Coal Slurry Atomization: Applications, Scaling and Unanswered Questions. GE Global Research Center One Research Circle Niskayuna, NY USA

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1 ILASS Americas, 23 rd Annual Conference on Liquid Atomization and Spray Systems, Ventura, CA, May 2011 Coal Slurry Atomization: Applications, Scaling and Unanswered Questions Ertan Yilmaz 1,*, Krishna Venkatesan 1, Ali Ergut 2, and Benjamin Steinhaus 2, 3 1 GE Global Research Center One Research Circle Niskayuna, NY USA 2 GE Energy 1333 West Loop South Houston, TX, Christopher Brown, Ulises Mondragon and Vincent McDonnell Energy Research Consultants South Pointe Drive, Suite E Laguna Hills, CA, USA Abstract Gasification plants react carbonaceous materials such as coal, heavy fuel oils or biomass with oxygen at high temperatures, converting them into hydrogen and carbon monoxide. The coal-water slurries used in these gasification plants are typically viscous, non-newtonian fluids containing high concentrations of suspended coal particles. Gasification feed injectors utilizing coal slurries typically run at high throughputs with atomization occurring in vessels at temperatures of several thousand degrees Fahrenheit and pressures up to 1450 psi. This paper provides an overview of the challenges encountered when scaling plant conditions down to the lab level, using the resulting spray characteristics to perform comparative analysis and predicting true injector performance. Current results include spray diagnostics on various Newtonian fluids at several length scales, using high-speed video, PDI and optical patternation. Industrial applications, challenges and areas for development in the field of coal slurry atomization are also emphasized. * Corresponding author 3 Author s current affiliation: General Vortex Energy, Missouri City, TX

2 ILASS Americas, 23 rd Annual Conference on Liquid Atomization and Spray Systems, Ventura, CA, May 2011 Gasification Overview Gasification plants utilize carbonaceous feedstocks such as coal, heavy fuel oils, or biomass to produce hydrogen and carbon dioxide. These gases can then subsequently be burned in gas turbines to generate electricity or used to produce chemicals such as butanol, methanol or acetic acid. The atomization of these feedstocks typically takes place in large pressure vessels, or gasifiers, at temperatures of several thousand degrees Fahrenheit and pressures of up to 1450 psi. The focus of this paper is on the atomization and gasification of coal slurries. Typical coal slurry injectors range from 6 to 10 inches in size and utilize an annular triplet design [1]. In this configuration, oxygen travels through the center and outer annular passages, while the coal slurry travels through the middle annulus. The three streams converge at the tip of the injector. The slurry is then atomized externally via air-blast atomization and combusts in the gasifier. The coal-water atomization is the first critical step of the gasification process and therefore deserves special consideration. This paper highlights the importance of slurry atomization to the gasification process, the industry requirements, and the unanswered key questions. General Coal Slurry Atomization Most injector designs are confidential, although a fair amount of knowledge can be gained from open literature. Most of the coal slurry injectors used in industry today are annular triplets as shown in Figure 1 [1]. Coal slurry and oxygen enters the injectors and travel parallel to one another down to the tip of the injector. The high-velocity oxygen subsequently atomizes and combusts with the coal slurry immediately at the exit of the injector tip. Gasification coal slurries have highest possible concentration of solids to maximize the throughput and efficiency of the reactor. Coal particle concentrations in excess of 60% are quite common. At such concentration levels, the slurry is highly viscous and exhibits non-newtonian characteristics [2]. When such slurry is atomized by oxygen, it is subjected to significant shear-rates. At the point of atomization, the viscosity of the slurry reduces significantly due to its shear-thinning characteristic. Factors Influencing Atomization Atomization of coal slurries is influenced by a wide variety of parameters and the ideal spray pattern for optimized gasification performance is not well understood in the industry. Several of the key spray characteristics and their influences on the overall performance of the injector as found in [3,4] are summarized in Table 1. Table 2 shows how the injector and spray performances are affected by different parameters such as fuel properties and injector flow conditions. Current Work Current work on air blast atomization of slurries has been aimed at understanding the effects of varying various non-dimensional numbers and flow properties on the spray characteristics. The effects of air-to-fuel ratio and shear-rate on atomization can be studied by varying the exit velocities, while varying the liquid viscosity allows for studying the effects of slurry Reynolds number (Re). General trends for the atomization of slurries have followed those that are well known and published in literature [6,7]. The droplet size has been shown to increase with increasing viscosity and length scale [6,7]. As the exit velocity and shear-rate increase, the droplet sizes decreases [6]. Each of these effects has been studied using a variety of experimental techniques, including high-speed video imaging, Phase Doppler Interferometry (PDI), and optical patternation. Figure 2 highlights a timeaveraged high-speed image of the injector spray field emanating from a scaled injector along with spray patternation images acquired at selected axial locations downstream of the injector exit. Despite the thorough characterization of sprays at ambient conditions, several effects remain to be elucidated Current efforts are aimed at the development of surrogate slurries and the analysis of the impact of their properties on atomization. Of particular concern is the diameter of the particles used in the surrogate slurries. Recent work has shown that the atomization of slurries is effected by the ratio of the mean slurry particle diameter to the atomizer orifice diameter as well as the particle volume fraction [8]. However, as the particle size decreases, the viscosity of the slurry increases [2]. Therefore, if the particle size is to be scaled down to the lab-scale while maintaining the slurry volume fraction, the slurry viscosity will become so large that the mixture becomes immovable. Atomization Research Needs The biggest concerns surrounding spray testing at the laboratory scale are its applicability at the plant level and whether the results can be scaled up with any confidence. While a large body of published literature exists for the atomization of non-newtonian fluids, only a small amount of this work focuses on slurries, and even less on the atomization of slurries at elevated pressures. Therefore, the designer is left with little to no historical data for use in model validation and design work. Specific gaps include:

