Stoichiometry and Natural Gas: LNG, LPG, and NGV
Stoichiometry
In chemistry, stoichiometry is the calculation of the quantitative relationships between reactants and products during a chemical reaction. These relationships can be deduced from the atomic theory, although historically spelled out without reference to the composition of matter, according to different laws and principles.
The first to set out the principles of stoichiometry was Jeremias Benjamin Richter (1762-1807) in 1792. He wrote:
Stoichiometry is the science that measures the quantitative proportions or mass ratios in which chemical elements are involved.
Mixture, Stoichiometric Ratios, and Conditions
When reactants of a reaction are in amounts proportional to their stoichiometric coefficients:
- The mixture is stoichiometric.
- The reagents are in stoichiometric proportions.
- The reaction takes place at stoichiometric conditions.
The three expressions have the same meaning.
Under these conditions, if the reaction is complete, all the reagents are consumed, giving the stoichiometric quantities of corresponding products. If not, the limiting reagent will be the one in lower proportion than needed for complete reaction. All calculations are based on the limiting reagent.
Example
How much oxygen is needed to react with 100 grams of carbon to produce carbon dioxide?
Atomic mass of oxygen = 15.9994.
Atomic mass of carbon = 12.0107.
The reaction is:
To form a molecule of carbon dioxide, we need one carbon atom and two oxygen atoms, or one mole of carbon and two moles of oxygen.
Solving for x:
Performed operations:
LNG (Liquefied Natural Gas)
Liquefied natural gas (LNG) is natural gas that has been processed to be transported in liquid form. It is the best alternative to monetize reserves in remote places where it is not economical to carry the gas to market directly either by pipeline or power generation. Natural gas is transported as a liquid at atmospheric pressure and -161°C, which reduces its volume approximately 600 times.
Reasons for Liquefaction of Natural Gas
Natural gas is usually transported using pipelines, but for long distances, it is more economical to use ships. It needs to be liquefied for transportation, as at room temperature and atmospheric pressure, it occupies a considerable volume. The liquefaction process reduces the volume of natural gas over 600 times.
About half the known fossil fuel reserves today are natural gas fields. They are often located in regions with low demand for gas. However, once liquefied, it can be transported safely to its target market using vessels, similar to crude oil.
Environmental Impact
LNG has the lowest environmental impact of all fossil fuels due to its high hydrogen content. LNG spills dissipate in the air and do not contaminate the soil or water. As a vehicle fuel, it reduces emissions of nitrogen oxides (NOx) by 70% and produces no sulfur compounds or particulates. For electricity generation, sulfur dioxide (SO2) emissions are virtually eliminated, and CO2 emissions are reduced by 40%.
All production, transport, and processing plant systems are designed to prevent leaks and fires, such as in LNG transfer systems to and from ships, shipping, or regasification. There are some design differences between gas plants, but environmental, safety, and health considerations are the same or more stringent. The main risks are the low temperature (cryogenic risk) and its flammability.
LPG (Liquefied Petroleum Gas)
Liquefied petroleum gas (LPG) is the condensable gas mixture present in natural gas or dissolved in oil. The components of LPG, although gases at standard temperature and pressure, are easy to condense, hence their name. In practice, LPG is a mixture of propane and butane.
Propane and butane are present in crude oil and natural gas, with some obtained during petroleum refining, especially as a byproduct of fluid catalytic cracking (FCC).
Uses of Liquefied Petroleum Gas
The major uses of LPG are:
- Collection of olefins, used for producing numerous products, including most plastics.
- Fuel for cars.
- Refinery fuel.
- Domestic fuel (via bottles or distribution networks).
Natural Gas (NG)
Natural gas is a non-renewable energy source formed by a gas mixture commonly found in oil fields, either non-associated (single), dissolved, or associated (accompanying) with oil or coal deposits. Although its composition varies depending on the extraction site, it is mainly composed of methane in quantities that commonly exceed 90% or 95% (e.g., non-associated gas from the West Sole well in the North Sea), and usually contains other gases such as nitrogen, CO2, H2S, mercaptans, and helium. Examples of contaminants include non-associated gas from Kapuni (NZ), containing up to 49% CO2. As additional sources of this natural resource, methane hydrate deposits are being investigated, which, according to estimates, could represent much greater energy reserves than current natural gas.
