Important Parameters to Consider When Evaluating Natural Gas Quality
Natural gas is a source of energy that is used all around the world for several purposes, such as heating, cooking, fuel for vehicles (CNG or Compressed Natural Gas), irrigation, electricity generation, and more. It is made up of naturally occurring hydrocarbons, primarily methane.
Natural gas is also referred to as a fossil fuel and is a nonrenewable source of energy. This means that, once consumed, this source of energy cannot be replenished. Thus, the reserves of natural gas in the world are limited because they take millions of years to get formed. Currently, we are using natural gas at a rate higher than they are being formed.
Natural gas is formed when layers of decomposing plant and animal matter form layers under the Earth’s surface and are then exposed to high levels of heat and pressure. Over the time of millions of years, this decomposing matter slowly converts to natural gas or fossil fuels. Since natural gas is used in almost all spheres – industrial, commercial, and even residential – it needs to be of top quality to make the most out of it. Impure natural gas does not combust properly, thus, not only hindering our work, but also causing higher levels of pollution. Additionally, it is also a waste of natural resources if it is not processed well.
Benefits of Using Natural Gas Over Other Sources of Energy
There are several benefits of using natural gas over other sources of energy. This has not only led to an increase in the demand for natural gas all across the globe, but has also caused extensive research development in this sector. Because of the combined efforts of research and development and the usage of high-class drilling and extracting equipment, the cost of natural gas has reduced over the years, thus making it available to a larger consumer base.
- Natural gas is more affordable than other alternatives, owing to the ease with which it can be extracted, processed, and transported.
- Energy generation from natural gas cannot be knocked out during storms (like is the case with electricity) and is thus reliable.
- Natural gas burns cleaner than other fossil fuels, such as diesel, coal, or petrol, and is thus very environmentally friendly.
- It is much easier to store and transport natural gas in storage tanks and pipelines, respectively, than it is to do the same with other fossil fuels.
Important Parameters to Consider When Evaluating the Natural Gas Quality
Owing to the multiple benefits and the high demand for natural gas all over the globe, it is crucial that natural gas is evaluated and transported for consumption at its top quality. This is why it is vital to know the important quality parameters to look into when ascertaining and evaluating a batch of natural gas. The parameters that affect the quality of natural gas are as follows.
- Water Vapor
- Wobbe Index
- Calorific Value (Net and Gross)
- Sulfur Compounds
- Density and Specific Gravity
- Methane Number
If you are studying or working in the field of natural gases, it is crucial for you to understand every one of these quality parameters. They not only help in maintaining the quality of natural gas, but also help you take care of safety concerns. To help you get started, here is a detailed look into each of these parameters.
The major constituent of natural gas is hydrocarbons. Hydrocarbon is an organic compound completely made of only hydrogen and carbon. Most hydrocarbons can be found naturally on the Earth on its surface as petroleum. Other than methane, natural gas has other constituents in minute amounts up to C14 (a compound with a total of 14 carbon atoms). When hydrocarbons are heated, they undergo a chemical reaction referred to as an exothermic reaction. This means that, along with the products produced upon combustion, hydrocarbons produce a huge amount of energy. It is this energy that is used for multiple purposes.
Now, hydrocarbons with up to six carbon atoms are referred to as light hydrocarbons, while any hydrocarbon with over six carbon atoms falls under the category of heavy hydrocarbons. During the processing stage of the natural gas, when all impurities are removed, heavy hydrocarbons are also gotten rid of. Since natural gas is mainly hydrocarbons, the quantity of the same can help us determine whether the natural gas is of good quality or not. The presence of any heavy hydrocarbons is an indicator that the natural gas will have a disbalanced knock resistance, thus affecting the air-fuel ratio. This leads to operational difficulties.
