Renewable heating with a lime-based DAC system

Abstract

Direct air capture (DAC) systems are currently further expanded, as they are able to extract CO2 from ambient air and make it available for subsequent processes or remove it permanently from the atmosphere by storing it underground. The remaining main challenge of the technology is the amount of energy needed, mainly required for regeneration of the CO2 sorbent. Thereby, the exothermic uptake of CO2 is not utilized at present. The presented concept aims to utilize (part of) this energy and thus improve the overall energy efficiency of such DAC systems. Basis is the technical lime cycle, which uses limestone as both, CO2 sorbent and thermochemical energy storage material. During regeneration and release of the CO2, the material is simultaneously thermally charged, forming burnt lime (CaO). The use of renewable energy requires a dynamic calcination, which works according to availability. Before using the material as CO2 sorbent material again, it can be stored and by the addition of water, an exothermic reaction starts, releasing heat. To utilize this heat, a reactor was developed at DLR, making it available for space heating and hot water. A more detailed description of the concept will be presented and can be found in literature. The uncharged storage material is Ca(OH)2, which is then used as CO2 sorbent as in other DAC systems. The reaction kinetics are quite slow in ambient conditions, but as large amounts of materials are required for the provision of heat, storage times are long enough to technically utilize the CO2 capture capacity of the material without additional energy input. The shift from fossil fuels to renewable sources for heat provision in private households develops quite slow. For example, in Germany, more than 80% of the final energy demand is required for space heating and hot water, currently with less than 20% provided from renewable source. Therefore, technologies providing renewable heat in addition to those which capture CO2 already emitted to the atmosphere are both urgently needed. In order to tackle this challenge, a test rig was developed in this work to identify main influencing parameters of the DAC absorption reaction in ambient conditions and subsequently optimize these reaction conditions. The experimental results of the study will be presented and are used for estimations of the potential of this concept and the overall efficiency of utilizing part of the regeneration energy in DAC systems.