Development of Nautral Gas Quality in Europe - 2.pdf

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Development of natural gasqualities in Europe

byKlaus Altfeld, Peter Schley

Natural gas qualities in Europe will become increasingly diverse and combustion characteristics (Wobbe index, methanenumber) will vary over wider ranges. The article presents the gas qualities to be expected over the medium term andanalyses and discusses their effects on future gas utilisation. Aside from rich (high-caloric) LNG qualities, future naturalgas and biomethane qualities are not expected to cause problems in gas utilisation in most European countries. Thisalso applies where up to 10 % of hydrogen produced from renewable surplus electricity is admixed except for threeimportant applications: tanks for compressed natural gas used as a motor fuel, gas turbines with premixed burnersand underground porous rock storage facilities; here further R&D input is still required. Biomethane produced fromcontaminated feedstock may carry undesirable trace substances. Particularly careful treatment and quality control arethen necessary. Hydrogen or methane produced from renewable surplus electricity will have a high purity level and,like biomethane, will contribute to further reducing CO2-emissions. This will make natural gas an even more climate-protecting fuel compared with other fossil fuels.

T he gases injected into the European gas transporta-tion system will become increasingly diverse: whileconventional pipeline gases from the North Sea,Russia, the Netherlands, Algeria and other producer regionsare not expected to change signicantly, liqueed naturalgas (LNG) as well as biomethane will be gaining importanceand, in the medium term, also hydrogen or methane fromsurplus renewable electricity as well as gases produced bythe gasication of solid biomass. The diversication trendis welcome as it enhances supply security. Moreover, gasesfrom renewable sources help reduce climate-harming car-bon dioxide emissions (greening of gas).

But, as a result, the market will see a greater variety ofgas qualities and gas qualities will vary over a wider range.Gas appliances will have to respond increasingly exibly.Continued reliable operation of the more than 160 milliongas appliances in Europe must not be affected.

The paper describes different gases and their effects onfuture gas utilization.

LIQUEFIED NATURAL GAS (LNG)LNG is available in lean, medium and rich qualities. While

lean qualities roughly correspond to today's pipeline gasesfrom Russia or medium qualities to those from the NorthSea, rich qualities have high superior caloric values andWobbe indices and low methane numbers (Table 1 ). This

may cause problems when the gas is used in boilers orengines. Because of the high Wobbe indices, direct useof rich LNG qualities may be problematic for safety rea-sons. But by adding small amounts of nitrogen the Wobbeindex can be easily reduced to values acceptable in mostcountries (55 to 56 MJ/m³) (Fig. 1 ; 25 °C/ 0 °C referencetemperatures). This is generally accepted practice and hasbeen applied successfully in many regasication terminals.

The methane number is an important gas propertywhich indicates knock resistance in engine combustionand is comparable with the octane number used forpetrol. Adding only a few percent of nitrogen has virtuallyno effect on the methane number of rich LNG. Highermethane numbers (and thus higher levels of knock resis-tance) can be obtained, for example, by mixing the gaswith lean LNG or pipeline gases containing only low levelsof higher hydrocarbons (Fig. 2 ). This has also becomeusual practice.

BIOMETHANEFollowing commissioning of the rst plants in 2006, thetreatment of biogas to obtain natural gas quality (bio-

methane) has seen noticeable growth in Germany. Morethan 50 plants have been injecting biomethane into thenatural gas network for quite some time now withoutcausing problems for networks or consumers.

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Biomethane from fermentation processes is a very simp-le gas: when injected into group-H gas networks it consistsof 96 to 98 % of methane as well as of carbon dioxide andair. Its superior caloric value and Wobbe index are low(Table 1). By conditioning with liqueed petroleum gas(LPG) the superior caloric value of biomethane can beadjusted to the superior caloric value prevailing in thenetwork, if necessary.

