Design Of Cement Plant Waste Heat Recovery

Design of Cement Plant Waste Heat Recovery Generation Totok R. Biyanto Department of Engineering Physics, Sepuluh Nopember Institute of Technology (ITS), Indonesia [email protected]

Ridho Bayuaji Department of Civil Engineering, Sepuluh Nopember Institute of Technology (ITS), Indonesia [email protected]

Sonny Irawan Department of Petroleum and Geosience Engineering, Universiti Teknologi Petronas, Malaysia [email protected]

Abstract—Waste heat in cement plant is a huge waste heat from kiln and raw mill plant. Such as ordinary power plant, waste heat recoverTy generation consist of various operation units ie., economizer, evaporator, super heater, steam drum, and turbine. In order to obtain the optimal heat recovery, proper operation units should be designed and arranged optimally. This paper describes heat recovery unit operations modelling using Aspen HYSYS software. HYSYS can be utilized as single tool for design and simulate WHRG system. The steady-state and dynamic process simulation, simulate pinch analysis result, area of heat transfer and roughly cost-estimation could be provided using this tool. The results show the Aspen HYSYS model could provide satisfied engineering design data that could be used in construction stage. Keywords: Waste Heat Recovery Generation, Cement Plant, Design, Modelling

I.

INTRODUCTION

Cement production requires approximately 3.2 to 6.3 GJ of energy and 1.7 tons of limestone as major raw materials per ton of produced clinker [1, 2]. Cost of energy is about 60% of the production costs. Therefore the cement industry is one of the most energy intensive industries. Modern portland cement process basically consist of quarry, raw meal preparation, preheating of raw meal, kiln, clinker cooling, grinding, storage and dispatch. In preheater tower chemical reaction starts with the decomposition of calcium carbonate (CaCO3) at around 900°C to leave calcium oxide (CaO, lime) and release CO2; this process is called calcination. In this process commonly, fossil fuels such as coal, petcoke and natural gas is burnt to get the required thermal energy consumption [3]. At the end of this process, 20 to 50% of the consumed energy is lost as waste heat that contained in the flow of hot gases, solids and liquids, as product or by product stream. An alternative approach to improve energy efficiency is to recover the waste heat. In some cases, such as industrial furnaces, the

efficiency increased about 10% - 50% by utilization of waste heat energy recovery. Refer to this purpose the simulation of heat recovery in cement plant is required. In current study, some reseacher utilized commercial simulation software such as Aspen HYSYS, Aspen Plus, etc. Zhang et al.[4], Rahman et al. [5], and Kaantee et al. [6] used Aspen family software to simulate the cement production due to ability to simulate chemical reactions within solid, liquid and vapour phases. The aim of this research is design and simulate the opportunies of benefit WHRG using single tool ie. model under HYSYS environment. The simulations are provided by HYSYS are steady-state and dynamic process simulation, advanced process controls, simulate pinch analysis result, area of heat transfer and roughly cost-estimation.The model is verified against measured data from plant. This paper presents the energy efficiency and CO2 reduction through waste heat recovery in cement plant using Aspen HYSYS software. II.

DESIGN OF WATE HEAT RECOVERY

The design method of WHRG is start from data collection from plant such as operating data, properties of flue gas, existing cement plant and available space, water resources, etc. Identify potential energy available from flue gas and usages, determine technology, WHRG modeling and simulation and equipment cost estimation . The main equipment in this system is a WHRG boiler that consist three heat exchangers ie. economizer, evaporator and superheater. Traditionally, the heat exchanger performance analysis and simulation are performed using steady-state energy balance across the heat exchanger. The energy balance on the hot and cold fluids together with the heat-transfer equation constitutes the model of heat exchangers. A simplified model generally uses an average driving force such

as log mean temperature difference (LMTD) and assumes uniform properties of the fluids along the length of the heat exchanger to determine the overall heat-transfer coefficient. Under the assumption that there is no heat loss to the surroundings, the heat lost by the hot fluid stream shall be equal to the heat gained by the cold fluid stream, thus (1) Qc  Qh where Qc = amount of heat received by cold fluid Qh = amount of heat released by hot fluid The amount of heat received by the cold fluid, Qc, is given by (2) Qc  mc c p ,c Tc,o  Tc ,i  where mc = mass flow rate of the cold fluid (crude oil) cp,c = specific heat of the cold fluid Tc,i = inlet temperature of the cold fluid Tc,o = outlet temperature of the cold fluid The amount of heat released (lost) by the hot fluid, Qh, is given by

