In the course of efforts to reduce emissions of carbon dioxide and to meet the global climate targets, the chemical looping combustion process (CLC) has increasingly received more attention over the years after originally being designed to produce pure CO2. In recent years the CLC has been investigated as a way to capture CO2 from the combustion without the need for any extra processing steps.
For conventional carbon capture a distinction is made between post-combustion capture, pre-combustion capture and oxyfuel combustion. In the post-combustion capture process, the flue gas is stripped of CO2 by scrubbing with aminomethanol or using the Carbonate Looping process. In the pre-combustion process gasification and reforming of the fuel is coupled with a watergas shift reaction. Carbon dioxide is extracted and the H2 is used in a gas turbine to produce power. The air, which is used in the combustion chamber for an oxyfuel combustion, is stripped of the N2. Therefore, pure oxygen is used during the combustion. The flue gas then only consists water and CO2 which can be separated by means of condensation.
The inherit CO2 capture of the CLC process has the potential to make the capture of CO2 more efficient, therefore more profitable. CLC is a promising technology to produce green energy whilst realizing negative CO2 emissions.
In CLC, two fluidized bed systems are interconnected and an oxygen carrier is looped between these two reactors. The fuel reactor provides oxygen for the combustion of the fuel by reduction. The oxygen carrier is then re-oxidized in the air reactor and used again for fuel combustion. By supplying the fuel reactor with pure oxygen, no nitrogen oxides can form during combustion and the flue gas consists of mainly CO2 and water vapor. The general process scheme of a CLC plant is shown in figure 1.
At the Institute of Solids Process Engineering and Particle Technology at the TUHH a Chemical Looping plant with a thermal power of 25 kW as well as a lab scale fluidized bed system are operated. The pilot plant has been operated for more then 200 hours with a variety of different oxygen carriers, fuels and operating conditions. The novelty of this plant is the installation of a two-stage fuel reactor for increased conversion rates.
The aim of this project is to further investigate CLC as a power plant process, especially to investigate the operating limits of the novel two-stage fuel reactor with different biomass types as fuel. For reasons of locality, woody biomass will be investigated, as it plays a major role as a biogenic solid fuel in Germany. Besides that, agricultural wastes and sewage sludge will be considered as fuels for CLC. Measurements of combustion- and CO2-capture efficiencies as well as residence time and dynamic conversion studies will be conducted.
In addition to these experimental studies a dynamic flowsheet simulation of the fluidized bed reactor systems will be set up using the open sources flowsheet simulation software DYSSOL (Dynamic Simulation if SOLids Processes), that has been developed at this institute. Furthermore, a CFD simulation will be conducted using the multiphase particle-in-cell-method (MP-PIC). This method shows great promise for the simulation of such a big plant and will be especially interesting in regard to the air reactor. A subproject of this implementation will be the integration of an enthalpy balance, considering reaction heats, heat losses and heat decoupling using heat exchangers.