Advances in small- scale LNG technology provide user options

http://www.ogj.com/display_article/224809/94/ARCHI/none/none/1/Advances-in-small–scale-LNG-technology-provide-user-options/

Ron Cascone
Nexant Inc.
White Plains, NY

World-scale cryogenic LNG liquefaction and handling facilities (400-14,000 tonnes/day) are designed to produce LNG for transportation by ship from remote high-volume fields to high-volume markets.

Similar liquefier designs have been adapted in cryogenic small-scale liquefaction (SSL) systems to make and store lesser amounts of LNG for “peakshaving,” that is, vaporizing to supply supplemental gas during the seasonal and shorter-term demand spikes that occur in most local distribution and transport gas pipeline systems.

Besides for peakshaving, SSL can provide LNG as a regular or switching fuel in industrial, commercial, or utility burners, or for fueling vehicles.

Peakshaving uses

LNG-based peakshaving plants are common in the US, Netherlands, Germany, and in other highly developed gas supply regions. Even in Japan, which depends on large LNG imports from the Middle East, North America, Indonesia, and other areas for most of its natural gas supply, small-scale LNG systems are used for strategic distribution of landed gas among the islands of its archipelago.

Royal Dutch/Shell Group claims that there are more than 240 LNG peakshaver facilities worldwide. A database maintained by the US Department of Energy’s Energy Information Administration (DOE-EIA) details 120 LNG units in the US that either make, store, and re-vaporize (“regasify”) LNG periodically; or only receive, store, and re-vaporize LNG periodically.

Peakshaving LNG plants typically have capacities of 10-200 tpd (but as high as 5,000 tpd), and operate 150-200 days/year. Vehicle-fuel LNG plants, on the other hand, typically have capacities of 10-400 tpd and operate 365 days/year. Other pipeline peakshaving strategies, which are usually integrated together in large systems (sometimes together with LNG storage) include:

# Aboveground and underground gas storage (UGS).

# Propane-air injection.

The US currently uses about 23 tcf/year of natural gas. Of this, total imports, mostly from Canada, have reached about 3.5 tcf/year, with less than 10% of this being LNG. Currently, LNG is imported into the US at only four terminals, although many more are under construction and planned. (See following article for an update.)

Many people, concerned about the future of the natural gas supply grid, are looking to both increased LNG imports in the long term as well as increased “distributed LNG” facilities, using peak-shaving type technology as a near-term solution to both supply continuity and price hedging concerns.

Interest in this strategy is among forward-looking natural gas industry midstream (gas transmission) and downstream (local distribution companies) operators and also significant natural gas customers.

Large gas customers (such as independent power producers, or IPPs, many now based on natural gas fueling) as well as LDCs could install SSL LNG as a hedging tool against natural gas price spikes or for their own fuel security (and, for some customers, as an alternative to using fuel oil or LPG in the same way).

For economy of scale and continuous utilization of invested capital, such firms could increase the size of their SSL LNG installations to supply others as well.

Vehicle fueling, other “distributed LNG”

LNG is increasingly of interest as an alternative vehicle fuel in California and other areas of the US that are struggling to comply with federal (Clean Air Act), regional, and local limits on vehicle tailpipe emissions, especially for heavy duty (currently diesel-fueled) fleets.

LNG can either be used as the fuel carried aboard the bus or truck or as a strategic fueling option known as “LCNG”(cryogenically storing, pumping to pressure, and vaporizing LNG to produce pressurized natural gas, as required for refueling vehicles).

LCNG avoids installing and operating large compressors and pressure buffer tank systems to accomplish high-rate refueling on demand. Potential customers for regular LNG supply could include truck or bus fleets that use LNG, taxi, service van, or passenger auto fleets served by LCNG, small industrial plants currently burning LPG but without pipeline gas supply and needing clean fuel, or others that burn fuel oil and wish to reduce their emissions for a number of possible reasons.

New SSL technologies

Some new SSL systems are being developed and commercialized.

Among the new systems are developments by the Gas Technology Institute, Des Plaines, Ill., and Brookhaven National Laboratory, Long Island, NY, and a radically simpler, more economical technology developed by Idaho National Energy and Environmental Laboratory, Idaho Falls.

One important version of INEEL’s technology is driven solely by the energy normally wasted in letting down gas from pipeline pressure to a distribution system or a large user such as a power plant or industrial facility (e.g., 450 psi to 50 psi). This is not a unique approach: There are more than 10 so-called “turboexpander” LNG peak shavers around the US.

Uniquely, however, by avoiding physical and chemical impurity removal before liquefaction and simply solidifying most impurities in the cryogenic cycle to form a slush that is mechanically separable from the LNG, the INEEL technology is simpler and potentially lower cost than competing approaches.

In it only a minor portion of the letdown gas is converted to LNG, with the impurities vented back to the main gas flow.

PG&E experience

Pacific Gas & Electric Co., San Francisco, is in the final phases of development, commercial demonstration, and operational shakeout (nearly 100%) with its first facility that uses the INEEL technology in Sacramento (nominal capacity of 10,000 gpd or 13,500 gal of LNG storage; nominal cost $500,000). The company is planning a second facility based on experience gained from the first.

This demonstration facility uses conventional expander technology and has tested several technical variations throughout the system. The LNG will serve PG&E’s strategic peak-shaving needs, as well as being potentially available for vehicle fueling, rather than trucking in LNG from other parts of the state or from other states, such as Arizona.

There is clearly a dynamic emerging between the needs of the utility and customer gas supply infrastructure (“distributed gas” applications of SSL LNG) and the growing potential to use compressed natural gas or LNG as an engine fuel alternative to diesel and gasoline for both clean air and fuel security purposes, as exemplified by PG&E’s development in Sacramento, Calif.

