Reduced-Order Modeling of a Commercial-Scale Gasifier Using a

Article pubs.acs.org/IECR

Reduced-Order Modeling of a Commercial-Scale Gasifier Using a Multielement Injector Feed System M. Hossein Sahraei,† Marc A. Duchesne,‡ Patrick G. Boisvert,‡ Robin W. Hughes,‡ and Luis A. Ricardez-Sandoval*,† †

Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada CanmetENERGY, Natural Resources Canada, Ottawa, Ontario K1A 1M1, Canada

‡

ABSTRACT: This study presents a reduced order model (ROM) that describes the behavior of a commercial-scale short-residence gasifier which uses a multielement injector feed system. The state-of-the-art injection technology disperses the feed across the crosssection of the gasifier to enhance the mixing efficiency, thereby allowing a reduction in the reactor size and capital cost. A reactor network is integrated into the ROM to capture the laminar and mixing zones formed by each nozzle and subsequently the merging point of the multiphase flow coming from all of the nozzles. The results of the ROM are in reasonable agreement with the limited data reported for a short-residence time commercial-scale gasifier, that is, residence time, carbon conversion, and cold gas efficiency. Moreover, the performance of the gasifier is examined under changes in the operating pressure, number of injectors, flow nonuniformity, and plugging in the fuel’s injection tubes. The ROM provides valuable insights on the operation of the commercial-scale gasifier and potential safety concerns that can be used to design suitable and safe operation policies for the system. Furthermore, sensitivity analyses on the model, design, and operational parameters are performed to assess the suitability of the model assumptions and identify the most important factors influencing carbon conversion, particle residence time, and temperature profiles. seconds;6 thus, large units with a significant capital cost investment are required for these systems. In recent years, the Gas Technology Institute (GTI), previously as Aerojet Rocketdyne or Pratt & Whitney Rocketdyne, has been developing a compact gasifier with a multielement feed injector which is capable of converting various solid fuels to syngas with a particle residence time of less than 0.5 s.7 The design features have reduced the volume of the gasifier by approximately 90% compared to conventional gasifiers in the market.7 An initial assessment of this gasifier by Fusselman et al. indicated that the cost of electricity generation can be potentially reduced by 18.5%.7 Thus, far, GTI has performed 18 tonnes per day (TPD) pilot-plant testing, risk reduction evaluations, solid pump testing and feed system testing to support development of this technology. The gasifier is now considered ready for demonstration at a scale of 400 TPD, which can support advancement toward the commercial scale at 1000−3000 TPD.8 To accelerate the development of compact multielement injector gasifiers, insights regarding their design and operability are desired through mathematical models to ensure an efficient performance under different operating conditions. In recent years, reduced order modeling of complex systems based on a reactor network showed promising potential for

1. INTRODUCTION Climate change has increased attention to emissions of carbon dioxide (CO 2 ) and other heat-trapping gases to the atmosphere. This has affected the operation of process industries, particularly fossil-based power plants which are responsible for almost a third of global CO2 emissions.1 The integration of CO2 capture technologies with fossil-fired power plants has been widely considered.2,3 The operation of the world’s first commercial-scale CO2 capture and storage facility in SaskPower’s Boundary Dam power plant, Canada, has resulted in a 20% reduction in the power plant’s electricity generation.4 High efficiency power production technologies with CO2 capture are desired to reduce global warming. Gasifiers are solid fuel-fired technologies that have been used in power plants. In a typical gasification plant, the carbonaceous fuel reacts with oxygen and steam to produce syngas, which is later passed through gas cleaning units and burnt inside a turbine to produce electricity. According to Buchan and Cao,5 integrated gasification combined cycle (IGCC) power plants have higher efficiency compared to combustion-based power plants integrated with CO2 capture. However, improvements in the initial investment and availability of gasifiers are still required to make this technology competitive with combustors. Most of the commercial-scale IGCC power plants use entrained-flow gasifiers developed by General Electric (GE) and Shell.6 In their designs, the fuel is injected from a single nozzle as slurry feed (GE) or dry feed (Shell). The residence time of particles in these gasifiers is approximately up to four © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

February 17, 2017 May 13, 2017 June 5, 2017 June 6, 2017 DOI: 10.1021/acs.iecr.7b00693 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Schematic of the gasifier system modeled in this study (36-element injector).

2. GASIFICATION SYSTEM

modeling of complex systems such as entrained-flow gasifiers.9−12 Our group has previously presented a ROM, implementing a reactor network based on computational fluid dynamic (CFD) simulation flow patterns, for CanmetENERGY’s pilot-scale entrained-flow gasifier.13 Simulation results generated using the ROM were in agreement with CFD simulation results13,14 and experimental data from petroleum coke gasification.15,16 This modeling approach is computationally efficient and enables modeling analyses that can otherwise need to be performed using computationally expensive CFD models. The present contribution aims to propose a ROM for a compact commercial-scale gasifier with a multielement injector feed system based on the insights and experiences gained through the reduced order modeling of CanmetENERGY’s pilot-scale gasifier. The commercial-scale gasifier has not been built yet and no detailed modeling/experimental study has been reported in the literature for this type of system. Therefore, this study intends to illustrate the performance, feasibility, and operating limitations of the gasifier under scenarios which are likely to occur. The ROM is initially used to investigate particle residence time, carbon conversion, and cold gas efficiency. Next, the performance of the gasifier is explored with different operating pressures, various numbers of injectors and nonuniformity in fuel distribution among the nozzles. A sensitivity analysis is also performed to identify the parameters which are most important to the accuracy of the model, and to indicate the effect of design parameters on the performance of the gasifier. The structure of this article is as follows: section 2 describes the commercial-scale gasification system, which is partly based on the design of GTI’s compact gasifier. A description of the gasifier model, CFD simulation results of a similar system and the proposed reactor network are presented in section 3. The results of case studies and sensitivity analyses are presented in sections 4 and 5, respectively. Concluding remarks are stated in section 6.

