RECYCLING OPTIONS FOR BIOMASS COMBUSTION ASH

The transition from fossil fuels to renewable energy is one of the pillars of the energy strategies that have been adopted by many governments in the last decades. A key role in the modern energy policies is covered by biomass, whose consumption for heat and power generation, particularly via the combustion technology, is dramatically increasing. This rises an environmental issue related to the wastes produced, the amount of which is rapidly growing and whose management requires to be addressed. At present, biomass ash is usually disposed in landfills, but this residue shows some physical and chemical characteristics that make it worthy to be reintroduced in the natural and production cycles as a valuable material, as the Circular Economy strategy suggests. The general objective of this work was, therefore, to maximize the potential of this kind of waste and study reuse options that are tailored on its characteristics. The specific objectives of this work were (i) to deeper the understanding of the biomass ash characteristics, and to study two innovative recycling options consisting in reusing ash (ii) as an additive in the composting process and (iii) as a substrate for carbon dioxide sequestration. To do so, three ash types (bottom, boiler and fly ash) generated from a thermal power plant, fueled entirely with biomass, were characterized and experiments were conducted with two selected ash types (boiler and fly ash). With the aim of studying ash as an additive for composting, two experiments at pilot- and laboratory-scale were conducted, where a blend of boiler and fly ash was added at several shares to a mixture of organic fraction of municipal solid waste and a bulking material, and then subjected to composting. On the basis of the results obtained, both the composting process and the final product quality were improved by ash addition; in particular, ash resulted to have a potential as a physical conditioner, mineral additive and improver of humification in the composts produced from organic waste. These results also allowed to identify a possible maximum amount of ash that might induce benefits to the compost quality, without being of harm for the health and the environment. In addition, similar process performance and final compost quality were attained even when a significant amount of structuring agent was replaced by a mass of the biomass ash blend, which suggests that ash could give a contribution in the partial replacement of the traditional bulking materials. As for the ash reuse as a substrate for CO2 sequestration, the experimental activities were conducted using a reactor for accelerated carbonation and aimed at accelerating and/or maximizing the CO2 uptake. Fly ash resulted to be effective in the carbon dioxide uptake, and could be a promising substrate in future for reducing the impact generated by the biomass used for energy purposes. Carbonation is also the most representative transformation occurring during the ash ageing process, and this study highlighted that a potential environmental hazard exists in treated/aged ash because of the varied leaching behavior of some heavy metals after the treatment. The directions for further research are many and are summarized at the end of the present work.