One-Dimensional Dynamic Modeling of a Single-Stage Downward

Jul 16, 2014 - Firing Entrained-Flow Coal Gasifier. Job S. Kasule,. † ... simulation results are presented for a single-stage down-fired entrained-f...
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One-Dimensional Dynamic Modeling of a Single-Stage DownwardFiring Entrained-Flow Coal Gasifier Job S. Kasule,† Richard Turton,*,† Debangsu Bhattacharyya,*,† and Stephen E. Zitney‡ †

Department of Chemical Engineering, West Virginia University, Morgantown, West Virginia 26506, United States National Energy Technology Laboratory, U.S. Department of Energy, Morgantown, West Virginia 26507, United States

‡

ABSTRACT: In the current paper, a one-dimensional partial differential equation (PDE)-based dynamic model and its simulation results are presented for a single-stage down-fired entrained-flow gasifier. The gasifier model comprises mass, momentum, and energy balances for the gas and solid phases. The initial gasification processes of water evaporation and coal devolatilization and the key heterogeneous and homogeneous chemical reactions have also been modeled. The resulting coupled system of PDEs and algebraic equations is solved using the well-known method of lines in Aspen Custom Modeler. In addition to the dynamic gasifier model, efficient control strategies that can satisfactorily perform both servo and disturbance rejection functions have been developed for the entrained-flow gasifier. The dynamic variations of key gasifier output variables in response to the disturbances commonly encountered in industrial operation are presented. Output variables of interest include gas and solid phase temperatures, synthesis gas compositions, and carbon conversion, while disturbances include ramp and step changes in input variables such as coal flow rate, oxygen-to-coal ratio, and water-to-coal ratio among others. Feedstock switchovers have also been studied by simulating transitions from one coal type to another. The gasifier model results are also compared to the dynamic data available in the literature.

1.0. INTRODUCTION Gasifiers are undoubtedly the most important components of the integrated gasification combined cycles (IGCC) that have emerged as promising power generation options due to their higher efficiency and environmental advantages over conventional coal utilizing technologies.1−3 In the gasifier, solid fuels, such as coal, petroleum coke, and biomass, are converted into synthesis gas (a mixture of mainly CO and H2), which, after being cleaned, can be subsequently combusted in a gas turbine to generate electricity or used as a feedstock to produce various chemicals. It is this crucial role that makes it imperative that gasifier operation is well-understood and optimized. However, the harsh, high-temperature and pressure environment within the gasifier has prompted numerical modeling as one of the major approaches for studying gasifier operation. Several mathematical models of the entrained-type gasifier have been developed and presented in the literature, ranging from one-dimensional (1D) plug flow4−11 to sophisticated multidimensional (two- or three-dimensional) computational fluid dynamics (CFD) models12−18 that describe coupled gas− solid hydrodynamics, heat and mass transfer, and reaction kinetics over the complex gasifier geometry. Additional details such as turbulence are generally included in the high-fidelity CFD models making them too computationally expensive to be used directly in operability and controllability studies. Even steady-state CFD models are not suitable for performing dynamic studies of gasifier operations, as they typically take from hours to days to execute on high-performance computers. Thus, computationally faster, lower-dimensional 1D dynamic models are useful and often necessary for these kinds of gasifier studies. Only a few dynamic gasifer models with corresponding simulation results have been found in the literature. A simple © 2014 American Chemical Society

dynamic entrained-flow gasifier model developed in Aspen Plus Dynamics has been reported by Robinson and Luyben.19 This process-type material and energy balance model, while only approximate in nature and using a high molecular weight hydrocarbon in the Aspen chemical component library to represent coal as a pseudofuel, has been simulated to obtain the temperature, flow, composition, and pressure dynamics. Recently, reactor network-based models (RNMs) have also been reported in the literature.20−23 These studies employ a network of different reactor models, for example, well-stirred reactors (WSRs), plug flow reactors (PFRs), and in some cases coupled WSRs and PFRs to model the different process zones within the gasifier. These include the combustion zone, internal recirculation zone, external recirculation zone, jet expansion zone, downstream zone, and in some cases the syngas cooler. The RNM-model approach has demonstrated flexibility in modeling various aspects of single- or two-stage gasifier operation. Moreover, such models have been suggested as a compromise approach for modeling dynamic gasifier operation at reasonable computation cost without compromising on model details.23 A dynamic RNM20,21 was developed for entrained-flow gasifier operation incorporating submodels for mixing and recirculation, particle properties, chemical kinetics, fluid dynamics, heat transfer, slag behavior, and syngas cooling among others. This model was subsequently used in the simulation of a commercial-scale entrained-flow gasifier under a wide range of realistic operating conditions.22 A similar approach has been used to develop a dynamic model for an oxygen-staged slagging entrained-flow gasifier.23 Received: May 4, 2014 Revised: July 16, 2014 Published: July 16, 2014 4949

