'THERMAL PROCESSING' - What are we talking about?
Updated: Apr 8, 2020
There are two different processes which come under the heading 'thermal processing' in this context. Hydrothermal liquefaction (HTL) and Hydrothermal carbonisation (HTC). Both subject the seaweed biomass to heat and pressure and use water as a catalyst to aid the reaction. This is useful as it means we can process wet biomass, so in theory, seaweed could be processed straight from the ocean.
So whats the difference?
Although similar in their methods HTC and HTL are conducted under different conditions. HTL is conducted under higher temperatures (300-370℃) and pressures (100-250 bar) with a short reaction time (15-30 minutes). Whereas, HTC is comparatively lower in temperatures (180-250℃) and pressure (10-40 bar) and takes longer to complete the reaction (1-6 hours). Both reactions result in different fractions being produced including CO2 gas, aqueous phase, bio-crude and solids. The reaction temperature and pressure influences the amount of each fraction that is produced.
Last week, I took a trip upto the University of Bath to see the processes in action and find out more.
Firstly the Sargassum is ground down and added to the reactor with water. The reactor is lowered into the furnace and monitored at a safe distance using probes to display the reaction conditions on a computer.
The Sargassum is dried and ground down
Once the optimum temperature and pressure is reached, the reactor is removed and allowed to cool overnight before the fractions are collected. Firstly, the gas is measured and released. In more continuous systems this can be reused to heat the system. The contents of the reactor are then poured into a filter to allow the aqueous phase to be collected.
Aqueous phase is filtered out.
Once this has filtered, the reactor is rinsed with chloroform and filtered. The chloroform binds to the bio-crude allowing it to pass through the filter paper - leaving the remaining solid char in the paper. The bio-crude is then spun in an evaporation separator to remove the chloroform leaving a thick, viscose bio-crude. All of the fractions are weighed so a mass balance can be calculated.
What are they used for?
The aqueous phase contains lots of nutrients, namely ammonia and phosphate and so can be used as crop fertilisers (we will be sending ours to Mexico to be trialled!). The bio-crude contains carbon, hydrogen and some oxygen and nitrogen. After stabilising it can be fed into the existing fuel infrastructure to produce biofuels and bulk chemicals. The solid char is interesting as it can be a great way to return carbon to soils. However, there is discussion about the heavy metal content of the char which could create some interesting extraction opportunities.
HTC fractions once separated.
We will continue to run these reactions over the coming months to optimise conditions and best use the products with the eventual goal of making this a continuous process which can be carried out in situ, contributing to the agricultural and industry infrastructures whilst removing a problem for Mexico's tourism industry.
Keep up to date on the latest developments by following us on twitter @PhycoMExUK.