Natural Gas

Fuel for the 21st Century

Vaclav Smil

• Editore: Wiley
• Anno: 2015
• ISBN:
  9781119012849
• Edizione: 1
• Argomento: Ciencias físicas
• Categoria: Energía

Natural gas is the world's cleanest fossil fuel; it generates less air pollution and releases less CO2 per unit of useful energy than liquid fuels or coals. With its vast supplies of conventional resources and nonconventional stores, the extension of long-distance gas pipelines and the recent expansion of liquefied natural gas trade, a truly global market has been created for this clean fuel.

Natural Gas: Fuel for the 21st Century discusses the place and prospects of natural gas in modern high-energy societies. Vaclav Smil presents a systematic survey of the qualities, origins, extraction, processing and transportation of natural gas, followed by a detailed appraisal of its many preferred, traditional and potential uses, and the recent emergence of the fuel as a globally traded commodity. The unfolding diversification of sources, particularly hydraulic fracturing, and the role of natural gas in national and global energy transitions are described. The book concludes with a discussion on the advantages, risks, benefits and costs of natural gas as a leading, if not dominant, fuel of the 21st century.

This interdisciplinary text will be of interest to a wide readership concerned with global energy affairs including professionals and academics in energy and environmental science, policy makers, consultants and advisors with an interest in the rapidly-changing global energy industry.

1
Valuable Resource with an Odd Name

Natural gas, one of three fossil fuels that energize modern economies, has an oddly indiscriminate name. Nature is, after all, full of gases, some present in enormous volumes, others only in trace quantities. Nitrogen (78.08%) and oxygen (20.94%) make up all but 1% of dry atmosphere’s volume, the rest being constant amounts of rare gases (mainly argon, neon, and krypton altogether about 0.94%) and slowly rising levels of carbon dioxide (CO₂). The increase of this greenhouse gas has been caused by rising anthropogenic emissions from combustion of fossil fuels and land use changes (mainly tropical deforestation), and CO₂ concentrations have now surpassed 0.04% by volume, or 400 parts per million (ppm), about 40% higher than the preindustrial level (CDIAC, 2014).

In addition, the atmosphere contains variable concentrations of water vapor and trace gases originating from natural (abiogenic and biogenic) processes and from human activities. Their long list includes nitrogen oxides (NO, NO₂, N₂O) from combustion (be it of fossil fuels, fuel wood, or emissions from forest and grassland fires), lightning, and bacterial metabolism; sulfur oxides (SO₂ and SO₃) mainly from the combustion of coal and liquid hydrocarbons, nonferrous metallurgy, and also volcanic eruptions; hydrogen sulfide (H₂S) from anaerobic decomposition and from volcanoes; ammonia (NH₃) from livestock and from volatilization of organic and inorganic fertilizers; and dimethyl sulfide (C₂H₆S) from metabolism of marine algae.

But the gas whose atmospheric presence constitutes the greatest departure from a steady-state composition that would result from the absence of life on the Earth is methane (CH₄), the simplest of all hydrocarbons, whose molecules are composed only of hydrogen and carbon atoms. Methane is produced during strictly anaerobic decomposition of organic matter by species of archaea, with Methanobacter, Methanococcus, Methanosarcina, and Methanothermobacter being the major methanogenic genera. Although the gas occupies a mere 0.000179% of the atmosphere by volume (1.79 ppm), that presence is 29 orders of magnitude higher than it would be on a lifeless Earth (Lovelock and Margulis, 1974). The second highest disequilibrium attributable to life on the Earth is 27 orders of magnitude for NH₃.

Methanogens residing in anaerobic environments (mainly in wetlands) have been releasing CH₄ for more than three billion years. As with other metabolic processes, their activity is temperature dependent, and this dependence (across microbial to ecosystem scales) is considerably higher than has been previously observed for either photosynthesis or respiration (Yvon-Durocher et al., 2014). Methanogenesis rises 57-fold as temperature increases from 0 to 30°C, and the increasing CH₄:CO₂ ratio may have important consequences for future positive feedbacks between global warming and changes in carbon cycle.

Free-living methanogens were eventually joined by archaea that are residing in the digestive tract (in enlarged hindgut compartments) of four arthropod orders, in millipedes, termites, cockroaches, and scarab beetles (Brune, 2010), with the tropical termites being the most common invertebrate CH₄ emitters. Although most vertebrates also emit CH₄ (it comes from intestinal anaerobic protozoa that harbor endosymbiotic methanogens), their contributions appear to have a bimodal distribution and are not determined by diet. Only a few animals are intermediate methane producers, while less than half of the studied taxa (including insectivorous bats and herbivorous pandas) produce almost no CH₄, while primates belong to the group of high emitters, as do elephants, horses, and crocodiles.

But by far the largest contribution comes from ruminant species, from cattle, sheep, and goats (Hackstein and van Alen, 2010). Soil-dwelling methanotrophs and atmospheric oxidation that produces H₂O and CO₂ have been methane’s major biospheric sinks, and in the absence of any anthropogenic emissions, atmospheric concentrations of CH₄ would have remained in a fairly stable disequilibrium. These emissions began millennia before we began to exploit natural gas as a fuel: atmospheric concentration of CH₄ began to rise first with the expansion of wet-field (rice) cropping in Asia (Ruddiman, 2005; Figure 1.1).

