A study of advanced RTO (Regenerative Thermal Oxidizer) technology by optimised combustor integration and carbon-free fuel for non-carbon emissions

Abstract

The control of emissions of VOCs (Volatile Organic Compounds) has been an important issue when environmental standards has been starting to implement. Thermal Oxidation which involves combustion processes of gases, liquids and solids has been a common technique to destroy VOCs and has a vast application prospect, especially the application of regenerative thermal oxidizer (RTO). In this thesis which is aiming to investigate advanced RTO technology by optimised integration with combustor(s) and by carbon-free fuel for non-carbon emissions, based on necessary literature review on advanced RTO technologies, main methodology to study and optimize RTO (Regenerative Thermal Oxidation) performances, current development of relevant numerical simulation, non-carbon combustion with carbon-free fuels, experimental investigation and CFD simulation have been carried out for examining effects of various design parameters and operation parameters on VOC conversion efficiency, energy application and non-carbon emissions. After the experimental equipment, instrumentations and testing conditions for initial experimental investigation are introduced, influences of operating temperatures and purging time on gas-out VOC concentration have been examined. Those results suggest that to maintain a lower gas-out VOC concentration but keep low fuel consumption and low combustion temperature still need significant work to do. The CFD numerical model including relevant sub-models have been introduced and developed. Based on those, the required meshes have been created and presented. Initial validations show the modelling results have very good agreement with the experimental results. It suggested the developed CFD model can be used for simulating the performance of three-bed RTO. Then the integration between combustor(s) and RTO has been investigated with CFD simulation. Five sections Including combustor protrusion, combustor diameter (or combustor exit velocity), combustor vertical position, combustor horizontal position, twin combustor were studied for examining their influences on temperature distributions, flow field, VOC concentration distributions, VOC concentration in gas-out flow, NO emissions etc. In summary, combustor horizontal position can provide a better solution for reducing both VOC and NO outputs, while twin combustor is not so promising for benefiting RTO performance improvement. As hydrogen can provide zero CO2 emissions and other emissions except NOx, it as fuel was studied with the main objective to explore the possibility for RTO to implement carbon-free combustion and emissions,. The same heat amount with hydrogen as fuel was supplied for comparing the difference between hydrogen as fuel and natural gas as fuel. When stoichiometric combustion is maintained for both natural gas fuel and hydrogen fuel, modelling cases for same combustor diameter and same combustor exit velocity are simulated. Results show that, although the same heat amount is supplied with hydrogen as fuel, both the same combustor diameter case and the same combustor exit velocity case produce higher VOC concentration in gas-out flows. The reason may be the reduced temperature in most RTO space due to more water condensation for hydrogen combustion. Reduced hydrogen amount/flowrate was also investigated for examining effects of reduced energy supply on RTO performance. Results show that reduced hydrogen amount will almost proportionally increase VOC concentration in gas-out flows. With the same heat amount for hydrogen as fuel, NO emissions have no big difference, compared to natural gas although VOC amount increased. With reduced hydrogen amount/flowrate, NO emissions amount has some slight reduction. It suggests that both reduced temperature in most RTO space due to water condensation and increased flame temperature of hydrogen combustion contribute to the results.