Posted: Posted on January 22, 2018 An Overview of CNG Storage Options

Above: A natural gas storage facility located in Boston, Massachusetts.

Expanded Applications and a Restructured Supply Chain

Amid increasing costs, ongoing political instability in nations with the largest proven reserves, and mounting concerns about environmental impact, crude oil/petroleum and its derivatives (e.g., gasoline/petrol, diesel, and propane/liquefied petroleum gas) face an ever-widening spectrum of competitive energy sources in the global energy market — including everything from mineral fuels like natural uranium to renewable sources like solar energy.

Natural gas, in particular, is playing an increasingly important role in addressing the world’s energy needs. In addition to cost and environmental-impact advantages, natural gas is now more plentiful than ever, due to previously unknown reserves being discovered … and previously unexploited technologies being utilized (e.g., leveraging fracking to extract shale gas, tight gas, and coalbed methane). In fact, given such concerted efforts to identify more natural gas resources, natural gas proven reserves worldwide in January 2012 totaled 300 trillion cubic meters (TCM) — a 71% increase from the January 2008 estimate of 175.5 TCM.

Accordingly, today’s natural gas applications are extremely diverse. For example, within the industrial and power-generation sectors, applications include heat- and electricity-generation, incineration, drying, food processing, and chemical feedstock for manufacturing processes.

Perhaps the most rapidly growing application, however, involves the use of compressed natural gas (CNG) — natural gas that is compressed to less than 1% of the volume it occupies at standard atmospheric pressure — as a fuel for private automobiles and fleet vehicles (i.e., vehicles that log high daily mileage, such as buses, garbage trucks, taxis, and airport shuttles).

Above: A natural gas vehicle with dispenser attached.

For several decades, stakeholders throughout the supply chain have sought solutions to meet their CNG containment equipment requirements. With today’s rapidly growing use of CNG as a vehicle fuel, however, the supply chain has evolved significantly. Specifically, stakeholders have expanded from distributors, suppliers, and end-users to include retailers (i.e., fueling stations), fueling station manufacturers and installers (e.g., infrastructure civil engineers), vehicle OEMs/manufacturers (i.e., companies that make vehicles with built-in CNG onboard storage tanks), and aftermarket vehicle technicians (i.e., businesses that retroactively convert gasoline-fueled vehicles to CNG-fueled vehicles). Furthermore, traditional transportation and storage application definitions have been somewhat modified, with “storage” now comprising not just ground storage receivers and stationary assemblies but on-vehicle storage tanks as well.

Implications for CNG Containment Equipment and the Decision-Making Process

The result of these developments is multifaceted. Not only is the demand for CNG containment equipment greater than ever, but the available options have become more sophisticated. To make the right decisions, it’s critical that purchasers understand the applicability of one packaging option versus another, as well as the specification differences and underlying benefits across all options. Equally important, purchasers must have an in-depth appreciation of factors specific to their  situation and anticipated usage — such as intended application, payload and space restrictions, weight, budget, relevant government regulations, safety and reliability considerations, and packaging life requirements.

Type I – Type IV Pressure Vessels

At present, the primary packaging options for CNG transportation and storage applications include pressure vessels Type I, Type II, Type III, and Type IV.  The key differentiator for each vessel type is its load-bearing element and associated material, whether a monolithic metallic pressure vessel, or a combination of metal or non-metallic liner and external wrap. These characteristics essentially determine overall vessel weight and cost, which are the primary considerations for driving a purchasing decision for a given application. For example, while cost is most often the overriding driver for any given application, the cost of a system has two essential elements for consideration, acquisition cost and life cycle cost. Generally, the heavier TYPE I and II vessels will provide lower acquisition cost, while the TYPE III and IV vessels have distinct advantages for life cycle savings due to the significant decrease in weight in comparison to TYPE I and II vessels. In general, for transport applications, where over the road gross vehicle weight and long distances are not significant factors, TYPE I vessels will most often provide a more economic solution.  Where long distance gas transport and/or vehicle gross weight limitations are considerations, TYPE III and IV vessels can significantly reduce cost over the life of the vehicle (i.e. lower vehicle maintenance costs and higher capacity gas storage). For on-vehicle fuel storage applications, TYPE III and IV tanks are generally more cost effective over the life of the vehicle due to their lighter weight which significantly reduces fuel consumption and vehicle maintenance, despite the initial higher acquisition cost of these type fuel tanks.

