SOLID MINERAL FUELS - DETERMINATION OF GROSS CALORIFIC VALUE BY THE BOMB CALORIMETRIC METHOD AND CALCULATION OF NET CALORIFIC VALUE
WARNING - Strict adherence to all of the provisions prescribed in this International Standard should ensure against explosive rupture of the bomb, or a blow-out, provided that the bomb is of proper design and construction and in good mechanical condition.
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 of 25°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 gases. 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 can also be used; equations are given for calculating both values.
General principles and procedures for the calibrations and the fuel tests 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 tests using specified types of calorimeters. Annex E gives example illustrating some of the calculations.
NOTE : Descriptors - solid fuels, coal, coke, tests, determination, calorific value, rules of calculation, calorimetry.
TERMS, DEFINITIONS AND SYMBOLS
For the purposes of this document, the following terms and definitions apply.
Gross Calorific Value at Constant Volume- absolute value of the specific energy of combustion for unit mass of a solid fuel burned in oxygen in a calorimetric bomb under the conditions specified.
Note 1 : The products of combustion are assumed to consist of gaseous oxygen, nitrogen, carbon dioxide and sulfur dioxide, or liquid water (in equilibrium with its vapor) saturated with carbon dioxide under the conditions of the bomb reaction, and solid ash, all at the reference temperature.
Note 2 : Gross calorific value is expressed in units of joules.
Gross Calorific Value at Constant Pressure - absolute value of the specific energy of combustion, for unit mass of a solid fuel burned in oxygen at constant pressure, instead of constant volume in a calorimetric bomb.
NOTE : The hydrogen in the fuel, reacting with gaseous oxygen to give liquid water, causes a decrease in the volume of the system. When the fuel carbon reacts with gaseous oxygen, the equal volume of gaseous carbon dioxide is formed and, hence, no change in volume occurs in combustion of the carbon. The oxygen and nitrogen in the fuel both give rise to an increase in volume.
Net Calorific Value at Constant Volume - absolute value of the specific energy of combustion, for unit mass of a solid 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 - absolute value of the specific heat (enthalpy) of combustion, 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.
Adiabatic Calorimeter - calorimeter that has a rapidly changing jacket temperature.
NOTE : The inner chamber and the jacket exchange no energy because the water temperature in both is identical during the test. The water in the external jacket is heated or cooled to match the temperature change in the calorimeter proper.
Isoperibol Calorimeter - (isothermal type) calorimeter that has a jacket of uniform and constant temperature.
NOTE : These calorimeters have the inner chamber surrounded by a water jacket in which the temperature is maintained at ambient temperature. The outer jacket acts like a thermostat and the thermal conductivity of the inter-space between the two chambers is kept as small as possible.
Automated Calorimeter - calorimeter system without fluid, where the calorimeter can, stirrer and water are replaced by a metal block and the combustion bomb itself constitutes the calorimeter.
NOTE : Characteristically, these calorimeter have a small heat capacity, leading to large changes in temperature. Therefore, smaller masses of sample are used. A calorimeter of this kind requires more frequent calibrations.
Reference Temperature - international reference temperature for thermochemistry, 25°C.
NOTE : The temperature dependence of the calorific value of coal or coke is small, about 1J/(g.K).
Effective Heat Capacity of the Calorimeter - amount of energy required to cause unit change in temperature of the calorimeter.
Corrected Temperature Rise - change in calorimeter temperature caused solely by the processes taking place within the combustion bomb.
NOTE : The change in temperature can 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 capacity of the calorimeter can be expressed in units of energy per such an arbitrary unit. Criteria for the required linearity and closeness in conditions between calibrations and fuel tests 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 tests 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 is 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 fuse(s) 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 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 pressure of the fuel are obtained by calculation from the gross calorific value at constant volume determined on the analysis sample. The calculation of the net calorific value at constant volume required information about the moisture and hydrogen contents of the analysis sample. In principle, the calculation of the net calorific value at constant pressure also required information about the oxygen and nitrogen contents of the sample.
Oxygen, at a pressure high enough to fill the bomb to 3 MPa, pure, with an assay of at least 99.5% volume fraction, and free from combustible matter.
NOTE : Oxygen made by the electrolytic process may contain up to 4% volume fraction of hydrogen.
Ignition Wire - of nickel-chromium 0.16mm to 0.20mm in diameter, platinum 0.05mm to 0.10mm in diameter, or another suitable conducting wire with well characterized thermal behavior during combustion.
Cotton Fuse - of white cellulose cotton, or equivalent, if required.
Crucible lining material - for use in aiding total combustion of coke, high ash coal and other less reactive fuels.
Paste - of fused aluminosilicate cement passing 6 63um test sieve and suitable for use up to a temperature of 1400°C, mixed with water.
Aluminium Oxide - fused, of analytical reagent quality, passing a 180um test sieve and retained on a 106um test sieve.
Silica Fibre - an ash-free, silica-fibre disc.
Standard Volumetric Solutions and indicators - only for use when analysis of final bomb solutions is required.
