Simulation and optimization on the regenerative thermal oxidation of volatile organic compounds

Volatile organic compounds (VOCs) are involved in the generation of ozone v ia reaction with atmospheric NO, in the atmosphere through a photochemical pathway. One of the popular treatment processes: for VOCs is thermal oxidat ion, since it shows high destruction and removal efficiency (DRE). A regene rative thermal oxidizer (RTO) is commonly used because of its high heat exc hange efficiency. The process uses ceramic beds to capture heat from gases exiting the combustion zone. Steady and unsteady flow field, distributions of temperature, pressure and compositions of flue gas inside an RTO were si mulated by computational fluid dynamics (CFD). The DRE of VOCs and the poll utant concentrations of CO and NO emitted from the RTO are estimated by inc orporating two-step incomplete combustion reaction of VOCs and Zeldovich's NO formation mechanism. The model system was the oxidation of benzene, tolu ene and xylene by the RTO, which was composed of three beds packed with cer amic beads to exchange heat. The height of the ceramic beds was varied and the effect of change of height was investigated in terms of DRE:and pressur e drop. The objective of the study was to simulate the performance of the R TO and optimize the height of ceramic beds. In a preliminary calculation, m ore than 95% of DRE is obtained when the premixed flow velocity of VOCs-air is less than about 3 m/s at the steady state at a constant fuel rate of 0. 469 m/s under the given conditions. A weak recirculation zone above the cen ter bed appeared and the intensity of recirculation decreases when no fuel is added in the normal direction. This region represents the putative site for the formation of NO, while most of the CO produced is converted to CO2 in the recirculation zone. The results show that a level of 1.0% of VOCs is sufficient to provide energy for the oxidation when heat is exchanged thro ugh the ceramic bed. A ceramic bed of 0.2 m in height is sufficient to oper ate properly at these conditions and 5 s is recommended as the stream switc hing time. The importance of this simulation study is that the performance of RTO can be calculated at any operating conditions without the need for a pilot stage. In addition, the:results can provide insight and practical re sponses involved in the design of an- industrial RTO unit. (C) 2000 Elsevie r Science S.A. All rights-reserved.