The compact gasification system modeled in this study is based on the system developed by GTI. It is a single-stage, pressurized, downward-fired entrained-flow gasifier (see Figure 1) that is anticipated to accommodate a range of solid fuels. According to a patent presented for a compact high efficiency gasifier and status update reports on GTI’s compact gasifier development, the diameter and length of a ∼3000 tonnes commercial-scale gasifier are expected to be 1 and 4.6 m, respectively.17,18 The state-of-the-art design features of this type of gasifier include: (1) a high pressure solid pump and dense phase flow splitter which enable distribution of pulverized fuel particles to the gasifier at high pressure, (2) a rapid spray quenching that enables cooling down to 600 K prior to passing gas to a cyclone, (3) a ceramic matrix composite liner actively cooled by a membrane which increases the life of the gasifier’s wall (shown in Figure 1) and (4) a rapid mix injector that uses multiple elements to enable a high conversion of solid fuel in a short residence time. Note that the focus of the present work is on the feed system and the other features are not considered in the ROM. A multielement injector feed system rapidly mixes the fuel with oxygen/steam and quickly disperses the feed across the reactor’s cross-section. This increases the mixing of fuel and oxidizing agents (i.e., oxygen and steam) and reduces recirculation within the gasifier. The feed system for GTI’s compact gasifier includes a solid pump which takes dried pulverized fuel at atmospheric pressure and delivers it to the multielement injector at pressures of up to 69 bar.19 As shown in Figure 1, each feed nozzle has a fuel injection tube (surrounded by six oxygen/steam impinging orifices) that is sized to inject up to 100 TPD of dry fuel to the gasifier.20 Therefore, the commercial-scale gasifier requires 18 and 36 nozzles to inject fuel for the capacities of 1200−1500 TPD and 2400−3000 TPD, respectively.19 Figure 1 illustrates the nozzle configuration proposed for a design with 36 injectors.19 The nozzles are placed on three ring layers, where the thickness of each layer (λ) is a key parameter in the design of the injector. B

DOI: 10.1021/acs.iecr.7b00693 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article


Figure 2. Gas-phase streamlines at a cross-section inside the combustor, with the bottom right image showing the cross-section plane from above.

zones. Due to the mixing behavior of the recirculation zones, a uniform distribution of composition and temperature are often observed in these zones. Accordingly, they can be accounted for as a single node. A ROM for an entrained-flow gasifier typically requires a reactor network composed of plug flow reactors (PFRs) and continuous stirred tank reactors (CSTRs). The zones with a laminar jet/plug-flow or uniform mixing conditions are modeled as PFRs or CSTRs, respectively. A method to design a reactor network is through CFD simulation since the streamlines, composition, and temperature profiles of CFD can provide good approximations for the boundaries and geometric parameters of the reactors. CFD simulation results for a gasifier with multiple feed nozzles were not available; therefore, CFD simulation results of a similar system, that is, a combustor with multiple injection nozzles, were used to study the flow fields created by multiple feed nozzles. This approximation was made due to similarities in reaction systems, hydrodynamics of multiphase flow, and the designs of the feed systems. This CFD model is described next. 3.1. CFD Model Description. An initial exploratory study of high pressure combustion of coal slurry with multiple injection nozzles (design similar to GTI’s compact gasifier20) was conducted to investigate particle residence time, coal reaction rates, and equipment sizing using the commercial computational fluid dynamics (CFD) code ANSYS CFX 14.5. Multiple 3D geometries were studied by CanmetENERGY and the best fit proxy for the ROM discussed herein was that of the simulation of a cylindrical combustion vessel with a 1 m inner diameter and 10 m length. The burner geometry was designed for 17.65 kg/s coal with 8.03 kg/s slurry water combusted with 25.48 kg/s O2 at the absolute pressure of 0.8 MPa. The computational grid used to represent this geometry was divided into an upper region (top 0.5 m) with 2 000 000 unstructured

There is no experimental data available for the gasifier described above; however, based on initial assessments in technology status update reports and the insights gained from the operation of similar systems, a set of operational constraints and limitations have been considered here to avoid infeasible operational regimes. Accordingly, the following considerations have been taken into account in the present study: • momentum ratio of oxygen to fuel of 2−5 to promote high mixing • particle residence time of 0.5 s21 • maximum operating pressure of 69 bar18 • maximum temperature near the water-cooled injectors of 1470 K • peak temperature (or flame temperature) of approximately 3000 K7

3. MODELING METHODOLOGY One of the challenges in modeling complex systems such as entrained-flow gasifiers is the computational time required to reach numerical convergence. To overcome this issue, the dimension of the differential equations can be reduced to specific domains that capture key process characteristics; thus, instead of solving the equations for a fine mesh in the entire grid, some sections of the grid with low gradients can be approximated by a single node. In addition, the complexity of mathematical submodels can be simplified by accounting for the most important features of the phenomena. Models developed based on such techniques are known as reduced order models (ROMs). For entrained-flow gasifiers, the governing equations describing the jet/plug-flow conditions created by the feed nozzles can be reduced to one-dimensional differential equations. Moreover, part of the flow is expected to recirculate toward the feed nozzles and create recirculation C


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Figure 3. Proposed reactor network for the multielement injector gasifier.

reduced order models and available correlations in the literature. As a result, the CFD simulation of the combustor cannot be used to validate the proposed gasifier ROM. 3.2. Reactor Network. Velocity field vectors obtained from CFD simulation of the multielement injector combustor described above are presented in Figure 2. The combustor’s flow field is divided into three sections, that is, top, middle, and bottom sections (see Figure 2a). The slurry fuel and oxygen were injected at the top of the combustor through 16 nozzles. Each nozzle consists of an inner fuel injection tube and an outer concentric oxygen-enriched flue gas injection tube. For the CFD simulation, 20% of the total injected gases are injected through the porous burner face as a cooling mechanism. For the ROM, the effect of gases passing through the porous burner face has been taken into account to calculate the gas velocity at the inlet boundary condition. Jet expansion regions of the combustor chamber are typically referred to as the jet expansion zones (JEZs). The low cooling gas momentum also enables recirculation of gases from JEZs toward the feed nozzles. This behavior was observed for all nozzles, however the recirculation flow closer to the side wall (i.e., with external nozzles) was higher compared to recirculation closer to the centerline (i.e., with internal nozzles) of the combustor (see Figure 2b,c). Accordingly, the recirculation regions were divided into internal recirculation zones (IRZs) and external recirculation zones (ERZs). In the middle section (see Figure 2b), the jet flows coming from the top section were merged together and formed a larger jet stream. As the larger jet flows approached the wall, some of the flow recycled toward the intersection of the top and middle sections, while the rest proceeded toward the outlet of the combustor through a laminar plug flow region in the bottom section. Because of the geometrical and feed system similarities of the combustor design simulated by CFD and the compact gasifier being modeled by the ROM, it was assumed that they have similar flow structures. Note that the previous studies have implemented similar approach, that is, usage of the CFD simulation for a combustor as the basis for developing the reactor networks of gasification systems.9,12 A reactor network is proposed to account for the flow structures in each section of the gasifier (Figure 3), which has been designed for an

