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A Study on coal combustion: experiments and modelling

Özer, Burak
Coal combustion involves multi-scale, multi-phase and multi-component aspects, in a process where both transport phenomena and reaction kinetics must be considered. The aim of the work is to investigate how the lignite characteristics and origin affect the combustion kinetics at different heating rates. Three Turkish lignites from different regions (Soma lignite, Tunçbilek lignite, Afşin-Elbistan lignite) and one German lignite (Rhenish lignite) were used. Combustion characteristics of these lignites are investigated experimentally and numerically. Experiments are conducted using a high temperature (1000ºC) and high heating rate (~104 ºC/s) drop tube furnace (DTF), along with a thermogravimetric analyzer (TGA) at non-isothermal conditions (5, 10, 15, 20 ºC/min). The numerical part of the study includes the computational fluid dynamic analysis of DTF and the predictive multi-step kinetic PoliMi model analysis of the fuel particle. TGA experiments show that the ratio of volatile matter over fixed carbon has an effect on the ignition times. Moreover, maximum reaction rates obtained by TGA experiments are inversely proportional to the ash contents of the fuels used. High heating rate DTF experiments shows similar combustion behaviors with TGA experiments. According to DTF experiments, RL has highest reactivity (RL: 7.8 s-1) among all fuels (AEL: 5.3, SL: 4.7, TL: 2.9 s-1). In comparison to experimental data, PoliMi model predictions on high temperature volatile yields are satisfactory with 5-vi 7 % errors whereas devolatilization rates are overpredicted. However, PoliMi model predictions on char oxidation rates are in agreement with the experimental data.