Recoverable Quantity of Waste Heat from Kiln and Preheater Systems and Economic Analysis (Case of Messebo Cement Factory)

This project entitled “Recoverable quantity of waste heat at Messebo cement factor y” has tried to quantify the amount of heat loss, the amount of energy and cost saved from the waste heat. Here both primary and secondary data collection methods were included to carry out the study. So, the projects present starts by identifying the main source of waste (which part of cement have high loss), following calculated heat lost from the identified places or machines and final calculating the possible money saved if the waste heat changed to use full form or if it recovered Successful recovery waste heat contributes to lower fuel cost, lower electricity consumption. Kiln surface zones, Preheater cyclone 4 and 5 are the main areas in which high waste was occur. From Kiln surface zones (959.13 kJ), Preheater cyclone 4 and 5(587.199 kJ) amount of heat is lost. From this lost we can recover around 62 kW power is recovered, 44155kwhr/month energy and 26492birr/month could be saved. It recommended the energy management department should invite and support others participation and to study on the heat recovery from the loosing of energy and to study on the alternative energy sources of the company.


INTRODUCTION
There are more than 10 cements factories in Ethiopia.Mesebo cement factory is among the largest and former plants which provides cement for the African largest dam which is known as great Ethiopian renaissance dam which has the capacity to produce 6050 mega Watt. In the country majority of cement factories their main source of power is electricity from of utility grid and imported fossil fuels like coal. Thus, they consume a lot of powers to perform their daily activity. As stated in [1 and 2] cement factories are among sector's which consumes high energy especially the clinker calcination process.
Due to the increase of human population and their demand of electricity increases parallelly. So, peoples are search an alternative option to meet their energy demand. Thus, they do either by introducing renewable source of energy, by hybridization different energy mixes or by optimization and increasing the efficiency of energy materials and equipment's. Besides these methods peoples are also started recovering the waste heat to generate power for different purposes. A model example of countries which are successfully produced power from waste of cement plants as indicated in [1] are India, China and South-east Asian countries.
As written in [3] the first heat recovery system was established in Japan in 1980 by Kawasaki Heavy Industry KHI at Sumitomo Osaka Cement. Then after, a key project with 15 MW capacity has been released in Kumagaya plant (Taiheiyo Cement). After almost two decades later as stated in [4] China was plant installed its first system in 1998 in partnership with a Japanese manufacturer. So, after many obstacles and modification we have reached in technology of waste heat recovery system to generate power from the flue gases. As described [5] some new generation of heat recovery installations in cement kilns producing up to 45 kWh per ton of clinker currently worldwide.
Messebo cement factory utilize a large quantity of fuel and electricity that ultimately produce heat for a process and generates large amounts of exhaust heat during these manufacturing processes, as much as of the energy consumed is ultimately lost via waste heat, that simply passes out through the gas tubes (chimneys) into the atmosphere or into the surrounding without any recovery.

waste heat
A lot of authors and researches define in different ways .for examples as defined in [3] Waste heat is heat generated in a process by way of fuel combustion or chemical reaction and then goes into the environment without using it .Reference [4] also define waste heat as the extra heat that escapes from the system .Reference such as [7] also says Waste heat is the energy associated with waste streams of air, exhaust gases, and/or liquids that leave the boundaries of an industrial facility and enter the environment. The amount of waste heat (Q) can be calculated by using equation (1) [6] Journal of Energy Technologies and Policy www.iiste.org ISSN 2224-3232 (Paper) ISSN 2225-0573 (Online) Vol.9, No.9, 2019 30 Q= C. m. ΔT……………………………………………………………………………………. [1] Where m= the mass of the heat carrying medium, C=the heat capacity of the medium and ΔT= the temperature difference between the waste heat and ambient temperature

Heat recovery
As defined in [3] Heat recovery is a method of reducing the overall energy consumption of your site and therefore reducing the running costs. [ 8] Waste heat to power (WHP) is the process of capturing heat discarded by an existing industrial process and using that heat to generate power (see Figure 1) [8] Figure 1: Waste Heat to Power Diagram [8] Waste heat recovery has the following advantages [3 and 9] • Reduces purchased power consumption (or reduces reliance on captive power plants), which in turn reduces operating costs.
• Mitigates the impact of future electric price increases • Enhances plant power reliability • Improves plant competitive position in the market • Lowers plant specific energy consumption, reducing greenhouse gas emissions (based on credit for reduced

Data collection
In order to conduct this study, methods and procedures have great contribution for reaching the final result and of the paper. The methods used are discussed below in detail.

Primary Data I. Direct measurement and observation
For this particular work the data were collected through measurements, formal and informal interview of company experts, machine manuals and direct observation. The temperatures of hot gases and air that loss to the atmosphere, heat loss by radiation were measured using the infrared thermometer,and direct observation from control class room (CCR) .

.Interview
For those conditions where the required information is not available written form or not directly measured (difficult to measure) making an interview is one part of data collection method.

Secondary data
Other sources are also used as complementary data source agents. Among these sources' internet and reference books, different literature survey and documents of the company were used.