Peakshaving plants, in supplying lower cost LNG for the CNG and LNG vehicle market that is currently cost or supply-constrained in many places, could also drastically improve the economics of maintaining a strategic cryogenic supply of LNG for energy security or price hedging.

For all alternative fuels, customers or fleet operators will not invest in fuel infrastructure without sufficient customers (in a public-access system) or economy of scale (for privately fueled fleets), while individual customers, fleet owners, and vehicle makers will not invest in vehicles unless the fuelling infrastructure is available.

LNG peakshaving plants could help break this vicious cycle for CNG and LNG because LCNG refueling systems can be much less expensive than CNG compression systems for on-demand (“fast”) fueling and much more scale-elastic.

Fig. 1 presents an economic and environmental performance analysis of competing clean vehicle and bus fueling options, based on discussions with PG&E.

This analysis shows that costs of operating either CNG or LNG buses, on an overall, annualized basis, compare well with costs of operating current diesel buses with, however, huge advantages in reduced emissions of particulate matter (“soot”), and oxides of nitrogen (NOx, a smog precursor), as well as reductions (not shown) in other emissions.

Emerging improvements, such as the currently available Cummins Westport CNG heavey-duty engine system with pilot diesel-fuel injection (indicated as “dual fuel” in the chart) are comparable to advanced diesel options in a fuel-saving electric hybrid configuration. Note that CNG or LNG is also feasible and high-performing in a hybrid configuration but is minimally commerical.
Other SSL systems

Still another SSL technology, also based on a turboexpander, is commercially aimed at exploiting smaller, mostly stranded-gas resources, including offshore natural gas reservoirs with floating production systems (in competition with other strategies for monetizing these resources, such as gas-to-liquids and methanol).

Offered by Randall Gas Technologies-ABB Lummus Global Inc., this approach is called the Dual Independent Expander Refrigeration Cycle. Like the INEEL approach, this system makes LNG with refrigeration generated by the isoentropic expansion of gases and without conventional mechanical refrigeration, which greatly simplifies the process. Unlike the INEEL approach, however, this process requires conventional gas cleaning ahead of liquefaction.

Isentropic gas turboexpanders have been proposed for many years for gas liquefaction, including by Linde AG and in several versions of the classic Claude cycle.

The gas is cooled by the extraction of work in the expander, and refrig-eration and shaft power from the expansion process are used to aid the liquefaction process. The work extracted is utilized to partially recompress the refrigerant gas.

Thermodynamically, turboexpansion cycles are theoretically as efficient as the most advanced (e.g., mixed refrigerant) cycles used in large conventional plants. While the efficiency of early turboexpanders was very low (60 to 70%), current expander efficiencies are exceeding 85%.

GE Power Systems and Cryostar SAS, among others, offer turboexpanders that can be used in these systems.

Other SSL applications

Other applications of SSL include those for making LNG to supply natural gas (in competition with LPG and fuel oil) to remote (“off-pipeline”) industrial or commercial customers, or “stranded-gas utilities” that serve remote communities such as in rural mountainous regions, or on islands, by shipping in LNG by truck, rail or barge.

Other, externally powered (non-turboexpander) versions of SSL are being developed to liquefy lower pressure gas streams, such as coalbed methane, biomethane, associated flare gas, and other small stranded-gas resources. Application opportunities depend on many economic and logistic factors, including geography, climate (affecting peakshaving requirements), and other conditions that include lack of a fully developed gas pipeline infrastructure.

Worldscale strategic LNG projects in various countries or US states can actually enable SSL LNG (and LNG-based natural-gas-fueled transportation) rather than competing with it, by providing a structure of experience and needed regulations and standards.

Applying SSL

With at least 240 facilities worldwide, LNG peakshaving is a standard, eminently feasible application and does not need to be analyzed here with respect to its operations. Fig. 2, however, presents typical economics (including alternative fuel grants and incentives) that could generally apply to SSL LNG systems that are available from a number of sources and developers.

Note that 1,000 gpd = about 88-93 million btu, depending on LNG composition and conditions. Therefore, liquefaction system costs range from about $1,500/million btu to about $2,500/million btu in this case of moderately clean gas being liquefied.

Most of the economies of scale are in the gas cleanup system and heat exchangers, rather than in the mechanical systems. Also, these costs are sensitive to gas quality. The liquefaction compressor can be driven either by a gas engine (ICE or microturbine) or by an electric motor.

Following are some suppliers of small LNG plants or designs on the market:

# Air Products and Chemicals Inc. (US).
# Black & Veatch Pritchard (US).
# BOC (UK).
# Chart Industries Inc. (US).
# Chicago Bridge & Iron Co. (US).
# GTI (US) – developmental, being licensed.
# Hamworthy KSE (Norway).
# INEEL (Idaho National Energy & Environmental Laboratory); developmental, being licensed.
# KryoPak Inc. (US).
# Linde AG (Germany). LNG

Author

Ron Cascone (rcascone@nexant.com) is manager of special projects, utilities and environmental, for Nexant Inc., White Plains, NY. He focuses on fuels and alternative energy, process, technology, and project development. He was a founder and vice-president of Technology Evaluation and Development Associates, a consultancy in Hoboken, NJ, 1985-88, after more than 18 years as a process manager with Scientific Design Co., New York City.

Cascone is a chemical engineer with more than 38 years of industrial experience in the process and energy industries. He holds a BChE from Manhattan College and pursues graduate study at Columbia University. He holds several patents in the energy field and is a member of AIChE and the ACS.

Volume 2 Issue 2 Apr 04, 2005