Natural gas is an excellent fuel for DG and CHP, explains Dr. Jacob Klimstra, but because of widening differences in its composition and the introduction of regional standards governing its quality concerns are growing.
Natural gas is an important fuel for distributed generation (DG) and cogeneration. Running generators on clean natural gas can result in large savings in fuel consumption by locally using the heat released during the production of electricity. On top of this, emissions will be realtively low.
Distributed generation also avoids the need for long transmission lines, plus transporting energy over long distances via pipelines with natural gas is said to be 5–25 times cheaper than transmitting electricity over power lines.
Energy can also be stored in the gas in the pipeline if it is at a pressure exceeding the value needed by the customers. So natural gas, is in effect, a large natural battery that is excellent for the long-term back-up of intermittent renewables, such as wind and solar.
Traditionally, the majority of natural gas consumers received it through pipelines from a single source. This meant that the composition of the gas remained fairly stable. This enables the users to achieve optimum performance and minimum emissions from their boilers, gas turbines or gas engines by tuning them to the prevailing composition of the fuel.
However, local gas reserves in industrialised countries are rapidly diminishing, at the same time as its popularity is increasing, due to its lower-specific greenhouse gas emissions and cleaner combustion.
In response to this, natural gas in increasingly being shipped as liquefied nartural gas (LNG) from areas, such as the Middle East, Indonesia, Africa and Australia, to many countries in Asia. The US has large shale gas resources, which might turn North America into a net gas exporter, while Europe increasingly depends on imports from Russia because domestic fields in the waters off the UK and the Netherlands are rapidly depleting.
However, dependence on a single foreign supplier is unattractive because it limits the possibilities for price negotiation. In addition, political tensions could affect security of supply.
It is for these reasons that the European Commission is now promoting the full integration of all European gas transmission systems.Such integration also aims to allow greater competition between gas suppliers, resulting in lower prices for customers.
However, when gas comes from multiple sources, its composition can vary widely and sometimes instantaneously, and thereby affecting the quality of the fuel.
Expressing the quality of natural gas is more complicated than doing the same thing for power. Customers are happy if electricity at 50 Hz or 60 Hz is at a voltage close to the rated value, has no excessive harmonic distortion and has a supply reliability of at least 99.99%. However, with natural gas, the definition of quality is more diverse.
Gas companies generally express the quality of their fuel through its composition, its Wobbe index (WI) and calorific value. Additional factors, such as combustion velocity, knock resistance, the absence of sulphur and siloxanes, as well as firmness in composition, can be important to users.
The WI is a measure of energy flow for a given pressure drop over a restriction. The majority of gas applications use a pressure drop when administering gas to a burner or carburettor. For the WI, the volumetric calorific value H (MJ/m3) of the gas has to be known, as does the relative density d = ρgas/ρair of the gas:
Because the quotient of the two densities ρ (kg.m3) is dimensionless, the WI has the same dimension as the calorific value: MJ/m3. If the WI changes, the power output of the gas application also changes unless corrective steps are taken. The same applies for the air-to-fuel ratio λ, because for most systems that consume gas, the air-to-fuel ratio varies in inverse proportion to the WI:
λ (new) = WI(initial)/WI(new) ∙ λ (initial)
The air-to-fuel ratio determines the temperature of the flame and the combustion velocity, so the combustion process will change with the WI, and thereby affecting fuel efficiency, thermal load and emissions.
For example, if the WI drops from 55 MJ/m3 to 50 MJ/m3, the initial λ value of 1.9 increases to 2.1. If the application is a gas engine with a venturi carburettor to prepare the fuel-air mixture, the engine would most probably misfire and stop fully.
If for the same initial λ value of 1.9, the WI increased from 50 MJ/m3 to 55 MJ/m3, the new value of λ would fall to almost 1.7, resulting in substantially higher NOx emissions and, most probably, knocking. In addition, the power output would increase by 10% and potentially leading to system overload
Less than a decade ago the US had big plans for importing LNG because its domestic resources were declining and it wanted to ensure security of supply, plus natural gas produces lower greenhouse gas emissions compared to coal.
Terminals for receiving LNG were built at major ports along the east and west coasts. Up to then the US had enjoyed reasonably stable gas compositions, but there were fears over the consequences of the differing compositions of the LNG. This led the Federal Energy Regulating Committee (FERC) to approach the US Natural Gas Council and other interested parties on how to deal with the anticipated problems.
A new committee, NGC+, was established, with members from equipment manufacturers, power plant companies, pipeline operators, gas distributors, feedstock companies and LNG suppliers.
