Carbon dioxide encapsulation in methane hydrates.
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Date
2022
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Abstract
Coal mining and petroleum refining processes face extreme pressure under climate change and
global warming threats. Hence alternative sustainable and renewable energy sources must be
made available for the rising energy demands. Natural gas found in permafrost and seabed
areas in the form of gas hydrates possess vast amounts of low-carbon methane gas, which can
replace fossil-based energy sources. The capture and storage of carbon dioxide gas in natural
gas hydrate beds with the release of methane gas is a sustainable route under intense research.
This study investigates the methane-carbon dioxide (CH4-CO2) replacement reaction
mechanisms and the improvement of the process using different techniques, namely, additives,
secondary gas, and thermal stimulation. Firstly, the gas hydrate dissociation measurements for
the former gases utilized in the study were performed. This was followed by kinetic
measurements with nanoparticles (aluminum oxide, copper oxide, and graphene nanoplatelets)
and chemical additives (zinc oxide powder, graphite powder, and magnesium nitrate
hexahydrate crystals) in the presence of sodium dodecyl sulfate (SDS) to affect kinetic or
thermodynamic improvement in hydrate formation. The kinetic parameters investigated were
induction time, hydrate storage capacity, water consumed in hydrate formation, fugacity of the
gaseous phase, and the ratio of gas consumed to moles of water. Graphene nanoplatelets were
selected for replacement reaction based on promising results obtained from the kinetic studies.
The CH4-CO2 replacement process was performed in a 52 cm3 equilibrium cell using deionized
water and nanoparticles. Also, a new experimental setup with a 300 cm3 reaction vessel was
designed and assembled for CH4-CO2 replacement in the presence of synthetic silica sand.
The results from kinetic studies showed an improvement in the hydrate formation kinetics due
to the presence of nanoparticles. The CO2 hydrate formation kinetics obtained a maximum
storage capacity of 51 (v/v), with 1.2 wt.% graphene nanoplatelets which also produced a
maximum water conversion of 25%. When nanoparticles were added, the induction time for
CO2 hydrate in deionized water was reduced from 9 minutes to less than one minute. Graphite
powder with a concentration of 1.2 wt.% had the highest rate of gas uptake of 0.0024 (mol of
gas/ mol of water. min). In CH4 kinetics, the induction time was reduced from 18 minutes with
deionized water to less than one minute due to addition of nanoparticles. A maximum storage
capacity of 28.5 (v/v), water-to-hydrate conversion of 13.09%, rate of gas uptake of 0.0089
(mol of gas/ mol of water. min), and gas consumption of 0.0238 moles were obtained with 0.1
wt.% CuO + 0.05 wt.% SDS. Also, CH4-CO2 replacement measurements showed that an 80
mol% N2/20 mol% CO2 gas mixture yielded a CH4 replacement efficiency of 17.04% at a temperature of 274.77 K and pressure of 5.34 MPa. The highest amount of CO2 sequestrated
was 57.03%, and 28.77% was the highest CH4 replacement efficiency. These results were
obtained using pressurized CO2 with application of thermal stimulation at a temperature of
275.90 K and pressure of 5.66 MPa. In the replacement reaction with silica sand, the maximum
amount of CH4 replaced was 37.49% with the pressurized CO2 at a pressure of 7.01 MPa and
temperature of 276.43 K. Applying thermal stimulation and adding secondary gas (N2)
improved CO2 sequestration from 51.73% to 76.63%. These outcomes are vital in applying
hydrates in gas storage and CO2 sequestration.
Description
Doctoral Degree. University of KwaZulu-Natal, Durban.