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The amount of heat released by a fuel during combustion is referred to as its heat value, energy value, or calorific value. This value is a measure of a fuel's energy density, and it varies depending on the type of fuel. For example, the combustion of hydrogen and oxygen into water vapour releases 242 kJ/mol of heat. The rate at which heat is released during combustion is called the heat release rate, and it is influenced by factors such as the type of fuel and the presence of oxidants. The heat release rate is important in understanding the efficiency of fuel combustion and can be calculated using equations such as Eh=cm∆T.
| Characteristics | Values |
|---|---|
| Heat value of a fuel | Amount of heat released during combustion |
| Heat value calculation | Energy (joules) per specified amount (e.g. kilograms) |
| Heat of combustion for CcHhOoNn composition | 419 kJ/mol × (c + 0.3 h − 0.5 o) |
| Lower heating value (LHV) | Energy losses considered, e.g. energy used to vaporize water |
| Gross heating value | Includes water in the fuel prior to combustion |
| Higher heating value (HHV) | Assumes all water is in a liquid state after combustion |
| Combustion | Conversion of hydrogen to water, carbon to carbon dioxide, and nitrogen to nitrogen gas |
| Incomplete combustion | Occurs when there is insufficient oxygen, resulting in carbon and carbon monoxide production |
| Pyrolysis | Occurs before combustion in some fuels, e.g. diesel oil, coal, or wood |
| Fuel examples | Wood, coal, natural gas, oil, gasoline |
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What You'll Learn
The heat value of a fuel
The heat of combustion is a measure of the heat released during the complete combustion of a compound in its standard state to form stable products in their standard states. For instance, hydrogen is converted to liquid water, carbon to carbon dioxide gas, and nitrogen to nitrogen gas. The heat of combustion is calculated using the formula: 419 kJ/mol × (c + 0.3 h − 0.5 o), where c, h, and o are the numbers of carbon, hydrogen, and oxygen atoms in the molecule, respectively. This formula provides a good approximation for most compounds, with an accuracy of ±3%. However, it may yield poor results for certain compounds, such as formaldehyde and carbon monoxide, and may be significantly off for compounds where the sum of o and n (number of nitrogen atoms) is greater than c, as in the case of glycerine dinitrate.
The heating value of a fuel can be determined using calorimeters, specifically bomb calorimeters. This involves combusting a stoichiometric mixture of the fuel and an oxidizer (e.g., hydrogen and oxygen) in a steel container at 25 °C. The combustion is initiated by an ignition device, and the reactions are allowed to complete. The vessel and its contents are then cooled back to the original temperature of 25 °C, and the higher heating value is calculated as the heat released between these identical initial and final temperatures.
It is important to distinguish between the lower heating value (LHV) and the higher heating value (HHV) of a fuel. The LHV considers energy losses, such as the energy used to vaporize water, and assumes that any water formed during combustion is released as vapour. In contrast, the HHV assumes that all water is in a liquid state after combustion. The LHV is particularly useful when comparing fuels where condensing combustion products is impractical or when heat below a certain temperature cannot be utilised.
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The higher heating value (HHV)
The combustion of a stoichiometric mixture of fuel and oxidizer (e.g. two moles of hydrogen and one mole of oxygen) in a steel container at 25 °C (77 °F) is initiated by an ignition device and allowed to complete. The vessel and its contents are then cooled to the original 25 °C, and the HHV is determined as the heat released between identical initial and final temperatures. HHV assumes that all the water in the combustion process is in a liquid state after the combustion process.
The heating value of a fuel can be calculated with the results of an ultimate analysis of the fuel. From this analysis, the percentages of combustibles in the fuel (carbon, hydrogen, sulfur) are known. Since the heat of combustion of these elements is known, the heating value can be calculated using Dulong's Formula:
> HHV [kJ/g]= 33.87mC + 122.3(mH − mO ÷ 8) + 9.4mS
Where mC, mH, mO, mN, and mS are the contents of carbon, hydrogen, oxygen, nitrogen, and sulfur on any (wet, dry, or ash-free) basis, respectively. HHV is a true representation of the actual energy content of a fuel and is useful in calculating heating values for fuels where condensation of the reaction products is practical. For example, in a gas-fired boiler used for space heating.
