A possible reactor design to determine the reaction kinetics of char during combustion is an experimental realization of a Perfectly Stirred Reactor (PSR). This concept is characterized by uniform state variables: temperature and composition are assumed to be constant through the entire reactor. This concept can be implemented as a small scale fluidized bed. The intense mixing inside the bed ensures a sufficiently uniform distribution of state variables.
The experimental rig at Aachen University bases on an inert sand-like bed material, which is fluidized by the reaction gas. A small fuel sample can be injected into the bed where it reacts with the fluidizing reaction gas. The intense mixing as well as the high temperatures of both the reaction gas and the inert bed material provide high heating rates as well as high temperatures comparable to the conditions found in pulverized fuel flames. The gaseous reaction products are entrained with the reaction gas. By sampling the gas above the fluidized bed and analyzing its chemical composition, the reaction kinetics can be derived from the change of gas composition in time. The solid ash particles remain within the fluidized bed and become part of it.
Figure 1 - Sketch of the fluidized bed reactor setup
A sketch of the experimental setup is shown in 1. The core of the setup is a small-scale fluidized bed with a diameter of 34 mm. The non-fluidized bed height is approximately 30 mm. Preliminary experiments with a cold fluidized bed of a comparable scale showed a bed height fluctuating around 70 mm, see 2. A small batch of particles dropped onto the fluidized bed is well mixed with the bed material within approximately one second. The inert bed consists of alumina (Al2O3) particles of sizes between 200 and 300 μm. The temperature of the fluidized bed is measured by means of a thermocouple (PtRh/Pt; type S), which is submersed into the fluidized bed and protected by a closed-tip alumina tube. A small batch of pulverized fuel can be dropped through another tube onto the fluidized bed where it reacts with the fluidizing reaction gas. The gas leaving the fluidized bed is sampled through a third alumina tube.
Figure 2 - Fluidized bed during first lab tests
The fluidizing gas consists of a base gas (CO2 or N2), which can be enriched with oxygen (O2). The O2 can be chosen freely between 0 % and 100 %. The mass flows of base gas and oxygen are each controlled by thermal mass flow controllers. Before entering the fluidized bed, the mixed gases pass through a distributor plate of sintered silica glass particles to distribute the gas flow equally across the cross section of the fluidized bed. This distributor plate serves also as bed plate for the fluidized bed.
To adjust the temperature of the fluidized bed, the whole experimental setup is mounted inside an electrically heated furnace. The flow of mixed gases (base gas and oxygen) is heated up while passing through an annular duct between the pipe containing the fluidized bed and an outside closed-tip pipe, see 1. This configuration gives a very stable temperature level of the fluidized bed, even at high temperatures (to give numbers: 1000.6 °C + 1.6 °C for one measuring day).
To analyze the sampled flue gas, a Fourier-transform infrared (FTIR) analyzer is used. It allows fast and thus time-resolved measurements of concentrations of those species which interact with infrared radiation. From these time-dependent measurements, the reaction rates can be derived. To protect the FTIR analyzer from being damaged by particles entrained in the gas sample, a fine ceramic filter is installed in the sampling line.