Pipeline gas typically has little variation in quality and composition on a day
to day basis and is normally made up of >90% methane.
Gas composition is an
important factor for Gaseous Fuel operation as the combustion characteristics of
methane (CH4) differ substantially from heavier hydrocarbon compounds.
Generally, as the methane content of the fuel decreases and the heavy-HC content
increases, the combustion characteristics of the fuel will change and may
require a lower substitution percentage of natural gas.
While the heating value
of pipeline quality natural gas will vary somewhat, it is generally in the range
of 1000 Btu/scf or 37.25 MJ/m3. A comparison of summer and winter gas
composition should be made to determine any seasonal variation in gas
composition.
Non-Pipeline Gases
Other methane-based gases can be utilized with the Gaseous 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 gas content
- Moisture content
- Caustics
- Particulates
For reasons explained above, it is important to determine the base composition
of the fuel gas as well as the possible range of composition prior to
installation of the Gaseous Fuel System.
Wellhead gas often consists of a
greater fraction of heavy HC’s, and in some cases, may have less than 50%
methane. The installer should be wary of so called "hot gas" which, due to high
HC concentrations, can have heat rates in excess of 1200 Btu/scf (44 MJ/m3).
Note: If the fuel has a heavy hydrocarbon concentration of >20% in the normal
gas stream, or alternately, can have periodic "slugs" of heavy-hydrocarbons
exceeding
>20%, it may be necessary to decrease the gas substitution percentage and/or
de-rate the engine during Gaseous Fuel operation.
Filtration
For non-pipeline gases (and some lower quality pipeline gases), it is important
to determine if 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 fuel grade standards.
It is recommended, at a minimum, to use
a high quality, coalescing type filter for all non-pipeline applications. 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 gas components of
the Gaseous Fuel System.
Additional filtration or treatment may be required in
order to protect against engine damage. For bio-gas fuels derived from
landfills, waste treatment facilities, etc., it is not uncommon to see high
levels of caustic compounds such as sulphur, which when combined with small
amounts of water can form damaging acids.
It is possible to filter-out these
types of contaminants, and filtration should be utilized if caustic compounds
are present in the fuel.
Note: It is the responsibility of the end-user to
ensure that the gas supplied to the engine is "fuel grade quality" and
sufficiently treated to prevent engine damage.
CNG Storage Requirements
For purposes of determining on board fuel storage requirements, assume 140
standard cubic feet (scf) of natural gas for each gallon of diesel no. 2 to be
replaced, or 1 m3 of natural gas for each liter of diesel no.2 to be replaced.
Example 1: a city bus consuming 75 gallons of diesel no. 2 per duty cycle will
require a total storage capacity of 6,300 scf of natural gas at an average
substitution rate of 60%
Example 2: a dump truck consuming 300 liters of diesel no. 2 per duty cycle will
require a total storage capacity of 210 m3 of natural gas at and average
substitution rate of 70%.
Note: The above estimates assume pipeline quality natural gas.