In short, choosing the optimal pressure vessel from among the four types can be a complex challenge. By providing an overview of Type I – Type IV CNG packaging options, the following discussion will hopefully alleviate some of the inherent difficulties.

Type I Overview

Constructed entirely of metal, Type I is the heaviest of the four vessel types, and has the lowest initial acquisition cost. As such, its primary applications include stationary ground storage and bulk transportation in situations where weight is not a restricting criterion. The standard design is a seamless steel pressure vessel ranging from 7 ft to 40 ft (2.1 m to 12.2 m). Type I is accepted globally as containment equipment for CNG.

Above: Type I pressure vessels assembled at a compressed natural gas fueling station.

Its ability to accommodate a wide range of pressures makes the Type I particularly desirable for permanent ground storage applications at CNG fast-fill fueling stations, where high pressure is critical to the fuel-dispensing process. At such stations — frequented most often by private automobiles and fleet vehicles requiring continuous filling — a large compressor is coupled with high-pressure ASME ground storage vessels arranged in a three-bank cascade; this enables refueling in approximately the same amount of time required to refuel a comparable gasoline-powered vehicle.

Type I vessels are also widely used for ground storage applications at slow-fill stations, many of which are installed as dedicated, on-site facilities for fleet vehicles not having time-sensitive refueling requirements.

Introduced in the 1900s, with larger versions designed in the 1950s, Type I has a history of proven reliability, unlimited life, and low acquisition costs. At the same time, Type I can be susceptible to stress-corrosion cracking if a maximum tensile strength/hardness to provide resistance is not maintained — and if the CNG is not scrubbed to reduce hydrogen sulfide and dried to an acceptable moisture level to ensure gas quality. This point is particularly important, since many documented ruptures can be attributed to the use of high tensile strength vessels in contaminated CNG service.

Type II Overview

Mostly metallic, Type II vessels are somewhat lighter than Type I vessels, and often times referred to as “hoop-wrapped” pressure vessels. The vessel port/head configuration is of metallic construction, while the cylindrical region of the vessel consists of two structural elements: an inner  metallic liner and an outer wrap of wire or fibrous composite material, typically glass fiber composite but occasionally carbon fiber composite. The vessel’s primary applications include high-pressure storage of medical oxygen for home oxygen therapy and air for firefighters’ self-contained breathing apparatus, as well as CNG for onboard vehicular fuel systems (particularly in geographic regions that desire a lighter weight alternative to Type I, yet not with such a higher acquisition cost as Type III or Type IV).

Above: Type II pressure vessels assembled for storing high pressure hydrogen.

TYPE II vessels are more vulnerable to cyclic fatigue than TYPE I, due to the thinner metallic wall in the cylindrical region of the vessel. However, the autofrettage manufacturing process, whereby the stresses in the liner are significantly reduced, generally provides for a vessel that meets or exceeds the cyclic fatigue requirements of most design standards.

Given the spate of parity Type II vessels measuring 10 ft. (3 m) or less, FIBA Technologies chose to manufacture a Type II that is unique in the industry so as to help pave the way for hydrogen-fueled vehicles. Specifically, mindful of the high-pressure requirements dictated by hydrogen’s energy density properties, FIBA developed a Type II that accommodates a higher pressure than any other manufacturer: 15,000 psi (1,034 bar).

