Urea Injection Design Analysis for NO x Control using CFD

Transcription

1 Urea Injection Design Analysis for NO x Control using CFD As resented at: 10th International Conference on Combustion and Energy Utilization, Mugla, Turkey, May 2010 Kathleen Brown*, Wojciech Kalata and Rudolf J. Schick Sraying Systems Co. Wheaton, IL USA Abstract There are many industrial ollutants roduced from coal fired ower generation lants. Recent legislation is ushing to reduce these emissions. Greenhouse gasses as well as nitrous oxides (NO X) and sulfur dioxide (SO 2 ) are the targeted ollutants. Governmental legislation outlined by Environmental Protection Agency (EPA) through its Maximum Achievable Control Technology (MACT) guideline is a major thrust. One method of NO x reduction is selective non-catalytic reduction (SNCR) which is imlemented rior to gases venting to the atmoshere. In SNCR systems a reagent is injected into the flue gas, in an effort to reduce NO x levels by u to 50%. The oeration of this equiment is highly deendent on the temerature and distribution of the gases requiring careful control of the system. Sray nozzles are an imortant factor in this rocess; sray nozzles add value by roviding controlled volumes of liquid with redictable dro size and consistent sray coverage. Knowing these values remains vital for the otimization of gas conditioning rocesses. In addition, the all-imortant lacement of nozzles is often times misunderstood and therefore inaccurate. While immense care is taken to otimize lacement, this often leads to failure due to the inability to redict sray erformance in the alication environment. This rocess can be facilitated by using comutational fluid dynamics (CFD). In this case study, CFD is used to simulate NO x levels roduced in the coal combustion rocess and estimate the NOx reduction that can be achieved with the urea injection. In the end, adjustments were made to original nozzle lacement suggestions in order to otimize gas conditioning rocess while minimizing the sli through the system. *Corresonding author Exerts in Sray Technology Sray Nozzles Sray Control Sray Analysis Sray Fabrication

2 Introduction An imortant concern in air ollution control is the treatment of exhaust gases from ower lants and industrial lants, so as to drastically limit the amount of ollutants entering the atmoshere. One of these ollutants is NO x, which is defined as the collective term for nitrogen oxide gases. There are several methods of NO x formation including: thermal NO x, romt NO x, and fuel NO x. The most revalent form is thermal NO x - which is formed at the areas of highest temerature during the combustion rocess. NO x is a comlex rocess which occurs in re-combustion, combustion and ost-flame regions. There are several aroaches to NO x abatement and reductions. This study focuses solely on emission control by flue gas treatment using selective non-catalytic reduction (SNCR). For SNCR systems, a reagent such as urea or ammonia is injection into the flue gas. NO x reductions of 30% 50% can be achieved. The reaction between reagent and NO x should occur within a temerature range of 900 C and 1,100 C deending on the reagent and condition of SNCR oeration [1]. Above this range there may be reductions in efficiency due to the thermal decomosition of ammonia. Below this temerature range, sli or carryover may increase. The aim is to injection the reagent with drolets that rovide a residence time in excess of one second. However a minimum residence time of 0.3 seconds is recommend for adequate effectiveness [1]. Ammonia sli or carryover is a revalent issue with SNCR control techniques. This occurs from injections at temeratures below the aforementioned temerature window or from too much mass er injector leading to uneven distribution. Current methods rely on using sray injectors to deliver the urea or ammonia fluid into the flue gas stream. Sray injectors add value by roviding controlled quantities of liquid and redictable dro size and sray coverage. This data is critical for efficient NO x control with minimal sli or carryover. While this method has yielded some success, it has resented a series of roblems due to the inability to accurately redict sray erformance in the comlex environment as required. Today, comutational fluid dynamics (CFD) is well suited to redict flow conditions and aid in the otimization of these exhaust systems. Today, there is no standard method in use for selecting and otimizing fluid srays for gas conditioning alications. This aer will outline a case study in which injectors were selected and ositioned based on the critical elements of combustion chamber design and injector characteristics. This method consists of injector selection and location considerations based on known laboratory measurements of dro size, velocity and sray distribution. Furthermore, this method will also evaluate the effects of normal oerating arameters such as flue gas flow rate, flue gas and combustion chamber temerature, and flue gas velocity on exhaust uniformity. Aroach The rocess of designing the gas conditioning rocess begins with determining the amount of liquid required for reaction with NO x levels measured. In addition the otimal dro size is also determined and the dro size in this case will be a function of the dwell time. Dro Size Secification Proer chemical interaction is deendent on liquid atomization and a controllable dro size distribution to ensure a raid evaoration rate and eliminate a wall wetting roblem causing down time (1). Wall wetting roblems can be caused by atomizers that roduce large dros that do not achieve full evaoration within the allowable dwell time in the duct. The rocess to determine the maximum allowable dro size is governed by the following equation: 2 (1) D( t) = Do λt 2

