Solid Mineral Fuels - Determination of Gross Calorific Value by the bomb calorimetric method, and calculation of net calorific value.
This International Standard specifies a method for the determination of the gross calorific value of a solid mineral fuel at constant volume and at the reference temperature 25 Deg C in a bomb calorimeter calibrated by combustion of certified benzoic acid.
The result obtained is the gross calorific value of the analysis sample at constant volume with all the water of the combustion products as liquid water. In practice, fuel is burned at constant (atmospheric) pressure and the water is not condensed but is removed as vapor with the flue gasses. Under these conditions, the operative heat of combustion is the net calorific value of the fuel at constant pressure. The net calorific value at constant volume may also be used; formulae are given for calculating both values.
General principles and procedures for the calibrations and the fuel experiments are presented in the main text, whereas those pertaining to the use of a particular type of calorimetric instrument are described in annexes A to C. Annex D contains checklists for performing calibration and fuel experiments using specified types of calorimeter. Annex E gives examples to illustrate some of the calculations.
For the purpose of this International Standard, the following definitions apply.
Gross Calorific Value at Constant Volume
The absolute value of the specific energy of combustion, in joules, for unit mass of a solid burned oxygen in a calorimetric bomb under the conditions specified. The products of combustion are assumed to consist of gaseous oxygen, nitrogen, carbon dioxide and sulfur dioxide, of liquid water (in equilibrium with its vapor) saturated with carbon dioxide under the conditions of the bomb reaction, and of solid ash, all at the reference temperature.
Net Calorific Value at Constant Volume
The absolute value of the specific energy of combustion, in joules, for unit mass of the fuel burned in oxygen under conditions of constant volume and such that all the water of the reaction products remains as water vapor (in a hypothetical state at 0.1 MPa), the other products being as for the gross calorific value, all at the reference temperature.
Net Calorific Value at Constant Pressure
The absolute value of the specific heat (enthalpy) of combustion, in joules, for unit mass of the fuel burned in oxygen at constant pressure under such conditions that all the water of the reaction products remains as water vapor (at 0.1 MPa), the other products being as for the gross calorific value, all at the reference temperature.
The international reference temperature for thermo-chemistry of 25 Deg C is adopted as the reference temperature for calorific values.
*Note : The temperature dependence of the calorific value of coal or coke is small (about 1 J/(g.K)).
Corrected Temperature Rise
The change in calorimeter temperature caused solely by the processes taking place within the combustion bomb. It is the total observed rise corrected for heat exchange, stirring power, etc.
*Note : The change in temperature may be expressed in terms of other units : resistance of a platinum or thermistor thermometer, frequency of a quartz crystal resonator, etc., provided that a functional relationship is established between this quantity and a change in temperature. The effective heat capacity of the calorimeter may be expressed in units of energy per such an arbitrary unit. Criteria for the required linearity and closeness in conditions between calibrations and fuel experiments are given.
Gross calorific Value
A weighed portion of the analysis sample of the solid fuel is burned in high-pressure oxygen in a bomb calorimeter under specified conditions. The effective heat capacity of the calorimeter is determined in calibration experiments by combustion of certified benzoic acid under similar conditions, accounted for in the certificate. The corrected temperature rise is established from observations of temperature before, during and after the combustion reaction takes place. The duration and frequency of the temperature observations depend on the type of calorimeter used. Water is added to the bomb initially to give a saturated vapor phase prior to combustion, thereby allowing all the water formed, from the hydrogen and moisture in the sample, to be regarded as liquid water.
The gross calorific value if calculated from the corrected temperature rise and the effective heat capacity of the calorimeter, with allowances made for contributions from ignition energy, combustion of the fuses and for thermal effects from side reactions such as the formation of nitric acid. Furthermore, a correction is applied to account for the difference in energy between the aqueous sulfuric acid formed in the bomb reaction and the gaseous sulfur dioxide, i.e. the required reaction product of sulfur in the fuel.
Net Calorific Value
The net calorific value at constant volume and the net calorific value at constant temperature of the fuel are obtained by calculation from the gross calorific value at constant volume determined on the analysis sample. The calculation of the new calorific value at constant volume requires information about the moisture and hydrogen contents of the analysis sample. In principle, the calculation of the net calorific value at constant pressure also requires information about the oxygen and nitrogen contents of the sample.
Oxygen, at a pressure high enough to fill the bomb to 3MPa, pure with an assay of at least 99.5% (V/V), and free from combustible matter.
Ignition Wire, if nickel-chromium 0.16mm in diameter, platinum 0.05mm to 0.10mm in diameter, or another suitable conducting wire with well-characterized thermal behaviour during combustion.
Cotton fuse, of white cellulose cotton, or equivalent, if required.
Benzoic Acid, of calorimetric-standard quality, certified by (or with certification unambiguously traceable to) a recognized authority.
