Oxyfuel fired combustion has the potential to increase the fireside corrosion rates comparing to air firing combustion as a result of the increasing amounts of aggressive combustion products due to recycling of flue gas. The changes in the combustion gas chemistry will also affect the chemistry and formation of deposits, with potentially increasing corrosion and internal attack of the boiler components that are in contact with the combustion and flue gas environment. As the currently available information on the corrosion rates under oxyfuel conditions is still limited, and partially also contradicting, corrosion testing of existing boiler materials under high carbon dioxide combustion environments with the relevant oxygen, water vapour and impurity concentrations and deposits is very much needed. The work describes the laboratory testing of selected alloys (EN 1.4922, UNS S34710, UNS S31042, UNS S31035, A263 and A617) under simulated oxy-and air-firing combustion conditions with and without calcium carbonate-calcium sulphate deposit at 600 and 650 8C. The results showed that the corrosion resistance increased when the chromium content increased but without added impurities like sulphur and chlorides, the simulated oxyfuel conditions did not result in more severe corrosion than under air-fired environment. No carburisation of the metal substrate was observed after exposure to simulated oxyfuel gas atmospheres without deposit, although some carbon enrichment was detected near the oxide-metal interface. With extended exposure time, the oxide scale properties may change to enable metal carburisation. The exposure with deposit at 650 8C resulted in corrosion of all tested alloys and clear carburisation of steels EN 1.4922 and UNS S34710. Corrosion and microstructural changes, like carburisation, may reduce mechanical performance such as creep and/or fatigue strength and ductility, and thus the expected component life.

Supercritical (SC) service at high operating values aims for good plant efficiency, but the waterside oxidation resistance can then become life-limiting. In this paper, selected materials and modeling options are compared for life assessment under waterside SC oxidation, particularly for thermal power plants that increasingly need to accommodate cyclic service, fast ramping, and low minimum loads to an extent to which the conventional design practices and materials solutions only partially accounted. For example, the life reduction by high-temperature oxidation and corrosion via lost load-bearing wall thickness is more easily accommodated than the impact on crack growth or material ductility.

Using biofuels in power and CHP boilers can pose a challenge for materials performance. Formation of deposits containing e.g. potassium, sulphur, calcium, sodium, and chlorine can result in severe corrosion of conventional steels and alloys at relatively modest temperatures. Given suitable component design and fabrication facilities, coatings may be considered to protect the fireside surfaces. This paper aims to present a systematic approach to the design and selection criteria for protective coatings of boilers. The approach includes modelling of the process and surface conditions, optimisation of the coating process and structure, and performance validation in the laboratory and plant scales. The applied examples have included iron and nickel based HVOF and arc sprayed coatings subjected to verification field testing in boiler testing under aggressive biofuel conditions. The coatings have shown good corrosion resistance in both laboratory tests and long-term harsh field tests. The paper discusses the used approach for finding a suitable and cost effective coating for biofuel boiler applications. The paper gives test results from microstructural, corrosion resistance and field testing experience for the selected coatings.

Oxy-fuel combustion combined with CCS (carbon capture and storage) aims to decrease CO2 emissions in energy production using fossil fuels. Oxygen firing changes power plant boiler conditions compared to conventional firing. Higher material temperatures and harsher and more variable environmental conditions cause new degradation processes that are inadequately understood at the moment. In this study, an Fe-Ni-Cr-Al alloy was developed based on thermodynamic simulations. The chosen composition was manufactured as powder by gas atomization. The powder was sieved into two fractions: The finer was used to produce thermal spray coatings by high velocity oxy-fuel (HVOF) and the coarser to manufacture bulk specimens by hot isostatic pressing (HIP). The high temperature corrosion properties of the manufactured FeNiCrAl coating and bulk material were tested in laboratory conditions simulating oxy-combustion. The manufacturing methods and the results of high temperature corrosion performance are presented. The corrosion performance of the coating was on average between the bulk steel references Sanicro 25 and TP347HFG. OPEN ACCESSCoatings 2015, 5 710

The advantage of using supercritical water (SCW) systems for power generation is based on the increased thermodynamic efficiency when operating at higher temperature and pressure. Steam oxidation has become an important issue for power plants as operating temperatures increase from current to 650°C and even higher. Three alloys, FeCrAlY, NiCrAl, and Sanicro 25, were investigated in an elevated steam oxidation condition. All three materials showed relatively good initial SCW oxidation resistance, but after 100 hr, the oxidation rate of FeCrAlY increased rapidly compared to NiCrAl and Sanicro 25, which both showed a steadier and lower rate of oxide growth and weight gain.