From laboratory-scale experiments to industrial-scale CFD simulations of entrained flow coal gasification

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Contents lists available at ScienceDirect

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From laboratory-scale experiments to industrial-scale CFD simulations of entrained flow coal gasification Michele Vascellari a,⇑, Daniel G. Roberts b, San Shwe Hla b, David J. Harris b, Christian Hasse a a

8 9

b

Numerical Thermo-Fluid Dynamics, ZIK Virtuhcon, Technische Universität Bergakademie Freiberg, Reiche Zeche, Fuchsmühlenweg 9, 09596 Freiberg, Germany CSIRO Energy Technology, PO Box 883, Pullenvale, QLD 4069, Australia

10 11

h i g h l i g h t s

1 3 14 15

 Four Australian coals characterized by laboratory-scale experiments.

16

 Intrinsic char conversion model calibrated by experiments in zone II.

17

 Char structural parameter determined from the calibration.

18 19

 Industrial-scale gasifier investigated by CFD using calibrated kinetics.  Calibration procedure shows good agreement with measured conversion.

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a r t i c l e 2 3 2 4 23 24 25 26 27

i n f o

Article history: Received 31 August 2014 Received in revised form 24 November 2014 Accepted 15 January 2015 Available online xxxx

28 29 30 31 32 33

Keywords: Coal gasification Char conversion CFD modeling Intrinsic kinetic

a b s t r a c t In this work advanced gasification models of four Australian coals were calibrated using laboratory-scale experiments with the aim of extrapolating these information for simulating large-scale gasification processes using CFD. In particular, the four studied coals, ranging from semi-anthracite to sub-bituminous, were extensively characterized using high-pressure bench and laboratory scale techniques. Coal devolatilization is modeled using the empirical competing two-step models, whose parameters are calibrated in a pre-processing step by means of the advanced CPD, FG-DVC and FLASHCHAINÒ pyrolysis models. The results of the advanced pyrolysis models are at first validated against true volatile yield data obtained at high pressures and heating rates from wire-mesh reactor experiments. Char gasification is modeled using a nth-order intrinsic kinetics model. At first, intrinsic char kinetics was measured in kinetic regime from experiments in pressurized thermogravimetric analysis. Then, gasification experiments in a laboratory-scale pressurized entrained flow reactor have been used for estimating the reactivity in pore-diffusion regime, defining the diffusivity inside the particle pores. Finally, the calibration of the same coals have also been tested in a 5 MW pilot-scale gasifier, offering a unique opportunity to apply our model over the continuum of laboratory and pilot scales. The model results again show good agreement with the experimental data of syngas composition and carbon conversion evaluated at the exit of the gasifier. The simulations of the pilot-scale gasifier demonstrates that industrial-scale gasification processes can be accurately predicted using advanced coal conversion models, adequately calibrated through laboratory-scale experiments. Ó 2015 Published by Elsevier Ltd.

35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

56 57

1. Introduction

58

The ability to predict coal behavior in different gasification technologies, or to understand the impact of coal blending or feedstock switching on the performance of a particular gasifier technology, supports the development and deployment of advanced gasification-based systems for power generation and chemicals

59 60 61 62

⇑ Corresponding author.

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E-mail address: [email protected] (M. Vascellari).

production. As the requirements of these modeling and simulation activities have increased in complexity, more detail in the fundamental submodels and processes is required in order for accurate and relevant outcomes. Furthermore, the increasing availability of high-performance computing resources has seen simulation tools used regularly for understanding and optimizing the complex reactive multiphase flow in coal gasification systems. CFD simulations in particular now play an important part in the design process of advanced reactors. Modeling coal gasification requires several mathematical submodels in order to describe the complex

http://dx.doi.org/10.1016/j.fuel.2015.01.038 0016-2361/Ó 2015 Published by Elsevier Ltd.

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Nomenclature Ac As As B cp D De dp Ec Es f h k kd ks ks;0 M m N ns P q Rs Rg Robs Sh

73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106

pre-exponential factor of CO/CO2 ratio specific particle surface, m2/s specific intrinsic surface, m2/kg blowing parameter for convective heat transfer specific heat, kJ/(kg K) diffusivity, m2/s pores effective diffusivity, m2/s particle diameter, m activation energy of CO/CO2 ratio, J/(kmol K) activation energy of intrinsic surface reaction,J/(kmol K) fraction of the total porosity in the feeder pores heat transfer coefficient, W/(m2 K) gas thermal conductivity, W/(m K) film diffusion transport coefficient, kg/(s m2 atm) intrinsic kinetic rate constant, kg/(s m2 atmn) pre-exponential factor of intrinsic surface reaction, kg/(s m2 atmn) molecular weight, kg/kmol mass, kg molar flux, kmol/(s m2) order of the reaction pressure, atm overall char reaction rate per particle surface unit, kg/ (s m2) intrinsic reaction rate, kg/(s m2) universal gas constant, 8314.33 J/(kmol K) observed reaction rate, kg/(kg s) Sherwood number

turbulent multiphase reacting flow system [1]; the relevance and applicability of the final simulation result directly depends on the quality and the applicability of these submodels and, especially in coal gasification, accurate description of the feedstock conversion is crucial. When one considers the extreme conditions that exist inside an industrial-scale gasifier it is almost impossible to perform in-reactor measurements as the high pressures and temperatures, together with slagging conditions, makes the measurements extremely challenging. For these reasons, the numerical models for coal gasification need to be calibrated and validated by means of laboratory-scale experiments, where it is possible to interrogate the gasification process under industrially-relevant conditions, providing insights into devolatilization yields, conversion reactivity, gas compositions, etc. The main transformation reactions requiring consideration by numerical simulations include pyrolysis, char formation, char conversion, and gas phase oxidation. First, during pyrolysis, the coal is thermally decomposed and the volatile matter released in the gas phase reacts with O2 in the flame zone. The remaining char then partially reacts with the remaining O2 in the gas phase (char oxidation) and with the gasification agents, mainly CO2 and H2O (char gasification). Pyrolysis and char oxidation are much faster than char gasification, which is the limiting step for determining the overall carbon conversion. For this reason the volatile yield under process conditions needs to be accurately estimated, as it defines the amount of carbon that is to be converted by the slow gasification reactions. However, the devolatilization rate also needs to be accurately reproduced in the simulations, because it influences the flame stabilization [2–4]. Char oxidation and gasification follow devolatilization. Oxidation reactions are generally faster than gasification, which is generally the limiting conversion step in gasification processes. For this reason the key to successful modeling coal gasification is

T X x Xc y

temperature, K char conversion factor gas mole fraction carbon conversion mass fraction

Greek letters a mode of burning coefficient DHr heat of reaction, J/kg g effectiveness factor m stoichiometric reactant to char molar ratio U Thiele modulus W structural parameter for the random pore model w stoichiometric coefficient of CO2 in reaction Rs;1 q density r Stefan–Boltzmann constant, 5.670 373 W/(m2 K4) s pores tortuosity e porosity, m3/m3 ep particle emissivity daf dry ash free Subscripts 0 initial state of char conversion 1 bulk phase c char m particle film layer mean conditions p particle

to accurately determine char gasification rates for the operating conditions existing in the reactor. Char particles are characterized by a highly porous structure [5,6]. The heterogeneous reactions occur on the internal surface of these porous particles. During char oxidation and gasification, quantifying the relative impacts of diffusion through the particle film layer, diffusion inside the porous structure of particle, and heterogeneous reactions on the surface of the particle is important. Three different regimes are generally observed [7]: (i) kinetic limited regime, (ii) pore diffusion regime, and (iii) film diffusion regime. Since industrial-scale entrained flow gasifiers operate at high temperatures with small pulverized particles, they mainly operate under Regime II conditions, although some char combustion reactions in the flame zone may be reacting under Regime III conditions, and fragmented, highly converted particles in the lower temperatures zones of the gasifier may be approaching Regime I. For this reason, the experiments required for calibrating char conversion models for use in industrial conditions should be performed in similar conditions, using chars with structures that are relevant to those made under entrained flow conditions. Char– gas reaction rate investigations have been made using drop tube reactors (DTR) [5,8–11], and similar studies have been made using entrained-flow reactors [11–14] which also allow some insights to be made into the overall gasification process, if they are configured correctly. Some measurements have also been made using flat flame burners [15]. These kinds of experiments allow investigations into char reaction rates (or, in some entrained flow reactors, gasification behavior), which depend on the combined effect of diffusion and heterogeneous reaction kinetics (Regime II). Such measurements are useful in support of specific technologies. However, the intrinsic char kinetics can be only measured in kinetic-diffusion regime (zone I), where diffusion effects can be neglected. Hence, to support truly transportable model development, kinetic submodels need to be based on zone I kinetics, and subsequently

