3 Fossil Fuel Classification and Composition

Dr. Dhanya M.S M.S

1. Introduction

The fossil fuel is the major contributor of the energy consumption in the world. It is a non-renewable and conventional energy source. These energy sources are originated from deposition of remains of living organisms buried under ground for millions of years by factors such as pressure, heat and time. These are finite resources that are limited and may deplete in future without replenishment.

2. Characteristics and advantages of fossil fuel

The fossil fuel has the following features

Combustible in presence of heat and oxygen Concentrated fuel High calorific value Formed naturally from plant and animal matter over millions of years Consists of hydrogen and carbon Highly stable with hydrogen and carbon bonding Limited amount of reserves are present in the world Non-renewable Cheap energy source (compared to non-conventional energy source) Easy in storage and transportation

3. Origin of fossil fuel

Sedimentary rock formations are the most common geologic environments for storing oil and gas. There are two theories of the origin of fossil fuels

Biogenic theory, and Abiogenic theory

3.1 Biogenic Theory

In the biogenic theory, a type of biochemical precipitation called organic sedimentation forms fossil fuels. Carbon rich organic material called peat can be formed when vegetation dies and decays in aqueous environments such as swamps. If peat is buried by subsequent geological activity, the buried peat is subjected to increasing temperature and pressure, and peat can eventually be trans-formed into coal by the process of coalification. An analogous process is used in the biogenic theory of the origin of oil and gas. Hydrocarbon mixtures can exist in solid, liquid or gas phase in the reservoir. The phase depends on the composition of the mixture and the temperature and pressure of the reservoir. The elemental mass content of naturally occurring hydrocarbon mixtures ranges from approximately 84% to 87% carbon and 11 % to 14% hydrogen, which is comparable to the carbon and hydrogen content of life. This is one piece of evidence for the origin of petroleum from biological sources.

The oil and gas formation begins with the death of microscopic organisms such as algae and bacteria. The remains of the organisms settle into the sediments at the base of an aqueous environment as organic debris. Lakebeds and seabeds are examples of favorable sedimentary environments.

Subsequent sedimentation buries the organic debris. The rise and fall of sea level, as well as other geologic processes, continue the process of burying the organic debris. As burial continues, the organic material is subjected to increasing temperature and pressure, and is transformed by bacterial action into oil and gas. In many cases the organic material is contained in sediment that originally had a muddy composition, such as silts and clays. The transformation of organic material into oil and gas often occurs in shale, which can be formed by the application of heat and pressure to muddy sediment. Consequently, shale is a common source rock for oil and gas. Petroleum fluids are usually less dense than water and will migrate upwards until they encounter impermeable barriers and are collected in geologic traps. The accumulation of a hydrocarbon mixture in a geologic trap forms an oil and gas reservoir. So in the biogenic theory, the origin’ of oil and gas begins with the death of organisms that live on or near the surface of the earth.

3.2 Abiogenic Theory

The abiogenic theory says that processes deep inside the earth, in the earth’s mantle, form petroleum fluids. Thomas Gold, an advocate for the abiogenic theory, argued that simple inorganic and organic molecules in the interior of the forming earth were subjected to increasing heat and pressure, and eventually formed more complex molecules. Crustal oil and gas reservoirs are formed by the upward migration of petroleum fluid until the fluid is stopped by impermeable barriers and accumulates in geological traps.

4. Classification of fossil fuel

The organic materials are heated and compressed over time to form oil, gas and coal. The fossil fuel can be classified as solid, liquid and gaseous fuel.

4.1 Coal (Solid fossil fuel)

Coal is a combustible rock that is composed primarily of carbon rich organic material. Coal is a heterogeneous sedimentary rock that reflects both the different sedimentological regimes within which peat forms and the varied vegetation types from which it is derived.

4.1.2 Origin of coal

The formation of coal from organic debris is by a process of coalification. When some types of organic materials are heated and compressed over time, they can form water, gas, and coal. In some cases, a high-molecular weight, waxy oil is also formed. For example, bog or swamp vegetation may be buried under anaerobic conditions and become peat. Peat formed by the decay of vegetation under anaerobic conditions is a coal precursor. It is an unconsolidated deposit of partially carbonized vegetable matter in a water-saturated environment. If peat is buried by rock in a depositional environment, it is subjected to increasing temperature and pressure. Volatile products and water migrate away from the formation. The resulting carbon-rich, compressed residue is coal. Organisms that form coal when subjected to coalification include algae, phytoplankton, and zooplankton. Coal can also be formed by the bacterial decay of plants and, to a lesser extent, animals. Organic debris is composed primarily of carbon, hydrogen, and oxygen. It may also contain minor amounts of other elements such as nitrogen and sulfur.

