Sulphidation resistance of composite boiler tube materials

Abstract

A lab-based testing program was undertaken to generate data to better define the sulphidation resistance of composite tubes installed in the lower-furnace section of black liquor recovery boilers. All composite tube cladding alloys tested were observed to have an acceptable corrosion rate at normal operating temperatures (up to 400°C) in the synthetic lower-furnace gaseous environment tested (1% H₂S–99% N2). This acceptable corrosion resistance is due to the expected formation of a relatively protective chromium-rich inner sulphide scale. An increase in temperature up to 560°C was found to significantly increase the corrosion rate. Of the various alloys tested, Alloy HR11N exhibited the lowest corrosion rate at each of the three temperatures tested. Moreover, the corrosion rate was found not to be strongly dependent on the fabrication route (weld overlay versus co-extruded). To minimize corrosion, operating conditions that promote prolonged exposure to elevated temperatures in the lower-furnace section of black liquor recovery boilers should be avoided, regardless of the type of composite tube installed.     

Probes to monitor in-plant material degradation

Abstract

Fireside corrosion in coal fired boilers has been well-investigated. The main causes of water wall fireside corrosion are: (1) impurities in the fuel, such as sulphur alkali metals and chlorine; (2) the lack of control of the combustion process resulting in a reducing gaseous environment at the tube surface; (3) flame impingement; and (4) overtemperature of tube metal.

Co-firing secondary fuels in coal fired boilers is becoming common practice in many power stations in Europe. Secondary fuels like wood, refuse derived fuels, meat and bone meal, straw, poultry litter or mixtures of several secondary fuels are co-fired up to 20-wt%.

Most of these biomass fuels contain high concentrations of alkali chlorides. Considering the composition of these fuels, limitations on the maximum amount of secondary fuels to be co-fired in coal fired boilers are expected.

In addition to the environmental benefits from biomass fired power plants, co-firing can result in “green” power labelling and governmental subsidy. Also savings on fuel costs may be a driving force for an increase of the amount of biomass or secondary fuels to be co-fired.

However, without corrosion monitoring, short-term policies concerning co-firing secondary fuels in large volumes can lead to high costs in the medium or long term. These costs can be due to corrosion damage both in the furnace and superheater sections and penalties due to unplanned outages in a highly competitive electricity market.

This paper summarizes practical experiences from corrosion monitoring programs with KEMA corrosion probes. The first prototype was successfully tested in 1997 at the Hemweg Unit 8 coal fired power plant of Reliant Energy in Amsterdam, the Netherlands. Other corrosion monitoring programs were carried out at coal fired power plants and at a waste incineration plant.

At present a large-scale corrosion monitoring and material testing program is in progress at the Maasvlakte power station Unit 1 near Rotterdam, the Netherlands. In this 520 MWe power plant of E.on Benelux more than 10-wt% of mixtures of secondary fuels are directly co-fired.

In addition to aspects such as emissions, fuel handling and fuel cost savings, co-firing secondary fuels requires corrosion monitoring to check the tolerance to different fuel types of coal fired boilers.     

电化学噪声技术检测核电环境材料的腐蚀损伤

探讨和解决了电化学噪声技术在核电环境材料腐蚀损伤检测应用的关键问题,建立了基于零阻电流(ZRA)检测的SCC电化学噪声测试体系.采用小面积的Pt或表面热喷涂陶瓷涂层的工作电极材料作为对电极,研制适用于核电现场检测的多种电化学传感器.运用Compact RIO模块化仪器和设计制作的基于ZRA电路的电化学噪声测试模块,实现电位-电流噪声的同步测量和采集.成功研制出便携式核电材料损伤检测系统.应用研制的测试系统和电化学传感器研究了高温高压和动态水环境304不锈钢的电化学噪声谱特征.并初步实现了在役核电站辅助车间不锈钢管道表面直接腐蚀检测和钢厂动力锅炉连续排污管的现场腐蚀检侧,取得了比较满意的结果.     

