By D. Levesque, M.P. Fard, and S. Morrison

Transcription

1 BLSpray: Understanding the effects of black liquor properties and splash-plate nozzle configuration on spray characteristics By D. Levesque, M.P. Fard, and S. Morrison Abstract: Black liquor spraying is an important parameter for proper combustion in a recovery boiler. Understanding the spray characteristics helps the operator optimize combustion. An ideal spray minimizes carryover while maintaining a char bed shape that maximizes chemical reduction. Since measuring spray distribution and droplet size in-situ is very difficult, work has begun to develop a computer code (BLSpray) that can accurately predict liquor spray characteristics. This paper will briefly describe the code, and will discuss results obtained from varying liquor properties and nozzle configuration. D. LEVESQUE, Alstom Canada Inc., Ottawa, ON denise.m.levesque@ power.alstom.com M.P. FARD, Simulent Inc., Toronto, ON S. MORRISON, Alstom Canada Inc., Ottawa, ON B LACK LIQUOR droplet size is an important parameter for proper combustion in a chemical recovery boiler, especially as boilers are pushed to higher and higher firing rates. Droplets that are too large do not have sufficient time to dry and partially pyrolyse before reaching the char bed, causing bed instability and possibly a blackout. Droplets that are too fine are entrained with the flue gas. This carryover can deposit on the upper furnace tubes, potentially plugging the flue gas passage. Drop size and flow distribution are also important in maximizing reduction efficiency. Droplets that are large enough to reach the bed partially pyrolysed provide carbon to the char bed, and maintain sodium sulphide (Na 2 S) in a reduced form. For droplets that are too small, Na 2 S is oxidized to sodium sulphate (Na 2 SO 4 ), thus decreasing the amount of sulphur leaving with the smelt in a reduced form. A well-distributed liquor spray also increases reduction efficiency. For a typical boiler, it is felt that the liquor spray is optimized when the nozzle provides an even, horizontal pattern with uniform droplets. This type of distribution promotes even coverage of droplets over the plan area of the lower furnace. This maximizes the surface area available for pyrolysis, combustion and reduction, which take place in the active top layer of the bed. A splash-plate nozzle design provides the spray pattern described above, and has the ability to provide spray coverage close to 180, without spraying back on the walls. Droplet mass mean diameter (MMD) is difficult to measure in-situ. Therefore, although the generally accepted optimum droplet mass mean diameter is approximately 3 mm [1], this has not been validated by extensive field testing. A computer code that could accurately predict black liquor spray droplet size and flow distribution based on specified operating conditions and nozzle configuration would be invaluable. The computer code has three main functions: To develop an improved nozzle based on understanding the effect of each change in nozzle configuration on spray characteristics. When given the operating parameters of a particular chemical recovery boiler operation, to predict the black liquor droplet size using a specific nozzle or, conversely, to determine the optimum nozzle selection for the specific operation. To provide input into boiler computational fluid dynamics (CFD) models so that the fate of droplets can be determined for a specific operation. It can be used to improve predictions of burn out, carryover, and combustion characteristics, resulting in better assessment of the effects of fuel changes on operating recovery boilers. Current CFD models cannot accurately predict droplet formation; they use droplet size distribution as an input value. In September 2000, work was initiated to develop such a code, entitled BLSpray. The code is currently being used to evaluate the effects of changes in black liquor properties and nozzle configuration. This paper will briefly describe the code, and will discuss some of the results seen to date. NUMERICAL TECHNIQUES BLSpray is based on a 3D model of liquid flow that predicts liquid free surface evolution and possible break-up following the fluid flow into the nozzle. The model is an extension of the one previously developed to track free surface deformation of a droplet during its impact on a flat substrate [2], and on a substrate of an arbitrary shape [3]. The flow governing equations are Navier-Stokes for an incompressible, Newtonian, and laminar flow. In addition to solving the flow equations within the liquid, the