Optimising the LNG process John Baguley, Liquefied Natural Gas Ltd, Australia, outlines the benefits of an innovative liquefaction process technology for mid scale LNG projects. The rapidly expanding global LNG industry continues to face challenging market conditions, especially with regards to CAPEX commitments. These challenges call for creative solutions to LNG project designs and delivery methods, to best enable LNG to maintain its position as a preferred fuel to meet the world s growing energy demand. Flexible, efficient mid scale liquefaction facilities are emerging as a means to effectively address the ongoing evolution in the energy market. This article highlights the Optimized Single Mixed Refrigerant (OSMR ) liquefaction process technology, which has been developed by Liquefied Natural Gas Ltd (LNG Ltd). The liquefaction process technology aims to deliver LNG efficiently and cost-effectively by using a combination of several well-proven, existing industrial technologies into one integrated system. This article summarises the process and the company s key projects where the technology is planned for use. Projects The technology s initial design application has targeted single LNG train capacities of 2 million tpy, while studies have demonstrated that it is economically scalable to single LNG train capacities ranging from 0.5 million tpy to 3 million tpy or more. Multiple LNG trains can be combined to produce plants of over 10 million tpy capacity. This mid scale approach enables developers to access the LNG market gradually, increasing LNG capacity in moderate steps with the installation of additional duplicate modular trains as available feed, markets or financial capacity grows.
The required plot space is less than that needed for traditional LNG plants and the execution strategy, using standard equipment and modular construction, can help to reduce construction cost and schedule risk. Currently, three LNG projects (each 100% owned by LNG Ltd) are proposing to utilise the liquefaction process technology: Magnolia LNG (MLNG) project in Lake Charles, Louisiana, US. Bear Head LNG project in Richmond County, Nova Scotia, Canada. Fisherman s Landing LNG project in Gladstone, Queensland, Australia. The US Federal Energy Regulatory Commission (FERC) released its Final Environmental Impact Statement (FEIS) for the MLNG project in November 2015. In the same month, MLNG executed a legally binding lump sum turnkey (LSTK) engineering, procurement and construction (EPC) contract for the project with the KSJV, a contractor joint venture (JV) between KBR Inc. and SK E&C Group (SKEC). CAPEX and OPEX benefits MLNG has achieved a LSTK EPC contract price in the range of US$495/t US$544/t (subject to final design capacity at FID). The design also delivers LNG plant fuel gas consumption at a guaranteed value of 8%, representing a 92% feed gas guaranteed production efficiency. Actual fuel gas consumption is expected to be in the range of only 6% during operation (94% production efficiency). The execution of the EPC contract for MLNG represents a critical milestone for the project. Importantly, this also reaffirms LNG Ltd s view that the business model of mid scale, modular-based LNG trains of nominally 2 million tpy using its liquefaction process technology is robust, delivering significant CAPEX and OPEX savings. The benefits of incorporating this process technology and modular construction approach are outlined in detail below. Efficiency The company s current projects have a train design capacity of approximately 2 million tpy each, configured in a two-in-one parallel design, in which there are two identical cold box exchanger units per train, each chilled by an independent closed loop mixed refrigerant (MR) system. Each MR loop has a dedicated gas turbine driven MR compressor. Having two parallel MR circuits within each LNG train provides an efficient turndown capability down to approximately 40% of design capacity per train. This design feature offers tollers (LNG purchasers) and plant operators flexibility to operate the LNG trains over a wide range of demand. Plant reliability also improves, as an LNG train can continue to operate at over 50% capacity when one MR circuit has tripped or is out for planned maintenance. Ambient air temperature directly affects LNG production in traditional LNG plants. The higher the ambient conditions, the lower the gas turbine power and, therefore, the lower the LNG production. Consistent gas turbine power over a range of ambient conditions can be achieved by pre-chilling the inlet air to the gas turbines. This inlet air chilling is proven and common in gas turbine power stations. The liquefaction process technology aims to maximise the energy efficiency of the LNG trains. Ammonia refrigerant precooling of the MR ahead of the cold box