Synthesis of Sintering-Resistant Calcium Based Sorbents for High Temperature CO2 Capture

Wenqiang Liu (2011). Synthesis of Sintering-Resistant Calcium Based Sorbents for High Temperature CO2 Capture PhD Thesis, School of Mechanical and Mining Engineering, The University of Queensland.

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Author Wenqiang Liu
Thesis Title Synthesis of Sintering-Resistant Calcium Based Sorbents for High Temperature CO2 Capture
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
Publication date 2011-11
Thesis type PhD Thesis
Supervisor Dr. Bo Feng
Professor João C. Diniz da Costa
Dr. Wang, Geoff Guo-Xiong
Total pages 130
Total colour pages 13
Total black and white pages 117
Language eng
Subjects 09 Engineering
Abstract/Summary Calcium looping process (CLP), which uses reversible reactions of CaO/CO2 for capturing CO2, has received considerable attention recently owing to abundant reserves of potential sorbents and its various applications in post-combustion, pre-combustion capture and thermal energy storage. The main advantage of the CO2 capture processes based on CLP is the much lower cost compared with other competing technologies. However, these processes face a major challenge, i.e., the loss-in-capacity problem of CaO-based sorbent after multiple cycles of carbonation/calcination. The calcium conversion of conventional CaO-based sorbents would decrease rapidly from nearly 80% in the first cycle to less than 20% in the 20th cycle. The loss-in-capacity problem increases the operating difficulty and costs and is one of the major obstacles to the application of the CLP-based systems and a solution is urgently needed to move the systems towards commercialization. Currently it is one of the hot topics in the research area of calcium looping. This work aims to find a solution to the problem and has been focusing on one particular strategy, i.e., synthesis of sintering-resistant calcium based sorbents that can sustain high calcium conversion over multiple cycles. First of all, this work attempted to find suitable calcium precursors for the synthesis of calcium based sorbents. The study of calcium precursors is very important for sorbent synthesis because it would directly affect the performance of final sorbent. Therefore, 9 different potential calcium precursors (including 4 new and unreported precursors and 5 conventional precursors) for the production of CaO sorbents were studied in this thesis. It was found that the CaO produced from one of the unreported precursors, i.e. calcium D-gluconate monohydrate (CG-CaO), exhibited the best capacity for capturing CO2 with a 30-minute calcium conversion of 83.8% at the 9th cycle, a few times better than the sorbent produced from conventional limestone. The sorbent CG-CaO also showed a much faster decomposition rate and higher predicted residual conversion after prolonged cycles, which are both beneficial for realistic applications, compared with the CaO produced from conventional CaCO3. Therefore, it was confirmed that the selection of the appropriate calcium precursors is an important step in synthesizing effective CO2 sorbents. Secondly, the work attempted to identify a method of synthesizing sintering-resistant sorbents. Following the hypothesis that separating nanosized CaO particles using inert metal oxides in the sorbent to prevent calcium sintering would produce highly effective sorbents, a simple and novel method (Wet Mixing) was discovered to produce sintering-resistant sorbents from soluable calcium and inert metal precursors including some precursors used in the precursor study. It was found that the calcium conversion of the sorbents was maintained at more than 90% over 24 cycles of utilization, under the conditions similar to those in practical systems. It was also found that a wide range of precursors could be used to produce highly sintering-resistant CO2 sorbents using the Wet Mixing method as long as they are soluable. This is believed to be because Wet Mixing can achieve a uniform distribution of calcium oxide nano-particles in the framework of inert metal oxide, and this is confirmed by high resolution images of the sorbents. A spray drying technique was also investigated as a quick manufacture process that can be used together with the Wet Mixing method to produce effective CO2 sorbent particles. Thirdly, theoretical work was conducted to understand the carbonation behavior of single sorbent particles in order to better design sorbent particles and identify optimum operating conditions. A modified grain size model has been developed to simulate the carbonation behavior of a synthetic sorbent particle (CGMG75, produced from calcium D-gluconate and magnesium D-gluconate with 75 wt.% of CaO content). It was found that the conversion of CaO mainly depends on particle size, reaction temperature, CaO content and its porosity. The carbonation reaction does not generate a significant temperature increase inside the particle with a maximum increment of less than 20K. Moreover, the diffusional effects seem to be negligible while the heat transfer due to convection is significant. From the simulation results it is clear that smaller CaO grain size is preferred and the well-known two stage behavior of carbonation (i.e. initial fast reaction followed by much slower reaction which was normally attributed to the more important diffusional effects in the second stage due to the formation of calcium carbonate product layer) could not be due to diffusional effects. Fourthly, the applicability of the synthetic sorbent (CGMG75) in an energy storage system was evaluated. Kinetic studies of CGMG75/CO2 and PbO/CO2 reactions were performed to examine the feasibility of utilizing the sorbents in a chemical heat pump system. The results showed that under the derived conditions for the chemical heat pump system, the synthetic sorbent did not have the problem of loss-in-capacity after multiple carbonation/calcination cycles, and therefore is believed to be applicable. In summary, this work discovered a new method that can be used to synthesize highly sintering-resistant CO2 sorbents that can be used in a chemical heat pump system without the problem of loss-in-capacity. The sorbents could be potentially utilized in other high temperature CO2 capture applications based on preliminary results, and further work needed has been identified.
Keyword High-temperature CO2 capture, sorbent, calcium oxide, energy storage
Additional Notes individual page numbers that should be printed in colour:14, 20,27,80,84,86,88,91,116,119,121-123

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Created: Tue, 07 Feb 2012, 15:12:31 EST by Mr Wenqiang Liu on behalf of Library - Information Access Service