Diesel gensets, long the backbone of decentralized energy applications worldwide, are facing new challenges around fuel price and environmental issues. Tildy Bayar speaks to genset makers about their fuel flexibility plans

Credit: Atlas Copco

Diesel-fuelled generating sets or generators have been utilized for many decades in power generation applications, but their days may well be numbered. Not only do the stubbornly high fuel price and, in many regions, transport issues make them expensive to run, their environmental credentials continue to raise eyebrows. However, the industry has responded and the diversity of fuels that can power gensets continues to grow.

Many genset ranges are already fuel-flexible according to the major manufacturers, with the list of acceptable fuels including syngas, pipeline gas, digester gas, landfill gas, coal mine gas, biodiesel, jet fuel and even plastic waste-derived fuel. However, Parag Vyas, Head of Technology at Aggreko, notes that ‘whether or not to adapt gensets to run on alternative fuels is only viable where there is an application or customer-specific need.’

Where there is such a need, some modifications are necessary for diesel gensets to run on these alternative fuels. Michael Yohe, Engineering Specialist at Caterpillar, notes that gensets’ ability to run on different fuels is highly dependent on the fuel type. No changes are required for a diesel genset to burn biodiesel, he explains, while for gas fuels, ‘variability in the fuel types has the largest impact on engine design, and fuel composition is critical to determining the impact on engine operation’.

To cope with various fuel types, he says, adjustments must be made to the engine, including ‘adjustments to the engine timing, changing compression ratio, thermodynamic combustion cycle, and changes to the exhaust system for different operating temperatures and corrosive elements associated with the various fuel types’.

With gaseous fuels, the most important consideration is gas composition. Yohe’s point is echoed by Udo Sander, Head of Gas Engine Development at MTU Onsite Energy, who notes that gas composition is crucial and that, ‘as opposed to diesel, we have no clear rules for gas composition. It must be burned how it comes out of the pipeline, which makes it more difficult compared to diesel.’ Sander explains that gas composition is characterized by a methane number between 1 and 100, with 100 being ‘pure methane, the best we can burn with the best engine efficiency. 100 means good gas, very stable, with high knock resistance.’

Credit: MTU Onsite Energy

Russian gas, he says, generally has a high methane number, between 90 and 95. ‘But if you follow discussions of the last years on European gas harmonization, it is a big issue and we don’t anymore have such good gases in our pipelines.’ In Germany, he says the gas’s methane number is usually between 70 and 90, ‘so not so good in some areas – but we can also run the engines down to 50 to 100, depending on customer needs – we can use the gas that’s available.’

Sander says natural gas is ‘our standard, and the basis for MTU’s engine development’, with one reason being its CO2 footprint, which is around 25% less than diesel’s. Natural gas also typically has a high methane number, between 80 and 90. Coal mine gas also ranks high on the methane scale, as do digester gases, ‘typical’ biogas and landfill gas – but these gases can contain up to 50% CO2.

To enable gensets to run on gaseous fuel, Sander says that if the gas’s methane number is higher than 90 or, in the case of biogas, 100 or higher, ‘we can use a high engine compression ratio and will end up with very good efficiency’. But although biogas tends to have a very high methane number, its negative side is its lower heating value, which is half that of natural gas. ‘So we have to change the gas mixing unit,’ he explains, ‘and flow double the amount of gas through the engine to get the same power output.’

Digester gas ‘can be burned with a standard engine and is easy to treat,’ he says, but ‘we don’t like landfill gas as it can cause contamination of the engine parts’. Landfill gas can cause ‘big problems’ during engine operation due to its siloxane content, with deposits forming in the combustion chamber and the oil lifetime reduced to one-half, or even one-third of the oil in a natural gas-fuelled engine.

Syngas from industrial processes has a low methane content, between 5% and 15%, and high CO, and requires strenuous treatment due to its toxicity. Other low-methane gases include flare gas, which generally has a methane number of around 50 to 60, but Sander says that decreasing the compression ratio by a factor of 20%-25% will allow the engine to run at full power, although with efficiency losses of two to three percentage points. But, he says, ‘it is better to use flare gas for energy than to burn it off’.

Coal mine gas is ‘good gas’ according to Sander, with a 90 (China) to 100 (Australia) methane number depending on the location. But there are challenges due to variable gas pressure, meaning that ‘control of the engine is a challenge – but we can solve it’.

Shale gas from the US is difficult to treat, he says, because of its low quality. ‘The challenge for the engineer is that you don’t know what quality of gas is coming out of the earth: it’s dirty, it has dust in it, so the engine must be prepared for fluctuations in quality’.

