I.- Ping Chung, PhD, is a senior development engineer in the Technology and Commercial Development Group at
3.4 Burners for Package Boilers
Package boilers by definition are substantially prefab- ricated and shop-assembled systems with a maximum size allowing shipment by some common means of transportation and with a minimum field installation cost. With rare exceptions, these boilers are designed for a single burner. The thermal capacity of burners varies from about 30 × 106 to 350 × 106 Btu/h (∼9 to 100 MW) that corresponds to about 25,000 to 250,000 lb/h (11 to 113 metric-ton/h) of steam production. There are several package boiler design configurations with respect to the layout of the drums, tubes, and radiant and convection section of the boiler. Package boilers may produce a relatively low-pressure saturated steam at 150–250 psig (10–17 barg) pressure or high-pressure superheated steam of about 600–1500 psig (∼40 to 100 barg) for driving steam turbines.
These boilers are typically designed for burning dif- ferent gaseous fuels, oil, and on occasion some fine coal or biomass. The radiant part of the boiler has a close to parallelepiped shape with the width varying from 5.5 ft to about 12 ft (1.5 to 3 m), the height varying from 7 to 12 ft (2 to 3.5 m), and the length of 15 to 40 ft (4.5 to 12 m).
In modern boilers, the furnace walls and their tops are usually formed by sheets of tubes each with two fins welded together forming a continuous membrane wall.
Older types of boilers were designed with walls formed by the tubes placed side by side without gaps and not connected by welding—tangent tubes walls.
The furnace space heat release at capacity varies from
∼40,000 to ∼120,000 Btu/(h-ft3) (400 to 1200 kW/m3) with the lower numbers more characteristic for small boilers.
These numbers can be approximately converted to an average residence time for the species passing through the furnace in the range from 0.5 to 1.0 s when fired at capacity. However, the actual residence time of a sub- stantial portion of the flow moving through the furnace will be much higher than average velocities resulting in lower residence times for this portion. The heat release per the unit of furnace cross section varies from 0.65 to 3.0 × 106 Btu/(h-ft2) (about 2–6 MW/m2). Boiler fur- naces usually operate at positive pressure defined by the hydraulic resistance of the boiler convection sec- tion including the economizer. The range of furnace pressure at peak loads varies from as little as 3 in. W.C.
(0.75 kPa) to as high as 1 psig (7 kPa).
Package boilers are also widely used in marine applica- tions. Marine package boilers are usually more compact than stationary boilers, fire only oil, and require very compact flames. The aforementioned considerations are important when determining the characteristic residence time of species inside the flame affecting the maximum allowable size of the evaporating fuel droplets.
Knowing the residence time and furnace pressure is also important for estimating the kinetic/mixing energy necessary to be transferred by the burner to the reacting gases entering the combustion zone.
Knowing the type of furnace walls, membrane or tan- gent type, has some relevance to the type of burner to be selected and regime of its operation. Tangent wall boilers are prone to short circuiting—the effects of a small portion of furnace gas surrounding the flame to pass between the tangent tubes into the convection sec- tion carrying with it a small amount of unburned fuel (UHC) and a CO. Controlling the UHC and CO emis- sions from tangent tube boilers often requires operation with elevated levels of excess air helping to reduce the concentration of UHC and CO reaching the tangent tube walls and selection of burners generating more compact flames—typically not very low-NOx burners unless of the premixed combustion type.
Firing into the relatively limited space of a package boiler often raises concerns regarding actual or per- ceived flame impingement on the boiler walls. The issue is important here because it is somewhat subjec- tive and often a source of debate when something goes wrong. The impingement is really defined only by its consequences.
The main concern of flame impingement is creating such a high heat flux onto the tube surface that the boil- ing process inside the tube starts developing a film of steam on the internal tube surface impeding tube cool- ing with otherwise two-phase steam-water flow. In boilers with properly designed water circulation and
good water quality, however, a soft impingement of vis- ible flame tails onto the tubes does not create a strong enough heat flux that causes any problems.
The other and more often occurring result of the flame being in contact with the tubes is soot deposition on the walls. The soot can be formed when products of incom- plete combustion containing carbon atoms or aggregates get in contact with the relatively low-temperature tubes.
The buildup of soot on the surfaces is usually limited as soot has a low thermal conductivity, so the conduc- tion heat transfer is low and that the surface of a thick soot layer acquires a higher temperature that causes the soot to oxidize. The effect, however, is still unwanted as it will interfere with the overall heat transfer from the flame to water the tubes and may create an accumula- tion of soot that will fall off onto the furnace floor.
