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Introduction

Natural gas can be used as a fuel for:
  • Spark ignited engines

  • Compression ignition (Diesel) engines

  • Gas turbines

  • Fuel cells

  • Others, such as steam turbines and rotary engines

For many decades, natural gas has been used as a dedicated fuel for spark ignited engines. In recent years, diesel engines have been modified in several configurations to use natural gas as a primary fuel by employing dual fuel (diesel/gas) technologies. In a duel fuel engine natural gas and air are introduced into the cylinder and ignited by a small pilot injection of diesel fuel. The pilot acts like a sparkplug and, in effect, turns the diesel engine into an Otto-cycle engine. Typically a dual fuel engine will run on 10-20% diesel pilot with the rest of the energy supplied by natural gas.

Natural gas has an Octane Number of 120 and an autoignition temperature in excess of 1200°F. These characteristics make natural gas compatible with the compression ratios normally used in diesel engines (17/1 and higher), making it possible to utilize most of the energy available in a dual fuel configuration. However, in spark ignited engines natural gas will not ignite with a compression ratio above 10/1 because of high resistance across the sparkplug, resulting in a loss in mechanical efficiency.

Dual fuel engines can retain their diesel fuel efficiency; whereas, spark-ignited engines tend to be more complex (i.e. more expensive) than compression ignition engines and markedly less efficient with losses of up to 20%.

Gas turbines are an interesting alternative to reciprocating engines when using natural gas for fuel, especially in workboats ferries and other marine vessels.

The Gas Research Institute offers a substantial library concerning the use of natural gas in engines and other applications.

Fuel Characteristics

Methane, the principal constituent of natural gas, has a gross, or higher, heating value (HHV) of 23,891 Btu per pound. The term “Lower Heating Value” or LHV refers to the energy of a fuel if the energy in the water vapor generated in combustion is not condensed. See Safety for more information about the characteristics of Methane and natural gas.

Fuel Comparison 

. Methane  Diesel  Propane (HD-5)
Higher Heating Value (HHV; Btu/lb.) 23,891 19,600 21,600
Lower Heating Value (LHV: Btu/lb.) 21,900 18,500 19,800
Density (lb./gallon) 3.5 7.0 4.3

The actual commercial product specifications for each of these fuels will cause variations in these numbers. As noted earlier, commercial natural gas typically has specifications permitting Methane, Ethane, Propane, Nitrogen and Carbon Dioxide and establishes a heat content specification for the gas. See Conversion Factors for a comprehensive set of equivalents.

Comparing the LHV of fuels can be helpful to establish the expected performance when using one fuel or the other. However, the combustion characteristics in engines or turbines can vary and cause further changes in efficiency. Brett & Wolff can help establish the economics of utilizing different fuels for customers having various engine configurations.

Spark Ignited Engines

Natural gas has been used as a dedicated fuel in spark ignited engines, including heavy-duty engines, for many years. Natural gas is compatible with gasoline powered engines and is increasingly being used as an alternative to gasoline. Many companies, such as Ford, Honda and others, now offer natural gas powered sedans as an option.

In stationary applications, businesses that have used dedicated natural gas powered engines include among others:

  1. Exploration and production companies that produce natural gas

  2. Gas transmission and distribution companies that deliver gas to customers

  3. Distributed power applications and, in a few cases

  4. Dredging companies.

Gas being produced or transported is used as fuel making the use of spark ignited engines economical even though they are less efficient and more expensive than comparable diesel engines. Natural gas engines are used in distributed power applications where low emissions are required in "non-attainment" areas and dredging companies have used natural gas where it is cheap and readily available by pipeline.

Caterpillar, Cummins and other firms offer dedicated natural gas engines as a major product line for the applications listed above and also for the trucking industry. Larger engines are available from Clarke, Cooper and others, primarily for stationary power applications.

Diesel Pilot Ignition

Diesel pilot ignition engines are becoming increasingly available as an effective means of replacing diesel fuel with natural gas. In compression ignition engines (diesel engines) air is compressed in the cylinder and becomes hot. The hot air ignites diesel fuel injected into the cylinder providing power. Because the autoignition temperature of natural gas is hundred’s of degrees higher than that of diesel fuel, natural gas is not readily ignited using the same technology. However, injecting a small amount of diesel fuel at the appropriate time, emulating a spark plug, can ignite a natural gas/air mixture. The engine is then said to be a dual fueled engine or, in a special situation, a "micropilot" engine.

The principal justification for a system using two fuels (diesel/gas) is that the fuel economy is better in dual fueled engines (by as much as 20%) than it is in spark ignited natural gas engines, particularly when an engine is running at partial power. In some instances, dual fueled engines may deliver even better thermal efficiency than an equivalent diesel engine (a graph follows here).

