Scholarly article on topic 'CO2 Capture in Cement Production and Re-use: First Step for the Optimization of the Overall Process'

CO2 Capture in Cement Production and Re-use: First Step for the Optimization of the Overall Process Academic research paper on "Chemical engineering"

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Abstract of research paper on Chemical engineering, author of scientific article — Nicolas Meunier, Sinda Laribi, Lionel Dubois, Diane Thomas, Guy De Weireld

Abstract Carbon Capture and Utilization (CCU) is one of the most widely studied technology to reduce anthropogenic CO2 emissions and particularly the ones coming from power plants and cement plants which are currently among the world's main industrial sources of carbon dioxide. As a result, this study focuses on the optimization of an overall CCU process that should be applied to an oxyfuel cement plant, and including the CO2 capture from flue gases and its purification in order to obtain a rich CO2 stream that will be further converted into methane, methanol, or other chemically valuable compounds. To investigate the feasibility of such as process, two units (namely sour compression and cryogenic units) have been modeled and simulated on Aspen Plus software. These simulations were conducted considering flue gases compositions coming from both power and cement oxyfuel plants in order to compare their respective energy demands with regard to the CO2 purity of the end-of-pipe product and to the CO2 recovery of the overall process. It was observed that such process applied to simulated oxyfuel cement plant flue gases has a global CO2 recovery range of 75.8 – 93.8% and that the CO2 molar purity of the final stream is between 94.8 and 98.4%. This process appears to be completely applicable for the treatment of oxyfuel cement plant flue gases with CO2 recovery and CO2 molar purity in agreement with requirements for the chemical conversion of carbon.

Academic research paper on topic "CO2 Capture in Cement Production and Re-use: First Step for the Optimization of the Overall Process"

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Energy Procedia 63 (2014) 6492 - 6503

GHGT-12

CO2 capture in cement production and re-use: first step for the optimization of the overall process

Nicolas Meunier, Sinda Laribi, Lionel Dubois, Diane Thomas, Guy De Weireld*

Thermodynamics and Chemical and Biochemical Departments, Faculty of Engineering, University of Mons, 20 Place du Pare, 7000 Mons,

Belgium

Abstract

Carbon Capture and Utilization (CCU) is one of the most widely studied technology to reduce anthropogenic CO2 emissions and particularly the ones coming from power plants and cement plants which are currently among the world's main industrial sources of carbon dioxide. As a result, this study focuses on the optimization of an overall CCU process that should be applied to an oxyfuel cement plant, and including the CO2 capture from flue gases and its purification in order to obtain a rich CO2 stream that will be further converted into methane, methanol, or other chemically valuable compounds.

To investigate the feasibility of such as process, two units (namely sour compression and cryogenic units) have been modeled and simulated on Aspen Plus software. These simulations were conducted considering flue gases compositions coming from both power and cement oxyfuel plants in order to compare their respective energy demands with regard to the CO2 purity of the end-of-pipe product and to the CO2 recovery of the overall process. It was observed that such process applied to simulated oxyfuel cement plant flue gases has a global CO2 recovery range of 75.8 -93.8% and that the CO2 molar purity of the final stream is between 94.8 and 98.4%.

This process appears to be completely applicable for the treatment of oxyfuel cement plant flue gases with CO2 recovery and CO2 molar purity in agreement with requirements for the chemical conversion of carbon.

© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Peer-reviewunderresponsibilityof theOrganizingCommitteeof GHGT-12

Keywords: CO2 capture ; CO2 re-use ; oxycombustion ; cement industry ; simulation.

* Corresponding author. Tel.: +32 65 37 42 03; fax: +32 65 37 42 09. E-mail address: guy.deweireld@umons.ac.be (G. De Weireld)

1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of the Organizing Committee of GHGT-12

doi: 10.1016/j.egypro.2014.11.685

1. Introduction

The cement industry is currently one of the world's main industrial sources of carbon dioxide emissions and, even if they have substantially reduced their emissions over the years through alternative fuels, clinker substitution, and energy efficiency improvement, there is still an increasing need to reduce them more[1]. However, further reduction is becoming limited (as CO2 is necessary emitted during the decarbonation step in the clinker burning process) and new technologies have to be considered to reach this aim.

