Scale-up of Fixed-Bed Chemical Looping Combustion

Fixed-bed chemical looping combustion (CLC) is a promising technology for the use of fossil energy in several applications while allowing for nearly 100% capture of CO₂.  Although the focus of much of the research on CLC systems is on fluidized-bed reactors, we have realized several advantages of using a fixed-bed system.  Without the requirements for bed fluidization, there is a greater range in the choice of operating parameters so that the system may be tailored to a specific application, e.g. production of H₂ or CO₂, without introducing new technical hurdles.  For example, power generation requires high flow rates and a specific temperature of the gases to drive a gas turbine, while production of gases, such as CO₂ or H₂, provides a much broader window of operation.  

One novel application that has been studied extensively at TNO is the use of fixed-bed CLC system in greenhouses, which requires steady production of CO₂ during the day and heat at night.  By using low flow rates of reactants—fuel to produce CO₂ and air to produce heat—a single fixed-bed CLC reactor can supply both the CO₂ and the heat at alternating time intervals, and each for several hours per cycle. Applying CLC in a greenhouse in this way will significantly reduce the use of natural gas (greenhouses are the number one consumer of natural gas in the Netherlands). Furthermore, the emissions, like ethylene, NO_x and methane, usually associated with the heating systems currently used in greenhouses are eliminated.

After successful operation of small (10 and 100 W) fixed-bed reactors, a larger 1 kW reactor is being tested and optimized.  Important parameters are the pressure drop through the bed, the heat management, the dynamics of the reactant switching, and the control of the reactant breakthrough at the outlet of the reactor as the bed becomes fully oxidized or reduced, resulting in impurities in the produced gas.  These factors are studied by measuring the outlet concentrations of the gases and the temperature distribution along the length of the reactor.  Results have shown that it is possible to achieve sharp breakthrough fronts of the reactant gases at the outlet of the reactor, allowing for a production of 92% of the maximum amount of CO₂ without contamination.  Pressure drop has also been shown to not be an issue, with a maximum pressure rise during operation of only 0.3 bar.  Modeling is used to interpret the data by predicting the temperature and reactant concentration profiles across the entire reactor bed and can be combined with a system model and will guide in the design of larger systems.  The realization of large, ~1 MW commercial fixed-bed CLC system for the greenhouse industry will then serve as a pilot and provide knowledge that is directly applicable to even larger systems with applications such as power generation.