Moisture significantly
affects the adsorption capability of gas
shales for methane (CH4), the main component of shale gas.
Primary moisture, i.e., the moisture that exists in in situ shale
reservoirs, is therefore crucial to estimate and produce shale gas
resource. Aiming to understand the influences of primary moisture
on CH4 adsorption on shale samples, the occurrence of primary
moisture in shales gathered from the Lower Silurian Longmaxi Formation
located in southern Sichuan Basin of China was experimentally investigated.
Additionally, the primary moisture dependence of CH4 adsorption
equilibrium and thermodynamics of shales was investigated. Results
indicate that the primary moisture contents of the four shale samples
vary between 0.64 and 0.82% (mass percentage), positively correlated
with the clay mineral content of shale samples. The adsorption equilibrium
behavior regarding water vapor on shales well follows the modified
Brunauer–Emmett–Teller (BET) equation. The water vapor
is typically adsorbed onto the primary adsorption sites of shale samples,
i.e., the oxygenic functional groups consisting of COOH, conjugated
CO, and highly conjugated CO, and the secondary adsorption
sites, i.e., the previously adsorbed water molecules and clay minerals.
The pores with pore diameter less than 4 nm of shales are the main
accommodation space for primary moisture. The adsorption equilibrium
of CH4 on primary moisture-containing shales well obeys
the Ono–Kondo lattice equation. On the basis of the modeling
results, the primary moisture causes a remarkable reduction in maximum
CH4 adsorption capacity of shale samples by 12.86–45.45%.
Moreover, the primary moisture reduces the isosteric heat of CH4 adsorption on shale samples. In summary, the primary moisture
in gas shales decreases the adsorption affinity between CH4 and shale samples. Therefore, focusing on the effects of primary
moisture on CH4 adsorption on shales is vital to better
estimate and produce shale gas resource.
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
NO
x
. 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,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.
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