This post is a part of BioBiz’s Bio-CNG Perspectives.
BioBiz, a division of EAI, is a leading market intelligence & strategic consulting firm for the Indian bio-based sectors.
This blog post uses the terms bio-CNG and renewable natural gas (RNG) interchangeably.
Bio-CNG or bio-compressed natural gas, also known as sustainable natural gas or biomethane, is a biogas which has been upgraded to a quality similar to fossil natural gas and having a methane concentration of 90% or greater. As the gas is derived from natural and renewable sources, it is also termed renewable natural gas (RNG).
Bio-CNG packaging and storage involves filling of the gas in cascades of cylinders which are stored under appropriate conditions for later use. As RNG is similar to CNG in its characteristics, CNG cylinders and cascades can be used for packaging RNG. CNG cylinders are generally manufactured from a special steel alloy and are seamless in construction. Their compact size allows them to easily fit into a small car. An empty CNG cylinder with a 50 litre-water-carrying capacity weighs 48 kg (approximately), and has a length of 835 mm and a diameter of 316 mm. A cylinder with a 50 liter water-carrying capacity is capable of carrying approximately 9 kg of CNG. This is equivalent to 12.5 liters of petrol and will allow a run of about 200 kms for a medium sized 1200 CC car.
A cascade filling system is a high pressure gas cylinder storage system which is used for the refilling of smaller compressed gas cylinders. The cascade system allows small cylinders to be filled without a compressor. CNG cascade consists of a series of natural gas cylinders, steel frame, valves, pipelines and high pressure fitting. Priority sequence control panel can also be installed according to customers. CNG cascade is used for ground storage of compressed natural gas. Different types of CNG cascades are available in the market which vary in materials used, weight and economics. For entrepreneurs, it would be good to have an understanding of different types of CNG cascades and their characteristics which could be an avenue for competitive advantage.
This blog post provides details on the major types of CNG cylinders and their characteristics. Details on the list of CNG cylinder manufacturing companies are also provided.
Types of CNG cascades
There are four types of tanks available, designed to hold natural gas at pressures up to 3,600 psi:
1. Type I cylinders:
Constructed entirely of metal, Type I is the heaviest of the four vessel types, and has the lowest initial acquisition cost. There is no covering other than paint on the outside of the cylinder. 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.
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.
2. Type II cylinders:
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).
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. This design weighs less than the type one and costs more.
3. Type III cylinders:
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:
- The high raw material cost of aluminum tubing,
- The high capital cost associated with equipment required to manufacture the aluminum liner (i.e. metal spinning and heat treatment) and
- The high raw material cost associated with the full wrap composite construction.
4. Type IV cylinders:
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.
It could thus be observed that through partnerships with leading technology providers, an investor could opt for the a more suitable RNG storage cascade after analysing its characteristics and economics to have a competitive advantage.
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