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Table 1. Defining Equations Used in the Current Dynamic Model gas phase mass

∂(ρg ε) ∂t

momentum

∂(ρg εug)

∂x

= (1 − ε)ΓS − g + mrgk + mmgk

∂(ερg ug2)

+

∂t energy

∂(ρg εug)

+

=−ε

∂x

∂(ερg Cp,gTg)

(1)

∂Pt + ερg g − (1 − ε)fs ∂x

(2)

∂(ugερg Cp ,gTg)

+ ∂t ∂x πDi = (egFw − gσ(Tw4 − Tg4) + hw − g (Tw − Tg )) AR

− (1 − ε)

6 (egFg − sσ(Tg4 − Ts4) + hg − s(Tg − Ts)) + dp

gr

∑ ε(−ΔHj)rj j

+ mrg hrg + mmg hmg species

(3)

∂ ∂ (ερ y ) + (ερ ugy ) = R gi + mrgik + mmgik ∂t g gi ∂x g gi

(4)

solid phase mass

∂(ρs (1 − ε)) ∂t

momentum

+

∂((1 − ε)ρs us) ∂t

∂(ρs (1 − ε)us)

+

∂x

∂((1 −

= − (1 − ε)ΓS − g

∂x ∂Pt = − (1 − ε) + (1 − ε)ρs g + (1 − ε)fs ∂x energy

∂((1 − ε)ρs Cp ,sTs) ∂t + (1 − ε)

species

+

(5)

ε)ρs us2)

∂(us(1 − ε)ρs Cp,sTs) ∂x

=

(6) πDi Fw − sσ(Tw4 − Ts4) AR

6 (egFg − sσ(Tg4 − Ts4) + hg − s(Tg − Ts)) + dp

∂ ∂ [(1 − ε)ρs xsj] + (ερ usxsj) = R sj ∂t ∂x s

sr

∑ (1 − ε)(−ΔHk)rk k

(7)

(8)

wall energy

ρw c p,w

4D ∂Tw = − 2 i 2 [qconv + qrad,w − s + qrad,w − g + qrad,w − w + qloss] j ∂t Do − Di

qconv = hw − g (Tw − Tg), qloss = heff (Tw − Tsurr), qrad,w − s =

Fw − sσ(Tw4

qrad,w − g = egFw − gσ(Tw4 − Tg4), qrad,w − w = egFw − wjσ(Tw4 − Tw4j) j

−

(9) Ts4)

(10)

2.0. MODEL DESCRIPTION

In the current study, a 1D PDE-based dynamic model of a single-stage, downward-fired entrained-flow gasifier has been developed and the results validated by comparing them with available experimental data. Unlike gasifier models reported previously,19−23 the current model captures larger dynamic changes typical of practical gasifier operation. By using various control schemes to manipulate input variables pertinent to gasifier performance, the impact of key controlled variables on gasifier outputs has been highlighted. In addition, the rigorous dynamic model has been used to study several important gasifier transients, which have not been reported before in earlier studies, including coal feed switchovers that are common operational changes in most gasification-based plants. In this current study, four different coal types, namely Illinois #6, Pittsburgh #8, Powder River Basin (PRB) and Utah coal, have been considered since these coals have significantly different chemical and physical properties. Previous studies involving 1D gasifier models have focused on either a single coal type or closely similar coals.

