In-situ fireside corrosion testing of superheater materials with coal, wood and straw fuels for conventional and asvanced steam temperatures

• An increase in the steam temperature of a power station increases the electrical efficiency of the turbine. This in turn leads to a reduction in the cost of electricity and in environmental emissions produced per unit of electricity generated. However, higher steam temperatures give rise to more aggressive corrosive environments and the choice of material becomes more important. In addition, the aggressiveness of the fuels also depends on their chemical compositions.  As part of a European research programme (COST 522) fireside corrosion tests of superheater and waterwall materials have been performed in-situ in industrial boilers or combustion test facilities, simulating conventional and higher steam temperatures. The fuels used were, straw, wood (logging residues) and coal. Goals were  set at different maximum steam temperatures and lifetimes according to the fuel. The targets were: coal 650°C/100,000 h,  wood 580°C/40,000 h and straw 580°C/20,000 h .A wide range of materials was tested.  An overriding constraint was that the materials must be economically viable and not impose exceptional fabrication requirements. Some materials were tested in a number of combustion environments, allowing useful comparisons to be made. The results showed that for a given superheater alloy, temperature and fuel, the corrosion rate depended on the alloy’s position in a superheater bank. Tubes on the outside, exposed to the flue gas, corroded faster than those positioned deeper in a bank.  Tubes experiencing a greater heat flux will corrode more rapidly. Poorly controlled combustion will also increase corrosion. Straw was much more corrosive than wood (logging residues). The difference in corrosiveness increased with increasing temperature above a metal temperature of about 520°C. The corrosion rates of the alloys tested during wood-firing were only a little higher than those from firing coal with a medium to high chlorine content. Straw and wood are often collectively known as “biomass”, but the corrosion they cause can vary by a factor of 5 or more. No difference could be found in the fireside  corrosion rates of the steels TP 347H and TP 347HFG, when tested under similar conditions. Esshete 1250 also showed similar fireside corrosion rates to347 instraw and wood boilers and at temperatures below 650°Cin coal boilers. Several alloys were identified to resist corrosion in coal-fired plant at metal temperatures of680°C(steam temperatures650°C) and high heat fluxes.  Adequate high temperature creep strength remains more problematic.  At lower heat fluxes (resulting in lower corrosion rates) more candidate alloys exist. It is expected that the goals set can be achieved with the use of suitable alloys in the case of wood, waste and coal. For wood (logging residues) TP347, Esshete 1250 and 50Ni50Cr coatings showed sufficient corrosion resistance at the target temperatures. In the coal case  HR3C fulfills the requirements of strength and corrosion resistance with a high heat flux and  Super 304H and SAVE 25 if the heat flux is low. HR11N and IN671 are suitable for use as claddings on a substrate with the appropriate creep strength. The goal set for straw firing was ambitious, a steam temperature of580°C, and in this case a combination of material, combustion technology and boiler design are needed.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Materialteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Materials Engineering (hsv//eng)

TEKNIK OCH TEKNOLOGIER  -- Kemiteknik -- Korrosionsteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Chemical Engineering -- Corrosion Engineering (hsv//eng)

Nyckelord

Thermal power

high temperature corrosion

superheaters

steels

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