Recovery of waste heat in cement plants for the capture of CO₂

Abstract

Large amounts of energy are consumed during the manufacturing of cement especially during the calcination process which also emits large amounts of CO₂. A large part of the energy used in the making of cement is released as waste heat. A process to capture CO₂ by integrating the recovery and utilization of waste heat has been designed. Aspen Plus software was used to calculate the amount of waste heat and the efficiency of energy utilization. The data used in this study was based on a dry process cement plant with a 5-stage preheater and a precalciner with a cement output of 1 Mt/y. According to the calculations: 1) the generating capacity of the waste heat recovery system is 4.9 MW. 2) The overall CO₂ removal rate was as high as 78.5%. 3) The efficiency of energy utilization increased after the cement producing process was retrofitted with this integrated design.

Key words： cement industry; waste heat; recovery; utilization; CO₂ removal

Introduction

The amount of CO₂ in the atmosphere has been increasing enormously in recent years, and CO₂ is the largest contributor to the greenhouse effect. Most CO₂ comes from industrial emissions, and the cement industry is the third largest source of CO₂ emission<FootNote>

IPCC. IPCC Fourth Assessment Report: Climate Change 2007. Intergovernmental panel on climate change, 2007

</FootNote>. In 2006, the global cement production was 2.55 billion tonnes (t), and 1.88 Gt of CO₂ was released from cement plants<FootNote>

WBCSD, IEA. Cement Technology Roadmap 2009 – Carbon emissions reductions up to 2050. World Business Council for Sustainable Development, 2009

</FootNote>. Therefore, CO₂ capture in cement plants is crucial. Much research has focused on the study of post combustion CO₂ capture technology. The energy consumption for the recovery of CO₂ is very large and can be about 3 GJ/t CO₂ [1]. Thus high energy consumption is a bottleneck for the post combustion capture of CO₂ [2].

Cement plants are high energy-consuming enterprises and the energy consumption of the cement industry accounts for about 11% of total industrial energy consumption. Currently their largest source of energy is the combustion of coal. A portion of the consumed energy is released as waste heat and carried away by vent gas or by ambient air. About 30% of the calcination heat in these processes is carried away by the exhaust gas<FootNote>

MIIT. Promotion and Implementation Program of the Waste Heat Power Generation for New Type Cement Dry Process. Ministry of Industry and Information Technology of the People’s Republic of China, 2010

</FootNote>. The recovery and utilization of waste heat should be an effective way to cut down on the energy needed to capture CO₂. So improving energy utilization efficiency by recovering waste heat from cement plants could not only play a significant role in energy conservation but also in emission reduction.

Most of the waste heat from cement plants is discharged along with the exhaust gas. The temperature of the flue gas leaving the preheater is about 330°C. This heat could be reused by a waste heat recovery system for power generation. Technologies for waste heat power stations have been reported by many researchers, and there are many different processes for different conditions. Sheinbaum and Ozawa [3] reported that using the waste heat to improve the cement plant process, increased the energy utilization efficiency to 28% and reduced the CO₂ emissions by 17%. After studying the cement industry in China, Liu et al. [4] reported that energy efficiency can be improved 10% to 30% by retrofitting the cement process. Özdoğan and Arikol [5] stated that using waste heat will increase the efficiencies according to the first and second laws of thermodynamics. Hasanbeigi et al. [6] proposed some technologies to improve the process and reported an average technical potential primary energy savings of 23% could be realized at best practical levels. Barker et al. [7] and Hassan [8] studied the feasibility of CO₂ capture by doing simulations of monoethanolamine (MEA) based processes for CO₂ removal in cement plants. However, the integration of waste heat recovery with CO₂ capture has not been mentioned in the literature.

The temperature in a cement kiln approaches 1500°C, and the mean temperature of the cement kiln shell surface can be as high as 270°C. Apparently, much energy is lost by the radiation of heat and by convection with ambient air. Qian [9] proposed a device for recovering and utilizing the waste heat from cement rotary kilns; the device uses water for heat exchanging and has little influence on the original process. This paper integrates the waste heat from a cement rotary kiln into a traditional heat recovery system, so that the efficiency of energy utilization can be improved.

