Power You Can Trust

Paper Overview

This paper looks at the impact of the Industrial Emissions Directive and Medium Combustion Plant Directive on Data Centre emissions permitting.


Air quality in the UK is a massively important issue, with pollutants such as oxides of nitrogen (NOx), known to be injurious to health are problem particularly in urban areas. Data centres, even those located within urban areas, make a negligible contribution to overall levels of pollutants like NOx, but certainly have the potential too. This potential is directly linked to the significant amount of standby diesel generating capacity required should there be a failure of the grid / DNO power supply. As this plant is installed for emergency / standby use only any impact from running the sets would only have a relative short-term impact on local air quality.

The Industrial Emissions Directive 2010 (IED) is an EU directive that aims to control pollution from a wide range of industrial emissions of air pollutants, discharges of wastewater and generation of waste which includes emissions from standby generators, but it is poorly designed for standby generators particularly when they are installed in large data centres. As a result, it is of itself disproportionately burdensome for operators to comply with particularly when combined with the requirements of other legislation such as the Medium Combustion Plant Directive (MCPD), The Clean Air Act other regional or local requirements e.g. ULEZ etc.

The Environment Agency (and other devolved organisations such as National Resources Wales) is rightly insisting that operators meet the requirements of these directives particularly those relating to NOx emissions which can be the costliest to implement but not always as effective as believed. Neither the Data Centre operators or Generator dealers / installers were not involved in the original consultation process and hence from an operational point of view these requirements have never been the subject to proper due diligence or the rigors of a thorough cost benefit analysis process, have not been formally discussed or consulted on with either the supply chain or the data centre industry, so there is no real clarity as to if these measures are working effectively or are appropriate for the sector. Operators do not appear to be arguing against the need to mitigate air quality impacts, nor against the additional costs that procuring compliant plant will impose in future, but against the lack of any formal communication, clarity of objectives, consistence in how rules are seemingly being applied and the lack of any formal notice all of which make it very difficult for UK operators to meet their formal obligations.

Executive Summary

Emissions controls and permitting of Data Centres in England is managed by the Environment Agency (EA), in Wales it is managed by National Resources Wales (NRW), in Scotland Environment Scotland and in Northern Ireland it is the responsibility of the Northern Ireland Climate Commissioner. Both of the legislative documents (IED and MCPD) discussed in this paper are “directives” and as such are open to interpretation as to how they are implemented from jurisdiction to jurisdiction and even within a jurisdiction.

Both the IED and MCPD were developed to achieve an overall reduction in levels of emissions (predominantly Nitrous Oxides or NOx from) power generation plant that is embedded within the countries’ infrastructure. Whilst this approach is hugely important and to be commended is often the case that there are some unintended consequences that flow from it. Some of those unintended consequences are linked to an inconsistent approach to either the advice given, or requirements imposed, a belts and braces” can’t get this wrong” approach to the problem or a type of installation which was never envisaged at the time of drafting becomes a challenge or misfit in the permitting system. It is the later that seems to apply in part to hyperscale data centres; they are some how a misfit in the system.

By way of some simple examples (details covered later in the text) give an indication to the nature of the problem.

  • Early recommendation of the adoption of generators able to meet TA-Luft2g as a bench make for meeting the emissions target. Engine manufacturers had for many years been focused on reducing fuel consumption. Meeting the TA-Luft2g meant burning more fuel to reduce NOx.
  • Producing engines that are fuel optimised (FO engines), rather than NOx optimised actually are better suited to the fitment of a Selective Catalytic Reduction (SCR) system. If you have to fit an SCR anyway you might as well reduce the amount of fuel you burn too
  • Many large data centres are now fitting SCRs’ as standard whilst at the same time being pushed to reduce overall fuel consumption and or hours run at full load by the generators for test purposes. Running a generating set at no load means that the engine and SCR will never achieve the operating temperatures required for the catalytic process to take place which enable the NOx to be removed from the exhaust by the SCR. Thus, negating SCR operation all togetherWe explore these and other point sin more depth throughout the balance of this paper.

What is a Data Centre?

