HK40042014B - Pressure reduction system - Google Patents

Description

Pressure reducing system

Technical Field

The present invention relates to a system for utilizing waste energy generated in a natural gas pipeline during depressurization.

Background

Typically, natural gas exits gas wells at pressures up to several hundred bar. The pressure is adjusted to the appropriate pressure for transport in the air supply grid. This pressure is maintained with the aid of a compressor. Thus, the transport system has a great potential energy which is lost as the pressure is gradually reduced in the distribution system to ensure that the customer receives the proper pressure.

Most countries use similar natural gas transportation systems in their territories and then perform regional distribution. In the uk, three broad groups of combinations with a depressurisation step reduce the pressure from 60-80 bar to a pressure of domestic millibar pressure. It is estimated that in the UK supply network (UK grid), the mechanical energy available from natural gas expansion is up to 1GW. This energy is lost mainly at the decompression stations and it is increasingly controversial that this energy can and should be recovered.

Therefore, it is expected that mechanical (kinetic) energy is captured as the natural gas depressurizes and loses potential energy.

Existing pressure reduction techniques include the use of simple orifices to reduce the pressure. This may take the form of a regulator valve and control system commonly referred to as a Joule Thomson valve. In natural gas pipelines, depressurization has different names. In the uk, higher pressure systems are known as Pressure Reduction Stations (PRS) or Transmission Regulator Stations (TRS). In the united states, such stations are called step-down stations (PLS).

As the natural gas travels through the joule thomson pressure relief valve, a temperature drop occurs that is associated with isenthalpic adiabatic expansion of the natural gas. If this temperature drop is uncontrolled, the cold pressurized natural gas may allow hydrates to condense and solidify, which may result in equipment damage or pipe blockage. As shown in fig. 1, this cooling currently requires the use of a preheating step.

Generally, the preheating technique is relatively rudimentary. The preheating technique is a combustion technique that heats the fluid in the reservoir. The natural gas passes through a heat exchanger in the accumulator to collect energy. The required preheating depends on the initial pressure, pressure variations and natural gas composition. For example, for a typical uk natural gas composition and an input pressure of 30 bar, the temperature will drop by about 0.6 ℃ for every 1 bar reduction in pressure. Thus, for a downstream pressure of 5 bar, a temperature drop of 15 ℃ will occur. If the temperature drop is not controlled, the output temperature will be made-5 ℃. The total amount of energy required to heat the natural gas is relatively small compared to the amount of chemical energy traveling in the pipe flow. However, in the case of over 14000 PRSs in the uk, this means that significant gas consumption and CO are undesirably incurred ₂ And (5) discharging.

Turboexpanders have been used for many years to recover energy from an expanded natural gas stream. Turboexpanders come in a variety of sizes and efficiencies. Most cases use high speed turbines, and the less common approach is to use a positive displacement system similar to the screw expander arrangement. Functionally, both high speed turbines and positive displacement systems have similar efficiencies (typically 30-85% isentropic efficiency) and similar effects on temperature and pressure. All designs can be coupled to a generator to convert the expanding natural gas into electrical energy. The electrical power to be taken from the generator is then typically conditioned for use or output. By nature, the turboexpander excites isentropic adiabatic expansion which reduces the temperature of the natural gas, typically by more than 5 times that of a joule thomson pressure reducing valve.

Several reasons why turboexpander or screw expander technology has not been widely implemented in natural gas pipeline networks are:

the electrical energy generated by the turboexpander is about 85% of the preheating requirement (although it is noted that the power generated is more per MWhr than the natural gas consumed).

The increased consumption of natural gas required for preheating results in increased carbon emissions from the distribution system when there is considerable incentive to reduce carbon emissions.

To allow for the output of electrical energy at low cost, the PRS needs to be very close to the sub-plant with sufficient production capacity to accept the electrical input from the turboexpander generator, thus greatly reducing the number of available sites.

Disclosure of Invention

The present invention solves the problem of exploiting the potential energy generated by the natural gas expansion (decompression) stage in natural gas pipelines or in the cryogenic industry. The use of a turbo-expander or screw expander only partially solves these problems, since the electricity generated is intermittent and often not positionally consistent with the grid. Therefore, outputting and storing electrical energy is not simple and is not sufficient to cover the additional heating required for preheating. The solution proposed by the present application addresses these deficiencies.