The dynamic entrained-flow gasifier model in this paper is an extension of the 1D PDE-based steady-state multiphase model presented earlier by the authors.24 Details of the model developed and solved in Aspen Custom Modeler (ACM) are presented below. 2.1. Model Equations. As shown in Table 1, the steady-state gasifier model equations for mass, momentum and energy in Kasule et al.24 are modified to include transient or accumulation terms for use in dynamic simulations in the current study. In eqs 1−10, ρs and ρg are the densities of the solid and gas phases, respectively; ε is the void fraction in the gasifier; Γs−g is the net rate of consumption of the solid phase (coal) by the heterogeneous reactions (grams per cubic centimeter times seconds); fs is the drag force per unit volume of particles given by the correlation given in Gidaspow;25 us and ug are the solid- and gas-phase velocities, respectively; Pt is the total pressure in the system, taken to be the same as the gas-phase pressure; the term (6)/(dp) is the ratio of the surface area of a particle (assumed spherical) to its volume; and the last two terms, mrghrg and mmghmg, in the gas-phase energy balance in eq 3 are the enthalpies of the recirculated gas as described in Kasule et al.24 In the energy balance for the gasifier wall shown in eq 10, the inner wall is considered to exchange energy with the gas and solid phases as well as with the top and bottom ends of the gasifier, which were 4950

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assumed to be flat plates. Thus, this balance considered radiation between wall and solids, convection between wall and gas, radiation between wall and top and bottom ends of the gasifier, and the energy loss to the surrounding environment while neglecting heat conduction along the length of the gasifier. The view factors between the various surfaces that exchange radiative energy were calculated similar to the work of Siegel and Howard.26 2.2. Chemical Reactions. In the current model, the initial stage physical and chemical processes of coal drying, coal devolatilization, and tar cracking have been incorporated as described in an earlier work.24 During devolatilization, the coal is thermally decomposed to release the volatile matter (VM) leaving behind a high carbon residue generally known as char. The VM releases a number of combustible gases, tar among other components as shown in eq 11.

2.3. Solution Methodology. The resulting set of partial differential and algebraic equations is solved in ACM by using the method of lines. The first-order backward difference method is used to discretize the PDEs in the spatial domain. The gasifier length is scaled to vary between 0 and 1.0 using the total gasifier length (6.62m) and is discretized into a total of 250 discrete grid points corresponding to a grid of uniform size of about 0.025 m. These were determined through a grid sensitivity study while obtaining the steady-state solution of the model, the results of which were presented in our earlier communication.24 Found to be satisfactory for capturing the main transient characteristics of the gasifier are 250 grid points. Since the resulting system of differential algebraic equations is extremely stiff due to the multiple time scales that exist in the gasifier, the Gears algorithm in ACM has been used for integration. To achieve a valid pressure-flow network for dynamic simulation, valve models (V_01, V_02 and V_03) have been added to the flow streams in and out of the gasifier, as shown in Figure 1. The coal slurry and the high purity oxygen are fed cocurrently to the top of the downfired entrained-flow gasifier. 2.3.1. Boundary Conditions. In order to solve the above system of equations, appropriate boundary conditions must be provided. The inlet gas and solid phase species concentrations, temperatures, and velocities, which are implicitly calculated from the coal slurry and gas phase flow rates and compositions are used as boundary conditions. The solid phase species concentrations were obtained from the proximate analysis of the coal(s), adjusted to account for the coal slurry water makeup. These are given in Table 3 for the Illinois #6 coal. To solve the dynamic model, the steady-state solution was provided as the initial conditions.

VM → αd Tar + βdCOCO + βdCO2CO2 + βdCH4 CH4 + βdH2 H 2 + βdH2OH 2O + βdH2SH 2S + βdNH3 NH3 + higher hydrocarbons (11) where VM is obtained from the proximate analysis of the coal. The tar is then cracked under increasing temperature, as shown in eq 12. Tar → αc FC + βcCOCO + βcC2OCO2 + βcCH4 CH4 + βcH2 H 2 + βcH2OH 2O + βcH2SH 2S + βcNH3 NH3 + higher hydrocarbons (12) where FC is the fixed carbon. The kinetics of devolatilization and tar cracking reactions vary from simple first order Arrhenius type to more complex multiple-step kinetic models. The phenomenological kinetics of Syamlal and Bisset27 were used in the current study. In addition, the main heterogeneous and homogeneous reactions taking place in the gasifier are also considered and outlined in Table 2.

