LBST Study

MOLECULAR HYDROGEN AND WATER VAPOUR EMISSIONS IN A GLOBAL HYDROGEN ENERGY ECONOMY

W. Zittel, M. Altmann, Ludwig-Bölkow-Systemtechnik GmbH

Published in the Proceedings of the 11^th World Hydrogen Energy Conference, Stuttgart, Germany, June 1996

ABSTRACT

In a global renewable energy economy including hydrogen as energy carrier, emissions of molecular hydrogen and water vapour will play the dominant role as all other emissions will be sharply reduced, most of them to zero.

Reviewing the natural and anthropogenic sources and sinks of molecular hydrogen and assuming ten percent losses of hydrogen over the whole production, distribution and utilisation chain it is estimated that in a hydrogen energy economy hydrogen emissions will be about four times higher than those caused by the use of fossil energy resources. Thus reducing the hydrogen losses to a level of 2 - 3 %, hydrogen emission levels will be in the same range as fossil hydrogen emissions.

Anthropogenic water vapour emissions on the other hand can be shown to be around 0.005 % of natural water evaporation. The present combustion of fossil energy carriers containing water and/or hydrogen as well as the evaporation of water in the cooling systems of electricity generation cause water vapour emissions exceeding the projected emissions due to the utilisation of hydrogen by roughly a factor of two.

1. INTRODUCTION

Today's fossil and nuclear energy economy has the drawback of nonsustainability: The ressources are finite as are the sinks of our combustion products. Both problems will force us to change our economy completely where the question is not if but when, and which problem (finite ressources or sinks) may trigger new techologies first.

A sustainable energy economy using the solar flux has the need of a mobile energy carrier and storage. Hydrogen can play a major role in such an economy whereever its qualities are needed: it is mobile and can replace oil as fuel for vehicle and aircraft, it is gaseous and can be transported and distributed in pipelines instead of natural gas. But in terms of sustainability its way of production has to be environmentally benign. Its combustion products (and evaporation losses) should not disturb natural cycles and balances.

It is the aim of this article to sketch the emissions of water vapour and hydrogen from a global hydrogen energy economy in the context of total hydrogen and water balances. Water vapour emissions of a hydrogen energy economy can be even smaller than those from the present fossil and nuclear energy economy and their share of 0.005 % of the total atmospheric water cycle is negligible. However, this does not hold for applications in stratospheric aircraft and rockets where these disturbations may be serious.

Hydrogen emissions of a future energy economy - when assuming 2 to 3 percent losses into the atmosphere - would be in the same range as those of present day hydrogen emissions from uncomplete fossil fuel combustion (under the assumption that half of the present energy supply would be covered by hydrogen).

2. RESULTS

2.1. CARBON DIOXIDE

Despite local emissions of NO_x, SO₂, CO, HC and particulates carbon dioxide and methane emissions represent the most serious problem caused by the combustion of fossil fuels resulting in a disturbation of the chemical content of the atmosphere with strong influence on its radiative characteristics.

Table 1 lists the CO₂ emissions from fossil fuel combustion. H₂ has no direct emissions. Indirect emissions due to its generation depend on the way of generation and - in the worst case - might even be higher than those listed for fossil fuels. Electrolysis, e.g. with hard coal as primary energy carrier would lead to emissions of around 1,400 g/kWh; electrolysis from hydropower on the other hand would lead to almost negligible emissions. As an example results of an analysis within the Euro-Québec Hydro-Hydrogen Pilot-Project [1] are presented in the following. Within that project the transformation and transportation chain of hydroelectricity at La Grande, Québec, via electrolysis and liquefaction at Sept Isles, Québec, as well as the maritime transportation of LH₂ to Hamburg, Germany, and the final distribution and application there were studied.

Figure 1 shows the resulting carbon dioxide emissions [2]. The right column shows the emissions due to electricity production. These result in 22 - 35 g CO₂ /kWh LH₂ (lower heating value) and are due to construction (concrete, steel and fossil fuels: ~ 2 g CO₂/kWh_el) and operation (CH₄-emissions of flooded organic matter resulting in 3.5 - 7 g CO₂-equivalent/kWh_el) of the La Grande hydropower complex. The middle column adds those emissions that are due to the construction of the electrolyser and liquefaction plant at Sept Iles, the hydrogen barges and ship as well as the port facilities and distribution hardware in Hamburg, assuming that these are built with conventional methods releasing CO₂ during materials production. The left column adds the emissions due to fossil fuel use during operation under today's conditions. The main increase of CO₂ results from the conventionally fuelled LH₂ tanker and nearly doubles the emissions reaching a total of 78 - 92 g CO₂/kWh LH₂ (lower heating value).

