Over the past decade, raised awareness and concern over the effect of engine exhaust emissions has led to the introduction of more and more stringent regulatory limits around the world, resulting in a significant reduction of pollutant emissions.

Despite this, the level of air pollution in many places is still problematic. Even though the impact of current legislation has been significant, there are still gaps and not all applications are regulated.

With this in mind, on 25 November 2015 the European Parliament signed what is called the Medium Combustion Plant Directive (2015/2193), with the aim of regulating emissions from combustion plants smaller than those covered by the Industrial Emissions Directive. The IED applies to “large” combustion plants, defined as 50 MWt energy input (Ein) and above, while medium combustion plants, those covered by the MCPD, are defined as being in the size range 1 MWt ≤ Ein < 50 MWt irrespective of the type of fuel they use. For generating sets this is equivalent to a power output roughly between 330 kWe and 17 MWe.

The MCPD focuses on the reduction of SO2, NOx and particulates, as agreed under the Gothenburg Protocol. It also covers the issue of possible future regulation of CO emissions and introduction of minimum energy efficiency requirements.

The MCPD does not apply to: combustion plants covered by the IED; combustion plants covered by Directive 97/68/EC (Non-Road Mobile Machinery, repealed earlier this year by regulation (EU) 2016/1628); any plants used in the propulsion of vehicles, ships or aircraft; gas turbines and gas and diesel reciprocating engines used on offshore platforms; and research, development or testing activities relating to medium combustion plants (subject to specific conditions that may be placed on them by member states).

Figure 1 summarises the emission limit values applicable to ‘new’ and ‘existing’ diesel and natural gas generating sets under the MCPD.

‘Existing’ plants are those put into operation before 20 December 2018 or for which a permit is granted before 19 December 2017, provided that the plant is put into operation no later than 20 December 2018. All other plants are ‘new’.

The Directive is applicable to Medium Combustion Plants regardless of the fuel they use. 

It is also applicable to aggregated plants, consisting of several new Medium Combustion Plants sharing the same stack, including those where total E is 50 MWt or more, unless the aggregated plant in is covered by the Industrial Emissions Directive. This addresses the issue that units of less than 15 MWt in an aggregated plant are not counted when determining whether such a plant is above the 50 MWt threshold that defines a Large Combustion Plant.

The MCPD entered into force on 18 December 2015, giving member states two years to “transpose” it (ie make it part of their own legislation).

The emission limit values set out in the Medium Combustion Plant Directive will have to be applied from 20 December 2018 for new plants and by 2025 or 2030 for existing plants, depending on their size.

There are flexibility provisions for district heating plants and biomass firing.

The MCPD addresses the potential need for member states to apply stricter emission limit values in areas where this can improve local air quality in a cost-effective way. The Commission says it “will help member states dealing with such hotspots by providing information on the lowest emissions achievable with the most advanced techniques.”

It is the responsibility of the operator to monitor and register the emissions of pollutants to the air and to take the necessary measures to ensure compliance, at all times, with the directive emission limit values (Article 7). Table 1 summarises the frequency of monitoring of emissions by the operator required by the Directive.

It should be borne in mind that this is a directive, which is essentially a guideline on the minimum requirements that must be achieved by individual member states. This means that, until it is transposed into laws and penalties by each country (a process currently underway) there is no clear definition of the requirements across the various countries, only an idea of the minimum.

Although it is expected that the directive is to be followed in most of the member states, including the UK (Brexit notwithstanding, which anyway will not happen before the transposition deadline), it is equally expected that stricter limits may be applicable in locations where the air quality is still problematic or in countries that are traditionally leading and pushing forward on lower emissions (Article 6, paragraph 9).

On the other hand, medium combustion plants may be exempted from complying with the emission limit values if operating less than 500 hours per year, averaged over a 5 year period.

For Medium Combustion Plants providing backup power in connected islands or used for heat production in cold climates this limit may be extended to 1000 hours. The exemption from compliance will be a decision for each member state (Article 6, paragraph 3).

Medium Combustion Plants located in the Canary Islands, French Overseas Departments, Madeira and Azores are not required to comply with the emission limit values of the Medium Combustion Plant Directive. These limits need to be determined by the member state concerned (Article 6, paragraph 1).

The current version of the directive was written to be flexible and allow the inclusion of future requirements. Examples of this are CO emissions and minimum energy efficiency.

The document signed on the 25 November 2015 does not restrict maximum carbon monoxide emissions. However, it clearly states that operators must carry out CO emissions measurements for all Medium Combustion Plants (Annex III, Part 1, paragraph 3).

Aligned with this approach is also the fact that by 2020 the European Commission plans to assess the benefits of setting minimum energy efficiency requirements, and that by 2023 the need to regulate CO emissions will also be reviewed (Article 12, paragraph 1 and 2).

Sulphur dioxide emissions are mainly a consequence of the quantity of sulphur in the fuel and can be reliably estimated without testing. This fact does not exonerate the operator from keeping SO2 emissions within the legislated limits. In the event of shortcomings in low sulphur diesel supply, a derogation for a maximum period of six months may be granted. Diesel fuel with less than 2000 ppm sulphur content meets the requirements in the MCPD (Article 6, paragraph 11).

Closing a gap

The Medium Combustion Plant Directive has closed a gap in the field of emissions legislation. Also, unlike Directive 97/68/EC covering non- road mobile machinery, the focus and responsibility for compliance is placed on the operator and application rather than on the equipment.

It is a “comply-at-all-times” directive that requires the combustion plant operator to continuously monitor and report the emissions, leaving an open door for future regulation of carbon monoxide emissions and minimum energy efficiency.

The strict limits imposed by the directive on NOx and dust emissions mean that for diesel generator sets, some form of after- treatment technology must be implemented.

The typical after-treatment solutions available on the market for this type of application are selective catalytic reduction systems, diesel particulate filters and diesel oxidation catalysts. Selective catalytic reduction is perhaps the most important in the context of the MCPD as the NOx levels presented in the directive are significantly below what is currently achieved by diesel engines used for power generation.

In order to meet the SO2 requirements, low sulphur or ultra low sulphur diesel must be used.

It should also be noted that both new and existing medium combustion plants, as defined in the directive, are impacted. Although existing plants will have more time to adapt, eventually all gensets will have to comply with some or all of the requirements. 

This article is based on Cummins Power Generation White Paper, EMERPT-6194-EN, Medium Combustion Plant Directive, by Pedro Ponte.