3.0. RESULTS AND DISCUSSION In this section, selected results from the response of the gasifier to input disturbances are presented. It should be pointed out that these results were obtained using Illinois #6 coal type as the gasifier feed, except for section 3.2.3 where coal feed switchover is considered. The gasifier model consists of a total of 32938 equations. When the model is solved on a Dell Optiplex 390 destkop machine with an Intel Core i3-2100 CPU @ 3.10 GHz Processor with 8.00GB Ram 64-bit operating system, the simulations took on average 3.5 times the real time for ramp rates of 1% change per minute. For other disturbances, the clock time varied but was not too far off. 3.1. Model Validation. Validation of the gasifier model over a wide operating range is difficult due to the scarcity of dynamic commercial-scale data, which are mostly proprietary. In an EPRI report,35 some data were reported from the dynamic studies of the gasifier in the Cool Water gasification project (hereafter referred to as the Cool Water gasifier). Results from load-following studies were reported for reduction in the plant power load from 100% to approximately 50% and then increase in the plant load back to 100%. The report states that, “The plant fuel feed variation from 65% to 100% corresponds to 50% to 100% power output”. Therefore, one way to validate the current model predictions is to implement the coal feed turndown from 100% to 65% to mimic the 100% to 50% plant load change and then compare the exit temperature profile to that in the EPRI report. In order to implement this scenario, the control scheme shown in Figure 1 is adopted to mimic that reported in the Cool Water Project35 and subsequently used to compare the observed model predictions. It should be noted that the slurry flow rate in not necessarily controlled by manipulating a control valve as shown in Figure 1 but usually by regulating the revolutions per minute of the slurry pump. However, for the dynamic studies, the simple schematic shown in Figure 1 is considered to be satisfactory.

Table 2. Homogeneous and Heterogeneous Reactions Taking Place in the Gasifier reaction

reference

heterogeneous reactions ⎛ ⎛2 ⎞ 1 2⎞ C + O2 → ⎜2 − ⎟CO + ⎜ − 1⎟CO2 ϕ ϕ⎠ ⎝ ⎝ϕ ⎠

C + H 2O ↔ CO + H 2

(R-2)

5

(R-1) 5

C + CO2 ↔ 2CO

(R-3)

5

C + 2H 2 ↔ CH4

(R-4)

5

homogeneous reactions 1 CO + O2 → CO2 (R-5) 2 CH4 + 2O2 → CO2 + 2H 2O

28

(R-6)

28

1 O2 → H 2O (R-7) 2 CO + H 2O ↔ CO2 + H 2 (R-8)

29

CH4 + H 2O ↔ CO + 3H 2

32

H2 +

1 3 N2 + H 2 ↔ NH3 2 2

(R-9)

30, 31

33

(R-10)

The kinetic rate parameters for the above homogeneous and heterogeneous reactions are obtained from the references shown in the right-hand column of Table 2. In the char-combustion reaction (R-1), ϕ is the mechanism factor that gives the ratio of CO2 to CO in the reaction products and is reported to vary markedly with reaction temperature.34 4951

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Figure 1. Gasifier pressure-driven flowsheet with control scheme.

Table 3. Boundary Conditions (Illinois #6 Coal) variable

inlet value

slurry flow rate (kg/s) gas flow rate (kg/s) Tg (°C) Ts (°C) pressure (atm) u/s of V_01 pressure (atm) d/s of V_03 gas phase species concentration (mass fraction) yo2 yN2 yCO , yCO , yH , yCH , yNH , yH S , yC 2

2

4

3

selexol unit is further downstream of the gasifier, it was desired to compare the dynamic response of the syngas flow at the outlet of the gasifier with this data in order to obtain a qualitative comparison. This comparison along with the transient in the slurry flow rate is shown in Figure 2. The