It is the oxygen content (0.1 to 0.5 %) that may be proble-matic when injecting biomethane into high-pressure trans-portation networks as even very low oxygen concentrations(e.g. as low as 0.01 %) may cause signicant problems in humidunderground storage facilities: corrosion on steel pipes as wellas detrimental effects on storage characteristics caused bythe formation of elementary sulphur and clogging of pores

[1]. Other undesirable accompanying or trace substances arenot known for biogas plants operated on quality-assuredfeedstock (energy crops, liquid manure, green waste) andequipped with a suitable gas treatment system [2,3].

But where contaminated feedstock is used, biomethanemay contain trace substances (such as silicon compoundsor halogens) which may cause problems in gas utilisation:for example silicon deposits on turbine blades. As it con-tains a vast number of potentially hazardous substances,landll gas should not be injected into the natural gasnetwork for safety reasons even after treatment.

HYDROGENWith the fast pace in developments, in particular in the eldof wind energy, the known problem of electricity storage hasgained a new dimension. Pumped storage power stationshave been used for decades to store electricity on a largerscale. The number of power stations and their potential arelimited in many countries. The concrete idea is being pursued

to use surplus electricity for the generation of hydrogen byelectrolysis and inject the hydrogen generated directly intothe natural gas network. This will cause natural gas and electri-city networks to become even more interdependent (Fig. 3 ).

Table 1: Gas qualities of different natural gases (pipeline), LNG and biomethane

Gas composition Symbol Unit RussianGroup H

North SeaGroup H

DanishGroup H

LibyaLNG (rich)

NigeriaLNG

(mean)

EgyptLNG

(lean)

Bio-methane

Bio-methane

+LPG

methane CH4 mol% 96.96 88.71 90.07 81.57 91.28 97.70 96.15 90.94

nitrogen N2 mol% 0.86 0.82 0.28 0.69 0.08 0.08 0.75 0.69

carbon dioxide CO2 mol% 0.18 1.94 0.60 2.90 2.68

ethane C2H6 mol% 1.37 6.93 5.68 13.38 4.62 1.80

propane C3H8 mol% 0.45 1.25 2.19 3.67 2.62 0.22 5.00

n-butane n-C4H10 mol% 0.15 0.28 0.90 0.69 1.40 0.20 0.50

n-pentane n-C5H12 mol% 0.02 0.05 0.22n-hexane n-C6H14 mol% 0.01 0.02 0.06

hydrogen H2 mol%

oxygen O2 mol% 0.20 0.19

total mol% 100 100 100 100 100 100 100 100

superior caloricvalue

Hsv MJ/m³ 40.3 41.9 43.7 46.4 44.0 40.7 38.3 41.9


Hsv kWh/m³ 11.2 11.6 12.1 12.9 12.2 11.3 10.6 11.6

relative density d – 0.574 0.629 0.630 0.669 0.624 0.569 0.587 0.641Wobbe Index Ws MJ/m³ 53.1 52.9 55.0 56.7 55.7 53.9 50.0 52.3

Wobbe Index Ws kWh/m³ 14.8 14.7 15.3 15.8 15.5 15.0 13.9 14.5

methane number MZ – 92 79 73 65 71 92 103 77


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If hydrogen from surplus renewable electricity is injectedinto the natural gas network, the enormous transportationcapacity and the huge storage capacity of the existing naturalgas infrastructure including underground storage facilities canbe used directly (Germany as an example: approx. 500,000 kmof pipelines and more than 20 billion m³ of working gas instorage facilities). This can make an important contribution tothe transportation and storage of surplus or non-transportablerenewable electricity and is particularly attractive if it helps toavoid construction of a new electricity line.

But the volume of hydrogen that may be added to natu-ral gas is limited. Studies [4] have shown that, with certainrestrictions, admixture of approx. 10 to 15 mol % is not cri-tical in most cases except for three important applications:■ Modern gas turbines with premixed burners (a great

number of manufacturers currently specify a limit valueof 5 %).

■ Tanks in NGVs and CNG fuelling stations (the currentlimit value is 2 %; but activities to increase the valueare under way).

■ Underground porous rock storage (studies have beeninitiated to determine a reliable limit value).