The present invention relates to the coupling of a load following electrolysis cell to a plant such as a turbo expander that recovers energy from a natural gas expansion step. The electrolyzer may be a Proton Exchange Membrane (PEM) or an alkaline based electrolyzer that will convert electricity to hydrogen. The hydrogen produced can be used in a number of efficient ways and therefore does not suffer from the drawback of having to use intermittent electricity and the energy can be stored indefinitely in a pressure vessel. The electrolysis cell also has the beneficial effect of generating waste heat that can be used in the preheating step. This has a significant beneficial effect on carbon reduction.

According to a first aspect, a system for reducing pressure and extracting energy from a natural gas pipeline or for use in the cryogenic industry comprises: an electrolyser for generating hydrogen, a heating device adapted to heat natural gas in the pipeline, and a device adapted to extract energy from the expansion of the natural gas, wherein the extracted energy is used to power the electrolyser.

According to a second aspect, the system as defined above may be used for extracting energy from natural gas expansion.

Drawings

Figure 1 illustrates a modular representation of a prior art pressure relief system.

Figures 2 and 3 show modular representations of two preferred pressure relief systems of the present invention.

Detailed Description

Figure 2 shows a preferred embodiment and involves coupling a PEM electrolyser and heat pump to a plant that recovers electrical energy from a natural gas expansion step, such as a turboexpander coupled to an electrical generator. Ideally, PEM electrolyzers will produce pressurized hydrogen and have the ability to load pressurized hydrogen from 0% to 100% of rated capacity. The energy required for natural gas preheating can come from a variety of sources including combustion. Preferably, the energy required for natural gas preheating is derived from a combination of waste heat from the electrolysis cell and thermal energy from the heat pump. The electrolyzer converts electricity into pressurized hydrogen gas, which is preferably injected into a natural gas supply network.

The hydrogen produced can be used in a number of effective ways. Thus, such an arrangement does not suffer from the drawback of being exported to the power supply grid, for example when production volumes are often insufficient to meet demand.

An alternative embodiment shown in fig. 3 is: the temperature of the natural gas is raised to near ambient temperature using technology from the cryogenic industry which regularly uses a turboexpander to cool the natural gas and the output of the turboexpander is fed directly into a large ambient heat exchanger specifically designed to minimize the formation of hydrates. The natural gas may then receive final heat gain by utilizing waste heat from the heat pump and/or the electrolyzer as previously described.

These methods have significant benefits, including:

the embodiment shown in fig. 2 consumes no natural gas, thereby reducing cost and carbon emissions, as compared to existing joule thomson valves.

The presence of carbon that will be used to remove the gas grid for the entire natural gas consumption process, due to the "environmental" nature of the hydrogen produced, would allow the invention to require a range of carbon abatement incentives.

There is no need for external sub-stations, increasing the number of available stations and reducing costs.

The system of the present invention includes a heater/heating device that increases the temperature of the natural gas. Suitable heaters will be well known to those skilled in the art. The term "heating device" should be interpreted broadly and refers to any method by which the temperature of natural gas can be increased. For example, the heating device may be anything that is capable of heating natural gas in a pipeline, such as a heat exchanger that uses ambient air or water to heat the natural gas.

In a preferred embodiment, the heating device is a heat exchanger, through one half of which the natural gas flows and on the other half of which the heating fluid is located. The heating fluid may be heated by combustion of natural gas or other fuels, and may also be heated by a combined heat and power system (CHP). The heating device may be an ambient air heat exchanger. The heating fluid may be heated by a heat pump, and may also be heated by waste heat from the electrolysis cell. The heat may be provided by an electric heater. Preferably, the heating fluid may be heated by a heat pump and waste heat from the electrolysis cell.

If a heat pump is used, the heat pump is preferably operated at as high an output temperature as possible. Several heat pump technologies and mediums are possible. The preferred embodiment is transcritical CO ₂ Heat pump, transcritical CO ₂ The heat pump may generate a temperature of 80 ℃ to 90 ℃. Another embodiment of the heating device is a subcritical ammonia heat pump. Various heat sources/heating devices are possible, including but not limited to air, ground, and water. Water and air are preferred heat sources due to the lower carbon footprint and capital expenditure costs.

