yuanshu_zhang@, phwang@
Abstract—This paper investigates on optimization problem of excess air ratio of coal-fired boiler by using the following three-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 purchase price of thermal power plants. When the pool purchase 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.
II. OPTIMIZATION OBJECTIVES AND OPTIMIZATION
MODELS
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 EAR increases the q2 and the power consumption of blowers and fans; however, too small EAR increases the heat loss due to unburned gas q3 and the heat loss due to unburned carbon in refuse q4. Therefore, the optimal EAR should be determined to make q2+q3+q4 minimum [7].
Keywords-excess air ratio optimization; variable condition;
I. INTRODUCTION
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.
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 ue the optimization of EAR, this paper proposes a novel 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)
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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. However, 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.
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
1 This study was supported by the key found from the National Natural Science Foundation of China (NO.51036002).
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.
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].