In praise of fossil fuels—Part 2: the remarkable legacy of ancient life

Over the past quarter century, the justifiably deep concern held by many of us about climate change has led to a shift in humanity’s relationship with fossil fuels—burning of which accounts for well over 60 percent of global anthropogenic greenhouse gas emissions. This relationship seems to have shifted from what  might be roughly characterised as ‘appreciative ambivalence’ (as long as the supply spigots remained open) towards uneasiness at first and more recently, even open animosity. Given the bad rap that fossil fuels get from many of us now, there’s more than a little irony in the fact that, even beyond the idea that the modern human identity is shaped—as I suggested last week—in the image of fossil fuels, we humans and the energy sources that enable our present ways of living are expressions of the same Earth-centred and carbon-based life continuum. As the remnants of vast accumulations of deceased organisms laid down over millions of years, fossil fuels are in a manner of speaking a gift left to us by our ancient selves. Granted, holding such a view requires that we first adopt a bio-centric sense of identity—an identity with all Earth-based life across evolutionary time. Assuming such a default mode of self-understanding may be taking things just a little too far for some readers. In the interests of advancing the systemic intent of our inquiry though, this is perhaps not an entirely unreasonable suggestion, at least as a provocation to new thinking about our collective situation i.e. a perspective with which to experiment in order to see what it might reveal.

At a lesser stretch perhaps, the observation that we and our primary energy sources are made of the same stuff, via the same basic organising process, is unlikely to strike most people as too controversial. From this point of view, a perspective is available in which we might appreciate the energy-related existential dilemmas of contemporary industrial society as a set of ever-present potentials built into the fabric of this living planet—and, it might be further argued, into the cosmos itself. I offer this view here as an opportunity to cut ourselves a little slack with respect to the self-disparagement that some of us are prone to casting upon the human species, and within which we are treated as a destructive force acting contrary to the well-being of nature, or ‘the planet’. We and our present situation are that nature in action, an expression of the ever present potentials enfolded within the cosmos, as it unfolds creatively across time. In this sense, to view our present situation as anomalous is not to see it as separate from or contrary to the processes of nature or of life by which we are also embraced.

While the geneses of fossil fuels are typically traced to their biological origins, the materials that we recover from beneath the Earth’s surface to use as energy sources may be more effectively regarded as bio-geological phenomena. It’s via processes of geo-sequestration that the remains of ancient organisms have first accumulated and then transformed to become coal, oil—more accurately, petroleum, but I’ll stick with the common terminology here—and  natural gas (conversion paths 2 and 3 in this diagram from earlier on in our inquiry). We recover these materials to exploit as energy resources at points across their transformation spectra and life spans—in the case of coals for example, from peat, really a pre-cursor to coal that is closer to compacted plant matter, through to anthracite, in which most of the hydrocarbon compounds that give away the substance’s origins have broken down leaving only the carbon in place. Despite the ways in which the materials that we categorise together as coal, oil or natural gas differ from place to place, there are two overarching characteristics that each shares and that are central to their collective economic importance.

Firstly, each exhibits what might be termed ‘stable instability’. The raw primary materials, or refined fuels produced from them, are sufficiently reactive (or unstable) to support vigorous continued combustion in air once ignited, while at the same time requiring relatively high ignition energy in order for combustion to initiate in the first place. For example, mechanical shocks associated with mining, handling and transport are insufficient to cause most conventional fossil fuels to burn. This is a very general statement—reactivity differs markedly between different fossil fuels. Lignite (brown coal), for example, is prone to spontaneous combustion, and so cannot be stored for extended periods or transported long distances. In the Latrobe Valley to the east of Melbourne where I live, brown coal is used to fuel most of the electricity generation for the state of Victoria. In order to avoid problems with spontaneous combustion during handling, the power stations are located immediately adjacent to the open cuts and the coal is essentially mined on a just-in-time basis. Even so, fires in the open cuts are a relatively regular occurrence, and can burn for weeks at a time. This poses a particular risk for electricity supply, especially in the summer bushfire season. Issues of this nature notwithstanding, it’s this relative stability that has allowed organic material from many tens of millions of years ago to be available to us today as fossil fuels. For example, the oil that we’re in the process of using over a period of perhaps a couple of hundred years formed from organic matter that accumulated between perhaps 20 and 300 million years ago. The manner in which the parent organic matter accumulated both prevented it from breaking down in the short term (resulting in its removal from the biosphere’s short-term carbon cycle), and provided the appropriate conditions of temperature and pressure, for the necessary period of time, to convert that original material to the forms that today serve us so well as primary energy sources.