The main aim of this study is to cut down on the energy needed to capture CO₂ in a cement plant by using an integrated system which recovers and utilizes waste heat. The data used in this study was based on a dry process cement plant with a 5-stage preheater and a precalciner with a cement output of 1 Mt/y [7,10]. Aspen Plus software (Version 10.1) was used to calculate the amount of waste heat and the energy utilization efficiencies. The reduction of CO₂ emissions was also estimated.

Waste heat from cement plants

Sources of waste heat

Large amounts of coal are consumed in the production of cement, especially in the calcination process. Figure 1 shows the flow diagram for the process of producing cement. The temperature of the exhaust gas from the kiln head to the back end is very high, so the recovery of the waste heat is necessary to improve the energy utilization efficiency. The places which release heat are discussed below.

Fig.1 General flow diagram for a cement plant

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Exhaust gas from the kiln terminal

The temperature of the exhaust gas from the kiln terminal is about 330°C. This heat can be reused by employing a waste heat recovery system. After the kiln, the gas is sent to the raw mill to remove moisture. But the flue gas leaves the raw mill at a temperature of approximately 100°C after heat exchange, which is too low for direct recovery and utilization. The CO₂ concentration in the exhaust gas is about 14%-33% (w/w) [10]. So CO₂ capture on this part of the exhaust gas is desirable. The gas must be cooled down to about 50°C for the CO₂ to be absorbed by MEA or other absorbents.

Exhaust gas from the head of the kiln

The temperature of the exhaust gas at the kiln head is about 290°C, which is high enough for a waste heat recovery system to be useful. This part of the exhaust gas can also be sent to the fuel preparation and combustion processes to boost the temperature of the inlet air.

Heat diffusion from cement kiln shell

The cement rotary kiln is the primary place where the calcination process occurs, so most of the energy is consumed here. The temperature in the kiln needs to be about 1450°C to ensure that the cement producing process is functioning properly. The temperature of the kiln shell surface is also very high and can be up to 350°C with a mean temperature of about 270°C. Between heat convection and radiation, much heat is released into the atmosphere. Fire bricks are set around the kiln to prevent heat dissipation; but the kiln shell can not tolerate temperatures higher than 400°C. So at the same time, excess heat in the kiln shell must be promptly radiated away.

Waste heat recovery system

Because the temperature of the exhaust gas is not high, the efficiency of the waste heat recovery system is limited. Organic cycles are thought to have better performance for electricity generation by recovering low-temperature waste heat than steam cycles [11]. But in this paper, CO₂ capture was also considered so a combined heat recovery and power generating system was designed. Therefore, a traditional waste heat recovery system using a steam cycle based on the cascade utilization theory was employed.

A waste heat boiler for cement plants has been studied by many researchers, and this mature technology is already used in the cement industry. In this study, an improvement in the original system was achieved by integrating the recovery of the waste heat from the cement rotary kiln shell as part of the “Waste heat recovery and generating system” shown in Fig. 2. For CO₂ capture, a “Coal-fire boiler”, “Heat pump system” and “CO₂ capture system” were also added to the process, and they will be introduced in Sect. 3. Only the “Waste heat recovery and generating system” is discussed in this section.

Fig.2 Schematic of a cement plant with post-combustion CO₂ capture integrated with waste heat recovery

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In this study, a suspension preheater (SP) boiler was placed at the back end of the kiln to recover heat from the exhaust gas at 330°C and an air quenching cooler (AQC) boiler was added to recover the heat of the exhaust gas from the clinker cooler at 290°C. An auxiliary super-heater (ASH) was also added. Part or all of the cooling air needs to be circulated between the AQC boiler and the clinker cooler to save energy. At present, the research on the waste heat boiler will be focused on improving the energy recycling rate and the steam grade.

The inlet air of the SP boiler comes from the final stage of the preheater (C1-stage), and its temperature is about 300°C-350°C. The temperature of the C1-stage inlet air is about 450°C-600°C, so extracting heat from the C2-stage is more economical and can improve the steam grade. Based on this consideration, the preheater was placed between the C1- and C2-stages, which reduces the temperature of the C1-stage inlet air by only 20°C-25°C and produces superheated steam at 1.6-3.8 MPa and 450°C.

Qian [9] proposed a device for recovery and utilization of the waste heat from the cement rotary kiln shell. This device is placed at the periphery of the shell. Water is fed into the device and exchanges heat with the shell to ensure that the temperature of the shell is well within the normal range. So the whole device has little influence on the original process. After heat exchange, the water is converted to low grade steam, with a pressure of about 0.2-0.3 MPa and a temperature of approximately 150°C-170°C.