2

61.23 36.22 29.85 29.85 26.00 23.69 0.95 0.05

2H 4

, yH O 2

solid phase species concentration (mass fraction) xash xFC xmoisture xVM

0.00 0.070811 0.313228 0.370001 0.245961

In this control scheme, flow controllers, COAL_FC and O2_FC, are used to control the flow rates of the coal slurry flow and oxygen gas flow into the gasifier, respectively. A ratio block (RB) is used to calculate the oxygen-to-coal ratio, which is controlled by a ratio controller, RATIO_C, which is cascaded with the oxygen controller. A dead-time/time delay block (ΔT or DTB) is also used to ensure that an oxygen-rich environment does not result when the gasifier throughput is varied (i.e., the oxygen flow is first turned down before turning down the coal slurry flow and vice versa). This is a necessary strategy that avoids temperature excursions in the gasifier during the ramping of the feed flow rate. The set-point of the COAL_FC controller is introduced into a comparator block (denoted by Δ) and sent to the coal flow controller via the time-delay block. All the controllers were tuned using the Ziegler-Nichols tuning rules. By using a flowsheet script in ACM, the gasifier throughput was ramped down from 100% to 65% at a rate of 5% change/min. In addition to the gasifier exit temperature, the Cool Water gasifier report35 has also provided transient response in the syngas flow at the outlet of the selexol unit. Even though the

Figure 2. Transient response of the scaled syngas product flow rate for 50% turndown in the coal feed flow rate.

responses compare well. It should be noted that the y axis is presented in terms of scaled variable that is obtained from V _scaled = (V _t − V _initial)/ΔV

(13)

where, V_scaled, V_t, V_initial, and ΔV are the scaled variable value, variable value at a given time, variable value at initial steady state condition (before slurry feed change), and the total change in the variable value after it reaches the new steady state. The response of the exit gasifier temperature (gas phase temperature) is shown in Figure 3. After a slight decrease, it increases and levels off at a new higher steady-state value. This response is qualitatively similar to that of the Cool Water gasifier temperature profile as shown in Figure 3. The main difference is in the time constant. In the view of the authors, this difference can be attributed to the following reasons. First, the coal types are different. The dynamic data reported in the Cool Water report are for Lemington coal, while the model results are for Illinois #6 coal. Second, no detailed design 4952

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is still increasing and has not reached steady state for the time duration shown in the figure. Therefore, the molten slag temperature is expected to have a time constant intermediate between the bulk gas phase temperature and the wall temperature. In conclusion, the comparison of the current model results with those reported for the Cool Water gasifier is considered to be satisfactory. 3.2. Other Gasifer Dynamic Model Responses. This section presents dynamic responses of key gasifer model variables to certain disturbances that are generally experienced during practical industrial operations. A number of efficient gasifier control strategies that can satisfactorily perform both servo and disturbance rejection functions have been developed in the current study. The disturbances considered include step or ramp changes in coal and oxygen flow rates and step/ramp changes in oxygen-to-coal ratio, among others. Such disturbances have also been used in previous studies.19,22,23 Whereas open-loop responses were also performed in the study, only closed-loop response results are presented in the current paper. 3.2.1. Responses to a Step Change in the Oxygen-to-Coal Ratio. As seen in the results from the steady-state study24 and those from studies in the open literature,8,9 one of the key gasifier operating variables is the oxygen-to-coal feed flow ratio, which impacts not only the temperature of the gasifier but also the carbon conversion and, subsequently, the product syngas composition. Transient responses for several key variables to a step decrease in oxygen-to-coal ratio are shown in Figure 4 (panels b−d). A step decrease in the oxygen-to-coal ratio (Figure 4a) results in a decrease in the ratio of CO to H2 in the syngas

Figure 3. Transient profiles of scaled temperature (gas phase) at the exit of the gasifier from the current model and that from the Cool Water gasifier in addition to wall temperature profile from the current model when a 50% turndown in coal feed slurry flow rate is implemented.

information is available for the Cool Water gasifier. The design considered in the model is that obtained from the work of Robinson and Luyben.19 Third, the exact location of the temperature measuring instrument in the Cool Water gasifier has not been reported. As the molten slag flows down the wall, if the tip of the thermocouple is close to the wall, it is likely to measure the temperature of the molten slag. The molten slag temperature is likely to have larger time constant than the bulk. It should be noted that the exit temperature from the model plotted in Figure 3 is that of the bulk gas phase. Figure 3 shows that while the gas phase temperature attains a new steady state value (after approximately 16 min), the wall temperature profile

