TERMINOLOGY

Gross Heat of Combustion - the quantity of energy released when a unit mass of fuel is burned in a constant volume enclosure, with the products being gaseous, other than water that is condensed to the liquid state.

Discussion - The fuel can be either liquid or solid, and contain only the elements carbon, hydrogen, nitrogen, and sulfur. The products of combustion, in oxygen, are gaseous carbon dioxide, nitrogen oxides, sulfur dioxide, and liquid water. In this procedure, 25°C is the initial temperature of the fuel and the oxygen, and the final temperature of the products of combustion.

Net Heat of Combustion - the quantity of energy released when a unit mass of fuel is burned at constant pressure, with all of the products, including water, being gaseous.

Discussion - The fuel can be either liquid or solid, and contain only the elements carbon, hydrogen, oxygen, nitrogen, and sulfur. The products of combustion, in oxygen, are carbon dioxide, nitrogen oxides, sulfur dioxide, and water, all in the gaseous state. In this procedure, the combustion takes place at a constant pressure of 0.1012 MPa (1 atm), and 25°C is the initial temperature of the fuel and the oxygen, and the final temperature of the products of combustion.

SUMMARY OF TEST METHOD

Heat of combustion is determined in this test method by burning a weighed sample in an oxygen bomb calorimeter under controlled conditions. The heat of combustion is computed from temperature observations before, during and after combustion, with proper allowance for thermochemical and heat transfer corrections. Either isothermal or adiabatic calorimeter jackets can be used. Temperatures can be measured in degrees Celsius. Temperatures can be recorded in either degrees Fahrenheit or ohms or other units when using electric thermometers. Use the same units in all calculations, including standardization. Time is expressed in calculations in minutes and decimal fractions thereof. It may be measured in minutes and seconds. Masses are measured in grams and no buoyancy corrections are applied.

SIGNIFICANCE AND USE

The heat of combustion is a measure of the energy available from a fuel. A knowledge of this value is essential when considering the thermal efficiency of equipment for producing either power or heat. The heat of combustion as determined by this test method is designated as one of the chemical and physical requirements of both commercial and military turbine fuels and aviation gasoline. The mass heat of combustion, the heat of combustion per unit mass of fuel, is a critical property of fuels intended for use in weight-limited craft such as airplanes, surface effect vehicles, and hydrofoils. The range of such craft between refueling is a direct function of the heat of combustion and density of the fuel.

APPARATUS AND FACILITIES

Test Room, Bomb, Calorimeter, Jacket, Thermometers and Accessories.

Test Room - The room in which the calorimeter is operated must be free from drafts and not subject to sudden temperature changes. The direct rays of the sun shall not strike the jacket or thermometers. Adequate facilities for lighting, heating, and ventilating shall be provided. Thermostatic control of room temperature and controlled relative humidity are desirable.

Oxygen Bomb - The oxygen bomb is to have an internal volume of 350 ± 50ml. All parts are to be constructed of materials which are not affected by the combustion process or products sufficiently to introduce measurable heat input or alteration of end products. If the bomb is lined with platinum or gold, all openings shall be sealed to prevent combustion products from reaching the base metal. The bomb must be designed so that all liquid combustion products can be completely recovered by washing the inner surfaces. There must be no gas leakage during a test. The bomb must be capable of withstanding a hydrostatic pressure test to a gauge pressure of 3000psi (20MPa) at room temperature, without stressing any part beyond its elastic limit.

Calorimeter - The calorimeter vessel shall be made of metal (preferably copper or brass) with a tarnish-resistant coating, and with all outer surfaces highly polished. Its size shall be such that the bomb will be completely immersed in water when the calorimeter is assembled. It shall have a device for stirring the water thoroughly and at a uniform rate, but with minimum heat input. Continuous stirring for 10 min shall not raise the calorimeter temperature more than 0.01°C starting with identical temperatures in the calorimeter, room, and jacket. The immersed portion of the stirrer shall be coupled to the outside through a material of low heat conductivity.

Jacket - The calorimeter shall be completely enclosed within a stirred water jacket and supported so that its sides, top, and bottom are approximately 10mm from the jacket wall. The jacket can be arranged so as to remain substantially constant temperature, or with provision for rapidly adjusting the jacket temperature to equal that of the calorimeter for adiabatic operation. It must be constructed so that any water evaporating from the jacket will not condense on the calorimeter.

A double-walled jacket with a dead-air insulation space may be substituted for the constant-temperature water jacket if the calorimeter is operated in a constant-temperature (±2F)(±1°C) room. The same ambient conditions must be maintained for all experiments, including standardization.

Thermometers - Temperatures in the calorimeter and jacket shall be measured with the following thermometers or combinations thereof:

Etched Stem, Liquid-in-Glass, ASTM Bomb Calorimeter Thermometer having a range from 66 to 95°F or 19 to 35°C, 18.9 to 25.1°C, or 23.9 to 30.1°C, as specified and conforming to the requirements for Thermometer 56F, 56C, 116C, or 117C, respectively, as prescribed in Specification E1. Each of these thermometers shall have been tested for accuracy at intervals no larger than 2.5°F or 2.0°C over the entire graduated scale. Corrections shall be reported to 0.005°F or 0.002°C, respectively, for each test point.

