Study on a carbon dioxide hydrate power generation system employing an unstirred reactor with cyclopentane

Abstract

A CO2 hydrate power generation system for cold regions has been proposed. In this system, CO2 hydrate is formed using cold heat from the outside air and a generator is driven by high-pressure gas obtained by dissociating the hydrate using low-temperature waste heat. In this study, we clarified the effect of cyclopentane (CP) as a formation reaction promoter in the proposed system. Cyclopentane shifts the phase equilibrium pressure curve to high temperature and low pressure, increasing the degree of supercooling that drives the formation reaction. Generally, as a means of promoting the formation of hydrate, stirring is performed in the reactor to increase the gas–liquid contact area. Interestingly, it has been reported that more CO2 is taken up in unstirred CO2 hydrates with CP added and formation starts from the water–CP interface in experiments that use a transparent resin reactor. However, no results have been reported when the formation–dissociation cycle is repeated in a practical metal reactor, such as that used in the proposed power generation system. In this study, experiments were conducted using a stainless steel unstirred reactor to clarify the effect of CP on the repetition of the CO2 hydrate formation–dissociation cycle. Carbon dioxide uptake, i.e., the amount of hydrate formation and the induction time, was investigated as an evaluation index of the above effects. The CO2 uptake was calculated from the temperature and pressure in the reactor, considering the compression coefficient of the CO2 gas and using the gas state equation. Because the timing at which formation starts cannot be visually confirmed, thermocouples were installed at four different depths in the reactor and the induction time was calculated from the time change of temperature. As a result, CP-added CO2 hydrate showed reproducibility in induction time and CO2 uptake with respect to repeated formation–dissociation cycles. The results of temperature measurements at multiple depths showed that the formation started at the water–CP interface, which was in good agreement with the visual observation results in a previous study. Furthermore, we conducted experiments by considering three different water/CP volume ratios (25 cm3/25 cm3, 37.5 cm3/12.5 cm3, and 43.8 cm3/6.2 cm3) and observed that the uptake of CO2 was the greatest at 37.5 cm3/12.5 cm3, which is the closest value to the theoretical mixing ratio of CP–CO2 hydrate.