The ZSW has been conducting research into CO₂ capture from the air for around 30 years and is one of the world’s leading research institutions in the field of Direct Air Capture (DAC). The institute develops and investigates both liquid-based and solid-based DAC processes.
In the field of liquid-based processes, methods using caustic soda (NaOH) and polyethyleneimine (PEI) have been developed, with prototypes achieving capture capacities of up to 10 kgCO₂/h. In particular, the PEI-based scrubbing process developed at ZSW is showing promising results in terms of reliability, cost-efficiency and scalability potential. In addition, continuous solid-based DAC concepts based on cellulose-immobilised amines have been developed and implemented as prototypes.
Specifically, the DACLab currently has around 10 test benches and pilot plants, ranging from sorbent characterisation on a laboratory scale to process validation on a kg/h scale. The test benches were designed, engineered and implemented in-house and can therefore be flexibly adapted to different research questions and customer requirements.
A particular focus of the DACLab is the investigation of various DAC technologies under realistic and globally varying climatic conditions. Whilst the expansion of renewable energy opens up many new sites for large-scale DAC plants, it often places considerable demands on DAC technologies, which react with varying degrees of sensitivity to fluctuations in temperature and humidity, depending on the specific technology. These factors have a decisive influence on efficiency, energy requirements and, ultimately, economic viability. With the aid of highly flexible air pre-conditioning, the DACLab can simulate almost any global climate situation – from dry desert climates to tropical humidity. The data obtained enables more precise process design, provides insights into site-specific limitations and creates a robust basis for technology comparisons and benchmarks.
In addition to developing its own DAC technologies and offering testing and consultancy services, the ZSW also promotes the establishment of networks and regional value creation. In the DAC-BW project, for example, a state-wide DAC network comprising around 40 companies and institutions was established to strengthen the development of DAC technologies in Baden-Württemberg.
With DACLab, the ZSW has a cross-technological and independent test platform for the development, optimisation and evaluation of direct air capture technologies. The infrastructure enables the investigation of both liquid- and solid-based technological approaches and covers the entire development chain, from material and component characterisation to process validation at prototype scale.
The approximately 10 test benches and pilot plants currently in operation were developed and constructed at the ZSW and can be flexibly adapted to different research questions and customer requirements. In addition to the characterisation of sorbents and individual components, DACLab enables the investigation of complete process chains under defined boundary conditions. Testing capabilities range from laboratory scale to the validation of DAC processes with capture capacities of up to 100 tonnes of CO₂ per year. Automated operating strategies and 24/7 continuous operation also allow for long-term and ageing studies under real-world conditions.
A particular focus of the DACLab is on investigating the influence of climatic boundary conditions on the performance and cost-effectiveness of DAC technologies. Using a highly flexible air conditioning system, air flow rates of up to 1,000 m³/h, temperatures between –15 and +40 °C, and absolute humidity levels of up to 28 g H₂O/kg of air can be set. This enables different global site conditions to be reproduced reliably, allowing technologies to be compared and evaluated under realistic operating conditions.
The test infrastructure is complemented by extensive analytical capabilities and end-to-end digital data acquisition. Online-capable measurement systems enable real-time access to process data, whilst material and gas analyses – including thermogravimetry and gas chromatography – allow for detailed characterisation of sorbents and process streams.
With its cross-technological approach, DACLab offers a flexible development and validation platform for industry, plant engineers, materials manufacturers and research institutions. This enables new DAC technologies to be further developed more quickly, more reliably and under realistic conditions, right through to industrial application.
The DAC technology developed by ZSW, which has so far been scaled up to a capture capacity of 100 tonnes of CO₂ per year, is based on an aqueous polyethyleneimine (PEI) solution. This technological approach combines the advantages of amine-based solid-phase processes (low desorption temperatures) with those of hydroxide-based liquid-phase processes (the possibility of continuous operation).
The continuous process control avoids downtime, high material loads and fluctuating CO₂ product gas qualities. At the same time, the control engineering requirements are comparatively low. This robust technology is also characterised by low maintenance costs and good scalability – key prerequisites for the future expansion of CO₂ capture capacity on a gigatonne scale. Due to its high purity, the captured carbon dioxide is suitable both for permanent storage and for material use, for example in the production of synthetic fuels (eFuels).
In the ZSW process, which has been successfully tested over more than 25,000 hours, the PEI washing solution is passed through an absorber designed as a gas scrubber in counter- or cross-flow to the air. The air is conveyed through the system by a fan, whilst an upstream filter prevents the ingress of coarse impurities. Structured packing or bed material increases the phase boundary between the air and the washing solution, thereby improving CO₂ absorption.
CO₂ absorption typically takes place at ambient temperature, whilst desorption requires temperatures between 80 and 100 °C. To increase energy efficiency, the absorber and desorber are coupled via a heat exchanger. This preheats the liquid emerging from the absorber and simultaneously cools the liquid stream returning from the desorber. This reduces both the desorber’s external heat demand and the cooling requirement in the absorber. Cooling the wash solution before it re-enters the absorber is crucial, as higher temperatures would reduce the CO₂ absorption capacity whilst simultaneously increasing water evaporation.
