Kinetics and Design Parameter Determination for a Calciner Reactor

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Kinetics and Design Parameter Determination for a Calciner Reactor in Unique Conditions of a Novel Greenhouse Calcium Looping Process Mohammad Ramezani, Priscilla Tremain, Kalpit Shah, Elham Doroodchi, and Behdad Moghtaderi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01882 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 18, 2017

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Energy & Fuels

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Kinetics and Design Parameter Determination for a Calciner Reactor in Unique Conditions

2

of a Novel Greenhouse Calcium Looping Process

3

Mohammad Ramezani1, Priscilla Tremain1, Kalpit Shah2, Elham Doroodchi1 and Behdad

4

Moghtaderi1* 1

5 6 7 8 9 10 11

The Priority Research Centre for Frontier Energy Technologies & Utilisation, Chemical Engineering, School of Engineering, Faculty of Engineering and Built Environment, The University of Newcastle, Callaghan, New South Wales 2308, Australia 2

Chemical and Environmental Engineering, School of Engineering, RMIT University, Melbourne, Victoria 3001, Australia

12 13

Abstract

14

The

greenhouse

calcium

looping

process

(GCL)

is

based

on

the

cyclic

15

carbonation/calcination reaction of limestone under mild operating conditions, e.g.

16

temperatures of 600 to 800 oC and CO2 partial pressures of 400 to 1600 ppm (0.04-0.16 %).

17

The GCL process can provide the required heat and CO2 enrichment to greenhouses with

18

limestone particles instead of fossil fuels such as natural gas. In this research, the calcination

19

reaction of the GCL process was studied in a thermogravimetric analyser (TGA) to determine

20

its kinetic parameters. The experimental results showed that the calcination reaction follows a

21

zero order reaction with a pre-exponential factor of 6.9 × 106 min-1 and an activation energy

22

of 103.6 kJ mol-1. Various models were analysed to find out the most appropriate model for

23

predicting the calcination reaction in the GCL process, of which the function G(x) = 1 - (1 -

24

x)1/3 was found most appropriate through the least-square linear fitting of experimental data.

25

The derived kinetic parameters were used in Aspen Plus® v7.3 to establish the minimum

26

calciner volume considering an RPlug unit. To obtain a practical overview of the energy

*

Corresponding Author: [email protected] (B. Moghtaderi). Tel.: (+61)2 4033 9062. Fax: (+61)2 4033 9095.

1 ACS Paragon Plus Environment

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efficiency of the calcination reaction in the GCL process, an exergy analysis was conducted

2

which highlighted an overall exergetic process efficiency of 85 %.

3

Keywords: Calcination reaction; Calcium looping process; Kinetic study; Low CO2 partial

4

pressure; Aspen Plus simulation; Exergy analysis

5 6 7


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Energy & Fuels

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1. Introduction

2

Plants need CO2 during the day for growth via photosynthesis, a reaction which utilises

3

light energy to convert CO2 and water to carbohydrate molecules like glucose1. Interestingly,

4

previous studies1, 2 reported an increase in growth and crop production by up to 40 % when

5

CO2 concentrations were increased from the atmospheric level of 390 ppm to levels as high

6

as 800-1600 ppm. Heating of greenhouses is also paramount particularly during cold nights

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even in mild climate zones such as Australia. According to American standards3, maintaining

8

greenhouses’ temperature in a range of 22 to 32 oC during the day and 16 to 24 oC during the

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night can boost crop yield and growth. To suffice the temperature requirement and CO2

10

enrichment of greenhouses, the majority of greenhouses use conventional natural gas burner

11

systems. The price of natural gas and the associated CO2 emissions4 have led to energy costs

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for greenhouses to be ranked third after labour and material costs5. This has created a push to

13

provide the heat requirement and CO2 enrichment for greenhouses from renewable energy

14

sources like biomass and solar energy rather than fossil fuels.

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Following this trend, a novel renewable process which is based on the calcium looping

16

process was proposed in our previous publications6, 7. This process, known as the greenhouse

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calcium looping (GCL) process, uses limestone (CaCO3) as a key sorbent in the cyclic

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carbonation/calcination reactions, providing the required heat as well as CO2 enrichment to

19

greenhouses. This study was involved in a broad area of chemical looping research which has

20

been carried out at the University of Newcastle since 20078, 9. These chemical looping based

21

processes need metal oxide oxygen carriers, while in the GCL process, proposed in our

22

previous study6, limestone was used as the key sorbent and worked via the cyclic

23

carbonation/calcination reaction mechanism. As mentioned before, the concept of the GCL

24

process was based on the principles of calcium looping technologies, which are capable of

25

reducing post combustion CO2 emissions from fossil fuel power plants10. For example, the


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calcium looping process for post combustion CO2 capture has been successfully operated at

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steady state conditions for more than 1800 hours in a 1.7 MWth pilot plant at CSIC-INCAR,

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Spain11. A CO2 capture efficiency of 90 % was reported to be successfully achieved if an

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appropriate amount of solid inventory was introduced to the reactor.

5 6

Two key reactions occur in the GCL process which are the carbonation and the calcination reactions (∆Hr, 298K values were determined using HSC Chemistry 6 software)6:

7

CaO+ CO2 → CaCO3

∆H r ,298K = −178.2 kJ mol-1

(R1)

CaCO3 →CaO+CO2

∆H r ,298K = 178.2 kJ mol-1

(R2)

8 9

During the night, the carbonation reaction (R1) occurs and due to its exothermic nature,

10

the produced thermal energy can be used to provide an appropriate temperature in

11

greenhouses. The produced heat might also be transferred to a waste heat recovery unit for

12

power generation (e.g. an organic Rankine cycle unit). In comparison, during day, the

13

calcination reaction (R2) occurs, through which the CO2 requirement for greenhouses (e.g.

14

800-1600 ppm) can be produced and used for photosynthetic reactions in crops.

15

In our previous simulation study6, the feasibility of the GCL process was confirmed using

16

Aspen Plus and it was concluded that a reduction in energy consumption of up to 70 % could

17

be achieved in comparison to conventional burner systems. The integrated options of solar,

18

biomass or natural gas systems with the GCL process were also taken into account in

19

simulation matrix (more details can be found in our past publication6). The conceptual design

20

of the GCL process was carried out in our previous work considering the carbonator and

21

calciner reactors as Gibbs reactors which operate on the basis of minimization of Gibbs free

22

energy. The reactions’ stoichiometry kinetics are not required when using a Gibbs reactor in

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Aspen Plus as this unit predicts the thermodynamic equilibrium of reactants and products at 4 ACS Paragon Plus Environment

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given process conditions (more details regarding the Gibbs reactor can be found in the Aspen

2

Plus user guide12). A plug flow reactor can be used instead of a Gibbs reactor to establish the

3

practical size of a reactor in a process. The plug flow reactor (RPlug unit in Aspen Plus)

4

works on the basis of an ideal plug flow reactor, assuming perfect radial mixing and no

5

mixing along length of the reactor13. To simulate the process with an RPlug unit, the

6

stoichiometry and kinetics of the reaction is required. Recently, we published7 our

7

investigation on deriving kinetic parameters of the carbonation reaction in the unique

8

conditions of the GCL process (low temperature and CO2 partial pressure ranges) and these

9

kinetic parameters were then utilised in Aspen Plus simulations of the carbonation reaction of

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the GCL process considering an RPlug reactor.