Figure 4. Responses to a step decrease in the oxygen-to-coal ratio (0.835 to 0.80): (a) step decrease in the ratio controller set point (SP) and corresponding process variable (PV); (b) steady-state CO/H2 ratio in the syngas product before and after the change; (c) carbon conversion and exit CO2 concentration; and (d) exit gas and solid phase temperatures. 4953

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product stream (Figure 4b) and a reduction in the overall carbon conversion in the gasifier of ∼1.5% (Figure 4c) due to the decrease in the outlet gas and solid phase temperatures (Figure 4d). For the produced syngas, this resulted in a decrease in CO2 concentration (Figure 4c) and an increase in H2 and CO concentrations (not shown). As mentioned before, the decrease in the ratio of CO to H2 in the syngas product stream would have a direct effect on the downstream process units in an IGCC power plant. However, if the exit gasifier temperature is controlled by manipulating the oxygen flow rate, or if the oxygen-to-coal ratio is controlled (as shown in the control scheme of Figure 1), it is observed that the CO/H2 ratio does not change (results not shown) significantly from the desired value when changes such as a ramp or step increase in the coal flow rate is introduced. For this case, although there is a noticeable reduction in the carbon conversion as well as some changes in the gas species concentrations. The reduction in carbon conversion is mainly due to the lower residence time and slight decrease in the gasifier temperature. 3.2.2. Responses to Changes in Gasifier Fuel Feed Properties. One of the desired attributes of an industrial gasifier is the flexibility to handle different types of coal feeds. The results presented above were obtained while using Illinois #6 coal. However, in this section results are presented from dynamic studies in which the coal feed type into the gasifier is dynamically changed to see if it results any undesired operational behavior. The additional coal types considered in this study are Pittsburgh #8, Utah, and Powder River Basin (PRB). In Table 4, the coal proximate and ultimate analyses

Figure 5. Coal feed type change from Illinois #6 to Pittsburgh #8 coal.

controller maintains the solid phase exit temperature and is cascaded with the oxygen flow controller. The output of the temperature controller is the remote set point of the oxygen controller, which adjusts the oxygen flow in order to maintain the exit temperature. A pressure controller (PC) controls the gasifier pressure. In Figures 6−10, the results from the studies of coal feed changes are presented. During the ramp change in the coal type

Table 4. Proximate and Ultimate Analyses (As Received) For the Alternative Coal Feed Types coal type proximate analysis moisture volatile matter (VM) ash fixed carbon (FC) ultimate analysis carbon hydrogen nitrogen sulfur oxygen

Illinois #6

Pittsburgh #8

PRB

Utah

11.12 34.7 9.99 44.19

2.63 35.78 9.21 52.38

27.42 31.62 4.52 36.43

8.43 39.71 12.74 39.13

63.75 4.50 1.25 6.88 2.51

73.15 4.97 1.49 2.36 6.22

50.23 3.41 0.65 0.22 13.55

63.74 4.82 1.22 0.44 8.61

Figure 6. Gas temperature response profiles within the gasifier during the coal type change from Illinois #6 to Pittsburgh #8 coal.

switchover from Illinois #6 to Pittsburgh #8 (the change of the coal type is introduced at time, t = 60 s), the gas and solid temperature profiles (not shown) exhibited a sharp increase and then decrease in value at times of approximately 0.65 and 0.9 h, respectively, before eventually leveling off at the end of the ramp. These sharp changes are attributed to the shifting of the maximum temperature front within the gasifier, as illustrated in Figure 6. The maximum gas temperature within the gasifer is initially at location #13, corresponding to the control volume number from the gasifier entrance. This is located at about 5.2% of the total gasifier length from the entrance. However, since the coal compositions are continuously being changed, the maximum temperature is seen to shift from location #13 to location #12, as shown at the point indicated by the circular label #1 in Figure 6. The second sharp change is attributed to the jump at circular label #2, at which the temperature at location #11 becomes higher than the temperature at location #14. These shifts in temperature fronts occur for coal types with compositions, especially the moisture content, that are significantly different from the original coal. For similar coals such as Illinois #6 and Utah, no shift in temperature is seen during the coal type change.

