[Blog 5] Assessing liquid hydrogen as an alternative fuel

The need for alternative fuel 

As previously mentioned in Blog 1, shipping is considered the most efficient transport mode of freight transport (on a tonne/km basis). It consumes less fuel and has a lower environmental footprint when compared to other transport modes (IMO, 2009). However, this does not discount the fact that the global shipping industry is still responsible for emitting a significant amount of carbon dioxide into the atmosphere. In 2018, according to the International Maritime Organisation’s 2020 greenhouse gas study, global shipping activity has been found accountable for emitting roughly 1.05 billion tons of carbon dioxide into the atmosphere, contributing approximately 2.9% of total global CO2 emissions (IMO, 2020). In addition to CO2 emissions, shipping is also a major contributor to sulfur dioxide (SO2) and nitrogen oxides (NOx) (Geels et al., 2021; Gong et al., 2018; Viana et al., 2014). 

This is largely in part due to the prevalent use of low-grade heavy fuel oil, or ‘Bunker oil’– produced from the blending of residual oil and refinery intermediates, in the shipping industry (Uhler et al., 2016). Figure 1 highlights the trend of the fuel mix of various vessel types (HFO= Heavy fuel oil; MDO/MGO= Marine diesel/Marine gas oil; LSH= Low sulfur heavy fuel oil; LNG= Liquefied natural gas). Based on this figure (published in 2019), it is apparent that a large percentage of shipping vessels still heavily rely on the use of heavy fuel oil (Schnurr & Walker, 2019). The combustion of such Bunker oil is the main culprit for the release of CO2, SO2, NOx, and other pollutive substances into the atmosphere (Schnurr & Walker, 2019). 

Figure 1: Fuel mix for various vessel types (HFO = Heavy Fuel Oil; MDO/MGO = Marine Diesel/Gas Oil; LSHFO = Low Sulfur Heavy Fuel Oil; LNG = Liquefied Natural Gas). (Schnurr & Walker, 2019).

In response, the International maritime organisation (IMO), through an update of the  International Convention for the Prevention of Pollution from Ships (MARPOL), mandated vessels to reduce their fuel sulphur content to 0.1% in Emissions Control Areas, and 0.5% globally from 2020 (Brynolf et al., 2014; Gilbert et al., 2018). Furthermore, in line with the Paris Agreement, IMO has, in 2021, set out its objective of a 40% reduction of CO2 emissions per transport work, when compared to 2008 levels by 2030, as well as a 50% reduction of total annual greenhouse gas emissions, when compared to 2008 levels by 2050 (IMO, 2021). As such, the combined pressure of reducing SO2 and NOx in the short term, and the longer need of reducing greenhouse gas emissions, has prompted the shipping industry to find alternative sources of fuel. Some examples include Liquefied Natural Gas (LNG), methanol, and hydrogen (Brynolf et al., 2014). These fuels offer a cleaner alternative to Bunker oil, given that they contribute to lower emissions. 

However, several authors have argued the need to critically consider the entire product life cycle of alternative fuels (Production → Storage → Transport → End Use), before determining whether or not a particular fuel can deliver meaningful emissions savings for the entire shipping industry (Atilhan et al., 2021; Gilbert et al., 2018). Additionally, there is a need to take into account various technological, economic and social factors, such as safety (Gilbert et al., 2018). Hence, in this week’s post, we would be critically assessing the potential one such alternative fuel, hydrogen. 

Hydrogen as an alternative fuel

Hydrogen has been considered one of the most promising potential alternatives to the replacement of heavy fuel oil, primarily because its combustion releases zero carbon emissions, producing only water in its process. This has the potential to significantly lower the amount of carbon dioxide emissions that are released into the environment (Jessop et al., 1995; Dorner et al., 2010). 

Hydrogen can be categorized based on the production method and raw materials used into grey, blue and green hydrogen. Grey hydrogen is formed through the reforming of fossil fuels. This is currently the most popular method in which liquid hydrogen fuel is obtained (~95%), often through the process of steam reforming of natural or shale gas (Martı́nez et al., 2014). The production of blue hydrogen is similar, except that in the reforming of fossil fuels, carbon emissions are captured, stored or further utilised (Noureldin et al., 2015). Lastly, liquid hydrogen fuel can be considered green when it is produced by utilizing renewable energy sources and feedstocks throughout (Atilhan et al., 2021). Figure 2 depicts the hydrogen fuel production chain and lifecycle. 

Figure 2: Production life cycle of various forms of liquid hydrogen. (Altihan et al., 2021).

Can all forms of hydrogen be considered clean? 

