High percentage (up to 70% energy based) biomass co-gasification tests have been carried out at the 253 MWe coal based Willem-Alexander Centrale (WAC), Buggenum in The Netherlands utilizing steam exploded wood pellets to assess feasibility of scaling up and to address stringent EU emission requirements in the coming decades. This principal article for demonstrating high percentage biomass co-gasification in large scale IGCC power plants, presents the obtained experimental results with a detailed and validated steady state thermodynamic model developed as an aid to assess future plant operations. The validated model is also used to predict plant performance involving 70% co-gasification with two fuel blends of torrefied wood pellets since the desired power output of 230 MWe could not be achieved with steam exploded wood pellets. The model predicts plant performance and process parameters with reasonable accuracy and gives a net power output of 173 MW and a net plant efficiency of about 37.2% with steam exploded wood pellets. A net output of 240 MWe and net plant efficiency of 41.7% is predicted for 70% co-gasification with high lower heating value (LHV) torrefied wood pellets. Exergy analysis indicates largest thermodynamic losses in the gasifier and during combustion, providing additional scope for efficiency enhancement. The demonstration of such a high percentage biomass co-gasification test at a large scale power plant is of vital importance for further development of low emission/carbon neutral power plants. The presented test data also serves as a reliable and prime data source for modeling studies. The validated models could serve as a strong platform to plan real plant operation with various biofuels and carry out studies involving novel technology integration, retrofitting and plant optimization. (C) 2016 Elsevier Ltd. All rights reserved.