Page 196 - Sustainable On-Site CHP Systems Design, Construction, and Operations

The Engineering Pr ocess 169

Other key issues discussed include CHP plant layout and the need to provide
operational flexibility by, if possible, dividing the load across a number of pieces of
equipment (e.g., use three smaller pumps instead of one large pump).

The Effects of Prime Mover Selection
As noted above and discussed in this book, the prime mover selection has a dramatic
impact on the heat recovery and on the type of systems employed. As discussed in
Chap. 4, when compared to internal combustion engines, CTGs typically have higher
thermal-electric ratios and thus typically produce much larger amounts of available
recovered heat at higher thermal qualities per unit of engine power output. Most CTGs
recover all of their heat in a HRSG, which can generate up to about 250-psig steam (the
maximum steam pressure is limited by the CTG exhaust temperature which typically is
around 1000°F or below, without duct firing, except for recuperated CTGs whose
exhaust temperatures are several hundred degrees Fahrenheit lower). Due to the large
quantity of excess oxygen in the CTG exhaust, additional high-pressure steam can be
produced by burning gas or liquid fuel in duct burners up to a maximum amount of
about 30,000 Btu/hp. This option is not available with IC reciprocating engines due to
the relatively low amount of excess oxygen.
Engine heat recovery, on the other hand, is usually in the form of low temperature
hot water from about 180°F up to a maximum of 250°F, although low-pressure steam—
less than 30 psig—can be produced with some engines. With IC reciprocating engines,
heat can be recovered from a number of sources including jacket water (JW) for engine
cooling, lube oil coolers, turbochargers, and the exhaust gas via an exhaust gas–to-
water heat exchanger.
Other differences between CTG and IC reciprocating engine CHP design include
vibration isolation requirements, which are typically more challenging with reciprocating
engines than with CTGs that use rotating shafts. Also, emissions reduction equipment
will be different for different prime movers. For example, fuel cells have very low emis-
sions and may require no exhaust gas treatment, while a rich burn engine may require
a three-way catalyst, and a CTG may require SCR.

Heat Recovery Options
With a CTG CHP system that uses a HRSG, which is an unfired boiler, the design
engineer should work with a number of HRSG manufacturers to properly layout and
specify the proposed HRSG unit(s). Most of the boiler systems and concerns that would
apply to any boiler also apply to a HRSG system. For example, a typical HRSG will
require: a condensate system and deaerator; feedwater system with feedwater control
valve; a properly designed steam outlet pipe and nonreturn valve; flue exhaust duct-
work to the outside; pressure and steam drum level control; monitoring, and alarm;
safety relief valves and vents; sample ports; and a blow-down system to remove total
dissolved solids. In order to properly size the HRSG, which is often custom-designed
and custom-built, the engineer will need to specify the following:
• CTG exhaust temperature
• CTG exhaust mass flow rate
• Minimum allowable exhaust stack temperature (the temperature leaving the
HRSG)