Schüppel, Ben (2012)
Impact of Improved Sorbents on the Performance of the Carbonate Looping Process.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

This thesis was considered with the Carbonate Looping (CL) process and in particular the impact of improved sorbents on the process performance. A broad overview about solid state chemistry was given. Crystal systems of CaCO3, CaO, Ca(OH)2 and CaSO4 have been presented. It was shown that CaO has the most dense package of all substances. However, CaO has the highest melting point in the group which indicates that sintering phenomena like loss of specific surface area and irreversible pore mouth blockage are much more likely to happen during the re-carbonation than during the calcination step. Impurities of the coal appear to become a challenge for the operation of the CL process: On the one hand impurities might get enriched in the loop causing high circulating mass flows and on the other hand melting phases are very likely to occur. Special emphasis should be put on the usage of a fuel containing very low amounts of alkali metals. As well, reducing conditions should be avoided in the calciner to avoid any undesired effects resulting of the system CaSO4-CaSO3-CaO. Theoretical investigations of the CaCO3 decomposition showed that the majority of sorbent particles should be calcined within seconds in the calciner. Low residential times and rather low mass loadings in the calciner seem to be achievable. The literature review on CL sorbents revealed a critical response of the CO2 capture ability of sorbents on changed TGA test conditions. In this perspective a lot of promising approaches, like special thermal pre-treatments became less promising. It was speculated that the good reported outcomes on these materials, was only an effect of randomly beneficial chosen TGA test conditions. Own experiments under harsh TGA test conditions with improved and synthetic products did not indicate any benefits for the CL process in terms of CO2 capture ability. Mechanical and chemical reactivation routes were presented. The only process discussed within literature offering both reactivations at the same time is the SD process. Increased mechanical stability and refreshed CO2 capture ability were reported by [86]. However, this process would have to be repeated after each third cycle leading to huge additional efforts for the sorbent reactivation. The sophisticated CL process model showed that optimisations could be done with a variety of targets. Optimising the economics, for example, would only require large utilities that should be operated with a sorbent as hard as possible. In contrast, an optimal efficiency could only be reached with special sorbents offering an optimal compromise between CO2 capture ability and hardness. Minimal CO2 capture costs were calculated to 20 €/t (including the compression of the CO2 produced by the power plant and the CL process). The calculations indicate that the handling of the huge amount of make-up and bleed might become a problem. Sorbent research was categorised into basic, advanced and synthetic materials. Basic materials are common, now available limestones, advanced materials are thermally treated oxides and synthetic products are tailor-made lab samples. Based on TGA experiments it turned out that neither advanced nor synthetic materials had any long term advantages under harsh TGA conditions compared to good performing natural sorbents. Hardness investigations with different experimental setups created a confusing picture in that no real correlations between the measurements of the individual test were achieved. Basically, two reasons were identified for this observation: First, depending on experimental conditions attrition by surface wear or impact stress was dominant. According to the nature of the products, the reaction on surface wear and impact might be completely different. Second, the cyclic change of sorbent properties that change from a carbonate shell to a soft CaO shell to a sintered particle. Additionally, the duration of the appearance of the different Ca-forms depends strongly on process conditions.

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