The present forms of the human socio-, econo- and techno-spheres have in a sense been shaped quite directly by those same serendipitous occurrences in the ancient geo-sphere; the ways that the Earth behaves geologically over periods vastly longer than the span of human history have profound sociological implications and consequences for us today. A key consequence of this stable geo-sequestration and slow geo-transformation is that fossil fuels have proven to be highly dependable primary energy sources. We’ve been able to map resources well ahead of their future production, with sufficient accuracy to allow us to—literally—bank on the materials being there as expected when producing them becomes economically viable and valuable. The characteristic reliability of fossil fuel resources provides a good fit with long term economic planning—and in fact enables such planning by bounding the uncertainties that it must deal with. Due to the abundance of fossil fuels, the limits on production rates have tended to be technological, financial, institutional and political in nature, rather than a function of physical resource constraints. In other words, as demand for fuels has grown, the limits to the rate of growth have tended to lie in the human sphere, and hence to be more-or-less subject to our influence if not control. Despite the institutional complexity of modern societies, the expansion of energy supplies has tended to be relatively straightforward—certainly in comparison with the challenges that we face as the age of fossil fuel abundance draws to a close. Germany’s present situation provides some insight into this—as nuclear electricity generation is taken off line and replaced by solar photovoltaic and wind generation, we’ll get a clearer picture of what it means to live in a world where variable resource characteristics that are not subject human influence play a much more prominent role than in the past. It’s worth highlighting in this context the way in which our present dependence on stable and secure base load electricity generation is in important respects a consequence of the energy sources upon which our electricity supply systems are based. In fact, the centralised generation model for electricity supply can itself be regarded as an artifact of abundant and reliable primary energy sources. Were our energy sources not of this nature, then our electricity infrastructure—had it developed at all—may have evolved in very different ways to what we now regard as the default supply model. From this point of view, the perceived need for stable base load electricity supply is not a fait accompli, but a contingent outcome of a particular process of social, economic and technological construction.

The second overarching characteristic of the conventional fossil fuels that accounts for their economic importance also arises as a consequence of the geo-transformation processes that shaped them. Through the application of elevated temperatures and high pressures over very long—in human terms—periods of time, the carbon-based compounds in the original accumulations of plant remains (coal) and microscopic plant and animal remains (oil and gas) were converted to the forms in which we find them today. These predominantly comprise a broad spectrum of hydrocarbon molecules—compounds comprising only carbon and hydrogen atoms—although small quantities of elements such as sulfur, nitrogen and oxygen mean that oil contains a range of other carbon-based compounds. Coal comprises related compounds, and is differentiated by the higher overall proportion of carbon to hydrogen, which depends in turn on the age of the coal. Lignite, the youngest coal, has the lowest proportion of carbon, while anthracite the oldest coal, has the highest. Coals falling between lignite and anthracite in age are typically differentiated into a range of sub-bituminous and bituminous grades. The bituminous compounds mainly comprise long-chain hydrocarbon molecules, though like oil, the presence of smaller quantities of elements such as sulfur, oxygen and nitrogen also results in other carbon compounds. The three types of fossil fuel are differentiated by the average molecular weight of their constituents. Coal is comprise of the highest molecular weight components—i.e. the largest molecules—followed by oil and then natural gas. In fact natural gas is predominantly comprised of methane, the lightest of all hydrocarbons—molecules containing a single carbon atoms bonded with four hydrogen atoms.

The nature of the chemical compounds of which coal, oil and natural gas are comprised is central to their value as fuels. As noted earlier, these compounds are relatively stable, but it is the relative instability of the bonds between carbon atoms and carbon and hydrogen atoms that makes them so useful as heat sources. The reactivity of hydrocarbon compounds in oxygen-rich environments results from the energy associated with the intra-molecular chemical bonds. This is the basis for regarding fossil fuels as stores of chemical potential energy. The chemical energy that we associate with the bonds is transformed to thermal energy associated with the combustion products that result when the constituents of the hydrocarbon molecules combine with oxygen to form more chemically stable compounds—in the presence of sufficient oxygen, predominantly carbon dioxide and water.