2.2. WATER VAPOUR

Table 2 shows the present contribution of several trace gases to green house warming [3]. Though atmospheric water vapour content is responsible for about 2/3 of the total greenhouse warming (20.6 °C of a total of 33 °C) its additional contribution of a few percent to anthropogenic greenhouse warming is small (for the stratospheric situation see below). Carbon dioxide with a share of 50 %, methane and CFC with at present about 15 - 20 % each are dominant.

The water vapour content of the atmosphere fluctuates between far below 1 % and several percent with an annual average of 2.6 %-vol, resp. 13×10¹⁸ g H₂O. With a tropospheric life time of 8 - 9 days this implies an annual evaporation and precipitation of 525×10¹⁸ g H₂O [3]. Due to the temperature increase of tropical sea surface water an annual concentration increase of 0.5 kg/yr per m² of air column has been measured [4]. From this annual concentration increase an annual increase of the evaporation of about 1.2×10¹⁸ g H₂O/yr is calculated [5].

In the context of this atmospheric water content of 525×10¹⁸ g H₂O and its annual increase due to tropical sea surface temperature rise the anthropogenic emissions of the present and future energy sector will be discussed. Today's world energy economy emitts around 20×10¹⁵ g H₂O/yr. About half of this stems from cooling water evaporation if we assume that about 80 % of the 10 PWh_el/yr are produced by thermal power plants with an average water consumption of 1.5 kg H₂O/kWh_el. The rest is due to the hydrogen content of fossil fuels and its release during combustion (table 1).

If we assume the same primary energy consumption of about 90 PWh/yr to be completely supplied by renewables then hydrogen would contribute only to a certain share. If we assume that 50 % of today's primary energy consumption would be covered by hydrogen, this would result in about 12×10¹⁵ g H₂O/yr. In contrast to the fossil fuel economy electricity production will mostly be done with fuel cells avoiding large cooling water consumptions. As a side effect this will simplify electrification of arid zones.

In total, the water vapour emissions of the already existing fossil based energy economy sum up to about 20×10¹⁵ g H₂O/yr. This is a share of less than 2 % of the estimated annual oceanic evaporation increase due to global warming (1,200×10¹⁵ g H₂O/yr) and less than 0.005 % of the whole annual evaporation (525,000×10¹⁵ g H₂O/yr). Even doubling or tripling these emissions would be negligible and would not influence global water vapour cycles. In contrast, the anthropogenic carbon dioxide emissions of about 6.5×10¹⁵ g C/yr hold a share of about 4 % of the total annual emissions (~160×10¹⁵ g C/yr) which is an order of magnitude that can influence the global budgets substantially when accumulating.

Besides global considerations, additional water vapour emissions might have local and regional effects as, for example, is seen by increased precipitation and fog formation around huge thermal power plants. These local emissions from a 1 GW power plant, for instance, reach about 10×10¹² g H₂O/yr. For comparison, if the whole indvidual traffic of the city of Munich was fuelled with hydrogen it would produce water vapour emissions of a factor of ten less (10¹² g H₂O/yr) or, distributed over the 400 km² of the city, specific emissions of 2.5 kg/m²/yr [5]. Today's traffic of Munich emitts around 1 kg H₂O/m²/yr. The annual natural precipitation of Munich is in the order of 950 mm H₂O/m² or 950 kg H₂O/m²/yr [6]. Therefore, the additional water vapour content will influence natural water cycles only marginally.

On the other hand, high flying aircraft will influence the stratospheric water vapour budget substantially. Today's lower stratosphere contains 1.3×10¹⁵ g H₂O with an annual average life time of one year. The water vapour sources are as follows [7],[8]:

- diffusion troposphere ® stratosphere (~220×10¹² g H₂O/yr)

- diffusion jets during storms (~880×10¹² g H₂O/yr)

- methane and C_nH_m destruction in the stratosphere (~110×10¹² g H₂O/yr)

- today's high flying aircraft (40×10¹² g H₂O/yr).

For the latter emissions it is assumed that about 20 % of all flights reach the lower stratosphere around 11 km height and mainly north of 40° N. According to [8] this contributes to the observed mixing ratio of 4 ppm for the lower stratosphere by about 5 percent.

According to the same reference, a ten percent increase of water vapour in the 10 - 15 km layer would increase the net radiation flux density by 0.75 W/m², where, for comparison, a doubling of CO₂ would lead to a net flux enhancement of 4 W/m² [8]. This effect may even be enhanced by the formation of condensation trails (contrails) where observations indicate that the present 20 g H₂O emissions per flight-meter trigger the contrail formation of undercooled stratospheric water vapour (depending on actual temperature and humidity conditions) of up to 10 km in width and 2 km in height [9].