Diesel’s uncertain future

As end users worldwide become more aware of environmental issues, there is growing concern in some regions about the long-term viability of diesel gensets, especially as renewables become a cheaper option for more remote regions. Will diesel gensets be pushed out of some markets?

‘There has been a shift in the past 10 years towards more environmentally friendly solutions and businesses across all sectors are more conscious than ever of their carbon footprint,’ says Dan Ibbetson, Aggreko’s Managing Director for Northern Europe. ‘This will be a continuing trend, which is why we are constantly striving to increase the efficiency of our engines and looking for ways to innovate.

‘Environmental issues will always be a consideration,’ he adds. ‘At the moment it is not possible to rely entirely on renewable energy sources, and diesel gensets provide a viable backup.

Credit: Caterpillar

‘We see diesel playing a continued role in the energy mix in the future. We do not rely exclusively on diesel, or any other fuel, and understand that each fuel source has its advantages and disadvantages.’

Vyas agrees: ‘The choice of fuel is site- and application-specific,’ he says. ‘Our customer needs are well defined, including their access to different types of fuel. Also, price of fuel, efficiency of the engine, load profile of the project all have a bearing on what fuels are preferred.’ But he stresses that ‘Diesel is the only viable alternative in some markets, and so we think it is unlikely that diesel will be replaced in the foreseeable future.’

Yohe notes that ‘the ability to operate on different fuels is very important. In the global markets that Cat generator sets serve, the availability of fuel (and associated costs) can vary greatly. It is important for Caterpillar to provide the correct generator set to a customer to operate on the lowest cost fuel available to them.

‘It is really based on customer preference, fuel availability, fuel cost, and location. Caterpillar and the local Cat dealer will work with a customer to identify the fuel type that provides the lowest cost of operation.’

Another option is dual-fuel gensets, especially in regions where a gas supply is expected in future but is not currently in place. Yohe notes that Caterpillar currently offers dual-fuel gensets, primarily in the oil & gas business. ‘Cat dual-fuel engines start with a diesel engine and the sensors and controls are added to ensure the engine will meet the reliability and durability expected of a Cat engine,’ he says.

Sander gives the example of a diamond mine project in Russia’s far east, where it is ‘a problem to get the diesel in the summertime when the earth is not frozen,’ making transport difficult, while in the winter transport by water is difficult as the rivers freeze. But, he says, ‘gas is under the earth everywhere in Russia, as in Brazil, Chile and South Africa, so it’s sometimes easier to use gas than diesel.’

‘If you observe the market and look at what’s happening in the world,’ he adds, ‘you’re quite right that diesel is being attacked by gas fuel – it may be slow, but it’s coming. I have a lot of literature on my desk that shows that the future is clear. MTU, MAN and all the other [OEMs] are working to develop engines operating on gas fuel, and I think in the near future – at the latest in the 2030s – we will have a major part of the sector.

‘Gas fuel is the better choice for constant operation,’ he continues, ‘and I am fully convinced that gas will replace a big portion of diesel in future. The lifetime reserves of gas in the earth will last, some people say, for 500 years, some say 700, some say 300… much longer than our reserves of oil.’

Yohe agrees. ‘We see natural gas – where available – starting to displace traditional diesel engine installations,’ he says. ‘Advances in gas engines have made the performance – start times, transient, and steady state – near to that of a diesel engine. Gas is a less expensive fuel than diesel and has lower emissions, making it easier for a gas engine to meet global emissions standards.’

But he also notes that Caterpillar sees ‘the migration from traditional diesel to natural gas as a regional issue, mainly driven by emissions regulations and cost of natural gas vs diesel. However, the availability of natural gas is a limiting factor in some areas. There are also some industries looking to migrate away from traditional diesel standby to gas standby to lower owning and operating costs and increase the flexibility of use – the lower emissions allow operating as a load management asset.’

Sander says MTU has recently developed what he calls a bi-fuel or gas mixing process, which allows an existing diesel engine to burn up to 70% gas. ‘The diesel engine stays unchanged,’ he says, ‘and we add the bifuel kit,’ which includes a gas regulation unit and valves. ‘We don’t touch the piston or the inner part of the diesel engine, which keeps its Tier II certification’ in the US, where the project is currently running.

He adds that his firm has just begun a prototype project called gas blending, which allows the user to mix different kinds of gases with different compositions. An installation in Slovakia mixes two gases with different heating values; ‘It’s working, maybe not so good, but it’s working,’ Sander says.

‘True fuel flexibility is, in principle, possible,’ he adds. ‘Demands from customers are not very much today, but they are growing and coming, and we’ve started these projects in response.’