A few examples of Industrial Problems that would benefit from Improved Understanding of Fluid Flow

Transcription

1 14 th Australasian Fluid Mechanics Conference Adelaide University, Adelaide, Australia December 2001 A few examples of Industrial Problems that would benefit from Improved Understanding of Fluid Flow Peter Mullinger Abstract Fluid flow plays an enormous, and underrated, role in industrial processes and its contribution is become more widely recognised. Topics such as flow in ducts, air distribution between burners, conveying of powders, mixing of fluids and powders, etc are becoming increasingly recognised as fluid flow problems. Many of these problems are not well understood, yet the cost of failure caused by incorrect design is very high. The author selects and describes a few industrial problems as examples where poor understanding of the fluid mechanical issues leads to poor or unpredictable performance and increased costs. Some of industry s needs could be met by better education, that is more effective application of existing knowledge, however achieving a better understanding of even a few of the more complex problems will not be easy and offers an exciting challenge to researchers. However the potential benefits from a better understanding of the issues are very significant and will often be applicable across a wide range of industries. Department of Chemical Engineering University of Adelaide, Adelaide, South Australia, 5005 AUSTRALIA Power Generation Power generation involves a wide range of fluid flow applications from simple airflow and gas flow issues to complex two-phase flow problems. A few examples will be considered below. Airflow Issues Most electricity is generated using fossil fuel fired steam turbine plant. Despite the increasing contribution of renewable energy sources and nuclear power, combustion of fossil fuel will continue to provide most of our electricity for many years ahead. Combustion takes place in the following four stages with the effectiveness of the fuel/air mixing determining the overall combustion rate and the heat transfer from the flame: Introduction Fluid flow plays an enormous, and often underrated, role in most industrial processes. It is only in recent years that the importance of understanding the contribution of fluid flow to industrial manufacturing processes has become more widely recognised. Topics such as flow in ducts, air distribution between burners, conveying of powders, mixing of fluids and powders, etc are becoming increasingly recognised as fluid flow problems. As a consequence of the lack of recognition of the importance of aerodynamics and hydrodynamics, the design of many of the items affected, such as ducting, pipework, etc is usually allocated to draught-persons with little or no knowledge of fluid mechanics. It is little wonder therefore that major problems have arisen from this approach. The need for improvements in this area is driven by economic factors because the cost of failure is so high. For example, a gas fired process air heater costing US$100,000 that was installed on a North Sea oilrig in the early 1980s failed as a consequence of flame impingement within 24 hours of start-up. It was out of service for over three months necessitating flaring of the associated gas. The consequential losses were in the order of US$100,000 per day. The problem was eventually traced to poor design of the combustion air ducting. A few splitters and turning vanes costing less than $5000 solved the problem! However the solution was only determined by extensive flow modelling. I shall try to illustrate the needs of industry for improved working knowledge of fluid mechanics by examining a few examples of fluid flow induced problems from some selected industries that I have experienced during my industrial career. Some of industry s needs could be met by better education, that is more effective application of existing knowledge, however to meet the remaining needs will require extensive research. Some of the problems are so complex that they may well defy a significantly improved understanding within the foreseeable future. Mixing > Ignition > Chemical Reaction > Dispersal of Products Both the mixing and the dispersal of the products are strongly dependent on the fluid flow patterns. The completeness of combustion and the emissions performance are critically dependent the way that the combustion air and fuel are supplied to the individual burners. For pulverised coal firing, which accounts for approximately 50% of power generation, aerodynamics also plays a crucial role in the fuel supply as is discussed below. Low NOx applications make special demands on the distribution of the combustion air supply, which typically needs to be controlled to within a tolerance of ±5% and ideally ±2%. An example of a typical low NOx burner is shown in figure 1. Its low NOx performance is dependent on the control of the airflow distribution between primary air, secondary air and tertiary air, all of which demands an excellent understanding of the aerodynamic flow patterns. Figure 1 Typical aerodynamics of a low NOx burner [1] 63