Barium Hydroxide Solution - prepared by dissolving 18g of barium hydroxide in about 1L of hot water in a large flask. Stopper the flask and allow the solution to stand for two days or until all the barium carbonate has completely settled out. Decant or siphon off the clear solution through a fine-grained (slow flow rate) filter paper into a storage bottle fitted with a soda-lime guard tube to prevent ingress of carbon dioxide. Standardize the solution against 0.1mol/l hydrochloric acid solution using phenolphthalein solution as an indicator.
Sodium Carbonate Solution - 0.05mol/L prepared by dissolving 5.3g of anhydrous sodium carbonate, Na2CO3, dried for 30 min to 270°C, but not exceeding 270°C, in water. Transfer the resulting solution quantitatively to a 1L volumetric flask and make up to volume with water.
Sodium Hydroxide Solution - 0.1mol/L, prepared from a standard concentrated volumetric solution as directed by the manufacturer. Alternatively, prepare from anhydrous sodium hydroxide by dissolving 4.0g of sodium hydroxide, NaOH, in water; transfer the resulting solution to a 1L volumetric flask and make up to volume with water. Standardize the resulting solution against 0.1mol/L hydrochloric acid solution using phenolphthalein solution as an indicator.
Hydrochloric Acid Solution - 0.1mol/L, prepared from a standard concentrated volumetric solution, as directed by the manufacturer. Alternatively, prepare by diluting 9ml of hydrochloric acid to 1L of water. Standardize the resulting solution against anhydrous sodium carbonate solution using a screened indicator solution.
Methyl Orange Indicator - screened 1g/l solution. Dissolve 0.25g of methyl orange and 0.15g of xylene cyanole FF in 50ml of 95% volume fraction ethanol and dilute to 250ml with water.
Phenolphthalein - 10g/l solution. Dissolve 2.5g of phenolphthalein in 250ml of 95% volume fraction ethanol or 2.5g of the water-soluble salt of phenolphthalein in 250ml of water.
Benzoic Acid - of calorimetric-standard quality, certified by (or with certification unambiguously traceable to) a recognized standardizing authority. 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 use mainly 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 should have a certified purity and a well-established energy of combustion. The benzoic acid is burned in the form of pellets. The benzoic acid is normally use without drying or any treatment other than pelleting; consult the sample certificate. The benzoic acid does not absorb moisture from the atmosphere at a relative humidity below 90%, but it is recommended that the benzoic acid be stored in a moisture-free environment (desiccator) until use. The benzoic acid shall be used as close to certification conditions as is 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 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 temperature measurement or control circuits. During measurements, the calorimeter is enclosed with a thermostat. The manner in which the thermostat temperature is controlled defines the working principle of the instrument and, hence, the strategy for evaluating the corrected temperature rise.
In aneroid systems (systems without a fluid), the calorimeter can, stirrer and water 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 tests, ratio of sample mass to bomb volume, oxygen pressure, bomb liquid, reference temperature of the measurements and accuracy of the results. A printout of some specified parameters from the individual measurements is essential.
Equipment, adequate for determinations of calorific value in accordance with this International Standard, is specified below.
CALORIMETER WITH THERMOSTAT
Combustion Bomb - capable of withstanding safely the pressures developed during combustion. The design shall permit complete recovery of all liquid products. The material of constructions shall resist corrosion by the acids produced in the combustion of coal and coke. A suitable internal volume of the bomb is 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. Manufacturers' 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. Colour coding is recommended. Swapping of parts can lead to serious accident.
CALORIMETER CAN - made of metal, highly polished on the outside and capable of holding an amount of water sufficient to completely cover the flat upper surface of the bomb while the water is being stirred. A lid generally helps reduce evaporation of calorimeter water but, unless it is in good thermal contact with the can, it lags behind in temperature during combustion, giving rise to undefined heat exchange with the thermostat and a prolonged main period.
STIRRER working at a constant speed. The stirrer shaft should have a low-heat-conduction and/or a low-mass section below the cover of the surrounding thermostat to minimize transmission of heat to or from the system. This is of particular importance when the stirrer shaft is in direct contact with the stirrer motor. When a lid is used for the calorimeter can, this section of the shaft should be above the lid. The rate of stirring for a stirred-water-type calorimeter should be large enough to make sure that hot spots do not develop during the rapid part of the change in temperature of the calorimeter. A rate of stirring such that the length of the main period can be limited to 10 min or less is usually adequate.
THERMOSTAT (water jacket), completely surrounding the calorimeter, with an air gap of approximately 10mm separating calorimeter and thermostat. The mass of water of a thermostat intended for isothermal operation shall be sufficiently large to outbalance thermal disturbances from the outside. The temperature should be controlled to within ±0.1K or better throughout the test. A passive constant temperature ("static") thermostat shall have a heat capacity large enough to restrict the change in temperature of its water.
NOTE : For an insulated metal static jacket, satisfactory properties are usually ensured by making a wide annular jacket with a capacity for water of at least 12.5L.
NOTE : Calorimeter surrounded by insulating material, creating a thermal barrier, are regarded as static-jacket calorimeters.