Beckmann Differential Thermomemter, range 6°C reading upward as specified and conforming to the requirements for Thermometer 115C as prescribed in Specification E1. Each of these thermometers shall be tested for accuracy at intervals no larger than 1C over the entire graduated scale and corrections reported to 0.001°C for each test point.

Calorimetric Type Platinum Resistance Thermometer, 25Ω.

Thermometer Accessories- A magnifier is required for reading liquid-in-glass thermometers to one tenth of the smallest scale division. This shall have a lens and holder designed so as not to introduce significant errors due to parallax.

A Wheatstone bridge and galvanometercapable of measuring resistance of 0.0001Ω are necessary for use with resistance thermometers.

Timing Device - A watch or other device capable of measuring time to 1s is required for use with the isothermal jacket calorimeter.

Sample Holder - Nonvolatile samples shall be burned in an open crucible of platinum (preferred), quartz or acceptable base metal alloy. Base metal alloy crucibles are acceptable if after a few preliminary firings the weight does not change significantly between tests.

Firing Wire - Use a 100mm length of No.34 B & S gauge iron wire or Chromel C resistance wire. Shorter lengths may be used if the same length is employed in all tests, including standardization tests. Platinum wire may be used if the ignition energy is small en reproducible.

Firing Circuit - A 6 to 16V alternating current is required for ignition purposes with an ammeter or pilot light in the circuit to indicate when current is flowing. A step-down transformer connected to a 115V 50/60Hz lighting circuit of storage batteries can be used (Warning - The ignition circuit switch shall be of the momentary contact type, normally open, except when held closed by the operator).

Oxygen Purification Device - Commercial oxygen produced form liquid air can generally be used without purification. Oxygen prepared by electrolysis of water should not be used without purification, as it can contain enough hydrogen to affect results by 1% or more. Combustible impurities can be removed from oxygen by passing it over copper oxide (CuO) at about 500°C.

REAGENTS

Benzoic Acid, Standard - Benzoic acid powder must be compressed into a tablet or pellet before weighing. Benzoic acid pellets for which the heat of combustion has been determined by comparison with the National Bureau of Standards sample are obtainable commercially for those laboratories not equipped to pellet benzoic acid.

Gelatin Capsules

Methyl Orange and Methyl Red Indicator.

Mineral Oil.

Oxygen - Commercial oxygen produced from liquid air can be used without purification. If purification is necessary, see A1.11 (Warning - Oxygen vigorously accelerates combustion.)

Pressure-Sensitive Tape - Cellophane tape 38mm (1.5in) wide, free of chlorine and sulfur.

Alkali, Standard Solution:

Sodium Hydrozide Solution - Dissolve 3.5g of sodium hydroxide (NaOH) in water and dilute to 1L. Standardize with potassium acid phthalate and adjust to 0.0866 mol/L as described in Practice E200 (Warning - Corrosive. Can cause severe burns or blindness. Evolution of heat produces a violent reaction or eruption upon too rapid mixture with water)

Sodium Carbonate Solution - Dissolve 3.84g of Na2CO3 in water and dilute to 1L. Standardize with potassium acid phthalate and adjust to 0.03625 mol/L as described in Practice E200.

2,2,4-Trimethylpentane, Standard - (Warning - Extremely Flammable. Harmful if inhaled. Vapors may cause flash fire)

STANDARDIZATION

Determine the Energy Equivalent of the Calorimeter - average not less than six tests using standard benzoic acid. These tests should be spaced over a period of not less than three days. Use not less than 0.9g nor more than 1.1g of standard benzoic acid (C6H6COOH). Make each determination according to the procedure described in Section 9 and compute the corrected temperature rise. Determine the corrections for nitric acid (HNO3) and firing wire as described and substitute.

Repeat the standardization tests after changing any part of the calorimeter and occasionally as a check on both calorimeter an operating technique.

Checking the Calorimeter for Use with Volatile Fuels - Use 2,2,4-trimethylpentane to determine whether the results obtained agree with the certified value (47.788 MJ/kg, weight in air) within the repeatability of the test method. If results do not come within this range, the technique of handling the sample may have to be changed. If this is not possible or does not correct the error, run a series of tests using 2,2,4-trimethylpentane to establish the energy equivalent for use with volatile fuels.

Heat of Combustion of Pressure-Sensitive Tape of Gelatin/Mineral Oil - Determine the heat of combustion of either the pressure-sensitive tape or 0.5g gelatin capsule/mineral oil in accordance with Section 9 using about 1.2g of tape or 0.5g gelatin capsule/mineral oil and omitting the sample.

Average the determinations, and redetermine the heat of combustion of the tape of gelatin capsule/mineral oil whenever a new roll or batch is started.

PROCEDURE

Weight of Sample - Control the weight of sample (including the auxiliary fuel) so that the temperature rise produced by its combustion will be equal to that of 0.9 to 1.1g of benzoic acid. Weigh the sample to the nearest 0.1mg.

