Gas turbines or gas engines?

Solid arguments for your future-proof technology choice

 

Business models and financing initiatives are shifting toward renewables in today's power generation market, while the current lack of long-term regulations is making investment decisions more difficult than in the past.

 

Under these circumstances, the right future-proof technology choice is essential for ensuring a project's long-term profitability and reducing exposure to environment-related risks that may result in stranded assets.

 

To provide you with solid arguments for making and explaining investment decisions, here we compare the relative merits of gas turbines (GT) and gas & dual fuel engines, also known as reciprocating internal combustion engines (RICE). Let's find out which technology has the smallest emissions footprint, burns next generation (cleaner) fuels, and is the best fit for your specific needs! Prepare for a rather complex answer, because the right technology choice always depends on your specific requirements and application type.

Use case

A reliable and highly efficient combined cycle power plant in the heart of the Brazilian Amazon forest

Coari, the third-largest city in the Amazon state, is isolated from the national power grid. All electricity was provided from diesel generators - until Siemens Energy built a gas and steam power plant within only 13 months.
Gas engine vs gas turbine

Technical comparison

Both technologies deliver many benefits when it comes to converting natural gas into power and heat.

Gas turbines are best for:

  • High combined cycle efficiency
  • High temperature cogeneration or combined heat and power (CHP): steam, hot air
  • Low combustion emissions
  • Gas fuel flexibility
  • Lowest maintenance costs

Gas engines are best for:

  • Open cycle efficiency
  • Low temperature CHP: hot water
  • Frequent starts and stops
  • Liquid fuel flexibility

Comparison of the main features

Looking to cut your emissions and become carbon-neutral?

Before providing funding, many large financing institutions dictate emission limits to power plant projects. These limits are getting lower and lower as the global warming crisis continues to raise attention to public’s opinion. 

 

To better understand the pollution problem, let’s group the harmful chemicals by their effects:

Global effects have all chemicals that drive the global warming, the so-called greenhouse gases. Two chemicals from the power industry contribute to this effect: CO₂ and methane (CH₄), major ingredient of natural gas. CH₄ has an 84 times higher global warming potential than CO₂ (averaged over 20 years, source: IPCC AR5 2013) and its slippage must be kept to a minimum.

 

Local effects have substances like nitrous oxides (NOx), carbon monoxide (CO), particulates (PM2, PM5, PM10…), sulfur oxides (SOx), heavy metals, and many others, harmful to humans and nature.

The proper combustion technology choice for electricity generation to reduce these pollutants to their absolute minimum and the fuel used, a very important emissions driver, will determine the content and amount of exhaust emissions during the total lifetime of the plant.

 

Emissions of NOx, CO, particulates, and many others are emitted in a significantly lower quantity by gas turbines compared to gas engines. The reason for this is the different combustion principle: while in internal combustion engines, like in cars, thousands of single explosions with very high temperatures in the cylinders generate the power, gas turbines have a continuous combustion process at a lower and more evenly distributed temperature profile.

To significantly reduce CO₂, the highest level of net efficiency is essential, because higher efficiency decreases specific CO₂ emissions in grams per produced kWh. It is therefore very important to not waste any energy. To extract high amounts of energy from still hot exhaust gases, heat recycle technologies offer solutions.

Engines have better open cycle efficiency than gas turbines and lower fuel consumption. Their CO₂ emissions  are lower but the concentration of pollutants overall in the emitted gas volume is higher. As their exhaust gas temperature is much lower, the potential to extract further energy from it is also much lower.

 

Where GTs have net efficiencies around 30-40%, engines show clearly higher values of up to 46%. By applying heat recycle solutions, the gas turbines net efficiencies increase up to almost 60% and for engines around 50%.

As the new European regulation reduces the current permitted limits by half, engines will have to operate with a NOx limitation of about 0.15g/kWh.

 

Gas turbines have due to their combustion process an advantage. As the cleanest conventional energy source, their use will be indispensable in the energy transition. We have proven technology at hand to efficiently gain electricity from fuels like natural gas and hydrogen. Natural gas is the cleanest of fossil fuels and produces much lower emissions compared to e.g. liquid oils.

 

For a substantial reduction in CO₂ emissions, we also recommend choosing a combined cycle power plant because it delivers the highest efficiency by far of all currently available fossil technologies and CH₄ slippage is negligible. If, for any reason, this is not possible, at least emission reduction technologies should be used to filter out certain chemicals like NOx and CO from the exhaust gas. Unfortunately, CH₄ emissions cannot be decreased easily.

Are you considering future fuels in your investments?

