SPRING 1998 DAVID No FRENCH! INCot METALLURGISTS ONE LANCASTER ROAD NORTHBOROUGH MASSACHUSETTS 01532 TEL: (508) 393-3635 VOL. mt O. 1 FAX: (508) 393-3914 A VIEW FROM THE PENTHOUSE: USEFUL INFORMATION FOR THE WORLD OF BOILERS SOME METALLURGICAL PROBLEMS WITH HRSGS For the past many years the install ation of choice for new electric-generating capacity has bee n a natural gas-fueled combustion turbine . On the back end is a heat- recovery steam generator (HRS G) to captur e the sensible heat in the turbine exhaust and improve overall thermal effici ency. These HRSG units are a series of heat exchangers that are designed to heat water in the feedwa ter heater or economizer, boi l wat er in the evaporator, and super- heat steam in the superheater sec tion . There is usually more than one operating pressure for economizers and evaporators as we ll. The HRSG will increase the output by more than 35%. The principal advantage of this combination is the favora ble heat rate, per haps as low as 7,500 btulmegawatt of electricity generated. A second important feature is the use o f natural gas as the primar y fuel . Oil is the back-up fuel . Natural gas is inherently less polluti ng than either coal or oil- fired boilers. Since methane (or natura l-_gas ) s one atom of _carbon to --_ four atoms of hydrogen, a significant portion of the heat (more than 50%) comes from combustion of hydrogen to water vapor which limits the amount of greenhouse gas (carbon dioxid e) emitte d per megawatt generated. For all of the thermodynamic advantages, howeve r, these HRSG devices come with some important, potential, metallurgical proble ms. Natura l gas is colorless an d odor les s. For safety reasons, a small quantity of an odorant, a sulfur compound, is adde d. Gas lea ks may then be detected by the smell. Eno ugh odorant is added that some sulfur dioxide and sulfur trioxide are par t of the combustion turbine exha ust . To prevent dew-point corrosion by sulfurous and sulfuric acids, the feedwater heaters have bee n fabricated of 304 or 304L austeni tic stainless steel. These alloys are also chosen to minimi ze the oxygen-pitting corrosion problem s on the water side of the tube s. The wat er inl et temperature to the feedwater heater is between 70° and 100°F, well blow the dew-point of sulfuric acid . The exact dew-po int depend s on the concentration o f sulfur trioxide in the exhaust bu t is around 280 °F . The dew-point of sulfurous acid (sulfur dioxide and water) is lower, around 230°F. Unexpe cted, however, were the failures o f the feedwater heaters by chloride-induced, stress-corrosion cracking that initiated on the outside or gas side. A combustion turbine is, in effect, a very large the atmospheric contaminants along with the combustion air. Cooling towers use chlorine or chlorine compounds as a biocide and contribute chlori ne a nd chlorides to the environ ment. When the wind is i n the I8 wr on gl 8 irection, cooling-tower spray becomes a part of the turbine inlet air . Other sources of chlorides are seawater and perhap s roa d deicing salt, as well as industr ial smog. Whatever the source, chlorine compounds then fin d their way into the combustion exhaust. The feedwater-heater inlet temperature is around 70-100°F, well belo w the hydrochloric-acid dew point of around 130°F. Inevitably the coldest portion of the inlet to the feedwa ter heaters is below the hydrochloric-acid dew-point. When the condi tions are just right (or wrong, dep ending on your