Power You Can Trust


This opinion piece seeks to focus on how “greening” or reducing emissions of the standby power sector might be achieved by giving thought to both existing and new infrastructure yet to be built. The author considers some of the options currently available to reduce elements such as Carbon Dioxide (CO2) with the use of Hydrotreated Vegetable Oil (HVO) fuels, the use of various catalysts to reduce some of the other pollutants such as Particulate Matter (PM) and Nitrous Oxides (NOx). In addition, the author also looks at alternative technologies and the impact of manufacturing and sourcing “fuels” for the future.

If a business is to be successful in reducing emissions generally from existing or new infrastructure it requires a cohesive strategy which has equal importance with all other business imperatives.

Key words: Standby Power, Reducing Emissions, CO”, HVO, Nitrous Oxide

1.0  Introduction

In this opinion piece, the author focuses on how “greening” or reducing emissions of the standby power sector might be achieved by giving thought to both existing and new infrastructure yet to be built. The author considers some of the options currently available to reduce elements such as

  • Carbon Dioxide (CO2) with the use of Hydrotreated Vegetable Oil (HVO) fuels
  • The use of various catalysts to reduce some of the other pollutants such as Particulate Matter (PM) and Nitrous Oxides (NOx)
  • So alternative technologies
  • The impact of manufacturing and sourcing “fuels” for the future.

In reviewing these aspects, the author discusses

  • How existing diesel generating standby plant can be adapted to reduce emissions
  • What adaptations can be made on new projects to ensure emissions are minimised
  • Sourcing alternative and future fuels and how their production will impact emissions

2.0 Reducing emissions for new and existing infrastructure

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.

Any emissions reduction strategy should give due consideration to all of the pollutants created in the combustion process. As well improving in cylinder combustion and overall engine efficiencies of the current crop of engines emissions remain a problem and so exhaust gas after treatment systems are available to be used to reduce other emissions such as NOx, PM, and HC many of which can be retrofitted to existing installations and should be considered for inclusion in new installations, these include:-

  • Oxidation catalysts
  • Diesel Particulate filters
  • Selective Catalytic Reduction

Diesel engines produce a variety of particles during the combustion of the fuel/air mix due to incomplete combustion. The composition of the particles varies widely dependent on engine type, age, and the emissions specification that the engine was designed to meet. Two stroke diesel engines produce more particulate per unit of power than four stroke diesel engines, as they burn the fuel-air mix less completely.

2.1 Social implications

Air pollution causes an estimated 29,000 early deaths in the UK and has annual health costs of £15 billion (1). The health effects of PM are more significant than those of other air pollutants with chronic exposure contributing to the risk of developing cardiovascular diseases and lung cancer with current evidence suggests that there is no ‘safe’ limit for exposure to fine particulate matter with the Report of the Committee on the Medical Effects of Air Pollutants (COMEAP) from 2008 (1). Although there had been improvements in pollutant levels, the average reduction in life expectancy as a result of airborne particulate matter across the population was 6 months. DEFRA Dec 2013) (2)

2.2 Hydrotreated Vegetable Oil (HVO)

The last four or five years have seen a significant increase in the availability and use of Hydrotreated Vegetable Oil (HVO) as a fuel in the standby generation market, particularly in the Data Centre (DC) sector. There are a number of factors which have brought about this change.

  • The push for reductions in the level of CO2 reduction globally and at a European level with the introduction of targets and legislation such as the “European Green Deal” (3) which includes “Fit for 55” has led the way. In the UK we have the “Climate Change Act” (4) and the UK budget of “April 2009” (5) which made achieving the targets a legal requirement.
  • The ramping up of production of HVO within a European context hence improving the availability of the product
  • The large volume of engine manufacturers approving the use of this product across an increasing number of their engine ranges. Most of the major manufacturers have a blanket approval
  • As further testing has been completed and field experience gained many manufacturers are opening this to engines on more recently installed generating sets too and where necessary identifying what if any checks need to be undertaken before HVO’s used.
  • The importance of ensuring securing supply
  • Widespread acceptance amongst HVO producers and engine manufacturers that HVO can satisfactorily be mixed with existing supplies of diesel fuel.

2.3 Catalysts and Filters

Products are available that facilitate the removal of CO and HC elements, and these are items such as Diesel Oxidation Catalysts (DOC) and Wall Flow Diesel Particulate Filters (DPF) which when working together significantly* reduce levels of CO/HC and assists with passive regeneration of the DPFs themselves.