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integrated with our knowledge of char structure and gasifier geometry. Generally, experiments in zone I are performed by means of thermogravimetric analysis (TGA) [9–13,16,17] or fluidizedbed reactors (FBR) [18–20], where truly differential conditions prevail. Such experiments require appropriate design to ensure that diffusion and heat transfer processes do not affect rate measurements, and that the char conversion step is properly isolated from any devolatilization or annealing processes that can occur during char formation and reaction. Such experiments are very useful to generate intrinsic reaction kinetics, which when combined with information regarding char structure, can be used in advanced, detailed models of coal conversion under gasification conditions. To sum up, both experiments in kinetic and pore diffusion regimes are necessary for carefully characterizing gasification rates in the whole range of operating conditions existing in industrial-scale reactors. Advanced detailed models have been developed in the last 20 years for integrating the results of these fundamental and applied studies in order to be predictive about coal conversion under gasification conditions. Pyrolysis models are generally based on a description of the coal molecular structure [21–24] and those for char combustion rates consider diffusion to the particle, and through the porous structure of the char remaining after devolatilization [7,25–28]. The correlations developed for pyrolysis [27,29,30] generally show a good agreement with experiments and they can be used for predicting the devolatilization rates and the composition of the volatiles [3] with a sufficient accuracy for CFD simulations. On the other hand, Liu and Niksa [28] and recently Shurtz and Fletcher [15] developed correlations for the char gasification reactivity using the CBK model based on the coal rank and on the coal molecular structure, respectively. The former showed a large data scattering compared to the experimental database used for calibrating the correlation, while the latter was calibrated only using a limited number of experiments and only for the CO2 gasification reaction. The large data scattering reinforces the importance of understanding the fundamentals of char gasification, and how they are impacted by char preparation conditions, presence of catalytic mineral species, thermal annealing, and others [28]. However, the correlations can be easily calibrated with a limited number of experiments, modifying the CO desorption rate and proportionally scaling all the other rates in order to match the overall carbon conversion. In fact, Liu and Niksa [28] demonstrated that all the other parameters can be scaled with a satisfactory accuracy. This procedure showed a good robustness, limiting the number of experiments required for calibrating the model. These advanced models are however generally too computationally expensive for direct use in regular CFD simulations. However, simplified models opportunely calibrated using the results of more advanced ones can be successfully used for simulating entrained flow coal gasification [3,31]. A similar approach was also used at NETL for fluidized bed gasification [32]. In previous work by the authors [31] it was demonstrated that the kinetic of the CBK model can be adjusted using only a limited number of experiments, correctly predicting the carbon conversion for 4 US coals with different ranks in an atmospheric laboratory-scale gasifier. Clearly, whilst useful in situations where coal gasification data are limited, calibrating models with other models is not an ideal way to develop sound, transportable tools for widespread application. It is always preferable to calibrate CFD submodels by means of more comprehensive characterization of the feedstock using pilot- or laboratory-scale tests. For this reason, char conversion is described through a 3-step nth-order intrinsic model, which can be fully characterized by laboratory-scale experiments. The model is designed to reflect the physical description of the main phenomena (heterogeneous reactions, film and pore diffusion) in char conversion, limiting the number of experiments necessary for its

3

characterization and the computational effort for the CFD simulations. In this work, laboratory-scale experiments [13] were used for accurately characterizing the CFD submodels for simulating coal gasification in industrial-scale reactors for four Australian coals. Volatile yields from wire-mesh reactor were used for selecting the best detailed pyrolysis model for each coal. Char conversion kinetics were estimated by means of pressurized TGA experiments in kinetic regime, while gasification behavior were measured in a Pressurized Entrained Flow Reactor (PEFR) in pore-diffusion regime. Finally, the results of the characterization by means of laboratory-scale experiments were validated against the gasification behavior in a 5 MW pilot scale facility [33] for the same Australian coals, providing a unique set of data well suited to calibrating fundamental CFD submodels and validating their outputs using industrial scale data.

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2. Numerical models

223

This section describes the numerical models used for the simulations of entrained flow coal gasification. The numerical setup was previously described in other papers of the authors [3,31,34,35]. The CFD code ANSYS Fluent solves the 2D axisymmetric RANS equations using pressure–velocity coupled algorithm [36]. Convective fluxes in all transport equations are discretized with a second-order upwind scheme and the pressure gradients are discretized with a second-order accurate scheme. Turbulence is modeled using the realizable k—e approach [37]. The Eddy Dissipation Concept (EDC) [38] accounts for the turbulence–chemistry interaction (TCI) in combination with a detailed kinetic mechanism [39] from the GRI-MECH suite, including 103 reactions among 22 species. This approach was used successfully for gaseous partial oxidation [40], for unconventional coal combustion [34,35], and for coal gasification [3,31]. In coal gasification, the heterogeneous char reactions are limiting in terms of the overall rate of conversion. However, correctly describing gas-phase chemistry is important for accurate estimation of flame temperatures, which play a significant role in determining these heterogeneous kinetics. Radiation was modeled with the P-1 model [41]. The radiating properties of the gas were modeled assuming a gray-band model, based on the Weighted Sum of Gray Gases (WSGG) model [42]. Absorption, emission, reflection and scattering from the coal particles were considered in the radiative heat transfer calculation. Coal particle trajectories are simulated using a Lagrangian approach. The Eulerian gas phase is coupled with the solid discrete phase exchanging mass, momentum and energy. The influence of the turbulent flow on the particle trajectories is accounted by means a stochastic method including a random component of the turbulent velocity. The coal conversion is modeled according to the following sequence: drying, pyrolysis and finally char burnout, where a sequential approach is used here. For particle ignition, various authors, e.g. [43,44], discussed non-sequential approaches under oxy-fuel conditions. However, considering the small particle sizes and high heating rates for the case investigated here (both leading to very fast pyrolysis), the standard sequential method is considered appropriate. Pyrolysis is modeled with the empirical Competing 2 Step Model (C2SM) [45]. The parameters required by C2SM are evaluated according to the calibration procedure developed by the authors [3] by means the in-house coal pyrolysis kinetic preprocessor (PKP). The results of one of the following detailed network-based models Chemical Percolation Model (CPD) [21], FG-DVC [22] and FLASHCHAINÒ [23] are used for calibrating the simplified C2SM. Volatile matter is considered as a mixture of light gases and heavy

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hydrocarbons (tar), which are released with a constant ratio during devolatilization. This is a reasonable hypothesis for entrained flow gasification, where pyrolysis occurs very quickly. In addition, it is assumed that char is composed only of pure carbon and that all the hydrogen, oxygen, nitrogen and sulfur fractions of the coal were released during pyrolysis.

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2.1. Modeling of char conversion using intrinsic kinetics

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After pyrolysis, the remaining is consumed by reacting with the surrounding gases. This involves heterogeneous reactions on the intrinsic surface of the porous particle. The following global reactions are usually considered as most relevant for inclusion in model development:

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278 279 280 281

282

wþ1 O2 ! wCO2 þ ð1  wÞCO 2 :CðsÞ þ CO2 ! 2CO

Rs;1 :CðsÞ þ

ð1Þ

Rs;2

ð2Þ

284

Rs;3 :CðsÞ þ H2 O ! CO þ H2

285

292

The first reaction Rs;1 (Eq. (1)) is a combination of the partial and complete oxidation of char with O2, producing CO and CO2 (combustion). Rs;2 and Rs;3 (Eqs. (2) and (3)) are the Boudouard and steam gasification reactions, respectively. Combustion is exothermic and several orders of magnitude faster than the gasification reactions, which are endothermic. The molar ratio between CO and CO2 produced from char oxidation (reaction Rs;1 ) is empirically expressed through an Arrhenius expression:

295

CO 1w  Ec ¼ ¼ Ac e Rg T p CO2 w

286 287 288 289 290 291

293

296 297 298 299

300

302 303 304 305 306 307 308 309

310

312

ð3Þ

ð4Þ

The values used are Ac ¼ 3  108 and Ec ¼ 60 kcal/mol [26], respectively. The overall observed reaction rates Robs are generally expressed as follows:

Robs ¼

3 X

Robs;i ¼ 

i¼1

1 dmc  mc dt

ð5Þ

and it can be measured directly from experiments. The overall observed reaction rate Robs is the sum of the observed rates for the heterogeneous reaction in Eqs. (1)–(3). The observed reaction rate express the overall char consumption scaled by the current mass of the char remaining (mc ). However, it is generally preferable to scale the reaction rate using the specific surface of the char particle As :

Rs;i ¼

mc 1X R ¼ R ; gi mc;0 As obs;i gi As obs;i

ð6Þ

316

where mc;0 is the initial char mass, X ¼ 1  mc =mc;0 char conversion ratio and gi the effectiveness factor of the reaction i. The intrinsic rate Rs;i for each reaction defined by Eqs. (1)–(3) is here defined by means of an empirical nth-order rate equation, [7,25]:

319

Rs;i ¼ ks;i Ps;is;i ¼ ks0;i e

320

where ks;i is the intrinsic rate coefficient for the reaction i, defined by the Arrhenius expression by means of the pre-exponential factor ks0;i and the activation energy Es;i . T p and P s;i are the particle temperature and the partial pressure of the reactant i on the particle surface, respectively. Finally, ns;i is the pressure exponent. In Eq. (6) the effectiveness factor gi is defined, as the ratio between the observed rate of reaction and the rate of reaction if not limited by pore diffusion [46] and reflects the degree to which intrinsic reaction kinetics are limited by the rate of mass transfer of reactants through char porosity at high temperatures. Effectiveness factors of unity indicate that the only process influencing char conversion

313 314 315

317

321 322 323 324 325 326 327 328 329 330

n

Es;i g Tp

R

n

P s;is;i

ð7Þ

rates are the chargas reaction kinetics. Effectiveness factors lower than unity reflect the degree of restriction of this rate by reactant diffusion through the pores, which is driven by concentration gradients arising from the rapid consumption of the reactants. Effectiveness factors are commonly used with the established empirical nth order rate equation [7,25] to estimate conversion rates at high temperatures using intrinsic kinetics and a knowledge of the pore structure of the char particles. The specific surface As in Eq. (7), which is defined as the intrinsic surface of the particle over the initial mass of char mc;0 , evolves during the char conversion process according to the Random Pore Model (RPM) as a function of the char conversion factor X ¼ 1  mc =mc;0 [47]:

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi As ¼ As;0 ð1  X Þ 1  W ln ð1  X Þ

332 333 334 335 336 337 338 339 340 341 342 343

344

ð8Þ

346

The evolution of the char apparent density qc respect to its initial value qc;0 is empirically defined as a function of the char conversion factor X [28]:

347

qc ¼ ð1  X Þa qc;0

ð9Þ

CFD modeling of entrained flow gasification has been extensively researched, e.g. [48–62]. Only Hla et al. [63] and recently Jeong et al. [61] incorporate directly the influence of the pore diffusion using an effectiveness factor approach coupled with intrinsic reaction kinetics, while Vascellari et al. [31] included it by means of an empirical Single Nth-Order Reaction (SNOR) model calibrated using the advanced CBK model. The latter shows an excellent agreement with experiments for an atmospheric pressure entrained-flow gasifier [48]. However, the prediction of high-pressure gasification behavior with the SNOR is difficult due to the large range of reactant partial pressures over which nth order models are often inadequate. In this work, the effectiveness factor gi is modeled according to the Thiele [64] theory, originally developed for catalytic porous media. The same approach has been used successfully for modeling char oxidation and gasification reaction processes [65,66] and it is based on the formulation derived by Bischo [67]. The effectiveness factor is calculated as follows:

1



1 1 gi ¼  U tanh ð3Ui Þ 3Ui

 ð10Þ

where Ui is the Thiele modulus for the reaction i:

Ui ¼

dp 6

"

ns;i 1 # c;0 As Rg T p P s;i

mi ðns;i þ 1Þks;i q

2M c De;i

De;i ¼ eDi

s

349

350 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370

371 373 374

375

;

ð11Þ 377

where dp is the particle diameter, mi is the stoichiometric coefficient of the reactant in Eqs. (1)–(3) and De;i is the effective diffusivity of the reactant inside the particle pores. Once the char intrinsic kinetics are defined, the Thiele modulus can be calculated from the reactant partial pressure (Ps;i ), from the particle temperature (T p ), from other char properties, such as As ; dp and qc , directly correlated to the char conversion (X) (Eqs. (8) and (9)), and from the effective diffusivity De;i . The effective diffusivity in the porous media is modeled similarly to the CBK model [27,28,65,68–70], neglecting the influence of the micro-porosity and of the Knudsen diffusivity and it is given by:

f

348

1 2

ð12Þ

where e ¼ 1  q=qt is the particle porosity, Di is the molecular diffusivity, f is the fraction of the total porosity in feeder pores and s is the tortuosity. The advantage of using this formulation is that all the parameters are defined, except the ratio s=f , which can be treated

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as a single empirical parameters, which lumps other factors neglected here. Recent studies have demonstrated the importance of accounting for specific char morphologies when estimating the char reaction rates at high temperatures and pressures [12,71–74]. However, this approach requires detailed knowledge of the char morphology, which is often difficult to obtain as it is dependent on a range of coal properties and devolatilization conditions. Clearly accounting for such aspects is important to the accuracy and applicability of models such as the one discussed here; however, there is not currently a quantitative, predictive framework that can be used to incorporate char morphological aspects into CFD models. For this reason, despite the fact that the morphologies of the chars studied in this work are very well characterized [12,74], the influence of the char morphology was not explicitly included with the main goal of developing a general approach, reducing as much as possible the number of experiments required for the calibration. Further work is clearly required to allow the interactions of coal properties and devolatilization conditions to be adequately modeled for quantitative inclusion into CFD applications such as this. The partial pressure of the reactants on the external particle surface can be evaluated including the effect of the Stefan flow [31] as follows:

422

    Ps;i P1;i kqd;ii cPi t ¼ 1  ci e ; 1  ci Pt Pt

396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418

420

423 424

425 427

428 429

430

432 433

434

where qi is the mass flux of i per external particle surface unit and Pt the gas total pressure. The transport coefficient kd;i is given by:

kd;i ¼

ShDi M i ; Rg T m dp

438

439 441

ð14Þ

and ci is defined as the molar ratio between the total molar flux and the molar flux of the i-species1:

ci ¼

N_ 00tot N_ 00i

ð15Þ

:

Finally, the heat balance of the coal particle is expressed by



  dT p B mp cp ¼ Ap h T 1  T p þ Ap ep r T 41  T 4p exp ðBÞ  1 dt þ mc

436 437

ð13Þ

3 X

Rv ;i DHr;i :

ð16Þ

B¼

cp dmp 2pdp k dt

3. Experimental setup

446

In this section, the main properties of the Australian coals and the setups of the laboratory- and pilot-scale experiments are briefly reported, mainly focusing on the required information necessary for the simulations. Detailed information on the experiments were reported more extensively elsewhere [13,33]. The coals have been analyzed extensively in laboratory scale studies of gasification behavior [13]. In particular, the slag viscosity, the

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3.1. Coal properties

482

Four Australian coals were investigated in this work. These coals cover a wide range of coal types, ranging from sub-bituminous to semi-anthracite, as shown by the Van Krevelen diagram in Fig. 1. CRC701 and CRC704 are sub-bituminous coals with high volatile content, CRC702 is a bituminous coal, and CRC703 is a semi-anthracite. The proximate and ultimate analyses of the coals used are reported in Table 1 [13,33]. The liquid slag temperatures for each coal are also reported. These temperatures were determined using standard ash fusion test [13]. The table reports also the properties of the fluxed CRC703 coal. In fact, based on the high ash fusion temperature, (1540 °C), the coal was fluxed with 30 kg

483

ð17Þ

445

447

453

where B is the blowing parameter, which corrects the convective heat transfer for a reacting char particle [75]:

444

443

volatile yield at high pressures, and the char reactivity were measured using laboratory facilities and gasification behavior measured under relevant entrained flow conditions using a pressurized entrained-flow reactor (PEFR) [13]. This allows to assess the effects of coal type and gasification conditions on gas composition, carbon conversion, and gasification efficiency and char properties to be studied. Finally, these coals were tested in the 5 MW Siemens pilot gasification facility [33] where the laboratory-scale results above were linked with the overall gasification behavior in a pilot-scale industrial entrained flow gasifier. In large-scale gasifiers in-reactor measurements of important gasification phenomena are extremely difficult and rarely done, because of the extreme operating conditions that exist (e.g. high pressures and temperatures, slagging conditions) and only global measurements at the reactor exit are possible. For this reason, CFD represents an important tool for understanding processes occurring inside industrial-scale reactors. CFD simulations are only accurate when detailed kinetic rates are used for describing the main coal transformations. However, these kinetics can be only calibrated by means of experiments in laboratory facilities and extrapolated to the operating conditions of the industrial-scale reactors. The aim of the present work is to demonstrate how laboratoryscale experiments can be used for calibrating the kinetics of advanced coal conversion models, obtaining accurate predictions of industrial-scale gasifier. Detailed information are reported only for the PEFR and Siemens pilot-scale setup, because CFD simulations of these reactors are reported for the model calibration and validation.

i¼1

The intrinsic model described above has been implemented in ANSYS Fluent using the UDF capabilities of the software in a similar way to previous works of the authors [31,35].

442

5

1 _ 00 are considered positive when is going The molar flux N_ 00 and the mass flux m from the particle surface to the bulk phase.

