Abstract
Pyrolysis and combustion behaviors of three coals (A, B, and C coals) were investigated and their combustion kinetics were calculated by the Freeman–Carroll method to obtain quantitative insight into their combustion behaviors. Moreover, the effects of coal size, air flow, oxygen content, and heating rate on coal combustion behaviors were analyzed. Results showed that the three coals have a similar trend of pyrolysis that occurs at about 670 K and this process continuously proceeds along with their combustion. Combustion characteristics and kinetic parameters can be applied to analyze coal combustion behaviors. Three coals having combustion characteristics of suitable ignition temperature (745–761 K), DTGmax (14.20–15.72%/min), and burnout time (7.45–8.10 min) were analyzed in a rotary kiln. Combustion kinetic parameters provide quantitative insights into coal combustion behavior. The suitable particle size for coal combustion in a kiln is that the content of less than 74 μm is 60% to 80%. Low activation energy and reaction order make coal, especially C coal, have a simple combustion mechanism, great reactivity, be easily ignited, and a low peak temperature in the combustion state. Oxygen-enrichment and high heating rates enhance coal combustion, increasing combustion intensity and peak value, thus shortening burnout time.
Highlights
Coal as a traditional fossil fuel is widely used in metallurgical production, such as iron and steel production
Thermal behavior of fuel has been widely studied by thermogravimetric analysis (TGA), which is an extensive and effective research method for studying thermal behavior [8,9,10,11,12]
Combustion kinetics of coal can be revealed by Equation (1) [20,21]
Summary
Coal as a traditional fossil fuel is widely used in metallurgical production, such as iron and steel production. Rotary kilns mainly use coal as an energy source to complete the preheating and roasting of green pellets. Coal consumption in rotary kilns is enormous and emits massive amounts of pollution gas into the atmosphere because million tons of steel is produced every year in China, leading to serious energy consumption and environmental pollution [1,2,3,4]. Mathematical models have been designed to forecast heat transfer and sintering temperatures in rotary kilns [5,6,7]. The results predicted by mathematical models are not accurate enough because coal combustion in a kiln is very complex. Since the thermal behavior of coal in kiln is unclear, Energies 2018, 11, 1055; doi:10.3390/en11051055 www.mdpi.com/journal/energies
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