 2.3.1 Diesel Oxidation Catalysts (DOC)

Diesel Oxidation Catalysts (DOC are catalytic converters designed specifically for diesel engines and equipment to reduce the levels of Carbon Monoxide (CO), Hydrocarbons (HC) and Particulate Matter (PM) in the exhaust gases emitted. DOCs are simple, inexpensive, maintenance-free and suitable for all types and applications of diesel engines.

Modern catalytic converters consist of a monolith honeycomb substrate typically coated with platinum, palladium and rhodium metal catalysts, packaged in a stainless-steel container. The honeycomb structure with many small parallel channels presents a high catalytic contact area to exhaust gasses. As the hot gases contact the catalyst, several exhaust pollutants are converted into harmless substances: carbon dioxide and water.

The diesel oxidation catalyst is designed to oxidise carbon monoxide, gas phase hydrocarbons, and the SOF fraction of diesel particulate matter to CO2 and H2O:

Diesel exhaust gases contain sufficient amounts of oxygen, necessary for the above reactions to take place. The concentration of O2 in the exhaust gases from diesel engines varies between 3 and 17%, depending on the engine load (load creates heat in the engine). Typical conversion efficiencies for CO and HC in the Nett® diesel oxidation catalyst are shown in Figure 2. The catalyst activity increases with temperature. A minimum exhaust temperature of about 200°C is necessary for the catalyst to “light off”. At elevated temperatures, conversions depend on the catalyst size and design and can be as high as 90%.

Dry particulate filter systems are capable of reducing particulate matter by up to 99%. Effective from start-up, the textured weave of the special fibre retains carbon particles as small as 20nm. Non-regenerative type filters can be fitted to new installations and as an addition to the existing exhaust system. At the end of the filter life, the cartridge is simply removed and replaced with a new one. The filter needs to be certified for the London Non-Road Mobile Machinery requirements (NRMM) (6) Ultra low emissions zone.


  • Over 99% Reduction in particulate matter (PM)
  • Operates from engine start-up
  • Suitable for standby generator sets, low-usage forklift trucks, side-loaders, excavators etc.
  • Quick cartridge replacement

 2.3.2   Particulate Matter and Particulate Filters

Particulate Matter (PM) is made up of a complex mixture of solid and liquid particles, including carbon, complex organic chemicals, sulphate, nitrates, ammonium, sodium chloride, mineral dust, water and a series of metals, which is suspended in the air. PM10 refers to particles with a diameter smaller than 10μm and PM2.5 to particles with a diameter smaller than 2.5μm. They may be produced directly from a source such as an engine – or formed from reactions between other pollutants (eg NO₂, SO₂, NH₃) in the air (secondary PM).

PM comprises nitrates, sulphates and ammonium which are the main drivers for acidification and eutrophication – 2 extremely damaging processes to natural ecosystems, which can cause habitat loss and affect biodiversity. The PM also contains black carbon, a known contributor to global warming. DPFs are essential in reducing current PM emissions to prevent these processes from occurring.

There are  several different types of DPF’s but the most popular is the Wall-Flow DPF’s which can capture up to 99% of all Diesel Particulate Matter (PM10/PM2.5) which can then be regenerated (combusted) passively on generator duty cycles where operating temperatures of the catalysts are above 300C. Operating temperatures of at least 300C are required for approximately 25% of the operational time to combust the soot passively with at least 10% of the time above 400C to prevent the DPF blocking with soot.

Once combusted the soot will turn to ash which will need to be removed periodically by removing the filter and blow cleaning with a low-pressure airline.

A diesel particulate filter (DPF) is a device fitted to a diesel vehicle which filters particulate matter (PM) from exhaust gases. It does this by trapping solid particles while letting gaseous components escape. This type of filter has been in use for over 20 years, and many variants exist. These filters enable reductions in emissions which help meet European emission standards, improving air quality and thereby health standards.

2.4 Regeneration of DPFs

DPFs need to be emptied of trapped particulate matter regularly. This is done by a process called regeneration, which involves burning the soot to gas at a very high temperature, leaving behind only a very small residue. Regeneration, If not carried out properly, can lead to a build-up of soot which can affect performance and ultimately lead to expensive repair costs. This has led to some diesel vehicles…………….

It is worth noting that whilst we continue to use the internal combustion engine, we will have issues with NOx.