Figure 2 shows the design of a “Waste heat recovery and generating system” which integrates the recovery of various sources of waste heat. The process takes full advantage of the waste heat from different exhaust gases based on the laws of thermodynamics. Steam at different pressures and temperatures are generated for different purposes.

Thermal calculation

The generating capacity of the independent “Waste Heat Recovery and Generating System” in Fig. 2 was first calculated without including the other systems. The basic data of the cement plant is given below. The data is based on standard engineering practices. The grades of steams were also calculated.

1) The data is based on a dry process cement plant with a cement output of 1 Mt/y.

2) The energy consumption of the clinker is 3140 kJ/kg. The temperature of the raw mill drying process is 210°C.

3) The flow rate of the exhaust gas from the kiln terminal is 1.77 × 10⁵ m³/h. The mean temperature is 330°C, but decreases to 210°C when it leaves the SP boiler.

4) The exhaust gas from the kiln head is 1.55 × 10⁵ m³/h and the average temperature is 290°C, which decreases to 100°C when it leaves the AQC boiler.

5) The mean temperature of the cement kiln shell is about 270°C and 5% of the calcination heat is radiated. Seventy percent of the radiated heat can be recovered.

6) The generator efficiency is set as 95%.

The simulation results show that the generating capacity of the whole system is 4.9 MW, which is about 15.6% higher than an ordinary waste heat recovery system. This increase is mainly due to the utilization of the waste heat from the kiln shell. The generated steam can be divided into three types, which are listed in Fig. 2. The first one has the highest temperature and pressure, 350°C-450°C and 1.6-3.8 MPa, respectively. The second one is 150°C-180°C and 0.2-0.5 MPa respectively and the last one is just hot water at 80°C-100°C.

CO₂ capture by integrating waste heat recovery

CO₂ emission from cement plants

In cement plants, CO₂ is mainly generated from three different sources [10]. Most of the CO₂ comes from the decomposition reaction of the limestone, which primarily occurs in the cement rotary kiln. About 525 kg of CO₂ is released per tonne of cement produced. During the combustion of the fossil fuel, about 335 kg of CO₂ is released per tonne of cement produced. The production of the electricity required by the process releases about 50 kg CO₂ per tonne of cement produced.

The concentrations of some of the components in the exhaust gas from a cement process are shown in Table 1.

Tab.1 The components in the exhaust gas from a typical cement plant

Component	Concentration
CO2	14%-33% (w/w), 10%-24% (v/v)
NO2	5%-10% (v/v)of NOx
NOx	<200-3000 mg·Nm-3
SO2	<10-3500 mg·Nm-3
O2	8%-14% (v/v)

There are many technologies for CO₂ capture, such as post-combustion capture, pre-combustion capture and oxy-fuel combustion capture [12]. In the cement producing process, much fresh air is blown into the kiln for cooling, so the CO₂ concentration in the exhaust gas is not very high. Under these circumstances, pre- combustion capture and oxy-fuel combustion capture are not suitable. However, the process of post-combustion CO₂ capture, which is a mature technology in other industries, could be used in cement plants

There are also many methods for post-combustion capture, such as absorption, adsorption, cryogenic separation, and membrane separation [13]. Because the mean concentration of CO₂ in the exhaust gas from cement plants is 17% (v/v), chemical absorption is more suitable than the other methods. Chemical absorption is already used in many industries and has a good performance for CO₂ capture.

MEA is widely used as an absorbent for CO₂ capture because of its fast reaction rate, low cost, stability and large CO₂ capacity. The study in this paper is based on using MEA. The ideal temperature for the reaction between MEA and CO₂ is about 50°C. Since the exhaust gas leaves the raw mill at about 100°C, a cooler must be placed before the CO₂ absorber as shown in Fig. 2.

SO₂ and NO_x in the exhaust gas cause degradation of MEA in CO₂ recovery systems. So before CO₂ capture, a selective catalytic reduction (SCR) unit and a flue gas desulfurization (FGD) unit are needed to remove NO_x and SO₂ respectively. These are also shown in Fig. 2.

