Coal and char properties for high temperature entrained flow gasification

Despite the wide application of entrained flow gasifiers worldwide the physical and chemical reactions at operation conditions (1400°C to 1600°C, up to 5.0 MPa) are almost unknown and gasifiers are designed mainly based on practical experience. With the objective to measure coal conversion at realistic operation conditions the Pressurised High Temperature Entrained Flow Reactor (PiTER) was developed. This novel pilot-scale reactor is designed to measure reaction kinetics of pulverized fuels in a defined gas atmosphere at very high temperature and pressure. The influence of temperature and pressure on the pyrolysis of Rhenish lignite was studied at temperatures up to 1600°C and pressures up to 2.5 MPa. At longer residence time (> 1.5 s) volatile yield is 66 wt% and independent of temperature and pressure. During PiTER operation char samples were extracted from the hot reaction zone and analysed by TGA experiments at lower temperature and specific char surface was measured. Both measurements are indicators for char reactivity also at higher temperature. At 1200°C intrinsic reactivity of char decreases with residence time, but surface area is hardly affected. At 1400°C and 1600°C intrinsic reactivity decreases considerably, but residence time only has a minor effect and surface area is decreased. The difference in deactivation can only be explained by two different mechanisms. Experiments at 1200°C are below the ash fusion temperature of lignite and graphitisation at the char surface leads to a reorganisation of carbon atoms and a loss of active sites. Above ash fusion temperature melting of mineral matter additionally blocks the micropore structure and results in a loss of specific surface area. The observations cannot be explained by current reaction models that are mainly based on thermal analysis at lower temperature. The high temperature data is the basis for the development of more detailed kinetic models.