Page 201 - Sustainable On-Site CHP Systems Design, Construction, and Operations

174 De s i g n

Like CTG exhaust, the IC engine exhaust pipe, which can reach 1200°F must be
insulated to retain heat and to protect operating personnel. The exhaust pipe should be
sloped down, away from the engine so that any condensation (which can form during
engine start-up and shutdowns) does not drain back into the engine.
Temperatures should be read by the control system (also provide local gauges for
operator use) at the engine exhaust, before and after a heat exchange device, and at the
inlet and outlet of the catalyst/SCR system. Some of these temperature sensors can be
combined, for example, the engine exhaust temperature might be the same as the cata-
lyst inlet temperature.

Emission Controls
Emission controls are a critical system for a CHP plant affecting the CHP systems ability
to operate and to be sustainable. The type of emissions control system required will, as
discussed, depend on the prime mover(s) to be used, the expected emissions levels
from the selected prime mover(s), as well as the maximum emissions allowable by
regulating authorities for the proposed CHP plant. Emission controls are discussed in
detail in Chap. 7 and permitting requirements are discussed in Chap. 12.
Key issues for the engineer include helping to ensure that catalysts are maintained
at the proper temperatures. Engine exhaust temperatures can vary with load and the
CHP plant system must be capable of maintaining the proper temperatures to prevent
damaging or destroying very expensive catalysts. Also, if minimum temperatures are
not achieved, catalysts will not perform effectively. Also, as part of the exhaust system,
provisions must be made for emissions equipment and its support and for operator
monitoring and maintenance. Another important design issues with respect to the
emissions control system is the storage of ammonia, often in the anhydrous form, which
may have code specified quantity and location limitations.

Thermal Uses
As discussed throughout this book, CHP derives its economic benefit from the recovery
and productive use of waste heat that otherwise would be rejected to the atmosphere.
Therefore, fully utilizing the heat as much as possible, not only on a peak design basis
but also throughout the year, is critical to achieving sustainable CHP. Of course the
amount of heat that can be effectively used at a facility is a function of the facility loads
and the size and type of the prime mover employed (and the resultant thermal-electric
ratio), which hopefully was well studied, planned, and sized as described earlier in this
book. Otherwise if, for example, the prime mover was oversized and there is not enough
thermal demand, there may be little the design engineer can do to avoid future heat
dumping. However, assuming a well-studied and planned CHP system, the CHP design
engineer still needs to plan for a variety of thermal uses and should account for periods
of low load. As noted, some thermal uses include
• Additional power (combined cycle)
• Space heating
• Space cooling (using an absorption chiller, for example)
• Domestic hot water production
• Swimming pool heat