Page 68 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Power Equipment and Systems 47

including catalytic conversion and selective catalytic reduction (SCR), aim to reduce
the amount of pollutants that are emitted into the atmosphere and to minimize the
CHP plant’s local environmental impact. NO , CO, and NMHC are main emissions to
x
be controlled. For rich burn engines, three-way catalyst are typically employed to
reduce NO , CO, and NMHC emissions to acceptable levels. With lean burn engines,
x
a SCR system is used to reduce NO levels; an oxidation catalyst reduces CO and
x
NMHC. With an SCR system, ammonia injection will be required and some form of
on-site storage will be needed (often in the form of an aqueous urea solution). All
catalysts must be kept at the proper operating temperature to function correctly and
not be damaged. If the exhaust temperature into the catalyst is too low, the proper
emissions reductions will not take place. Higher than allowable exhaust temperatures
can cause damage to expensive catalyst requiring its replacement.

Noise and Vibration
Due to the reciprocating motion of the internal components, IC engines tend to produce
significant vibration and noise. The noise includes low-frequency rumble denoted by a
continual, loud, thumping sound emanating from the engine block. This noise can be a
problem and noise is typically addressed by sound attenuation inside an enclosure
(either the building housing the CHP plant or a special dedicated enclosure, or both).
Additional noise can occur from improperly positioned or designed air intake and
discharge systems and/or inadequately muffled exhaust systems. Positioning exhaust
discharges and air louvers and vents away from places where noise will cause problems
can reduce some of the observed noise. Not all of the engine noise can be mitigated
through these techniques. Installing reciprocating engines in sound-insulated engine
rooms is one good way to help mitigate unwanted sound. Engines are usually mounted
on vibration isolators to greatly reduce vibration transmission from the engine to sur-
rounding areas. Where vibration is likely to cause problems, an isolated inertia base is
often used in addition to vibration isolation on the engine. While a heat recovery heat
exchanger will help reduce translated engine noise, an exhaust muffler may also be
required. Chapter 10 addresses various design solutions intended to minimize or mitigate
unwanted noise and vibration associated with IC reciprocating engines.

Controls
Apart from being able to safely start and stop a reciprocating engine generator set, addi-
tional controls are required to protect the mechanical integrity of the engine and installed
CHP system. For example, alarm and shutdown controls are required for adverse situ-
ations, such as low oil level, low oil pressure, high oil temperature, high coolant tem-
perature, low coolant temperature (to help prevent thermal shock), low coolant level, or
inadequate coolant flow. Correct air intake temperature and air mass flow, proper fuel
delivery, and correct spark timing (in the case of spark ignition engines) are also impor-
tant factors in proper engine function.
Engine speed controls are typically designed to keep the engine at a constant rota-
tional speed in order to maintain the correct generator frequency. Often, the generator
must stay in phase with either utility power or with another generator (see Chap. 11).
Governors are installed on engine generators to regulate fuel delivery and to keep rota-
tional speeds within a narrow design range. However, the engine governor’s ability to
respond effectively to a changing load condition is a function of the nature and magni-
tude of the load change. Virtually all governors can maintain steady-state conditions