ORGIN AND TIME SCALE OF FOSSIL FUELS
It produces, by refining and other treatments, products that can be used to supply domestic or industrial heat, or else transformed into mechanical energy or electricity in converters, motors or power plants.
Fossil fuels currently provide just over 80% of the world's primary energy.
Their consumption, first of all that of oil, will be forced to decrease during this century because of exhaustion of the deposits.
The functioning of highly consuming societies will be deeply disturbed if they are unable to relay them in time by other sources of energy.
The use of fossil fuels is responsible for 82% of current anthropogenic CO2 emissions (coal 35%, oil 31%, gas 16%).
It is also responsible for numerous serious accidents and water and air pollution of concern to public health and ecosystems.
In particular, coal is by far the most dangerous of the energies used by humans, with 1 to 2 million deaths caused each year worldwide, about half of which are due to atmospheric pollution from coal-fired power plants.
The solid waste from its operation and use is in the hundreds of millions of tonnes each year.
It produces, by refining and other treatments, products that can be used to supply domestic or industrial heat, or else transformed into mechanical energy or electricity in converters, motors or power plants.
Fossil fuels currently provide just over 80% of the world's primary energy.
Their consumption, first of all that of oil, will be forced to decrease during this century because of exhaustion of the deposits.
The functioning of highly consuming societies will be deeply disturbed if they are unable to relay them in time by other sources of energy.
The use of fossil fuels is responsible for 82% of current anthropogenic CO2 emissions (coal 35%, oil 31%, gas 16%).
It is also responsible for numerous serious accidents and water and air pollution of concern to public health and ecosystems.
In particular, coal is by far the most dangerous of the energies used by humans, with 1 to 2 million deaths caused each year worldwide, about half of which are due to atmospheric pollution from coal-fired power plants.
The solid waste from its operation and use is in the hundreds of millions of tonnes each year.
Formation, varieties, uses, energy
Fossil fuels all have their origin in the very slow transformation over geological times of organic debris (kerogens) contained in certain sediments.
Rich in carbon and hydrogen, their combustion produces heat, used as such or transformed into mechanical energy or electricity.
They supply about 82% of the world's primary energy consumption, and this proportion is even higher in many industrialized and rapidly industrializing countries.
Thanks to nuclear power, this share is only around 50% in France.
In international accounting, energy production and consumption are valued in tonnes of oil equivalent (toe), the amount of energy supplied by one tonne of standard quality oil, i.e. 41.86 GJ (11.630 MWh) according to International Energy Agency (IEA).
☆☆☆ FORMATION ☆☆☆
All fossil fuels are formed from kerogens, that is to say debris from organisms (especially unicellular algae, bacteria and terrestrial plants) more or less altered, preserved in clayey and clayey sediments - limestone (marl).
These sediments have accumulated in vast depressions of the earth's crust overgrown with water, the sedimentary basins.
A sedimentary basin can be compared to a huge physicochemical reactor fed from above by sediments.
It deepens and deforms very slowly over geological times.
A "tectonic inversion" then ends the deepening and brings the accumulated sediments up to the surface, where they erode.
What remains of the basin then becomes a fossil basin.
The reactions taking place there are extremely slow on a human scale, but because of their enormous length, their effects end up being sensitive.
If a sediment contains kerogen, the latter is buried slowly and is subjected to increasing temperatures: it gradually degrades under the effect of heat, in the absence of oxygen since there is only very little in free form in sediments, this is called pyrolysis.
Kerogens mainly contain carbon, hydrogen and oxygen atoms, and incidentally sulfur and nitrogen, the proportions of which vary according to the biomass from which they are derived and the transformations undergone in the sediments .
During this pyrolysis, they gradually lose most of their oxygen, then their hydrogen, and the residual kerogen is therefore enriched in carbon.
This process is called carbonification or maturation.
The products formed are first carbon dioxide and water, then petroleum, and finally natural gas.
Oil and natural gas are expelled from the kerogenic sediment, it is then called bedrock, by the strong pressures that prevail there to invade porous and permeable rocks called reservoirs.
But most of the oil and gas disperses in the sedimentary series and eventually ends up on the surface of the basin. Some of it also remains in the parent rock.
What remains of the initial kerogen, that is to say the solid residue of this natural pyrolysis, of course remains in the bedrock.
The reservoirs are most often porous or cracked sandstones or carbonates.
Oil and gas gradually accumulate there, giving rise to future oil and gas deposits.