2 Large boiler installations require combustion air ducting to supply air to many burners, typically more that forty and often up to seventy individual burners. The limited space available usually require the ducting to follow tightly constrained routes and usually involves several bends, with the ducting splitting several ways to feed the individual burners, figure 2. This makes prediction of the mass flow of air to individual burners very difficult and prediction of the flow distribution within the burner virtually impossible. Sometimes flow balancing dampers are installed to permit adjustment of the flows with the objective of equalising the flows at the burners. This approach is fundamentally flawed because it is virtually impossible to measure the actual airflow at the burner and the adjustment of the flow to one burner affects the flow to all the other burners. Furthermore partially closed dampers cause a major disturbance of the flow that adversely affects the air flow distribution within individual burners. of two-phase flow that results from air conveying of the pulverised coal. Most power generation PF systems are lean phase, that is the ratio of air to coal is in the order of 2:1 kg/kg or greater. One of the major problems of pulverised coal conveying is that the coal is not evenly distributed in the duct. Typically 80% of the mass flow of the coal is concentrated in only 20% of the duct area [5]. This phenomena, known as roping, leads to great difficulties in ensuring equal distribution of coal to the different burners, even if the airflow is equally distributed. Attempts to achieve equal distribution leads to the use of complex junctions at ducting splits known as riffle boxes, figure 3. These are only a palliative solution because they are subject to intense wear with abrasive coal. Little research has been undertaken on the cause of roping and the phenomenon does not readily lend itself to modelling either physically or using CFD. Combustion air ducting Figure 3 Riffle box to promote even split of pulverised coal at conveying duct junctions [6] Figure 2 Typical power boiler combustion air ducting [2] Whilst, in principle such ducting can be modelled using either physical modelling or CFD modelling [3,4] and the design modified to optimise the air distribution, the sheer complexity of the ducting means that physical modelling is extremely time consuming and hence very expensive. On the other hand the effectiveness of CFD modelling of such complex systems is still limited by computing power. Furthermore the very high cost of modelling is a major deterrent to the use of the technique by boiler designers. Many of the flow distribution problems are created by poor design of bends and splits that set up an asymmetrical flow pattern, which then persists throughout the system. The need is for a simple design technique that allows designers to design bends and junctions so that the flow distribution remains uniform. Pulverised Coal Conveying Pulverised coal (PF) conveying suffers from all the problems associated combustion air ducting with the additional complexity Significantly different velocities have been observed between the particles and the conveying air. Frompovicz [5] stated that typically the particles move at approximately half the velocity of the conveying air stream no doubt as a consequence of the gravitational forces acting on the particles. This means that all attempts to measure the mass flowrate of the coal by measuring air velocity and density of the suspended particles are fundamentally flawed, especially when such measurements are made in vertical ducting. Despite all the research efforts lavished on clean coal technology such as fluid bed combustion and coal gasification as a means of increasing efficiency and improving environmental performance pulverised coal fired power stations are still being constructed and will continue to be used well past the middle of the century. Indeed many of today s existing units will be operational until at least 2040, so research in this area is well worthwhile. Jet Flow Burners are, in effect, a complex system of jets and the mixing behaviour of these jets largely determines the combustion performance both in terms of emissions and completeness of combustion. Whilst the performance of cylindrical co-annular jets has been extensively studied and is now relatively well understood [7] many PF burners consist of a series of rectangular jets, figure 4. Very little work has been undertaken on such jets, largely being confined to a study of the downstream velocity profiles [8] yet it is known by practical experience that quite 64

3 small changes in jet geometry can have a major effect on down stream behaviour [9]. The author and his co-workers in the CRC for Clean Power from Lignite have recently commenced work in this area. De-volatilising Particles Pulverised coal combustion involves the rapid de-volatilisation of the particles in the flame. Photographs of single particles burning show that the volatile combustion takes place in the wake behind the particle, figure 6. Figure 6 Single particle combustion of pitch/water mixture [11] Figure 4 Typical slot type power boiler burners Adjacent burners interact with each other as a consequence of both the interaction of the jets and the effects of transmitted and reflected radiation between the flames. This has the effect of increasing flame length and temperatures, figure 5. Again little research has been undertaken on the phenomena of the interaction of adjacent jets. It might be expected that such behaviour could affect the drag coefficient of the particle and in particular lead to a reduction in drag. Again little work has been done in this area but there is quantitative evidence from the cement industry that the drag coefficient of burning particle is lower than for non-burning particles. In rotary kilns the fuel/air mixing is controlled by jet entrainment, figure 7. The coal is injected at high velocity, ( m/s) with the primary air, a small proportion of the total combustion air, typically 8-20%. The remaining air or secondary air is introduced at a much lower velocity, typically 2-10 m/s. Figure 7 Principle of jet entrainment pulverised coal burners used in the cement industry[12] Moles & Jenkins [13] found that increasing the primary jet velocity firstly reduces the flame length as expected owing to better fuel/air mixing until it reaches a minimum length with further increases in velocity leading to a lengthening of the flame, figure 8. Figure 5 Effect of spacing of multiple burners on flame temperature [10] Since fuel/air mixing continues to improve with increasing primary air velocity the only possible explanations of the increasing flame length are that the particles are taking longer to burn or that their residence time in the early part of the flame is reduced. The reduction in residence time is by far the most likely reason for the longer flame and is probably caused by the burning coal particles continuing to travel at, or close to, their injection velocity rather than being slowed by drag forces to the bulk flow velocity (typically m/s).. 65

