Energy production based on combustion of biomass is environmentally attractive both according to future fuel dependency and in order to keep CO2-emissions down. However, combustion of biomass holds several challenges like uncontrolled ash deposit build up and corrosion due to the nature of the ash-forming elements in the biomass. This paper presents results from full-scale experiments on a 250 MWth straw pellet fired suspension drum-type boiler in Denmark: Amager Power Station Unit 2. At the power station problems with severe deposit build up and following increased maintenance costs are experienced. To investigate possible solutions to this problem, fuel additives were tested. As such, the work is applied research to try and solve a significant problem at the power station. The objective of this work was to evaluate the ability to decrease ash deposit formation by utilizing additives together with the straw pellets, hence altering the ash-forming mechanisms in the boiler. During a period of 14 days four additives were added to the boiler during (full-scale) operation of the power station. The additives used were: sand, dicalciumphosphate (DCP), chalk and bentonite, thereby representing both Al-Si and P-Ca type substances. Each additive was added to the straw before the mills and comprised approximately 5% of the straw on a mass basis. The experiments showed that chalk and DCP were unsuitable as additives, due to their physically white appearance which produced white re-radiating deposits, leading to poor operation of the boiler.Sand and bentonite are more promising additives, but further and longer testing is needed to conclusively evaluate their effect on deposit build up, ash-handling systems and so on. Bentonite is prohibitively expensive, and a cheaper product with similar characteristics would be required. (C) 2007 Elsevier B.V. All rights reserved.