numerical model must also track the location of the liquid free surface. The approach used is a Volume of Fluid (VOF) based technique called Youngs algorithm [4]. The body of the splash-plate nozzle in the computational domain is an internal obstacle that affects the fluid flow. The fluid flow and VOF equations are modified in presence of the splashplate nozzle. Surface tension at the liquid free surface is considered in the model. The numerical techniques used in BLSpray code have been validated by a direct comparison of numerical results with those of the measurements for various experimental settings [5,6,7]. The fluid flow equations are solved on a computational domain that encompasses the volume :10 (2005) T 198 Pulp & Paper Canada

2 FIG. 1. Computational domain for BLSpray. FIG. 2. Description of break-up point and break-up length. of the splash-plate nozzle and sufficient volume to cover liquid film deformation and break-up after exiting the nozzle. An example of the computational domain is seen in Fig. 1. The size of the computational mesh was determined by a mesh refinement study on BLSpray results. To facilitate mesh generation and nozzle shape introduction, BLSpray code is equipped with a pre-processor interface that links a commercial mesh generator called ICEM CFD to BLSpray input data. BLSpray is also facilitated with an interface for post-processing; the interface embedded in the code generates binary results that can be directly displayed using Tecplot software. The results of BLSpray are in the form of digital files that include all necessary information regarding the liquid location, velocity and pressure in the whole computational domain. The post-processing interface extracts all specific film and spray information from these results. TEST CASES BLSpray is an important tool for determining the effect of various parameters on the formation of a liquid sheet and subsequent droplets. The following describes results of test cases where black liquor properties and nozzle configuration were varied. Black Liquor Properties: At the preliminary stages of the development of BLSpray, three key black liquor properties were varied to determine their significance on droplet size. Jet velocity was also varied during this study. The results were analysed in terms of droplet size and break-up length. Break-up length is the length from the tip of the splash plate to the point of film break-up, Fig. 2. Table I summarizes the results of varying black liquor viscosity, surface tension and density as well as jet velocity. Cases where droplets did not form, even with extended simulations, were excluded from the table and corresponding graphs. As the liquor impacts on the splash plate, it makes a liquid film that spreads to the front and sides of the nozzle. Surface waves formed on the liquid film break it FIG. 3. Effects of viscosity (cases 1, 2 and 3) and velocity (cases 2, 4, 5 and 8) on mean droplet size. TABLE I. Summary of simulation results for black liquor properties, including jet velocity. into liquid ligaments shortly after it leaves the rim of the plate. The ligaments later break up to form the final droplets. The results show that when the viscosity is increased, the liquid ligaments remain as ligaments for a longer period of time before breaking up into droplets. The formation of droplets is therefore delayed, and the resulting droplets are larger in size. When the velocity is increased, the film thickness at the film break-up point is reduced. The resulting mean droplet size is decreased. Surface tension has a significant effect on mean droplet size. When the surface tension is doubled, the mean droplet size is increased nearly 40%. This is because at higher surface tension, many ligaments do not break up, and hence form large single droplets. When the liquor density is increased, the mean droplet size is decreased due to higher momentum, and thus higher kinetic energy, of the liquid prior to impact. Figures 3 and 4 summarize the relationship between mean droplet size and the corresponding black liquor property. Nozzle Configuration: Understanding the fundamentals of black liquor spraying is important for designing an optimized splashplate and ensuring proper black liquor spraying into the recovery boiler. The following parameters were varied to gain an insight on how these changes Pulp & Paper Canada T :10 (2005) 35

