Through the assistance of oxygen carrier particles, the chemical looping processes convert carbonaceous fuels while producing a sequestration ready CO2 stream. Two chemical looping gasification processes, the syngas chemical looping (SCL) process and the coal direct chemical looping (CDCL) process, are developed for hydrogen and electricity co-production from carbonaceous fuels. Both processes involve the reduction of a metal oxide with a fuel followed by regeneration of the reduced metal oxide with steam and air in a cyclic manner. The syngas chemical looping process converts gaseous fuels such as syngas and methane while the coal direct chemical looping process converts solid fuels such as coal and biomass.
A novel iron oxide based composite oxygen carrier particle is currently being developed for the aforementioned chemical looping gasification process. The physical and chemical properties of the particles including compressive strength, attrition rate, reactivity and recyclability are tested. Reduction of the particles with syngas in an integral bed reactor is performed followed by oxidizing the reduced particles in the same reactor with steam and then air. More than 99.7% syngas is converted during the reduction step. During the regeneration step, hydrogen with an average purity of 99.8% is produced. The results indicate that the particle is suitable for the chemical looping gasification processes.
In the SCL process, the oxygen carrier particle is first reduced by a gaseous fuel in a first reactor, the reducer. Next, the reduced oxygen carrier is partially regenerated with steam to produce hydrogen in a second reactor, the oxidizer. The partially regenerated particle is further oxidized to its original oxidation state by air in a third unit, the combustor. The SCL process is extensively studied both analytically and experimentally. Thermodynamic analysis shows that a countercurrent moving bed design is suitable for both the reducer and the oxidizer. ASPEN Plus® simulation further suggests the optimum operating conditions and pollutant control strategies for the SCL process. Experiments are carried out in a bench scale (2.5 KWth) moving bed reactor to validate the reducer and oxidizer operations. A quartz fixed bed reactor and TGA are used to mimic the combustor operations. The experimental results match well with the simulation outcomes. More than 99.5% of the syngas is converted during the reducer test. The hydrogen generated during the subsequent oxidizer test has an average purity in excess of 99.95%. The particles can also sustain the high operating temperature of combustor without losing its reactivity and recyclability. ASPEN Plus® simulation shows that the SCL process can improve the efficiency of the current coal to hydrogen process by 4 – 10% with 100% CO2 capture. When integrated with the indirect coal-to-liquid process, “the Chemical Looping system proposed by OSU has the potential to significantly (~10%) increase the yield of the conventional cobalt based F-T process and allow more efficient heat recovery and much lower (~19%) carbon emissions.”
The CDCL process has a fuel conversion scheme similar to that of the SCL process. However, the CDCL process faces additional challenges resulting from direct solid fuel conversion. These challenges include ash and pollutant handling, solid fuel conversion enhancement, and heat management and integration. A solid fuel conversion enhancement scheme is proposed and tested in the bench scale moving bed reactor. More than 90% conversions for various types of coal chars are achieved. ASPEN Plus® simulation is used to both analyze the fate of the pollutants involved and the optimization of energy integration. The process simulation using ASPEN Plus® shows that the hydrogen production efficiency for the CDCL process can reach nearly 80%.
A 25 KWth sub-pilot scale chemical looping demonstration unit is designed and constructed. It is capable of demonstrating the chemical looping gasification processes in an integrated, continuous manner. The unit, which is comprised of six sub-systems, has been fully assembled. Preliminary tests including reactor leakage tests, solid flow calibration, particle hydrodynamic studies, and integrated reactor operations are performed. The test results show that the sub-pilot unit meets the design standard and is ready for the SCL process demonstration. It can also be used for CDCL process demonstration with minor modifications.