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Gas Engines

Bi-Fuel System

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    Gaseous Fuel System™

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    Glossary

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    Gaseous Fuel Systems

  • General
  • Gas Variation
  • Non-Pipeline Gases
  • Filtration
  • CNG Storage Requirements
  • Gases Fuel System

    Gas Supply

    General

    The term "natural gas" generally refers to a combustible, gaseous mixture of simple hydrocarbon (HC) compounds, usually found in deep underground reservoirs. Natural gas is primarily composed of methane (CH4/C1), but can also contain small amounts of other gases, including ethane, propane, butane and other compounds.

    At room temperature and pressure, methane is a colorless and odorless gas. Gas distributors/processors typically add odorant to the natural gas in order to alert operators of gas leaks. Natural gas is typically distributed via pipelines, but may also be transferred or stored in the form of LNG (Liquid Natural Gas) or CNG (Compressed Natural Gas).

    Gas Variation

    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 HCs, 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.

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