Liquid and gaseous fuels are to be differentiated by their constituents, energy content and many other properties, also their carbon intensity. Fossil fuels consist mainly of hydrocarbons. Combustion of these fuels create CO₂. If decarbonization is one of the main investment drivers, future fuels should be considered in the evaluation of the best technology.

 

Future fuels can be also differentiated into carbon neutral ones, like 

e-methane and e-methanol, and carbon free, like green hydrogen or 

green ammonia, depending on the production process. Fuel flexibility 

will grow in importance in the transition to a decarbonized energy 

system. The use of less carbon intense or carbon free e-fuels is very 

promising to achieve carbon neutrality in power generation. Due to

the rapid surge in growth of the intermittent renewable power 

generation, the security and affordability aspects of the energy 

trilemma are increasingly challenging. Reliable (backup) power 

generation with a low carbon footprint is crucial in supporting 

consumer needs. 

 

Gas turbines are the cleanest conventional energy source, and their fuel flexibility is ideally suited to support the transition on both centralized as well as decentralized grids. Compared to the gas engines, gas turbines have a significantly lower concentration of air pollutants (CO₂, NOx, SOx, particulates) in their emissions. Engines consume less fuel and emit a lower volume of gas but produce a higher concentration of pollutants.

 

Gas turbines can operate using a wide range of fuels, with on-line fuel switching to ensure security of the energy supply. These fuels are not only conventional fossil fuels like natural gas, LPG, and diesel but also process off-gases like coke oven gas (COG) and refinery gas (RFG) and low- and zero-carbon fuels like hydrogen, biogas, and renewable natural gas (RNG). Many of these can be burned without a significant performance impact while still maintaining the lowest possible environmental footprint.

 

Gas engines can run on very low heating value (LHV) fuels like syngas (4.5 MJ/Nm³). They can also burn biogas, landfill, and higher LHV gases (flare gas), propane, and LPG that have an LHV of about 110 MJ/Nm³, although performance may differ from that achievable on natural gas.

 

Every investment into power generation, every purchased gas engine or gas turbine today, will see hydrogen as a fuel in its lifetime. Customers should be sure to purchase future-ready products to avoid the possibility of being left with stranded assets.

Looking to maximize plant profitability?

Power plant efficiency isn’t just a major driver of plant profitability, it’s also directly linked and proportional to CO₂ emissions. Increasing a plant’s efficiency reduces its fuel consumption, and with less fossil fuels burned, CO₂ emissions will be lowered.

 

Do you plan to operate more in full load or in part load or in residual load operation?


To find the best technology and solution for your project, the expected operating profile is essential.

The comparison of gas turbine and gas engine efficiencies presents an ambiguous picture: For small simple cycle plants with a lower power output, engines deliver the best electrical efficiency. For example, the standard electrical efficiency of 300 to 2,000 kW gas engines is 40-45% and up to 85-92% total efficiency in low temperature CHP applications.

 

For large power only plants with a higher power output, gas turbines in combined cycle are a step ahead because they can achieve the highest electrical efficiency at higher power, up to 63%. For plant outputs below 100 MW, combined cycle plants with an electrical net efficiency close to 60% are available, while even small, combined cycle plants down to 20 MW have competitive efficiencies compared to open cycle engines. Combined cycle plants have the potential to boost the fuel utilization up to 90% or higher and add new revenue streams.

When it comes to profitability, reducing downtime and maximizing availability are crucial. Gas engines can provide an availability average of over 96%, while industrial and aero-derivative gas turbines can average over 97% availability.

 

OPEX can also be minimized with improved maintenance schedules: 60,000 hours of operation until a major overhaul and even longer (90,000 hours) with more advanced engines, although there are more frequent outages throughout the year for routine maintenance. Gas turbines require less annual routine maintenance, with first significant maintenance interventions occurring typically between 25,000 and 32,000 operating hours (OH). Gas turbine maintenance tends to be lower on a €/MWh basis.

Need flexible backup power for residual load or additional revenue streams?

A short start-up time, high ramping rates, good part-load efficiency, and low emissions are key requirements for providing backup power during periods with low solar and wind power production

Due to the increasing renewable penetration in the grid, not only the flexibility in fuels is essential for the future success of operating a reliable power plant. Offering operational flexibility to the power market increase the revenue streams from selling power also into ancillary services.

 

In the classic power markets mainly the increase of power output has been the focus of the generators. Providing increased power on demand became a key business which was also considered as mandatory reserve within the grid codes. The intermittency of renewable power and the capability of highest possible ramp rates to be utilized with the shortest possible response time became a key aspect for frequency stability which can only be achieved with rotating equipment when online.