3 FIG. 4. Effects of surface tension (cases 2, 6, 7 and 10) and density (cases 2 and 11) on mean droplet size. FIG. 5. Data was analysed at three cross-sections of the spray. affect the spray characteristics. Nozzle diameter: 0.5-, 0.88-, 1.25-in. ( and 32 mm); Jet angle: 35, 45, and 55 degrees; Jet velocity: 5, 10, 18 m/s (16, 33, 59 ft/s); Bore shape and location: round, oval, fish mouth; centred, off-centre; Transition section between pipe and plate; and Plate surface: convex, flat. These simulations were carried out at varying stages of the development of BLSpray. Nozzle diameter and jet angle were evaluated using a flat plate with only a small backsplash, Fig. 5. This type of plate was used so that the investigation could focus on the main part of the flow, and not the backward flow. The results were analysed at three different cross-sections, namely =0, 45 and 90, as Fig. 5 shows. Nozzle Diameter: Three nozzles diameters were tested (12, 22, 32 mm), with all other parameters, including jet velocity, remaining constant. The film thickness increased in all directions with increasing nozzle diameter, Fig. 6. This is to be expected since the flowrate increases as the nozzle diameter increases (jet velocity is held constant at 10 m/s). The sheet breaks up at a distance further from the plate as the nozzle diameter increases. As the liquid film leaves the plate, the velocity direction is parallel to the plate at all cross-sections. Top surface velocity is significantly lower for the smallest diameter than the other two cases. This is because most of the liquid kinetic energy is lost due to viscous dissipation. As the nozzle diameter increases, the film thickness also increases, but the boundary layer thickness remains constant. As a result, the difference in the top surface velocity between the two larger diameters is less pronounced. Increasing nozzle diameter has minimal effect on radial distribution. Jet Angle: Three jet angles were tested (35, 45, 55 ), with all other parameters, including jet velocity and nozzle diameter, remaining constant. Results indicate the effect of jet angle varies depending on the cross-section FIG. 6. Effect of nozzle diameter on liquid film spray shape at the plane of symmetry. FIG. 7. Effects of varying jet velocity. location: At =0: For every 10 degrees increase in jet angle, the film thickness decreases by nearly 20%. This indicates that for nozzles with higher angles, the atomization at the plate centreline occurs at a shorter distance from the splash plate because the film thickness approaches the limit value for the break-up more quickly. At =45: The effect of jet angle is negligible. The film thickness remains the same. At =90: The film thickness increases slightly as the jet angle increases. The film breaks up at a distance further from the splash plate for the higher jet angle. We can conclude, therefore, that as the jet angle increases, more atomisation occurs towards the sides of the splash plate, and the atomisation zone at the plane of symmetry ( =0 ) will be closer to the splashplate. The radial distribution of the film flowrate is, therefore, affected by the jet angle. At lower jet angles, the film thickness is thicker at the plane of symmetry ( =0 cross-section) and thinner at the cross-section of =90. When the jet angle is increased, the film thickness along the plate tip becomes more uniform. Since droplet size distribution is directly related to film thickness and velocity distributions, it can be concluded that using a lower jet angle will result in a wider range of droplet sizes. The magnitude of the velocity at the splash-plate exit for all cases is close to that of the jet velocity. Jet Velocity: Three jet velocities were tested (5, 10, 18 m/s), with all other parameters, except flowrate, remaining constant. The sheet thickness and velocity results are summarized in Fig. 7. As the figure shows, the nozzle jet velocity has no effect on sheet thickness. Even at =90, there is only a very slight difference between the three cases. As the jet velocity increases, the velocity boundary layer thickness decreases :10 (2005) T 200 Pulp & Paper Canada

