Flare gas is a mixture of various gases sourced from associated gas in oil fields, natural gas processing plants, refineries, and other oil and gas production facilities such as well testing. The average methane content in flare gas is approximately 50%. Flaring or venting flare gas releases large amounts of CO₂, NOx, and methane, exacerbating the greenhouse effect. By deploying flare gas recovery systems (FGRS), flare gas can be converted into fuel gas, feedstock gas, or commercial gas, offering both environmental and economic benefits.
Gas-to-liquid conversion synthesizes greenhouse gases into liquid fuels via the Fischer-Tropsch process. This method requires significant initial investment but delivers good economic returns for large-volume flare gas recovery.
Power generation involves collecting flare gas and using it in gas turbine generators or other power generation facilities. Studies show this method has lower initial investment and shorter payback periods compared to GTL but is less economical than the compression method.
The compression method uses different compressor types to collect low-pressure flare gas, pressurize it, and inject it into pipelines for delivery to other process systems for treatment or sale. This mature and widely applied technology offers high reliability.
Among these flare gas recovery methods, the compression method has proven to be mature and reliable in oil and gas production facilities. Key components include compressors, heat exchangers, three-phase separators, and compressor control systems, with different compressor types being the core equipment. The comprehensive technical-economic ranking is: compression method > power generation > GTL.
Various types of compressors can be used in FGRS. The selection for a given operating condition depends on factors such as suction pressure, discharge pressure, gas molar flow rate, throughput, efficiency, and footprint. Understanding the working principles, advantages, and limitations of each types of compressor is essential for selecting the most suitable gas compressor for a flare gas recovery system.
Advantages: Reciprocating piston compressors are the most widely used and mature type for flare gas recovery. They offer high efficiency, high compression ratios, and high discharge pressure. Before the 1980s, reciprocating piston compressors dominated flare gas recovery in the global petrochemical industry.
Limitations: However, flare gas often contains solid and liquid components that accelerate wear on valves and piston rings. Reciprocating piston compressors are typically suitable for suction pressures above 400 kPa, whereas flare gas is usually low-pressure. To address this, roots blowers are often installed upstream to boost inlet pressure and reduce the impact of cylinder clearance. Additionally, reciprocating piston compressors have complex auxiliary systems, rapid temperature rise, and frequent maintenance requirements. Since the 1980s, their use in FGRS has gradually declined.
(Reciprocating Piston Compressor)
As reciprocating compressors became less common in FGRS, screw compressors gained popularity. Screw compressors, as rotary positive displacement machines, adapt well to gas composition variations, making them particularly suitable for FGRS. Based on lubrication methods, screw compressors are categorized into oil-injected and oil-free types.
Advantages: In oil-injected screw compressors, bearings and the rotor compression chamber are directly connected, allowing lubricant circulation without seals between them. Sliding valves enable flow regulation, achieving high pressure ratios and efficiency.
(oil injected screw compressor)
Defects: Since lubricant directly contacts the gas medium, impurities in flare gas can contaminate the lubricant, leading to degradation. Thus, lubricant replacement is frequent, increasing operating costs. Additionally, the cooling effect of lubricant limits rotational speed, restricting gas throughput.
Features: Unlike oil-injected screw compressors, oil-free screw compressors employ mechanical seals between bearings and the compression chamber, preventing lubricant contamination. This allows them to handle gas with impurities and achieve high discharge pressure. When equipped with VFDs, they enable dynamic flow regulation for energy savings.
Limitations: Despite their advantages, oil-free screw compressors have high construction costs, limited material options, high failure rates, and relatively low discharge pressure. High-speed operation generates more vibration and noise compared to liquid ring compressors. Heat generated during compression requires cooling via softened water injection, and multi-stage compression is needed for high discharge pressures.
Liquid ring compressors are currently the most widely used type in FGRS. They feature a cylinder with suction and discharge ports on both end covers and an eccentrically installed impeller.
Advantages:
Simple structure with only one rotating part (impeller), facilitating easy disassembly and maintenance.
Tolerant of dust, particles, and liquids in the gas.
Wide adaptability to gas composition and temperature.
Low speed, resulting in minimal vibration and noise.
Slow temperature rise due to heat absorption by the working liquid.
For acidic gases like H2S - containing flare gas, amine solutions can serve as the working liquid to absorb acidic components.
Limitations: Liquid ring compressors have low compression ratios, requiring multi-stage compression for high discharge pressures. Their efficiency is significantly lower than screw compressors, impacting lifecycle costs. Liquid ring compressors also require a water supply for the working liquid, auxiliary cooling systems, and lack speed adjustability.
(Liquid petroleum gas)
Advantages: Rotary vane compressors are rotary positive displacement compressors with a rotor, vanes, cylinder, and end covers. They feature compact footprints, low cost, insensitivity to inlet pressure fluctuations, higher efficiency than liquid ring compressors, minimal lubricant consumption, and easy field maintenance.
Limitations: Similar to liquid ring compressors, rotary vane compressors have limited discharge pressure capabilities. Reliability decreases with gas containing impurities or liquids, and maintenance frequency is high.
Integrally geared compressors are multi-shaft centrifugal compressors with a central bull gear driving multiple pinion gears, each equipped with two impellers. Different combinations of gear speeds and impeller sizes are possible. Their compact footprint, high efficiency, low mechanical vibration, and ability to handle inlet pressures as low as partial vacuum make them ideal for offshore platforms, often serving as air compressors or hydrocarbon recovery compressors.
Historically, offshore platforms flared tail gas, but recent designs collect and compress tail gas (including flare gas) for injection into other processes. Tail gas components typically have low molecular weights and require significant pressurization before injection into processes like gas lift or gas injection compressors. Integrally geared compressors are ideal solutions, especially when inlet temperatures are above the boiling point or in supercritical states.
Limitations: As multi-shaft machines, integrally geared compressors require mechanical seals for each impeller stage under flare gas conditions, increasing construction costs. Complex rotor-bearing dynamics raise manufacturing challenges, and numerous intercoolers demand large cooling water volumes. Additionally, these centrifugal compressors' performance becomes unreliable with wide gas composition ranges or entrained solids and liquids.
(centrifugal compressor)
Technical factors: Technical requirements include process conditions, flare gas composition, compressor efficiency, cooling medium availability, footprint, reliability, and more.
Non-technical factors: Non-technical considerations include familiarity with the types of compressors, delivery time, cost, operational maintenance needs, and spare parts. Compressor sizing must also account for performance during non-design conditions, including startup and shutdown.
Hybrid configurations: For example, Esso Australia's FGRS at the Longford Crude Stabilization and Gas Processing Plant uses a liquid ring compressor for the first stage and reciprocating piston compressors for the second and third stages to recover liquid phases from flare gas.
Compressor selection requires balancing technical feasibility and economic viability. Specific recommendations include:
Low-pressure gas with impurities: Prioritize liquid ring compressors.
High-pressure requirements: Opt for screw compressors or reciprocating compressors.
Acidic gases: Use liquid ring compressors with amine working fluid.
Offshore platforms: Choose integrally geared compressors to accommodate space constraints.
Through proper compressor sizing, FGRS can maximize environmental and economic benefits, aiding the oil and gas industry in achieving "zero flaring" targets.
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