3 t is dwell time in seconds D 0 is the initial diameter of the dro in microns 8κ ( T Twb ) λ = L ρ. liq ĸ is the Thermal Conductivity of the gas. T is the gas temerature T wb is the Wet-Bulb Temerature of the system L is the Latent Heat of Vaorization of the liquid ρ liq is the density of the liquid. Nozzle Secification Generally, large volume, two fluid atomizers are used in large scale industrial rocesses. These tyes of nozzles are referred because they are able to control large and/or frequent variations in gas temerature or volume, rovide consistent redictable dro size and are energy efficient. Based on the required flow rate and dro size values determined above, a Sraying Systems Co. WhirlJet BD-5 nozzle was used in this case study. The WhirlJet nozzle is a hydraulic atomizer, featuring an inline hollow cone design for ease of installation and reduced footrint. It consists of a main liquid inlet in line with the orifice. The flow is diverted into a swirl chamber and the liquid is discharged tangential to the orifice. The orifice is relatively large minimizing the risk for clogging. The rincial of oeration is as follows. The liquid enters the swirl chamber and exits tangential to the orifice. The liquid exeriences a high shear and is broken into sheets. The sheets exerience turbulence and additional shear causing further atomization. All testing was erformed with each WhirlJet injector mounted on a single lance body. A schematic of the WhirlJet BD-5 nozzle used in this test is shown in Fig. 1. Sray characterization testing was erformed for several nozzles to determine otimal injection roerties for the CFD model, based on the theoretical requirements. For dro sizing, the nozzles were mounted on a 3-axis traverse. Dro size testing was erformed in a single lume of the sray. Dro size measurements were executed at multile locations, based on nozzle erformance. A two-dimensional Artium Technologies PDI-200MD instrument was used to make dro size and velocity measurements, as shown in Fig. 2, the test setu is shown in Fig. 3. The solid state laser systems (green 532 nm and red 660 nm) used in the PDI-200 MD are Class 3B lasers and rovide about 50-60mW of ower er beam. This is an intense enough laser ower to hel offset dense sray effects. [2, 3, 4]. The test results are shown in Table 1 and Fig. 4. Figure 2. Artium PDPA Figure 3. Emirical Setu Figure 1. Nozzle Inline WhirlJet BD5 Figure 4. Injection Location 3

4 Inline WhirlJet Nozzles (hollow cone) units BD5 Velocity magnitude m/s 30 Q gm 1.6 ṁ a kg/s Pressure sig 100 Temerature C 20 Sray Angle 76 D V minimum μm 23 D V average μm 200 D V maximum μm 298 N 3.0 Lance Insertion Deth m a. Sray dros treated as inserted urea/water articles. Wet combustion modeling used with urea as devolatizing secies and water as evaorating secies Table 1. Injector roerties as acquired emirically The DV0.5 and D32 diameters were used to evaluate the dro size data. The dro size terminology [5, and 6] is as follows: DV0.5: Volume Median Diameter (also known as VMD or MVD). A means of exressing dro size in terms of the volume of liquid srayed. The VMD is a value where 50% of the total volume (or mass) of liquid srayed is made u of dros with diameters larger than the median value and 50% smaller than the median value. This diameter is used to comare the change in dro size on average between test conditions. DV0.1: is a value where 10% of the total volume (or mass) of liquid srayed is made u of dros with diameters smaller or equal to this value. DV0.9: is a value where 90% of the total volume (or mass) of liquid srayed is made u of dros with diameters smaller or equal to this value. All ressures were monitored immediately ustream of the nozzle body using a 0-7 bar, class 1A ressure gauge. Liquid flow to the nozzle was delivered using a ositive dislacement um. The flow rate was measured reviously using a MicroMotion D6 flow meter and was correlated to nozzle ressure settings. The MicroMotion flow meter is a Coriolis Mass flow meter that measures the density of water to determine the volume flow. The meter is accurate to ±0.4% of reading. The Rosin-Rammler distribution function is used to convert raw measured dro data into a dro size distribution function for CFD. The Rosin-Rammler distribution function (2) is a reresentation of the dro oulation and size in a sray. The exact size for every volume fraction F(D) in the sray can be calculated using the X and N arameters. (2) F(D) = 1- ex - D X CFD Analysis Method Comutational Fluid Dynamics (CFD) is the science of redicting fluid flow, heat and mass transfer, chemical reactions, and related henomena by solving numerically the set of governing mathematical equations. For the comutation the general CFD code Fluent is alied. Fluent solvers are based on the finite volume method. The fluid region is broken into a finite set of control volume (mesh). The general conservation equations are alied. Each volume within the mesh is simultaneously solved to render the solution field. The conservation equations are shown below (3 5)[7]. Conservation of mass: n (3) α ρ + α ρ u = q q q q q m t = 1 Conservation of momentum: (4) α qρquq + ( α qρquq uq ) = α q P + α qτ q + α qρqfq + t Conservation of enthaly: (5) dq ( α qρqhq ) + ( α qρquqhq ) = α q + τ k : uq. qq + sq + t dt N n q = 1 n = 1 ( R q ( Q q + m u ) q + m h q q q ) 4