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Regarding the calcination reaction, Table 1 summarizes the literature investigations for

12

the derivation of kinetic parameters for the calcination reaction. A number of studies have

13

been published at a CO2 concentration of 0 % where the reaction was conducted in a purely

14

inert gas such as N2. For instance, Borgwardt14 determined the kinetics of the calcination

15

reaction of two natural limestone containing 95 % CaCO3 in a reactor (considered as a plug

16

flow reactor) capable of operating with higher sweep gas (N2) throughput and smaller particle

17

dispersion densities than when operating with a TGA. The calcination reaction was

18

conducted in a temperature range of 515 to 1000 oC and in presence of pure N2. The reaction

19

was reported to be zero order regarding the partial pressure of CO2 with conclusion of

20

activation energy of both limestones to be 201 and 205 kJ mol-1, for 10 and 1 µm particles,

21

respectively. Dennis and Hayhurst15 established the calcination reaction to be first order

22

regarding the CO2 partial pressure which was varied from 0 to 20 % in a mixture of air or N2.

23

The natural limestone particles with the size of 0.4-2.0 mm were heated in a fluidized bed

24

reactor in a temperature range of 800-975 oC. The overall pressure of the reactor was changed

25

from atmospheric pressure to 18 bar. The activation energy was reported to be the enthalpy of


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decomposition of CaCO3 which is 169 kJ mol-1 and the pre-exponential factor was 1.0×10-5

2

(kmol m-2 s-1). As can be seen in Table 1, a larger number of studies have focused on deriving

3

kinetic parameters of calcination reaction in operational conditions which are applied mostly

4

in post combustion CO2 capture in fossil fuel power plants. Generally, the calcination

5

reaction kinetics reported in previous studies were carried out in a CO2 partial pressure of 0

6

% or more than 10 % and at temperatures more than 800 oC. Accordingly, there is a

7

knowledge gap when investigating the calcination kinetics at the GCL processes operational

8

conditions e.g. in a CO2 partial pressure range of 0.04-0.16 % (or 400-1600 ppm) and a

9

temperature range of 600-800 oC.

10

The decomposition of CaCO3 was reported by several authors to be chemically controlled

11

regime for both small particles (1-90 µm)16 and larger particle sizes (less than 2 mm)15. Two

12

kinds of models have been considered to predict the calcination reaction, namely, grain size

13

models and pore models. For grain size models, the particles are assumed to have a porous

14

structure consisting of a matrix of non-porous small grains, having spherical shapes. For

15

example in the calcination reaction, it is assumed that the product CaO consists of grains

16

around the unreacted core grain CaCO3. Therefore, the unreacted core radius of CaCO3

17

shrinks as the reaction proceeds, the result is an increase in porous structure of CaO because

18

of its lower molar volume compared to CaCO317. For pore models or the shrinking core

19

model, it is assumed that the decomposition of CaCO3 occurs at the CaO-CaCO3 interface,

20

followed by diffusion of CO2 through the CaO to the particle surface, then releasing CO2

21

from the particle surface to the bulk gas, and finally continuous shrinking of the unreacted

22

core of CaCO318.

23

Accordingly, there is a knowledge gap considering various kinds of kinetic models in the

24

unique conditions of the GCL process for predicting the calcination reaction. The derivation

25

of kinetics for dilute CO2 streams is not only useful the GCL process but also has


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applicability in scenarios such as CO2 capture from fossil fuels as well as the capture of CO2

2

in areas such as ventilation air methane (VAM) in which CO2 concentrations are often below

3

0.5%. Furthermore, it would be practically important to use the experimentally derived

4

calcination kinetics in a simulation study to find out the appropriate calciner reactor size and

5

energy efficiency of the novel GCL process. Therefore, the main objectives of this study are

6

as follows:

7

•

8

Establish the kinetic parameters of the calcination reaction at the unique operating conditions of the GCL process.

9

•

Derive a suitable kinetic model for the calcination reaction process.

10

•

Study the minimum reactor size required and conditions for the GCL process in

11 12

Aspen Plus, using the derived kinetic parameters. •

Conduct an exergy analysis on the GCL process.

13 14

2. Experimental methods

15

To establish the calcination kinetic parameters, a set of experiments were conducted in a

16

thermogravimetric analyser (TGA). 5 mg of raw Omya limestone sorbent (75-150 µm),

17

provided by a local Australian supplier, was inserted into a platinum crucible of a Q50 TGA.

18

The elemental analysis of this sorbent, carried out in X-ray fluorescence (XRF) apparatus

19

(Spectro X-lab 2000), can be found in our previous publication 7. At ramp rate of 20 oC min-1,

20

the limestone sample was initially heated to 700 oC, in a N2 environment, and held

21

isothermally for 10 minutes to ensure that the limestone sorbent was completely converted to

22

CaO. For the carbonation/calcination reaction, industrial grade CO2 diluted in air supplied by

23

Coregas, was introduced to the TGA by a mass flow controller (MFC). The total flow rate of

24

gases was set at 200 mL min-1 at atmospheric pressure inside the TGA. Experiments were

25

conducted at a high flow rate of 200 mL min-1, to minimise external mass transfer resistance 7 ACS Paragon Plus Environment

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due to the formation of gas films around the particles. The carbonation reaction was

2

conducted at a temperature of 450 oC and CO2 partial pressure of 1000 ppm (0.1 %). When

3

the carbonation reaction was completed (there would be an insignificant change of sample’s

4

weight due to the carbonation reaction), the gases inside the TGA (air and CO2) were

5

switched to 100 % CO2 to stop the incidence of the calcination reaction before reaching the

6

desired calcination temperature. Once the calcination temperature was reached (600-800 oC),

7

the gases were reverted to CO2 partial pressures of 400-1600 ppm. To be assured of the

8

completion of the calcination reaction, experiments were only concluded when the sample

9

weight did not change for a minimum of 20 minutes. It should be noticed that pure CO2

10

environment caused the sorbent to be converted to pure CaCO3 and therefore the kinetic

11

study of the calcination reaction in this study is applied for one cycle. All experiments were

12

repeated 3 times to obtain reliable data. The conversion of CaCO3 particles via the calcination reaction was calculated on a mass

13 14

basis using Equation (1):

15 x=

m CaCO3 − m CaCO3 (t )

Eq (1)

m CaCO3 − m CaO

16

where x is the CaCO3 conversion, mCaCO3 (t) is the weight of the sample at time t, mCaCO3 is

17

the initial weight of CaCO3 just before the start of calcination reaction and mCaO is the sample

18

weight at the end of the calcination reaction.

19 20

The general equation for a gas-solid reaction is a function of temperature, reaction mechanism and equilibrium gas pressure ( PCO,eq)19: 2

21

dx = kf ( x )( PCO 2 , eq − PCO 2 ) n dt

Eq (2)

22 8 ACS Paragon Plus Environment

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where t is time, f(x) is a function for the reaction mechanism, n is the reaction order and k is

2

the reaction rate constant based on the Arrhenius equation:

3

−E k = k0 exp   RT 

Eq (3)

4 5

where k0 represents the pre-exponential factor, E is the activation energy, R is the gas

6

constant and T is the temperature. Baker20 postulated an empirical correlation to calculate the CO2 equilibrium partial

7 8

pressure which is dependent on the carbonation/calcination reaction temperature:

9

8307.83 T [k ]

log PCO2,eq [kPa] = 9.079 −

Eq (4)

10 11 12

G(x) function represents different reaction mechanisms resulted by integrating Equation (2):

13 x

G (x ) = ∫

(

0

= k PCO ,eq − PCO 2

(

t 1 dx = ∫ k PCO ,eq − PCO f (x ) 0 2

) dt n

2

Eq (5)

)t n

2

14 15

The calcination reaction mechanisms, used to determine the calcination reaction rate, are

16

summarised as G(x) functions in Table 2.

