The synthesis reaction in a chemical heat pump reactor filled with chloride salt impregnated carbon fibres: the NH3–CoCl2 system

Bench-scale experiments demonstrated power densities of 235 kW/m3 during the synthesis reaction of ammonia with carbon fibres impregnated with CoCl2 salt. This established the suitability of impregnating CoCl2 into carbon fibres as a simple and effective preparation method. At least 75% of the power was generated as the process cycled through large transient variations in temperature, ammonia pressure, and overall heat transfer coefficient. By using a load cell to weigh the reactor, the reaction of the ammonia with the salt could be measured instantaneously. The instantaneous rate of reaction with the salt was almost identical to the rate of heat transfer to the cooling water flowing through the jacket on the reactor exterior. This indicated that for the experiments reported in this work, the rate of the synthesis reaction was not controlled by the rate of heat transfer. Therefore the synthesis and decomposition reactions are controlled by different phenomena, since previous work had shown that the heat transfer rate controls the decomposition reaction. The rate of ammonia diffusion within the salt crystals was proposed as the rate-controlling step for the synthesis reaction, after discarding other alternatives. The overall heat transfer coefficient was found to increase from 0 to 350 W/(m2 K) as the temperature driving force decreased during the cycle. Although overall heat transfer coefficients have constant values for steady-state processes, these results show that their values change during the transient conditions of chemical heat pump cycles. Quantitative overall heat transfer coefficients that are a function of heat flux and temperature driving force will be required for the design and scale-up of chemical heat pump reactors.