Fig. 1. Location of the coals used in the van Krevelen diagram.

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Table 1 Analysis of the subsamples of the Australian coals used in this work. CRC703 is also reported as fluxed with limestone (CRC703f). Rank

CRC701 Subbituminous

CRC702 Bituminous

CRC703 Semi-anthracite

CRC703f

CRC704 Subbituminous

Proximate analysis, % as received Moisture Ash Fix. carb. Vol. matt.

6.4 5.6 51.8 36.2

1.3 8.7 55.6 34.4

0.9 9.6 82.3 7.2

0.9 10.9 79.5 8.7

2.3 11.7 42.8 45.2

Ultimate analysis, % daf C H O N S

75.8 4.98 16.73 1.38 1.12

86.35 5.22 7.7 1.89 1.20

92.81 3.69 0.89 1.90 0.71

93.41 3.96 0.00 1.92 0.70

78.80 5.8 14.04 1.01 0.35

HHV as received, MJ/kg

25.672

30.610

32.066

31.120

27.525

Liquid slag temperature, °C

1290

1310

1540

1430

1440

499

of limestone to 1000 kg of coal in order to lower the ash melting temperature. This sample has been labeled CRC703f to differentiate it from the unfluxed CRC703. Properties of unfluxed CRC703 are used for simulations of PEFR, while the composition of the fluxed CRC703f for the pilot-scale gasifier. Other properties are kept constant between the fluxed and the unfluxed coal.

500

3.2. Pressurized Entrained Flow Reactor (PEFR)

501

The Pressurized Entrained Flow Reactor (PEFR) is a facility designed to allow the interrogation of high pressure gasification processes under conditions where key conversion phenomena can be isolated and studied. The reactor is fed with the four coals described in Section 3.1. The coal and gas inlet of PEFR is shown in Fig. 2. The coal particles are injected into the reactor together with inert gas (N2) through the central nozzle, while the preheated reactant stream is blown in the external annular section. Coal devolatilization, combustion and gasification occurs in a 70 mm internal diameter and 2100 mm long tube. The reactor walls can be electrically heated up to a temperature of 1500 °C. For the

494 495 496 497 498

502 503 504 505 506 507 508 509 510 511

Fig. 2. Schematic view of the PEFR inlet.

experiments used here, the reactor was operated at pressure of 20 bar, a range of (controlled) wall temperatures (1100–1400 °C) and a range of O:C ratios (0.60–2.00). The feed coal and carrier gas (N2) temperatures are around ambient and the diluted oxygen is pre-heated to a temperature of about 900 °C. Details of the reactor design and additional background can be found elsewhere [76]. Syngas composition was measured with an isokinetic oil-cooled sampling probe at different heights of the reactor. Carbon conversion (X c ) was determined from the analysis of the syngas composition by relating the measured carbon contained in the product gas (determined as CO, CO2 and CH4) to the carbon introduced in the feed coal, using the equation:



Xc ¼

MC xCO þ xCO2

 þ xCH4 N_

daf _ daf m coal yC

;

ð18Þ

513 514 515 516 517 518 519 520 521 522 523

524

526

where x are the molar fractions of the main chemical species containing carbon, N_ is the overall syngas molar flux in the probe secdaf tion, mdaf the daf carbon coal is the input daf coal feed rate and yc fraction. The PEFR geometry was discretized using a 2D axisymmetric computational grid composed of about 10 000 rectangular cells.

527

3.3. Siemens pilot scale entrained flow gasifier

533

The Siemens 5 MW pilot gasification facility in Freiberg, Germany was used for pilot-scale testing of the Australian coals. The facility and test program were previously described [33]. It is a down-fired entrained flow gasifier feeding 300–500 kg/h of coal operating as a slagging gasifier with a water-cooled wall and partial water quench of syngas. The reactor is fired with the four coals described in Section 3.1. The reactor is characterized by a volume of 200 L. The CFD simulations of the pilot-scale reactors were performed using the input conditions measured during the tests and the detailed geometry of the reactor. The gasifier was fired with the four Australian coals previously described (Section 3.1). The gasification tests in the Siemens pilot-scale facility were divided in different ‘‘balance phases’’, during which the reactor was operated in stationary mode. For each balance phase the operating conditions of the reactor were kept constant in terms of feedstock and input feed rates. The composition and the feed rate of each input and output stream was measured during the balance phase. The input conditions measured during the balance phase were finally used as input for the CFD simulations, while the output values were used for testing the CFD results. Different C:O ratios were investigated during each balance phase in order to identify the preferred and minimum gasification conditions for each coal.

534

Q2 Please cite this article in press as: Vascellari M et al. From laboratory-scale experiments to industrial-scale CFD simulations of entrained flow coal Q1 gasification. Fuel (2015), http://dx.doi.org/10.1016/j.fuel.2015.01.038

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Gasification tests were performed at a pressure of 26 bar. The pulverized coal was fed at controlled rate (between 280 and 360 kg/h) transported by a flux of inert gas, mainly composed of N2 (about 87% by vol.). The main oxidant stream, composed by almost pure O2, is also introduced in the reactor with feed rates comprised between 270 and 330 N m3/h. Finally, a pilot stream of natural gas of 50 N m3/h is introduced for stabilizing the flame. A minimum feed rate of 30 N m3/h is generally necessary to ensure plant stability. Dry syngas composition was measured and averaged for each balance phase, sampling the syngas after the quench section. Since in the numerical simulations only the gasification reactor was calculated, the syngas composition is extracted at the outlet section. In order to compare the measured and the calculated syngas composition, it is assumed that the dry composition of the syngas does not change during the quench cooling. In fact, the fast cooling freezes the chemical reactions in the quench cooler. The carbon conversion is calculated from the syngas composition Eq. (18), in a manner similar to that used for the PEFR data. The geometry of the Siemens pilot-scale gasifier is discretized using a 2D axisymmetric computational grid composed of about 32 000 rectangular cells. The temperature of the slagging wall is assumed to be equal to the liquid slag temperature reported in Table 1.

581

4. Numerical results

582

601

In this section the numerical results of the CFD simulations of the PEFR and of the pilot-scale gasifier are reported for the different coals used. At first, the calibration of the pyrolysis model is reported. For the char conversion model, most of the parameters required by the model previously described (Section 2) can be directly measured from the laboratory-scale experiments. In fact, the intrinsic kinetics are directly measured by TGA experiments, while the specific surface areas are directly measured by BET analysis for different carbon conversion, estimating the parameters for the random pore model. The main undefined parameter is the structural parameter s=f , which is used in Eq. (13) for evaluating the effective diffusivity in the porous particle. It should be considered as a lumped parameter, representing an average property describing the porous structure behavior, which fits the intrinsic model to the measured reaction rates in zone II. The carbon conversion measured from the PEFR experiments will be therefore used for calibrating the structural parameter s=f for each coal. Eventually, the Siemens pilot-scale gasifier will be simulated using the structural parameter s=f , obtained from the calibration using the PEFR experiments.

602

4.1. Calibration of the pyrolysis model

603

The kinetic parameters of the empirical Competing 2 Step Model (C2SM) are calibrated using the results of a detailed network-based pyrolysis model by means of the procedure developed by the authors [3]. For each coal, different detailed models are tested (CPD, FG-DVC and FLASHCHAINÒ) and the one giving the best agreement with the experimental volatile yield measured by means of a Wire-Mesh Reactor (WMR) is finally used for calibrating the empirical C2SM model. Fig. 3(a) compares the volatile yields evaluated by CPD, FG-DVC and FLASHCHAINÒ models, respectively, against the values measured from WMR [13] for the four coals examined. WMR experiments were performed heating the coal from ambient temperature up to 1373 K with a rate of 1000 K/s at 20 bar. The same conditions were applied to the numerical models. FG-DVC shows the best agreement for CRC701, while CPD and

583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600

604 605 606 607 608 609 610 611 612 613 614 615 616 617

60

Volatile yield, %

558

(a)

WMR exp

FG-DVC

CPD

Flashchain

40

20

0

CRC701

(b) 60

CRC702

CRC703

PA

PEFR

WMR exp

Siemens

CRC704

WMR pred

Volatile yield, %

557

7

M. Vascellari et al. / Fuel xxx (2015) xxx–xxx

40

20

0

CRC701

CRC702

CRC703

CRC704

Fig. 3. Comparison of the measured and predicted volatile yields (wt.% daf) for the different coals studied. (a) Shows the comparison between the prediction by CPD, FG-DVC and FLASHCHAINÒ, respectively and the experiments for the WMR (1373 K, 1 K/ms, 20 bar). (b) Shows the measured values for the Proximate Analysis (PA), WMR and the predicted values for PA, WMR and for the PEFR (1673 K, 10.73 K/ms, 15 bar) and the Siemens pilot-scale gasifier (2500 K, 733 K/ms, 25 bar) conditions. Numerical results are obtained using the CPD model for CRC701, FG-DVC for CRC702 and FLASHCHAINÒ for CRC703 and CRC704.