Considering the three most advanced capture technologies, post-combustion and oxy-combustion capture processes seem to be useful to the cement industry as pre-combustion techniques are inadequate because CO2 is produced during the conversion of limestone to calcium oxide in the clinker burning process[2]. As a result, Carbon Capture and Storage (CCS), and Carbon Capture and Utilization (CCU) are the most widely studied technologies to reduce anthropogenic CO2 emissions. Recently, new progresses and/or concepts related to CO2 capture techniques (amine-based solvents scrubbing, ionic liquids scrubbing, solid sorbents adsorption (amine-, metal-, or carbonate-based), etc.), and CO2 reuse techniques (CO2 conversions into various valuable compounds such as carbonates, propylene glycol, methanol, methane, etc.) have been investigated[3].

Nowadays, oxy-combustion and the related CO2 capture techniques are trending and widely studied topics especially in the case of power plants[4]. However, applying oxy-combustion to the cement industry is highly challenging due to the significative differences between cement plant and power plant flue gases. Indeed, cement plant flue gases have a higher CO2 concentration essentially because of the decarbonation step during the burning process, and have also higher pollutants concentrations as cement plants usually use alternative fuels in their kilns. As a result, oxycombustion in the cement industry will require specific researches and developments as many of the fundamentals of oxyfuel cement plant still need to be better understood. The topic is thus brand-new leading to a clear lack of data about the CO2 purification from oxyfuel cement plant as none of this kind of plants (neither industrial nor pilot unit) currently exists.

To treat CO2 coming from oxy-combustion plants, and especially to remove SOx and NOx components, several methods were investigated: the alkaline scrubber designed for the removal of nitrites and the reduction of thermal nitrites[5]; the "CRYOCAP" technology developed by Air Liquide[6]; the "Lead Chamber Process" concept used for the removal of SOx and NOx components to produce acid solutions'7^ and the "CO2 Processing Unit" developed by Praxair[8] and using activated carbon beds as adsorbents/catalysts for the removal of SO2 and NO to respectively produce H2SO4 and HNO3.

However, the usual procedures to remove classical impurities (H2O, O2, Ar, N2, SOx, and NOx) are mostly based on three main steps: a desulphurization/denitrification step (with compression of the flue gas), a water adsorption step, and a separation step using either a distillation column or a double flash unit, to remove the residual inert gases. Pipitone et al.[9] described this methodology with two different cases presenting the differences between the use of a distillation column or a double flash unit in the last inert separation step. They also chose seawater as scrubbing medium in the first step to desulphurize the flue gas coming from two different oxy-combustion power plants fired with either natural gas or pulverized fuel. Posch et al.[10] also optimized both these configurations (distillation column versus double flash unit) and showed the impact of main design parameters on performance features such as specific power requirement, specific cooling duty, separation efficiency, and CO2 purity.

Based on this principle, this work focuses on an interesting 3-step purification process of oxyfuel-derived CO2 in which the raw CO2 coming from power plants flue gases is compressed and purified in a first unit called "sour compression uni/"[11],[12]. This configuration can be used to remove SOx (up to 100%) and NOx (up to 90-99%) from the feed CO2 stream by controlling the formation of acids, potentially saving expensive upstream control options and minimizing potential downstream corrosion. Considering that the stripping medium (water) is quite simple to provide and that the temperatures of absorbers are relatively low (and could lead to smaller energy demands) this technology has been applied, in this work, to CO2-rich flue gases coming from oxyfuel cement kilns and from oxyfuel power plants. The performances of this process applied for both flue gases (considering similar flow rates) were then compared keeping all operating and design parameters unchanged. The third unit (namely cryogenic unit) was also investigated especially regarding removal performances and energy consumption in terms of CO2 purity of the final product and the CO2 recovery of the overall process.

2. Description of the CO2 purification unit

Considering a block diagram of an oxyfuel cement plant (Figure 1), the CO2 purification unit is integrated between the pretreatments of flue gases (mainly de-dust) and before the CO2 conversion unit. Moreover, this purification process is divided into three main units which are respectively the desulphurization/denitrification (DeSOx/DeNOx) (also called "sour compression unit'), the dehydration and the cryogenic units.