Result and discussion
3.1 Existing system 3.

Use of hot gases in existing system
In Messebo Building Materials Production Plc the exhausted gases from Rotary kilns, pre-heater and Calciners are used to heat the incoming feed material and gases then exhausted to the atmosphere. The exhaust gas temperature is averagely around 325℃.Part of this gas is used in raw mills & coal mills for drying purpose. The solid material (i.e. Clinker) coming out of the Rotary kiln is at around 1300-1650 °C and is cooled to 100-120 °C using ambient air. This generates hot air of about 280-300 °C which simply is exhausted to the atmosphere.

.2 Numerical calculation and thermodynamic analysis
In this portion we are calculating the amount of energy (heat) lost from 1kg of clinker the system and heat inputs to system required to 1kg of clinker .Besides the above figure 5 information we are also consider different data's on each stage of calculation which recorded from CCR and we have used different thermo dynamics laws and principles to analyze the collected data such as Ideal gas laws, first law of thermodynamics thus we take the following thermodynamic assumptions .  Our reference is 1kg  Steady state working conditions.  The change in the ambient temperature is neglected.  Cold air leakage (false air) into the system is negligible i.e. no false air enters to the system and exit from the system  Consider the mass flow rate of air through tertiary air duct is negligible  the system is control volume or open system  The coal used is south Africa coal which has calorific value(heating value) of 4000-6500kcal/kg  Figure 6: control volume of pyro processing system The general formula for heat output is given in equation (1) above is .

Sensible heat in kiln feed
Where mkf= mass of kiln feed ,Ckf =specific heat of kiln feed With kiln feed of typical lime saturation and calcium carbonate content the kiln feed to clinker factor would be expected to be around 1.54 due to loss of CO2 from the CaCO3. That factor is increased by the dust losses from the preheater to the raw mill and dust filters. If the factor is 1.75 then you must have high dust losses from the preheater so improving collection efficiency will reduce the factor. The true raw meal to clinker factor is given by: 1/(1 -Loss on ignition of kiln feed) the given percentage loose of material chemical lab. There is 34%-36% lost. 2. Sensible heat due to cooling air (Q2)=mco Cpair (Ta-Tr)……………………..…………….……….. (3) Where mco= mass of cooling air, kg/kg clinker 3.