A data centres is highly resilient facility which forms part of a global network of such facilities. data centres are places which underpin our modern digital economy by processing, storing and transacting digital data and when combined with communications networks, form our core digital infrastructure. As well as underpinning all “online” activity, data centres enable the day-to-day use of social media platforms, the general public, retailers and banks to process financial transactions, supermarkets to resupply, delivery companies to manage logistics, and public authorities to deliver services and messaging. Some sites are officially deemed Critical National Infrastructure (CNI) so that they reflect the nature of the type of activity being managed within.

Why are standby generators needed?

Data centres are large users of Electrical power, so the primary reason standby generators are included within a data centre is to provide back-up power in the event of a prolonged power utility failure. This utility failure can be anything from just a few seconds to hours or possibly days. Some sites do have a secondary mains supply, but diesel generators are the “go to” solution because they are able to provide the largest amount of power in the shortest time, and diesel fuel can be safely stored on site.

Emissions from a diesel-powered internal combustion engine are multifaceted and as such, there isn’t any one single solution but several options available which we consider in this piece.

There has, over the last 20 years, been a significant push to reduce levels of CO2 with this global objective framing both international and local legislation.

Whilst reducing CO2 remains an ongoing challenge other elements present in the exhaust line have been of concern for many years and this too has been driving the industry to respond with innovations and targeted responses.

The Diesel engine and generating set manufacturers have worked to develop engines that comply with new regulations and as a result, there have been significant advances made in the reduction of Particulate Matter (PM) Hydrocarbon (HC/soot) etc these the adoption of common rail fuel induction systems and manufacturing engines to much tighter tolerances which significantly reduce heavier lubrication oil seeping past the piston rings. With older installations and engines of an older design, these issues will continue to be a problem until they are replaced.

Which regulations cause the problem?

The Industrial Emissions Directive (IED) speaks directly to installations having a thermal input of 50MWth and above and includes standby plant. There are already a good number of “hyperscale” data centres in the UK which have sufficient on-site standby generating capacity such that they require IED permits. The current trend towards fewer, larger facilities, more operators are likely to find themselves needing to comply with this regulation. Awareness is growing but still patchy, mainly because IED wasn’t widely consulted over in the data centre world, it seems to be being implemented differently in different EU nation states, (and from jurisdiction to jurisdiction within the UK) the legislation has been poorly communicated to interested parties and the government guidance that is available is extremely difficult to interpret.

What is IED all about?

IED is one of a number of regulatory requirements targeted at reducing/ controlling pollution. IED is an EU Directive and as such is implemented through domestic legislation in the UK; in this case the Environmental Permitting Regulations or EPR. Its requirements are wide ranging and aim to set controls on all forms of emissions to air, water and ground. It is air quality element which impacts data centre operations. Standby diesel generator exhaust gases are complex and include CO, CO2, oxides of nitrogen, sulphur, soot and particulate matter. Oxides of Nitrogen (NOx) are of particular concern as they are proven to be injurious to human health and levels in the UK are higher than they should be.

Data centres with installed diesel generating capacity are potential point sources of NOx emissions. The generators themselves are rarely run in anger but they have to be tested periodically emitting NOx as they do so. In the event of a grid outage, they may have to run continuously for a prolonged period. While emergency running due to a grid outage is exceptionally rare, the generators have the “potential” to emit greater levels of NOx into their local environment in this operation mode. It must be stated that whilst the overall contribution of data centres to UK NOx levels is very small, generators installed in a data centre could have short term impacts at local level. This is particularly true if they are located in more urban areas where local NOx emissions are approaching, or regularly breaching, legal thresholds and/or where there are areas of particular sensitivity such as busy pedestrian areas, schools, hospitals, care homes. We should also not overlook people in the general population suffering with respiratory problems. In some areas for example Slough or Hemel Hempstead (there are others) data centres are also clustered and there is concern that in the event of a major power outage multiple facilities could be utilising their emergency generators at the same time. In order to reduce NOx emissions diesel generating sets are fitted with Selective Catalytic Reduction (SCR) equipment.

Why is IED difficult?

Data centres and in particular the “hyperscale” data centres all have a large amount of standby capacity which is used infrequently. The low level of usage presents a number of challenges for operators that require IED permitting. To demonstrate all systems are working correctly the generating set are run regularly for testing and maintenance. This testing, which is often for relatively short periods (30 minutes), can often be undertaken with no or very low levels of load. This well-established practise has traditionally been done to keep fuel consumption and hence CO2 emissions to a minimum. (See WB white Paper on “wet stacking”) Running at low levels of load means that the SCR’s fitted to reduce NOx never reach their optimal operating temperature and hence do not function.