show that while the Utah coal is somewhat similar to Illinois #6 coal, the Pittsburgh #8 and PRB coals are significantly different from the Illinois #6 coal. To change the coal feed type, a “task” block in ACM is used to introduce a ramp change in the proximate and ultimate assays from those of Illinois #6 to those of the new coal type. Figure 5 shows the feed switchover to Pittsburgh #8 coal starting at time = 60 s and continuing for a period of 1 h. In the figure, only the proximate analysis change is shown but the ultimate analysis is simultaneously changed when the task is initiated. Whereas a 1 h ramp duration has been used in the study, shorter ramp durations as short as 15 min were also possible for the Utah coal which was not so different from the initial Illinois #6 coal. In performing the coal feed switchover studies, a flow controller is used to maintain the coal flow rate. A temperature 4954

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Pittsburgh #8 coal in comparison to the PRB coal, as seen in Table 4. The increase in volatile matter results in more combustible gas species such as H2, CH4, and CO which, after combustion, results in an increase in temperature while the reverse is true with a decrease in the volatile matter content. It is interesting to note that the shifts in the location of the maximum temperature and the sharp increases or decreases as shown above are not observed in the temperature profiles of the gasifier wall as shown in Figure 9. In the figure, responses of

This is further supported by the steady-state temperature profiles along the gasifier length once the coal type has been completely switched to a new coal type, as shown in Figure 7.

Figure 7. Comparison of gas temperature profiles along the gasifier for Illinois #6, Pittsburgh #8, Utah, and PRB coals.

In the figure, it is clear that there is negligible deviation in the steady-state gas temperature profiles for the Illinois #6 and Utah coals as compared to the significant deviations in the gas temperature profiles for Pittsburgh #8 and PRB coals in terms of the maximum temperature and the location at which the maximum temperature takes place inside the gasifier. From Figure 7, it is seen that the ignition point as well as the position of the maximum gas temperature shifts toward the entrance of the gasifier for Pittsburgh #8 coal, while that of PRB coal shifts in the opposite direction. In addition, the maximum temperature is seen to increase and decrease for Pittsburgh and PRB coals, respectively. This is attributed to the decrease in the water content for the Pittsburgh #8 coal and an increase in the water content of the PRB coal, as shown in Figure 8. The

Figure 9. Wall temperature response profiles within the gasifier during the coal type change from Illinois #6 to Pittsburgh #8 coal.

the inside wall temperature are plotted at given positions along the gasifier. The maximum wall temperature occurs at location #36 before and after the coal type change unlike in the case of the gas and solid phase temperature profiles in Figure 6, where the maximum temperature occurs at location #13. It is also noted that whereas the gas and solid phase temperatures reach new steady-state values at ∼1.2 h, this is not the case with the wall temperature, which is continuing to gradually increase as seen in Figure 9. This is mainly due to the higher thermal time constant of the wall compared to the gas and solid phases. The coal switchover studies have been performed while controlling the exit temperature of the solid phase, resulting in an almost constant gas phase temperature at the gasifier exit as seen in Figure 7. Table 5 shows the final oxygen-to-coal feed Table 5. Carbon Conversion Before (Illinois #6) and after the Coal Feed is Changed to Different Coal Types and the Oxygen-to-Coal Ratio Required if the Exit Gasifier Temperature Is Maintained

Figure 8. Moisture content mass fraction profiles for Illinois #6, Pittsburgh #8, and PRB coal feeds.