4 Figure 8 Effect of primary quantity and air/fuel injection velocity on flame length in a simple pulverised coal fired cement kiln [13] 1 Outer cone 2 Adjustable guide vanes, 3 oversize collection cone 4 Product outlet, 5 Vane adjuster 6 oversize outlet Minerals Processing Minerals processing encompasses a wide range of applications where solids are processed as suspensions in either air or gases or suspended in liquids. These processes include grinding & classification processes, powder mixing & blending, calcination, flotation processes, sedimentation, etc. The effectiveness and energy consumption of many of these processes is dependent on fluid mechanical issues. Figure 10a Typical stationary classifier [14] Grinding and Classification Grinding is used to reduce the size of hard materials such as bauxite, limestone, quartz, metal ores, etc. It is normally undertaken in water or air/gas swept mills and can be open or closed circuit, figure 9. Feed Grinding mill Open circuit milling system Product Feed Grinding mill Oversize return Closed circuit milling system Figure 9 Open and closed circuit milling systems Product With closed circuit mills the ground material is conveyed to a classifier where the oversize is separated from the product and returned to the mill for re-grinding. Classifiers are complex devices and may be either stationary or rotary, figure 10. Separation is achieved using a combination of the inertial, gravitational and drag forces on the particle. 1 Fan 2 Distribution disc 3 Oversize cone 4 Oversize outlet 5 Outer Cone Figure 10b Typical rotary classifier [15] 6 Counter vanes 7 Feed 8 Air slide 9 Collector 10 Product outlet Classifier performance controls the size distribution of the final product and its efficiency has a large influence on the energy consumption of the grinding circuit. Ideally a classifier would allow only the specified particle size to pass through and return only the oversize to the mill. In practice the cut is much wider than this and often a significant proportion of oversize passes the 66

5 classifier as product unless the cut is set unduly fine in which case excessive material is returned to the mill for re-grinding, increasing the power consumption and reducing the capacity of the milling circuit. With the power consumption of industrial milling circuits typically in the range of 1-5 MW with some mills up to 20MW the potential financial returns on improved classifier performance are significant. Little fundamental work has been undertaken on this topic, most of the knowledge arising from trial and error development undertaken by the commercial manufacturers. This is probably because the complexity of the process makes it difficult to study from a fundamental perspective and also difficult to model effectively. Without doubt physical modelling of classifiers is very difficult owing to the difficulty of scaling and maintaining dynamic similarity while the effectiveness of CFD modelling is still limited by computer performance for such a complex system. Powder Mixing & Blending Certain processes, such as cement manufacture require the blending of naturally variable materials in multi-component mixtures to close tolerances. Owing to the difficulty of blending powders this was often achieved in the past by grinding wet and mixing the resulting slurry. This method is no longer acceptable owing to the huge energy consumption involved in evaporating the water so the raw materials are now ground dry and blended in large concrete silos, figure 11. Compressed air is blown in though jets or distributor plates in the base to agitate the powder with the objective of mixing it thoroughly. The blending performance is defied by the homogenising effect: H = S in /S out Where: S in is the standard deviation of variability of the incoming feed S out is the standard deviation of the blended material A typical target for the homogenising effect is 8 to 10. However such effective blending is rarely achieved for example, a recent study of the performance of a large blending silo indicating that it had an homogenising effect of only [17]. As for classifiers, modelling the performance of blending silos is extremely challenging but would have significant benefits, both in terms of product quality and, for the cement industry, in significantly reduced kiln energy consumption. Calcination Calcination involves the use of heat to drive off gas or water to produce a stable oxide such as CaO (lime) or Al 2 O 3 (alumina). It is usually undertaken in rotary kilns, fixed beds, fluid beds or flash calciners. The choice of equipment depends on the raw material and product type, particle size, fuel available, etc. All processes involve combustion and its related fluid flow and mixing problems whilst both fluid beds and flash calciners involve devolatilising particles suspended in a hot gas stream. In the case of oil fired units both the fuel and product are devolatilising simultaneously! Extensive modelling of both combustion and fluid flow has already been undertaken on these units using both acid/alkali and CFD modelling, figure 12. [18] However investigations into the product flow and residence time are very limited and, to date, devolatilisation has not been accounted for at all. Chemical Manufacture Most chemical processes involve the transport and mixing of reactants, followed by the separation of products. In many cases the reactants and products are in a similar physical state ie all gases, all liquids, or all solids but often the systems are heterogeneous, ie gases and vapours reacting with solids, liquids reacting with solids, or gases reacting with solids. Again devolatilising particles often play a major role in the reaction and flow processes taking place in vessels. Figure 11 Typical continuous blending silo [16] One of the major fluid flow problems of interest is the flow distribution and residence time in vessels. Where solid materials are present as particles in the flow then the distribution and residence time of the particles will also be of interest, and in many cases of greater interest than the gas distribution and residence time. Brown [19] has shown that the vessel geometry and the inlet geometry have enormous and different influences on both the gas and particle residence time. It is clear that fluid flow has a controlling influence on many vessels and should be the major tool used in vessel design including the sizing, overall geometry, location of inlet and outlets and sizing of inlets and outlets (choice of velocity). Unfortunately this is rarely the case with geometry, inlets and outlets being designed on the basis of mechanical convenience rather than process flow needs. Where vessels have internal fittings such as trays, packing, catalyst beds, etc, then again fluid mechanics should play a major role in the design of the vessel. 67

