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The Future Role of in Petrochemistry and Energy Supply DGMK Conference October 4-6, 2010, Berlin, Germany

Decarbonisation of Fossil Energy via Pyrolysis G. Kreysa, D. W. Agar, I. Schultz Technische Universität Dortmund, Germany

Abstract Despite the rising consumption of energy over the last few decades, the proven reserves of fossil have steadily increased. Additionally, there are potentially tremendous reserves of methane available, which remain to be exploited. The use of fossil energy sources is thus increasingly being dictated less by supply than by the environmental concerns raised by climate change. In the context of the decarbonisation of the global energy system that this has stimulated, new means must be explored for using methane as energy source. Non- catalytic thermal pyrolysis of methane is proposed here as a promising concept for utilising methane with low to zero dioxide emissions. Following , only the energy content of the hydrogen is used, while the carbon can be stored safely and retrievably in disused mines. The thermodynamics and different process engineering concepts for the technical realisation of such a carbon moratorium technology are discussed. The possible contribution of methane pyrolysis to carbon negative geoengineering is also addressed.

Introduction Ever since the first oil crisis in 1973 at the latest, the conventional wisdom has gained ground in politics, the media and public opinion that the current basic structure of the global energy supply is unsustainable. The famous Reports of the Club of Rome [1,2] on the situation of mankind mainly broached the issue in terms of the apparent scarcity of resources. At the same time, however, they disregarded the fact that exploration is an expensive business that can only be financed from current production revenues, which explains why this apparent contradiction has still not been totally resolved. In the 30-year update of the first Club of Rome Report [3] this fact was acknowledged. In the years following 1973, therefore, the amounts of new fossil resources discovered annually almost always exceeded the amounts being produced. This fact is reflected in Table 1, in which the known reserves and supplies of fossil raw materials in 1973 are compared with those from 2006.

World inventories of fossil raw meterials 1973 2006 reserves production reserves resources production raw material EJ EJ/a EJ EJ EJ/a crude oil 3.732 101,5 6.805 3.430 163,9 non-conv. oil 2.761 10.460 2.026 49,3 6.891 7.866 111,4 non-conv. gas 76 58.335 coal 121.760 70,6 21.286 255.194 141,8 sum 127.518 221,4 37.819 335.285 417,1 Table 1: Development of world inventories of fossil raw materials between 1973 [2] and 2006 [4]

DGMK-Tagungsbericht 2010-3, ISBN 978-3-941721-07-4 31 The Future Role of Hydrogen in Petrochemistry and Energy Supply

Although the annual production of the three fossil energy sources, crude oil, natural gas and coal, has almost doubled in the period under consideration, today’s reserves (i.e. those permitting economic recovery using established technology) amount to 182 % and 340 % of the values for 1973 for crude oil and natural gas respectively. In order to assess these reserves and resources properly, it should be noted that in 2006 the world consumption of totalled 455 EJ. Table 1 clearly shows that the prophesised crisis of a raw material shortage has failed to materialise, which by no means alters the finiteness of reserves. It may, however, explain why, in the wake of the “oil crises”, the large-scale and successful R&D programmes addressing the problem of securing the supply of raw materials [5] made little impact on the raw materials base and the energy mix.

A further dramatic change in the resource situation is on the horizon if and when the ongoing intense development work on the recovery of methane hydrates from the ocean [6] bears fruit. Methane hydrates have been found in depths of 100 to 500 m along the continental coasts of almost all the world’s oceans. Wallmann at the Leibnitz Institute of Marine Sciences (IFM-GEOMAR) [6] estimates their reserves to be twice the amount of all the world’s known oil, natural gas and coal deposits (prior to the coal increase of 2006 [4]). This would correspond to total methane reserves of almost 7,000 Gt C. Admittedly, these figures are extremely uncertain; in other studies they vary between 76,000 Gt C and 3,000 Gt C. Even the lowest estimate of 3,000 Gt C still corresponds to huge energy reserves of 200,000 EJ though. Based on the annual consumption of fossil energy sources of 399 EJ in 2006 [4], this suggests a production life expectancy of around 500 years.

