Fuel Analysis PDF

Fuels, Combustion, and Flue Gas Analysis • Chapter 3 OBJECTIVE 4 Explain proximate analysis, ultimate analysis, and heating value of a fuel, and describe the use of calonmetry to determine heating value. Explain higher and lower heating values. FUEL ANALYSIS Analyzing a fuel to determine its const...

Fuels, Combustion, and Flue Gas Analysis • Chapter 3 OBJECTIVE 4 Explain proximate analysis, ultimate analysis, and heating value of a fuel, and describe the use of calonmetry to determine heating value. Explain higher and lower heating values. FUEL ANALYSIS Analyzing a fuel to determine its constituents helps determine the fuels burning characteristics, the amount of air required for combustion, and the heating value of the fuel. Two standard methods of analysis are used, the proximate analysis and the ultimate analysis. Proximate Analysis This analysis is performed on a solid fuel, such as coal, to determine the percentages of moisture, volatile matter, fixed carbon, and ash. The results of this analysis indicate the behaviour of the fuel in the furnace and help determine the best method of firing the fuel. Proximate analysis has a three part procedure in which each part uses a separate sample of the fuel. All three samples have the same starting weight. 1. Part one: The first sample is dried for one hour in an oven at 110°C and then weighed again. The percentage of moisture in the original sample is the loss of mass divided by the original mass, and this result multiplied by 100%. 2. Part two: The second sample is heated to 950°C and held for seven minutes in a covered, oxygen-free container. This drives off all moisture and volatile matter, so the total loss of mass represents both moisture and volatile matter. The percentage of moisture plus volatile matter is calculated. Then the percentage of volatile matter in the original sample is obtained by subtracting the percentage of moisture that was determined in part one. 3. Part three: The third sample is heated to 760°C and held for two hours to achieve complete combustion. This stage removes all moisture, volatile matter, and carbon. The residue (ash content) is weighed and calculated as a percentage of the original sample weight. The percentage affixed carbon can now be calculated as the difference between 100% and the sum of the ash, volatile matter, and moisture percentages. An example of a proximate analysis is: Fuced carbon 57.43% Volatile matter 34.67% Moisture 2.71% Ash 5.19% 3rd Class Edition 3 • Part A2 127 ^ Chapter 3 • Fuels, Combustion, and Flue Gas Analysis Ultimate Analysis When a more detailed analysis is required for combustion calculations, the ultimate analysis is used. This analysis uses chemical methods to determine the elemental components of the fuel, including carbon, nitrogen, oxygen, hydrogen, and sulfur. An ultimate analysis is performed in a laboratory by a qualified chemist. An ultimate analysis of the same coal in the proximate analysis above may be: Carbon 79.71% Hydrogen 5.29% Sulfur 1.26% Oxygen 7.13% Nitrogen 1.42% Ash 5.19% Since the proximate and ultimate analyses are based on mass percentage, the ash content is the same for both. However, in the ultimate analysis, the carbon content includes the fixed carbon plus the carbon contained in the volatile material. Therefore, the carbon percentage is greater in the ultimate analysis. The ultimate analysis may be expressed in three ways: 1. As received or as fired: The constituents are listed as found in the raw fuel when received at site or as they exist before being fired in the furnace. In this case, the moisture content is included in the hydrogen and oxygen content. 2. Dry or moisture free: The moisture is first removed, and the constituents are then determined and listed as a percentage of the remaining fuel. 3. Moisture and ash free: The total moisture and ash content is determined; then the other constituents are determined and listed as a percentage of the remaining fuel. Heating or Calorific Value When a unit mass (one kilogram) of a fuel is burned completely, the heat produced is called the heating value (or calorific value) of the fuel. This value is expressed in units of: • kj/kg for solid and liquid fuels • kj/m3 for gaseous fuels. The volume of gas is at standard conditions of 15.5°C and 101.3 kPa. Two methods are used to determine the heating value of a fuel: 1. Calculation based on the ultimate analysis of the fuel. If the amounts of carbon, hydrogen, and sulfur in a fuel are known (from an ultimate analysis), the heating value of the fuel can then be calculated using Dulongs formula. This formula is covered in the next objective. 2. Calorimetry, as explained below. 128 3rd Class Edition 3 • Part A2 Fuels, Combustion, and Flue Gas Analysis • Chapter 3 7® CALORIMETRY Calorimetry is the science of measuring quantities of heat, as opposed to temperature. Quantities of heat are measured with instruments known as calorimeters. The standard instrument for measuring the calorific values of solid and liquid combustible samples is the oxygen bomb calorimeter, shown in Figure 4. Figure 4 - Bomb Calorimeter (Exterior) (MEDlAMAG/Shutterstock) The calorific value (heat of combustion) of a sample is the number of heat units liberated by the sample when burned with oxygen in a container of constant volume. The following conditions apply to the combustion reaction: • The sample and the oxygen are initially at the same temperature. • The products of combustion are cooled to within a few degrees of the initial temperature. • The water vapour formed by the combustion is condensed to the liquid state. The heat of combustion determined in a bomb calorimeter refers to the heat liberated by the combustion of all carbon and hydrogen with oxygen to form carbon dioxide and water. This also includes the heat liberated by the oxidation of other elements, such as sulfur, which may be present in the sample. Heat energy is measured with units such as the calorie (cal), British thermal unit (BTU), or joule (J). One calorie is the heat energy required to raise the temperature of one gram of water by one degree Celsius at 15°C. The British thermal unit is the heat energy required to raise one pound of water by one degree Fahrenheit at 60°F. The joule is an energy unit equivalent to the work done by a force of one newton (N) acting over one meter (m). Conversion factors: 1 cal = 4.1868 J, 1 BTU = 1055 J. On Track The term calorifi'c value may be used interchangeably with the term heating value. While the term originated from the measurement of heat in calorie units, it has become accepted terminology even when heat is indicated in kilojoules. 