Figure 1: Block diagram of an oxyfuel cement plant including the CO2 purification unit (CPU) upstream of a CO2 conversion unit

2.1. "Sour compression unit"

A part of the flue gas coming from either the power plant or the cement plant is taken after the de-dust unit. The flue gas is then compressed to 15 bar in an isentropic two-stage compressor (COMP-1 and COMP-2), and enters the first absorber tower (DESOX) of the "sour compression unit' at the bottom and flowing counter currently with water. This first absorber column has an internal diameter of 0.15 m and consists of consecutive packed-beds with a total packing height of 12 m and filled with IMPT 25 random packing. This absorber was designed for gas and liquid flow rates of respectively 120 Nm3/h and 0.62 m3/h, and leading to a liquid/gas ratio of 0.082 and a gas velocity of 0.12 m/s (which represents 70% of the flooding velocity). A splitter is used to recycle a part of the liquid flow (REC-1) to the top of the absorber and a water make-up is also provided (WATER-1). The washed gas then leaves the column by the top and is compressed to 30 bar (COMP-3) before entering the second absorber tower (DENOX) at the bottom and also flowing counter currently with water. Another splitter is used to recycle a part of the liquid flow (REC-2) to the top of the second absorber and a water make-up (WATER-2) is also provided. This second absorber column has the same geometry (in terms of diameter, height and random packing) than the first one. The washed gas then leaves the column by the top and flows through the dehydration unit. A detailed flow sheet of the unit is present in Figure 2.

Figure 2: Detailed flow sheet of the sour compression unit

Operating parameters (split ratios of circulation loops and water make-ups) were set as following to match the range value of recycled flows and water make-ups specified by White et al.[11] for both columns. These parameters

are kept unchanged for all simulations to compare the performances of the designed process applied to both power plant and cement plant flue gases. These specifications are summarized in Table 1.

Table 1 : Operating parameters for absorption columns in the sour compression unit

Parameter Units Value

15 bar Column Make-up Water Flow Rate (WATER-1) kg/h 60

15 bar Column Recycle Water Flow Rate (REC-1) kg/h 550

30 bar Column Make-up Water Flow Rate (WATER-2) kg/h 32

30 bar Column Recycle Water Flow Rate (REC-2) kg/h 791

To compare the performances of the process applied to power plant and cement plant flue gases, two representative compositions were chosen to model these flue gases. These compositions are summarized in Table 2, where Case A represents a power plant flue gas composition saturated with water[13], and Case B a cement plant flue gas composition coming from ECRA simulations on oxyfuel cement kilns (Courtesy of ECRA).

Table 2: Inlet flue gas compositions for power plant (Case A) and cement plant (Case B)

Flue gas composition (%mol) Case A Case B

CO2 72 83.13

N2 14 11.11

O2 5.9 3.27

H2O 5.6 1

Ar 2.39 1.34

NO 320 ppm 861 ppm

CO - 397 ppm

SO2 700 ppm 156 ppm

NO2 51 ppm 96 ppm

Total mole flow (mol/h) 4748 4764

It can be seen that the main differences between both compositions are the higher concentration of CO2 (83%) and the low concentration of H2O (1%) for the cement plant flue gases composition, and that more NOx (957 ppm) and less SOx (156 ppm) are present. Furthermore, residual CO is also present in cement plant flue gases.

In both cases, the inlet flue gas was set to enter the sour compression unit with a flow rate of 120 m3/h at temperature and pressure of respectively 30°C and 1 bar.

2.2. Dehydration unit

The dehydration unit is composed by a Temperature Swing Absorption (TSA) dual-bed and water is absorbed at high pressure (30 bar) onto a solid adsorbent which can be silica gel, activated alumina or molecular sieve alumina.

Typically, regenerative desiccant dryers supply a dew point of -40°C to -70°C if required[14]. At these temperatures, the vapor pressures of ice are respectively 12.84 Pa and 0.261 Pa which leads to a water concentration range of 4.28 - 0.087 ppm in the gas phase. As a result, water present in the flue gas at the exit of the "sour compression unit' (typically about 0.9 mol%) is supposed to be completely removed before entering the cryogenic unit.