FLASHCHAINÒ overestimate the volatile yield. The opposite situation is observed for CRC702, where CPD shows the best agreement, while the other models underestimate the measured value. On the contrary, FLASHCHAINÒ gives the best agreement for CRC703 and CRC704. As no clear trends can be observed, the choice of pyrolysis model should be performed individually for every coal modeled, whenever it is possible. Generally, CPD is characterized by higher release of volatile matter than FG-DVC, while FLASHCHAINÒ shows lower yields for high-rank coals (CRC703) and higher for low-rank coals (CRC704), showing the best agreement for these coals. The pyrolysis rates predicted by all these models are similar, as already observed [3], while differences in the final yield are observed. Since the correct prediction of the overall carbon conversion during the gasification process is strongly depending on the final volatile yield, it is important to correctly estimate this value for modeling the coal gasification process [3,28]. The devolatilization conditions existing in the PEFR and in the pilot-scale Siemens gasifier are quite different from the WMR, and different devolatilization rates and volatile yield can be observed. Hence, the calibration of the empirical C2SM are performed using representative operating conditions for the PEFR and the Siemens gasifier, respectively. Fig. 3(b) shows the comparison between the volatile yield obtained for different conditions either from experiments and numerical results. In particular, the daf volatile yields from the proximate analysis and measured from WMR are reported [13]. The yields obtained from the best pyrolysis model are also reported for the WMR conditions. In addition the

Q2 Please cite this article in press as: Vascellari M et al. From laboratory-scale experiments to industrial-scale CFD simulations of entrained flow coal Q1 gasification. Fuel (2015), http://dx.doi.org/10.1016/j.fuel.2015.01.038

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Table 2 Kinetic parameters of C2SM used for the simulations of PEFR and the Siemens pilot-scale gasifier. The parameters are calibrated using the detailed network model giving the best agreement with the measured volatile yield from WMR. Measured swelling ratio [74] are also reported. CRC701

CRC702

CRC703

CRC704

1:0522  105 67.20 0.337

1:02626  105 62.455 0.2875

1:1910  105 60.206 0.5292

8:1967  107 126.69 0.578

7:9933  107 118.972 0.6261

2:108173  104 63.900 0.1210 –

1:1730  105 5 56.06 0.417

1:3586  105 53.536 0.3441

E2 , kJ/mol y2

7:4818  107 110.67 0.484

7:8052  107 94.397 0.4653

Model

FG-DVC

Swelling ratio [74]

1.5

PEFR A1 , 1/s E1 , kJ/mol y1 A2 , 1/s E2 , kJ/mol y2 Siemens pilot-scale A1 , 1/s E1 , kJ/mol y1 A2 , 1/s

645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668

– –

2:1746  108 153.435 0.9951

CPD

FLASHCHAINÒ

FLASHCHAINÒ

1.33

1.35

1.51

calculated volatile yields are also reported for the operating conditions of the PEFR and of the Siemens pilot-scale gasifier. The PEFR representative conditions are a maximum temperature of 1673 K with a heating rate of 10 730 K/s at 20 bar, while the representative conditions of the Siemens gasifier are a maximum temperature of 2500 K with a heating rate of 733 000 K/s at 25 bar. These operating conditions were estimated from preliminary CFD simulations. As expected, the volatile yield calculated for the operating conditions existing in the PEFR and the Siemens gasifier are higher than yields measured and calculated. In fact, the PEFR is characterized by a maximum operating temperature of 1400 °C, while the pilot-scale gasifier is characterized by flame temperatures higher than 2000 °C, increasing the overall amount of volatile matter released during pyrolysis. Since the empirical C2SM is applicable only for a limited range of operating conditions, two different calibrations for each coal are performed for the PEFR and the pilot-scale gasifier. The kinetic parameters of C2SM obtained from the calibration with the detailed pyrolysis models are reported in Table 2 for PEFR and Siemens gasifier conditions. For the high-rank CRC703 coal only one devolatilization rate is used because of the small amount of volatiles released during pyrolysis. In addition, the swelling ratios used during the simulations are also reported in Table 2. They were measured, based on image analysis of feed coals and sampled chars [74].

669

4.2. Calibration of the char intrinsic kinetic model

670

The char nth order intrinsic model reported in Section 2.1 was completely calibrated using data from laboratory-scale tests. Pressurized TGA experiments together with intrinsic surface measurements were used for calibrating the nth order intrinsic kinetics (Eq. (7)) and the specific surface (Eq. (8)). More details about the experiments can be find in [12,74,77,78] and the main information are reported in Table 3. Char porosity diffusion for the different coals is modeled through the structural parameter s=f , as described in Section 2.1. For each coal four different runs of PEFR are used for calibrating s=f . In particular, the wall temperature of the PEFR is fixed at 1100 °C and 1400 °C and two different gas composition are used for the gasification agents: the first with 2.5% by volume of O2 and the second with 1.3% of O2 and 2.2% of H2O. The diluent gas is N2. The experimental carbon conversion is compared with the calculated carbon conversion at different heights of the reactor.

672 673 674 675 676 677 678 679 680 681 682 683 684 685

2:9335  105 48.781 0.5942

1:3951  106 86.206 0.1230 –

Table 3 Measured and estimated coal, char properties, kinetic data and char structural parameter s=f for the coals used in this study.

Aint;0 , m2/g

W

CRC701

CRC702

CRC703

CRC704

Refs.

250 2.9

250 3.6

230 0.3

250 3.8

[74] [74]

3:02  104 153 0.8

C þ 1=2wO2 ! wCO2 þ ð1  wÞCO A, g/(s m2 atmn) 3:33  104 1:12  104 E, kJ/mol n C þ CO2 ! 2CO A, g/(s m2 atmn) E, kJ/mol n

[77]

136 0.8

136 0.8

1:97  105 153 0.8

1:79  109 335 0.35

1:46  105 278 0.32

5:69  103 242 0.46

2:77  107 294 0.35

2:41  105 263 0.46

1:21  106 253 0.35

16.80

7.38

[74]

C þ H2 O ! CO þ H2 A, g/(s m2 atmn) 5:37  109

[74]

E, kJ/mol n

335 0.35

8:4  104 254 0.32

s=f

1.00

7.13

–

The parameter s=f is chosen by minimizing the following average error function:

  s 1 ¼ E Nruns  Nprobes f

671

9:9824  107 138.106 0.8869

– –

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2 XNruns XNprobes  exp cfd s X  X ; c;ij c;ij i¼1 j¼1 f

687

688

ð19Þ

690

where the index i is ranging from 1 to the total number of tests for each coal Ntest ¼ 4, the index j is ranging from 1 to the number of measurement points Nprobes for each test and X c are the measured exp and calculated cfd carbon conversion. The calculated carbon conversion values X cfd c;ij depend on the values of the structural parameter s=f . The carbon conversion for the numerical solutions is calculated from the syngas composition, similarly to the experiments, using Eq. (18).

691

4.3. CRC701 coal

699

CRC701 is a high-volatile sub-bituminous coal (Table 1 and Fig. 1), characterized by high-reactivity char [13]. The structural parameter s=f is calibrated minimizing the average error function in Eq. (19), obtaining an optimal value of 1.0, as reported in Table 3, with an average error of 3.82%.

700

Q2 Please cite this article in press as: Vascellari M et al. From laboratory-scale experiments to industrial-scale CFD simulations of entrained flow coal Q1 gasification. Fuel (2015), http://dx.doi.org/10.1016/j.fuel.2015.01.038

686

692 693 694 695 696 697 698

701 702 703 704

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PEFR - CRC701

PEFR - CRC701

100

120

(b) 100

80

Carbon conversion, %

Carbon conversion CFD, %

(a)

60

40 1100 C O 2 1100 C O 2/H2O 1400 C O 2 1400 C O 2/H2O

20

0

0

20

40

60

80

100

120

706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724

40

0

0

0.4

0.8

1.2

1.6

2

Reactor length, m

Fig. 4. Comparison between experiments and numerical results of PEFR fired with CRC701: (a) Parity plot. (b) Carbon conversion along the reactor length.

