Professor Tony Ryan and Professor Rachael Rothman have recently proposed a novel method to continue producing chemicals without the need for fossil fuels. Here, they explain how agricultural waste can be repurposed to meet carbon requirements in the chemicals industry whilst utilising durable plastics as a carbon sequestration method.

COP26 has set a precedent to begin tackling climate change with a seriousness not previously given to mitigating change beyond the scientific community. But many of the declarations are based on technologies that do not yet exist, revolving around the idea of carbon sequestration. 

A key challenge we are all familiar with is to do with plastics, which are being produced at an unsustainable rate out of finite fossil fuels. These two words carry with them a slew of issues, both in the fact that we will eventually run out of stock and that when we do, all carbon within the stored fuel has been used and may degrade into the atmosphere. 

This is especially true in the chemical industry. Petrochemical derived chemicals for medicines, furniture polish and food additives often contain ‘forever chemicals’ (that never degrade in the natural environment). These are often in such small quantities that they cannot be collected, and instead flow down the drain to cause problems elsewhere. 

As these are often derived from petrochemical feedstock (ie fossil fuels), carbon is released when producing the chemicals, contributing to climate change. The longevity of ‘forever chemicals’ also pose threats to ecosystems as these run into delicate water systems.

Mitigating climate change requires more than just a move away from using fossil fuels, however. So much carbon has been released that we now require carbon sequestration technologies alongside converting over to fossil-free energies to reduce atmospheric carbon concentrations and reduce global warming. This has been relied on as a main way of reacting to climate change and has pushed forward the planting of forests and relying on as yet non-existent carbon sequestration technologies. 

Infographic of agricultural waste use in creating neo-fossils. Plant biomass is converted into petrochemical feedstock, which can then either be used as a fuel or in creating plastics which are buried as a carbon sequestration method.

How can we begin to address this?

This is a problem of two halves: 1) the use of fossil fuels and release of stored carbon from thousands of years ago and 2) the lack of viable carbon storage methods.

Professors Tony Ryan and Rachael Rothman propose a novel method to address these two problems using chemistry as the founding principle. 

In the search for an alternative source of carbon that does not tap into a thousand year old carbon cycle, they came across waste agricultural biomass as a potential source. 

Waste agricultural biomass (including stalks, leaves and husks) are a byproduct of agriculture. This plant material is formed of carbon harnessed through photosynthesis. This acts as a carbon store, but is often left to rot or burnt in the fields.

Based on a model of an American Prairie, collecting just 20% of this waste agricultural biomass would meet the carbon demand currently supplied by the fossil fuel industry. 

Using plant biomass as an alternative would replace the need for fossil fuel combustion and release of carbon, instead relying on a source with a much shorter time scale carbon cycle – ie taking carbon out of the air rather than out of the ground. 

The problems and solutions of using plant biomass

Plants at their most fundamental are constructed of carbon chains. These are very long and difficult to break, hence why cows have four stomachs to get the nutrients out of grass. In chemical terms, these are not very useful for building chemicals unless broken up into smaller chains that are more versatile.

This is, however, easily done. Simple refining techniques are taught at university, with some chemical conversions covered at school.

Excess heat energy from these processes can also be used to heat greenhouses and carbon rich flues can be pumped into growing containers to boost plant growth.

The question is when will it be a viable process in terms of carbon emissions as these processes all require energy inputs to break up the carbon chains.

Is this more carbon-efficient than just using fossil fuels?

Using plant biomass instead of fossil fuels can only be more environmentally friendly if renewable energy powers conversion. As energy sources have not yet fully converted to a defossilised system, this does mean that there will be a time delay, but we will be converting to renewables in the near future in line with the climate declarations. 

One way to assess when this process will be viable is by using a ‘Life Cycle Assessment’ (LCA). This is an assessment of the carbon footprint of the entire life-time of the product. In other words, an LCA calculates how damaging to the environment a process or product will be. Whilst we rely on fossil fuel energy systems, the LCA of using plant-carbons is higher than just using fossil fuels in the chemical industry. As we convert to sustainable energy sources, the LCA will show that using plant-carbons (known as bicarbon) produces fewer emissions.

Using bicarbon to solve a bigger crisis

This bicarbon also has its place in the carbon sequestration debate. Plastics, known for their problematic long life, are essentially a  carbon store. This can be used to our advantage to sequester carbon. This would take carbon, harnessed by photosynthesis into plants and sequester it in a non degradable resource, essentially creating anthropogenic neofossils. Simply put, making and storing plastics out of plants could become a carbon sequestration technique. 

This sounds counterintuitive… shouldn’t we be trying to get rid of plastic?

Plastic in the oceans and soils pose a great threat to our ecosystems and food supply. This proposal is not supporting the casual landfill of plastic objects, but instead to store them underground in a curated fashion with a known location. Single-use plastics then become a source of highly refined carbon for future generations, and a carbon sequestration technology that’s feasible today.

For example, a wind turbine could be manufactured using bicarbon from agricultural waste via refining processes powered by renewable energy. Excess heat energy from this process will be used to heat greenhouses and carbon emissions will be channelled to feed plants.  At the end of the turbine’s structural life, it will be buried, storing the carbon it is made from.

Alongside this is a move towards turning those ‘forever chemicals’ into fully biodegradable products which are harmless to the environment.

Chemistry at the heart of climate mitigation

Changing the petrochemical supply and sequestering carbon are both crucial to addressing the climate crisis. Adopting agricultural waste has the potential to work towards mitigating climate change. Chemistry was at the heart of COP26, and even if the politicians didn’t notice it, the policymakers did.

To read the full article, please find it here.