Modeling of Combustion Systems A Practical Approach 10 ppsx

101 2 Introduction to Combustion Chapter Overview This chapter overviews some practical and theoretical aspects of combustion. We first discuss burners generically, including fuel and air metering, flame stabilization and shaping, and some fun- damental techniques for control of emissions such as NOx. So prepared, we move on to consider archetypical burners — those representative of the traditional classes of burners one might find in a refinery or petrochemicals plant, or that provide heat for steam. We build upon this foundation by next considering arche- typical process units such as boilers, process heaters of various types, and reactors such as hydrogen reformers and cracking units. In order to lay the groundwork for more detailed combustion modeling, the chapter considers important combustion-related responses such as NOx emissions, flame length, noise, etc., and the factors that influence them. Historically, practitioners have defined a traditional test protocol for quantifying these effects, which we present. We also consider some aspects of thermoacous- tic instability; this has become a more important topic with the advent of ultralow NOx burners employing very fuel lean flames. In the latter third of the chapter, we develop stoichiometric and mass balance relations in considerable mathematical detail. We also consider energy-related quantities such as heat and work, adiabatic flame temperature, and heat capacity. As well, we relate the practical consequences of a mechanical energy balance as applied to combustion equipment. Such things include draft pres- sure, incompressible airflow, compressible fuel flow, and practical representations thereof — capacity curves for air and fuel. © 2006 by Taylor & Francis Group, LLC 102 Modeling of Combustion Systems: A Practical Approach 2.1 General Overview Combustion is the self-sustaining reaction between a fuel and oxidizer charac- terized by a flame and the liberation of heat. Usually, but not always, the flame is visible. A flame is the reaction zone between fuel and oxidizer; it typically comprises steep thermal and chemical gradients — the flame is often only a millimeter or so thick. On one side of the flame, there is fuel and oxidizer at low temperature; on the other side are combustion products at high tempera- ture. Hydrogen and hydrocarbons in some combination are the typical fuels in the petrochemical and refining industries. Occasionally, due to some special refining operations, we find carbon monoxide in the fuel stream. Oxygen (in air) is the usual oxidizer. In practice, combustion reactions proceed to comple- tion with the fuel as the limiting reagent — that is, with air in excess. A burner is a device for safely controlling the combustion reaction. It is typically part of a larger enclosure known as a furnace. A process heater is any device that makes use of a flame and hot combustion products to pro- duce some product or prepare a feed stream for later reaction. Examples of such processes are the heating of crude oil in a crude unit, the production of hydrogen in a steam–methane catalytic reformer, and the production of eth- ylene in an ethylene cracking unit. A boiler is a device that makes use of a flame or hot combustion products to produce steam. The furnace is the portion of the process unit or boiler encompassing the flame. The radiant section comprises the furnace and process tubes with a view of the flame. In contrast, the convection section is the portion of the furnace that extracts heat to the process without a line of sight to the flame. Every industrial combus- tion process has some thermal source or sink. 2.1.1 The Burner A burner is a device for safely controlling the combustion reaction. A diffusion burner is one where fuel and air do not mix before entering the furnace. If fuel and air do mix before entering the furnace, then the device is a premix burner. Premix burners may mix all or some of the combustion air with the fuel. If one desires to distinguish between them, a partial premix burner is one that mixes only part of the combustion air, with the remainder provided later. Most pilots are of the partial premix type to ensure that they will light under high excess air conditions typical of furnace start-up. Diffusion burners supply most of the heating duty in refinery and boiler applications; therefore, we discuss them first. Figure 2.1 shows the main features for accomplishing this. The particular version of burner shown in Figure 2.1 is a natural draft burner. That is, a slight vacuum in the furnace (termed draft — 0.5 in. water column below atmospheric pressure is a