The impetus behind this unique Type II design concerns consumer behavior. Since private automobile operators are accustomed to getting 150 miles to 300 miles (241 km to 483 km) per tank of gasoline, their acceptance of hydrogen as a vehicle fuel is contingent upon getting similar mileage with a full tank of hydrogen. However, doing so requires that their automobile tanks can store hydrogen at approximately 10,000 psi (689 bar) … and enabling fast-fill refueling — that is, the “three-minute fill” consumers have come to expect — under those conditions in turn requires cascading hydrogen into the on-vehicle tank from a 15,000 psi(1,034 bar)  ground storage vessel.

Type III Overview

The TYPE III vessel consists of a load bearing metallic liner (typically aluminum alloy) and a fully wrapped composite shell, and is often referred to as “full-wrap” composite vessel. Type III pressure vessels were originally developed for aerospace applications, with commercialization as breathing apparatus for firefighters. Today, in addition to breathing apparatus, they are primarily used for CNG on-vehicle fuel tank applications.

Approximately 70% lighter than a Type I and 40% – 60% lighter than a Type II, the Type III aluminum liner is gas-impermeable and has a significantly higher capacity-to-weight ratio over TYPE I and II vessels. Designed to bear the majority of the pressure load, the composite material comprising the shell provides 75% – 90% of the vessel’s strength.

TYPE III vessels are also more vulnerable to cyclic fatigue, due to the very thin metallic wall in the cylindrical region of the vessel. However, the autofrettage manufacturing process generally provides for a vessel that meets or exceeds the cyclic fatigue requirements of most design standards.

TYPE III vessels generally have the highest manufacturing cost due to: 1) the high raw material cost of aluminum tubing, 2) the high capital cost associated with equipment required to manufacture the aluminum liner (i.e. metal spinning and heat treatment) and 3) the high raw material cost associated with the full wrap composite construction.

Type IV Overview

The Type IV is generally considered an all-composite vessel and is often referred to as a “full-wrap composite plastic lined vessel”. The TYPE IV vessel consists of metallic bosses (end fittings) integrally attached to a polymeric liner, typically high-density polyethylene (HDPE), and wrapped with a carbon fiber or carbon/glass fiber composite shell. The polymeric liner is non-load bearing and the metallic bosses and composite shell are the primary structural load bearing components of the vessel.

With its light weight, high capacity to weight ratio and lower cost, all of which are somewhat superior to those of TYPE III vessels, the Type IV vessel has seen increasing use for CNG on-vehicle storage tank applications. In addition, TYPE IV vessels have recently made significant inroads in CNG transportation applications, particularly in South East Asia.

With the use of a non-load bearing liner and carbon composite material, the TYPE IV vessel is the least sensitive to cyclic fatigue and largely exceeds the cyclic fatigue requirements of the current design standards. Unlike a metallic liner, the HDPE liner is not gas impermeable. However, the gas permeation characteristics of the HDPE material are well within the permeation rates established in the prevailing design standards, ensuring a leak tight vessel.

The FIBA TYPE IV vessel is designed with an aluminum or steel alloy end fitting (pole piece or boss) with HDPE injection molded around the pole piece to form the vessel head structure. The head is then fusion welded to HDPE pipe (a readily available and low cost liner component which is widely used in waste water treatment applications). The liner assembly is then filament wound with carbon and glass composite material to form the completed pressure vessel. This manufacturing method insures the lowest cost (i.e. lowest liner raw material and capital equipment costs) for full-wrap composite construction.

Key Differences Between Type III and Type IV

While the Type III and Type IV are both fully wrapped composite vessels used for CNG on-vehicle fuel tank and transportation applications, there are important differences between them that can drive purchasing decisions, including:

• The Type III is more expensive to produce than the TYPE IV.

The cost to acquire the necessary sophisticated manufacturing technology and equipment for metallic liner fabrication is high. For example, spinning equipment required to form aluminum tubing into a liner is expensive, as is the process itself. In addition, the liner heat treat process, both equipment acquisition and manufacturing process itself, is an inherently expensive technology.

