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1 Introduction & Background to Skid Mounted LNG Liquefaction for Flare Gas

Skid Mounted LNG Liquefaction For Flare Gas: Natural gas is the cleanest of all fossil fuels, but it is quite expensive to transport and store when compared to oil. In terms of energy, litres (1m3) of gas has an energy content equivalent to 1 litre of oil in ambient conditions. Thus to transport gas economically, it is either liquefied or compressed to increase energy content per unit volume. Consequently, “gas-only” exploration and production is limited to economically recoverable quantities where transport to consumption centres is feasible.

For associated fields, producing oil and gas, there are instances when gas is either not produced (re-injected into reservoir) or flared, owing to commercial considerations. Re-injection of gas into a reservoir is often a sunk cost, and so is gas flaring, which contributes to environmental degradation. Technological developments in gas production coupled with advances in gas transportation methods are gradually lowering the costs and increasing the contribution of this clean hydrocarbon fuel in the world economy. For example, when gas is converted to LNG, its volume is reduced to six-hundredths of the original volume, so gas transmission costs are significantly reduced.

Until recently, flaring was a frequent and unobjectionable practice in the upstream petroleum sector. In , the World Bank took the global initiative[1] of “Zero Routine Flaring by ” to end more than a century-old tradition of petroleum industry related to gas flaring at petroleum production sites. A growing number of governments, IOCs / NOCs and development institutions recognize and participate in the on-ground realization of this initiative.

Given previous, commercial interest in implementing solutions for flare gas utilization has significantly improved, with a range of technology applications. One of the most promising options is converting flare gas to LNG through mini or small scale LNG liquefaction plants – modular (skid-mounted) LNG liquefaction plants. These skid-mounted LNG liquefaction plants can also be used to produce stranded gas reservoirs far from the pipeline network, conveniently transporting produced gas to consumption points. LNG, thus produced through modular LNG liquefaction plants, can be shipped conveniently via road tankers for regasification and supplied to the consumer location or a pipeline network.

1.1 Enabling Environment

Various governments around the world are in the process of putting in place legal frameworks and policy initiatives/guidelines to incentivize flare gas utilization through the use of skid-mounted LNG liquefaction plants in the following forms:

• E&P companies have the option of commercializing flare gas quantities in-house through the establishment of a downstream subsidiary or by using internal gas to meet processing plant requirements.
• The government takes natural gas produced (free of cost and free from all encumbrances) at the flare gas site location from Petroleum producers (E&P companies) and appoints a qualified party (often selected through a competitive bid process) to commercially utilize such flare gas and save environmental degradation.

In addition to policy support initiatives, technology development and maturation of small-scale LNG businesses are key enablers for flare gas utilization through skid-mounted LNG liquefaction plants. More efficient and cost-effective small-scale liquefaction processes are commercially developed with supporting developments in LNG road tankers for LNG transport and cheap availability of modular regas technologies.

1.2 Skid Mounted LNG Liquefaction for Flare Gas: LNG Market Trend

The LNG market that started as a secretive wholesale market (managed through long-term supply contracts) has been opening up to short-term (5-year supply contracts), spot purchases and cargo re-directing options. Small Scale LNG (SS LNG) operations, including skid-mounted LNG liquefaction plants, have been identified during IGU-Conference and have now been actively pursued. The recent decline in energy supplies due to COVID-19 also adversely affected long-term supply contracts between LNG suppliers and purchasers, highlighting the importance of small-scale LNG business and the potential of modular LNG liquefaction plants.

According to IGU- report, key observed drivers for SSLNG developments including the use of flare gas to modular LNG liquefaction plants are:

• Economics: LNG offers an energy cost advantage over alternative energy sources for end-users, including gas, in the absence of pipeline infrastructure. The Figure below gives an example of the use of LNG as transport fuel compared to diesel.
• Environmental: Small-scale LNG can bring attractive environmental benefits compared to alternative fossil fuels, both in terms of gas production (preventing flaring) and end-customer use (LNG for transport/power & heating generation). These include CO2, SOx, NOx, particles, and noise emissions.
• Governmental decisions to increase the level of energy independence for a country or region by developing an alternative energy supply.

2 Skid Mounted LNG Liquefaction for Flare Gas: LNG Liquefaction  Process

Standard land-based LNG liquefaction plants take raw gas directly from the wellhead for gas processing and liquefaction to -162°C temperature for convenient storage/transportation in low pressure (50 – 100 PSIG) vacuum-insulated road tankers specially designed to preserve low temperature for weeks. Such plants have the following process schematic:

2.1 LNG Liquefaction Process Schematic

Modern land-based LNG liquefaction plants are usually built as multiple trains with a capacity of an individual train in the range of 3 – 5 mtpa (million tons per annum). Larger LNG trains up to 7.8 mtpa are operational in Qatar, supplying LNG equivalent to a natural gas supply of 1,120 MMSCFD[1] (1.12 BSCFD) per train. These LNG trains are individual processing facilities, with a combined capacity of all trains giving the liquefaction capacity of the plant.

