Influence of Condensed Water Temperature on Thermal Efficiency of Coal-fired Power Plant and Energy strategy

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1 Advanced Materials Research Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Influence of Condensed Water Temperature on Thermal Efficiency of Coal-fired Power Plant and Energy strategy Deliang Zeng,Shuwen Diao, Yong Hu, Shan Gao,Qing Li North China Electric Power University, Beijing Key Laboratory of New Technology and System on Measuring and Control for Industrial Process, Changping District, Beijing, China Keywords: Non-grid-connected wind power; Dynamic mathematical model; Condensed water temperature; Thermal economic analysis; Energy-saving Abstract. For the problem of the abandoned large-scale wind power currently, in order to improve the utilization of wind power and make full use of wind resources, a method of using wind power to heat the condensed water through electrothermal furnace is proposed. On the basis of mass and energy conservation, through analyzing variable condition of the regenerative heating system and water-cooled condenser, the dynamic mathematical model is created and then the dynamic influence of condensed water temperature on the economy of thermal power plant is studied. This paper, taking a typical 660MW water-cooled unit as an example, discusses the dynamic characteristics of steam turbine and energy-saving effects of this method. Results show that obvious economic effects can be obtained through non-grid-connected wind power heating the low-pressure condensed water, which can reduce the steam extraction flow from steam turbine, improve the thermal efficiency of thermal power plant and save the amount of coal. It has positive significance to digging energy-saving potential of plant. Introduction Energy-benefit scholars have favored renewable energy in power generation applications where energy savings have alleviated the energy crisis compared to traditional thermal power generation. Wind power as the renewable energy resources are inexhaustible and definitely clean, but because of the instability and discontinuity, wind power is constrained to be integrated into the grid. The contribution rate of wind power is hard to reach 8%-10%, which has become an internationally technical problem. Because of the peak shaving restraint, a large part of wind power is abandoned. The wind power, which was not integrated in to grid, is a tremendous waste. In recent years, many scholars have conducted extensive research on non-grid-connected wind power field. For example, Gu Weidong and others research on non-grid-connected wind power/water-electrolytic hydrogen production system, so that the utilization rate of wind power is increased and explore a diversified development way of large-scale wind power. Domestic and foreign scholars have invented the wind power energy storage technology [4, 9], the remaining unused energy can be stored, which can effectively solve the imbalanced problem between the wind power generation system and the grid load. In this paper, if the wind power is utilized for heating the low pressure condensed water through electrothermal furnace, which not only can avoid the waste of wind power resources, but also can save a portion of coal resources, reducing the environment pollution caused by coal combustion as well as improving the thermal efficiency of coal-fired power plant. On the basis of mass and energy conservation, combined with the operating characteristics and the actual variable condition of the water-cooled unit, the dynamic models of low pressure heater and water-cooled condenser are created. Taking a typical 660MW water-cooled unit as an example, based on the thermal economic state equation, the thermal characteristic of steam turbine is analyzed and influence of condensed water temperature on the steam extraction flow of steam turbine, condenser pressure and turbine power is studied. The new way of making use of large-scale wind power can break through the bottlenecks of wind power utilization. It has positive guiding significance in engineering practice. 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-13/05/16,00:38:51)

