Download Furnace Heat Utilization: Extracting Energy from Fossil Fuels & Heat Exchanger Principles and more Study notes Materials Physics in PDF only on Docsity! Lecture 38: Industrial Furnaces Contents: What is a furnace? Source of energy Types of furnaces How thermal energy is obtained from fossil fuel? Variables affecting heat utilization Heat Utilization: Concepts Thermodynamic principles of capture and re‐use of heat of POC: Efficiency of heat exchangers Illustration Keywords: Heat recovery, heat utilization What is a furnace? A furnace is essentially a thermal enclosure and is employed to process raw materials at high temperatures both in solid state and liquid state. Several industries like iron and steel making, non ferrous metals production, glass making, manufacturing, ceramic processing, calcination in cement production etc. employ furnace. The principle objectives are a) To utilize heat efficiently so that losses are minimum, and b) To handle the different phases (solid, liquid or gaseous) moving at different velocities for different times and temperatures such that erosion and corrosion of the refractory are minimum. Source of energy a) Combustion of fossil fuels, that is solid, liquid and gaseous. b) Electric energy: Resistance heating, induction heating or arc heating. c) Chemical energy: Exothermic reactions Types of furnaces: Furnaces are both batch and continuous type. In the continuous type for example in heating of ferrous material for hot working, the furnace chamber consists of preheating, heating and soaking zones. The material enters through the preheating zone and exits the soaking zone for rolling. But the flow of products of combustion is in the reverse direction. Furnace design is recuperative type in that material exits at the desired temperature from the soaking zone and the products of combustion discharge the preheating zone at the lowest possible temperature. Different types of continuous furnaces are in use, like walking beam type, pusher type, roller hearth type, screw conveyor type etc. In the batch furnaces, the load is heated for the fixed time and then discharged from the furnace. There are different types of batch furnaces like box type, integral quench type, pit type and car. In many cases the furnace is equipped with either external heat recovery system or internal heat recovery system. In the external heat recovery system a heat exchanger like recuperator is installed outside the furnace. Here heat exchanger must be integrated with the furnace operation. In the internal heat recovery the products of combustion are recirculated in the furnace itself so that flame temperature is somewhat lowered. The objective is to reduce the NOx formation ottom type How thermal energy is obtained from fossil fuel? All fossil fuel contain potential energy. On combustion potential energy is released in the products of combustion. The products of combustion exchange energy with the sink to raise its temperature to the required value and then exit the system. The sensible heat in POC at the critical process temperature is not available to the furnace. The higher the process critical temperature higher would be the sensible heat in POC. This sensible heat in POC is very important from the point of view of fuel utilization. We define gross available heat (GAH) as GAH = Calorific value of fuel + sensible heat of reactants − Heat carried by POC 1) GAH represents the heat available at the critical process temperature; it may not represent heat available to perform a given function due to the various types of losses. GAH may be used as a criterion for comparing different fuel‐combustion system. limit the transfer of heat to the cold fluid. In such a situation for an adiabatic process .∆Ti = (dThi − dTci) will be non‐zero, but will have constant value when mh x CPh = mc x CPc . The practical result of the irreversibility is that the heat exchange is not complete and there is always some heat which is left with the POC on leaving the heat exchanger. Difference in heat capacities of fluid will influence the heat exchange process. For example if CPh > CPc, cold fluid can be heated nearly to the entering temperature of hot fluid provided mh = mc. Efficiency of heat exchangers Thermodynamically thermal resistance of the wall, heat leaving the exchanger with POC influences the thermal efficiency of the heat exchanger. Overall thermal efficiency S S f 8) According to the definition of overall thermal efficiency, it appears that the air can be preheated to the temperature above the flue gas temperature since no upper limit is assigned to the temperature of the preheated air temperature. Thermodynamically, in heat exchange between hot flue gas and cold air, air can not be preheated to the temperature above the flue gas temperature. Efficiency limit S f S f 9) Relative efficiency O f f 10) Relative efficiency S S f 11) Consider a heat exchanger which receives hot flue gas at 1600K and cold air at 298K. The hot flue gases leave the exchanger at 900K and cold air at 1373K. About 15% of the heat in flue gases is lost to the surroundings. The ratio of specific heat of flue gas to air is 1.2. Calculate the various efficiencies. Heat balance gives = 0.55 Overall thermal efficiency by equation 8 is 45.4% Efficiency limit by equation 9 is 55% and relative efficiency by equation 10 or 11 is 82.5% Illustration Regenerator receives hot flue gases at 140 and cold air at 25 , the flue gases leave at 750 and the air is preheated to 1100 . As estimated 15% of the heat given up by the flue gases is heat lost to the regenerator surroundings, and the rest 85% is recovered in the preheated air. It may be assumed for estimating purposes that for flue gases and for air, independent of temperature. Estimate over all thermal efficiency, efficiency limit, and relative efficiency for this heat exchange operation. 0 CP 0.3 CP 0.25 U B Suppose now that the depth of the regenerator is increased to 2.5 times in such a way to double the heat exchange area while keeping constant the over‐all heat transfer coefficient entering temperatures of the flue gases and air will be kept the same. Heat losses are same as that in a). Estimate for the enlarged regenerator (a) air preheat temperature, (b) over‐all thermal efficiency and relative Solution: Heat balance: reference temperature 25 P P 750 CP CP .The quantities and thermal efficiency a) m C 1100 25 0.85 m C 1400 0.514 Overall thermal efficiency 40.18%. Efficiency limit 51.4%. Relative efficiency 79.4%. b) Air preheat temperature and exit perature of flue gas are not known. Since quantities and ln T tem entering temperatures of flue gas and air are same. We can write T 2.5 ln T 25 12698 9.07T (1) Heat balance for the enlarged (2) In equation 2, T and T are air and flue gas te e exit of the regenerator. 15 1 1 and 3 We get T 1335.8 and T 557 . regenerator: m CP T 25 0.85 m CP 1400 T mperature at th Or 0.605 T .11 400 T (3) By solving Overall thermal ncy efficie 49 %. Relative efficiency 96 %.