https://doi.org/10.1016/j.fuel.2017.08.105
“Carbonate looping or calcium looping (CaL) is an efficient post-combustion CO2 capture technology using limestone based sorbents, and which was initially proposed by Shimizu et al. [1]. CaL is particularly suited for retrofitting existing power plants. As shown in Fig. 1, the CO2 contained in the flue gas from an emission source, e.g. an upstream power plant, is absorbed by calcium oxide (CaO) in the carbonator in an exothermic reaction at around 650 °C forming calcium carbonate (CaCO3) according to Eq. (1).(1)CaO(s)+CO2(g)↔CaCO3(s)+178.2kJmol-1”
“Fig. 1. Principle of the carbonate looping process.”
“A semi-industrial scale CaL pilot plant consisting of two interconnected CFB reactors and a combustion chamber with a thermal capacity of 1 MWth each is located at Technische Universität Darmstadt. In previous tests [15], [27], the main focus was proof of operation in 1 MWth scale. Therefore, CaL tests were accomplished decarbonizing synthetic flue gas, a mixture of air and CO2. The calciner was operated with oxygen enriched air in these tests not representing realistic operating conditions in terms of calcination. As a consequence from these results, the pilot plant was upgraded to operate with coal originated flue gas from the furnace and oxy-combustion in the calciner.
The scheme of upgraded 1 MWth CaL pilot plant is shown in Fig. 2. The CaL reactor systems consists of a carbonator with an inner diameter of 0.6 m and a height of 8.6 m. The dimensions of the calciner are 0.4 m and 11 m, respectively. Both reactor loops include high efficiency cyclones separating the particles from the decarbonized flue gas and CO2 rich stream leaving the carbonator and the calciner, respectively. The separated solids fall down in loop seals where a part is internally recirculated or transferred to the other reactor (from carbonator to calciner and vice versa). Both reactors are equipped with all conventional components of industrial CFB systems, i.e. refractory lining, start-up burners fired by propane, two-pass heat exchangers to cool down the off-gases, bag filters to remove entrained solid particles, water-cooled screw conveyors to extract solids. Additionally, the carbonator is equipped with five axially arranged internal cooling tubes. The cooling tubes allow the extraction of released heat in the exothermic carbonation reaction to operate at various reactor temperatures. The carbonator flue gas is provided by the combustion chamber and is fed to the reactor with a fan. Sorbent is continuously fed as make-up to the carbonator by a gravimetric dosing system. The calciner is operable under air and oxy-fuel combustion conditions. Coal is continuously fed by a gravimetric coal dosing system either in the return leg or in the bottom of the calciner. Different types of coal, i.e. hard coal or lignite, in various particle size fractions from pulverized to coarse, can be fed to the system. For start-up, air combustion is used to heat up the reactor. Oxy-firing is applied in regular CaL operation. Therefore, liquefied oxygen provided by a tank storage is diluted with recirculated flue gas, coming from the cyclone after cooling and filtration. The solid mass flow from carbonator to calciner is controlled by a screw conveyor. The solid mass flow vice versa is balanced by the entrainment in the calciner reactor, controlled by the fluidization velocity. The calciner reactor was operated in once through without internal recirculation.”
“Fig. 2. Scheme of the 1 MWth CaL pilot plant.”
“Table 2. Range of operating parameters during long-term pilot tests.”