2 Advanced Materials Research Vols Dynamic characteristic analysis of regenerative heating system Regenerative heating system is an important part of the thermal system of the power plant which is shown is Fig. 1. Because the inlet temperature of heater rises, the performance of low pressure heater changes and the steam extracted from low pressure (LP) cylinder of steam turbine to heat the condensed water is reduced, saving the amount of steam extracted from steam turbine to work. Assuming that the main steam flow is constant, thus this method can gain extra power so as to improve the thermal efficiency of the unit. h zr reheater h zr h1 h h D zr 2 3 ' h hw1 hw 2 h w h 3 w 4 h 4 h5 h w 4 h h 6 h7 w5 w6 h h w7 wc t wc hd1 hd 2 hd 3 hd 5 hd 6 h d 7 NO.1 NO.2 NO.3 NO.4 NO.5 NO.6 NO.7 Fig.1.Regenerative heating system of thermal power plant According to mass and energy conservation, the dynamic model of heater can be written as: s M sc ps = Di ( hi - hs ) w2 Dwc cpf = Dh i i + Ddin hdin -Ddout hdout - Dwcc pf (t w2-t w1) l Lnk LNcpl = Ddinhdin + Dihs -Douthout d L n k L N = D i + D d in -D d o u t d t (1) (2) (3) (4) The Equation (1) is the heat released by the condensation of steam; Equation (2) is the energy conservation equation of feedwater; Equation (3) is the energy conservation equation of drain side; Equation (4) is the dynamic characteristic equation of water level. Where, Di is the column vector consisting of steam extraction flow of each stage; hi is the enthalpy of steam extraction; Dwc is the condensed water flow rate, tw 1 is the inlet temperature of heater; tw2 is the outlet temperature of heater; D is the higher level hydrophobic flow rate; D is hydrophobic flow dout rate ; hdin din and h dout are the hydrophobic enthalpy; L is the water level of heater; t n s is the saturated water temperature of heater; hs is the enthalpy of saturated water. The steam extraction flow of each stage is determined by the difference between saturated water pressure and extraction pressure as shown is equation (5). The saturated water pressure can be obtained by saturated water temperature of heater and extraction pressure can be obtained by curve fitting. D = f ( p - p ) (5) i i s While the condensed water temperature changes, the state parameters of final-stage heater change, and it must influence the performance of each low pressure heater. Therefore, the steam flow extracted from turbine to heat feedwater changes. When the condensed water flow rate is 1960t/h, the relationship between condensed water temperature and final stage steam extraction flow rate is shown as Fig. 2.With the increase of the condensed water temperature, it is required to reduce energy from final stage heater, so that the final stage steam extraction flow rate will slowly decrease.

3 820 Advanced Energy Technology Dynamic Characteristic analysis of condenser Turbine exhaust steam flow has always been a concern for condenser pressure. While steam extraction flow rate of the regenerative heating system has reduced, assuming the main steam flow constant, so that the low pressure cylinder of steam turbine exhaust steam flow will be: Dc = D0 -( Di [1] + Di [2] + Di [3] + Di [4] + Di [5] + Di [6] + Di [7] + Di [8] + DBFPT ) (6) Where: D is the steam flow extracted from the 4th stage for the boiler feed pump turbine (BFPT); 0 BFPT D is the main steam flow. Fig.2. Influence of condensed water temperature on final stage steam extraction flow rate Dynamic Characteristic analysis of condenser The heat transfer process occurs between the steam and the water in the water cooling condenser, according to the heat balance equation, we can get: k M scp = D ( h - c t ) C c p k (7) m2 Dwcp = ( hc - cptk ) Dc - Dwcp ( tm2 - tm 1) (8) Where, t k is the saturation vapor temperature corresponding to the condenser pressure; hc is the steam turbine enthalpy of exhaust steam; D is the circulating cooling water flow; c p is the specific heat capacity of condensed water, generally taken to be kj/ (kg ); The condenser pressure is generally determined by experience formula [8]. w p k 7.46 tk = Kpa ( ) (9) The condenser pressure of water-cooled unit can be approximated as the final stage turbine exhaust pressure through which the enthalpy of exhaust steam can be calculated. In this paper, the enthalpy of exhaust steam is calculated by using an approximate order estimation algorithm [6].The final stage steam extraction efficiency of the steam turbine is: η = ( h7 hc) / ( h7 h% c) (10) Type: h 7 the seventh stage enthalpy of steam extraction. h c the enthalpy of exhaust steam h % the final stage ideal enthalpy of exhaust steam c η the stage steam extraction efficiency, it is the function of pressure ratio.