6 Similarly separation processes, hydrodynamic performance of fluidised beds, residence time of gases and particles in vessels, etc. are all significantly influenced by fluid mechanics issues that should be taken into account during the design process. Acknowledgments I should like to thank the organisers of this conference for the opportunity to present this highly personal view of industry s needs. I should also like to thank those process plant operators who allowed my to implement many of the pioneering (from an industrial viewpoint) modelling techniques that my company employed. References [1]International Flame Research Foundation [2]Modern Power Station Practice, Volume 2, p 9, Pergamon Press, Oxford 1971 [3]Mullinger, PJ, The contribution of modelling to several industrial problems, Inst E. Midland Branch Symposium: Modelling Profitable applications to industrial processes [4]Warren, S & Jones, C, Practical utilisation of physical modelling at reduced scale, Inst E. Midland Branch Symposium: Modelling Profitable applications to industrial processes Acid/alkali modelling of fuel/air mixing Figure 12 Acid/alkali modelling of flash calciner Concluding Remarks Flow interpreted from water/bead modelling Recirculation zone The above examples are but a small selection of the many fluid mechanics related problems that I have experienced over the years. Better education has a role to play in providing industrial designers with greater understanding of the fluid mechanics issues that affect their work. There simply is not sufficient information available in a readily assimilated form on the topics of duct and vessel design or gas/particle interaction. For example a standard text on fluid flow for chemical engineers has a mere 16 pages on Fluid motion in the presence of particles [20]. Achieving a better understanding of even a few of these very complex problems will not be easy; indeed many of the problems described above offer an enormous challenge to researchers. However the potential benefits from a better understanding of the issues are very significant and will often be applicable across a wide range of industries. Hopefully this paper has raised the reader s awareness of a few of the many industrial problems that still await serious study, enjoy the challenge! [5]Frompovicz, ZJ, The New International Standard Method for Sampling Pulverised Coal, Current Developments in Solid Fuel Combustion Systems, Council of Industrial Boiler Owners, Cleveland, Ohio, May [6]Modern Power Station Practice, Volume 2, p 31, Pergamon Press, Oxford 1971 [7]Jenkins, BG, Moles, FD, Design of burners for kiln applications using modelling techniques, 5 th International rotary kiln conference, London 1988 [8]Perry, JH, & Pleasance, GE, The influence of nozzle geometry on the development of a three-jet isothermal flow field, 8 th Australasian Fluid Mechanics Conference, Newcastle, 1983 [9]Hart, J, Private communication 2001 [10]Apak, G, University of Sheffield report 1971 [11]Matthews, KJ, & Street, PJ, Observations on the combustion of pitch water fuel, Jnl. Inst. E, Sep 1988, pp [12]Mullinger, PJ, Rotary kiln firing: The problems and the solutions, 3 rd International rotary kiln conference, Ocho Rios, Jamaica, 1987 (Published by World Cement, London 1987) [13]Moles & Jenkins Private Communication to Rugby Cement, June 1977 [14]Helming, B, Cement Manufacture Vol 2 pp 104 [15]Helming, B, Cement Manufacture Vol 4 pp [16]Helming, B, Cement Manufacture Vol 2 pp 146 [17]Gorden, GD, Simulation of the Effect of Variability of Raw material composition on product Quality for a cement plant, Dept. of Chem. Eng., University of Adelaide,

7 [18]Jenkins B.G. & Mullinger P.J. Optimising pre calciner design and performance using combustion process modelling. World Cement, April [19]Brown G.J CFD Prediction of Particle and Fluid Residence Time in Industrial Vessels, 6 th World Congress of Chemical Engineering, Melbourne, Australia, 2001 [20]Holland, FA, & Bragg, R, Fluid flow for chemical engineers, 2 nd Edition, 1995, Edward Arnold, London 69

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