Liquid Fuel Production Using Solar-Thermal Energy: Process Development and Technoeconomic Evaluation

We have previously described a novel solar-based concept for the production of methanol from carbon dioxide and water. This system applies concentrated solar energy to thermochemically reenergize CO₂ into CO, and then water-gas shift (WGS) to produce syngas (a mixture of CO and H₂) that is fed to a methanol synthesis reactor. Aside from the thermochemical reactor, which is currently under development at Sandia National Laboratories, the approach is based on well-established industrial processes and component designs. Our previous work included an assessment of energy efficiency and economic feasibility of this baseline configuration for an industrial-scale methanol plant. We determined that an overall process energy efficiency (solar-to-fuel) of 7.1% could be achieved and a break-even price of the methanol produced using this approach would be 1.22 USD/kg. Our current work focuses on improvements to the baseline system. We propose several avenues for improvement: 1) alternative pathways to syngas including water splitting and combined CO₂ and H₂O splitting, 2) improved separations, and 3) alternative hydrocarbon products.

For alternative pathways and products, we studied three systems in addition to the baseline system previously reported:
(i) H₂O water splitting, followed by reverse water-gas shift (RWG) reaction, followed by methanol synthesis (MS)
(ii) Simultaneous CO₂ and H₂O splitting (mixed pathway) followed by MS
(iii) Simultaneous CO₂ and H₂O splitting (mixed pathway) followed by Fischer-Tropsch (FT) synthesis

In all designs, we employed amine-based separation systems to remove CO₂ from CO/CO₂ or H₂/CO/CO₂ mixtures. However, we also studied the effect of replacing the CO₂ separations system with an analogous CO separations system. We found that CO separations could provide a significant improvement in energy efficiency and utility costs for the CO₂ splitting and mixed pathways.

To compare the improved systems to our baseline, we performed rigorous process simulations to determine equipment sizing and costing, as well as operational expenses (e.g., raw material and utility costs). We then performed detailed profitability calculations. We determined that the mixed pathway (i.e., simultaneous splitting of CO₂ and H₂O) has a number of advantages over the CO₂-splitting or H₂O-splitting pathways. Also, the production of methanol is slightly more attractive than the production of FT diesel.

Using the mixed pathway with MS as our new baseline, we then carried out sensitivity analysis studies to identify the main cost drivers. We found that the primary economic drivers are the high capital investment associated with the solar concentrator/reactor sub-system which accounts for more than 90% of the capital expenditure, and the high utility consumption for CO/CO₂ separation. Economic parameters such as tax rate and interest rate are also important.

Rather than use a cost of CO₂ as had been done previously, we expanded the boundaries of our system to include CO₂ capture at a power plant. We determined that the effect of CO₂ capture leads to small increases in the minimum selling price (i.e., break-even price) of methanol. We also studied how the minimum selling price (MSP) of methanol increases as a function of the distance between the power plant and the solar facility and the transportation technology.

The energy efficiencies of the different systems were also analyzed. Along with lower cost, the mixed pathway with MS demonstrated the highest efficiency. The primary energy efficiency (= chemical energy out (HHV) over total primary energy in) was calculated as 12.7% if process heat and electricity are provided from fossil fuels and 10.7% if solar power is the sole primary energy source. This 10.7% system efficiency is significantly higher than what can currently be achieved with photosynthesis-based processes, and illustrates the potential for solar thermochemical based strategies to overcome the resource limitations that arise for low-efficiency approaches.

Finally, the analysis indicates that a solar-thermochemical pathway to fuels has significant potential, and points towards future research opportunities to increase efficiency and reduce cost. Particularly, it is evident that there is much room for improvement in the development of a less expensive and more efficient solar concentrator/reactor sub-system, an opportunity that will benefit from the increasing deployment of concentrated solar power. In addition, significant advances are achievable through improved separations, different end products, and different degrees of process integration and distribution. Our investigation establishes a methodology for comparison and assessment of impact on both the economics and the efficiency for advanced system designs.

"This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, in the form of a Grand Challenge project entitled Reimagining Liquid Transportation Fuels: Sunshine to Petrol. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000. "