Thermal regeneration of metallic fibrous particulate filter

Abstract

Diesel particulate emitted from diesel vehicles is carcinogenic and a major air pollutant in a big city. The problem can be solved effectively by installing suitable after-treatment devices to the vehicle-tailpipes. A diesel particulate filter using stainless steel fibres as the filtering elements, has been developed by The Hong Kong Polytechnic University for application to light-duty diesel vehicles for such purpose. It has been found to be a very effective device, however, it should be cleaned regularly to recover its usefulness once it has been over-accumulated with particulates. Thermal regeneration, a process to burn off the accumulated particulate, has been suggested to be a possible method to clean up the filter. Literatures on experimental and numerical simulation studies on the thermal regeneration of ceramic wall-flow particulate filter are available, but those on metallic fibrous filter were rare. The present study, which is conducting to fill the gap, contains two integrated pans: an experimental investigation and a numerical simulation study. In the experimental study, an experimental rig had been designed and built to simulate the regeneration process in the metallic fibrous particulate filter. A gas burner was used to heat up the diluted air to ignite the particulate in the filter. The experimental study involved different configurations of the filter including length of the filter cartridge, and packing densities of the filter element, and different particulate loading conditions. The effects of different conditions of the heated air mixture including its temperature, oxygen concentration, and volume flow rate were studied. Temperature distributions along the particulate filter before, during and after the regeneration process were measured by twelve K-type thermocouples inserted along the length of the particulate filter. The present experimental study provided very comprehensive data under different operation conditions. It was found that the temperature increased sharply during the regeneration process throughout the entire length of the filter. Thermal regeneration proceeded from the upstream to the downstream regions and excessive heat might accumulate at the downstream region leading to local melting of the filtering element. A mathematical model had also been developed to simulate the thermal regeneration process of the filter. The model consisted of two parts, with one part concerning with the particulates oxidation in the filter and the other part concerning with the heat transfer processes. The model included the conduction heat transfer within the solid phase, the change in internal energy of the gas and solid phases inside the filter, and the heat released during the oxidation of particulate. The particulate oxidation was controlled by the kinetic reaction rate of oxygen and carbon, thus the particulate consumption rate, which cannot be obtained experimentally, can be simulated and provide insights into the regeneration process. Numerical simulations were performed to obtain the filter temperatures at different locations along the filter at different time steps. The thermal regeneration characteristics with respect to the same parameters of the experimental study had been simulated. It was found that the simulated results were in good agreement with the experimental results with an acceptable degree of deviation. The deviation was small in the heating up and cooling down periods and was relatively large in the heat release period. A significant contribution of the present study is to achieve a better understanding of the thermal regeneration process of a metallic fibrous particulate filter, which is a very important process to maintain its best performance on a continuous basis.