Fig. 4(a) and (b) report the parity plot of the carbon conversion and the comparison between the numerical and measured carbon conversion, respectively, for CRC701. These show a generally-good agreement of the numerical carbon conversion with the experiments, except at the beginning of the reactor, where the carbon conversion is underestimated by the numerical simulations. Since the carbon conversion in the first measurement point is mainly due to mass loss from devolatilization, the lower calculated conversion is related to an inaccurate estimation of the volatile yield in the simulations. As previously discussed (Section 4.1), the devolatilization behavior is predicted by the FG-DVC model for CRC701, which shows the best agreement with the measured yield in the WMR. However, there is no experimental evidence for the FG-DVC model correctly predicting the volatile yield under the more complex conditions in the PEFR. The analysis of the remaining measurement points shows that the subsequent gasification behavior is adequately predicted by the simulations. The value of the structural parameter (s=f ¼ 1:0) evaluated from the calibration procedure described above is therefore used

Siemens - CRC701 2000

100

1800

80

60

1600

40

1400

1200

20

0 0.55

Temperature, C

705

60

20

Carbon conversion experiments, % Q4

80

0.6

0.65

0.7

0.75

1000 0.8

for simulating the pilot-scale Siemens gasifier. Fig. 5 shows the comparison between the measured and calculated species on a dry-basis at the exit of the reactor. In particular, H2, CO and CO2 were reported as a function of the C:O molar ratio in the reactor. The main experimental trends are reproduced by the numerical results, showing slightly increasing concentrations of CO and H2 and decreasing CO2 moving away from the stoichiometric conditions (C:O = 1:2). In particular the amount of CO and H2 predicted are slightly higher than the measured values, while CO2 shows the opposite trend. However, the gas compositions are analyzed after the partial quench, and in this work compared to the modeled gas composition directly extracted from the outlet section of the reactor before the quench section. As a consequence, the differences between the experimental and modeled results might partially depend on the validity of the assumption that the gas composition does not change significantly during the quench, which may be unrealistic. For this reason, the carbon conversion also reported in Fig. 5 is a better estimation of the gasification process, because the sum of the molar fluxes of CO and CO2 does not change during the quench process. The calculated carbon conversion shows an excellent agreement with the measured data with deviations ranging from 1% to 2.8%, lower than the standard deviation of the measured values.2 Fig. 5 shows also the calculated temperature at the exit of the reactor, before the quench. The outlet temperatures range between 1358 and 1432 °C. The maximum temperature is observed for a C:O ratio of 0.692, and it mainly depends on the different fluxes existing during the balance phases. In fact, the feed rates measured during the balance phases are used as input for the CFD simulations. For instance, the run at C:O = 0.692 is characterized by a lower coal feed rate (349 kg/h) with respect to the other cases (356–361 kg/s). The lower velocities for this case explains the reduced heat flux and consequently the higher temperature at the exit. Models such as this allow further insights to be gained into some of the details of specific reactions that are occurring under complex gasification conditions. Fig. 6 shows the calculated observed rates (Robs ) and the effectiveness factors (g) for the Boudouard reaction (Eq. (2)) as a function of the particle temperature (T p ) in Arrhenius form (i.e. using a log scale for the rate

C:O molar ratio Fig. 5. Comparison between experiments and numerical results of the pilot-scale gasifier fired with CRC701: syngas composition, carbon conversion and temperature at the reactor exit are reported as a function of the C/O molar ratio.

2 Since the carbon conversion is not a directly measured parameter, its standard deviation is estimated from the standard deviations of the single measurements using the uncertainty propagation theory.

Q2 Please cite this article in press as: Vascellari M et al. From laboratory-scale experiments to industrial-scale CFD simulations of entrained flow coal Q1 gasification. Fuel (2015), http://dx.doi.org/10.1016/j.fuel.2015.01.038

725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763

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10 4

Observed rate, 1/s

10 3

(a)

10 2 10 1 10 0 10 -1

1100 C O2 1400 C O2 Siemens

10 -2 10 -3 10 -4 2700

2000

1400

1100 1000

Tp, C

the particle size (dp ) influences the Thiele modulus (Eq. (11)), the partial pressure directly the reaction rates (Eq. (7) and the Thiele modulus (Eq. (11)), and the carbon conversion X indirectly the Thiele modulus through the intrinsic surface (Eq. (8)) and the porosity e in the effective diffusivity equation (Eq. (12)). Effectiveness factor numbers are useful indicators regarding the manner in which chars are reacting. Numbers close to 1 suggest no limitation from pore diffusion, and as they decrease towards zero the impact of diffusion on overall rates becomes more significant. Numbers above 0.1 but below 1 (such as those in Fig. 6(b)) suggest that the system is well within the pore-diffusion-controlled regime, where both intrinsic kinetics and pore diffusion are important. Values as low as 0.03 begin to suggest that a the highest flame temperatures we are approaching where bulk diffusion is the primary process affecting conversion.

774

4.4. CRC702 coal

789

CRC702 is a low-volatile bituminous coal (Table 1 and Fig. 1), characterized by low char reactivity [13]. The same calibration procedure described for CRC701 was performed also for CRC702, obtaining a optimal value of the parameter s=f ¼ 7:13 with an average error of 0.82%. The parity plot (Fig. 7(a)) and the carbon conversion comparison along the reactor length (Fig. 7(b)) show an excellent agreement between the experiments and the numerical simulations for all the runs examined, as demonstrated by the low average error. Unlike with CRC701, the first measurement points are correctly predicted, demonstrating a good estimation of the overall volatile yield by the CPD model also for the operating conditions in the PEFR. Fig. 8 report the experimental and predicted gas composition and the overall carbon conversion at the exit of the pilot-scale gasifier fired with CRC702 as a function of the C:O molar ratio. The CFD results are obtained with the optimal value of s=f , calibrated using the results from the PEFR experiments, as discussed above. Similarly to CRC701, the outlet composition shows some differences between the numerical results and the experiments. In particular, H2 and CO are both underestimated from the simulations of about 3.7% and 2.6% respectively, while CO2 is overestimated of about 2.7%. As previously discussed the difference in the outlet composition may partially depend on the assumption of frozen chemistry during the quench. However, the differences are also evident in the different carbon conversion predicted, with the numerical

790

775 776 777 778 779 780 781 782 783 784 785 786 787 788

Effectiveness factor

100 (b)

10-1

CRC701 10-2

2700

2000

1400

1100 1000

Tp, C Fig. 6. (a) Observed rates (Robs ) and (b) effectiveness factor (g) for the Boudouard reaction in the PEFR (2.5%O2 1100 °C and 1400 °C) and in the Siemens pilot-scale reactor fired with CRC701 coal.

766 767 768 769 770 771 772 773

PEFR - CRC702

PEFR - CRC702

100

100

(a)

(b)

80

80

Carbon conversion, %

765

and an inverse scale for the particle temperature). The figure reports also the theoretical reaction rate, showed by the dashed line, i.e. that which would be achieved in the absence of limitations arising from diffusion inside the pores, and assuming a constant partial pressure of 1 atm. The theoretical reaction rate is reported only as reference. In fact, higher reaction rates may be observed in the reactor, because of the partial pressure higher than 1 atm. The figure shows a large scattering for the observed reaction rates for a given temperature, which mainly depends on the different particle size, partial pressures and carbon conversions (X). In fact,

Carbon conversion CFD, %

764

60

40

20

0

20

40

60

80

Carbon conversion experiments, %

40

20

1100 C O2 1100 C O2/H2O 1400 C O2

0

60

0 100

0

0.4

0.8

1.2

1.6

2

Reactor length, m

Fig. 7. Comparison between experiments and numerical results of PEFR fired with CRC701: (a) Parity plot. (b) Carbon conversion along the reactor length.

Q2 Please cite this article in press as: Vascellari M et al. From laboratory-scale experiments to industrial-scale CFD simulations of entrained flow coal Q1 gasification. Fuel (2015), http://dx.doi.org/10.1016/j.fuel.2015.01.038

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10 4

Siemens - CRC702

40

1400

20

1200

Observed rate, 1/s

%

1600

60

Temperature, C

1800

80

0 0.55

10 3

2000

100

(a)

10 2 10 1 10 0 10 -1

1100 C O2 1400 C O2 Siemens

10 -2 10 -3 10 -4 2700

2000

1400

1100 1000

Tp, C 0.6

0.65

0.7

0.75

1000 0.8

10 0

(b)

Fig. 8. Comparison between experiments and numerical results of the pilot-scale gasifier fired with CRC702: syngas composition, carbon conversion and temperature at the reactor exit are reported as a function of the C/O molar ratio.