3 Skid Mounted Flare Gas to LNG Liquefaction Plants

The traditional practice of flare gas usage through re-compression for re-injection use related to enhanced oil recovery adds cost without any tangible benefit. Water injection, steam injection and down-hole steam generation technologies are the right applications for enhanced or tertiary oil recovery. Profitable use of flare gas is its use as an energy source, which can be achieved through careful analysis of location, flare gas composition, and available infrastructure. Development of modular LNG liquefaction plants has opened new avenues for generating additional revenue from existing reservoirs. Studies have indicated that substantive quantities of currently flared gas resources can be converted to valuable LNG with a payback period as low as five years (actual payback period is required to be calculated for each location as it significantly varies with a calorific value of flared gas).

3.1 Commercial Considerations – Skid-Mounted LNG Liquefaction Plants

Modular / Skid-mounted LNG liquefaction plants are operated on the premise that flare gas (or stranded gas where applicable) is a resource that should be put to use for profitable company operations. Commercially viable modular LNG liquefaction plants for flare gas utilization must balance CAPEX, OPEX, and the revenue stream (including environmental benefits/incentives). The following considerations are essential:

• Simple energy input requirements – utilizing available gas stream for power generation and other energy inputs
• Efficient power consumption (kWh / kg of LNG) for reduced OPEX
• Commercially proven efficient processing and refrigeration technology for reduced CAPEX
• Plug and play philosophy incorporated for the deployment of individual modules, control systems and power generation
• Designed for unmanned operation and easy-to-handle LNG export mechanism
• Easy containerized shipment to facilitate the relocation

Relocation and redeployment (with minimal modifications) are key commercial benefits offered by modular LNG liquefaction plants. In this way, the CAPEX requirement gets diluted over geographically scattered assets and maximises shareholder value.

3.2 Design Considerations – Skid-Mounted LNG Liquefaction Plants

Modular / Skid-mounted LNG liquefaction plants are designed to meet the criteria of commercial viability, compactness and lighter weight. For varying requirements, they are classified as:

• Mini LNG liquefaction plants – Capacity 10 – 25 tons per day (tpd) LNG production. Can handle 500 – 1,300 MSCFD flare gas quantities. Can fill standard 48,000 litres LNG road tanker[2] in 1-2 days.
• Small LNG liquefaction plants – Capacity 60 – 800 tpd LNG production. Can handle 3 – 40 MMSCFD flare gas quantities. It can fill several standard 48,000 litre LNG road tankers in a day.

3.3 Gas Processing

A requirement of liquefaction is clean, dry gas that is free of contaminants. Accordingly, processing modules are required along with liquefaction, power generation, and control modules depending on the composition of flare gas. These modules act as mini processing plants for water separation, acid gas removal, water dew-point control, heavy hydrocarbon removal, hydrocarbon dew-point control, and delivering gas that is ready for the refrigeration process.

• Separator / Scrubber Module

This is usually the first step in a processing plant whereby bulk gas separation is achieved from oil and water. This module is usually not required for flare gas applications and may be required for stranded gas applications where raw gas is directly coming from wells.

• Acid Gas Removal (Sweetening) Module

H2S and CO2 are acid gases that can corrode the pipelines and gas vessels used for gas storage and transportation. Additionally, H2S is harmful to health and must be reduced to levels of less than 10 ppmv. Various technologies are commercially available for removing H2S and CO2 from gas.

Amines

Amine sweetening is a proven technology for removing H2S and CO2 from sour gas. Commonly used amines are mono-ethanol-amine (MEA), di-ethanol-amine (DEA), and methyl-di-ethanol-amine (MDEA). Amine systems consist of an absorber column in which the H2S and CO2 are absorbed in the amine. The regeneration section consists of a regeneration column with a re-boiler for evaporation of contaminants mixed with an amine. Amine thus processed in the regeneration section is re-injected into the absorber column.

Molecular Sieves

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Molecular sieve systems are multi-use modules for removing water, hydrocarbons, H2S, and CO2 from gas. These modules handle gas with low concentrations of H2S and CO2.

Solid Bed Scavengers

This gas sweetening technology is very efficient and can reduce the H2S content to ultra-low levels. Solid bed scavenger technology efficiently removes H2S from gas by chemically bonding it to non-regenerable adsorbent granules.

• Dehydration Module

These modules remove entrained and dissolved water from the gas, an essential liquefaction requirement. Technological solutions include Glycol-based Dehydration and Molecular Sieve-based Dehydration.

Glycol-Based Dehydration

Various commercially available glycols, including di-ethylene glycol (DEG), tri-ethylene glycol (TEG), and tetra-ethylene glycol (TREG), are used to control water dew point. Wet gas is brought into contact with glycol in the contact column.
After absorbing the water from the gas stream, the glycol is regenerated for reuse in a closed-loop system.

