2011 Second International Conference on Intelligent System Design and Engineering Application2012 International Conference on Intelligent Systems Design and Engineering Application
Optimizing the Excess Air Ratio of Coal-Fired Boiler1
Zhang Yuanshua , Wang Peihonga ,Yin Jieb
a
Southeast University of Energy & Environment, Nanjing,Jiangsu, 210096, China
b
Nanjing Nari-Relays Electric Co., Ltd Abstract —This paper investigates on optimization problem of excess air ratio of coal-fired boiler by using the following th ree-step strategy. Firstly, the net efficiency of boiler (NEB) and the Cost of Boiler Heat Production (CBHP) are both taken as the objective functions, and the excess air ratio (EAR) is taken as the input variable. Secondly, the relationship between the objective functions and the EAR is established based on the model of variable working conditions. Finally, the implicit enumeration algorithm is employed to obtain the optimal EAR value that optimizing the objective functions, under the constraints that the EAR must be committed. Three cases of boiler load, such as 100%, 75% and 50%, are studied. The results show that NEB has the optimal value by using different EAR values. The cost of the boiler thermogenesis is affected not only by the EAR, but also by the coal price and the pool purch ase price of thermal power plants. When the pool purch ase price is fixed, the EAR has the decreasing trend corresponding to the lowest cost of the boiler thermogenesis in keeping with the economics laws. The calculation result was strongly by the result of the performance optimization test of the boiler.
Keywords-excess air ratio optimization; variable condition;
are both taken as the objective functions, and the excess (EAR) is taken as the input variable. Secondly, the relationship between the objective functions and the EAR is established based on the model of variable working conditions. Finally, the implicit enumeration algorithm is employed to obtain the optimal EAR value that optimizing the objective functions, under the constraints that the EAR must be committed.
II.
OPTIMIZATION OBJECTIVES AND OPTIMIZATION
MODELS
I.
I NTRODUCTION
Utility boiler is one of the most important equipment in the thermal power plant.Generally, there are two methods to enhance the boiler efficiency. One is to optimize the EAR and the other is to strengthen the heat transfer.This paper studies on the EAR optimization.
To obtain the optimal EAR value, one main way is oxygen optimization [1].There exists some literature on the oxygen optimization of utility boiler [2-4]. Currently,thermal test is a common method used to determine the best oxygen or EAR for guiding the operation. The advantage of such method is that the results are reliable and accurate.H owever, the disadvantage is that it costs a lot of time, money as well as human resources. Due to the reasons that the season or equipment performance changes,the effectiveness of the optimal solution may be weaken.
To solve the optimization of EAR,this paper proposes anovel method. The proposed method is based on the following three-step strategy. Firstly, the net efficiency of boiler (NEB) and the Cost of Boiler Heat Production (CBHP) This study was supported by the key found from the National Natural Science Foundation of China (NO.51036002).
1
Because the EAR directly affects the combustion process
and the flue gas loss q2, it is one of the most important factors affecting the boiler efficiency. In a certain load range,too much EARincreases the q2and the power consumption of blowersand fans;however, toosmall EAR increasesthe heat loss due to unburned gas q3 and the heatloss due to unburned carbon in refuse q4. Therefore, theoptimal EARshould be determined to make q2+q3+q4 minimum[7].
The optimal EAR also affected by the boiler load. Under a high load, the EAR could be reduced appropriately because of the high furnace temperature and the suitable combustion conditions; however, the EAR should be increased appropriately to raise the boiler efficiency for the bad combustion conditions under a low load condition.
Based on the above analysis, author selects the EAR at the exit of the furnace as the optimization variable.
A. The Choice of Optimization Objectives
As boiler thermal efficiency is a major comprehensive technical index that illustrate the economics of power plant boiler, the most important purpose of boiler unit optimization is to maintain the highest thermal efficiency of boiler under certain parameters and load; subsequently,when the boiler unit is running, boiler auxiliaries need to consume part of plant-power, such as :mill, fan, blower, etc. This branch will affect economy of boiler unit to some extent [8].Therefore, the optimization of boiler unit must maintain a minimum of auxiliary power consumption; lastly, coal price and the pool purchase price are also important factors to the cost of the boiler thermogenesis, so the optimization objective should contains they two [9].
Therefore, the NEB and the CBHP are chosen to be the optimization objectives.
1) The net efficiency of boiler Kj
The NEB is that converting the energy consumption of the equipment which includes the total power consumption of blowers and fans into standard coal consumption, and then converting into fuel calorific value, finally adding to the input heat for the calculation. If the boiler thermal efficiency is fixed, to raise the net rfficiency of boiler, the energy consumption of auxiliary equipment must be reduced [10]. According to the definition of the NEB, the expression of the NEB is:
(1)KQ y
Kj
b
d
CBH P is the fuel calorific value that consumed by
providing turbine unit effective use of heat and the cost of the power consumption of auxiliary equipment. It can be calculated as follow:
BC fuel W sy C power (7)
C b
BQ d y Kj
Where,
C b ——the cost of boiler heat production, Yuan/GJ;
C fuel ——coal price, Yuan/kg;
C power ——electricity price,Yuan/Kwh.