3rd Class Edition 3 • Part A2 0 129 ?& Chapter 3 • Fuels, Combustion, and Flue Gas Analysis Bomb Calorimeter As shown in Figure 5, four essential parts are required in any bomb calorimeter: 1. A sealed bomb or vessel in which the combustible charges can be burned. The bomb itself is a strong, thick-walled metal vessel which can be opened for inserting the sample, for removing the products of combustion, and for cleaning. Valves are provided for filling the bomb with oxygen under pressure and for releasing residual gases at the conclusion of a test. The ignition system is comprised of a fuse wire and electrodes to carry an ignition current. Since an internal pressure of up to 10 MPa can develop during combustion, most oxygen bombs are constructed to withstand pressures of at least 20 MPa. 2. A container or bucket for holding the bomb in a measured quantity of water. The calorimeter bucket must have enough capacity to hold the bomb completely submerged in water. The bucket is equipped with a probe to read temperature and a stirrer to ensure uniform temperature. In Figure 5, this is labelled as an insulated container. 3. An insulating jacket to protect the bucket from transient thermal stressors during the combustion process. The bomb and bucket are held in a calorimeter jacket to control any heat transfer between the bucket and the surroundings. The jacket minimizes the effects of drafts, radiant energy, and changes in room temperature during the test. In Figure 5, this is labelled as an insulated container. 4. A thermometer or other sensor for measuring temperature changes within the bucket. Precise temperature measurements are essential in bomb calorimetry. Mercury-in-glass thermometers, platinum resistance thermometers, quartz oscillators, and thermistor systems can all be used to successfully measure the temperature rise in the bucket. Figure 5 - Bomb Calorimeter (Interior) Ignition box Fuse wires Motorized stirrer rn n Sealed bomb Water Insulated container' ignition coil Thermometer Sample cup reader (VectorMine/Shutterstock) 130 3rd Class Edition 3 - Part A2 Fuels, Combustion, and Flue Gas Analysis • Chapter 3 On Track Sl units for energy and heat James PrescottJoule (1818-1889) was an English physicist, mathematician, and brewer noted for his discovery of the relationship between heat and mechanical work. The Sl ?S 0 unit for energy and heat/ called the joule (J), is named after him. Joule's experiment showed that work and heat are interchangeable and, in every case, a given amount of work generates the same amount of heat. The mechanical equivalent of heat is a constant given by the ratio of a unit of work to an equivalent unit of heat. By conducting a series of experiments, Joule found that when the falling weights lost 4.186 kJ of energy, the temperature of water increased by one degree Celsius. The value of J (the mechanical equivalent of heat) obtained by this experiment was 4.186 kJ/kcal. In Sl units, 1 J = 1 Nm (Newton-metre). This means that 1 Nm of work can be converted to 1 J of heat. Bomb Calorimeter Procedure Heat of combustion, as determined in an oxygen bomb calorimeter, is obtained by burning a fuel sample in a high-pressure oxygen atmosphere within a metal pressure vessel or "bomb." The heat of combustion is absorbed by the calorimeter and a temperatire change within the absorbing medium occurs. The following procedure is used to acquire the heating value: 1. Approximately one gram (carefuUy weighed to four decimal places) of the fuel is placed in a crucible inside the bomb. An ignition wire is placed just above the sample. 2. The bomb is closed and charged with oxygen to a pressure of 2000 to 2500 kPa. 3. The bomb is then placed in the bucket and a measured mass of water, generally two kilograms, is poured into the bucket. The calorimeter cover, containing the mixer and thermometer, is installed and the mixer is started. 4. When the temperatire has stabilized, after about five minutes, power is applied to the ignition wire and an explosive combustion occurs. 5. The heat produced by the combustion of the fuel is transferred to the water, causing an increase in temperature. The temperature increase is applied to the formula supplied with the instrument and the value calculated is the higher heating value (kj/kg) of the fuel. HIGHER vs. LOWER HEATING VALUE The heating value of a fuel, determined through calorimetry or by Dulongs formula, is called the higher heating value (HHV). This higher value includes the latent heat of the water vapour in the products of combustion. Therefore, the higher heating value represents the total heat energy released by complete combustion of a unit quantity of the fuel. However, in actual boiler operation, the water vapour in the combustion gases does not condense (i.e., lose its latent heat) before it leaves the boiler. Therefore, the latent heat of evaporation contained in this water vapour is lost up the stack and not utilized to produce boiler steam. This loss means that the portion of heat energy in the fuel that produced this latent heat is wasted. If the lost latent heat is subtracted from the higher heating value, it gives the lower heating value (LHV). The reduction in heating value (HHV minus LHV), in units ofkj/kg of fuel, is equal to the total mass of water vapour per kilogram of fuel (the moisture in the fuel plus the vapour formed by combustion of hydrogen in the fuel) multiplied by the latent heat of evaporation. Terminology The terms higher calorific value (HCV) and lower calorific value (LCV) can also be used to refer to the higher and lower heating values respectively. 3rd Class Edition 3 ' Part A2 0 131