Numerical design of a reactor-heat exchanger combined unit for AmmoniaSrCl2 thermochemical storage system

This work presents the design of a reactor-heat exchanger combined unit using COMSOL transient simulations in 2D for a thermochemical storage (TCS) system of NH3-SrCl2 (absorption-desorption of SrCl2·NH3 and SrCl2·8NH3). TCS with NH3-metal halide reactions is emerging, with many packed-bed reactors often incorporating salt with expanded graphite (EG) to improve thermal conductivity.

Similarly, a packed-bed reactor is chosen here using SrCl2-EG composite, for a reaction pressure of 8 bar, accommodating desorption above 82 ? and absorption below 79 ?. The aim here is to find a simple and cost-effective reactor-heat exchanger (HEX) combined unit to store 0.5 kWh heat (1.1 kg of SrCl2·NH3 forming 1.8 kg of SrCl2·8NH3).

From several HEX configurations, the first configuration is modelled here. This is contains three reaction media cylinders (a composite of 87.5% w/w of SrCl2·8NH3 in EG) sandwiching two units of tube-in-tube (TinT) HEXs. A stationary study for the heat transfer fluid (HTF) velocity analysis is coupled with a time-dependent study for the reaction phenomena, respectively for the salt-alone and salt-EG composite.

For 15 hours reaction time, the reaction advancement (above 0.85) was enhanced significantly in the salt-EG composite, which was only above 0.55 for the salt-only case. The reaction progression is the slowest in the bottom of the innermost reaction media, where the HTF temperature is the lowest. Thus, an additional HEX unit along the center-axis of the reactor appears suitable to reduce the reaction time.

As next, the effect of HEX thickness, number of TinT units, as well as other reactor-HEX configurations will be analyzed to choose the optimal setup.