Is HTC The New Opportunity For Renewable Energy?

Fossil fuels, as an energy source, accounted for 79.7% of the world’s total energy consumption in 2015.

02 September 2019

Man holding wooden pellets

By Eloise Bevan, Technical Analyst at ABGi UK

Fossil fuels, as an energy source, accounted for 79.7% of the world’s total energy consumption in 2015. However, this rate of consumption is unsustainable in present day due to the continual depletion of world reserves, the growing demand for energy and the negative environmental impacts that have been associated with their use. Each of these factors individually and collectively demonstrate the importance and need for alternative renewable energy sources, and how their development is paramount for sustainable future development.

The relevance of Biomass to Renewable Energy
One renewable energy source that is available in abundance worldwide is biomass, the official term denoted to organic matter. Traditional processes that convert biomass into usable energy includes incineration, anaerobic digestion, gasification, torrefaction and pyrolysis. However, each of these technologies are only energetically favourable for biomass with moisture contents of <10-20%, limiting the range of biomass feed that can be processed through them efficiently.

What is HTC and how does it work?
This being said, a relatively new technology that has only applied on an industrial scale since the start of the 21st century has recently been developed and caught the attention of many academics and innovation funding programmes worldwide. This process has been coined hydrothermal carbonisation (HTC), or wet pyrolysis, which unlike other biomass conversion processes HTC can efficiently transform high moisture (75-90%) feeds. The biomass is subjected to elevated temperatures and pressures (~200 °C and ~20 bar; varies between reactor size, feedstock content, product application and manufacturer) in order to change the chemical composition and increase the contained energy content through converting the processed biomass into pellets of hydrochar.

Hydrochar is similar to coal in its final chemical structure and composition and is of a similar calorific density, the process can be -to some extent- interpreted as a rapid coal formation process. The fundamental reason that the processes technology is of specific interest today is because of the positive net energy-balance it possesses, this means that the energy contained in the pellets notably exceeds the energy required to run the process. Additionally, the carbon dioxide produced as a result of burning the pellets for energy realisation returns back into the natural carbon cycle, making this process completely renewable, unlike the burning of coal. Furthermore, due to the reactors ability to process feeds with a much higher moisture content than traditional methods could justify, some of the industry/commercial wastes can now be diverted from landfills and converted into usable energy. This includes high moisture biomass wastes such as agricultural and garden waste, biodegradable municipal (food) waste (BMW), organic waste from the industrial food sector (e.g. orange peel and brussel sprouts!) and sewage sludge.

The benefits
Technical Analyst, Eloise Bevan investigated the application of a HTC plant to a small village in Scotland (2,000 residents), and the opportunities and challenges this technology could present. In her study she found that the annual food and sewage waste produced by the town could be alternatively disposed of and used to produce 1452 MWhr of energy per year, which equates to 35.6% of the towns predicted energy demand.

This demonstrates that the technology has huge opportunities for industrial and commercial applications. In comparison to the traditional processes outlined above, a HTC plant has a considerably lower CAPEX and OPEX and comes second only to composting and landfilling. However, neither of these produce an energetically valuable end-product and the reduction of BMW sent to landfill is an objective in the European Commission’s Landfill Directive due to the uncontrolled production of landfill gases such as methane and carbon dioxide. Comparison of the pellets to other fuel products including coal, wood pellets, diesel and natural gas, for categories price, price variability, sustainability and energy content ranked the hydrochar pellets in 1st place. However, a challenge presented by current HTC pellets is their market price, which is currently 4 times higher than that of coal (per GJ). This being said, the production of pellets from waste can provide price stability unlike its fossil counter-parts and can also provide energy security to countries who currently import their fuels from overseas.

The future of HTC
The research and development of HTC technology is continually and increasingly being supported through European innovation programmes such as FP7 and Horizon 2020 to further improve process efficiency, commercial ability and competitive pricing. The UKs first HTC plant was commissioned by the collaboration efforts of the University of Nottingham and CPL Industries for an estimated £4 million. The plant completed construction in 2018 and their investment into this HTC plant, with future plans for commercialisation, is the first step on the UKs Hydrothermal Carbonisation journey and a leap towards a sustainable future.