The Comparison and Analysis of the System Utilizing the General Boiler Flue Gas Waste Heat

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1 Advanced Materials Research Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland The Comparison and Analysis of the System Utilizing the General Boiler Flue Gas Waste Heat Yanfeng Liu a, Pengcheng Wang b and Shaoshan Zhang c School of Energy and Power Engineering, North China Power University, Baoding , China a liuyf100@sina.com, b wpc515@126.com, c zssbbc@163.com Keywords: Waste heat utilization, Energy conservation, Cryogenic economizer. Abstract. Flue gas recycling system is an effective way of saving energy and improving efficiency for coal-fired power plant. In this paper, the general low-temperature economizer, heat pipe type low temperature economizer, composite phase change heat recovery system are introduced. Combined with a 600MW unit parameters, the economies of various waste heat recovery system are compared. Introduction With the rapid development of the power industry, higher energy utilization efficiency, less energy consumption and lower cost of production are becoming increasingly important. In thermal power unit, the efficiency of the boiler is the key factor that influences the economic operation of the power plant. Boiler heat loss during normal boiler operation is unavoidable. The smoke exhaust loss is one of the biggest loss of boiler. The exhaust gas temperature and flue gas volume are the factors that affect the smoke exhaust loss. For stable and continuous operation of the boiler, flue gas volume can be seen as a constant. Thus, the direct factor affecting the loss is the exhaust gas temperature. High exhaust temperature will make boiler exhaust loss increased, and result in a decline in boiler efficiency and an increase of power plant coal consumption. In addition, it will affect the operation of the plant budget [1]. System of flue gas waste heat recovery Low-temperature economizer. Low-temperature economizer principle of waste heat recovery system is shown in figure 1. The low-temperature economizer is arranged in the boiler tail flue. The condensed water from the cooler flows into the low-temperature after being heated in several low-temperature heater. When the flue gas flows through the rear flue, part of the heat transfer to the cold water in the economizer. Since the heat is absorbed, the boiler flue gas temperature will decrease and the boiler flue gas loss will reduce, then the efficiency of the boiler is improved. The outflows from the economizer will be transported to the thermal system. Low-temperature economizer acts as the role of a low pressure heater through removing the heat of boiler tail flue gas. Therefore, the original steam extraction of the low-temperature heater is reduced and the unit economy is improved. A new type of low-temperature economizer waste heat recovery system of the coal whose H is available to calculate by the equivalent enthalpy drop method: m 1 H = αd ( td tm 1) ηm + τ rηr (1) r= x Low-temperature heat pipe economizer. One of the advantages of Heat pipe type low-temperature economizer[2,3] is that the endothermic and exothermic process are separated. Since the endothermic and exothermic process isrelatively independent, the temperature can be controlled through changing the endothermic area or exothermic area. For example, while the inlet water temperatuer is low, the flue gas temperature will drop to a reasonable level by reducing the heat transfer area in the water side. So tail flue is protected in this way. Working principle of the separated type heat pipe type low pressure economizer is shown in Fig 2. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-10/05/16,16:33:15)

2 830 Progress in Applied Sciences, Engineering and Technology Fig. 1 Low-temperature economizer schematics Fig. 2 Heat pipe type low-temperature economizer schematics Heat pipe type low-pressure economizer flue gas side wall metal thermometer formula is: tg tw tτ = tg α g Sg 1+ β (2) α w Sw Heat pipe type low-temperature economizer has a high practical value. The value incarnates that this economizer can greatly ease the low temperature corrosion and the ash problem. What s more, the economizer can be provided with better utilization of waste heat recovery. Fig. 3 Composite phase change heat recovery system schematics Composite phase change heat recovery system. Composite phase change heat exchanger [4] is usually installed under the air preheater in the boiler tail flue. It can run simultaneously with low temperature economizer. Phase-change media absorbing the waste heat evaporates into vapor, then the medium flows into phase change heat drum. The heat of vapor is transferred to the condensate water coming from low pressure heater in the drum, then the vapor becomes liquid and return back to the composite phase change heat exchanger. Phase change heat drum and low-temperature heaters are in parallel state. According to the load of boiler changing, the temperature of composite phase change

