POWER SOLUTIONS COMPACT GENSETS BI-FUEL
RANGE – 2006
The Bi-Fuel System has been designed to allow conventional diesel engines to operate
on a mixture of natural gas and various grades of fuel oil. Typical applications for this
technology include power generation, pumps, compressors, marine propulsion and
electromotive. The Bi-Fuel System is easily installed, simple to operate and has been
designed for heavy-duty uses such as prime power production, oil field water injection,
co-generation and distributed power. Depending on factors such as gas quality, engine
condition and engine rating, the quantity of gas used will vary from a low of 40% to a
high of over 70%. For example, a given engine may operate at 70% gas using pipeline
quality fuel, but may only be able to achieve 50% substitution using associated wellhead
gas. Maximum gas percentage is typically governed by the knock limit of the air-gas mix
for a given engine rating. With pipeline quality gas, the Bi-Fuel System typically allows
operation of the engine up to the prime rating with gas mixes between 65% and 70%. If
the engine is run above prime power, i.e., between 100% and its 110% standby rating,
gas is automatically cut off and reverts to diesel fuel only.
HOW DOES A BI-FUEL SYSTEM WORK ?
The Bi-Fuel System provides gas to the engine using a technique known as fumigation.
In a process similar to that utilized by carburetors and throttle bodies found in spark
engine applications, the Bi-Fuel System supplies gas to the engine utilising the original
air intake system of the engine. In the typical configuration, the gas is introduced at a
location upstream (relative to engine intake-air flow) of the turbocharger and
downstream of the engine air cleaner. The gas is introduced at approximately
atmospheric pressure using a proprietary air-fuel mixing device that allows for a high
level of fuel atomization with the smallest possible air restriction.
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After exiting the mixer, the fuel-air charge is compressed in the turbocharger and then
distributed to each cylinder by the engine air-intake manifold. At this point, the intake
valves deliver the mixture to individual cylinders at the proper moment according to the
normal valve timing of the engine. Upon entering the combustion chamber, the air-gas
mix is compressed during the compression stroke and ignited when the diesel injector is
activated. Because a majority of the fuel and air is already pre-mixed prior to activation
of the injector, the combustion differs slightly from the normal “stratified charge” process
of the diesel engine. During bi-fuel operation, the combustion is slightly accelerated and
the pressure rise in the cylinder is slightly “steeper” than normal diesel combustion.
Peak cylinder pressures reached during bi-fuel operation, however, are within normal
limits. Although natural gas has a lower energy density than diesel fuel, the lean-burn /
excess-air operation of the diesel engine combined with the compression provided by
the turbocharger, allow the Bi-Fuel System to supply adequate quantities of natural gas
to the combustion chamber, therefore assuring that equivalent power can be made.
The Bi-Fuel System monitors various engine parameters (depending on engine size and
application) such as manifold air pressure (boost pressure), exhaust gas temperature,
intake manifold air temperature, inlet air restriction, supply gas pressure and engine
vibration. This data allows the Bi-Fuel System to monitor critical engine parameters in
order to determine when to activate or deactivate bi-fuel operation depending on load
level, ambient conditions, knock limits, low or high gas pressure or a detected
malfunction in the engine. The Bi-Fuel System incorporates a programmable controller
that allows the operator to select various limits for monitored engine data and customize
the load window for bi-fuel operation. Depending on the size of the application, the
sophistication of the controller will vary from a 3-channel analyser all the way to a 20
channel PLC.
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Bi-Fuel System Control Panel
IS THE DIESEL ENGINE PERFORMANCE AFFECTED ?
Engine performance during bi-fuel operation is typically on par with normal diesel levels.
Heat rejection levels to the exhaust and water jacket systems are kept within normal
operating parameters published by the manufacturer. As such, engine operating
temperatures such as coolant temperature, exhaust gas temperature, oil temperature
and intake air temperature remain within allowable limits while in bi-fuel mode.
