The Medium Combustion Plant Directive and the future of diesel

The Medium Combustion Plant Directive (MCPD) has heralded a new raft of rules and regulations aimed at slashing emissions from a wide range of mid-sized equipment – a category that includes many of the diesel-fired generators used in hospitals, schools and other large public buildings. Jason Harryman, Sales and Business Development Manager – Electric Power-Diesel at Finning UK & Ireland (Finning), discusses the future of these generators under the new regime and how to prepare your existing machines for any changes.

Found in the plant rooms of many large developments, diesel engines are commonly used as standby generators. If the mains power is cut off for any reason, the generator automatically starts up and keeps the building’s vital electrical systems running until the grid connection can be restored.

Diesel generators are particularly prized in this role because of their rapid start-up time – usually a matter of seconds – and reliability. However, the introduction of the MCPD brings one major challenge to this technology. Established by the European Union, the Directive (EU) 2015/2193 limits emissions of certain pollutants into the air from medium combustion plants, and regulates pollutant emissions from the combustion of fuels in plants with a rated thermal input equal to or greater than 1 MWth and less than 50 MWth.

Simply put, the new 190mg/Nm3 limit the regulation imposes on nitrogen oxide (NOx) emissions is simply not feasible for a typical four-stroke diesel engine. This is obviously rather concerning news for the countless businesses that rely on their existing plants for continuous or standby power, and for those who plan to invest in new equipment in the future.

While regulations march on, however, so does engineering, and many of the advances made in recent years can ensure that diesel technology will remain a sound investment for the foreseeable future.

A happy medium

If a business does operate a diesel generator – or any of the other equipment covered by the MCPD such as turbines or boilers – the first step to complying with the regulations is checking that they actually apply.

The term ‘combustion plant’ covers a huge range of equipment, taking in anything that burns fuel in order to produce energy. This includes generators, boilers and heaters. The ‘medium’ part of the MCPD refers to the fact that it’s designed to fill in the EU’s current regulatory gap between large plants and smaller appliances. Bigger plants, with a rated thermal input greater than 50 MWth, are already covered by the Industrial Emissions Directive. Smaller appliances, such as heaters and boilers, usually fall under the Ecodesign Directive.

The new regulations only apply to units with a thermal input greater than 1 MWth but lower than 50 MWth. Each piece of equipment is considered individually, so if a site operates a single 1 MW generator it would fall under the MCPD while one operating a trio of 500 kW units would not.

Importantly, it is also possible for some generators to obtain an exemption from the regulations if they operate for less than 50 hours per year. This is measured on a three-year rolling average for new plants and a five-year rolling average for existing ones.

Obviously, this is a rather limited amount of running time, working out to just under an hour per week. This is not as large a barrier as it may seem, however, as the majority of diesel generators based in public buildings that fall under the MCPD are commissioned to provide standby power. These units only come online when they’re needed, and so should comfortably fit within this restriction.

Of course, this will not apply to every generator out there, which is where the many engineering advances developed by the power sector over recent years begin to factor in.

Reducing the issues

As the biggest issue that the MCPD introduces to diesel generators are its concerns over NOx emissions, one of the most important technologies for meeting its requirements is selective catalytic reduction (SCR). This is a process already widely used in marine and automotive applications, and one that can pay huge dividends when used with diesel generators, dramatically cutting NOx emissions.

The system works by injecting a urea-based diesel exhaust fluid (DEF) into the generator’s exhaust air stream. The DEF sets off a reduction reaction with the NOx, converting the harmful pollutant into nitrogen, water vapour and a bit of CO2 – still a harmful gas, but much less damaging than NOx. When properly installed, SCR technology can reduce an engine’s NOx emissions up to 90 per cent, while also reducing carbon monoxide emissions by 50-90 per cent and particulate emissions by 30-50 per cent.

Most of the time you would expect that any significantly reduced emissions would come with some kind of drawback, usually in the form of reduced efficiency. However, when engines are fitted with SCR technology the combustion process can be optimised without fear of increasing emissions, actually boosting the overall fuel efficiency.

Of course, all these benefits don’t come for free. SCR technology does consume DEF while the generator is operating, and this does need to be considered in the running costs. The actual amount of fluid being used will vary substantially depending on a wide range of factors, ranging from the annual running hours, the concentration of the DEF and its duty cycle.

If planning to invest in SCR technology, operators should speak to a specialist to determine how much DEF a generator will require, but in most cases, the cost of the DEF will be more than offset by the reduced fuel costs thanks to the boosted efficiency. It’s also vital that DEF is obtained from a trusted partner. The pre-mixed solutions should be mixed, stored and tested to ISO standards, as any errors could cause irreversible damage to the engine it’s used in.

Hot and cold

This isn’t the only way to tackle emissions and bring generators in line with the MCPD, however. It’s also possible to cut back on NOx by altering the type of cooling used.

Most modern diesel engines run with high temperatures in the combustion chamber, as this is most efficient and helps to reduce the production of soot and particulates. However, these hotter conditions also increase the engine’s levels of nitric oxide (NO), which oxidises into nitrogen dioxide (NO2). Lowering the temperature in the combustion chamber reduced the amount of NOx produced, but this also increases soot emissions that can cause their own set of problems.

However, new technology means this can be overcome. Air-to-air after cooled and charge-air cooled engines are both designed to cool the engine air after it has passed through a turbocharger, but before it enters the combustion chamber.

This decrease in air intake temperature enables a denser intake charge to the engine, allowing for more air and fuel to be combusted per engine cycle. This enables the engine to increase output while lowering the combustion temperatures. With cooler combustion comes reduced NOx production, allowing engines to move closer to meeting MCPD targets.

Looking to the future

When investing in a new diesel generator rated for more than 1 MW, these are all the technologies that should be considered when making purchases from this point onwards – the deadline for new plants being registered with MCPD passed in December 2018.

There is still some room to manoeuvre if a facility has existing generators on a site, however, as the deadline for registration and permits hits on 1 January 2024 for existing medium combustion plants greater than 5 MWth. For those between 1 MWth and 5 MWth, the deadline is 1 January 2029. While it can be tempting to put off a decision, it makes sense to start thinking – and planning – right now.

It’s also important to note that generators connected to the UK’s National Grid via a DNO supply to export also need to comply with the emissions standards in the directive.

There are plenty of retrofit technologies available to keep your diesel generators running for years to come, so make sure to begin speaking to experts as soon as possible.

For more information on diesel generators and the MCPD, please visit