Chemical looping combustion – a sustainable combustion technology for negative emission energy production

Abstract

Chemical looping combustion (CLC) is considered as a sustainable combustion Technology for negative emission energy production. Thereby an inherent CO2 separation is one of the main benefits of this technology. Unlike a conventional combustion, the combustion process is separated into two areas, so that fuel and combustion air are never mixed. To realize that, a metal oxide, the so-called oxygen carrier (OC), is used to deliver the necessary Oxygen for oxidizing the fuel. The typical CLC system consist of two reactors which are designed as circulating fluidized beds. The oxidation of the OC takes place in the so-called air reactor and is reduced in combination with the used fuel in the so-called fuel reactor. The Technology readiness level (TRL) is high for gaseous fuels, but when using solid fuels, such as biomass or coal, the developments are not so far advanced. To contribute to the development of CLC of biomass, in this thesis experiments on two different scales of reactor systems have been conducted. Especially to evaluate the influence of fuel impurities typical for solid fuels in CLC, such as sulfur or nitrogen, investigation in a pilot plant for gaseous fuels have been performed. Additionally a novel batch reactor for gaseous and solid fuels have been developed and build, to accelerate the screening of suitable OCs with both kind of fuels. The experiments included experiments with a cooper based oxygen carrier (Cu15), perovskite and ilmenite. Experiments in the pilot plant with natural gas as fuel and gaseous impurities, such as hydrogen sulfide (H2S) and ammonia (NH3) showed that, concentration of impurities and solid inventory have a big influence on the performance. During all experiments a stable operation was possible regardless which OC or impurity have been combined. Only with large amounts of H2S the long-term operation suffer when using perovskite as OC. Fuel conversion and CO2 yield decreased from the presence of sulfur in the system with Cu15 and perovskite. On the other hand NH3 caused no effect on the fuel conversion or CO2 yield with both OCs. Separate evaluation of the emissions of the air and fuel reactor showed that, at any time of the experiments the air reactor exhaust gas was not polluted. The only emissions have been in the fuel reactor off gas as SO2 when using Cu15 and perovskite or NO only when using perovskite. To further assess the capability of potential OCs, experiments in a novel fluidized bed Batch reactor with ilmenite as OC have been performed using biomass as fuel. During the commissioning and the very first experiments, methane has been used as fuel. In a second step wood and chicken manure pellets have been used to investigate the influence of bed temperature, fluidization rate and solid fuel dosing speed on the performance of the OC. To conclude from the batch reactor to the pilot plant, a method have been developed to evaluate the results. Due to the carbon loss in the exhaust gas, the in- and outgoing carbon could not be properly balanced. The carbon loss varies from 11 % when using methane, up to 15 % with chicken manure and 18 % with wood pellets as solid fuel. Higher hydrocarbons such as ethylene, ethane and acetylene could be confirmed, using a gas chromatograph in additional to the online gas analysis.

Chemical looping combustion (CLC) is considered as a sustainable combustion Technology for negative emission energy production. Thereby an inherent CO2 separation is one of the main benefits of this technology. Unlike a conventional combustion, the combustion process is separated into two areas, so that fuel and combustion air are never mixed. To realize that, a metal oxide, the so-called oxygen carrier (OC), is used to deliver the necessary Oxygen for oxidizing the fuel. The typical CLC system consist of two reactors which are designed as circulating fluidized beds. The oxidation of the OC takes place in the so-called air reactor and is reduced in combination with the used fuel in the so-called fuel reactor. The Technology readiness level (TRL) is high for gaseous fuels, but when using solid fuels, such as biomass or coal, the developments are not so far advanced.