3 Advanced Materials Research Vols heat exchanger can be controlled by adjusting the amount of condensed water to ensure that the temperature of exchanger wall will be above the flue gas acid dew point. The temperature of the wall is roughly in the same level, which is the superiority of this system. Composite phase change heat recovery system diagram is shown in Fig. 3. When the composite phase change heat exchanger was put into operation, boiler efficiency variation η : g ( ) ( 1 ηg ) ty t q py f ηg = K tk tk + t t 1+ q y k f (3) Economy calculation and analysis Above this paper, three different kinds of flue gas waste heat recycling system are introduced, each system has its own properties and technical advantages and disadvantages. According to a thermal power plant model for DG1900/25.4 Ⅱ type 1 unit, the specific technical parameters is shown in Table 1. Combined with the principle of equivalent enthalpy drop and energy-saving quantitative analysis theory, recycling economic benefits of above-mentioned flue gas waste heat recovery system will be calculated under the exhaust temperature 120 [5]. Table 1 Thermal System Parameters Name Numeric The unit load/mw 600 Intake air/t. h The unit heat rate/kj. kwh Boiler efficiency /% 93.6 Exhaust gas temperature/ 140 Economic benefits of system before and after adding waste heat recycling device are shown in Table 2. Table 2 Economic calculations Name Low-temperature economizer Heat pipe economizer Phasechange exchanger temperature/ Inlet water(air) Outlet water(air) Crowding extraction efficiency 0.26and Heat rate decrement/kj. kwh Save standard coal/g. kwh Boiler efficiency/% The results show that: Flowing through the ordinary low-temperature economizer, condensate water can obtain higher outlet water temperature. While, the heat loss rate and economizer amount of heat pipe type low-temperature economizer are lower than the average low temperature economizer with the limit of heating water temperature. For a given unit, the utilization of low temperature economizer can save standard coal 2.43g.kWh-1 and is better than other two systems. For different units and different operating conditions, the above-mentioned three systems have different performance and fuel economy. Thus, for different units and working conditions, suitable system should be utilized.

4 832 Progress in Applied Sciences, Engineering and Technology Conclusion These three kinds of flue gas waste heat recovery system have its own pros and cons. the average energy saving effect of low temperature economizer is better, but the equipment is large, expensive and complex. According to the design parameters, the temperature of the heat transfer tubes of heat pipe type low-temperature economizer can be set to completely avoid the flue gas acid dew point corrosion and possible corrosion temperature area. This system works in a safe and reliable way, but the equipment could be easily damaged. The minimum wall temperature of the rear heating surface can be adjusted in the composite phase change heat exchanger. But this system is also complex and has higher operation and maintenance costs. References [1] H. Xu, L.Y. Wu and C.F. Ma: Further Improve the High Temperature Air Combustion Waste Heat Recovery (Industrial Heating, China 2002), p.1-4. [2] W.P. Lu, Y.T. Shi and F.Z. Sun: Performance Analysis of Separate Type Heat Pipe Heat Exchanger with Low-pressure Economizer (Journal of Shandong University, China 2012), p ,(in Chinese) [3] Z.Y. Zhang, J.X. Song and P. Yu: Composite Phase Change Heat in A Circulating Fluidized Bed Boiler (Industrial Boiler, China 2001), p [4] C.M. Shi, L.X. Chen and B. Li: Inflatable Heat Pipe Air Preheater Design and Application (Industrial Boiler, China 2004), p [5] X.Y. Huang, F.Z. Sun and Y.T. Shi: Additional Thermal Power Plant System Low Pressure Economizer Energy Savings (Thermal Power Generation, China 2008) p , (In Chinese)

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