Over the course of 19 meetings, the 71 stakeholders discussed all aspects of combustion efficiency, emissions, flame stability and appliance performance. As a result, a White Paper on natural gas interchangeability and non-combustion end use1 was issued on 28 February 2005.
Table 1 gives the agreed values for some gas indices, while Figure 2 shows how these values affect the upper calorific value and WI.
In the White Paper, the WI is allowed to vary in the range ±4% around the traditional average value of 53.16 MJ/m3, while the upper calorific value can vary by ±6% around 41.17 MJ/m3. It is important to note that the upper calorific value is specified here for a reference temperature of 25°C, while Table 1 uses reference conditions for a m3 of 101.25 kPa and 273.15 K. These reference conditions often differ depending on the country or the organization, and care should be taken to take this into account when comparing different gas quality standards.
In Europe, the EASEEgas consortium, made up of primarily members from the gas sector, has been working for almost a decade on specifications for the transborder transfer of natural gas.
Table 2 lists the gas quality index values set for this. Based a mandate from the Commission, the organization of gas transmission operators, ENTSO-G, and the normalisation committee, CEN, are now turning this into a standard.
It appears that in the EASEEgas proposal, almost any natural gas available in the world can be accepted. This is welcomed by gas traders and shippers but has significnatly negative consequences for gas consumers.
A range in the WI between 49 MJ/m3 and 57 MJ/m3 means that a gas-consuming device can suddenly experience a decrease of 14% in fuel supply. In such a case, an initial air-to-fuel ratio λ of 2 in a gas engine or a gas turbine combustor will instantaneously become λ = 2.3, resulting in combustion instability and misfiring. The gas standards do not exclude so-called plug flow, which means that a sudden change in composition of the gas supplied can always occur.
A change in the opposite direction – in other words a sudden jump in the WI from 49 MJ/m3 to 57 MJ/m3 – will decrease the air-to-fuel ration λ from 2 to 1.7, resulting in 16% more power, a higher combustion velocity and higher combustion temperatures.
The power output controller of a gas engine can normally handle a rapid change in output caused by a change in WI. In engines that feature a carburettor, the throttle valve will readjust the amount of mixture flowing to the engine, and in gas engines with electronic gas admission valves, the readjustment in power output will be even faster. However, the air-to-fuel ratio of carburettor-based engines takes longer to control because of the adjustment in the carburettor setting.
In gas engines, a gas with a higher volumetric calorific value will generally have a lower knock resistance – the knock resistance of gaseous fuels is expressed by the methane number (MN).
The MN method was initially developed at the laboratories of AVL in Graz, Austria, with a consortium of German and Austrian engine manufacturers in the early 1970s. In that programme, no hydrocarbons higher than butane were taken into account. Subsequently, the initial method was improved to fit the actual performance of modern engines. The effects of higher hydrocarbons, such as pentane, hexane and heptane on the methane number are now included.
Gas engines in stationary applications for cogeneration and on-site power production demonstrate optimum performance with a MN of 80 or higher. This also applies to natural-gas-fuelled trucks and ships. Fuel efficiency, power output and load-step-response capability are negatively affected by low MNs.
Some gases within the EASEEgas range, such as LNG from Libya, have a MN as low as 63. Figure 4 shows MNs for a selection of natural gases that lie in the EASEEgas range. Gases with an MN of less than 60 might even occur if the specifications contain no lower limit for the MN. The specifications for gas in the US guarantee that the MN is always above 73.
Shale gas in the US3 varies widely in composition from site to site. To comply with the NGC+ limits, the concentration of higher hydrocarbons is reduced by condensing them out (Figure 5) as natural gas liquids (NGLs). These NGLs help to make shale gas production profitable. According to Valerie Wood, president of EnergySolutions3: ‘NGLs are priced in accordance with crude oil prices. The production of high-value NGLs helps to lower natural gas break-even prices.’
However, gas transmission operators in Europe refuse to see removal of higher hydrocarbons at LNG terminals as a solution for obtaining narrower gas specifications. Their excuse is that European and national legislation prohibits gas transmission companies from selling NGLs to refineries. Such an aberration can easily be rectified.
Also, rich gases might occur only occasionally, resulting in a low utilisation factor for a treatment installation. However, that is no excuse. In electricity supply, peaking plants necessary to keep the system stable also have a limited number of operating hours per year. Keeping the WI in a narrow range, even with a large number of gas sources, is not a technical problem. Gasunie in the Netherlands has maintained the WI of the L-gas and H-gas within a range of ±2%.