17 18

3. Process design and simulation

19

Process simulation and reactor modelling are key and initial steps before cost intensive

20

process demonstration. This is due to the fact that process efficiencies and optimisation or


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1

selectivity strategies can be revealed through accurate and reasonable simulation

2

assumptions21. As previously mentioned, a conceptual study of the GCL process was fulfilled

3

in our past research6 utilising Gibbs reactors as carbonator/calciner reactors which simulate

4

the reactor on the principle of Gibbs free energy minimization. In our recent publication7 , the

5

carbonation reaction of the GCL process was simulated using plug flow reactor (RPlug unit

6

in Aspen Plus) as a carbonator with the kinetic parameters derived experimentally.

7

Accordingly, in the current study, the calcination reaction of the GCL process is simulated

8

using RPlug reactor and experimentally derived kinetic parameters. Details about the RPlug

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unit function, mass and energy balance equations used in RPlug blocks and associated

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property database for conventional and solid compounds can be found from Aspen Plus

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manual12. The kinetic parameters of the calcination reaction, derived experimentally, were

12

introduced to the reaction tools of Aspen Plus. The assumptions and input parameters such as

13

components, property method and units are the same as our previous research7 focusing on

14

the carbonation reaction of the GCL process (please refer to this publication7 for more

15

details).

16

As mentioned previously, the calcination reaction takes place during day in the GCL

17

process to suffice the CO2 enrichment of greenhouses. A CO2 enrichment level of 1000 to

18

2000 ppm was fulfilled in our previous study6, focusing on the feasibility of the GCL process

19

using RGibbs reactors. In this work, like our recent publication7, the simulation matrix

20

contains 18 cases considering three different greenhouse sizes (bench-, pilot- and typical-

21

scale) and six different air changes per hour varying from 10 to 60 hr-1 as detailed in Table 3.

22

Figure 1 shows the process flow diagram (PFD) for case L (refer to Table 3), as an example

23

out of the 18 cases, for the calcination reaction in the GCL process. As can be seen in the

24

PFD of the calcination process, 0.00035 kmol hr-1 of fresh calcium carbonate (CaCO3) was

25

converted to calcium oxide (CaO) in an RPlug reactor at 860 oC. The optimization tools of


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Aspen Plus were used to establish the minimum RPlug reactor size at which at least 94 % of

2

reaction conversion could be reached. A conversion target of 94% was chosen as the

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equilibrium limit of conversion for the calcination reaction was 95%, after which the reactor

4

size proceeded to infinity. Hence at these conditions, the minimum size of the calciner for

5

case L was established to be 0.008 m in length and 0.001 m in diameter. The CO2 enrichment

6

of the greenhouse was achieved by the exhaust gas from the RPlug calciner, consisting of

7

1700 ppm (0.17 %) CO2. To reduce the thermal energy of the exhaust gas leaving the

8

calciner, a heat exchanger was used in which the thermal energy was transferred to 0.4 kmol

9

hr-1 of ambient air (with a CO2 concentration of 400 ppm), leading to an appropriate

10

temperature for the greenhouse. High temperature gases leaving the heat exchanger were

11

recycled to the calciner to compensate the thermal energy requirement of the reactor. This

12

was fulfilled by a mixer in which the recycled gas was mixed with CaCO3 inventory,

13

followed by a blower to increase the inventory pressure from 100 kPa to 125 kPa to

14

overcome the pressure drop in the calciner. The required energy for the endothermic

15

calcination reaction can be sufficed using natural gas, biomass or a solar system, with the full

16

details of these hybrid GCL systems presented in our previous publication6. The calcination

17

reaction kinetic parameters required for the simulation of RPlug reactor were derived

18

experimentally and introduced to the reaction tool of Aspen Plus. In addition, the kinetic

19

parameters of the reverse reaction, the carbonation reaction, were also taken into account by

20

using the results of our recent publication regarding determination of the carbonation reaction

21

parameters7. The conversion of MgO to MgCO3 was also taken into account as there was a

22

small amount of MgO in the inventory. The related kinetic parameters were derived from the

23

study performed by Helpa et al.22 and set accordingly to the reactions tool of Aspen Plus.

24

Table 4 summarizes molar and energy balances as well as thermodynamic properties of each

25

stream of the PFD presented in Figure 1.


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An exergy analysis of the calcination reaction in the GCL process was also conducted,

2

using Aspen Plus and the method proposed by Querol et al.23, of which the outcomes can be

3

seen at the end of Table 4. Exergy analysis can reveal the irreversibility of a process using

4

both the first and second laws of thermodynamics. The exergy of a process consists of three

5

categories: exergy of work transfer, exergy of heat transfer and exergy related to a steady

6

stream of matter. All of the equations and assumptions applied to calculate the exergy

7

efficiency of each unit in the process and the overall process were the same as our recent

8

publication7. Like our previous work, for every stream a heater block was considered to be

9

working at reference conditions of 25 oC and 1 atm and according to the method suggested by

10

Querol et al.23, there was not requirement to calculate the mixing exergy term when

11

determining the chemical exergy of a stream of matter. The Aspen Transfer utility option was

12

also utilised in the current study, which transferred all the thermodynamic properties of each

13

stream to a new feed stream of the heater block operating at reference conditions. Since the

14

methods and assumptions used to determine the exergy analysis of the calcination process of

15

current research were the same as our previous publication7, please refer there for more

16

details.

17 18 19 20

4. Results and discussion At first, the experimental results will be discussed, followed by the outcomes of the Aspen Plus simulations and exergy analysis.

21 22

4.1. Derivation of kinetic parameters of calcination reaction rate

23

Figure 2 illustrates the CaCO3 conversion versus time, determined by Eq. (1), at a CO2

24

partial pressure of 400 ppm (0.04 %) and temperature range of 600 to 800 oC. As expected,

25

with increasing temperature from 600 to 800 oC, the calcination reaction rate increased. 12 ACS Paragon Plus Environment

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According to Eq. (4), proposed by Baker20, an increase in temperature causes an increase in

2

CO2 equilibrium partial pressure and similarly in the driving force of PCO ,eq − PCO

3

resulted in a higher calcination reaction rate. At temperatures greater than 600 oC, the

4

calcination reaction was completed in less than 12 minutes, whereas at 600 oC the reaction

5

was completed after 40 minutes. This is due to the fact that 600 oC is close to the equilibrium

6

temperature at which the calcination reaction occurred for a CO2 partial pressure of 400 ppm.

7

It is worth mentioning that when the calcination reaction occurs at low temperature (e.g. 600

8

o

9

operational cost and the related issues such as sintering which happens at high temperatures

10

(

2

2

) which

C), a larger reactor is required as the reaction rate is comparatively slow. However, the

would be decreased.

11

The reaction rate constant, k, in the right hand term of Eq. (5) depends on temperature

12

and, therefore, by plotting the G(x) functions (summarized in Table 2) versus time for an

13

isothermal temperature enables the determination of the most appropriate reaction

14

mechanism. Accordingly, R2 values for the least square fits for each reaction mechanism are

15

tabulated in Table 5 for CO2 partial pressures of 400-1400 ppm and a temperature range of

16

600-700 oC. The averaged R2 values of each reaction mechanism are represented in Table 6.