increases plant capacity, as well as efficiency. The impact of ammonia precooling on plant capacity, and the fact that it consumes little additional fuel, is fundamental to the overall energy balance of the process. A combined cycle steam system using the gas turbine waste heat to generate steam largely powers the ammonia precooling system. Ammonia precooling increases the LNG plant capacity without increasing the size and cost of the major components of the cryogenic liquefaction plant (cold box, gas turbine and MR compressor). These two simple enhancements cooling gas turbine inlet air and precooling the MR, coupled with the selection of ammonia as a precooling refrigerant contribute towards the reduction in plant cost per unit of LNG produced, and also enhance the overall plant efficiency. Using proven technology A liquefaction train using OSMR process technology incorporates a low equipment count and a simple configuration. The equipment used does not require significant unique specifications and is readily available in the marketplace from multiple vendors, reducing long lead times and allowing for competition throughout the procurement process. The following components, applied and proven in LNG and other industries, comprise the core liquefaction process: Single mixed refrigerant (SMR) liquefaction, using plate-fin cold box liquefaction units. Ammonia as a precooling refrigerant, as it has excellent refrigeration properties, allowing for smaller condensers, exchangers, piping and general plant size. Gas turbine waste heat steam generation (combined cycle process) providing motive power to the ammonia refrigeration system. This is achieved by utilising a highly efficient once-through-steam-generator (OTSG) harnessing the waste heat from the turbine exhaust. The closed loop ammonia refrigeration circuit itself, driven by steam recovered from waste heat (above), precools the MR and directly cools inlet air to the gas turbines. The ammonia also provides additional and efficient process cooling within the feed gas treatment systems. Efficient and reliable gas turbines selected for the MR compressor mechanical drive that serves the MR circuit. Inlet air chilling to the gas turbines to ensure a consistent power output and to avoid significant power loss at high ambient conditions. Integration of these components in the process technology enables high overall performance levels. The liquefaction process technology s patents include a boil-off gas (BOG) handling system, in which the BOG is lightly compressed, reliquefied by passing it through the cold box and then into the liquid methane separator. Flash gas separation precedes liquid methane delivery to the LNG storage tank via the LNG rundown line. The lean vapour phase flash gas from the liquid methane separator, containing a high proportion of nitrogen and some methane, provides low pressure fuel gas in the steam plant auxiliary boiler. Together, this system enables recovery and LNGINDUSTRY REPRINTED FROM APRIL 2016
reliquefaction of the low temperature BOG, while minimising compression losses. A modular construction approach allows repeatability with respect to the liquefaction trains. Use of a modular fabrication approach translates into inherently safer construction sites and reduced on-site labour and associated temporary facilities, while providing a high degree of quality and schedule control. Figure 1 illustrates the component capital cost savings from applying the company s liquefaction process design and modular construction. approach allows repeatability with respect to the liquefaction trains. Delivery of the process modules to the site in a pre-determined sequence allows optimised assembly of the LNG train, as shown in Figure 3. A material offloading facility (MOF) and a heavy haul road enable transfer of the modules from the barge to the final position, using self-propelled modular transporters (SPMTs). Why ammonia? The selection of ammonia as the precooling refrigerant is a significant element of this process technology. Ammonia is a commonly used, environmentally friendly, naturally occuring and efficient industrial refrigerant. It has a lifecycle in the atmosphere of less than one week and, therefore, has a global warming potential (GWP) and an ozone depletion potential (ODP) of zero. When compared to commonly used propane precoolant, the use of ammonia in refrigeration cycles demonstrates excellent thermodynamic qualities, resulting in excellent efficiency and, therefore, reduced emissions from the power generation required for refrigeration. Compared to propane, anhydrous ammonia vapour is harmful in relatively low concentrations. Despite this important consideration, which must be managed effectively, ammonia has the following characteristics: It is not readily flammable or explosive. It has a