 

To keep the emissions as low as possible and simultaneously with low operating costs, a low, emission compliant turn down with a high part load efficiency (see Efficiency) becomes more and more important. In case the generation units are offline, a fast and reliable start-up becomes essential for a successful operation. These operating properties are in many countries payed services and so, additional revenue stream can be created to increase profitability of the power plant.

 

As there are different technologies with different unique properties, we recommend you identify the best technology and solution for your operating profile. As an example for decision criteria, we discuss the start-up capability more in detail.

 

Fast-start capability is valued by customers because they can realize additional revenue streams. In markets with capacity mechanisms, merit order rankings, for secondary and tertiary frequency response, plant operators can offer power in five or 15 minutes at high prices.

Gas engine and gas turbine start-up time depends on the initial conditions. Gas turbines require only the lube oil to be at or above 20° Celsius. Gas engines require the cylinder heads to be at or above 60° Celsius, and the lube oil to be at the correct operating temperature. This is achieved by heating and circulating cooling water, which can take several hours starting from the ambient temperature. That’s why gas engines are often maintained at fast-start conditions, and the standby power consumption is factored into the overall operating cost.

In general, the starting and loading phases from warm standby are similar for gas turbines and gas engines, usually five to 10 minutes. Both fast-start gas engines and gas turbines are available with the ability to achieve full load within one to two minutes. Both engines and turbines can run both at part-load and full load to adapt to specific duties. Both technologies can be used for emergency power/ standby power applications, backup-peaking applications with low annual operating hours (<2,000 h) or run for 8,500 hours per year for baseload applications.

 

A combined cycle plant’s start-up time is much longer than that of simple cycle plants. A state-of the-art gas turbine in a combined cycle plant needs less than 30 minutes to full power for a hot start. With a bypass stack, operators can first fast start the gas turbine and later synchronize the steam turbine.

How can we operate the grid safely?

Impact of the momentary share of intermittent renewables on the grid operation.

A safe and reliable operation of the grid requires to balance power generation and demand at any time. Short-circuit voltage providers or reactive power compensators are essential for balancing the grid’s synchronous rotating masses (inertia). Short circuit power is required to be able to locate failures and in case of a power outage restore the grid.

 

Historically, almost all power grids provided a large proportion of fossil power from coal and gas plants as well as from nuclear and hydro plants (the latter, of course, isn’t fossil power), and they offered a high grid stabilization potential due to their very high rotating masses and high short circuit power. Only a few grid events occurred that required interventions like re-dispatch.

 

Today’s green power sources like wind and solar don’t offer grid-stabilizing properties (dynamic stabilization of the frequency) as they are liable to fluctuations. Therefore, those plants providing residual load power must generate as much inertia for dynamic frequency control as possible. As fossil energy production decreases, at a certain point the balancing capacity won’t be sufficient to prevent grid failure.

For future grids synchronous inertia becomes a paid good. TSOs will need to modify their merit-order dispatch rankings in order to account for the marginal cost of energy (COE) and to bring their plants to dispatch, which provides high grid stability and averts the risk of grid failures and blackouts.

 

In general, the larger the synchronous electrical generator is, less equipment in operation is necessary to have a significant impact on the grid stability.

 

Gas turbines provide up to an order of magnitude higher inertia than gas engines as they operate at much higher speeds and the whole power train contributes to the mechanical energy. Especially in combined cycle power plants, gas turbines offer high grid-balancing capabilities and have the lowest emission footprint of all fossil-power generation equipment. Gas engines have very low inertia, primarily because of the lightweight crankshaft in the engine and the electric generator rotor that spin at low rotations per minute.

 

For more information on the importance of grid-stabilizing capabilities, read our white paper.

Power generation scenarios for Countries and Regions

To offer you guidance on the most appropriate technology for your requirements, we investigated the most common scenarios and/or solutions for specific customer needs.

The German government committed to dramatic reductions in CO₂ emissions in the near future. To achieve these targets, the power sector will have a strong contribution by introducing new technologies for reducing its carbon footprint: perhaps even carbon-free or -neutral power generation.

 

Imagine that we could simply replace all coal-powered facilities with clean combined cycle gas-powered plants. This would immediately reduce Germany’s overall CO₂ emissions by 17 percent. For the power sector, this reduction would mean halving its CO₂ emissions (see Figure.) The CO₂ reduction targets for 2020 would be overachieved, and it would be an important step towards the more stringent goals set for 2030 and 2040.