elements with a higher concentration of elements near the nozzles, and a lower region (remaining 9.5 m) with 200 000 structured elements. The average element volume in the near burner region was 1.92 × 10−7 m3. The two zones were coupled using a general grid interface able to conserve interface flux. Conservation equations of mass, momentum, and energy were solved simultaneously. For turbulence closure of the Navier− Stokes equations, the standard k−ε model was utilized. Radiation heat transfer was included and modeled using a discrete transfer radiation model. Slurry injection was modeled using Lagrangian particle tracking. To simulate the combustion process, the CFD model implements submodels for heterogeneous and homogeneous reactions. The heterogeneous reactions occurred sequentially; moisture release, followed by coal devolatilization, and then char oxidation. The volatiles, once released from the coal, were modeled as a single component gas, and would partially oxidize to a product mixture of six species: CO, CO2, H2, H2O, NO, and SO2. CO oxidation was included to complete the reactions. Moreover, due to the high injected volume of slurry water, the excess oxygen, and the available heat, the water gas shift reaction and H2 oxidation were also included in the model. Note that the homogeneous gas phase reactions were modeled using a minimum rate approach, either eddy dissipation or finite rate chemistry. Coal slurry was injected through 16 nozzles based on an injector design similar to Pratt & Whitney Rocketdyne’s gasifier injector.20 Burner face cooling was achieved by specifying 20% of the total injected gas flow to be introduced through the top reactor face. All injection boundary sites were modeled as walls with local mass sources instead of inlet boundaries to prevent the Lagrangian particles from exiting through these faces. The side walls were assumed to be covered by running slag; therefore, the wall temperature was set to 1800 K and its emissivity was set to 0.58.22 The fuel injection tube’s orifice diameter was set to 11.2 mm to approach the velocity described in the gasifier injector patent20 and ensure good mixing between the reactants. Note that the key information used from the CFD simulation to build the proposed ROM is the general flow pattern. The rest of the model parameters have been adjusted based on the authors’ experience in developing D


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Industrial & Engineering Chemistry Research Table 1. Geometry Parameters of the Reactor Network gasifier diameter (m) gasifier length (m) fuel nozzle diameter (m) wall-space, λw (m) section number of nozzles nozzle layer thickness (m) recirculation ratio jet angle (deg) length (m) recirculation zone diameter (m)

top int. 6 0.35 0.95 14.3 0.32 0.10

18 element injector

36 element injector

0.7 4.6 0.0127 0.07

1 4.6 0.012 0.15

top ext. 12 0.28 1.04 15.8 0.32 0.14

middle NA NA 0.26 7.8 0.83 0.28

top int. 6 0.30 0.53 10.5 0.32 0.08

top mid. 12 0.25 0.75 12.3 0.32 0.09

top ext. 18 0.3 0.98 15.8 0.32 0.16

middle NA NA 0.28 12.6 0.83 0.42

λwall is evenly added to the volume of ERZ-1s corresponding to the external layer of nozzles. • The outlet diameter of the JEZ-2 and diameter of the DSZ are set to the gasifier’s diameter. One the most important parameters for a reactor network is the recirculation ratio. This parameter is defined as the mass flow entering an IRZ/ERZ divided by the mass flow entering the corresponding JEZ. In our previous work, we proposed an empirical correlation to calculate the recirculation ratio for a confined jet based on the flow rates of the feed.16 The correlation was derived by fitting the ROM’s parameters (in terms of conversion and temperature) to experimental data obtained from CanmetENERGY’s pilot-scale gasifier, which uses a single injector. The proposed correlation improved the ROM’s accuracy with respect to experimental data when compared to the method proposed by Thring and Newby which uses a fixed recirculation ratio for different flow rates. To the authors’ knowledge, a correlation for the recirculation ratio of a multielement injector is not available in the literature; hence, the correlation proposed in our previous work (eq 17 in Table 2) was used to calculate the recirculation ratio for each nozzle (Table 1). The impact of recirculation ratios on the results of the ROM are presented in section 5. 3.3. Mathematical Model. Table 2 presents the governing equations and constitutive relations used in the ROM. The notations for the equations presented in Table 2 are provided in the nomenclature section. The momentum transfer equations for gas and solids are expressed as eqs 1 and 2, by including terms for convection, gravity, pressure drop, and friction forces between gas, particles and the reactor wall. The mass transfer within the system is given by eqs 3 and 4, which include terms for convection, diffusion, and homogeneous and heterogeneous reactions. Moreover, the heat transfer in multiphase flow is considered via eqs 5 and 6, which take into account convection, conduction, radiation, and heat of reactions. The radiation terms for gas phase and interparticle interaction are neglected as gas phase is assumed to be transparent and the gasifier system’s solid phase is diluted. Similar assumptions were made in the pilot-scale gasification ROM.13 Note that the wall friction forces, radiation, and convection terms between particle-slag and gas-slag are not considered for the internal and middle zones of the top section. The solid volume fraction (εp) is given by a particle density/ concentration balance (eq 7). To solve the governing equations, the unknown terms must be evaluated through the use of submodels for friction forces (eq 8−11), energy sources (eq 12−16) and reactions. As shown in Figure 1, a slag layer

operating pressure of 69 bar. As shown in Figures 1 and 3, the nozzles in the top section are classified as internal, middle, and external nozzles. To account for the jet and mixing zones of each nozzle in the top section, two reactors are implemented for each nozzle: a conical PFR to simulate the JEZ (JEZ-1) produced by that nozzle and a cylindrical CSTR to simulate the IRZ or ERZ (ERZ-1). The fundamental differences between the reactors associated with internal, middle, and external nozzles are the jet angle, recirculation ratio, size of reactors, and heat loss. Therefore, one set of PFR/CSTR is not sufficient to effectively capture the behavior of the system. Although the axial direction is considered as the only domain of the differential equations, the radial domain is also captured by the proposed reactor network since the composition/pressure/temperature varies radially between jet expansion and recirculation zones of each nozzle. Owing to the assumption of uniform feed distribution among the nozzles and high velocity jet flow in the top section of the gasifier, it is assumed in the ROM that no significant heat and mass are transferred among the recirculation zones of each nozzle. The design parameters for the different zones of the gasifier (presented in Table 1) are calculated based on the following procedure: • The inlet diameter of the JEZ-1s is calculated based on the diameters of the fuel tube and oxygen orifices as described,20 that is, 12.7 mm. • The summed length of the top and middle sections is assumed as 25% of the gasifier’s length. • The length of the top section is assumed as half of the length of the middle section. • To ensure that the entire gasifier volume is accounted by the reactor network, the outlet diameter of the JEZ-1s is calculated based on the cross-sectional area formed by the corresponding λ (thickness of each layer) divided by the number of nozzles in each layer. • The jet angle of each nozzle is calculated based on the outlet diameter and length of the JEZ-1. • The inlet diameter of the JEZ-2 is calculated by summing the outlet diameters of the JEZ-1s in an axial plane passing through the diameter of the gasifier. • The length of each recirculation zone is equal to the length of its corresponding JEZ. • The volume of each recirculation zone is calculated by assuming that each pair of JEZ and ERZ/IRZ forms a cylindrical reactor with a diameter equal to the JEZ’s outlet diameter. Note that the volume associated with the E


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Industrial & Engineering Chemistry Research Table 2. Mathematical Model for Gas and Solid Phases in the Gasifier Momentum Balance

