Gas Compression Emission Management: Everything You Need to Know
There is a growing production and use of natural gas across North America. While this comes with many advantages, it also poses other threats. The environmental impact is at the top of the list of concerns, which calls for gas compression emission management.
Sources of Gas Compression Emissions in the Industrial Sector
Managing your gas compression starts with understanding your compression system. Notably, centrifugal compressors use wet gas seals or dry gas heals. The seals help prevent methane gas or process gas leakage at the points where the shafts and compressor casings meet.
The configuration of gas compressor systems utilizes packing technology to prevent leakage. The packing includes nose cup seals, gaskets, and a stuffing box. However, this is never wholly preventable. Thus, it calls for the need for specific changes in the design of compressors and their blowdown units.
Among the sources of methane and other greenhouse gas compression emissions include:
The Counter Surface of the Stuffing Box- Over time, the counter surface of the stuffing box will degrade. The degradation is often due to unwanted deposits, corrosion, and minor indentations from previous gaskets. These irregularities will prevent the creation of a tight interface that can prevent leakage.
Nose Gasket Overuse- Gaskets should plastically deform when put under clamping pressure. This characteristic ensures that gaskets can fill any irregularities between mating services. Overuse reduces the ability of gaskets to deform plastically. The result is that the gasket will fail to fill all the irregularities, leading to gas leakage.
Misapplication of Gasket- Ideally, copper, aluminum, iron, and stainless steel are among the materials often used for manufacturing gaskets. Each material has unique clamping load, pressure, and process gas specifications. The material used should always be compatible with the application since misapplication could cause leakage.
Cup-to-Cup Leakage- Finally, defects can cause gas compression emissions between two flat surfaces. Usually, this could be due to clamping force and size disparities.
How to Effectively Manage Compression Gas Emissions
Regardless of the source of leakage of liquid gases in the industrial sector, they are pretty manageable. Among the ways through which you can manage gas compression leakage include;
Reinjection into Turbine Inlets
Controlling carbon emissions from compression units can be expensive and demanding. But a more cost-effective method of burning fugitive gas emissions is to reroute the vents emanating from the dry gas seals into the inlet of your turbine.
Your worry could be about the flammability of the resulting air (oxygen) and methane mixture. Interestingly, this mixture falls below the flammability limits, which could often cause concern.
This emission management method does not require extensive maintenance. However, you may have to improve certain design aspects. The aim is to improve the mixing of intake air with the vented air to maintain low flammability. Of course, this is necessary both when the compressor is in operation and when it is not working.
Reciprocating Compressor Units
An electric reciprocating compressor that recompresses gas emissions is arguably the most widely used emission management method. This method reliably recovers and recaptures greenhouse gases leaked across seals and injects them back into the stream of process gas. Of course, this process happens at the discharge header or station suction.
Apart from injecting this fugitive gas into the process gas stream, it is also possible to directly inject it into the fuel inlet, the electrical generator, or the gas turbine. The fugitive gas is also usable as heating fuel. The size of the reciprocating compressor used in this method is dependent on the flow rate and volume of the emitted gases.
Recompression comes with two benefits. Firstly, it allows for the successful recapturing of emissions caused by station depressurization during planned shutdowns and maintenance activities. Again, recompression is pretty scalable, and a single unit is useable across multiple centrifugal compressors.
Frameless Thermal Oxidizer
Frameless thermal oxidizers will premix methane with auxiliary fuel and ambient air and pass it through a preheated ceramic media bed. The system effectively transfers heat to the gaseous mixture from the media bed.
The high temperature oxidizes the methane in the mixture into carbon dioxide and water. This method requires gas burners or electric heaters to preheat the ceramic media.
Thermal oxidizer units are vital in handling emissions resulting from compressor settle-outs and blowdowns. Besides, media beds used in this method are quite stable and resistant to fluctuations in temperatures. This feature enhances more reliable temperature controls.
Enclosed Vapour Combustion Units
Another interesting method of managing greenhouse emissions from compressor units is through enclosed vapour combustion units. Notably, vapour combustion units use enclosed stacks to collect and burn compression gases. The combustion in these systems happens at the bottom and is completely invisible.
These systems are pretty easy to set up and work flawlessly. They do not require any mechanical devices to help move fugitive gases into the stack inlet. Instead, they naturally draw in the heat generated from the combustive process.
Combustion stacks are can accommodate high temperatures to allow more fugitive gases to burn as they rise in the stacks. Enclosed vapour pressure combustion units work differently. Their burners are not ideal for high-pressure applications. Moreover, their enclosed designs could lead to too much flame on exposure to too much gas. Of course, very intense flames could damage the stuck insulation.
Supersonic Ejector Systems
Ejector systems are devices used for sucking in fugitive gas emissions from primary dry seals. This emission management mechanism does not feature any moving parts. Thus, it requires little or no maintenance.
In this method, the energy created by the motive fluids pressure will convert to velocity energy. The result is the creation of a low-pressure region which will easily capture emitted greenhouse gases. The captured air will mix with ambient air and move through the diffuser. At the diffuser, the air pressure increases while the velocity decreases to recompress the gas effectively.
You can then re-inject the recompressed methane-air mixture for various applications. It is useable in compressor inlets, heater inlets, and fuel system inlets. Unfortunately, you cannot use ejector systems to capture fugitive gas emissions from settle-out or blowdown emissions.
Double Opposed Gas Seals
Double opposed gas seals can also help you manage gas compression emissions. But unlike all the other methods discussed above, this method does not rely on recapturing fugitive gas emissions for combustion or reinjection into the system. Instead, it is a preventive method that ensures that there is no leakage altogether.
The method uses primary and secondary seals in a back-to-back arrangement. The mechanism then supplies inert gas such as nitrogen between the two seals. Any leakage will comprise nitrogen only, which is inert and harmless.
The reliability of this method depends on the consistent availability of sealed nitrogen gas. Again, it is impossible to rely on this method to address methane gas leakage from compressor blowdown.
Putting it All Together
Gas companies across the supply chain are under intense pressure to reduce the emission of greenhouse gas into the environment. Of course, the pressure and concerns are pretty much in order. Natural gas compressor stations are responsible for a sizeable amount of the emissions. Focus now shifts from reducing emissions from combustion processes to cutting down gas compression emissions. The above technologies are very helpful.
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