Response to load variation while in bi-fuel mode is typically as good or better than 100%
diesel performance. This is due to the unique design of the Bi-Fuel System as well as
the combustion characteristics of the air-gas mixture. For large block loads, most
engines converted to Bi-Fuel System actually recover more quickly in bi-fuel mode as
compared to 100% diesel. When it is necessary to switch from bi-fuel mode to diesel
mode, the transition has little or no effect on engine speed or stability. This feature
allows for bi-fuel operation in speed sensitive paralleling applications where engine
stability is most critical.
IS THE DIESEL ENGINE OPERATING EFFICIENCY AFFECTED ?
The Bi-Fuel System has been designed to preserve engine operating efficiencies. In
most cases, specific fuel consumption in bi-fuel mode will equal or exceed 100% diesel
performance. Unlike most rich-burn, spark-ignited gas engine designs, the Bi-Fuel
System does not employ an air-throttle device. The low restriction, “fixed venturi” air-gas
mixers utilized in the Bi-Fuel System prevent efficiency draining “pumping losses”
typically caused by air-throttle devices. In addition, this lack of air restriction maintains
the lean-burn operation of the engine, again maintaining fuel efficiency. When
calculating specific fuel consumption in bi-fuel mode, the total fuel burn of the engine
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must be considered. For each 1.0 litre of diesel fuel displaced in bi-fuel mode,
approximately 1.0 m³ of gas will be consumed by the engine (see calculations below).
IS THE DIESEL ENGINE DURABILITY AFFECTED ?
Engines converted to bi-fuel will typically meet or exceed projected overhaul intervals as
published by the manufacturer. During bi-fuel operation, there is a reduction in post-
combustion contaminants as natural gas leaves little or no particulate residue when
burned. Reduction of these contaminants reduces cylinder liner and ring wear and also
reduces fouling of engine lubricating oil. A significant portion of ring and liner wear is
caused by the “scouring” effect of these carbon particles, as they are ground between
the rings and cylinder wall. This action causes small scratches that allow compression
levels to fall (lowering efficiency). By reducing the post combustion particle levels, the
on-set of reduced compression and ring blow-by is slowed. Since reduced compression
and ring blow-by lead to ever worsening combustion residue, and an acceleration of
engine wear, the slowing of this process can lead to less frequent engine overhauls and
reduced maintenance costs.
A significant operating cost for diesel engines is the requirement for frequent oil and oil
filter changes. Due to the reduction in combustion contaminants, engine lubricating oil
becomes fouled at a slower rate while operating on bi-fuel. As such, operators of bi-fuel
engines typically find that they can extend intervals between oil and filter changes. This
leads to savings in oil, filter and labor costs, as well as savings in used oil disposal fees.
It is important to note that any changes in oil interval schedules be dictated by the results
of oil analysis.
Because engine peak cylinder pressures and exhaust gas temperatures remain within
limits specified by the manufacturer, engine components such as pistons, valves, valve
seats and exhaust manifolds exhibit normal wear levels associated with 100% diesel
operation. It is important however, that these items be replaced as per
recommendations of the engine manufacturer. Although components such as pistons
and valves may look in better condition after a high number of hours on bi-fuel, the
actual stresses and wear rates will be similar and therefore require that the parts be
replaced at the recommended time.
HOW ARE THE DIESEL ENGINE EMISSIONS AFFECTED ?
Conversion of most diesel engines to bi-fuel will generally provide for reductions in
harmful emissions. The extent of these reductions will depend on a number of factors
including age and condition of the engine, engine make and model, load factor and the
quality and composition of the supplied natural gas. In general terms, bi-fuel operation
will reduce NOx, NMHC (non-methane hydrocarbons), SOx, PM and opacity. Typical
reductions are as follows:
NOx: 15% - 30% decrease
HC (reactive): 20% - 80% decrease
PM-10: 20% - 50% decrease
Opacity: 30% - 50% decrease
SOx: 50% - 70% decrease.
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CO output will increase during Bi-Fuel operation as well as the total hydrocarbon (THC)
output. If the lowest possible CO and THC are required, a variety of catalysts can
supplied which will decrease these emissions approximately 70% to 95%.
GAS FLOW AND PRESSURE REQUIREMENTS
Natural gas is sold in cubic metres (m³) and its energy measured in MegaJoules (MJ)
per m³ (metres cubed).