Plants for the expansion extraction of energy from natural gas will be well known to those skilled in the art. In the present invention, the device for extracting energy from the expansion of natural gas is preferably a turbine or a positive displacement device such as a screw expander. More preferably, the means for extracting energy from the expansion of natural gas is a turbine, such as a turboexpander. The device for extracting energy from the expansion of natural gas extracts kinetic energy from the natural gas as the natural gas expands during depressurization. The apparatus is preferably mechanically coupled to a dc motor or alternator that generates electricity. Preferably, the output is Direct Current (DC), and in this case, the solution can reduce costs by eliminating AC-DC power conditioning to the electrolyzer. Because the pressure reduction station experiences both diurnal and seasonal flows and changes in pressure, all equipment connected to the generator must be able to accept time-varying inputs.

The electrical output from the generator can be used to power the heat pump and the electrolyzer. In the preferred embodiment, because the input and output pressures of the turboexpander vary, the power should be split dynamically between the two devices to ensure that the total production of hydrogen is maximized while providing sufficient heat for natural gas preheating.

The system of the invention uses an electrolytic cell, preferably a water electrolysis cell, which generates hydrogen gas. The skilled person will be able to select an electrolysis cell suitable for use in the system of the invention. Water electrolysers have been manufactured for many years, however, only recently has it been possible to repeatedly cycle electrolysers in the power range from 0 to 100% without damage.

The electrolysis cell may be a Proton Exchange Membrane (PEM) electrolysis cell, a solid oxide electrolysis cell or an alkaline based electrolysis cell, the electrolysis cell being based on a solid polymer or liquid electrolyte. Preferably, the electrolyzer is a PEM electrolyzer. Preferably, the PEM is anionic. The membrane may be a hydrophilic cross-linked polymer. In one embodiment, the membrane is a perfluorinated polymer.

PEM technology is able to respond quickly to fluctuating electrical inputs without a large penalty to performance or lifetime. In particular, the present invention uses hydrophilic ionic PEM membranes of the type described, for example, in WO03/023890, which is incorporated herein by reference in its entirety. The hydrophilic ion PEM membrane can be used in a large current range (preferably 0.1-3A/cm) ² ) The operation is carried out significantly higher than in alkaline liquid electrolysers. The increase in current density makes the cell less demanding, contributing both to the carbon footprint and to the cost.

The oxygen generated by electrolysis may be discharged, used or stored. Oxygen may be used to improve the combustion performance of the supplemental device. The oxygen may be used in a combustion reaction to heat natural gas in a pipeline and/or be transported to a heating facility for use in the facility. The oxygen may be preheated prior to combustion or being delivered to the heating device.

The electrolysis cell for use in the present invention should generate hydrogen at a pressure of greater than 1 bar. The hydrogen gas output from the electrolysis cell can be pressurized by a compressor. Preferably, the electrolyzer should generate hydrogen at a pressure that allows direct injection into the natural gas pipeline without the need for a compressor.

A mechanism for hydrogen storage may be required to allow for buffering of the cell output. Several storage technologies are possible, including compressed gases and metal hydrides. The storage technique used in the preferred embodiment is compressed gas.

The natural gas may be heated before or after it enters the plant for extracting energy.

The natural gas may be heated by any of the following:

a) A heat pump;

b) Waste heat from the electrolysis cell;

c) Waste heat from a plant for extracting energy from natural gas expansion;

d) Energy derived from the combustion of natural gas;

e) Energy from a combined heat and power plant; and/or

f) A combination of two or more of the foregoing.

In the uk, the mixing and dilution of hydrogen inside natural gas pipelines should be demonstrated. This may be achieved by a mixing device such as a static mixer prior to injection into the natural gas pipeline. Alternatively, dilution may be achieved by turbulence in the natural gas pipeline. Hydrogen may be introduced into the high pressure side prior to the plant that extracts energy from the expansion of natural gas. In this case, the reduction in pressure generates the turbulence required for mixing. Preferably, the hydrogen is introduced into the low pressure side after the device for extracting energy from the expansion of natural gas, the downstream turbulence from the device for extracting energy from the expansion of natural gas providing mixing of the two gases. Measurements may need to be taken downstream and upstream of the pressure reduction station to meet statutory requirements. Such measurements may include, but are not limited to: flow rate, pressure, wobbe (Wobbe), calorific value and hydrogen content.

Instead of returning the hydrogen to the natural gas grid, the hydrogen may be diverted for use in other applications. In these applications, higher prices can be achieved for these natural gases. For example, hydrogen may be stored for use in transportation applications (as a fuel), or hydrogen may be used as a reactant in chemical reactions such as ammonia production, methanation, liquid fuel synthesis, and/or oxygen applications.

