Developing small-scale LNG plants

With the perpetual advancement of technology, traditional fuel sources for both domestic and industrial uses are evolving around the world. LNG can be produced and distributed quickly and easily. As a result, Brazil, China, the United States and Australia are utilising this technology to meet their increasing fuel needs.

A diversification of fuel sources is taking place across the globe. One of the key resources is natural gas, which can be used to fuel vehicles, as an energy source for household and industrial uses, and for power generation. A key method of supplying this gas is through the use of LNG. LNG can be produced at remote locations and distributed to end-users quickly and easily. Brazil and China have been increasingly utilising LNG to meet basic fuel needs. Australia also uses LNG as a portable fuel.

Brazil: leaders in LNG technology

Brazil has been a world leader in the transformation of its vehicle fuel infrastructure from traditional petroleum fuels to ethanol and other alternative sources. LNG fits into this transformation by providing a clean fuel that can be produced from a variety of gas feed streams from both natural gas reservoirs and other sources, such as refinery streams. The first of the LNG fuel plants is now operational and is the prototype for future expansions of this fuel source.

LNG is used for basic energy supply, either for distribution or as a clean vehicle fuel. The facility in Figure 1 is such a facility. Built by White Martins in Brazil, this facility uses Black & Veatch’s (B&V’s) PRICO LNG technology to produce 14 million cubic feet per day (MMcf/d) of LNG for use as vehicle fuel. The LNG is stored in a 6,000 cubic metre tank shown in Figure 2 which provides about 9 days’ storage.

The White Martins facility is designed to handle six different feedstocks of varying pressure and richness to produce an LNG product for vehicle use. This plant started operations in 2006 and has been steadily increasing the LNG market with this first train. Construction of a second train at this site is being considered.

LNG supply facilities

Distributed LNG supply facilities tend to be small in scale. In facilities in the United States, the LNG produced from a liquefier is stored in a large LNG tank for use in peak demand periods. When additional gas supply is needed, such as during winter, the LNG is pumped from storage, vaporised and then sent to customers.

Typical plants in the US have liquefaction capacities of 5-20 MMcf/d and are intended to fill a large LNG tank in 150-200 days. These plants are set up to send out large volumes of gas at a moment’s notice. One such plant is the Alabama Gas Pinson Plant. Using the PRICO LNG technology from B&V, this liquefaction plant has a capacity of 12 MMcf/d. Storage of up to 1.2 Bcf is used to hold the LNG until it is ready for use in the winter heating season.

Technology selection

Existing LNG production facilities in the small-scale area employ a variety of LNG production technologies. These older plants are either:

  • Cascade refrigeration
  • Expander
  • Single mixed refrigerant (SMR)
  • Nitrogen refrigeration
  • Other multiple refrigerant schemes.

After the initial developments in the 1960-70s, the predominant technology has been the mixed refrigerant system with over 60 per cent of the installations. Modern LNG production facilities are either expander, nitrogen refrigeration or SMR designs. The cascade and multiple loop systems have proven to be too complex and too costly to operate.

Expander

The expander process (see Figure 3) uses the feed gas pressure expanding to a lower pressure to drive the liquefaction process. In many instances, plants are located where the natural gas is sourced from a high pressure main line and is to be delivered to a local distribution system. In these cases, the liquefaction of a portion of the feed gas can be accomplished with little or no compression or mechanical refrigeration.

A typical facility would liquefy approximately 15-20 per cent of the feed gas with the balance going to the downstream system. These expander plants have been used frequently, where the pressure drop is available. If the tail gas could not be dumped to a low-pressure system, this type of process would not be considered. Recompression of the tail gas would result in a high cost and low efficiency process.

Nitrogen refrigeration

The nitrogen refrigeration process (see Figure 4) has been used on a limited basis for liquefaction, especially on smaller systems. The process is similar to the expander process in that it uses a vapour refrigeration stream with an additional large compressor for nitrogen circulation. Since the process relies on nitrogen vapour for condensing, the refrigeration flow is quite large and the system will be significantly larger than a mixed refrigerant system. The amount of power needed to run this system is much larger than other technologies.

The SMR process (see Figure 5) is normally the lowest cost design for small-scale liquefaction systems. Also, unless free pressure drop is available for the expander process, the mixed refrigerant system will have the lowest power requirement. B&V’s PRICO technology is such a mixed refrigerant process and it has now been applied in 20 facilities around the world. This technology, in contrast to the others discussed, has only a single compression system for the refrigeration. The main exchanger is a simple plate-fin unit with a minimal number of connections, designed to offer a liquefaction system that is easy to operate. During a shutdown the refrigerant inventory is maintained in the system so that no venting or pressure relieving is needed.

SMR

Improvements in the SMR process by B&V over the years have resulted in a 25-35 per cent reduction in power compared to older facilities. Generally, these processes require about 260-370 kilowatts (kW) per MMcf/d of LNG capacity. The exact value depends on the system design parameters, such as feed gas pressure, ambient conditions, and process specifications. Besides the refrigerant compressor, the main exchanger is the key piece of equipment in the liquefaction system.

