1. Background

The concepts of fuel characterisation are outlined in Combustion File (CF) 48 and augmented with respect to basic fuel characterisation in CF120. The basic combustion calculations required for industrial combustion engineering are introduced in CF227.

A basic property of an industrial [GLOSS]fuel[/GLOSS] is its [GLOSS]calorific value[/GLOSS]. The concept of calorific value in its various forms and nomenclatures is introduced in CF21.

Calorific Value ([GLOSS]Higher Calorific Value[/GLOSS]) can be measured and from the results of the measurement, the more practical [GLOSS]Lower Calorific Value[/GLOSS] can be calculated as described in CF21.

The Lower Calorific Value can also be calculated from the [GLOSS]ultimate analysis[/GLOSS] of the fuel, adjusted for example to allow for as-received or as-fired moisture. This calculation is introduced in the present CF.

2. Definitions

For the definitions of Higher Calorific Value Q_H and Lower Calorific Value Q_L and their relationship, see CF21.

The composition of solid fuels, given by the ultimate analysis, is usually given in weight fractions (kg/kg fuel), which are specified with the lower-case character of the substance, or in percent by weight.

As an example of a solid fuel, a typical steam [GLOSS]coal[/GLOSS] is shown in Table 1:

Table 1: Analysis of a coal

3. Calculation Procedure

Since the elements of solid fuels can be bound in various ways, it is not possible to calculate exactly the Lower Calorific Value. Therefore an empirical equation, developed by Boie [1] can be used instead:

Q_L = 34.8c + 93.9h + 6.3n + 10.5s– 10.8o –2.5w (in MJ / kg fuel)        (1)

Note that, inorganic materials such as ash are omitted because they do not significantly produce or consume heat. Some energy is required to heat these materials from ambient [GLOSS]temperature[/GLOSS] to the operating temperature, but this is considered of minor importance.

For the example coal specified in Section 2, the Lower Calorific Value is calculated according to Table 2 below:

Table 2: Example calculation of the Lower Calorific Value of a coal

A calculator based on this table can be downloaded from the IFRF ONLINE Combustion Handbook via CF228.

4. Further Data

The Solid Fuel Characterisation Database within the IFRF Combustion Handbook is regularly augmented.  Readers are advised to activate the search function (Search Combustion Files – Left Hand Side Bar) and enter the keyword “data” with a keyword search. Examples are CF106 and CF107.

Sources

[1] Schramek, Taschenbuch für Heizung und Klimatechnik, Munich, 1999

[2] Perry, Perry’s Chemical Engineers’ Handbook 7^th ed., 1997