ISO 18125 Solid Biofuels - Determination of Calorific Value

The standard “ISO 18125 Solid biofuels - Determination of calorific value” developed by the International Organization for Standardization (ISO) describes a method for determining the gross calorific value of a solid biofuel at constant volume and 25 degrees reference temperature in a calibrated bomb calorimeter by burning certified benzoic acid. The result obtained is the gross calorific value of the analysis sample at constant volume, where all the water of the combustion products is liquid water.

In practice, biofuels are burned at constant (atmospheric) pressure and the water is either not condensed (removed as vapour in the flue gases) or is condensed. In either case, the working heat of combustion to be used is the net calorific value of the fuel at constant pressure. The net calorific value at constant volume can also be used. This standard contains formulas for calculating both values.

General principles and procedures for calibrations and biofuel testing are presented in the main text. Those pertaining to the use of a specific type of calorimetric instrument are included in the standard annexes (Annexes A to C). Annex D contains checklists for performing calibration and fuel tests using the specified types of calorimeters. Annex E gives examples to illustrate some calculations.

For the purposes of this standard, in addition to the terms and definitions given in “ISO 16559 Solid biofuels - Vocabulary”, the following definitions apply:

• The gross calorific value at constant volume expresses the absolute value of the specific energy of combustion in joules per unit mass of solid biofuel burned in oxygen under the conditions specified in the calorimetric bomb. The combustion products are assumed to consist of gaseous oxygen, nitrogen, carbon dioxide and sulfur dioxide, liquid water saturated with carbon dioxide under the bomb reaction conditions (in equilibrium with its vapor), and solid ash.
• Net calorific value at constant volume expresses the absolute value of the specific energy of combustion in joules per unit mass of biofuel burned in oxygen under constant volume conditions. All water of the reaction products remains as water vapor, the other products as in the gross calorific value.
• Net calorific value at constant pressure is the absolute value of the specific heat of combustion in joules per unit mass of biofuel burned in oxygen at constant pressure. All water from the reaction products remains as water vapor, the other products remaining as in the gross calorific value.
• The reference temperature is the international reference temperature for thermochemistry, the reference temperature for calorific values ​​is 25 degrees. The temperature dependence of the calorific value of biofuels is small.
• The effective heat capacity of the calorimeter expresses the amount of energy required to cause a unit change in the temperature of the calorimeter.
• The corrected temperature rise expresses the change in calorimeter temperature resulting solely from the processes taking place within the combustion bomb. The corrected temperature rise is the total observed temperature rise corrected for heat exchange, mixing power, etc. The change in temperature may be expressed in other units: the resistance of a platinum or thermistor thermometer, the frequency of a quartz crystal resonator, provided that a functional relationship is established between this quantity and the change in temperature. The effective heat capacity of the calorimeter may be expressed in units of energy per such arbitrary unit. The necessary criteria of linearity and closeness in conditions between calibrations and fuel tests are given in the standard.

A weighed portion of the solid biofuel sample is burned in high-pressure oxygen in a bomb calorimeter under specified conditions. The effective heat capacity of the calorimeter is determined in calibration tests by burning certified benzoic acid under similar conditions as specified in the certificate. The corrected temperature rise is determined from temperature observations before, during and after the combustion reaction takes place. The duration and frequency of temperature observations depend on the type of calorimeter used. Water is initially added to the bomb to obtain a saturated vapor phase prior to combustion, ensuring that all water in the sample, consisting of hydrogen and moisture, is considered liquid water.

The gross calorific value is calculated from the corrected temperature rise and the effective heat capacity of the calorimeter, making allowances for contributions from ignition energy, the combustion of fuses, and thermal effects from side reactions such as nitric acid formation. In addition, a correction is applied to account for the energy difference between the aqueous sulfuric acid formed in the bomb reaction and the gaseous sulfur dioxide, i.e. the required reaction product of the sulfur in the biofuel. The corresponding energy effect between aqueous and gaseous hydrochloric acid can be neglected due to the generally low value for the correction for solid biofuels.

The net calorific value of biofuel at constant volume and net calorific value at constant pressure are obtained by calculating the gross calorific value at constant volume determined in the analysis sample. The calculation of the net calorific value at constant volume requires information on the moisture and hydrogen contents of the analysis sample. In principle, the calculation of the net calorific value at constant pressure also requires information on the oxygen and nitrogen contents of the analysis sample.

Our organization has a strong staff that closely follows the developments in the world in the field of science and technology and constantly improves itself. Among the numerous test, measurement, analysis and evaluation studies provided for businesses in various sectors, there are also testing services in accordance with the "ISO 18125 Solid biofuels - Determination of calorific value" standard.