MYTH: ALL LIQUID FUELS ARE THE SAME

Rainer drinks copious amounts of strong coffee, while I prefer Diet Coke to get the required caffeine intake to wake me up in the morning. Both liquids work perfectly well to get us started for the day, although one can argue which of us is more productive.

However, unlike coffee and cola, not all liquid fuels for a gas turbine produce similar outcomes in performance and reliability. Liquid fuels, such as diesel, light distillate oils, crude oil, or other hydrocarbon liquids, are often used as the primary or backup fuel in gas turbines. And these fuels are more like beer or wine:  There are good ones and then there are really bad ones.

Whereas most gaseous fuels can be heated, treated, and filtered to be appropriate for almost all gas turbine combustors, liquid fuel constituents and contaminants can significantly impact hot-section part life and combustion characteristics and thus, indirectly, the gas turbine’s output power and efficiency. A number of contaminants — soluble solids or emulsified liquids — can be present.

Physically, the parameters of interest of a fuel include specific gravity, viscosity, flash point, pour point, and wax content. Chemically, the prime interests are the amounts of carbon residues, ash, and corrosive trace metals present.

One should remember that all fuels, including liquid fuels, must be converted to gaseous form to work in a gas turbine combustor. This is usually achieved through pressurized atomization but for some very clean fuels such as methanol, the fuel can also be evaporated and then used in the combustor.

The operation of gas turbines with medium API distillates, light natural gas hydrocarbon liquids, and alcohols, such as methanol, ethanol, diesel, and even heating oil, is well established, and the effects of these fuels on operation and maintenance are documented and understood.

For most gas turbines, operation on these fuels will have no functional impact, but they will have some operational impacts. So this is like drinking a German Pilsner beer – it may impair you if you have too much of it, but it usually will not give you a headache.

Specifically, all oil distillate products are derived from crude oil in the refining process. As such, they will always contain some undesirable trace materials that originate from the crude oil and are difficult to completely remove.

This may include sulfur as well as metal salts that are water-soluble, such as sodium, potassium, calcium, and several others, but also the oil-soluble metals like vanadium, lead, nickel, and others. Oil-wetted materials, such as rust and metal particles, may also be present.

Diesel, a very common gas turbine fuel, can be purchased with various levels of sulfur, including ultra-low sulfur diesel with less than 15ppm sulfur content. Sodium, sulfur, vanadium, lead, and potassium contribute to hot corrosion when in a liquid or molten state.

Furthermore, although the fuel as specified by the vendor may meet the fuel specification as required by the gas turbine manufacturer, impurities introduced during the forwarding and transportation of the fuel to the site can add significantly to the total fuel quality.

Fuel may sometimes come into contact with salts and seawater, especially in salt-laden atmospheres and when transporting by ship or barge. The only effective methods to control and limit these impurities are to perform frequent chemical analyses of liquid fuel and to add a fuel washing and filtration system (centrifuges and cartridge filters) to the plant’s liquid-forwarding system.

Also, proper storage tank maintenance (bottom draining and corrosion control) and a floating fuel intake reduce the chance of accidentally introducing seawater or other non-diesel liquids into the gas turbine.

A totally different problem

Heavy oils, such as crude oil, represent a totally different problem. Here the main problem is that carbon residues can cause coking. Coking refers to solid deposits of fused carbon-ash that are left over after the more volatile constituents in a hydrocarbon fuel or lubricant are distilled.

This process can occur inside the gas turbine’s fuel and lube-oil system in sections when the local temperature exceeds the boiling point of the liquid and the lighter hydrocarbon constituents evaporate.

Other gas turbine deposits from corrosion, dirt fouling, ash, and other solid impurities are often misdiagnosed as coking, but should really be classified by their correct source to avoid the wrong maintenance solution approach.

Another source of problems can be aromatics and olefins in the fuel. Not only can they result in increased smoke, but they can also lead to carbon or soot depositions and increased combustor metal temperature. High levels of olefins may cause injector plugging.

In the liquid fuel system, the primary factor that determines a fuel’s coking is its carbon content. Fuel coking is usually limited to the fuel piping, valves, and combustor injectors. Deposits inside the combustor are not usually coke, as coke burns at the temperatures encountered in the hot section of the gas turbine; these deposits are usually ash, silicates, or iron-oxides from upstream corrosion.

Most manufacturer’s specifications limit the carbon residue content of allowable fuels for their gas turbines, but many of the heavier liquid fuels such as crude oil, heavy fuel oils, refinery waxes, and even certain diesel and heavy naphtha fuels may have a carbon residue content that can go as high as 15%.

For these applications, fuel system coking cannot be completely avoided and can only be treated using frequent fuel system cleaning, as well as the disassembly and solvent soaking of components. Using heavy oils and crudes as a gas turbine fuel is more like drinking a cheap light lager beer — it will impair you and give you a massive hangover if you have too much of it.

The gas turbine OEM always defines minimum fuel quality standards that, if not met, will negatively affect the life of the gas turbine’s combustor and turbine hot sections. Strict adherence to these specifications is always recommended.

However, even if all the requirements of the fuel specifications are met, gas turbines operating on liquid fuel will still have increased maintenance factors. Some of these effects, such as the corrosion rates of hot-section metal and hard-surface deposits can be controlled and limited by proper fuel transport, washing, and maintenance activities. But in most cases they cannot be completely eliminated.

So unlike the eternal discussions about the health benefits of beer versus wine or cola versus coffee, with liquid fuels, the impact of a specific fuel on gas turbine life and performance can actually be fairly accurately quantified. Always choose your liquid fuels and drinks wisely.