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ELECTRIFICATION

Electrification in its simplest form is the process of powering by electricity. In the specific context of turbine replacements, electrification involves replacing turbines with electric systems. This replacement is preferred because of its benefits like increasing process efficiency and shutdown maintenance interval hence lowering operational costs. The idea of electrification is not specific to turbine replacements, depending on the project and its components, several mechanical devices can be replaced with electrical motors and drives. This article focuses on the process of electrifying gas compressor sites which is done by replacing the gas turbines with electric motor drives (EMDs).

Gas Compressor Sites

They are also known as gas compressor stations. Compressor stations provide energy needed to move natural gas through the pipelines to reach our homes and provide everyday comforts, such as heat and cooling [1]. A typical station contains the following:

Piping and valves: The piping brings natural gas through the pipeline to the compressor units and also transports it back after compression.
Gas scrubber: They remove liquid, dirt or any other particle from the natural gas.
Compressor units: They are the most important devices in the entire station. A compressor increases the pressure of a gas by reducing its volume. Power is provided by an engine (usually a turbine) which is then transferred to the compressor that uses an impeller to push gas down the line.
Mufflers: Used to decrease the amount of sound produced by compressor units.
Air system: As part of a control system for instrumentation, pressurized air is taken from the air compressor and is used throughout the site.
Generators: Electric Generators are used as primary power sources in these sites as an alternative to utility power (i.e power from the grid).
Alternative power supply: An uninterrupted power supply source is installed in the site as an alternative in case of power failure.
Fuel gas system: The compressor units are fueled by natural gas from the pipeline [2].

In gas compressor sites the sole purpose of turbines is to provide power to the compressor engine like we see in jet engines. Alternatively in gas turbines, a compressor is used to increase the pressure of the incoming air before it enters the combustor [3].

GAS TURBINES

A gas turbine is simply a type of turbine that uses pressurized/compressed gas to spin in order to power a load.

This process specifically uses a turbine that receives gasses from the combustor. This energy then drives a generator that produces the electrical energy that moves along power lines to homes and businesses [4].

The Process

Combustion of Fuel: The compressed air is mixed with fuel that is then burned at very high temperatures to provide hot air.
Spinning of Blades: The blades in the gas turbine spin quickly as a result of the mixed hot air and fuel.
Turning Drive Shaft: The spinning blades of the turbine rotate the drive shaft. The turbine shaft connects the turbine to the generator, turning at the same speed as the turbine [5].
Powering the Generator: A rod in the generator connects the spinning turbine which turns a large magnet surrounded by coils of copper wire.
Revolving Magnets: The magnets in the generator turning very fast create a magnetic field that causes the electrons to line up around copper coils which causes them to move.

Advantages

Costs: Gas turbines historically have a lower operational cost. Compared to more sustainable alternatives like nuclear and renewable energy, setting up and operating gas turbines costs significantly less.
Durability: With stringent carbon emission regulations, volatility in fuel costs, as well as emphasis on high performance at low costs, gas turbines have emerged as the most viable option [6]. Its high performance at low operation cost makes it efficient.
Environmentally Friendly: Compared to combustion engines and previously common alternatives, turbines are more environmentally friendly. Water is also used less compared to steam power plants.
Flexibility: They offer flexibility by supplying electricity for power generation as well as by supplying compressed air for process needs [7]. In general power technologies that can be distributed are flexible and can operate both in standalone and as part of an integrated system.
Variety of Fuel Usage: Turbines are capable of using various types of fuels including gaseous, liquid and even synthetic fuels. The specifications are typically set during the design and construction phase.

Disadvantages

Efficiency: Overall efficiency is approximately 20% as a result of the exhaust gasses that contain heat (steam). The only exception being the combined heat and power (CHP) systems where the heat from the exhaust is repurposed.
Vibrations: Gas turbine plants have less vibrations compared to reciprocating engines. The lower frequency tone of about 60 Hz however causes turbines to make less noise.
Temperature: As expected when the combustion chamber gets as hot as 1900°C, a lower lifespan is inevitable. The heat also causes strict restrictions on servicing and operating conditions.
Incompatibility: Gas turbines are incompatible with fuels in their solid forms like biomass and coal. This incompatibility makes these turbines less versatile and adaptable.
Cooling: Heat is generated from the rapid spinning of the turbine blades. Heat as high as 1100°C - 1260°C needs to be counteracted with a suitable cooling system which would add to the operation costs.

ELECTRIC MOTOR DRIVES (EMD’s)

An EMD is an electric motor that is supplied with direct and alternating current. The goal of the electrification process is to use a high speed electric system driven compressor to eliminate the need for a gas turbine. As already seen in modern electric trains, the operating speed , efficiency and greenhouse gas emission is reduced in the process.