Furthermore, per-unit costs of the Type III are greater than that of the Type IV, since the former vessel utilizes aluminum tubing while the latter vessel utilizes relatively inexpensive wastewater-treatment piping and a much less expensive liner manufacturing process. The per unit cost of TYPE III’s is hugely higher than TYPE IV’s for large diameter designs (greater than 16 inches (406 mm)). For example, the diameters of bus/truck on-vehicle storage tanks typically measure 18 inches or 26 inches (457 mm or 660 mm). When tubing size exceeds approximately 16 inches (406 mm), the cost of aluminum increases dramatically. Therefore, to convert 40-gallon (151 L) diesel on-vehicle saddle tanks — those tanks commonly found on tractor-trailers — to CNG-fueled systems would require the manufacturing of very expensive 26-inch (660 mm) diameter aluminum tubing.

• The Type III is more appropriate for fast-fill fueling and the Type IV is more appropriate for slow-fill fueling.

The Type IV is particularly appropriate for fleet vehicles, since they typically use slow-fill fueling facilities. Natural gas heats rapidly when compressed (termed “heat of compression”) and the TYPE IV vessel is largely constructed of polymeric materials (liner and composites) which are less conductive than metals. When the vessel reaches full pressure during filling, and as the temperature of the gas cools, the eventual settled pressure of the gas in a TYPE IV vessel can be less than that of TYPE III, requiring gas “topping off” to achieve full vessel rated capacity.  Fleet vehicle operators often have well-established fueling station infrastructures in place — dedicated on-site facilities available to them on a continuous 24×7 basis — to maximize vehicle operational time during daylight hours. As such, fleet vehicles like garbage trucks are usually refueled with CNG overnight (i.e. “trickle-filled”) to the rated capacity of the vessel. Conversely, the Type III, with a higher rate of conductivity due to the metallic liner, reaches the settled temperature within the vessel with a lower drop in pressure than the Type IV. The result is more gas in the Type III at first fill, and the need to perform “top off” dispensing operations is minimized. Accordingly, the Type III is more suitable for operators wanting to maximize their CNG capacity at initial fill under fast filling conditions.

Considerations in the Decision-Making Process

As mentioned, there are numerous factors to be considered when deciding which CNG packaging option is right for you. It’s important to discuss your unique requirements (e.g., weight, payload, and space restrictions, as well as safety, reliability, and packaging life requirements), situation (e.g., budget), and anticipated usage (e.g., intended application) of CNG containment equipment with an industry professional — a recognized expert well-versed not only in the specification differences and underlying benefits across all options, but also in the country-specific standards and international codes/regulations governing the safe transportation and storage (whether permanent/stationary ground storage or on-vehicle tank storage) of CNG throughout the geographic scope of your business operations.

At its most simplistic level, the decision-making process is driven by the following major criteria (all of which, by the way, are interrelated and frequently overlap one another):

• First and foremost, intended application.
• Secondly, weight. Your intended application has immediate implications for the weight of your CNG pressure vessels — with a ground storage application necessitating a Type I and an on-fleet-vehicle storage tank application strongly suggestive of a Type III or Type IV.
• Thirdly, applicable government codes, standards, and regulations.
• Second-tier criteria, such as: size (Can I get the vessels in the size(s) I need — and in the size(s) approved by the countries in which I will conduct business?); cycle cost (Is it more cost-efficient to purchase a vessel with a finite life span at cost X — or purchase a heavier vessel at cost Y?); and anticipated life span (With proper care and maintenance, do I expect vessel A or vessel B to have a longer service life?).

Summary

The use of natural gas in general — and CNG in particular — as a worldwide energy source is projected to grow at an ever-increasing rate. This trend, in turn, will continually heighten the demand for CNG containment equipment for transportation, ground storage, and on-vehicle tank storage applications.

While the ultimate demand and usage levels for CNG vis-à-vis other primary energy sources (i.e., coal and crude oil/petroleum) over the next 20 years are impossible to predict with certainty, the cost of crude oil will most definitely be a predominant influence. Nonetheless, all indications at present foreshadow a 21st century in which CNG will play a huge role in meeting the world’s energy needs.