5 General Alication Requirements In the framework of the Eulerian-Lagrangian modeling aroach, coal articles are inserted using the non-remixed model. The arameters of the coal and gas conditions are shown in tables 1-2. Meshing was erformed with a GAMBIT re-rocessor. Dense mesh was incororated in the areas very near the injection locations and coal injection locations. Size functions were used to further refine mesh size. The 3D mesh consisted of mixed elements with aroximately 3.2 million cells. Figure 5 rovides a schematic of the CFD model setu and defines the coordinate system referenced in both the comutational and exerimental results. Ultimate Proximate Carbon Ash 10.6 Hydrogen 5.13 Volatile Matter 32.0 Oxygen 8.78 Fixed Carbon 46.2 Nitrogen 1.29 Moisture 11.2 Sulfur 2.13 Table 2. Chemical and Thermo Physical Proerties of the Fuel Figure 5. Geometry Overview Gas arameters Symbol Units Total Primary Air (mass flow) ṁ1 lb/hr Secondary Air (mass flow) ṁ2 lb/hr Oerating ressure P sia 14.7 PA oer. Temerature T1 F 160 SA oer. Temerature T2 F 550 Coal (mass flow) ṁ3 tons/hr Secific heat caacity C J/kg K PWL (6) Thermal conductivity k W/m K Table 3. Physical Proerties of the Gas Figure 6. Mesh Detail 5

6 The CFD model was set u with mass flow inlet boundary conditions at the rimary and secondary air inlets. The outlet side of the chamber was defined with a constant ressure boundary condition. The combustion chamber and lance walls were secified as rigid with no-sli and adiabatic conditions. Throughout all simulations the following models were included: k-ε Realizable Turbulence Model, DPM for LaGrangian tracking of coal articles and urea drolets, and Secies Transort Model to include combustion of coal based on volumetric reaction rates and urea reaction with NO x secies created during combustion. Sray attern is redicted by tracing drolets trajectories in the simulation. In the DPM model, a drolet s trajectory is obtained by numerical integration of Newton s second law: du (6) = M / ρ + g dt Interfacial force density accounts for the couling between the movement of gas and the liquid drolet; and g is the gravitational acceleration. In the flow analyzed there, the drag force is a rimary mechanism of gas-drolet interaction, which can be comuted as: 18µ CD Re (7) M = (U U) 2 D 24 where U is the gas viscosity, D is the drolet diameter, Re is the drolet Reynolds number and C D is the drag coefficient. The drag coefficient was comuted from the law of Moris and Alexander [7]. After a drolet s velocity is solved, its location can be integrated out from the trajectory equation: dx (8) = U dt Initial states (velocity, injection direction) of liquid drolets from the sray nozzle are determined by lab exeriments and are given as inuts in the simulation. The Rosin-Rammler distribution is used in the simulation to reresent the drolet size variation [8]. The gas hase, combustion and article tracking were erformed in steady state. The P-1 radiation model was used due to the high temeratures associated with the combustion. This is the simlest formulation of the more general P-N radiation model, which is based on the exansion of the radiation intensity I into an orthogonal series of sherical harmonics [9]. RESULTS & DISCUSSION Results Many different simulations have been made to determine where and how to lace the injectors to achieve otimal NO x reduction. Fig. 7 shows the gas flow athlines and exected travel of the coal articulate. From this, area of recirculation and high swirl can be avoided as otential injection locations. Figure 7. Gas Flow Pathlines 6

