Module 5: Combustion Technology. Lecture 34: Calculation of calorific value of fuels

Transcription

1 1 P age Module 5: Combustion Technology Lecture 34: Calculation of calorific value of fuels

2 2 P age Keywords : Gross calorific value, Net calorific value, enthalpy change, bomb calorimeter 5.3 Calculation of calorific value of fuels The heating value or calorific value of a combustible material is an important property, which may be used to evaluate its effectiveness for using as a fuel and also for the design of chemical equipments where it is to be used. The calorific value may be defined as the quantity of heat liberated by the complete burning of a unit mass of the fuel with oxygen at constant volume process. In case of gaseous fuel, the heat released during the complete combustion of one cubic meter of gas at N.T.P (normal temperature and pressure) i.e, 1 atm pressure at 0 C is the measure of calorific value. Whereas, the calorific value for solid fuel is measured per gram or per kg of solid fuel. In general, the calorific value of a solid or liquid fuel is the gross calorific, which is determined at constant volume for a liquid fuel and for gaseous fuels at constant pressure. If the water formed and liberated during combustion is in the liquid phase, then the corresponding calorific value is called gross calorific value. The net calorific value corresponds to the process when the water formed during combustion remains as steam. The calorific value of fuel depends on the type of exothermic reaction and the heat of reaction. Heat of combustion is measured from the heat of reaction of the reaction. It is determined from the value of enthalpy change for the reaction at constant pressure and temperature. At constant pressure system, the enthalpy change is obtained from the equation. (1)

3 3 P age Therefore, the enthalpy change for the reaction may be determined from the internal energy and number of mole changes in the reaction. The internal energy change with the change of temperature is given as or, (2) From the knowledge of thermodynamics, and, -, (3) = (4) The heat of reactions may be determined from Eqn. (2) and (4), where the subscripts 1 and 2 are designated as reactants and products. and are the average temperature of reactants and products respectively. and are the heat capacities at constant pressure and constant volume respectively. The use of average heat capacities in the above equation is a well approximation. The heat of reaction may be determined from the heat capacities of all reactants and products. The heat capacity is usually the temperature dependent extensive property in thermodynamics. It may be expressed as The values of the constants,,,.etc are available in the literature. The calorific value of solid or liquid fuel may be measured by the bomb calorimeter.

4 4 P age From the summation of all heat of reactions of the possible reactions in the burning process of a fuel may give an idea of the heating value or the calorific value of the fuel. This procedure is easily applicable for the gaseous fuels. For gaseous fuels, such as, natural gas, LPG and producer gas, the heat of combustion is sufficient to be used as an approximate calorific value for natural gas. The composition of the fuel gas should be known. Then the values for the heat of reaction for oxidation of each constituent to CO 2 and H 2 O at 25 0 C are to be determined. The heat of reaction also can be determined from the standard heat of formation data of products and reactants. The summation of all these heat of reactions are made to obtain the heat of combustion in kcal or kj per mole of the gas and further it may be converted in per unit mass or volume. The values of the standard heat of formation ( H f ) of reactants and products are available in the literature. The water formed during combustion may be either in liquid phase and vapor state, an addition amount of heat is required to vaporize the water present in the fuel. Then the heat generated by combustion, known as the gross calorific value if water is in liquid state after condensing the vapor. Otherwise, the water will be in the vapor state. Then the heating value is called the net calorific value. This is called the net calorific value is obtained by subtracting the latent heat of vaporization ( H v ) from the gross calorific value. Example: Calculating the heating value of Methane It may be assumed the methane is burnt in pure oxygen and does not contain any water vapor. The reaction stoichiometry is CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) The H comb of methane at 298K is the heat of reaction between CH 4 and O 2 to form CO 2 (g) and H 2 O(g), The heat of formation data are as follows:

5 5 P age H f of CO 2 (g)= kj/mol, H f of H 2 O(g) = kj/mol and H f of CH 4 (g) = 74.8 kj/mol Then the heat of combustion of methane is calculated from the equation,, kj/mol So, the heat of combustion of methane at 298 K is kj/mol assuming water formed is in vapor phase, this is same as the net calorific value of methane. If water is in the liquid phase, then the heat H of H O liquid kj/mol kj/mol The heating value or heat of combustion is kj/mol, which 86.8 kj/mol more than the value obtained for water in vapor phase. The heat of vaporization of water = 86.8/2=43.1 kj/mol. This heating value is identical to gross calorific value. The calorific value for gaseous fuel may be experimentally determined using Junker s gas calorimeter. The calorimeter consists of a combustion cylinder surrounded by a water jacket and fuel burner is kept below the combustion cylinder. The flow of cooling water may be adjusted by a control valve. The temperature of the gas exhaust, cooling water inlet and outlet temperatures are measured. The burner is set in such a way so that a complete combustion of the gaseous fuel is occurred. The flow rate of water is then measured. Temperature of the exhaust gas is brought down to the ambient temperature by the flow of cooling water. Water vapour

6 6 P age contained in the flue gas is condensed. The heat released by the combustion process is used to heat up the gases inside the combustion chamber (i.e. air and fuel). Then the gases are cooled by the cooling water and the outlet water temperature is increased. If the flue gas is cooled down to ambient temperature, then the heat of the hot gas is completely transferred to the cooling water. Assuming the continuous water flow rate, a steady state heat balance may be written as:.. If the heat loss from the calorimeter body to surrounding is negligible for the temperature of wall of the instrument is same as the ambient temperature. So, 0.,. /. Where, and are the mass flow rate of fuel and water respectively. and are outlet and inlet temperature of water respectively. is the specific heat of water. If water is condensed and collected from the gas outlet for a specified time interval, then the net calorific value is,, where, = mass of water condensed, and = heat of condensation of water vapor. The calorific value of solid or liquid fuel may be experimentally determined in a bomb calorimeter. The sketch of a bomb calorimeter is given in Fig.1 of Lecture-6 in Module-1. The total quantity of heat generated by combustion including the heat needed to vaporize the water is obtained, which is called gross calorific value. These measurements are obtained by burning a representative sample in a high pressure oxygen atmosphere within a stainless steel pressure

7 7 P age vessel or bomb. The heat released by this combustion is absorbed by water within the calorimeter and the resulting temperature change of water is noted. The heat absorbed by the water in the calorimeter, Where, = water equivalent of the calorimeter, = mass of water in the calorimeter, = specific heat of water, = mass of fuel, = initial temperature of water and = final temperature of the water.

8 8 P age Reference 1. Fuels and Combustion, S. Sarkar, 3 rd Edition, University Press, India, Physical Chemistry, P. C. Rakshit, 6 th Edition, Sarat Book Distributers, India, Chemical Process principles, Part-I, Materials and Energy Balances, O. A. Hougen, K. M. Watson and R. A. Ragatz, 1 st Edn, (Reprint), Asia Publishing House, Calcutta, 1976.