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Kinetics, Catalysis, and Reaction Engineering

Kinetic Model for Syngas Chemical Looping Combustion using a Nickel-Based Highly Performing Fluidizable Oxygen Carrier Imtiaz Ahmed, and Hugo de Lasa Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05880 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on February 10, 2019

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Industrial & Engineering Chemistry Research

Kinetic Model for Syngas Chemical Looping Combustion using a NickelBased Highly Performing Fluidizable Oxygen Carrier Imtiaz Ahmed and Hugo de Lasa* Chemical Reactor Engineering Centre (CREC), Chemical and Biochemical Engineering, University of Western Ontario (UWO), 1151 Richmond St, London, Ontario, Canada N6A 5B9, *Corresponding author: T: 519-661-2144 F: 519-850-2931; E-mail: [email protected]

Abstract

The study reports a kinetic model for a nickel based highly performing oxygen carrier (HPOC). This HPOC is free of nickel aluminates and can perform very efficiently at lower temperatures (i.e. 550650°C). In this study, a kinetic model is proposed to predict syngas chemical looping combustion (CLC) using a HPOC. The syngas used in this study, emulates the syngas that can be derived from biomass gasification containing H2, CO, CH4 and CO2. To establish the kinetic model, isothermal runs are developed in the CREC Riser Simulator, which is a mini-batch fluidized bed reactor. The operating conditions are varied between 2-40s reaction times and 550-650°C, with the H2/CO ratio being 2.5 and 1.33 and with the ψ parameter (fuel stoichiometric oxygen to HPOC oxygen ratio) being 0.5 and 1. For the 2.5 and 1.33 H2/CO ratios, a 92% CO2 yield with 90% CO, 95% H2 and 91% CH4 conversions are achieved. As well, the HPOC shows an oxygen transport capacity ranging between 1.84-3.0 wt% (gO2/gOC) with a 40-70% OC oxygen conversion. The solid-state kinetics considered uses of a Nucleation and Nuclei Growth Model (NNGM). The proposed kinetics leads to a model with ten independent intrinsic kinetic parameters. The various kinetic parameters are determined via numerical regression within a 95% confidence interval and small cross correlation coefficients. According to the frequency factors obtained, the reactivity of the species with a HPOC can be expressed following the H2>CO>CH4 order. Given that the HPOC shows high performance and stability, it is anticipated that the developed kinetic model could contribute to establishing large-scale reactor CLC for syngas combustion.

Keywords: CLC, Syngas, Kinetic Modeling, Oxygen Carrier, Nickel Oxide

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1

Introduction

Chemical looping combustion (CLC) is a promising technology to achieve complete combustion of fossil fuels and the production of essentially pure CO2 exhaust during power generation 1–4. The CO2 can be further sequestrated, leading to zero emissions into the atmosphere 5. Biomass plays a natural role in capturing existing CO2 from the atmosphere via photosynthesis. The synthesis gas derived from biomass gasification contains mostly H2 and CO. Hence, using this syngas in the context of chemical looping combustion can lead to negative CO2 emissions 6. The CLC process consists of a fuel reactor and an air reactor. The syngas combustion occurs in the fuel reactor with a fluidizable oxygen carrier (OC). The OC is further oxidized in the air reactor and circulated again to the fuel reactor. For industrial applications, CLC requires stable OCs able to perform under repeated redox cycles. OCs must have a high surface area, be resistant to agglomeration and attrition and be economically feasible. According to the technical literature, nickel oxide appears to be a promising OC that can provide these features 7–9. As an OC support, gamma alumina has received much attention due to its high surface area (190 m2/g) 10,11

. However, the potential formation of nickel aluminate may be an eventual issue, reducing OC

reactivity. In addition, high temperature (>750°C) CLC operation may be required to reduce nickel aluminate 12,13. Preventing the formation of NiAl2O4 by incorporating a cobalt additive in the OC, was reported by Quddus et al. 14. It was found however, that controlling temperatures during OC preparation and CLC operation has a great effect on OC performance. Considering these issues, the authors of this manuscript recently reported a HPOC where the NiAl2O4 is excluded very effectively. The stable performance of this HPOC was demonstrated using a syngas with H2/CO ratios of 2.5 and 1.33 , at 550-650°C and with at least 90% CO2 yield 15. Hence, a major contribution of this manuscript is to develop and demonstrate a CLC reaction kinetics for a free of nickel aluminate HPOC. An OC reaction rate model should include mechanistic solid state kinetic models accounting for : a) nucleation b) geometrical contraction, c) diffusion d) reaction order and e) power law 16,17. In this respect, OC and CLC operating conditions must be selected to obtain an overall CLC reaction rate unaffected by either external or internal mass transfer.


