At Stora Enso’s Heinola fluting mill, bio sludge is a by-product of wastewater treatment. The mill used to burn the bio sludge at the power station for thermal power, but because it is so wet, it needed fossil-based fuel to support the burning process. This was not in line with the mill’s long-time goal to achieve carbon neutrality in production.

In 2019, an industrial scale pilot plant was built at the mill area, using a technology developed and patented by the Swedish company C-Green Technology AB, one of the ventures supported by InnoEnergy. With C-Green’s solution, bio sludge is dried in an energy-efficient way with pressure and heat, and it becomes clean and odourless biofuel. It can then be burned without additional fossil-based fuel.

“Our process, OxyPower HTC, uses an innovative application of hydrothermal carbonisation, which converts complex organic compounds into sterile odorless biofuel,” says Erik Odén, CEO of C-Green.

The pilot plant processes 16 000 tonnes of biowaste per year. The resulting biofuel is used in the mill’s power boiler and to heat the nearby town of Heinola, with about 20 000 inhabitants. Heinola Mill has been providing district heat to the city and its residents since the 1980s.

“The treatment of bio sludge produced at our mill offers opportunities for a bio-based circular economy, which supports Stora Enso’s environmental goals. With this technology, we can save both the environment and costs”, says Mikael Sillfors, Development Manager at Heinola Mill.

With the pilot plant, Stora Enso is investigating how much CO₂ emissions can be reduced with the treatment of bio sludge. Stora Enso’s goal is to reduce the use of fossil fuels and get as close as possible to zero levels of CO₂ emissions by technically and commercially feasible means. A high proportion of biomass is already used in the internal energy production at most of their mills. In 2018, the share of biomass in energy generation at Stora Enso’a mill sites was 81%.