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.
This work evaluated the effects of inherent alkali and alkaline earth metals on nitrogen transformation during steam gasification of Shengli lignite at the temperature of 873-1173 K in a fluidized-bed/fixed-bed quartz reactor. The results indicated that the alkali metal Na and alkaline earth metals Ca, Mg in coal have different effects on inherent nitrogen transformation to NH 3 , HCN and char-N during the lignite steam gasification. Specifically during the steam gasification of Shengli lignite, Na and Ca, Mg not only catalyze the inherent nitrogen conversions to NH 3 , but also promote the secondary reactions of the nascent char-N as well as the generation of NH 3 from the generated HCN, meanwhile they also inhibited the inherent nitrogen conversion to HCN and char-N. The presence of Na, Ca and Mg hindered the formation of oxidized nitrogen (N-X) functional groups, but enhanced pyridinic nitrogen (N-6) and quaternary nitrogen's (N-Q) formation in char.
This study aims to investigate the effects of calcium on the migration of nitrogen in coal (coal-N) to N-containing gas species, particularly, NH 3 and HCN (volatile-N) in volatiles, as well as the chemical transformation of the N in char during coal pyrolysis under different temperatures. The pyrolysis experiments of Shengli brown coal and its derived coal samples loaded with different contents of calcium were conducted under 600-800°C in a novel fluidized bed reactor. The experimental results showed that during coal pyrolysis, the generation of NH 3 is mainly derived from secondary reactions among volatiles, tar and char with the catalytic effect of mineral matter, especially calcium in coal. Increasing pyrolysis temperature from 600 to 800°C could enhance the release of N in coal to volatiles. Meanwhile, the increased pyrolysis temperature could also inhibit the generation of NH 3 while facilitating the formation of HCN. The release of HCN is more sensitive to pyrolysis temperatures. Specifically, under higher pyrolysis temperatures, more N-containing structures in coal would become thermally unstable and crack into HCN; On the other hand, higher pyrolysis temperature could also enhance the decomposition of N in coal to N-containing species in tar or N 2 , thus reducing the release of HCN and NH 3. Nitrogen in tar could either undergo secondary decomposition reactions, generating NH 3 , HCN, N 2 and other N-containing species in gas phase, or experience condensation polymerization by forming macromolecular structure and be retained in char at high pyrolysis temperatures. Calcium could significantly restrain the release of N from coal, thus reducing the yields of NH 3 and HCN. During coal pyrolysis, calcium catalytically enhances the fracture and combination of chemical bonds, generating abundant free radicals. These free radicals could continuously attack N-containing structures and consequently release the N-containing gaseous products, such as NH 3 , HCN, N 2 etc., resulting in the decrease of N in char. Calcium also plays important roles in nitrogen transformation in char during coal pyrolysis by catalytically intensifying the transformation of N in char from pyridinic nitrogen (N-6) and pyrrolic nitrogen (N-5) to quaternary type nitrogen (N-Q) during coal pyrolysis.
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