Structural Evolution of Maize Stalk Particles during Pyrolysis

The structure evolution of maize straw (MS) particles during pyrolysis was studied. The samples were characterized by ultimate analysis, Fourier transform infrared spectroscopy (FTIR), true density measurement and N₂ isothermal adsorption/desorption method. The H/C and O/C atomic ratios decreased from 1.59 and 0.84 in the raw MS to 0.21 and 0.33 at 1173 K respectively, which implied that the char became progressively more aromatic with increasing temperature. Above 773 K, the H/C ratio decreased dramatically compared to the O/C ratio, suggesting a direct dehydrogenation of the char at high temperatures. FTIR results showed that the hydroxyl, methylene, carbonyl and olefinic C=C functional groups were lost at high temperatures. The aromatization process started below 673 K and continued to higher temperatures. The BET surface area reached a maximum value at 773 K, and at higher temperatures, the specific area dropped significantly, probably due to thermal annealing. The moderate shrinkage of the carbon structure occurred when the temperature was higher than 773 K, which was concurrent with the aromatization process. The loss of volatile matter produced pore opening, whereas the structural shrinkage was responsible for pore narrowing.