2.3. Cryogenic unit

The gas coming from the dehydration unit is cooled (COOLER-1) and flashed in a first flash (FLASH-1) at 30 bar. The vapor stream is then cooled (COOLER-2) and flashed anew in a second flash (FLASH-2) with a lowest temperature of -55°C to avoid the formation of dry ice (solid CO2) at this pressure. The vapor stream from the second flash goes then through a turbine (VENT) which lowers its pressure to 1 atm and electrical energy is recovered from. Liquid streams from both flashes are finally mixed (MIX) and compressed (COMPR) to 110 bar for transport or storage. A detailed flow sheet of the unit is presented in Figure 3.

Figure 3: Detailed flow sheet of the cryogenic unit

Temperatures for both flash units are operating parameters which have to be optimized regarding the CO2 molar purity required for further reuse and/or CO2 recovery of the installation. The molar CO2 recovery of the cryogenic unit is defined as (1):

1C02,out

recovery = —-— (1)

Qco2,in

where qco2,out and qCo2,in are respectively the CO2 molar flow rate at the exit (CO2-HP) and inlet (CO2-DRY) of the cryogenic unit in kmol/h.

3. Modeling principles

All the simulations of the sour compression and cryogenic units were performed using Aspen Plus v8.4. 3.1. "Sour compression unit"

Simulations of the sour compression unit were performed on Aspen Plus v8.4 and Electrolyte NRTL[15] was selected for thermodynamic properties calculations as various electrolytes are considered.

A chemical mechanism has been chosen to study the reactions of the SOx and NOx compounds in the gas/liquid absorbers (reactions of oxidation, hydrolysis and equilibrium between NOX species), and the solubilities of gases in water were calculated by means of the Henry's law[16]. Concerning these reactions, they could either occur in the gas or liquid phase, and be considered either as equilibrium or kinetic reactions. Table 3 presents the considered reactions during the absorption of SOx and NOx compounds and the generation of acidic solutions (HNO2, HNO3, and H2SO4).

Nicolas Meunier et al. / Energy Procedia 63 (2014) 6492 - 6503 Table 3: Kinetic and equilibrium reactions considered for the sour-compression unit

N° Reaction Type Phase Reference

1 2 NO + O2 NO2 Kinetic Gas [17]

2 2 NO2^N2O4 Equilibrium Gas [18]

3 NO + NO2 + H2O^2HNO2 Equilibrium Gas [18],[19]

4 NO2+NO^N2O3 Equilibrium Gas [19]

5 N2O3+H2O^2 HNO2 Equilibrium Liquid [20]

o 6 N2O4 +H2O ^ HNO3 + HNO2 Kinetic Liquid [21]

w z 7 2NO2+H2O^ HNO3 + HNO2 Kinetic Liquid [22]

8 NO+NO2+H2O^2HNO2 Kinetic Liquid [23]

9 2HNO2^NO+NO2+H2O Kinetic Liquid [23]

10 3HNO2^H2O+2NO+HNO3 Kinetic Liquid [24]

11 HNO2+H2O^ NO2-+ H3O+ Equilibrium Liquid *

12 HNO3+H2O^ NO3-+ H3O+ Equilibrium Liquid *

13 SO2 + 0,5 O2 SO3 Equilibrium Gas *

14 SO2 + 2 H2O^ H3O+ + HSO3- Equilibrium Liquid *

y. 15 HSO3- + H2O + SO32- Equilibrium Liquid *

GO 16 SO3 + 2 H2O H3O+ + HSO4- Equilibrium Liquid *

17 HSO4- + H2O H3O+ + SO42- Equilibrium Liquid *

18 H2SO4 + H2O H3O+ + HSO4- Equilibrium Liquid *

^Equilibrium constants were calculated by Aspen Plus by means of the Gibbs free energy equations

3.2. Cryogenic unit

Simulations of the cryogenic unit were also performed on Aspen Plus v8.4 and UNIQUAC was selected for thermodynamic properties calculations as only thermodynamic equilibria are considered in this unit.

4. Results and discussion

In this section, results for the "sour compression" and the cryogenic units are presented and analyzed especially regarding removal performances and energy consumption.

4.1. "Sour compression unit"

The sour compression unit was firstly simulated considering the treatment of a power plant flue gas (Case A) and the resulting compositions of gases from the DeSOx and DeNOx columns are presented in Table 4.