4.1.3 Classification of Coal

Coals are classified by rank. Rank is a measure of the degree of coalification or maturation of carbonaceous material. The coal is classified into lignite, sub-bituminous coal, bituminous coal, anthracite and graphite

The lowest rank coal is lignite, followed in order by sub-bituminous coal, bituminous coal, anthracite and graphite. Table 1 summarizes the relative properties of coal for four coal ranks that are used as energy sources. The moisture content of lignite is high compared to the moisture content of anthracite. High moisture content is associated with low heating value, while low moisture content is associated with high heating value. Coal rank is correlated to the maturity, or age, of the coal. As a Coal matures,the ratio of hydrogen to carbon atoms and the ratio of oxygen to carbon atoms decrease. The composition of the highest rank coal, graphite, approaches 100% carbon. Coal becomes darker and denser with increasing rank.

Coals burn better if they are relatively rich in hydrogen; this includes lower rank coals with higher hydrogen to carbon ratios. The percentage of volatile materials in the coal decreases as coal matures. Volatile materials include water, carbon dioxide and methane. Coal gas is primarily methane with lesser amounts of carbon dioxide. It is absorbed in the coal structure or coal matrix, and adsorbed on the surface of the coal. Absorption is the penetration of one substance into another substance. By contrast, adsorption is the accumulation of one substance on the surface of another. The amount of gas that is present in the coal matrix depends on rank. As rank increases, the amount of methane in the coal matrix increases because the molecular structure of higher rank coals forms a structure that has a greater capacity to absorb gas and can therefore contain more gas.

Coals are organic, sedimentary rocks. A coal seam is the stratum or bed of coal. It is a collection of coal matrix blocks bounded by natural fractures. The fracture network in coal beds consists of micro-fractures called “cleats.” An inter-connected network of cleats allows coal gas to flow from the coal matrix blocks when the pressure in the fracture declines. This is an important mechanism for producing coal gas.

4.1.4 Composition of coal

The organic origin of coal provides an explanation for the elemental composition of coal, which ranges from almost pure carbon to compounds of carbon with other elements, notably hydrogen, oxygen, and sulfur. The chemical composition of coal has a strong influence on its combustibility.

The chemical composition of a bituminous coal is

Carbon content : 84%

Hydrogen content : 5%

Oxygen:10%

Sulfur and Nitrogen :0.5 -1%

4.1.5 Gradation of Indian coal

The gradation of Indian coal based on its calorific value is as follows:

In the Indian Industry, the most widely accessible grades of coal are D,E and F coal grades .

4.2. OIL (Liquid fuel)

Oil and gas are terms that refer to mixture of hydrocarbon molecules in the liquid phase and gas phase, respectively. Hydrocarbon molecules are compounds of carbon and hydrogen. The United States Energy Information Administration defines conventional oil and natural gas production as “crude oil and natural gas that is produced by a well drilled into a geologic formation in which the reservoir and fluid characteristics permit the oil and natural gas to readily flow to the wellbore” [US EIA Glossary, 2012]. Unconventional oil refers to hydrocarbon production from low permeability shale (shale oil) and tar sands. Unconventional gas refers to gas production from coal (coal gas), low permeability sands (tight gas), and low permeability shale (shale gas). The primary difference between conventional and unconventional oil and gas is the ability of the fluid to flow through rock. Most crude oil is refined for use in transportation, while gas is used to generate electric power and as a fuel in industrial, commercial, and residential sectors of the economy.

4.2.1 Composition of crude oil

Crude oil is a mixture of hydrocarbons that exists in liquid phase in natural underground reservoirs and remains liquid at atmospheric pressure after passing through facilities on the surface that separate gas and liquid (US EIA GLOSSARY , 2012). Table 2 enlists the most common elements found in crude oil and natural gas.

TABLE 2: Elemental Composition of Crude Oil

The actual elemental composition depends on factors such as molecular composition of the source, reservoir temperature and reservoir pressure. Hydrocarbon molecules in oil are compounds of carbon and hydrogen. Inorganic molecules such as nitrogen, carbon dioxide, and hydrogen sulfide can also be found in oil and natural gas.