*Note : Benzoic Acid is the sole substance recommended for calibration of an oxygen-bomb calorimeter. For the purpose of checking the overall reliability of the calorimetric measurements, test substances, e.g. n-dodecane are used. Test substances are mainly used to prove that certain characteristics of a sample e.g. burning rate or chemical composition, do not introduce bias in the results. A test substance shall have a certified purity and a well-established energy of combustion.
The Benzoic Acid is burned in the form of pellets. It is normally used without drying or any treatment other than pelletizing : consult the sample certificate. It does not absorb moisture from the atmosphere at relative humidities below 90%.
The Benzoic Acid shall be used to certification conditions as a feasible; significant departures from these conditions shall be accounted for in accordance with the directions in the certificate. The energy of combustion of the benzoic acid, as defined by the certificate for the conditions utilized, shall be adopted in calculating the effective heat capacity of the calorimeter.
The calorimeter consists of the assembled combustion bomb, the calorimeter can (with or without the lid), the calorimeter stirrer, water, temperature sensor, and leads with connectors inside the calorimeter can required for ignition of the sample or as part of the temperature measurement or control circuits. During measurements the calorimeter is enclosed in a thermostat. The manner in which the thermostat temperature is controlled defines the working principle of the instrument and hence the strategy for evaluation of the corrected temperature rise.
In aneroid systems (systems with a fluid) the calorimeter can, stirrer and ware are replaced by a metal block. The combustion bomb itself constitutes the calorimeter in some aneroid systems.
In combustion calorimetric instruments with a high degree of automation, especially in the evaluation of the results, the calorimeter is in a few cases not as well-defined as the traditional, classical-type calorimeter. Using such an automated calorimeter is, however, within the scope of this International Standard as long as the basic requirements are met with respect to calibration conditions, comparability between calibration and fuel experiments, ratio fo sample mass to bomb volume, oxygen pressure, bomb liquid, reference temperature of the measurements and repeatability of the results. A print-out of some specified parameters from the individual measurements is essential.
Calorimeter with Thermostat
Combustion bomb, capable of withstanding safely the pressure developed during combustion. The design shall permit complete recovery of all liquid products. The material of construction shall resist corrosion by the acids produced in the combustion of coal and coke. A suitable internal volume of the bomb would be from 250ml to 350ml.
Warning - Bomb parts shall be inspected regularly for wear and corrosion; particular attention shall be paid to the condition of the threads of the main closure. Manufacturer's instructions and any local regulations regarding the safe handling and use of the bomb shall be observed. When more than one bomb of the same design is used, it is imperative to use each bomb as a complete unit. Swapping of parts may lead to serious accident.
The electrical supply shall be 6V to 12V alternating current from a step-down transformer or direct current from batteries. It is desirable to include a pilot light in the circuit to indicate when current is flowing.
Where the firing is done manually, the firing switch shall be of the spring-loaded, normally open type, located in such a manner that any undue risk to the operator is avoided.
Crucible, of silica, nickel-chromium, platinum or similar non-reactive material. For coal, the crucible should be about 25mm in diameter, flat based and not more than 20mm deep. Silica crucibles should be about 1.5mm thick and metal crucibles about 0.5mm thick. A low-mass crucible of nickel-chromium foil about 0.25mm thick is recommended when testing high-ash coals, in order to reduce any error from incomplete combustion.
For benzoic acid, either of the crucibles specified is suitable. If smears of unburned carbon occur, a small low-mass platinum or nickel-chromium crucible, for example 0.25mm thick, 15mm in diameter and 7mm deep, may be used.
Pressure Regulator, to control the filling of the bomb with oxygen.
Pressure gauge, to indicate the pressure in the bomb with a resolution of 0.05 MPa.
Relief valve or bursting disk
Relief valve or bursting disk, operating at 3.5MPa, and installed in the filling line, to prevent overfilling the bomb.
Caution - Equipment for high-pressure oxygen shall be kept free from oil and grease. Do not test or calibrate the pressure gauge with hydrocarbon fluid.
The calorimetric determination consists of two separate experiments, combustion of the calibrant (benzoic acid) and combustion of the fuel (coal or coke), both under specified conditions. The calorimetric procedure for the two types of experiment is essentially the same. In fact, the overall similarity is a requirement for proper cancellation of systematic errors caused, for example, the uncontrolled heat leaks not accounted for in the evaluation of the corrected temperature rise.
The experiment consists of carrying out quantitatively a combustion reaction (in high-pressure oxygen in the bomb) to defined products of combustion and of measuring the change in temperature caused by the total bomb process.
