Investigation on transferring and release characteristics of chlorine during pyrolysis of low rank coal

In order to reveal the release mechanism of chlorine(Cl) in low rank coal during pyrolysis process, a low rank coal from northern Shaanxi was pyrolyzed at different temperatures, particle sizes and heating rates in a tubular reactor and a Thermo Gravimetric-Infrared-Mass spectrometry (TG-IR-MS) to investigate the transferring and release characteristics of Cl during pyrolysis process. The dynamic model of Cl transferring and release was built based on the process of coal pyrolysis. The results show that the chloric species from the pyrolysis of low rank coal is HCl and little Cl₂, and the temperature strongly influences the release rate of Cl during pyrolysis in the tubular reactor. With the pyrolysis temperature increasing from 300 to 800 ℃, the release rate of Cl increases significantly, reaching to 49.5% at 800 ℃, while the fraction of Cl in the char decreases and the fraction of Cl in coal tar and pyrolysis gas increases gradually. The release rate of Cl has a maximum of 35.8% as the particle size is 3.0-4.0 mm. The particle size also affects the distribution of Cl in pyrolysis products. Moreover, increasing the heating rate is beneficial to the devolatilization, and the release rate of Cl in coal changes obviously as the heating rate is 15-25 ℃/min, but much higher heating rate will result in the decrease of the release rate of Cl in coal. Most Cl species in the coal tar is in form of water-soluble inorganic chlorides, and therefore the ultrapure water can present a better performance to remove chlorides. The release activation energy of chlorine in pyrolysis is about 20 kJ/mol.