There are multiple uses for this heat during cold winter months in northern climates. It can be used to heat nearby buildings and even provide district heating for small and remote northern communities. It can aslo be used to energise engines that can operate from low-grade heat. The table below provides data from a pumping station. It is based on the pressure ratio of the pump (85% isentropic efficiency) and temperature to which the gas will rise. The ambient air temperature and the temperature of the natural gas are assumed to be 40-degrees F (4.4445-degrees C). The heat exchangers nomimally operate at 80% effectiveness.
There are occasions when the ambient temperature in northern locations can drop to below zero degrees F (-17.777-degrees C). The table changes as follows:
Natural gas has a density of 0.5979-pounds per cubic foot at a pressure of 200-psia and temperature of 40-degrees F. A million cubic feet per hour were pumped through the pipeline would weigh 597,944-pounds. A heat exchanger at the pumping station that rejects 100-BTU/lb of heat would process 59,794,456-BTU of heat per hour from a high temperature of over 200-degrees F. It would be possible to transfer this heat into district heating systems in northern communities during winter months.
There are locations where the communities may be too small to use all the heat that could be made available to it from pumping stations during winter. The difference in temperature between the newly re-pressurised gas and the ambient winter air would be sufficient to energise a low-heat engine. Honeywell’s Genetron division now supplies a chemical that can be used in such an engine. Such an engine could fulfill all or part of the power generation requirements of a northern remote community during winter.
Pressure in natural gas pipelines had to be reduced prior to the gas being distributed to customers. Power can be extracted from the pressure differential in natural gas pipelines where the gas pressure needs to be reduced. A previous article by this author covered that possibility. The gas that exists a pressure reduction is notable cooler than the ambient air temperature. The specific heat of natural gas is about twice that of air (0.5099 vs 0.24) and there are locations where air conditioning and cooling can be extracted from pressure reduction stations.
The following table illustrates the cooling effect of a turbine (isentropic efficiency of 90%) in pressure reduction station on the natural gas (at 100-degrees F) and over a range of pressure engine ratios.
About 100,000-pounds per hour of natural gas could pass through the engine of a pressure reduction station with a pressure ratio of 5:1. The natural gas passing through the turbine would yield a cooling effect of some 4,207,000-BTU/hour. This cooling effect could be distributed through the pipelines of a district heating system during hot summer months. An alternative would be to use the cooling effect in a commercial refrigeration system such as a food terminal. The cooling effect on the natural gas could also serve as the heat sink of an engine that would operate from low-grade heat. The heat source would be atmospheric heat or concentrated solar heat.
Natural gas pumping stations produce thermal energy that is rejected so as to maximize the amount of gas that can be pumped through long distance pipelines. The companies earn revenue from the sale of the gas. There is much downstream thermal energy that is available from the pumping stations throughout the year. There are productive uses to which that heat energy may be put.
The pressure of natural gas needs to be reduced prior to being distributed to the final customers at low pressure (3-psi or 17.7-psia). Power can be generated at the pressure reduction stations from the pressure drop. The temperature of the natural gas would drop as it passes through the turbines at a pressure reduction station. This drop in temperature can be used for air conditioning purposes or as a heat sink for an engine that runs on small differences in temperature.