Table 4: Gaseous compositions at the outlets of the DeSOx and DeNOx units for Case A

Gas composition (%mol) DeSOx outlet DeNOx outlet

CO2 76.05 76.07

N2 14.87 14.95

O2 6.2 6.24

Ar 2.54 2.56

H2O 0.34 0.19

NO2 18 ppm 2 ppm

NO Trace Trace

SO2 Trace Trace

Total Flow Rate (mol/h) 4471 4444

It can be observed that 100% of the SOx and almost 95.2% (371 ppm down to 18 ppm) of the NOx present in the initial flue gas are removed in the first column, and that the NOx concentration is further reduced in the second column (from 18 ppm to 2 ppm NOx) leading to a 99.5% NOx removal over the sour compression unit. This is in great agreement with White et al.[12]: the first compression unit (working at 15 bar) removes all the SO2 and the bulk of NO to produce a dilute solution of H2SO4, and the second one (at 30 bar) removes the excess NO2 to produce a dilute solution of HNO2/HNO3.

The energy requirements of the sour compression unit were also investigated during the simulation and results are presented in Table 5 with specification of electrical and thermal needs.

Table 5 : Energy requirements for Case A

Power type Operation Power (kW) %

Electrical 1 - 15 bar compression 14.26 81

15 - 30 bar compression 3.27 19

Total 17.53 100

Thermal 1 - 15 bar cooler (30°C) - 18.01 82

30 bar cooler (30°C) - 3.92 18

Total - 21.93 100

As expected, the first compression step (1-15 bar) is the most demanding operation in terms of electrical needs requiring up to 14.26 kW which represents 81% of the electrical energy demand of the sour compression unit. In the meantime, the thermal energy demand of both the first coolers is predominant (18.01 kW) and represents together 82% of the thermal energy requirements for this unit.

The sour compression unit was then simulated considering a cement plant flue gas (Case B) and the same approach was conducted. The compositions of gases from DeSOx and DeNOx scrubbers were recalculated and results are presented in Table 6.

Table 6 : Gaseous compositions at the outlets of the DeSOx and DeNOx units for Case B

Gas composition (%mol) DeSOx outlet DeNOx outlet

CO2 83.82 83.89

N2 11.25 11.32

O2 3.21 3.23

Ar 1.36 1.37

H2O 0.35 0.19

NO2 28 ppm 3 ppm

NO Trace Trace

SO2 Trace Trace

Total Flow Rate (mol/h) 4703 4674

Once again, it can be observed that 100% of the SOx and almost 97.1% (957 ppm down to 28 ppm) of the NOx present in the initial flue gas are removed in the first column and that the NOx concentration is further reduced in the second column (from 28 ppm to 3 ppm NOx) leading to a 99.7% NOx removal over the sour compression unit. The same conclusion can thus be drawn about the removal of the SO2 and the bulk of NO in the first absorption column, and the removal of excess NO2 in the second one.

The energy requirements of the sour compression unit were also re-evaluated for a cement plant flue gas and the results are presented in Table 7 with specification of electrical and thermal needs.

Table 7: Energy requirements for Case B

Power type Operation Power (kW) %

Electrical 1 - 15 bar compression 14.58 81

15 - 30 bar compression 3.42 19

Total 18 100

Thermal 1 - 15 bar cooler (30°C) - 16.47 80

30 bar cooler (30°C) - 4.19 20

Total - 20.66 100

Similar observations can be noticed in comparison with results from the power plant flue gas simulations: the first compression step is still the most demanding in terms of electrical needs requiring up to 14.58 kW which also represents 81% of the electrical energy demand of the sour compression unit. The thermal energy of both the first coolers is still predominant and reaches 16.47 kW which represents 80% of the thermal energy requirements for this unit. Moreover, the net differences for the electrical and thermal requirements between power plant and cement plant flue gases can be estimated and these results show that the treatment of cement flue gases requires 18 kW (increase of 2.68% in comparison to 17.53 kW for power plant flue gases) for the electrical energy needs, and -20.66 kW (decrease of 5.8% in comparison to -21.93 kW for power plant flue gases) for the thermal energy needs.

In conclusion, the sour compression unit seems to be applicable for the treatment of cement plant flue gases with some minor adaptations. However, the optimization of operating parameters will be a crucial point in our further developments to refine the energy demand of the process and to estimate its operating and investment costs.

4.2. Cryogenic unit

As the good application of the sour compression unit has been pointed out for the treatment of cement plant flue gases, simulations of the cryogenic unit were performed considering only the cement plant flue gases composition

(Case B).

