Page 50 - Energize July 2021

TECHNICAL

including combustion control and post combustion treatment. A
method often used with it is exhaust gas recirculation, which sends
part of the exhaust gases back into the combustion chamber.
This lowers the adiabatic flame temperature, allowing for lower-
temperature combustion and thus lower NOx production. Selective
catalytic reduction (SCR) has been successfully used on new diesel
gensets to reduce NOx emissions by as much as 95%.

Figure 1: High pressure common rail fuel injection system (Cummins)

Electronic engine controls
Sophisticated technology requires more elaborate controls, so
traditional analogue systems are giving way to advanced digital
control systems. Digital controls are essential for the newest Figure 2: A combination of after treatment technologies (Cummins)
gensets which rely on HPCR fuel systems and precise control
of ignition and combustion. Many of the parameters previously Operational developments
measured only during routine tests are now permanently monitored In addition to improvements to the basic diesel generator set, there
and recorded. This enables a dynamic record to be kept, showing have been a number of operational improvements which expand the
trends which can be uploaded, identified, and analysed. Modern flexibility and scope of operation of the unit.
diesel generator sets will have an array of sensors and monitors
covering all operational parameters. Variable speed diesel generator sets
Another advantage is that digital controls can monitor the real- Many diesel generator (DG) applications require a daily cycle which
time state of a wide variety of operating parameters and display calls for operation at low loads for extended periods, while delivering
them on a centralised panel, as opposed to analogue systems which the maximum load during other times. Typical applications would be
rely on less-sophisticated alarms and indicator lights, as well as PV/DG hybrids, construction sites, remote lodges and clinics. Diesel
manual recording. Digital systems allow operators to identify and generation is a logical partner for renewable generation, given the
correct faults much more quickly, leading to more reliable power and technology is often pre-existing and dispatchable. However, while
less downtime. They also allow for remote monitoring and operation the DG is easily scheduled to parallel with renewable generation
(though proper cybersecurity is necessary), another advantage for in a hybrid system, operating characteristics constrain partial load
gensets which might be located in remote areas. Remote operation approaches.
and monitoring expand the possibilities of diesel generator sets. The classical DG, where the alternator is connected directly to
the generator, is designed to run at a fixed speed to maintain voltage
Fuel flexibility and frequency. When running at a fixed speed, engine efficiency
An advantage of diesel generator sets is the ready availability of decreases at low loads. This is understandable, as the internal
diesel or heavy fuel oil, and the ability to run on a wide variety of losses are constant at a fixed speed. Figure 3 shows the variation of
fossil and renewable fuels, including refined vegetable oil, biodiesel, specific fuel consumption with varying load for a typical DG.
depolymerised plastic and a variety of other natural and synthetic This increases both emissions intensity and fuel consumption.
fuels such as dimethyl ether. Other future possible fuels include Running a fixed speed DG at low load can also damage the engine
hydrogen, or a hydrogen carrier such as ammonia. There are four and requires more frequent maintenance. For this reason, an engine
alternative fuels which could be used in conventional compression loading of less than 30% of the rating is generally prohibited,
ignition engines (CIE): vegetable oil, biodiesel, Fischer-Tropsch (F-T)
diesel, and dimethyl ether (DME). Vegetable oils, such as palm,
soybean, sunflower, peanut, and olive, can be used as alternative
fuels for diesel engines. Both F-T and DME can be manufactured
from natural gas and are therefore not limited by feedstock availability.

Emission controls
Emission control measures are being increasingly applied to
stationary diesel generator sets, whether for standby or continuous
operation. Industry has responded with a number of solutions, Figure 3: Variation of SFC with load for a typical DG set (Varipower)

energize | July 2021 | 48