The influence of granular flow rate on the performance of a moving bed granular filter

Abstract

The goal of this study is to evaluate the effect of granular flow rate on the performance of a moving bed granular filter designed for hot gas filtration of fine char particles (dust) produced during fast pyrolysis of biomass. The filter employs a counter-current configuration, in which down-flowing granular material spreads out at the bottom of the filtration vessel to form an interfacial area where the dusty gas enters the granular bed and much of the gas cleaning is hypothesized to occur. This study uses a real-time particle counter to measure the instantaneous filtration efficiency during cold flow tests of the filter. Differential pressure measurements at various locations within the granular bed are used to assess the level of char dust hold-up over time. These experiments reveal a critical granular residence time below which the filter must be operated to achieve filtration efficiencies exceeding 99%. Operating above the critical value causes the filter to “clog” and decrease in efficiency. The clogging is characterized by a critical dust volume fraction as determined through a fixed bed filtration test. The filter is found to accumulate most of the dust at the interfacial region. Also the interfacial region is more efficient than the downcomer section of the granular bed in removing dust. Decreasing residence time of granular material in the filter reduces the hold-up of char dust in the filter, which is expected to mitigate coking reactions of organic vapors when the filter is used to remove char from fast pyrolysis gas streams.