Steam Methane Reforming with Chemical-Looping Combustion: Scaling of Fluidized-Bed-Heated Reformer Tubes

Abstract

The combination of chemical-looping combustion (CLC) and steam methane reforming (SMR) bears the potential for quantitative and energy-efficient CO2 capture along with hydrogen production from natural gas. A 2-dimensional axisymmetric model of a SMR tube was used to estimate the possibility to adapt the tube dimensions to better fit into fluidized-bed heat exchangers. A constant surrounding fluidized-bed temperature was set as the boundary condition. There are two phenomena that affect the reactor performance: the effective heat-transfer rate from the fluidized bed to and through the tube wall and onward to and into the catalyst bed on the one hand and the effective reaction rates of the governing chemical reactions on the other hand. Literature models were used for the heat-transfer description assuming a state-of-the-art reforming catalyst and classical SMR kinetics were formulated. The simulation results show a temperature decrease toward the tube center in steady state operation. The gas phase composition at the tube outlet reflects the radial temperature distribution as the chemical equilibrium is approached well. Simulations with smaller tube diameters indicate that the necessary tube length for equivalent gas conversion is significantly reduced. In a 1/2-scale setting with 63 mm inner diameter (ID) of the tube instead of 126 mm ID in full scale, the necessary tube length is only 6.0 m instead of 12.5 m, and in a 1/4-scale setting with 31.5 mm ID, the necessary tube length is only 3.3 m. A temperature increase of the fluidized bed from 900 to 950 and 1000 °C may reduce the necessary tube length in the 1/4 scale from 3.3 to 2.6 and 2.2 m, respectively. These indications are promising with respect to the possibilities for fluidized-bed immersed reformer tube dimensioning and arrangement. More detailed reactor design studies will be necessary to judge the industrial feasibility of the CLC-SMR combination.