Page 32 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Overview 11

• Engine(s) or prime mover(s)
• Generator(s) and electrical paralleling/distribution system
• Heat recovery boiler(s) (e.g., HRSG and HRHWG—heat recovery hot water
generator)
• Thermally activated components and/or facility thermal uses
• Emission control system
The following paragraphs provide some basics on CHP components. Additional
details on prime movers can be found in Chap. 3 and on heat recovery devices and
thermal technologies in Chap. 4.

Engine Types
There are a variety of engine types and sizes which can be used as the prime mover for
electric power generation. Prime mover choices include internal combustion (IC) recip-
rocating engines, combustion turbine generators, microturbines, and fuel cells (which is
not really a prime mover per se). The following paragraphs provide a brief description
of the different CHP engines.

Internal Combustion Reciprocating Engines
As shown in Chap. 2, IC engines (both spark ignition and compression ignition) are the
principal prime movers used in smaller (typically less than 1 MW) CHP plants. Most
people are familiar with the IC engine as one powers their automobile; key components
include the pistons and rods, heads, valves, crankshaft, and engine block. Reciprocating
IC engines are available in a wide range of sizes from 50 kW to more than 5 MW and are
able to use all types of liquid and gaseous fuels, including methane from landfills or
sewage treatment plant digesters. Reciprocation engines are classified as either rich-burn
engines or lean-burn engines depending on the fuel-air ratio. Internal combustion engines
that use the diesel cycle (compression ignition) can be fueled by a wide range of fuel
oils, and today there is a move to use biodiesel in place of petroleum diesel which
improves the CHP plant eco-footprint.
Waste heat, in the form of hot water or low-pressure steam (maximum of 30 psig but
typically 15 psig or less), can be recovered from the IC engine jacket manifolds, the
lubrication system, and the flue exhaust.
Combustion Turbine Generators
CTG are typically used in larger facilities with electric loads larger than 1 MW. A com-
bustion turbine is similar to a jet engine; the key components include the compressor,
combustor, and turbine. CTG are commercially available in sizes ranging from approx-
imately 1 MW to more than 100 MW for utility power plants. CTG are also able to operate
on a wide variety of fuels, although some fuel treatment may be required.
For combustion turbine cycle engines, average fuel to electrical shaft efficiencies
generally range from less than 20 percent to more than 35 percent. The remainder of the
fuel energy is discharged in the exhaust, with some loss through radiation or internal
coolants in large combustion turbine generators, and the exhaust heat is recovered in a
HRSG. Because combustion turbine exhaust contains a large percentage of excess air,
duct burners may be installed in the exhaust for supplementary firing to generate addi-
tional steam. Duct burners can be very efficient, exceeding 90 percent.