−

⎛ dP ⎞ ∂ + εgρg g − Fg′→ wall − Fg′→ p⎟ = 0 (Acsεgρg ug 2) + Acs⎜− ⎝ dz ⎠ ∂z

−

∂ (Acsεpρp u p2) + Acs(εpρp g + Fg′→ p) = 0 ∂z

−

∂(εgC totalxi) ⎞ ∂(AcsεgugC totalxi) ∂⎛ ⎜⎜AcsDg,eff ⎟⎟ − + Acs(R hom + R het) = 0 ∂z ⎝ ∂z ∂z ⎠

(1)

(2)

Mass Balance

∂(Msolid) − + Acs(R het) = 0 ∂z

(3)

(4)

Energy Balance

−

∂Tg ⎞ ∂⎛ ⎟− ⎜Acskg,eff ∂z ⎝ ∂z ⎠

∂(AcsεgugC totalc p Tg) g

∂z

′ →p + Acs(R homHR ) + Q conv,g

′ → slag = 0 − Q conv,g −

∂(Acsεpu pρp c p Tp) p

∂z Particle Density/Concentration Balance

∂(AcsNpu p) ∂z

=0

(5)

′ → p − Q radiation,p ′ + Acs(R hetHR ) − Q conv,g =0 → slag

(6)

(7)

Friction Forces

′ = 3εpC Dρg εg −2.65(ugas 2 − u particle 2)/d p Fg,p C D = 24/Red,p(1 + 0.15Red,p 2

′ = fρg εgug /(16rgasifier) Fg,w

0.687

)

(8)

(9)

(10)

⎛ Ω 2.51 ⎞⎟ wall 1/f 0.5 = − 2 log⎜⎜ + 0.5 ⎟ 7.4 r Re ⎠ ⎝ gasifier d,w f

(11)

Energy Sources per Axial Length

Q ′conv,g → p = A phpg (Tg − Tp)

(12)

Q ′conv,g → slag = πdgasifierhgw (Tg − Tslag) 4

4

Q ′rad,p → slag = A pσ ϑwϑp(Tp − Tslag ) Heat of Reactions Nusselt Number

(13) (14)

13

Nu p − g = 1.32Rep,d 0.5Pr 0.33

(15)

⎡ ⎤ ⎛ f ⎞0.5 Nu fluid − wall = f /8Red,wPr /⎢1.07 + 12.7⎜ ⎟ (Pr 2/3 − 1)⎥ ⎝8⎠ ⎢⎣ ⎥⎦

(16)

Recirculation Ratio

RR = 0.47(dreactor /d jet)(2.38Moxygen /(M fuel + Msteam))0.53 − 0.5

(17)

Reactions Homogenous reactions (Rhom) were incorporated by gathering kinetic data from the literature13 Kinetic data for heterogeneous reactions (Rhet) for the petroleum coke fuel were obtained from PC Coal-lab13 The devolatilization process was simulated by using PC Coal-lab software for petroleum coke at high pressures13

forms on the interior surface of the refractory. A slag layer of negligible thickness and with an inner surface temperature of 1880 K was assumed. This slag layer exchanges heat by convection and radiation with multiphase flow in the ERZ-1, ERZ-2, and DSZ. Its inner surface temperature was estimated by applying a slag viscosity model and setting the viscosity to a value of 10 Pa-s.23 Note that the sensitivity of the ROM’s outputs to the slag temperature is studied in section 5. More details regarding the submodels can be found in our previous work13 and are not discussed here for brevity.

The governing equations and submodels were solved using the implicit finite difference method. The spatial derivatives were estimated on a spatial one-dimensional grid. Accordingly, the momentum transport was solved initially to calculate the velocities and pressure drop at each grid point by assuming an average gas density in each zone of the gasifier. The resulting velocity profiles were passed as inputs to solve for the mass and heat transfer equations simultaneously at each grid point inside the gasifier. To obtain steady-state results, the spatial domain was discretized for the JEZ-1, JEZ-2, and DSZ reactors. The F


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Industrial & Engineering Chemistry Research grid sizes of each reactor were selected based on a sensitivity analysis that was performed with different numbers of nodes so that the relative error of the velocity, temperature, and conversion are less than 0.1%. Accordingly, a nonuniformly distributed grid size was obtained in the axial domain: 20 nodes for each JEZ-1, 40 nodes for the JEZ-2, and 40 nodes for the DSZ. Overall, a system of 2592 nonlinear equations is solved to obtain the steady-state results by using a nonlinear solver in MATLAB, which implements a modified version of Newton’s method. Note that the Jacobian matrix was evaluated analytically to speed-up the calculations. The physical properties of the gases have been calculated by using the thermodynamic package of a process simulator, that is, Soave−Redlich−Kwong equation of state. A summary of the information used to develop and validate the ROM for the commercial-scale short-residence time gasifier is presented in Table 3.

Table 4. Operating Conditions and Fuel Properties flow rate (TPD)

Table 3. Summary of Information Used for the CommercialScale Gasifier ROM ROM features

source

velocity flow field

CFD simulation of a combustor with multielement feed injector (Figure 2) experimental tests for pilot-scale gasification system15 proposed correlation by Sahraei et al. for confined jet flows16 Sprouse et al.18 mass, energy and momentum balance for solid and gas phases (Table2) conversion, cold gas efficiency and residence time reported in the literature7,25

feed ratio recirculation ratio system geometry mathematical equations model validation

case I

fuel 3000 oxygen 2795 steam 779 nitrogen 600 Proximate Analysis of Fuel (As Received) ash 0.046 volatiles 0.127 moisture 0.005 carbon 0.822 Ultimate Analysis of Fuel (Dry, Ash-Free) hydrogen 0.042 sulfur 0.061 nitrogen 0.018 oxygen 0.015 carbon 0.864

