Ashes from challenging fuels in the circular economy.

In line with the objectives of the circular economy, the conversion of waste streams to useful and valuable side streams is a central goal. Ash represents one of the main industrial side-products, and using ashes in other than the present landfilling applications is, therefore, a high priority. This paper reviews the properties and utilization of ashes of different biomass power plants and waste incinerations, with a focus on the past decade. Possibilities for ash utilization are of uttermost importance in terms of circular economy and disposal of landfills. However, considering its applicability, ash originating from the heat treatment of chemically complex fuels, such as biomass and waste poses several challenges such as high heavy metal content and the presence of toxic and/or corrosive species. Furthermore, the physical properties of the ash might limit its usability. Nevertheless, numerous studies addressing the utilization possibilities of challenging ash in various applications have been carried out over the past decade. This review, with over 300 references, surveys the field of research, focusing on the utilization of biomass and municipal solid waste (MSW) ashes. Also, metal and phosphorus recovery from different ashes is addressed. It can be concluded that the key beneficial properties of the ash types addressed in this review are based on their i) alkaline nature suitable for neutralization reactions, ii) high adsorption capabilities to be used in CO2 capture and waste treatment, and iii) large surface area and appropriate chemical composition for the catalyst industry. Especially, ashes rich in Al2O3 and SiO2 have proven to be promising alternative catalysts in various industrial processes and as precursors for synthetic zeolites.

NO and SO2 emissions from combustion of raw and torrefied biomasses and their blends with lignite.

The impact of torrefaction on the NO and SO2 emissions from combustion of biomass was investigated. Combustion experiments were carried out with two torrefied biomass fuels, i.e., poultry litter and olive tree pruning and their blends with lignite using a bench scale single particle reactor. For comparison, NO and SO2 emissions from tests with untorrefied biomasses and their blends with lignite were also investigated. The total release of SO2 and NO for each fuel was determined at three different temperatures: 900, 1000, and 1100 °C. The NO release from the untorrefied biomasses was found to be lower than those from torrefied biomasses, despite their higher fuel- N content. In case of co-combustion of both raw and torrefied biomass with lignite, the NO release was lower than the anticipated one. On the other hand, in the co-combustion experiments, blends with torrefied biomass showed a larger reduction in SO2 release than the blends with raw biomass. The study revealed that the SO2 emissions from blends are not proportional to the mixing ratio of the fuels and to the emissions properties of the respective fuels. No clear correlation was detected between the NOx emissions and fuel-N content. In addition to the NO and SO2 emissions, the sintering propensity of the ash residue were investigated using scanning electron microscopy (SEM).