2.5 Medium Combustion Plant Directive (MCPD) – Reduction of Nitrous Oxide

The MCPD is EU legislation that became UK law in December 2017 (Directive (EU) 2015/2193) (7) and covers diesel engines with ratings between 1 and 50MWth requiring them to meet a level at or below 190mg of NOx /Nm3. These “medium sized engines” represent an important source of the emissions of Nitrous Oxide (NOx) and dust (Particulate Matter). The directive seeks to regulate their emissions in order  to reduce the production of these substances which are known to be harmful to human health.

 2.5.1 Selective Catalytic Reduction (SCR)

Selective catalytic reduction (SCR) systems are active post-combustion NOx emission control systems. These units work in a similar way to how such a catalytic converter works to reduce NOx emissions in a car. Catalytic converters circulate unoxidised portions of the exhaust stream in an oxidising environment to break down additional hydrocarbons and react with NOx. SCRs are different in so far as they incorporate a gaseous or liquid reductant, generally ammonia or urea in this case, to the exhaust gas stream. The exhaust gas mixes with the reductant and travels through several catalytic layers, where a reaction between NOx emissions and the injected reductant occurs. The reaction converts the NOx emissions into N2 and water vapour. The benign elements are then released into the air through an exhaust /flue arrangement specifically designed for the application.

One common problem with selective catalytic reduction systems (SCR) or many other catalytic devices is that they operate effectively within a relatively narrow temperature band. In the case of standby generating plant specifically it means that the set(s) must be working at relatively high levels of load for at least ten minutes (and consistently thereafter) in order for the optimum operating temperatures to be achieved that will allow the catalytic reaction to occur. A closed loop management system is essential to ensure there is no ammonia slip produced in the exhaust discharge.

2.6 Current and potential future standby power options

In this section, we consider current and developing solutions that can be built into new infrastructure currently in the early stages of development. We also consider the limitations as we currently know them to be. These products include

  • Battery storage systems such as Lithium–Ion. These are available today and being trialled on the grid, data centres and other large-scale projects.
  • Other more compact and efficient battery / chemical storage types are under development
  • Mechanical Energy storage systems such as pumped water storage have been around for a long time. Others such as compressed air storage and flywheels are also under development
  • Fuel Cells have been around for many years and continue to see performance improvements. They have a lower footprint compared to batteries, and the possibility of quick refuelling with more readily available pressurised or liquid H2 opens up the possibility of wider acceptance. These factors mean fuel cells could be adapted for backup applications and more extended storage periods.
  • A new range of standby generator engines that can run on hydrogen rather than diesel or HVO alternatives. Whilst this technology is available today on smaller engines it will be some time maybe ten years or more before medium and larger engines are available for applications such as hospitals and data centres

2.7 Sourcing current and future fuels

HVO and hydrogen are  current and potential future fuels to power standby generation as we know it to be today. This section of the disposition looks at the care that needs to be taken in the selection and sourcing of these alternative fuels.

2.7.1 HVO

HVO is a liquid fuel that is a paraffinic bio-based liquid fuel produced using a special hydrotreatment process. It is manufactured from many kinds of vegetable oils, such as rapeseed, sunflower, soybean, and palm oil (7). Some or all of these sources can be from waste products such as used frying oils or animal fats and hence can be made from entirely renewable energy sources that do not impact crop resources.

Unlike first-generation biodiesels such as EN590 B7 which we currently use in road vehicles and standby generation contains just 7% of bio content. The adoption of HVO (EN 15940) could translate into a widely claimed maximum 90% reduction in CO2 emissions over its entire lifecycle. It has not been possible to find a reliable source to back up this claim and how it might be realised. Some HVO however is manufactured either as an entirely “new” product or a mix of products. Such products use newly harvested oils such as sunflower, palm oils etc which remove these from the food chain and in the case of palm oil production is known to be responsible for widespread deforestation. Care should be taken when selecting a source of supply to ensure that the up to 90% (claimed by some suppliers 8 and 9) CO2 lifecycle reductions are maximised.

I have only been able to locate one manufacturer of HVO in the UK. This source is using totally recycled products. It must there be assumed that all other supplies are manufactured outside the UK.

Other points to note are that

  • HVO has a slightly higher energy density than fossil diesel by weight but, it has a lower energy density by volume. Recent tests results undertaken by an engine manufacturer indicate that engine fuel consumption by volume is higher by 4% in l/hr (latest Kohler test data)

HVOs (or HWCOs) are straight chain paraffinic hydrocarbons with the chemical structure CnH2n+2, free of sulphur and aromatics (Aatola et al., 2008) (10). As HVO contains no oxygen, the oxidation stability is higher compared to market diesel, resulting in very good storage behaviour; its much higher Cetane Number (CN) (EN15940 Class A = 70), when compared with EN590 B7 diesel, translates into a much cleaner burning fuel (10).