Molecular Sieve-Based Dehydration

Molecular sieve-based technology can remove water from gas as well as CO2 and produce gas streams with low water and hydrocarbon dew points. Water dew points obtained by molecular technology are far lower than those achieved with absorption (glycol) technology—as low as -80°C or even -100°C.

• Hydrocarbon Dew Point Control (heavy hydrocarbon removal) Module

Strict hydrocarbon dew-point control is required for LNG liquefaction applications. Depending on feed gas composition, Low-Temperature Separation, Silica Gel-based Dew Point Control or a combination of both technologies can be used in the same module.

Low-Temperature Separation

Low-Temperature Separation (LTS) is widely used to remove both water and hydrocarbons from gas. This technology is also helpful in increasing the methane number of gas for compatibility as fuel gas for LNG plant power generation. LTS technology depends on the allowable pressure drop required to achieve the Joule Thompson effect and is often supported by a chiller.

Silica Gel-based Dew Point Control

Silica is a solid desiccant that removes water and hydrocarbons from gas. Solid desiccant technology can achieve very low dew points—as low as -80°C or even -100°C. In the regeneration mode, gas is obtained directly from the process or an external source. Repeated cycles are performed using consecutive heating and cooling of the saturated column.

Fractionation

Fractionation technology is a conventional method for removing light hydrocarbons, such as Liquefied Petroleum Gas (LPG) and (condensate) C5+, from the gas stream. This separation process is based on distillation using controlled heating and cooling, which exploits the difference in boiling points of the light hydrocarbons.

3.4 Liquefaction

Traditionally, land-based LNG liquefaction plants usually deploy a varying combination of Propane-Precooled Mix Refrigerant (C3MR) Cycle with Nitrogen recycle expansion process sub-cooling to produce LNG. Liquefaction has three basic steps:

1. Pre-cooling the treated gas to about -30 to -40 °C
2. Liquefaction of pre-cooled gas to about -120 to -135 °C
3. Sub-cooling of LNG to about -140 to -165 °C

To optimize for space and weight for modular LNG liquefaction plant applications, different variations in refrigeration cycles are commercially used, as listed below:

• The Dual Mixed Refrigerant (DMR) Cycle replaces Propane Precooling with Warm (high boiling point) Mixed Refrigerant in a Coil Wound Heat Exchanger (CWHE) using counter-current flow to improve heat transfer performance.
• A Single Mixed Refrigerant (SMR) Cycle occurs when one MR loop performs pre-cooling, liquefaction, and sub-cooling. This reduces the number of pieces of equipment required but increases the size or number of CWHE.
• The N2 Recycle method is another development that can either be used with the C3MR Cycle for sub-cooling or standalone by bleeding Nitrogen at different expansion levels for use in pre-cooling, liquefaction, and sub-cooling.

Note: Under normal atmospheric pressure, Nitrogen exists as a liquid between the temperatures of -210°C and -196°C, and hence, it is significant in LNG production. Additionally, Nitrogen is abundantly available in air and can be separated through proven Nitrogen Recycle Expander technology.

3.5 LNG Storage and Loading Module

LNG is stored at the production site in ISO vacuum-insulated tanks with provisions for handling boil-off vapours. A dispatch requirement is the transfer of LNG to road tankers through tank gauging applications. GIIGNL[3] provides necessary guidelines for LNG transfer and measurement requirements.

4 Skid Mounted LNG Liquefaction for Flare Gas: Key Challenges

Operating modular LNG liquefaction plants has unique challenges that must be carefully overcome for successful operations. These were identified in the IGU Triennial Report published in and reproduced below:

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• Cost: The Main challenge of modular LNG liquefaction plants relates to the costs due to the lack of economies of scale and expensive materials (cryogenic). The absence of policies should be considered when developing a new modular LNG liquefaction plant project, mainly for less developed markets. In a country without experience in LNG, the developers should refer to and use the international set of standards and guidelines. Time and experience are expected to offset these challenges as the SSLNG market develops.
• Fit-for-purpose engineering: Modular LNG liquefaction plants have attracted big and smaller players to the market. For the larger players, an observed challenge is to develop cost-effectively and fit-for-purpose technological solutions, while not compromising company and safety standards.
• Safety: For new players entering the modular LNG liquefaction market, maintaining a safe and reliable operation can be challenging when lacking LNG experience. Additionally, modular LNG liquefaction plant operation is a part of SSLNG network. It involves many parties and smaller parcel sizes, requiring a framework of standards and guidelines to maintain the current safety level in the industry.
• Modular LNG liquefaction opportunities can be more feasible and sustainable with the development of a complete supply chain, from source (gas field) to end consumer or pipeline network. The challenge is to operate and design all elements of such a supply chain effectively and competitively.