Understanding Amine Chemistry in Combined Cycle Power Plants

许多联合循环发电厂已制定了介于9.6和10.0之间的pH值准则,以尽量减少热回收蒸汽发生器和冷凝器的腐蚀。Neutralizing amines are used to achieve these pH targets.本文研究了胺的选择,the pros and cons of ammonia-only versus organic blends,有机胺的最终命运,以及喂入中和胺的数量。

Proper water chemistry is vital for combined cycle power plants to operate reliably.Without it,corrosion will damage systems and force unscheduled downtime.The following case study will help readers understand some of the challenges.

Defining the Problem

The subject facility is a 2 x 1 x 1 (two blocks of one gas turbine by one steam turbine) combined cycle power plant.Commissioned in 2016,the plant consists of two Siemens H-class gas turbines,总发电量830兆瓦。蒸汽是通过air-cooled condenser (ACC).The heat recovery steam generator (HRSG) configuration is a standalone low-pressure (SALP) drum (64 psi),中间压力(IP)鼓(479 psi)以及一个高压(HP)鼓(2,390磅/平方英寸,管道点火)。重要的是,循环化学是全挥发性的,氧化程序;没有固体碱(磷酸盐或苛性碱)进入余热锅炉。自施工以来,已使用氨和乙醇胺(ETA)的混合胺,targeting a pH of 9.4–9.8.It is controlled using specific conductivity.

The author became involved with the subject plant when concerns arose about the level of amine usage,which was 22 gallons per day (gpd).Theoretically,在纯水中,大约需要10 ppm氨/ETA混合物(取决于活性百分比)才能达到9.6的pH值。假设凝结水流量为150万lb/hr,this would equate to 43 gpd of chemistry.However,95%到99%的冷凝水回流在发电厂是典型的。At this return percentage,2.1 gpd should maintain a condensate pH of 9.6.The subject plant was using 10 times that amount,凝析油和LP/IP鼓的阳离子导电率在1.3μmhos和大于5.0μmhos时非常高,分别。

一项彻底的调查发现,大量的胺液通过真空泵排气口和溢流口流失。通过真空泵敲出罐确认胺丢失后,a plan to recycle the overflow drain back to the condensate receiver tank was developed.To eliminate the need for an additional pump and subsequent flow balancing,a slip stream was taken off the discharge of the heat exchanger forwarding pump.By throttling the slip stream,可以控制分液罐的液位,同时也回收胺。

The plan worked.Recycling reduced amine usage from 22 gpd to 3.5 gpd.Adding to that success,整个系统的阳离子导电率迅速下降(图1)。However,all was not well with this approach.While amine usage appeared to be optimized,pH control throughout the HRSG appeared to have suffered.如图2所示,prior to modifications on August 23,整个圆桶的酸碱度分布是一致的,averaging 9.6.8月23日之后,HP转鼓的pH值升高,同时,考虑到flow-accelerated corrosion (FAC)potential.所以,发生了什么事?为什么在修改之后才开始?这就是中和胺的讨论开始的地方。

1。Intermediate-pressure (IP) and high-pressure (HP) drum cation conductivity.Courtesy: ChemTreat Inc.
2.8月23日修改前后的汽包pH值,2017年。Courtesy: ChemTreat Inc.

胺的讨论

It is common for newly commissioned combined cycle plants to use straight ammonia for cycle chemistry pH control,owing to ammonia's zero contribution to cation conductivity.相反地,有机胺,比如ETA,通常由于分解成有机酸而被禁止,有助于阳离子导电。

由于氨在低压下的挥发性,it does not tend to remain in the LP drum,which in turn causes a depressed pH (less than 9.4) relative to the IP and HP drums—a significant drawback to an ammonia-only program.已确定四个环境因素导致了FAC的发生:PH值低于9.4,a temperature range of 250F to 450F,碳钢结构,在还原气氛中操作(0 ppb溶解氧)。HRSG LP circuits,以及IP和HP省煤器入口,experience temperatures in the 295F to 305F range,are constructed of carbon steel,并且经常在还原环境中操作四个因素中的三个。When coupled with an ammonia-only program,FAC发生的概率很高。

3.给水泵吸入前管道内部的单相流加速腐蚀(FAC)。Courtesy: Electric Power Research Institute (EPRI)
第四章。Two-phase FAC in deaerator interior.Courtesy: EPRI

图3和图4显示了单相和两相FAC的示例。它可以出现光滑,波纹状的,black,black/red,or shiny silver,根据环境条件。Flow-related corrosion has also been observed in areas where initial condensation occurs,such as at the back end of an LP turbine or inside an ACC.这种类型的FAC发生在较低的温度下(从90F到130F),通常表现为光滑的,闪亮的,silver surface (Figures 5 and 6).

