Fluidized bed boilers are often the technology of choice to fire biomass fuels, and circulating fluidized bed boilers (CFB) are most advantageous for large-scale applications, including utility boilers. During the recent years public subsidies supporting renewable fuels increased the opportunities and thus the demand for highly efficient boilers capable to co-fire woody biomass with large shares of agricultural residues (agro). The ash chemistry of such fuel mixtures is notoriously challenging for highly efficient boilers, where it can lead to intense bed agglomeration, fouling, and corrosion. This paper list the technical difficulties in co-combustion of woody biomass with agro biomass, the R&D conducted to verify the challenges and countermeasures, and the distinguishing features of the Advanced Biomass CFB (ABC) technology newly developed by Foster wheeler for utility-size CFBs. ABC boilers have been recently booked to co-fire biomass with up to 20 wt.% agro-biomass including willow, oat husk, palm kernel, straw, dried fruits, sunflower, and rapeseed residues. The largest of these units (447 MWth) is designed to produce 158 kg/s of superheated steam at 128 bar(a) and 535 degrees C. (C) 2011 Elsevier B.V. All rights reserved.