In view of the resource situation described, it is evident that this alone has not exerted any pressure for a dramatic change in the world’s energy supply. This only arose with the increasingly clear appreciation of the anthropogenic contribution to the greenhouse effect of atmospheric . Arrhenius [7] was the first to provide a quantitative description of the natural greenhouse effect of CO2, but it was not until the end of the 1980s that the focus shifted to the role of anthropogenic emissions, mainly attributable to the of fossil fuels [8,9]. Of all the numerous studies since that have dealt with the anthropogenic contribution to climate change and the consequences for our energy systems, only a few of the more recent ones are cited here [10-15].

A presently popular strategy for the reduction of CO2 emissions [10,11] is CCS technology (carbon capture and storage) for removing and disposal of CO2 from fossil-fired power plants and other centralised sources of CO2 (e.g. cement and steel production). The possible use of the ocean for long term CO2 storage is viewed sceptically by many scientists and the capacity of the world’s remaining available storage sites, such as oil and gas reservoirs, saltwater aquifers, coal beds and mines [9], is hardly likely to suffice for the CO2 generated from methane hydrate combustion. So far, the technology required has neither been fully developed nor is it available on the scale necessary. The industries concerned tend to regard CO2 absorption merely as a retrofitting measure, since the technical, legal and regulatory framework for CO2 storage, which also fall outside the remit of the companies, still need to be clarified.

In view of all the known difficulties in controlling the CO2 problem caused by carbon-based combustion technology, all the relevant studies and recommendations are unanimous in that a sustainable global energy supply will be based on a progressive decarbonisation of our energy systems. Generally, however, the chances for the transformation of the world energy system in this direction are not seen to be realistic until after 2050. The construction of solarthermal power plants in , around the Mediterranean [10] and in other sunny locations worldwide [14] has been strongly recommended as an important step towards this goal. If such unanimity exists on the objective of a solar energy economy, then all the technologies representing a step on this path should treated with high priority,

DGMK-Tagungsbericht 2010-3 32 The Future Role of Hydrogen in Petrochemistry and Energy Supply

because they are compatible with the ‘asymptotic’ future energy scenario and in particular, because they will make it possible to establish a hydrogen based energy system..

In addition to the vast methane hydrate reserves of 200,000 EJ, other natural gas stocks amount to a further 73,168 EJ (i.e. 37 % of the methane hydrate reserves) if the reserves, resources and non-conventional sources, such as , are included in the calculation (cf. Table 1). In relation to the total consumption of fossil energy sources in 2006, this extends the prospective production life span by a further 175 years. These figures call for a reassessment of the role of methane in a future energy portfolio.

Thermodynamics of a carbon moratorium based on methane pyrolysis A carbon moratorium based on methane pyrolysis has already been proposed previously [16,17] and will therefore only summarised briefly in the following section.

The conventional use of methane for energy today (mainly in the form of natural gas) is based on its complete combustion.

CH4 + 2 O2  CO2 + 2 H2O ΔH1000C = - 803.5 kJ/mol (1)

Methane, however, possesses two interesting thermodynamic features which pave the way for a climate-friendly alternative utilisation.

The production of hydrogen by the of water is only feasible at temperatures significantly above 3,000 K and is, moreover, constrained by the fact that, above 2,000 K, endothermic dissociation of hydrogen and molecules into their component atoms takes place [18]. Methane, by contrast, can be thermally cracked at temperatures above 500 0C, which are technically easily attainable.

CH4  C + 2 H2 ΔH1000C = + 91.7 kJ/mol (2)

Figure 1 shows the calculated temperature dependency [19,20] of the reaction enthalpy and free enthalpy for methane cracking. The data presented here, which were recalculated for reasons of consistency, are in good agreement with older data from the literature [21].

150,00

100,00 heat of reaction

50,00 CH4 C + 2 H2

Methan Kohlenstoff und Wasserstoff 0,00 energy / kJ/mol -50,00 free enthalpy change

-100,00

-150,00 25 200 400 600 800 1000 1200 1400

temperature / 0C Figure 1: Temperature dependence of heat of reaction and free enthalpy change for methane decomposition

DGMK-Tagungsbericht 2010-3 33 The Future Role of Hydrogen in Petrochemistry and Energy Supply

The second thermodynamic feature stems from a consideration of the combustion reactions for carbon and hydrogen.

