Page 64 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Power Equipment and Systems 43

stoichiometric air-fuel ratio has on engine emissions. Note that both carbon monoxide
(CO) and nonmethane hydrocarbons (NMHC) have their lowest value at an equiva-
lence ratio near 1.0.
In general, except for NMHC depending on the air-fuel ratio, the lean combustion
engine provides much lower levels of atmospheric pollutants. The lean combustion engine
is sometimes capable of meeting emissions requirements without the aid of exhaust treat-
ment and air-fuel ratio controllers depending on the location of the CHP plant.
Although lean combustion engines offer benefits such as lower pollutant emissions,
lean combustion engine’s emissions performance degrades significantly to sometimes
unacceptable values when they are operated at part load. Therefore, load management
and exhaust treatment may also be necessary for lean burn combustion engines. Addi-
tionally, lean burn engines may be more temperamental and require more maintenance
than naturally aspirated rich burn engines.
Reciprocating engines are cooled by a jacket water cooling system. Jacket water
cooling systems are either pump driven, ebullient, or a combination of both. Pump-
driven cooling systems tend to operate at lower temperatures than ebullient cooling
systems. Ebullient means “with boiling” and such systems use density differences
between steam and water in the cooling water to circulate water in the engine. Ebullient
cooling systems require operation at fairly high temperatures. The temperature must be
high enough for water to begin a phase change within the engine. This type of system
is used to produce low-pressure steam (approximately 15 psig) by connecting the jacket
water to a tank (steam drum) above the engine where the steam and hot water separate.
The tank has a jacket cooling water supply connection on the bottom of the tank to the
engine and a heated jacket steam/water return up near the water/steam interface in the
tank. Heat in the engine causes steam bubbles to form and the mixture of water and steam
bubbles is much lighter than water alone, which results in a pressure difference across the
engine jacket causing water circulation. The height of the water level in a tank above the
engine determines the temperature and pressure of steam produced. The steam can
be used for heating processes or for use in a single-stage absorption chiller (see Chap. 4).
Condensate and feed water systems are needed with an ebulliently cooled IC engine CHP
system. Ebullient systems require a separate system to cool engine oil.

Size Ranges
IC engine generators are available in a wide range of sizes to meet many applications.
Both natural gas and diesel fuel engines can have electrical outputs from 50 kW to 15 MW.
Automotive derivative engines supply the lower end of the power production scale,
generally less than 10 MW, while the larger engines in the 15 MW and above range are
typically derived from marine applications. Various off-road equipments and station-
ary power engines derived from truck engines provide CHP alternatives between the
automotive and marine derivative power output ranges.

Useable Exhaust Temperatures/Useable Heat
As discussed, waste heat in the form of hot water or sometimes in the form of low-pressure
steam can be recovered from reciprocating engine jacket manifolds, after-coolers, lubri-
cation systems, and engine exhaust. In terms of temperature, the highest potential is
from exhaust gases, then from the engine jacket, and the lowest temperature potential
is heat recovery from lube oil cooling systems. Lube oil cooling systems are particularly
temperature sensitive due to oil breakdown at high temperature.