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Given the above, the Nucleation and Nuclei Growth Model (NNGM) 11,18,19 and the Shrinking Core Model (SCM) 20–22 have been considered to describe solid state changes of the OCs. The SCM describes the CLC reaction initiation at the outer surface of the NiO crystallite, where NiO is converted into Ni0. This process continues until the crystallite metal core is reached. As an alternative and according to the NNGM, the gas-solid reaction proceeds by nucleation, which means that once the reduction of a site is achieved, the reduction of neighboring unreacted NiO is initiated. Metal crystals dispersed in the porous support may have imperfections (e.g. cracks, surface, and point defects) 23. It is at these metal sites, known as nucleation sites, that the activation energy displays a minimum value

24

. Therefore, the

NNGM model has been found to be more suitable, especially with OCs where there is no influence of internal mass transfer on the overall CLC rate 17. Regarding the CLC, it can be described as a function of the degree of OC reduction and as a function of the partial pressure of the gas phase chemical species25,26. On this basis, several possible reactions including non-catalytic and catalytic reactions can be accounted for 4. This choice is critical to develop an adequate kinetic model and to determine intrinsic kinetic parameters. Given the promising results obtained with the new HPOC 15, a comprehensive kinetic model which describes the CLC using syngas was investigated in the present study. The kinetic parameters were evaluated using experimental data obtained in the CREC Riser Simulator. It is anticipated that the proposed kinetic model may facilitate the evaluation of large scale CLC reactor performance, which is likely to be implemented in a CLC Riser-Downer configuration, as shown in Figure 1.

Figure 1: Schematic Diagram of the Syngas CLC Process with a Riser-Downer Configuration



The continuous CLC process of Figure 1, is under active investigation by CREC-UWO researchers. This CLC system involves a riser air reactor, where the HPOC is re-oxidized and a downflow fuel reactor where the oxygen carrier is reduced. The gas-solid stream containing oxygen depleted air, leaves the air reactor and feeds a cyclone separator. Here, the solid oxygen carrier is disengaged from the gaseous phase, which descends via the neck of the cyclone towards the entry of the fuel reactor. It is in the vicinity of the reactor fuel entry where distributed jets of syngas contact the oxidized oxygen carrier. This contact between the syngas rich stream and the oxygen carrier continues in the downflow section. The resulting combustion products are allowed to separate from the reduced oxygen carrier, with the oxygen carrier being returned to the air reactor via a L-type loop seal. 2

Experimental Methods

2.1 Oxygen Carrier (OC) and Feedstock This study considers the HPOC as an oxygen carrier. It consists of 20wt% Ni over γ-alumina with an added 5%wt La and 1%wt Co 15. To synthesize the HPOC, a new preparation technique was implemented, with essentially all NiAl2O4 being converted into NiO and over 90% of the NiO being activated. The prepared OC was characterized by X-ray diffraction (XRD), N2-adsorption isotherm, BET surface area, H2-temperature programmed reduction (TPR), temperature programmed reduction (TPO) and NH3-temperature programmed desorption (TPD). A detailed description of the HPOC preparation steps and characterizations are reported in a recent publication by Ahmed et al. 15. Two types of syngas obtained from biomass gasification using a La2O3 promoted Ni/γ-Al2O3 catalyst were considered: a) a gas mixture designated as “Syngas-250” with a H2/CO ratio of 2.5 and 50v% H2, 20v% CO, 10v% CH4, 20v% CO2 (∆Hco=-258 kJ/mol 27, b) a gas mixture designated as “Syngas-133” with a H2/CO ratio of 1.33 and 44v% H2, 33v% CO, 11v% CH4, 12v% CO2 (∆Hco=-288 kJ/mol). 28. 2.2 Syngas CLC Studies in CREC Riser Simulator The CLC runs with the HPOC were carried out in the CREC Riser Simulator under different conditions. The CREC Riser Simulator is a bench-scale mini-fluidized bed reactor (a capacity of 50.7 cm3) invented by de Lasa 29. It operates under batch conditions and can be adapted for catalyst/oxygen carrier evaluation. This experimental unit allows kinetic studies under fluidized bed (riser/downer) conditions. Figure 2 illustrates the value of experimentation with the CREC Riser Simulator for the determination of a reaction mechanism and the development of a kinetic model. Additional details describing how the CREC Riser Simulator can be used for OC evaluation can be found in 15,30.