Appreciating the second characteristic that I want to draw attention to requires though that we shift focus from the molecular to the macro-scale. It is not just the reactivity of the bonds at the molecular scale that is important: it is the overall concentration of this reaction potential. As a result of the changes to the source organic material brought about by geological heating and compression, fossil fuels have associated with them particularly high specific energy, or energy per unit mass, as measured by their heating values. At the end of this post, I’ll take a look at what it means to characterise an energy source in such terms, and in particular, how this further illustrates the energy concept’s systems basis.

While heating values provide a measure of energy per unit mass, in appreciating the special economic status of fossil fuels—coal and oil in particular—energy density, the energy per unit volume, is perhaps even more significant. To appreciate why this is so, it’s necessary once again to reflect on the systemic basis of our inquiry. A systemic view will take into account the entire infrastructural context within which we access energy. Energy supply itself accounts for a significant proportion of all economic activity. As a very rough indication of this, global financial expenditure on fuels and electricity is in the order of 8 percent of world GDP. This energy-related economic activity is associated with a similarly significant proportion of our total physical infrastructure, and represents capital investment of a commensurate scale. The energy density of our primary sources and the refined fuels derived from them has direct implications for the scale and hence the capital value of this infrastructure. As the volume of primary energy sources that we use increases, so too must the quantity of the infrastructure and associated economic activity to produce it, transform it to fuels and electricity, and distribute these to their points of use. The total infrastructure requirements and the associated quantity of economic activity are therefore sensitive to the volumes of source material that must be processed. Primary sources and derived fuels with higher energy densities require less infrastructure, and less associated economic activity, to make a given quantity of energy available at its point of use. Moreover, the end use tasks enabled by that energy are significantly influenced by fuel density. We see the consequences of this in relation to proposals to replace road transport fueled by automotive gasoline (petrol) and diesel with hydrogen or natural gas fueled or electrically powered vehicles. Whichever way we come at this, we are confronted by the consequences of lower energy density. In order for vehicles to carry fuel or batteries with an equivalent energy value to those with which our present infrastructure—and the expectations and habits that go along with it—have evolved, a greater proportion of the overall transport task must be taken up with carrying the fuels or batteries themselves. Over the better part of a century, in the industrialised world at least, we have made vast investments—both economic and socio-cultural—in an intricate web of arrangements the form of which is intimately bound up with the energy density of our predominant fuels. Were we starting from scratch with primary sources and fuels of lower energy density, then there would be little issue: the world we know now would be shaped differently, and we’d be proceeding along with the lives so shaped. Transitioning to fuels and other energy carriers with lower energy density after the fact though is another matter entirely. We simply can’t escape our present world’s energetic path dependence—the ways in which our present options are constrained in significant ways by the decisions made under past circumstances.

At this stage, in focusing on considerations related to infrastructure and overall economic activity associated with energy supply and use, I’ve left aside discussion of what is perhaps the most important issue related to the energy density of primary sources and fuels—the energy use required to make energy available to us. I’ve made brief mention of this previously in introducing the concept of energy return on energy invested, or EROI. This is a much larger topic in its own right, which I’ll take up in detail as the inquiry proceeds. For now, I just make the observation that the dominant role of fossil energy sources in industrial societies means that all economic activity, including that related to energy supply from non-fossil sources, is inevitably subsidized by fossil fuels, and benefits from their particular properties, including their high energy density.

Incidentally, the macro-perspective on the role of energy density in the physical economics of energy supply discussed above extends also to electricity, despite its apparent immateriality. Electricity supply voltage can in this respect be regarded as analogous to energy density of primary sources and fuels. Higher supply voltages and lower currents reduce transmission and distribution losses for given cable sizes, and hence reduce the quantity of cable material required for a given transmission and distribution task. The relationship is a little more complex though, as higher voltages also necessitate greater separations between cables—hence the greatly increased size of support towers as voltage increases. Overall though, this is driven by economic considerations—specifically, the total life-cycle costs of the infrastructure and its operation, and the general rule tends to hold that higher energy densities support lower costs.

I’ve presented representative data on specific energy and energy density for the major fossil energy sources, and some of the important petroleum-based fuels, in the table below. Three bio-fuels are also included for comparison: hardwood; softwood; and bagasse, the fibrous material left over when sugarcane is processed to remove its sugar-containing juice. These latter fuels give a very rough sense of how energy density changes when plant-matter is geologically sequestered and transformed to coal.