Among large diesel engine providers, Wartsila Diesel, Inc. and Fairbanks Morse Diesel Engines have offered engines using dual fuel technology for many years. A dual fuel system for Cummins engines is under development by Westport Innovations, Inc. A number of firms offering kits to convert engines from diesel to dual fuel include:

Energy Conversions Inc. supplies conversions for Caterpillar D390 series engines and for EMD 645 engines. They cite a 1.5:1 LNG/Diesel conversion factor in field tests with railroad engines instead of the expected 1.7:1 conversion factor. According to their observations, the fuel efficiency of dual fueled EMD engines using 90% natural gas and 10% diesel pilot can be higher than an equivalent diesel engine running 100% diesel fuel.

In addition to dual fuel engines, "micropilot" engines are being developed and some have reached the marketplace. Dual fuel engines use a diesel pilot to ignite the natural gas/air mixture in the cylinder, but have standard diesel injectors that allow the engine to run on 100% diesel fuel if necessary. In contrast, micropilot engines have special diesel fuel injectors that inject a much smaller volume of diesel pilot into the cylinder and lack the capability to run full power on diesel fuel alone. They become, in effect, dedicated natural gas engines with a “limp home” capability on diesel fuel. Micropilot engines, however, have the advantage of displacing more diesel fuel than dual fuel engines while running cleaner with no loss of efficiency. There is also the potential to use lube oil from the crankcase as pilot fuel eliminating the need to change oil. A constant supply of clean oil in the crankcase could be maintained.

Gas Turbines

Gas turbines are growing in popularity as marine engines and can be purchased with the capability of dedicated operation on various fuels, including natural gas. The option to purchase gas turbines as dual fuel engines is available, making it possible to affect a realtime changeover to either natural gas or distillate as the situation dictates. Many land based power generation projects using turbines are designed specifically to use natural gas, so significant turbine development is related to producing turbines that run especially well on natural gas. Although the thermal efficiency of gas turbines declines markedly at partial loads, technologies are now available to offset this shortcoming, especially when coupled with high-load steady-state operations frequently required in the marine industry. Natural gas powered turbines are deemed to be very safe prime movers.

Many vessel operators are now choosing to use gas turbines for special applications, and, as a result, more and more new applications are being discovered. Although gas turbines are initially more expensive than equivalent power reciprocating engines, the tradeoff for some vessel operators has been that turbines have low emissions, low maintenance costs and attractive power to weight characteristics. High-speed weight-sensitive vessels such as fast ferries and military combatants use gas turbines. Cruise ships have incorporated gas turbines because they offer a compact design, allow more space for revenue generating uses and essentially eliminate vibration. Coupled with combined-cycle steam turbines, gas turbines on cruise ships can provide a thermal efficiency of 50% (exceeding that of reciprocating engines) while providing electricity and steam for ship's services. Similar configurations could be practical for large workboats and transports.

Gas turbines in on-shore power plants commonly use natural gas with distillate fuel as a standby backup. The turbines are designed to handle either fuel, having two different sets of fuel injectors, and are, in effect, dual fuel engines. When required to utilize backup distillate, some operational allowances have to be made. Because natural gas burns more cleanly in gas turbines than distillate fuels, maintenance associated with burning natural gas is significantly reduced compared to the maintenance regularly performed for turbines burning distillate fuel and can make gas turbines competitive with reciprocating engines on a "life-cycle" basis in some cases.

Thermal efficiency of gas turbines available for use in marine vessels has been improving. Turbine thermal efficiency of gas turbines is largely a function of unit size and whether heat recuperators or combined-cycle packages are used. Small gas turbines (2000-4000 hp) characteristically deliver simple-cycle thermal efficiencies of 24-28%. Large gas turbines (25,000-55,000 hp) will return simple-cycle thermal efficiencies in the range of 35-42%. When paired with a Rankin bottoming accessory, combined-cycle efficiencies can reach 52% on the largest members of the group.

Brett & Wolff and Chancellor Systems (770-948-8312) have developed a conceptual design to incorporate an integrated-cycle dual fuel gas turbine/fuel cell assembly for marine and other applications. Power modules are to produce 3,000 and 5,000 hp each with optimal fuel efficiency ratings projected to reach the 48-51% level. The power modules are intended to serve as prime movers in simplified, highly efficient, electric propulsion/powering systems for a broad range of vessel types having total power requirements of 3000-15,000 hp. Special focus, however, is being concentrated on the latest (and upcoming) generation of high-speed commercial merchantmen.

Prominent attributes of the integrated-cycle, electric propulsion/powering systems are (see schematic diagram):

  • High peak thermal efficiency

  • High part-throttle thermal efficiency

  • Both prime movers (FC/GT) capable of operating on either #2 distillate or natural gas

  • Fuel Cell provides ship's service/hotel load during off-line operations

  • Inherent functional redundancy in case of either turbine or fuel cell failure

  • Eliminates the requirement for equilateral duplication of prime movers

  • Cascade multiple power module bringup/shutdown capability

  • Sequenced Module rotation for extended TBO intervals

  • All gearboxes eliminated

  • Enables the displacement of traditional diesel applications

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