A novel preheating system in pressure reduction stations—natural gas directly flowing inside deep borehole heat exchangers.

To prevent hydrates from forming during the pressure reduction process in natural gas pressure reduction stations (i.e., city gate stations, CGS), fuel-intensive preheating methods are normally adopted, leading to high fuel consumption and carbon emissions. In this study, an innovative preheating system for natural gas is proposed, which is called the "Direct Geothermal Heating System" (DGHS). The main idea is to inject high-pressure natural gas into geothermal wells before the throttling valve; thus, natural gas is directly heated by the surrounding strata when it flows through the geothermal well. A proper temperature for natural gas should be ensured, as natural gas goes back to the ground for pressure regulation. The idea is performed based on the preheating configuration using coaxial deep borehole heat exchangers (DBHE). A mathematical model describing the flow and heat transfer processes of DGHS through DBHE is established to study its heat transfer characteristics. Additionally, the environmental and economic benefits of the proposed configuration are also investigated. Through the CGS case in this paper, the findings indicate that, over the fifteen years of preheating with a DBHE depth of 2000 m and inlet flow rate of 4.39 × 107 Nm³/year, the outlet temperature of the throttling valve remains within the range of 17 °C–37 °C, meeting the heating requirement of CGS. Among fifteen preheating cycles, the average peak heating power of the DBHE reaches 105.78 kW. In addition, in the 15th year, the proposed system has a 1/3 cost of a traditional line heater, along with a reduction of 46.43 × 105 kg in CO 2 emission. The DGHS for natural gas is easy to establish and operate; once it is promoted on a large scale, it will produce huge economic and environmental benefits. [Display omitted] • A new Direct Geothermal Heating System for preheating natural gas is introduced. • Coaxial deep borehole heat exchangers replace traditional line heaters for natural gas preheating. • Analyzed the heat transfer characteristics and environmental-economic benefits of the system. • After 15 cycles, the system costs one-third of traditional heaters and cuts CO 2 emissions by 4.643 million kg. [ABSTRACT FROM AUTHOR]