Gas Turbine Handbook : Principles and Practices

Microturbines 175
gas turbines) with a radial stage. The selection of turbine wheel
design types is primarily a function of parentage (that is, the
origin of the microturbine design).
While current turbine wheels are made from the same materials
used in the larger gas turbines, development of turbine wheels made
from monolithic silicon nitride material (a ceramic) is in progress.
These new materials will provide improved high temperature creep
rate and oxidation resistance.
When developed these designs and manufacturing techniques
will undoubtedly be incorporated into the multi-megawatt gas turbines—just
as advances made in jet engine technology was, and
continues to be, transferred to the large industrial gas turbine engine
designs.
Recuperator
Recuperators transfer heat from the turbine exhaust to the
combustor inlet (or compressor discharge). In a recuperator the heat
transfer occurs through the passage walls (hot exhaust gases on one
side and cool compressor air on the other—Figure 11-5). Recuperators
are essential to achieving increased efficiency from microturbines.
“With recuperation electric efficiencies are typically 26-32% versus
15-22% for non-recuperated units.” 3 Another advantage of recuperators
is the reduction in NO x . This reduction is due to the decrease
in the amount of fuel (evidenced as an increase in efficiency) and the
subsequent decrease in fuel-bound nitrogen (See oxides of nitrogen
in Chapter 9 for a detailed discussion of exhaust emissions).
The renewed interest in recuperators is extending into the
multi-megawatt sizes. Recuperated gas turbines can take advantage
of their lower compressor pressure ratios and combustion temperatures
to reduce NO x formation. “Recuperated turbine DLE combustors
show potential to meet ultra-low emissions levels without any
after-treatment.” 4
High temperature recuperator materials are being developed
in order to achieve greater than 40% efficiency at a cost less than
$500.00 per kilowatt. These materials are expected to operate between
1290 o F (700 o C) and 1830 o F (1000 o C). Among the materials
being studied are 347SS, Alloy 230, Modified Alloy 803, Alloy 120,
Thermie Alloy, Alloy 625 and Alloy 214. 5