1、Energy auditing and recovery for dry type cement rotary kiln systemsA case studyAbstractCement production has been one of the most energy intensive industries in the world. In order to produce clinker, rotary kilns are widely used in cement plants. This paper deals with the energy audit analysis of
2、a dry type rotary kiln system working in a cement plant in Turkey. The kiln has a capacity of 600 ton-clinker per day. It was found that about 40% of the total input energy was being lost through hot flue gas (19.15%), cooler stack (5.61%) and kiln shell (15.11% convection plus radiation). Some poss
3、ible ways to recover the heat losses are also introduced and discussed. Findings showed that approximately 15.6% of the total input energy (4 MW) could be recovered.Keywords: Cement plant; Rotary kiln; Energy audit; Heat balance; Heat recovery1. IntroductionCement production is an energy intensive p
4、rocess, consuming about 4 GJ per ton of cement product. Theoretically, producing one ton of clinker requires a minimum 1.6 GJ heat 1. However, in fact, the average specific energy consumption is about 2.95 GJ per ton of cement produced for well-equipped advanced kilns, while in some countries, the c
5、onsumption exceeds 5 GJ/ton. For example, Chinese key plants produce clinker at an average energy consumption of 5.4 GJ/ton 2. The energy audit has emerged as one of the most effective procedures for a successful energy management program 3. The main aim of energy audits is to provide an accurate ac
6、count of energy consumption and energy use analysis of different components and to reveal the detailed information needed for determining the possible opportunities for energy conservation. Waste heat recovery from hot gases 4 and hot kiln surfaces 5 in a kiln system are known as potential ways to i
7、mprove overall kiln efficiency. However, it is still fairly difficult to find a detailed thermal analysis of rotary kiln systems in the open literature. This paper focuses on the energy audit of a horizontal rotary kiln system, which has been using in the Van Cement Plant in Turkey. A detailed therm
8、odynamic analysis of the kiln system is first given, and then, possible approaches of heat recovery from some major heat loss sources are discussed.2. Process description and data gatheringRotary kilns are refractory lined tubes with a diameter up to 6 m. They are generally inclined at an angle of 3
9、3.5o, and their rotational speeds lie within 12rpm. Cyclone type pre-heaters are widely used to pre-heat the raw material before it enters the kiln intake. In a typical dry rotary kiln system, pre-calcination gets started in the pre-heaters, and approximately one third of the raw material would be p
10、re-calcined at the end of pre-heating. The temperature of the pre-heated material would be of the order of 850 oC. The raw material passes through the rotary kiln towards the flame. In the calcination zone (700900 oC), calcinations, as well as an initial combination of alumina, ferric oxide and sili
11、ca with lime, take place. Between 900 and 1200 oC, the clinker component, 2CaO SiO2, forms. Then, the other component, 3CaOSiO2, forms in a subsequent zone in which the temperature rises to 1250 oC. During the cooling stage, the molten phase, 3CaOAl2O3, forms, and if the cooling is slow, alite may d
12、issolve back into the liquid phase and appear as secondary belite 6. Fast cooling of the product (clinker) enables heat recovery from the clinker and improves the product quality.The data taken from the Van Cement Plant have been collected over a long period of time when the first author was a proce
13、ss engineer in that plant. The plant uses a dry process with a series of cyclone type pre-heaters and an incline-kiln. The kiln is 3.60 m in diameter and 50 m long. The average daily production capacity is 600 ton of clinker, and the specific energy consumption has been estimated to be 3.68 GJ/ton-c
14、linker. A large number of measurements have been taken during 2yr, and averaged values are employed in this paper. The raw material and clinker compositions are given in Table 1. The rotary kiln system considered for the energy audit is schematically shown in Fig. 1. The control volume for the syste
15、m includes the pre-heaters group, rotary kiln and cooler. The streams to and from the control volume and all measurements are indicated in the same figure.3. Energy auditing and heat recovery3.1. Mass balanceThe average compositions for dried coal and pre-heater exhaust gas are shown in Fig. 2. Base
16、d on the coal composition, the net heat value has been found to be 30,600 kJ/kg-coal.It is usually more convenient to define mass/energy data per kg clinker produced per unit time.The mass balance of the kiln system is summarized in Fig. 3. All gas streams are assumed to be ideal gases at the given temperatures.3.2. Energy balanceIn order to analyze the kiln system thermodynamically, the following assumptions are made:1. Steady state w
