2脱除效率和>90%的NO脱除效率。该技术有4个特点:① 有效利用压缩过程能量,无须外部压缩能耗;② 具有较高SO2和NO脱除率,省去传统脱硫脱硝装置;③ 吸收后可以回收H2SO4和HNO3产品,降低系统成本;④ 烟气中的Hg以Hg(NO3)2形式脱除。

图10 富氧燃烧杂质去除机理[66-67]

Fig.10 Mechanism diagram of impurity removal in oxy-fuel combustion[66-67]

图11 富氧燃烧CO2压缩脱硫脱硝系统示意[67]

Fig.11 Schematic diagram of desulfurization and denitrification system in CO2 compression of oxy-fuel combustion[67]

除了上述公司开发的设备外,国内外学者也对联合脱除技术展开研究。SOx和NOx的去除取决于铅室反应的发生,可通过压缩和直接接触酸洗来去除。帝国理工学院[72]在实验室规模的工作中使用质量流量控制器合成模拟烟气,然后通过干式和湿式反应器进行高压下SO2和NOx的脱除。结果表明,压力的增加会促进NO氧化为NO2,在1.5 MPa和3.5% O2条件下,转化率可达87%,最终使得99%的SOx和90%的NOx被去除,并讨论了压力、温度、停留时间和水对脱除结果的影响。阎维平等[73]在NO单独氧化吸收试验中发现,随着压力的升高NO转化为HNO3的比率增大,达到3 MPa以上时,大于90%的NO转化为了稀硝酸。SO2与NO同时存在时,在SO2氧化过程中,NO只起到催化剂的作用。李伟等[74]在富氧燃烧下对鼓泡反应器高压联合脱除烟气中NOx和SOx进行了建模分析,通过龙格-库塔法进行求解,阐述了联合脱除过程的吸收机理,发现压力提高有利于和HAMS(N-S化合物)的生成,同时也导致了N2O生成量增多。

如果元素汞可以在富氧燃烧烟道气体压缩过程中与NOx气体结合捕获,这就提供了一种成本相对较低的清洁方案。Ting等[75]采用实验室规模的试验研究气态元素汞在硝酸中的吸收,以及2.5 MPa压力下由NO氧化生成的NO2与汞发生的气相反应。观察到硝酸对汞的吸收有限,降压后可能有部分汞发生解吸。另外,汞在气相中易与NO2发生反应,NO2对气态元素汞的氧化显著。Stanger等[69,76]对富氧燃烧压缩过程中同时去除SOx、NOx和Hg进行研究,结果表明,在压缩过程中,降低温度可以产生最大的NOx和Hg捕获。高压下反应主要发生在气相,且压力、停留时间以及NO2浓度对Hg的脱除都有影响,还发现没有NO2,在压缩系统中无法观察到Hg的脱除。

Hall等[77]与Snider等[78]对NO2与Hg的气相反应进行研究,虽然表面体积比不同,但2项研究的动力学结果一致,如图12所示(k′为速率常数),从本质上证明了Hg-NO2反应是气相反应。关于Hg与NO2反应生成的产物,一些文献认为只有Hg(II)盐形成,而另一些文献认为只有Hg(I)盐或同时存在Hg(I)盐和Hg(II)盐。汞盐受热容易分解并形成中间产物,一些学者对热解产物进行了研究[78-84],不过目前关于汞化合物的文献还不能够阐明Hg和NO2反应的产物。

图13为根据现有文献和Ting等[85]试验结果推测的汞产品成分分布,其中7%为I价或II价硝酸盐,6%为II价氧化汞,剩余87%为暂时未知的中间态汞化合物。

图12 Snider与Hall动力学拟合结果[78]

Fig.12 Kinetic fitting results of Snider and Hall[78]

图13 可能形成的Hg-NO2反应产物[85]

Fig.13 Possible formed Hg-NO2 product[85]

近年来,低温碳捕集技术(Cryogenic carbon capture,CCC)也有相关报道,低温碳捕集是去除二氧化碳的有效方法,该方法运用了烟气各成分的物理性质在加压低温下分离,有潜力取代洗涤器和其他烟气处理方法。Baxter等[86]用Aspen Plus对低温碳捕集系统进行了比较理想化的模拟,表3为CCC过程中简单体系(N2、O2、CO2)和复杂体系(烟气中包含S、N、Hg、Cl)下各数据的模拟结果,可知采用低温碳捕集技术各部分的能耗,且该技术使烟气中SOx、NOx、Hg的捕集效率达到100%,并能捕集到高纯度CO2。对于低温碳捕集技术目前还缺乏可行性的研究。

表3 CCC过程中简单体系(N2、O2、CO2)和复杂体系(烟气中包含S、N、Hg、Cl)下的模拟结果[86]

Table 3 Simulation results under simple system(only N2,O2,CO2)and complex system(flue gas containing S, N,Hg,Cl impurities) in CCC process[86]

变量简单体系复杂体系CO2进入/(kg·h-1)706 073730 057(13.5%)CO2捕集/(kg·h-1)702 122657 108压缩器能耗/kW175 626160 300膨胀器能耗/kW-58 340-57 444泵能耗/kW1 9881 486补充的制冷能耗/kW—17 600比能/(GJ·t-1)0.6010.620CO2捕集效率0.9550.900SOx捕集效率—1.000NOx捕集效率—1.000Cl2/HCl捕集效率—0.001Hg捕集效率—1.000可用H2O回收—0.91捕集的CO2纯度1.000 01.000

7 结语与展望

本文详细描述了富氧燃烧碳捕集过程中的烟气净化工艺,包括SOx、NOx、Hg和惰性气体的单独脱除技术,以及利用富氧燃烧碳捕集过程特有的烟气压缩液化环节,实现多种污染物的联合脱除技术。SOx脱除技术中,石灰石/石膏湿法脱硫技术脱硫效率高并能产生可回收的石膏,是目前应用最广泛的烟气脱硫技术。NOx脱除技术中,SCR运行费用高、氨易泄漏而且催化剂易失活;SNCR脱硝效率较低;氧化吸收法工艺简单、能耗少、成本低,已有较多研究;加压条件下的脱硝能产生具有经济效益的硝酸,有较高的研究价值。Hg脱除技术中,不同烟气成分对Hg的氧化有一定作用,利用活性炭及对活性炭改性控制Hg排放是目前主要研究方向。惰性气体净化技术中,采用变压吸附方法吸附和解吸附,降低了CO2气流中惰性气体去除的成本。联合脱除技术在烟气压缩液化系统中有效利用压缩过程的高压条件将SOx、NOx、Hg分别以硫酸、硝酸、Hg(NO3)2形式协同去除,该技术是富氧燃烧烟气净化技术的研究热点。

根据现有文献报告后续的研究重点为:

1)深入研究高压下的NO氧化动力学,对不同压力下NO氧化速率常数的变化趋势进行解释。

2)高压下NOx与SOx联合脱除的产物还需要进一步分析,包括N-S化合物HADS和HAMS,以及如何减少N2O生成,如何使高压下同时吸收的NOx和SOx以HNO3和H2SO4形式分离进行回收。

3)NO2与Hg的反应体系在高压下生成的产物尚不清楚,可以展开后续研究。

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