However, while the combustion of liquid hydrogen does not emit any carbon dioxide, it does not mean that no GHG emissions are released in the other stages of its lifecycle. Critical analysis of the production of various types of liquid hydrogen (broadly classified into grey, blue and green) by Atilhan et al. (2020), have found that while grey liquid hydrogen is the most cost-effective (costing approximately ¼ of the cost currently required to produce green hydrogen), its total carbon footprint has been measured to be between 120-155g CO2 eq per megajoule of energy contained in the fuel. This exceeds that of the production of heavy fuel oil, at about 90g eq/MJ (Brynolf et al., 2014). Next, depending on the carbon capture or other technology used, blue liquid hydrogen, while produced in similar ways as grey liquid hydrogen, has a lower carbon footprint of between 40-90g eq/MJ. Lastly, while most expensive, green liquid hydrogen, when obtained through the liquefaction of hydrogen by renewable sources such as wind and solar energy, can have a carbon footprint as low as 4.6 and 11.7g eq/MJ respectively. Figure 3 below compares the greenhouse gas emission released by various types of hydrogen production. It can be seen that the method of hydrogen production largely affects the environmental footprint of the hydrogen fuel produced. 

Figure 3: Greenhouse gas emissions data comparison (g CO2e/MJ fuel) for LH2 to grey, blue and green H2 production. (Altihan et al., 2021).

Techno-economic evaluation of liquefied hydrogen

As explored above, green hydrogen has the lowest overall carbon footprint, making it a promising replacement fuel for the shipping industry. However, in its production, large amounts of electricity (generated from renewable sources such as solar or wind power) are required for electrolysis, where water molecules are split into hydrogen and oxygen. The large amount of energy required for electrolysis significantly increases the cost of production (4 times more than the production of grey hydrogen), hindering the wide-scale implementation of this technique (Altihan et al., 2021). Figure 4 compares the cost of producing grey hydrogen, with current green hydrogen production methods. 

Figure 4: Comparing the cost of producing grey liquid hydrogen (LH2), with various green liquid hydrogen producing methods. (Altihan et al., 2021).

Safety evaluation of liquefied hydrogen

Next, according to US National Fire Protection Association (NFPA) 704 standards that categorize various fuels from 0-4 to determine its risk level, liquid hydrogen has been assessed to have high flammability, assigned a severity of 4 (Table 1). Furthermore, the high energy intensity of hydrogen, demonstrated by its lower heating value (LHV) of 120 MJ/kg (2.8 times the LHV of Heavy fuel oil), as well as its wider flammable limit range and lower boiling and flash temperature, is expected to make hydrogen an inherently more dangerous fuel (Table 2). Furthermore, the storage of liquid hydrogen requires cryogenic conditions as the liquefaction temperature of liquid hydrogen is -253°C (Table 3). This creates multiple hazards as exposure to skin can cause cold burns, while any leakage can potentially create an explosive mixture of liquefied air and hydrogen (Edelia et al., 2018). 

Table 1: Comparison of fuel hazardous characteristics via the US National Fire Protection Association (NFPA) 704. (Altihan et al., 2021).

Table 2: Comparison of the potential of explosion and fire. (Altihan et al., 2021).

Table 3: Comparison of different marine fuels. (Altihan et al., 2021).

Other logistical and infrastructural concerns pertaining to the use of liquid hydrogen

As seen from the table above, liquid hydrogen has a significantly lower volumetric density when compared to liquid natural gas (LNG) (Table 3). For instance, compared to LNG, liquid hydrogen requires 2.8 times the volume to store. This results in infrastructural and logistical concerns, as this can result in the reduction of cargo space available, to create room for the fuel (Atlihan et al., 2021). 

Moving forward 

However, there is still potential for green hydrogen to become a replacement fuel for the shipping industry in the future. With the advancement of technical capabilities in green energy production, the cost of green renewable energy production is projected to lower by 70% in the next decade. This would render the uptake and production of green hydrogen more economically feasible (Atlihan et al., 2021). To facilitate the commercial adoption of green hydrogen, Atlihan et al. (2021) further argue that codes and standards of hydrogen bunkering and onboard storage need to be well-defined before widespread implementation can occur, in order to ensure safety and public acceptance. 

In conclusion, the shipping industry currently runs on highly pollutive ‘Bunker oil’. There is therefore a need for the shipping industry to transit towards cleaner sources of fuel. However, when considering the adoption of alternative fuels, there is a need to consider the environmental footprint not only when it is in use, but also its footprint along all stages of its production life cycle. Furthermore, various technological, economic and social factors regarding the production and use of the fuel, will also need to be considered. 

References

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Brynolf, S., Fridell, E., & Andersson, K. (2014). Environmental assessment of marine fuels: Liquefied natural gas, liquefied biogas, methanol and bio-methanol. Journal of Cleaner Production, 74, 86–95. https://doi.org/10.1016/j.jclepro.2014.03.052

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Edelia, E. M., Winkler, R., Sengupta, D., El-Halwagi, M. M., & Mannan, M. S. (2018). A computational fluid dynamics evaluation of unconfined hydrogen explosions in high pressure applications. International Journal of Hydrogen Energy, 43(33), 16411–16420. https://doi.org/10.1016/j.ijhydene.2018.06.108

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Gilbert, P., Walsh, C., Traut, M., Kesieme, U., Pazouki, K., & Murphy, A. (2018). Assessment of full life-cycle air emissions of alternative shipping fuels. Journal of Cleaner Production, 172, 855–866. https://doi.org/10.1016/j.jclepro.2017.10.165

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