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Theoretical and Numerical Analysis of Convective Recuperator for an Oil Fired Water Tube Boiler to Improve the Boiler Performance

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Theoretical and Numerical Analysis of Convective Recuperator for an Oil Fired Water Tube Boiler to Improve the Boiler Performance ISSN (Print) : 0974-6846 Indian Journal of Science and Technology, Vol 9(42), DOI: 10.17485/ijst/2016/v9i42/102725, November 2016 ISSN (Online) : 0974-5645 Theoretical and Numerical Analysis of Convective Recuperator for an Oil Fired Water Tube Boiler to Improve the Boiler Performance V. Narendran1, Seralathan Sivamani1*, V. Hariram1, M. Gnanaprakash1, A. Mohammed Raffiq2 and D. Sathish Kumar3 1Department of Mechanical Engineering, Hindustan Institute of Technology and Science, Padur - 603103, Tamil Nadu, India; [email protected], [email protected], [email protected], [email protected] 2Department of Automobile Engineering, Hindustan Institute of Technology and Science, Padur - 603103, Tamil Nadu, India; [email protected] 3Department of Aeronautical Engineering, Hindustan Institute of Technology and Science, Padur - 603103, Tamil Nadu, India; [email protected] Abstract Objectives: This paper deals with the theoretical and numerical analysis of a convective recuperator used to recover waste Methods/Statistical Analysis: heat from the stack flue gases and use it to pre heat the air before it is fed into the combustion chamber of an oil fired water tube boiler. Three different tube diameters are considered in the present study. Theoretical analysis is carried out to determine the efficiencyFindings: and other parameters of the boiler with and without recuperator. To support the theoretically obtained results on improved boiler performance by pre heating the air using recuperator,oC numericalo simulation studies is carried out. The study showed that 10” inch (0.254 m) pipe diameteroC with arranged a rise in in a 3 x 3 array gave a flow ovelocity of air at around 15 to 25 m/s. The stack flue gas exit temperature decreased from 200 to 135.50 C whereas the inlet air temperature after pre heating through the recuperator is found to be 85 temperature of around 50 C. After pre heating the air, the efficiency of the boiler improved from 71.90% to 74.60%. The quantity of furnace oil saved per day is 3.76% with respect to its present consumption level. CFD simulationsApplication/ are carried Improvements:out using ANSYS CFX. A good agreement is found between the data obtained from simulation and theoretical analysis. Payback period by incorporating the recuperator to the existing boiler is estimated to be 60 working days. Optimized performance improvement of existing boiler used in industries is possible without extensive redesign by recovering waste heat leading to energy conservation. Keywords: Air, Boiler, Convective Recuperator, Flue Gas, Performance, Pre Heat 1. Introduction Demand for fuel is expected to be tripled by the year 2025. The fossil fuel generating capacity has increased at Energy is the key input for economic development and a slower rate in comparison with the increasing demand. growth of all the countries in the world and it is the basic Among the fossil fuels, oil is the principle source of com- need for all the industrial facilities. Around 35% of total mercial energy. So, energy conservation is of greater energy consumed in the world is used in industrial sectors importance both in economic and environmental aspect. and in India it is, around 45%. In the present world sce- In industry, energy can be conserved by ensuring nario, consumption of fossil fuel is increasing rapidly and optimum utilization of energy. So, the equipments are also this leads to environmental pollution significantly. designed to operate efficiently. For existing machineries, *Author for correspondence Theoretical and Numerical Analysis of Convective Recuperator for an Oil Fired Water Tube Boiler to Improve the Boiler Performance efficiency improving modifications can be made without by 1% by raising the combustion air temperature to recourse to extensive plant redesign. Boilers find most around 20oC. application in various industries and it is widely used in various processing industries, textile industries and used 2.4 Excess Air in steam power plants as a power generation unit. The Excess air is the quantity of air available in a furnace recovery of heat from exhaust flue gases is one of the most which is more than the requirement for combustion to popular energy saving measures1. In boilers with an oil occur. For every 1% reduction in excess air, an improve- fired burner type, reduction in consumption of oil leads ment of 0.6% rise in efficiency is possible. The optimum to reduction in total cost of production. excess air level varies depending upon the furnace design, A boiler is a closed vessel in which steam is gener- fuel, burner type and other operating process variable. ated due to exchange of heat generated from combustion Based on the National Productivity Council