Petroleum is another term for a hydrocarbon mixture found in nature. Petroleum liquid typically refers to crude oil and natural gas plant liquids.

Natural gas is typically methane with lesser amounts of heavier hydrocarbon molecules like ethane and propane. If the natural gas contains liquids dissolved in the gas phase at high temperatures and pressures, the liquids can condense out of the gas phase when the temperature and pressure of the gas is reduced. The change in temperature and pressure typically occurs when gas is produced from a subsurface formation to a facility on the surface. The liquids obtained from the gas are referred to as condensates or natural gas plant liquids. The total energy utilization in the world relies around 30-35 % on petroleum liquid and 20-25% on natural gas.

4.2.2 Classification of liquid fuel (Crude oil)

Hydrocarbon fluids that are one phase at the temperature and pressure in the reservoir often become two-phase fluids when they are produced to the surface where the temperature and pressure are lower. Natural gas is a hydrocarbon mixture in the gaseous state at surface conditions. Crude oil is a hydrocarbon mixture in the liquid state at surface conditions. Heavy oils do not contain much gas in solution at reservoir conditions and have a relatively large molecular weight. By contrast, light oils typically contain a large amount of gas in solution at reservoir conditions and have a relatively small molecular weight.

Table 3-3 Rules of Thumb for Classifying Hydrocarbon Fluid Types

A classification of hydrocarbon fluid types is given in Table 3-3. Separator gas-oil ratio (GOR) is a useful indicator of fluid type. It is the ratio of gas produced at the surface to the liquid produced. The unit SCF/STB equals one standard cubic foot (SCF) of gas per stock tank barrel (STB) of oil. The gas volume in SCF is at surface temperature and pressure and the oil volume in STB is at stock tank temperature and pressure.

The hydrocarbon liquids are classified based on the properties of the hydrocarbon liquid such as viscosity and density. The ratio of hydrocarbon liquid density (HO liquid density) to water density is the specific gravity of the hydrocarbon liquid. Specific gravity of a hydrocarbon liquid is calculated as

HC liquid density = (water density)/(specific gravity)

American Petroleum Institute (API) gravity is calculated using specific gravity:

API = (141.5/specific gravity) —131.5

Table 3-4 shows a hydrocarbon liquid classification scheme using API gravity and viscosity. Water properties are included in the table for comparison. Bitumen is a hydrocarbon mixture with large molecules and high viscosity. Light oil, medium oil, and heavy oil are different types of crude oil. Crude oil is less dense than water, while extra heavy oil and bitumen are denser than water. In general, crude oil will float on water, while extra heavy oil and bitumen will sink in water.

Medium oil: 0.870 -0.920 g/cm3

Heavy oil: 0.920- 1.00 g/cm3

The crude oil is separated into different fractions by fractional distillation method in the refinery. It works on the principle that top portion of fractionating column have lower boiling points than that at the bottom.

4.2.3 Shale Oil, Tar Sands and Extra Heavy Oil

Shale oil is solid bituminous material contained in low permeability shale. An oily liquid is obtained when the material is heated. Tar sands, also known as natural bitumen, are a combination of clay, sand, water, and bitumen. The World Energy Council [WEC, 2010] estimated that the shale oil resource base in at the end of 2008 was approximately 4.8 trillion barrels. Nations with the largest volume of shale oil are listed in Table 3-5.

Table 3-5 Nations with Largest Shale Oil Resources (billion barrels) at the end of 2008 [WEC, 2010]

4.3 Classification of Gaseous fossil fuels

Gaseous fuel in common use are liquefied petroleum gases (LPG), Natural gas, producer gas, blast furnace gas, coke oven gas etc.

4.3.1 Liquified petroleum Gas

LPG is a predominant mixture of propane (C3H8) and Butane (C4H10) with a small percentage of unsaturates (propylene – C3H6 and butylenes- C4H8) and some lighter C2 as well as heavier C5 fractions.

LPG may be defined as those hydrocarbons, which are gaseous at normal atmospheric pressure, but may be condensed to the liquid state at normal temperature, by the application of moderate pressures. Although they are normally used as gases, they are stored and transported as liquids under pressure for convenience and ease of handling. Liquid LPG evaporates to produce about 250 times volume of gas. LPG vapour is denser than air: The butane is about twice as heavy as air and propane is about one and a half times as heavy as air. Consequently, the vapour may flow along the ground and into drains sinking to the lowest level of the surroundings and be ignited at a considerable distance from the source of leakage. In still air vapour will disperse slowly. Escape of even small quantities of the liquefied gas can give rise to large volumes of vapour / air mixture and thus cause considerable hazard.

