S6C1: Fossil Fuels

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a) The origin of hydrocarbons

This sixth series returns to the theme of energy we covered extensively in the first series titled “Building Blocks and Energy” but this time the focus is not about physics and chemistry, though I will still provide some kind of refresher on this topic in section e), rather it is about its use in the complex social and economic organism we call society. Following this metaphor, in Part B, we will cover the key systems ensuring its homeostasis, including the flows of food (Chapter 6), commodities and manufacturing (Chapter 7), the financing of the economy (Chapter 8) and the challenge in balancing it all at the sovereign level (Chapter 9).

From the early days of humanity and our mastering of fire, the main source of energy we have tapped on, besides that provided by the movement of water under the effect of gravity to power mills, have been hydrocarbons and carbon compounds. An example of the latter would be the cellulose making up wood, which consists of chains of carbon, hydrogen and oxygen atoms. As for hydrocarbons such as oil, coal and natural gas, like their name suggests, they contain hydrogen and carbon atoms. According to the International Energy Agency, by the end of the 2010s decade these hydrocarbons we call fossil fuels represented just over 80% of the total worldwide energy production. But why call them fossil fuels?

Essentially, because that is what they are. Hydrocarbons are the result of the anaerobic decomposition of dead organisms that started as sediments on the ocean floor or mixed with the soil. In the case of marine plankton, over geological times, incremental layers of sediments deposited over them so they became exposed to high pressure and temperature, causing chemical alterations and turning them into kerogen and bitumen within sedimentary rocks. This is the material present in oil shales. Some of these were eventually metamorphosed into hydrocarbons as part of a cracking process called catagenesis.

Cracking is the breaking of the inter-carbon bonds, thus shortening the hydrocarbon chains, and catagenesis sees a change in the phase characteristics under high temperature and catalysis by minerals. Depending on the exact parameters, the result might be natural gas (very small chains) or oil. This liquid hydrocarbons will then migrate into porous rock formations where they accumulate but, being less dense than water (or rock, of course), they tend to make their way to the surface unless they are trapped under an impermeable layer. Those are the conditions that need to be met for petroleum deposits and why we need to drill to find them.

As for coal, it is the result of the decay of dead plants with limited exposure to oxygen, such as in swamps with stagnant water. This decomposition gives us peat and under the effect of heat and pressure, over millions of years, it will change into lignite, then sub-bituminous coal and into bituminous coal with higher calorific content, the highest echelon in the classification belonging to anthracite with a carbon content of over 85%. Unlike the liquid hydrocarbons, coal is solid and therefore doesn’t have a tendency to rise to the surface. This means it can be found deep underground as well as close to the surface, a particularity that will become apparent in the next section.

b) Coal mining

There is archaeological evidence of coal being mined as long as 25,000 years ago. Certainly, the tools and technique were primitive and mostly consisted of surface pits with potentially some horizontal digging following a vein of coal. As we have seen in S5 Section 1.a on the steam engine, the Industrial Revolution provided the impetus for an explosion in the volume of coal being mined and, necessity being the mother of invention, new techniques were developed to mine deeper and more effectively above and underground.

In surface mining called open-cut, large excavators, earthmovers and trucks are combined to remove the overburden and then mine the ore seams. The economics are mainly a function of the overburden to ore ratio, the cost of transporting it from the mine to a port and then shipping it to destination in a dry bulk carrier (S5 Section 4.b), and the quality of the coal being extracted with calorific content being the main variable although important characteristics such as ash content also play an important role in terms of determining suitability and price.

Underground mining is more challenging than open-cut due to the inherent risks of collapse, but it permits companies to reach and follow the richer seams of ore, without having to remove as much overburden with low or no ore in it. In order to avoid collapse, pillars and timber roofing can be used to stabilize the soil above the passageways. Modern heavy equipment such as hydraulic jacks can also be used, in particular for longwall mining which sees large shearing machines shaving slices of coal following long straight cuts of several hundred metres or even a few kilometres. Back and forth at 10 to 30 metres per minute. The coal is then automatically loaded into conveyor belts and back up to the surface through a series of tunnels or shafts. This is capital intensive but the operating cost are advantageous.

The second main technique is called room and pillar mining; it consists in excavating only parts of a horizontal section by leaving some untouched natural pillars to support the soil overhead. The size and layout of the rooms and pillars will be a function of the mine plan, itself optimized based on the sampling data. When an area has been mined, it is possible, though somewhat risky, to mine some of the pillars and then retreat before the ground above caves in.