An important negative aspect of a wide range of gas compositions is the variability in volumetric calorific value. As mentioned earlier, the EASEEgas specifications allow an upper calorific value of between 36 MJ/m3 and 48 MJ/m3. However, commercial and domestic gas consumers use a gas meter that is based on volume flow without a correction for calorific value.
Gas distribution companies have a policy of correcting gas bills for the average calorific value over a certain time span. However, under the proposed regulations, the gas composition can change instantaneously and frequently. Proof of this has already been seen at a cogeneration installation at a point at where three gas streams met.
An owner of a local generating set, such as a cogeneration plant for a greenhouse, might use the installation to sell electricity to the grid during times of peak demand. With today’s gas prices, the profitability of such plants is only marginal. If the calorific value at a given time is only 36 MJ/m3 and the gas company charges the CHP plant for a calorific value of 40 MJ/m3, it appears that the electrical efficiency of the CHP plant has dropped from 45% to 40.5%.
Instantaneous monitoring of plant performance based on the quotient of electricity production and gas flow will be flawed under such circumstances.
Optimum adjustment for minimum NOx emissions is also not possible with a wide range in WI.
It is not only the cogeneration and on-site power sector that is worried about the proposed wide range in gas quality. BDH, the German association of energy and environmental industries, and Figawa, the country’s association of gas and water companies, have voiced their concerns in a letter to stakeholders.
Most existing gas appliances are not able to cope with a wide range in gas composition. In the UK, the allowed WI is restricted to between 47.2 MJ/m3 and 51.2 MJ/m3, which is about the same range as that of the USA’ NGC+. Research has shown that expanding this range is extremely costly because the required scale of investment is factors higher than any profits that come from acquiring cheaper gas.
A paper from Jackson, Finn and Tomlinson4 propose an effective method for extracting higher hydrocarbons from LNG. Ballasting rich gases with nitrogen is ofter proposed to reduce the WI and the calorific value. This, however, is of no use for gas engines because nitrogen in the fuel gas does not improve the knock resistance in modern, high-performance, lean-burn engines.
Arguments by the gas sector that engines and turbines are just a small segment in the gas market does not bear any relationship to the reality and the future.
Better insulated homes and solar heat collectors will drastically reduce the use of gas for heating purposes. In contrast, gas use in engines with the ability to rapidly respond to the intermittency of renewable energy from wind and sun will substantially increase. Next to that, gas-fuelled cogeneration is still a favoured way of saving fuel and reducing greenhouse emissions.
Unfortunately, the gas industry is also now trying to convince countries outside Europe to adopt the gas quality range as proposed for that region. Hopefully, democratic processes will prohibit the interests of consumers from being ignored.
In 1986, a major gas quality conference5 was held in the Netherlands in which experts from gas companies from all over the world participated. The main message was clear: gas quality should be user-led, not supplier-led, and care has to be taken for it not to become politician-led.
In a nutshell
The proposed wide range in transboundary gas composition by the gas industry in Europe has negative consequences for fuel efficiency, power capacity and emissions of gas-fuelled equipment. And the aspirations of European policy makers on security of supply and open markets for natural gas will ultimately result in higher costs for most gas users.
The economic benefits for Europe of accepting all gas available on the world market regardless of its quality may well be lower than the extra costs incurred by adapting gas consuming equipment for efficiency loss and for emission increases.
Solutions for reducing the large range in gas quality available on the market are standard, proven and globally widespread.
In Europe, gas companies have so far dominated all policy making on gas quality without taking into account the expertise of equipment manufacturers and users of gas-fuelled equipment. The US, in contrast, has followed a more democratic path.
Finally, a wide range in calorific value will further deteriorate and obscure the way gas energy deliveries are measured with gas meters.And legislation in Europe should allow gas transmission companies to sell NGLs.
- Natural Gas Council, White Paper on Natural Gas Interchangeability and Non-Combustion End Use, 28 February 2005.
- Leiker M, Cartelliery W, Christoph K, Pfeifer U & Rankl M, ‘Evaluation of the Anti-knocking Property of gaseous Fuels by means of the Methane Number and its Practical Application to Gas Engines’, ASME paper 72-DGP-4, April 1972.
- Darin L George & Edgar B Bowles, ‘Shale Gas Measurement and Associated Issues’, Pipeline & Gas Journal, pp38–41, July 2011.
- G J van Rossum, editor, ‘Gas quality’, Proceedings of the Congress of Gas Quality, Groningen, the Netherlands, 22–25 April 1986, ISBN 0-444-42628-0.