TERMINOLOGY

Calorific Value - the heat of combustion of a unit quantity of a substance. It may be expressed in joules per gram (J/g), British thermal units per pound (BTU/lb), or calories per gram (cal/g) when required.

Gross Calorific Value - the heat produced by combustion of a unit quantity of solid fuel, at constant volume, in an oxygen bomb calorimeter under specified conditions such that all water in the products remain in liquid form.

Net Calorific Value - a lower value calculated from the gross calorific value. It is equivalent to the heat produced by combustion of a unit quantity of solid fuel at a constant pressure of one atmosphere, under the assumption that all water in the products remains in the form of vapor.

Calorimeter - describes the bomb, the vessel with stirrer, and the water in which the bomb is immersed.

Energy Equivalent - The energy required to raise the temperature of the calorimeter system 1°C (or 1°F) per gram of sample. This is the number that is multiplied by the corrected temperature rise in degrees and divided by the sample weight in grams to give the gross calorific value after thermochemical corrections have been applied.

Refuse-Derived Fuels - solid forms of refuse-derived fuels from which appropriate analytical samples may be prepared are defined as follows in ASTM STP 832. RDF-1 : Wastes used as fuel in as-discarded form with only bulky wastes removed. RDF-2 : Wastes processed to coarse particle size with or without ferrous metal separation. RDF-3 : Combustible waste fraction processed to particle sizes, 95% passing 2-in. square screening. RDF-4 : Combustible waste fraction processed into powder form, 95% passing 10-mesh screening and RDF-5 : Combustible waste fraction densified (compressed) into the form of pellets, sligs, cubettes, or briquettes.

SUMMARY OF TEST METHOD

Calorific value is determined in this method by burning a weighed analysis sample in an oxygen bomb calorimeter under controlled conditions. The calorific value is computed from temperature observations made before and after combustion, taking proper allowance for thermometer and thermochemical corrections. Either isothermal or adiabatic calorimeter jackets may be used.

SIGNIFICANCE AND USE

The calorific value, or heat of combustion, is a measure of the energy available from the fuel. Knowledge of this value is essential in assessing the commercial worth of the fuel and to provide the basis of contact between producer and user.

APPARATUS AND FACILITIES

Test Room - The apparatus should be operated in a room or area free of drafts that can be kept at a reasonably uniform temperature and humidity for the time required for the determination. The apparatus should be shielded from direct sunlight and radiation from other sources. Controlled room temperature and humidity are desirable.

Oxygen Bomb - constructed of material that 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 shall be designed so that all liquid combustion products can be completely recovered by washing the inner surfaces. There shall be no gas leakage during a test. The bomb shall be capable of withstanding a hydrostatic pressure test of 21 MPa (3000 psig)at room temperature without stressing any part beyond its elastic limit.

Calorimeter - 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 (0.02°F) 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 walls. The jacket may be arranged so as to remain at constant temperature or with provisions for rapidly adjusting the jacket temperature to equal that of the calorimeter for adiabatic operation. It shall be constructed so that any water evaporating from the jacket will condense on the calorimeter.

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 walls. The jacket may be arranged so as to remain at constant temperature or with provisions for rapidly adjusting the jacket temperature to equal that of the calorimeter for adiabatic operation. It shall be constructed so that any water evaporating from the jacket will condense on the calorimeter.

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

Mercury-in-Glass Thermometers - Conforming to the requirements for Thermometers 116°C or 117°C (56°F or 57°F) as prescribed in Specification E1. Other thermometers of equal or better accuracy are satisfactory. These thermometers shall be tested for accuracy against a known standard (preferably by the National Bureau of Standards) at intervals no greater than 2.0°C (3.6°F) over the entire graduated scale. The maximum difference in correction between any two test points shall not be more than 0.02°C (0.04°F).

Beckmann Differential Thermometer - having a range of approximately 6°C in 0.01°C subdivisions reading upward and conforming to the requirements for Thermometer 115C, as prescribed in Specification E1. Each of these thermometers shall be tested for accuracy against a known standard at intervals no larger than 1°C over the entire graduated scale. The maximum difference between any two test points shall not be more than 0.02°C.

Calorimetric-Type Platinum Resistance Thermometer, 25- , tested for accuracy against a known standard.

Other Thermometers - A high precision electronic thermometer employing balanced thermistors or a quartz thermometer may be used, provided the temperature rise indication is accurate within ± 0.003°C per 1°C rise.

Thermometer Accessories - A magnifier is required for reading mercury-in-glass thermometers to one tenth of the smallest scale division. This shall have a lens and holder designed so as to introduce no significant errors due to parallax. A Wheatstone bridge and galvanometer capable of measuring resistance to 0.0001 Ω is necessary for use with resistance thermometers.

