(259f) Process Performance and Techno-Economic Analysis for Methane Mitigation from Low Concentration Sources

CH₄ is the biggest contributor to global warming among non-CO₂ GHGs [1]. The majority of CH₄ emissions are associated with low-concentration sources, which are notoriously hard to abate with agricultural sector being the largest source of anthropogenic CH₄ emissions [1, 2]. The technological options for methane mitigation via oxidation are present for confined sources of relatively high CH₄ concentrations (> 1000 ppmv) [1]. In addition to the existing preventive measures to limit CH₄ emissions, technological options to oxidize/convert them to a lower global warming potential gas can assist in mitigating the otherwise hard-to-abate or the already emitted gases. This is important in a larger context of achieving the net-zero GHG targets of respective sectors. The current work combines the mitigation of CH₄ followed by the capture of CO₂. The resulting process concept (s) can be applied to a multitude of low concentration CH₄ sources to reactively capture and mitigate the emitted greenhouse gas.

In our previous work [1], we presented process concepts to oxidize CH₄ followed by CO₂ capture for low concentration CH₄ sources. We presented two cases, namely, with CO₂ capture (co-removal of CH₄ and CO₂) and without CO₂ capture (CH₄ conversion), and analysed the effect of process parameters on energy penalty required to mitigate per ton of CO₂ equivalents in the system. The specific energy demand metric enables the comparison of these process concepts to direct air capture of CO₂ processes. The CH₄ oxidation process consists of a blower, a dehumidification unit, a recuperating heat exchanger, catalytic reactor, cooler, followed by CO₂ capture unit for the case with CO₂ capture [1]. CH₄ oxidation happens catalytically via thermal – and photocatalytic routes. A steady state model was developed and simulated in Aspen Plus to estimate the specific energy demand for CH₄ conversion. A model for CO₂ capture was not developed in this study but an energy estimate corresponding to a solid sorbent-based vacuum temperature swing adsorption (VTSA) process model from the literature [3] was considered. In the case with thermal catalytic route, a constant CH₄ conversion on 6.5 wt% Pd/Al₂O₃ [4] reported in literature is assumed. The amount of catalyst for the required conversion was later calculated using the kinetic model developed by Alyani et al. [4]. A similar process analysis for photocatalytic route using 0.8 wt% CuO/ZnO [5] as photocatalyst was performed. The results of energy demand by both the thermal- and photocatalytic routes were presented in [1].

Techno-economic assessment (TEA) for CH₄ oxidation for the thermal catalytic route yields a cost of 1258 $/t-CO₂eq for the reference case with 300 ppmv of inlet CH₄ concentration in air. Figure 1 shows the results of TEA with different inlet CH₄ concentrations for CH₄ oxidation. Results for base case in CH₄ oxidation show that the recuperator and dehumidification unit dominate the capital costs with share of 41% and 45%, respectively. The costs of electricity and catalyst combined make up over two-thirds of the operating expenses. The costs presented in Figure 1 suggests a strong dependence on inlet CH₄ concentration, a result which is similar to energy demand (also shown in Figure 1). These results support a conclusion that for CH₄ oxidation to be economically feasible, either the catalysts which are insensitive to water vapor or higher inlet CH₄ concentrations are required. A higher CH₄ concentration can be achieved by including a pre-concentration step.

The current estimates for CO₂ removal through direct air capture were in the range of 200-600 $/ton [6]. These estimates suggest that the co-removal (of CH₄ and CO₂) costs are likely to be lower than the cost estimate by just CH₄ oxidation (1258 $/t-CO₂eq). In addition, a potential process heat integration between CH₄ oxidation and CO₂ capture unit was highlighted in [1] which in theory could further bring down the costs of co-removal. Alternatively, as noted before, the cost of CO₂ equivalents mitigation can also be lowered with higher inlet CH₄ concentrations. A CH₄ pre-concentration step (until 1 vol% CH₄) based on solid sorbent technology could potentially lower the overall cost of CO₂ equivalents mitigated. The TEA results for co-removal will be presented in the final work. The results will be compared to the process integrated with the pre-concentration step.

Sirigina et al. [1] reported the energy demand for inlet CH₄ concentrations varying from 100 ppmv to 1 vol% CH₄, and for CH₄ conversion in the range of 20%-100% in the reactor (Figure 4 in the reference [1]). The study however considers the catalytic oxidation of CH₄ to take place at 330 °C based on experimental results reported in literature for the 6.5 wt% Pd/Al₂O₃ [4]. The catalyst was reported to have a higher conversion at 380 °C [4]. In addition, a lower loss of catalytic activity by water vapor and thereby lower loss in CH₄ conversion in the experiments by adding Ce to 6.5 wt% Pd/Al₂O₃ catalysts is reported. As catalysts exhibit different activity with temperature (in addition to the effect of support, H₂O concentration etc,), and with temperature and conversion as proxy to capture the performance of a catalyst, a process performance map can be created reflecting the steady state activity of different catalysts. The process performance map in conjunction with a range of inlet concentrations representative of different emission sources could be used to identify the process performance of catalytic CH₄ oxidation on different catalysts. The available process model for CH₄ oxidation (which is largely independent of catalyst) will be used to analyse the effect of different temperatures on the performance in terms of specific energy demand per ton CO₂ equivalents mitigated. At higher temperatures (from the reference case at 330 °C), the waste heat from CH₄ oxidation will increase the potential for process heat integration between CH₄ oxidation and CO₂ capture. We will evaluate and compare the CH₄ oxidation and co-removal to identify the dynamic interplay at different temperatures. The performance map in terms of energy demand with temperature, concentration, and conversion will be presented in the final work. This will help in identifying an optimal operating region for a given catalyst system or be used to choose a catalyst system based on given inlet stream conditions. In addition, the final work also showcases the techno-economic potential of CH₄ oxidation, CH₄ oxidation with CO₂ capture (co-removal), and integrates both the pathways with a CH₄ enrichment step to assess their feasibility against the existing technologies.

Acknowledgements

The work is funded by:

(i) Energimyndigheten (Swedish Energy Agency) for the project “Energieffektiv negativa utsläpp från jordbrukssektorn” [In English: Energy efficient negative emissions from agriculture and farming] (project number 50340-1)

(ii) "The Norwegian Continental Shelf: A Driver for Climate-Positive Norway" (NCS C+) project funded by the Research Council of Norway (328715) under the green platform program.

References

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