The IGF project 17796 N of VDZ gGmbH is supported by the AiF within the framework of Industrial Collective Research (IGF) of the Federal Ministry of Economic Affairs and Energy on the basis of a decision of the German Bundestag.
Energy-efficient dry CO₂ capture from exhaust gas based on the example of the cement industry
CO2 capture in the clinker burning process is basically possible with the use of so-called post-combustion methods. The Advantage of these is that such systems are installed at the end of the process chain and thus do not interfere with the actual production process. As an alternative to the already relatively advanced amine scrubbing technique (wet method), a chemical absorption process with an extremely high energy demand for regeneration of the sorbent, it is possible to employ dry CO2 adsorption. This involves using a solid adsorbent to bind the CO2, which is then liberated again in highly enriched form in a regeneration operation.
To investigate this method, a research project supported by the AiF was conducted with the cooperation of the IUTA (Institute of Energy and Environmental Technology, Duisburg) and the TU Dortmund. In the course of the project between the end of 2013 and the start of 2016, fundamental studies were performed in the laboratory and in a pilot centre, then in a Westphalian cement works. The work focused on both the development of new adsorbents and the testing of commercially available adsorbents, including trials under real conditions in a cement works. In a further step, these findings were used as a basis for modelling to map the mass and heat transfer phenomena, as well as to produce a feasibility study on the integration of the adsorption unit into the overall cement clinker production process.
The tests at the cement works were performed using a triple cyclically operated pressure-temperature swing adsorption (P-T-SA) fixed bed system, suitable for handling 11 m3/h gas (Fig. 3.2.6-1). The adsorbent used was a styrene-divinylbenzene copolymer with aminomethylene groups in free base form. The utilisation of such amine-functionalised adsorbents for capturing CO2 from process gases has already been the subject of numerous studies, which however to date have primarily concentrated on thermogravimetric analyses or experiments on a laboratory scale, and to a lesser extent on a pilot centre scale, and generally only involved a few adsorption/ desorption cycles. By contrast, the investigations conducted here focused on the suitability of the method for practical use.
In the course of the several weeks of experimentation with the real exhaust gas of a cement works, with an average CO2 clean gas concentration of 13 to 15 vol. %, it proved possible to achieve stable plant operation. Adsorption brought about a reduction in the CO2 concentration of the exhaust gas to a residual content of 0.6 to 1.5 vol. % (Fig. 3.2.6-2; the higher concentrations at the end of the adsorption phase are the result of opening valves on switchover). As with the pilot centre experiments conducted previously, these tests however again revealed only incomplete CO2 desorption on sorbent regeneration, i.e. only up to one third of the bound CO2 was liberated. A CO2 purity of up to 79 vol. % was obtained in the gas flow extracted with a vacuum pump. The majority of the remaining CO2 was only released from the adsorbent and discharged when flushed with air. With cyclical operation of the reaction vessels, the loading capacity achieved with desorption and subsequent flushing permitted cycle times of up to eight minutes without CO2 adsorption breakthrough. A welcome aspect for the adsorbent used was the low level of unwanted co-adsorption of water from the exhaust gas saturated with water vapour. Another positive finding was the absence of degradation of the adsorbent even after several months of trial operation with more than 1 000 adsorption/desorption cycles with real flue gas.
As a final step, studies into the energy demand for regeneration of the adsorbent and the total energy demand of the plant were taken as a basis for calculating the cost-effectiveness of the method examined and for initial assessment of dry adsorption in comparison with alternative methods. It became evident that, so far, the specific capture costs are far higher than for other CO2 capture processes. This can be attributed to the as yet inadequate desorption, for example. The method-based process modelling revealed that it would be no problem to incorporate dry adsorption at the end of the clinker burning process (post-combustion process). With single-stage adsorption it would not however be possible to achieve sufficient CO2 reduction – more or less complete CO2 capture would only be feasible with three-stage adsorption. The models for a BAT (Best Available Techniques) plant further revealed that, with an assumed raw material moisture level of 2 %, the entire heat of regeneration could be supplied by the clinker burning process. It also became apparent that the incorporation of dry adsorption would have scarcely any effect on the actual clinker burning process, so that retrofitting in existing plants would basically be possible. Extensive further studies would however have to be performed beforehand, with better adsorbents for example.