It may also be obtained by decomposing organic debris (garbage, plants—marsh gas) in treatment plants (urban wastewater treatment plants, garbage processing plants, etc.). The gas thus obtained is called biogas.
CO2 Generation in Comparison With Other Gases
Natural gas produces much less CO2 than other fuels such as oil and especially coal. It is also a fuel that burns more cleanly and efficiently.
The reason it produces little CO2 is that the main component, methane, contains four hydrogen atoms and one carbon atom, producing two molecules of water for each CO2 molecule, while long-chain hydrocarbons (liquid) yield only one water molecule per CO2 molecule (remember that the heat of formation of water is very high).
As an added benefit, it is a more versatile fuel that can be used in more efficient generation systems, such as combined cycle or fuel cells, and its acquisition is easier compared to other fuels. However, its energy content per unit volume is low compared to other fuels.
Natural Gas Liquids (NGL = LPG + NG)
Natural gas liquids (NGL), consisting of ethane, propane, butane, and other heavier hydrocarbon components, are used in the domestic market as fuel and raw materials (including petrochemicals).
To convert natural gas into liquid, the treated gas is cooled to about -161°C, the temperature at which methane, the main component, becomes a liquid. The liquefaction cooling process is similar to common refrigerants; gases are compressed to produce cold liquids such as propane, ethane/ethylene, methane, nitrogen, or mixtures thereof, which then evaporate as heat is exchanged with the natural gas stream. Thus, natural gas is cooled to the point where it becomes liquid. Once liquefied, the gas is subjected to a Joule-Thompson expansion or work extraction process so that it can be stored at atmospheric pressure. The produced LNG is stored in special tanks and then transferred to special transport tankers.
Natural gas extracted from deposits is a colorless, odorless, non-toxic product lighter than air. It comes from the decomposition of organic matter sediments trapped between rock layers and is a mixture of light hydrocarbons in which methane (CH4) is found in large proportions, together with other gases whose concentration depends on the site’s location.
Natural Gas Vehicles (NGV): Advantages and Disadvantages
LPG, or Autogas, consists mainly of butane and propane and is obtained from both natural gas and oil refining. This gas is available in large quantities worldwide, so in principle, it will not be easily exhausted.
Its main advantage is that any gasoline car can use it with a slight modification, becoming bi-fuel and able to run on either fuel.
Advantages
- Lower cost: Due to its lower production cost, natural gas will always be cheaper than traditional fuels. This economy is not only due to its price but also to savings in vehicle maintenance costs, as it increases the lifespan of certain items such as spark plugs, exhaust systems, carburetors, and lubricants.
- Existing infrastructure: It can harness existing gas pipelines, industrial and domestic natural gas networks, and existing gasoline service stations, which can become mixed gasoline/NGV outlets.
- Reliable supply: The supply of NGV is much safer and more reliable than other automotive fuels since the product is received directly through gas networks and not by trucks. Moreover, the product’s characteristics make it difficult to adulterate.
- Additional foreign exchange income: It generates additional foreign exchange earnings by exporting the volumes of liquid hydrocarbons released into the domestic market.
Disadvantages
- Weight/volume of cylinders: The gas storage cylinder adds weight and takes up space, reducing vehicle load capacity. This is particularly critical for small cars but not for commercial vehicles (minibuses, buses, pickups, trucks, etc.), as these can support the weight of the tanks and have more space available for storage cylinders.
- Loss of acceleration: Due to its characteristics, natural gas produces a power loss in the vehicle of approximately 15%, which is more noticeable during acceleration, especially in vehicles with low engine displacement.
Safety
Due to its properties, NGV is a safer fuel than traditional fuels because it is lighter than air and dissipates quickly, while gasoline vapors are five times heavier and can accumulate in poorly ventilated low-lying areas, creating potentially explosive mixtures.
The range for forming explosive mixtures is wider in gasoline than in NGV. The lower flammable limit is lower in gasoline compared to NGV, meaning it is more likely to form flammable air-fuel mixtures than air-NGV mixtures. Moreover, NGV requires a higher ignition temperature in air than gasoline.