case II 1500 1397 389 300

sharply due to the combustion of volatiles/carbon and recirculation of hot gases. As oxygen was depleted, the heat required for endothermic reactions overcame the heat released by exothermic reactions and the temperature decreased as the gases moved toward the gasifier’s middle zone. Furthermore, the temperatures at the outlet of the external JEZ-1s and in the external recirculation zones were lower than their internal/ middle counterparts due to heat loss to the gasifier’s wall. The highest peak temperature was observed in the JEZ-1s of internal nozzles; 3085, 2987, and 2934 K for operating pressures of 20, 40, and 69 bar, respectively. The obtained peak temperature at 69 bar is consistent with the initial data reported for the peak temperature of GTI’s compact gasifier, that is, 3000 K.7 In the present work, the average residence time of particles was estimated by using the particle’s velocity profile in the axial domain. At lower pressure, the average residence time of particles was shorter (see Table 5) as the particles moved faster. Consequently, at lower pressure the peak temperature was shifted toward the end of the JEZ-1s. The molar percentage profiles of CO and H2 are presented in Figure 4d−f. The concentration of these two species initially decreased at the beginning of the JEZ-1s as they reacted with oxidizing agents (i.e., oxygen and steam) to produce H2O and CO2. After the particles passed through the combustion zone, the gasification reactions became dominant and the molar fractions of CO and H2 increased. Among the JEZ-1s, more carbon was converted in the JEZ-1s (and corresponding recirculation zones) of external nozzles since they have a larger size, and hence a longer particle residence time, compared to the JEZ-1s of internal/middle nozzles. As a result, the molar percentages of these two species were slightly higher at the outlet of the external JEZ-1s. The temperature and molar percentage profiles for the middle and bottom sections are presented in Figure 5. Overall, at elevated pressure more fuel was converted to CO and H2 as the partial pressures of reactants were higher and particles have longer residence times. A summary of the gasifier’s performance with varied pressure is presented in Table 5. At 69 bar, the ROM predicted a carbon conversion, cold gas efficiency (CGE) based on the higher heating values, and particle residence time of 99.7%, 83.7%, and 0.52 s, respectively. These results are in agreement with the claims reported for GTI’s compact gasifier, that is, a conversion of >99% and a CGE of 80−85% with a particle residence time of less than 0.5 s.7,25 As presented in Table 5, the residence

4. RESULTS AND DISCUSSION: CASE STUDIES This section presents the results obtained with the proposed reduced order model for multielement injector gasification systems. A comparison between the responses obtained from the proposed model to preliminary studies available for a shortresidence time gasifier at commercial-scale is presented first. This is followed by a discussion on the performance of the system at different operating pressures and using different numbers of injector elements. In addition, the effect of flow nonuniformity in fuel nozzles of the multielement injector is assessed by performing three case-studies, presented at the end of this section. The proposed case-studies can provide insight regarding the operation and availability of the system in the scenarios that are likely to happen for the gasifier. 4.1. Operating Pressure. Most gasifiers operate in the pressure range of 10 to 100 bar.24 Gasifying at high pressure reduces equipment sizes and compression requirements for downstream operations. The operating pressure of GTI’s compact gasifier is reported to be 69 bar. To evaluate the effect of pressure on the performance of a multielement injector gasifier with a throughput of 3000 TPD, and to illustrate the operational feasibility of such a system, simulation of petroleum coke gasification was performed under three operating pressures (20, 40, and 69 bar). The operating conditions and fuel properties used to model the gasifier are listed in Table 4 (Case I). Note that a uniform flow distribution among 36 nozzles was assumed in this case study. The temperature and molar fraction profiles of the top section are presented in Figure 4. As shown in Figure 4a−c, within each individual JEZ-1, the temperature initially increased G


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Figure 4. Top section simulation results with different pressures: (a−c) temperature profiles; (d−f) molar percentage profiles for JEZ-1s.

K. Such high temperature may damage the injector. Moreover, the inlet velocity of the gas at 20 bar was increased by a factor of 3.2 compared to the velocity at a pressure of 69 bar. As a result, the combination of the proposed operating conditions and design parameters may not be suitable for operation of the gasifier at 20 bar. It should be noted that the ROM reactor network may not be suitable for this pressure. The aim of performing this case study was to illustrate the capabilities of the ROM to predict such potential operational problems. Moreover, varying the pressure affected numerical convergence of the ROM. To test the capabilities of the present ROM, we explored the performance of the gasifier at a pressure of 15 bar, that is, the normal operating pressure of CanmetENERGY’s pilot-scale gasifier; however, low particle volume fractions (calculated based on the nozzle size and inlet flow rates) resulted in convergence difficulties. Similar numerical insta-

Table 5. ROM Results with Different Operating Pressures and Capacity 3000 TPD pressure (bar) carbon conversion (%) cold gas efficiency (%) residence time of particles (s) Near Injector max T (K) internal nozzle external nozzle

1500 TPD

20 84.4 61.2 0.16

40 91.0 71.2 0.31

69 99.7 83.7 0.52

69 98.4 82.5 0.55

1537 1601

1327 1400

1299 1364

1380 1273

time of particles was strongly affected by the operating pressure. At low pressures, the gases were less compact and the particles moved faster. Therefore, the carbon conversion was the lowest at 20 bar (84.4%). The temperature surrounding the injector at 20 bar exceeded the considered temperature limit, that is, 1470

Figure 5. Middle and bottom section simulation results with different pressures: (a) temperature profiles; (b) molar percentage profiles. H


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Figure 6. Simulation results for gasifiers with 18 and 36 injector elements: (a) temperature profiles in the top section; (b) temperature profiles in the middle/bottom sections; (c) molar percentage profiles in the top section; (d) molar percentage profiles in the middle/bottom sections.

bilities were observed by Monaghan when the volume fraction of particles approached zero.26 On the basis of the above results, the proposed ROM was able to predict reasonable results for the peak temperature, conversion, CGE, and residence time of the gasifier at 69 bar. The required computation time of the ROM was approximately 17−30 min for each simulation (Core i7, 3.4 GHz with 8 GB of RAM), while CFD simulation had computational time on the order of multiple days with the use of a multiprocessor computer. 4.2. Number of Injectors. The GTI compact gasifier has been designed to process 1500 to 3000 TPD of dried coal.19 In this case study, the performance of a similar gasifier is compared at two fuel capacities: 1500 (lower scale) and 3000 (higher scale) TPD which requires 18 and 36 fuel injector nozzles, respectively.19 The oxygen/fuel and steam/fuel ratios were kept constant for both gasifiers, with each nozzle capable of injecting 83.33 TPD of dried fuel to the gasification chamber at 69 bar. As the inlet flow rates for the lower-scale gasifier were decreased by 50%, the cross-sectional area of the large-scale gasifier was reduced by 50% and only two layers of nozzles were considered. The geometry parameters for the two gasifiers are presented in Table 1. Note that the optimal design of the gasifier at different scales is the subject of future work. Figure 6 presents the simulation results for the two gasifiers. As illustrated in Figure 6a,b, the peak temperature in both gasifiers were the same; note that the outlet temperature of the lower-scale gasifier was only 25 K less than the higher-scale gasifier. This was due to higher heat-loss area per volume in the lower-scale gasifier. The molar fraction of CO was slightly higher in the higher-scale gasifier, however the molar fraction of