7 RESULTS & DISCUSSION Gas velocity is another imortant factor in determining locations of injections inside of a large system. Velocity is exected to control article tracks due to the relatively high velocity of the gas flow comared to the momentum of the drolet injected into the system. Gas velocity is shown in Fig. 8 below. Temerature K Gas Velocity m/s Figure 9. Temerature Profiles 10 0 Figure 8. Velocity Profiles Velocity eaks are found at the rimary and secondary air inlets. The velocity magnitude and turbulence remain fairly low above the burners. There is one elevated velocity area with recirculation directly above the burners (gas flow inlet). For this reason the injectors were located outside of the recirculation zone. Another imortant factor for efficient NO x removal is temerature. The reaction between reagent and NO x should occur within a temerature range of 900 C and 1,100 C deending on the reagent and condition of SNCR oeration [1]. Above this range there may be reductions in efficiency due to the thermal decomosition of ammonia. Below this temerature range, sli or carryover may increase. Temerature rofiles are shown in Fig. 9. The injection locations were selected based on temerature rofiles. Injector locations were located in areas of desirable temerature and velocity rofiles. Some restrictions on injector locations were set based on hysical constraints and accessibility. A simulation was erformed to evaluate the NO x levels rior to the reagent injection. Fig. 10 contains the NO x rofiles calculated by the simulation. The outlet surface is used to determine the aroximate NO x levels that could be exected to be exhausted. The average NO x level across the outlet is 156.3mvd. NOx levels mvd Figure 10. NOx Level Profiles 7

8 RESULTS & DISCUSSION Fig. 11 shows the injection locations and article tracking. Particles are colored based on dro size. In this simulation, the injections are comrised of 93% water and 7% urea. Liquid water concentration contours and article tracking results indicated that all sray articles are evaorating without a contact with walls. Evidence of this can be seen from evaluation of the injection tracking as shown in fig. 11. Though the injections are all oerating at the same arameters, injection lumes vary based on variations in temerature and velocity rofiles of the environment. NOx levels mvd Particle Size (µm) Figure 11. Injection Tracking Dro Size Post analysis was erformed to determine the imact of the injected reagent on temerature and NO x rofiles. The results of these simulations are contained in fig Temerature K Figure 13. NOx Profiles Post Urea Injection The resulting temerature rofile indicates a slight reduction in temeratures in the injection region. The water injected was found to be 100% evaorated rior to the turn into the exit. There was found to be no contact with the walls in the combustion chamber, from both the water and urea agent. No foreseeable areas of recirculation indicate that there is essential no otential issues with wall wetting or wall buildu/ damage. The resulting NO x values indicate a favorable result in NO x reduction. The outlet surface is used to determine the aroximate NO x levels that could be exected to be exhausted with urea injections. The average NO x level across the outlet is 87.5mvd with urea injections. This would indicate an aroximate 44% reduction in NO x levels. This would indicate a fairly efficient reduction based on the inut level of the urea. The outlet surface was also examined to determine the exected carryover of urea. The value of carryover was calculated to be 2.325e-21kg/s through the outlet. This would indicate that carryover is negligible. 533 Figure 12. Temerature Profiles Post Urea Injection 8

9 Conclusion Comutational Fluid Dynamics (CFD) was used to otimize injection location and orientation in a coal combustion alication. Through a case study, many different factors which imact the NO x reduction were quickly investigated and rioritized. The aim of this study was to demonstrate effectiveness of alying CFD tools in such alications to reduce risk and evaluate environmental imact of designs. References 1. IEA Clean Coal Centre, Mar Abrams, J.Z., Baldwin, A.L., Higgins, S., Rubin A.G., Demonstration of Bechtel s Confined Zone Disersion Process at Pennsylvania Electric Comany s Seward Station, International Power Generation Conference, October 1991, San Diego, CA. ASME Paer No: 91-JPGC-EC Bachalo, W.D. and Houser, M.J., Phase Doler Sray Analyzer for Simultaneous Measurements of Dro Size and Velocity Distributions, Otical Engineering, Volume 23, Number 5, Setember-October, Bachalo, W.D. and Houser, M.J., Sray Dro Size and Velocity Measurements Using the Phase/Doler Particle Analyzer, Proceedings of the ICLASS (3rd Intl.), July R. J. Schick, A Guide to Dro Size for Engineers, Sraying Systems Co. Bulletin E : Standard Terminology relating to liquid article statistics Annual Book of ASTM Standards, General Methods and Instrumentation, Volume Morsi, S.A. and Alexander, A.J., An Investigation of Particle Trajectories in Two-Phase Flow Systems, Journal of Fluent Mechanics, Vol. 55, , Li, G.L., Messah, H., Schick, R.J., Numerical Simulation of Sray Pattern in a Liquid Flashing Column, ILASS Americas 20th Annual Conference, May 2007, Chicago, IL. 9. Fluent Inc. Fluent 6.3 User s Guide, Lebanon, NH,