Dual Fuel Engines for Emergency Generators
by Starlight Generator dieselgeneratortech2.3.2.3
Dual Fuel
Engines
Dual fuel engines are
predominantly fueled by natural gas with a small percentage of diesel oil
added.
There are two main
configurations for introducing the gaseous fuel in a dual fuel engine. These
engines can be purpose built or conversions of diesel engines. Such engines can
be switched to 100 percent diesel operation. Dual fuel engines provide a
multi-use functionality. Operation on predominantly cheaper and cleaner burning
natural gas allows the engine to be used in CHP and peak shaving applications,
while operation on 100 percent diesel allows the engine to also meet the onsite
fuel requirements of emergency generators. The dual function adds benefit in
applications that have specific emergency generator requirements such as in
hospitals or in public buildings.
There are three main
configurations for introducing the gaseous and pilot diesel fuel: 1) low
pressure injection with the intake air, 2) high pressure injection after the
intake air has been compressed by the piston, and 3) micropilot prechamber
introduction of the diesel fuel. New dual-fuel engines are offered in oil and
gas production markets to reduce operating costs. Dual-fuel retrofits of
existing diesel engines are also offered as a means to reduce both operating costs
and emissions for extending the hours of use for limited duty engines such as
emergency and peaking applications. Dual fuel is not widely used for CHP
applications.
2.3.2.4
Heat Recovery
The economics of engines
in on-site power generation applications often depend on effective use of the thermal
energy contained in the exhaust gas and cooling systems, which generally
represents 60 to 70 percent of the inlet fuel energy. Most of the waste heat is
available in the engine exhaust and jacket coolant, while smaller amounts can
be recovered from the lube oil cooler and the turbocharger's intercooler and
aftercooler (if so equipped). As shown in the previous table, 45 to 55 percent
of the waste heat from engine systems is recovered from jacket cooling water and
lube oil cooling systems at a temperature too low to produce steam. This
feature is generally less critical in commercial/institutional applications
where it is more common to have hot water thermal loads. Steam can be produced
from the exhaust heat if required (maximum pressure of 400 psig), but if no hot
water is needed, the amount of heat recovered from the engine is reduced and
total CHP system efficiency drops accordingly.
Heat in the engine
jacket coolant accounts for up to 30 percent of the energy input and is capable
of producing 190 to 230 °F hot water. Some engines, such as those with high
pressure or ebullient cooling systems, can operate with water jacket
temperatures of up to 265°F. Engine exhaust heat represents 30 to 50 percent of
the available waste heat. Exhaust temperatures for the example systems range
from 720 to 1000°F. By recovering heat in the cooling systems and exhaust,
around 80 percent of the fuel's energy can be effectively utilized to produce
both power and useful thermal energy.
Closed-loop cooling
systems – The most common method of recovering engine heat is the closed-loop cooling
system as shown in Figure 2-2. These systems are designed to cool the engine by
forced circulation of a coolant through engine passages and an external heat
exchanger. An excess heat exchanger transfers engine heat to a cooling tower or
a radiator when there is excess heat generated.
Closed-loop water
cooling systems can operate at coolant temperatures from 190 to 250°F.
Depending on the engine and CHP system’s requirements, the lube oil cooling and
turbocharger after-cooling may be either separate or part of the jacket cooling
system.
Ebullient Cooling
Systems – Ebullient cooling systems cool the engine by natural circulation of a
boiling coolant through the engine. This type of cooling system is typically
used in conjunction with exhaust heat recovery for production of low-pressure
steam. Cooling water is introduced at the bottom of the engine where the
transferred heat begins to boil the coolant generating two-phase flow. The
formation of bubbles lowers the density of the coolant, causing a natural
circulation to the top of the engine.
The coolant at the
engine outlet is maintained at saturated steam conditions and is usually
limited to 250°F and a maximum of 15 psig. Inlet cooling water is also near
saturation conditions and is generally 2 to 3°F below the outlet temperature.
The uniform temperature throughout the coolant circuit extends engine life and
contributes to improved combustion efficiencies.
Exhaust Heat Recovery –
Exhaust heat is typically used to generate hot water of up to about 230°F or steam
up to 400 psig. Only a portion of the exhaust heat can be recovered since
exhaust gas temperatures are generally kept above temperature thresholds to
prevent the corrosive effects of condensation in the exhaust piping. For this
reason, most heat recovery units are designed for a 250 to 350°F exhaust outlet
temperature.
Exhaust heat recovery
can be independent of the engine cooling system or coupled with it. For
example, hot water from the engine cooling can be used as feedwater or
feedwater preheat to the exhaust recovery unit. In a typical district heating
system, jacket cooling, lube oil cooling, single stage aftercooling, and
exhaust gas heat recovery are all integrated for steam production.
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Created on Apr 23rd 2019 00:43. Viewed 256 times.