Using the Quasiturbine to Regulate Natural Gas Pipeline Pressure and Flow-rate
- September 27, 2005
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Natural Gas Transportation
In anticipation of this expected increase in power demand, new exploration for natural gas has been underway in Canada's Arctic. New pipelines are planned to move large volumes of natural gas to Canadian and American markets. Clean-burning natural gas is the preferred fuel for thermal power stations. It is collected from a multiplicity of wells using pipes as small as 3-inches in diameter, then transported over long distances through pipelines that measure between 16-inches to 48-inches in diameter. Interstate pipes that branch off from the main pipeline measure 24-inches to 36-inches diameter, while lateral pipes would measure 6-inches to 16-inches. Local delivery pipes would measure 0.5-inches to 3-inches diameter, with pressure as low as 3-psi.
Compressor stations are located every 100-miles to 245-miles along main and interstate pipelines, where natural gas pressure may be pumped to between 200-psi and 1500-psi using centrifugal compressors. These compressors may be driven by gas turbine engines, by piston engines or by electric motors to move the gas at a typical speed of 20 miles / hour. Control valves to regulate line pressure may be installed at intervals between 5 to 20-miles along main pipelines. Such valves would be installed where main pipes connect to interstate pipes, where interstate pipes join lateral pipelines and where lateral pipes connect to local low-pressure pipes. The control valves may be used to reduce line pressure, gas velocity and/or mass flow-rate in the pipeline. When gas pressure is reduced by a control valve the gas temperature drops and heat is transferred to the surroundings through the control valves' metal structure and sometime through direct cooling.
Typically, the pressure potential energy in a 1000 psi gas pipeline is about 3% of the thermal BTU (21,500-BTU/lb) gas flow content. Gas at 1,000-psi and 32-degrees F or zero-deg C has a density of 3.083-lb/cu.ft. When the gas flows at 30-ft/sec in a 36-inch diameter pipe, the mass flow rate of 653-lb/sec carries a combustion heating value of over 14,000,000-BTU's over a 1-hour period or 19,860,000-Hp. Centrifugal compressors raise pipeline pressure at pumping stations and can add over 207,000-Hp to the gas flowing inside a major intercity pipeline, after raising pressure from 200-psi to 1000-psi. Compressor stations located along collector pipelines systems that feed into the mainline, can add some 196,000-Hp to the gas flow as a result of raising gas pressure from 3-psi to 200-psi and using a mass flow-rate of 653-lb/sec.
Theoretically, an estimated 20% of the mainline pumping energy can be extracted at the main pipeline control valve, where the pressure drops from 1,000-psi to 200 psi as it enters a local pipeline system. This can be accomplished by replacing the control valve with an engine. Of the over 196,000-Hp that was expended to raise gas pressure in short-distance collector lines from 3-psi to 200-psi, an estimated theoretical maximum of 85,000-Hp could be extracted in local lines at customer transfer points. At these points, pressure-drop engines could be installed to reduce line pressure from 200 psi to 3 psi prior to entering customers' premises. However only very large purchasers of natural gas would have an interest in installing such engines on their premises.
During an earlier time, no suitable pressure-reduction energy recovery engines were available to install at natural gas pipeline transfer points. New positive-displacement rotary engines have recently been developed that have little need for lubrication can be installed at pipeline transfer points that involve a large flow-rate of gas as well as large drop in line pressure. These engines could provide base-line electrical power for numerous large customers of natural gas.
The Quasiturbine in the Gas Line
At several locations along the gas pipeline pressure and flow-rate may be reduced using engines (rotary expanders) instead of control valves. Heat energy that the control valves release into the atmosphere could become electric power. The Quasiturbine is a low-friction, positive-displacement rotary engine that can be installed where high-pressure pipelines connect to local low-pressure lines. Because conventional turbines cannot be widely modulated in rpm, they are not suitable for gas flow and pressure control. The Quasiturbine is essentially a closed valve at zero rpm. It and has high efficiency and high torque and over a wide rpm.
High-Pressure (200-psi) Release
Quasiturbine pressure-reduction engines may be installed where a main gas pipeline (1000-psi) connects to a regional pipeline (200-psi). The engine at such a location would use a pressure ratio of 4.74, allowing a Quasiturbine to operate using full (no dominant restriction) inlet ports, which means that there is no significant gas expansion within the chambers. To reduce the need for lubrication, patented roller carriages may be installed into the engine. When operating with natural gas, the engine will have a volume expansion ratio (V2/V1) of 3.24, meaning an engine cut-off ratio would be set between 20 % to a maximum of 24 % depending on engine clearance volume.
Before inlet cut-off occurs, an inlet pressure of 1,000-psi will push on to the rotary cylinders. Gas expansion will occur after inlet cut-off, reducing pressure to 200-psi. For gas flowing at 30-ft/sec inside a 36-inch diameter pipe, volume flow rate will be 212-cu.ft/sec. For a pressure differential of 800-psia, this will give 800-lb/sq-in x 144-sq.in/sq.ft x 212-cu.ft/sec = 24,430,000-lbf-ft/sec or 44,400-Hp. The rest of the engine expansion will involve gas temperature dropping by a factor or 1.46 and an estimated isentropic efficiency of 79 %. With gas temperature at 32-degrees F, its density would calculate to 3.08-lbm/cu.ft (density = pressure / (gas constant x absolute temperature)).