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The HPOC reduction runs using the Syngas-250 15 and the Syngas-133 were developed at three different temperatures (i.e. 550, 600, and 650°C), six residence times (i.e. 2, 5, 10, 20, 30 and 40s), using two fuel stoichiometric oxygen to supplied oxygen ratios (ψ=0.5, 1) and 175 mg OC loading. To ensure the reproducibility of the results, all experiments were repeated at least three times. The reactor effluent analysis procedure and the data post processing methods for CLC syngas conversion, including CO2 yield were reported in a recent publication 15. Carbon mass balances, accounting for all carbon product containing species such as carbon monoxide, methane and carbon dioxide, closed in excess of 90%. Carbon deposited over the HPOC was found to be negligible in all cases. This can be explained given that carbon deposition is inhibited by the steam present 31.

Figure 2: Schematic Diagram Illustrating the Value of CREC Riser Simulator Experiments for the Development of a Kinetic Model Applicable in a Downer Reactor Unit. 3

Kinetic Modeling

3.1 Rate Limiting Steps Gamma alumina is a good porous support reported for Ni. The CLC reduction of this heterogeneous oxygen carrier involves a multistep process. The solid reduction rate may also include: a) internal and external diffusion, b) reacting species adsorption and intrinsic reaction steps. Thus, it is important to determine if the rate-limiting step is dependent on particle size and physicochemical properties.



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The potential OC particle external mass transfer influence can be neglected under the CREC Riser Simulator reactor operating conditions. This is the case, given the high gas superficial velocities 30, with fr,i>>(-rexp), where, fr,i refers to the radial flux of the gaseous species “i” as described by eq. (1): 𝑓𝑟,𝑖 = 𝑘𝑔 (𝐶𝑏,𝑖 − 𝐶𝑠,𝑖 ) (−𝑟𝑒𝑥𝑝 ) =

(1)

𝑁0 𝑑𝛼 𝑤𝑆𝑒𝑥 𝑑𝑡

(2)

with Cb,i and Cs,i represent the gas species concentrations at bulk and at the external surface of the particle, respectively.

The calculation of the fr,i also requires one to know the kg mass transfer coefficient. This can be evaluated as kg=Sh DiA/dp; where, Sh is the Sherwood number, DiA stands for the species diffusion coefficient in argon and dp denotes the mean OC particle diameter. DiA can be evaluated using the Fuller Correlation 32,33

. The Sherwood number can be calculated using the Frossling’s correlation, Sh=2+0.6Re0.5Sc0.33,

where, Re is Reynolds number and Sc is Schmidt number.

One possible approach is to set kg at its lowest possible value, using Sh=2. This corresponds to the stagnant fluid around an OC particle model. Furthermore, the (-rexp) as in eq. (2), can be calculated from the experimental data, with N0 being the number of consumable oxygen moles in the OC, α standing for the OC oxygen conversion, Sex denoting the external surface area per gram of sample and w representing the weight of OC. On this basis, the fr,i>>(- rexp) condition can be confirmed and the influence of the external mass transfer neglected.

In order to calculate the possible internal mass transfer influence on the CLC rate, the Weisz-Prater measure can be used 34,35. According to this, there is no internal mass transfer resistance when CWP(-rexp). Thus, no influence of the external mass transfer can be assigned to the CLC rates. Furthermore, Cs,i=Cb,i can also be postulated. Given that CWPrCO>rCH4. However, it was observed that for longer reaction times, this order of reactivity was altered, with rH2 becoming, in all cases, smaller than the rCH4 and rCO consumption. This change in the CLC rates was assigned to much lower oxygen availability in the OC, at extended reaction periods. 4.2 Estimated Kinetic Parameters In this study, the kinetic parameters were assessed based on HPOC experimental data for CLC using both Syngas-250 and Syngas-133, separately. Table 2 reports the estimated parameters determined with low 95% confidence spans. One should note that the ten kinetic parameters were determined using 250 average observed data points, with at least 3 repeats for each experiment and a degree of freedom (DOF) of 240.