A diluent is a light hydrocarbon mixture (carbon atoms less than six), which is added to natural gas. This is done so that the natural gas is made less viscous to make it flow better. Diluents are produced by refineries where crude petroleum is processed. Diluents are also produced when impure and raw natural gas undergoes condensation. While this is important for eased transportation, it can also cause issues when natural gas is made to undergo combustion to generate heat. Moreover, the presence of sufficient amounts of diluents (especially Hydrogen) can cause the storage tanks to become brittle and weak. Even though diluents are in the category of light hydrocarbons and not heavy hydrocarbons, diluents can still cause quality problems because, unlike methane, they do not have any energy content.
Since they do not produce any amount of energy when combusting, they reduce the heating value of the natural gas. This means that more natural gas will be burnt to produce the same amount of energy as when the same amount of 100% pure natural gas is burnt. Because of this, consumers do not get full value for the natural gas that they are paying for. This also increases pollutants level in the environment. The most common diluents found in natural gas are H2O (Water), N2 (Dinitrogen), CO2 (Carbon Dioxide), He (Helium), H (Hydrogen), CO (Carbon Monoxide), and O2 (Oxygen). Amongst these, water vapors have additional issues, which will be discussed next in detail.
Natural gas has trace amounts of H2S and CO2. Water vapor (H2O) is present in natural gas as a diluent. When these two combine, they form the respective acids. Since natural gases are stored in high-pressure storage tanks and transported to the city and end-consumers in large and lengthy pipelines, the presence of these acids in the natural gas can cause the material of these storage tanks and pipelines to corrode. The acid seeps into even the tiniest of fissures, causing the boundaries of the storage tanks and pipelines to not only corrode, but also burst. Even if it does not combine to form these acids, water vapor alone can precipitate out and cause issues.
Moreover, the water vapors in natural gas can also freeze if the outside temperature falls drastically. The resulting ice formation not only reduces the quality of natural gas by a great margin, but it can also cause the flow of natural gas in the pipelines to get restricted, and thus it cannot be transported to the end-consumer in its pure form. If this ice gets stuck in the valves or regulators, it can cause the machinery to jam, thus causing operational issues. These operational issues take a long time to get fixed, and if the issue is in pipelines, the entire situation not only becomes very time-consuming, but also very expensive.
Wobbe Index is a much more technical parameter to evaluate the quality of natural gas than others mentioned in the list above. The measure of the rate of thermal input through a fixed nozzle in a stationary burner is referred to as the Wobbe Index. The natural gas undergoes combustion in a closed stationary burner chamber, and then the Wobbe Index for that amount of natural gas is calculated. It usually has the units of Btu/scf or MJ/m3. When trying to evaluate the quality of natural gas, the Wobbe Index is now a universally accepted and widely used parameter.
Now, if a certain amount of natural gas has a very high value for Wobbe Index, it means that the heating effect of the natural gas is very high, and so is the load on the burner. If the value for the Wobbe Index of a certain amount of natural gas is low, it means that the heating effect of the natural gas is low, and so is the load on the burner. Thus, by figuring out the load on the burner, we can not only obtain natural gas that has a high permissible value of heating effect, but also prevent mishaps from happening due to overheating. Thus, the Wobbe Index is not only a quality evaluating parameter, but also a safety parameter.
Calorific Value (Net and Gross)
In simple terms, the calorific value (CV) for a certain amount of gas is the amount of heat it can generate upon combustion. The amount of heat generated by a unit amount of natural gas at a constant pressure of one atm (0.1 MPa) is referred to as the Net Calorific Value (NCV), given that the water in the resulting products remains in the form of vapors. Gross Calorific Value (GCV) refers to the amount of heat generated when one cubic meter of gas undergoes combustion at constant pressure and room temperature. It is also sometimes referred to as heating value.
The calorific value obtained is then multiplied by the delivered amount of gas to calculate the amount of energy delivered. Thus, for the same level of delivered amount of gas to multiple points, the calorific value of the natural gas can work as a good parameter to ascertain how much energy is being delivered to all those multiple points. In case they do not receive the same amount of energy, the measure of calorific value can be used to ascertain which natural gas is of low quality. Moreover, if the maximum heating value is being reached quickly, diluents need to be added to the natural gas to make sure it does not exceed it, thus further lowering the quality of the natural gas.