The situation is different when tubes of the super- heater are experiencing impingement. The super- heated steam does not have the same cooling intensity as the two-phase flow even when it moves with high velocity. The superheater tubes may actually overheat even without flame impingement by fully oxidized high-temperature combustion products that still emit substantial thermal radiation that visually can be mis- taken as flame impingement. It is more often that burn- ers making longer flames or burners operating with FGR (higher mass flow flames) will generate higher temperatures in the area of the superheater at the back of the furnace. Burner suppliers need to be aware if there are some specific requirements for the temper- ature in the area of the superheater and whether the boiler is equipped with the proper means of control- ling it. Problems like this usually happen when an old burner needs to be replaced with a new low-NOx burner using FGR or when a new burner generates a more transparent (lower luminosity) flame.
3.4.1 Conventional round Burners and Burners with air Staging
Round burners are the most common burners histori- cally used in package boilers. The term refers to the burners with a predominantly round shape of the com- bustion air passage or several concentric air passages for combustion air through the burner. The burners are typically equipped with one or two sets of adjustable or fixed register louvers placed at the air inlet into the burner for imposing a swirling motion to the combus- tion air and a bluff body or another swirler/spinner placed at the burner centerline close to its exiting plane.
With all these features, the air-flow pattern through the burner can be described predominantly as 2D, with all velocity vectors a function of only radial and axial coor- dinates, but not the angular/circumference coordinate.
The presence of the louvers or spinner blades, or slots
in the flame-stabilizing shields, creates some patterns of periodic disturbance over the flow circumference, but these patterns are usually weak, fast dissipating, and not always essential for the burner operation.
The very first round burner designs had a predomi- nantly uniform distribution of fuel over the burner cir- cumference and were designed for high stability and flame intensity. The burners trace their lineage back to the days of ship-mounted boilers, where boilers had to be kept as small and light as possible, thus making the requirement for short flames very important. Without these boilers in operation, a ship could be left dead in the water resulting in the need for the burners to reliably operate regardless of changes to the fuel or air supply.
The maneuvering demands of the ship also require that these burners be able to operate across a wide range of firing rates and to be able to operate within a wide range of air to fuel ratios essential for maintaining good flame stability even with a very crude setting of single-point positioning combustion controls and deviations in fuel pressure, etc., explained in Volume 2, Chapter 2.
To accomplish all of these goals, the design resulted in burners that mix air and fuel and ignite the mixture as rapidly as possible to create a highly stable and compact flame. A strong swirling motion imposed on the flow facilitated the mixing and compactness of the flame. For operation on gaseous fuels, the fuel could be injected through a gas ring surrounding the burner flow exit- ing cross section at the entrance to the flared refractory throat. The gas rings were drilled with a series of holes grouped in small clusters injecting fuel in the direc- tion of about 45°–60° to the burner centerline. Other gas injection methods were also used. For oil firing, the burner was equipped with an atomizer placed through the center of the flame-stabilizing shield.
Register-type swirling devices and some spinners often had a tendency to produce an overly excessive swirling motion at the burner centerline and starve this region on air or create backflows. This effect was unde- sirable, especially for oil firing when fuel was injected from the point close to the burner centerline causing oil gun coking and other problems. To avoid the effects of over-swirling, some burners were designed with a small air zone surrounding the atomizer that did not have a swirling motion.
The overall swirling and burning of the fuel–air mix- ture typically exits the burner through a conical refrac- tory throat with a length of about 50% of the burner diameter. This divergent throat helps to further facili- tate the development of a large-size recirculation zone in the wake of the spinner delivering hot combustion products to the area of ignition at a location close to the fuel injection. The glowing parts of the throat also provided a source of ignition for the peripheral parts of the flow.
The Coen DAZ™burner—dual air zones burner sche- matically shown on Figure 3.5—is an example of such a burner. The burner has two sets of adjustable register- type louvers typically set for spinning the flow in oppo- site directions, achieving very intense stable flames of the necessary shape, operating with a wide range of air–fuel ratios, and making it a perfect burner at the time for many applications including marine and pack- age boilers. When the burner was not in use, both sets of louvers could be closed to reduce the draft through the burner.
Applying the burner to stationary boilers firing natu- ral gas with reduced NOx requirements was found to be difficult as the burner generates relatively high NOx emissions of about 70–160 ppm (ref. 3% O2 dry) on natu- ral gas and its ability to tolerate FGR was very poor. The problem stemmed from the pattern of gas injection from the periphery of the air flow.
A new class of low-NOx round burners appeared then as a first response to the demand of simplifying the burner design that also allowed operation with reduced NOx emissions on gas and oil in boilers. These burners were equipped with multiple fuel gas injectors positioned around the spinner or a bluff body provid- ing patterns of fuel gas injection beneficial for operation with FGR or patterns delivering delayed mixing with air—air staging. The following are a few examples of modern air-staged low-NOx burners for package boilers designed to fire both gaseous and liquid fuels.
