Higher heating values (HHVs) of vegetable oils range between 39,000-48,000 kJ/kg, or 16,770-20,650 BTU/lb, depending upon the kind of vegetable oil. The HHV of vegetable oil is higher than that of anthracite, spent tire rubber, or wood. Table 2.4 shows a comparison of higher calorific values of common fuels and energy sources [7].

The higher heating value is also known as the gross calorific value, higher calorific value, gross energy, or gross heat. The HHV of a fuel is defined as the amount of heat released per unit mass (initially at 25°C) once it is com­busted and the products have returned to a temperature of 25°C. This means that the HHV value is the total heat recoverable, including the energy con­tained in water vapor released due to combustion reaction. In other words, the higher (or gross) heating value is the gross calorific value (gross CV) when all products of combustion are cooled back to the precombustion tem­perature, water vapor formed during combustion is also condensed, and necessary corrections have been made.

On the other hand, the lower heating value (LHV) is also known as net calorific value, or net CV. The LHV of a fuel is defined as the amount of heat released due to combustion of a unit mass of fuel (initially at 25°C or another reference state) and returning the temperature of the combustion products to 150°C. As such, the energy contained in water vapor released during com­bustion is not wholly included in the LHV. A major portion of the energy amount excluded from the HHV, in obtaining the LHV, is the latent heat of vaporization of water.

As shown in Table 2.4, the heating value of vegetable oil is quite high, higher than other naturally derived fuels such as coal and wood. Vegetable oils have long been used for cooking, lighting, and heating throughout the world. Residential furnaces and boilers that are designed to burn heating oil No. 2 can be modified to burn vegetable oils, including filtered waste vegetable oil. The required modification is based on a similar ground as

TABLE 2.4

Higher Heating Values (HHVs) of Common Fuels

Higher Heating Value
(Gross Calorific
Value—GCV)

that for diesel engines for SVO, in which the viscosity of vegetable oil is reduced by appropriate preheating. Although this method results in sub­stantial cost savings, it has not been popularly practiced in the United States.