Admissible Hydrogen Concentrations in Natural Gas Systems
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Reprint: gas for energy 03 / 2013 ISSN 2192-158X DIV Deutscher Industrieverlag GmbH www.gas-for-energy.com Admissible hydrogen concentrations in Reprint natural gas systems by Klaus Altfeld and Dave Pinchbeck There are proposals to inject hydrogen (H2) from renewable sources in the natural gas network. This measure would allow the very large transport and storage capacities of the existing infrastructure, particularly high-pres- sure pipelines, to be used for indirect electricity transport and storage. 1. SUMMARY AND CONCLUSIONS Investigations have been conducted to evaluate the impact of hydrogen as related to the above topics. At The results of this study show that an admixture of up to present it is not possible to specify a limiting hydrogen 10 % by volume of hydrogen to natural gas is possible in value which would generally be valid for all parts of the some parts of the natural gas system. However there are European gas infrastructure and, as a consequence, we still some important areas where issues remain: strongly recommend a case by case analysis. Some prac- ■ underground porous rock storage: hydrogen is a tical recommendations are given at the end of the paper. good substrate for sulphate-reducing and sulphur- reducing bacteria. As a result, there are risks associ- 2. INTRODUCTION ated with: bacterial growth in underground gas stor- age facilities leading to the formation of H2S; the con- In certain parts of Europe we have the situation already sumption of H2, and the plugging of reservoir rock. A where the generation of 'renewable' electricity from limit value for the maximum acceptable hydrogen wind and solar energy has led, from time to time, to pro- concentration in natural gas cannot be defined at the duction plants being shut down because the electricity moment. (H2-related aspects concerning wells have generated exceeds local requirements and the transpor- not been part of this project); tation or storage capacities are inadequate. It's a problem ■ steel tanks in natural gas vehicles: specification UN ECE that will become even more severe in the future because R 110 stipulates a limit value for hydrogen of 2 vol%; construction of new electricity lines and high-capacity ■ gas turbines: most of the currently installed gas tur- pumped storage power plants is a costly and very lengthy bines were specified for a 2H fraction in natural gas of process. Projects are therefore being discussed in which 1 vol% or even lower. 5 % may be attainable with the surplus electricity is used to power electrolysers that minor modification or tuning measures. Some new or will split water into its component parts, with the hydro- upgraded types will be able to cope with concentra- gen being directly injected into natural gas pipelines for tions up to 15 vol%; both storage and transportation. The concept has ■ gas engines: it is recommended to restrict the hydro- become known as "Power to Gas" or P2G. gen concentration to 2 vol%. Higher concentrations It is becoming more widely accepted that hydrogen up to 10 vol% may be possible for dedicated gas could become an important energy carrier in the energy engines with sophisticated control systems if the mix in the quest for sustainability, because it offers several methane number of the natural gas/hydrogen mix- benefits related particularly to the potential for energy ture is well above the specified minimum value; storage. Indeed it's possible that, with the existing infra- ■ many process gas chromatographs will not be capa- structure, hydrogen/natural gas mixtures could be trans- ble of analysing hydrogen. ported, stored and converted into electricity where REPORTS Gas quality most cases. However, there are bottlenecks which this project sets out to identify, proposing, where possible, solutions so that the natural gas infrastructure can be developed sufficiently to support the storage and trans- port of hydrogen-natural gas mixtures in a move towards a low carbon economy. This will, of course, cause natural - Coal- gas and electricity networks to become even more inter- dependent, as shown in Figure 1, and the project high- lights the R&D that will be necessary to achieve a robust solution based on the existing natural gas grid and its various constituent components. The project was initiated by E.ON New Build and Tech- nology GmbH, under the auspices of GERG, the European Gas Research Group, and has been conducted on behalf of a consortium of interested parties from a range of relevant industry sectors, including; gas industry; energy transmis- sion, power generation, equipment manufacturers, insti- tutes, etc. (See annex 1). Most of the work, which has con- sisted of reviews of existing work and literature searches, was carried out by a few, specially selected 'active' partners. uctuating No experimental work was involved but the collated, exist- ing information has been substantially augmented by in- house knowledge from both 'active' and project partners. - uctuating 3. COMBUSTION OF DIFFERENT GASES: GENERAL REMARKS The most important combustion parameters are Wobbe index, methane number and laminar flame speed. H-gases only are considered in this study. 