17

The highest value represents the most appropriate G(x) function which is R3 or G(x) = 1 - (1 -

18

x)1/3.

19

(

According to Eq. (5), to establish the k P − Peq

)

n

term of the calcination reaction, a plot

20

the function, G(x), versus time is required and the slope of each curve determined. These

21

functions are demonstrated in Figure 3, for a CO2 partial pressure of 400 ppm and a

22

temperature range of 600-800 oC. Using the same method, the values of k P − Peq

23

determined for CO2 partial pressures of 400 to 1600 ppm and are presented in Figure 4 and 5,

24

for temperatures of 700 oC and 800 oC, respectively. The slope of curves in Figure 4 and 5,

(


)

n

were

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[(

) ] versus ln(Peq − P) , can be used to establish the order of calcination

1

showing ln k Peq − P

2

reaction and the intercepts are the reaction constant (k). The slopes derived from Figure 4 and

3

5 (0.022 and 0.023, respectively) clearly show that the calcination reaction follows a zero

4

order reaction based on the CO2 partial pressure in the unique operational conditions of the

5

GCL process (low temperature range of 600-800 oC and low CO2 partial pressure of less than

6

0.2 %). The independence of the calcination reaction rate on the CO2 partial pressure was

7

also reported by other authors14,

8

potential rate controlling processes: (1) heat transfer from the bulk gas to the surface of the

9

particles and then through the CaO product layer to the reaction interface, (2) chemical

10

reaction at the interface and (3) mass transfer of the CO2 product from the interface to the

11

bulk gas through the porous structure of the CaO layers. It was reported that for particles

12

more than 1 cm in diameter, the mass and heat transfer resistance are significant which

13

should be taken into account. Ar and Dogu24 suggested that for particles more than 1.7 mm in

14

size, the mass transfer diffusion might have an effect on the calcination reaction. In this

15

study, the particle size was between 75 to 150 µm and during the experiments high

16

throughput of feed gas and sweep gas was assured. Therefore, the decomposition of CaCO3

17

was assumed to be controlled chemically not by mass or heat transfer resistance. On the other

18

hand, a first order reaction was reported by other researchers25. Abanades’s team25 concluded

19

that the calcination reaction follows a first order reaction on the operational conditions of a

20

temperature range from 825-920 oC and a CO2 partial pressure of 0 to 100 %. It is worth

21

mentioning that the level of CO2 examined in their experiments was considerably more than

22

our study. A CO2 content of 25 % could cause a significant film resistant layer which hinders

23

CO2 mass transfer from the interface to the bulk phase. In contrast, experiments conducted in

24

this study used a maximum CO2 content of 0.2 % which causes a very thin layer of CaCO3 on

n

24

. The thermal decomposition of CaCO3 follows three


Page 14 of 43

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Energy & Fuels

1

the product CaO and consequently has an insignificant effect on the mass transfer between

2

the interface and bulk phase.

3

Using the reaction constants (k) obtained from Figure 4 and 5, ln (k) versus 1/T was

4

plotted in Figure 6. This Arrhenius plot covers the calcination reaction rate in operational

5

temperature conditions of 600-800 oC and CO2 partial pressure of 400-1600 ppm. A least-

6

square linear fit was then carried out, where the intercept of the fitted line determined the pre-

7

exponential factor and the slope represented the activation energy of the calcination reaction.

8

The calculated kinetic parameters of the calcination reaction over the unique operational

9

conditions of the GCL process are summarized in Table 7. The calculated activation energy

10

of 103.6 kJ mol-1, was less than those reported in previous literatures which concluded that

11

the activation energy for the calcination reaction was in the range of 147-212 kJ mol-1 14, 24, 26,

12

27

13

related to the difference in the porous structure of the limestone samples and/or the

14

operational conditions of which the calcination reaction occurred (i.e. temperature of 850-

15

1150 oC and CO2 partial pressure of 10-100 %). High temperature (more than 850 oC) and

16

high CO2 partial pressure are associated with higher occurrences of sintering in calcined

17

particles14 which increase the required activation energy. Sintering causes the morphology of

18

the particles to change such as changes in pore structure, pore shrinkage and grain growth28.

19

Impurities in the sample may also act as catalyst during the calcination reaction, resulting in a

20

lower activation energy compared to pure samples. On the other hand, the current calculated

21

activation energy (103.6 kJ mol-1) is in a good agreement with studies conducted by Rao29,

22

Calvo et al.30 and Martinez et al.25 who reported the activation energies of 92 to 112 kJ mol-1

23

for different commercial and natural limestones (please refer to Table 1 for more details).

. The discrepancy between literature results and the current activation energy might be

24


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

4.2. Aspen Plus simulation results

2

The experimentally derived kinetics of the calcination reaction, reported in Table 7, were

3

introduced into the reaction tool of Aspen Plus to simulate the calcination reaction in the

4

GCL process. An RPlug reactor was used to model the calciner and the model analysis tools

5

in Aspen Plus were used to determine the minimum reactor size to achieve the maximum

6

possible conversion of CaCO3 (94 %) in the calciner. The sensitivity tools in Aspen Plus

7

helped to determine the optimum reactor temperature at which the minimum heat duty was

8

required. Please refer to our previous publication6 for more details. The minimum reactor

9

size, heat generated from the calciner and the work consumption of the blower are

10

summarized in Table 8 for every case of the simulation matrix, which was chosen

11

considering three different greenhouse sizes and six different air changes per hour varying

12

from 10 to 60 hr-1 (see Table 3). Based on our past experience with chemical looping

13

reactors, the actual reactor volume will be anywhere between 1.5 to 2 times greater than that

14

predicted by the plug-flow model due to hydrodynamic considerations7. It can be noticed that

15

as the number of air circulations per hour increases, both the calciner’s heat generation (- sign

16

means generation) and the blower’s consumption of work (+ sign means consumption)

17

increased. This is because as the air circulation increases the amount of CaCO3 as inventory

18

required increases, which resulted in more required work for blower to intensify the pressure

19

of the increased amount of CaCO3 inventory from 100 kPa to the objective pressure of 125

20

kPa suitable for the calciner. The products of the blower contain inventory at a higher

21

temperature and pressure than that of the calciner (e.g. for case L, the inventory leaves the

22

blower at a temperature of 934 oC and total pressure of 125 kPa, while the calciner is

23

operated at 860 oC and 100 kPa), this led the calciner to produce heat instead of requiring

24

heat. The same trend can be seen when enlarging the greenhouse’s size from bench-scale to


Page 16 of 43

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Energy & Fuels

1

the typical-scale. Like the previous case, this is due to an increase in the required amount of

2

CaCO3 inventory with an increase in the size of the greenhouse.

3

The effect of calciner temperature on the minimum reactor size required is shown in

4

Figure 7 in which case L of the simulation matrix was selected as an example. As described

5

before, the optimization tool in Aspen Plus was used to establish the minimum reactor size

6

required with the objective of achieving the highest possible conversion of CaCO3 which was

7

more than 94 %. Due to the fact that at the unique conditions of the GCL process (e.g. low

8

temperature and CO2 partial pressure) the calcination reaction was controlled primarily

9

kinetically, increasing the temperature resulted in a faster calcination reaction. This resulted

10

in CaCO3 needed less residence time to be converted to CaO and CO2 and therefore reducing

11

the required reactor volume.