5.FAC inside an air-cooled condenser.Courtesy: John Monahon / Panda Liberty
6。在水冷式冷凝器内。Courtesy: ChemTreat Inc.

To minimize all forms of FAC,research and experience has led many plants to convert from an ammonia-only program to a blended-amine program,特别是氨/ETA,once commissioning is complete.是什么使得混合胺程序在减少与流量有关的腐蚀方面表现得更好?是吗?

The answer has to do with胺挥发性,也就是说,胺在余热锅炉中的移动方式.每种胺都有汽液比(v:l)。表示特定胺在蒸汽/液体混合物中的分布。例如,an amine with a high V:L ratio prefers to go with the vapor phase relative to the liquid phase,while an amine with a low V:L ratio prefers to remain in the liquid phase.使事情更加复杂,V:L ratios change with pressure.

当只给氨气程序喂食时,it is difficult to elevate LP drum pH due to ammonia's high V:L ratio at the lower pressure.相反地,在高压鼓中,由于氨的v:l比低得多,液相氨的百分比明显增加;因此,高压汽包的酸碱度要高得多,相对来说。Recall,经验表明,为了将FAC最小化,it is necessary to maintain a pH of,至少,9.4在冷凝液中,给水,and LP drum.At these circuits' operating pressures,由于氨不停留在液相中,因此需要相对较多的氨才能达到目标的pH值。

有机胺就是在这里出现的。相对于氨,ETA has a low V:L ratio at all HRSG operating pressures.For comparison,在低压操作温度和压力下,氨的挥发性是ETA的10倍。这意味着ETA在整个余热锅炉循环化学过程中倾向于停留在液相,包括冷凝液,给水,低压磁鼓以及任何初始冷凝区域的液相,也就是说,water-cooled condenser structures,ACCs,and feedwater drain coolers.

It is ETA's ability to maintain a consistent pH in the liquid phase of the cycle chemistry that makes it valuable when combating all forms of FAC.ETA在将低温/低压区域缓冲到高于9.4的pH值方面做得更好,thus eliminating one of the four factors necessary for FAC.由于波动性低,the pH of any liquid phase developed during the initial condensation (such as the back end of the LP turbine) is considerably higher than with the ammonia-only treatment.当液体开始在相变区(靠近最后三排叶片的区域)形成时,using an ammonia-only program means that most of the amine present is going to go into the vapor phase,因此,离开液相时几乎没有pH缓冲。Hence the reason initial condensation has been found to have a very low pH.This high-velocity,低pH值,high-purity liquid is sent to the water cooled-condenser or ACC unit,在哪里?upon contact,it causes dissolution of the carbon-steel components.When using a blended amine,ETA保持初始凝结,further buffering the pH and making the water less aggressive.

v:l比率的极端差异意味着答案不仅仅是eta或氨,它是一种混合体。电力研究所的研究和实践经验表明,在典型的公用事业运行温度和压力下,适当的混合可以提供更多的碱度。provides more consistent distribution throughout a multi-pressure HRSG,改善低压和给水回路的pH缓冲,在任何有初始凝结的区域改善了pH缓冲,and offers better protection from two-phase FAC because the ammonia buffers the vapor phase while the ETA buffers the liquid phase.

The biggest problem with organic amines is the degradation byproducts—acetic acid (which breaks down further into formate and carbon dioxide),甲酸,and ammonia.这些分解产物有助于阳离子导电,which is the main concern in practical applications.The rate of amine degradation is very dependent on the maximum temperature to which the amine is subjected.In many HRSGs,过热蒸汽温度可达到1,华氏000度为1,050F;temperatures that have resulted in organic amine degradation.

Blended amines have proven to be very effective tools in maintaining HRSG cycle chemistry.However,由于化学性质,这些混合物的有机部分最终降解为乙酸,甲酸,propionic acid,and CO.It is how and where these acids accumulate that is important to anyone tasked with maintaining cycle chemistry.

Organic Acid Movement and Accumulation

As organic amines are subjected to superheat temperatures,they degrade into acids,with acetic and formic acid being the dominant species.这两种酸在整个余热锅炉中相对运动和积累不同。在实际应用中,作者发现,在前馈低压汽包结构中,这些酸在中压汽包中的积累程度大于高压汽包。In a SALP drum configuration,acids accumulate in the LP drum to a greater extent than in the IP and HP drums.