typical figure) and a relatively large opening (burner throat) allow enough air to enter the combustion zone to © 2006 by Taylor & Francis Group, LLC Introduction to Combustion 103 support the full firing capacity. The diffusion burner comprises a fuel mani- fold, risers, tips, orifices, tile, plenum, throat restriction, and damper. Each diffu- sion burner type may differ in detailed construction, but all will possess these main functional parts. We discuss each in turn. 2.1.1.1 The Fuel System A fuel manifold is a device for distributing fuel. In the figure, one fuel inlet admits fuel to the manifold while several risers allow the fuel to exit. A riser is a fuel conduit. (In the boiler industry, risers are sometimes termed pokers, but the function is the same.) Each riser terminates in a tip; for some boiler burners the tip is called a poker shoe or just a shoe. A tip is a device designed to direct the fuel in a particular orientation and direction. It has one or more orifices, holes, or slots drilled at precise angles and size. An orifice is a small hole or slot that meters fuel — for a particular design fuel pressure, compo- sition, and temperature, the orifice restricts the flow to the specified rate. Together, these parts comprise the fuel system of a gas burner. FIGURE 2.1 A typical industrial burner. The typical industrial burner has many features, which can be classified in the following groups: air metering, fuel metering, flame stabilization, and emissions control. Refer to the text for a discussion of each. Burner Throat Primary Fuel Tip Flame Secondary Fuel Tips Burner Front Plate Cone (Throat Restriction) Inlet Damper Noise Muffler Refractory Tile Furnace Floor Fuel Risers Air Plenum © 2006 by Taylor & Francis Group, LLC 104 Modeling of Combustion Systems: A Practical Approach 2.1.1.2 About Fuels There are two main gaseous fuels for combustion processes: natural gas and refinery gas. Commercially available natural gas has a stable composition the other hand, is capable of considerably more variation. In a sense, refinery gas is the “garbage dump” for the gas products in the refinery. Generally, whatever the refinery cannot use for some higher value-added process it consumes as fuel. It is quite typical for refineries to specify several different refinery fuels for combustion equipment — one representing normal condi- tions, another representing a normal auxiliary case, and perhaps two or three upset scenarios. The scenarios will vary in hydrogen concentration, typically from 10 to 60%, giving the fuels quite different combustion properties. It is very important that the refinery weigh the likelihood of scenarios that represent widely varying heating values on a volumetric basis. For example, suppose a particular process unit can receive fuel according to four different scenarios: • Fuel A: 620 Btu/scf, normal fuel representing 10% of run time • Fuel B: 760 Btu/scf, stand-by fuel representing 84% of run time • Fuel C: 840 Btu/scf, start-up fuel representing 5.98% of run time • Fuel D: 309 Btu/scf, high hydrogen upset case representing 0.02% of run time Since fuel C is a start-up case, it does not matter how infrequently it occurs, the burners must operate on fuel C. However, the difference in volumetric flow rate among the fuels A, B, and C is small. On the other hand, fuel D represents a significant difference in hydrogen concentration. This will mark- edly affect major fuel properties such as flame speed, specific gravity, and flow rate through an orifice. Later, we shall develop the flow equations that show that fuel D represents the maximum flow (and maximum fuel pressure) condition. One can obtain a burner to meet all these fuel conditions. However, for fuels A, B, and C, the pressure will necessarily be lower. Some possible consequences are lower fuel momentum and “lazier” longer flames when the facility is not running fuel D. In severe cases, the flue gas momentum will control the flame path. Thus, flames may waft into process tubes and will be generally poorer in shape — all for the sake of preserving good operation for an operating case representing only 0.02% of the run time. A far better scenario would be to design the burner to handle fuels A, B, and C. Then fuel B is the maximum pressure case, but the facility will not have enough pressure to make maximum capacity with fuel D. Therefore, 99.98% of the time the flames will be fine and the unit will operate properly; 0.02% of the time the unit will not be able to fire the full firing rate. In the opposing scenario, the operators will struggle with the unit 99.98% of the time, and the burner will run optimally only 0.02% of the time. © 2006 by