Comparison on process efficiency for CLC of syngas operated in packed bed and fluidized bed reactors

Abstract

Chemical-looping combustion can be carried either in circulating fluidized bed reactors or in dynamically operated packed bed reactors. In this work, the influence of the reactor selection on the overall process efficiency has been estimated. The CLC reactors are fed with syngas at 20bar, which is produced from coal gasification.In the fluidized bed system, the oxygen carrier particles are transferred from the air to the fuel reactor and vice versa. Uniform reactor temperatures can be maintained due to internal solids circulation and vigorous mixing. Due to the large solids circulation and low reaction enthalpy in the syngas-fed fuel reactor, small temperature differences result between the reactors. A case where NiO/Al2O3 is used as oxygen carrier has been considered and an LHV efficiency of 41.4% has been calculated, assuming the fuel conversion at chemical equilibrium, leading to some unconverted CO and H2 in the reaction products. If full gas conversion is obtained by using another oxygen carrier operating at the same maximum temperature of 1200°C, this efficiency can be increased to 41.8%. In the packed bed case, large temperature gradients inside the reactors are obtained and the CO2 and steam is produced at a lower (time-)average temperature. In addition, thanks to the lower average reaction temperature during fuel oxidation, the selectivity of the reduction reactions with nickel is not an issue. On the other hand, in this case a significant amount of steam has to be added or recycled to avoid carbon deposition, leading to an LHV efficiency of 41.1%. If a carrier is selected with a low kinetic activity for the Boudouard reaction, the efficiency can be increased by 1.1% point up to 42.2% of LHV. Other differences between the two systems are related to the number of vessels needed and the volume of the solids inventory, which is expected to be lower in the circulating fluidized bed system. An initial investment cost estimation demonstrates that the oxygen carrier costs are important, so that a low cycle time and a cheap oxygen carrier is preferred.The process efficiency for both reactor configurations is at the same level, 41–42% of LHV. Therefore, it can be concluded that the reactor selection will not be made based on the process efficiency, but on the availability, operability and cost of high temperature and high pressure reactor systems, that are still under development and both present pros and cons.