Regan Brown
The heat value of a fuel refers to the amount of heat released during its combustion. This is also referred to as its calorific value and is a measure of a fuel's energy density. The most common unit for comparing energy sources or fuels is British thermal units (Btu). One gallon of diesel fuel or heating oil with a sulfur content of less than 15 parts per million has a Btu of 137,381. The Btu content of fuels varies slightly from country to country, depending on the energy sources used in that country.
| Characteristics | Values |
|---|---|
| Unit of measurement for energy | British thermal units (Btu) |
| Btu content of 1 barrel (42 gallons) of crude oil | 5,689,000 Btu |
| Btu content of 1 gallon of diesel fuel or heating oil (with sulfur content <15 parts per million) | 137,381 Btu |
| Btu content of 1 gallon of heating oil (with sulfur content 15-500 parts per million) | 138,500 Btu |
| Heat value of a fuel | The amount of heat released during combustion, expressed in energy (joules) per specified amount (e.g. kilograms) |
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Heat value of diesel fuel
The heating value of a substance, typically a fuel or food, is the amount of heat released during the combustion of a specified amount. The calorific value is the total energy released as heat when a substance undergoes complete combustion with oxygen under standard conditions. The chemical reaction is typically a hydrocarbon or other organic molecule reacting with oxygen to form carbon dioxide and water and release heat.
The heating value of a fuel can be calculated with the results of an ultimate analysis of the fuel. From the 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.
There are two types of heating values: high(er) and low(er) heat(ing) value, depending on how much the products are allowed to cool and whether compounds like H2O are allowed to condense. The high heat values are conventionally measured with a bomb calorimeter. Low heat values are calculated from high heat value test data.
Gasoline and diesel fuel are complex mixtures of hydrocarbons, mainly alkanes, and additives. The actual composition of a fuel can vary widely, due to feedstock variability, refinery capability, supply and demand factors, climate, and seasonality. For this reason, statements about ‘gasoline’ or ‘diesel fuel’ properties must be understood as average or typical values only.
The heating values for fuels are generally measured empirically, especially for mixed fuels such as gasoline and diesel fuel. The higher heating value of diesel exceeds the lower heating value by about 7%. According to the US Energy Information Administration, diesel fuel produces about 11% more heat energy per litre than gasoline when burned.
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Energy units
The British thermal unit, or Btu, is a measure of heat energy commonly used in the United States for comparing energy sources or fuels. The Btu content of fuels can vary slightly from country to country, depending on the energy sources available in those countries. For instance, 1 barrel (containing 42 gallons) of crude oil produced in the United States equates to 5,689,000 Btu, whereas 1 gallon of diesel fuel or heating oil (with a sulfur content of less than 15 parts per million) amounts to 137,381 Btu.
The heat value, or calorific value, of a fuel refers to the amount of heat released during its combustion. This value is expressed in energy units per specified amount, such as joules per kilogram. For instance, one tonne of oil equivalent (toe) is equal to 41.868 GJ (gigajoules).
To compare the fuel prices of different heating sources, one can compare the average fuel prices over a given period in dollars per million Btu. This is calculated by dividing the average natural gas and heating oil monthly prices per unit by the Btu content of the fuels in million Btu per unit. For instance, the average price per million Btu for natural gas during the winter of 2023-2024 was found to be less than the price of heating oil per million Btu.
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Btu content
The British thermal unit (Btu) is a measure of heat energy commonly used to compare energy sources or fuels in the United States. The Btu content of fuels varies slightly from country to country.
The Btu content of diesel fuel depends on its sulfur content. One gallon of diesel fuel or heating oil with a sulfur content of less than 15 parts per million contains 137,381 Btu. If the sulfur content is between 15 and 500 parts per million, the Btu content increases to 138,500.
According to the Bureau of Transportation Statistics, diesel motor fuel has a Btu value of 138,700 per gallon. This value is higher than that of automotive gasoline (125,000 Btu/gallon) and aviation gasoline (120,200 Btu/gallon) but lower than that of residual fuel (149,700 Btu/gallon).
Comparing the Btu content of different fuels can help determine which fuel provides more energy for heating. For example, by converting the consumption of natural gas and heating oil into Btu, one can determine which fuel source provides more energy for heating between the two.
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Fuel prices
The price of diesel fuel is influenced by several factors, including the cost of crude oil, regional and international demand and supply, transportation costs, and local market conditions. Diesel fuel prices tend to fluctuate throughout the year, with higher prices during the fall and winter due to increased demand for heating oil, which is produced simultaneously with diesel fuel.
The cost of crude oil is a significant factor in diesel fuel prices, accounting for approximately 61% of the retail price. Refining, distribution, marketing, and tax costs make up the remaining proportion. Diesel fuel prices in certain regions, such as the West Coast, especially California, tend to be higher due to taxes and supply issues. California's taxes on retail, on-highway diesel fuel were 96.32 cents per gallon as of January 1, 2024, significantly higher than the national average of 34.74 cents per gallon.
Transportation costs also impact diesel prices, with areas farther from production sources, such as the Gulf Coast, typically facing higher prices. Additionally, the demand for diesel fuel in agricultural regions can influence prices, with seasonal swings in farming activities leading to price variations.
When comparing fuel prices for heating homes, it is essential to consider the energy content of different fuels. The British thermal unit (Btu) is commonly used in the United States to compare energy sources, and fuel prices can be compared in dollars per million Btu. For instance, natural gas and heating oil prices can be compared by dividing their average monthly prices by their respective Btu content.
In terms of heating values, diesel fuel produces approximately 11% more heat energy than gasoline. This is due to the higher carbon percentage in diesel, which results in 11% more carbon dioxide production and a corresponding impact on carbon pricing. The heating value of fuels is typically measured empirically, especially for mixed fuels like gasoline and diesel.
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Energy density
The energy density of a fuel relates the stored energy to the volume of the storage equipment, for example, the fuel tank. The higher the energy density of the fuel, the more energy can be stored or transported for the same amount of volume. The energy of a fuel per unit mass is called its specific energy. The gravimetric and volumetric energy density of fuels can be compared using a convenient table of HHV and LHV values. However, the density values for chemical fuels do not include the weight of the oxygen required for combustion.
Liquid hydrocarbons, such as gasoline, diesel, and kerosene, are currently the densest way to store and transport chemical energy on a large scale. For example, 1 kg of diesel fuel burns with the oxygen contained in approximately 15 kg of air.
In engines, the specific fuel consumption is always greater than the rate of production of kinetic energy due to inefficiencies and thermodynamic considerations. Energy density is also distinct from energy conversion efficiency (net output per input) and embodied energy (the energy output costs to provide).
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Frequently asked questions
The heat rate of a diesel generator is represented by the thermal efficiency of the generator, which is usually expressed in British thermal units per kilowatt-hour (BTU/kWh). It is calculated by multiplying the energy output of the fuel used to generate electricity with the quantity of electrical energy produced.
The heat rate of a diesel generator is calculated by multiplying the heat rate with the cost of fuel per BTU. The heat rate signifies the total amount of energy required by an electric generator to produce 1 kilowatt-hour (kWh) of electricity.
The heat rate of a diesel generator is influenced by its efficiency or capacity to generate electricity, which improves over time. The heat rate of simple cycle units decreases at a rate of 0.2% annually, while that of combined cycle units decreases by 0.05% per year.
