(24e) 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.