Gas Turbine Handbook : Principles and Practices

The Gas Turbine's Future 283
to 30%—and all of it is petroleum based. Ultimately the gas turbine
will be required to burn hydrogen, which is the leading candidate to
replace gasoline. Presently hydrogen is made most often be reforming
methane. The use of hydrogen eliminates the fuel bound nitrogen that
is found in fossil fuels. With combustion systems currently capable
of reducing the dry NO x
level to 25 ppmv; and catalytic combustion
systems demonstrating their ability to reduce emissions to “single
digits”; the use of hydrogen fuel promises to reduce the emission levels
to less than 1 ppmv. The gas turbine will need to achieve this low
level in order to compete with the fuel cell. Today the largest fuel cell
manufactured is rated at 200 kilowatts. This technology is available
and it will be developed.
Hospitals, office complexes, and shopping malls will become the
prime sales targets for 1 to 5 megawatt power plants. Residential
homes, small businesses, and schools will turn to the smaller gas
turbines—the microturbines. Operating these small plants, on site,
may prove to be more economical than purchasing power through the
electric grid from a remote power plant. This is especially true with the
current restructuring of the electric utility industry. In the very small
gas turbines, manufacturers are returning to the centrifugal design
in both the compressor and the turbine. Centrifugal compressors
and radial inflow turbines are used in most microturbines. The
advantage is that this design can be produced as three distinct
parts (compressor, turbine, and shaft). The compressor and turbine
components are then pressed onto the shaft forming one part. This
small gas turbine generator design entered the low power market
in the mid-to-late 1990s with units from Capstone (Figure 18-3),
Allied Signal, Ingersoll Rand, and Elliott Energy Systems, Inc. The
capital cost of the microturbine is already at $1.00 per watt and it is
targeted to reach $0.50 per watt. This is possible primarily due to the
design of the compressor and turbine components. These components
are relatively easy and inexpensive to cast. With precision casting
methods, finish machining is limited and in some cases unnecessary,
which further reduces cost. These units operate on gaseous or liquid
fuel and generate from 20 to 500 kilowatts.
To reduce the expense of units under 5 megawatts, the next
breakthrough must be in the production of axial blade and disc
assemblies as a single component. The greatest single obstacle to
reducing gas turbine costs is the manufacturing process—machining