(1. Hefei University of Technology, Hefei, Anhui 230000; 2. Datang Huaibei Power Plant, Huaibei, Anhui 235000) 1. 2 multi-level analysis and evaluation
Abstract: The gray comprehensive evaluation analysis method is introduced into the comprehensive evaluation of flue gas heat exchanger of wet desulfurization system. The method and process of comprehensive evaluation and optimization of flue gas heat exchanger are discussed from two aspects of technology and economy. The actual calculation of the flue gas heat exchanger technology is given as an example, and the evaluation results are given.
Key words: wet flue gas desulfurization; flue gas heat exchanger; grey comprehensive evaluation
introduction
The flue gas heat exchanger is an important equipment in the wet flue gas desulfurization system. Its function is to reduce the temperature of the flue gas from the electrostatic precipitator and prevent the high temperature flue gas from entering the desulfurization tower, causing damage to the equipment and the anticorrosion layer; The flue gas in the desulfurization tower can be brought to a temperature that is favorable for SO2 absorption. The second is to increase the temperature of the flue gas after desulfurization to enhance the diffusion capacity of the pollutants in the flue gas, reduce the visibility of the flue gas and avoid corrosion of the equipment downstream of the desulfurization tower.
The flue gas heat exchanger in the wet flue gas desulfurization system has two forms of regenerative type and non-regenerative type, and the regenerative flue gas heat exchanger is currently used more. The regenerative heat exchanger achieves the purpose of heating and cooling the flue gas through the heat transfer between the original flue gas and the net flue gas. At present, many types of applications include rotary type, heat pipe type and water medium type.
Based on the grey system theory, this paper discusses the method of comprehensive evaluation and optimization of flue gas heat exchanger in wet flue gas desulfurization system by using grey correlation analysis method. The research results can be economically practical and technically feasible for coal-fired power plants. The flue gas heat exchanger type provides a reference.
1 Theory and steps of grey comprehensive evaluation method
The grey comprehensive evaluation method is a new analysis method developed on the basis of the grey system theory, and can perform single-level or multi-level analysis and evaluation as needed. That is to say, using the advantages of effective comprehensive qualitative judgment and quantitative calculation by the Analytic Hierarchy Process (AHP), the gray correlation degree is used as the measure to compare the advantages and disadvantages of each scheme, and the calculation method is simple, and the comprehensive evaluation error is small.
1. 1 Single-level analysis and evaluation
(1) Determine the evaluation indicators. The evaluation index is the attribute or performance of the evaluation object and is the basis for evaluation of the evaluation object.
(2) Determine the reference sequence. The optimal set of numbers is selected from the same index of each evaluation object, which is a reference for comparison of each evaluation object, and is recorded as {x0(k)}, (k = 1, 2, ..., n).
(3) Non-dimensional processing of data. A common method is to average the data. There are m flue gas desulfurization technical schemes to be evaluated, and there are n evaluation indicators, then:
Where x'i(k), xi(k) are the raw data and the dimensionless processed data of the kth index of the i-th technical solution, respectively. (4) Determine the evaluation matrix. Substituting the non-dimensionalized data into equation (2), the gray correlation coefficient of the kth indicator when the i-th evaluation index series is compared with the reference series can be obtained: (2)
Where Ï is the resolution coefficient, which is a number between 0 and 1, generally taking the value 0.5; i = 1, 2, ..., m; k = 1, 2, ..., n.
Finally, each technical solution is evaluated according to the principle of maximum relevance, that is, the best solution corresponding to the largest one of di.
When the evaluation targets have different hierarchical structures, it is necessary to carry out multi-level analysis and evaluation. The main idea is to use single-level comprehensive evaluation multiple times, and the second-level evaluation results form the first-level evaluation matrix. 4) Calculate the results of the first layer of evaluation.
2 Gray comprehensive evaluation of flue gas heat exchanger in wet flue gas desulfurization system
According to the actual situation in China, and with reference to the literature [10], this paper selects three kinds of flue gas heat exchangers for evaluation: S1—swirl heat exchanger; S2—heat pipe heat exchanger; S3—water medium exchange Heater. The evaluation indexes of the three types of flue gas heat exchangers are shown in Table 1.
2. 1 Determine the evaluation index system
This paper adopts a multi-level grey comprehensive evaluation method, which is evaluated in two levels. The first layer is technical performance and economic performance.
The technical performance evaluation mainly reflects the comprehensive performance of the flue gas heat exchanger technology, including the applicable unit, the degree of flue gas leakage, the maintenance frequency and the service life of the heat exchange components. The economic performance evaluation mainly includes the proportion of the flue gas heat exchanger to the total investment of desulfurization. , floor space, operating power consumption and flushing water consumption.
2. 2 Quantification of evaluation indicators
It can be seen from Table 1 that the evaluation index of flue gas desulfurization technology is both qualitative and quantitative (the quantitative indicators give more data ranges). In the evaluation of grey comprehensive technology, these evaluation indicators need to be quantified separately.
The quantification of qualitative indicators can be determined by expert review. The qualitative indicators are divided into five levels, which are excellent, good, medium, poor, and inferior, and the evaluation values ​​corresponding to the factor indicators are 0.9, 0.7, 0.5, 0.3, 0.1, respectively. When the factor indicator is between two levels, the rating takes the value between the two rating values.
For the quantification of the quantitative indicators giving the data range, the following methods can be used:
(1) When the data range is x ~ y, take ( x + y ) /2;
(2) When the data range is less than x, take x;
(3) When the data range is greater than y, take y.
According to the quantitative method of qualitative and quantitative indicators mentioned above, the values ​​of the evaluation indicators given are shown in Table 2.
According to reference [10], the weights are as follows:
2. 5 Evaluation results and analysis
(1) From the above calculation results, and evaluating the three types of flue gas heat exchangers according to the principle of maximum correlation, it can be known that the hydrothermal heat exchanger is the best only from the technical performance evaluation result D21; From the economic performance evaluation result D22, the heat pipe heat exchanger is the best. It can be seen that different results will be obtained from different aspects of evaluation, so it is necessary to comprehensively measure and comprehensively consider various factors when optimizing the flue gas heat exchanger scheme. It can be seen from the overall evaluation result D11 that the comprehensive evaluation values ​​of the three types of flue gas heat exchangers are in the order of heat pipe heat exchanger, rotary heat exchanger and water medium heat exchanger. This sequence is the order of heat exchanger type. Therefore, heat pipe flue gas heat exchangers should be the first choice.
(2) The literature [10] used the fuzzy comprehensive evaluation method to judge the flue gas heat exchanger in the wet flue gas desulfurization system. The analysis results in this paper are basically the same, but there are still small differences. The reasons for discrepancies may be related to the factor values ​​of the respective evaluation indicators and the differences in weight distribution. In summary, fuzzy comprehensive evaluation and grey correlation evaluation are good choices for evaluating and optimizing flue gas heat exchangers in wet flue gas desulfurization systems.
(3) The evaluation results of this paper are obtained on the basis of corresponding indicators and weights. When comprehensively evaluating the flue gas heat exchangers in the specific wet flue gas desulfurization system, these indicators should be determined according to the actual situation. The weight value is obtained to obtain an accurate evaluation result in a specific environment.
3 Conclusion
This paper aims to introduce a new research method for the comprehensive evaluation of flue gas heat exchanger in wet flue gas desulfurization system. The method is simple in principle, clear in thinking, simple in algorithm, objective and reliable in evaluation results, and has strong practicability. Although in the actual program decision, there are various influencing factors to choose the solution, but it is reasonable and scientific to use this method to make the decision.
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