Development of a loop seal for the recirculation of granular solids
The PyRos plant in the ThE laboratory is a pilot plant that is able to convert biomass into pyrolysis oil. In order to increase the efficiency of the cyclonic PyRos reactor an upgrade has been proposed. The rotating valve currently employed for the transportation of the solids from the PyRos reactor to the fluidized bed combustor should be replaced by a valve better equipped for this particular situation.
Based on design requirements several mechanical and non-mechanical valves have been evaluated. The loop-seal is found to be the best valve for application underneath the PyRos reactor. In order to gain a better understanding of the operating characteristics of the loop-seal, an experimental setup has been built for tests at room-temperature. Various parameters have been varied in order to determine the optimal dimensions of the loop-seal. Besides changing the physical dimensions of the loop-seal, the influence of the fluidization velocity, particle size and fluidization location have been investigated. The experimental setup proved that the loop-seal can satisfy the set of requirements. In order to predict the operating characteristics of the loop-seal at elevated temperature, a mathematical model of a loop-seal has been developed. The model shows a good agreement with the experimental results at room temperature. Using the model and the experimental results a design for a loop-seal underneath the PyRos reactor has been proposed. Product gas instead of air has to be used for fluidization, since the oxygen inside air will disturb the pyrolysis process inside the PyRos reactor.
Another advantage of the loop-seal concerns the biomass conversion in the pyrolysis reactor. Approximately 10% of the biomass leaves the PyRos reactor unconverted via the bottom exit. The solids inside the standpipe may be operated as a moving bed reactor in order to convert these last biomass particles into oil. In this way an increase between 8 and 9% in oil yield can be achieved.