Converting the whole aircraft fleet to hydrogen operation would result in about 500×10¹⁵ g H₂O/yr. Not changing cruise altitude would increase the H₂O mixing ratio of the lower stratosphere by about 20 % with the above described consequences. Therefore it is a "must" that hydrogen powered aircraft reduce their cruising altitude below the tropopause. Calculations within the former Deutsche Airbus Industries [10] indicate that reduction to 9 km height (28,000 ft) in summer and to 7 km height (21,000 ft) in winter are possible when accepting increased operation costs of about 1 % and 4 %, respectively.

2.3. MOLECULAR HYDROGEN

The hydrogen content of the earth´s atmosphere amounts to about 0,51 ppm or 180 - 200×10¹² g H₂ with a tropospheric life time of 4 - 9 years depending on the actual OH-concentration. In recent years a concentration increase of 0.5 %/yr or 3.2 ppb/yr corresponding to 0.6 - 1.6×10¹² g H₂/yr has been observed.

The sources of hydrogen may be split into three groups [11], [12]:

- natural sources: These are exhaust emissions from volcanos, geothermal steam, oceans and soils with aerobic bacteria. The source strenghts are estimated to lie between 2 and 5×10¹² g H₂/yr.

- photochemical sources: These are both natural and anthropogenic and are due to the destruction of formaldehyde (CH₂O ® CO + H₂) where CH₂O itself is a destruction product of hydrocarbons. Methane oxidation is estimated to contribute by 11 - 16 ×10¹² g H₂/yr. Another source of 10 - 35 ×10¹² g H₂/yr is the destruction of natural isoprene and terpene emissions from soils. Additional CH₂O might be due to other hydrocarbons.

- anthropogenic sources: These are mostly hydrogen shift reactions from uncomplete combustion processes (CO+H₂O ® CO₂+H₂). They are estimated to be 9 - 21×10¹² g H₂/yr due to biomass burning and 11 - 57×10¹² g H₂/yr due to the combustion of fossil fuels. Today's hydrogen supply with 500 bill. Nm³/yr results in a net source of 0.5×10¹² g H₂/yr when assuming a 1 percent loss of the whole production volume.

The above-mentioned annual increase of the atmospheric H₂ concentration is believed to be mostly due to the increase in fossil fuel combustion and methane destruction.

Most of this hydrogen is destroyed in the troposphere. Chemical destruction (OH + H₂ ® H₂O + H) is estimated to amount to 10 - 24×10¹² g H₂/yr, where the resulting products again are involved in chemical processes (e.g. H + O₂ + M ® HO₂ + M and H₂O + O(¹D) ® 2 OH). Bacterial destruction in soils amounts to 20 - 107 ×10¹² g/yr. Only a small portion of between 0.6 and 1.6 ×10¹² g/yr enters the stratosphere.

The H₂-content of stratosphere and mesosphere amounts to about 30×10¹² g with a strongly varying life time of between 1 and 100 years (depending on chemical composition and temperature). Measurements of the isotopic composition indicate that the hydrogen and water vapour of the upper stratosphere result mostly from hydrocarbon decomposition, not from tropospheric diffusion of H₂ or H₂O [13].

In a height of 40 - 80 km the decomposition H₂ + OH ® H₂O + H is the dominant destruction mechanism. Above 140 km atomic hydrogen remains the most abundant chemical "compound" where a loss flux of between 16 - 31 ×10⁷ particles/cm² is calculated from experimental data [14]. This flux gives a total loss of 50 - 100 ×10⁹ g H/yr. Most of this hydrogen is assumed to result from hydrocarbon destruction [15]. For comparison, if the whole H-losses came from oceanic H₂O this would reduce oceanic sea surface levels by 10 atomic layers per year or by 1 mm in 1000 years [12].

Assuming, as in the preceeding chapter, that 50% of the present primary energy consumption of 90 PWh/yr are supplied by hydrogen a total amount of about 1.3×10¹⁵ g H₂/yr would be produced. Partly it would be consumed at its production site, partly transported as liquid hydrogen with evaporation losses. Today's losses of gaseous hydrogen are far below 1 % (about 0.1 % are reported from the existing hydrogen distribution grid in Germany; about 0.7 % gas losses are reported from the existing natural gas grid in Germany), those of liquid hydrogen depend on the method of handling and are reported to be from around 1 % to up to 10 %. At the worst, a ten percent loss of 45 PWh/yr hydrogen would result in 130×10¹² g H₂/yr and would nearly double today's world wide hydrogen emissions. On the other hand a loss of 2 - 3 % (which seems to be more realistic) would result in 26 - 40×10¹² g H₂/yr, emissions comparable to today's hydrogen emissions from fossil fuel combustion (11 - 57×10¹² g H₂/yr).

