Marine

Production of Blue Diesel from marine plastics with hydrothermal liquefaction


Plastic waste in the oceans is a significant problem that will require innovative solutions to tackle. A research article published by PNAS demonstrated the feasibility of creating fuel from the collected waste, which will significantly improve the entire process of plastic waste disposal.

To study: Thermodynamic feasibility of the on-board conversion of marine plastics to blue diesel for autonomous ocean cleaning. Image Credit: Rich Carey / Shutterstock.com

Plastic waste: a growing problem

It is estimated that there are over 5,000 billion pieces of plastic in the oceans, with millions of tonnes of trash entering them each year. The problem is almost immeasurable, with the large pieces of plastic breaking down into smaller and smaller pieces which then enter the food chain. As more and more trash is created, the problem worsens and cleanup efforts become a moving target that goes in the wrong direction.

Plastics production is expected to continue for several years, so there is an urgent need to dispose of current waste to prevent the problem from spiraling. This will help protect vital ecosystems and ensure the safety of marine food production.

The problem with current waste disposal methods

Currently, several technologies have been developed to clean plastic waste from the oceans. However, despite some progress, clean-up operations are limited by the amount of plastic waste that can be stored on board ships and the refueling and resupply needs.

Round trips can be time consuming, especially if the port where the disposal occurs is thousands of miles from the cleanup site. Add to that the problem of fuel emissions, and it is evident that the process is lacking in key areas of efficiency and sustainability.

Designing an Onboard HTL-Based Process to Convert Ocean Plastics to Usable Fuel: Process Flowchart.  The entire process is designed to fit in a standard 20ft shipping container.

Designing an Onboard HTL-Based Process to Convert Ocean Plastics to Usable Fuel: Process Flowchart. The entire process is designed to fit in a standard 20ft shipping container. Image Credit: Belden, ER et al, PNAS

A better method is needed for the sector to realize its full potential. One way to eliminate storage problems and the time it takes to refuel is to use the plastic waste itself to provide fuel for on-board operations.

Using plastic waste to create blue diesel

Using plastic waste to create fuel is a new idea, but one that means the disposal and sanitation process has a circular element in the production and use of fuel. Plastic waste has an energy profile similar to that of hydrocarbons, and by using this infamous resource, the need to produce petroleum-based fuels is eliminated.

The article proposed a process based on hydrothermal liquefaction, which uses heat and pressure to turn plastics into fuel, recovering the polymers and removing them from the environment. The oil yield is over 90%, even without catalysts. In addition, the yield of solid by-products is low. Another method explored by the research team was pyrolysis, but this has significant drawbacks compared to hydrothermal liquefaction.

Overview of the GPGP plastic removal process showing (A) the system overview, (B) part of the collection ramp system, (C) a single collection ramp and (D) the reactor HTL.

Overview of the GPGP plastic removal process showing (A) the system overview, (B) part of the collection ramp system, (C) a single collection ramp and (D) the reactor HTL. Image Credit: Rich Carey / Shutterstock.com

To make the process commercially viable, the energy produced must be greater than the plastic required to produce fuel. However, advancements in technology and data analysis will help achieve this. The research ran simulations for common plastics – polyethylene and polypropylene – as well as a mixture of the two. A mixture of PP and PE has been shown to be the most promising, having the highest yield while generating no solid by-products.

Using this process, recovery rates of between 230 and 11,500 tonnes of plastic waste per year were modeled by the research team. The range of models depended on the surface concentration of ocean plastics. Further studies will be needed to assess this.

Benefits and main challenges

There are several key advantages to creating fuel on board ships. Besides the aforementioned benefits for operational efficiency, the process is much more sustainable than current fuel production. Emissions from blue diesel (so called because it has a marine origin) are a fraction of the emissions produced by traditional fuel manufacturing.

In addition, congestion in ports and the risk of an oil spill near the coast are reduced by self-supply vessels.

However, the technological challenges will have to be met. Besides producing energy in excess of the waste required for fuel, scalability is a vital consideration if the process is to be commercially viable. Other considerations include how the technology can be implemented safely on board ships, given physical and economic constraints.

Estimated time required to completely clean the GPGP based on high (2,500 g.km-2) and low (50 g.km-2) (1) concentration estimates for the plastic in the GPGP for a distance of deployment between dams of 1 km to 50 km and the corresponding number of dams deployed.

Estimated time to completely clear the GPGP based on the highs (2,500 g.km-2) and low (50 g.km-2) Concentration estimates (1) for plastic in the GPGP for a deployment distance between dams of 1 km to 50 km and the corresponding number of dams deployed. Image Credit: Rich Carey / Shutterstock.com

A framework for assessing the implications of this on-board process was also presented by the research team. This will need to be developed as new data is collected. In addition, the gaps in scientific knowledge on the nature of marine plastic litter will need to be filled.

The future

The oceans are increasingly polluted by human activity. Efforts must be intensified to increase removal rates if the worst effects of environmental damage are to be mitigated. The ability of ships to convert existing plastic waste into fuel will be a big step forward for the industry, creating more efficient, sustainable and safer waste disposal operations in the future.

Further reading

Belden, ER et al. (2021) Thermodynamics feasibility on-board conversion of marine plastics to blue diesel for autonomous ocean cleaning [online] PNAS.org. Available at: https://www.pnas.org/content/118/46/e2107250118

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