Taylor & Francis Group, LLC comprising mostly methane (see Appendix A, Table A.9). Refinery gas, on Introduction to Combustion 105 2.1.1.3 Fuel Metering A fuel control valve upstream of the burner generally does the fuel metering. A riser or poker delivers fuel to the burner tip. The facility specifies the pressure the burner will receive at maximum (design) firing rate. The burner manufacturer then sizes the fuel orifices in the tips to ensure that burner will meet the maximum fuel capacity at the specified conditions. The burner manufacturer provides a series of capacity curves (one for each fuel) that show the firing rate vs. the pressure. Figure 2.2 gives an example. In natural draft burners, air is generally the limiting factor, and its con- trolling resistance is the burner throat. Thus, the only way to increase the overall firing capacity of the burner is to increase the throat (i.e., burner) size. This may or may not be possible depending on the available space in the heater and if the heater can handle the extra flue gas and heat that result. If not, the entire unit may require modifications, not just the burner. 2.1.1.4 Turndown Burners operate best at their maximum capacity. One measure of the flame stability of a burner design is the turndown ratio. The turndown ratio is the FIGURE 2.2 A typical capacity curve. Fuel capacity curves give heat release as a function of fuel pressure. They are quite accurate for a given fuel composition. However, if there are a range of fuels, each needs its own capacity curve. This example shows three. The typical range is two to five fuel scenarios, all represented on the same graph. As the flow transitions to sonic, the capacity curve becomes linear. Fuel Pressure, bar(g) Heat Release, MW 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 subsonic flow sonic flow Fuel A Fuel B Fuel C transition to sonic flow © 2006 by Taylor & Francis Group, LLC 106 Modeling of Combustion Systems: A Practical Approach ratio of the full firing capacity to the actual firing capacity. The maximum turndown ratio is the max/min firing ratio. The turndown ratio is higher if one modulates the air in proportion to the fuel. If the air dampers are manually controlled, then one is interested in the maximum unmodulated turndown ratio, because this is the more conservative case. A typical maxi- mum turndown ratio for premix burners is 3:1. Diffusion burners often have turndown ratios of 5:1 without damper modulation, i.e., leaving the damper fully open despite lower fuel flow. One typically achieves 10:1 turndown ratios with automatic damper and fuel modulation. Multiple burner furnaces usually require a turndown ratio of somewhere between 3:1 and 5:1 with the air dampers fully open. To achieve greater turndown for the unit as a whole, one isolates some of the burners (fuel off, dampers closed). Turndown is easiest for a single fuel composition. Multiple fuel compositions always reduce the available pressure for some fuels and reduce the maximum turn- down ratio. 2.1.1.5 The Air System For natural draft burners, air is the limiting reactant. That is, sufficient fuel pressure is available to allow the burner to run at virtually any capacity, but only so much air will flow through the burner throat at the maximum draft. The burner throat refers to the minimum airflow area; it represents the con- trolling resistance to airflow. Therefore, the airside capacity of the burner determines the burner’s overall size. Air may enter from the side (as shown in Figure 2.1) or in line with the burner. Analogous to the fuel orifice, there are two metering devices for the airside. The first is the damper, upstream of the plenum. A damper assembly is a variable-area device used to meter the air to the burner. This is necessary because the maximum airside pressure drop is limited and the firing rate modulates. Therefore, one must modulate the air to maintain the air/fuel ratio. The damper may require manual adjustment, or one may automate it by means of an actuator. The inlet damper unavoidably creates turbulence and pressure fluctuations behind it. The plenum is the chamber that redistributes the combustion air- flow before allowing it to enter the burner throat. This redistribution does not need to be perfect, and there is a trade-off between uniform flow (larger plenum) and burner cost. In some cases, one may shorten the plenum by means of a turning vane — a curved device designed to redirect the airflow (not present in the figure). Burners are available in discrete standard sizes. To accommodate the infi- nite variety of potential capacities, manufacturers adjust the limiting airflow by means of a restriction in the