| Abstract | Water scrubbing is a physical absorption method which uses absorption to remove impurities from biogas. The feasibility of water scrubbing to upgrade biogas was analyzed for two fluids (tap water and digestate liquid), and in two settings (U.S and Nigeria). Experiments were carried out to test the absorption capacity of carbon dioxide using these fluids. A techno-economic model was created to simulate the performance of a biogas upgrading system, and the model was used to estimate the viability of the system.
Results indicate that biogas upgrading for a small-scale farm in Pennsylvania yields a negative net present value (NPV) of -$795,246, with all but the most optimistic scenario producing negative values. Sensitivity analysis indicates that pipeline cost, number of cows, and biogas production costs have the greatest impact on NPV, while utility costs have the least influence.
Digestate liquid can be used to absorb carbon dioxide; however, its performance is lower than that of water. It was found to be most effective at 4°C and 1440 minutes. The techno-economic analysis of using digestate liquid for upgrading also results in a slightly improved but still negative NPV (-$65,240), compared to using water, despite no associated cost for the liquid. This outcome can be attributed to the required height of the column, which is approximately twice the height needed when using water to achieve the same carbon dioxide removal efficiency.
Techno-economic analysis of biogas upgrading in Nigeria using biomethane as a substitute for liquefied petroleum gas (LPG), with a base case of 60 cows, also yields a negative NPV of -?10,253,874, only the most optimistic variations of this scenario result in a positive NPV. While the NPV of these systems tends to be negative, their use may be justifiable in favorable settings and conditions. Sensitivity analysis shows that the biogas production cost has the greatest impact on NPV, followed by compressed biomethane cost and the number of cows. However, biogas production costs could be reduced through funding incentives, grants, and financing mechanisms. The farming system in Nigeria also affects the availability of manure, as cows are not typically confined to a single location.
Overall, this thesis provides an assessment of the economic and technical feasibility of biogas upgrading using water and digestate liquid in two different continents, showing the significant variables assessing the adoption of the use of biomethane as electricity, renewable natural gas or liquified petroleum.
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