Sample Holder - Samples shall be burned in an open crucible of platinum, 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 tasks.

Firing wire shall be 100mm of No. 34 B & S nickel-chromium alloy wire or 100mm of No. 34 B & S iron wire. Equivalent platinum of palladium wire may be used provided constant ignition energy is supplied, or measured, and appropriate corrections made.

Firing Circuit - A 6 to 16V alternating or direct 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 an alternating current lighting circuit or batteries may be used.

Caution - The ignition circuit switch shall be of momentary double-contact type, normally open, except when held closed by the operator. The switch should be depressed only long enough to fire the bomb.

REAGENTS

Purity of Reagents - Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents shall conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society, where such specifications are available. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.

Purity of Water - Unless otherwise indicated, references to water shall be understood to mean reagent water. Type III, conforming to Specification D1193.

Benzoic Acid, Standard (C6H5COOH) - Use National Bureau of Standards SRM (Standard Reference Material) benzoic acid. The crystals shall be pelleted before use. Commercially prepared pellets may be used provided they are made from National Bureau of Standards benzoic acid. The value of heat of combustion of benzoic acid, for use in the calibration calculations, shall be in accordance with the value listed in the National Bureau of Standards certificate issued with the standard.

Methyl Orange, Methyl Red, or Methyl Purple Indicator - may be used to titrate the acid formed in the combustion. The indicator selected shall be used consistently in both calibrations and calorific determinations.

Oxygen - free of combustible matter. Oxygen manufactured from liquid air, guaranteed to be greater than 99.5% pure, will meet this requirement. Oxygen made by the electrolytic process may contain a small amount of hydrogen rendering it unfit without purification.

Sodium Carbonate, Standard Solution (0.34 N) - one milliliter of this solution should be equivalent to 20.0J in the nitric acid (HNO3) titration. Dissolve 18.02g of anhydrous sodium carbonate (Na2CO3) in water and dilute to 1L. The Na2CO3 should be previously dried for 24H at 105°C. The buret used for the HNO3 titration shall be of such accuracy that estimations to 0.1ml can be made. A more diluted standard solution may be used for higher sensitivity.

PRECAUTIONS

Due to the origins of RDF in municipal waste, common sense indicates that some precautions should be observed when conducting tests on the samples. Recommended hygienic practices include use of gloves when handling RDF and washing hands before eating or smoking.

The following precautions are recommended for safe calorimeter operation:

• The weight of solid fuel sample and the pressure of the oxygen admitted to the bomb must not exceed the bomb manufacturer's recommendations.
• Bomb parts should be inspected carefully after each use. Threads on the main closure should be checked frequently for wear. The bomb should be returned to the manufacturer occasionally for inspection and possibly proof of firing.
• The oxygen supply cylinder should be equipped with an approved type of safety device, such as a reducing valve, in addition to the needle valve and pressure gauge used in regulating the oxygen feed to the bomb. Valves, gauges, and gaskets must meet industry safety codes. Suitable reducing valves and adapters for 2 to 3.5 MPa (300 to 500 psig) discharge pressure and obtainable from commercial sources of compressed gas equipment. The pressure gauge shall be checked periodically for accuracy.
• During ignition of a sample, the operator shall not permit any portion of his body to extend over the calorimeter.

SAMPLING

RDF products are frequently non-homogeneous. For this reason significant care should be exercised to obtain a representative laboratory sample for the RDF lot to be characterized.

The sampling method for this procedure should be based on agreement between the involved parties.

The laboratory sample must be air-dried and particle size reduced to pass a 0.5mm screen as described in Practice 829.

STANDARDIZATION

Determine the energy equivalent of the calorimeter as the average of a series of ten individual runs, made over a period of not less than 3 days or more than 5 days. To be acceptable, the standard deviation of the series shall be 6.9kJ/C (6.5 BTU/C) or less. For this purpose, any individual run may be discarded only if there is evidence indicating incomplete combustion. If this limit is not met, repeat the entire series until a series is obtained with a standard deviation below the acceptable limit.

The weight of the pellets of benzoic acid in each series should be regulated to yield the same temperature rise as that obtained with the various samples tested in the individual laboratories. The usual range of weight is 0.9 to 1.3g. Make each determination in accordance with the procedure described in Section 11, and compute the corrected temperature rise. Determine the correction for HNO3, and firing wire.

Standardization tests should be repeated after changing any part of the calorimeter and occasionally as a check on both calorimeter an operating technique.