New model evaluates efficiency of pistachio and walnut shells as low-carbon fuels

Researchers from the University of Nottingham and CSIRO Australia have developed a pioneering combined milling and combustion performance model to improve the selection of low-carbon fuels for power generation.

This model allows power generators to predict how well fuels will break down in mills, and how this then impacts how well they burn. This knowledge helps optimize energy efficiency and reduce waste in power plants.

Published in the Journal of the Energy Institute, the study evaluates the milling and combustion properties of five biomass types—pistachio shells, walnut shells, rice husks, palm kernel shells, and wood pellets.

Biomass is the second largest contributor of renewable energy to the U.K., providing 31.1 terawatt-hours of electricity generation in 2022. Currently, around 8.3 million metric tons of biomass is used annually for pulverized fuel (PF) combustion in the U.K., which accounts for 21% of the global wood pellet market.

This is dominated by wood pellets and wood chips, followed by recycled and waste wood, with other types of biomasses making up only around 1% of the total. The majority of the U.K. wood pellet demand is also met by imports, with the United States and Canada being the major suppliers.

Selecting new fuels for combustion requires the evaluation of numerous material characteristics, which includes milling performance.

Four agricultural residues and one commercially sourced white wood pellet (Brites) were used for all tests. The agricultural residues were pistachio shells, walnut shells, rice husks, and palm kernel shells, which were sourced from a U.K. power generator.

The team behind the study developed a unique burnout prediction tool that, for the first time, uses char morphology, where image analysis is used to assess the shape and structure of the unburnt fuel after combustion, to anticipate burnout efficiency, offering power plants a quick method to assess the carbon loss of different fuels.

The milling metric allows power generators to rank potential fuels by their milling performance and has been used in industry to identify new sources of low-carbon fuels, assisting with the U.K. aims to reduce carbon emissions to Net Zero by 2050.

By combining these tools, the research provides a holistic view of how particle size and composition impact both milling efficiency and combustion outcomes. This breakthrough allows energy producers to make better-informed decisions on fuel selection, enhancing both performance and sustainability in biomass and low-carbon fuel energy production.

“This research, which builds upon years of research in biomass processing at the University of Nottingham, is really exciting as it opens the door to using underutilized agricultural residues, such as pistachio and walnut shells, as viable low-carbon fuels,” says Dr. Orla Williams, from the University of Nottingham’s Department of Mechanical, Materials and Manufacturing Engineering.

“By combining milling and combustion modeling, we can now predict fuel performance more accurately, helping power plants improve efficiency and reduce emissions, while diversifying their supply chains.

“It’s a significant step forward in diversifying and decarbonizing the UK’s bioenergy portfolio but can also be applied to any country that generates electricity on a large-scale using coal, enabling other coal-dependent economies to decarbonize.”