burner throat. A choke ring is an annular blockage from the outside diameter inward. A baffle plate is a flat restriction originating from the center outward. A cone is an angled restriction originat- ing from the center of the burner throat. Figure 2.1 shows a cone, but baffle plates or choke rings are also very common. The purpose of the throat © 2006 by Taylor & Francis Group, LLC Introduction to Combustion 107 restriction is to provide a point of known minimum area and pressure drop characteristic. Some codes actually specify what portion of the draft (airside pressure drop across the burner) that the damper and the burner throat must take. For example, one guideline says that the burner must use 90% of the available pressure and take 75% of the pressure drop across the throat.* The 90/75 rule, as it has come to be known, aims to create turbulence in the burner throat rather than at the damper to aid fuel–air mixing. In the author’s opinion, this is a misguided approach and the industry should abandon it for several reasons. First, if the burner performs well, the degree of turbulent mixing in the throat is immaterial. Second, if the burner performs well, it does not matter where the pressure drop occurs so long as the total pressure drop is correct. Third, this rule increases the time and cost of burner testing. More importantly, changes to the burner for the sake of meeting the 90/75 rule may actually make the burner perform worse. Therefore, one is consuming resources to meet an essentially useless rule. Very often, the end user will have to clean or inspect tips while the furnace is running. In multiple-burner furnaces comprising many burners, there is usually little danger in shutting off a single burner. In petroleum refineries, the usual practice is to specify a burner having replaceable tips that do not require burner removal from the heater (of course, one must still shut off fuel flow to the individual burner before removing the tips). One must also take care to close the air damper during this procedure; otherwise, the furnace will admit tramp air. Tramp air is air admitted out of place. Air entering the furnace should participate fully in the combustion process, and tramp air enters the com- bustion process too late to oxidize the fuel properly. Tramp air may come not only through unfired burners, but also through leaks in the furnace. One possible sign of tramp air is a high CO reading even with supposedly sufficient excess oxygen. CO is a product of incomplete combustion. Depend- ing on the furnace temperature, 1 to 3% oxygen should represent enough excess air, and CO should be quite low under such conditions. But with tramp air, significant CO (>200 ppm) may still occur — even with 3% oxygen (or more) in the flue gas. The oxygen has entered the furnace somewhere, but it is not participating fully in the combustion reaction. As long as there is tramp air, the furnace will require higher oxygen levels — enough to provide both effectual air through the burner and ineffectual tramp air. In severe cases of tramp air leakage, even a wide-open damper cannot provide enough air to the burner and CO persists, though stack oxygen levels are 5% or more. The exit of the furnace radiant section is the relevant place to measure emissions for combustion purposes. The exit of the stack is the relevant place to measure emissions for compliance purposes. The stack exit is not generally useful for understanding what is happening in the combustion zone. * This requirement appears as a footnote to API Standard 560, Fired Heaters for General Refinery Service, 3rd ed., Washington, DC, American Petroleum Institute, May 2001, p. 65. © 2006 by Taylor & Francis Group, LLC 108 Modeling of Combustion Systems: A Practical Approach 2.1.1.6 The Flame Holder Anchoring the flame to the burner is essential for the sake of performance and safety. A flame holder is a device designed to keep the leading edge (root) of the flame stationary in space. There are many different devices for accom- plishing this. The most common device is the burner tile. A burner tile is a refractory flame holder designed to withstand the temperature of direct flame impingement. The tile ledge is the portion of the tile that anchors the flame (Figure 2.3). One type of flame holder is the bluff body — a nonstreamlined shape in the flow path — to present an obstruction to some of the flowing fuel–air mixture; the tile ledge qualifies. This obstruction generates a low-pressure, low-velocity zone at its trailing edge. The flame holder affects only a small portion of the flow, reducing its velocity to well below the flame speed. The velocity upstream of the flame holder is very low. Thus, the hot combustion products recirculate there, continually mixing fresh combustion products with an ignition source — the hot product gases. In this way, the flame holder anchors and stabilizes the flame over a wide turndown ratio. Figure 2.4 shows a 2-MW round-flame burner employing a tile-stabilized flame. The shape of the burner and position of the fuel ports mold the flame into the desired shape. 