Cooling air fans Cooler fans:
Instead of cooling the air itself, fans circulate air inside an ambient space. Circulation of air speeds up the evaporation of sweat on our body, hence giving a cool feeling. So, where a fan is only circulating the air, an air cooler is actually providing cool air for relief from hot weather As stated in [10] Heat Capacities of different Gas mixtures(Cp) is the average of the heat capacities of the components: given by the equation below C ig Pmixture =YAC ig PA+ YBC ig PB + YCC ig PC + YDC ig PD …………………………………...………… (4) Where, y is mole fraction or molar fraction (yi) is defined as the amount of a constituent (expressed in moles), ni, divided by the total amount of all constituents in a mixture (also expressed in moles),ntot [11] Yi=∑ ni = ntot; ∑ Yi …………………………………………………………….………. …. (5) Therefore Cmixture=(m1/mmixture) Cp1+(m2/mixture) Cp2+(m3/m/mixture) Cp3………...…………... (6) So let us calculate and Lets assume the gases will be modeled as ideal gases with constant specific heats. The molar masses of N2, O2, H2O, and CO2are 28.0, 32.0, 18.0, and 44.0 kg/kmol respectively The constant-pressure specific heats of these gases at room temperature are 1.039, 0.918, 1.8723, and 0.846 kJ/kg.K, respectively. The air properties at room temperature are cp= 1.005 kJ/kg.K , cv= 0.718 kJ/kg.K, and k =1.4 which is the ratio of Cp and Cv 1. Sensible heat in kiln feed( Q1): Q1 = mkf  CPkf  (Tkf -Tr) ℃ mkf = Kiln feed rate/ Kiln output rate clinker With kiln feed of typical lime saturation and calcium carbonate content the kiln feed to clinker factor would be expected to be around 1.54 due to loss of CO2 from the CaCO3. That factor is increased by the dust losses from the preheater to the raw mill and dust filters. If the factor is 1.75 then you must have high dust losses from the preheater so improving collection efficiency will reduce the factor. The true raw meal to clinker factor is given by: . But the total clinker out let from the burner includes Ash from coal. But the percentage of Ash is 2.43%-3.01% of the total coal feed see the table 2 below .   Cooling air fans Cooler fans Cooling air fans are centrifugal fans used to cool the clinker by sucking atmospheric fresh air before crushed by the clinker crusher. As the atmospheric air is meet with the clinker heat exchange takes places. Then the atmospheric air becomes hot air. Part of this hot gas is suck to the kiln burner by the ID fan. But they remain is goes through the cooler Chimney by pulling of EP fan. See the below data that the system has nine fans and data was recorded from CCR computers of the cooler funs volume flow rate and pressure when the system runs. 36 But mass flow rate of a given system is given by the ration of volumetric flow rate to specific volume. As stated in equation (7) below. mco= ∑ ῡ F G H ………………………………………………………………………………………. (7) but from ideal gas law PV = nRT where P is the pressure of the gas, V is the volume of the gas, n is the amount of substance of gas (also known as number of moles), T is the temperature of the gas and R is the ideal, or universal, gas constant. From this the specific volume of a substance is the ratio of the substance's volume to its mass. It is the reciprocal of density specific volume ( v) = v/m = RT/p…………………………………………………………….…… (8) p v = RT thus v = RT/P here T = 20 ℃ which is the reference temperature and the universal gas constant (R) = 287 N.m/kg.k P is the average of the 9 cooler funs: p = 495.9/9 = 55.1mbar = 5510pa = 5510N/m 2 So v = RT/P =   before directly goes to calculation see the following necessary date's or points  The coal temperature is increasing from atmospheric temperature to 75℃ during drying by absorbing heat from hot gas  The temperature of the hot gas used for coal mill is maximum-minimum(220-210℃) and the outlet after drying is from(90-80)℃ The temperature of the hot gas used for raw mill is the same as the temperature outlet from pre heater (mostly 325℃) and the out let after drying the raw material is mostly 110℃.Having these points in mind let us calculate Heat out put 6. Heat formation clinker (∆`R): Heat of formation of clinker: This is the heat to convert the raw material to clinker. This is termed the theoretical heat of formation of the raw meal, from first principles by using heat of reaction data. A more rapid estimation of this heat can be done by using a formula developed by ZurStrassen The exhaust gases through the preheater cyclone are O2, CO and NOx let as calculate their density on their dry bases because the moisture from kiln feed and coal in preheater is very small. Average of O2 is 26.36 and total average of CO is 0.62257 and the amount of NOx is 2000-3000ppm which is averagely around 0.0025.so the totally is 26.98  Q9 = 1.6875 kcal/min of clinker 11. Heat loss due to the radiation from the preheater cyclones and kiln surface: Radiation loss = σ ×(Ts 4 -Tr 4 ) × surface area  Radiation loss from pre heater cyclones surface considered as cylindrical in geometry. So formula for finding the surface area of a cylinder is, with h as height, r as radius, and S as surface area is S=2(π)rh+2(π)r That is Surface area = 2πr 2 + 2πrh Qc5 =120020kcal So the total Radiation loss from pre heater cyclones surface = Qc4 + Qc5 = 140276.95 kcal = 587.199kJ  Radiation loss from kiln system: the kiln found in the company is 57m in length and 3.7 m in diameter and is divided in to 3 main zones. Kiln is cylindrical in geometry. So formula for finding the surface area of a cylinder is, with h as height, r as radius, and S as surface area is S=2(π)rh+2(π)r That is Surface area = 2πr 2 + 2πrh Journal of Energy Technologies and Policy www.iiste.org ISSN 2224-3232 (Paper) ISSN 2225-0573 (Online) Vol.9, No.9, 2019 39 Figure 5: kiln of the company As shown in the above figure the kin system have cylindrical shape.so we calculating the formula which is used to calculate surface area of cylinder's as shown in the table below So the heat loss due to radiation from the kiln surface calculated as follow using equation ( So the total heat loss from the kiln surface by radiation = Qiz + Qtz + Qhbz =229143.28kcal = 959.13MJ Then total amount of heat output = ∆Hr + Q6 + Q7 + Q8 + Q9 + Radiation loss from cyclone4 and 5+radion loss from kiln surface There for the total amount of heat output = 612.48kcal/clinker+1.079kcal/min clinker+1103267802kj/min+ 19.3kcal/kgclinker+1.6875kcal/min clinker+587.199kJ +959.13kJ = 2649.267kJ Let us calculate the power gained from the heat loss for one month period of time. Power is the rate of using or supplying energy and is given by the following formula: Power (p) = ' !=x …………………………………………………………………………… (11) Where: Power is measured in watts (W) : Energy is measured in joules (J) : Time is measured in minute N.B 1kcal=4186J 3.1.5 summary of powers which is gained from heat loss and the amount birr saved P1 (from exhaust dust) = Q6/1 month (hr) = 1.079kcal/kg min clinker/43200min = 0.1045w P2 (from pre heater exit gases) = Q7/1month (min) =1103267802kj/min/43200min = 25538w

CONCLUSION
The cement sector is one of the most energy highly uses industries. The clinker Calcination process is the most energy consuming in cement production, because of the exit gases from the clinker cooler and pre-heater and from the kiln surface.
The aim of this study was to determine the amount of heat loss energy and to identify the main source of the heat losses, and to set possible solutions The amount of heat loss and major heat losses for the system were identified as the preheater exhaust gases and heat carried away by cooler vent air (grate cooler). In addition to this the power generated and cost saved was discussed. See the table below main parts. The power generated is used to powered 2480 light bulbs with 25-watt capacity. Generally important efforts are being made to continue for saving the energy for the cement industry, Successful reduction of fuel consumption contributes to lower fuel cost, higher clinker production, lower electricity consumption by recover the waste heat.