Currently the total volume of coffee waste sent to landfill is estimated to be 408,000 tonnes/year. A promising disposal path is to convert the spent coffee grounds to biochar through the process of Pyrolysis. This could release valuable chemical compounds from the waste and the resulting biochar could then be added to soil to improve function and sequester carbon.
Matthew Algie are the UK’s largest independent coffee roaster. They have their roaster based in Glasgow and offices in Glasgow, London and Dublin. They supply a number of major retailers, independent cafes, universities and hotels. Although they don’t have a direct problem with brewed coffee waste many of their customers are looking for an environmentally friendly solution to spent coffee ground disposal and as an innovative company Matthew Algie was interested in exploring a solution for them.
Pyrolysis is a thermochemical conversion of biomass, typically produced in temperatures between 350 and 850 Degrees Celcius, in an oxygen depleted atmosphere. The result is biochar, a solid carbon-rich material. Biochar can be the main product or produced as a co-product with liquid and gaseous biofuels.
The project examined both the biochar and the chemicals and gasses produced. It was found that a number of valuable compounds can be found in the pyrolysis liquid produced from spent coffee grounds such as furfual, phenol and volatile fatty acids. Some were found to be in relatively high concentrations, however, most were diluted by the water content in the liquid. To overcome this a system capable of collecting concentrated pyrolysis liquid for commercial applications would need to be investigated.
The project looked at the potential for the use of spent coffee ground biochar as a seed propagation and plant growth medium. Using the coffee biochar as a component to horticultural media showed that biochar can be used as a safe additive/ replacement in growing media without reducing performance with additional benefit of sequestering carbon.