A large amount of energy, about 3 GJ/t CO₂, is needed for the CO₂ recovery process. Therefore, large amounts of steam at about 0.35 MPa and 140°C-150°C are required. Because a large steam source does not exist in cement plants, an extra coal-fired combined heat and power (CHP) plant is required to generate the needed low pressure steam and electric power. A 40 MW coal-fired CHP plant has to be installed to meet this requirement and this is shown in Fig. 2 as the “Coal-fire boiler”. Although the waste heat recovery system can provide steam and electric power, the scale is too small to meet all the demands.

After separation and recovery, the CO₂ must be compressed to about 11 MPa by a multi-stage compressor so it can be transported and further utilized. This system is shown in Fig. 2 as part of “CO₂ capture system”.

Theoretically, the CO₂ recovery rate should be higher than 90%. However, considering the many complex factors in cement plants, the CO₂ recovery rate was set as 85% [7]. Thermodynamic calculations were carried out to determine the utilization of waste heat and the effectiveness of CO₂ removal.

Synthesis of the heat network

A pinch analysis was carried out to optimize the system for CO₂ capture and waste heat utilization. The composite curves of a CO₂ capture system with integrated waste heat recovery is shown in Fig. 3. The mean pinch temperature of the system was 105°C (110°C for hot steam and 100°C for cold steam), which is the point closest to the hot and cold composite curves. This is a critical point for the maximum conservation of energy.

Fig.3 The composite curves of a CO₂ capture system with integrated waste heat recovery

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Based on the composite curves, a heat exchange network for the system was developed as shown in Fig. 4. Heat transfer across the pinch point has been avoided. So the energy utilization efficiency increases because of the reduction of heating and cooling requirements, which are still very large as Fig. 3 shows. A further decrease in the heating and cooling requirements could be achieved by using a heat pump system [14].

Fig.4 The heat exchange network of a CO₂ capture system with integrated waste heat recovery

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From Fig. 4 it can be seen that the heat from the exhaust gas at 100°C has not been recovered by the system, but is taken away by the cooling water. To make full use of the waste heat, a heat pump system needs to be employed to recover this part of the heat and to generate low-temperature steam for CO₂ recovery. The heat sources for the heat pump are the exhaust gases from the raw mill and from the kiln head at 100°C, which will be cooled down to 50°C. So the evaporating temperature of the refrigerating medium is approximately 40°C. The heat from the heat pump is transferred to the recovered liquid at about 130°C. NH₃ was selected as the optimum refrigerating medium because it has the highest coefficient of performance (COP) value. Thermodynamic calculations show that the electric power required for compression of NH₃ in the heat pump system is 2.6 MW. The COP value of the heat pump is 2.71. So the heat provided by the heat pump for the CO₂ recovery process is 7.2 MW, which accounts for 7.5% of the heat requirement. Obviously, the heat pump system is advantageous for raising the energy utilization efficiency. As the “Heat pump system” in Fig. 2 shows, the heat contained in the exhaust gas, which is carried away by the cooling water in the traditional process, is reused in the improved process by the heat pump for heating the regenerated liquid of CO₂.

The pinch analysis was used to propose a cement plant with post-combustion CO₂ capture integrated with waste heat recovery and the schematic is shown in Fig. 2. The results of the simulation show that the generating capacity with waste heat recovery drops to 3.7 MW, due to the consumption of low-grade steam in the CO₂ recovery process. Therefore, the generating capacity of the whole system is 43.7 MW.

Results and discussion

To demonstrate the superiority of an integrated system, several cases were calculated using Aspen Plus software (Version 10.1). The results are listed in Table 2. Data from literature are also listed.

Tab.2 Calculation results

Options	Unit	Case 1a) [7]	Case b)2	Case 3 c) [7]	Case 4 d)
Fuel and power
Coal feed	kt/y	63.3	63.3	291.6	266.2
Petroleum coke feed	kt/y	32.9	32.9	32.9	32.9
Average power consumption	MW	10.2	10.7	42.1	41.7
Average on-site power generation	MW	-	4.9	45.0	43.7
Average net power consumption	MW	10.2	5.8	-2.9	-2.0
CO2 capture
CO2 captured	kt/y	-	-	1067.7	1017.9
CO2 emitted on-site	kt/y	728.4	728.4	188.4	179.5
CO2 emission associated with power<FootNote> Brown M A, Jackson R, Cox M, Cortes R, Deitchman B, Lapsa M V. Making industry part of the climate solution: policy options to promote energy efficiency. Department of Energy (Internet). May 2011 (cited 2011 June 13). Available from http://www.osti.gov/energycitations/product.biblio.jsp? uery_id=1&page=0&osti_id=1016041 </FootNote>	kt/y	55.7	31.7	-15.8	-10.9
Overall CO2 emissions	kt/y	784.1	760.1	172.6	168.6
Overall CO2 reduction rate	%	-	3.1	78.0	78.5