12 13

4.3. Exergy analysis results

14

At the end of Table 4, the exergy calculations of the calcination reaction in the GCL

15

process can be seen which was conducted for case L of the simulation matrix (the exergy

16

analysis related to other cases in the simulation matrix were similar to case L). Table 4

17

presents the detailed information of each stream’s exergy including the physical and chemical

18

exergy, heat exergy of the calciner and consumed exergy of the blower. The exergetic

19

efficiency of every unit and the whole process, presented in the Table 4 which was calculated

20

by related equations (please refer to this publication7 for more details), and shows that the

21

exergetic efficiency of the blower was the least at 98 % while for the other three blocks it was

22

more than 99.8 %. One of the reason for the lower exergetic efficiency of the blower might

23

be related to its assumptions for the running of the simulation involving the isentropic

24

efficiency of 72 % and mechanical efficiency of 95 %. The overall exergetic efficiency of the

25

GCL process was 85.3 % for case L. In addition, the overall exergetic efficiency of the 17 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

calcination reaction was more than that of carbonation reaction in the GCL process which

2

was reported to be 83.9 %7. This is due to the operation of the calcination reaction at higher

3

temperature than the carbonation reaction, resulting in a higher exergy ratio of the leaving

4

streams to entering streams for the whole process. Furthermore, the ratio of the heat

5

generation of the calciner to the work consumption of the blower was also increased in the

6

calcination reaction of the GCL process compared to the carbonation reaction. It is worth

7

mentioning that the exergy efficiency of the calcination reaction in the GCL process in the

8

real plant would be less than 85.3 % due to the associated heat losses to the environment from

9

reactor and piping.

10

The effect of temperature on exergetic efficiency of the whole process and every unit

11

including the Mixer, Blower, Calciner and SSplit for case L of the calcination reaction in the

12

GCL process. While the exergetic efficiency of three units: Mixer, Calciner and SSplit did

13

not changed with temperature, the blower’s exergetic efficiency increased insignificantly

14

from 97.9 % at 800 oC to 98.4 % at 1100 oC. This trend resulted in an insignificant increase in

15

the overall exergetic efficiency of the process from 85.2 % at 800 oC to 86 % at 1100 oC.

16

Since increasing the temperature does not have a significant positive exergetic effect on the

17

calcination reaction, it is recommended that the calcination reaction of the GCL process be

18

carried out at lower temperatures (800-900°C) where the calcination reaction occurs

19

kinetically.

20 21

5. Conclusion

22

The kinetic parameters of the calcination reaction of the GCL process were determined

23

via TGA experiments and least square linear fitting of experimental data over the temperature

24

range of 600 to 800 oC and CO2 partial pressure of 400 to 1600 ppm (0.04-0.16 %). The most

25

appropriate model for predicting the calcination reaction in the unique operational conditions 18 ACS Paragon Plus Environment

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Energy & Fuels

1

of the GCL process was found to be a phase boundary reaction function or G(x)=1-(1-x)1/3.

2

As expected, with an increase in temperature, the calcination reaction rate increased while the

3

calcination reaction followed a zero order reaction rate based on the CO2 partial pressure, in

4

the unique operational conditions of the GCL process. To establish the minimum reactor size

5

required with the objective of reaching more than 94 % CaCO3 conversion, the calcination

6

reaction was simulated in Aspen Plus using an RPlug reactor and the experimentally derived

7

kinetic parameters. The optimization tool of Aspen Plus was used to determine the minimum

8

reactor volume for three different sizes of the greenhouse and 6 different air circulation rates.

9

To obtain a practical and precise efficiency of the GCL process, an exergy analysis was

10

conducted as well as the energy footprint of the calciner and blower used in the simulation

11

process. The main outcomes from this research are:

12

•

factor of 6.9 × 106 min-1 and an activation energy of 103.6 kJ mol-1.

13 14

The calcination reaction followed zero order reaction rate with a pre-exponential

•

RPlug blocks and the optimization tool of Aspen Plus helped to establish the

15

minimum reactor volume which increased with enlarging greenhouse size and

16

increasing air changes per hour.

17

•

The overall exergetic efficiency of the calcination reaction in the GCL process was

18

found to be 85 % while an increase in temperature had an insignificant effect on

19

efficiency.

20

For future investigations, it is recommended that research on multiple carbonation and

21

calcination cycles in a lab-scaled reactor are conducted as well as a study of the morphologic

22

structure of the CaCO3 and/or CaO particles in the unique conditions of the GCL process.

23 24

Acknowledgement


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

The authors appreciate the financial support provided by the University of Newcastle, Australia.

3 4

Nomenclature Exch

Chemical exergy, (W)

Exeff, overall

Overall exergetic efficiency of a system, (%)

Exeff, unit

Exergetic efficiency of each unit, (%)

Exph

Physical exergy, (W)

f (x)

Function of reaction mechanism in Eq. (2)

G(x)

Function of reaction mechanism in Eq. (5)

h

Molar enthalpy of a stream at its initial conditions, (kJ kmol-1)

h0

Molar enthalpy of a stream at reference conditions, (kJ kmol-1)

∆H

r , 298 K

Heat of reaction at temperature of 298 K, (kJ mol-1)

k

Reaction rate constant in Eq. (3), (min-1)

k0

Pre-exponential factor, (min-1)

n

Order of reaction

mCaCO3

Initial weight of CaCO3 just before the start of calcination reaction, (g)

mCaCO3 (t)

Weight of sample at time t, (g)

mCaO

Sample weight at the end of calcination reaction, (g)

PCO

Partial pressure of CO2 at time t, (kPa)

eq PCO 2

Equilibrium partial pressure of CO2, (kPa)

ppm

Parts per million

R

Gas constant, 8.314 × 10-3, (kJ mol-1 K-1)

s

Molar entropy of a stream at its initial conditions, (kJ kmol-1 K-1)

s0

Molar entropy of a stream at its reference conditions, (kJ kmol-1 K-1)

t

Time, (min)

T

Temperature, (K)

x

CaCO3 conversion, (%)

2

Abbreviations CLAS

Chemical looping air separation 20 ACS Paragon Plus Environment

Page 20 of 43

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Energy & Fuels

GCL

Greenhouse calcium looping process

IGCC

Integrated gasification combined cycle

MFC

Mass flow controller

PFD

Process flow diagram

RGibbs

Gibbs reactor

RPlug

Plug flow reactor

TGA

Thermo-gravimetric analyser

XRF

X-ray fluorescence analyser

1 2 3

References

4

1.

Nelson, P. V., Greenhouse operation and management. Prentice Hall: 1991; p 612.

5

2.

Lawlor, D.; Mitchell, R., The effects of increasing CO2 on crop photosynthesis and

6

productivity: a review of field studies. Plant, Cell & Environment 1991, 14, (8), 807-818.

7

3.

8

https://elibrary.asabe.org/abstract.asp?aid=45682&t=3&dabs=Y&redir=&redirType=

9

4.

Annual Energy Review. In U.S. Energy Information Administration: 2012.

10

5.

Geoff

11

http://www.geoffmiller.com.au/products/greenhouse-products/greenhouse-plastic.html, 2016.

12

6.

13

calcium looping process for production of heat and carbon dioxide enrichment of

14

greenhouses. Energy Convers. Manage. 2015, 103, (0), 129-138.

ASABE Heating, Ventilating and Cooling Greenhouses. ANSI/ASAE EP406.4.

Miller

Property

Limited

In

Greenhouse

Plastic

Ramezani, M.; Shah, K.; Doroodchi, E.; Moghtaderi, B., Application of a novel


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1

7.

