Hydrochar: the value cycle
We talked about the various aspects of a sustainable hydrochar carbon economy. But how do the individual pieces fit together?
In the previous newsletters we have offered various entry points into hydrothermal carbonization (HTC), the process that produces hydrochar. Today, we offer you the entire process in the form of a value cycle to show how the various inputs and outputs of the process fit together.
Everything in this process can be realized in a carbon neutral to carbon negative way – the atmosphere will benefit from reduction of carbon dioxide or keeping it constant. And the process has a great potential as the bio-based residues are readily available in billions of tons and they are free of charge or it is paid for their removal.
Let's start at the top left of the diagram.
Biomass is produced by photosynthesis in green plants. It does not matter whether these plants use photosynthesis on land or in the sea. In the process, biomass is built up from CO2 from the atmosphere — the gas we want to remove from there — water and sunlight.
This biomass consists of carbohydrates, lignin and related substances. The plants also need some minerals, such as nitrogen, phosphorus, potassium, magnesium and trace elements in bioavailable form.
Output biomass and intermediate products
This biomass can be used directly, as in the case of wood. This is the direct use of bio-based materials, for example as building materials.
A share is taken from the biomass flow as food, e.g. for animals or humans. These will build up their own biomass from it. What is left over plus the remains of humans and animals goes back into the material flow. This is what we call bio-based residues.
Biogas can be produced from these in fermenters with the help of suitable bacteria, or biogas can also be produced in a sewage treatment plant from wastewater that also contains these bio-based residues. This is converted into electricity and waste heat of approx. 200–400°C in combined heat and power plants.
In both biogas production processes, a certain amount of sewage sludge or fermentation residue remains and this is now transferred to an HTC plant.
Together with the waste heat from the biogas power plant, HTC coal is now produced in the HTC plant from the fermentation residues and sewage sludge and the waste heat. In addition, waste heat is produced that can be used further. And a process water that contains minerals and some organic material.
Nutrients are then precipitated from the process water, and the rest of the water is returned to the fermenter so that further biogas can be produced there. The nutrients recovered from the HTC process water, such as nitrogen, phosphorus, potassium and magnesium, can now be applied to the fields as fertilizer and the nutrient cycle is closed.
The recovered HTC carbon can now be used further, for example as fuel, to generate emission certificates after safe sequestration or for soil improvement.
When seen from above, we have a process that builds up biomass from CO2 in the air and sunlight, which is split into carbon via the HTC process and thus removes CO2 from the atmosphere. Here we have an open process — which is desirable here, because we want to remove CO2 and sequester carbon. The second parallel process concerns nutrients. This process is completing the cycle of nutrients and allows the plants to grow quickly and healthily.
How to create value from hydrothermal carbonization
In this whole process, there are now some opportunities to generate revenue:
Production of bio-based materials
Production of food
Biogas and electricity that can be generated from the biogas produced
Waste heat for housing estates and factories
Recovery of nutrients that can also be used to produce fertilizers
Last but not least, HTC coal: direct material uses are feasible here, e.g. as an energy source or for soil improvement, but also for generating emission certificates to compensate for CO2 emissions or to sequester carbon dioxide from the atmosphere.
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