(630c) Assessing the Potential of CO2 Utilization with an Integrated Framework for Producing Power and Chemicals

Energy is an indispensable part of our daily lives. The last decade of development has resulted in a sharp rise in energy demand. Reducing CO₂ emissions urgently is a global issue, and the stakes are increasing with the passage of time. The major focus so far has been on sequestering the captured CO₂, but this has its own problems and concerns. Thus, the idea of â??usingâ? or â??convertingâ? CO₂ rather than â??disposingâ? seems more attractive! The direct use of CO₂ being limited, we need ways to convert CO₂ into something useful such as valuable chemicals and fuels. Carbon dioxide utilization (CDU) is seen as a strategic measure to mitigate CO₂ emissions. In CDU, CO₂ is viewed as a carbon source for producing chemicals and fuels instead of treating CO₂ as a mere waste. However, a quantitative assessment of the global impact of CDU is a crucial yet unaddressed issue.

In this work, we propose the concept of CO₂ neutral chemicals and power production. We propose a network of reactions that begins with oxy-combustion of methane (the energy source) and ends with production of chemicals that have significant global demands. Dimethyl ether and methanol are considered as potential alternatives for conventional fuels. The reactions in the network are categorized as combustion reactions, hydrogen (H₂) producing reactions, CO₂ consuming reactions and secondary reactions. CO₂ consuming reactions use any CO₂ produced within the system. Secondary reactions produce any other reactants required to achieve CO₂ neutrality. The superstructure of all possible reactions is solved as an optimization problem to obtain the reaction network that achieves CO₂ neutral chemicals and electricity production. The process is entirely self-sufficient in its thermal and electrical energy needs with a net surplus of electricity, and completely utilizes all CO₂ produced.

The only caveat in achieving CO₂ neutrality mentioned above is the availability of hydrogen, as hydrogen produced internally is not sufficient to achieve both CO₂ neutrality and meet global demands for the chosen chemicals. Hydrogen can either be produced within the proposed framework (i.e. fossil based), or obtained from electrolysis of water using renewable energy resources. Thus, based on the source of hydrogen, we present three different scenarios. Firstly, we assumed availability of unlimited renewable energy to produce hydrogen by electrolysis of water. In this scenario, the estimated CO₂ avoidance was 59% as compared to the 2013 emission level. In the second scenario, we assumed the availability of renewable energy at current global levels. CO₂ avoidance dropped dramatically to 6% in this scenario. Finally, in the third scenario, we assumed zero external hydrogen and the avoidance of CO₂ emissions reduced to a mere 1%. Our analysis quantitatively establishes the fact that availability of renewable hydrogen will play a pivotal role in determining the future of CDU. Note that our framework does not advocate centralized production of bulk chemicals in one place. We also tested the idea of producing individual chemicals within our framework, but it obviously results in lower CO₂ avoidance.

Our work has taken a process systems viewpoint to show that utilizing CO₂ to produce alternate fuels along with bulk chemicals has the potential to combat CO₂ emissions at the global scale. Our framework allows us to obtain an upper bound for the global impact of CO₂ utilization by taking into account the maximum current and projected global demands for all chosen chemicals. Market demand of chemicals derived from CO₂ will have a significant impact on CDU. With the rising concerns about CCS, CDU may well be the next strategy for consideration in the long run. However, availability of abundant renewable hydrogen will be necessary to realize the full potential of CDU.