molecular weight of 17.0 and is lighter than air (molecular weight of 29), so it tends to rise and naturally dissipate. High volumetric air flow from the air coolers within the company s liquefaction trains assists in the dissipation of any ammonia vapour present. Ammonia releases can be readily detectable at relatively low concentrations. Importantly, mitigation of ammonia release exposure is reliable and effective through simple application of automated detection systems, automatically isolatable sections, and water sprays due to ammonia s high affinity and solubility in water. Modular construction The liquefaction process technology s compact mid scale design configuration allows each LNG train to be broken down into only five main process modules, as shown in Figure 2. Offsite fabrication of the modules in a specialty fabrication yard, with transport to the project site via ocean-going barge or specialty carrier, represents the construction base case strategy. This modular approach is safe and reduces onsite labour, whilst providing a high degree of quality and schedule control during module construction. The modular construction Figure 1. Optimized Single Mixed Refrigerant (OSMR ) CAPEX improvements chart. Figure 2. Schematic of modules. Figure 3. Schematic of assembled modules. REPRINTED FROM APRIL 2016 LNGINDUSTRY
detailed analysis of the range of components in the feed gas. Coal bed methane (CBM) feedstock, for example, does not contain heavy hydrocarbons and, therefore, a unit is not required. Careful consideration of the range of possible heavy hydrocarbon contaminants is necessary for locations using pipeline specification gas. The design may include a turboexpander as well as a scrub column. The auxiliary steam boiler burns any resultant heavy liquids, which, alternatively, may be exported for sales. Figure 4. Schematic of OSMR liquefaction process technology. The process Figure 4 shows a schematic representation of the liquefaction process technology. Pretreatment Feed gas routes from the gas gate station to each LNG train and initially passes through an inlet filter coalescer to separate any liquids/solids to prevent foaming in the acid gas removal unit (AGRU). Removal of acid gases (CO 2 and H 2 S) using a proprietary amine solution occurs in an absorber column. Removal of CO 2 to approximately 50 ppm avoids freezing in the downstream liquefaction unit. An ammonia refrigeration system precools the water-saturated sweet gas exiting the absorber column, which passes through a knockout separator to remove bulk water from the gas. The condensed water and trace amounts of amine are recycled to the amine system as make-up water. Depending upon the design of the dehydration unit, the gas recycled back to the process will be minimised, or even eliminated. The dry gas stream also accommodates any shortfall in fuel gas. A mercury removal unit, provided after the molecular sieve dust filters (typical position), removes any trace mercury in the gas prior to entering the liquefaction unit. A treated gas filter downstream of the mercury removal unit is also in place to capture any loose dust particles from the mercury removal system. Heavy hydrocarbon removal plant The final pretreatment unit involves removal of heavy hydrocarbons, such as pentanes and benzene. Benzene must be removed to avoid freezing in the liquefaction plant. The heavy hydrocarbon removal plant design is dependent upon a Ammonia refrigeration plant The ammonia refrigeration is used to precool the dry feed gas to approximately 18 F (-8 C) prior to entering the liquefaction plant. The ammonia system is comprised of a single or two-stage closed loop refrigeration cycle (depending on plant capacity), utilising two parallel steam turbine driven compressors powered by steam from the waste heat recovery plant. The ammonia refrigeration improves the output and efficiency of the SMR process. It also provides stable operation of the plant, since it dampens the impact of variations in ambient air temperatures on the MR gas turbines, which would otherwise greatly affect plant operation and capacity. Optimising the temperature of the ammonia refrigerant tunes overall performance of the plant consistent with the operating environment and plant capacity. The ammonia refrigerant cools a number of units around the LNG train, including the following: Precooling dry feed gas prior to entering the liquefaction unit. MR in the ammonia/mr precooler. Inlet air for the gas turbines. Wet gas exiting the amine contactor. Cooling requirements, when necessary, within the heavy hydrocarbon liquid removal system. Liquefaction plant The liquefaction plant cools and liquefies the feed gas from approximately 18 F exiting the ammonia precoolers to -260 F (-162 C). The liquefaction process comprises a single-stage high pressure