Numerical modelling of the thermal decomposition of methane to hydrogen and carbon black
Creators: Punia, Ambuj
Publisher: name: University of Alberta Library
Year: 2024

<p>Fossil fuel combustion results in carbon dioxide (CO$<em>{2}$) and particulate emissions which are linked to climate change and health problems, respectively. Hydrogen can be an alternative zero-emission fuel for future energy systems. However, hydrogen is not sufficiently available in nature in its pure form, so it needs to be extracted. Methane pyrolysis enables the production of hydrogen through the use of methane while eliminating its combustion thereby eliminating CO$</em>{2}$ and particulate emissions. Methane, heated to high temperatures in the absence of oxygen, converts to hydrogen, solid carbon, and a small fraction of intermediate hydrocarbons. The amount of products generated depends on the temperature and pressure of the reactor.</p>

<p>Methane pyrolysis is complex and results in many products in addition to hydrogen, such as ethane, ethylene, acetylene, naphthalene, and pyrene. Furthermore, the quality of carbon depends on the accurate prediction of intermediate species and carbon formation because of a number of reasons: a) first, the quantification of the intermediates helps in a better understanding of the pathways a fuel goes through while decomposing; b) the decomposition process can be controlled and optimized if the details about the intermediates are known; c) improved understanding towards carbon formation results in the development of a catalyst; and d) avoiding the formation of intermediates that are unsafe or reactive. A reaction mechanism reflecting the decomposition chemistry and a soot model is needed. Therefore, in the present work, we developed a detailed methane pyrolysis model capable of predicting intermediates and carbon formation. </p>

<p>To achieve this goal, different reaction mechanisms available in the literature were integrated in a 0D isothermal, isochoric batch reactor model and the numerical predictions were compared to experimental results in literature at different temperatures and pressures. Results showed that almost all the detailed reaction models struggled to accurately predict the decomposition products at above-atmospheric pressures irrespective of carbon formation. The observed discrepancy was attributed to the slow rate parameters of the reaction mechanisms considered. Therefore, a methodology was developed to obtain a more accurate mechanism.</p>

<p>First, the mechanism showing the best agreement with the experimental data was selected and reduced using a graph-based method. Then, the rate parameters of the reduced mechanism in the high-pressure limit were recalculated by least-square parameter estimation until they accurately tracked methane and hydrogen mole fraction profiles while taking into account solid carbon formation. The optimized model was tested against the available experimental data in the high-pressure regime and shows significant improvement in its prediction capabilities compared to the original kinetic model. A reaction pathway analysis tracking carbon element shows that due to the change in the rate parameters, additional pathways, such as C$<em>{2}$H$</em>{5}$ $&lt;=&gt;$ IC$<em>{3}$H$</em>{7}$ $&lt;=&gt;$ NC$<em>{3}$H$</em>{7}$ $&lt;=&gt;$ C$<em>{2}$H$</em>{4}$ $&lt;=&gt;$ C$<em>{2}$H$</em>{3}$ $&lt;=&gt;$ C$<em>{2}$H$</em>{2}$, appeared at high-pressure that previously were considered unimportant in driving up the decomposition process. </p>

<p>Finally, a transient 0D monodisperse population balance model was implemented to track soot formation during methane pyrolysis. The model accounted for particle formation due to soot nucleation, and its evolution due to soot agglomeration and surface growth. The model was validated against benchmark results from the literature and then it was coupled with the optimized gas-phase model. The combined model shows that soot nucleation acts for a very short duration and the particle evolution is mainly governed by soot agglomeration and surface growth. A compact and dense particle was predicted at lower and higher temperatures of 892~K and 1292~K, respectively, whereas at an intermediate temperature of 1093~K, a porous particle structure was predicted by the model. Additionally, it was found that the surface growth model  altered acetylene kinetics and increased its consumption rate compared to pure nucleation. A parametric study on the effect of soot nucleation for different residence times on particle formation revealed that extended soot nucleation leads to lower particle number concentration and the primary particle diameter at the end of the reaction. The model developed can be used to quantify the amount of soot generated during methane pyrolysis.</p>