Adsorption and Desorption Behaviors of Nitrous Oxide on Various Rank Coals: Implications for Oxy-coal Combustion Flue Gas Sequestration in Deep Coal Seams

摘要

Injecting oxy-coal combustion flue gas into deep coal seams is viable to simultaneously reduce the main anthropogenic greenhouse gas (GHG) CO2 and gaseous contaminants SO2 and NOx. This paper investigates the adsorption and desorption behaviors of N2O on different rank coals from peat to anthracite. The potential adsorption mechanism is also elucidated. The results show that the Sips model can well describe the equilibrium relationship of N2O adsorption on coals. The fitting results derived from the Sips model indicate that the adsorption affinity of N2O on coals decreases with the increasing coal rank, while the heterogeneity of the adsorption system tends to be stronger with the decreasing coal rank. The micropore surface area of coals greatly determines the maximum adsorption capacity of N2O derived from the Sips model. The kinetics process of N2O adsorption on coals follows the simplified bidisperse model, and it is controlled by the micropore diffusion. The apparent diffusion coefficient in micropores mainly depends upon micropore surface area of coals. The adsorption and desorption process of N2O on the high-rank Fumin (FM) coal (R-o,R-max = 2.59%) is a completely reversible and physical adsorption process. In contrast, the adsorption and desorption hysteresis of N2O on coals becomes more significant with the coal rank decreasing from 0.83 to 0.23%, indicating that the chemical adsorption of N2O exists for the low-rank coals. The X-ray photoelectron spectroscopy characterization further reveals that the oxygenic and nitric speciation compositions of the high-rank FM coal after N2O adsorption remain unchanged. However, the oxygenic functional groups in the low-rank coals act as the main active sites for the chemical adsorption of N2O. Interaction with N2O only increases the total nitrogen content of the three low-rank coals but also changes their nitric speciation compositions, which are characterized by the increasing content of pyridine N and oxide N and the decreasing content of pyrrole/pyridone N. The aforementioned chemical adsorption is beneficial for stable storage of N2O in the target coal seams with a low metamorphic degree.