C + O2  CO2 ΔH1000C = - 395.8 kJ/mol (3)

2 H2 + O2  2 H2O ΔH1000C = - 499.4 kJ/2mol (4)

By using reaction (4) to provide the endothermic reaction enthalpy needed for methane cracking (reaction 2), the energy yield from the hydrogen combustion is reduced to 407.7 kJ. After autothermal cracking of methane, of the original heat of combustion (803.5 kJ), 49.3 % (= 395.8 kJ) resides in the carbon and 50.7 % (= 407.7 kJ) is to be found in the remaining hydrogen. Given sufficient availability of methane, as is the case with non-conventional natural gas reserves and, in particular, in all probability, with methane hydrates, these thermodynamics open up an entirely new option for the energetic utilisation of methane. In contrast to technologies like CCS, it is relatively simple to sequester the solid carbon generated by methane cracking, either temporarily or permanently. Former coal mines, particularly open-cast lignite mines, would be eminently suitable for this purpose (for lignite power plants, only the direction of the freight trains would need to be reversed!). The energy content of carbon would not be lost irretrievably, because it could still be used in the future, when further technological developments or reduced atmospheric carbon dioxide levels permit. Hydrogen can be used as in flexible centralised gas power stations for or it can be employed in a variety of decentralised applications, such as fuel cells in domestic heat and power systems and for transportation. This novel CO2-free concept is referred to as the carbon moratorium, because it basically means putting a stop to paying for our energy demands with carbon combustion and its detrimental consequences for the climate.

Reaction Engineering for Methane Pyrolysis Methane pyrolysis for the production of , mainly used in the manufacture of tyres, is a well established, large-scale industrial process [22]. However, product quality rather than energy consumption is the primary concern in such processes. The energy required for methane decomposition (reaction 2) is supplied either by a plasma, as in the Kvaerner process [23], or by the partial oxidation of the feedstock and products. Such energy sources are, however, unsuitable for the decarbonisation of methane for because of the poor overall thermal efficiency (161.4 kJ/mol Methane for Kvaerner process) or accompanying carbon dioxide emissions. Quenching the product mixture directly following the reaction zone to prevent the reverse reaction is similarly an aspect of carbon black synthesis to be avoided in the interests minimising exergy losses.

Several catalysts have been identified, which enhance the rate of methane decomposition at lower temperatures, including carbon itself [24]. The application of catalysis to methane pyrolysis is precluded by both the unfavourable equilibrium position for the reaction at lower temperatures (figure 1) and the rapid blockage of the active sites by the solid carbon formed. The oxidative regeneration of deactivated catalysts, which has be proposed for the free generation of hydrogen from methane [25], would, of course, result in precisely the carbon dioxide emissions one is trying to avoid. The reconditioning required to maintain the activity of carbon catalysts also generates some carbon dioxide and entails extensive solid handling operations at very high temperatures.

Non-catalytic thermal decomposition of methane on the other hand poses the challenge of introducing of large amounts of heat at temperatures in excess of 600 °C. Furthermore, since the reaction occurs preferentially at the locations exhibiting the highest temperatures, the use of surfaces in recuperative or regenerative heat exchange processes would be expected to give rise to severe difficulties with carbon fouling. An obvious choice for in situ

DGMK-Tagungsbericht 2010-3 34 The Future Role of Hydrogen in Petrochemistry and Energy Supply

reactive heat generation – the oxidation of some of the hydrogen formed - can be rejected because of the difficulties in suppressing concomitant carbon formation, as is apparent in the existing carbon black processes employing this principle. Two possible techniques for supplying heat remain, which meet the demanding criteria imposed by methane pyrolysis: convective heating with an inert medium or heating by adiabatic compression.