vapour compression cycle using a mix of refrigerants, providing a close fit of cooling curves in the main plate-fin heat exchanger (cold box). The main liquefaction exchanger is a multi-core brazed aluminium plate-fin exchanger using a minimal number of exchanger streams. Enhancement of main exchanger performance results from the ammonia precooling refrigerant, which cools the MR in the ammonia/mr precooler as noted previously. This allows cooler low pressure mixed refrigerant (LPMR) to exit the cold box. The cooler LPMR feeding the MR compressor improves its performance. Within each train, two separate independent parallel refrigeration circuits each include an MR compressor, MR air cooler, ammonia/mr precooler, cold box, and suction scrubber. The dual parallel refrigeration/liquefaction circuits provide added reliability and availability, while allowing use of commonly available equipment sizes. The high pressure precooled MR is further cooled in a cold box pass, and is then flashed to low pressure across a Joule-Thomson expansion valve, producing a very cold two-phase liquid-vapour refrigerant. A liquid knockout separator is used to provide consistent remixing of the two-phase MR refrigerant stream, which is then fed back into the cold box to liquefy the feed gas. The precooled dry feed gas itself splits into two streams, LNGINDUSTRY REPRINTED FROM APRIL 2016
feeding the two cold boxes in parallel and exits the cold box as LNG. This liquefied gas is then flashed to low pressure to achieve the final cryogenic temperature of -260 F (-162 C) as it flows into the storage tanks. The MR for each cold box is compressed by a single-stage centrifugal compressor directly driven by a highly fuel efficient, low emissions gas turbine. Air coolers remove the heat of compression from the MR prior to ammonia precooling, while inlet air to the gas turbines is cooled to approximately 44 F (7 C) using an ammonia-to-air exchanger, to increase the power output and efficiency of the gas turbine, particularly at high ambient temperature conditions. BOG system The BOG system for a 4 x 2 million tpy LNG plant would typically comprise three to five low pressure gas compressors to recover flash gas, BOG and ship vapour from the LNG tank, and a simple reliquefaction and nitrogen rejection system to ensure the required LNG composition is met. The compressed BOG vapour is reliquefied in the cold box and sent to the liquid methane separator, where it is separated with the liquid methane stream returning to the LNG storage tank. The lean vapour flash gas from the liquid methane separator, containing a high proportion of nitrogen and some methane, is used as lean low pressure fuel gas in the Waste Heat Recovery (WHR) and steam plant auxiliary boiler. Normally, only one BOG compressor is used to handle BOG from one LNG train. However, during ship loading, additional BOG compressors are used to recover the additional BOG generated. WHR and steam plant The WHR and steam plant is comprised of the following: WHR from the two gas turbines using OTSGs. Two steam turbines for the auxiliary refrigeration plant (ammonia) compressor drives. An auxiliary boiler for start-up and supplemental control steam. Process and utility steam heating system. Air cooled condensers. All associated systems required for a WHR and steam plant. Ammonia compression power and heat for the plant is provided by waste heat from the gas turbine exhausts as well as from the auxiliary boiler, which is fuelled by three sources: feed gas in the plant; lean flash gas from the methane separator in the BOG system; and heavy hydrocarbon waste stream. The high pressure steam powers the two ammonia refrigeration steam turbines. A portion of this steam is attemperated and used as low pressure process heat for the amine reboiler, fuel gas heater and other process duties. The ammonia auxiliary refrigeration plant is sized to consume all available power that can be generated from the waste heat and lean flash gas. During ship loading, generation of additional BOG occurs, thereby producing additional low pressure fuel gas, which reduces the use of feed gas used in the auxiliary boiler. This alleviates the need for flaring of BOG during ship loading. Conclusion The efficiency of LNG Ltd s liquefaction process technology results from integrating proven, highly efficient gas turbine drives for the main refrigerant compressors with combined cycle heat and power technology, and efficient ammonia refrigeration. Gas turbine inlet air cooling and low pressure BOG reliquefaction are employed. The innovative configuration of these proven technologies results in competitive capital and process efficiencies. The compact modular design strategy and mid scale production targets are also features of the process technology. REPRINTED FROM APRIL 2016 LNGINDUSTRY