To aid in the detection of atmospheric leaks, all LPG’s are required to be odorized. There should be adequate ground level ventilation where LPG is stored. For this very reason LPG cylinders should not be stored in cellars or basements, which have no ventilation at ground level.

4.3.2 Natural Gas

Natural gas is a source of relatively clean energy that can be obtained from such sources as conventional oil and gas fields, unconventional gas resources, landfill gas, and municipal solid waste gas (MSW gas).Methane is the main constituent of Natural gas and accounting for about 95% of the total volume. Other components are: Ethane, Propane, Butane, Pentane, Nitrogen, Carbon Dioxide, and traces of other gases. Very small amounts of sulphur compounds are also present. Since methane is the largest component of natural gas, generally properties of methane are used when comparing the properties of natural gas to other fuels. Natural gas is a high calorific value fuel requiring no storage facilities. It mixes with air readily and does not produce smoke or soot. It has no sulphur content. It is lighter than air and disperses into air easily in case of leak.

4.3.3 Unconventional Gas

Unconventional gas re-sources include gas hydrates, tight gas sands, coal gas, and shale gas.

Gas hydrates, tight gas sands and shale gas are discussed below. Gas from landfills and municipal solid waste (MSW) gas are obtained from the decay of organic waste. Landfill gas and MSW gas are not fossil fuels; they are renewable energy sources.

4.3.3.1 Gas Hydrates

Gas hydrates are naturally occurring, crystalline complexes that are formed when one type of molecule completely encloses another type of molecule in a lattice. In the case of gas hydrates, hydrogen-bonded water molecules form a cage-like structure in which molecules of gas (such as methane, ethane and propane) are absorbed or bound. Methane hydrates contain a relatively large volume of methane in the hydrate complex. The hydrate complex contains about 85-mole percent water and approximately 15 mole percent guests, where a guest molecule is methane or some other relatively low molecular weight hydrocarbon.

Methane hydrates are found around the world. They exist on land in sub-Arctic sediments and on seabeds where the water is near freezing. They can be found in Arctic sands, marine sands, fractured muds, and shale.

4.3.3.2 Tight Gas Sands and Shale Gas

Coal gas, tight gas sands, and shale gas are characterized by low permeabilities. Conventional gas reservoirs have permeabilities on the order of millidarcies. Tight gas sands have permeabilities on the order of micro-darcies. Gas shale is a lowpermeability source rock found throughout the world. It has enough porosity to hold large amounts of gas but permeability on the order of nanodarcies makes it difficult to economically produce hydrocarbons. Gasproduction from shale has flow mechanisms that are similar to those required for gas production from coal. Shale gas must diffuse through the shale until it reaches a permeable path that allows it to flow to a well. Permeable pathways are typically natural or induced fractures.

5. Oil and gas reservoirs and reserves

Several key factors must be present to allow the development of a hydrocarbon reservoir:

Source rock for the hydrocarbon

For example, the biogenic theory says that oil and gas are formed by the decay of single-celled aquatic life. Shale formed by the heating and compression of silts and clays is a good source rock. Oil and gas can form when the remains of an organism are subjected to increasing pressure and temperature.

A flow path must exist between the source rock and reservoir rock.

Once hydrocarbon fluid has migrated to a suitable reservoir rock, a trapping mechanism must exist. If the hydrocarbon fluid is not stopped from migrating, buoyancy and other forces will cause it to move towards the surface.

Time. A source rock can provide large volumes of oil or gas to a reservoir, but a trap must exist at the time oil or gas enters the reservoir so that fluid can accumulate.

6. Limitations of fossil fuel

Environmental implications such as greenhouse gas emissions, acid rain, global warming Carbon emissions  Pollution.Biodiversity loss Affects marine life Health impacts Ecosystem imbalance

Conclusion

The global energy demand is currently met with different types of fossil fuel which are finite and non-renewable in nature. The high energy content and relatively cheaper than renewable energy sources makes fossil fuel consumption in all the energy sectors preferable. The conventional and unconventional fossil fuel reserves are concentrated in small portion of the world. So the judicious utilization of fossil fuel is very essential for meeting energy demands in the future.