In Part 1 of the current series “Gases in the 21st Century” (see gasworld, June 2008, p.50), compressed hydrogen was described as a potential future source of fuel for automobiles, and transport in general, for a cleaner, ‘greener’ future.

Well Compressed Natural Gas (CNG) has enjoyed that accolade for almost a century, though its momentum in the last couple of decades has been rapidly rising on account of growing worldwide anxiety about the quality of urban life.

CNG is essentially a mixture of the gas methane (CH4), with other hydrocarbons such as ethane, propane and butane in varying amounts. It also contains small quantities of several substances including nitrogen, hydrogen sulphide, carbon dioxide, helium (which is highly sought in the off-shore oil industry), water and even mercury.

The latter element is a major source of problems for equipment used to purify CNG on account of the mercury forming amalgams within the process vessels.

The exact composition of CNG is very hard to define accurately since it varies from one gas field to another particularly as far as the impurities are concerned. Though CNG is refined prior to its use as a fuel, it is often the presence of the trace impurities which are left in the post-refined product which are responsible for the various incidents experienced around the world.

In particular, the presence of hydrogen sulphide with some entrained moisture has caused serious stress corrosion cracking in natural gas vehicle (NGV) cylinders with a high (>1100 MPa) tensile strength.

Its properties
Somewhat akin to hydrogen, CNG has quite a low density, being lighter than air, a narrow explosive range of between 5–15% of CNG in air and a high auto ignition temperature of 540°C. All these data mean that CNG is a safe gas for most operations and is reckoned by many observers to be even safer than petrol/diesel.

Its low density means that it has to be compressed into high pressure cylinders for its use as an on-board fuel. This was not always the case, in the early days CNG was stored in somewhat unconventional and clumsy looking packages.

Today however, the vast majority of CNG, on board, fuel, storage vessels are seamless steel (Type 1) cylinders. Not only cars but other forms of transport vehicles are also attracted by the qualities of CNG as a fuel.

CNG is used on buses, fork-lift trucks, go-karts, public utility vehicles, boats, and trains amongst other transportation forms.

The Type 1 NGV cylinder
Today there are almost 8 million CNG cylinders in use as on board fuel storage vessels. Since 90% of these are Type 1, that makes over 7 million seamless steel cylinders in worldwide NGV service. Such a disproportionate percentage merits a special mention.

Users of Type 1 cylinders are attracted to them by their comparative low price, but since the cylinder will be carried on the vehicle for its lifetime, they also require such cylinders to be technically efficient - to be able to carry as much CNG as possible, per unit weight of the cylinder.

Readers of gasworld magazine are referred to the April 2007 issue p.30, where the comparative production techniques leading to differing technical efficiencies for Type 1 cylinders are described.

But in summary, the most efficient cylinders are those manufactured using a steel plate as the starting material, followed by the billet piercing process, with the tube formed cylinders languishing a poor third.

The plate route is by far the best, as it is based on a cold, deep-drawing technique which ensures that the final wall thickness of the cylinder is only just greater than the minimum required by the design calculation.

This thus ensures that the overall weight of the cylinder is at its minimum, hence maximising the cylinder’s technical efficiency.

Regrettably, there are only a few manufacturers who have mastered this intricate process of producing the highest quality cylinders.

But with the world’s NGV market expected to grow, from the current 7 million figure, to 65 million NGV’s by 2020, Type 1 cylinders, hopefully made from the plate route, will abound.

Of greater interest to vehicle manufacturers could be Type 2 cylinders which utilise liners formed using the plate route, and which could be up to 30% lighter than the conventional Type 1 cylinders.

Life after manufacture
Once a CNG cylinder is manufactured it has to be safely installed in a vehicle and then maintained for its lifetime. These are both critical areas which if not meticulously executed could result in a cylinder incident.

a) Installation
The installation can either be effected at the time of manufacture of the vehicle or as a retrofit in an existing vehicle. The retrofit programme has been very popular particularly in Asian countries for public taxes.

It is primarily in the field of retrofitting, that the NGV industry has been brought into disrepute. Whilst in several countries retrofitters adhere to regulations and have well trained and certified staff, elsewhere there are no enforcement mechanisms in place which result in non-observance of regulations with work undertaken by incompetent staff.

To ensure a safe and incident-free retrofit industry, cylinder manufacturers must take greater responsibility. They must only sell to organisations that have the necessary controls and know-how in place and at all times resist the temptation of using ‘middlemen’ in an industry where high volumes of cylinders need to be sold.

b) Lifetime maintenance
It is a requirement in most countries that NGV cylinders shall be periodically examined to ensure continued safe use. A number of international standards exist for this operation e.g. ISO 19078, which is intended to be used in conjunction with cylinders manufactured to a dedicated standard for NGV cylinders, notably ISO 11439.

Additionally, in Europe there is Regulation 110 for NGVs. But while these latter documents all describe what tests need to be performed, the stipulated periodicity within the documents varies.

The inconsistency between the documents is all due to the newness of this CNG/NGV subject area. An all-embracing unifying document to pull this subject area together is desperately required. An added complication for NGV cylinders is that the user does not wish to remove the cylinder from the vehicle to undertake the inspection.

Hence, a robust in-situ test methodology is sought. Bearing in mind that the cylinders can be both metallic and non-metallic, for example Type 4 plastic lined composites; the eventual test method needs to be capable of dealing with all such materials.

A final point which needs a mention in this section concerns the fate of a cylinder if,and when, the vehicle is scrapped. Type 1 cylinders can last several decades and can easily outlive a car! Hence once more the NGV industry relies very heavily on the competence of the retrofitters.

What next?
The safety aspects of CNG cylinders in NGV’s are seldom questioned. The rigorous sets of tests make sure that very few safety aspects are not simulated in the laboratory first! Witness an accident when the engine of a bus caught fire and the vehicle was wrecked. But the cylinders remained intact.

It would require a magician to forecast the future in this rapidly changing environment. Oil prices are shooting up daily and with it the price of CNG. But CNG still remains very attractive, as in most countries the taxation implications are low for CNG. That is the status quo for now.

But what will happen to this low tax regime, when governments around the world see diminishing returns from petrol as consumers slowly swap over to CNG, is anyone’s guess!!

Will refilling plants continue to abound and thus maintain the impetus for the NGV market? Attention is also being focussed on ‘Biogas’, particularly the form derived from the gas generated from waste water treatment and anaerobic digestion (Biogas), where Sweden has been a pioneer.

Overall, methane in its various forms remains a most interesting substitute as a fuel, for oil based derivatives in the foreseeable future.