A Fuel-Free Trigeneration Plant at Stations for Technological Reduction of Transported Natural Gas Pressure

A new original basic process circuit of a fuel-free trigeneration plant simultaneously producing electricity, heat, and cold is considered. The plant can be used at technological transported gas pressure reduction stations instead of throttle devices conventionally used for this purpose. The plant process circuit involving, as its key components, an expander–generator unit and a vapor compression thermotransformer (VPTT) configured for simultaneously producing heat and cold, along with its operating principle, is described. The flow of transported gas (without combusting it) serves as the primary energy carrier supporting the plant operation. The gas flow energy is converted in mechanical work (as its pressure is decreased in the expander from the initial level at which gas arrives to the technological gas pressure reducing station to the level necessary according to the requirements of the gas utilization technology at the consumer end), and the generator connected to the expander converts this work into electricity. Part of the produced electricity is supplied to an external consumer, and its other part is used for driving the VPTT. The gas flow downstream of the expander supplied to the consumer and the flow of the VPTT working fluid, which takes heat from the cold carrier in the VPTT evaporator as it is transferred from a liquid to a gaseous state serve as the sources of cold in the plant. The working fluid downstream of the VPTT compressor serves as the source of heat; part of this is supplied to the consumer and its other part is used for heating gas upstream of the expander. The article presents the results from studying the effect the temperature to which gas upstream of the VPTT expander is heated by the heat of the VPTT working fluid has on the plant thermodynamic efficiency. The exergetic efficiency is taken as the thermodynamic efficiency criterion. The processes occurring in the plant when changing the gas heating temperature are subjected to a qualitative analysis. The results of calculations carried out using the plant mathematical model described in the article are presented. The obtained calculation results made it possible to determine the effect that the gas heating temperature upstream of the VPTT expander has on the specific (per unit flowrate of transported gas) electric, heating, and refrigeration capacities of the plant; on the specific exergy values of the same flows; and on the exergetic efficiency subject to the conditions adopted in the calculations.