3 Length Scale Effects Although previous studies have investigated the effect of length scale on slurry atomization [4,7], no published literature is available on length scales of typical gasification process injectors, which are on the order of 6 to 10 inches. Current lab-scale facilities are capable of reaching approximately 1/2 to 1/3 rd of these sizes. Pressure Effects While some work has been conducted toward examining pressure effects on sprays [6], the majority of the efforts reported in the literature have been conducted at atmospheric conditions. Gasifier pressures, however, can reach as high as 1450 psi. Careful scaling of non-dimensional numbers must be performed to accurately re-create high-pressure atomization at atmospheric conditions. However, due to the excessive pressures seen at the plant level, it becomes difficult to conserve all non-dimensional numbers. For example, consider the scaling of a 10 inch injector at 1000 psi from the plant scale down to an atmospheric test at 1/5th scale. Due to the high density of the gas at the plant scale, once the oxygen velocities exceed only 10 ft/s in the plant injector, the flow would need to become supersonic at the lab scale to maintain the Reynolds (Re) and Weber numbers (We). Due to these difficulties, special care must be taken for the selection of dimensionless numbers to be conserved. Spray Characteristic Definition Effect Dispersion Patternation Radial Patterantion - Circumferential Percentage of the spray volume which contains liquid Radial distribution of liquid in a spray Circumferential distribution of liquid in a spray Large dispersion leads to a fuel lean mixture, too dense leads to a fuel rich mixture. Uniform distribution prevents fuel lean or fuel rich regions. Uniform distribution prevents fuel lean or fuel rich regions. Cone Angle Angle of spray spreading Wider angle leads to a highly dispersed spray. Average SMD Average sauter mean diameter Table 1. Key spray characteristics. Small SMD s increase the surface area available for mass transfer and reaction [5,7]. Factor Particle Size Solids Loading Oxygen Exit Velocity [4] Slurry Exit Velocity [2] Flow Splits Effect on Performance Particles, which are too small, increase viscosity. Particles, which are too large, may block passages. Too high of a solids ratio can cause the slurry to become too viscous. Too low of a solids ratio makes the design inefficient. Too low exit velocities can lead to burn-back and poor atomization. Too high exit velocities can result in equipment over sizing. Too high velocities can increase erosion and wear. Too large of a flow split can cause an excessively large cone angle (impingement on gasifier walls). Too low can lead to poor atomization. Table 2. Key factors effecting slurry spray performance. Slurry Rheology At the point of atomization, the shear-rate experience by the coal slurry is estimated to be on the order of s -1. However, current rotational viscometers are capable of measuring slurry viscosities at shear-rates only up to approximately 100 s -1. Obviously, a large gap exists between experimental capabilities and measurement needs. To date, only a limited amount of literature has been published utilizing capillary viscometry to generate shear-rates on the

