The natural gas market world-wide is huge. Although there is a need to provide a standardized gas such that all the end users’ gas burners will function as intended, there are regional differences in specifications. The US market has this challenge that makes the interchangeability of gases difficult, and the cost and feasibility of standardizing has been considered but discarded.
In the UK, however, a similar conversion was done area by area in the 1960s and 1970s as the market was converted from ‘town gas’ to ‘North Sea gas’. (Town gas was synthesized by gasification of coal.) Town gas was common in Europe until the advent of gas finds in the North Sea. Pipelines from these and Russian fields serves this market today. North America has had a change of fortune in recent years by technology enabling the production of so-called shale gas. There have also been LNG projects developed, with more coming on stream in the next few years. Gas is challenged by other forms of energy.
Although existing users are to an extent ‘sitting ducks’ due to investments made, provision costs of gas must be kept in check to keep its market share. Electricity is the immediate competitor in the retail market, and that in turn could be provided through the combustion of gas, coal or oil, and other sources are nuclear power plants and hydroelectricity. The more alternatives that are available in any one market, the more the focus on provision cost of energy in the market. Deeper discussions of these issues may be found elsewhere (BP, 2011; IGU, 2013a,b; Natural Gas Supply Association, 2005).
Specifications of natural gas as a product is a very interesting topic in many ways and the specifications really determine what treatment a gas eventually needs. There are two dimensions to this. One is the transport system that supplies a market and what treatment the gas needs to uphold flow assurance in the supply chain. The other is the end market with its appliances where gas burners have been fitted with certain gas properties in mind.
Interchangeability of gas cannot be taken for granted. There are many stumbling blocks to this (IGU, 2011). Methane, or natural gas, is less reactive than their heavier analogues like ethane, propane and so on. As feedstock for making hydrogen as in the ammonia process it is the preferred starting point as the ratio of hydrogen to carbon is highest in methane. For this reason, and because of the pricing, natural gas is the feedstock of choice for this purpose.
The C2+ fraction of the natural gas has in the main a higher market value as feedstock than as fuel. Hence the opportunity to separate these components from the gas is often taken. The economics of this has varied over time though.
For various assessments it is valuable to have a feel for sizes of plants and associated variables. The question being, what is big, what is small, what is a challenge and what is trivial. Plant sizes and complexities will vary widely. Perhaps the simplest gas treating plant to be encountered in this context will the end-of-pipe solution scrubber where some contaminant is to be removed from an effluent gas stream before being released. Maybe this scrubber has a packing height of 3m and a diameter of 2 m, and furthermore when the absorbent has done its job, it may be returned to the process without further ado. A 400MW CCGT (Combined Cycle Gas Turbine) power plant that needs CO2 abatement will have a gas stream in the order of 1.8millionm3/h, and the absorber would have a diameter around 17m if there is one train only.
A large synthesis gas train may have a gas flow in the order of 10 000 kmol/h. This would be 224 000Nm3/h. However, the pressure could be around 25 bar if this was an ammonia plant, and this would imply a real gas stream in the order of 10 000m3/h. In natural gas treating there is a wide range of plants.
A fairly small one might be 10 MMSCFD. This is a typical way of specifying plant size in North America. MMstands for ‘mille-mille’, which is Latin inspired, meaning 1000 × 1000 (or a million). SCF is Standard Cubic Feet, and D implies per 24 hours (a Day). In North America ‘Standard’ means the gas volume is at 60ºF and an absolute pressure of 14.696 psi (psi =pounds per square inch).
the ‘standard’ pressure may also be 14.73 psi, which is based on a pressure of 30 in. of a mercury column. Beware; if you are buying gas the difference in what you get is 0.23%, which is not to be given away easily in negotiations. A large gas plant could be in the region of 2million Sm3/day. This is typical of a gas field in the North Sea. This is in metric units, and the ‘standard’ now implies 15ºC and 1.013 bar. If this was indeed the gas’s temperature and pressure it would be at its ‘standard
conditions.’ Note that 15ºC and 60ºF are not identical. European and American standard conditions are not equal: something to be kept in mind when selling and buying.
An often used specification for H2S allowed in natural gas is 0.25 grain per 100 SCF. This is a US term. One ‘grain’ is 1/7000th of a pound (lb). LNG plants are usually referred to in million tonnes of LNG per year. A plant of 3million tonnes per year was considered big less than 10 years ago, but one-train capacities have been stretched to 5–7 and there is a new generation of plants with a third refrigeration loop that could take the capacity to 10 million or more.
A large ammonia plant today would typically be 2000 tonnes per day. This is almost the double of what was usual around 1970. Cryogenic air separation units (ASU) could be as big as 3500 tonne of oxygen per day, but this size of plant is rare. Traditionally they have been built to provide oxygen for steel works. However, they figure in present day studies on oxy-fuel plants. That is, power plants where hydrocarbons, or coal more likely, is combusted with oxygen to make the CO2 resulting more easily accessible for capture and storage.
It is good to develop an intuitive sense for plant sizes and put them into perspective. The ability to distinguish between the various ‘standard’ units of gas quantity is a must. To help in this direction and to summarize the earlier discussion of plant sizes.
Plants have been built in all sorts of places. Some are hot, some are cold and some are to be found at a high altitude where the air is thin. When comparing plant costs and efficiencies, this must be kept in mind. An LNG plant will of course have a better efficiency if the heat sink is at 5ºC compared 35ºC. On the other hand winterization may be costly. Special precautions must be made if it is to be operated for weeks on end at −40ºC.
1. Gas Treating – Absorption Theory and Practice – DAG A. EIMER
2. Fundamentals of Natural Gas, Arthur J. Kidnay & William R. Parrish