What is hydrothermal carbonisation?

By Sarah Farthing

Hydrothermal carbonisation (HTC) is a renewable energy method where a carbon-dense solid product, hydrochar, is are produced from wet biomass by heating it in a sealed container.

 

As targets for the UK to be carbon neutral by 2050 become more pressing and over 41% of the electricity generation in January to March 2021 being generated by renewables,  alternative energy solutions are most definitely a growing trend. But how exactly does HTC work and how is it a sustainable source of energy?

 

How HTC works

Hydrothermal carbonisation (HTC) is the process by which the heating of wet feedstock and subcritical water (water which is held at a temperature above 100 degrees Celsius under pressure) produces hydrochar. The matter is kept under pressure by the rising temperature in a sealed container. Essentially, it is a large-scale pressure cooker which reaches temperatures of around 180 – 250 degrees Celsius.

Hydrochar, the resultant product of the pressure-cooked wet feedstock, can then be dried down and compressed into pellets which can be used as a biofuel.

What kind of feedstock can be used?

As a technology that is relatively cheap to set-up and run, HTC permits the use of ‘lower-value’ waste feedstock. This provides an opportunity to use waste products such as garden waste, most types of farming agricultural waste, sewage and digestate from an anaerobic digester.

The feedstock used does not have to be waste, however it is most cost-efficient to use a waste product.

Often wet wastes are used, but it is possible for other feedstocks to be utilised in HTC. In particular, there is an increased interest in the HTC of clothes and textile wastes. Drier feedstocks can be processed simply by adding water, making it a truly universal system. Essentially, almost anything can be used in HTC, but it doesn’t make monetary sense to put in feedstock which is more valuable in its original state, for example.

Why is the HTC of ‘wet feedstock’ beneficial?

Pyrolysis systems operate by burning biomass to produce biochar. These systems require the feedstock to be dry and therefore add an extra step of ‘drying’ the waste before burning. HTC removes this preparation step, as there is often no need to dry the feedstock down to low moisture levels before it is used, therefore creating a more energy efficient process.

Digestate, the remaining material produced by anaerobic digesters, is another wet feedstock which is compatible with HTC. Initial studies indicate that the integration of HTC with the anaerobic digestion process can improve the overall efficiency.

What are the benefits of using hydrothermal carbonisation with anaerobic digestion?

Digestates, a slurry like by-product of anaerobic digestion, must meet restrictive specifications before being spread across the land as a renewable fertiliser. However, HTC can produce a more stable by-product, therefore, putting digestate through an HTC system can bypass the issues which anaerobic digesters may have.

 

Digestates are also extremely wet which makes them expensive to transport. This isn’t a problem for on-farm plants as digestate is often spread as an organic fertiliser on the land it is situated on. However, for city-based plants the cost of transportation can outweigh the benefits of the AD process. A combined HTC and AD unit could offer the potential for digestate from the AD to be used as feedstock for the HTC plant, with heat being recovered from the process to warm the digester.

 

What can hydrochar be used for?

Currently hydrochar’s primary use is as a biofuel, but research is ongoing as to alternative uses, including its benefits for carbon storage and exploring its uses as a fertiliser.

The amount of ash contained in the hydrochar largely depends on the feedstock used and affects the suitability of its usage. High ash levels, which are commonly found in sewage feedstock, are not suitable if the resultant product is going to be used as a biofuel, as the ash can foul and corrode the sides of the burner.

 

Whereas on the other hand, in theory a higher ash content can supplement potassium and phosphorus levels in soils thus making it beneficial as a fertiliser. Although research has shown that direct application of hydrochar into soil is phytotoxic and therefore it may need to be accompanied with a post-treatment.

 

Looking forward, HTC can offer benefits as a source of renewable energy by itself but also in conjunction with other processes. It’s an exciting prospect for sustainable energy development and I’m looking forward to seeing how it is used alongside other renewable energy technologies in the future.