Both gases and molten metals have been suggested as convective heating media for the endothermic methane decomposition reaction [26,27]. Problems with carbon deposition can be circumvented by introducing the heating gases through a porous reactor wall, thus ensuring that neither methane nor carbon comes into contact with hot reactor surfaces [26]. The selection of suitable gaseous heating media is problematic though: using a hydrogen recycle stream for heating purposes adversely affects the reaction equilibrium, while alternative gases must be subsequently separated from the hydrogen produced. In order to recover the heat from the hot reaction products, the carbon must be removed at high temperatures immediately after the reactor, in a cyclone for example, to prevent the reverse reaction of methane formation from occurring upon cooling. When molten metal is employed as the reaction medium [27], the continuous renewal of the contact surface between gas and liquid means the carbon deposits do not impede heat transfer and the carbon formed can be separated off from the melt by decantation. It is even conceivable that one can exploit possible catalytic activity of the metal in this manner. Nevertheless, ensuring intimate gas liquid contact for a sufficient period with such an aggressive media represents a considerable technological challenge.

In contrast to most other techniques of volumetric heat introduction, adiabatic compression provides the opportunity of recovering at least some of the energy supplied during the subsequent expansion phase, which can also be conducted rapidly so as to ‘freeze’ the high temperature equilibrium. Both free and driven piston arrangements have been proposed for such high temperature crack reactions [28,29]. The operation of such systems in the presence of carbon and the possible need for additional diluent remain to be clarified.

To summarise, non-catalytic methane pyrolysis is an extremely challenging reaction system and, while there are several promising reaction engineering solutions available, considerable development work is still required, in particular to guarantee the high thermal efficiency demanded by an economically and environmentally competitive hydrogen manufacturing process.

From carbon neutral to carbon negative energy Worryingly, the ever increasing carbon dioxide emissions from newly industrialising countries together with the lifetime of carbon dioxide in the atmosphere, estimated at around 100 years [30], mean that many of the measures presently being discussed to ameliorate climate change will be ‘too little, too late’. Simply fixing carbon dioxide in , for instance, a favourite form of emission offset, represents a postponement rather than a solution of the problem, since an overwhelming proportion (c. 99%) of biomass is simply reconverted to carbon dioxide during the decay process at a not too distant later date. It is thus necessary to look beyond carbon neutral to carbon negative approaches.

One such concept is the localised production of ‘’ [31] by low temperature catalytic hydrothermal carbonisation of biomass. It is proposed to use the inert carbonaceous residue generated as a soil conditioner. Whilst the principle is sound, in that carbon is transferred, hopefully irreversibly, from the atmosphere via the biosphere to the geosphere and other elements in the biomass are recycled back into the soil, the energy content of the original biomass, which is released as low temperature heat in what is an exothermic conversion process (reaction 5), is not really exploited effectively and the environmental impact of the multifarious by-products of a low temperature carbonisation remains unclear.

DGMK-Tagungsbericht 2010-3 35 The Future Role of Hydrogen in Petrochemistry and Energy Supply

“CH2O”  C + H2O ΔH298K = - 74 kJ/mol (5)

Were the biomass first to be disproportionated to a roughly equimolar mixture of methane and carbon dioxide, via biogas fermentation, aqueous phase reforming [32] or supercritical hydrothermal [33], the carbon content of the methane could be rendered into a truly inert form by the pyrolysis process described above, and a large part of the energy content of the biomass recovered as hydrogen. Half of the carbon in the biomass must be sacrificed as carbon dioxide for the conversion process (reaction 6) to be slightly exothermic. This carbon dioxide could, however, be used a sink for hydrogen generated from sources by converting it to methane, in a simple process which has been suggested for resolving the considerable discrepancies between supply and demand with sustainable energy [34]. In this way, further carbon would be made available for sequestration in the geosphere and the important buffer function of gas power plants in a future energy mix underscored.

“CH2O”  ½ CO2 + ½ CH4 ΔH298K = - 22.4 kJ/mol (6)

The localised conversion of biomass to a mixture of methane and carbon dioxide offers favourable logistics, since the presumably innocuous residues from the gasification process can be returned to the soil as fertiliser while an existing pipeline infrastructure can be exploited to methane to favourable sites for methane pyrolysis, and thus , and for hydrogen utilisation elsewhere.

In this concept, terrestrial biomass is primarily used to extract carbon dioxide from the atmosphere and only secondarily as an energy source. This reflects the strengths and weaknesses of photosynthesis and represents one of the more promising and less controversial geoengineering countermeasures proposed for mitigating climate change [35].