In Table 2 the k10, k20, k30 frequency factors correspond to (3.1), (3.2) and (3.3) forward reactions. This shows that k10< k20< k30 is consistent with the initially observed CLC reactivity difference described above, with this being rH2>rCO>rCH4. Furthermore and regarding the backward reactions, the k40 and k50 display a k40 < k50 difference, showing that H2O displays a higher reactivity than CO2 on nickel sites. In other words, the retardation rates due to the reverse reactions are more significant for H2 than for CO. Table 2: Kinetic Parameters Determined for Syngas-250 and Syngas-133, Separately



Syngas-250 Syngas-133 Apparent Parameters Value 95% CI Value 95% CI a -2 -3 -2 k0,1 3.03×10 ±4.85×10 5.62×10 ±10.1×10-3 -2 -3 -2 k0,2 12.5×10 ±18.7×10 10.5×10 ±12.2×10-3 k0,3 29.8×10-2 ±21.3×10-3 39.7×10-2 ±42.4×10-3 -2 -3 -2 k0,4 2.94×10 ±9.42×10 1.54×10 ±5.59×10-3 -2 -3 -2 k0,5 19.1×10 ±22.1×10 8.9×10 ±12.6×10-3 b -13 -14 -14 E1 3.6×10 ±2.1×10 4.02×10 ±1.1×10-14 E2 13.02 ±5.8 5.13 ±2.9 E3 20.10 ±6.9 7.36 ±3.6 E4 40.17 ±12.2 36.90 ±14.4 E5 30.31 ±10.8 21.23 ±7.8 m 250 DOF 240 a -3 -1 -1 b -1 0 mol.m psi s ; kJmol ; Tc=873K; N NiO=390 x10-6 mol, Degree of Freedom (DOF)= data points (m) – parameters (p)= 250 – 10= 240

Figure 7: Parity Plot Comparing Experimental Chemical Species Partial Pressures with Model Predictions using Syngas-250 and HPOC. Data: 250 average data points for at least 3 repeats runs. Note: Standard Deviation=2.9%.


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Figure 8: Parity Plot Comparing Experimental Chemical Species Partial Pressures with Model Predictions using Syngas-133 and HPOC. Data: 250 average data points for at least 3 repeats runs. Note: Standard Deviation=3.9%. Figure 7 and Figure 8 present parity plots for Syngas-250 and Syngas-133, respectively. These figures compare predicted and experimentally observed values for chemical species partial pressures at different reaction times, temperatures and ψ ratios. One can observe that the Syngas-250 shows a standard deviation error of 2.9% while Syngas-133 shows a deviation error of 3.9%. Both of these deviations are considered acceptable and within the anticipated experimental error level. Thus, the estimated parameters and the kinetic model using either of the two syngas samples are considered adequate for the CLC using the HPOC.



Figure 9: Comparison of Experimental Chemical Species Partial Pressures with those of Model Predictions at Various Conditions using the Syngas-250 Feed and a HPOC

Figure 10: Comparison of Experimental Chemical Species Partial Pressures with those of Model Predictions at Various Conditions using Syngas-133 Feed and a HPOC


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Furthermore, Figure 9 and Figure 10 illustrate the predicted partial pressures using the estimated kinetic parameters for both Syngas-250 and Syngas-133 and compare with experimental results. This comparison was developed at various reaction times. Thus, it appears that the proposed kinetics and the estimated kinetic parameters are able to fit the chemical species partial pressures well. This is the case for the entire range of reaction times, with no error bias observed for some specific reaction times. 4.3 Model Verification and Validation This research considered the applicability of a CLC kinetic model for a HPOC using both Syngas-250 and Syngas-133 with H2/CO ratios of 1.33 and 2.5, respectively. In principle, if the proposed kinetic model is adequate, it must be applicable for both Syngas-250 and Syngas-133, with a single set of kinetic parameters. To validate this, a single set of kinetic parameters was estimated using all of the experimental data of Syngas-250 and Syngas-133, simultaneously.