H2 was the same for both cases (Figure 6c,d). Overall, the carbon conversion of the lower-scale gasifier was 98.4%, which was 1.3 percentage points lower than with the higher-scale gasifier, even though the particle residence time of the lowerscale gasifier was slightly higher (Table 5). Both gasifiers were able to maintain a near-injector temperature below the 1470 K limit. On the basis of the insights gained from this case study, a 50% reduction in the gasifier’s cross-sectional area for the lower-scale gasifier was reasonable for gasifier scaling and obtaining a high carbon conversion and CGE. 4.3. Fuel Distribution. To achieve a high carbon conversion, uniform distribution of fuel is required within the elements of the injector to ensure that the solid particles are well mixed with steam and oxygen. As the residence time of the particles becomes smaller, poor mixing may reduce the efficiency of the system. An initial design requirement for GTI’s compact gasifier feed system was a relative standard deviation (RSD) of less than 2% for the fuel split nonuniformity.19 However, testing of GTI’s feed system indicated a RSD of 4.5% and 10% for 6 and 18 element injectors, respectively.27 In addition to flow nonuniformity, plugging is another concern that may occur to fuel nozzles during operation. In this study, the performance of a gasifier with 36 injector elements is explored under flow nonuniformity and plugging conditions. The measurement of temperature, flow, and composition at the gasifier’s outlet to indicate the presence of feed flow nonuniformity is also discussed. It should be noted that the flow nonuniformity scenarios discussed below may drastically affect flow patterns in the gasifier, causing them to deviate from the patterns assumed for the design of the present ROM’s reactor network. I


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Figure 7. Simulation results with flow nonuniformity, Scenario 1: (a) temperature profile of the internal (high-flow) JEZ-1s; (b) temperature profiles of the external (low-flow) JEZ-1s; (c) carbon conversion with different relative standard deviations (RSDs) of fuel flow; (d) dry gas molar flow with different RSDs of fuel flow.

Figure 8. Simulation results for flow nonuniformity, Scenario 2: (a) temperature profiles of external (high-flow) JEZ-1s; (b) temperature profiles of internal (low-flow) JEZ-1s.

4.3.1. Flow Nonuniformity in Nozzles. The nominal flow rate of each fuel nozzle is 83.3 TPD, while they are designed to inject up to 100 TPD of dry fuel to the gasifier. For the present case study, the maximum fuel flow rate within a nozzle is assumed to be 100 TPD. To assess the effect of flow nonuniformity, the flow rates through the nozzles of a given layer are assumed to be equal; however, flow nonuniformity between each layer of nozzles is considered through two scenarios. Note that flow nonuniformity was only considered for the fuel, while the oxygen/steam flow rates were assumed to remain at their nominal values (Table 3). In the first scenario, the internal nozzles were assumed to have higher flow rates compared to the external nozzles, while the middle nozzles operated with their nominal flow rates.

Based on such a distribution of the flow rates, a maximum RSD of 20% was calculated. The results of this scenario are presented in Figure 7. As shown in Figure 7a, as the fuel flow rate increased in the internal nozzles, the oxygen/fuel ratio was reduced which led to a lower temperature profile for internal nozzle JEZ-1s. On the other hand, the temperature profile of JEZ-1s for low flow nozzles was increased with greater feed flow nonuniformity (Figure 7b). Since the number of external nozzles is higher than the number of internal nozzles, the effect of flow nonuniformity was more significant on the temperature profile of internal nozzle JEZ-1s. As indicated in Figure 7c, the carbon conversion and CGE of the gasifier were reduced by 0.4 and 0.5 percentage points, respectively, with a feed flow RSD of 20%. Since the overall feed ratio of the gasifier J


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Figure 9. Simulation results of the plugging scenario (data for nonplugged nozzles are shown): (a) temperature profiles in the top section; (b) molar percentage profiles in the top section; (c) temperature profiles in the middle/bottom sections; (d) molar percentage profiles in the middle/bottom sections.

remained constant and the fuel flow nonuniformity was evenly distributed among the nozzles, it is reasonable to expect small changes in the carbon conversion and CGE of the gasifier. The molar fraction of CO and H2, and the temperature at the outlet did not change significantly with a feed flow RSD of 20% compared to the base case, that is, deviations of 0.22 and 0.18 percentage points for CO and H2, respectively, and 7 K for temperature. As a result, measurements of the temperature and composition may not indicate the presence of flow nonuniformity. As shown in Figure 7d, compared to the base case, the dry molar flow rate of gas exiting the reactor was decreased by 57 kmol/h with a fuel feed RSD of 20%. Therefore, in this scenario, gas flow rate is likely the most sensitive variable at the outlet of the gasifier in the presence of feed flow nonuniformity. The fuel distribution in the second scenario is the opposite of the first scenario, that is, internal nozzles are assumed to have a lower flow rate and external nozzles have a higher flow rate. Figure 8 demonstrates the trend of temperature profiles in higher/lower flow rate nozzles for feed flow RSDs of up to 20%. As shown in this figure, the peak temperature of internal JEZ-1s (which have lower fuel flow rates) increased significantly with higher feed flow RSDs. Accordingly, the peak temperature increased by 350 K with a feed flow RSD of 20% compared to the base case. As the temperature increased within the internal JEZ-1s instead of the external JEZ-1s, there is less risk of

damage to the gasifier’s walls in the second scenario compared to the first. In the second scenario, the carbon conversion increased to 100% for flow nonuniformity RSDs of 5% and greater. Gas flow measurements may not indicate the presence of flow nonuniformity in this scenario. On the other hand, gas temperature is the most sensitive variable (at the outlet of gasifier) in this scenario, as the outlet temperature was increased by 40 K with a feed flow RSD of 20% compared to the base case. On the basis of the above results, no significant changes were observed in the carbon conversion and CGE of the gasifier with flow nonuniformity RSDs of up to 20%. It is expected that the effect of nonuniformity increases when the feed system has fewer nozzles. Furthermore, two potential risks could affect the operation of the gasifier during flow nonuniformity: the temperatures surrounding the injector elements and the gasifier’s wall could increase. Regarding temperature surrounding the injector elements, the maximum temperature surrounding the injector elements in the first scenario was 1431 K which is lower than the considered constraint (1470 K). In the second scenario, the maximum temperature surrounding the injector elements was 1480 K and 1612 K for feed flow RSDs of 15% and 20%, respectively. Therefore, with feed flow RSDs of 15% and greater (when the external nozzles have higher flow rates), injector elements may be damaged. K


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Figure 10. Results of sensitivity analyses on the reactor network parameters.

