Optimization for Reduction of NOx and Heat Loss of Parawood Chips Boiler Process Combined with Air Preheater and Controller Using Aspen Dynamics

Abstract

In a small scale of boiler process, a typical problem is an improper control of air to fuel ratio during the combustion process. Such poor control causes heat loss and undesired emission production. In this study, the main aim was to make the process green in the sense of energy recovery and emissions regulation (CO, NOx, and SOx) in an optimum manners, and to design the control system to maintain the optimal conditions using Aspen Plus and Aspen Dynamics. The external flue gas recirculation technique was applied to the process to reduce thermal NOx. Air preheater was also designed and applied to preheat the feed air combustion temperature for energy recovery purpose. Dynamic simulation scheme was proposed to overcome a limitation of Aspen Dynamics for solid process, and the air to fuel ratio control was then designed to regulate CO, NOx, and SOx emission. Boiler level and boiler pressure controllers were also designed for safety purpose. In addition, the steam production controller was also designed for achieving the required steam. Rejection of uncertain moisture content of wood chips was considered. The results have shown that the simulated trajectories were consistent with the measured data collected by wood manufacturing company in the south of Thailand. The optimal operating condition of the process was 5% excess air and 25% flue gas recirculation with 40.5 wt% moisture content of wood chips. The optimal condition with the designed air preheater provided 29% heat recovery in this case, and 35.4% of steam production higher than original process. The proposed control scheme can regulate successfully the CO, NOx, and SOx emission under their limitations, and can handle disturbance changes very well.