Rates of fireside corrosion of superheater and reheater tubes: making sense of available data

Abstract

Interpretation and use of a body of field data for corrosion of superheater and reheater (SH/RH) tubes in coal fired boilers was found to be unsatisfactory without some indication of how the corrosive environment encountered differed when different coals were burned. A new factor intended to represent the relative corrosion potential (RCP) of the coal burned is suggested, based on the accepted mechanism of accelerated corrosion of SH/RH tubes by the development of low melting complex sulphates beneath ash deposits. Initial testing of the RCP concept was encouraging, given the difficulty in some cases of ensuring accurate representation of the actual coal burned during the corrosion exposures. However, some data suggested that new boiler operating modes, such as various approaches to emissions reduction through staging of the combustion process, appeared to be capable of causing significantly more rapid corrosion of SH/RH tubes than would be expected from the RCP value for the coal. Although increased corrosion with some emissions control systems has been recognised in practice, understanding of the particular corrosion process involved is lacking. Some degree of mechanistic understanding of the key features of this form of corrosion is needed to provide a firm basis for application of concepts such as RCP or development of improved versions.     

Field test corrosion experiences when co-firing straw and coal: 10 year status within Elsam

Abstract

In Denmark, straw is utilised for the generation of energy and district heating in power plants. Combustion of straw gives rise to high contents of potassium chloride and some sulphur dioxide in the flue gas. These compounds can lead to deposits with high content of potassium chloride and potassium sulphate on superheater tubes resulting in increased corrosion rates. From field experimental results this paper show, that by co-firing straw with coal, corrosion rates can be brought down to an acceptable level.

This paper firstly deals with the results from a demonstration program co-firing coal and straw at the 150MW pulverized coal fired boiler Studstrup unit 1. Two exposure series lasting 3000 hours each were performed for co-firing 10 and 20% of straw (% energy basis) with coal. Using built in test tubes in the hot end of the actual superheaters and air/water cooled corrosion probes, the corrosion during these experiments was monitored. Various ferritic and austenitic materials were investigated at steam temperatures ranging from 520 to 580°C and flue gas temperatures ranging from 925 to 1100°C.

The results obtained in the demonstration program led to the rebuilding of the 350MW pulverized coal fired boiler, Studstrup unit 4, into a co-firing boiler with straw in 2002. During the rebuilding, test tube sections of X20CrMoV12 1 and TP347H FG were built into the superheater and the reheater loops. The temperature ranges during these exposures was for the steam from 470 to 575°C and for the flue gas from 1025 to 1300°C. All these test tubes have been removed during the last three years at one year intervals for corrosion studies.

The corrosion studies performed on all investigated tubes included measurements of the corrosion attack, light optical microscopy and scanning electron microscopy of the corrosion products.     

The effect of deposits on waterwall corrosion in fossil fueled boilers

Abstract

Corrosion of water walls in fossil fueled boilers and gasifiers has traditionally been considered the result of gaseous corrodants, such as H₂S and HC1, reacting with the heat exchanger tube surfaces. Under reducing conditions these corrodants prevent the formation of a protective oxide scale, leading to increased metal loss. Recent field experience in boilers, using staged combustion systems, has shown much greater corrosion rates than predicted by simple gas/solid corrosion processes. The presence of large quantities of unoxidized iron sulfide in deposits in areas where high corrosion rates were found, suggests that deposits play a role as well. Subsequent laboratory corrosion studies found that the presence of FeS can indeed lead to very high corrosion rates, but only under oxidizing conditions. Since FeS usually deposits only where reducing conditions are present, the accelerated corrosion observed requires alternating reducing and oxidizing conditions. These are usually found in load following boilers. It is also shown that chlorine corrosion may be caused or at least accelerated by chloride containing deposits in fossil fueled boilers, similar to waste incinerators, where the role of chloride rich deposits has been well established. Due to the relatively high sulfur content of fossil fuels, chloride deposits most likely form under reducing conditions only. However, once formed they can be highly corrosive under oxidizing conditions.     