Table 3 reports the single set of kinetic parameters including a 95% confidence interval. The ten model parameters are determined by 500 observed data points, averaged with at least 3 repeats of each experiment with a degree of freedom (DOF) of 490. One can notice that the 95% confidence spans are limited. Table 3 also presents the cross-correlation matrix for the kinetic parameters with most values being below 0.21 and none being more than 0.75. This shows the lack of dependence of calculated kinetic parameters. Thus and as a result, one can conclude that the kinetic model proposed can predict the HPOC performance when using a significant range of syngas compositions. Table 3: Summary of Apparent Intrinsic Kinetic Parameters for the Proposed Kinetic Model and Crosscorrelation Matrix Apparent Parameters k0,1a k0,2 k0,3 k0,4 k0,5 E1b E2 E3 E4 E5 m DOF

Value

95% CI

4.16×10-2 10.3×10-2 33.2×10-2 1.86×10-2 13.4×10-2 4.4×10-7 3.89 9.85 37.13 23.48 500 490

±7.3×10-3 ±13.5×10-3 ±28.9×10-3 ±6.87×10-3 ±17.2×10-3 ±3.2×10-8 ±2.1 ±4.4 ±11.5 ±9.7

k0,1a 1 0.14 0.06 0.00 -0.13 -0.06 -0.01 -0.01 -0.01 0.00

amol.m-3psi-1s-1 bkJmol-1 ; ;

Correlation matrix k0,5 E1b

k0,2

k0,3

k0,4

1 0.16 0.76 0.06 -0.01 -0.01 0.01 -0.16 0.01

1 0.13 0.70 -0.01 0.01 0.05 -0.02 0.02

1 0.21 -0.01 -0.15 -0.02 -0.39 -0.04

1 0.01 0.02 -0.04 -0.06 0.01

1 0.14 0.07 -0.01 -0.13

E2

E3

E4

E5

1 0.16 0.75 0.05

1 0.12 0.70

1 0.21

1

Tc=873K; N0NiO=390 x10-6 mol, Degree of freedom (DOF)= data points (m)

– parameters (p)= 500 – 10= 490



Table 3 reports a very low E1 energy of activation for CH4 in CLC. Furthermore, it has been reported in the technical literature that E1 may change in the 5-50 kJ/mol range 22,26,38, with the lowest observed values being for OCs free of nickel aluminate. This is consistent with the low E1 observed for the HPOC free of nickel aluminate. Concerning the E2 and E3 constants for CO and H2 CLC, one can note that the magnitude of the determined energies of activation are in the range of reported values for the OC free of nickel aluminate 22,44

as well. Furthermore, and for the reverse reactions (eqs. (3.2) and (3.3)), the CO2 conversion

displayed a higher activation energy than the H2O conversion.

Figure 11: Parity Plot Comparing the Experimental Chemical Species Partial Pressures with Model Predictions for both Syngas-250 and Syngas-133. Data base: 500 averaged data points for at least 3 repeat runs. Note: Standard Deviation=5.4%. Figure 11 compares the predicted partial pressures with the experimental observations, using the proposed kinetic model predictions for the HOPC and both Syngas-250 and Syngas-133. One can notice that the ensemble of data points including the 500 data points show a standard deviation of 5.4%. Nevertheless, one can also observe that when one eliminates the 15 data points considered outlier data from the analysis, the standard deviation is reduced to 2%.


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Figure 12: Comparison of Experimental Data and Model Predictions using the HPOC of this Study and Syngas-250. Note: Parameters estimated use both Syngas-250 and Syngas-133 data.

Figure 13: Comparison of Experimental Data and Model Predictions using the HPOC of this Study and Syngas-133. Note: Parameters estimated use both Syngas-250 and Syngas-133 data.