All fossil fuel deposits are formed this way .
Their rate of formation was 4 to 5 orders of magnitude lower than the speed at which we consume them: we will probably end by the end of this century by exhausting most of the recoverable stock, which put dozens millions of years to form.
☆☆☆ Varieties of fossil fuels:
The main fossil fuels are coal, petroleum, natural gas and their varieties.
There are also what are called oil shales: - Coals are sediments very rich in kerogen, 40% of their dry weight and more.
Their kerogen, particularly rich in oxygen, comes essentially from debris of terrestrial plants (trees, herbaceous plants) accumulated in very specific sedimentary environments, in particular swampy deltas of rivers located in regions with very high plant productivity.
They are classified using physico-chemical criteria according to the successive stages of transformation, known as coalification, which they have reached during carbonification: the peat stage designates coals that have not been buried or very little.
You can still see plant remains there with the naked eye.
Next come the lignite, subbituminous coal, bituminous coal, then anthracite stages, in order of increasing coalification.
The coals are found in coal basins whose depth can reach several kilometers, in the form of veins whose thickness varies from ten centimeters to several tens of meters and the extension from one hundred meters to ten kilometers.
These veins alternate with argillites containing, but in much smaller proportion, a kerogen of the same nature as that of coals, and with other sediments, most often sandstones.
They are mined underground to maximum depths of around 1,500 meters, and for deposits whose depth does not exceed 200 to 300 meters, very often in the open (uncovered).
After their extraction, the coals must be freed as much as possible of the debris of their host rocks (the wallwalls) which are extracted at the same time as them.
This is done by grinding and washing. She uses a lot of water and chemicals.
Despite this, the commercial coal contains on average 15% of its weight in ash, that is to say non-combustible minerals.
Most of it comes, not from the wall, but from minerals intimately mixed with kerogen. We are thinking, but we have not passed the pilot stage so far, in exploiting the coal seams which cannot be exploited, because they are too deep or too thin, by underground gasification, that is to say - say by practicing horizontal drilling, then by injecting oxygen and steam at high temperature.
We could thus produce from the gas formed, composed mainly of carbon monoxide and hydrogen, energy or synthetic fuels, or molecules, methanol, ammonia, etc., which can be used for chemical synthesis.
This method is currently very expensive, and its environmental risks very poorly evaluated.
Exploitable oils and natural gases are fluids formed in source rocks and generally accumulated in reservoir rocks of high permeability.
Oils contain mainly liquid hydrocarbons, that is to say molecules composed only of carbon and hydrogen having a number of carbon atoms greater than 4, but also in very variable proportions of resins and asphaltenes, compounds high molecular weight organic also containing sulfur, oxygen and nitrogen, elements which must be eliminated during refining.
Natural gases contain hydrocarbons which are gaseous once on the surface, mainly methane (CH4), accompanied in smaller quantities of hydrocarbons having from 2 to 4 carbon atoms, but also in variable proportions of carbon dioxide, nitrogen, hydrogen sulfide, sometimes a little argon or helium, and even traces of mercury or arsenic compounds.
Oil and natural gas are frequently associated in the same tank, and often form separate phases there, depending on the thermodynamic conditions of the tank: an oil phase containing dissolved gas (called associated gas) and a less dense gas phase, containing oil dissolved, located above, the whole being above a water phase called aquifer.
There are also oil fields without associated gas, and gas without associated oil ("dry" gas)
They are exploited by drilling wells. The productivity of a well depends on the speed of flow, which is itself governed by the permeability of the rock-reservoir to oil or gas and by the pressure difference between the reservoir and the surface.
To maintain the oil pressure in the reservoir, water must be injected into the aquifer. This is what is known as conventional oil and gas.
There are varieties of so-called unconventional oil and gas: - The so-called extra heavy oils (bitumens, extra heavy oils) contained for the most part in the tar sands of Athabasca in Canada, or in the shallow deposits of the Bituminous Belt (Faja bituminosa) of the 'Orinoco in Venezuela, are found in deposits brought to the surface or in its vicinity following a tectonic inversion.
They have been affected by oxygen dissolved in surface water and by microorganisms.
These oils are richer in oxygen and much more viscous than conventional oils, and are therefore of lower value and much more difficult to exploit.
In Canada, part of it is extractable by surface mining, which is, as well as the subsequent techniques of separation of bitumens from their mineral matrix, very polluting and very destructive of landscapes.