Prediction and real-time monitoring techniques for corrosion characterisation in furnaces

Abstract

Combustion modifications to minimise NO_x emissions have led to the existence of reducing conditions in furnaces. As regulations demand lower NO_x levels, it is possible (to a degree) to continue to address these requirements with increased levels of combustion air staging. However, in most practical situations, a number of adverse impacts prevent the application of deep combustion air staging. One of the more important limitations is the increased corrosion that can occur on wall tubes exposed to fuel rich combustion environments. Current boiler corrosion monitoring techniques rely on ultrasonic tube wall thickness measurements typically conducted over 12 to 24 month intervals during scheduled outages. Corrosion coupons are also sometimes used; typically require considerable exposure time to provide meaningful data. The major drawback of these methods is that corrosion information is obtained after the damage has been done. Management of boiler waterwall loss and system optimisation therefore requires a real-time indication of corrosion rate in susceptible regions of the furnace. This paper describes the results of a program of laboratory trials and field investigations and considers the use of an on-line technology in combination with innovative applications, also modelling and precision metrology to better manage waterwall loss in fossil fuelled boilers while minimising NO_x emissions.     

镁水泥混凝土中钢筋的电化学腐蚀研究

为解决分布在西部盐湖地区出现的煤矿矿区钢材腐蚀问题,结合煤矿矿区实际环境,采用实验、理论分析等方法研究了镁水泥混凝土中钢筋的腐蚀.采用CS350电化学工作站,研究镁水泥混凝土中钢筋的电化学腐蚀,结合SEM微观实验和EDS元素扫描结果,对镁水泥混凝土中裸露钢筋腐蚀状况进行描述.研究表明:美加力涂层钢筋状态良好,未出现腐蚀状态;锌美特涂层钢筋出现了低腐蚀现象;而裸露钢筋表面出现了严重腐蚀.钢筋涂层防腐蚀性美加力涂层较好,锌美特涂层次之;涂层钢筋虽然防腐效果不同,但对裸露钢筋的防腐蚀性能有明显有利影响.     

钢筋混凝土腐蚀的电化学检测研究现状

综述了混凝土中钢筋腐蚀的几种重要的电化学检测技术,主要包括线性极化电阻的测量、电化学阻抗谱的应用和电化学噪声的测量与分析,重点介绍了电化学噪声技术在钢筋混凝土失效方面的应用,并比较了各种方法的优缺点,同时展望了钢筋混凝土腐蚀的电化学检测的发展方向。     

Development and application of a methodology for the measurement of corrosion and erosion damage in laboratory,burner rig and plant environments

Abstract

Fuel gases derived from solid fuels such as coal, biomass and waste and their mixes have the potential to cause both erosion and corrosion damage to components in gas turbines and diesel engines. To allow the statistically valid assessment of materials performance in short term plant runs, burner rig tests and laboratory simulated environments a methodology has been developed to collect compatible quantitative data on materials degradation. Accurate measurement techniques based on pre-exposure contact metrology and post-exposure optical microscopy/image analysis have been developed. These take into account both the low level of damage required for practical systems and the localised nature of hot corrosion damage. The data produced have been used to derive and test quantitative models for the prediction of the performance of candidate materials in such power systems. For these models to be used with confidence, similar damage morphologies must be produced in both the real and simulated conditions, as well as similar damage rates.     

Degradation of heat exchanger materials under biomass co-firing conditions

Abstract

Co-firing biomass in conventional pulverised coal fired power stations offers a means to rapidly introduce renewable and CO₂ neutral biomass fuels into the power generation market. Existing coalfired power stations are both much larger and more efficient than current designs of new biomass combustion systems, so feeding a few percent of biomass feed into an existing large coal fired station will give more biomass derived power than a new dedicated biomass station. Co-firing levels started at ～2% biomass, but this has increased to ～5–10% biomass, with higher levels of biomass co-firing being investigated, although supply of biomass becomes an issue with increasing co-firing levels. The lower levels of biomass co-firing (up to ～5%) can be achieved with relatively minor modifications to existing plants, so avoiding the large capital costs and risks of building new biomass-only fired power systems. However higher levels of co-firing are more difficult to achieve, requiring dedicated biomass supply systems and burners. For existing coal-fired power stations, the co-firing of biomass causes some practical problems, e.g.: the control of co-firing two fuels; changes to bottom/fly ash chemistry; changes to deposition (fouling and slagging) within the boiler; reduced reliability of key high temperature components (e.g. heat exchangers) due to increased corrosion problems relative to those experienced with coal alone.