Regarding temperature near the wall, since there are 16 nozzles in the external layer, the effect of nonuniformity on the temperature distribution of the ERZ-1s was minor and no significant risk was observed in both scenarios discussed above. However, if the RSD is not evenly distributed between the external nozzles, the risk of wall damage may increase. Note that during significant nonuniformity of fuel among the nozzles, there may be other effects that are not captured by the ROM due to flow field assumptions. 4.3.2. Plugging. Multielement injectors have flow splitters to evenly distribute the feed flow. To assess the performance of the gasifier during plugging, failure of a splitter providing flow to six nozzles is considered in the present case study. The six nozzles include one internal nozzle, two middle nozzles, and three external nozzles. In this scenario it was assumed that the fuel flow rate of the plugged nozzles is redistributed among the remaining nozzles. Therefore, during the failure of a splitter, the fuel flow rate of nonplugged injection tubes was increased from 83.33 TPD to 100 TPD. As described in section 2 of the manuscript, each fuel nozzle is sized to inject up to 100 TPD of dry fuel to the gasifier. Since this fact is considered in the simulations, increasing the fuel flow in other nozzles is not expected to cause any potential damage to the injection system. Note that the plugging was only considered for the fuel injection nozzles, meaning the oxygen/steam impinging orifices of the failed nozzles were still in operation at their nominal values. Figure 9 presents the simulation results of the plugging scenario. According to results in Figure 9a, the temperature of nonplugged nozzles at the top section of the gasifier are lower than in the base case scenario, since the oxygen/fuel ratio was lower in those nozzles. Moreover, the molar fraction of CO was

reduced (see Figure 9b) compared to the base case, since less fuel carbon was converted. On the other hand, the molar fraction of H2 was slightly increased as the volatiles, which have a higher H/C ratio than the fuel, were still fully converted. Note that the results of middle and external nozzles followed the same trends as internal nozzles, and are not presented for brevity. One of the most important effects of plugging was observed in the middle section where the multiphase flows coming from all nozzles merge together. At the beginning of this section, the unused oxygen (coming from the impinging orifices of the nozzles with dis-functioned fuel tubes) reacted with H2, CO, and carbon, resulting in a second peak temperature (Figure 9c). Accordingly, the rates of char gasification reactions were increased, which resulted in complete carbon conversion of fuel at 2.5 m from the gasifier’s inlet. Therefore, the outlet molar fractions of gas species in the plugging scenario were almost the same as the base case (Figure 9d). The CGE of the gasifier during plugging was 80.5%, that is, 3.2 percentage points less than the CGE of the base case, while the dry gas flow rate was decreased by 3.5% compared to the base case. The peak temperature, now located at the middle section of the gasifier instead of the top section, reached 2592 K. Depending on the flow patterns inside the gasifier during plugging, this may result in undesirable higher temperatures near the gasifier wall. If the corresponding oxygen/steam impinging orifices of the nozzles with plugged tubes do not inject any oxygen to the system, such a problem might be avoided. Plugging reduced the predicted carbon conversion and CGE to 74% and 62%, respectively. Note that proportionally increasing the oxygen/steam flow rates of the nonplugged nozzles (which increases fuel flow rates) may result L


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Figure 11. Results of sensitivity analyses on the design and operation parameters.

particles. According to results presented in Figure 10b−d, the length of JEZs is a critical parameter in the reactor network. Increasing the length of JEZs has two opposite effects on the temperature profile of the system: (1) it reduces the heat loss through the DSZ and therefore contributes to a higher temperature profile, and (2) it reduces the average temperature of recirculation gases and therefore contributes to a lower temperature profile. In the case of JEZ-1s’ length, the second effect was dominant since the internal/middle zones were not affected by the heat loss, therefore lower carbon conversion was obtained (Figure 10a) when the JEZ-1s’ length was increased. However, in the case of JEZ-2’s length, the first effect was dominant; thus, it contributed to higher temperature and increased carbon conversion. The proposed ROM is aimed to be a practical modeling tool for initial studies/ideas, and narrow the scope of exploration for subsequent experimentation and CFD analysis. It is not expected that a single value of key model parameters (such as the length of JEZs) will work for a wide range of operating conditions. Such parameters depend on the operating conditions and the geometry of the actual gasification system. In addition to determining the most influential parameters for the proposed ROM, sensitivity analyses are useful to predict the effect of design and operating parameters on the performance of the system. In the present work, the sensitivity to the oxygen, steam, and nitrogen flow rates, the inlet temperature of oxidizing agents, the percentage of volatiles in the fuel, the temperature of the slag layer, the thickness of nozzle layers, and the size of the fuel nozzles were considered.

in undesirably high temperatures near the injectors and/or gasifier wall.

5. SENSITIVITY ANALYSIS Because of the lack of information in the literature regarding the operation of commercial-scale gasifiers with multielement injectors, the proposed reactor network was based on assumptions that could not be completely verified. To provide insight on the expected behavior of these systems, it is useful to identify the most influential parameters affecting the results of the proposed ROM. In this section, univariate sensitivity analyses of the following reactor network parameters are considered: the recirculation ratio (RR) of the internal, middle, and external nozzles, and the length of the JEZ-1s and JEZ-2. To perform the sensitivity analyses, the input parameters were individually varied by up to ±20% relative to their base-case values. To maintain a fixed length of the gasifier, the length of the DSZ was adjusted based on the changes in the lengths of the JEZ-1s and JEZ-2. Figure 10 presents the sensitivity of the carbon conversion, particle residence time, peak temperature and outlet temperature to changes to reactor network parameters. As depicted in this figure, the selected ROM outputs were not significantly affected by the recirculation ratio developing around each of the nozzles. Since multiple feed nozzles reduced the recirculation flow in the system, the recirculation ratio is less impactful on the results compared to a gasifier with a single-element feed injector.9,14 As shown in Figure 10b, the considered parameters did not significantly increase the average residence time of M


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conversion since a higher nitrogen/fuel ratio reduces the partial pressure of reacting species, and therefore the rate of reactions. The residence time of particles was mostly affected by the nitrogen/fuel ratio and diameter of the feed nozzle (see Figure 11b). On the basis of the results presented in Figure 11c,d, in addition to the oxygen/fuel ratio, the inlet temperature of oxidizing agents and the steam/fuel ratio were the other major parameters that affected the peak and outlet temperatures. In addition, the effect of operating parameters on the H2/CO ratio at the outlet of the gasifier is presented in Figure 12. According to this figure, oxygen/fuel ratio is the most important parameter affecting the H2/CO ratio. The key factor which plays a role in reducing the H2/CO ratio at higher flow rates of oxygen is enhanced reaction rate between hydrogen and oxygen. On the other hand, steam flow rate and volatile percentage of fuel are directly proportional to the H2/CO ratio. The rest of operating parameters considered in the sensitivity analysis did not have a significant effect on the H2/CO. The changes in the temperature profile of the gasifier for the most influential parameter, that is, oxygen flow rate, are presented in Figure 13. As shown in this figure, changes in oxygen flow rate by ±10% did not affect the position of peak temperature in the JEZ-1 and the overall temperature profile of the gasification system. The significant nonlinear relation between these variables may be noted. The thickness of nozzle layers (λ) affect several reactor network parameters: all jet angles, the initial diameter of the JEZ-2, and the volume of recirculation zones. Therefore, univariate sensitivity analyses were not performed on the thicknesses. Instead, the effect of these parameters was examined in the three scenarios presented in Table 6. As shown in this table, as the value of λ1 increased, the value of λ3 was reduced by the same amount, while λ2 and the wall space thickness (λ4) were kept constant. Such variation of parameters resulted in three scenarios which can be analyzed through the size of jet angles (θ). In case 1, the jet angle of the JEZ-1s increased in the radial domain, that is, θinternal < θmiddle < θexternal. In case 2, the three nozzle layers have equivalent jet angles. In case 3, the jet angle of the JEZ-1s reduced in the radial domain. Note that since there are more nozzles in the external layer, λ3 is more impactful on the results compared to λ1. According to the results presented in Table 6, as the jet angle of nozzles in the external layer was decreased, the velocity of gases was increased which resulted in a lower particle residence time and carbon conversion. Overall, the results of the sensitivity analysis

The effects of these design and operating parameters are presented in Figures 11−12 and Table 6. Results from the

Figure 12. Results of sensitivity analyses on the operation parameters for H2/CO.