Of course, hydrogen could also be used to produce metha-ne, the main constituent of natural gas. But the processwould involve further capital expenditure and energy los-ses. This option will therefore only be used to a limited

extent for economic reasons.What does it mean to inject 10 % of hydrogen into thenatural gas network? The two examples below illustratethe situation:

■ In Germany almost 1,000 TWh (1012 kWh) of energy inthe form of natural gas are transported annually; this isalmost twice as much as the electricity consumed. 10 %of hydrogen admixed to natural gas would correspondto an energy quantity of approx. 30 TWh. For comparison:the total capacity of the pumped storage power plantsin Germany is 0.04 TWh per cycle (40,000 MWh).

■ A medium-sized natural gas transportation pipelinehas a capacity of, for example, 1 million m³/h. Injectionof 10 % (100,000 m³/h) of hydrogen would require anelectrical input of more than 400 MW for the electrolysisreaction, which corresponds to the maximum outputof several large wind farms taken together.

The examples make it clear that injection of a hydrogenvolume into the natural gas network seemingly as low as10 % would signicantly contribute to solving the problemof transporting and storing surplus electricity generatedfrom renewable resources.

GASES FROM SOLID BIOMASSGASIFICATION This option for generating renewable gases is also gainingimportance; some test plants are being built or in opera-tion [5]. Gas composition may vary greatly depending onprocess control. Aside from methane, the gas may alsocontain hydrogen, carbon monoxide and carbon dioxide or

other undesirable trace substances. Stringent quality con-trol is therefore required prior to injection into the naturalgas network. Comprehensive operational experience withlarger plants is not yet available.

Fig. 1 : Wobbe index for a mixture consisting of rich LNG(Libya) and nitrogen as a function of the amountof nitrogen admixed

Fig. 2 : Methane number for a mixture of rich LNG (Libya)and lean LNG (Egypt) as a function of mixture ratio


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Fig. 3 : Converging electricity and gas infrastructures


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COMBUSTION CHARACTERISTICSRANGES The most important combustion characteristics are Wobbeindex, relative density, superior caloric value and metha-ne number. Table 1 lists these characteristics for selectedgroup-H gases as used in Europe today.Table 2 shows thecombustion characteristics for the gases listed in Table 1following admixture of 10 % of hydrogen. The data werecalculated with the GasCalc program [6], the methane num-bers in line with [7, 8] (25 °C / 0 °C reference temperatures).

Fig. 4 shows superior caloric value as a function ofWobbe index including the EASEE-gas recommendations[9] for the Wobbe index range (49/57 MJ/m³) (red lines). Theblue symbols stand for the gases with hydrogen admixture.

Fig. 4 conrms that, prior to hydrogen admixture, allgases listed comply with with the EASEE-gas recommen-dations. But, as already mentioned, the very high Wobbeindices of rich LNG (just under 57 MJ/m³) are not acceptablein most European countries for safety reasons. Biomethanewithout LPG (approx. 96 % methane) is in the lower Wobbeindex range. Admixture of 10 % of hydrogen reduces theWobbe index for all gases. In the case of gases with very

high methane content relative densities may be slightlylower than the minimum value recommended by EASEE-gas (0.555) (Table 2). But according to our experience andndings from [4] this is not problematic with respect to

combustion behaviour in residential gas appliances.Fig. 5 shows methane number as a function of Wobbe indexcalculated on the basis of the AVL method [7] using a DGCprogram [8]. The accuracy is within approx. ±2 methanenumbers. The wide range with values from 103 (biometha-ne without LPG) to 62 (rich LNG with 10 % of hydrogen) isremarkable. But even without hydrogen admixture, someLNG qualities and pipeline gases are in the range from 65to 75. This must be taken into account when designing gasengines for packaged cogeneration plants and vehicles. The design could be based on a methane number of 70while methane numbers are usually higher in practicaloperations, but can also be as low as 65 in some cases.