Another parameter that should be considered when trying to evaluate whether the quality of the natural gas is low or high is the presence or the amount of sulfur compounds in it. H2S, mercaptans, sulfides, thiophenes, elemental sulfur (S8), and iron sulfide are some of the sulfur compounds that are mostly present in natural gas. These are sometimes present in the form of impurities in trace amounts or are added on purpose for several safety concerns. The pungent smell of sulfur is helpful in getting to know if there has been a leak of the gas, especially in residential households. However, excessive amounts of these sulfur compounds in natural gas can cause its quality to drop.
When present in natural gas in high amounts, sulfur compounds not only reduce the amount of heat generated upon combustion of the natural gas, but also deactivate certain chemical reactions owing to their catalytic nature. Additionally, they also cause corrosion in storage tanks and pipelines like water vapors and can thus be the cause of accidental storage tank or pipeline bursts. Such accidents not only cause harm to human lives, but also the environment.
In its basic nature, siloxane is a functional group that has the Si – O – Si linkage. It is an organosilicon compound that does not occur naturally in natural gas. Siloxanes are famously used as very effective fire retardant agents. The most common siloxanes that are found in varying amounts in natural gas are trimethylsilanol, octamethylcyclotetrasiloxane (D4), and decamethylcyclopentasiloxane (D5). Siloxanes are also very commonly used in cosmetics, deodorants, water repellent windshield coatings, food additives, and soaps. The presence of siloxane in processed natural gas, especially in high amounts, can cause several operational and functional issues.
Upon combustion of natural gas containing siloxanes in gas turbines, boilers, or combustion engines, solid silica or silicates are produced. These get deposited in various parts of the machinery, and they not only interfere with other processing chemical reactions, thus inhibiting the formation of relevant products, but they also are very harmful for exhaust catalysts. They can cause the entire processing and transportation process to be severely inhibited. They also cause the calorific value of the natural gas to go down, thus producing lower amounts of energy than the natural gas ideally should. Siloxane can be removed from natural gas by using high-quality, non-impregnated activated carbon.
Density and Specific Gravity
Density is a measure of mass per unit volume. Thus, if you divide the total mass of a certain amount of gas to its total volume, you will get the density for it. Now, at the same level of temperature and pressure, if you divide the density of that certain amount of natural gas by the density of air, you will obtain the specific gravity for it. Density and Specific Gravity are very good indicators or parameters for when you are trying to evaluate the quality of natural gas. If a certain amount of natural gas is showing values for density and specific gravity different from what it should be ideally, this means that the natural gas has impurities that are changing its mass and volume.
The presence of these impurities reduces the quality of natural gas. It leads to certain issues, such as low energy levels produced upon combustion, natural gas not combusting at all, or in some cases, the natural gas ignites way before it should, thus causing accidents. The presence of such impurities can also lead to chemical reactions in the processing tanks to get inhibited and not be carried out properly. In rare but possible cases, these unwanted impurities can also corrode pipelines and storage tanks, causing them to burst, which is again hazardous. All this can be prevented by checking the parameters for density and specific gravity and taking appropriate measures to get these measures at the correct values.
The Methane Number of natural gas is similar to the Octane Number of petrol. Methane Number is a measure for the anti-knock behavior of the natural gas or its tendency to resist detonation. Pure methane has a Methane Number of 100, and the Methane Number of all other hydrocarbons is calculated by comparing it to the Methane Number of pure methane. All hydrocarbons heavier than methane have a Methane Number less than 100. The typical Methane Number of natural gas is kept at a level over 80 so that the gas does not ignite prematurely.
Thus, if the Methane Number of natural gas is not maintained at above 80, the anti-knock characteristics of the gas begin to degrade. While the natural gas should have ignited to produce energy at a much higher temperature and pressure, in this scenario, the lower Methane Number of natural gas causes it to ignite way before than it should. This can lead to catastrophic accidents and cause loss of lives. Additionally, the premature ignition of natural gas also causes high levels of pollutants to be released in the environment, thus furthering the already degrading ecosystem of the Earth.