Energy Analysis Using Matlab Graphical User Interface of Cement Rotary Kiln for Alburge Cement Plant: A Case Study

Cement manufacture has been one of the most energy intensive industries in the world. In order to produce clinker, rotary kilns are widely utilized in cement plants. This work deals with the energy analysis of a rotary kiln system working in a cement plant in Alburge Cement Plant (ACP), at Arab Union Cement Company (AUCC), Zliten city, Libya. The kiln has a capability of producing 4200 ton-clinker per day. The main objective of this study is to examine heat loss of various components of the rotary kiln system. In the present study, a Matlab program with a graphical user interface (GUI) has been used for energy auditing of ACP as a case study. The GUI inputs and outputs showed through the paper as an interface widows. Result shows that 30% of the total energy is being lost. An amount of 20.54% missed through hot flue gas, 5.25% across cooler stack, and 4.56% by kiln shell convection plus radiation.

The energy audit has arisen as one of the most useful ways for a successful energy managing method and it has received great attention among researchers working in this area [6]. For instance, Karellas and coworkers [7] have carried out energetic and exergetic analysis of waste heat recovery systems. Two different cycles have been investigated; an organic Rankin cycle (ORC) with isopentane as the working fluid and a water-steam cycle. The water steam cycle outperforms the ORC in terms of energy and exergy, with a system efficiency of 23.58 percent compared to 17.56 percent for the ORC. Morteza and colleagues [8] have performed an energy audit in the Momtazan cement plant in Kerman, Iran. They proposed a decision-making procedure to help the decision maker to choose the best strategy in order to reduce energy loss. Another energy auditing study conducted by Engin and Ari [4]. They indicated that with an input heat of 3686 kJ/kg of clinker (kJ/kg-cl) (95.47 %), the energy utilized for clinkering was 1795 kJ/kg-cl with an efficiency of 48.7 % with almost around 40 % of the total input energy being lost through hot flue gas (19.15 %), Cooler stack (5.61 %) and kiln shell (15.11 % convection plus radiation).

NOMENCLATURE
In this paper, energy audit of the ACP in Zliten, Libya performed. This analysis considered an important step in any cement plant before conducting heat recovery in order to improve overall efficiency. Since the Calculations to achieve energy, analysis would be time and effort consuming. Therefore, (GUI) in Matlab software was proposed to achieve these calculations in easy way for users. It will also reduce the error of calculation and the time of complex calculation. The other benefit of using GUI in energy auditing is that it can be used for other different cement plants by only changing the input data related to the studied plant.

PROCESS DESCRIPTION
The reference cement plant is Alburge Cement Plant (ACP), is one of the plants followed to (AUCC). The average daily production capacity is 4200 ton of clinker and the used fuel is heavy oil. The process based on the dry type kiln, which consists of a five-stage cyclone preheater, pre-calciner, rotary kiln and clinker cooler. First, the raw material entered to  process where homogenizing and grinding actions performed. The grounded material is stored, as raw meal, in storage silos. Second, the raw material came from the silos entered into cyclones (five stage preheaters) from the top. Flue gases came from the rotary kiln used to heat up the raw meal. After that, the raw material then transferred to rotary kiln. Rotary kiln is refractory lined tubes with a diameter up to 4.35 meter and 75 m long. It is inclined with an angle of 3-3.5°, and it rotated with speed of 1-2 rpm. Cyclone pre-heaters usually used to preheat the raw materials before it enters the kiln. In this process, the pre-calcination step gets started in the preheaters. At the end of preheaters, the temperature of the raw material would be around 850 °C. After that, the materials enters the rotary kiln and passes towards the flame. At the first zone of the kiln where the temperature around (700 -900 °C), calcination and initial combination of alumina, ferric oxide, silica, and lime takes place. In the second zone where the temperature is around 900 -1200°C, clinker and 2CaO.SiO 2 will be formed. At the final zone of the rotary kiln, 3CaO.SiO 2 will be formed at temperature of 1250 °C. The product out of the rotary kiln enters the clinker cooler where the air at 30°C used to cool down the product. During the cooling stage, where the molten phase of 3CaO.Al 2 O 3 formed, the cooling should be fast to improve the product quality. Figure 1 represent simple schematic diagram of cement manufacturing process.