This paper reports the results of assessments carried out to evaluate the potential operating conditions of heat exchangers in combustion systems with biomass (wood or straw) and coal cofiring, as well as laboratory corrosion tests that have been carried out to give an initial assessment of potential effects of biomass-co-firing.

The corrosion tests have been carried out using the deposit recoat method in controlled atmosphere furnaces. A series of 1000 hour tests have been carried out at typical superheater and evaporator metal temperatures using simulated deposit compositions and gaseous environments (selected on the basis of plant experience and potential fuel compositions). Five materials were exposed in these tests: 1Cr steel, T22 steel, X20CrMoV121, TP347HFG and alloy 625. In order to produce statistically valid data on the actual metal loss from the materials, the performance of the materials in these tests was determined from dimensional metrology before and after exposure. For each material, these data have been used to determine the sensitivity of the corrosion damage to changes in the exposure conditions (e.g. deposit composition, gas composition) thereby producing initial models of the corrosion performance of the materials. The corrosion data and model outputs have been compared with data available from power plants operating on coal, straw or wood fuels.     

Stress Assisted Corrosion of Waterwall Tubes in Recovery Boiler Tubes: Failure Analysis

Waterside cracking of carbon-steel boiler tubes is one of the major safety and efficiency concerns in kraft recovery boilers in the pulp and paper industry, because any water leak into the furnace could cause a smelt-water explosion in the boiler. Failed carbon-steel boiler tubes from different kraft recovery boilers were examined to understand the role of carbon-steel microstructure on crack initiation and crack morphology. A number of carbon-steel tubes showed a deep decarburized layer on the inner surface (water-touched) and also an unusually large grain size at the inner tube surface. In some boiler tubes, cracks were found to initiate in areas with large-grained-decarburized microstructure. However, tubes without such microstructure were also found to have stress assisted corrosion (SAC) cracks. It was found that the decarburization and large grained microstructure may facilitate initiation and growth, but it is not necessary for SAC of carbon-steel boiler tubes.     

Waterwall corrosion in pulverized coal burning boilers: root causes and wastage predictions

Abstract

Waterwall corrosion has become a serious problem in the USA since the introduction of combustion systems, designed to lower NO_x emissions. Previous papers have shown that the main cause of the increased corrosion is the deposition of corrodants, iron sulfides and alkali chlorides, which occurs under reducing conditions. In this paper, the contribution of various variables such as the amount of corrodant in the deposit, the flue gas composition and the metal temperature, is further quantified in laboratory tests, using a test furnace allowing thermal gradients across the deposit and the metal tube samples. Approximate deposit compositions were calculated from the coal composition, its associated ash constituents and corrosive impurities. A commercially available thermochemical equilibrium package was used, after modifications to reflect empirical alkali availability data. Predictions from these calculations agreed reasonably well with the alkali chloride and FeS content found in actual boiler deposits. Thus approximate corrosion rates can be predicted from the chemical composition of the coal using corrosion rates from laboratory tests, adjusted to account for the short duration (100 hours) of the laboratory tests. Reasonable agreement was again obtained between actual and predicted results.     

聚苯胺防腐蚀电化学方法研究

本文综述了国内外在聚苯胺 (PANI)防腐研究中一些常用和新颖的电化学方法 ,指出其测试原理、过程和结果 ,表明PANI有良好的防腐效果。     

Laser-induced breakdown spectroscopy at high temperatures in industrial boilers and furnaces

Laser-induced breakdown spectroscopy (LIBS) was applied (1) near the superheater of an electric power generation boiler burning biomass, coal, or both; (2) at the exit of a glass-melting furnace burning natural gas and oxygen; and (3) near the nose arches of two paper mill recovery boilers burning black liquor. Difficulties associated with the high temperatures and high particle loadings in these environments were surmounted by use of novel LIBS probes. Echelle and linear spectrometers coupled to intensified CCD cameras were used individually and sometimes simultaneously. Elements detected include Na, K, Ca, Mg, C, B, Si, Mn, Al, Fe, Rb, Cl, and Ti.     

