Page 145 - Sustainable On-Site CHP Systems Design, Construction, and Operations

118 CHP B a s i c s

• The fourth column provides the threshold levels for sites that are considered
major modifications to existing sources.
• The fifth column provides the threshold levels for sites that are considered new
major sources.
The right side of the printout provides similar information for the case when the
prime mover is located and operated in a nonattainment area.
It should be noted that the emissions calculator only calculates the emissions for the
selected pollutants, which may be critical for CHP facilities. However, emission limits
for other pollutants have to be observed as well.

Emission Control Technologies for CHP
Emissions of CO from CHP systems are not presently controlled and depend primarily
2
on the type and quality of fuel used. Except for coal, gaseous fuels used most often in
CHP systems are quite low in sulfur and generally do not require post-combustion
treatment to reduce SO . The emission control technologies for CHP systems control the
x
emissions of NO , CO, and unburned hydrocarbons. The applications of these technolo-
x
gies depend on the type of prime mover deployed for power generation.
Reciprocating Internal Combustion Engines
As discussed in Chap. 3, two types of ignition systems are used in four-stroke recipro-
cating internal combustion engines (IC engine): spark ignition and compression igni-
tion. Spark-ignited engines can use natural gas, biogas, propane, or gasoline as fuel where
as compression-ignited engines can only use diesel, biodiesel, or a combination of diesel
and natural gas.
IC engines are designed to operate in one of the two modes:
1. Rich burn
2. Lean burn

10
Typical effect of rich and lean burn operation of IC engines on the emissions is
shown in Fig. 7-1.
Rich burn operations use fuel-air ratios (or the inverse of the air-fuel ratios) that are
higher (or the air-fuel ratios are lower) than the stoichiometric ratio (defined as the fuel-air
ratio theoretically required for complete combustion of the fuel). Generally, rich burn mode
is more common for engine capacities <500 kW (670 hp). Emissions of NO from rich burn
x
engines are in the range of 30 to 50 lb/MWh (or 625 to 1060 ppm at 15 percent oxygen). In
order to put the emissions of engines in some perspective, it is important to note that the
average emissions from all central power plants in the United States are approximately 3 lb
11
of NO per MWh according to the EPA eGRID data for the year 2000. The emissions from
x
rich burn engines are generally not acceptable at most locations especially in Europe and in
the United States. Therefore, most installations using rich burn engines require post-com-
bustion treatment of exhaust gases.
Exhaust from rich burn engines is generally treated by a three-way catalyst. A three-
way catalytic converter accomplishes the following three simultaneous tasks:

1. Reduction of nitrogen oxides to nitrogen and oxygen: 2NO → xO + N
x 2 2
2. Oxidation of carbon monoxide to carbon dioxide: 2CO + O → 2CO
2 2