As using gas as a motor fuel has become increasinglyimportant over the past few years, methane number as afuel property should be included in international gas qua-lity specications and will also be an important parameterin European gas quality standardisation.

ACCOMPANYING AND TRACESUBSTANCES The EASEE-gas recommendations for accompanying and

trace substances are an important basis for European gasquality standardisation (CEN TC 234, 408):■ Total sulphur: 30 mg/m³■ H2S+COS: 5 mg/m³ (S)

Fig. 4 : Superior caloric value as a function of Wobbeindex for different gases with or without 10 %hydrogen admixture (25 °C/0 °C)

Fig. 5 : Methane number as a function of Wobbe indexfor different gases with or without 10 % hydrogenadmixture (25 °C/0 °C)


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Table 2 : Gas qualities of different natural gases (pipeline), LNG and and biomethane with admixtures of 10 mol % of hydrogen

Gas composition Symbol Unit RussianGroup H

North SeaGroup H

DanishGroup H

LibyaLNG (rich)

NigeriaLNG

(mean)Egypt

LNG (lean)Bio-

methane

Bio-methane

+LPG

methane CH4 mol% 87.26 79.84 81.06 73.41 82.15 87.93 86.54 81.85

niotrogen N2 mol% 0.77 0.74 0.25 0.62 0.07 0.07 0.67 0.62

carbon dioxide CO2 mol% 0.16 1.75 0.54 2.61 2.41

ethane C2H6 mol% 1.23 6.24 5.11 12.04 4.16 1.62

propane C3H8 mol% 0.41 1.13 1.97 3.30 2.36 0.20 4.50

n-butane n-C4H10 mol% 0.14 0.25 0.81 0.62 1.26 0.18 0.45

n-pentane n-C5H

12 mol% 0.02 0.05 0.20

n-hexane n-C6H14 mol% 0.01 0.02 0.05

hydrogen H2 mol% 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00

oxygen O2 mol% 0.18 0.17

total mol% 100 100 100 100 100 100 100 100


Hsv MJ/m³ 37.5 39.0 40.6 43.0 40.9 37.8 35.7 38.9


Hsv kWh/m³ 10.4 10.8 11.3 12.0 11.4 10.5 9.9 10.8

relative density d – 0.523 0.573 0.574 0.609 0.568 0.519 0.535 0.583Wobbe Index Ws MJ/m³ 51.8 51.5 53.5 55.1 54.2 52.5 48.8 51.0

Wobbe Index Ws kWh/m³ 14.4 14.3 14.9 15.3 15.1 14.6 13.6 14.2

methane number MZ – 83 74 68 62 67 83 97 71

■ Mercaptans (RSH): 6 mg/m³ (S)■ Oxygen: 0.001 mol% (0.01 mol% in exceptional cases)■ Carbon dioxide: 2.5 mol %■ Water dewpoint: - 8 °C at 70 bar■ Hydrocarbon dewpoint: - 2 °C (1 to 70 bar)

Future discussions will focus on the following aspects:■ Total sulphur limit value The value of 30 mg/m³ seems too high today as most

non-odorised natural gases transported in Europe as wellas biomethane are virtually sulphur-free (1 to 3 mg/m³).

When used as a motor fuel, similar requirements areexpected in Europe for natural gas as for diesel andpetrol (10 mg/kg; this corresponds to approx. 8 mg/m³for odorised natural gas).

■ Oxygen When coming out of the well, natural gas does notcontain oxygen. Oxygen might be entrained during

treatment; but this is usually not the case. When pro-ducing biomethane, on the other hand, oxygen iscontained in small volumes (e.g. 0.2 %) for process-inherent reasons.Separation of oxygen to obtain values of 0.01 % or0.001 % requires additional capital and operationalexpenditure. It should therefore be carefully examinedwhich oxygen limit value is really required, in particularwith respect to humid underground storage facilities.

■ Hydrogen As mentioned in Section 4, the natural gas infrastruc-ture still includes some sensitive elements where evenhydrogen volumes of less than 10 % may cause prob-lems. Further R&D input in the elds of CNG tanks, gas

turbines and underground storage facilities is thereforerequired. Once the results are available, future gas quality speci-cations and standards should include hydrogen.