Table 6. Results of Sensitivity Analysis for the Thickness of Nozzle Layers internal layer thickness, λ1 middle layer thickness, λ2 external layer thickness, λ3 wall-space, λw carbon conversion % particle residence time(s) outlet T (K)

case 1

case 2

case 3

0.3 0.25 0.3 0.15 95.1 0.53 1613

0.35 0.25 0.25 0.15 94.9 0.52 1617

0.4 0.25 0.2 0.15 94.7 0.50 1622

sensitivity analysis (Figure 11) showed that increasing the slag temperature resulted in a higher outlet temperature since less heat was lost through the wall. Consequently, a higher slag temperature profile increased the carbon conversion. However, the impact is small; varying the slag temperature by 190 K changes the carbon conversion by less than 0.5%. As illustrated in Figure 11a, the oxygen/fuel ratio has the greatest impact on the carbon conversion. Moreover, the nitrogen/fuel ratio is the only parameter which was inversely proportional to the carbon

Figure 13. Effect of oxygen flow rate on the temperature distribution of the gasifier: (a) Internal JEZ-1; (b) JEZ-2/DSZ. N


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Industrial & Engineering Chemistry Research d = diameter (m) D = diffusivity (m2/s) F′ = volumetric force (N/m3) g = gravitational acceleration (m/s2) h = convection coefficient (W/m2/K)) H = enthalpy (J/kg) k = thermal conductivity (W/m/K) m = mass (kg) m′ = mass flux (kg/m2/s) M = mass flow (kg/s) N = particle density/concentration (particles/m3) P = pressure (Pa) Pr = Prandtl number Q′ = heat flux (W/m2) r = radius (m) R = reactions Re = Reynolds Number RR = recirculation ratio T = temperature (K) u = velocity (m/s) w = weight fraction x = molar fraction z = axial domain (m)

indicated the major outputs of the gasifier model did not change significantly by variation of the thickness of nozzle layers.

6. CONCLUSIONS This study has analyzed the performance of a commercial-scale gasifier with a multielement injector feed system by means of using a reduced order model (ROM). A reactor network consisting of 75 reactors is integrated into the ROM to capture the streamlines of the multiphase flow inside the gasifier with 36 feed nozzles. The results indicated that the ROM is able to predict the values reported for fuel conversion and cold gas efficiency (CGE) at 69 bar with a short computational time, that is, 17−30 min. The ROM was further used to evaluate the gasifier at lower pressures. The proposed ROM suggested that the design parameters (e.g., nozzle size and inlet flow rates) were not suitable for 20 bar operation due to a high temperature surrounding the injectors which may increase the risk of damaging the equipment. The operation of the gasifier was then compared at two scales (1500 and 3000 TPD) using 18 and 36 injector elements. On the basis of the results, the lower-scale gasifier conversion and CGE were 1.3 and 1.2 percentage points lower, respectively, compared to the higherscale gasifier. Furthermore, the flexibility of the gasifier was tested during two case studies with nonuniform distribution of fuel among the nozzles. According to the first scenario, no significant change was observed in the carbon conversion when flow nonuniformity is distributed among the nozzles. In addition, the results indicated that the measurements of outlet flow and temperature may express the presence of flow nonuniformity in the inlet, while the measurement of composition cannot. In the second scenario, the plugging of six fuel injection tubes was considered through failure of one of the flow splitters. The results illustrated that if the impinging orifices of the plugged nozzle continue to inject oxygen, a second peak temperature will be formed in the middle section of the gasifier which may increase the risk of damaging the wall. Finally, the results of the sensitivity analyses performed in this study showed the suitability of the assumptions made in the ROM and revealed that the oxygen/fuel ratio and the length of the JEZ-1s and JEZ-2 are the most influential operation and model parameters.

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Greek Symbols

ε = volume fraction ρ = density (kg/m3) λ = thickness ϑ = emissivity Ω = Stefan−Boltzmann constant Subscripts

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REFERENCES

(1) IGCC State-of-the-art Report; University of Stavanger: Norway, 2010. (2) Power plant carbon dioxide capture and storage projects at MIT. http://sequestration.mit.edu/tools/projects/index_capture.html (accessed August 1, 2016). (3) He, Z.; Ricardez-Sandoval, L. A. Dynamic modelling of a commercial-scale CO2 capture plant integrated with a natural gas combined cycle (NGCC) power plant. Int. J. Greenhouse Gas Control 2016, 55, 23−35. (4) Banks, B.; Bigland-Pritchard, M. SaskPower’s carbon capture project: What risk? What reward?; Canadian Centre for Policy Alternatives, 2015. (5) Buchan, B.; Cao, C. Coalfired Generation: Proven and Developing Technologies; Office of Market Monitoring and Strategic Analysis Florida Public Service Commission, 2004. (6) Hossein Sahraei, M.; McCalden, D.; Hughes, R.; RicardezSandoval, L. A. A survey on current advanced IGCC power plant technologies, sensors and control systems. Fuel 2014, 137, 245−59. (7) Fusselman, S. P.; Sprouse, K. M.; Darby, A. K.; Tennant, J.; Stiegel, G. J. Pratt & Whitney rocketdyne/DOE advanced single-stage gasification development program; National Energy Technology Laboratory, 2007.

AUTHOR INFORMATION

Corresponding Author

*Tel: +1-519-888-4567 ext. 38667. Fax: 1-519-888-4347. Email: [email protected]. ORCID

Marc A. Duchesne: 0000-0002-5403-5888 Luis A. Ricardez-Sandoval: 0000-0001-9867-6778 Notes

The authors declare no competing financial interest.

■ ■

conv = convection cs = cross section eff = effective g = gas het = heterogeneous reactions hom = homogeneous reactions i = ith gas phase component p = particle w = wall

ACKNOWLEDGMENTS This study was supported by the Government of Canada’s Program of Energy Research and Development. NOMENCLATURE

Symbols

A = area of each zone (m2) C = concentration (mol/m3) cp = heat capacity (J/kg/K) O


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Reduced-Order Modeling of a Commercial-Scale Gasifier Using a

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