When the thermostat (water jacket) is required to follow closely the temperature of the calorimeter, it should be of low mass and preferably have immersion heaters. Energy shall be supplied at a rate sufficient to maintain the temperature of the water in the thermostat to within 0.1K of that of the calorimeter water after the charge has been fired. When in a steady state at 25°C, the calculated mean drift in temperature of the calorimeter shall not exceed 0.0005K/min.
TEMPERATURE-MEASURING INSTRUMENT, capable of indicating temperature with a resolution of at least 0.001K so that temperature intervals of 2K to 3K can be determined with a resolution of 0.002K or better. The absolute temperature shall be known to the nearest 0.1K at the reference temperature of the calorimetric measurements. The temperature-measuring device should be linear, or linearized, in its response to changes in temperature over the interval it is used. As alternatives to the traditional mercury-in-glass thermometers, suitable temperature sensors are platinum-resistance thermometers, thermistors, quartz crystal resonators, etc., which, together with a suitable resistance bridge, null detector, frequency counter, or other electronic equipment, provide the required resolution. The short-term repeatability of this type of device shall be 0.001K or better. Long-term drift shall not exceed the equivalent of 0.05K for a period of six months. Sensors with inear response (in terms of temperature) are less likely to drift, causing bias in the calorimetric measurements, than are non-linear sensors.
For adiabatic systems, a suitable arrangement is as follow: Mercury-in-glass thermometers in accordance with ISO 651, ISO 652, ISO 1770 or ISO 1771 satisfy the measurement requirements. A viewer with magnification about 5x is needed for reading the temperature with the resolution required. Also, mechanical vibrator to tap the thermometer is suitable for preventing the mercury column from sticking. If this is not available, the thermometer can be tapped manually before reading the temperature.
IGNITION CIRCUIT - The electrical supply shall be 6V to 25V alternating current from a step-down transformer or direct current. 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 nonreactive 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. The crucible should be lined with an ash-free, silica-fibre disc for coke, anthracite, high-ash coal and other less reactive fuels. A low-mass, shallow 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 coke, the nickel-chromium crucible, as described for use with coal, should be lined with a commercially produced ash-free, silica-fibre disc. The mass of the disc is not included as part of the sample mass. Alternatively, line the crucible with a paste of fused aluminosilicate cement. After drying at 50°C to 60°C, the excess cement shall be scraped off to leave a smooth lining about 1.5mm thick; the crucible shall then be incinerated at 1000°C for 2h. Before use, 0.3g of aluminium oxide shall be spread over the base of the lined crucible and compacted with the flat end of a metal rod.
For other substances with a high moisture content, such as bio-oils, the ash-less disk is placed on top of the sample in the crucible. This helps to absorb the moisture, and easy burning occurs without misfires.
For Benzoic Acid, either of the crucibles specified for coal 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.
ANCILLARY PRESSURE EQUIPMENT
PRESSURE REGULATOR, to control the filling of the bomb with oxygen.
PRESSURE GAUGE (e.g. 0MPa to 5MPa), to indicate the pressure in the bomb with a resolution of 0.05MPa.
RELIEF VALVE or BURNING 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.
TIMER, indicating minutes and seconds.
BALANCE, capable of weighing the sample, fuse, etc. with a resolution of at least 0.1mg; 0.1mg is preferable and is recommended when the sample mass is of the order of 0.5g or less.
BALANCE, capable of weighing the calorimeter water, with a resolution of 0.5g (unless water can be dispensed into the calorimeter by volume with the required accuracy).
THERMOSTAT (optional), for equilibrating the calorimeter water before each test to a predetermined initial temperature, within about ± 0.3K.
PREPARATION OF TEST SAMPLE
The coal and coke used for the determination of the calorific value shall be the analysis sample ground to pass a test sieve with an aperture of 212um. In some circumstances, it has been shown that a maximum particle size of 250um is acceptable for low-and medium-rank coals.
The sample shall be well mixed in reasonable moisture equilibrium with the laboratory atmosphere. Either the moisture content shall be determined on samples weighed within a few hours of the time that samples are weighed for the determination of calorific value, or the sample shall be kept in a small, effectively closed container until moisture analysis are performed, to allow appropriate corrections for moisture in the analysis sample.
Determination of the moisture content of the analysis sample shall be carried out in accordance with one of the methods specified in ISO 687, ISO 11722 or ISO 5068-2.
The calorimetric determination consists of two separate tests: 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 tests is essentially the same. In fact, the overall similarity is a requirement for proper cancellation of systematic errors caused, for example, by uncontrolled heat leaks not accounted for in the evaluation of the corrected temperature rise.
The test 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 (equals the "reaction") period and an after-period. For the adiabatic-type calorimeter, the fore- and after-periods, in principle, should 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 calorimeters, 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. It is then necessary for the fore- and after-periods to be longer.
The power of stirring shall be maintained constant throughout a test that calls for a constant rate of stirring. An excessive rate of stirring results in an undesirable increase in the power of stirring with ensuing difficulties in keeping it constant. A wobbling stirrer is likely to cause significant short-term variations in stirring power.