Ramezani, M.; Tremain, P.; Shah, K.; Doroodchi, E.; Moghtaderi, B., Derivation of

2

Kinetics and Design Parameters for a Carbonator Reactor in a Novel Greenhouse Calcium

3

Looping Process. Energy Technol. 2017, 5, (5), 644-655.

4

8.

5

solid mixtures in bubbling fluidized beds under conditions pertinent to the fuel reactor of a

6

chemical looping system. Powder Technol. 2013, 235, 823-837.

7

9.

8

looping air separation: a thermodynamic approach. Energy Fuels 2012, 26, (4), 2038-2045.

9

10.

Peng, Z.; Doroodchi, E.; Alghamdi, Y.; Moghtaderi, B., Mixing and segregation of

Shah, K.; Moghtaderi, B.; Wall, T., Selection of suitable oxygen carriers for chemical

Hanak, D. P.; Anthony, E. J.; Manovic, V., A review of developments in pilot-plant

10

testing and modelling of calcium looping process for CO2 capture from power generation

11

systems. Energy Environ. Sci. 2015.

12

11.

13

Alvarez, J.; Sánchez-Biezma, A., Demonstration of steady state CO2 capture in a 1.7 MWth

14

calcium looping pilot. Int. J. Greenh. Gas Control 2013, 18, 237-245.

15

12.

16

States 2003.

17

13.

Schefflan, R., Teach yourself the basics of Aspen plus. John Wiley & Sons: 2011.

18

14.

Borgwardt, R., Calcination kinetics and surface area of dispersed limestone particles.

19

AlChE J. 1985, 31, (1), 103-111.

20

15.

21

calcination of limestone and dolomite particles in fluidised beds. Chem. Eng. Sci. 1987, 42,

22

(10), 2361-2372.

Arias, B.; Diego, M. E.; Abanades, J. C.; Lorenzo, M.; Diaz, L.; Martínez, D.;

Aspen plus user guide. Aspen Technology Limited, Cambridge, Massachusetts, United

Dennis, J. S.; Hayhurst, A. N., the effect of CO2 on the kinetics and extent of


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16.

Hu, N.; Scaroni, A. W., Calcination of pulverized limestone particles under furnace

2

injection conditions. Fuel 1996, 75, (2), 177-186.

3

17.

4

reactions. Chem. Eng. Sci. 1979, 34, (8), 1072-1075.

5

18.

6

calcination of dispersed calcium carbonate and calcium hydroxide particles. Ind. Eng. Chem.

7

Res. 1989, 28, (2), 155-160.

8

19.

Szekely, J.; Evans, J. W., Gas-solid reactions. Academic Press: UK, 1976.

9

20.

Baker, E., The calcium oxide-carbon dioxide system in the pressure range 1-300

Georgakis, C.; Chang, C. W.; Szekely, J., A changing grain size model for gas-solid

Silcox, G. D.; Kramlich, J. C.; Pershing, D. W., A mathematical model for the flash

10

atmospheres. J. Chem. Soc. (Resumed) 1962, 464-470.

11

21.

12

Reactors by Considering the Bubble Size Distribution. Ind. Eng. Chem. Res. 2012, 51, (16),

13

5705-5714.

14

22.

15

Reaction kinetics of dolomite rim growth. Contrib. Mineral. Petrol. 2014, 167, (4), 1-14.

16

23.

17

application for exergy and thermoeconomic analysis of processes simulated with Aspen

18

Plus®. Energy 2011, 36, (2), 964-974.

19

24.

20

83, (2), 131-137.

Ramezani, M.; Mostoufi, N.; Mehrnia, M. R., Improved Modeling of Bubble Column

Helpa, V.; Rybacki, E.; Abart, R.; Morales, L.; Rhede, D.; Jeřábek, P.; Dresen, G.,

Querol, E.; Gonzalez-Regueral, B.; Ramos, A.; Perez-Benedito, J. L., Novel

Ar, I.; Doğu, G., Calcination kinetics of high purity limestones. Chem. Eng. J. 2001,


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1

25.

Martínez, I.; Grasa, G.; Murillo, R.; Arias, B.; Abanades, J. C., Kinetics of

2

Calcination of Partially Carbonated Particles in a Ca-Looping System for CO2 Capture.

3

Energy Fuels 2012, 26, (2), 1432-1440.

4

26.

5

Chemistry and biotechnology 1973, 23, (10), 733-742.

6

27.

7

Deactivation of CaO in a Carbonate Loop at High Temperatures of Calcination. Ind. Eng.

8

Chem. Res. 2008, 47, (23), 9256-9262.

9

28.

Barker, R., The reversibility of the reaction CaCO3⇄ CaO+ CO2. Journal of applied

González, B.; Grasa, G. S.; Alonso, M.; Abanades, J. C., Modeling of the

Blamey, J.; Anthony, E. J.; Wang, J.; Fennell, P. S., The calcium looping cycle for

10

large-scale CO2 capture. Prog. Energy Combust. Sci. 2010, 36, (2), 260-279.

11

29.

12

19, (4), 373-377.

13

30.

14

decomposition. Thermochim. Acta 1990, 170, 7-11.

15

31.

16

of calcium carbonate alone and in the presence of some clays using the rising temperature

17

technique. Thermochim. Acta 1982, 54, (1), 187-199.

18

32.

19

calcium-based sorbents at pressure in a broad range of CO2 concentrations. Chem. Eng. Sci.

20

2002, 57, (13), 2381-2393.

21

33.

22

capture at a constant high temperature. Energy Fuels 2013, 28, (1), 307-318.

Rao, T. R., Kinetics of calcium carbonate decomposition. Chem. Eng. Technol. 1996,

Calvo, E. G.; Arranz, M.; Leton, P., Effects of impurities in the kinetics of calcite

Guler, C.; Dollimore, D.; Heal, G. R., The investigation of the decomposition kinetics

Garcıa-Labiano, F.; Abad, A.; De Diego, L.; Gayan, P.; Adanez, J., Calcination of

Yin, J.; Qin, C.; Feng, B.; Ge, L.; Luo, C.; Liu, W.; An, H., Calcium looping for CO2


Page 24 of 43

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Energy & Fuels

1 2


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Table Captions

2

Table 1. Summary of kinetics studies of calcination reaction reviewed.

3

Table 2. G(x) equation for different reaction mechanism.

4 5

Table 4. Stream table of calcination reaction of case L in GCL process considering RPlug reactor and the related exergy calculations.

6 7

Table 5. R2 values of linear fitting of different reaction mechanism in the CO2 partial pressure range of 400-1400 ppm and the temperature range of 600-700 oC.

8

Table 6. Average R2 values of linear fitting of reaction mechanisms.

9 10

Table 7. The calcination’s kinetic parameters for CO2 partial pressures of 400-1600 ppm and the temperatures of 600-800 oC.

11 12 13

Table 8. Required CaCO3 inventory, RPlug’s heat duty and dimensions, and Blower’s work consumption for different cases using the derived calcination/ reaction kinetics while keeping RPlug unit temperature at 860 oC.

14 15


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Energy & Fuels

1

Figure Captions

2 3

Figure 1. Process flow diagram (PFD) of the calcination reaction using RPlug reactor for case L.

4 5 6 7

Figure 2. The calcium carbonate (CaCO3) conversion, versus time for different calcination temperatures with the following conditions: CO2 partial pressure: 400 ppm (0.04 %), total pressure: 1 atm, total flow rate: 200 mL min-1, sample size: 5 mg, particle size: 75-150 µm.