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■ Siloxanes, halogens and other trace substances As mentioned, biomethane produced from conta-minated feedstock may involve the risk of unwantedtrace substances. Careful gas treatment and qualitycontrol are therefore indispensable prior to injectioninto the natural gas network. Today’s very high naturalgas quality levels must not be affected to protect boththe consumer and the image of natural gas. Tests haveconrmed that the purity level of natural gas is similarto the level of breathing air with respect to the contentof metals, halogens or similar [10].

CONCLUSION The natural gas qualities in Europe will become increa-singly diverse involving greater variations in combustioncharacteristics (Wobbe index, methane number). Except forrich LNG qualities, natural gases expected to come to themarket and biomethane will not pose any utilisation pro-

blems in most European countries as their Wobbe indicesare in a range from 49 MJ/m³ (13.6 kWh/m³) to just under56 MJ/m³ (15.5 kWh/m³). With certain restrictions this alsoapplies where up to 10 % of hydrogen is admixed exceptfor three important applications (CNG tanks, gas turbineswith premixed burners, underground storage facilities). These areas still require R&D input. Biomethane producedfrom contaminated feedstock may contain undesirabletrace substances. Particularly careful treatment and qualitycontrol are then necessary.

Hydrogen or methane produced from surplus renewa-ble energy has a high level of purity and, similar to bio-methane, contributes to further reducing carbon dioxideemissions. This will make natural gas an even more climate-protecting fuel compared with other fossil fuels. Standar-disation (harmonization) of gas quality specications willhelp to ensure smooth natural gas trading across borders.

LITERATURE

[1] Gronemann, U.; Forster, R.; Wallbrecht, J.; Schlerkmann, H.: OxygenContent in Natural Gas Infrastructure. gwf International 2010, pp.26-30.

[2] Graf, F.; Köppel, W.: Ergebnisse des DVGW Messprogramm „Bio-gaserzeugung und -aufbereitung“. gwf Gas/Erdgas 151 (2010), pp.110-119.

[3] Graf, F.; Bajohr, S.: Biogas-Erzeugung, Aufbereitung, Einspeisung.Oldenbourg Industrieverlag GmbH, 2010.

[4] Florisson, O. et al.: NaturalHy – Preparing for the hydrogen eco-nomy by using the existing natural gas system as a catalyst;An integrated project, Final Publishable Activity Report: http://www.naturalhy.net/docs/project_reports/Final_Publishable_Activity_Report.pdf

[5] Kopyscinski, J.: Production of synthetic natural gas (SNG) from coaland dry biomass – A technology review from 1950 to 2009. PaulScherrer Institut; Fuel, 89 (2010) 8, pp. 1763 – 1783.

[6] www.gascalc.de

[7] Christoph, K.; Cartellieri, W. und Pfeiffer, U.: Bewertung der Klopf-festigkeit von Kraftgasen mittels der Methanzahl und deren prak-tische Anwendung bei Gasmotoren. MTZ 33, (1972) No 10, pp.389-429.

[8] DGC – Danish Gas Technology Centre. Methane number calcula-tion of natural gas mixtures. Software Version 1.0.

[9] EASEE-gas Common Business Practice Nr. 2005-001/02, (harmoni-sation of gas quality) EASEE-gas European Association for theStreamlining of Energy Exchange – gas.

[10] van Almsick, T.; Kaesler, H.: Bestimmung von Spurenkomponen-ten in Erd- und Biogasen. gwf Gas/Erdgas 150 (2009).

AUTHORSDr.Klaus AltfeldE.ON Ruhrgas AGEssen, GermanyTel.: +49 (0)201/ [email protected]

Dr.Peter SchleyE.ON Ruhrgas AGEssen, GermanyTel.: +49 (0)201/ [email protected]

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Development of Nautral Gas Quality in Europe - 2.pdf