Decarbonizing Mission-Critical Power

­Back-up power plays a central role in the mission-critical systems that support our essential infrastructure such as data centers, smart grids, hospitals, utilities and airports. But we also need to drastically cut carbon dioxide emissions and other greenhouse gases as part of the ongoing fight against climate change.

Balancing these two requirements has traditionally been a difficult task, especially when many backup power systems use diesel generators as an enabling technology.

However, while achieving reliable, low emission back-up power might seem like a long-term ambition, exciting advances in areas such as renewable fuels means simple and affordable solutions are in fact available right now. And crucially, these new fuels can be used with existing generators without modification, meaning that adoption is likely to be rapid.

Engine optimization and after-treatment

Before we take a closer look at the impact of renewable fuels, it is worth taking a step back and re-assessing how diesel generators have become more sustainable over the years. While diesel generators play a crucial role in ensuring that mission-critical applications never go offline, the climate crisis has made diesel generator manufacturers acutely aware of the need to continually improve their products' environmental performance. This recognition has led to rapid advances in emissions reduction technologies that can be grouped into either in-cylinder or after-treatment categories. In-cylinder techniques reduce the pollutants emitted by an engine, and after-treatment further reduces these pollutants by treating the engine's exhaust stream.

Advancements in computer-aided engineering tools and computational fluid dynamics enable improved engine behaviour modelling, allowing optimization of the entire system for fuel consumption, pollutant emissions creation, torque, power, and transient performance. Finer piston and ring assembly tolerances reduce the amount of fuel escaping the combustion chamber, resulting in increased engine burn efficiency, significantly mitigating the conditions that lead to problems such as wet stacking. Common rail fuel injection systems coupled with engine monitoring systems ensure better fuel atomization and enable “fuel mapping,” where the combustion process is better tailored to the requirements of emissions and/or cylinder temperatures.

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Figure 2. Design optimization has increased engine burn efficiency in generators, reducing problems such as wet stacking caused by fuel escaping from the combustion chamber

In-cylinder control technologies can only reduce pollutant formation to a certain point, given the inverse relationship between nitrogen oxides emissions, (NOx) and particulate matter, (PM). NOx forms with high cylinder temperatures and PM forms with low cylinder temperatures; therefore one, or sometimes both, must be treated within the exhaust stream to meet local regulations. After-treatment devices including Diesel Oxidation Catalysts, Diesel Particulate Filters, and Selective Catalytic Reduction have therefore become an integral part of the engine system, reducing emissions further still.

New generation biofuel

But engine optimization and after-treatments for diesel generators can only be taken so far. Other technologies – such as the use of next-generation renewable fuels – are required to further reduce net carbon dioxide emissions. A vital tool for achieving this decarbonization is the use of Hydrotreated Vegetable Oil (HVO) - replacing fossil fuel diesel with HVO in existing generators can reduce greenhouse gas emissions by up to 90%.

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Figure 3. The production process uses hydrogen and results in renewable diesel that can be used in existing engines

So, what is HVO? And how is it produced? HVO comes from waste vegetable oil, used cooking oil, and waste animal fats. Other feedstocks are fish fat from processing waste and inedible technical corn oil generated when ethanol is produced from corn.

After impurities are removed, the raw materials undergo hydrogenation and hydrocracking using hydrogen at high temperatures and pressure. The end product is straight-chained hydrocarbons (paraffin) of consistent quality, which have similar chemical properties to fossil diesel. The process is sufficiently flexible to convert a wide range of low-quality waste and residue materials to hydrocarbon-based drop-in fuels, making HVO a diesel substitute for a broad range of diesel engine applications. In the near-to-medium term, it will also be possible to make HVO from photosynthetic organisms such as algae.

HVO holds exciting potential because it overcomes many problems typically associated with first-generation biofuels. Firstly, it originates from waste products that don’t impact agricultural land use. It also overcomes the challenges of instability and ageing when stored over long periods. While first-generation solutions have a storage life as short as six months, HVO can be stored for up to ten years without any notable degradation. There is also no susceptibility to other factors such as oxidation, water absorption or bacterial growth. 

To help reduce emissions, HVO burns cleaner than first-generation biofuels. It also has a higher cetane number than existing biofuels and fossil diesel, falling in the range of 70-90. Cetane number acts as a measure of the quality or performance of diesel fuel. As a rule of thumb, the higher the number, the better the fuel burns within the engine of a diesel generator – with advantages such as better cold start and better combustion. HVO can be used in temperatures as low as -32ºC, while a minimum flashpoint of 61 degrees C means it is safe to use in warmer parts of the world.

As the benefits of HVO become recognized, its supply base is increasing worldwide. For example, the US is expected to reach an output capacity of six billion gallons of HVO annually by 2024, while global companies, including Shell, are building new facilities in Europe. In China, multiple HVO plants are under construction, as illustrated by a new biorefinery in Rizhao, which produces biofuel from used cooking oil and palm oil mill effluent. 

Overall, using waste feedstocks to produce HVO also reduces the need to ship raw materials worldwide, so production can be closer to the end-user, which means lower carbon emissions from transportation.

The only possible performance downside to HVO is a slight fall in power output due to lower volumetric mass. However, this does not result in less power electrical output for mission-critical generator users. Instead, the fuel consumption will rise by 3-5% to compensate for the difference in volumetric mass. Meanwhile, generator transient response time with HVO is similar to performance with fossil diesel in operation.

HVO-ready generators

Generator manufacturers have made tremendous efforts to ensure that mission-critical diesel generators are more efficient. Now, HVO can be used as a direct, ‘drop-in’ replacement for fossil diesel without any engine modifications to installed generators, allowing for the immediate rollout of renewable fuel to end users who want to reduce their carbon footprint. There is also no requirement for additional maintenance.

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Figure 4. HVO can be used as a drop-in replacement for fossil diesel without any engine modifications

HVO can also be mixed with fossil diesel, directly in the tank, in any proportion. This means that it can be used immediately as the sole fuel supply for most generators or blended as required.

As more and more generator manufacturers are working towards reducing emissions irrespective of which fuel is used, HVO has enabled a huge step in the journey to net zero greenhouse gas emissions.

Kohler Power Systems