2.1.1.7 Stabilizing and Shaping the Flame A flame has a very fast but finite reaction rate. One measure of the reaction rate is the flame speed. The laminar flame speed is the flame propagation rate [L/θ] in a combustible mixture of quiescent fuel and air. If the air and fuel mixture exceed the flame speed, then the flame will travel in the direction of the stream — a phenomenon known as liftoff. If the liftoff continues, the FIGURE 2.3 The tile ledge as flame holder. (From Baukal, C.E., Jr., Ed., The John Zink Combustion Handbook, CRC Press, Boca Raton, FL, 2001.) © 2006 by Taylor & Francis Group, LLC Introduction to Combustion 109 flame will be transported to a region of high flue gas concentration that cannot support combustion and the flame will extinguish. Air and fuel velocities in a typical industrial burner far exceed laminar flame speeds. But in the vicinity of the bluff body, the fluid speed is low; therefore, the flame anchors over wide turndown range and the burner is quite stable throughout its entire operation. 2.1.1.8 Controlling Emissions In the past, the only emission of concern was CO because it indicated incom- plete combustion, combustion inefficiency, or a safety hazard. Nowadays, life is more complex and other emissions such as nitric oxides are important due to their role in the formation of ground-level ozone and photochemical smog. Technically, noise is also a regulated emission (e.g., <85 dBA). Emis- sions control is an active subject of interest. Staging the combustion into distinct zones is one strategy (termed staging) to lower certain emissions such as NOx. If NOx emissions are not a concern, then the secondary fuel tips of Figure 2.1 may be unnecessary and the burner carries out combustion using only primary fuel tips. 2.1.2 Archetypical Burners When taxonomists classify things, they speak of slots and filler. Slots are the classes or categories, and filler is the stuff that populates the class. With respect to major considerations that affect burner design, we shall list five: FIGURE 2.4 A gas burner in operation. © 2006 by Taylor & Francis Group, LLC 110 Modeling of Combustion Systems: A Practical Approach • Fuel state: – Gas – Liquid – Solid • Flame shape: – Round – Flat • Fuel–air mixing strategy: – Fuel and air premixed (premix burner) – Separately metered fuel and air (diffusion burner) • Firing orientation – Upfired (the burner fires from the floor upward) – Downfired (the burner fires from the roof downward) – Side-fired (the burner fires from the wall sideway) • Emissions – The burner design reduces combustion-related emissions. – The burner design has no special features for reducing combus- tion-related emissions. These five characteristics generally fix the burner design, and they will define an archetypical burner. Neglecting solid-fired burners for now (e.g., wood, municipal solid waste, pulverized coal, etc.), each of the above cate- gories has two possibilities, except for firing orientation, which has three. This leads to 2 4 ·3 = 48 different slots. However, as is typical, not every slot has a filler, and some burner models fill more than one slot. For our purposes, about a dozen burner types are of importance. We should add that there are many kinds of esoteric designs for special reactions, but as regards traditional how one burner manufacturer has chosen to fill them. It is, of course, possible to entertain other considerations. For example, • Type of draft: Is the motive force for air due to natural convection in the heater (natural draft), from a fan outlet upstream of the burner (forced draft), from a fan inlet downstream of the burner (induced draft), or both inlet and outlet fans (balanced draft)? • More fuel-state variations. Will the burner fire liquid and gaseous fuels at the same time or separately? • Service: Will the heater serve in a boiler to generate steam, or will it serve in a process heater to refine petroleum or make petrochemicals? However, for the most part, design variants of the enumerated burner types will accommodate all of the above categories. So, we will describe the © 2006 by Taylor & Francis Group, LLC fuel–air combustion, these categories will do. Table 2.1 shows the slots and [...]