coal type

oxygen-to-coal ratio

carbon conversion

Illinois #6 Utah Pittsburgh #8 PRB

0.8347 0.8241 0.9024 0.7326

0.9973 0.9998 0.9785 0.9936

ratios and the carbon conversion for different coal types. The highest oxygen-to-coal ratio is observed for Pittsburgh #8 coal, while the lowest is observed for the PRB coal. Near-complete carbon conversion is observed for Utah coal, which is attributed to the decrease in the fixed carbon content as seen in Table 4. Conversely, an appreciable drop in conversion is observed for Pittsburgh #8 coal, which is attributed to the significant increase in the fixed carbon content of the coal feed. The slight decrease in conversion for the PRB coal is mainly due to the reduction in

reduction in water content results in rapid evaporation and thus quick ignition while the reverse happens for PRB coal whose water content is higher. It should be noted that the heat of evaporation for water must be provided to evaporate off the water and thus an increase in water content results in a quenching effect thereby reducing the maximum temperature within the gasifier. This phenomenon is also partly attributed to the volatile matter content for the coals, which is more for 4955

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Figure 10. Temperature response for the case when the coal feed is changed from Illinois #6 to Pittsburgh #8, while fixing the carbon conversion. Also shown are the set point (SP) and process variable (PV) for the carbon conversion controller.

compositions, including the CO/H2 ratio which impacts the performance of downstream process units in an IGCC power plant. However, the variation in the syngas CO/H2 can be reduced by controlling the oxygen-to-coal ratio or the gasifier exit temperature. The dynamic gasifier model has also been used to study feed switchover from one coal type to another. Results show that even though the gasifier exit temperatures for the solid and gas phases are maintained, the temperature front inside the gasifier can shift depending on the composition of the new coal type in comparison to the original coal type, particularly on its water content. It is observed that by maintaining the phase temperatures at the gasifier exit at a given value, the carbon conversion can be maintained for the new coal type, assuming that this carbon conversion can be reliably estimated. However, this may result in undesired or unallowable change in the gasifier temperature.

the overall temperature profile as a result of the high water content of the coal feed. To maintain the same carbon conversion as the Illinois #6 coal, the temperature at the gasifier exit should be maintained at a value different than the case when Illinois #6 coal is used. To simulate this, a conversion controller is added to the control scheme of Figure 1 that maintains the carbon conversion by manipulating the oxygen flow. This study is performed for a changeover of coal from Illinois #6 to Pittsburgh #8. It should be noted that the carbon conversion in a gasifier is not a variable that is available in real time, but this study is performed on the assumption that the carbon conversion is estimated by some model-based approach. Figure 10 shows that if the carbon conversion can be estimated, the gasifier exit temperature for the solid phase should be increased from 1511 to 1610 °C when the coal is changed over to Pittsburgh #8 to maintain the same level of conversion. The maximum temperature attained by the gas phase within the gasifier is found to be ∼2084 °C. Such high temperature is expected to result in faster degradation of the refractory. In contrast, lower temperatures not only negatively impact carbon conversion but can also cause problems by restricting the flow of the slag due to higher viscosity.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +1-3042939364/ 9335. Fax: +1-3042934139. *E-mail: [email protected]. Notes

4.0. CONCLUSIONS In this work, a 1D dynamic model of a single-stage downwardfiring entrained-flow gasifier has been developed. A lower-level control structure has been designed and a number of transient studies have been performed. Step/ramp changes in the oxygen-to-coal ratio and the coal slurry/oxygen feed flow rates have been introduced, and the transient responses have been observed for a number of key gasifier variables, including gas and solid phase temperatures, carbon conversion, and synthesis gas compositions. Both open- and closed-loop responses have been studied to get better insights into gasifier dynamics. Dynamic simulation results show that changes in the oxygento-coal ratio in the gasifier feed (and changes in either the oxygen or coal slurry feed flow rates) affect gas and solid phase temperatures, overall carbon conversion, and syngas product

Disclosure: This project was funded by the Department of Energy, National Energy Technology Laboratory, an agency of the United States Government, through a support contract with URS Energy & Construction, Inc. Neither the United States Government nor any agency thereof, nor any of their employees, nor URS Energy & Construction, Inc., nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe on privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do 4956

dx.doi.org/10.1021/ef5010122 | Energy Fuels 2014, 28, 4949−4957

Energy & Fuels

Article

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not necessarily state or reflect those of the United States Government or any agency thereof. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS As part of the National Energy Technology Laboratory’s Regional University Alliance (NETL-RUA), a collaborative initiative of the NETL, this technical effort was performed under the RES contract DE-FE0004000.

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