During combustion, the bomb head becomes appreciably hotter than other parts of the bomb, and it is important to have enough well stirred water above it to maintain a reasonably small temperature gradient in the calorimeter water during the rapid part of the rise in temperature. For aneroid systems, the particular design determines to what extent the hot spots may develop.
Certain less reactive fuels may persistently leave residues that contain significant amounts of unburned sample or soot. By mixing these samples with known amounts of an auxiliary material, complete combustion can, in my instances, be achieved. Wrapping samples in tissue or rice paper, in addition to providing a combustion aid, gives and opportunity to affect the configuration of the sample in the crucible at the moment of ignition.
The auxiliary material shall be chemically stable, have a low vapor pressure and a well-established energy of combustion. The energy should be known to within 0.10% for the particular material used. Benzoic acid appears to be the ideal compound, even though n-dodecane or paraffin oil, for example, being liquids, are easier to distribute evenly. The amount used should be limited to the minimum amount required to achieve complete combustion of the sample. The amount used should not exceed an amount that contributed half the total energy in a test. The optimum proportion of sample to auxiliary material depends on the properties of the fuel, and it is necessary that it be determined experimentally.
For coals having ash values exceeding approximately 35%, there is a possibility of incomplete combustion, and a sufficient, known mass of auxiliary materials should be added to ensure a temperature rise similar to that obtained in benzoic acid calibrations.
When the auxiliary material is a liquid, it can wet the sample more thoroughly if it is added to the crucible before the fuel sample.
PREPARING THE BOMB FOR MEASUREMENT
Weigh the sample in the crucible, with an accuracy of 0.01% of the mass of sample or better. For 1g samples, this means weighing to the nearest 0.1mg. Weigh the combustible fuse and/or ignition wire, either with a precision comparable to that for weighing the sample, or keep its mass constant,within specified limits, for all tests.
Fasten the ignition wire tautly between the electrodes in the bomb. Check the resistance of the ignition circuit of the bomb; for most bombs, it should not exceed 5 ohm to 10 ohm, measured between the outside connectors of the bomb head, or between the connector for the insulated electrode and the bomb head.
Tie, or attach firmly, the fuse to the ignition wire, place the crucible in its support, and bring the fuse into contact with the sample. Make sure that the position of the crucible in the assembled bomb is symmetrical with respect to the surrounding bomb wall.
When the ignition wire is combustible as well as electrically conducting, an alternative procedure may be adopted. A longer piece of wire, enough to make an open loop, is connected to the electrodes. After mounting of the crucible, the loop is brought close to the sample; for samples in pellet form, the loop shall be in contact with the sample (in some cases, the ignition process is better controlled when the wire is kept at a small distance above the sample). Care should be taken to prevent any contact between ignition wire and crucible, in particular when a metal crucible is used, since this would result in shorting the ignition circuit. A special fuse is superfluous under these conditions. The resistance of the ignition circuit of the bomb will be increased by a small amount only.
Add the prescribed amount of distilled water to the bomb, for example (1.0 ± 0.1 ml) for 1g of sample. Assemble the bomb and charge it slowly with oxygen to a pressure of (3.0 ± 0.2) MPa without displacing the original air. If the bomb is inadvertently charged with oxygen above 3.3MPa, discard the test and begin again.
WARNING - Do not reach over the bomb during charging.
The bomb is now ready for mounting in the calorimeter can.
USING A COMBUSTION AID
Use a low-mass crucible. Weigh the auxiliary material as accurately as possible so that is contribution can be correctly accounted for. This is particularly important when a hydrocarbon oil is used, as its specific energy of combustion is considerably higher than that of the fuel.
When the auxiliary material is, for instance, rice paper or a liquid, it is weighed before the fuel sample. Weight the benzoic acid last when it is used as the combustion aid. Mix the solid materials without removing any of the mixture; check by weighing. Compact the mixture by tapping the bottom of the crucible against a clean table. A flat, polished rod can be used for additional compression of the mixture.
ASSEMBLING THE CALORIMETER
Bring the calorimeter water to within ± 0.3K of the selected initial temperature and fill the calorimeter can with the required amount. The quantity of water in the calorimeter can shall be the same to within less than 0.5g in all tests. Make sure that the outer surface of the can is dry and clean before the latter is placed in the thermostat. Mount the bomb in the calorimeter can after the can (containing the correct amount of water) has been placed into the thermostat.
Alternatively, the system may be operated on a constant total-calorimeter-mass basis. The bomb is then mounted in the calorimeter can before this is weighed with the water. The total mass of the calorimeter can, with the assembled bomb and calorimeter water, shall then be at least within 0.5g in all tests.
The assembled calorimeter shall contain enough water to thoroughly cover the flat, upper surface of the bomb head and cap.
NOTE : Weighing the water to within 0.5g applies when the effective heat capacity is in the order of 10kJ/K.
Check the bomb for gas leaks as soon as its top becomes covered with water. If the gas valves are not fully submerged, check for leaks with a drop of water across the exposed opening. Connects the leads for the ignition circuit and mount the thermometer.
