Terminology

Gross Heat of Combustion

Expressed as megajoules per kilogram. The gross heat of combustion at constant volume of a liquid or solid fuel containing only the elements carbon, hydrogen, oxygen, nitrogen, and sulfur is the quantity of heat liberated when a unit mass of the fuel is burned in oxygen in an enclosure of constant volume, the products of combustion being gaseous carbon dioxide, nitrogen, sulfur dioxide, and liquid water, with the initial temperature of the fuel and the oxygen and the final temperature of the products at 25°C. Gross heat of combustion is represented by the symbol Q.

Net Heat of Combustion

Expressed as megajoules per kilogram. The net heat of combustion at constant pressure of a liquid or solid fuel containing only the elements carbon, hydrogen, oxygen, nitrogen, and sulfur is the quantity of heat liberated when a unit mass of the fuel is burned in oxygen at a constant pressure of 0.101 MPa (1atm), the products of combustion being carbon dioxide, nitrogen, sulfur dioxide, and water, all in the gaseous state, with the initial temperature of the fuel and the oxygen and the final temperature of the products of combustion at 25°C.

Temperatures

Temperatures are measured in degrees Celsius.

Time

Time is expressed in minutes and decimal fractions thereof. It can be measured in minutes or seconds, or both.

Masses

Masses are measured in grams. No buoyancy corrections are applied except to obtain the mass of benzoic acid. The energy unit of measurement employed in this test method is the joule with the heat of combustion reported in megajoules per kilogram.

Summary of Test Methods

The heat of combustion is determined by burning a weighed sample in an oxygen-bomb calorimeter under controlled conditions. The temperature increase is measured by a temperature reading instrument which allows the precision of the method to be met. The heat of combustion is calculated from temperature observations before, during and after combustion, with proper allowance for thermochemical and heat transfer corrections. Either isoperibol or adiabatic calorimeters may be used.

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 mass heat of combustion, that is, the heat of combustion per unit mass of fuel, is measured by this procedure. Its magnitude is particularly important to weight-limited vehicles such as airplanes, surface effect vehicles, and hydrofoils as the distance such aircraft can travel on a given weight of fuel is a direct function on the fuel's mass heat of combustion and its density.

The volumetric heat of combustion, that is, the heat of combustion per unit volume of fuel, can be calculated by multiplying the mass heat of combustion by the density of the fuel (mass per unit volume). The volumetric heat of combustion, rather than the mass heat of combustion, is important to volume-limited craft such as automobiles and ships, as it is directly related to the distance traveled between re-fueling.

Reagents and Materials

Benzoic Acid - The acid must be pelleted before use.

Firing Wire - 0.127mm (No.36 gauge) platinum wire, No.34 B & S gauge iron wire or Chromel C resistance wire, cut in 100mm lengths.

Oxygen - Commercial oxygen produced from liquid air can be used without purification (Warning - Oxygen vigorously accelerates combustion. (See A3.1). Oxygen prepared by electrolysis of water cannot be used without purification as it can contain some hydrogen. Combustible impurities may be removed by passage over copper oxide at 500°C.

Preparation of Apparatus

Arrangement of Apparatus - Install the thermometers as recommended by the manufacturer of the calorimeter. Position the mercury-in-glass thermometer so that the bulb is halfway to the bottom of the bucket and locate the thermistor with its sensing elements at about the midpoint of the thermometer bulb. Mount these elements to that exactly the same length is immersed each time the calorimeter is used. Install a thermistor in the water jacket with the element immersed to the same depth as in the bucket. It is helpful, but not necessary to have mercury-in-glass calorimetric thermometers in both the bucket and jacket for quick temperature observations. Thermistors can be taped to these thermometers. If the thermistors are taped to the thermometers, it can be done in such a manner that the sensing elements are at the midpoint of the thermometer bulbs. The thermometer bulbs and temperature-sensing elements shall not touch the bomb, bucket, or water jacket.

Standardization

Energy Equivalent of the Calorimeter - Benzoic Acid shall be used as the primary standard (Warning - Oxygen vigorously accelerates combustion). Choose a sample mass so that the temperature rise is approximately equivalent to an energy change of 30 000 J. Initially determine the energy equivalent by averaging six determinations made using benzoic acid over a period of at least 3 days.

A relative standard deviation (RSD) of 0.1% or less for the six determinations must be achieved. If not, continue to run until six determinations establish a value that has a RSD of 0.1% or better. If this degree of precision cannot be achieved, review the procedure, critical measurements, mechanical operations and everything that may contribute to scatter in the results. After establishing an energy equivalent value, determine the value at frequent intervals using benzoic acid (every 1 or 2 days of testing) with the average of the last six determinations being used for the energy equivalent as long as the last six determinations have a RSD of 0.1% or less.

If any part of the equipment is changed or any part of the procedure is altered, redetermine the value.

Apparatus

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 exact temperature is not important as long as it is in the range from 23 to 26C (74 to 78F) and is held constant. The temperature must be constant, not only throughout the day, but from one time of the year to another. The direct rays of the sun shall not strike the calorimeter jacket, bridge, and galvanometer. Adequate facilities for lighting, heating, and ventilation should be provided. Thermostatic control of room temperature and controlled humidity are desirable.

Oxygen Bomb

The oxygen bomb shall meet the requirements specified in Practice E144 and have an internal volume of 350 +- 50ml. All parts shall be constructed of materials that are not affected by the combustion process or products sufficient 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 20.7 MPa (3000psi) at room temperature without stressing any part beyond its elastic limit. If necessary, modify the bomb such that the feet are 12mm high to allow for better water circulation under the bomb.

Calorimeter Bucket

The calorimeter bucket 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 temperature more than 0.01C 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 bomb, bucket, and water shall be completely enclosed within a stirred water jacket and supported so that its sides, top and bottom are approximately 1cm from the jacket walls. The jacket may be arranged so as to remain at a constant temperature, or with provisions for automatically adjusting the jacket temperature to equal that of the calorimeter bomb, bucket, and water for adiabatic operation. It must be constructed so that any water evaporating from the jacket will not condense on the calorimeter bucket.

Thermometers

Temperatures in the jacket and the calorimeter, respectively, shall be measured with the following thermometers: Etched stem, mercury-in-glass, ASTM Bomb Calorimeter Thermometers.

Automatic Calorimeter Microprocessor Controller

In place of manually recording temperature and calculating the energy equivalent, an automatic controller may be used. It shall be capable of storing temperature readings take at accurate intervals, firing the bomb and ending the experiment. The stored information along with the sample mass, nitric acid correction and sulfur content, when entered into the instrument, shall be used to calculate the desired energy equivalent or heat of combustion.

Firing Circuit

A 6 to 16V alternating or direct current is required for ignition purposes with 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 or storage battery may be used (Warning - The ignition switch shall be of the momentary contact type, normally open, except when held closed by the operator).

Timing Device

A watch or other timing device capable of measuring time to 1s is required for use with the isoperibol method.

Sample Cup

Alow-form platinum cup, 26mm in diameter and 11mm deep with a spun rim. Base metal alloy crucibles are acceptable if after a few preliminary firings the weight does not change significantly between tests.