Multi-objective optimization of combined synthesis gas reforming technologies

Abstract

Synthesis gas (syngas) is a mixture of H2, CO and occasionally CO2, whose main application is as a building block of chemical compounds. The desired product dictates the syngas characteristics, which are also affected by the employed syngas synthesis technology. In this work, we study the process of producing syngas under desired specifications while consuming CO2 in the synthesis. We propose a superstructure that includes seven reforming technologies for the syngas production, as well as a variety of auxiliary units to control the final composition of the syngas. Each potential solution is assessed, in terms of the economic and environmental performance, by the Total Annualized Cost (TAC) and the Global Warming Potential (GWP) indicator. As the problem statement involves discrete decision, we use disjunctions to model the system. The resulting MINLP multi-objective problem is solved by the epsilon constraint method. Results show that at low syngas H2/CO ratios and pressures, dry methane reforming (DMR) is capable of net consuming CO2. Partial Oxidation (POX) is the technology that exhibits the minimum TAC, although shows the maximum value for the GWP. Synergistic combination of two processes allows reducing the cost and CO2-equivalent emissions through the pairing of DMR and bi-reforming (BR) and BR with steam methane reforming (SMR). Furthermore, increasing the CO2 content in the syngas at a fixed (H2 − CO2)/(CO + CO2) ratio proves that TAC and GWP decrease as the CO2/CO ratio increases.