Qh  mh c p ,h Th ,i  Th ,o  (3) where mh = mass flow rate of the hot fluid cp,h = specific heat of the hot fluid Th,i = inlet temperature of the hot fluid Th,o = outlet temperature of the hot fluid The amount of heat transferred from the hot fluid to the cold fluid, Q, across the heat exchanger surface would be equal to Qc and Qh and is given by

Tg1 Tg2 Tg3

Ts2

Q  UAFTlm where

Various correlations for individual heat-transfer coefficients are predict highly varying heat-transfer coefficient values for a given application and determining the appropriate correlation is a challenging task. Since variations in the fluid properties and hence heat-transfer coefficients take place along the length of the heat exchangers, distributed-parameter models are more appropriate to the nature of heat exchangers. Such models are represented by a set of partial differential equations. Commercial process simulation software packages such as HYSYS, ASPEN, PETROSIM®, iCON, etc., employ distributed-parameter models that have capabilities to incorporate several considered variables in heat exchangers design. There are several considered variables in designing the WHRG boiler systems, such as pinch point, approach point, the temperature of flue gas and steam pressure. Pinch point is the temperature difference between the flue gas outlet temperature of the evaporator and saturated steam (Figure 1). while the approach point out is the difference in the water inlet temperature at evaporator and saturated steam at outlet economizer. Therefore, in order to obtain desinged WHRG in good performance, the boiler must be designed and simulated at the first stage to determine the temperature profile of flue gas and steam [7,8,9]. Typical of gas and steam temperature profile can be seen in Figure 1 [7]:

(4)

Tg4

U = overall heat-transfer coefficient A = heat-transfer surface area ∆Tlm = Log Mean Temperature Difference (LMTD) F = LMTD correction factor.

The overall heat-transfer coefficient, U, is determined using empirical correlations of the individual film heattransfer coefficients and the resistance due to fouling as given by

d o R f ,i d 1  o  U d i hi di

d d o ln o  di  2k w

where: Rf,i Rf,o hi ho U kw do di

inside fouling resistance outside fouling resistance tube-side film heat-transfer coefficient shell-side film heat-transfer coefficient overall heat-transfer coefficient thermal conductivity of the tube metal outside diameter of the tube inside diameter of the tube

= = = = = = = =

  R

f ,o



1 ho

(5)

Ts

Pinch point = Tg3 -Ts

Tw2

Aproach point = Ts -Tw2

Tw1 Superheater

Evaporator

Economizer

Figure 1. Flue gas and steam temperature profile

As Shown in Figure 1, Tg1, Tg2, Tg3 and Tg4 are inletoutlet temperatures entering the boiler flue gas temperature from superheater until economizer. While TW1 and TW2 are the inlet and outlet water temperature economizer. Ts is the temperature of saturated steam and superheated steam tS2 is the temperature. The amount of allowable pinch point and approach point of WHRG in the boiler can be seen in Table 1 below [7]. In the other hand, the outlet temperature of the boiler flue gas should be follow best practice tabulated in Table 2.

TABLE 1. Pinch and approach point Pinch point T oC

Inlet gas, oC

Bare tube

Finned tube

Approach point, T oC

649 - 982

72 – 83

17 - 33

22 – 39

371 – 649

44 – 72

5.5 - 17

5.5 - 22

TABLE 2. The temperature of flue gas out as a function of the steam conditions Pressure (Mpa)

Steam temperature, oC

Saturated temperature, oC

K= (Tg1-Tg3)/ (Tg1-Tg4)

Flue gas outlet temperature

0.79

saturated

170

0.904

149

1.12

saturated

186

0.8754

156

1.83

saturated

208

0.8337

167

2.86

saturated

231

0.7895

178

2.86

315

232

0.8063

186

4.24

Saturated

254

0.7400

189

4.24

399

256

0.7728

203

Using operating data from plant and calculation about alowable pinch point and flue gas outlet temperature, model of WHRG under Aspen HYSYS was built. The schematic of Process Flow Diagram (PFD) of WHRG is shown in Figure 3.

