Hydrocarbons – What are they?

As discussed in blog 3 solar energy, water and carbon dioxide create carbohydrates by photosynthesis in plants.  Animals eat the plants creating more complex carbohydrates (carbon, hydrogen and oxygen molecules). When these decay without oxygen they form hydrocarbons (carbon and hydrogen molecules). More complex molecules are also formed which integrate nitrogen and various minerals.  Decay products can also include pure carbon (coal) and oxocarbons including carbon dioxide 

Methane is the simplest hydrocarbon with a formula CH4. In Australia it is also known as Natural Gas and is reticulated directly into homes for cooking and heating. It is also used to generate a large portion of our electricity, used as the base for fertiliser production and can be used as rocket fuel. (Elon Musk is using methane in his Raptor engine). It is also used to produce hydrogen gas. 

Methane can be compressed to form CNG (Compressed Natural Gas) and used for transport fuel.  Large tanks are needed making it generally unsuitable for domestic car use.

It can also be compressed further into liquid form (LNG) so it can be transported without pipelines.  LNG is also Elon’s rocket fuel.

As methane has no smell, other chemicals (eg metcaptins – fart smell) are added to domestic gas for safety reasons. Methane does not harm humans if breathed in (unless its enough to asphyxiate you). Methane is created naturally by biological processes (biogenic), pretty much by anything undergoing decomposition and is what we know as biogas. Gases released by animals often contain some methane. Rotting plant material, including grasses produce methane as well (so don’t blame the cow – she’s the middleman, so to speak).

As organic material (anything containing carbon chains) is buried more deeply it is cooked in thermogenic processes creating huge amounts of methane amongst other chemicals. Methane is very light and is partially water soluble. As it is expelled, it rises to the Earth’s surface where it can. Otherwise it gets trapped underground.  Artesian waters will generally contain dissolved methane which escapes when the water surfaces.

Methane is a significant greenhouse gas, coming in third after water vapour and carbon dioxide. It is relatively short lived breaking down to oxocarbons and water vapour. In Australia, leakage at well sites and flaring gas are well regulated and are a minimal source of “fugitive emissions”.  Gas reticulation pipes however are a concern with gas leaks common. These need regular maintenance !  Methane emissions from waste disposal, land fill, farming, compost, swamps, coal fields, animal breath are by far the greatest source of atmospheric methane.  These can and should be captured for fuel use wherever possible.

Methane is the most efficient of the hydrocarbon fuels, creating the most energy per gram.  It also releases the least carbon dioxide as it burns four atoms of hydrogen for every carbon atom. It has always been considered clean and green as its combustion products are not harmful. 

Ethane is the next simplest saturated hydrocarbon. Saturated hydrocarbons ( also known as alkanes) have only single chemical bonds. These are the well known hydrocarbons and are often referred to as C1 (methane has 1 carbon and 4 hydrogen atoms), C2 (ethane has 2 carbon atoms and 6 hydrogen ), C3 (propane has 3 carbon and 8 hydrogen) etc.  Once double bonds are introduced complex hydrocarbons form ( eg benzene). 

Ethane can be used as fuel, but in Australia it is generally separated out. Moomba has a separate pipeline which transports ethane to the Orica factory in Sydney where it is used to make ethylene, the basic component to make plastics and synthetic rubber.  Our synthetic clothes, plastics ( in your smart phone etc), car tyres are made from this.

Ethane is also non poisonous, but is also a greenhouse gas.

Propane (C3), together with Butane (C4) make up LPG or Liquid Petroleum Gas. These are highly volatile liquids under normal temperature and a little pressure (eg as cigarette lighter fuel).  This is what we use as barbecue or transport gas (not the US gasoline gas!). It is found as a small percentage in numerous Australian gas wells but is generally produced as a refinery by-product.

Propane is heavier than air and will tend to sink when released.  It is not poisonous but is dangerous to inhale as it displaces air.  It is used as a propellant in most spray cans, as a refrigerant, in blow torches, as a chemical feedstock (ie the base for making industrial chemicals) and in semiconductor and solar cell manufacture.

Butane (C4) has many similar uses to propane and is often mixed with it to form LPG. Butane cartridges are a popular camping and cooking fuel.   

Butane is the most abused of the hydrocarbons and has been the cause of many “solvent” deaths.  When burnt, propane and butane will produce carbon dioxide and water. However if there is not enough oxygen accessible, toxic gases will form making it dangerous to use lpg in enclosed spaces.

Pentane (C5) is a colourless liquid at room temperature.  It looks like water but has an offensive smell, is toxic and its combustion products are toxic.  C5 together with C6 form a product called Naptha while C5-C10 form petrol ( otherwise known as gasoline).

Pentane is also used to create polystyrene foam. 

C1-C5 are the likely products produced in gas wells.  Dry gas wells will be nearly 100% C1 and C2.  Wet gas wells will include the “condensates” C3-C5.  Oil wells will produce C6 – C70 (and may have associated gas).  Oil and gas are often found in the same reservoir with the gas sitting above the oil which will sit above any water.  Petroleum is generally considered as the oil component which is refined into its various components. Diesel and petrol are both components of petroleum.

Hexane (C6)  is used as a solvent, paint thinner and in particular it is used to extract edible oils from seeds and vegetables (cold pressed oils do not use hexane). Probably not poisonous in small doses but may have long term effects with chronic exposure. 

Petrol (C5-C10) is a mix of hydrocarbons within this range ( although butane is sometimes added).  A variety of unsaturated hydrocarbons are present in petrol and other solvents in this range.  Some of these, in particular benzene, are particularly poisonous and known carcinogens.  Exposure should be limited.  Principle exposure is from cigarette smoke, bushfires and petrol / solvents.  Yes cigarette smoke is the greatest concern as organic material is burned, forming benzene (and other toxic gases) and delivered direct to your lungs!

Kerosene / Avgas (C10-C16).  Fuels with this range of hydrocarbons include avgas, kerosene and avjet fuel.  They are liable to still contain lead pollutants.  There are many products with different proprietary mixtures.  These are not simple hydrocarbons and also contain many different saturated and unsaturated hydrocarbons plus other additives.  Combustion products are also complicated but are invariably greenhouse gases.

Diesel (C14-C20) is essentially any fuel that can be burned in a diesel engine.  This might include light crude direct from an oil well with no refining.  Diesel fuel is not volatile and does not use a spark for ignition.  Diesel can be natural, synthetic ( using Fischer Tropsch process on coal etc) or biodiesel.

There has been significant discussion on  which is less polluting – diesel or petrol.  It is a difficult question as there are many parts to the pollution.  Diesel was considered the cleanest fuel for most of the last ten years after turbo diesels became common and sulphur content was reduced.  However recent evidence suggests that diesels create more dangerous pollution including particulates (very small particles). Nitrous oxides are also a big component of smog.  VW has not helped the situation by hiding the actual pollution levels of diesel engines.

Lube Oils ( C20-C50) – industrial and transport lubricating oils. 

Fuel Oil (C20-C70) – used in shipping and heating. 

Tar / Bitumen (C70 plus)  – a by product of refining and also found naturally in tarpits and tar sands.  Large amounts of our bitumen roads are dug up in Canada as part of the massive Athabasca tar sand deposit.  Amazingly we have put bitumen across much of our prime farming country to facilitate transport.  While it is not particularly poisonous, heated bitumen does give off somewhat toxic fumes ( as you can smell).  Also water capture after it has run down dirty bitumen roads is problematic.

Phew!! That’s a very basic review.  And Elon even fitted in! 

Will do oil and gas geology and production another time!


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