VSD/VFD

A variable frequency drive is also known as a VSD (variable-speed drive), ASD (adjustable-speed drive) and AFD (adjustable-frequency drive). It is a type of motor drive used to control AC motor speeds and torque by varying motor input frequency. VFDs depend on topography to control voltage and current variation. The variable frequency power supply uses solid state components to produce a pulse-width modulated current that varies the power and frequency supplied to the motor [8].

The VFD displayed in the figure above is used to regulate the speed/frequency of the induction motor. Induction motors operate at a speed less than their synchronous speed i.e the speed of rotation of the magnetic field in a rotary machine. In an induction motor, the electric current in the rotor needed to produce torque is obtained via electromagnetic induction from the rotating magnetic field of the stator winding [9].

Advantages

Environment factors: The carbon emissions that are produced from turbine-driven compressor sites are higher than those produced from the electrified solution. About 30% of the emissions are reduced. Noise emissions produced by an electric motor are also lower than those produced by a turbine, typically -12 dB(A) [10].
System Efficiency: All the components working seamlessly together create a system improving efficiency by 15%. Part of the electric power is used to supply the compressor driving motor through a VSD and a transformer.
Operational Cost: Higher efficiency contributes to the lower operational costs compared to turbines. Another factor is maintenance, electric motors do not typically require much maintenance especially when they are fitted with active magnetic bearings.
Safety: Certain hazards that are associated with fossil fuels, hot temperatures and lubricating units are eliminated which leads to a safer site. Other risks associated with electrical wiring are easily preventable and not as risky.
Motor Efficiency: Specifically, the efficiency of the electric motor driven by a VSD is very flexible. It only takes a few seconds for the motor to start operating at full speed and full torque, cutting out the usual time used to wait for any thermal cycle. The lifetime is further increased by the starting current being limited by the VSD to the motor rated current.

Disadvantages

Integration: Figuring out what capacity each of the electrical components (motor, drive, etc) are required to fit seamlessly into the existing site is an important step. Also factoring the slope and other existing components of the site, some changes might be needed which would add to the setup costs.
Compatibility: As a result of the limited information we have on projects involving turbine replacement, we can tell that not all turbine-driven compressor sites can be easily electrified. Knowing this and finding the right components to fit these sites is relatively difficult.
Electrical Hazards: With systems like this that are electricity dependent, a possible issue in the wiring or power supply can cause the entire system to crash or halt ongoing processes. Although rare, multiple steps in the design and construction stages can be taken to prevent this.
Setup Cost: The amount typically needed to integrate these electrical components into an already existing turbine site is quite high. If the replacement is done by rebuilding the entire station to be fully electric, it would cost much more and also take longer to accomplish.

Replacement Options

According to the work done by engineers and designers in the past years, there are two main ways that they go about replacing a turbine in a compressor site.

Option 1:Constructing a new electrified compressor station.

Takes more time
Not feasible in most cases
More setup costs
Involves replacing turbine and compressor
Most auxiliaries will be changed
More plot space is needed

Option 2: Replacing the turbine with electric motors and drives.

Takes less time
More efficient
Less setup costs
Involves only replacing the turbine
Reusing existing auxiliaries
Previous turbine location is re-used

From the options presented above for how the electrification of compression sites can be done, we can see why option 2 is preferred. Although, depending on the site aspirations and other components involved, option 1 might be explored more.

CONCLUSION

Electrification is not the solution for replacement in every turbine-driven compression site. In specific cases like electric trains with their turbine engines converted to be fully electric, the pros outway the cons when the integration is seamless. With all of us aiming for a more sustainable future with near zero carbon emissions, considerations for electrified stations should be made. As a temporary solution aiming at a more sustainable option, energy companies should focus on replacing select turbines with electric motor devices.

REFERENCES

[1] https://www.tcenergy.com/siteassets/pdfs/discover-energy/transcanada-natural-gas-compressor-stations-canada.pdf

[2] https://sites.google.com/site/metropolitanenvironmental/basics-of-gas-compressor-stations

[3] https://www.grc.nasa.gov/www/k-12/airplane/compress.html

[4] https://www.ge.com/gas-power/resources/education/what-is-a-gas-turbine

[5] https://www.somersforge.com/sectors/turbine-shaft/#:~:text=The%20turbine%20shaft%20connects%20the,a%20usable%20form%20or%20medium.

[6] https://blog.technavio.org/blog/5-major-benefits-gas-turbines

[7] https://www.brainkart.com/article/Advantages-and-disadvantages-of-Gas-turbine-power-plant_5600/

[8] https://www.sciencedirect.com/topics/engineering/variable-speed-drives

[9] Induction Motor: How Does it Work? (Basics & Types) | Electrical4U

[10] *EUR21_08_PIDv.pdf