As organic acids are generated in the superheat circuits,他们先去蒸汽轮机,然后去冷凝器,一部分酸通过冷凝器的空气排出部分排出。The remaining acids are ionized in the condensate and ultimately end up in the HRSG drums.然后酸要么积聚在桶里要么挥发,在空气清除部分清除一些酸后再次进行循环。如果这些酸的积累是基于几个因素,带着压力,temperature,ph是本文的重点。

三者都影响有机酸的挥发性和降解。挥发性受酸碱度影响,因为酸碱度决定了挥发性酸或电离盐的形成。例如,在77F(25C)下,乙酸和甲酸的pka值分别为4.76和3.76。分别but are significantly higher at 662F (35OC),7.97 and 5.23,分别。这意味着在各自的PKA,50% is acid and 50% is salt.pH值越高,盐的主宰作用越大;pH值越低,挥发酸越占主导地位。As it pertains to IP and HP drums,9.5的汽包酸碱度(25c)与6.15至6.25的酸碱度(t)有关。Meaning,acetate tends to volatilize in the HP drum and ionize (accumulate) in the IP.Theoretically,相对于高压汽包,IP汽包中的醋酸盐浓度更高,这已经通过实际的循环化学分析得到了证实。

Given formate's pKa of 5.23 at 662F,it would be expected that a large portion would ionize and remain in the liquid phase,leading to high concentrations in drum water analyses.However,在IP鼓样品中发现的甲酸盐相对较少,更不用说HP滚筒样品,这都是因为甲酸盐缺乏热稳定性。甲酸盐在IP和HP桶中的温度或高于IP和HP桶中的温度时表现出较差的热稳定性。甲酸盐的挥发性比乙酸高很多,尤其是在更高的温度和压力下,这就解释了为什么高压蒸汽样品中的甲酸盐浓度比高压汽包样品高。External research has confirmed acetate dominates relative to formate production as temperatures and pressures increase.Therefore,the result of feeding organic amines or blended amines tends to be acid accumulation in IP drums.在严重情况下,这种积聚伴随着轻微到显著的酸碱度降低。

But who cares?What is the point of this discussion?Are these byproducts causing any real adverse effects within HRSG cycle chemistry that warrant consideration and action?It is well-researched that these byproducts increase cation conductivity to above the standard upper limit of 0.2 µmhos.但这真的是个问题吗?In the author's experience,答案是肯定的。授予,examples of turbine failures directly attributed to organic acids are hard to find,but these acids do in fact cause operational issues.

关键是,毫无疑问,these acids will be generated when feeding an organic amine.ACC装置中的冷凝阳离子导电率通常会增加到1.0μmhos以上,取决于所用的pH目标和中和胺/混合物。This elevated cation conductivity may not be an issue from a technical standpoint,但从人类的角度来看,这是一个问题。When people,who possibly have little HRSG/ACC operating experience,看看这些阳离子导电率,cycle chemistry discussions can get tense.根据自己的经验和知识基础,可以决定只进行氨气项目,which has other problems as previously discussed.

Furthermore,这些酸容易积聚在HRSGS的SALP和IP桶中。They accumulate to a lesser extent in HP drums,but in a HRSG's LP and IP drums,有机物很容易积聚,有时浓度很高。It has been the author's experience that the amount of acids accumulated can exceed 30,000磅,这会降低汽包的酸碱度。This leads to the feed of more amine,contributing more acids—then the vicious cycle begins.这种现象反映了余热锅炉中乙二醇的污染,因为发生了相同的化学反应。

这并不是为了说明前面讨论过的好处,但重要的是,工厂人员必须接受专业人员的良好教育,cons,以及喂养这种化学物质时的期望。

将科学与现实世界联系起来

Having a better understanding of neutralizing amines and their pros and cons,它与案例研究有什么关系?Recall,prior to the mechanical modifications that took place on August 23,the pH control across the triple-pressure HRSG was very consistent and stable between 9.5 and 9.7.After modifications,pH control was very inconsistent,with the HP drum pH rising to nearly 10.0,and the IP and LP drum pH dropping below 9.0.所以,what is happening,why is it happening,and most importantly,what can be done to correct the situation?是吗?