A normal value is approximately 34.7 MJ/m³ (34,700 kJ/m³)
In order to determine gas flow rates, it is necessary to compare diesel fuel to natural gas
on an energy equivalency basis. As diesel fuel is a liquid fuel typically measured in litres
a direct comparison of equal units is not possible. It is therefore better to compare
approximately equivalent amounts to arrive at a GAS EQUIVALENT LITRE (GEL).
The following typical values are used..
Diesel = 36.1 MJ/litre
Gas = 34.7 MJ/m³
Calculating from these values shows that 1.04 m³ of natural gas is equivalent in heat
(energy) content to 1.0 litre of diesel, i.e., its GEL is 1.04.
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The gas flow requirement for any given generator set and kiloWatt load can therefore be
calculated as follows.
CALCULATION FOR TYPICAL GENERATOR SET MODEL P1250
PRIME POWER RATING 1250kVA – 1000kW
Fuel rate (diesel): 264 litres/hour
Approximate gas %: 70%
264 litres x 0.70 = 185 litres of diesel to be replaced by gas
185 litres x GEL (1.04) 192 m³ gas
As a general rule; For natural gas flow estimating purposes a ratio (GEL) of 1.0 m³ of
natural gas to 1.0 litre of diesel fuel may be used with little error.
Therefore, if gas is to replace 70% of diesel at full power, multiplying the consumption of
diesel fuel (litres/hour) by 0.7 will give the equivalent gas flow per hour. It is normal
practice to add 20% to the calculated figure to ensure pipelines are adequately sized.
In the example above for P1250 generating set
185 litres/hour x 1.2 = 222 m³/hour gas
When estimating gas flow requirements for other gas types (bio-gas, wellhead gas, etc.),
the GEL must be recalculated based on the heat rating of the fuel. The lower the calorific
value the larger the volume required for a given amount of energy.
For example, if a biogas has a heat rating of 26.0 MJ/m³ its GEL is;
34.7 divided by 26.0 x 1.04 = 1.39
Once the gas flow requirement has been calculated, this value should be supplied to the
gas supplier along with the service pressure requirements. The Bi-Fuel System requires
a regulated, low-pressure gas supply of between 14 and 35 kPa. Once the gas supplier
has been given gas flow and pressure requirements, they will be able to calculate gas
line size well as gas meter and primary regulator specifications. The Bi-Fuel System
requires stable gas supply pressure with little or no fluctuation. Ideally, there should be
no more than 0.7 to 2.0 kPa variation in gas supply pressure. For applications where
multiple engines are being converted at a single site, it is imperative that each engine
have a dedicated gas supply regulator, as trying to supply gas to each engine from a
common manifold (with a single regulator) causes unstable supply pressures. Please
consult with FG Wilson technical personnel for detailed information on gas supply
regulation and design issues.
FUEL CONSIDERATIONS
Natural gas is a combustible, gaseous mixture of simple hydrocarbon compounds,
usually found in deep underground reservoirs. Pipeline quality natural gas is composed
almost entirely of methane, but does contain small amounts of other gases, including
ethane (C2H6), propane (C3H8), butane (C4H10) and pentane (C5H12). A methane
molecule is composed of one carbon atom and four hydrogen atoms (CH4).
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Bi-Fuel System has been designed for optimum performance using pipeline quality
natural gas. Pipeline gas typically has little variation in gas quality and composition on a
day to day basis and is normally made up of more than 90% methane (CH4). The MN
(Methane Number) of the fuel is important for Bi-Fuel operation as the combustion
characteristics of methane differs substantially from ethane, butane, pentane,
(collectively referred to as heavy-hydrocarbons) etc. Methane has a high auto-ignition
temperature (approximately 650 ºC) and a high amount of resistance to detonation,
thereby allowing its use in high compression diesel engines. As the MN of the fuel
decreases, and the heavy-hydrocarbon number increases, the combustion
characteristics of the fuel will change and may require a lower substitution percentage of
natural gas and/or a reduction in engine rating in bi-fuel mode.