B. Objective function
As mentioned above, boiler performance can be assessed from the NEB and the CBHP. In order to meet the needs of different optimization, considering the above two evaluation indicators, the objective function is set as:
§1· (8) max MK 1 M f DccˈC ˈC
Where,
Dl cc——the excess air coefficient of furnace outlet;
¨
j
Q d y 29310¦W
B
u100
Where,
Kg ——the NEB;
b ——standard coal consumption, kg/(kW
B ——fuel consumption, kg/h
h) ;
;
¦W ——the actual power of auxiliary of boiler unit,which includes Mill, blower, induced draft fan, flue gas recirculation fans, forced circulation pump, slag and ash handling system, electrostatic precipitator, etc.kw. Todivide ¦W into two parts, one is the power consumption of blower and fans, W sy ; the other is the total power consumption of all other equipment ,W 0. And ¦W can be written as:
¦W W sy W 0 (2)Put (2) into (1),the NEB can also be written as:
Kg (3)K
j
¸
C b ¹
l fuel power
M——weight coefficient,0dMd1. When M=0, means only assess the CBHP; when M=1, means only assess the
NEB.
III.
O PTIMIZING M ODEL AND A LGORITHM
Based on the above analysis, the optimization model of excess air ratio for SG-2093/17.5-M910 boiler can be established as:
§1·max ¨MKj c 1 M ¸ f Dl ccˈC fuel ˈC power
C b ¹
s.t. 1.1dDl ccd1.3(100%load ǃ75%load )
1.2dDl ccd1.4
jw
D zr 0t/h
Where,
[sy ——the ratio of the fuel calorific value of thepower consumption ofblower and fansand the heat input of the boiler:
29310bW sy (4)[
sy
1 [sy [0
(50%load )
jw
0t/hdD gr d215t/h
BQ d y
-l cc 1170ć
t h 580ćt h 500ć
§the import and export bundle ·
¨¸of suerheaters and reheaters¹ other heating surfaces
[0——the ratio of the fuel calorific value of the power
consumption of all other equipment and the heat input of the boiler:
29310bW 0 (5)
[0
BQ
y d
300ćdt rk d400ćt py !t sld 101.4ć
In the boiler operation, the changes of the EAR mainly affect the blowers and fans power consumption. In order to simplify the calculation and better describe the effect that caused by the changes of the blowers and fans’ power consumption to the NEB, the[0 is omitted.Formula (3) can be simplifiedas:
K (6) K g
j
1sy
2) The cost of boiler heat productionC b
As it is a single parameter and single objective
optimization problem, author uses the implicit enumeration optimize whose Algorithm is simple and stable.Generally, the range for the power plant boiler EARis limited. Combined with boiler heat calculations,the difficulty of calculatingcould bereduced with a reasonable step size. The block diagram of enumeration method forsolving optimization model of boilerperformance is shown in figure 1.
2) The analysis of CBHP optimal solution
Besides NEB, the coal prices and electricity priceare also main factors to the cost of heat productionof a boiler.In this paper, the electricityprice is 0.42 Yuan/kwh, and two different coal prices are used in thecalculation to show the influencefrom the prices.
Table 3. The CBHP of 50%load, 100%load, 100%load
item
11.10
21.1226.20113.29926.14213.251
31.1426.20313.30326.10913.237
41.1626.22613.31726.09813.234
51.1826.24713.33126.09713.237
61.2026.29913.36026.09913.24026.09313.246
71.2226.37513.40226.11013.24826.09213.245
81.2426.46013.44926.12913.26126.09113.250
91.2626.51213.47826.15613.27726.09213.253
101.2826.55413.50226.18913.29726.09613.258
111.3026.58313.52026.23113.32126.11113.268
Dl cc
100%load
C fuel =0.6yuan/kgC b 1
26.20413.29726.16813.261
C fuel =0.3yuan/kgC fuel =0.6yuan/kg
75%load
C fuel =0.3yuan/kgC fuel =0.6yuan/kg
fuel
According to table 3, when the pool purchase price is 0.42yuan/kw, the EAR values for the lowest CBHP should H be:
50%load
C
=0.3yuan/kg
C b 2
C b 1C b 2C b 1C b 2
(a) When the coal price is 0.6yuan/kg, the lowest CB Ps for 100%load, 75%load, 50%load are: 26.201yuan/GJ, 26.097yuan/GJ and 26.091yuan/GJ, and the EAR values are 1.12, 1.18 and 1.24.
(b) When the coal price is 0.3yuan/kg, the lowest CB Ps for 100%load, 75%load, 50%load are: R EFERENCES 13.297yuan/GJ, 13.234yuan/GJ and 13.245yuan/GJ, and the [1]Chen chunyuan, “Optimum design for large capacity power boiler
and method of improving boiler efficiency,” Power System EAR values are 1.10, 1.16 and 1.22.
Engineering, 1993(02).7-14.(in Chinese)
3) Comparison of optimization results
[2]Yan shunlin,Zhang bin, Wu huanying,Wu qinyuan, “Study on
To compare the three optimization results of the EAR, optimum excess aircoefficient for power plant boilers,” Power as shown:
Equipment, 2010(04).237-240.(in Chinese)
1.301.281.261.241.22
(2)The optimization results shows that the optimum
EAR is one-way trend changes(rise) with the unit load changes(decrease).
(3)The optimum EAR forthe purely economic
optimization objective is largerthan the one whoseobjective includes NEB and CBHP.When the coal price is lower, the optimization results tend to lower values.
[3]
E A R
1.201.181.161.141.121.10
load
Figure 3. The three optimization results of the EAR
As we can see from figure 9,when the electricity price is fixed, the EAR has the decreasing trend corresponding to the lowest cost of the boiler thermogenesis in keeping with the economics laws.
V.
CONCLUSION
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(1)It established a optimization model whose
objectives function are the NEB and the CBHP.