Development of a Carbon Stripper Particle Separation System for Chemical Looping Applications

摘要

The development of chemical looping combustion technology for electrical power generation is of great interest due to its ability to generate highly concentrated CO_2 exhaust gas streams that more easily facilitate carbon capture and sequestration efforts. A significant amount of research has been devoted towards the study of chemical looping of coal due to the abundance and relatively cheap cost of coal as a fuel source. In its most simplistic form, chemical looping involves the cycling of a metal oxide (hence forth referred to as an oxygen carrier) between to fluidized bed reactors. In one reactor, the oxygen carrier is mixed with the fuel coal under conditions that lead to the reduction of the oxygen carrier. The oxygen released by the carrier then reacts with the coal. This is commonly known as the fuel reactor. Upon exiting the fuel reactor, the oxygen carrier is then separated from any remaining unburnt carbon and introduced into the second reactor, known as the air reactor. In this reactor, the carrier compound is oxidized, thus regaining the oxygen lost in the fuel reactor. This cycle of passing the oxygen carrier between the two reactors is repeated, thus the reason behind the name "chemical looping". In this process, it is critical that only the reduced oxygen carrying metal oxide is passed into the air reactor. If any unburnt carbon (aka coal) is introduced into the air reactor, combustion is likely to occur. Among other adverse consequences, this results in the inability to efficiently capture the CO_2 produced. Because of this, it is absolutely critical that an efficient means of separating the oxygen carrier compound from coal char, or unburnt carbon be developed. In fact, a recent report from the U.S. Department of Energy on chemical looping combustion determined that the lack of an effective means of separating char from the oxygen carrier would prevent implementation of this technology. To address this issue, researchers at National Energy Technology Laboratory (NETL) have proposed a novel carbon stripper unit for use with chemical looping combustion systems. The proposed unit is a conical spouted bed that combines an annular opening between the top of the cone-shaped spout inlet and the inner wall of the bed (See figure 1), and the use of a sweep gas to restrict the flow of particles through this opening. A mixture of granular oxygen carrier and coal particles are introduced into the bed above the spout cone via a feed screw. The spouting gas is injected from the bottom of the spout cone, forming a vertical jet along the longitudinal axis of the bed riser and inducing an eddy-like recirculating motion within the bed of oxygen carrier and coal particles (see figure 2).