(602e) Scale-Up and Experimental Investigations of Loop-Seal in Chemical Looping Combustion (CLC) Process: Solid Flowrate Regulation and Gas Tightness

The current work is defined in the frame work of CLC project at IFPEN, a cooperative development project joint by TOTAL.

 Chemical looping combustion (CLC) is a novel combustion process with inherent CO₂ capture resulting in a minimum capturing energy penalty. CLC process is an oxy-combustion technology in which the oxygen required for the combustion is supplied by metal oxides in so called "Fuel Reactor". The metal oxides, known as oxygen carriers, are then sent to a separate reactor (Air Reactor) where they are re-oxidized in contact with air. The oxidized particles are sent back to Fuel Reactor, hence forming a loop.

 Control of solid flow rate and gas-tightness are two essential parameters in chemical looping combustion process. This paper presents experimental results for use of loop-seals as a pneumatic solid flow rate controlling device which minimizes gas leakage between two regions in a circulating fluidized bed. The pressure drop variations in different elements of the loop-seal are analyzed as function of gas and solid flow rate and pressure balance across the solid circulation loop. Moreover, minimization of gas leakage and formation of the moving solid column in the supply chamber is investigated in terms of gas flow rate, solid flow rate, external aerations, and pressure drop downstream of a small loop-seal. The horizontal solid and gas flow in the loop-seal and the impact of the first and second loop-seal aerations are also investigated.

 Loop-seal of different internal diameters (0.021 m and 0.150 m) are used in this work. Ilmenite and two sand particles are used with corresponding density of 4750 kg/m³ and 2650 kg/m³ and average diameters ranging from 128 to 368 µm for the small loop seal. In case of large loop-seal, a metallic oxide called BMP and a mixture of BMP and FCC catalyst are used with corresponding density of 3300 kg/m³ and 3100 kg/m³ and average diameters ranging from 150 to 200 µm. The mock-up used in this study is a circulated fluidized bed, constituted of a reactor (dense bed), a L-valve that fix the solid flowrate, a riser and a loop-seal.

 Loop-seals are commonly used in circulating fluidized bed systems to convey particles from a low pressure region to a high pressure region [1]. One of the principal functions of a loop-seal is to avoid the undesirable inverse gas flow and to provide gas tightness. Formation of a moving solid bed in the supply chamber of the loop-seal provides the primary functions of the loop-seal in terms of pressure drop balance and gas leakage minimization. Pressure drop of the moving bed in the supply chamber of the loop-seal is developed as results of the relative solid and gas flow and is a function of the overall pressure balance.

Three types of pressure drops can be distinguished in this part of the system including: reactor pressure drop (DR) which is a function of solid inventory in the reactor, pressure drop in the horizontal pipe (HP), recycle chamber (RC), recycle pipe (RP) of the loop-seal, and cyclone which are function of solid and gas flow rate, and finally pressure drop in the supply chamber (SC) of the loop-seal which is a dependent variable, that varies to adjust the overall pressure balance in the circulation loop.

Calculation of the pressure drop in the horizontal pipe of the loop-seal is a key parameter which is characteristic of a loop-seal. Various correlations have been developed in the literature on the pressure drop in the loop-seals as listed by the reference [2]. These correlations were compared with the actual experimental results of 21 and 150 mm diameter loop-seals. Arena et al. [3] have correlated the pressure drop in horizontal section of an L-valve in terms of solid flow rate (W_s), solid bulk density (ρ_b), pipe diameter (D_ls), and particles diameter (d_P). This correlation resulted in the best prediction between the experimental results for all 4 solids and 2 tested loop-seals.

 Impact of the first and the second loop-seal aeration (Q_ls1 and Q_ls2) on the loop-seal operation are studied in a continuous solid circulation test for the 150 mm loop-seal. The input solid flow rate was maintained constant by aid of the L-valve. The solid flow rate in the current tests is calculated based on the pressure drop in the riser. A constant pressure drop in the riser is an indication of the constant solid flow rate. The variation of the second aeration (Q_ls2) shows no impact on the solid circulation. However, change of the first aeration (Q_ls1) has considerable influence on the stability of the loop-seal. Increase of the Q_ls1 decreases the height of the solid column in the dipleg (DP_SC). Accordingly, more particles will be accumulated in the downstream reactor and its pressure drop increases.

The variation of the solid repartition in the loop-seal is related to the variation of the solid flow rate in the horizontal section of the loop-seal as well as the change of the gas flow rate in the dipleg (SC) which results in a unique solid height in the dipleg for each pressure and aeration condition in the loop-seal. The impacts of Q_ls1 and Q_ls2 are studied in details for the small loop-seal [4].

 Solid flow rate in the current loop-seal is controlled in the horizontal section of the loop-seal similar to a L-valve [4]. The solid flow rate through the horizontal section can be directly linked to the gas velocity in the horizontal pipe U_HP. The gas flowrate in the horizontal pipe results of the sum of the first loop-seal aeration flowrate and the flowrate in the supply chamber, that can be calculated from the Ergun equation (knowing the linear pressure drop in the SC) [5]. The solid flux in the loop-seal is measured as a function of fluidization number in the horizontal pipe of the loop-seal (U_HP/U_mf) for 4 solids and 2 loop-seals. This work shows the linear dependency of the solid flux in the loop-seal to fluidization number as the principal solid flow controlling parameter in the loop-seal.

References :

 [1]      A. Johansson, F. Johnsson, and B. A. Andersson, “The Performance of a Loop Seal in a CFB Boiler,” Journal of Energy Resources Technology, vol. 128, no. 2, pp. 135–142, 2006.

[2]      M. M. Yazdanpanah, “Investigation of a Chemical Looping Combustion (CLC) Configuration with Gas Feed,” Dissertation, IFP Energies nouvelles, Lyon, France, 2011.

[3]      U. Arena, C. B. Langeli, and A. Cammarota, “L-valve behaviour with solids of different size and density,” Powder Technology, vol. 98, no. 3, pp. 231–240, 1998.

[4]      M. M. Yazdanpanah, A. Forret, T. Gauthier, and A. Delebarre, “An experimental investigation of loop-seal operation in an interconnected circulating fluidized bed system,” Powder Technology, vol. 237, no. 0, pp. 266–275, 2013.

[5]      P. Basu and J. Butler, “Studies on the operation of loop-seal in circulating fluidized bed boilers,” Applied Energy, vol. 86, no. 9, pp. 1723–1731, 2009.