This is to confirm that the thesis entitled “Design of Waste Heat Recovery System in A Sponge Iron Plant” submitted by Saurav Kumar Sahu as an academic project in the Department of Chemical Engineering, National Institute of Technology, Rourkela, is a report of bona fide work carried out by him under my guidance and supervision. In the present work, a waste heat recovery system is designed to integrate the heat of the waste gas into the sponge iron process. The aim of this project is to design a suitable heat recovery system for the above cases that can efficiently remove and utilize the sensible heat.
INTRODUCTION
Thus, the energy saving in sponge iron plants has also sought the attention of many investigators. To identify the possible areas of sponge iron process where energy is available and can be utilized in the process. To design suitable waste heat recovery systems for sponge iron process using the possible areas.
LITERATURE REVIEW
DRI PROCESS OF SPONGE IRON MAKING
- Introduction
- Theory
- Commercial Processes
- Advantages of DRI process
Iron ore, coal and dolomite are cut to size and sieved to the right size. The raw materials are loaded into the rotary kiln using a conveyor belt. Iron and CO2 are formed gradually. The rotary kiln is held at an angle, causing the materials to flow under the influence of gravity. The unique feature is that 55-60% coal is injected from the discharge end and remaining with ore injection.
WASTE HEAT RECOVERY
- Waste Heat
- Low temperature heat recovery
- Benefits of Waste Heat Recovery
- Development of a Waste Heat Recovery System
- Heat Exchangers
- Recuperators
- Waste Heat Boilers
- Factors affecting the waste heat recovery feasibility
When considering the potential of heat recovery, it is useful to consider all options and classify the waste heat according to its potential value. Heat losses when providing chilled water or when discharging .. chilled water. a) High rate if it can be used to reduce cooling demand. High-temperature heat sources range from 500 to 1500 C. Some high-temperature sources and their temperatures are shown in Table 2.4 [18].
A number of toxic combustible wastes such as carbon monoxide gas, acid gas, etc., which are released into the atmosphere if/when burned in the incinerators, serve a dual purpose, i.e. recover heat and reduce environmental pollution levels. This leads to reduction in equipment sizes of all flue gas handling equipment such as fans, stacks, ducts, burners, etc. Waste heat recovery methods include the transfer of heat between gases and/or liquids (e.g. combustion air preheat and boiler feed water preheat), the transfer of heat to the charge entering furnaces (e.g. batch/spot preheat in glass furnaces), the generation of mechanical and /or electric power, or using waste heat with a heat pump for heating or cooling.
Recuperators recover residual heat from exhaust gases in applications with medium to high temperatures, such as soaking or annealing furnaces, melting furnaces, afterburners, gas incinerators, radiant tube burners and reheating furnaces [14]. Waste heat boilers are available in various capacities, allowing gas inlets from 1000 to 1 million cu.ft/min. In cases where the waste heat is not sufficient to produce the desired steam levels, auxiliary burners or an afterburner can be added to achieve more steam.
Evaluating the feasibility of waste heat recovery requires the characterization of the waste heat source and the stream to which the heat will be transferred.
UTILISATION OF WASTE HEAT OF FLUE GASES IN SPONGE IRON PLANT
These parameters allow analysis of the quality and quantity of the flow and also provide insight into possible materials/design limitations [18], [(Cook, 1979)]. The preheated raw material is then fed to the rotary kiln for further heating and reduction process to produce sponge iron and thus reduce coal consumption for the same production quantity. Without the project activity, an equivalent amount of coal would have been consumed in the main rotary kiln to raise the temperature of the raw material mixture to C.
The project activity therefore helps in reducing coal consumption per tonne of sponge iron produced in the sponge iron furnaces, and thus leads to greenhouse gas (GHG) reduction.
CONVENTIONAL METHODS OF HEAT RECOVERY FROM STACK GASES
- Feed Water Preheating
- Organic Rankine Cycle
- Direct Contact Heat Exchanger
22 economizer, or 200 C increase in combustion air temperature through an air preheater, there is a 1% saving of fuel in the boiler. The Organic Rankine Cycle (ORC) is named for its use of a high molecular mass organic liquid with a liquid-vapor phase change, or boiling point, that occurs at a lower temperature than the water-vapor phase change. The fluid enables Rankine cycle heat recovery from lower temperature sources such as biomass combustion, industrial waste heat, geothermal heat, solar ponds etc.
The working principle of the organic Rankine cycle is the same as that of the Rankine cycle: the working fluid is pumped into a boiler where it is vaporized, passed through a turbine and finally recondensed. In the ideal cycle, the expansion is isentropic and the evaporation and condensation processes are isobaric. The cost of the heat exchange surface is a major cost factor when the temperature changes are not large.
Each plate is separated from the next by gaskets and the hot current passes in parallel through alternate plates while the. Hot fluid passing through a lower port in the head is allowed to pass upward between every other plate, while cold fluid at the top of the head is allowed to pass downward between the odd plates. When the directions of hot and cold fluids are opposite, the arrangement is described as countercurrent.