Energy source Typical density*kg/m3

Specific energy

(higher heating value)

MJ/kg

Typical energy density

(higher heating value basis)

MJ/m3

Bagasse 120

10

1,200

Softwood (dry) 420

15

6,300

Hardwood (air dried) 600 (varies greatly with type of wood, moisture content, and size of pieces)

16

9,600

Peat (damp) 800

6

4,800

Brown coal (Victorian) (wet) 850

10

8,500

Black coal (NSW, electricity generation) 830

23

19,000

Crude oil (petroleum) 850

45

38,000

Automotive gasoline 740

46

34,000

Automotive diesel fuel 850

46

39,000

Aviation turbine fuel 800

46

37,000

Natural gas (Victoria) 0.75 @ 101 kPa and 15 oC

52

39

Liquified natural gas (LNG) 460

54

25,000

*For solid materials, this is the bulk density, and is a rough approximation only—actual densities vary significantly in practice. Bulk density is the mass for a given volume of a solid material taking into account space between separate pieces of the material. For example, wood or coal must be handled and transported as large numbers of small pieces, so the density for the total volume being handled will be less than the density for an individual piece of material. For coal, in situ densities prior to mining are higher than the bulk density of produced fuels, and hence energy density is higher also.  For gases and liquids, density and bulk density have the same value.

Sources: Energy in Australia 2012; www.simetric.co.uk; The Engineering Toolbox

There are a number of observations worth making here, particularly given the very dramatic difference in energy density between natural gas and the other fossil energy sources. In light of the discussion above about the relationship between energy density and the scale and extent of energy-related infrastructure, this stands out as a significant anomaly. Some further reflection is needed in order to understand why natural gas is now such an economically significant fuel.

Firstly, there’s a general increase in specific energy (energy per unit mass) as we move from solid fuels (comprising materials with high average molecular weight) to liquid fuels (intermediate molecular weight) and finally to gaseous fuels (lowest molecular weight). There’s a correspondence here too between the organic material from which each of the fossil fuels formed, and their specific energy. The land-based woody plant matter—vegetation from ancient forests—that became coal was cellulose-based (associated with long, stringy fibres), whereas the source organic matter from which oil and gas formed—marine plankton and freshwater algae—comprised mainly lipids (the group of organic molecules including fats and waxes) and proteins. We can see here a relationship between the initial form of the source matter, and the hydrocarbon molecules in the fossil matter. Smaller molecules in the source matter generally lead to smaller molecules and higher heating values for the fossil fuel material, although the geological transformation process plays an important role here also—the elevated temperature and pressure to which the molecules are subjected breaks them down and changes their structure, increasing the proportion of smaller molecules over time. It’s this process that leads to the evolution and accumulation of natural gas, with its very high specific energy, in both oil and coal formations.

A second observation is that in moving from wood to coal to oil, the energy density increases, and the volume of material associated with a given quantity of energy decreases. As discussed earlier, this has important implications for the utility of each energy source, and for the range of applications for which each is suitable. This trend is broken, though, when we get to natural gas. In the case of natural gas, although specific energy is the highest of any or the sources, the energy density is three orders of magnitude lower. In order to make natural gas easier to handle, it must be compressed or chilled to very low temperature, which requires both energy and special techniques. The significance of this becomes more apparent if we consider the historical sequence of the exploitation of the energy sources listed in the table.

Wood, the first major energy source for humans, occurs naturally in our immediate surrounds (i.e., it is visible and requires no special techniques for its exploitation other than a knowledge of fire and simple tools for harvesting). This was followed by peat and lower quality brown coal, available just below the surface of the Earth, and hence, like wood, requiring little in the way of special techniques for its exploitation. Black coal followed brown coal. This required more sophisticated mining techniques, including development of the earliest practical heat engines to pump water from the mines. Black coal was followed by oil, which requires special exploration technology (without which, it would remain ‘invisible’) and sophisticated and capital intensive refining and distribution systems. It also requires—and in turn, influences the development of—specialised social, political and economic institutions.

