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azda Motor Corporation (Mazda) has conducted a demonstration operation of a cupola melting furnace (cupola), the core casting facility installed at its Hiroshima Plant, by fully replacing fossil fuels that emit CO2 during combustion with bio-coal briquettes, a biomass fuel derived from coconut shells, and confirmed the stable operation.

Cupola furnaces are vertical, cylindrical furnaces used primarily in foundries to melt metals—most commonly cast iron, as well as some bronzes and nickel-resistant irons. Its design date back centuries. The furnace typically operates by burning coke as its primary heat source. The structure consists of a steel shell lined with refractory bricks to withstand extreme heat, typically ranging from 20 to 35 feet tall, with diameters between 1.5 and 13 feet, depending on the scale of operation. It’s supported by legs over a concrete base and has a drop bottom design: hinged doors at the base that swing open to release leftover material after a melting cycle.

In operation, layers of coke, metal (such as pig iron or scrap), and a flux (usually limestone) are loaded from the top. The fuel is ignited at the bottom, and air is blown in through openings called tuyeres, creating a hot blast that intensifies combustion. As the coke burns, it generates heat (often exceeding 1,000°C) and carbon monoxide, which helps reduce oxides in the metal. The metal melts, drips through the coke bed, and collects in a well at the bottom. A tap hole releases the molten metal into ladles for casting, while a higher slag hole drains off impurities (slag) separated by the flux.

The operation of a cupola that uses 100% biomass fuel had not yet been confirmed for implementation, according to Mazda, and the company took on the challenge of conducting the demonstration experiment in collaboration with the foundry industry and local partners. In the future, Mazda will expand the circle of regional cooperation and promote an energy circulation scheme for local production for local consumption, aiming for carbon neutral (CN) operation of cupolas using waste-derived biomass fuels such as bio-coal briquettes by fiscal year 2030.

In March 2023, Mazda established the Cupola CN Co-creation Working Group by inviting volunteer companies and organizations to conduct research and development for the shift to biomass fuels, the establishment of production methods, and studies on the local procurement of raw materials. While bio-coal briquettes derived from coconut shells were used in the demonstration experiment this time, Mazda has been working to establish a locally produced, locally consumed energy circulation scheme in Hiroshima and neighboring areas, from biomass waste collection to production, with the aim of revitalizing local industries and contributing to the local communities.

As part of such activities, Mazda has been collecting spent coffee bean husks from Tully’s Coffee Shop (operated by Tully’s Coffee Japan Co., Ltd.) and vending machines in the premises of Mazda Headquarters since November 2024, and confirmed through demonstration experiments that it is possible to produce bio-coal briquettes from these bean husks.

Going forward, the company will work together with the local community to develop the local production for local consumption energy circulation scheme into a cooperative effort among industry, government, academia, and the private sector to realize stable raw material procurement of and implementation of fuel conversion.