4 FIG. 8. Standard nozzle with a 3 / 4 -in (19-mm) centred bore, with simulation results. FIG. 9. Standard nozzle with a 3 / 4 -in (19-mm) off-centre bore, with simulation results. This means that the effect of splash plate on deceleration of the jet flow is more significant at lower velocities. Because of the lower initial kinetic energy of the jet and higher percentage of energy loss due to viscous dissipation, the size of the atomised droplets are larger for jets with lower velocity. This confirms the results discussed in the previous section. Figure 7 indicates that the rate of velocity change with respect to the angle from plate centreline is nearly the same for all cases. In terms of the black liquor radial flow distribution, the distribution remained almost the same for all three velocities. Testing various nozzle configurations using the pre- and post-processor functions of BLSpray began in November 2003, and is continuing. The results from these simulations are analysed in terms of film thickness, mean droplet size, film velocity and mass distribution at various angles from the plate centreline. The following sections describe some of the results obtained to date. Bore Shape and Location: Four simulations were run using a Standard nozzle 3/4-in. (19-mm) bore. The bores were as follows: Round, centred on the pipe; Round, with the bottom of the bore flush with the transition section to eliminate any step at the bore exit; Oval (wide); and Oval (tall). A fifth simulation was run adjusting the lower portion of the oval bore to make a smooth transition, giving the bore the appearance of a fish mouth. Figure 8 indicates a range in droplet size of 2.8 to 4.2 mm. The film velocity is higher near the plate centreline, and decreases gradually by 44% as one moves toward the side. The spray flow distribution graph also indicates a decrease as one moves toward the side: 55% of the mass flow is located between =0-30. Moving the bore off-centre, Fig. 9, flattens the film velocity, mean droplet size, and spray flow distribution. However, a much larger percentage of the flow is concentrated between 40 and 50 from centreline (30.5% vs. 8.5% of the mass flow for the centred bore), due to black liquor propagating upward and forward off the sides of the transition section between the pipe and plate (rooster-tailing). In addition, there is no flow between = A width-wise (wide) oval bore creates a surprisingly uneven flow distribution, with very little liquor reaching the sides. A height-wise (tall) oval bore, on the other hand, creates a flow distribution that is much more concentrated on the sides. Neither produces the desired distribution. The nozzle and simulation results for the height-wise (tall) oval bore are depicted in Fig. 10. The fish mouth nozzle causes the film velocity profile to flatten, but results in a sharp decrease in droplet size as one moves radially out from the plate centreline. In addition, 75% of the flow is concentrated between =0-30, and there is no flow between = Pulp & Paper Canada T :10 (2005) 37

5 FIG. 10. Standard nozzle with a 3 / 4 -in (19-mm) oval (tall) centred bore, with simulation results. FIG. 11. Standard nozzle with a 1.25-in (32-mm) bore at time =0 and at steady state. Transition Section: There are two fundamental designs that were examined with BLSpray: the Standard nozzle, as Fig. 11 shows, has a transition section between the pipe and the plate. The sides of this transition are called sideskirts. The Flush design has no transition section, as Fig.12 shows. BLSpray results indicate a better mass distribution and flatter velocity distribution with the Standard nozzle design. The droplet distribution is similar for both nozzles. As Fig. 11 shows, the Standard nozzle sideskirt causes the flow at the sides to propagate upwards, called rooster-tailing, under certain operating conditions. Even though the percentage of mass contained in the rooster-tails is typically low, this is still an undesirable effect. Modifications were made to the design of the transition section to minimize rooster-tailing. Using BLSpray, simulations were run to see the effect of the different designs. Preliminary results are promising. The use of BLSpray was invaluable in troubleshooting several sideskirt designs. Plate Surface: The plate surface was curved to determine the effect on distribution. A concave surface was not simulated, as this would produce an undesirable spray by propagating the droplets toward the upper furnace. A convex surface was modelled. The film velocity profile was very flat and the mass distribution profile is favourable. However, there is no mass flow beyond =60. It was noted that the convex shape increased the MMD slightly. This is likely due to the fact that the convex surface reduced the impact of the incoming black liquor on the plate. Simulation results are summarized in Table II. In summary, preliminary analysis of the latest test cases on nozzle configuration indicated the following: Oval and fish mouth shaped bores did not produce the desired spray distribution. A centred bore produces a better mass distribution than an off-centre bore, but may promote rooster-tailing. This will be further investigated. A convex surface causes a slight increase in