Eventually we come to natural gas, originally a nuisance by-product of crude oil recovery. Natural gas has become an economically important primary energy source only relatively recently in historical terms, requiring very sophisticated processing and transport systems in order to use it far from its point of production. Prior to the availability of such systems, natural gas use tended to restricted to locations where it was available relatively close to its point of use. This allowed it to be transported economically via pipelines at relatively low pressure. To some extent, the low molecular weight of natural gas—the basis for it being gaseous at atmospheric pressure and ambient temperature in the first place—offsets transport implications of its low energy density. For instance, gases can be transported in pipelines at much higher velocity than liquids, and so for a given pipe size, a larger volume of gaseous fuel than liquid fuel can be conveyed economically. Even so, this doesn’t fully compensate for the low energy density. In order for long distance transport by sea to be economic, the gas must first be chilled to a liquid at around -162 degrees Celsius, with the refrigeration process requiring significant energy input. Once liquefied, the energy density is much higher, and so for a given volume, fuel with a much higher quantity of associated energy can be transported compared with the gaseous state. Liquefied natural gas (LNG) facilities can cost billions of dollars to establish, and their original development benefited from the full spectrum of industrial-world technological and institutional developments that coal and oil had previously enabled.

There’s another important entailment of the reduction in average molecular weight, and increase in specific energy, across the historical evolution from biomass fuels to coal, then oil and then natural gas. As noted earlier, with each of these energy sources, the ratio of hydrogen to carbon atoms varies. When fossil fuels are burnt to provide heat, the carbon reacts with the oxygen in air to produce carbon dioxide gas (CO2) and the hydrogen reacts with oxygen to produce water vapour (H2O).

The greater the ratio of carbon atoms to hydrogen atoms in the compounds that make up the energy source, the greater the quantity of carbon dioxide that will be produced for a given quantity of thermal energy. The table below provides indicative figures for carbon dioxide emissions per unit of thermal energy associated with combustion of the major primary energy sources.

Fuel

kg of CO2 per GJ of thermal energy

Coal

120

Oil

75

Natural gas

50

Biomass

77

Source: Elliott, D (2003), Energy, Society and Environment, Routledge, London, p.29, table 2.2.

The data in the table makes apparent that, in general, as the specific energy of hydrocarbon energy sources increases, the quantity of carbon dioxide produced for a given quantity of thermal energy reduces significantly. This provides a ready illustration of why natural gas has such a prominent role in most responses—proposed or actual—to anthropogenic climate change.

What do we mean by heating value?

In wrapping up this post, I’ll return to the question I raised previously about just what it means to characterise an energy source in terms of its heating value. As I noted earlier in this post, to better appreciate this, we need to return once again to the energy concept’s systemic basis: energy is always a property of an appropriately defined system rather than a property of a material substance per se. The energy that we associate with a fuel—such as a given quantity of coal—is available only in the context of specific configurations of appropriate system components that include but are not limited to the coal itself. A quantity of coal in isolation does not contain energy, despite the way that a fuel’s specific energy or heating value is often described as its energy content. Rather, when we specify the heating value of a quantity of coal, we’re using the energy concept to describe the behaviour of a physical system undergoing transformations that involve oxidation of the coal’s constituent molecules in a combustion reaction with oxygen, and that result in the formation of combustion products (predominantly carbon dioxide and water, but also a range of other compounds associated with minor elements included in the coal) at elevated temperature. We’re saying, in effect, that when certain quantities of compounds containing particular configurations of certain elements of matter are excited sufficiently (i.e. if the intra- and inter-molecular motion of the reactants is increased above a given threshold), these compounds will undergo a self-sustaining set of reactions, resulting in the formation of a different set of compounds with more stable configurations, and for which the overall quantity of  motion will be greater than that of the reactants by a certain amount.

Couching this process now in energetic terms, in order for the chemical energy that we associate with a quantity of combustible fuel to be made available to us as thermal energy, the fuel must be burnt under appropriate conditions, requiring the presence of oxygen, and an ignition source, as well as a suitable apparatus for mediating heat exchange between the combustion products and a heat transfer medium such as a mass of water. It is just such a set of arrangements that is used to determine the heating value of a fuel. The device that is used for this purpose is a bomb calorimeter. A bomb calorimeter allows a quantity of fuel and a sufficient quantity of oxygen for the fuel’s complete combustion (i.e. for all of the fuel molecules to react with oxygen to form products of combustion, leaving behind none of the original fuel) to undergo a contained combustion reaction whereby all of the reaction products remain inside the device. By preventing any loss of combustion products, we know from the conservation of energy (energy law 1) that the total energy associated with the calorimeter and its contents after the combustion reaction is completed must be equal to the total energy associated with the calorimeter and its contents before the reaction takes place. It’s on the basis of this principle that we can use the transformation associated with the combustion reaction to characterise the fuel in terms of its heating value or specific energy.

