Page 197 - Sustainable On-Site CHP Systems Design, Construction, and Operations

170 De s i g n

• Required steam pressure and flow rate
• Condensate return temperature

Of course, the selection must be thermodynamically and economically feasible.
Additional consideration should be given to what are the worst-case conditions and
minimums required at those conditions. For example, the exhaust gas temperatures can
change depending on engine load and inlet-air temperatures. The engineer may want
to use the minimum expected exhaust gas temperature for sizing of the HRSG to make
sure that peak steam flow rates can be achieved at those conditions. If hotter exhaust
temperatures are experienced, the selected HRSG (or heat exchanger) will only perform
better. Note that the hottest anticipated exhaust temperature should also be provided to
the HRSG manufacturer for proper material selection.
As discussed below, the exhaust ductwork from the CTG exhaust to the HRSG must
be carefully designed for minimum pressure drop and should allow for uniform flow
and velocity of exhaust gasses across the HRSG (or alternative heat recovery device).
The total exhaust system pressure drop needs to be kept as low as possible and below
the engine manufacturer’s allowable maximum pressure drop. However, as the HRSG
(or any heat exchanger) increases in size to achieve a lower pressure drop, the capital
cost requirements increase. Likewise, as discussed, as much of the heat must be extracted
as possible to achieve an economic, sustainable CHP design. However, to achieve more
heat recovery, more heat exchanger surface is required and heat exchanger capital costs
increase. The engineering team must balance these competing factors of performance
and capital costs. As a practical matter, exhaust temperatures should normally be kept
above 300°F to prevent the formation of carbonic acid in the exhaust stream, which can
lead to rapid exhaust duct/pipe corrosion and eventual exhaust duct/pipe failure (if
condensing is anticipated, stainless steel exhaust ductwork/piping or other noncorrod-
ible materials should be used). Note that diesel engine manufacturers have a lower
minimum exhaust temperature limit of 250°F to prevent corrosion from condensation
of exhaust. Also, sometimes, in order to be located in the proper temperature zone, the
SCR must be installed in the middle of the HRSG tube bank and this system require-
ment must be coordinated with the HRSG manufacturer.
With an IC engine, as discussed, one source of heat recovery is from the jacket water
(JW) typically at about 200°F, which represents about 30 percent of the fuel input energy.
The percent depends on the type engine with turbocharged engines having a greater
percent in the exhaust gases and naturally aspirated having more in the jacket water.
Ebullient (with boiling) jacket water cooling systems operate at higher temperatures.
The actual temperatures are dependent on the height of the steam separator above the
engine and typically produce 5- to 15-psig steam. In order to minimize thermal stresses,
engine manufactures typically limit the temperature differential (delta-T) across the
engine to a maximum of 15°F. Controls must be included to prevent thermal shocking
the engine from returning “cold” water back to the engine, where cold water is defined
at a temperature less than allowed by the maximum delta-T. As an example, if the JW
supply temperature leaving the engine is 200°F, the JW return temperature to the engine
must be no lower than 185°F.
Another source of heat is the IC reciprocating engine exhaust which can be as high
as 1200°F and represents almost 30 percent of the fuel energy. Approximately 60 percent
of the exhaust heat can be recovered in an exhaust gas heat exchanger. While the exhaust
pressure drop for an IC engine is not as critical in resultant prime mover performance