Abstract
Methanol is an example of a valuable chemical that can be produced from water and carbon dioxide through a chemical process that is fully powered by concentrated solar thermal energy and involves three steps: direct air capture (DAC), thermochemical splitting and methanol synthesis. In the present work, we consider the whole value chain from the harvesting of raw materials to the final product. We also identify synergies between the aforementioned steps and collect them in five possible scenarios aimed to reduce the specific energy consumption. To assess the scenarios, we combined data from low and high temperature DAC with an Aspen Plus® model of a plant that includes water and carbon dioxide splitting units via thermochemical cycles (TCC), CO/CO2 separation, storage and methanol synthesis. We paid special attention to the energy required for the generation of low oxygen partial pressures in the reduction step of the TCC, as well as the overall water consumption. Results show that suggested synergies, in particular, co-generation, are effective and can lead to solar-to-fuel efficiencies up to 10.2% (compared to the 8.8% baseline). In addition, we appoint vacuum as the most adequate strategy for obtaining low oxygen partial pressures.
Highlights
It can be seen that the configuration with the lowest heat consumption is Scenario A thanks to the more efficient sHT-direct air capture (DAC) approach
The oxygen obtained as a by-product in the thermochemical cycles (TCC) doubles the needs for producing the oxyfuel mix used in scenario B, which means that no air separation unit (ASU) needs to be powered
It should be noted that Scenario D relies on the same technology as scenarios C and E, it shows a neutral point in the plot because it is assumed that no water collected via low temperature DAC (LT-DAC)
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The last approach, sHT-DAC, is a modification of the HT-DAC in which the calcination step is performed with aid of CSTE instead of combustion or electric heating This concept is inspired by previous works on solar rotary kilns for cement production, where exactly the same reaction takes place [34]. On the one-hand, HT-DAC (and subsequently sHT-DAC) is claimed to have a consumption of 4.7 water tons per ton of captured CO2 at 20 ◦ C and 64% relative humidity due to the water evaporated in the contactor This value can double at lower humidity and higher temperatures [32]. Not directly related with DAC nor quantified in this analysis, HVAC systems can provide considerable amounts of water if condensation under humid environmental conditions is collected [38,39]
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