The calorimeter’s reaction chamber is surrounded by a bath containing a known quantity of water. The heat of combustion for the fuel is determined by measuring the increase in the water’s temperature following the combustion reaction. Once again, this has important implications for appreciating the systems basis of the energy concept—it is the relative temperature difference that is important here i.e. the heating value for the fuel is defined in terms of a system-relative change. The unit of measure for thermal energy (‘heat’) is itself defined in terms of temperature change in a given quantity of water—the calorie, the original unit for thermal energy, is defined as the quantity of heat transfer that will increase the temperature of 1 gram of liquid water by 1 degree Kelvin. For reference, 1 calorie is equivalent to 4.184 joules. This allows the thermal energy associated with the combustion reaction to be determined directly from the increase in temperature and the mass of water (in practice, this is slightly more involved, as the heat capacity—the energy associated with a given temperature increase—of the calorimeter must also be taken into account).

There’s one other consideration that’s important here also. To date, the specific energies of fuels that I’ve discussed have all been higher heating values or gross heating values. Specific energy can also be specified in terms of lower heating values or net heating values. The difference between these two heating values is the energy associated with vaporisation of water in the combustion products. For the higher heating value, the combustion products are brought back down to the temperature of the reactants prior to the combustion process before measuring the increase in the temperature of the calorimeter’s water bath, with the result that all water in the products is in the liquid state. As a result, the temperature of the water bath in the calorimeter will be elevated by an amount equivalent to the energy associated with the change in state of the water in the combustion products from a gas to a liquid.

For the lower heating value, the increase in the calorimeter’s water bath temperature is measured while the combustion products are at a temperature sufficient for the water component to be present as a gas. As a result, the water bath temperature increase will be slightly lower than it would be if the products were returned to the starting temperature of the reactants.

The significance of this is hopefully clear: the heating value does not give us an absolute measure of ‘energy content’ in the fuel—it provides us with a system-relative characterisation of what happens to a particular configuration of components undergoing a given set of transformations. This is very useful for comparing different fuels, and for designing energy conversion and supply systems, but it doesn’t imply that energy is literally contained in the materials that we use as energy sources, or that the energy that is metaphorically contained in materials according to standard, system-relative definitions is necessarily available to us for the purposes that we find useful.

In practice, the thermal energy available from the combustion of a fuel is dependent on the temperature at which the combustion products are disposed of, usually by expelling them from the energy conversion system to the atmosphere. The higher the temperature of the combustion products when they leave the system, the lower the proportion of the fuel’s nominal heating value that is channeled to the ends that we wish to serve by burning the fuel. This is important, because in any practical application involving fuel combustion, the products of combustion will by necessity be expelled at a higher temperature than that associated with the higher heating value as measured by a calorimeter. For example, if we’re using natural gas to provide domestic hot water, the fact that the higher heating value of our fuel is 52 MJ per kilogram doesn’t mean that if we burn 1 kilogram of natural gas, we will transfer 52 MJ of thermal energy to the water in our hot water system. A proportion of the nominal energy available based on the fuel’s higher heating value will be lost to the system’s surrounds (the atmosphere) along with the combustion products themselves, and so this heat transfer process must always take place with efficiency significantly less than 100 percent. This energy associated with the combustion products is required to drive the overall operation of the water heater—a small proportion of the energy associated with the fuel is required to maintain the flow of air into the heater, and the flow of combustion products through the heater’s flue and out into the surrounding environment. The combustion products must have some non-zero velocity in order to leave the flue system, and this requires that their temperature be greater than that of the surrounding air. Further to this, there are practical considerations associated with the material from which the heater is made. If the temperature of the combustion products is sufficiently low, water condenses out of the combustion product stream while still inside the device, leading to corrosion. In order to minimise corrosion, it’s often important to ensure that the water in the exhaust stream exits as vapour, which requires designing the system so that the exhaust temperature is higher than might otherwise be required. This is especially important in industrial applications, where exhaust fans, equipment for cleaning the combustion product stream and ducts can all be susceptible to corrosion that would shorten their operating lives and significantly increase plant costs.

I’ll finish here, and pick up the story next time by taking a first look at how discussion of energy densities for fossil fuels might be extended to get a sense of the infrastructure scales associated with different energy sources performing the same energy supply task.

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