The main exchanger is an aluminum plate-fin core or cores in a carbon steel box. The box is filled with perlite for insulation. All connections are external to the box, eliminating any leak potential inside the box. Modern SMR plants have much more efficient main exchanger designs.

Facility developments in China

China is a large country with vast underdeveloped areas and large population centres that lack clean, reliable energy supplies. Electricity may be available, but only to light the home at night. In many remote areas of China, a propane tank is often stored inside living quarters for cooking and heating purposes. While gas is available from small and newly developed fields in many areas, the infrastructure required to distribute the gas is non-existent. The use of LNG for distribution and supply of gas to people in China is one method for expanding the use of clean fuel.

LNG production facilities in China are based on the same liquefaction technology as other facilities. However, the LNG product is handled differently. The portability of LNG makes it an ideal fuel to be supplied from a single point to multiple customers. In addition, users can access supply from a variety of sources. The model for the developments in China is a central plant for distribution of LNG to multiple gas users in industrial and residential markets.

B&V and partner Chemtex International are developing eight facilities in China. Through this partnership, LNG supply solutions can be provided across a geographically diverse area. The eight facilities under development are spread out across China as far as the Xinjiang province in far western China.

These facilities use the same PRICO process but vary in both capacity and in the driver for the main refrigeration compressor. The selection of the driver is dependent on the economics of gas versus electricity supply, as well as the availability of reliable electrical supply. In the case of the Xinjiang project, steam for the turbine drivers is provided from an upstream coal gasification plant.

The Erdos and China National Offshore Oil Corporation (CNOOC) plants came on stream near the end of 2008. Erdos uses an electric motor for the main refrigeration compressor.
Figure 6 shows the fleet of LNG trucks being used to transport the LNG from this plant. Up to 33 trucks per day can be loaded at this facility. The Erdos plant is located in remote Inner Mongolia, and many of the trucks travel a significant distance with their cargoes.

CNOOC’s Zhuhai facility is a gas turbine driven unit located on the coast near Hong Kong. Both of these plants include a complete process for gas treating, dehydration, liquefaction and boil off gas handling. These facilities came on stream in record time and passed their performance in less than one month following gas introduction. A third plant, Dazhou began operation on 3 July 2010.

LNG feedstocks

Most of the feedstocks for the various facilities developed by B&V and Chemtex rely on feed gas from traditional sources such as gas fields and pipeline gas. However, a new type of facility is being developed, which is based on LNG production from coal gasification. The Xinjiang project is designed to take 10.9 x 106 cubic metres per day of feed gas from the gasification unit to produce 1,214 tonnes per day (t/d) of LNG product and produce 4,040 t/d of carbon dioxide or hydrogen gas for downstream methanol production. The process for this is similar to other liquefiers, but requires an additional step to separate the products. This plant will be a blueprint for other such facilities across China.

A second feedstock being considered is coal seam gas (CSG). The CSG tends to be very lean gas, with a small amount of carbon dioxide and varying amounts of nitrogen. The same liquefaction process can be used for this gas with the only variation being the possible need to reduce the nitrogen in the product gas. This nitrogen reduction can be handled by flashing of the LNG or with nitrogen stripping of the LNG similar to what is used in base load facilities. For higher nitrogen contents, a nitrogen rejection step can be integrated with the PRICO liquefaction process or added downstream of the liquefaction unit.

LNG down under

With its vast undeveloped CSG resources, Australia is positioned to increase its existing LNG fuel market by deploying LNG as a portable fuel. CSG reserve holders and developers are able to monetise these reserves by deploying distributed LNG, which has been successfully implemented in China and Brazil. Australia can reduce its carbon dioxide emissions by switching from diesel fuel to clean burning LNG for long haul trucking, power generation, and domestic gas supply. Further, offshore markets exist for development of mid-scale LNG liquefaction, which are far less costly and require less time to develop than the large base load projects under development in Queensland. These projects are ideal for smaller reserve holders.

B&V has worked with clients to evaluate various LNG production opportunities for both domestic markets in Australia as well as export of LNG from Australia. The developments have covered a broad range of plant sizes from 100 t/d to 1 million tonne per annum of LNG. These projects will require infrastructure developments including construction of transmission pipelines as well as port facility improvements for export facilities. These developments in Australia parallel the ongoing work that B&V has undertaken in China for the past five years.

An innovative approach

The small-scale LNG markets in Brazil and China are examples of an innovative approach to providing vehicle fuel and gas supply to remote and distributed users in a rapid development model. These types of facilities will supply basic needs of many people while the longer term pipeline distribution systems are developed. Also, clean fuel for vehicles is available from these units to displace conventional fuels. Future developments will also focus on non-conventional feedstocks, such as coal gasification and CSG feeds.

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