For installation in colder climates, it is important to determine the gas composition during
the winter months when gas companies may utilize “propane peak-shaving” to boost the
heat level (MJ/m³) of the fuel to meet high demand. Prior to installation of the system, a
comparison of summer and winter gas analyses should be made to determine what
allowance should be made, if any, in the calibration and/or operation of the system due
to expected changes in the gas MN. In recent years, liquid natural gas (LNG) peak-
shaving has become the preferred method of meeting winter demand for many gas utility
companies and this type of enrichment process maintains a more stable methane
number throughout the year.
Other methane-based gases can be utilized with the Bi-Fuel System such as wellhead
gas and bio-gas. When utilizing gases other than pipeline quality, the following factors
must be considered:
Methane content
Heavy hydrocarbon content
Heating value
Inert gases
Moisture content
Caustics
Particulates
For reasons explained above, it is important to determine the methane and heavy-
hydrocarbon content of the fuel, and possible ranges thereof, prior to installation of the
Bi-Fuel System. If the fuel has heavy-hydrocarbons in excess of 20% in the normal gas
stream, or alternately, can have periodic “slugs” of heavy-hydrocarbons, it may be
necessary to decreases the gas substitution percentage and/or de-rate the engine
during bi-fuel operation. The user should be wary of so called “hot gas” which due to
large concentrations of heavy hydrocarbons can have heat rates in excess 41.6 MJ/m³.
Once again, this type of gas may result in limitations relative to engine rating and gas
percentage, and in extreme cases may preclude the use of Bi-Fuel.
It is common for some non-pipeline gas types to have a relatively low heat rate, usually
resulting from a low methane number coupled with a high concentration of inert gases.
The inert gases in low calorific value (CV) fuel do not have a negative impact on Bi-Fuel
operation from a combustion stand point, however this gas will require over-sizing of the
gas supply components of the Bi-Fuel System (see Calculating Gas Flow Requirements)
in order to attain viable gas substitution percentages. In this case, the CV rating of the
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gas is compared with pipeline quality gas (CV = 34.7 MJ/m³) and a correction factor is
applied.
For non-pipeline gases, it is important to determine that sufficient filtering means have
been incorporated in the gas supply line such that particulate and liquid contents in the
fuel are kept to a level approximating pipeline quality standards. It is also important to
determine what caustic compounds, if any, are present in the fuel, which may potentially
cause harm to the engine and/or components of the Bi-Fuel System. For bio-gas fuels
derived from landfills and waste treatment facilities, it is not uncommon to see high
levels of caustic compounds that when combined with water can form damaging acids.
It is possible to filter-out these type of contaminants, and filtration should be utilized if
caustic compounds are present in the fuel.
SAVINGS
FORM BFCC 2006
Savings realized during bi-fuel operation are the
result of the cost differential between the diesel fuel
displaced and the gas used. When the difference in
cost between the two fuels is large, the savings will
be similarly large. When the cost differential is low,
the savings will be smaller. A significant factor in
the operational savings is the number of hours that
the engine is operated per year. For example, an
engine that is operated on a continuous basis and
accumulates over 8,000 hours per year will not
require a large differential in the cost of gas and
diesel to realize significant savings. As a general
rule, the lower the operating hours, the larger the
fuel cost differential must be in order to generate
savings. Likewise, the higher the operating hours,
the smaller the cost differential need be to
accumulate significant savings for the user.
Using the 8,000 hour example and assuming a generator output of 500 kW, the total fuel
burn per year would be about 1,040,000 litres of diesel fuel. With a 70% substitution gas
for diesel and a typical GEL of 1.04 the total gas required would be…
0.26 (litres/kWh) x 500 (kW) x 8000 (hrs) = 1,040,000 litres of Diesel
0.7 x 0.26 (litres/kWh) x 500 (kW) x 8000 (hours) x 1.04 (GEL) = 757,120 m³ of Gas
Knowing the application, the Bi-Fuel generating set size, expected
operating parameters, and the relative prices of diesel and gas fuel
please refer to the calculation sheet BFCC 2006 to provide the
estimated fuel cost savings.
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