The properties of renewable fuels, e.g. biomass, can be improved by processing them before the combustion process. This preprocessing consists of different types of activities, among other, adding fuel additives. This paper describes the selection of a specific fuel additive (halloysite) for fuels with a high content of chlorine, sodium and potassium. The selection of the appropriate dose must be related to the analysis of the primary reactions occurring with the additive, and also include the interaction of the additive with all ash and exhaust components. To determine properly an impact of the halloysite on the fuel and ash properties the fuels should be recognized as a potentially high-risk fuels with a low ash melting temperature. Different types of biomass fuels have been selected. The investigation was extended by additional halloysite dose selection that includes primary reactions between additive and fuel. This paper presents the methodology for the selection of a fuel additive, including chemical reactions of the additive and ash compounds. The choice of fuel additive and its dose should be preceded by a comprehensive analysis of the additive's impact on the entire combustion process. (C) 2020 Published by Elsevier Ltd.