... exit An advantage of this © 2006 by Taylor & Francis Group, LLC 112 Modeling of Combustion Systems: A Practical Approach Secondary air Primary air Pilot Gas FIGURE 2.5 A gas premix floor burner (Courtesy of the American Petroleum Institute, Washington, DC.) arrangement is that increased fuel flow results in increased airflow However, premixed burners also have major disadvantages in the wrong application... diameter — smaller orifices are more effective than larger ones Appendix A, Table A. 4 gives critical diameters and minimum slot widths for various fuels Generally, for a given area, a circular orifice is more efficient for quenching flames than a rectangular one However, smaller rectangular slots may have manufacturing advantages In practice, both geometries are commercially available The removal of heat... Francis Group, LLC 132 Modeling of Combustion Systems: A Practical Approach acts as a piston and the furnace volume as a spring Equation 2 .10 represents such a case: ν= 1 ⎛ AS ⎞ ⎛ γRT ⎞ c AS ⎜ V L ⎟ ⎜ W ⎟ = 2 π V L Helmholtz resonator 2π ⎝ F S ⎠ ⎝ ⎠ F S (2 .10) where AS is the cross-sectional area of the stack, LS is the length of the stack, and VS is the volume of the furnace For a cylindrical furnace... and Figure 2 .10 show, the fuels travel down a central riser; the slotted tip projects the fuel in a radial plane parallel to the wall Preheated air comes through the large annular gap and enters the furnace in the same orientation At the high furnace temperatures, the separate fuel and air streams react to generate a flame with very low NOx and uniform radiation 2.1.2.4 Flat-Flame Premix Burners Flat-flame... compressed air for atomization This is the case if steam atomization could be detrimental or there is insufficient fuel oil pressure for mechanical atomization For © 2006 by Taylor & Francis Group, LLC 118 Modeling of Combustion Systems: A Practical Approach FIGURE 2.11 A flat-flame premix burner The flame heats the refractory, which in turn radiates heat to the process tubes inside the furnace Secondary air allows... concentration because the laminar flame speed of hydrogen is about three times that of hydrocarbons Various techniques moderate flashback One important consideration is the quench distance — a characteristic length for a given orifice geometry through which a flame cannot propagate For small-diameter orifices, the edges will abstract sufficient heat from a propagating flame to extinguish it The quench distance varies... acoustic wave superimposed on the mean flow will alternately enhance or attenuate the flow as the velocity alternates Now in the case of enhanced flow, the flue gas is colder than the mean temperature because it comprises a greater portion of cool influent air In the case of attenuated flow, the flue gas temperature is warmer than average because less cold air flows into the furnace during that period Clearly,... important for initial installation and because one requires sufficient room to extract, clean, and reinsert fuel oil guns, risers, and tips © 2006 by Taylor & Francis Group, LLC 120 Modeling of Combustion Systems: A Practical Approach REGEN TILE GAS RISERS (FOR COMBINATION FIRING) OIL GUN TERTIARY AIR CONTROL PRIMARY TILE GAS PILOT AIR INLET PLENUM PRIMARY AIR CONTROL SECONDARY AIR CONTROL GAS RISER MANIFOLD... In a refinery, they comprise hot-oil heaters, crude heaters, vacuum heaters, and the like A fired reactor is a process combustion unit © 2006 by Taylor & Francis Group, LLC 124 Modeling of Combustion Systems: A Practical Approach designed to effect some thermochemical transformation One major distinction among fired units is their shape Figure 2.1 8a to 2.18i gives a sampling of the many process heater and... the tubes and out the stack, transferring heat to surrounding water Nowadays, firetube boilers are generally smaller units generating saturated steam They are usually fully automatic and unattended These provide facility steam and heat for schools, hospitals, and other commercial needs The household water heater is a firetube configuration However, because it does not generate steam, it is not a boiler . quantities such as heat and work, adiabatic flame temperature, and heat capacity. As well, we relate the practical consequences of a mechanical energy balance as applied to combustion equipment Modeling of Combustion Systems: A Practical Approach 2.1 General Overview Combustion is the self-sustaining reaction between a fuel and oxidizer charac- terized by a flame and the liberation of heat Francis Group, LLC 104 Modeling of Combustion Systems: A Practical Approach 2.1.1.2 About Fuels There are two main gaseous fuels for combustion processes: natural gas and refinery gas. Commercially