Figure 1.1 Methanogens in rice fields (here in terraced plantings in China’s Yunnan) are a large source of CH₄.

Reproduced from http://upload.wikimedia.org/wikipedia/commons/7/70/Terrace_field_yunnan_china_denoised.webp. © Wikipedia Commons.

Existence of inflammable gas emanating from wetlands and bubbling up from lake bottoms was known for centuries, and the phenomenon was noted by such famous eighteenth-century investigators of natural processes as Benjamin Franklin, Joseph Priestley, and Alessandro Volta. In 1777, after observing gas bubbles in Lago di Maggiore, Alessandro Volta published Lettere sull’ Aria inflammabile native delle Paludi, a slim book about “native inflammable air of marshlands” (Volta, 1777). Two years later, Volta isolated methane, the simplest hydrocarbon molecule and the first in the series of compounds following the general formula of C_nH_2n+2. When in 1866 August Wilhelm von Hofmann proposed a systematic nomenclature of hydrocarbons, that series became known as alkanes (alkenes are C_nH_2n; alkines are C_nH_2n−2).

The second compound in the alkane series is ethane (C₂H₆), and the third one is propane (C₃H₈). The fossil fuel that became known as natural gas and that is present in different formations in the topmost layers of the Earth’s crust is usually a mixture of these three simplest alkanes, with methane always dominant (sometimes more than 95% by weight) and only exceptionally with less than 75% of the total mass (Speight, 2007). C₂H₆ makes up mostly between 2 and 7% and C₃H₈ typically just 0.1–1.3%. Heavier homologs—mainly butane (C₄H₁₀) and pentane (C₅H₁₂)—are also often present. All C₂–C₅ compounds (and sometimes even traces of heavier homologs) are classed as natural gas liquids (NGL), while propane and butane are often combined and marketed (in pressurized containers) as liquid petroleum gases (LPG).

Most natural gases also contain small amounts of CO₂, H₂S, nitrogen, helium, and water vapor, but their composition becomes more uniform before they are sent from production sites to customers. In order to prevent condensation and corrosion in pipelines, gas processing plants remove all heavier alkanes: these compounds liquefy once they reach the surface and are marketed separately as NGL, mostly as valuable feedstocks for petrochemical industry, some also as portable fuels. Natural gas processing also removes H₂S, CO₂, and water vapor and (if they are present) N₂ and He (for details, see Chapter 3).

1.1 METHANE’S ADVANTAGES AND DRAWBACKS

No energy source is perfect when judged by multiple criteria that fully appraise its value and its impacts. For fuels, the list must include not only energy density, transportability, storability, and combustion efficiency but also convenience, cleanliness, and flexibility of use; contribution to the generation of greenhouse gases; and reliability and durability of supply. When compared to its three principal fuel alternatives—wood, coal, and liquids derived from crude oil—natural gas scores poorly only on the first criterion: at ambient pressure and temperature, its specific density, and hence its energy density, is obviously lower than that of solids or liquids. On all other criteria, natural gas scores no less than very good, and on most of them, it is excellent or superior.

Specific density of methane is 0.718 kg/m³ (0.718 g/l) at 0°C and 0.656 g/l at 25°C or about 55% of air’s density (1.184 kg/m³ at 25°C). Specific densities of common liquid fuels are almost exactly, 1,000 times higher, with gasoline at 745 kg/m³ and diesel fuel at 840 kg/m³, while coal densities of bituminous coals range from 1,200 to 1,400 kg/m³. Only when methane is liquefied (by lowering its temperature to −162°C) does its specific density reach the same order of magnitude as in liquid fuels (428 kg/m³), and it is equal to specific density of many (particularly coniferous) wood species, including firs, cedars, spruces, and pines.

Energy density can refer to the lower heating value (LHV) or higher heating value (HHV); the former rate assumes that the latent heat of vaporization of water produced during the combustion is not recovered, and hence it is lower than HHV that accounts for the latent heat of water vaporization. Volumetric values for methane are 37.7 MJ/m³ for HHV and 33.9 MJ/m³ for LHV (10% difference), while the actual HHVs for natural gases range between 33.3 MJ/m³ for the Dutch gas from Groningen to about 42 MJ/m³ for the Algerian gas from Hasi R’Mel. Again, these values are three orders of magnitude lower than the volumetric energy density of liquid fuels: gasoline’s HHV is 35 GJ/m³ and diesel oil rates nearly 36.5 GJ/m³. Liquefied natural gas (50 MJ/kg and 0.428 kg/l) has volumetric energy density of about 21.4 GJ/m³ or roughly 600 times the value for typical natural gas containing 35–36 MJ/m³.

Methane’s low energy density is no obstacle to high-volume, low-cost, long-distance terrestrial transport. There is, of course, substantial initial capital cost of pipeline construction (including a requisite number of compression stations), and energy needed to power reciprocating engines, gas turbines, or electric motors is the main operating expenditure. But as long as the lines and the compressors are properly engineered, there is no practical limit to distances that can be spanned: multiple lines bring natural gas from supergiant fields of Western Siberia to Western Europe, more than 5000 km to the west. Main trunk of China’s West–East pipeline from Khorgas (Xinjiang) to Guangzhou is over 4,800 km long, and eight major branches add up to the total length of 9,100 km (China.org, 2014). Moreover, pipelines transport gas at very low cost per unit of delivered energy and can do so on scales an order of magnitude higher than the transmission of elec...