ETA在过热温度下会分解为有机酸。This is the first clue into what is taking place in the case study.Consider this,混合胺被送入冷凝泵排放口。That amine travels to the drums,where a portion stays with the drum water and a portion volatilizes to the steam.Depending on conditions,a percentage of the volatile amine breaks down into organic acids.As the steam performs work in the turbine,已经排到机场指挥中心了。

在ACC内部,一些挥发性胺和有机酸通过空气去除系统去除。有些通过冷凝物返回。If the amine is ammonia-only,没有退化,胺回收率应该很高。如果胺为ETA或混合氨/ETA,在余热锅炉的每个循环中,一部分会降解。随着氨的循环利用,the amount of fresh blended-amine fed to the system continues to be reduced until the system is essentially an ammonia-only system.根据氨挥发背后的科学原理,这就解释了为什么酸碱度开始扩散,随着高压、低压和中压的酸碱度的升高。

ETA的损失也可以通过阳离子导电率读数来确认。8月23日之后,the drum cation conductivities decrease,随着IP鼓的迅速下降。What this indicates is that less organic acids are accumulating,这是有意义的,因为向系统提供的预计到达时间减少了。这里是第22条。Feeding a blended-amine at the necessary dosage to minimize FAC in an ACC would appear to lead to elevated cation conductivities and high amine usage rates.当只给氨气项目喂食时,通过所讨论的改性回收胺,allowed for the significant reduction in amine feed,but set the plant up for FAC conditions.

治疗方案

The author has struggled in assisting ACC plants with balancing pH control and cation conductivity throughout HRSGs.Personal experience has shown that when maintaining a pH of 9.6 to 10.0 with a blended amine,阳离子导电率很少低于1.0μmhos。However,使用纯氨程序,the LP drum,LP generating banks,and IP/HP economizers are at serious risk for FAC given the inevitable low pH of these circuits.

增加了选择纯氨和混合胺的复杂性,there is the question of how these chemistries should be fed.The case study provided shows very clearly that mechanical modifications can optimize amine usage.But at what expense?In practical experience,作者确定了几种选择中和胺的方法,并用ACC装置将其投喂给HRSGS。

Feed Ammonia-Only to Target 9.8 pH in the LP Drum.这需要大约25 ppm的19%氨才能达到9.8的pH值。If the vacuum pump drain is recycled,氨损失应最小,并应降低进料速度。即使真空泵的气相会损失一些氨,它不应该是重要的。When recycling the vacuum pump drain,the case study shows that when HP drum pH is above 9.9,the LP drum pH is at 9.2 due to ammonia volatizing in the steam phase.The upside to an ammonia-only program is that there are no organics present,so there will be no cation conductivity addition.作为旁注,重要的是要了解样品板上的阳离子柱会由于铵离子与氢的交换而更快地排出。

向低压罐中的目标9.6 ph注入氨/ETA混合物,不回收真空泵排水。This has proven to require a significant amount of amine given the losses.这种混合物在整个余热锅炉汽包中提供了更好的pH曲线,but at what cost.是否可以通过优化真空泵操作来控制损失?In other words,人为地降低真空泵的容量(在泵前产生空气泄漏或提高密封水温度)是否能最大限度地减少胺从系统中的抽取?If the vacuum pump drain is to be recycled,不需要喂这种混合物。The data indicates the ETA degrades over time and the system is left essentially an ammonia-only program.This is confirmed by not only the pH profile,但是整个系统阳离子导电率的降低是因为氨满足特定的导电率设定值。

Feed Blended Amine to the IP/LP Turbine Crossover Piping.This would mean the ACC sees an elevated pH to minimize FAC prior to any degradation of the organic portion of the amine.A condensate pH target of 9.6 to 10.0 would still be necessary to minimize FAC in the LP circuits,and IP/HP economizers.If the amine were fed to the IP/LP turbine crossover it could allow for a reduced feed rate of the blended amine.Because the amine is not subjected to the elevated final superheat steam temperatures initially,very little degradation would occur.应该注意的是,胺最终会循环到汽包和过热蒸汽中,which will cause a portion of the amine to convert to organic acids.所以,有机酸的生产将会发生,but the ACC is going to see a larger amount of intact amine;因此,ACC内的pH控制将更加一致。

Regardless of the chosen amine and feed point,HRSG complexities,尤其是那些有ACCS的人,使化学管理人员难以充分保护这些系统。The key is monitoring.仔细准确地监测系统(阳离子导电率,pH,而铁)是决定循环化学计划有效性的唯一方法。■

-Kevin Boudreaux(Kevinb@chemtreat.com) is an 金莎线上开户industry technical consultant with ChemTreat Inc.特别感谢John Monahonand the plant personnel at Panda Liberty,和马克·麦金泰尔对Chemtreat的所有帮助和支持在本文的发展中。