Combustion of Turkish lignites and olive residue: Experiments and kinetic modelling

- Combustion characteristics of two lignites and a biomass are investigated.
- Lignites present lower reactivity and combustibility compared to the biomass.
- Lignites present higher burnout temperatures compared to the biomass.
- Tunçbilek lignite presents the highest apparent activation energy.
- Soma lignite and olive residue had similar apparent activation energy values.
- Increasing heating rate increased combustibility and burnout temperatures.

This study investigated the combustion behavior and kinetics of Turkish fuels. Two lignite coals from Tunçbilek and Soma region, and olive residue, were used, all within a size range of 106-125 µm. Experiments were performed in a thermogravimetric analyzer (TGA) coupled with a differential thermal analyzer (DTA), under three different heating rates, namely 15, 20, and 40 °C/min. Based on the weight loss (TG) and derivative weight loss (DTG) curves, the characteristic temperatures were determined, three different conversion stages were identified, and a combustibility index was calculated for the major stage of combustion. In addition, combustion kinetics parameters of each fuel were determined using the Coats-Redfern method. Experimental results revealed that all three fuels went through a decomposition stage followed by the combustion stage(s). Tunçbilek and Soma lignite had one major combustion stage at an approximate peak temperature of 500 °C, while olive residue had two distinct stages for combustion at peak temperatures of 290 °C and 423 °C. Burnout temperatures of olive residue were always lower than either of the lignites, and an increase in heating rate from 20 to 40 °C/min shifted the burnout to higher temperatures. Combustibility index of lignites was similar at low heating rates, whereas at higher heating rates the combustibility of Tunçbilek was approximately twice that of Soma lignite. Olive residue presented values of combustibility which were at least fivefold those of the lignites. During the major stage of combustion and at 20 °C/min, Tunçbilek lignite had the highest apparent activation energy of approximately 100 kJ/mol, while Soma lignite and olive residue had similar apparent activation energies of approximately 40 kJ/mol. Increasing heating rate clearly increased the reactivity, combustibility, and burnout temperatures of all fuel samples. Apparent activation energies decreased with a shift in the heating rate from 20 to 40 °C/min, whereas between 15 and 20 °C/min the values were approximately constant.