(586a) Experimental Studies on Unmixed Combustion – Packed Bed Reactor Technology for Heat Transfer Applications

The demand for energy in modern world is increasing and meeting this demand has become a priority worldwide. Currently power production largely depends on a “premixed” process wherein fossil fuel is “premixed” with excess air and the produced flue gases exchange energy via convection and radiation with water flowing inside tubes, thereby generating steam for power plant applications. This “premixed combustion” process used in existing power plant steam boilers has 3 limitations:

1. Excess Entropy generation
2. NO_x emissions
3. Difficulties in carbon dioxide (CO₂) capture from flue gases

There is therefore an urgent need to investigate alternative methods of combustion and heat transfer in boilers to improve energy transfer efficiency and reduce the impact on environment simultaneously. A large number of researchers in recent times have attempted to address the issue of CO₂ capture and sequestration. However, very few attempts have been made to change the fundamental nature of the combustion process itself and synergizing it with heat transfer applications. A novel form of combustion process called “Unmixed Combustion (UMC)” was proposed by Lyon in 1993. This process occurs when fuel and air alternately pass over a catalyst or Oxygen Storage and Release Material (henceforth referred to as OSRM). The OSRM undergoes oxidation and reduction, storing oxygen from the air in one cycle and delivering it to the fuel in another. Nitrogen (N₂) and unreacted oxygen (O₂) exit during the oxidation step while the flue gas produced during reduction of OSRM contains CO₂ and steam. The hot N₂ stream from the oxidation step can be used for preheating applications or running a turbine. Water can easily be separated from the flue gas by condensation and pure CO₂amenable for sequestration and storage or reuse can be obtained. UMC can be carried out in fluidized or packed bed reactors, the former being referred to as Chemical Looping Combustion (CLC). Recent studies on the use of packed bed technology for CLC (UMC) confirmed it as a better alternative to fluidized bed system.

The heat generated during oxidation/reduction of OSRM in unmixed combustion can be transferred at near isothermal conditions radially by increasing the ratio of radial heat transfer rate to axial heat transfer in the packed bed. In order to investigate this possibility and related practical aspects, an experimental “proof of concept” test-rig comprising of an annular packed bed SS reactor has been designed and fabricated. Cu/CuO supported on alumina was used as an OSRM in this study and the same was packed in the annular area of the reactor whereas the fluid (air) to be heated was passed through the inner pipe. Zero air and methane (mixed with N₂) were injected as reactive gases cyclically for oxidation of Cu and reduction of CuO respectively.

The experimental runs showed that it was possible to transfer heat radially while maintaining the bed temperatures to near isothermal conditions (desired temperature +/-10 ^oC) if cycle times of oxidation and reduction reactions are controlled within a certain range. For chosen amount of OSRM (500 gm), several cycle times were tested to get the optimum values and found to be 5 minutes for oxidation and 3 minutes for reduction within the chosen practical ranges of flow rates of reactive gases (air and methane). Higher values of cycle time results in lowering the bed temperatures (upto 50^oC) while lower cycle time lead to incomplete conversion of OSRM. Thus, cycle time of oxidation and reduction reactions play a very important role in maintaining near isothermal conditions in the reactor bed while transferring the heat in the radial direction.

The effect of following parameters on the packed bed heat transfer was tested.

1. Flow rate of process air
2. Initial bed temperature

With the increase in the flow rate of air, the temperature at the outlet of the bed (and hence the heat transfer in axial direction) was observed to increase while radial heat transfer rate was almost the same. The optimum flow rate for the current experimental set-up was then found to be 30 SLPM. For different initial bed temperatures (550 ^oC to 650 ^oC) selected based on practical range of interest, amount of radial heat transfer did not differ much and near isothermal condition in the bed could also be maintained.   

The effect of reducing gas (CH₄) flow rate and study of heat transfer in case of larger amount (1 Kg) of OSRM are currently being investigated.