The temperature measurements required for the evaluation of the corrected temperature rise are made during a fore period, a main (=reaction) period, and an after period. For the adiabatic type calorimeter, the fore and after periods need, in principle, be only as long as required to establish the initial (firing) and final temperatures, respectively. For the isoperibol (isothermal jacket) and the static-jacket type calorimeter, the fore and after periods serve to establish the heat exchange properties of the calorimeter required to allow proper correction for heat exchange between calorimeter and thermostat during the main period when combustion takes place. The fore and after periods then have to be longer.
Combustion of certified benzoic acid under specified conditions to gaseous carbon dioxide and liquid water serves to make a change in temperature of the calorimeter of one unit interpretable in defined units of energy. The classical type of combustion calorimeter can be maintained unchanged over extended periods of time in terms of mass (heat capacity), geometry and heat exchange surfaces. This allows for calibration of the instrument to be carried out as a separate series of measurements, establishing the effective heat capacity of the calorimeter.
This calibration constant should not change significantly over time, provided minor repairs or other changes in the system were correctly accounted for. Some of the fully automated calorimetric instruments are, however, physically less well-defined and therefore require more frequent calibrations, for some systems even daily.
Systematic errors may arise, for example from evaporation of calorimeter water, from uncontrolled heat exchange along various paths and/or imperfections in an adiabatic temperature control system during the reaction period. Cancellation of this type of error depends largely on the similarity between the calibration experiments and combustion of the fuel samples with respect to time-temperature profile and total change in temperature of the calorimeter. Systematic variation in the mass of benzoic acid used in the calibration experiments is an expedient way of establishing the requirements for "similarity" for a particular calorimetric system.
For the ordinary series of calibrations, five satisfactory combustions on benzoic acid shall be carried out. The sample shall be burned as pellets. The calorimetric procedure described in clause 8 shall be followed. Recommendations concerning sample mass and initial amount of bomb water are given. It is advantageous to use a crucible of low mass for the benzoic acid combustions. The initial temperature shall be chosen such that the reference temperature of the experiment is within the chosen range for the reference temperature.
The design of the calibration experiment, in terms of oxygen pressure, amount of bomb water, reference temperature, duration of the fore, main and after periods, etc., defined the detailed procedure for sub-sequent fuel combustions.
When the effective heat capacity of a calorimeter cannot be regarded as constant over the required working range, but needs to be expressed as a function, the number of calibration experiments shall be increased to eight or more. The mass of sample for the individual experiments is chosen to yield values for the change in temperature over the entire intended working range, with a few replicate measurements around the end points, to define the slope for the relationship.
Gross Calorific Value
The calorimetric conditions for the fuel combustions shall be consistent with those of the calibration experiments. With the calorimetric procedure under satisfactory control, ascertaining complete combustion of the fuel is the most important issue.
Fuels with a low content of volatiles, e.g. coke, tend to be difficult to burn completely in the bomb and may have to be burned in a crucible of low mass, preferably in poor thermal contact with the crucible support. An alternative strategy, particularly useful with coke, is to mix the fuel sample with a combustion aid, e.g., benzoic acid or a hydrocarbon oil of low volatility. Benzoic Acid has the advantage of a well-established energy of combustion.
The variation in the correction for nitric acid is often on the borderline of significance. When the sulphur content is determined separately on the sample, the nitric acid correction may be assigned a constant per-gram-of-sample value. A similar strategy shall then be adopted for the calibration experiments. As nitric acid formation largely depends on the combustion temperature and is enhanced by nitrogen in the sample, the nitric acid correction is normally different for fuel and benzoic acid combustions, respectively. It may also vary significantly for different types of fuels.
When analysis of the bomb washings for sulfuric and nitric acid is required, the procedure described may be used.
Duplicate combustions shall be made. A representative sample shall be taken from the analysis sample, which is used without further pretreatment. The amount shall be used that the observed temperature rise is within the range of the calibration experiments. The calorimetric procedure described shall be followed, with the same calorimetric conditions as in the calibration experiments.
Usually 1g of coal is the appropriate test portion. For high-ash coals the use of for example 0.75g of sample and a shallow, low-mass crucible (foil) usually facilitates complete combustion. If the observed temperature rise fails outside the valid range, the calibration shall be confirmed for the extended range.
The same conditions as prescribed for coal apply for coke. The use of a lined crucible, or something equivalent, is recommended. The coke sample shall be distributed evenly in the crucible. Certain non-reactive cokes may persistently leave residues that contain significant amounts of unburned sample or soot. Optimum conditions for clean combustions may be investigated by varying the amount of sample. A superior strategy may be to use a combustion aid to promote complete combustion of the sample. The optimum proportion of combustion aid to coke sample depends on the properties of the particular coke and needs to be determined by experiment. The nitric acid correction for coke is usually smaller than that for most coals. When an auxiliary material is used, the correction for nitric acid per experiment will normally be larger than in combustions with coke alone.