MASc Oral Exam | Real Impact of CO2 Utilization: A Dynamic LCA Approach, by Majd Tabbara

You are invited to attend Majd Tabbara's MASc oral exam, where they will discuss their research into the environmental costs and benefits associated with the conversion of already-captured CO₂ into final chemicals or energy products.  

Abstract

Global warming has received widespread attention in recent years due to increasing levels of carbon dioxide (CO₂) and other pertinent greenhouse gases in the atmosphere. Various solutions have been proposed to reduce the net CO₂ emissions, including switching to renewable energy and CO₂ capture and sequestration. A probable and attractive alternative to CO₂ storage could be CO₂ utilization, which is defined as the conversion of already-captured CO₂ into final chemicals or energy products. Customarily, a static life cycle analysis (LCA) has been employed to comprehend the environmental costs and benefits associated with CO₂ utilization processes. Essentially, the LCA procedure retains a crucial aspect in understanding the extent to which CO₂ is truly being mitigated within a CO₂ utilization process. However, the scope and extent of the LCA procedure requires careful attention. Although ultimately all of the CO₂ utilized will likely end up in the atmosphere, a comprehensive dynamic LCA needs to be conducted in order to encompass a time scale to represent the time that CO₂ is being displaced by within a proposed CO2 utilization process.

The research presented within this thesis primarily focused on assessing two products, methanol (MeOH) and dimethyl carbonate (DMC), through the application of the dynamic LCA procedure. Initially, a model schematic, inclusive to the necessary equations, was established so as to compute the net CO₂ emissions within a given system. This included the use of a 620 MW natural gas combined cycle power plant, that accounted for de-rating, to produce the required amount of CO₂ necessary for the utilization process.

Additionally, the conventional process and the so-called CO₂ utilization process for manufacturing MeOH, were developed and simulated in Aspen Plus^TM.

Normalizing the values to 1 (tonne MeoH/hr), the cumulative amount of CO₂ emitted within both the conventional and utilization processes is 1.878 (tonne CO2/hr) and 1.703 (tonne CO2/hr) respectively. These results were then utilized so as to compute the net CO₂ emissions within each respective approach. Subsequently, the values attained were then used as an input to the dynamic LCA framework yielding in the necessary environmental results. After an in-depth comparison, the utilization approach proved superior, from an environmental perspective, when contrasted against the conventional route of manufacturing MeOH. This is seen as the cumulative impact on radiative forcing, at year 100, was computed for both routes yielding in 4.328*10^-5

(W/m^2) for the conventional approach and 3.613*10^-5 (W/m^2) for the utilization approach. Notably, implementing the utilization approach would result in a 16.51 % percent reduction in the cumulative impact of radiative forcing at year 100. Furthermore, a sensitivity analysis was also conducted on the utilization route and this showed that an increase in the CO₂ storage duration within the MeOH product results in a diminished environmental impact.

Similarly, an environmental comparison-based assessment was conducted to analyze both a conventional and CO₂ utilization approach of manufacturing DMC. The conventional approach analyzed the partial carbonylation route, whilst the utilization approach assessed the urea route through reactive distillation. Subsequent to the cradle-to-grave computations, the obtained CO₂ emissions within both approaches were further inputted into the dynamic LCA framework. Overall, the cumulative impact on radiative forcing, at year 100, was calculated for both routes resulting in 5.118*10^-5 (W/m^2) for the conventional approach and 5.859*10^-5 (W/m^2) for the utilization approach.

From an environmental standpoint, employing this utilization approach to manufacture DMC results in a 14.46 % increase in the cumulative impact of radiative forcing at year 100. A sensitivity analysis was also performed to study the effect of increasing the CO2 storage duration, in the DMC product, on the cumulative impact on radiative forcing. An inverse relationship was observed showing that an increase in the CO₂ storage duration yields a relative decrease in the cumulative impact on radiative forcing.

Supervisors: Professors Peter Douglas and Eric Croiset