Coal mining was a defining feature of the 19^th and early 20^th centuries, generating much literature such as Germinal by Emile Zola and taking a terrible toll in terms of occupational health due to inhaled coal particles, besides the risk of death. Since then, it has dwindled in most industrialized countries as other developing countries with more economical resources have seen their production and exports grow – adynamic we will take a look at in S6 Section 7.a titled “mapping hydrocarbon production and refining”.

c) Oil & gas extraction

Since petroleum reserves can seldom be detected from the surface, they have to be probabilistically inferred based on a collection of geological data fed into computer models. Consequently, even before the decision of drilling for oil or gas, this data must be created and collected through exploratory geophysical methods such as seismic reflection and this often requires expensive, specialized equipment such as seismic survey ships for offshore exploration and production. I include a link to the Wikipedia entry for reflection seismology in the final section if the topic is of further interest.

Once potential geologic targets have been mapped, a series of wells will be drilled to determine the presence, or not, of the oil and gas reserves and their assumed extent (if no hydrocarbon is encountered it is called a dry well). Note that this overarching principle is also at play for other type of mining and is important in assessing the volume and therefore value of economically mineable reserves, categorized as proven (1P), proved and probable (2P), and proved + probable + possible reserves (3P).

Drilling an oil well is the creation of an access to a deposit from the surface, whether onshore or offshore. It is achieved by rotating a drill bit located at the end of a drill string which burrows further into the ground. At the same type, drilling fluid mostly consisting of water and clay is pumped down to both cool the drill bit and flush out the rock cuttings back to the surface, and at regular intervals sections of steel pipes are lowered to create the casing. All the equipment and material, including the motor providing the torque for the drilling, are contained on what is called a drilling rig.

If the area moves to production, a tubing will be added within the casing through which the hydrocarbons will flow, potentially with the assistance of pumps – these are the iconic pumpjacks driving reciprocating piston pumps. I include a link to the Wikipedia entry for pumpjack at the end of this chapter.

Land rigs could be said to be relatively simple and cheap to deploy and operate. Not so when it comes to the offshore production of oil and gas. In its simplest incarnation, an oil rig can be standing on firm ground using legs resting on the seabed, this would be a jackup rig. However, this concept quickly becomes impractical as depth increases (the tallest jackup rig has legs of just over 200m) so the alternative is to build floating platforms that can either be tethered to the seabed using tension-legs made of cables, as deep as 1500m, or semi-submersible structures using water ballast tanks to adjust their buoyancy and either anchoring ropes or dynamic positioning to maintain the fixed position required for safe operations – something not always easy to achieve when confronting heavy seas, such as hurricanes in the Gulf of Mexico or the giant waves experienced during North Sea storms.

Production requires supervision, ongoing maintenance and frequent repairs, hence offshore oil platforms often comprise living quarters for crew rotating in and out every few weeks either by helicopter or ship, the latter also ensuring the supply of food and material.

Regarding the oil or gas, the volume brought to the surface needs to be channelled all the way onshore or to a loading facility through pipelines or, alternatively, stored onsite and then transhipped into oil tankers. The latter option is the domain of the phenomenally large and expensive-to-build floating production storage and offloading units, better known under their acronym FPSO. At its simplest, a converted tanker will do for acting as storage, but FPSO can do it all, including some partial processing, and can therefore operate independently or in conjunction with other oil platforms in frontier regions without the presence of existing pipeline infrastructure.

Since the early 2000s, the increase in the price of oil coupled with a shift towards “cleaner” energy sources has heralded a period of greater investments towards the production of natural gas. Clean is a relative concept, the combustion of natural gas still releases greenhouse gases such as carbon dioxide, just less of it compared to petroleum products due to its higher ratio of hydrogen to carbon atoms. In fact, methane, the primary constituent of natural gas, is a greenhouse gas. You may want to refer to S3 Section 3.e on the causes and effects of climate change if you are unfamiliar with the concept and dynamics of greenhouse gases.

The primary differentiator between the production of natural gas compared to oil, is that the former is gaseous and the latter liquid. Being fluids, both are easy to pipe, but the economics of shipping or trucking gas just don’t work, so it has to be liquefied to become much denser and therefore more valuable on a per-ton basis. The whole infrastructure investment, including the liquefaction and regassification equipment and shipping terminals, runs into several billion dollars and the ships designed to transport these internationally, the LNG carriers, are not inexpensive affairs either, as we have seen during S5 Section 4b.

d) Oil refining

The upshot of the metamorphosis from dead organic matter into hydrocarbons described in section a) is that not only the content of each oil and gas field will be different depending on the specific conditions it was subjected to, in particular with regards to pressure and temperature, but there will also be variations within the field itself so that each barrel of crude oil or cubic metre of natural gas contains a mix of different chemical compounds. This is theoretically problematic because those compounds all exhibit different properties such as, inter alia, viscosity, flash point (the temperature threshold at which a liquid starts vaporizing), or sulphur content, and the operation of various types of combustion engines or other manufacturing processes where petroleum is made use of requires a consistent and particular set of properties.