MMD. Rooster-tailing, which occurs under certain operating conditions can be minimized/eliminated by modifying the sideskirts. This will be further investigated. FUTURE WORK Although BLSpray is a functional and useful program, work is continuing to develop the code to include the effects of secondary droplet break-up, flashing, heat transfer/evaporation, and flue gas air flow fields. Work is also in progress to couple BLSpray with CFD modelling programs. This will improve the model s ability to accurately predict droplet size for any black liquor firing condition. BLSpray will continue to be used as a tool to further understand the effects black liquor properties and nozzle shape have on droplet size and radial distribution. It will also be used to model current :10 (2005) T 202 Pulp & Paper Canada

6 Table II. Summary of simulation results. recovery boiler nozzle installations to understand how the recovery boiler operation can be improved by making adjustments to the firing philosophy. CONCLUSION A preliminary code has been developed and validated against experimental results [5,6]. The code is being used to understand the fundamentals of black liquor spraying, improve the design of splashplate nozzles, and model current nozzle installations on recovery boilers. Work is continuing to couple BLSpray with CFD modelling. Work is also continuing to improve the model to include in-situ influences such as vaporisation and flue gas. This will improve the model s ability to accurately predict droplet size for any black liquor firing condition. These efforts will help to improve combustion within the recovery boiler, resulting in a better operation. The benefits of this include longer run times, improved reduction efficiency, reduced tube corrosion, and the ability to operate at higher loads. LITERATURE 1. ADAMS, T.N. Kraft Recovery Boilers. Chapter 4. Atlanta: TAPPI Press (1997). 2. BUSSMAN, M., MOSTAGHIMI, J., CHANDRA, S. On a three-dimensional volume tracking model of droplet impact. Phys. Fluids 11(6): (1999). 3. PASANDIDEH-FARD, M., BUSSMAN, M., CHAN- DRA, S., MOSTAGHIMI, J. Simulating droplet impact on a substrate of arbitrary shape. Atomization and Sprays 11(4): (2001). 4. YOUNGS, D.L. An interface tracking method for a 3D Eulerian hydrodynamics code. Technical Report 44/92/35, AWRE (1984). 5. FARD, M.P., ASHGRIZ, N., MOSTAGHIMI, J., LEVESQUE, D.M., MORRISON, S.A. Film thickness and velocity distribution in a splash plate atomizer: FIG. 12. Flush nozzle with a 1.25-in (32-mm) bore at time =0 and at steady state. comparison between simulations and experiments. Proc., 9th International Conference on Liquid Atomization and Spray Systems, Sorrento, Italy (July 2003). 6. LEVESQUE, D.M., MORRISON, S.A., FARD, M.P., MOSTAGHIMI, J. Simulation of the Spray Characteristics of Black Liquor. Proc., PAPTAC 89th Annual Meeting, Montreal (January 2003). 7. LEVESQUE, D.M., FARD, M.P., MORRISON, S.A. BLSpray: Understanding the Effect of Black Liquor Properties and Splash Plate Nozzle Configuration on Spray Characteristics. Proc., 2004 International Chemical Recovery Conference, Charleston, SC (June 2004). Résumé: Le mode de pulvérisation de la liqueur noire est un paramètre important dans l obtention d une bonne combustion dans une chaudière de récupération. Comprendre les caractéristiques de la pulvérisation permet d optimiser la combustion. Une pulvérisation idéale réduit l entraînement de liqueur tout en maintenant le lit de charbon en bon état afin de maximiser la réduction chimique. Puisqu il est très difficile de mesurer la répartition de la pulvérisation et la dimension des gouttelettes in situ, des travaux ont été amorcés afin de développer un code de recherche informatisé (BLSpray) pouvant prédire précisément les caractéristiques de la liqueur pulvérisée. La présente communication décrit brièvement ce code et discute des résultats obtenus à partir de diverses propriétés de la liqueur et configurations de la buse de pulvérisation. Reference: LEVESQUE, D., FARD, M.P., MORRISON, S. BLSpray: Understanding the effects of black liquor properties and splash plate nozzle configuration on spray characteristics. Pulp & Paper Canada 106(10): T (October 2005). Paper presented at the 2004 International Chemical Recovery Conference in Charleston, SC, on June 6 to 10, Not to be reproduced without permission of PAPTAC. Manuscript received on October 13, Revised manuscript approved for publication by the Review Panel on April 18, Keywords: BLACK LIQUORS, SPRAYING, COMBUSTION, OPTIMIZATION, SPRAYERS, NOZZLES, MACHINE DESIGN, COMPUTER PROGRAMS. Pulp & Paper Canada T :10 (2005) 39