815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857

results also showing a lower conversion than the experiments. In fact, the predicted carbon conversions have deviations ranging from 3.5% to 8.6%. These values are lower than the standard deviation of the measured conversion, except for the run at C:O = 0.628, which is characterized by a measured conversion above 100%. The overall carbon conversion calculated for CRC702 coal (93.4– 94.4%) is generally lower than CRC701 (96.7–97.3%), as it can be expected comparing the results of the PEFR. The lower carbon conversion depends on the lower reactivity of CRC702 respect to CRC702. However, difference in carbon conversion between CRC702 and CRC701 observed in the pilot-scale reactor is much lower than in the PEFR. It should be noted that similar levels of conversion in the pilot-scale gasifier were obtained even though the coals have different reactivities, because C:O ratios are adjusted in order to obtain optimum conversion levels. In the laboratory PEFR coals behavior was investigated under the same experimental conditions and so coal-specific behavior is revealed from conversion and efficiency results. Fig. 8 reports also the calculated temperatures at the exit of the reactor, before quench. The temperatures (1495–1519 °C) are generally higher compared to CRC701 (1358–1427 °C), because of the lower C:O ratios and the higher heating value (Table 1). In fact, considering the lower reactivity of CRC702 with respect to CRC701, more oxygen was used for optimizing the carbon conversion. The maximum temperature is obtained for a C:O ratio of 0.633, which corresponds to a lower carbon conversion (93.71%). Fig. 9 reports the observed rate (Robs ) and the effectiveness factor (g) for the Boudouard reaction (Eq. (2)) for CRC702 as a function of the particle temperature (T p ) for the PEFR runs with 2.5% O2 (1100–1400 °C) and for the pilot-scale gasifier. The effectiveness factor plot shows that the gasification reactions for CRC702 takes place in kinetic-controlled regime at the lower temperature in the PEFR and with pore diffusion limitation for higher temperatures (1600 °C). Gasification reactions in the pilot-scale gasifier occur in the pore diffusion regime for a range of particle temperatures between 1400 °C and 2500 °C, corresponding to effectiveness factor above 3  102 . In the PEFR the reaction rates of CRC702 are about two order of magnitude lower (1  101 1/s) than coal CRC701 (1  101 1/s), as demonstrated by the different carbon conversions achieved. In fact, conversion in the PEFR for CRC702 is always lower than 50% (Fig. 7(b)),), while almost complete conversion is achievable for CRC701 (Fig. 4(b)). On the contrary, in the pilot-scale gasifier

Effectiveness factor

C:O molar ratio

10 -1

CRC702 10 -2 2700

2000

1400

1100 1000

Tp, C Fig. 9. (a) Observed rates (Robs ) and (b) effectiveness factor (g) for the Boudouard reaction in the PEFR (2.5%O2 1100 °C and 1400 °C) and in the Siemens pilot-scale reactor fired with CRC702 coal.

CRC702 has similar reaction rates (1  102  1  103 1/s) to CRC701, despite of the slower kinetics, because of the higher char particle temperatures. The maximum rates for CRC702 are obtained at higher temperatures (2600 °C) than CRC701 (1800 °C). In fact, the CRC702 HHV is higher (30.610 MJ/kg) than CRC701, enhancing the carbon conversion in the flame zone. However, the high-temperature flame zone is limited and the lower temperatures existing in the rest of the reactor prevent to obtain complete conversion. In fact at the exit temperature (1560 °C) the reaction rates are sensibly lower than CRC701. In summary, the high temperatures reached in the flame zone allow CRC702 to obtain quite high carbon conversion (90–92%), despite the low char reactivity.

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CRC703 is a semi-anthracite coal (Table 1 and Fig. 1), characterized by low volatile yields and char reactivity [13]. The experimental run at 1400 °C with O2 and H2O shows the highest carbon conversion for CRC703 tests (Fig. 10(b)). The experimental data for CRC703 have higher uncertainties than the other coals and the results of the calibration need to be carefully used. For this reason two values of the structural parameter s=f are examined here. The first value s=f ¼ 50:4 is obtained by minimizing the error function (Eq. (12)) for all the runs, while the second value s=f ¼ 16:8 comes from the minimization only of the test at 1400 °C with O2 and H2O. The errors are 2.67% and 2.07%, respectively. The parity plot (Fig. 10(a)) reports the results of the simulations with s=f ¼ 16:8 and s=f ¼ 50:2, using solid and hollow symbols, respectively. The carbon conversion at 1400 °C with O2 and H2O is largely underestimated, while it is overestimated for the other tests. In fact, the calculated conversion is always higher for the

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Fig. 10. Comparison between experiments and numerical results of PEFR fired with CRC703: (a) Parity plot. Results are reported with solid and hollow symbols for s=f ¼ 16:8 and s=f ¼ 50:2, respectively. (b) Carbon conversion along the reactor length obtained for s=f ¼ 16:8. Only the 1400 °C O2/H2O case (highlighted with thicker line and symbols) is used for the calibration.

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cases with 2.5% O2. Opposite results are observed for s=f ¼ 16:8, where the carbon conversion is correctly predicted for the test at 1400 °C with O2 and H2O, while is further overestimated for the other tests. The results obtained for both the calibrations show that the model cannot reproduce the experimental trends for any value of s=f using the given kinetic rates (Table 3). Since, the value of s=f ¼ 50:4 is non-coherent with the results obtained for the other coals, which is ranging between 1% and 7.4%, the value obtained from the calibration using only the 1400 °C with O2–H2O test (s=f ¼ 16:8) is used for the discussion.The carbon conversion comparison along the reactor length is reported in Fig. 10(b) for s=f ¼ 16:8. The numerical simulations tend to over predict the carbon conversion in the first measurement point of the reactor (0.275 m). As discussed for CRC701 coal, this difference may be related to the overestimation of the volatile yield in the simulations. The remaining measurement points show that carbon conversion calculated is generally higher than the experiments. Fig. 11 reports the experimental and predicted gas composition at the exit of the pilot-scale gasifier fired with CRC703 as a function

Fig. 11. Comparison between experiments and numerical results of the pilot-scale gasifier fired with CRC703: syngas composition and temperature for s=f ¼ 16:8, carbon conversion for s=f ¼ 16:8 and s=f ¼ 50:4 at the reactor exit are reported as a function of the C/O molar ratio.

of the C:O molar ratio. The CFD simulations were obtained for the value of s=f ¼ 16:8, calibrated using the 1400 °C with O2 and H2O PEFR test, as discussed above. It should be noted that the composition of the fluxed CRC703f is used for the calculations. Other parameters reported in Tabs. are assumed the same as the unfluxed coal. The simulations predict lower concentrations of CO and H2, corresponding to lower carbon conversion, similarly to CRC702. On the contrary CO2 is over predicted by the simulations. The same consideration previously made for CRC702 may be extended also to CRC703. In fact, the numerical simulations generally predict a lower carbon conversion than the experiments, as reported in Fig. 11. The conversion obtained for s=f ¼ 16:8 and 50:2 are both reported in the figure, showing a better agreement with the experiments for the first case. On one hand, the deviation (2.7–3.9%) from the experimental conversion is lower than the measurement standard error for s=f ¼ 16:8. On the other hand, the deviations from the experiments are larger than the standard values for the simulations with s=f ¼ 50:2, further demonstrating that the entire PEFR data set for CRC703 cannot be used for correctly calibrating the intrinsic char conversion model. Finally, Fig. 11 shows the temperature at the exit of reactor, before the quench. CRC703 is characterized by the highest exit temperatures (1669–1708 °C) respect to the other coals, because of the highest heating value (31.12 MJ/kg) and ash fusion temperature (1540 °C) (Table 1). The observed rate (Robs ) and the effectiveness factor (g) for the Boudouard reaction (Eq. (2)) are reported as a function of the particle temperature (T p ) for the PEFR runs with 2.5% O2 (1100–1400 °C) and for the pilot-scale gasifier in Fig. 12 considering s=f ¼ 16:8. The gasification behavior of coal CRC703 is similar to coal CRC702. In fact, they are characterized by similar gasification reactivity and conversion in the PEFR. Although CRC703 is known to produce a very dense, microporous structure compared with CRC702 [13], which tends to make char with a less dense, more open structure. This behavior is confirmed by the values of s=f obtained from the calibration. In fact, the high s=f of CRC703 determines effectiveness factor below one, also at low temperatures, despite of the low-reactivity of the char. In the pilot-scale reactor gasification reactions occur mainly in the pore diffusion regime for a wide range of particle temperatures from 1350 °C to 2700 °C. However, the observed reaction rates even at the higher temperatures are lower than the maximum observed rates for CRC701 and CRC702, because of the lower reactivity of

Q2 Please cite this article in press as: Vascellari M et al. From laboratory-scale experiments to industrial-scale CFD simulations of entrained flow coal Q1 gasification. Fuel (2015), http://dx.doi.org/10.1016/j.fuel.2015.01.038

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the CRC703 coal and of the high s=f . In fact, the overall carbon conversion achieved for CRC703 ranges between 89.8% and 92.5%. The differences observed between the measured and the calculated carbon conversion are slightly higher than for the other coals. It might be explained by the large uncertainties in the PEFR experiments. Additionally, the calibrated s=f is obtained from the experiments in the PEFR, which only partially take place in pore diffusion regime (see Fig. 9). Hence, the extrapolation of the gasification behavior at the higher temperature may be not accurate enough for correctly predicting the gasification behavior in the pilot-scale gasifier. Again, this highlights the importance of understanding coal-specific coal-to-char transformations under a range of relevant process conditions.

