Making graphite in a renewable way

Louise Smyth

The transition to renewable energies and the bioeconomy are two concepts that have become a constant amongst politicians, the scientific community and the public. But what do they mean, and what do they have to do with materials? In this article Dr Catalina Rodriguez Correa, writing on behalf of materials search engine Matmatch, uncovers potential solutions to the impact that growing consumer economies have on raw material supply lines.

It turns out that with population growth and the sinking prices of different commodities, the use of raw materials has increased, many of which have entered the critical raw materials list of the European Union.

Furthermore, due to their scarcity, the extraction of these materials is becoming more energy-intensive, leading to an increase in their CO2 footprint. Therefore, it is necessary to find renewable and sustainable alternatives not only for energy sources but also for raw materials. This is the core objective of the energy transition and the bioeconomy.

Bioeconomy is a new model that proposes to use renewable instead of fossil resources to produce energy and industrial commodities whilst ensuring a sustainable agricultural production, securing global nutrition and avoiding the food-vs.-fuel dilemma, i.e. the problematic of prioritising energy crops over food.

Bioeconomy proposes biomass in the form of agricultural residues and other biological wastes as its main feedstock basis to shift towards a system independent from fossil sources. This model is supported by the European Union and Germany, for example strives towards understanding the feasibility and implications of the bioeconomy by 2030 to start structural changes on an industrial scale.

Bio-based carbon materials

Graphite is one example of a critical material with high commercial or strategic importance. It can be found in its natural state or it can be produced artificially. Both, however, involve the use of fossil-sources either as feedstock, to power the mining extraction process, or because naturally occurring graphite is itself a fossil resource.

Graphite plays a major role in energy applications such as batteries, supercapacitors, fuel cells or as an anode in metallurgical applications. Additionally, it is used as a lubricant and in seals for high-temperature applications. The graphite demand is expected to increase in the following years due to the production increase of lithium-ion batteries and other energy storage systems involved in the electrification process.

Companies like Syrah Resources expect an increase of 400 metric tonnes in the graphite demand by 2021 driven solely by the battery sector. Luckily, bio-based carbon materials could replace graphite in most applications.

Carbon is one of Earth’s most abundant and versatile elements. Depending on how it is bonded, it is present in various structural configurations, which results in materials with completely different properties. Graphite is a form of crystalline carbon in which the atoms are organised in honeycomb-like stacked layers. Diamond is also a carbon crystal, however, the atoms are more closely packed together. Graphite is a soft and electrically conductive material whereas diamond is extremely hard and insulating.

Hydrothermal carbonisation and hydrochar

When a carbon-rich precursor (e.g., biomass) is exposed to high temperatures in the absence of oxygen, the carbon atoms begin to rearrange and build graphite-like structures. This process is called carbonisation. The two most widespread carbonisation processes are pyrolysis and hydrothermal carbonisation (HTC). During both processes, the main biomass components, such as cellulose, hemicellulose and lignin, are decomposed to different degrees depending on the process temperature, residence time and reaction medium.

HTC is a thermochemical conversion process that mimics the natural coalification process of biomass; however, it occurs in a matter of hours instead of centuries. During this process, a mixture of water and biomass is brought up to relatively mild temperatures (150 – 250 °C) in a closed reactor, known as an autoclave, that can tolerate pressures of up to 100 atmospheres. As a result, a solid product known as hydrochar with similar properties to those of lignite is obtained.

Since the carbonisation process takes place in water, the energy-intensive process of drying is avoided, making HTC an appealing process to carbonise wet biomass. Thus, it has been gaining importance in the waste management industry to process waste streams like sewage sludge or biogas digestate since after HTC, the product is more easily dewaterable and free of pathogens.

Companies like HTCycle are making proper use of this technology not only to provide a solution to the waste management problem but also by increasing the added value of the hydrochar. The hydrochar can be used to produce activated carbon, which in turn can be used in water treatment plants to remove micropollutants, heavy metals and other contaminants, in gas separation processes and in energy storage systems.

If HTC is conducted in the low-temperature range and for short reaction times, not only is hydrochar obtained but also hydroxymethylfurfural (HMF). HMF is an extremely valuable platform chemical for the production of bioplastics like polyethylene furanoate (PEF), the bio-based alternative to PET.

Pyrolysis and pyrochars

Pyrolysis is a dry thermochemical process that occurs at temperatures higher than 500 °C and in the absence of oxygen. This way, biomass decomposition is favoured over combustion. During pyrolysis, the main biomass components are thermally cleaved and as a result, a gaseous, a liquid tar-like phase and a solid product are obtained. This process is similar to the one used for charcoal production.

The solid product, also known as biochar or pyrochar, has an extremely high carbon content and with higher pyrolysis temperatures, its microstructural properties resemble more those of graphite. Yet, biomass is a not-graphitisable material, i.e., independently of the carbonisation temperature, it will never become graphite.

Pyrochars belong to a category of carbon materials known as amorphous or hard carbons, which enjoy many of the properties of graphite and can even perform better than graphite in many applications. During HTC, biomass is not as thoroughly decomposed as during pyrolysis.

Therefore, hydrochars have a rich superficial chemistry that can be advantageous in energy storage or adsorption applications. Furthermore, coupling HTC and pyrolysis opens a whole new area to modify and tailor the properties of bio-based carbon materials based on the intended application.

Is the future of carbon bio?

For bio-based carbon materials to be economically feasible and to overcome the possible hurdles related to the intrinsic heterogeneity of biomass, it makes sense to make a paradigm shift and start rethinking the concept of economy of scale. For the bioeconomy to work, it will be better to move from large centralised plants towards smaller decentralised reactors close to the biomass source.

One example of this is the proposal from the company carbonauten. With decentralised technologies, they produce bio-based carbon materials from wood as well as agricultural and forest residues via pyrolysis. These products can be used as additives in plastics, to produce activated carbon and in metallurgical applications.

Bio-based carbon materials can be possible replacements for graphite and other carbon materials. By using renewable resources such as biomass as feedstock, these materials have a (theoretically) neutral or even negative CO2 footprint. The reason being that the carbon fixed by biomass during photosynthesis is coming from CO2 from the atmosphere.

As a result of their physicochemical and structural properties, bio-based carbon materials can perform similarly or even better than graphite in various applications. Furthermore, it is not only technically and economically feasible to achieve this, but also this will contribute to the development of a well-rounded and robust bioeconomy. This leads to the conclusion that the future can indeed be bio!