Hot Corrosion of Nickel-Base Alloys in Biomass-Derived Fuel Simulated Atmosphere Page: 2 of 14
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Introduction
General Background
Fuels derived from peat, forest and agricultural wastes, or from plants grown
specifically for their calorific value are of interest as a renewable source of energy
for future power generation systems for ecological as well economical reasons.
Today biomass provides about 14% of the energy needs worldwide, thus ranking
fourth as an energy resource [1]. It is quite evident that biomass resources offer
environmental advantages over many other fossil fuels. If grown and consumed
on a sustained-energy basis, biomass feedstocks do not contribute to carbon dioxide
enrichment in the atmosphere over their total life cycle. The viability of biomass-
fueled power generation depends on the ability to sustain the mechanical integrity
of the plant, and on the economics of the process. Recent data on the projected
economics of a cogeneration system consisting of a biomass gasifier combined with
gas turbine power production suggested power costs comparable with those from
conventional coal-fired power plants. Therefore, biomass feedstocks are considered
to have the potential to contribute significantly to the world's energy mix [1].
In principle, biomass fuels can be utilized directly as solid fuel via simple
combustion processes in steam boilers or via more sophisticated routes such as
liquefaction or gasification followed by combustion in gas turbines [2]. In order to
meet the 60 percent thermal efficiency goal of future electric power plants targeted
by DOE's Advanced Turbine Systems (ATS) program [3], conversion of solid
biomass to an essentially clean gaseous fuel is necessary. However, the highest
efficiency levels achieved in large utility scale systems today are on the order of
58%; so that the possible interaction of biomass-derived fuels with the advanced
turbine design features necessary in ATS engines must be considered. For smaller
plant sizes (5-20MW), for which the efficiency is generally lower, combined cycle
efficiencies up to 42.5% seem to be feasible using biomass-derived fuels, essentially
without any necessity to modify the current gas turbine design [4].
Materials Issues
At the operating temperatures of gas turbines, the hot gas-path components must
be capable of forming a thermally grown oxide layer to protect against rapid metal
loss due to oxidation. The oxides of Al and Cr are the most effective oxidation
barriers as they form slow-growing scales, thus keeping the oxidation rate in a
technically reasonable range. Therefore all nickel- or cobalt-based superalloys rely
on the formation of either alumina or chromia scales. In service, loss of the
protective oxide scales by spallation, either induced by growth stresses after
reaching a critical scale thickness, or as a result of imposed thermal and
mechanical stresses, is a critical factor limiting the useful lifetime of the
components. Lost protective scale can be reformed as long as the activity of scale-
forming elements in the underlying alloy is higher than some critical level needed
for an external protective scale to form. Repeated scale loss and reformation,
however, depletes the alloy of these elements such that less-protective scales of the
base metal may become dominant, resulting in a rapid increase of the oxidation
rate which finally leads to component failure. From an oxidation standpoint, high
levels of Al and/or Cr are therefore highly desirable. However, the demand of
maximum high-temperature strength and creep resistance for superalloys has led
to the development of alloys containing relatively low levels of Al and Cr
(compared with the levels required to form protective scales of exclusively
alumina or chromia). Thus, protective coatings are universally used for
components in the hot sections of gas turbines; several coating application
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Leyens, C.; Pint, B. A. & Wright, I. G. Hot Corrosion of Nickel-Base Alloys in Biomass-Derived Fuel Simulated Atmosphere, article, February 28, 1999; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc688528/m1/2/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.