8 9 10 11 12

Figure 3. G(x)=1-(1-x)1/3 reaction mechanism versus time for determination of k of the calcination reaction in a temperature range of 600 to 800 oC and CO2 partial pressure of 400 ppm. Symbols represent experimental data calculated by the R3 model and the continuous lines are the linear fits of the data which are time based on the linear least-square regression method.

13 14

Figure 4.

15 16

Figure 5.

17 18

Figure 6. Arrhenius plot for the calcination reaction at 600-800 oC and CO2 partial pressure of 400-1600 ppm.

19

Figure 7. Effect of temperature on the RPlug unit volume for case L in simulation matrix.

[

ln k (Peq − P )

n

] versus ln(P

eq

− P)

in the temperature of 700 oC and 400-1600 ppm

CO2 partial pressure.

[

ln k (Peq − P )

n

] versus ln(P

eq

− P)

in the temperature of 800 oC and 400-1600 ppm


20


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 28 of 43

Table 1. Summary of kinetics studies of calcination reaction reviewed. Investigator

System

Barker 26

Commercial limestone (99.5 % CaCO3) in a TGA Natural limestones in a TGA Natural limestones in a differential reactor Natural limestones in a fluidised bed reactor

Guler et al. 31 Borgwardt 14 Dennis and Hayhurst 15 Calvo et al. 30

Rao 29

Ar and Dogu 24

Commercial limestones (99.5 % CaCO3)

Commercial limestone in a TGA Natural limestone in a TGA

CO2 partial pressure (%)

Temperature (oC)

Order of reaction

0%

866

Nil

0%

650-770

First order

0%

515-1000

Zero order

20 %

800-975

First order

0%

860-920

Zero order

0%

680-875

0.67

0%

900

Zero order

32

Natural limestones and dolomite particles in a TGA and a thermobalance for pressurized experiments

0-80 % with increment of 5%

775-900

Nil

Gonzalez et al. 27

A commercial limestone (99.5 % CaCO3) in a TGA

10 %

950-1150

Nil

Martinez et al. 25

Natural limestones in a TGA

0-100 % with increment of 25 %

825-920

First order

Yin et al. 33

Pure CaO and synthesised limestones in a TGA

0%

550-800

Zero order

Garcia et al

28

ACS Paragon Plus Environment

Activation energy (kJ mol-1) 147 203.8 201-205 169 193.8 110.5

109.8-175.5 155.6-212.5 Blanca limestone: 166 Mequinenza limestone: 131 Dolomite: 114

205 91.7 112.4 173.7 (CaO) 148.5-159.5 (Synthesised)

Pre-exponential factor Nil

1.1×108 (s-1) 1.6×10-5 (kmol m-2 s-1) 1.0×10-5 (kmol m-2 s-1) 1.9×109 1.3×106 (min-1) 2.2×102 - 3.3×104 (min-1) 3.65×107 - 14.3×107 (kmol m-2 s-1) Blanca limestone: 6.7×106 Mequinenza limestone: 2.5×102 Dolomite: 29.5 (mol m-2 s-1) 5.3×104 (min-1) 2.5×105 2.0×106 (m3 kmol-1 s-1) 2.1×103 (CaO) 2.0-6.5 (mol m-2 s-1)

Page 29 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 2. G(x) equation for different reaction mechanism a. Reaction mechanism

G (x )

D1

x2

D2

(1 − x ) ln (1 − x ) + x

D3

1 − (1 − x )13    1−

D4

2 2x − (1 − x )3 3

C1

− ln (1 − x )

C2

(1− x)−1 −1 1

P1

[− ln(1 − x )]2 1 [− ln (1 − x )]3 1 1 − (1 − x )2 1 1 − (1 − x )3 x

P2

x2

P3

x3

P4

x4

A2 A3 R2 R3

a

2

1

1

1

D1 to D4: diffusional function, C1 and C2: first and second order chemical reaction, A2 and

A3: Avrami-Erofe’ev random nucleation and subsequent growth function, R2 and R3: phase boundary reaction, P1 to P2: Mampel power law function.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 3. Simulation cases examined, adapted from Ramezani et al.7.

Case A B C D E F G H I J K L M N O P Q R

Number of air changes per hour (hr-1) 10

Greenhouse’s dimensions (length×width×height) (m) 30×9×3 (Typical) 6×1.8×0.6 (Pilot-scale) 2×0.6×0.2 (Bench-scale) Typical Pilot Bench Typical Pilot Bench Typical Pilot Bench Typical Pilot Bench Typical Pilot Bench

20

30

40

50

60


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Energy & Fuels

Table 4. Stream table of calcination reaction of case L in GCL process considering RPlug reactor and the related exergy calculations. Physical & chemical data

CaCO3

HP-LIME

CaO-CO2

CaO

CO2

AIR-SUR

Stream description

CaCO3 feed to calciner

High pressure CaCO3 from blower

CaO+gases production of calciner

CaO out from cyclone

Gases out from cyclone

Ambient air to heater

Mole flow (kmol hr-1) O2 N2 CO2 H2O CaO CaCO3 MgCO3 MgO Total flow gas (kmol hr-1) Total flow solids (kmol hr-1) Temperature (oC) Pressure (bar) Vapour fraction Enthalpy gas (J kmol-1) Enthalpy gas (W) Enthalpy solids (J kmol-1) Enthalpy solids (W) Entropy gas (J kmol-1 K-1) Entropy solids (J kmol-1 K-1) Heat duty of calciner (W)

0 0 0 0 0 5.31×10-4 2.80×10-5 0 0 5.59×10-4 10 1.00 0 0 0 -1.20×109 -187.0 0.0 -2.68×105

0.084 0.315 1.60×10-4 3.99×10-4 0 5.31×10-4 2.80×10-5 0 0.40 5.59×10-4 934.41 1.25 1 2.83×107 3139.7 -1.10×109 -171.1 45887.8 -1.17×105

0.084 0.315 6.90×10-4 3.99×10-4 5.30×10-4 8.97×10-7 2.80×10-5 1.57×10-19 0.40 5.59×10-4 860 1.25 1 2.53×107 2807.1 -6.14×108 -95.4 43826.8 -4.28×104

0 0 0 0 5.30×10-4 8.97×10-7 2.80×10-5 1.57×10-19 0 5.59×10-4 860 1.25 0 0 0 -6.14×108 -95.4 0.0 -4.28×104 -256.8

0.084 0.315 6.90×10-4 3.99×10-4 0 0 0 0 0.40 0 860 1.25 1 2.53×107 2807.1 0 0.0 43826.8

0.084 0.315 1.60×10-4 3.99×10-4 0 0 0 0 0.40 0 10 1 1 -8.46×105 -93.7 0 0.0 2893.2

31


CO2-GH Air + CO2 (1700 ppm) appropriate for greenhouse 0.084 0.315 6.90×10-4 3.99×10-4 0 0 0 0 0.40 0 22 1.25 1 -1.02×106 -113.1 0 0.0 2319.4

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Physical & chemical data Exergy calculation h (J kmol-1) gas h (J kmol-1) solids h0 (J kmol-1) gas h0 (J kmol-1) solids s (J kmol-1 K-1) gas s (J kmol-1 K-1) solids s0 (J kmol-1 K-1) gas s0 (J kmol-1 K-1) solids Exph (J kmol-1) gas Exph (J kmol-1) solids Exph (W) Exch (J kmol-1) gas Exch (J kmol-1) solids Exch (W) Extotal (W) Calciner’s exergy (generation) (W) Blower’s exergy (consumption) (W) Exeff, unit (%)

