Papers by Keyword: Fireside Corrosion

Abstract: 1¹/₄Cr¹/₂Mo³/₄Si (T11) and 2¹/₄Cr1Mo (T22) grade steels are extensively used in critical parts of sub-critical thermal power plants like superheater, reheater tubes or headers. The tubes are designed to last for a very long time in service. These tubes are exposed to hot flue gas on the outer side and consequently, the inner side heats the steam passing through the tube. The outlet temperature of the steam ranges from 500-550°C. Therefore, not only the temperature on the outer side is much higher than the steam outlet but also the environment which the tube faces is extremely corrosive thereby leading to hot corrosion/ fireside corrosion. Some parts of the tubes are also exposed to fly ash coming from the combustion of coal leading to the formation of deposits on the tubes. In the present work, the tubes are cut and taken out during overhaul shutdowns and they are characterized by X-Ray Diffraction (XRD) for the likely phases that are formed on the surface of the fireside corroded layer. The deposits/scales are also qualitatively characterized. The cross-sections of the tubes are used for characterization of fireside corroded product layers using Optical Microscopy and Field Emission Scanning Electron Microscopy (FESEM) coupled with Energy Dispersive X-Ray Spectroscopy (EDS). This work will be extended further to develop rate laws and rate mechanisms of the corrosion product layers formed on the service exposed tubes.

Abstract: Japanese government project of the A-USC technology was started in 2008 August. 700°C class boiler, turbine and valve technologies, which include high temperature material technology, will be developed. This report provides the present state of the art and technical background of this development effort for A-USC in Japan, especially focusing the high temperature corrosion and the steam oxidation behavior of available and developmental materials for boiler.

Abstract: Increase of the energy recovery efficiency facilities is one of the challenges fixed recently by UE to Municipal Solid Waste Incineration (MSWI) operators. To achieve this target, one option consists in optimizing the water/steam cycle to increase electrical efficiency. Nevertheless, higher steam temperature into heat exchanger tube is expected to increase the risks of fireside corrosion, particularly on superheater tubes, along with important loss of materials, frequent shutdowns for repairs and high operational costs. In this study, fireside corrosion test had been performed using an innovative laboratory-scale corrosion pilot. Effect of increase in metal temperature from 400°C to 450°C on corrosion performances of SA192 carbon steel tube and AISI 316L stainless steel had been examined. Mechanisms and corrosion rates are discussed with regards to lifetime reported for both materials after 1 year in service in the same plant.

Abstract: There is growing concern over the effects of global warning. In response the power generation sector is having to consider a wider range of systems and fuels for use in generating heat and power. One of the classes of solid fuels that is being increasingly developed is biomass, which is regarded a both sustainable and carbon neutral. In fact, the term biomass covers a wide range of fuels from waste products, such as straw, forestry wastes and sawdust, through to purpose grown energy crops, such as coppiced willow and miscanthus. To maximise combustion plant efficiency it is necessary to use high temperature/pressure steam turbines. However, to generate such steam conditions, the high heat exchanger surface temperatures can interaction with the various potential products of biomass combustion to cause excessive deposition and corrosion of these surfaces. This paper considers the range of heat exchanger operating environments that can be produced by the combustion of different potential biomass fuels, especially the effects of the higher K and Cl contents of the faster growing biomass fuels. This paper reports the results of a series of laboratory corrosion tests that have been carried out to assess the effects of various types of biomass on the corrosion of high temperature heat exchanger materials in combustion plants. The corrosion tests have been carried out using the deposit recoat method in controlled atmosphere furnaces. Six 1000 hour tests have been carried out at typical superheater / reheater and evaporator conditions (450-600°C) using simulated deposit and gas compositions, which have been selected on the basis of potential biomass fuel compositions. The five metals exposed in this study are widely used in power plant heat exchangers: 1% Cr steel, 2.25% Cr steel (T23), 9% Cr steel (T91), X20CrMoV121, TP347HFG and alloy 625. During the course of the tests, the material degradation was monitored using traditional mass change measurements. 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: pre-exposure measurements were made using a micrometer; post-exposure measurements were made using an image analyser system. SEM/EDX and XRD analyses have been used to confirm corrosion mechanisms and their association with corrosion damage levels. For each material, the dimensional metrology data have been used to determine the sensitivity of the corrosion damage to changes in the exposure conditions (e.g. deposit composition, gas composition) to generate 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.

Abstract: Fireside corrosion is since a long time the main limitation to increase efficiency of energy recovery boilers of waste to energy (W-t-E) facilities. Nevertheless, the increase of steam conditions in addition with the variation of feeding fuels composition imply greater risks of fouling and corrosion along with heat exchanger failure, loss of plant availability and high maintenance costs. Fireside corrosion mechanisms had already been widely treated in the literature and this paper will review the main critical factors that enhance fireside corrosion of superheater. Recent failure cases will be developed in regards with recent studies that provide interesting routes to predict corrosion failure and develop maintenance strategy.

Abstract: The aim to reduce the CO2-emissions has triggered the evaluation of new cycle concepts for power plants. For the coal-fired power plants, the oxy-fuel firing is a promising option for CO2- emission reduction. Here, the combustion takes place in a nitrogen-free atmosphere. The oxygen is separated from the air and burned in near-stoichiometric conditions with the fuel. The gas composition is significantly changed, when the combustion is changed from air-fired to oxy-fuel fired condition. For lignite, the carbon dioxide content is raised from 15 to 59vol% and the watercontent from 10 to ~32%. For the same fuel, the SO2-content in the flue gas increases by a factor of 3-4 to ~0.5%. These changed environmental boundary conditions will affect corrosion life of the materials especially on the water walls and the heat exchanger surfaces. Considering the significant changes in the combustion gas, the composition and the occurrence of the corrosive deposits has been evaluated with the thermodynamic modelling program ‘FactSage’. The chemical compositions of the deposits have been modelled for dried lignite from Germany. The results exhibit that the oxy-fuel firing will give a significant change in the atmosphere as well as in the deposit composition. Consequently, the corrosion rates of current used materials in air fired boilers need to be evaluated for the application in oxy-fuel fired boilers.