Valorisation of biomass bottom ash for road construction

By project INERTWASTE

Published on 10 March 2025

Spain

La Rioja

About this good practice

The practice addresses the challenge of valorising inert waste generated by biomass power plants. ACCIONA Energía operates 3 biomass power plants in Spain, producing 61 MW and generating 32,000 tons of waste yearly. 30,000 tons of these are bottom ash, which require innovative solutions for valorisation. With the upcoming construction of a new power plant, the waste generation will double, requiring new valorisation strategies and the identification of additional applications and markets for the ash. To tackle this challenge, ACCIONA tested biomass bottom ash in a public road construction, aiming to replace quarry materials and promote the circular economy (CE) principles.
A pilot project was conducted on the A68 highway, incorporating biomass bottom ash in the subbase layer of a 365-meter-long auxiliary road. The project included stockpiling, material preparation, and compaction tests. It was aslo important to ensure that the implementation complied with environmental regulations and technical road standards.
This initiative was approved by the Ministry of Transport. The new Spanish Law on Waste and Polluted Soils foresees the implementation of pilot projects by simplifying the permitting process, allowing R&D projects to be authorised automatically through a Statement of Responsibility.
The reuse of biomass ash supports ACCIONA goal of giving waste and by-products a second life, promoting CE, and reducing CO2 emissions in construction, as well as industrial symbiosis.

Resources needed

The project required technical expertise, stockpiling space, as it is impossible to source the construction project with the daily ash generation, and field trial funding, through the public Innovation Centre (CDTI).
Additionally, regulatory approvals were essential for its implementation.

Evidence of success

This initiative successfully valorised 1,145 tons of biomass bottom ash waste in road construction, saving 1,400 tons of quarry material. The lab scale analysis showed good mechanical performance, chemical properties compatible with road regulations, and high quality performances stabilised with lime/cement. The road pilot achieved successful compaction, particularly under optimal moisture conditions. The carbon footprint was reduced, being an effective example of waste valorisation.

Potential for learning or transfer

This practice demonstrates how inert waste such as biomass bottom ash can be effectively repurposed in road construction, reducing reliance on quarry materials. Other regions with biomass power plants can adopt similar approaches. The concept is scalable for various road types and infrastructure projects, contributing to waste valorisation and CE principles. Furthermore, the integration of biomass ash in public procurement policies could further encourage the adoption of this sustainable practice in large-scale infrastructure development. A favourable legislative scenario facilitates valorisation, as well as the demand from authorities, such as in public procurement.
The results showed that biomass ash is considered a marginal material due to its soluble salt content, but it meets selected soil characteristics. Load plate tests showed all moduli above 100, which is feasible for any low-medium heavy traffic intensity roads and for any roadside.