WARNING - If gas escapes from the bomb, discard the test, eliminate the cause of leakage and begin again. Apart from being a hazard, leaks inevitably lead to erroneous results.
Cooling water, temperature controls, stirrers, etc. are turned on and adjusted, as outlines in the instrument manual. Make sure that the calorimeter stirrer works properly. A period of about 5 min is normally required for the assembled calorimeter to reach a steady state in the thermostat or jacket, irrespective of the type of calorimeter. The criteria for when a steady state has been attained depend on the working principle of the calorimeter.
COMBUSTION REACTION AND TEMPERATURE MEASUREMENTS
Start taking temperature readings, to the nearest 0.001K or better, as soon as the calorimeter has reached steady-state conditions. Readings at 1 min intervals normally suffice to establish the drift rate of the fore-period or check the proper functioning of an adiabatic system. When a mercury-in-glass thermometer is used for the temperature measurements, tap the thermometer lightly for about 10s before each reading and take care to avoid parallax errors.
At the end of the fore period, when the initial temperature has been established, the combustion is initiated by firing the fuse. Hold the switch closed only for as long as it takes to ignite the fuse. Normally, the current is automatically interrupted as the conducting wire starts burning or partially melts. As long as the resistance of the ignition circuit of the combustion bomb is kept at its normal low value, the electrical energy required to initiate the reaction is so small that it is not necessary to measure and account for it separately.
WARNING - Do not extend any part of the body over the calorimeter during firing, nor for 20 seconds thereafter.
Continue taking temperature readings at 1 minute intervals. The time marks the beginning of the main period. During the first few minutes after the charge has been fired, when the temperature is rising rapidly, readings to the nearest 0.02K are adequate. Resume reading temperatures to the nearest 0.001K or better as soon as is practicable, but no later than 5 minutes after the beginning of the main period. Criteria for the length of the fore-, main, and after-periods, and hence the total number of temperature readings required, are given in Annexes A and B.
ANALYSIS OF PRODUCTS OF COMBUSTION
At the end of the after-period, when all the required temperature readings have been completed, remove the bomb from the calorimeter, release the pressure at a moderate rate and dismantle the bomb. Examine the interior of the bomb, the crucible and any solid residue carefully for signs of incomplete combustion. Discard the test if unburned samples of any soot deposit are visible. Remove and measure any unreacted pieces of combustible ignition wire.
NOTE : Another symptom of incomplete combustion is the presence of carbon monoxide in the bomb gas. Slow release of the gas through a suitable detector tube reveals any presence of carbon monoxide and indicates the concentration level. 0.1ml/l of carbon monoxide in the combustion gas from a 300ml corresponds to an error of about 10J.
Wash the contents of the bomb into a beaker with distilled water. Make sure that the underside of the bomb head, the electrodes and the outside of the crucible are also washed.
In the case of calibration tests, dilute the combined washings to about 50ml and analyze for nitric acid, e.g. by titration with the sodium hydroxide solution to a pH of about 5,5 or by using the screened methyl orange solution as an indicator.
When the "sulfur" and/or nitric acid corrections are based on the actual amounts formed in the bomb process, the bomb washings from fuel combustion are analyzed by the procedure described below, or by an equivalent method. If the sulfur content of the fuel and the nitric acid correction are known, analysis of the final bomb liquid may be omitted.
Dilute the combined bomb washings to about 100ml. Boil the washings to expel carbon dioxide and titrate the solution with barium hydroxide solution while it is still hot, using the phenolphthalein solution as an indicator. Add 20.00ml of the sodium carbonate solution, filter the warm solution and wash the precipitate with distilled water. When cold, titrate the filtrate with the hydrochloric acid solution using the screened methyl orange solution as an indicator, ignoring the phenolphthalein colour change.
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 interpret-able 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 carrying out the calibration of the instrument as a separate series of instruments, establishing the effective heat capacity, i.e. the calibration constant of the calorimeter.
This calibration constant, should not change significantly over time, provided minor repairs or other changes in the system are 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 can arise, for example, from evaporation of calorimeter water, from uncontrolled heat exchange along various paths and/or imperfections, and lag in an adiabatic temperature control system during the reaction period. Cancellation of this type of error depends largely on the similarity between the calibration tests and the 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 tests is an expedient way of establishing the requirements for "similarity" for a particular calorimetric system.
The certificate value for the energy of combustion of benzoic acid refers to a process where the mass, expressed in grams, of the sample and the initial water, respectively, is equal to three times the volume of the bomb, expressed in liters (3g/l), the initial pressure of oxygen is 3.0MPa and the reference temperature is 25°C. The products of combustion are defined as gaseous carbon dioxide, liquid water and an equilibrium amount of carbon dioxide dissolved in the aqueous phase. Any nitric acid formed is corrected for by the energy of the process, where the acid it decomposed to form a liquid water and gaseous nitrogen and oxygen.
When calibrations are performed under different conditions, the certificate value shall be adjusted. A numerical expression to correct for such deviations is given in the certificate.