In relation to NOx, there are a number of conflicting effects which influence the formation of NOx. The absence of oxygen and aromatics in HVO generally prevents the formation of NOx. Aromatic compounds typically have a higher adiabatic flame temperature which leads to a higher local combustion temperature in the cylinder. (Glaude, 2010) (10). The very high cetane number of HVO may promote NOx formation, as it leads to a decrease in ignition delay, meaning that the start of combustion is earlier (well before the top dead centre), which results in earlier pressure and temperature rise which of itself could be overcome by changed engine mapping. As a result, no clear conclusion concerning the use of HVO has on NOx emissions can be drawn, as mixed effects have been observed. (10)

2.7 Hydrogen (H2)

Several combustion engine manufacturers are working on projects related to either modifying existing engines or developing new engines to run on hydrogen. These are likely to be “smaller” engines up to 10l in capacity (diesel equivalent) for the mass market ie trucks and buses. These will then be adapted for the standby generator market. Some early modified engines are currently coming to the market but there is a long way to go before these are likely to be widely adopted and even longer before mid-range (10-25l diesel equivalent) and large engines (30-100l+ diesel equivalent) are available. Time scales are likely to be in the range of 10 to 15 years. Regardless of the fuel used, NOx will always be present in the exhaust of an internal combustion engine. As with the production of HVO consideration needs to be given to the environmental impact of production and the operational constraints such as

  • Availability of hydrogen
  • Storage of hydrogen given that it would currently require over 50 tonnes of hydrogen to support an all-electric major teaching hospital for 48 hours or 100 tonnes

Fuel cells or hydrogen fuel combustion engines can only really be considered ‘green’ if the hydrogen used to power them comes from sustainable sources such as renewables, and nuclear. Research suggests that using energy generated by wind turbines to make hydrogen by the hydrolysis method is the “greenest” means of production. At the time there is a surplus of wind generation which could be readily redirected to this purpose.

Currently,  around 95 per cent of hydrogen comes from natural gas and making 1kg of this ‘grey’ H2 emits 11 tons of CO2. ‘Blue’ hydrogen is produced the same way, but the CO2 is captured and stored. Suitable storage sites are currently few and far between, so availability is limited. The long-term solution comes with “green” H2 made from renewable energy such as wind, solar or nuclear. It should be recognised that this is many years away from being practically available at scale. Any H2 produced is hard to store in bulk without significant investment in associated infrastructure.

Efficient methods of hydrogen storage and distribution are key to enabling the development of technology for the advancement of hydrogen and fuel cell technologies in applications which include transport, stationary and portable power. Hydrogen has the highest energy per mass of any fuel; however, its low ambient temperature density results in a low energy per unit volume, which necessitates the development of advanced storage methods that have the potential for providing a much higher energy density. The US  Department of Energy (11) identify four specific methods of hydrogen storage and these are:-

  • Compressed Gas
  • Cold / Cryo Compressed Gas
  • Liquid H2
  • Materials based solutions – five options in number
  • Reduced running of generating sets for regular maintenance and testing

All such methods require the use of additional energy for example maintaining the necessary very low temperature required to keep hydrogen in a liquid form.

The UK Government has set out a plan to include the large-scale production and wide scale distribution of both “blue” (with carbon capture) and “green” hydrogen as a way by which the country’s climate change objectives can be reached. Recommendations for hydrogen production have been set out in a recently published report to The House of Commons (12)

3.0     Conclusions

It is clear that good progress has and is being made in reducing some of the key pollutants created in the burning of diesel fuel in an internal combustion engine, such as HCs and PMs but there is still a way to go. The two “unavoidable consequences” of this method of combustion are CO2 and NOx which will continue to be persistent problems whilst we continue to use conventional hydrocarbon diesel burnt in an internal combustion engine.

The use of HVO when combined with the addition of an SCR can bring a packaged standby generator solution to the point of operation with a potential 90% reduction in CO2 emissions and NOx levels well inside the requirements of the MCPD for NOx and other key emissions requirements. This applies to both existing and installations still in the design stages. Care must be exercised

  • When modifying the exhaust system of generating sets with older engines (typically older than 8 to 10 years depending on model and manufacturer). This should be done in conjunction with both the manufacturer and emissions experts
  • When modifying the exhaust system of generating sets more recently installed (typically less than 5 years depending on model and manufacturer). This should be done in conjunction with both the manufacturer and emissions experts
  • When designing and planning new installations to ensure the latest developments in engine design, fuels and emissions treatments are used
  • In the selection of a supplier for HVO to ensure that as close to the 90% “claimed lifetime reduction in CO2 emissions” are achieved with the use of sustainable sources derived from waste and residue oils not from “virgin” oils.