Figure 12 and Figure 13 describe the chemical species partial pressure changes for Syngas-250 and Syngas-133 CLC when using the HPOC of this study at various reaction times. It appears that for the CLC of the two types of syngas studied, a single set of kinetic parameters can provide excellent fitting with no observable increased errors at some specific reaction times or conditions. Thus, the proposed phenomenological kinetics appears to be a trustable model for syngas CLC rates using the HPOC of this study. This kinetic model is demonstrated to be applicable in an ample range of operating conditions. This makes the model a valuable tool for the CLC simulation of larger circulating fluidized bed units. 5

Conclusions 

A new 20wt%Ni 1wt%Co 5wt%La /γ-Al2O3, free of aluminate high oxygen carrier (HPOC) was successfully developed and tested in a CREC Riser Simulator reactor in the 550-650°C range. The syngas composition was the one anticipated from biomass gasification.



Based on the experimental data obtained, a kinetic model was proposed for syngas CLC using the HPOC of the present study. The depletion of the lattice oxygen was described using the AvramiErofeev NNGM Model with random nucleation, while the CH4, H2 and CO consumption was accounted for via simultaneous heterogeneous gas-solid reactions.



The resulting kinetic model involved ten parameters consisting of frequency factors (k0,i) and activation energies (Eapp,i). These parameters were determined using non-linear least-squares regression with 95% confidence intervals and very limited cross-correlation between parameters.



It is anticipated that the proposed kinetic model could have significant value for the development of large scale downer reactor units for CLC.

6

Acknowledgments

We would like to gratefully acknowledge the financial support provided by the NSERC-Canada Strategic grant program (2015-H. de Lasa grant), Canada and RECAT Technologies Inc. to Imtiaz Ahmed at the University of Western Ontario. We would also like to thank Florencia de Lasa who provided valuable assistance in the editing of this manuscript.


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7

Nomenclature Cb,i

concentration in bulk phase(mol/m3)

Cs,i

concentration at external surface of particles (mol/m3)

CWP

dimensionless number

dp

mean oxygen carrier particle diameter (m)

DiA

diffusion coefficient in medium

Eiapp

apparent activation energy (kJ/mole)

f1

conversion factor=101325/RT/14.7 where, R(J.mol-1K-1), T(K)

f(α)

function to describe lattice oxygen depletion at solid state

fobj

objective function

∆Hco

heat of combustion (kJ.mol-1)

k0,iapp

pre-exponential or frequency factor (mol.m-3psi-1s-1)

k(T)

reaction rate constant as a function of temperature

kg

mass transfer coefficient (m/s)

n

Avrami’s equation exponent

N0

consumable oxygen amount in the oxygen carrier (mol)

0

N

NiO

oxygen amount at initial time of reaction (mol)

ni

moles of “i” in the gaseous product/feed

p

kinetic parameters

Pi

partial pressure of species “i” (psi)

Pi,exp

experimentally observed partial pressure of species “i” (psi)

Pi,th

model predicted partial pressure of species “i” (psi)

(-rn)

rate of nth reaction (mol.m-3s-1)

(-ri)

reaction rate of species “i” (mol.m-3s-1)

R

universal gas constant (J/mol/K)

Re

Reynolds number

(-rexp)

experimental rate of reaction (mol/m2/s)

Sc

Schmidt number

Sh

Sherwood number

Sex

external surface area (m2/g)

Syngas-250

syngas with a H2/CO molar ratio of 2.5

Syngas-133

syngas with a H2/CO molar ratio of 1.33

T

operating temperature (K)

Tc

centering temperature to reduce cross-correlation factor (K)



t

reaction time (s)

w

oxygen carrier weight (g)

Symbols α

oxygen carrier oxygen conversion

αi,exp

experimentally observed solid-phase oxygen conversion

αi,th

model predicted observed solid-phase oxygen conversion

ρOC

apparent particle density of oxygen carrier

ψ

fuel stoichiometric oxygen to supplied oxygen ratio

Abbreviation

8

BET

Brunauer–Emmett–Teller

CLC

Chemical Looping Combustion

CREC

Chemical Reactor Engineering Centre

DOF

Degree of Freedom

HPOC

Highly Performing Oxygen Carrier

NNGM

Nucleation and Nuclei Growth Model

OC

Oxygen Carrier

SCM

Shrinking Core Model

TPD

Temperature Programmed Desorption

TPO

Temperature Programmed Oxidation

TPR

Temperature Programmed Reduction

XRD

X-ray Diffraction

References

(1)

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(4)

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