Application of Rotating Fluidized Bed to Solar Gasification

Abstract

The development of sustainable energy technologies such as Concentrated Solar Thermal (CST) is attracting growing attention. One of the applications of CST technology is solar thermal gasification of carbonaceous feedstocks. This thermochemical process combines a gasification agent, either steam or CO2, with widely available carbonaceous feedstocks to produce a useful mixture of H2 and CO, commonly known as syngas. Syngas can be burned directly in power generation cycles to produce electricity or used as a chemical feedstock in the Fischer-Tropsch process to produce hydrogen based chemicals. The present thesis reports the development of a directly irradiated solar receiver concept termed Rotating Fluidized Bed Receiver (RFBR) for solar thermal gasification. The RFBR concept involves rotating a cylindrical cavity containing feedstock particles about its axis of symmetry and injecting a radially inward gas flow through the porous cylindrical wall. The centrifugal acceleration generated through rotation forms an annular particle bed on the cylindrical wall for solar radiation absorption, and the radially injected gas flow fluidizes the particle bed with a drag force that counters the centrifugal force acting on the particles. A comprehensive analytical model was developed to track the movement of a single biomass char particle undergoing gasification in the RFBR under typical solar receiver conditions. The analytical assessment found that: the particle residence time was highly dependent on the rate of particle gasification; the centrifugal force generated through rotation could effectively retain char particles in the receiver cavity until sufficient particle mass is lost to gasification conversion. It was shown that operating the RFBR at a rotational speed of 70 rad/s or greater could result in gasification conversion extents greater than 85% for char particle sizes between 100 and 450 microns. A CFD analysis of the flow field in the RFBR concept was also conducted to determine the effects of control parameters such as receiver rotational speed and velocity of the radially injected gas on the propensity of particles depositing on the receiver window and investigate the aerodynamic mechanisms involved. The analysis found the receiver rotational speed to be the most effective parameter in preventing entrained particles from entering the receiver aperture and depositing on the window. Operating at a relatively low rotational speed of 42.1 rad/s could limit the rate of particle deposition on the window to 0.25 % of the injected 10 μm diameter particles, which is comparable to the deposition rate in other solar receivers. Lastly, an experimental campaign was conducted to investigate the fluidization characteristics and bed surface profile in a non-reacting rotating fluidized bed (RFB) which is integral to the RFBR concept. It was found that at relatively low rotational speeds between 21 rad/s (200 RPM) to 31 rad/s (300 RPM), there was insufficient centrifugal force to create a uniformly distributed annular bed in the investigated vertical RFB. The greatly varied radial bed thickness in the axial direction led to highly non-uniform fluidization quality. A bed redistribution procedure was devised to redistribute the bed particles and reduce the variation in radial bed thickness prior to low rotational speed fluidization. The implementation of this redistribution procedure was observed to significantly improve the uniformity of radial bed thickness and fluidization quality. This confirmed that the RFBR could be operated at relatively low rotational speeds that are unlikely to introduce excessive mechanical energy loss or component wear.