MYTH BUSTERS: GAS TURBINES VERSUS GAS ENGINES: WHICH ARE BETTER?

When following press releases from the recent past, we find frequent discussions about the future of fossil-fueled engines. There is public debate about the viability of gas engines in automotive transportation as well as other applications. In our myth-buster column we usually do not write about automotive issues. But we care about fossil-fueled engines for oil & gas and power generation applications, which currently face similar public discussion and scrutiny from environmental regulatory agencies. In this context, it is important to understand the fundamental principles of the oil & gas industry’s two primary fossil fuel burning drivers: gas engines and gas turbines.

Specifically, we want to highlight the technical and operational differences of gas turbines and gas engines that impact their performance for industrial applications. Both systems are basic thermodynamic heat engines and work by compressing the working fluid air, adding and burning fuel for heat input, and extracting useful work by expanding the hot exhaust gas resulting from the combustion of air and fuel. The difference lies in the fact that a reciprocating engine performs combustion intermittently, while a gas turbine performs it as a continuous process.

This means that a gas turbine runs at significantly higher speeds, thus achieving a higher power density. For example, a 10,000 hp gas turbine weighs less than a 1,000 hp gas engine. Continuous combustion allows for better control of the combustion process, which results in lower exhaust emissions of all criteria pollutants such as Nitrous Oxides (NOx), Carbon Monoxide (CO), Unburned Hydrocarbons (UHC), and Particulate Matter (PM).

A gas turbine’s combustion system further allows for a complete separation of the combustion process from the engine’s lubrication system. Therefore, the combustion process can- not contaminate the lube oil, nor can the lube oil participate in combustion. This not only avoids frequent oil changes or the need to replenish oil that is consumed, but it also prevents hydrocarbon soot emissions from burned lube oil.

In a gas turbine, all parts of the turbo- machinery train rotate rather than oscillate as they do in a reciprocating gas engine. This results in a significantly lower vibration load introduced to the machine’s foundation. Particularly for offshore applications, this can lead to a reduction in the amount of structural steel required. But even on- shore, there is a difference in the complexity of the required foundations between gas engines and gas turbines. Finally, the difference in the level of vibrations and the number of moving parts also impacts the reliability of these machines. One of the key ingredients of emission reduction is the improvement of operational efficiency. This includes the efficiency of components like the driver and the driven compressor of a system.

Arguably, the biggest efficiency enhancements come from the capability for smart operation. This requires optimal matching of the compressors and their drivers with the pipeline system, gas storage system, or any other compression operation system. Here, fast adaptability of the machinery system is a key requirement. There is a noticeable difference between gas engines and gas turbines. For compatibility reasons, gas turbines are usually matched to centrifugal compressors, while gas engines are typically the drivers of reciprocating compressors. So, while gas engines offer attractive levels of driver-only efficiency, this advantage is reduced when the efficiency level of the overall package (i.e., the compressor and its driver) are compared, especially for pipeline applications, or if options with waste heat recovery are compared.

Also, hydrocarbon leakage is lower for centrifugal compressors compared to reciprocating compressors. And yes, size matters. For low-power applications, reciprocating systems have advantages regarding cost and efficiency. The bigger the systems become, the more the advantage turns to gas turbine systems with lower maintenance cost and higher achieved availability. Furthermore, because gas turbines have much higher mass flows than gas engines, they are better suited for waste heat recovery applications.

Finally, gas turbine noise emissions are easier to control since they create noise in a much narrower frequency band, and avoid low frequency noise. So yes, there are differences between gas turbines and gas engines. Each have their advantages and disadvantages, depending on the application. Regardless, when deciding between gas turbines and gas engines one should always look at the whole system performance, including total system efficiency, reliability, avail- ability, maintenance, emissions, footprint, weight, modularity, noise, and so forth, rather than simply and singularly the driver’s efficiency.