Evolution behavior of reactivity and microstructure of lignite char during in-situ gasification with steam

The pyrolysis of brown coal and the in-situ gasification of "hot" char were carried out in a newly-designed two-stage fixed-bed reactor. Firstly, the brown coal was pyrolyzed in ultrapure argon for 5 s under the temperature of 400 ℃, 600 ℃, and 800 ℃ at the pyrolysis stage. Then the reactor was shifted to the "gasification stage" immediately and the "hot char" obtained at the pyrolysis stage was gasified in-situ for 2~30 min at 900 ℃ with a 15%H₂O balanced with argon to obtain a so-called in-situ gasification char. TGA and Raman spectroscope were used to characterize the reactivity and microstructure of char, respectively. Results show that the yields of in-situ gasification char decrease significantly within the initial 10 min, and then decrease slowly with prolonging the holding time further (10~30 min). Plenty of O-containing functional groups are released in the first 2 min during gasification. The ratio of small aromatic ring systems to big aromatic ring systems decreases dramatically within 2 min, and then decreases slowly from 2 min to 30 min. Microstructure of char has an effect on its reactivity. With the char gasification, the amorphous carbon as well as small aromatic ring systems with high activity decreases, leading to the char reactivity decreaseing consequently.