The calibration conditions determine the overall calorimetric conditions for the subsequent fuel determinations. For bombs with an internal volume of about 300ml, 1g of calibrant and 1ml of water initially in the bomb are normally used. For bombs with a volume nearer to 200ml, 0.5g of benzoic acid is preferable; the amount of water should then be reduced accordingly.
NOTE : The correction terms (per gram of benzoic acid) for deviations from certificate conditions, quoted from a typical benzoic acid certificate, are for an initial pressure of 5 J/MPa, a mass-of-sample-to-bomb-volume ratio of 1.1 J/g/l, an initial mass-of-water-to-bomb-volume ratio of 0.8 J/g/l, and a reference temperature for the test of -1.2J/K.
NOTE : As long as the initial pressure of oxygen and the reference temperature are kept within (3.0 ± 0.3) MPa and (25 ± 2)°C respectively, the departure from certification conditions caused by pressure and/or temperature deviations is within ± 3 J/g and is not necessary to account for it.
NOTE : If larger ratios of water to calibrant, e.g. 5ml/g are used, this is usually the most significant deviation from the certification conditions. For a 300ml bomb, this causes an increase in the certified value of 11 J/g. If 1.0g of benzoic acid and 5.0ml of water is used in a 200ml bomb, the certified value increases by 20 J/g. The change is caused mostly by an increase in the fraction of carbon dioxide dissolved in the bomb liquid.
NOTE : When the total heat capacity of the calorimeter is small, for example in aneroid systems, it can be necessary to reduce the sample mass in order to limit the total change in temperature.
VALID WORKING RANGE OF THE EFFECTIVE HEAT CAPACITY
It ought to be possible to vary the amount of calibrant at least ± 25% without getting a significant trend in the values obtained for the effective heat capacity. If this is not the case, the working limits for a constant value shall be defined in terms of total temperature rise measured. All subsequent measurements of calorific value shall be kept within these limits.
A plot of the values of the effective heat capacity, as a function of the mass of calibrant used reveals whether there is a significant trend in the effective heat capacity for a particular calorimeter. In this test, the calibrant mass should be varied from 0.7g to 1.3g, or an equivalent relative amount, and a minimum of eight tests should be performed. It is not necessary to vary the initial amount of water in the bomb.
A convenient way of checking a system already calibrated by combustion of, for example, 1.0g samples is to use the benzoic acid as an unknown. The mean values from triplicate runs on 00.7g and 1.3g sample masses, respectively, are compared with the certificate values. This normally suffices to ascertain whether the effective heat capacity is constant for the range of heat produced. Deviations are generally expected to be in the direction of "low" calorific values for larger sample masses, equivalent to obtaining values on the high side when derived from large samples. Using benzoic acid as a test substance is particularly useful in checking the performance of highly automated systems.
In addition to the energy from the combustion of benzoic acid, there are contributions from the combustion of the fuse(s) and the formation of nitric acid (from "air" nitrogen in the gaseous phase). The contribution from a fuse is derived from the amount involved and the appropriate energy of combustion. It is necessary to take into account any unreacted fuse wire, i.e. subtracting it from the initial amount.
The amount of nitric acid formed is determined on the final bomb solution, for example, by acid-base titration.
In most systems, the contribution from the fuse(s) can be kept nearly the same in all tests (fuel and calibration) and can, consequently, be assigned a constant value. For a given bomb configuration, the amount of nitric acid formed in calibration tests is approximately proportional to the amount of benzoic acid burned.
GROSS CALORIFIC VALUE
The calorimetric conditions for the fuel combustion shall be consistent with those of the calibration tests. With the calorimetric procedure under satisfactory control, ascertain 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 it can be necessary to burn then 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 ot a hydrocarbon oil of low volatility. Benzoic Acid has the advantage of having a well established value for the energy of combustion.
The variation on the correction for nitric acid is often on the borderline of significance. When the sulfur 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 tests. 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 combustion. The nitric acid correction can also vary significantly for different types of fuels.
When analysis of the bomb washings for sulfuric and nitric acid is required, the procedure described above, or an equivalent one, may be used.
Duplicate combustion shall be made. A representative sample shall be taken from the analysis sample, which is used without further pretreatment. The amount shall be such that the observed temperature rise is within the range of the calibration tests.
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. The use of an ash-free, silica-fibre disc to line the crucible, or something equivalent, is recommended. If the observed temperature rise falls outside the valid range, the calibration shall be confirmed for the extended range.
The same general conditions that are prescribed as prescribed for coal apply for coke. The use of an ash-free, silica-fibre disc to line the crucible, or something equivalent, is recommended. The coke sample shall be distributed evenly in the crucible. Certain unreactive cokes can persistently leave residues that contain significant amounts of unburned sample or soot. Optimum conditions for clean combustion may be investigated by varying the amount of sample.
NOTE : Lower sample mass and the addition of one or two drops of distilled water to the sample after weighing can lead to the complete combustion of some cokes that are difficult to burn.
An alternative method is 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 it is necessary to determine it experimentally.