The desulphurized and denitrified gas enters the cryogenic unit and has the normalized composition of the DeNOx outlet stream where water is removed after the dehydration unit. The NO2 remaining from the sour compression unit (3 ppm) is also neglected as it will be captured during the adsorption process (dehydration unit). In the worst case, 3 ppm NO2 is far below the security limit of most CO2 conversion catalysts which are mainly based on CuO/ZnO/Al2O3 compounds[25].

The resulting inlet gas composition and flow rate which will be considered as the inlet of the cryogenic unit are presented in Table 8.

Table 8: Cryogenic inlet gas composition and flow rate

Gas composition (%mol)

CO2 84.05

N2 11.34

O2 3.24

Ar 1.37

Total Flow Rate (mol/h) 4551

Principal parameters of this unit are the temperatures of both flash units. With the composition stated in Table 8, the dew point of the gas is about - 18°C. As a consequence, the temperature of the first flash (T1) should be lower than -18°C to liquefy CO2 from the mixture. Moreover, the temperature of the second flash (T2) should always be higher than -55°C to avoid solid CO2 (dry ice) formation which can degrade the installation.

Considering the temperature range of the flash units, two parametric studies were conducted to quantify the influence of these temperatures on the CO2 molar purity of the final product stream and on the CO2 recovery of the unit. The results of these parametric studies are presented in Table 9 and in Table 10.

Table 9: Parametric study: Temperatures influence on CO2 molar purity (%) of the product stream

T1 | T2 (°C) - 25 - 30 - 35 - 40 - 45 - 50 - 55

- 20 98.4 98.2 98.0 97.8 97.6 97.3 97.1

- 25 97.9 97.8 97.7 97.5 97.3 97.2

- 30 97.3 97.3 97.2 97.1 96.9

- 35 96.7 96.7 96.6 96.5

- 40 96.1 96.1 96.0

- 45 95.5 95.4

- 50 94.8

Table 10: Parametric study: Temperatures influence on the CO2 recovery of the unit T1 | T2 (°C) - 25 - 30 - 35 - 40 - 45 - 50 - 55

- 20 78.5 84.9 89.1 91.9 94.0 95.5 96.6

- 25 85.2 89.2 92.0 94.0 95.5 96.6

- 30 89.5 92.2 94.1 95.5 96.6

- 35 92.4 94.3 95.7 96.7

- 40 94.5 95.8 96.8

- 45 96.0 97.0

- 50 97.1

These results highlighted the antagonist effects of the temperatures on the CO2 molar purity of the final product and on the global recovery: a low temperature in the flash units causes an improvement of the recovery (up to 97.1%) but a decrease of CO2 molar purity of the final product (from 98.4% to 94.8%), where a high temperature gives a very good CO2 molar purity (up to 98.4%) of the final stream but a lower recovery (78.5%).

To estimate the electrical and thermal energy requirements of the cryogenic unit, two different "operating points" were considered. In the first case (Case I), considering the CO2 molar purity of the product stream as the parameter to maximize, the temperatures of both flashes are -20°C and -25°C respectively and will lead to a 98.4% CO2 molar purity and a 78.5% CO2 recovery of the unit. In the second case (Case II), considering the CO2 recovery as the parameter to maximize, the temperatures of both flashes are -50°C and -55°C respectively and will lead to a 94.8% CO2 molar purity and a 97.1% CO2 recovery of the unit. The energy requirements of both cases were calculated and are presented in Table 11.

Table 11: Energy requirements of the cryogenic unit for Case I and II

Power type Operation Power (kW) %

Thermal 1st cooler (-20°C) - 11.57 87

2nd cooler (-25°C) - 1.68 13

Electrical 110 bar pump 1.00

1 bar turbine - 1.38

Thermal 1st cooler (-50°C) - 19.25 99

a 2nd cooler (-55°C) - 0.18 1

q Electrical 110 bar pump 1.16

1 bar turbine - 0.50

It can be observed that Case II has a higher energy demand in terms of both electrical and thermal needs. This observation can be explained by the fact that, in Case II, the liquid flow rate at the outlet of the cryogenic unit is higher due to the low temperatures of the flashes leading to a major increase of the thermal energy needs of the unit (+41% in comparison to Case I), and to an increase of electrical energy requirements for pumps.