A Variflame™ burner shown in Figure 3.6 features a venturi-shaped passage for the air flow, providing a well-balanced air-flow pattern just upstream of the combustion zone and two sets of gas fuel injectors con- nected to the same fuel plenum. The first smaller set
delivers fuel directly into the wake of a large spinner developing where the overall flame is stabilized. The second main set placed adjacent to the spinner pro- vides fast initial mixing of fuel and air at each injec- tion point and delayed mixing further in the main body of the flame. The number of main fuel injec- tors varies from 3 to 8 depending on the application.
Another feature of this burner is that a small portion of combustion air can be supplied around the venturi passage. The amount of this flow can be adjusted. This flow diversion around the venturi has a strong effect on the burner draft loss and the overall shape of the flame. The burner is simple and efficient and can be optimized for operation with FGR.
Figure 3.7 shows another air-staged burner with both types of fuel injectors placed around the spinner—the
Figure 3.6
Coen Variflame™ burner.
Throat
Gas ring
Pilot
Flat blade stabilizer and oil atomizer
Figure 3.5 Coen DAZ™ burner.
Division rolling
Low NOx baffles
Swirler
Main gas nozzle
Pilot gas nozzle
Figure 3.7
Hamworthy DFL® burner.
DFL® burner. On the air side, the burner design includes multiple concentric passages some with the fixed turning vanes and a concentric bluff body filled with ceramic. When properly applied, the burner generates low baseline NOx and is capable of operating with low excess air and low CO emissions.
Figure 3.8 shows a picture of one of the most versa- tile air-staged burners for a package boiler—the Coen DAF™ burner. The burner is mechanically more com- plicated than the Variflame burner. It has two major air passages and the ability for external adjustments to the air distribution between the passages. It has a set of externally adjustable louvers—register—that control the swirling motion imposed onto the air flow in either the clockwise or the counterclockwise direction. A set of fuel injectors around the spinner is connected to the external or internal gas ring header. If the burner needs to fire multiple gas fuels separately or in combination, additional sets of fuel injectors and ring gas headers are added. The fuel injectors deliver a small portion of fuel into the wake of the spinner while most of the fuel is delivered in several lobes to gradually mix with the combustion air. The drilling pattern of all fuel injectors is usually customized for each application. The avail- able substantial adjustments to the air flow allow effec- tive additional control over the flame pattern as well as the ability to detune the operation from some unwanted acoustical interactions with the system. The burner can also operate with good stability with the FGR rates on natural gas up to 20% with the NOx emissions reduced to 25–30 ppm level. When firing oil, the maximum amount of FGR is limited to 15% with a corresponding reduction in thermal NOx of about 65%.
3.4.2 Burners with Fuel Staging
Figures 3.9 and 3.10 provide examples of fuel-staged burners: the Coen Delta-NOx™ burner and the Ham- worthy ECOjet® burner. In both burners, the combustion air is delivered through a single venturi-shaped pas- sage, and the staged fuel is injected into the flame from a series of fuel injectors placed outside of the diverging refractory throat.
In the Delta-NOx burner, the primary fuel is delivered through a series of small gas headers—spuds, placed in radial directions around a centrally located spinner, or
Figure 3.8 Coen DAF™ burner.
Figure 3.9
Coen Delta-NOx™ burner.
Figure 3.10
ECOjet® gas-only burners.
a combination of a bluff body and a spinner. Fuel injec- tion from each spud is distributed along the radius.
The Delta-NOx burner ignition points are stabilized by the action of the spinner or the bluff body and by the fuel jet arrangement that generates flame patterns suffi- cient to ignite the staged fuel at some distance from the burner refractory throat. When firing natural gas with- out FGR, the burner delivers NOx performance in the range of 45–60 ppm depending on the relative furnace to the heat input size. With about 10% FGR, the emis- sions can be typically reduced by about 45%.
In the ECOjet burner, all of the primary fuel is deliv- ered through a center-fired gas gun. The primary fuel forms a highly stable primary flame zone. The flame is stabilized by the action of the spinner. The primary zone is large enough to provide ignition of the staged fuel combustion zone. Figure 3.11 shows the burner flame when operating on natural gas without the FGR.
Both burner designs are very simple and are usually designed with an external means of adjusting the air distribution between the zones. The NOx performance of the ECOjet burner is slightly better than that of the Delta-NOx burner but less forgiving to the deviations in the excess air ratio. In some cases, the ECOjet burner per- formance on natural gas can achieve 10–12 ppm NOx.
