Combustion engine vs gas turbine: fuel flexibility

Liquid fuels present many challenges for gas turbines because they can contain water-soluble salts, high concentrations of heavy metals and other impurities. Crude and residual oils are more viscous and contain higher concentrations of trace metals than distillates. Metals and salts are abrasive to turbine blades and can create ash deposits which lead to fouling and corrosion in hot gas path components. Because combustion occurs continuously in gas turbines, the unit must be taken offline for inspection and maintenance. A combination of fuel conditioning (cleaning, blending, heating and pressurization) and more frequent maintenance cycles are required for gas turbines running on fuel oil. Catalysts may be added to improve combustion, and in some cases, heavy fuel oils (HFO) or crude may be blended with higher purity liquid fuels to achieve permissible sulfur, ash and metals content. Fuels containing vanadium or lead, which are oil-soluble and cannot be removed by washing or centrifuging, require corrosion inhibitors for use in gas turbines. Generally distillate fuels are considered to be relatively free of contaminants, but contamination during fuel transportation and delivery has led to occurrences of corrosion in gas turbines.

Overhauling a gas turbine that was designed for natural gas to burn liquid fuels is costly and requires adjustment of the firing temperature control, revised startup and shutdown procedures, and offline cleaning cycles to remove ash deposits. As a result, the availability of the gas turbine power plant is decreased. Because certain fuel oils contain volatile components with low flash points (such as naphtha), explosion protection is also often required for gas turbines. Thus, the ability of most gas turbines to operate on liquid fuels is very limited, in terms of the characteristics of fuels oils that can be used and the amount of time the turbine can operate on such fuels.

Liquid fuel options for gas turbines vary by manufacturer and model, with some gas turbines only able to use No. 2 distillate. Multiple fuel delivery systems and combustors are employed to accommodate different fuels. GE offers an HFO package for their 7E and 9E gas turbines; the Siemens SGT-500 gas turbine can burn crude, HFO and bio-oils; and Alstom offers fuel oil capability on their GT24 and GT26 models.

Wärtsilä engine maintenance is not affected by fuel type as the engines are not sensitive to metals or salts in fuel oils. No corrosion inhibitors are needed and only minimal fuel conditioning (centrifugal separators and filters) is required to burn lower quality fuels including HFO/RFO and crude. Because combustion occurs intermittently in combustion engines with the expulsion of combustion products during the exhaust stroke, the buildup of ash deposits is prevented.

While the use of ash-forming fuels (such as HFO) reduces gas turbine output by 4 to 5 percent compared to natural gas operation, Wärtsilä multi-fuel engines retain the same output and high efficiency whether running on natural gas, LFO or HFO. If the natural gas supply is interrupted, a Wärtsilä multi-fuel power plant instantaneously switches to a backup fuel oil and maintains load without incurring any maintenance penalty. When routine maintenance is required, the modular architecture of Wärtsilä power plants allows an engine to be taken offline while maintaining the bulk of plant output.

Wärtsilä dual-fuel (DF) engines use lean-burn combustion technology when operating on gas and a normal diesel process when operating on fuel oil. Wärtsilä DF engines have three fuel delivery systems that work in parallel: a pilot fuel injection system, a liquid fuel supply, and a gas admission system. The liquid backup fuel system allows the engine to transfer automatically and instantaneously from gas operation to liquid fuel operation at any load. The tri-fuel delivery allows instantaneous switching from LFO to HFO as well. Fuel flexibility was a major factor in the selection of Wärtsilä multi-fuel engine technology to help solve energy supply problems in Jordan. The 573 MW IPP3 plant, comprised of 38 Wärtsilä 50DF engines that can utilize natural gas, LFO and HFO is the largest tri-fuel power plant in the world, providing Jordan with dependable power.

While gas turbines require about 10 minutes to switchover from baseload gas to fuel oil, Wärtsilä multi-fuel engines can switch from natural gas to fuel oil instantaneously. Switching back to gas from liquid fuel takes approximately 90 seconds with no load reduction. As shown in Table 1 below, Wärtsilä multi-fuel engines offer numerous advantages over gas turbines for flexible fuel solutions including the ability to operate on a wide range of fuels without sacrificing power plant availability or incurring additional maintenance costs. This fuel flexibility provides cost savings because a Wärtsilä power plant can ensure a secure power supply as fuel supplies change over time.