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 test is normally larger than in combustion with coke alone.
The results of duplicate determinations, carried out in the same laboratory by the same operator with the same apparatus within a short interval of time on the same analysis sample, shall not differ by more than 120J/g.
The means of the results of duplicate determinations carried out in each of two laboratories, on representative portions take from the same sample at the last stage of sample preparation, shall not differ by more than 300J/g.
SAFE USE, MAINTENANCE AND TESTING OF CALORIMETER BOMBS PRESSURE TEST
The bomb shall not show visible signs of leakage. The closure ring shall couple and uncouple smoothly and shall not show signs of galling. The application and release of pressure shall not cause permanent deformation of the body or of the closure ring, subject to a tolerance of 0.02mm on either the increase in diameter of the body or the increase in height of the ring to allow for the estimated measurement uncertainty.
Subject the bomb to internal pressure as recommended by the manufacturer and maintain this pressure for 10 min. If the oxygen inlet valve is of the self-sealing type, remove the valve for this test.
MEASUREMENT OF BODY DIAMETER
Measure the diameter of the body at a minimum of eight located positions, evenly distributed. Check the consistency of reading and continuity, taking readings on a setting rod at the outset of the test and during a series of measurements. To permit the necessary accurate measurements on body and ring, ensure that their external surfaces are protected from damage at all times.
MEASUREMENT OF HEIGHT OF CLOSURE RING
With the lower narrow face of the ring standing on a datum surface, such as a surface plate complying with the requirements of BS 817:2008, grade 1, determine the height at a minimum of eight positions, four along each of the two diameters at 90 degrees and alternately positioned as neat as practice-able to the outer and inner circumferences of the top annular face. Check the consistency of the readings and continuity with gauge blocks or other precision-setting pieces at the initial and later stages of the proving test.
GAS LEAKAGE TEST
The bomb shall be tested and during this test period no bubbles attributable to leakage shall appear.
If the requirements are met, each calorimeter bomb is subjected to a gas-leakage test using air or oxygen at a pressure of 4 MPa. When the bomb is at this pressure remove it from the gas supply and immerse it in a bath of cold water for a period of 10min.
Take care not to confuse air escaping from inter-component spaces with leakage. If leakage is suspected, empty the bomb, examine the suspect region and tighten or replace seals as necessary, and repeat the test.
SAFEGUARDING, PERIODIC RE-CHECKING, OVERHAUL, AND RE-TEST
WARNING - The calorimeter bomb is a high-pressure vessel and should be handled with care at all times to avoid damage.
The following shall apply:
- Each calorimeter bomb shall be marked indelibly on the cap, closure ring and the base of the bomb body with an identification code. If engraving or stamping is used for marking the calorimeter body, it shall be confined to the positions indicated and shall be completely within the thickness of the base and collar. Marking by electric etching is also permissible. Colour-coding of bomb components may be used as an additional identifier. A certificate provided by the testing authority shall be supplied for each calorimeter bomb and shall contain the following information: identification code used; this can be engraved, stamped or etched marking, maximum pressure for testing according to manufacturer's instructions, date of pressure test and name of testing authority, compliance or non-compliance with this International Standard. Some test samples, such as those that release chlorine, have been found to corrode steel bombs. Users are advised to check beforehand the likely reactions of test samples, to avoid running the risk of damaging their combustion bombs. Deterioration of the vessel can be minimized by keeping the parts clean. In particular, the products of combustion should be cleansed from the inside of the body as soon as the observations are completed. Any deterioration of the surface should be reported by the operator so that the circumstances can be investigated. Any screw threads should be cleaned carefully and thoroughly with a brush and damage to the threads should be rectified by a skilled operator.
- It is recommended that the user of the calorimeter bomb keep a log of the number of firings and a record of the dates and results of inspections and measurement checks. The outside diameter of the body should be measured regularly so that any distortion that can result in the withdrawal of the bomb from use is detected when it occurs. The frequency of such measurements should be related to the frequency of firing; they should be made weekly if a bomb is in continual daily use. NOTE : For bombs having threads, test the screw thread for wear.
- It is essential that the development of slackness of fit of a bomb be checked after not more than 1000 firings, and, subsequently, at intervals not exceeding 500 firings (or as per manufacturer's recommendations), and the vessel should be withdrawn as soon as the threads are too slack. The surveillance may be undertaken within the user's organization, provided that the equipment and expertise are available and responsibility is defined. It is imperative that a calorimeter bomb is re-certified by the manufacturer or by an independent testing authority at intervals not exceeding one year.
- When the calorimeter bomb is overhauled and a new ring fitted, it is imperative that the assembly be subjected before use to a further proving pressure test, a report recording the latest information be supplied, and the bomb be marked with the date of the latest test.
- The following information shall be supplied with the calorimeter bomb : identification number of the bomb, standard reference, the date of the proving test and the gas-leakage test, the test pressures, the testing authority and the result of the test, standard reference (if any), the analysis and the mechanical properties of the material from which the closure ring is made.
The report for the pressure-proving test shall be valid for not more than four years. A new report shall be issued on re-testing.