Two preferred embodiments, as shown in figures 2 and 3, will be discussed in more detail below.

Embodiment mode 1

The technology utilizes an integrated natural gas preheating, energy recovery system, heat pump and hydrogen electrolysis equipment kit to replace the existing static expansion valve (joule thomson) system. The system will preheat the natural gas, extract energy during depressurization, and use this energy to power the heat pump and electrolyzer. The heat from the electrolyzer and the heat pump will provide energy for preheating, while the electrolyzer will generate low carbon hydrogen. This hydrogen can be injected rapidly into the gas grid, thereby reducing the carbon footprint of the natural gas. The system of the present invention is shown as the "proposed system" in the schematic shown in fig. 2.

Embodiment mode 2

The technology replaces the existing static expansion valve (joule thomson) system with an energy recovery system, a cryogenic heat exchanger, a heat pump, and a hydrogen electrolyzer package.

The system extracts energy during depressurization and then uses that energy to power the heat pump (heating device) and the electrolyzer. The sub-cooled natural gas may then enter a heat exchanger for warming by the atmosphere. The heat from the electrolyzer and the heat pump will provide any additional energy required for the natural gas to be admitted into the natural gas system. The electrolyzer will generate "environmentally friendly" hydrogen. Hydrogen can be injected immediately into the gas grid, thereby reducing the carbon footprint of the natural gas. The system of the present invention is shown as the "proposed system" in the schematic shown in fig. 3.

After expansion, the natural gas enters an ambient air heat exchanger. The ambient air heat exchanger may be specifically designed for cryogenic natural gas and transfer heat from the air. Typically, there will be two parallel systems that cycle to thaw ice that is present on the blockage, and thus will provide most of the heat. For example, during cold weather, further heating may sometimes be required. This will preferably be provided by a variety of sources including air, earth or water heat pumps, electrical heating, waste heat from the CHP plant or conventional combustion processes. More preferably, the heating is performed by air or running water using a heat pump.

The electrical output from the generator is used to power the electrolysis cell. Waste heat from the electrolysis cell is preferably used to provide additional heating to the natural gas via a heat exchanger. If these additional heats are insufficient, the heat pump may also be powered by a generator, and the output of the heat pump is used to further warm the natural gas via a heat exchanger. In a preferred embodiment, because the input and output pressures of the turboexpander vary, the power should be split dynamically between the two devices to ensure that the total production of hydrogen is maximized while providing sufficient heat for the natural gas.

A mechanism for hydrogen storage may be required to allow for buffering of the cell output. Several storage technologies are possible, including compressed gases and metal hydrides. The preferred embodiment is compressed gas.

In yet another aspect of the invention, the hydrogen and oxygen producing electrolyzer cell is adapted to be attached to a natural gas pipeline and to be powered by energy derived from the expansion of natural gas in the pipeline.

The following examples describe the invention.

Example 1

This example is a medium sized pressure reduction station that delivers natural gas. The amount of natural gas is 15840m per hour ³ And the inlet temperature was 10 ℃. The inlet pressure was 70 bar and the outlet pressure was 30 bar.

In a typical prior art system using joule thomson technology, the natural gas would be heated to 24 ℃ so that the natural gas would reach the pressure relief valve at 34 ℃. The pressure relief valve will reduce the temperature by 24 c and the natural gas will exit the valve at 10 c. The heat required to raise the temperature of the natural gas by 24 ℃ is 160kW. This required 197kW (chemical) methane in view of the inefficiency of the heat exchanger and the burner. Assuming a wholesale price of 0.02/kWhr, this means a working cost of 35000 pounds per year and 342 tons of C0 are produced ₂ 。

This system is replaced by the system of the present invention (same flow rate, input pressure and output pressure). At typical uk natural gas composition and outlet pressure of 30 bar, our model shows that hydrates can start to form at 8.8 ℃. Therefore, for safety, the output temperature should be kept 5 ℃ above this level, so the natural gas output temperature is 13.8 ℃.

The inlet air was preheated by 69.2 c to provide an inlet temperature of 79.2 c to the turboexpander. This requires 461kW of heat to be applied to the natural gas. This would require 512kW of heat input due to the inefficiency of the heat exchanger.

The natural gas then enters a turboexpander where 262kW of energy is extracted as electricity. The natural gas was then cooled down to 13.8 ℃.