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1100 C O2 1400 C O2 Siemens 2000

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Tp, C 4.6. CRC704 coal

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CRC704 is a sub-bituminous coal, characterized by a high volatile content and low heating values, (Table 1, Fig. 1) and by high char reactivity. The optimal agreement between the experimental carbon conversion and the numerical results is obtained for s=f ¼ 7:38 with an average error of 4.14%. Large variations (1.0–16.8) of the optimal s=f were observed for the coals used, although in the range of the values reported in literature ([27,28,65]). The large variation of s=f mainly depends on the large differences existing in the char structural morphology of the coal used in this work [12,78]. In fact, s=f is a lumped parameter including different effects, such as char morphology, Knudsen diffusivity in mesopores, etc. The comparison between the numerical carbon conversion and the experiments in PEFR are reported in Fig. 13(a) and (b), showing the parity plot and the carbon conversion, respectively. The comparison shows that the simulations underestimate the carbon conversion in the runs with O2, while they overestimate the conversion in the runs with O2 and H2O. A non-accurate kinetics of the gasification reaction may be the reason of the non-accurate estimation of the carbon conversion for CRC704. The value of the structural parameter (s=f ¼ 7:38) evaluated from the calibration procedure is than used for simulating the pilot-scale Siemens gasifier. Fig. 14 shows the comparison of the measured and calculated species on a dry-basis at the exit of the reactor. The amount of CO and H2 predicted are slightly higher than the measured values, while CO2 shows an opposite trend. As expected, the higher concentrations of CO and H2 correspond

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Effectiveness factor

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10 -1

(b)

CRC703 10 -2 2700

2000

1400

1100 1000

Tp, C Fig. 12. (a) Observed rates (Robs ) and (b) effectiveness factor (g) for the Boudouard reaction in the PEFR (2.5%O2 1100 °C and 1400 °C) and in the Siemens pilot-scale reactor fired with CRC703 coal.

to a slightly higher carbon conversion in the numerical results with deviations ranging from 2.6% to 7%, lower than the experimental standard deviation, except for C:O = 0.585. Fig. 14 additionally reports the calculated temperature at the exit of the reactor, ranging between 1563 and 1605 °C. Fig. 15 finally reports the observed rate (Robs ) and the effectiveness factor (g) for the Boudouard reaction (Eq. (2)) as a function of the particle temperature (T p ) for the PEFR runs with 2.5% O2 (1100–1400 °C) and for the pilot-scale gasifier. Gasification reac-

Fig. 13. Comparison between experiments and numerical results of PEFR fired with CRC704: (a) Parity plot. (b) Carbon conversion along the reactor length.

Q2 Please cite this article in press as: Vascellari M et al. From laboratory-scale experiments to industrial-scale CFD simulations of entrained flow coal Q1 gasification. Fuel (2015), http://dx.doi.org/10.1016/j.fuel.2015.01.038

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Fig. 14. Comparison between experiments and numerical results of the pilot-scale gasifier fired with CRC704: syngas composition, carbon conversion and temperature at the reactor exit are reported as a function of the C/O molar ratio.

10 4

Observed rate, 1/s

10 3 10 2

(a)

10 1 10 0 10 -1

1100 C O2 1400 C O2 Siemens

10 -2 10 -3 10 -4

2700

2000

1400

1100 1000

Tp, C

Effectiveness factor

10 0

(b)

10 -1

CRC704 10 -2 2700

2000

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1100 1000

Tp, C Fig. 15. (a) Observed rates (Robs ) and (b) effectiveness factor (g) for the Boudouard reaction in the PEFR (2.5%O2 1100 °C and 1400 °C) and in the Siemens pilot-scale reactor fired with CRC704 coal.

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tions totally occur in pore diffusion regime in the PEFR at either at 1100 °C and 1400 °C, because of the high-reactivity of the coal. Gasification reactions also occur in pore diffusion regime for the pilot-scale reactor with very low values of the effectiveness factor ð1  103 Þ. The particle temperatures mainly range between 1500 °C and 2100 °C, which are similar to CRC701. The high gasification rates observed in the reactor allow to reach almost completely carbon conversion (98–99%), which are similar to CRC701 (96.4–97.3%) and slightly higher than CRC702 (93.4–94.4%) and

CRC703 (89.8–92.5%). In fact, comparable gasification rates are calculated for CRC701 and CRC704 with similar temperatures in the reactor. Despite of the lower reactivity, similar reaction rates are also reached for CRC702 and CRC703, although for higher temperatures in the reactor, resulting in similar overall conversions.

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5. Conclusions

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In this work advanced gasification models of four Australian coals were calibrated using laboratory-scale experiments with the aim of extrapolating these information for simulating large-scale gasification processes using CFD. In particular, the four studied coals, ranging from semi-anthracite to sub-bituminous, were extensively characterized using high-pressure bench and laboratory scale techniques (a wire mesh reactor, thermogravimetry and a pressurized entrained flow reactor). The advanced models for devolatilization and char conversion were coupled to a commercial CFD code and calibrated using the laboratory-scale work. The same models were finally used for the CFD simulations of the pilot-scale gasifier. Pyrolysis kinetics and overall volatile yield were predicted by means of detailed network-based devolatilization models (CPD, FG-DVC and FLASHCHAINÒ) which are able to predict devolatilization behavior for a wide range of coal ranks. The overall volatile yield predicted by these models were validated using measured values. The accurate prediction of the volatile yield is fundamental for correctly predicting the overall carbon conversion in a gasification reactor, because it characterizes the repartition of the coal conversion between the fast pyrolysis and the slow gasification transformations different models were found to be suitable for different coals. The results of the detailed pyrolysis models were eventually used for calibrating the empirical Competing 2-Step Model (C2SM), which is suitable for CFD simulations because of its low computational demand. Char oxidation and gasification were simulated using a intrinsic-based kinetic model, able to combined gassolid reaction kinetics with limitations arising from the diffusion of the reactants inside the porous structure of the coal particles. Char gasification experiments are required for calibrating the intrinsic model. At first, the kinetic rates were estimated from TGA experiments in kinetic-diffusion regime. Higher-temperature tests in the entrained flow reactor were used for calibrating the char gasification behavior in pore diffusion regime. In particular, the pore structural parameter s=f was obtained from the calibration for each coal. The results of the calibration were finally tested against the experimental data in the 5 MW Siemens pilot-scale entrained flow gasification facility. The simulations adequately reproduce the experimental syngas composition and the carbon conversion at the exit of the reactor, with deviation of the carbon conversion generally lower than the standard deviation of the measured values. Although the pilot-scale experiments allows only measurements at the exit of the reactor, the CFD models were also extensively tested for the large range of operating conditions in the laboratory-scale facilities, adequately describing gasification in kinetic and pore diffusion regimes. Generally, better results were obtained for the lower-rank coals. In fact, the PEFR experiments for these coals clearly occurred in pore diffusion regime, allowing a correct extrapolation of the effectiveness factor also to the higher temperatures existing in the pilot-scale gasifier. On the other hand, the carbon conversion of the high-rank coals in the PEFR only partially occurred in pore diffusion regime, and the extrapolation of the effectiveness factor to the higher temperatures of the pilot-scale reactor was generally unsatisfactory, dealing to lower overall carbon conversion with

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respect to the experiments. In particular, CRC703 was characterized by the higher deviations from the experiments, because of the poor calibration of the s=f parameter due to the combined effect of the large uncertainties of the measured carbon conversions with the limited diffusion inside the char porous structure for the PEFR. This highlights the importance of coal-specific char morphological data, and the ability to be predictive regarding coal-to-char transformations under realistic gasification conditions. These outcomes can be used to direct ongoing research whereby relationships between coal properties, gasification conditions, and char morphology can be understood in more detail.

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Acknowledgments

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This research has been funded in the framework of Virtuhcon by the Federal Ministry of Education and Research of Germany (Project number 03Z2FN11) and by the Saxon Ministry of Science and Fine Arts (Project number 4-7531.50-02-0390-09/1). The authors also would like to thank Achim Moser, Tino Just, Alexander Tremel and Frank Haneman from Siemens Fuel Gasification Technology GmbH for providing the geometry of the pilot-scale gasifier and for the helpful discussion about the results.

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Q2 Please cite this article in press as: Vascellari M et al. From laboratory-scale experiments to industrial-scale CFD simulations of entrained flow coal Q1 gasification. Fuel (2015), http://dx.doi.org/10.1016/j.fuel.2015.01.038

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Q2 Please cite this article in press as: Vascellari M et al. From laboratory-scale experiments to industrial-scale CFD simulations of entrained flow coal Q1 gasification. Fuel (2015), http://dx.doi.org/10.1016/j.fuel.2015.01.038

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