Materials performance in waste to energy plants

Abstract

The concept of phasing out of landfills has made mass burning of refuse an attractive alternative. Even more popular is the advent of the “garbage in, energy out” philosophy. This has led to the development of boilers that convert the heat generated from burning refuse into revenue-producing process steam and electricity. Because refuse is non homogeneous in nature, the furnace zone must accommodate the combustion of a particularly aggressive and complex fuel. It must be designed to withstand the potential damaging effects of corrosion and erosion caused by the ash and flue gas generated as well as the methods used to remove the ash once it has deposited on the tube surfaces. The early mass-burning units, built with carbon and low-alloy steel waterwalls and convective passes, have experienced substantial wastage rates from corrosion and erosion. Parameters affecting the operation of the unit can be changed to minimize tube wastage, but at the expense of boiler efficiency. Upgrading the materials to more corrosion-resistant alloys, however, should not affect efficiency and should prove to be an economical solution to the problem.     

Simulated fireside corrosion of T91 in oxy-combustion systems with an emphasis on coal/biomass environments

Oxy-combustion is the burning of a fuel in oxygen rather than air for the ease of capture of CO₂ for reuse or sequestration. Corrosion issues associated with the change in heat exchanger tube operating environment (replacement of most of the N₂ with CO₂ and potentially higher SO_x levels) from air- to oxy-combustion were examined. The ferritic-martensitic alloy T91 was used in accelerated fireside corrosion tests using several different gas compositions and ash deposit overcoats to simulate air-fired, oxy-fired coal, and oxy-fired co-fired coal/biomass conditions. Initial corrosion was observed after 240 h of exposure by examining cross-sections with retained ash. Metal section losses were determined after exposures of up to 1440 h at 600–700°C. Severe corrosion was observed, and a corrosion response with respect to ash deposit chemistry was observed. Corrosion response differences with respect to gas phase chemistry were minimal. Alloy-oxide scale-ash morphologies were consistent with oxide fluxing mechanisms.     

P7412陶瓷涂层的制备与腐蚀机理的研究

利用等离子喷涂技术制备NiCr作为粘接层的P7412陶瓷涂层。采用失重法研究了P7412涂层在3.5%NaCl溶液中的腐蚀速率。采用SEM观察涂层腐蚀前后的表面形貌,并用动电位极化曲线和电化学阻抗谱(EIS)技术研究涂层腐蚀过程中的电化学特征。根据陶瓷涂层的特殊结构,建立等效电路图并进行拟合分析,同时结合涂层的失重曲线研究涂层的腐蚀机理。失重曲线显示陶瓷涂层质量在3.5%NaCl溶液中出现周期性增减变化。电化学研究结果表明,基体表面腐蚀钝化膜以及腐蚀产物等都会影响涂层失重。氧化膜的生长与溶解以及腐蚀二次产物的积累共同导致了涂层质量出现增减交替的变化。     

某锅炉水冷壁腐蚀原因分析及其改进措施

针对某电厂锅炉水冷壁管的严重腐蚀问题,通过宏观形貌检查、化学成分分析、微观组织分析及能谱分析等方法进行了系统分析。结果表明:该锅炉燃用高硫煤时,炉膛火焰中心偏斜导致水冷壁局部出现还原性气氛,产生硫化物型高温腐蚀。最后针对具体原因,提出了控制燃煤质量、调整燃烧、增加贴壁风等改进措施,有效地解决了该锅炉水冷壁高温腐蚀问题。     

电流密度和添加剂对镁合金电化学磷化膜耐蚀性的影响

电化学磷化可以快速获得磷化膜,提高镁合金的耐蚀性,目前就电化学磷化工艺条件对膜层的影响研究尚不深入。为此,采用扫描电镜和电化学方法研究了电流密度和添加剂对镁合金电化学磷化膜耐蚀性的影响。结果显示：电流密度为4．oA／din。时基础磷化液中所得磷化膜表面致密均匀,具有良好的耐蚀性;以0．5g／L酒石酸和5．Og／L磷酸二...