R2Chem: Renewables-to-Chemicals Process Networks
To reduce CO2 in process and energy supply industries, the massive use of renewable energy and the substitution of fossil-based feedstocks through the implementation of efficient Renewables-to-Chemicals (R2Chem) conversion systems are of key importance. Due to the multitude of alternative raw materials and process technologies, there is a large number of different potential pathways for converting renewables into valuable target products.
In  we introduce an elegant problem formulation in terms of continuous process extent variables to avoid binary decision variables. All constraints (equalities, inequalities) as well as the objective function are formulated as linear expressions in terms of the (purely continuous) decision variables, namely the fluxes of mass, energy, heat and work as well as the extent variables [2-3]. The objective function contains the Total Annualized Costs (TAC) as well as penalty terms for direct and indirect carbon dioxide emissions.
For the target product methanol, we have shown that a very good trade-off between production cost and emissions can be achieved by using natural gas or biogas as a feedstock source, especially if the required energy is supplied from renewable sources. A net consumption of CO2by the overall production system is possible, if renewable energy sources are exploited while using CO2as a feedstock source at the same time. If using fossil energy sources, a negative carbon footprint is unavoidable due to the high indirect CO2emissions caused by the energy supply (electricity, heat). Thus, in addition to the economic challenges of using CO2as a feedstock, the ecological impact also strongly depends on the energy source used. The main advantage of the proposed methodology is the ability to quickly determine an optimal process system within a superstructure in which many alternative process configurations are embedded. So far, the method has been used for optimizing process systems for the production of methanol  and formic acid . More recently, we have extended our network approach to the level of a single process as well as to single process units . This allows detailed process design and also more precise estimation of entropy production inside each process and process unit.
The conversion of CO2into syngas is another important research direction for the PSE group in the field of energy conversion. Syngas is widely used in the chemical industry as building block for the production of many bulk chemicals and chemical intermediates. Replacing the traditional syngas production routes from fossil fuels with sustainable production routes based on CO2and renewable electricity has the potential to drastically lower CO2emissions and help to close the carbon loop to pave the way for a circular economy.
An auspicious option for direct syngas production from CO2is the reverse water-gas shift (RWGS) reaction. The RWGS reaction is interesting within the context of sustainable syngas production, because of its higher technological readiness compared to dry reforming and solar thermochemical looping. In the PSE group, RWGS chemical looping (RWGSCL) was investigated as a potential process for sustainable syngas production from CO2. By applying the chemical looping concept to the RWGS reaction, a partial product separation is realized within the process, thus reducing the energy penalty associated with downstream product separation . This also enables the production of syngas with a defined H2/CO ratio for a variety of processes (e.g. methanol synthesis, Fischer-Tropsch, etc.). Our group was able to show, that the energy efficiency of RWGSCL is comparable to solar thermochemical looping, but it avoids the problem of heat recuperation because it can be operated isothermally . Lab-scale experiments were conducted to show the feasibility of the RWGSCL process and to provide kinetic data for process design and optimization [7-8]. An important aspect of the chemical looping concept is that it always leads to a cyclic batch process which is detrimental for large scale syngas production since continuous processes are favored in process industry. We investigated how fixed bed and fluidized bed reactor designs can be used to continuously produce syngas by RWGSCL . This work focuses on the dynamic aspects of the process that were not analyzed as detailed before due to the high complexity of the equilibrium limited gas-solid reactions involved. From our results it can be concluded that both the conventional RWGS process as well as the novel RWGSCL process are viable options for the sustainable production of syngas.
With regard to CO2methanation using hydrogen generated via water electrolysis, the PSE group systematically compared different reactor-separator configurations as part of the project “Altmark Energy” [10-11]. The results of our analysis demonstrate that biogas, a mixture of methane and carbon dioxide from anaerobic digestion, can be fed directly into the methanation reactor. No prior removal of biogenic methane is necessary. This configuration is the most efficient process in terms of exergetic efficiency. It combines water electrolysis, CO2methanation, separation of CH4via pressure swing adsorption (PSA) and separation of water via temperature swing adsorption (TSA), and gas conversion back to electricity. The whole process has an overall energetic efficiency of 23.4%, without utilizing the excess heat, covering the complete cycle from electricity via chemical storage back to electricity.
 Schack, D., Rihko-Struckmann, L., & Sundmacher, K. (2018). Linear programming approach for structure optimization of Renewables-to-Chemicals (R2Chem) production networks. Industrial & Engineering Chemistry Research, 57(30), 9889-9902.
 Schack, D., Rihko-Struckmann, L., & Sundmacher, K. (2016). Structure optimization of Power-to-Chemicals (P2C) networks by linear programming for the economic utilization of renewable surplus energy. In: 26th European Symposium on Computer Aided Process Engineering, pp. 1551-1556.
 Schack, D., Rihko-Struckmann, L., & Sundmacher, K. (2017). Economic linear objective function approach for structure optimization of Renewables-to-Chemicals (R2Chem) networks. In: 27th European Symposium on Computer Aided Process Engineering, pp. 1975-1980.
 Schack, D., & Sundmacher, K. (2018). Techno-ökonomische Optimierung des Produktionsnetzwerkes für die Synthese von Ameisensäure aus erneuerbaren Ressourcen. Chemie-Ingenieur-Technik, 90(1-2), 256-266.
 Liesche, G., Schack, D., Rätze, K.H.G., & Sundmacher, K. (2018). Thermodynamic network flow approach for chemical process synthesis. In: 28th European Symposium on Computer Aided Process Engineering, pp. 881-886.
 Wenzel, M., Rihko-Struckmann, L., & Sundmacher, K. (2017). Thermodynamic analysis and optimization of RWGS processes for solar syngas production from CO2. AIChE Journal, 63(1), 15-22.
 Rihko-Struckmann, L., Datta, P., Wenzel, M., Sundmacher, K., Dharanipragada, N., Poelman, H., Galvita, V., & Marin, G. B. (2016). Hydrogen and carbon monoxide production by chemical looping over iron-aluminium oxides. Energy Technology, 4(2), 304-313.
 Wenzel, M., Aditya Dharanipragada, N. V. R., Galvita, V., Poelman, H., Marin, G. B., Rihko-Struckmann, L., & Sundmacher, K. (2017). CO production from CO2via reverse water–gas shift reaction performed in a chemical looping mode: Kinetics on modified iron oxide. Journal of CO2Utilization, 17, 60-68.
 Wenzel, M., Rihko-Struckmann, L., & Sundmacher, K. (2018). Continuous production of CO from CO2by RWGS chemical looping in fixed and fluidized bed reactors. Chemical Engineering Journal, 336, 278-296.
 El-Sibai, A., Rihko-Struckmann, L., & Sundmacher, K. (2017). Model-based optimal Sabatier reactor design for Power-to-Gas applications. Energy Technology, 5(6), 911-921.
 Uebbing, J., Rihko-Struckmann, L., & Sundmacher, K. (2019). Exergetic efficiency assessment of CO2methanation processes for the chemical storage of renewable energies. Applied Energy, 233-234, 271-282.