Page 442 - Compression Machinery for Oil and Gas

Downstream Chapter 10 421

Large combustion turbine power plants have high initial costs, coupled with
increasingly common long delays from permitting, financing, and local and/or
environmental opposition reasons. The high cost of associated power lines and
real estate are also factors that inhibit new construction, especially in the north-
east and California. The trend is to build many more, but much smaller power
plants, often referred to as (combined heat and power (CHP), distributed gen-
eration (DG), or microgrid (MG) plants. The driver of the gen-set can either be a
small turbine or a reciprocating engine. However, most engines require low-
pressure fuel gas readily available at many sites, and would not require an FGC.
The CHP, DG, and MG plants are much less expensive, and are physically
smaller and much quicker to build. In this age of potential terrorist attacks, they
may be installed at a government facility, military base, or wastewater treatment
plant for security reasons to ensure power to these sites cannot easily be cut off.
However, the driving force is also simple economics with a reasonable return on
investment (ROI). This is due to the current low cost of natural gas, which
should continue to be inexpensive over the next decade or more, coupled with
the relatively high cost and increasing demand of electricity. The ROI for CHP
plants are the best, because these are a smaller version of a large combined cycle
plant, with the waste exhaust heat typically used to heat water to replace boilers
and/or for hot water usage in hospitals or industrial facilities.
Fuel gas compressor designs can be reciprocating, rotary screw, or centrif-
ugal. Other chapters in this book cover the mechanical details of each. However,
there is really not the one design that always best fits this application, and each
design has unique features which may make it better suited for a specific power
plant project.
Oil-free fuel gas compressors are not an important consideration, even
though nearly “oil-free” fuel gas quality is often specified by turbine manufac-
turers. The pipeline quality natural gas coming into most sites would have likely
gone through several lubricated compressors on its journey to the power plant.
Even without requiring compression at a power plant, high-efficiency filtration
is required to ensure the fuel gas is nearly “oil-free” at most sites. So, there is no
real benefit to an oil-free FGC, since filtration will be also typically be needed.
Large power plants typically require large FGC’s with electric motors that
are several hundred or even several thousand horsepower. Operating power cost
for all the auxiliary equipment sized for a large turbine is an overall power plant
evaluation point, commonly referred to as the “AUX LOAD” guarantee. Con-
tractually, this typically has liquidated damages associated with it, in case the
guarantee in kilowatts cannot be demonstrated during the start-up of the power
plant. Since the FGC is one of the largest components of the “AUX LOAD”
guarantee, the most efficient compressor at the design point or anticipated oper-
ating points may be big factor in the compressor selection.
The compressor capacity control system is also an important selection fac-
tor. Typically, the capacity control system is designed to maintain a constant