Transient simulation of heat and mass transfer in a parallel flow regenerative lime kiln with pulverized coal injection

Abstract

Quicklime, which is an essential raw material in various industries, is primarily produced by thermally calcinating limestone in kilns. Among the various kiln designs, parallel flow regenerative (PFR) kilns with pulverized coal combustion are the predominant technology. The performance of the PFR kilns is determined by the interactions between the gas, limestone, quicklime, and coal particles inside the chamber. Additionally, the two chambers in the PFR kiln alternate after a short period during the operation. Consequently, the transfer behaviors between the various phases inside the chamber are in a transient state. A three-dimensional transient multiphase model was created to investigate the transient multiphase behavior. The entire operation cycle, which consists of calcination and regeneration processes, was simulated using a coupling method. The predicted CO2 concentration in the exhaust gas was compared with the industrial measurement data, and good agreement was obtained, with an error of less than 6%. Furthermore, variations in the gas and limestone temperatures and limestone concentrations with residence time during each period were obtained. During the calcination period, the gas temperature field stabilized after 100 s. After stabilization, the zone where intensive limestone decomposition occurred was located in the range 0–5 m from the lance tip. The maximum gas temperature in the calcination process was 1845 K, which was approximately 700 K higher than that of a PFR kiln fueled with natural gas. CO2 gradually accumulated in the middle part of the chamber; the average concentration of CO2 in the off-gas was 27.7%. In the regeneration period, the transfer process stabilization required 400 s. Furthermore, the maximum gas temperature was higher than that of a PFR kiln fueled with natural gas, with a difference of approximately 90 K. Contrary to the calcination period, CO2 accumulated in the peripheral region at the end of regeneration. The total CO2 emission is 1.2024 t CO2/t product for the investigated coal-fueled PFR kiln. These transient transfer characteristics, during the PFR operation, could aid the optimization of coal utilization and control of CO2 emissions in the future.