With the objective to measure coal conversion at realistic operation conditions the Pressurised High Temperature Entrained Flow Reactor (PiTER) is developed. The pyrolysis of Rhenish lignite is studied at temperatures up to 1600 degrees C and pressures up to 2.5 MPa. At longer residence time (above 1.5 s) volatile yield is 68 wt% and independent of temperature and pressure. Char samples are extracted from the hot reaction zone and their reactivity is analysed by weight loss in TGA experiments at defined conditions. Furthermore, specific char surface area is measured. At 1200 degrees C the intrinsic reactivity of char decreases by a factor of almost 7 from 0.5 s to 2 s residence time, but surface area (approximately 500 m(2)/g) is hardly affected. At 1400 degrees C and 1600 degrees C, the intrinsic reactivity also decreases, but simultaneously the surface area is reduced to below 300 m(2)/g. The difference in deactivation can only be explained by two different mechanisms: (i) experiments at 1200 degrees C are below the ash fusion temperature and graphitisation at the char surface may lead to a reorganisation of carbon atoms; (ii) above the ash fusion temperature, the melting of mineral matter additionally blocks the micropore structure and results in a loss of specific surface area. (C) 2012 Elsevier Ltd. All rights reserved.