A high temperature solid looping cycle (HTSLC) is a type of chemical process carried out in twin reactor system. The hot solid particles are transferred from first reactor to second reactor and same particles are transferred back to the original reactor in the continuous and endless cyles. The purpose of the solid transfer is either to provide heat to carry out the desired reaction or to regenerate the particle reactivity. In some operations solid transport is required for both purposes. The calcium looping process, steam gasification process and chemical looping combustion are the examples of high temperature solid looping cycles. All these processes are well acknowledged for their potential in carbon capture development. Although these processes differ on the basis of chemistry but they require the use of same reactor system called dual fluidized bed (DFB) system.
These HTSLCs are currently under the demonstration phase at pilot scale. A 200 kWth test plant is built at University of Stuttgart to demonstrate calcium looping and sorption enhanced reforming process. This thesis presents hydrodynamic studies carried out on the cold model of the test plant. This study includes foundation of reactor schematic, proving feasibility of the schematic and suggesting improvements for the reactors. Various combinations of DFB systems are in use. A twin circulating fluidized bed (CFB) CFB-CFB combination is used to investigate calcium looping process (CaL mode). Two CFBs namely carbonator and regenerator are coupled with individual cone valve to facilitate solid transport between them. Within the same test plant another bubbling fluidized bed (BFB) and CFB combination is used for investigating Sorption- enhanced reforming process (SER mode). Based on the preliminary test plant design a cold model is built with a geometric ratio of cold/ hot as 1/ 2.5. The particles used in the study are as per the Glicksmann`s simplified scaling rules. These rules enable to extrapolate the results of cold model to predict the test plant performance. The results from cold model related to pressure drop, inventory and entrainment rates are important for extrapolation. Once the feasibility studies are carried out, the cold model is dedicated to detailed hydrodynamic studies.
In a CaL mode, two CFBs are interconnected with cone valves, a long term steady state operation is feasible in this schematic, with both cone valves delivering equal magnitude of solid flow rates. The dynamic pressure balance between the two CFB makes it possible. For CaL mode the predictions from extrapolation of cold model results show that most of the required boundary conditions are met in the test plant, i.e. pressure profiles, inventories and carbonator entrainment rates, except the regenerator entrainment rates. The modifications are suggested: a regenerator with hopper like bottom, loop seals with increased weir height and riser exit shape. Some of these suggested modifications upon testing again resulted in improved performance.
In SER mode cold model, a CFB (regenerator) and BFB (gasifier) are coupled with an L-valve and a loop seal situated at the bottom of BFB. The forethought DFB set up works in highly stable manner. The extrapolation of the cold model results show that most of the required boundary conditions are met in the test plant. The solid flow patterns and segregation tests in the gasifier confirmed the suitability of the gasifier design.
The standpipe and the loop seal stability is a crucial in a CFB as well as in a DFB operation. However, little is known about such an important part of the CFB system. The gas solid flow is studied in a CFB operation. It is found that, the amount of loop seal aeration influences the gas solid flow in loop seal and standpipe and entire riser hydrodynamics. The most of the loop seal aeration flow enters the recycle side of the loop seal and only up to 5-7 % is observed to enter the standpipe side of the loop seal. The slugging in standpipe is a common problem in small scale CFB risers. Selecting low solid downflow velocity can improve the slugging behavior in CFB standpipes. This study can help set proper guidelines for loop seal and standpipe design for HTSLC.
The accurate knowledge of particle inventory in a CFB riser is important in the case of HTSLC. Some process related parameters, such as space time, turnover ratio are dependent on the riser particle inventory. The pressure drop to inventory co-relation is normally used to calculate the particle inventory in a fluidized bed. However, in a CFB the pressure drop is significantly influenced by the friction and acceleration phenomenon. This phenomenon can cause error in inventory measurements and very little is known about the magnitude of the error. The experiments are performed in a small scale CFB unit to estimate the magnitude of the friction and acceleration pressure drop, by using quick closing valves method. The friction and acceleration pressure drop increases with increasing the riser velocity. However in turbulent regime, the riser contains more inventory than the pressure drop showing zero or negative influence of the friction and acceleration phenomenon. The core annulus flow structure in the CFB riser is supposed to cause this influence. Further studies to find accurate co-relations are required.