Advanced Characterization of Available not Conventional Mediterranean Biomass Solid Fuels for Ash Related Issues in Thermal Processes

Abstract

According to the European Biomass Association, bioenergy represents about 60% of the EU’s total consumption of renewables. The EERA Bioenergy, section of the European Energy Research Alliance, indicates that agricultural residues and non-food crops should be used at farm- and village- scales, while low quality biomass and waste could be used for medium- and large- scale power and CHP plants. Among the barriers that hamper the efficient processing of solid fuels in thermal processes, such as combustion and gasification, ash related issues are still challenging for plant operators. These issues include agglomeration in fluidized bed combustion, slag formation, and fouling of convective heat exchangers. Furthermore, advanced characterization methods for solid biomass fuels, especially predictive approaches for ash related issues, are not fully developed. Consequently, the use of unconventional biomass solid fuels is limited because of operating risks. In addition, the availability of the detailed chemical analyses of unconventional fuels is often limited. In this investigation, a validated fuel characterization tool to define the agglomeration - slagging propensity of biomass fuels for their use in thermal processes, is applied to an advanced database of mediterranean (Greece, Italy, Spain) opportunity fuels. The database is built by means of an extensive literature review, including the detailed ash chemical compositions. The input of the tool is the specific fuel ash chemical composition, and the output is the agglomeration - slagging propensity computed as a parameter in the range (0 – 100). In this work it is shown that a high ash fusion temperature (from the Ash Fusion Test) is a necessary but not sufficient condition to low agglomeration and slagging in combustion applications. Among the fuels analyzed, wood, shrubs and citrus industry agro-residues evidenced a low-to-medium median propensity to agglomerate and slag, contrarily to the high propensity of agro-residues (various), wine industry residues, and crops. About 20 opportunity fuels, with low-to-medium agglomeration and slagging propensity and for which environmental, economic and social sustainability has to be further explored, are suggested for thermal applications. Some countermeasures to efficiently process the more challenging fuels are mentioned, including the use of specific combustion additives.