Research on the Decomposition Characteristics of Methane Hydrates Exploited by the NH4Cl/NaNO2 System

Considering the influence of the system conversion rate on hydrate decomposition kinetics and energy utilization during the decomposition process of pure methane hydrate in the NH4Cl/NaNO2 system (an in situ chemical heat generation system), this study carried out hydrate decomposition experiments in the NH4Cl/NaNO2 system under different decomposition conditions at low temperature and high pressure (3 °C, 8 MPa) and calculated the decomposition efficiency, reaction conversion rate, and methane energy efficiency. The results showed that, based on the differences in the kinetic behavior of hydrate decomposition, the decomposition process was divided into an unstable stage, a stable stage, and a decay stage. When the chemical reaction entered the stable stage, the hydrate decomposition process became stable, and it formed a stable dynamic response mode consisting of an exothermic chemical system and endothermic decomposition of hydrate. Four reactant concentrations (3 mol/L, 4 mol/L, 5 mol/L, and 6 mol/L) and three hydrochloric acid concentrations (0.0178 mol/L, 0.0225 mol/L, and 0.0356 mol/L) were designed. This proved that increases in the reactant concentration and H+ concentration both improved the decomposition efficiency and energy efficiency of pure methane hydrate, but reactant concentrations up to 6 mol/L reduced the decomposition efficiency due to the formation of side reactions, and H+ concentrations up to 0.0356 mol/L produced toxic reddish-brown nitrogen oxides. The overall decomposition efficiency of Cases 1–6 was up to 72.92%, the conversion rate was 25–45%, and the methane energy efficiency was higher than 3.5. The experiment proved the feasibility of exploiting pure methane hydrate in this self-generating heat system, which provides a new idea for the application of this system in hydrate exploitation.