CaCO3 9

-1.20×10

9

-1.20×10

-2.68×105 5

-2.64×10

5

2.41×10 0.04

1.70×107 2.64 2.68

HP-LIME

CaO-CO2

2.83×107 -1.10×109 -4.07×105 -1.20×109 4.59×104 -1.17×105 4.29×103 -2.64×105 1.63×107 5.70×107 1819.17 1.42×106 1.70×107 159.70 1978.87

2.53×107 -6.14×108 -9.29×105 -6.60×108 4.38×104 -4.28×104 4.37×103 -1.15×105 1.45×107 2.43×107 1608.35 1.44×106 1.22×108 178.91 1787.26

Page 32 of 43

CaO

CO2

AIR-SUR

CO2-GH

2.53×107

-8.46×105

-1.02×106

-9.29×105

-4.07×105

-9.29×105

4.38×104

2.89×103

2.32×103

4.37×103

4.29×103

4.37×103

1.45×107

-2.12×104

5.21×105

1604.57 1.44×106

-2.35 1.42×106

57.79 1.44×106

159.93 1764.5

157.06 154.71

159.93 217.72

8

-6.14×10

8

-6.60×10

-4.28×104 5

-1.15×10

7

2.43×10 3.78

1.22×108 18.98 22.76 189.2 346.34

Mixer 99.83

Blower 98

Exeff, overall (%)

Calciner 99.88 85.3

32


SSplit 100

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Energy & Fuels

Table 5. R2 values of linear fitting of different reaction mechanism in the CO2 partial pressure range of 400-1400 ppm and the temperature range of 600-700 oC. Temperature (°C)

CO2 (PPM)

D1

D2

D3

D4

C1

C2

A2

A3

R2

R3

P1

P2

P3

P4

400

0.972

0.989

0.926

0.988

0.956

0.839

0.986

0.962

0.989

0.997

0.923

0.814

0.738

0.672

600

0.989

0.979

0.914

0.954

0.992

0.975

0.979

0.936

0.997

0.996

0.958

0.871

0.827

0.784

800

0.985

0.963

0.874

0.936

0.986

0.953

0.985

0.957

0.987

0.989

0.961

0.887

0.860

0.825

1000

0.973

0.964

0.921

0.947

0.971

0.852

0.993

0.955

0.993

0.985

0.965

0.873

0.787

0.790

1200

0.970

0.979

0.958

0.977

0.992

0.951

0.979

0.928

0.966

0.986

0.913

0.852

0.803

0.749

1400

0.966

0.972

0.978

0.993

0.994

0.944

0.970

0.939

0.967

0.984

0.857

0.761

0.747

0.699

CO2 (PPM)

D1

D2

D3

D4

C1

C2

A2

A3

R2

R3

P1

P2

P3

P4

0.996

0.967

0.853

0.949

0.987

0.958

1.000

0.993

0.989

0.962

0.995

0.984

0.965

0.933

0.991

0.993

0.982

0.990

0.973

0.953

0.992

0.965

0.998

0.993

0.988

0.929

0.893

0.868

0.972

0.989

0.926

0.988

0.956

0.839

0.986

0.962

0.989

0.997

0.923

0.814

0.738

0.672

700

Temperature (°C) 600 650 700

400

33


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 6. Average R2 values of linear fitting of reaction mechanisms. Reaction mechanism Average R2 of linear fitting 0.987 R3 0.986 R2 0.985 A2 0.982 C1 0.980 D1 0.976 D2 0.967 D4 0.954 A3 0.945 P1 0.928 C2 0.926 D3 0.871 P2 0.827 P3 0.790 P4


Page 34 of 43

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Energy & Fuels

Table 7. The calcination’s kinetic parameters for CO2 partial pressures of 400-1600 ppm and the temperatures of 600-800 oC. Order of reaction (n)

Activation energy (E) (kJ mol-1)

Pre-exponential factor (k0) (min-1)

0.02

103.6

6.9 × 106


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 43

Table 8. Required CaCO3 inventory, RPlug’s heat duty and dimensions, and blower’s work consumption for the different cases examined using the derived calcination/ reaction kinetics while keeping the RPlug unit temperature at 860 oC.

Case

Greenhouse scale

CaCO3 (kmol hr-1)

RPlug’s volume (m3)

A B C D E F G H I J K L M N O P Q R

Typical Pilot Bench Typical Pilot Bench Typical Pilot Bench Typical Pilot Bench Typical Pilot Bench Typical Pilot Bench

0.3 0.0024 0.00009 0.55 0.0045 0.00017 0.83 0.0066 0.00027 1.09 0.0088 0.00035 1.36 0.011 0.00045 1.8 0.013 0.00051

0.089 0.00073 0.00003 0.18 0.0015 0.00005 0.27 0.0022 0.00008 0.37 0.0029 0.00011 0.46 0.0037 0.00014 0.55 0.0044 0.00016


RPlug’s produced heat duty (kW) -217.5 -1.7 -0.1 -434.3 -3.5 -0.1 -651.0 -5.2 -0.2 -867.7 -6.9 -0.3 -1084.2 -8.7 -0.3 -1301.9 -10.4 -0.4

Blower’s consumption work (kW) 292.5 2.3 0.1 584.9 4.7 0.2 877.4 7.0 0.3 1169.9 9.4 0.4 1462.4 11.7 0.4 1754.8 14.0 0.5

Page 37 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

851 10

25

H-CACO3

AIR-HEAT CACO3-PE

CACO3-E

10

Heater

CO2-GH

AIR-SUR

Mixer Calciner

860

22

CO2

860

10

SSplit LIME-AIR

848 HP-LIME

934

25

H-AIR-HE

AIR-HE-E

848

AIR-H-PE

CAO

860

LIMEA-PE

934

25

H-HP-LIM HP-LI-E 860

HP-LI-PE

H-CAO-CO

CAOCO2-E

Blower

25

H-LIME-A

LIME-A-E

CAO-CO2 CACO3

851

25

CAOCO-PE

Temperatu re (C)

Exergy Analysis

Figure 1. Process flow diagram (PFD) of the calcination reaction using RPlug reactor for case L.

37


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. The calcium carbonate (CaCO3) conversion, versus time for different calcination temperatures with the following conditions: CO2 partial pressure: 400 ppm (0.04 %), total pressure: 1 atm, total flow rate: 200 ml min-1, sample size: 5 mg, particle size: 75-150 µm.


Page 38 of 43

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Energy & Fuels

Figure 3. G(x)=1-(1-x)1/3 reaction mechanism versus time for determination of k of the calcination reaction in a temperature range of 600 to 800 oC and CO2 partial pressure of 400 ppm. Symbols represent experimental data calculated by the R3 model and the continuous lines are the linear fits of the data which are time based on the linear least-square regression method.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[(

Figure 4. ln k Peq − P

) ] versus ln(Peq − P) in the temperature of 700 oC and 400-1600 ppm n



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Energy & Fuels

[(

Figure 5. ln k Peq − P

) ] versus ln(Peq − P) in the temperature of 800 oC and 400-1600 ppm n



Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. Arrhenius plot for the calcination reaction at 600-800 oC and CO2 partial pressure of 400-1600 ppm.


Page 42 of 43

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Energy & Fuels

Figure 7. Effect of temperature on the RPlug unit volume for case L in simulation matrix.


Kinetics and Design Parameter Determination for a Calciner Reactor

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