III. RESULTS AND DISCUSSION Simulation of WHRG under Aspen HYSYS environment utilize the operational data from plant such as in cement plant for Suspension Preheater (SP) boiler and Air Quencing Cooler (AQC) boiler. Maping of temperatures in SP and AQC boiler under Aspen HYSYS simualtion are shown at Figure 3 and 4, under conditions without and with 10% heat loss, repectively.

Figure 2. Waste heat racovery flow diagram under HYSYS environment

The results of simulation SP and AQC boiler in WHRG under Aspen HYSYS, with and without 10% of heat loss and 40% of bypass due to changing in process and environment that it considered in this research are tabulated ini Table 4. The pinch point and outlet gas temperature decrease if bypass and losses is affected. Disturbance in rainy season is also reduce the steam production due to the water content in raw material increase. Simulation of Boiler coupled with turbin-generator was performed under different operating condition. The results under efficiency 80% – 98% are shown in Table 4,

Figure 3. Temperature profile of SP and AQC boiler (without 10% heat loss)

Figure 4. Temperature profile of SP and AQC boiler (with 10% heat loss) TABLE 3. Pinch point and outlet gas temperature at different heat loss and bypass Flow of steam, (kg/h) Max SP Boiler AQC Boiler

40440 10640

No loss

10% loss

No loss

Loss 10%

109

101

329.2

321

49

43

168

148.5

40% +10% loss

No loss

10% loss

28970

93

80

317

300

8010

37

33

159

143

No loss

10% loss

No loss

Loss 10%

37810

109

101

329

321

8010

37

33

162

146

Dry SP Boiler AQC Boiler

Outlet gas temperture, oC

40% bypass

Rainy SP Boiler AQC Boiler

Pinch point, oC

TABLE 4. Generated power at different turbine eficiency and operating condisions Operating Effisiency Effisiency Effisiency Effisiency conditions 80% 90% 95% 98% Max 20.67+ 23.25+ 24.55+ 25.32+ 0.83+ 0.93+ 0.98+ 1.01+ 0.12 0.13 0.14 0.15 =21.62 =24.31 =25.67 =26.48 Rainy 15.3+ 17.21+ 18.17+ 18.74+ 1.02+ 1.15+ 1.21+ 1.25+ 0.15 0.17 0.17 0.18 =16.47 =18.53 =19.55 =20.17 Dry 20.01+ 22.51+ 23.76+ 24.51+ 0.8+ 0.9+ 0.95+ 0.98+ 0.11 0.13 0.14 0.14 =20.92 =23.53 =24.85 =25.63

There are some possibilities of amount generated power from WHRG due to operating conditions and disturbances. The variables that influence the generate power are as follow: 1. Inlet temperature of SP boiler flue gas of 386-400 0C 2. Flow ratio to the boiler flue gas of 0.8 - 1 3. Turbines efficiency 80, 90, 95 and 98% In the other hand, implementation of WHRG resulted in the fuel savings that could be represented as reduction of CO2 emissions. The calculation of Green House Gas (GHG) can utilize the formula from eGRID that it convert the reduction of electrical into units of carbon dioxide emissions. The converting equation is 1 kWh = 7.0555.10-4 MT CO2. Hence, for emissions reductions due to the utilization of WHRG about 20 MW is 14 MT CO2. IV. CONCLUTION HYSYS can be utilized as single tool for design and simulate WHRG system. The steady-state and dynamic process simulation, simulate pinch analysis result, area of heat transfer and roughly cost-estimation could be provided using this tool. The simulation results show that the amount of generated power is about 20 MW and WHRG could reduce 14 MT CO2 emision per year ACKNOWLEDGMENT The authors gratefully thank to Sepuluh Nopember Institute of Techology (ITS) Surabaya, Universiti Teknologi Petronas, Malaysia and DIKTI for providing the facilities for conducting this research.

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