Experimental investigation on the promotion of CO2 hydrate formation for cold thermal energy storage – Effect of gas-inducing stirring under different agitation speeds

Abstract

To promote the formation of CO2 hydrate for cold energy storage, the influence of gas-inducing agitation at varying operating speeds were studied experimentally. A comparison was made with normal stirring (without gas inducing) from the perspectives of deviation from equilibrium condition, subcooling, agglomeration, and hydrate production. The test results revealed that gas-inducing agitation contributed to a closer shift of the hydrate formation profiles towards equilibrium conditions when compared to normal stirring. However, this advantage became less pronounced as the stirring speed increased. Notably, a substantial improvement in subcooling phenomena was observed when transitioning from 250 rpm normal stirring to 500 rpm, decreasing the induction time to 19.3%. Comparing normal stirring, the incorporation of a gas-inducing stirrer further reduced the induction time by 68.6% at 400 rpm. Nevertheless, further increasing agitation speed for both sets did not yield apparent improvement in the subcooling phenomenon. In contrast to normal stirring, gas-inducing agitation effectively prevented hydrate agglomeration at a lower speed and led to increased hydrate production at the same rotation speed. An ascending trend in hydrate production was achieved as agitation accelerated from a low speed to a specific speed, e.g., 400 rpm for gas-inducing stirring and 500 rpm for normal stirring. However, further elevating the stirring speed did not stimulate greater hydrate production. The findings of this study indicated the existence of double-sided effects in using gas-inducing stirring for hydrate promotion and a crucial speed range (e.g., 400∼450 rpm in this study) essential for the efficient implementation of gas-inducing technology. Operating at this prescribed speed range was recommended to improve the energy Return on Investment, maintaining high hydrate production, and enhancing the controllability of cold storage systems. This study provides practical insights for applying gas-inducing technology in gas hydrate reactors, contributing to the development of green cold energy storage.