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The lower heating value (LHV)
The LHV is a useful benchmark for most applications that burn fuel, as these applications produce water vapour that wastes its heat content. Engine manufacturers typically rate their engines' fuel consumption by the LHV since the exhaust is never condensed in the engine. The LHV is also used to determine the fuel flow rate required when going into the engine.
The LHV is calculated differently from the higher heating value (HHV). The HHV assumes that all of the water in a combustion process is in a liquid state after combustion, and it takes into account the latent heat of vaporization of water in the combustion products. The HHV is considered to be a true energy calculation for some specific cases, such as natural gas burned in condensing boilers and power plants with flue-gas condensation.
The difference between the LHV and HHV depends on the chemical composition of the fuel. For hydrogen, the HHV is 18.2% above the LHV, while for hydrocarbons, the difference depends on the hydrogen content of the fuel.
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The combustion of fossil fuels
The combustion process involves burning the fuel in the presence of an oxidant, typically oxygen from the air. The reaction between fuel and oxygen produces water and carbon dioxide, along with the release of energy in the form of heat. For instance, the combustion of hydrogen and oxygen yields water vapour, releasing 242 kJ/mol of heat. Incomplete combustion occurs when there is insufficient oxygen for the fuel to completely react, resulting in the formation of carbon and carbon monoxide instead of carbon dioxide.
The heating value of fossil fuels can be determined through experimental methods, such as using a bomb calorimeter. The higher heating value (HHV) is obtained by measuring the heat released when the combustion products are cooled back to the original temperature, assuming all water is in a liquid state. The lower heating value (LHV) considers energy losses, such as the energy used to vaporize water, and is calculated by cooling the products to 150°C, resulting in water vapour as a waste product.
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The combustion of renewable fuels
The heat value of a fuel refers to the amount of heat released during its combustion, also known as its energy or calorific value. This value is a measure of a fuel's energy density and is expressed in energy per specified amount (e.g. joules per kilogram).
Renewable fuels, such as biofuels, differ from fossil fuels in that they are produced at the same rate at which they are consumed. Biofuels are often produced from living biological matter, or biomass, and can be converted directly into liquid fuels to meet transportation fuel needs. The two most common types of biofuels are ethanol and biodiesel. Ethanol is an alcohol made from various plant materials, which can be used as a blending agent with gasoline to increase octane and reduce emissions. Biodiesel, on the other hand, is a liquid fuel produced from renewable sources such as vegetable oils and animal fats, and it serves as a cleaner-burning replacement for petroleum-based diesel fuel.
In contrast to renewable fuels, fossil fuels such as coal, petrol, diesel, and natural gas, take millions of years to form and are being consumed at a much faster rate. The combustion of fossil fuels involves the oxidation of hydrocarbons, which are molecules containing primarily carbon-hydrogen bonds. During combustion, these hydrocarbon molecules are converted into carbon dioxide and water, with the amount of energy released depending on the oxidation state of the carbons, which is related to the hydrogen-to-carbon ratio. The more hydrogen per carbon, the lower the oxidation state, and the more energy is released during the oxidation reaction.
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Frequently asked questions
The heat of combustion is the amount of heat released when a compound in its standard state is burned to form stable products. For example, when methane is burned, it reacts with oxygen to produce carbon dioxide, water, and energy (heat).
The heat value of a fuel is the amount of heat released during its combustion. It is also referred to as the energy or calorific value of the fuel and is expressed in energy (joules) per specified amount (e.g. kilograms).
Fuel releases heat energy through combustion, a chemical reaction that occurs when a fuel is burned in the presence of an oxidant, usually oxygen. The energy released during combustion comes from the chemical energy stored within the fuel molecules.