The use of hydrogen fuel in an internal combustion engine as an alternative to BS590 B7 diesel or HVO for large standby power installations is still some years away. Significant progress needs to be made on the production, storage and distribution of both “blue” and “green” hydrogen before this fuel becomes a possible alternative to HVO, especially for large scale installations.

Fuel Cells have been with us for a number of years but their use in larger scale standby applications is still minimal due to the limited capacity of the devices and the availability of hydrogen. As they differ from the internal combustion engine as neither CO2 nor NOx is emitted during the conversion of hydrogen to electrical power.

Large capacity batteries, mostly Lithium-Ion, are currently being deployed as grid balancing devices (mopping up surplus wind power capacity for example) and also on some early standby applications. Physical size is a major limitation and in the years ahead as Lithium is used more widely across many different applications such as cars it could be a limiting factor in their adoption.



  • Committee on the Medical Effects of Air Pollutants. PHE publication gateway reference: 2018537 published 22 October 2018. http://gov.uk/government/publications/air-pollution-and-cardiovascular-disease-mechanistic-evidence [Accessed 13th December 2022]
  • Department for Environment, Food and Rural Affairs. Air Pollution in the UK 2013 published Sept 2014. http:// uk-air.defra.gov.uk/ assets/documents/reports/cat05/1409261329_air_pollution_uk_2013_issue_1.pdf  [Accessed 13th December 2022]
  • European Parliament and of the Council. Establishing the framework for achieving climate neutrality and amending Regulations (EC) No 401/2009 and (EU) 2018/1999 (‘European Climate Law’). https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32021R1119 [Accessed 13th December 2022]
  • UK Government. Climate Change Act of 2008. https://www.legislation.gov.uk/ ukpga/2008/27/contents [Accessed 13th December 2022]
  • UK Government. Building Britain’s Future / Budget of “April 2009” section 1.31 Building a low carbon economy https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/250681/0407.pdf [Accessed 13th December 2022]
  • Mayor of London. Non-Road Mobile Machinery (NRMM) Practical Guide v5. https://www.london.gov.uk/ programmes-and-strategies/environment-and-climate-change/pollution-and-air-quality/nrmm. [Accessed 12th December 2022]
  • European Commission. Directive (EU) 2015/2193 of the European Parliament and of the Council 25th Nov 2015. https://ec.europa.eu/ industry/stationary/mcp.htm#:~:text=Directive%20%28EU%29%202015%2F2193%20on%20the%20limitation%20of%20emissions,Megawatt%20thermal%20%28MWth%29%20and%20less%20than%2050%20MWth. [Accessed 12th December 2022]
  • Speed Fuels. 90% reduction in CO2 using HVO fuel. https://www.speedyfuels.co.uk/ products/hvo-fuel/?utm_keyword=hvo&msclkid=4b11a02266661092fb46c39647753696&utm_source=bing&utm_medium=cpc&utm_campaign=*Search%20-%20HVO&utm_term=hvo&utm_content=*HVO [Accessed 30th December 2022]
  • Crown Oil. 90% reduction in CO2 using HVO fuel https://www.crownoil.co.uk/  hvo-fuel-hydrotreated-vegetable-oil/. [Accessed 30th December 2022]
  • Frontiers in Mechanical Engineers.Evaluation of a Hydrotreated Vegetable Oil (HVO) and Effects on Emissions of a Passenger Car Diesel Engine. https://www.frontiersin.org/ articles/10.3389/fmech.2018.00007/full [Accessed 14th December 2022]
  • US Department of Energy – Hydrogen and Fuel Cell Technologies office. Hydrogen Storage. https://www.energy.gov/ eere/fuelcells/hydrogen-storage. [Access 14th December 2022]
  • UK Government. The role of Hydrogen in achieving Net Zero. https://publications.parliament.uk/pa/cm5803/cmselect/cmsctech/99/report.htm. [Accessed 14th December 2022]l


The author acknowledges the support and information provided by

  • Industrial and Marine Silencers Ltd
  • Kohler Generators (France)