Effect Of Particle Size On The Process Of Coal Pyrolysis And Gasification

Efficient use of coal and controlling serious environmental pollution in processes ofChina’s traditional coal mining, transport and transformation are becoming a very importantissue. As a potential advantage clean coal technology, underground coal gasification （UCG）technology has begun to be focused on research. The mechanism of UCG is similar as thetraditional coal gasification, but the processes and influencing factors are different. Hence,based on the research of the traditional coal gasification, the kinetics and thermodynamicsanalysis method can be used to study the characteristics of the underground coalgasification process.In this paper, Dayan coal and Eerduosi coal were used to study the effects of particlesize on the coal pyrolysis and gasification. There are four sizes of coal samples:20mm*20mm*20mm,10mm*10mm*10mm,1.25mm-2mm and74μm-105μm. Firstly, theeffect of particle size on the dynamic mechanism of coal pyrolysis and gasification wasinvestigated. The results show that coal gasification reaction rate decreases withthe increase of particle size which is controlled by heat and mass transfer process.Homogeneous model and shrinking core model can make good simulations on the coalgasification with different scales of coal. It’s found that there is a kinetic compensationeffect between apparent activation energy and pre-exponential factor for different sizes ofcoal samples, and under the same conditions, the kinetic compensation effect of differentkinds of coal char scale is the similar.Then, based on the research on the pyrolysis mechanism between different scales andits associated factors, it’s found that homogeneous mode can make good simulations on thepyrolysis process with different scales. The pyrolysis process of different scales is almostthe same. Through the analysis of internal char morphology and microcrystalline structure,it’s found that the coal crushing and mesoporous structure forming in the pyrolysis processmake the mainly impact on devolatilization. And the pyrolysis of large-scale coal at slow heating process forms large cracks inside, while the part near outside forms small surfacecracks and small holes and channels. This morphological structure makes impact on thegasification process.Finally, the char at rapid heating and slow heating process were used to react with CO₂at1000℃. For the char at fast heating process, it’s found that gasification kinetic model ofall the scales is very suitable by simulation using the shrinking core model, and with theincreasing of the scale, the homogeneous model is becoming better. For the char at slowheating process, it’s found that with the increasing of the scale, there is a trend that theshrinking core model is usable rather than the homogeneous model. At the same time,according to the different flow rates and reaction gas concentrations in the experiment, it’sfound that the gasification reaction rate of different scales is mainly due to the differentinternal diffusion process, the impact of external diffusion and chemical reaction is verysmall.