Numerical investigation on the combustion behaviour of pre-dried Greek lignite

Dry coal firing is expected to play an increasingly important role in future lignite power plants. The planned evolution from the conventional lignite drying concept with hot recirculated flue gas to the “fluidized bed drying with internal heat utilization, WTA”, technology in the next generation of lignite power plants is estimated to bring an additional efficiency increase of 2–4% points compared to the today’s state of the art. In this framework NTUA/LSB and CERTH/ISFTA has performed experimental investigations at a semi industrial scale 1 MWth facility on the characterization of Greek pre-dried lignite’s combustion behaviour in terms of temperature fields, heat transfer, emissions, slagging and fouling tendency and residues quality. The present work focuses on the numerical investigation of Greek dry lignite combustion firstly by post-processing and evaluation of available experimental data and secondly by combustion simulations. Q–T plots describing the heat transfer in the experimental facility are derived and specific cases of the performed tests are simulated with a commercial CFD tool, in order to estimate flow, temperature fields, NOx emissions and compare with the available experimental data. A good agreement between simulated and experimental results will support the further work on large scale boiler simulations in raw and dry coal co-firing mode, where the possibility of validation with experimental data is limited. The obtained Q–T diagrams are used to evaluate the influence of co-firing on the heat transfer in the facility and to further extrapolate the conclusions of the performed semi industrial tests on the large scale. The overall results of the CFD simulations, including predictions of temperature and NOx profiles, are in good agreement with the available experimental data at the reference case, while at the dry coal co-firing cases succeed on reproducing the basic trends of the performed experiments.