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

Authors: 
Farooq, S., National University of Singapore
Karimi, I. A., National University of Singapore
Khan, S. A., National University of Singapore
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.

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