3.4.3 Burners with Partial lean Premixed Combustion
A description of burners utilizing partial lean premixed combustion is given here using the Coen QLN™ burner as a classic example. Partial lean premixed combustion
is fundamentally different than partial premixed fuel- rich combustion, the most common example of which is utilized in gas burners for all cooking ranges. The burner is schematically shown in Figure 3.12. The burner was the first in the industry to generate simu- lated lean premixed combustion in a substantial part of the flame. The burner was developed in the early 1990s15–17 with a goal to efficiently fire natural gas in package boilers with less than 30 ppm NOx without using FGR. It had several new revolutionary features.
Visually, the main unique feature is an air distribu- tion plate or a number of bluff bodies shaping the
(a) (b)
Figure 3.11
(a) Hamworthy ECOjet® flame. (b) Natural gas firing at 30 MW (100 × 106 Btu/h) at Hamworthy test facility.
Core gas Premixed
gas
Staged gas Throat
Figure 3.12
Schematic of a Coen QLN™ burner.
combustion air flow in a series of radially aligned pas- sages separated by bluff bodies and spaced uniformly around another centrally located bluff body, whereas in all prior burners for package boilers, the air flow exiting the burner was predominantly circumferen- tially uniform. Another departure from convention at the time was the injection of fuel into the middle of the air-flow streams at a substantial distance from the pos- sible area of fuel ignition to form a uniform very fuel lean mixture prior to its possible sources of ignition.
It was known at the time that such premixing would result in a high-intensity acoustic response; however, the design of the QLN burner solved the problem with desynchronizing these instabilities and reducing them to well below the would-be critical levels.
The burner has three zones of fuel delivery: the flame-stabilization zone that burns a small portion of fuel in the wake of the bluff bodies, the premixed zone, and the staged fuel zone that gets the majority of its fuel from injectors placed through the ports of the short straight cylindrical refractory throat. The oxida- tion of the premixed fuel and staged fuel takes place gradually as the premixed fuel and air mix with hot combustion products from the flame-stabilization zone.
The oxidation of the staged fuel also takes place very gradually as it is first mixed with medium-temperature low-oxygen combustion products around the flame (FGC effect described in Section 3.3.5) and then mixes with the remaining oxygen of the products of premixed combustion. In the QLN flame, most of the fuel burns at locally fuel lean or close to stoichiometric conditions minimizing the production of prompt NOx. The ther- mal NOx is also substantially reduced due to very fuel lean conditions for burning the premixed fuel and FGC effect created by the staged fuel.
The NOx emissions with QLN burners depend on the furnace size and the burner peak heat input. In small boilers rated to less than 75,000 lb/h (34,000 kg/h) of steam production (∼32 ton/h) firing natural gas, QLN burners achieve typically less than 25 ppm of NOx with low excess air and no FGR. QLN burners in larger boilers with a capacity of up to 100,000 lb/h (45 m-t/h or 45,000 kg/h) of steam emit higher NOx up to about 40–45 ppm.
A small amount of induced FGR can be used in QLN burners to reduce NOx emissions by about 20%–25%.
Better results, down to 15 ppm NOx, can be achieved with forced FGR delivered to the plenum around the burner and injected into the furnace around the staged fuel injectors. A typical appearance of a QLN flame is shown in Figure 3.13.
QLN burners can also fire oil. The NOx emissions on oil are also reduced due to the effects of FGC enhanced by the star-shaped pattern of air injection. However, NOx reduction with the atomizer design by creating the
air-staging effects is reduced as only a narrow range of atomizer designs can be applied with the burner.
Over almost 20 years of its use to date, the QLN was successfully adapted for applications with multiple fuels firing simultaneously or one at a time proving to be a very versatile and highly tunable to the specifics of the application. The product always delivered the expected low NOx performance. QLN burners utilizing small amounts of forced FGR can operate with NOx emissions down to 15 ppm level in small- or medium-size package boilers and in larger package boilers with up to 250,000 lb/h (113,000 kg/h) steam production if the furnace of the boiler is slightly oversized.
3.4.4 Premixed Burners
Burners with all premixed and predominantly premixed combustion were developed in middle of the 1990s. The first burner of this kind utilized rapid mixing of filtered fuel gas passing through hundreds of tiny laser-punched orifices uniformly distributed through the flow of combus- tion air mixed with flue gas. The burner name, the RMB™ burner, is an abbreviation for rapid mix burner. The fuel dis- tribution in the RMB was accomplished using a multitude of radially placed airfoil-shaped miniature fuel gas risers imposing a swirling action onto the combined flow of air fuel and flue gas.18 The flame was stabilized by a strong recirculating flow of hot combustion products developed as the result of swirling flow in combination with a diverg- ing refractory throat and another refractory bluff body at the burner centerline. Figure 3.14 shows an overall view
Figure 3.13
Coen QLN burner flame with 20 ppm NOx firing natural gas without FGR.