Lecture 39: Furnace efficiency and carbon credit. Key words: furnace efficiency, carbon credit, carbon offset, fuel economy, energy conservation

Transcription

1 Lecture 39: Furnace efficiency and carbon credit Contents: Furnace efficiency Installation of preheater and efficiency Carbon offset Conservation of energy resources Key words: furnace efficiency, carbon credit, carbon offset, fuel economy, energy conservation Furnace efficiency Consider a fuel fired furnace used to heat the charge at the temperature T u. The furnace attains a steady state temperature at certain ratio of mass of fuel/mass of air. At steady state operation of the furnace, a certain amount of heat H in is generated at the adiabatic flame temperature by combustion of fuel. Heat is being carried out by the products of combustion (H POC ) at temperaturet P. The heated charge carries heat (H u ) at temperature T u. Some amount of heat H is lost at temperature T through the walls of the lining. The inputs and output of heat are shown in the figure Figure 39.1: Heat balance of a furnace at steady state Steady stale heat balance at the constant furnace temperature T and constant fuel input rate is H P H H (1) Other heat losses are due to opening of furnace door for charging and discharging of the products, cooling of water etc. which are included in H oth e r. It is important to note that at the constant furnace temperature, temperature of products of combustion leaving the furnace and the wall heat loss temperature are also fixed. Thermal efficiency of the furnace is defined as

2 η T E E 1 H P H (2) Efficiency of the furnace can also be measured by the effectiveness of the insulation. η F η F H H H 100. (3) H (4) η F 1 H (5) n F n T Furnace efficiency indicates the amount of fuel required during the unsteady state heating period of the furnace. During the unsteady state period a certain quantity of fuel is required to raise the temperature of the furnace to the temperature of the charge T u. Higher furnace efficiency as attained by better insulating refractory will require less amount of fuel during the unsteady period. Installation of preheater and efficiency We install a preheater to preheat air from the heat captured by incoming products of combustion as shown in the figure: Figure39.2: Installation of a preheater The efficiency of the preheater is defined as Preheating of air brings sensible heat into the furnace in addition to the heat of combustion. Two possibilities are considered to use the additional amount of sensible heat of air. In one, the fuel input rate is not changed which means H in is unchanged. Revised Heat balance of the furnace:

3 H POC H H POC H It must be noted that the furnace has to operate at temperature T which means that T POC and T l remains also unchanged. As a consequence, extra amount of charge is to be added to utilize the sensible heat in air. Now the heat output by the charge will be H such that H H H POC. η T H H POC η T H POC Thermal efficiency of the furnace increases in proportion to amount of sensible heat in air. In another, we keep the amount of load constant so that heat carried by the load is H u. Sensible heat in air will increase the temperature of the furnace, if the furnace is operated at the same fuel input (H i ) as was without preheating air. Since the temperature of the furnace should remain at T u, thus fuel input has to be reduced such that H H POC H is saving and can be expressed in percent of fuel in uel saving (%) = H POC F Carbon offset Carbon offset is a financial instrument aimed at reduction in greenhouse gas emissions. This concept is introduced to motivate and encourage the entrepreneurs to reduce carbon emissions from their furnaces. Carbon offset is measured in terms of metric ton of CO equivalent. One carbon offset = Reduction of 1 metric ton of CO or its equivalent in other greenhouse gases. One metric ton of CO reduction is equivalent to reduction of around 270 Kg Carbon. There are several ways to generate carbon offset; some of the ways are given below: I) Use of renewable source of energy: The basis of generating carbon offset is to reduce the carbon consumption. Any source of energy which is free from carbon, for example solar energy etc. will generate carbon offset. In this connection, use of hydrogen and to replace carbon to the extent possible is another way to reduce carbon emissions.

4 II) Fossil fuel reduction Most of the thermal energy in high temperature processes is derived directly from consumption of carbonaceous fuels. In processes where electric energy is used, there is indirect consumption of carbon (in thermal power plants, potential energy of the fossil fuel is converted to electric energy). It is to be noted that around 70% of the electric energy requirements are met through thermal power plants. In the above scenario, reduction in the fossil fuel consumption will lead to reduction in carbon emission. One of the ways to reduce the carbon content is to capture the sensible heat of products of combustion and reuse to preheat the air. This method has dual benefits; utilization of sensible heat of products of combustion and reduction in carbon consumption. The fuel saving resulting due to preheating of air is F S % H POC 100 If H POC 0.5 at 1240 & 0.3. Fuel saving =15 %. This fuel saving in terms of carbon offset corresponds to Carbon offset = % % C Conservation of energy resources: Natural energy resources are limited and every attempt must be made to conserve them. One of the ways could be to substitute the lower quality fuel. The quality of the fuel can be judged by its capability to generate flame temperature. Quality of the fuel is the relative term. The fuel which produces higher adiabatic flame temperature on complete combustion with stoichiometric amount of air is of higher quality than a fuel which produces lower flame temperature. The following table gives the adiabatic flame temperature for some fuels (the readers can calculate the flame temperature as illustrated in lectures 12 and 13 of this course. Also you may consult the video lectures of the fuel,furnace and refractory course) Type of fuel Adiabatic flame temperature (K) Fuel oil 2300 Natural gas 2275

5 Blast furnace gas 1667 Producer gas 1550 According to the above table it can be said that fue l oil is high quality fuel than producer gas. If it is possible to substitute the lower quality fuel to perform a high temperature operation, high quality fuel can be conserved. The question is to what extent this substitution is possible. In combustion of fuel, flame temperature of the products of combustion controls the furnace temperature. Whereas heat requirement of a process can be met by increasing the amount of fuel temperature requirement needs fuel of a particular quality for given combustion conditions. This is the basis of calculation the percent substitution of lower quality fuel. Consider a furnace originally operating with natural gas. We want to substitute it by blast furnace gas. The following procedure may be adopted: First calculate the flame temperature of the natural gas. At this flame temperature the furnace was operating. Make a mixture 90%natural gas and 10% blast furnace gas and calculate the composition of the mixture and its calorific value. Recalculate the flame temperature and analyze. If the flame temperature of the mixture is lower than that of natural gas, preheating of the fuel is required to attain the same flame temperature as that with natural gas. The following table illustrates the effect of preheating efficiency of substitution of blast furnace gas in a natural gas fired furnace BF gas (%) (AFT) BF + NG /AFT NG Preheating efficiency (%) In the table BF= blast furnace gas, NG = natural gas. (AF T) BF + NG/AFT NG = 1 means %BF is zero

6 The table indicates that a mixture of 40%BF+60%NG would have adiabatic flame temperature lower than natural gas. A preheater efficiency of 9% would make the adiabatic flame temperature of the mixture to that of natural gas. Ref: H A fine and G.H. Geiger: Hand book on material and energy balance in Metallurgical Processes