In conclusion, the temperatures of both flashes will depend on the targeted objective based on:

• The required CO2 purity of the final product which depends on the following applications requirements. The range of CO2 molar purity reachable with the considered cryogenic unit is 94.8 - 98.4%. This range of purity is in agreement with CO2 purity of stream products coming from post-combustion CO2 capture which are typically ranged between 95 - 98%[26].

• The required CO2 recovery which depends on industry politics and/or country's laws. The range of CO2 recovery of the cryogenic unit is 78.5 - 97.1% and would lead to a global CO2 recovery range of the overall process of 75.8 - 93.8%. Once again, this recovery range is in agreement with those of post-combustion CO2 capture plants which are typically between 85 - 90%[26].

However, according to the precise objective, additional economic aspects (e.g. energy integration among the units, cooling mediums to use, etc.) will have to be considered.

4.3. Global results comparison

Considering the cement plant flue gas composition and flow rate at the inlet of the process (Case B), and the highest reachable CO2 molar purity for the cryogenic unit (Case I), the results of simulations are presented in terms of compositions and flow rates in Table 12 and in terms of energy demands in Table 13.

Nicolas Meunier et al. /Energy Procedia 63 (2014) 6492 - 6503 Table 12: Compositions and flow rates of gases over the process for cement plant flue gas

Gas composition (%mol) Inlet process DeSOx outlet DeNOx outlet Cryo inlet Product CO2

CO2 83.13 83.82 83.89 84.05 98.4

N2 11.11 11.25 11.32 11.34 1.0

O2 3.27 3.21 3.23 3.24 0.4

Ar 1.34 1.36 1.37 1.37 0.2

H2O 1 0.35 0.19 - -

NO2 96 28 ppm 3 ppm - -

NO 861 Trace Trace - -

SO2 156 Trace Trace - -

Total Flow Rate (mol/h) 4764 4703 4674 4551 3051

Table 13: Energy requirements for simulated oxy combustion cement plant flue gas

Power type Operation Power Energy *

(kW) (kWh/kg CO2)

Electrical Sour compressions 18 0.111

Cryogenic compressions - 0.38 - 0.002

Total 17.62 0.108

Thermal Sour compression coolers - 20.66 ** -

Cryogenic coolers - 13.25 0.082

* Required energy per kg liquid CO2 produced at the outlet of the cryogenic unit

** In the sour compression unit, no supplementary thermal energy is needed as the flue gases are cooled at room temperature.

These results have to be considered with care as it is only a partial approximation of the energy demand of the installation as the dehydration unit still need to be modeled.

5. Conclusion and perspectives

In the context of the optimization of the overall process of CO2 capture and reuse, it has been presented that the application of a CO2 purification process based on three different units (namely sour compression unit, dehydration unit, and cryogenic unit) can be applied to treat the flue gases coming from an oxyfuel power plant and an oxyfuel cement plant. The respective performances and energy demands have been compared for both the different compositions and it has been pointed out that the process could be applied to the purification of cement plant flue gases with some minor changes especially concerning the optimization of operating parameters in order to reduce the global energy demand.

Considering the composition of flue gases coming from a simulated oxyfuel cement plant, it has been presented that the SOx present in these flue gases could be completely removed inside the sour compression unit and that almost 99.5% of the NOx could also be eliminated by both absorption columns. However, further investigations will be conducted in future works concerning the interactions between SOx and NOx compounds in order to improve the description of the reaction system occurring in both absorption columns.

It has also been observed that the range of CO2 molar purity of the final product of the cryogenic unit and the range of CO2 recovery of the overall process could respectively reach 94.8 - 98.4% and 75.8 - 93.8% according to the temperatures set in the flash units which are directly related to the maximization of (or trade-off between) CO2 purity and CO2 recovery.

Finally, this study provides a first approximation of energetic costs for the purification of CO2 coming from oxyfuel cement plant and the high CO2 purity (>98%) reachable at the end-of-pipe of the process is very encouraging and could allow the reuse of the CO2 product in other applications such as its catalytic conversion into methane, methanol, or other chemically valuable compounds.

Acknowledgements

UMONS acknowledges the European Cement Research Academy (ECRA) for the technical and financial supports accorded to the ECRA Academic Chair.

References

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