| Reciprocating
Engines
Applications
OverviewNatural gas reciprocating engines used for industrial applications fall into two broad categories: auto derived and diesel derived. Meaning, was the initial technology (major components) developed for gasoline or diesel engines? Auto/gasoline derived engines are less than half the initial cost of diesel derived engines of the same horse power. This makes them very attractive for light duty applications under about 150 horse-power. Auto derived engines run at 1,800 to 3,000 RPMs and use 9,000 to 11,000 BTUs/Horse Power - Hour. Diesel derived engines are available in such a large size range that there are some other categories that are used to describe them, such as 'low speed' (the largest engines with very low RPMs, less than about 1,000 RPMs) and various duty ratings. Diesel derived engines use 7,000 to 11,000 BTUs/H.P.-Hr, with the larger, slower engines being more fuel efficient. There are also options for various fuels and combinations, such as:
For more information about some of these fuel technologies, see the links below. For applications under about 250 hp, there have been some attempts to develop a new engine category for natural gas that gives the long-life of a diesel derivative at a cost closer to that of a gasoline derivative. So far, this has been difficult to achieve. For applications below about 150 hp, a developer must choose between durability and initial cost. For larger applications, diesel derivatives are currently the only option. NOTE: Waukesha does NOT build a diesel engine; however, their engines are considered diesel derivative technology. All other manufacturers make both diesel and natural gas engines using the same block and major components.
Economic InformationAlmost any electric motor driven equipment with an open drive can be driven by an engine. This includes generators, compressors, vacuum pumps, blowers, dust collectors, bag houses, refrigeration equipment, conveyors, grinders, and so forth. However, just because it is physically possible to do it, doesn't mean that it's economically feasible. In every application, the gas engine will cost more than an electric motor. Therefore, operating costs and other benefits must be considered carefully to determine the best applications for an engine over a motor.
Part LoadingEngines generally benefit from part loading more than electric motors. However, this varies with the application. The gas engine must be controlled such that the load is reduced when the running speed RPMs are reduced. If the horse power demand of the load remains constant, gas usage will not change much with small changes in RPMs. The larger the engine, the more benefit from small changes in loads/RPMs as small percentages add up to bigger numbers with larger engines. Electric motors can now be equipped with variable speed drives. While this substantially increases the cost of the electric equipment, it is still likely to be less cost than the engine equipment and gains similar savings at part loads. It is also easier to turn
on and off an electric motor than it is a gas engine. Loads that can
be cycled between on and off often, are better applications for an electric
motor. Engines perform better when they can be run for long periods
of time, and most control systems do not allow engines to be cycled
more than about once per hour. Heat RecoveryThe best gas engine applications are in areas of high electric costs and where there is a simultaneous use for hot water captured off the engine's cooling system. If the hot water can all be used, and energy would have been purchased for another piece of equipment to make hot water anyway, then the fuel cost for the engine has been almost off-set. If the fuel cost is off-set by heat recovery, then the capital cost difference can be more quickly recovered by electric savings. Engines provide about 60% of their input energy as recoverable hot water, where most boilers are in the 80% range. A general rule-of-thumb for heat recovery is about 30% of input energy can be recovered from engine jacket water (the radiator) and about 30% from the exhaust, for a total of about 60%. Generally engine jacket water is available 'free' or is a very low cost option, as the engine must be cooled and is already equipped with heat-exchangers to do it, although it may be just a water-to-air radiator. Some engines can produce low pressure steam from the engine coolant, called 'ebullient' cooling, but hot water is the most common. Ebullient cooling combines heat recovery of the exhaust and engine by using low pressure steam saturated in water to move the coolant through the engine. When the pressure is reduced at the exit of the engine exhaust, the coolant produces some flash steam, which is captured for the steam process. Exhaust heat recovery can easily produce low pressure steam (up to about 15 psi on large engines) but is much more expensive to install. Therefore, if considering exhaust heat recovery, the capital cost of the heat exchanger must also be considered. For more information about the application of Heat Recovery, see the Applications Guide , Industrial Market Section.
Emissions InformationThe 'problem' emissions from engines are CO and NOx, and in some cases CO2. Better controls on engine fuel/air ratios and closed-loop feedback systems on the engine's exhaust system has made great improvements in emission levels. New engine designs such as Lean Burn and ARIES have taken emissions reduction to the next level. Currently, the most common
method of dealing with engine emissions in the non-attainments areas,
is the addition of Selective Catalytic Reduction - SCR, to the exhaust
system. These are costly, high maintenance additions, but must be added
for permitting of large engines in some locations. SCRs chemically reduce
the CO and/or NOx to acceptable levels. ManufacturersA list of Manufacturers and Vendors of Reciprocating Engines is located within the Applications Guide, Manufacturers Section.
Source: TechPro DTE Energy Bob Fegan 5/02 |