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ENERGY AUDITING Thermal energy balance in cement plant is a practical method for determining the energy consumed during the process by various equipment operational activities, as well as the source of losses. Considering design parameters of the cement plants by keeping adequate safety factors of calculation and the physical properties of equipment can be found in Perry's handbook [10]. It covers mass and energy balances.

Mass Balance
The mass balance developed for the clinker produce and across the boundary of the kiln system, which involve the preheater, kiln and cooler. Figure 2 shows the main building block of development mass and energy balance (figure 2a) as well as the GUI input data required for mass balance (figure 2b). The input data, including composition of raw meal, ultimate analysis of the fuel, and dust content in the exhaust gas, loss of ignition (LOI) and exhaust gas computations in the suspension preheater employed for the mass balance computation. Based on the heavy oil configuration, the net heat value rate has been found to be 43000kJ/kg-fuel. It is typically more suitable to express mass/energy data per kg clinker produced yielded per unit time.

Figure 2. A graphical User Interface (GUI) screens: a) main menu; b) input data for material balance
The mass balance of the kiln system summarized in figure 3. All gas streams are assumed to be ideal gases at the given temperatures [4]. While the raw material and clinker compositions have been taken from the operation datasheet of (ACP) and listed in table 1 Inputs: kg/kg-clinker Outputs: kg/kg-clinker  The mass balance obtained based on the following assumptions [3]:  The raw material factor is equal to the amount of raw material which enter to the boundary kiln system per hour/clinker produced per hour.  Amount of the required fuel to produce 1 Kg of the clinker is equal to the ratio of specific energy to the heating value of fuel.  Amount of air required for cooling the clinker to 110 o C is equal to the air volumetric flow rate times density of the air divided by the clinker produced per hour.  The amount of dust exist with the clinker from the cooler are 0.6%.  The amount of hot air exit from the cooler is equal to the ratio of the product of the hot-air volumetric flow rate and the hot-air density at 220 o C to the clinker produced per hour.  Amount of dust exist with exhaust gas from the preheater are 4.5%.

Energy Balance
To achieve the energy balance in the cement plant, information about some parameters such as temperature, amount of fuel, fuel heating value, fuel heating capacity, and energy consumption of the utility equipment is required (Figure 4).  The kiln system considered for the energy auditing summarized in figure 5. All gas streams are assumed to be ideal gases at the provided temperature. The control volume for the system includes the pre-heaters group, rotary kiln and cooler. The streams coming to and from the control volume and all measurements indicated in the Figure 5. To analyze the kiln system thermodynamically, steady state working conditions assumed with neglecting the change in the ambient temperature and cold air leakage into the system. Also, According to collected data [11], an energy balance submitted to the kiln. All equations used for energy input and output calculations summarized in table 2. While equations regards calculations heat loss from various equipment such as rotary kiln, cyclones, cooler, air duct, and precalciner presented in table 3. Regarding the preheater group, there are five stages of cyclone difference in dimensions and outside temperature which are measured using infrared instruments. Table 4 shows the outside temperature and physical properties for the five cyclones.

RESULTS AND DISCUSSION 4.1 Input and Output Energy of the Kiln System
The total heat input to the kiln system given in table 5. These include the total heat of the coal, the sensible heat of coal, raw material, and air. It is important to notice that the reference enthalpy is assumed to be zero at 0 o C for the calculations [3]. As indicated in table 5, all the energy employed in the process is 3493.416 kJ/kg-clinker. The key heat source is heavy oil, providing an overall heat of 3319.6 kJ/kg-clinker (95%). As clearly shown in Table 6, there is a consistency between the input and output energy. Since most of the heat loss sources have been considered, there is only a 274 kJ/kg-clinker of energy difference from the input heat. It is about 8% of the total input energy and could be attributed to the assumptions and nature of data. The heat loss calculations of each individual component exhibited a good agreement with other published reports [12,13]. As depicted in table 6, there are a few major heat loss sources. Heat of formation of clinker was 1749.5 kJ/kg-clinker. It is in an acceptable industrial range where the major share of thermal energy in cement production is required for the endothermic chemical-mineralogical reactions forming cement clinker phases at temperatures up to 1450 °C with gas temperatures up to 2000 °C. It amounts to 1,590 to 1,840 kJ/kg-clinker, in the kiln dry type system. Heat of formation of clinker depending predominantly on the kiln feed chemical composition. The kiln feed considered as an indicator for soft or hard burning [14].