Case 1: base case without waste heat recovery or CO2 capture; b) Case 2: waste heat recovery system; c) Case 3: CO2 capture without waste heat recovery integration; d) Case 4: CO2 capture with waste heat recovery integration

In Table 2, Case 1 is the base case without waste heat recovery or CO₂ capture. The average net power consumption is 10.2 MW, which is the largest power consumption among the four cases. The overall net CO₂ emission of the base case is also the largest.

Case 2 is for a waste heat recovery system. The electric power generated by the waste heat recovery system is 4.9 MW, which provides 45.8% of the electric power requirement. So a waste heat recovery system is effective for reducing the electric power consumption. However, fuel consumption remains the same. Due to the reduction of electric power consumption, the overall CO₂ emission drops 3.1%.

Case 3 is the CO₂ capture process without the integration of waste heat recovery. To generate the low-temperature steam for the CO₂ recovery process, a 45 MW coal-fired CHP plant has to be added. So the coal consumption increases greatly but the overall CO₂ reduction rate is as high as 78.0%. This reduction of CO₂ emission is based on the higher fuel consumption. Here, the high energy consumption is the bottleneck for CO₂ capture.

Case 4 is the CO₂ capture process with the integration of waste heat recovery. Because of the utilization of waste heat, a lower capacity coal-fired CHP plant (40 MW) is enough for the whole system. So the coal consumption decreases by 8.7%. The overall CO₂ reduction rate rises to 78.5%. So the recovery and utilization of waste heat has a very significant effect on energy saving and on emission reduction.

The results show that an integrated process has an obvious effect on reducing the energy consumption of the CO₂ capture process, which achieves the original intention of the design. The technology of waste heat recovery could also be applied to other industries for CO₂ capture, such as power plants [15]. The thermal system integrations that were carried out in this study may not be the optimal situation. An integrated transfer network of heat, mass and momentum should be analyzed to obtain an optimal solution, which will the primary focus of our future research.

In summary, a post-combustion CO₂ capture process consumes much coal and electrical power. The integration of waste heat recovery cuts down on the electrical power consumption by 8.8% and reduces the coal consumption 8.7%. There are also many other methods to reduce energy consumption, such as using internal heat-integrated distillation columns for the separation of mixtures with close boiling compounds and recovering inter-stage compression heat [16]. The oxy-combustion CO₂ capture process consumes less energy than the post-combustion CO₂ capture process, but the overall CO₂ reduction rate is lower [7]. Using the methods mentioned above, further increases in energy utilization efficiency could be achieved.

Conclusions

In this paper, the recovery and utilization of the waste heat from a cement plant was studied. An improved process for waste heat recovery and the integration of post-combustion CO₂ capture with waste heat recovery is proposed. For a typical cement plant with a production capacity of 1 Mt/y, the generating capacity of the isolated waste heat recovery system was calculated to be 4.9 MW, which is about 15.6% higher than an ordinary waste heat recovery system. A post-combustion CO₂ capture process integrated with waste heat recovery was also developed. Calculations indicate that the utilization of the waste heat would provide 3.7 MW of electrical power in the integrated process. The heat provided by the heat pump system for the CO₂ recovery process is 7.2 MW, which accounts for 7.5% of the total heat requirement. The overall reduction of CO₂ emission achieved by the integrated CO₂ capture process is 78.5%, which is higher than that achieved for a normal CO₂ capture process. The utilization of the waste heat in the post-combustion capture of CO₂ could reduce the total electrical power consumption by 8.8% and reduce coal consumption by 8.7%.

CO₂ capture is a high energy-consuming process. Great amounts of coal and electrical power are consumed, especially in the CO₂ recovery process. The recovery and utilization of the waste heat is just one way to save energy. More technologies and further research are still required to raise the energy utilization efficiency of cement plants.

Acknowledgements

Financial support from the National Natural Science Foundation of China (Grant Nos. 50976090 and 20936004) is greatly appreciated.

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