3. CONCLUSION

Assuming a future renewable energy economy with today's total energy consumption but completely supplied by renewables with a 50 % share of hydrogen results in water vapour and molecular hydrogen emissions which are comparable to today's emissions of these trace gases from fossil fuel combustion. From the environmental point of view, molecular hydrogen emissions will react mostly in the troposphere. There are indications that hydrocarbon emissions - besides other effects - could increase the stratospheric and exospheric hydrogen content more seriously through their decomposition in high altitudes.

4. LITERATURE REFERENCES

[1] Euro-Québec Hydro-Hydrogen Pilot-Project, Phase II Feasibility Study, Hydro-Québec, Montreal, and Ludwig-Bölkow-Foundation, Ottobrunn, 1990

[2] W. Zittel, Investigation and Damages by Hydro-Hydrogen on Environment and Men, Euro-Québec Hydro-Hydrogen Pilot-Project, Phase III.0-1, Final Report, Ludwig-Bölkow-Systemtechnik GmbH, Ottobrunn, February 1992

[3] Protecting the Earth, Third Report of the Enquête Commission of the 11th German Bundestag "Preventive Measures to Protect the Earth´s Atmosphere", Bonn 1991

[4] H. Flohn, A. Kapala, Changes of tropical sea-air interaction processes over a 30-year period, Nature, vol. 338 (1989), p.244

[5] W. Zittel, M. Altmann, Der Einfluß von Wasserdampf auf das Klima, Energie, April 1994, p. 25

[6] Statistical Yearbook of the Federal Republic of Germany, 1992, Wiesbaden.

[7] U. Schumann, Auswirkungen des Luftverkehrs auf die Umwelt, Vortrag am 12.12.1990, DLR, Oberpfaffenhofen

[8] H. Graßl, Impact of increased water vapour emission on the atmosphere, EQHHPP contract No. 330 final report, Hamburg, 1992

[9] U. Schumann, Klimatische Auswirkungen von Schadstoffen des hochfliegenden Luftverkehrs, in "Flugverkehr und Umwelt", Jornalistenseminar, Bd. 8, GSF-Forschungszentrum für Umwelt und Gesundheit, Neuherberg, 1991

[10] Volkhausen, Klug, DASA, unpublished, 1992

[11] U. Schmidt, G. Kulessa, E.P. Röth, The atmospheric H₂-cycle in "Proceedings of the NATO advanced Study Institute on atmospheric Ozone: Its variation and human influences", ed. by A.C. Aikin, Aldeia das Acoteias, Portugal, 1 - 13 October, p. 307, 1979

[12] W. Zittel, Molekularer Wasserstoff in der Atmosphäre, Ludwig-Bölkow-Systemtechnik GmbH, Ottobrunn, december 1990

[13] C.P. Rinsland et al., Stratospheric Profiles of Heavy Water Isotopes and CH₃D From Analysis of the ATMOS Spacelab 3 Infrared Solar Spectra, J. Geoph. Res., vol.96, No. D1, 1057 - 1068 (1991)

[14] S.C. Liu, T.M. Donahue, Realistic model of hydrogen constituents in the lower atmosphere and escape flux from the upper atmosphere, J. Atmos. Sciences 31, (1974), 2238 - 2242

[15] D.H. Ehhalt, On the consequence of a tropospheric CH₄ increase to the exospheric density, Jour. Geophys. Res. 91 (1986), 2843

Table 1: Specific carbon dioxide and water vapour emissions from fossil fuels and hydrogen (based on lower heating value). Note that hydrogen as secondary energy carrier has no direct CO₂-emissions. Indirect emissions are discussed in the text. For comparisons, the energy efficiency of the final application also has to be taken into account.

Table 2: Atmospheric share of trace gases and their absolute and specific contribution to the total greenhouse effect (natural and anthropogenic)

Figure 1: CO₂-Emissions of LH₂ within the Euro-Québec Hydro-Hydrogen Pilot-Project; Emissions in g CO₂/kWh LH₂ (LHV) after receipt and distribution of hydrogen in Hamburg. The first column gives the figures as achievable within the planned project; the figures in the second column are achievable if all fuels consumed would be carbon free. The figures in the last column could be achieved in an advanced energy economy, where also steel and concrete would be produced without releasing CO₂.

Figure 2: Estimated natural, photochemical and anthropogenic H₂-emissions into the atmosphere (based on ref. [11], [12])