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 energy terms, at ambient conditions, 1000 litres (1m3) of gas has energy content equivalent to 1 litre of oil. 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 as well as gas, there are instances when gas is either not produced (re-injected into reservoir) or flared, again owing to commercial considerations. Re-injection of gas into a reservoir is often a sunk cost and so is gas flaring which additionally contributes to environmental degradation. Technological developments in gas production coupled with advances in gas transportation methods are gradually lowering the costs and increasing 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, and so gas transmission costs get significantly reduced.
Until recently flaring was a frequent and unobjectionable practice in the upstream petroleum sector. In 2015 the World Bank took the global initiative of “Zero Routine Flaring by 2030” to end more than a century-old tradition of petroleum industry related to gas flaring at petroleum production sites. There are a growing number of governments, IOCs / NOCs and development institutions who 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 the conversion of flare gas to LNG through use of mini or small scale LNG liquefaction plants – modular (skid-mounted) LNG liquefaction plants. These skid-mounted LNG liquefaction plants can also be used for producing stranded gas reservoirs which are far from pipeline network to transport produced gas to consumption points conveniently. LNG thus produced through modular LNG liquefaction plants can be shipped conveniently via road tankers for regasification and supply to 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 are given the option for in-house commercialization of flare gas quantities through the establishment of a downstream subsidiary or through internal gas usage 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 appoint a qualified party (often selected through competitive bid process) for commercial utilization of such flare gas as well as for saving environmental degradation.
In addition to policy support initiatives, technology development and maturation of small scale LNG business 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.
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 2014 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 supplier and purchasers, highlighting the importance of small scale LNG business and the potential of modular LNG liquefaction plants.
Source: Triennium Work Report – SS LNG – IGU 2015
According to IGU-2015 report, key observed drivers for SSLNG developments including the use of flare gas to modular LNG liquefaction plants are:
Economics: energy cost advantage of LNG over alternative energy sources for end-users, including gas in the absence of pipeline infrastructure. An example is given in Figure below for the use of LNG as transport fuel compared to diesel.
Environmental: small scale LNG can bring attractive environmental benefits both to the gas production (preventing flaring) as well as end-customer use (LNG for transport/power & heating generation), compared to alternative fossil fuels. This includes 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.
Source: Triennium Work Report – SS LNG – IGU 2015
2 Skid Mounted LNG Liquefaction For Flare Gas: LNG LIQUEFACTION PROCESS
Standard land-based LNG liquefaction plants take raw gas directly from wellhead for gas processing and liquefaction to -162oC 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
Source: Petrowiki website
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 going up to 7.8 mtpa are operational in Qatar supplying LNG which is equivalent to natural gas supply of 1,120 MMSCFD (1.12 BSCFD) per train. These LNG trains are individual processing facilities with a combined capacity of all trains giving 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 adding 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 and 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).
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 plant for flare gas utilization must strike a balance among CAPEX, OPEX and revenue stream (including environmental benefits/incentives). 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 results in shareholders’ value maximization.
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 in 1-2 days.
Small LNG liquefaction plants – Capacity 60 – 800 tpd LNG production. Can handle 3 – 40 MMSCFD flare gas quantities. Can fill several standard 48,000 litres LNG road tankers in a day.
3.3 Gas Processing
A requirement of liquefaction is clean, dry gas which is free of contaminants as such 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 refrigeration process.
Separator / Scrubber Module
This is usually the first step in a processing plant whereby bulk separation of gas 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 which can corrode the pipelines and gas vessels used for gas storage and transportation. Additionally, H2S is also harmful to health and is necessarily required to be reduced to levels of less than 10 ppmv. Various technologies are commercially available for removal of H2S and CO2 from gas.
Amine sweetening is a proven technology for removing H2S and CO2 removal 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 absorber column in which the H2S and CO2 are absorbed in the amine. The regeneration section consists of a regeneration column with re-boiler for evaporation of contaminants mixed with an amine. Amine thus processed in regeneration section is re-injected into the absorber column.
Molecular sieve systems are multi-use modules for removing water, hydrocarbons, H2S and CO2 from gas. These modules handle gas that contains low concentrations of H2S and CO2.
Solid Bed Scavengers
This gas sweetening technology is very efficient and is capable of reducing 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.
These modules remove entrained and dissolved water from gas which is an essential requirement for liquefaction. Technological solutions include Glycol-based Dehydration and Molecular Sieve-based Dehydration.
Various commercially available glycols are used including di-ethylene glycol (DEG), tri-ethylene glycol (TEG) and tetra-ethylene glycol (TREG) for water dew-point control. 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 sieves based technology can remove water from gas as well as CO2 and Can produce gas streams having low water and hydrocarbon-dew points. Water dew points obtained by molecular technology are far lower than 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 (LTS) is widely used to remove both water and hydrocarbons from gas. This technology is also helpful in increase the methane number of gas for compatibility as fuel gas for LNG plant power generation. LTS technology is dependent on allowable pressure drop required for achieving Joule Thompson effect and is often supported by a chiller.
Silica Gel-based Dew Point Control
Silica is a solid desiccant used for removing both water and hydrocarbons from gas. Very low dew points can be obtained using solid desiccant technology – as low as -80°C, or even -100°C. For the regeneration mode, gas is either obtained directly from the process itself or an external source. Repeated cycles are performed using consecutive heating and cooling of saturated column
Fractionation technology is a conventional method used to remove light hydrocarbons, e.g. Liquefied Petroleum Gas (LPG) and (condensate) C5+ from the gas stream. This separation process is based on distillation using controlled heating and cooling, thus exploiting the difference in boiling points of the light hydrocarbons.
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:
Pre-cooling the treated gas to about -30 to -40oC
Liquefaction of pre-cooled gas to about -120 to -135oC
Sub-cooling of LNG to about -140 to -165oC
To optimize for space and weight for modular LNG liquefaction plant applications, different variations in refrigeration cycles are commercially used, as listed below:
Dual Mixed Refrigerant (DMR) Cycle replaces Propane-Precooling with Warm (high boiling point) Mixed Refrigerant in Coil Wound Heat Exchanger (CWHE) using counter-current flow for improving heat transfer performance.
Single Mixed Refrigerant (SMR) Cycle wherein one MR loop performs pre-cooling, liquefaction and sub-cooling. This reduces the number of equipment required but increases the size or numbers of CWHE.
N2 Recycle method is another development which can either be used with C3MR Cycle for sub-cooling or can be used standalone through 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 hence has significant use 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 provision for handling boil-off vapours. A requirement for dispatch is the transfer of LNG to road tankers through tank gauging applications. GIIGNL provides necessary guidelines for LNG transfer and measurement requirements.
4 Skid Mounted LNG Liquefaction For Flare Gas: KEY CHALLENGES
Cost: Main challenge modular LNG liquefaction plant relates to the costs due to the lack of economies of scale and expensive materials (cryogenic). Mainly for less developed markets, the absence of policies should be considered when developing a new modular LNG liquefaction plant project. In a country without previous experience in LNG, the developers should refer to and use the available international set of standards and guidelines. Time and experience are expected to offset these challenges as SSLNG market develops further.
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 a challenge 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 a complete supply chain development, from source (gas field) to end consumer or pipeline network. The challenge here is to operate and design all elements of such a supply chain effectively and competitively.