• Energy Research

The present work provides an overview of technological measures to increase the self-sufficiency of wastewater treatment plants (WWTPs), in particular for the largely diffused activated sludge-based WWTP. The operation of WWTPs entails a huge amount of electricity. Thermal energy is also required for pre-heating the sludge and sometimes exsiccation of the digested sludge. On the other hand, the entering organic matter contained in the wastewater is a source of energy. Organic matter is recovered as sludge, which is digested in large stirred tanks (anaerobic digester) to produce biogas. The onsite availability of biogas represents a great opportunity to cover a significant share of WWTP electricity and thermal demands. Especially, biogas can be efficiently converted into electrical energy (and heat) via high temperature fuel cell generators. The final part of this work will report a case study based on the use of sewage biogas into a solid oxide fuel cell. However, the efficient biogas conversion in combined heat and power (CHP) devices is not sufficient. Self-sufficiency requires a combination of efficient biogas conversion, the maximization the yield of biogas from the organic substrate, and the minimization of the thermal duty connected to the preheating of the sludge feeding the anaerobic digester (generally achieved with pre-thickeners). Finally, the co-digestion of the organic fraction of municipal solid waste (OFMSW) into digesters treating sludge from WWTPs represent an additional opportunity for increasing the biogas production of existing WWTPs, thus helping the transition toward self-sufficient plants.

Abstract This work conveys the study of the production of synthetic fuels, in this case, methane and methanol, by means of comparing two processes that employ high-temperature water splitting based on solid oxide electrolysis cells (SOEC) technology. In both cases, the process consists of mixing hydrogen produced by electrolysis with carbon dioxide in order to achieve hydrogenation synthesis via a catalytic reactor. An energy analysis was performed with special care on thermal integration (minimization of external heat requirements) via pinch analysis, as well as a final estimation of power-to-fuel overall efficiency. The study demonstrates that power-to-methane and power-to-methanol process can achieve efficiency of up to ≈77% and ≈59%, respectively. The energy analysis (based on the process modelling developed for both the systems) and the heat exchange network design enabled the development of capital expenditure estimation. An economic analysis comparison for the production cost of both synthetic fuels was performed with the purpose of highlighting any potential risk associated with the systems. The economic analysis considered the impact on synthetic fuel cost of some parameters as electrolysis specific costs, the expenditure for carbon dioxide, electricity price, and yearly operating hours. The results show that for both systems, as expected, the SOEC electrolyzer is the greatest capital expenditure of the design. Methanol synthesis plant showed lower efficiency and higher investment costs; on the other hand, fossil-based methanol has higher costs ($/MWh) than fossil methane; thus, the breakeven point of electricity price (i.e., that making economically comparable synthetic and fossil fuel) is similar for the two considered cases. It was concluded that to produce an economically attractive market for methane and methanol, the production plants should maintain a utilization factor of approximately 50%, the cost of SOECs should be near to 1050 €/kW and the electricity required to run the system needs to be supplied from renewable sources at a very low cost (below 40–50 $/MWh).