Some fuels contain water and particulate matter (ash) that will degrade calorimetric values. If the heat of combustion is required on a clean fuel, filter the sample to remove free water and insoluble ash before testing.

For highly volatile fluids, reduce loss with use of tape or gelatin capsule mineral oil.

Tape - place a piece of pressure-sensitive tape across the top of the cup, trim around the edge with a razor blade, and seal tightly. Place a 3mm by 12mm strip of tape creased in the middle and sealed by one edge in the center of the tape disk to give a flap arrangement. Weigh the cup and tape. Remove from the balance with forceps. Fill a hypodermic syringe with the sample.

Add the sample to the cup by inserting the tip of the needle through the tap disk at a point so that the flap of tape will cover the puncture upon removal of the needle. Seal down the flap by pressing lightly with a metal spatula. Reweigh the cup with the tape and sample. Take care throughout the weighing and filling operation to avoid contacting the tape or cup with bare fingers. Place the cup in the curved electrode and arrange the fuse wire so that the central portion of the loop presses down on the center of the tape disk.

Gelatin/Mineral Oil - Weigh the cup and gelatin capsule. The capsule should only be handled with forceps. Add the sample to the capsule. Reweigh the cup with capsule and sample. If poor combustion is expected with the capsule, add several drops of mineral oil on the capsule and reweigh the cup and contents. Place the cup in the curved electrode and arrange the fuse wire so that the central portion of the loop contacts the capsule and oil.

Water in bomb - Add 1.0ml of water to the bomb from a pipette.

Oxygen - with the test sample and fuse in place, slowly charge the bomb with oxygen to 3.0 MPa (30 atm) gauge pressure at room temperature. Do not purge the bomb to remove entrapped air (Warning - be careful not to overcharge the bomb. If, by accident, the oxygen introduced into the bomb should exceed 4.0 MPa, do not proceed with the combustion. An explosion might occur with possible violent rupture of the bomb. Detach the filling connection and exhaust the bomb in the usual manner. Discard the sample, unless it has lost no weight, as shown by reweighing).

Lower or higher initial oxygen pressure can be used within the range from 2.5 to 3.5 MPa, provided the same pressure is used for all tests, including standardization.

Calorimeter Water - Adjust the calorimeter water temperature before weighing as follows: Isothermal Jacket Method (1.6 to 2.0°C below jacket temperature), Adiabatic Jacket Method (1.0 to 1.4°C below room temperature). This initial adjustment will ensure a final temperature slightly above that of the jacket for calorimeters having an energy equivalent of approximately 10.2 kJ/C. Some operators prefer a lower initial temperature so that the final temperature is slightly below that of the jacket. This procedure is acceptable, provided it is used in all tests, including standardization.

Use the same amount (±0.5g) of distilled or deionized water in the calorimeter vessel for each test. The amount of water (2000g is usual) can be most satisfactorily determined by weighing the calorimeter vessel and water together on a balance. The water can be measured volumetrically if it is measured always at the same temperature.

Observations, Isothermal Jacket Method - Assemble the calorimeter in the jacket and start the stirrer. Allow 5 min for attainment of equilibrium, then record the calorimeter temperatures at 1-min intervals for 5 min. Fir the charge at the start of the sixth minute and record the time and temperature. Add to this temperature 60% of the expected temperature rise, and record the time at which the 60% point is reached. After the rapid rise period (about 4 to 5 min), record temperatures at 1-min intervals on the minute until the difference between successive readings has been constant for 5 min.

Observations, Adiabatic Jacket Method - Assemble the calorimeter in the jacket and start the stirrers. Adjust the jacket temperature to be equal to or slightly lower than the calorimeter, and run for 5 min to obtain equilibrium. Adjust the jacket temperature to match the calorimeter within ± 0.01C and hold for 3 min. Record the initial temperature and fire the charge. Adjust the jacket temperature to match that of the calorimeter during the period of rise, keeping the two temperatures as nearly equal as possible during the rapid rise, and adjusting to within ±0.01C when approaching the final equilibrium temperature. Take calorimeter readings at 1-min intervals until the same temperature is observed in three successive readings. Record this as the final temperature. Time intervals are not recorded as they are not critical in the adiabatic method.

Analysis of Bomb Contents - Remove the bomb and release the pressure at a uniform rate such that the operation will require not less than 1 minute. Examine the bomb interior for evidence of incomplete combustion. Discard the test if unburned sample or sooty deposits are found.

Wash the interior of the bomb, including the electrodes and sample holder, with a fine jet of water and quantitatively collect the washings in a breaker. Use a minimum of wash water, preferably less than 350ml. Titrate the washings with standard alkali solution, using methyl orange or methyl red indicator.

Remove and measure the combined pieces of unburned firing wire, and subtract from the original length. Record the difference as wire consumed.

Determine the sulfur content of the sample if it exceeds 0.1%. Determine sulfur by analyzing the bomb washings remaining after the acid titration, using the procedure described in Test Methods D129, D1266, D2622, D3120, D4294, or D5453.