4 order of 10 5 inverse seconds [9]. Due to this dearth of data, the experimentalist is left to use available empirical correlations and models to curve fit the data collected at lower shear-rates and to extrapolate to higher values. In order to have confidence in high shear-rate viscosities of the coal slurries employed in gasification injectors, actual experimental data in these ranges is needed. Non-Newtonian Effects Typical gasifier slurries contain high coal concentrations in excess of 60%. At such coal concentration levels the slurry is highly viscous, and exhibits a non-newtonian behavior. Moreover, the slurry viscosity will vary significantly between the inside of the injector and the atomization region due to its shear-thinning characteristics and the significant shear rate variation it is exposed to. Inside the injector, low shear-rates result in relatively high slurry viscosities compared to the atomization region where high shear-rates significantly decrease the viscosity. Testing with a fluid exhibiting this shear-rate range becomes essential for accurately recreating the atomization of real coal slurries in the plant. In addition to the changes in viscosity of the fluid, the particles themselves impart instabilities to the fluid. Several studies have shown that at low Re, the break-up of fluids is enhanced by the presence of particles [7,10,11,12]. This phenomenon is believed to be caused by increasing instabilities generated from the presence of the particles. Due to the limited amount of information available on each of the above effects, the validation of any spray modeling becomes challenging. In addition, the results of lab-scale tests must be used cautiously. While they may serve as a good general basis for comparison, differences will likely remain between atmospheric spray characteristics and those found at the plant level. Conclusions The atomization of coal water slurries presents several complex problems that can be excellent research opportunities for academia. It draws on knowledge from several different fields including non- Newtonian fluids dynamics, atomization and multiphase flow. Work remains in developing experimental techniques and models for use in advanced injector designs. Advances will result in more efficient plant operation and cost savings for gasification. References 1. Higman, C., and Van der Burgt, M., Gasification, Elsevier, Abulnaga, B., Slurry Systems Handbook, MacGraw Hill, Gill, G.S., and Nurick, W.H., Liquid rocket engine injectors, NASA Space Vehicle Design Criteria, NASA Lewis Research Center, NASA SP-8089, March Lefebvre, A. H., Atomization and Sprays, Taylor and Francis, Turns, S., An Introduction to Combustion, 2 nd Ed., McGraw-Hill, Rizkalla, A., and Lefebvre, A.H., The Influence of Air and Liquid Properties on Air Blast Atomization, ASME J. Fluids Eng., 97(3): (1975). 7. Lefebvre, A.H., Airblast Atomization, Prog. Energy Combust. Sci., 6: (1980). 8. Furbank, R., and Morris, J., An experimental study of particle effects on droplet formation, Physics of Fluids, 16(5) (2004). 9. Turian, R., Attal, J.F., Sung, D., Wedgewood, L.E., Properties and rheology of coal-water mixtures using different coal, Fuel, 81: (2002). 10. Fristching, U., Mulhem, B., Kurt, O., Schulte, G., Influence of Suspended Solid Particles on Suspension Atomization Processes, Twenty first International Conference on Liquid Atomization and Spray Systems, Vail, USA July Kihm, K., Terracina, D., Payne, S., Caton, J., Synchronized Droplet Size Measurements for Coal Water Slurry Spray Generated from a High Pressure Diesel Injection System, Journal of the Institute of Energy, Vol. LXVII(470):2-9, March Hecht, J. P., and Bayly, A.E., Atomization for Spray Drying: Unanswered Questions and Industrial Needs, Twenty First International Conference on Liquid Atomization and Spray Systems, Vail, USA July Nomenclature Re SMD We Reynolds Number SauterMean Diameter Weber Number

5 Figure 1. Typical coal slurry feed injector tip details (from United States Patent ). Figure 2. Time-averaged high-speed spray image and optical patternation planes of a scaled injector.