CO2 hydrogenation from a process synthesis perspective: Setting up process targets

- Use of fundamental thermodynamics to analyse CO2 hydrogenation processes.
- Mass, energy and chemical potential conservation targets for CO2 hydrogenation.
- Relatively low H2 efficiency by high chemical potential efficiency.
- CO2 hydrogenation to CO and CH4 combined has highest performance targets.

The conversion of CO2 and H2 to valuable products, such as methanol (CH3OH), methane (CH4), hydrocarbon fuels (CH2) and carbon monoxide (CO) has attracted a great deal of research. In general, the focus of most work done in this field has been the development of better catalysts for the production of fuels and chemicals. In this article we look at CO2 hydrogenation from a process synthesis point of view, adopting a systems approach to evaluate and compare the performance of selected routes for the conversion of CO2 and H2 to valuable products namely methanol, methane, hydrocarbon fuels and carbon monoxide. The various routes are assessed in terms of defined mass and energy balance targets. More particularly, the paper introduces the concept of chemical potential efficiency target, which can be expressed as the ability to conserve the chemical potential of the feed materials when they are converted into products. It is shown that the chemical potential efficiency can be translated into work or useful energy conservation. It is also shown that the maximum chemical potential efficiency targets of the considered products are relatively high. They vary from 86% to 100%, with the production of CH4 having the lowest and that of CH3OH having the highest efficiency target, even though the hydrogen efficiency is low due to water production. The analysis also takes into account the combustion efficiency of the products of CO2 hydrogenation relative to that of hydrogen, and it is found that production of CO has the highest chemical potential target of 96%, while that of CH4 remains the lowest at 83%. Another notable finding is that most two-process combinations can be targeted for 100% chemical potential efficiency. However, such processes come with a penalty that heat has to be rejected or supplied externally. Nevertheless, the COCH4 combined process offers 100% chemical potential efficiency, with no external heat or work transfer required. This indicates that CO2 hydrogenation to CO and CH4 is possibly the most efficient way for storing renewable energy (solar, wind and hydroelectric) via water electrolysis, to produce H2 as an intermediate product.