For many years government policy was to strive for ever (and still are) lower emissions of CO2 which takes you in the direction of higher efficiency / low fuel consumption engines. IED then poses a challenge for generator manufacturers and data centre operators because it represents, in part, a change in direction of government policy. For many years manufacturers have been focused on improving fuel efficiency to reduce carbon impacts associated with scope 1 emissions from plant. This new policy now requires manufacturers to supply generators on the basis of their air quality performance and the regulator is seeking to impose a 2g limit (see below) for NOx, a major pollutant. This is something of a “cake and eat it” moment where there needs to be a trade-off between fuel efficiency, and lower CO2 emissions and air quality, in this case NOx. Simplistically, you cannot have your cake and eat it. Generator manufacturers and data centre operators alike accept that the policy priorities have changed and need to change, which is to be compromised more CO2 through on load testing or less NOx.

It is important to note that the Environment Agency has taken something of a pragmatic approach when permitting older installation which are only part way through their life cycle with plant yet not requiring replacement.

What is the Medium Combustion Plant Directive?

From a regulator basis, a data centre operator can confront several different licencing, permitting and regulatory requirements, varying from legal jurisdiction, location to location within a given jurisdiction and size / rating of generator installation etc. Much is covered by the Medium Combustion Plant Directive (MCPD) but other requirements will be locally derived and often dependant on other factors. Due to the relative complexity and local variations available, this article does not seek to cover all aspects just to highlight some key considerations. Some of these are:

  • Total capacity of installed generating sets
  • Thermal input of generating sets (below 3MWth can bring benefits)
  • NOx emissions
  • If generators are used for other purposes other than standby i.e. STOR of Grid demand response

In relation to NOx emissions specifically, it is important to study the information provided by each individual manufacturer. The results provided (as with all of the other elements which are present in the gas stream) are measured under specific conditions, including engine load and operating temperature, ambient temperature and distance from engine gas flow. All these parameters will vary when at site, hence it is important that each installation is treated and assessed on its own relative merits. The MCPD indicates a requirement to achieve a NOX level better than 190mm/m3 at 100% load and after a max of 20 mins operation.

It is also important to ensure that the units of measurement and remedy are the same. NOx levels are usually given in mg/Nm3 @ 5% O2 at 100% PRP. The established method of NOx reduction is by the introduction of a Selective Catalytic Reduction unit (SCR) into the exhaust gas flow. NOx reduction/ elimination is achieved by injecting ammonia into the gas flow prior to it passing over the catalyst. The use of a precise closed loop ammonia injection system ensures that there is no discharge of unused ammonia. It is important that the exhaust gases (350 – 450C) are at a high enough temperature to ensure the catalysation process can occur; this is achieved by good design and a generating set working at load. As the NOx produced in the engine cylinders directly linked to engine pressure and temperature it should be noted that as the exhaust gases expand and cool then the NOx levels will fall on their own. The longer the exhaust flue the lower the level of NOx is present at the point of discharge.

See WB White Paper standby Generators in a Data Cantre Environment. Also see WB White Paper “green standby power generation ”.


What is this TA-Luft2g ?

The introduction of the Medium Combustion Plant Directive (MCPD) in 2019 set a threshold for new generating sets of 2000mgN of NOx per M3 at 5% Oxygen (O2). In practice this defines the maximum allowable concentration of NOx in the exhaust for a given oxygen level. This can be calibrated for other oxygen concentrations by a series of calculations. The Environment Agency has also previously stated (and this was picked up in many consultant specifications) that the applicable standards for a data centre is 2gTA-Luft or Tier II USEPA. TA_Luft2g sets requirements of 2000mg/m3N for NOx; 650 mg/m3 for CO; particulates and dust 130 mg/m3 and 150 mg/m3 for hydrocarbons (all at reference conditions and 5% O2).

Is TA-Luft2g relevant?

The simple answer is NO!

