Page 83 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Power Equipment and Systems 61

operate on natural gas, biogas, and propane, while diesel IC engines can be designed to
operate on diesel fuel (fuel oil No. 2), biodiesel, or residual oil. Fuel cells can be designed
to operate on natural gas, propane, or pure hydrogen gas (H ). CHP plant installations
2
that include equipment involving the use of dirty fuels, like digester gas, require addi-
tional fuel-treatment equipment as this fuel requires drying and cleaning before it can
be used in combustion engines.
The fuel pressure required to operate fuel-to-power prime mover equipment in CHP
plants varies from between 0.5 and 45 psig for fuel cells to between 120 and 500 psig for
CTGs. Natural gas IC engines are designed to operate on low-pressure gas. CHP plant
prime mover equipment installations that require high-pressure fuel, including CTGs and
microturbines, require gas compression equipment to increase the pressure of the utility-
delivered fuel to the required pressure.

NO Emissions
x
Emissions characteristics are important to consider in CHP plant prime mover selec-
tions. Permitted allowable limits vary widely depending on where the CHP system is
installed. As discussed in Chap. 12, any proposed project must work closely with the air
quality regulation agencies and permitting an installion is sometimes a challenge.
Untreated NO emission levels vary from very high, more than 30 lb/MWh for diesel
x
engines, to almost nonexistent, below 0.02 lb of NO per megawatthour for fuel cells.
x
Natural gas IC engines have NO emission levels in a range of 2 to 30 lb/MWh. NO
x x
emission levels for CTGs and microturbines vary between about 0.3 to 4 and 0.4 to
2.2 lb/MWh, respectively.
Power Density
The amount of area needed for a CHP plant is a major consideration. Knowing the
2
2
power density (kW/ft or kW/m ), that is, how much area is needed for the plant can
help engineers estimate the CHP plant square footage that will be required for a calcu-
lated facility or building electrical load. The footprint for CHP plant fuel-to-power
prime mover equipment installations does not vary widely between all systems consid-
ered. CTG and microturbine energy footprints are small, varying from about 0.2 to
2
2
0.6 ft /kW and 0.15 to 1.5 ft /kW, respectively. IC reciprocating engines can take more
room that CTG for the same power output, and energy footprints for natural gas and
2
diesel engines varying from about 0.2 to 0.3 ft /kW. Fuel cell installations can have
some of the largest energy footprints of all CHP plant fuel-to-power equipment at up to
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4 ft /kW. Of course, the balance of plant can require extensive area.
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STG energy footprints are noted to be extremely low, typically less than 0.1 ft /kW;
however, it should be noted that STGs represent the thermal-to-power classification of
CHP prime mover equipment and additional square-footage considerations must
account for the steam producing equipment that is installed to power the thermal-
to-power equipment.
Online Availability and Time between Overhauls
Electric power or thermal production capability is often only as good as the availability
of the installed CHP plant prime mover equipment’s ability to consistently operate to
meet the facility electric and thermal loads. All of the noted fuel-to-power prime
mover equipment has online availability between 90 and 98 percent, while the noted