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oxy‐fuel combustion of solid particulate fuels such as the work presented by Wu et al. [49], where the Eulerian approach discussed earlier was implemented in order to track the movement of the solid phase, and the work of Bhuiyan and Naser [50], who applied the Eulerian–Lagrangian method. Table 2.3 summarizes some recent CFD studies in oxy‐fuel technologies. Also, a look at the literature shows that some reported studies on oxy‐fuel CFD simulations have been combined with process simulations in co‐simulation strategies. Such is the work published by Edge et al. [54] and Fei et al. [55]. Co‐simulation is the object of the Section 2.8, where it will be discussed further because it can give way to enhanced numerical predictions with implications also in control engineering.
2.5 CFD for Carbon Storage and Enhanced Oil Recovery (EOR): The Link Between Advanced Imaging Techniques and CFD
In terms of carbon storage and enhanced oil recovery, CFD simulations can be applied at various scales in a way similar to the methodology discussed earlier in this chapter for amine scrubbers. The most interesting scale appears to be (according to the number of articles published) the small scale though, where it is possible to utilize the VOF method to analyze the flow across a small portion of a solid porous medium representing the geometry of the pores. In this direction, He et al. [56] studied the two‐phase flow between supercritical CO2 and water in the walls of a saline aquifer. The porous rock was assumed to be formed by detached spheres in a body‐centered cubic (BCC) arrangement. Their simulation setup allowed visualization of the displacement process (the porous medium was initially filled with water). The effect of wettability (i.e. contact angle), surface tension, and viscosity ratio was also assessed, obtaining the permeability saturation curves. The latter certainly constitutes essential information in terms of carbon sequestration, but the approximation of considering the porous medium as a network of perfect spheres with an ideal BCC arrangement might lead to undesired errors. Similar examples are the simulations reported by Dezfully et al. [57] and Gharibshahi et al. [58] (Figure 2.4), which also considered spheres but in a random arrangement.
Table 2.3 Summary of published CFD studies concerning oxy‐fuel technologies.
| Authors | Aspect studied | Short comment on findings |
|---|---|---|
| Gharebaghi et al. [51] | Single‐phase combustion simulation for a test facility | Comparison between turbulence modeling strategies, i.e. large eddy simulation (LES) and Reynolds averaged Navier–Stokes (RANS), and experimental data |
| Mayr et al. [52] | 3‐D steady‐state simulation of a natural gas furnace including radiation models and the eddy dissipation concept (EDC) model. Effect of O2/N2 ratios on furnace efficiency | To increase the O2‐to‐N2 ratio resulted in better furnace efficiency. Good matching between simulated and experimental results |
| Bhuiyan and Naser [50] | Co‐firing biomass + coal using the Eulerian–Lagrangian approach | The authors included factors describing the irregular shape of the biomass particles. The effect of changing the fuel ratio combustion atmosphere in the performance parameters of the furnace |
| Carrasco‐Maldonado et al. [53] | Single‐phase approach to simulate the effect of integrating oxy‐fuel technologies in a cement production plant | Validation against experimental data accomplished. The k–ω turbulence model gave way to the best results |
| Wu et al. [49] | Study of oxy‐fuel combustion in a circulating fluidized bed (CFB). The model uses the Eulerian approach and thus this is a multiphase case that deviates from common oxy‐fuel CFD studies in the literature | Detailed profiles of temperature and hydrodynamic variables are obtained, which match the experimental results. Gas hold‐up is also studied, resulting in identification of gas accumulation spots |
Figure 2.4 (a) Details of the volume fraction map describing the liquid flow within the porous medium.
Source: Dezfully et al. [57]. © Trans Tech Publication.
(b) Details of the pore geometry considered.
Source: Gharibshahi et al. [58]. © Elsevier.
The main hurdle to get accurate simulations of CO2 trapping in porous rocks appears to be the correct representation of the intricate geometry of the pore. To overcome this, a recent trend in the simulation of the flow through porous media microchannel networks consists in applying imaging techniques, which are subsequently exported into computer‐aided design (CAD) files and spatially discretized. This approach has been applied in other fields where CFD simulations are of great help to gain insight into the flow within such geometries. For instance, Sznitman et al. point out the possibility of using micro‐computed tomography (μCT) or scanning transmission X‐ray microscopy (STXM) to reproduce the alveoli network within the lungs. Other examples are the use that Owens et al. [23] did of X‐ray computed tomography (CT) to obtain a geometric representation of a structured packing, or the work of Isoz and Haidl [21] also applied to structured packing columns. The use of such imaging techniques to be coupled with VOF‐CFD simulations remains rather unexplored and will surely constitute an important research avenue in the future.
2.6 CFD for Carbon Utilization with Chemical Conversion: The Importance of Numerical Techniques on the Study of New Catalysts
Utilization of carbon dioxide with chemical conversion entails its use as a feedstock in order to transform it into other valuable products such as polymers, fuels, methanol, pharmaceuticals, urea, etc., which otherwise would need to be manufactured by using petrochemicals.
A reaction of particular importance in carbon dioxide utilization with chemical conversion is the Sabatier reaction, whereby COx is converted to methane by hydrogenation and subsequently introduced into the gas grid:
(2.3)
CFD simulations of the Sabatier reaction are single phase, which is an advantage from the perspective of the computational resources needed, although they require a multispecies approach and a careful selection of the turbulence model in those cases where the Reynolds number is high. CFD can play an important role in research oriented towards the implementation of different catalysts to accelerate the production of methane. The general approach followed in that case is to obtain experimental data on the reaction kinetics first and introduce them subsequently into the simulation set‐up.
A recent example of the modeling of the Sabatier reaction combined with the water gas shift reaction in a microchannel reactor was presented by Engelbrecht et al. [59], who used the commercial software COMSOL Multiphysics to perform their simulations. They assumed equally distributed flow within the microreactor, and therefore, only one microchannel was considered. To successfully model the process, the computational domain was divided in two zones: a free flow region,