NOTE : See BS 3643-2, which valuates the limits of tolerance on diametrical difference between internal and external threads in assembly.
METHOD OF MEASUREMENT OF SLACKNESS OF FIT OF CLOSURE RING AND BODY
REQUIREMENTS, FIT OF SCREW THREAD
When tested, calorimeter bombs having threads shall conform to the manufacturer's instructions.
The threads should first be cleaned and inspected for damage, such as burns, bruising of the metal or roughness due to galling. Local damage should be corrected by skilled attention before further tests.
The procedure shall be as follows:
- The closure ring and cap should be assembled fully, with the body and the number of turns of the ring required to close the bomb counted. The ring and cap should be removed and the ring, without the cap, should then be assembled onto the body using four turns less than the full number of turns previously counted.
- The bomb body should be placed on a surface plate and held down firmly. An engineer's dial gauge capable of reading to an accuracy of at least 0.02mm and mounted on a sturdy stand should be set in contact with a finished surface of the ring. With the bomb body held immovable, the closure ring should be moved between its two extreme positions diametrically or axially and the amount of slackness observed on the dial gauge.
- If there is a plain surface on the side of the ring, it is preferable to measure diametrical slackness. If, however, knurling of the side prevents side registration, then axial slackness may be measured.
- The ring should not be rotated; when conducting the axial test, a plastic ring should be used with a free-running fit on the bomb body to displace the closure ring.
- To measure the diametrical slackness (where this is possible), five readings should be taken at a position on the periphery of the ring and then a further five readings at a position 90 degrees to the original. The mean of the total of 10 readings should then be taken to establish the diametrical slackness.
- To measure the axial slackness, readings should be taken on the top surface of the closure ring, with one reading of each of 10 positions evenly distributed around the ring. The mean of the 10 readings should be ascertained for the purpose of establishing axial slackness.
MATERIALS AND MINIMUM DIMENSIONS
BOMB CAP AND BODY
The bomb cap and body shall be made of a material that is capable of withstanding the pressures generated by the combustion process and they shall not be corroded by the products of combustion of the test samples.
The bomb cap and body shall each be machined from solid or hollow forgings of bar; they shall not be fabricated from components welded or brazed together.
If the bomb cap and body are made of a material other than stainless steel, the material supplier should certify that the material has passed a correspondingly suitable test for resistance to inter-crystalline corrosion.
The closure ring shall be made of material, such as aluminium bronze, capable of withstanding the pressure generated by the combustion process.
Materials should be chosen to minimize galling or seizing of the thread engaging the bomb body.
The closure ring shall be machined from a solid or hollow forging or bar. It shall not be fabricated from compounds welded or brazed together.
The cylindrical wall thickness of the bomb shall not be less than 0.10 times the internal diameter at any point, including the roots of the knurling and the closure threads and any undercuts at the threaded portions.
SAFE USE OF A CALORIMETER BOMB AND ITS ANCILLARY EQUIPMENT
WARNING - It is essential that attention be paid to the safety measures given in any method for the determination of a calorific value and that particular attention be paid to prescribed precautions when testing volatile liquid fuels.
OXYGEN FILLING EQUIPMENT
- It is imperative that the filling system include a control valve, a pressure-operated safety device and a pressure gauge.
- The control valve may be of the single-stage diaphragm type or of the needle type.
- The safety device may be either a valve or a bursting disc. Set the device to operate at 0.25 MPa above the working pressure specified in the test method for the sample, provided that the setting does not exceed 4.25 MPa.
- The pressure gauge should be of the safety pattern described in EN 837-1, with a range of at least 5 MPa and an accuracy of ± 0.2 MPa at 5 MPa.
It should be checked annually and also any time its accuracy is suspect. It is essential that oil not be used when checking this gauge, which should be marked "USE NO OIL".
DANGER - Do not fire the charge if a calorimeter bomb has been inadvertently overcharged with oxygen; abandon the test.
DANGER - Do not fire the charge if there are gas leaks when the bomb is immersed in the water in the calorimeter.
- To reduce the risk of explosion, it is strongly recommended that the oxygen cylinder be situated outside the room or enclosure containing the bomb.
QUANTITY OF SAMPLE
In every case, the quantity of sample placed in the bomb should be kept within the limit specified in the relevant test method. As a general guide, the heat released should not exceed 100J per milliliter capacity of the bomb.
The ignition circuit shall observe the following:
- The firing circuit should be controlled by a switch that is spring-loaded (biased) to return to the "contacts open" condition when its operating level is released.
- The ignition voltage should not exceed 24V. If the ignition voltage is derived from the mains supply, a double wound (isolating) transformer having an earthed inter-winding screen should be used.
- The firing button should be in such a position that the operator can stand back and fire the bomb without having to reach over it. A remote firing position, e.g. behind a protective wall or in another room, is recommended, especially when testing liquid fuels.
- It is desirable to include an ammeter or a pilot light in the firing circuit to indicate when current is flowing. A 5A fuse should also be included.
- Do not approach the bomb until 20s after firing.