Trans critical C0 ₂ The Air Source Heat Pump (ASHP) consumes 262kW of heatAnd generates 485kW of heat. Assuming an efficiency of 70% (total heat generated =512 kW), the cell consumed 89kW of heat and generated 27kW of heat.

Electrolytic cell generation of 195Nm ³ O/day ² And 390Nm ³ H/day ² This is equal to 0.1% of the natural gas flow as the uk legal limit for hydrogen concentration in natural gas networks.

This compares to the £ 35 kpa loss associated with systems with traditional joule thomson technology, which generate 106000 pounds of RHI (Renewable Heat accounts) per year, 96000 pounds of ROC (blistering Certificates) per year, and 10000 pounds by sale of hydrogen gas, providing a total revenue of 214000 pounds per year. This represents a savings of 248000 pounds per year (all prices were based on 2013 september). By reacting zero-carbon H ₂ Injecting into the natural gas pipeline and diluting the natural gas saves 108t of carbon each year. Thus, 450t of carbon is saved each year compared to the existing joule thomson technology.

Claims (25)

1. A carbon abatement system for reducing pressure and extracting energy from a natural gas pipeline, the system comprising:

an electrolytic cell to produce hydrogen;

a heat pump;

a low temperature first heat exchanger for warming natural gas by ambient air or water;

a second heat exchanger disposed downstream of the cryogenic first heat exchanger and adapted to heat natural gas in the pipeline; and

an energy extraction device adapted to extract energy from the expansion of the natural gas, the device being disposed upstream of the cryogenic first heat exchanger,

wherein the extracted energy is used to power the electrolysis cell and the heat pump, and wherein the second heat exchanger obtains its energy from the heat pump and the electrolysis cell.

2. The system of claim 1, whereinThe heat pump is transcritical CO ₂ A heat pump or a subcritical ammonia heat pump.

3. The system of claim 1, wherein the energy extraction device is a turboexpander or a screw expander.

4. The system of claim 1, wherein the natural gas is heated after exiting the energy extraction plant.

5. The system of claim 4, wherein the natural gas is heated by, in addition to energy derived from the heat pump and the electrolyzer:

a) Waste heat from the energy extraction plant;

b) Energy derived from the combustion of natural gas;

c) Energy from a combined heat and power plant; or

d) A combination of two or more of the foregoing.

6. The system of claim 1, wherein the electrolyzer is a proton exchange electrolyzer.

7. The system of claim 1, wherein the electrolytic cell is anionic.

8. The system of claim 7, wherein the electrolyzer is a liquid alkaline electrolyzer.

9. The system of claim 8, wherein the electrolyzer is a solid polymer alkaline electrolyzer.

10. The system of claim 7, wherein the membrane of the electrolytic cell is a hydrophilic cross-linked polymer.

11. The system of claim 7, wherein the membrane of the electrolytic cell is a perfluorinated polymer.

12. The system of claim 1, wherein the electrolyzer produces hydrogen at a pressure greater than 1 bar.

13. The system of claim 1, wherein the hydrogen produced by the electrolyzer is pressurized by a compressor.

14. The system of claim 1, further comprising a gas mixing device adapted to mix hydrogen from the electrolyzer with natural gas in the pipeline.

15. The system of claim 14, wherein hydrogen produced by the electrolyzer is injected into the natural gas pipeline.

16. The system of claim 14, configured such that the mixed gas is injected into the natural gas pipeline.

17. The system of claim 16, configured such that the natural gas enters the gas mixing device after a device for extracting energy from expansion of the natural gas.

18. The system of claim 1, configured such that oxygen produced by the electrolyzer is used to heat the natural gas in combustion and/or for use in the second heat exchanger.

19. The system of claim 18, configured such that the oxygen is preheated prior to combustion.

20. Use of the system of claim 1 for extracting energy from natural gas expansion.

21. Use according to claim 20, wherein the extracted energy is divided dynamically between the heat pump and the electrolysis cell as the input pressure and output pressure of the energy extraction apparatus vary.

22. Use according to claim 20, wherein the energy extraction device is a turboexpander.

23. Use according to claim 20, wherein the energy extraction device is a screw expander.

24. Use according to claim 20, wherein the equipment for extracting energy is connected to equipment for converting said energy into electricity.

25. Use according to claim 24, wherein the energy is used to power the electrolysis cell, heating means and on-site auxiliary means.