Optimization design based on the thermal resistance analyses of heat recovery systems for rotary kilns
Qian Yin, Wen-Jing Du, Xing-Lin Ji, Lin Cheng uArr;
Institute of Thermal Science and Technology, Shandong University, Jinan 250061, Shandong Province, China
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Article history: Received 3 June 2016 Revised 18 August 2016 Accepted 23 August 2016 Available online 26 August 2016 Keywords: Energy conservation Rotary kiln Heat recovery system Thermal resistance Optimization |
Heat loss from the rotary kiln accounts for a certain amount of the total energy input in cement production. In order to reduce the heat loss, a heat recovery system with nine heat recovery exchangers is proposed in this paper. Experimental measurements are carried out to investigate the heat transfer characteristics of each heat recovery exchanger. Then, integration of thermal resistance and fluid flow performance analyses yields the mathematic relations connecting the heat transfer areas and the mass flow rates of heat recovery exchangers to the system requirements. Based on the relations, two optimization problems are formed to deduce the optimization equation groups by the conditional extremum principles. Solving the equation groups gives the optimal arrangement of the structural and operating parameters of the heat recovery exchangers. Finally, the optimization cases of a series-parallel system with practical heat recovery exchangers are studied. The results illustrate that the optimization method based on thermal resistance analyses can obtain the optimal area allocation and mass flow rate distribution of each heat recovery exchanger compared with the traditional method such as the control variate method. The total heat transfer area and power consumption are reduced by 10.8% and 12.1%, respectively. 2016 Elsevier Ltd. All rights reserved. |
1. Introduction
Cement production is one of the most energy-intensive sectors in industries. Some conventional plants consume about 3.6–3.8 GJ to
uArr; Corresponding author. produce one ton of cement [1–3]. The energy consumption occurs
E-mail address: cheng@sdu.edu.cn (L. Cheng). mainly in raw mills [4], grate clinker coolers [5], pyroprocessing
http://dx.doi.org/10.1016/j.applthermaleng.2016.08.148 1359-4311/ 2016 Elsevier Ltd. All rights reserved.
towers [6] and rotary kilns [7], where about 27% of the total energy input is lost by exhaust gas, cooler and radiation from kiln surfaces [5–7]. Therefore, to achieve the efficient use of energy has become one of the key issues in cement industry.
In order to reduce heat loss in cement plants, some theoretical analyses and practical applications are proposed. Khurana et al. [8] studied the energy balance of a cement plant and proposed a waste heat recovery steam generator system to utilize the exhaust gas. Engin and Ari [5] use the waste heat to preheat the raw material for energy conservation. Madlool et al. [9] summarized some energy saving measures from previous literatures, which include improving grinding media and high efficiency classifier. Engin and Ari [5] proposed an energy analysis method for a cement plant and found that the total energy input was lost mainly through kiln shell and hot flue gas. Although the energy analysis method was used to reduce heat loss, it did not reflect the degradation of energy in the cement production. The energy degradation was assessed through exergy analyses methods [10] for the trass mill [11], the raw mill [4,12] and the pyroprocessing system [6]. Some researchers [2,7] used energy and exergy analyses to investigate the energy consumption and exergy efficiency in practical cement plants, respectively. These studies indicate that the rotary kilns, pyroprocessing towers and raw mills have larger amount of energy and exergy losses than other components. Atmaca and Yumrutas [7] studied a cement plant in Turkey and found that the highest energy and exergy losses occur in the rotary kiln. Therefore, to improve the thermal performance of the rotary kiln could be a potential way for saving the energy consumption. The heat loss from the surface of a kiln accounts for a certain proportion of the total energy input. Atmaca and Yumrutas [13] calculated the heat loss from kiln shell which was 11.3% of the total energy consumption. Some other studies [5,14] also showed that this loss accounted for 5–15%. Therefore, heat recovery from the surfaces of kiln shell could be an effective way for saving the energy consumption.
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Nomenclature A C0, k0 cp D F, F0 f G g H h K k L l m Nu P Q R Re S T u v X |
heat transfer area (m2) undetermined coefficient constant pressure specific heat (J/(kgK)) pipe diameter (m) Lagrange function Darcy friction-factor equation group acceleration of gravity (m/s2) head loss (m) installation height of the pipelines (m) or the heat transfer coefficient of the tube-side or shell-side (W/(m2K)) minor loss coefficient The total heat transfer coefficient (W/(m2K)) characteristic length (m) pipeline length (m) mass flow rate (kg/s) Nusselt number power consumption (W) heat transfer rate (W) entransy dissipation-based thermal resistance (K/W) Reynolds number cross-sectional area of the pipelines (m2) temperature (C) cold fluid velocity of water tubes (m/s) fluid velocity in pipelines (m/s) variable matrix |
x distribution ratio Subscript
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