This is where refining comes in; it consists in processing crude oil or natural gas so as to yield a basket of refined products made of similar compounds and therefore exhibiting homogenous and consistent properties, regardless of the history of the molecules. The two main processes involved are distillation and cracking.

The distillation process involves the heating up of petroleum products in a distillation column with a vertical temperature gradient so that the compounds with the lowest boiling point vaporize first and make their way to the top where they are extracted while, a certain distance below, it is a product with a slightly higher boiling point that will vaporize and be removed. Thus, the distillation process fractionates the crude oil into separate products with different economic value.

Those products with the lowest boiling point are described as being lighter since they exit the column at or near the top and they consist of short-chain hydrocarbons. Because they are in great demand relative to the heavier products, it may make economic sense to crack some of the long-chain hydrocarbons and transform them into short-chain molecules by splitting their inter-carbon-atoms bonds. There are various cracking methods such as hydrocracking or steam cracking depending on which end products one desires to obtain; if you are interested in these technologies, I have included a link to the relevant Wikipedia entry in the last section.

Not all refineries have the same degree of complexity and those with more complex distillation, desulphurization and cracking capabilities are much more expensive to build and operate but are able to process heavier, sourer crude oil which is sold at a potentially very significant discount compared to the light, sweet crude with little sulphur and a higher pre-cracking yield of light petroleum products.

Below is a non-exhaustive list of the key refined products, their formula, and their main usage:

The lightest compounds are the gaseous fuels such as propane (C₃H₈) and butane (C₄H₁₀). They are often compressed in order to increase their volumetric energy density and then sold as LPG (liquid petroleum gas) in cylinders used for heating or cooking.
Gasoline or petrol is one of the major refining products and it is primarily used in spark-ignited internal combustion engines (refer to S5 Section 2.b for this as well as for diesel). It includes a blend of naphtha, some additive to increase the octane rating and therefore the resistance to compression before ignition, and various hydrocarbons with molecules comprising between four and twelve carbon such as alkanes or cycloalkanes.
Naphtha gets further processed, undergoing cracking and desulfurization, especially the heavier stream with longer chains. Besides its use in gasoline (as mentioned above), it is an essential feedstock in the petrochemical industry.
Kerosene is the main product used as fuel in the aviation industry and is also quite widespread among households in developing markets for cooking and lighting. Most of the hydrocarbon molecules within this middle distillate have 10 to 15 carbon atoms.
Diesel (also known as gas oil) is preferred over petrol for trucks and utility vehicles requiring more torque partly due to the mechanics of the compression-ignition internal combustion engines and their high calorific value. Sulphur is removed through a catalytic process called hydrodesulfurization and most of the hydrocarbon chains contain 10 to 15 carbon atoms per molecule.
Heavy fuel oil or bunker fuel is pretty much the leftover liquid at the bottom of the distillation process. It is very viscous, high in sulphur and consists of long-chain hydrocarbons, most of them with more than 20 carbon atoms per molecule. It is the fuel of choice for cargo ships because it is the cheapest.
Lubes (or lubricants) help reduce friction, mostly for industrial applications. They are somewhat viscous, have a high boiling point, and consist of predominantly long-chain hydrocarbons (from 15 to 50 carbon atoms per molecule). Depending on the end properties required, manufacturers will also blend in some additives.
Asphalt or bitumen is literally the bottom of the tank. It is extremely viscous and more than two-thirds of it finds its way to the road construction industry where it is used for surfacing (refer to S5 Section 7.b). Bitumen consists of different types of compounds, including naphthalene (C₁₀H₈), aromatic compounds and alkanes (saturated hydrocarbons with formula C_nH_2n+2).
The main feedstocks for the petrochemical industry are naphtha and gaseous fuels like propane and ethane. The former undergoes catalytic reforming to yield aromatics such as benzene and xylene isomers – that is with the same molecular composition but varying spatial organization – whereas the natural gas liquids are steam-cracked to produce olefins such as ethylene and propylene. Olefins are defined by the presence of a carbon-carbon double bond, which is a covalent bond involving four electrons (refer to S1 Section 2.b on chemical bonds); they are one of the key building blocks in the manufacturing of many types of plastics such as polyethylene.

e) Hydrocarbons and chemical energy

By now we have a decent grasp of the technologies involved in searching for and producing oil and gas, we also understand the need for refining those raw products into refined commodities with specific properties and, at a very high level, of the refining process. The description of some of those products in the previous section unequivocally points to the pivotal aspect of the chemical makeup and in particular the nature of the bonds between various atoms within the sometimes short and sometimes very long molecules of hydrocarbons. This provides a good segue to examine, as promised in section a), the reason why hydrocarbons are such popular compounds, in particular as fuel used in internal combustion engines or thermal power plants in the case of coal and methane.

