Copper smelter slag as bed material in fluidized bed combustion

Granulated copper-smelter slag can improve biomass combustion and eliminate the need for virgin silica sand

By replacing silica sand with granulated copper-smelter slag, a by-product of the metal industry, in fluidized bed biomass combustion, not only is the extraction of a virgin resource avoided, but combustion performance is also improved thanks to the oxygen-carrying properties of granulated copper-smelter slag.

Granulated copper-smelter slag is a granular by-product of copper production whose main constituents are iron and silicon oxides. Today, approximately 250,000 tonnes of iron sand are produced annually, and this volume is expected to increase in the future. Granulated copper-smelter slag is currently a by-product with relatively low economic value.

When used in combustion processes, granulated copper-smelter slag acts as an oxygen carrier. This means that the material can absorb and release oxygen, effectively serving as an oxygen buffer, which is beneficial in fluidized bed biomass combustion. However, this application currently relies on the use of up to 200,000 tonnes of virgin silica sand, which is subsequently disposed of in landfills.

This project demonstrated that granulated copper-smelter slag from Boliden can replace silica sand as a bed material in fluidized bed biomass combustion. In addition to reducing the need for virgin silica sand, the iron oxide content of granulated copper-smelter slag improves combustion performance through a process known as oxygen carrier-aided combustion.

This was demonstrated during a two-week semi-industrial-scale test campaign conducted as part of the project. The results were highly promising: operations ran smoothly, and emissions of CO and NO were reduced compared with the use of silica sand.

The project also included preparatory and follow-up laboratory experiments examining factors such as material calcination, its chemical interactions with critical ash components such as potassium, and aging effects within the boiler. These experiments also yielded positive results. They confirmed that potassium becomes bound within the material rather than forming potentially sticky ash deposits on its surface. Furthermore, the material did not lose significant reactivity or undergo substantial degradation during boiler operation.

The next logical steps following the project are to evaluate granulated copper-smelter slag as a bed material in waste incineration, develop a business model for its commercial-scale introduction, and investigate opportunities to valorize the bottom ash saturated with ash-forming elements.