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, initially in the Data Centre (DC) sector but now more widely in the construction sector on mobile plant and some of the UK’s new low emissions zones, such as in London where the London Non-Road Mobile Machinery requirements (NRMM) (1) Ultra low emissions zone.
There are several factors which have brought about this change.
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 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 in the case of palm oil production are 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 most suppliers on their websites) CO2 lifecycle reductions in the production and consumption of HVO are to be maximised. Therefore, care needs to be taken in the selection and sourcing of HVO if the maximum possible CO2 emissions reductions are to be achieved.
In the production process, the raw materials are saturated with hydrogen at high temperatures and pressures removing any esters and oxygen. A positive result of this process is that HVO degrades at a far slower rate than ordinary biodiesel and diesel products generally.
It is worth noting that currently around 95 per cent of hydrogen needed as part of the HVO production process comes from natural gas and making 1kg of this ‘grey’ hydrogen (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.
At the time of writing the author has 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.
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 HVO effect on NOx emissions can be drawn, as mixed effects have been observed. (6)
It is clear that good progress has/is being made in reducing some of the key pollutants created in theburning of diesel fuel in an internal combustion engine, such as HCs and PMs but there is still a way to go. There are however two “unavoidable consequences” of using this method of combustion and they 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