Table5. Total heat input of the kiln system
The energy output from the kiln exhaust gas was 717.713 kJ/kg-clinker, where the gases released are CO 2 , NOx, and SO 2 . Due to high carbon content in raw material, the most released gas is a carbon dioxide. The carbon dioxide emitted from calcination and combustion process increased by increasing calcium carbonate content. Increase in carbonate means increase in fuel consumption and results in increase in exhaust gases and consequently increase in heat losses [15].
Heat loss through the cooler stack was in an acceptable industrial range. It depends on the efficiency of cooling process and the efficiency of exhaust gas fan to drawing hot gases from cooler. In addition, the hot kiln shell outer surface is another significant heat loss source. The outer shell temperature depend on the coating formation inside the kiln and its optimum value should not exceed 350 ℃. On the other hand, the use of a secondary shell on the kiln surface can significantly reduce this heat loss. Clinker discharge was 85.8 kJ/kg-clinker. Clinker temperature at the discharge of the cooler should be as low as possible because high temperatures endanger the transport equipment and waste valuable heat. However, fast cooling of the product (clinker) enables heat recovery from the clinker and improves the product quality. The tertiary air is a duct covered by certain insulation, used for transfer hot air from cooler to precalciner. Heat loss from tertiary can be reduced by using material with high insulation quality. Heat losses due to dust in the kiln exist gas was 17 kJ/kg-clinker. It depends on the efficiency of kiln filters (electric precipitate), that separate dust from kiln exhaust gases and recycle the dust to the kiln as cement kiln dust (CKD).

Heat Losses from Preheater Group
There are five stages of cyclone difference in dimensions and outside temperature, which measured by using infrared instrument. The total heat losses from preheater group are 32.373 kJ/kg-clinker. Figure 6 represents the heat losses from the preheater group. The main source of the heat, in this section of the cement production process, is came from the hot flue gases that are passes through the cyclones. The flue gases were been generated by burning of fossil fuel in the kiln. Consequently, the heat lost by radiation and convection to the ambient surroundings. As shown in this figure, the heat losses from the cyclones is not uniform. However, it is varies from one cyclone to another through the preheater group. This variation in the heat losses is due to position of the cyclones with regarding to kiln where the heat come from. The closer to the kiln, the more heat losses to surrounding. The overall system efficiency can be defined by This is considered relatively low. Based on current dry process methodology, some kiln systems operating at full capacity would declared an efficiency of 55% [4]. By recovering some of the heat losses, the overall efficiency of the kiln system can be improved. The recovered heat energy can then be used for a variety of applications, including heating raw materials and generating electricity. The following are the primary heat loss sources that would be considered for heat recovery: (1) kiln exhaust gas (20.54%), (2) hot air from the cooler stack (5.25%), and (3) radiation from kiln surfaces (4.56%).

CONCLUSION
The GUI was used for energy analysis of ACP Rotary Kilns. After all data regarding the plant was collected and entered, GUI Matlab have been developed to help cement process engineers to calculate mass and energy balances. However, the distribution of the input heat energy to the system components exhibited good agreement between the total input and output energy and gave significant insights about the reasons for the low overall system efficiency. According to the results obtained, the system efficiency was around 50%. The total amount of heat input is 3493.416 kJ/kg-clinker, about 95% of this amount is obtained from fuel combustion (3319.6 kJ/kg-clinker). The major heat loss sources have been determined as kiln exhaust (20.54% of total input), cooler exhaust (5.25% of total input) and combined heat losses from raw material and fuel as well as radiative and convective heat transfer from kiln surfaces (9.68% of total input).The overall efficiency of the plant can be improved by reducing the heat losses. One way to reduce heat loss is by implementation of the annular duct type of the heat exchanger a secondary shell on the kiln surface. This will reduce fuel consumption and increase the overall system efficiency.