Those who query the apparently schizophrenic logic of mining coal at one location and burying carbon at another should bear in mind that mankind is unlikely to kick its fossil energy addiction, inextricably linked as it is to higher living standards, without being offered an alternative. Only alternatives which can replace most, if not all, of our present and future total energy demands within years rather than decades have the potential to avert serious climate change. This is not to deny the importance of energy saving or renewable energy, it is simply to recognise their intrinsic limitations in terms of both scale and implementation times.

1

ch4CH4/H2 0,8

h2CO2/H2 2 2 2 H H 3 3 0,6 /m /m 2 4 CH CO 0,4 3 3 m m3CH4/m3H m 0,2

0 SR SR + PD TD TD Sequestr. (CH4-fuel) (H2-fuel)

Figure 2: Comparison of methane consumption and CO2 emission per produced hydrogen for various hydrogen production processes [25]: SR - , PD – plasmochemical decomposition, TD – thermal decomposition

DGMK-Tagungsbericht 2010-3 36 The Future Role of Hydrogen in Petrochemistry and Energy Supply

The particular appeal of methane pyrolysis for providing decarbonised energy lies in its excellent compatibility with the existing energy infrastructure and the trend toward increasing renewables. It is a technology which can both supply large quantities of energy in the short to medium term and even help to reduce atmospheric carbon dioxide levels in the longer term. Analogous pyrolysis processes could also be used to decarbonise other energy feedstocks. The underlying compromise involved is the simplification of carbon sequestration task at the expense of sacrificing roughly half of the energy available in methane. Extensive investment in new efficient energy technologies, such as high temperature solid oxide fuel cells and heat pumps can actually recuperate the original calorific value of the methane. Nevertheless, the economics of hydrogen production by methane pyrolysis can be presently said to be foreseeable and ‘affordable’ but not competitive with more traditional routes, despite the clear environmental superiority (figure 2). Utilisation of the carbon in manner that avoids it ultimately being converted to carbon dioxide, for example as a building material [36], would certainly improve the process economics, but can hardly be envisaged on the scale required. It can be argued, however, that, in the final analysis, nature pays little attention to human foibles such as economics.

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[21] G. Collin, J. Schmalfeld et al.: ‚Chemierohstoffe aus Kohle’; in :R. Dittmeyer, W. Keim, G. Kreysa, A. Oberholz (Hrsg.): Winnacker  Küchler: Chemische Technik – Prozesse und Produkte, p 667, Volume 4: Energieträger, Organische Grundstoffe; WILEY-VCH, Weinheim (2005). [22] Voll, M.; Kleinschmit, P., Carbon, 6. Carbon Black. Wiley-VCH Verlag GmbH & Co. KGaA (2000) [23] Gaudernack, B., Lynum, S., Energy Conversion and Management, 38, p. 165 (1997) [24] Muradov, N. Z., Veziroglu, T. N., Int. J. of Hydrogen Energy, 33, p. 6804 (2008) [25] Muradov, N. Z., Energy & Fuels, 12, p. 41 (1998) [26] Matovich, E., U.S Patent No 4,056,602 (1977) [27] Steinberg, M., Int. J. of Hydrogen Energy, 24, p. 771 (1999) [28] Matturro, M., U.S Patent No 5,162,599 (1992) [29] Kronberg, A., Glouchenkov, M., Proceedings of 16th World Hydrogen Energy Conference, Lyon (2006) [30] Blasing, T.J., Carbon Dioxide Information Analysis Center, DOI: 10.3334/CDIAC/atg.032 [31] Antonietti, M., New Journal of Chemistry, 31, p. 787 (2007) [32] Dumesic, J.A., Applied Catalysis B: Environmental, 56, p.171 (2005) [33] Elliott, D. C., , Bioproducts and Biorefining, 2(3), p. 254 (2008) [34] Specht, M., Stürmer, B., Fach-Seminar 2010 des WBZU, Energiespeicherung – Zukunftskonzepte im Zeitalter Erneuerbarer Energien (2010) [35] Moore, J. C., Jevrejeva, S., Grinsted, A., Proc. Nat. Acad. Sci., 107(36), p. 15699 (2010) [36] Muradov, N. Z., International Journal of Hydrogen Energy, 18(3), p. 211 (1993)

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