The direct contact of the sprayed water with the flue gases turns out to be quite an efficient low-energy washer.
PROBLEM STATEMENT
CASE STUDY
- Feed Stream
- Dust Settling Chamber
- Waste Heat Boiler
- Electrostatic Precipitator
- Process description
The flue gas temperature at the boiler inlet and outlet is shown in table 3.4. The flue gas temperature at the inlet and outlet of the electrofilter is shown in table 3.5. The flowchart of the production process of sponge iron with material and energy is shown in Figure 3.1.
Screened iron ore (6 tph), coal (3.3 tph) and dolomite (0.2 tph) are loaded into the rotary kiln using a conveyor. Compressed air (18 tph) is injected into the rotary kiln at various points with the help of blowers. This reacts with CO2 to produce CO, which acts as a reducing agent for the iron ore.
The rotary kiln is held at an angle from - causing the material to flow under gravity. After being discharged, the material enters a rotary cooler where water is sprayed over the cooler and the temperature is brought down to about 1 C. The iron particles are separated from carbon and other non-magnetic impurities by electromagnetic separation.
The gases from the rotary kiln are burned in a chamber, making the presence of CO negligible.
AREAS OF INTEREST FOR HEAT INTEGRATION
- Rotary Cooler
- Stack Gases
ENERGY CONSERVATION MEASURES
POTENTIAL AREAS FOR HEAT INTEGRATION
- Air-Preheater
- Flue Gas-Water heat exchanger
- Gas-solid heat exchanger
- Return Duct
- Cost Estimation
The purpose of the duct is to carry the flue gases from the ESP to the Rotary K iln & Cooler. In general, mild steel can be taken as the material for the construction of the channel. As the duct diameter increases, the fixed costs (material costs) increase, but the pressure drop along the duct decreases and vice versa.
Details are shown in Table A.1. 3) Film coefficients for flue gas (hi) & air (ho) were calculated &. Air temperature was assumed to be constant at 45 C. 6) Log mean temperature difference, heat loss, area and diameter were calculated for each temperature drop using the weight average Ui. 32 9) The actual heat transfer coefficient was calculated for this diameter and then the corresponding heat loss and outlet temperature were calculated.
Details are shown in Table A.5. 10) Glass insulation was chosen and calculated thickness was applied to limit the temperature drop in the duct to 1 C. Details are shown in Table A.6. The purpose of this heat exchanger is to cool the flue gas to a temperature of 60 C so that it can be used in the rotary cooler to cool the hot products. Since the heat transfer coefficient on the gas side is usually smaller, a fin-type Shell&tube heat exchanger was designed.
The return duct will lead the flue gases from the Gas-Solid heat exchanger to the chimney.
RESULTS & DISCUSSION
DESIGNED EQUIPMENT
- Duct
Annual Fixed Cost
Annual fixed costs (duct and compressor depreciation) were plotted against Diameter and the diameter corresponding to the least cost was selected. Glass wool was chosen as the insulation material because it is suitable for the temperature range of 100-400 C. The thickness was taken to be 15.56 mm to limit the temperature drop to 1 degree.
The cost of insulation is so low compared to the cost of coal that the savings add up much faster even as the cost of insulation increases.
Savings
COST ESTIMATION
- Capital Cost Estimation
- Operating Cost Estimation
The duct costs were calculated by calculating the weight of the required material and multiplying by the cost per weight unit. Insulation volume, density and weight were calculated and the insulation price was determined by multiplying by price per weight unit. The costs of air preheater and flue gas cooler were determined by comparing with plots of area versus cost [10], [11].
The solid-gas heat exchanger cost was found by adding the material cost and refractory cost. The new coal consumption was found by subtracting the annual coal savings (equivalent to heat duty from Air Preheater) from current coal consumption. The water cost is calculated using the water consumption data in the Flue Gas Cooler.
For the existing system, the cost of the rotary cooler and the cost of refractory material are added, and the capital costs are shown in Table 5.9. The existing operating costs are based on the current consumption of coal, water and the cost of pumping water. The pump power was taken from practical installation data and the operating costs were calculated.
CONCLUSIONS
BIBLIOGRAPHY
APPENDIX A DESIGN OF DUCT
51 The value of diameter was calculated for each pressure drop and shown in table A.3. For each diameter, pressure drop, FD Fan requirement and Net expenses were calculated and shown in Table A.4.
Net Expenses
54 The thickness of the insulation was calculated for different temperature drops and the values are shown in table A.6. For each insulation thickness, net costs and savings were calculated and the values are presented in Table A.7.
APPENDIX B
DESIGN OF AIR PREHEATER
APPENDIX C
DESIGN OF FLUE-GAS COOLER
APPENDIX D
DESIGN OF SOLID-GAS HEAT EXCHANGER
62 The equivalent conductance of the wall and the refractory was calculated and the values are shown in Table D.3. 63 Using a range of diameters, an average U-coefficient was calculated and the values are shown in Table D.4. Keeping the length to diameter ratio as 40/3, the diameter and length were calculated and the values are shown in Table D.5.