report, an of fuel. In a typical water tube boiler, fuel is burned in excess air of 15 to 20% by weight is permissible for fuel oil. the furnace and hot gases generated by combustion flows over the water tubes and heat the circulating water in 2.5 Fuel Specification the tubes. The heated water converts itself into steam due to sensible and latent heat transfer and its stored in Different types of fuels like coal, pulverized coal, fuel the steam drum as saturated steam. This saturated steam oil and natural gas are used in the furnace or burner of is converted into superheated steam by exposing this a boiler and its performance depends upon the type of steam again to the flue gas path by using a super heater. fuel as well as its composition. Accordingly, the thermal To increase economy of the boiler, exhaust gases are also efficiency also varies. used to pre-heat the air blown into the furnace and warm the feed water supply. 2.6 Ambient Temperature Ambient air prevailing temperature also affects the per- 2. Factors Affecting the Boiler formance of a boiler. Boiler efficiency varies by 1% for o Performance every 40 C variations in ambient temperature. Energy saving does not always imply efficiency improve- ment in boiler. Therefore, it is necessary to understand the factors that influence the efficiency. 2.1 Stack Temperature Also known as flue gas temperature, the stack tempera- ture above 200oC indicates the potential for recovery of waste heat. By reducing the flue gas temperature by 22oC, the efficiency of the boiler improves by 1%. 2.2 Feed Water Preheating For an older shell boiler with a flue gas exit temperature of around 260oC, an economizer can be used to reduce the temperature of flue gas to 200oC with an overall improve- ment in thermal efficiency by 3%. One percent savings in fuel consumption is possible by raising the feed water 23 temperature by 6oC using an economizer. Figure 1. Fluid flow in a typical convective recuperator . 2.3 Combustion Air Preheating 2.7 Radiation and Convection Losses Combustion air is preheating is an alternative to feed External surfaces of the boiler shell are hotter than the water heating. The thermal efficiency of boiler increases surroundings. These surfaces thus lose heat to the sur- 2 Vol 9 (42) | November 2016 | www.indjst.org Indian Journal of Science and Technology V. Narendran, Seralathan Sivamani, V. Hariram, M. Gnanaprakash, A. Mohammed Raffiq and D. Sathish Kumar roundings depending on the surface area and difference those for a heat exchanger with helical baffles. The -per in temperature between surface and surroundings. In formance of heat exchangers with helical baffles based on general, irrespective of the boiler output, the heat loss test results of various baffles geometries was discussed in from the boiler shell is a fixed energy loss. This may be detail7. Heat transfer analysis of recuperative air pre heater around 1.5% on gross calorific value at full rating which was studied with gas entering the domain in tubes loca- may increase to 6% if the boiler operates at 25% of its out- tion with temperature around 200oC. The gas entered the put. With proper insulation, these losses through boiler tubes domain and passed over the heat exchanger tubes. walls and piping can be reduced. Baffles were hindering the flue gas path and changed the flow path for effective heat transfer. Baffles were designed 2.8 Reduction of Scaling and Soot Losses such a way that entire length of the tubes had to exchange the heat by conduction and convection process8,9. In oil fired boilers, soot built up on tubes acts as an insu- The importance of air pre heater in enhancing the lator against heat transfer. Elevated stack temperature is efficiency of thermal power plant using boiler was dis- an indication of excessive soot built up. When the flue cussed10. Also, the benefits in recovering the heat of flue gas temperature rises about 20oC above the temperature gas and the implications in lowering the temperature of is indicator for removing soot deposits. A built up of 3 flue gas and its associated problems were discussed which mm thick soot causes an increase in fuel consumption by was again emphasised by latter work11. The flow behav- 2.5%. iour within an air pre heater with focus on improving the overall heat transfer coefficient was studied numerically12. 3. Literature Review and Optimizing the air pre heater design using staggered as Objective of the Present Study well as inline tube arrangement was discussed13. The pres- ent status of using an air pre heater in extracting energy Various heating process involved in Turkish textile indus- from the flue gases in power generation was elaborated tries and steps taken to recover the waste heat using waste in brief14. Similar studies based on experimental as well heat recovery systems like recuperator was studied. The as numerical analysis were carried out earlier by several authors revealed that consumption of energy decreased researchers15–21. by using waste heat recovery systems.
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