Power resulting from expansion would calculate to 3.08-lb/cu.ft x 212-cu.ft/sec x 0.5099-BTU/lb-deg-F x ((32-deg F + 460-deg F)-((32-deg F + 460-deg F)/1.46)) x (3600-sec/hr)/2545-Hp-hr/BTU) = 132.7-Hp at 100%-isentropic efficiency. This would raise total engine output to 44,532-Hp or 33,176-Kw. Overall expansion engine power may be increased if the gas were heated prior to pressure reduction and an intercooler cooled engine exhaust gas. If engine inlet temperature were raised to 540-deg F (1000-deg R), expansion engine power would be raised to 269.79-Hp and overall engine power would be raised to 44,670-Hp or 33,279-Kw. The added complexity would result in raising power output by less than 1 %; however, it would serve to keep gas pipeline pressure above the freezing point of water. Ignoring gas expansion and considering only the gas pressure flow, a 36 inches diam. gas pipeline at 700 psi carry typically a pressure power in excess of 30 MW - 25 millions of pound-ft/sec - of zero pollution pure mechanical energy.
Low-Pressure (3-psi) Release
The low-pressure release system would be used where gauge pressure in the local lines may read as low as 3-psi and burners receive this gas at atmospheric back- pressure. This means that the absolute pressure in the local line system would be 17.7 psia. Local lines would typically be located several miles downstream of a compressor station, where the gas pressure would be near to 215-psia and temperature near the freezing point of water. Pressure reduction from the main pipeline can be cascaded into 2 or more steps into the local service line. The Quasiturbine could operate using inlet- ports dominant restriction to make sure some expansion actually occurs within the chamber.
The following table can give an idea of relevant data to the gas transfer point:
Engine performance is calculated based on natural gas having a gas constant of R = 96.33 ft-lbf/lbm-deg R, a specific heat Cp = 0.5099 Btu/lb-deg R, a specific heat ratio of 1.321 and a lower heating value of 21,500-Btu/lb. The following table gives data on engine performance based on a gas mass flow-rate of 1-lb per second and a counter-flow heat exchanger effectiveness of 75%:
The term Tex' is the engine outlet temperature after the isentropic efficiency of the engine has been taken into account. Gas used is the amount of natural gas that will be burned to heat the natural gas (from the freezing point of water) that is to be expanded in the gasline pressure / flow-rate control engine. The engine power is calculated from the temperature difference across the engine (Th - Tex) x Cp x 3600-seconds-per-hr/2545-Btu-Hp-hr (Eg; (540 - 135) x 0.5099 x (3600/2545) = 292 Hp).
A High Efficiency Application
When 1-lb/sec of natural gas is combusted in a turbine engine operating at 35 % efficiency, 30,412 Hp in thermal energy would be released and converted to 10,644 Hp on the power turbine drive-shaft. The pressure control engine processing 1-lb/sec natural gas would yield between 2.7 % to 3 % of the power of a gas turbine that burns 1-lb/sec natural gas. On extensive natural gas fired power grids, Quasiturbine pipeline energy-recovery engines could provide low-level power to small markets at remote locations along the gas pipeline at competitive cost. The heat absorbed along the pipelines by the control valves reducing line pressure, would otherwise be dissipated.
Inter-Cooling and Reheat
The "thermal efficiency" of the Quasiturbine pipeline pressure control engine would be high because it would use no energy compressing the gas. That work has been done at the pipeline compressor station, the operating cost of which would be recovered from the sale of natural gas to customers. Thermodynamic efficiency could be improved by raising engine inlet temperature; however, the temperature that enters the local gas distribution network would need to be kept low. If the engine exhaust temperature is too high for local distribution, the exhaust can be cooled in an inter-cooling heat exchanger. Combustion will occur in the main heat exchanger.
The high-pressure natural gas coming from the main pipeline will receive primary heat in the intercooler then be superheated in the main heat exchanger before being expanded in the engine. Before entering the local distribution network, the low-pressure engine exhaust gas will release its heat to the cold incoming gas in the intercooler. Engine efficiency can be improved and power output could be raised by 45% using higher engine inlet temperature and the intercooler. The use of the intercooler and a re-heater will allow for cascade (high-pressure, reheat, low-pressure) operation of 2-Quasiturbines.
The table below shows data with regard to engine performance using inter-cool:
The use of the intercooler increases the pressure-reduction engines output at a flow-rate of 1-lb/sec to between 4% and 5% of the power output of a gas turbine burning natural gas at a rate of 1-lb/sec. Smaller pressure-reduction engines that flow under 0.1-lb/sec of natural gas could be used at industrial buildings.
Natural-gas fired industrial boilers as well as building heating systems usually operate with a heat-transfer effectiveness of 85%. If the boiler or heating system supplied 1,500,000-BTU's per hour (589.39-Hp) of heat, 82-lb/hr of gas would be burned. To reduce outside pipe pressure from 200-psi to 3-psi inside the building, an inter-cooled Quasiturbine could be installed into the supply line. It would produce 82/3600 x 426 = 9.7-Hp or 7.2-Kw of power to an alternator and supply at least 6.5-Kw of useable electric power to maintain basic level building operations during winter months.
If regulators and natural gas companies were willing to allow externally heated Quasiturbine engines to be used to reduce line pressures at gas pipeline system transfer points, a substantial amount of low-cost electric power could be generated for local use. As an example, a survey (M. Dehli, GWF Gas-Erdgas 137/4, p.196, 1996) showed that in Germany alone, the potential for utilizing this pressure was 200-700 MW in 1996, and the gas consumption has increased since then... This is the equivalent of tens of mega windmills on kW-h basis!
Strategic Local Electrical Backup
At a time where electrical networks suffer some instability an their reliability are questioned following spectacular black-out, this could be of great interest as a stable strategic local power source, not only for the Gas Pipeline companies themselves, but also for priority local needs like hospitals and public services.