Combustion behavior and kinetics of a Turkish lignite blended with biomass/magnesite dust

2017
Yousefzad Farrokhi, Farshid
This study investigated the effect of blending on the combustion behavior of Turkish lignite blended with biomass or magnesite dust using a thermogravimetric analyzer (TGA) under air atmosphere. The lignite used in this study is Tunçbilek lignite (TL), which is blended with the biomass types; olive residue (OR) and almond shell (AS), and the inorganic industrial waste, magnesite dust (MD). The blends are composed of various weight fractions of fuels, with a constant weight fraction of molasses (10 wt. %) as a binding agent. In addition, neat fuels, with and without molasses, are analyzed for comparison purposes. TGA weight loss trends are used to obtain characteristic temperatures and to define weight conversion stages. Experimental results show three distinct stages of conversion during combustion of biomass fuels and two stages for lignite. The stages are represented as stage A, B, C in the text. Stage A represents decomposition of the fuel, stage B shows first stage of combustion, and stage C represents the major stage of combustion. Four main characteristic temperatures obtained from TGA trends are decomposition temperature (Td), ignition temperature (Tig), peak temperature (Tp), and burnout temperature (TB). Burnout temperature of lignite is 630 ºC which is shifted to a higher temperature at approximately 723 ºC when blended with molasses. This is an indication of a synergistic effect of molasses on combustion characteristics of lignite. On the other hand blending of biomass with lignite results in lower decomposition and ignition temperatures. Furthermore, CO2, CO, H2O and SOx emissions are monitored during the combustion of lignite and its blends using a Fourier transform spectrometer (FTIR) coupled with TGA. FTIR results show the positive effect of olive residue in reducing the CO and SOx emissions when blended with lignite, whereas almond shell does not show a consistent trend. Magnesite dust addition causes a decrease in gaseous emissions for all blending ratios having the maximum reduction at 10 wt. %. On the other hand, the binder (molasses) addition to neat fuels effect the gaseous emissions. CO2, H2O, and SOx emissions increase slightly by adding molasses to olive residue, whereas, they decrease by adding to almond shell. Moreover, kinetic parameters (activation energy, pre-exponential factor) for each fuel are obtained using a model fitting method (Coats-Redfern). Regarding to the model, stage B of relevant fuels is controlled by reaction order mechanism, whereas, the responsible mechanism for stage C is diffusion control. Addition of biomass/magnesite dust to the lignite caused a decrease in activation energy of stage C which is an indication of increased reactivity. Specifically, activation energy of lignite decreases from 105.6 kJ.mol-1 to 81.6 kJ.mol-1 by adding molasses and finally reaches to 20 and 22.4 kJ.mol-1 by adding olive residue and almond shell with a blending ratio of 70 wt.%, respectively. The activation energy increases as follows for stage B: 20TL70OR, 90OR, and 100OR, whereas, it decreases for the same portions of samples for almond shell. 

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Citation Formats
F. Yousefzad Farrokhi, “Combustion behavior and kinetics of a Turkish lignite blended with biomass/magnesite dust,” M.S. - Master of Science, Middle East Technical University, 2017.