Decarbonisation of Fossil Energy Via Methane Pyrolysis G
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The Future Role of Hydrogen in Petrochemistry and Energy Supply DGMK Conference October 4-6, 2010, Berlin, Germany Decarbonisation of Fossil Energy via Methane 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 fuels have steadily increased. Additionally, there are potentially tremendous reserves of methane hydrates 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 carbon dioxide emissions. Following cracking, only the energy content of the hydrogen is used, while the carbon can be stored safely and retrievably in disused coal 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 natural gas 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 primary energy 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 hydrate 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 carbon dioxide. 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 combustion 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 southern Europe, 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 shale gas, 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 thermal decomposition 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 oxygen 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.