Structure and CO2 Gasification Reactivity of Char Derived through Pressured Hydropyrolysis from Low-Rank Coal

Abstract

Chinese long flame coal named Naomaohu was pyrolyzed at 600 °C under hydrogen pressures of 0.1–3.0 MPa. The resultant chars were gasified at 900 °C under carbon dioxide to illustrate the correlations between structure and reactivity of pyrolyzed chars. For comparison, experiments under the same conditions in the nitrogen atmosphere were also conducted. The char yield decreased markedly with increasing hydrogen pressure, whereas the char yield under nitrogen stayed constant regardless of nitrogen pressure, being certainly larger than that of hydropyrolysis. The structure of chars was analyzed by Raman spectroscopy and X-ray diffraction, which informed that more content of larger aromatic sheets and higher graphitic order was found, respectively, in the char pyrolyzed under higher pressures. Hydropyrolysis certainly led to more content of larger aromatic sheets and higher graphitic order in its char than those of chars pyrolyzed under the nitrogen atmosphere. The gasification reactivity of hydropyrolyzed char decreased with the increase of hydrogen pressure, indicating that the higher graphitic order and more content of larger aromatic sheets in char are related to the lower char gasification reactivity. Higher hydrogen pressure in the pyrolysis can enhance the hydrocracking of the reactive char components, decreasing yield, and gasification reactivity of the chars. The intrinsic structure features of raw char obtained at 600 °C have good correlations with its CO2 gasification reactivity at 900 °C. The successive combination of hydropyrolysis and CO2 gasification has been promised to provide an effective conversion scheme of the low-rank coals. The most efficient balance of hydropyrolysis and gasification conversions can be designed or higher reactivity of hydropyrolyzed char can be explored.