This paper describes the practical combustion issues encountered with biomass co-firing on a large scale trial in a 500 MW down fired utility boiler at Aberthaw power station. It also investigates and discusses the effect of biomass particle size and physical properties on devolatilisation; flame stability; and slagging by using the biomass energy crop miscanthus. During large scale biomass co-firing, the air flow around the injectors plays an important part in the fuel combustion and stable boiler operation. Secondary air flow at the point of biomass injection was adjusted during the experiments to provide higher airflows around the biomass injectors, hence the mixing, and air available for biomass combustion, was improved producing less unburnt material in the ash and reducing potential filter blockages. However, higher biomass air flows disrupt the temperature distribution in the boiler causing a wider variation of superheater temperatures compared to the baseline condition firing coal only. This reduces the combustion efficiency and can lead to localised hot spots, in the superheaters, with the potential to cause equipment damage. The trials indicated during the experiments that a secondary biomass air flow of 50% of its maximum, gave an optimum balance between air/biomass mixing and variations in the superheater temperatures. (C) 2012 Elsevier Ltd. All rights reserved.