TA-Luft2g isn’t actually a standard but more of a contractual specification. It was introduced in Germany to address a specific pollution problem mostly from legacy plant. It arrived into the UK back in the late 1990’s driven by a German manufacturer of large capacity generating set engines keen to set the emission performance credentials of his product above others in the market place. Many engine and generating set manufacturers have continued to develop products with reduced emissions aimed at meeting advancing directives and legislation leaving TA-Luft2g behind long ago.

Operators with legacy 2gTA Luft compliant plant may not be materially different from other “!non-compliant” plant in the typical data centre operation, where generators are usually only run for testing and maintenance purposes. Such plant should be tested for current performance by a specialist and exhaust after treatment fitted if necessary.

How can emissions be further reduced?

Organisations that are looking to reduce emissions further are often turning to Hydrotreated Vegetable Oil (HVO). HVO is a paraffinic drop-in diesel fuel substitute that provides a cleaner alternative to power standby diesel generators, as well as other diesel-powered machinery. Conforming to EN15940 standards, HVO is manufactured from many kinds of vegetable oils, such as rapeseed, sunflower, soybean, and palm oil. Some or all of these sources can be from waste products such as used frying oils or animal fats hence can be made from entirely renewable energy source that do not impact crop resources. The adoption of HVO (EN 15940) could translate into a widely claimed maximum 90% reduction in CO2 emissions over its entire lifecycle (1). There is currently no conclusive evidence that the use of HVO has any significant impact on the levels of NOx produced by the generating set. See WB White Paper “Greening standby Power”

Some designers and operators have looked to gas as an alternative as it is cleaner in emissions and particulates than diesel.  However, on-site storage is a space and safety problem, with having a local source of sufficient capacity and reliability another.

Gas, having a lower calorific value than diesel, means the engine swept volume must increase in order to deliver the same power, meaning higher initial capital costs.  Starting a gas engine can take much longer and the set is much slower at accepting site loads. Those issues, combined with higher maintenance costs, have ensured gas generation is very rarely adopted.

Many of the larger manufacturers are currently working on a hydrogen fuel solution, running test programmes to assess long term reliability issues. This option looks to offer a promising future but there is some way to go with work still to be done in order to provide a truly “green” source of hydrogen fuel, workable local safe storage and distribution methods. In the long term the data centre sector (and others) will need to transition away from diesel plant, but at the moment these potential alternative solutions are still at trial stage. In the short to medium term diesel generators remain are the default solution for ensuring resilience and business continuity. Mitigating air quality impacts will continue to be a priority for the sector.

Reducing the time that generators run for testing and maintenance purposes will also add to the reduction of emissions and consequently permitting arrangements to. See WB white paper on Wet Stacking” and the impact on Diesel Generator Maintenance

Data centre operators along with the supply chain take meeting their responsibilities to minimise air quality impacts seriously. The need to minimise NOx emissions is clear, meeting the obligations is being committed to with organisations investing in appropriate solutions. With growth continuing in the sector more sites will need to comply with IED requirements in future. The IED legislation as currently enacted is not wholly appropriate for the Data Centre market as it imposes a regulatory burden disproportionate to the outcomes achieved. This isn’t particularly a failure of the legislation itself but more that the regulations haven’t kept up to date with market developments and particularly the rapid growth in large and hyperscale data centres. The legislation therefore urgently requires a more bespoke approach to minimise and mitigate data centre air quality impacts. There is no need to compromise on either the requirements or ambition, but it is essential that the requirements are proportionate and above all deliver meaningful outcomes.

Development schedules need to meet challenging deadlines and long order items like generators are often procured years in advance if project completion deadlines are to be met. It is essential then that these requirements are communicated clearly to operators, developers, designers, construction teams and the supply chain and with sufficient time to ensure that all are aware and hence able to comply. This is currently not happening on a consistent enough basis.

What is Needed?

The sector needs a collective voice on how best we can move forward to achieve a common objective within a defined and clear timetable. It is the authors view that:-

  • All stakeholders need to work together to develop a bespoke Industry Code of Practice for Generating Plant
  • The code of practice needs t be presented to the appropriate government body for approval and formally announced to operators and the supply chain through government websites and industry associations.
  • A clear timetable for compliance needs to be agreed with operators and supply chain
  • Government to confirm full details of how site testing and monitoring is to be implemented and run

G R Halliday

Business Consultant

WB Power Services Ltd



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