3.1 Wobbe index Figure 1. Convergence of power and gas infrastructures The Wobbe index (W) is an indicator of the interchange- ability of different fuel gases. Regardless of calorific value, gases with the same W produce the same heat load in a required. However, if the addition of small quantities of gas burner. Therefore W is by far the most important hydrogen, up to 10 %, to natural gas pipelines is to be combustion parameter for gas appliances (except accepted, it must guarantee a technically feasible, eco- engines) and is specified in all countries. Admixture of nomically viable and, crucially, safe system of storage, hydrogen slightly decreases W (10 % hydrogen lowers W transportation and use. by some 3 %). Figure 2 illustrates the W of pure methane, It's clear that the European natural gas pipeline network biomethane (simplified analysis: C1=methane: 96 %; CO2: has the potential to offer such a solution and several studies, 4 %) and a "medium rich" LNG (C1: 92 %; C2: 5 %; C3: 2 %; including the EC-supported NaturalHy [1] project, have C4: 1 %). (N.B.: % always means vol% in this paper). examined the feasibility of using it as a means of widespread The W range is 13.8 kWh/m3-15.4 kWh/m3 for the hydrogen storage and transportation. However a number of gases without hydrogen admixture and 13.5-15.0 kWh/m3 crucial aspects were not sufficiently addressed in earlier if 10 % hydrogen is admixed (Wobbe index reference studies and work remains to examine these bottlenecks in temperatures: 0 °C (volume), 25 °C (combustion), 1.01325 the interaction between hydrogen and the wider European bar). It is obvious that the variations caused by the differ- natural gas network, including aspects of utilisation. ent gases are significantly higher than the effects caused The volume of hydrogen that may be added to natu- by 10 % hydrogen. However, if biomethane is considered, ral gas is limited. There are already some very low 'ad hoc' local Wobbe specifications can prevent hydrogen injec- limits in place, but studies [1] have shown that, with cer- tion because biomethane has already a low Wobbe index tain restrictions, admixture up to 10 % is not critical in (H-gases only are considered in this study). 2 gas for energy Issue 3/2013 Gas quality REPORTS 3.2 Methane number 20 The methane number (MN) describes the knock behav- iour of fuel gases in internal combustion engines and strongly depends on the specific gas composition and 15,4 14,9 14,5 15 15 especially the amounts of higher hydrocarbons (C3, C4, 13,8 13,5 ] C5) and hydrogen in the fuel gas. The MN of pure meth- 3 ane is 100, for pure hydrogen it's 0 and for rich LNG: without hydrogen 65-70, according to the AVL method [2]. [kWh/m 10 Figure 3 shows the MN of the gases described in gas with 10% hydrogen admixture chapter 3.1 without/with 10 % hydrogen admixture. Again it can be concluded that the MN of different 20 Wobbeindex gases without H2 show a greater variation (from 100 to 5 e.g. 74) than the effect of 10 % hydrogen (reduction by ≤ 10). However, if the natural gas already has a low MN 15,4 14,9 14,5 15 (e.g. rich LNG) the admixture of 10 % hydrogen15 can 0 13,8 13,5 ] result in an unacceptably low MN from a3 gas engine methane biomethane "medium rich" LNG operator's perspective (combined heat and power without hydrogen plants, dedicated natural gas vehicles). [kWh/m 10 Although the MN is an important parameter for gas Figure 2. Wobbe index of different gases without / gas with 10% engines it has not been specified in most of the EU with 10% hydrogen admixture hydrogen admixture member states. This may be changed as a result of the CEN TC 234 activities in developing a EuropeanWobbeindex gas 5 quality standard which includes the methane number. 110 103 100 3.3 Laminar flame speed 100 94 0 90 90 Flame speed is a complex combustion parametermethane biomethane "medium rich" LNG 80 related to flash back and flame stability. Both laminar 74 70 and turbulent flame speeds may be defined but, unfor- 70 tunately, they are difficult to measure and are not, there- without hydrogen 60 number fore, specified in technical rules and standards. e 50 gas with 10% Figure 4 illustrates that the experimental data for lami- hydrogen admixture nar flame speed, from different authors [3, 4, 5, 6, 7, 8], have methan 40 significant deviations but there is a trend to increasing 110 30 103 flame speed with increasing hydrogen addition.