Energetic operations
25 Jan 2005
We are all used to our domestic gas bill reporting consumption in megajoules. However, the gas meter actually records volumetric consumption, usually by cubic metres. The conversion to energy consumption is done at billing time, based on the calorific value (CV) of the gas.
This is also true for commercial and industrial customers, as hydrocarbon gases are traded in terms of their energy value. Electricity bills report consumption in megajoules, and electricity meters provide a direct output of consumption in megajoules, but no single metering device is currently available that directly measures the energy stream in a natural gas pipeline.
In practice this requires a combination of flow measurement and a secondary determination of the CV of the gas, since the CV can vary depending on the type and source of the gas.
At present, the bulk of the UK's gas comes from offshore fields. However, this is likely to change over the next three to five years.
New sources of gas for the UK include LNG from the Middle East and Africa (high CV); gas currently going to Norway (high CV); gas from depleted fields which have become economic again (low CV); mine and landfill gas (variable CV); and gas through the interconnector from the European grid.
Apart from mine and landfill gas, the effect of the other gas sources is likely to be a step change on introduction, simply moving the mean value of the CV of the gas in the distribution system.
However, while the assumption of a step change in CV followed by establishment of a new mean value is valid for the continuous delivery of gas from a new source, there remains the questions of the magnitude of the fluctuations (if such supplies are introduced on an as-required/as-available basis, probably driven by wholesale prices) and their potential impact on billing and process operations.
Even at present, fluctuations of the order of +/-1M to 2 MJ/m3 occur over comparatively short periods (of the order of an hour or less) throughout the distribution system. With the introduction of gas from other sources (including higher CV gas such as LNG and lower CV gas from depleted fields), the magnitude of the fluctuations is unlikely to decrease. If the fluctuations are randomly distributed about the mean, the net effect on billed energy will be zero, but when longer-term step changes occur there will be offsets.
Direct energy metering of hydrocarbons would overcome these limitations and provide customers, in particular commercial and industrial users, with significant benefits.
For example, for those processes which are either controlled on energy content (such as glass and pottery) or whose emissions are strictly controlled, a 2 MJ/m3 change in CV will alter the combustion conditions, which could lead to a change of 1 per cent in the oxygen content of the flue gas. The net result would be to affect both product quality (for example, the colour of the glass, due to changes in the flame emissivity and furnace heat transfer) and stack emissions (for example, increased CO, due to incomplete combustion).
At present, the most widely used technique for determining energy content of a gas stream is the combination of flow measurement and a separate determination of the energy content of the gas. This depends on a compositional analysis by gas chromatography and an equation of state or industry-standard correlation.
Through the use of an automated field gas chromatograph, this approach can provide a highly accurate ( less than 0.2 per cent uncertainty) determination of the energy content and is the closest approach to 'direct' energy metering currently used, but is effectively limited to large energy users.
Although the potential benefits of direct energy metering of gases have long been recognised, little real progress has been made despite some innovative developments.
As part of DTI's National Measurement System Directorate's 2002-2005 Flow Programme, NEL (the main contractor) therefore undertook a review of prospective techniques for direct energy metering of liquid and gaseous hydrocarbons.
The original aim of the project was to form the basis for future development of methods of direct energy metering of hydrocarbons, for both liquids and gases. However, during the initial review process, it became apparent that the technology is still at a very early stage, with essentially no developments for liquid hydrocarbon energy metering.
True direct energy metering for hydrocarbons would involve on-line calorimetry. All other techniques are derived, to greater or lesser degrees. The only technique currently being actively pursued for gases is the correlative technique.
This method is based on the assumption that natural gas can be modelled as a four-component mixture consisting of nitrogen, carbon dioxide and an equivalent hydrocarbon gas treated as two components (methane + higher hydrocarbons). Such a mixture can then be characterised by three independent physical or chemical properties of the mixture and an iterative method developed to infer the gas composition from the measured properties without performing a detailed composition assay, as summarised in the diagram below.
The correlative technique has a number of advantages over other techniques. By careful selection of the input parameters, combined with good design and measurement, it appears to be possible to obtain an uncertainty in calorific value close to that obtainable with a gas chromatograph-based approach, but potentially at around half the cost of a field gas chromatograph.
Alternatively, by accepting a higher uncertainty in calorific value, it is possible to produce a device at much lower cost (around a quarter of the cost of a traditional system). The technique is clearly promising and, if the claims for its accuracy and cost can be substantiated, it should certainly make it possible to carry out direct energy metering much further down the gas distribution system than at present.
The initial market is therefore likely to be amongst medium to large industrial users wishing to check their bills, or those for whom knowledge of energy content fluctuation is vital for their process control. As with the uptake of other technologies, once there has been sufficient penetration at a particular level, there will be pressure from potential users at the next level down for lower-cost devices. This in turn may stimulate further technical developments.
Given that hydrocarbon gases are traded in terms of their energy value, the need for direct energy metering devices appears to exist throughout the potential customer base. It seems strange at first sight that such devices have not been more widely used.
However, the gas supply and distribution arrangements within the UK have historically been such that the costs of such devices have outweighed the benefits. New technology and changes in gas supply and distribution should mean that the cost / benefit balance swings more in favour of direct energy metering further down the gas distribution system.
Normal Glen is a senior consultant at NEL.