4 Advanced Materials Research Vols Thus the enthalpy of exhaust steam is expressed as: hc = h7 η ( h7 h% c) (11) Assuming that the seventh stage steam extraction flow is the superheated steam, pressure and temperature expressed respectively as p7 an 7, and then s7 are obtained. s7 = f ( t7, p 7 ) (12) Meanwhile, due to expansion of the steam in steam turbine to do work is seen as the ideal adiabatic process, the final stage of steam turbine ideal entropy equals to the eighth stage entropy, combined with the pressure of the final stage of steam turbine, and then the final stage ideal enthalpy of exhaust steam is eventually obtained. h % = f ( s, p ) (13) c 7 k Where, s7is the seventh stage entropy; pk is the final stage turbine exhaust steam pressure. Steam extraction efficiency on variable conditions can be obtained by curve fitting. The relationship between condensed water temperature and condenser pressure is shown in Fig. 3.With the increasing of condensed water temperature, final stage steam extraction flow rate of low-pressure heater decreases gradually, and then the steam turbine exhaust steam flow increases. Under the circumstance where the circulating cooling water flow is constant, it would lead to saturated steam temperature of condenser increasing. At the same time, the condenser pressure increases gradually. Thermal Economic Analysis According to the affection of thermodynamic system state and system structure, the turbine power equation and heat absorption equation can be obtained. The turbine power equation is: 2 8 N = D ( h + σ - h ) - D ( h + σ - h ) - D ( h - h ) - D ( h - h ) (14) 0 0 c i i c i i c B F P T 4 c i = 1 i = 3 The heat absorption equation is: 2 D 0 ( 0 - fw ) D zrσ - Diσ i= 1 Q = h h + (15) σ = h - h zr 2 (16) Fig.3. Influence of condensed water temperature on condenser pressure Where, σ is the enthalpy rise of reheat steam; h i is the enthalpy of each steam extraction, hz reheat steam enthalpy.

5 822 Advanced Energy Technology The thermal efficiency increment is: N+ N N N η= - = Q Q Q (17) The wind power is converted to heat the condensed water, and the amount of fuel needed to heat the boiler feed water decreases. we can obtain: Q/ ( N* η ) = Q b e net.v.ar s Where, ηeis the generation efficiency; consumption. Application example Qnet.v.ar (18) is the low calorific value of coal; bs is the coal The following chart shows the algorithm to obtain the thermal economy of this new method. When condensed water make full use of wind power converted heat, resulting in condensed water enthalpy changing and then affecting the steam extraction flow rate of the regenerative heating system, so that the steam turbine exhaust steam flow change, which has an influence on condenser pressure. Eventually, the thermal economy improves. The calculation of this method is a process of iterative loop, which is shown in Fig.4. Assuming that using 35% of disposable air volume of the 100MW wind farm and condensed water flow is 1500t / h, therefore the condensed water can be from 32.5 Celsius up to 52.5 Celsius. The energy-saving efficiency of wind power heating condensed water is shown in Fig. 5and Fig.6.The steam amount of regenerative heating system extracted from steam turbine makes a reduction, in the main steam flow constant case, more steam in the turbine to do work, so that turbine work capacity increases about 1.5MW and thermal efficiency is up to 44.52%. The variation of coal consumption is about g/ (kw h) through saving coal.throughout the year by full load,the annual savings of standard coal are about 250 tons, per ton of coal charging 1000 Yuan, and then fuel cost saving is up to more than two million. twc'-twc < 0.01 Fig. 4.The calculation procedure chart

6 Advanced Materials Research Vols Conclusions If the wind farm directly discards the wind power which is not integrated into the grid, not only a lot of energy resources, but also the manufacturing cost of wind power generation is wasted. By using wind power to heat condensed water reduces the waste of wind energy resources, therefore the actual utilization of wind energy efficiency achieves to the maximization. Based on thermo-economic state equations, through calculation and analysis, this method can improve the economy of thermal power plant, compared with the original unit in the same conditions. The temperature of condensed water increases more, and economic benefit is acquired more remarkable. The combination of wind power and thermal power plant is an important research direction of wind-coal multi energies system, which can effectively economize the coal resources and reduce the greenhouse gas emissions of coal combustion to the air polluting the atmosphere. For the case study, it attaches positive significance to the thermal power plant energy-saving emission reduction. In short, the combination of wind power and thermal power units can solve the long-standing problems and improve the economic and environmental benefit, which will contribute to the development of green economy and low carbon economy in China. Fig.5. Relationship between condensed water temperature and turbine power Fig.6. Relationship between condensed water temperature and thermal efficiency Acknowledgements This work was supported by a grant from the The National Key Technology R&D Program(2011BAA04B03).

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8 Advanced Energy Technology / Influence of Condensed Water Temperature on Thermal Efficiency of Coal-Fired Power Plant and Energy Strategy /