 

Siemens Energy has committed itself to supporting governments in achieving their ambitious CO₂ reduction goals – and now it’s time to deliver.

 

Let’s work together to develop roadmaps for efficiently and economically decarbonizing Countries, Regions, and ultimately the world.

The UK faces a particular situation that forces the National Grid to make a special configuration of the electrical distribution along the country. The energy providers can’t supply at 100% of availability and there may be a lack of coal-fired plants in the near future.

The Ukrainian government recently announced ambitious goals: to massively increase the share of renewables in its power sector, replace inflexible coal-power generation with cleaner gas-based technology, and connect the power grid to the European ENTSO-E. The optimal solution for decreasing carbon emission levels and at the same time efficiently backing up renewables is to install gas turbine technology. Why? See our white paper on highly efficient and grid-stabilizing gas turbine solutions.

The main economic resource in the Anzoategui area of Venezuela is oil and gas production. Associated gas from crude oil provides abundant energy. Large quantities of oil are extracted and the associated petroleum gas (APG) can be used as a fuel for power generation, instead of being flared into the atmosphere (as it previously was).

Newly available fuels, new technologies, and ambitious renewable goals are bringing a disruptive change to utilities in the Caribbean. They’re opening the way to a more sustainable, reliable, resilient, and affordable energy future, but they’re also adding more complexity to the planning process.

These presentations focus on optimized generation solutions, taking criteria like project size, available fuels, flexibility, stability, reliability, availability, and emissions into account. In particular, they discuss where reciprocating engines and where gas turbine technology can each offer their advantages and how to make sure that newly built power plants will complement the renewable growth story in the Region.

References

Siemens Energy Products and Solutions

Heavy-duty gas turbines

Heavy-duty gas turbines

Siemens heavy-duty gas turbines are robust and flexible engines, designed for large simple or combined cycle power plants. They are suitable for peak, intermediate, or base load duty, as well as for cogeneration applications. Customers benefit from our extensive validation and testing capabilities. Our engines are proven in commercial operation and provide outstanding efficiency.

(Power generation in MW(e))

Industrial gas turbines

Industrial gas turbines

Siemens industrial gas turbine models with their compact and rugged design make them an ideal choice for both industrial power generation and mechanical drive applications. They also perform well in decentralized power generation applications. Their high steam-raising capabilities help achieve overall plant efficiency of 80 percent or higher. They are proven with more than 2,250 units sold at small utilities, independent power producers, and in the oil and gas industry.

(Power generation in MW(e) / mechanical drive in MW)

Aeroderivative gas turbines

Aeroderivative gas turbines

Originally developed for use in aviation, Siemens aeroderivative gas turbines are flexible, compact, and lightweight designs that are ideally suited for power generation and mechanical drives in the oil and gas industry. Their high efficiency and fast start capabilities mean that our aeroderivative gas turbines also perform well in decentralized power generation applications. With a fleet of 2,500 installed units, millions of operation hours in different environments have been generated in numerous reference projects.

(Power generation in MW(e) / mechanical drive in MW)

Gas turbines for oil and gas applications

Gas turbines for oil and gas applications

Oil and gas will continue to be the backbone of the global energy supply, and natural gas will become even more important in decades to come. With Siemens as a partner, you benefit from proven expertise and experience in oil and gas applications.

For power generation in upstream, midstream and downstream applications as well as mechanical drives for compressors and pumps, Siemens gas turbines are proven and trusted in the industry. Since many years, they ensure outstanding reliability for oil and gas production and processing as well as pipeline, LNG and refinery applications.

Gas turbines for industrial power generation

Gas turbines for industrial power generation

Benefit from our expertise and experience in industrial power generation: Siemens gas turbines can be used for power generation and cogeneration (Combined Heat and Power, CHP) in many industries, such as the chemical and fiber, cement, metals and mining, as well as other manufacturing industries.

Our engines ensure an uninterrupted supply of power and heat, high energy efficiency, low energy costs and a high return on investment.

Gas turbines for power production

Gas turbines for power production

Siemens is as a trusted project partner for utilities, municipalities and independent power producers. Our gas turbines are the heart of the most modern and efficient power plants.

They provide low power generation costs, flexible and short start-up times as well as high efficiency, and will make your business even more profitable.

S Series Engines

S Series Engines

A robust and reliable solution for power generation, CHP and mechanical drive applications

H Series Engines

H Series Engines

This series represents a new concept in engine design with the advanced technology incorporated into the cylinder heads, valves, camshafts, and turbochargers. 

E Series Engines

E Series Engines

The new best-in-class solution for more efficient power generation