The release of energy occurs when a fuel burns, this is called combustion and it also requires the presence of an oxidant such as the oxygen present in the atmosphere. So, a reader who has not been through the delights of the first and second series would think the energy released is that contained within the chemical bonds being broken, but that is quite the opposite because the creation of bonds results in the freeing up of energy (refer to S1 Section 5.a on metabolism for instance) and it takes energy to break bonds. This is the reason why energy needs to be injected into the system to ignite the process of combustion. Therefore, the reason hydrocarbons are an excellent fuel is that their intramolecular links are not very strong and the product they form, such as water (H₂O) and carbon dioxide (CO₂), have very strong internal bonds due to the elevated electronegativity of oxygen (being its propensity to attract shared electrons). In other words, it is the differential in intramolecular bond strength between the fuel and oxidant versus the products of the oxidation process which determines how exothermic the chemical reaction is (the expression in the jargon would be a reduction in enthalpy). I include a link to the Wikipedia entry for combustion at the end of this chapter if you wish to dig more into this subject.

The above combustion process is what we know as fire and the heat released enables a self-sustaining chain reaction, without further injection of energy into the system. So to put out a fire, you can remove the fuel source, or the oxidizer (by throwing a blanket on it to avoid atmospheric contact), or heat (pouring water will dilute the kinetic energy and blowing hard will temporarily displace the heat source and break the chain reaction). As for chemical retardants, they either reduce the flammability of fuels or slow down the combustion, which result in a drop of the level of heat being released and can also stop the chain reaction.

Why do we see flames? This is thermal radiation, the emission of electromagnetic waves (light) we have already come across in S3 Section 4.b titled sun power, and spectral band emission. Accordingly, the colour of the light we perceive is a function of the rate of combustion, which depends on the nature and mix of fuel and oxidant. This determines both the temperature and the exact chemical reactions taking place, and thus the nature of the resulting products, each with their own spectral bands.

f) Thermal power plants

The objective of power plants is to produce electricity which can then be transmitted across distances and distributed to residential, commercial and industrial consumers through the electrical grid, the forthcoming topic of Chapter 4. We have already seen how an electric motor works in in S5 Section 1.b on the electric locomotive and in effect power plants seek to produce the opposite: translate mechanical energy into electricity.

In a thermal power plant, it is actually a two-step process. First, heat is generated through the combustion of hydrocarbons such as coal, and this heat is used to boil water into high pressure steam which then drives the rotation of blades in a turbine. Second, the rotation of the turbine drives the rotation of one part of an electric generator whilst another part remains static. Because one of those two parts is made of an electromagnetic material (a magnet or a field coil), this creates an oscillating magnetic field which induces a back-and-forth flow of electrons, also known as alternating current.

In the case of natural gas-fired power plants, the fuel combustion can take place directly in gas turbines, thus bypassing the requirement to generate steam. In a gas turbine, the fuel such as methane is mixed with compressed air containing oxygen, the oxidant. This mixture is then ignited with a spark and the exothermic reaction taking place in the combustion chamber is then self-sustaining provided an adequate air-to-fuel ratio is maintained. The kinetic energy of the high-pressure exhaust gas produced by the combustion then drives the blades of the turbine. Same as with a propeller engine mounted on an aircraft (S5 Section 5.a on the physics of flight).

Either way, the production of electricity entails the conversion of chemical energy stored in fuels and oxidants into mechanical energy and this energy is then converted into electrical energy. No surprise then that the energy efficiency is a modest 30% to 45% depending on the technologies being used.

One way to improve efficiency is by recovering some of the waste heat dissipated during the electricity generation process. This less intense energy dissipation is recovered and used to heat either air or water which is then distributed to consumers in the immediate vicinity of the plant. This is called cogeneration and such plants are called combined heat and power (CHP) plants and the energy efficiency of smaller scale CHP plants can exceed 70%.

g) Trivia – CNG

Compressed natural gas (CNG) is a mildly popular fuel source used in transportation with significant penetration in public bus services and several developing or middle-income countries. The main drivers for adopting CNG rather than more traditional liquid fuels such as diesel or petrol are the lower cost per kilometre driven and the materially lower emission of greenhouse gases and other atmospheric pollutants such as particulate matter, nitrogen oxides and carbon monoxide. Unfortunately, the fuel has its drawbacks, especially the lack of refuelling infrastructure in many geographic markets, the need for modified engines, and its lower volumetric energy intensity – meaning the storage of fuel to drive a given distance requires more space compared to the main alternatives.

CNG is natural gas, mostly methane, compressed at 200 to 250 bars (one bar is 100 kPa, or almost one standard atmosphere; 1 atm is defined as 101,325 Pa). It is burned in a modified internal combustion engine. Hence, CNG should not be mistaken with LNG which is cooled to -162 °C where it experiences a phase transition into liquid and a dramatic reduction in volume. So, unlike LNG, it is quite inexpensive to produce, transport and store.