Another part is exploited by horizontal drilling and steam injection to reduce the viscosity.
In Venezuela, so-called extra heavy oils are accumulated under bitumen plugs formed in reservoirs by the degradation of petroleum.
The oil and gas contained in reservoirs of very low permeability (tight) or remained in their source rocks, which can only be exploited by horizontal drilling followed by hydraulic fracturing.
The term shale oil is used, but wrongly as we will see below, to designate “source rock oil” recoverable in source rocks, that is to say kerogen rocks that have naturally produced oil, but not having completely expelled it.
In fact, this oil is not found elsewhere in the parent rocks proper, but in reservoir rocks of small thickness and very low permeability (tight) located within certain parent rocks or in their immediate contact .
Shale gas is found in source rocks that produced gas, but did not completely expel it.
This gas is sometimes recoverable by horizontal drilling and hydraulic fracturing when the bedrock contains relatively permeable levels (microcracks, larger particle size, etc.), and now constitutes in the United States an important contribution to the production of natural gas.
When the bedrock is coal, we speak of coal seam gas (in English CBM for coal bed methane).
Oil and gas cannot be sold directly at the exit of the wells, because they must be separated from the water and sludge extracted at the same time as them and also remove unwanted compounds such as carbon dioxide, nitrogen or hydrogen sulfide.
On the other hand, the variety of molecular weights of their constituents leads them to be separated into various categories to allow their transport.
The gas frequently contains, in addition to methane and a little ethane which will be transported by pipeline or, after liquefaction, by LNG carrier (liquefied natural gas, LNG), propane and butane which is liquefied in the form of liquefied petroleum (LPG), and liquid hydrocarbons. All this is the subject of what is called field treatment.
LPG and liquid hydrocarbons extracted from natural gas are recognized in unconventional oil under the heading natural gas liquids (NGL).
International statistics of the quantities produced are made in tonnes (metric) for coal, in barrels for oil (2), and in thousand m3 under normal conditions (N.m3) or in millions of British Thermal Units (MBTU, 1 BTU = 1055 joules) for natural rich in kerogen, usable in the open air, which have never been brought to depths sufficient to have produced natural oil or gas.
They are pyrolysed in ovens at temperatures of 500 to 800 ° C, therefore much higher than the temperatures prevailing in sedimentary basins, to artificially produce shale oil, a poor quality imitation petroleum, from kerogen that they contain.
It is a potentially considerable resource, particularly in the United States (Green River Shales), but the current methods of exploitation are not economical, and are very polluting.
uses On a global scale, coals have three main uses: in% by weight, the production of electricity (70%), the manufacture of metallurgical coke used to produce the energy necessary for the production of cast iron in blast furnaces (15%), and domestic heating (5%), the latter mainly in the countries of Asia, China and India in particular.
The rest is used by various industries as fuel to provide heat at high temperatures. The main uses of petroleum are, after refining, in% by weight: 65% fuels (petrol and diesel for vehicles, kerosene for airplanes, heavy fuel oil for boats), 22% fuels (of which 15% fuel oil domestic for heating and 7% heavy fuel for the production of industrial heat and the production of electricity), and at 8% the bases of synthesis for petrochemistry. The rest consists of road bitumens, solvents, etc.
These various products are produced in refineries from crude oil.
Those of natural gas are mainly electricity production (40%), fuels for domestic heating and cooking (28%) and heat production for industry (22%), the rest being used as raw material or energy source for making fertilizers and various chemicals and also as fuel in the form of natural gas vehicle (NGV) used by gas cars.
Energy content The energy produced is the heat given off by oxidation of the carbon and hydrogen in the air by oxygen.
Large-scale accounting of energy production is done in tonnes-oil equivalent (toe). One toe is the energy released by the combustion of one ton of oil.
Since this quantity can vary quite significantly from one oil to another, it has been arbitrarily set at an average value of 41.86 billion joules (GJ), or 11.630 MWh. In 2008, world consumption of primary energy was around 12 billion toe (Gtoe), of which 81.7% came from fossil fuels: 34% for petroleum, 20.9% for natural gas and 27% for coal (table).
This consumption has since increased at a rate of around 1% on average per year, without much variation in the proportion of the total contribution of fossil fuels.
On the other hand, the proportion of natural gas and especially that of coal has increased, to the detriment of that of oil. On the other hand, China's consumption has increased significantly, making it the number one consumer of fossil fuels.
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