I.- Ping Chung, PhD, is a senior development engineer in the Technology and Commercial Development Group at
3.7 Gas and Oil Burners for Utility Boilers Steam cycles of fossil fuel fired utilities have much
3.7.1 Combustion Systems for Oil and gas Wall-Fired utility Boilers
The wall-fired utility boilers are usually equipped with a number of burners ranging from 6 to 36 that are placed on one or two opposing walls of the boiler. The furnaces are generously sized with a significant part of the fur- nace volume above the burner levels. A typical average residence time of the combustion products in the fur- nace at high fire is about 1–1.5 s. All boilers operate with air preheated to 450°F–650°F (230°C–340°C).
The operation of utility boilers is continuously closely monitored by the operators overseeing the performance.
Spinner spuds Gas
Circular gas header Heavy duty
rollers Air damper
cylinder
Insulated front plate damperAir
Figure 3.32
Schematic of Coen Delta Power™ burner.
(a) (b)
Figure 3.31
Flames of Coen DAF™ burner firing (a) natural gas and (b) syngas (baking soda was added to the airflow for better visualization of syngas flame).
When necessary, the operators can add biases to set- points of different controlled parameters, one of which is excess oxygen at the stack that normally is set in the range of 1%–2%. The boiler controls also monitor mul- tiple parameters and notify operators if anything devi- ates outside of the normal range.
Most of the wall-fired utility boilers were designed orig- inally for firing coal—a fuel producing high- temperature very luminous flames. Firing faster burning fuels like natural gas creates a different temperature pattern and more transparent flames with a typically higher rela- tive rate of heat transfer to the steam tubes in the area of the superheater and reheater and less in the saturated steam tubes zone. For the efficiency of the steam turbine cycle, it is important to maintain the design temperature of the steam coming out of the superheater and reheater.
Excessive temperature in the superheater can be con- trolled with water injection—also called attemperation.
However, attemperation in the reheater is very undesir- able as it changes the ratio of flows in different stages of the turbine and reduces the cycle efficiency. The conver- sion of utility boiler operation to gas shifts the heat dis- tribution to the superheater and the reheater requiring more attemperation and loss of efficiency or the inability to operate at full load due to tube overheating.
The use of forced FGR recirculation back into the fur- nace through a hopper on the bottom is also a common feature of the boiler serving to control the loss of steam temperature coming out of the superheater and reheater at partial boiler loads.
When evaluating a multi-burner combustion system, it is important to understand that burners are only one of many elements affecting the system performance. The overall performance of the combustion system substan- tially depends on the design of the fuel distribution and air distribution to the burners. The difficulties of design- ing a uniform air and fuel distribution system are quite substantial. The relative magnitude of the buoyancy effect on the air distribution and the hydrostatic effects on the pressure of liquid fuel delivered to the burners vary with load and often are not properly accounted for. Addressing potential air distribution problems with compartmentalizing the wind box and measuring and controlling the air flow to individual burners may look attractive at a glance. However, it often creates more problems than it solves as the complexity of the system is increased and the reliability reduced. The potential severity of oil maldistribution between the burners depends on the piping design and on the type of atom- izers and available margins of the supply pressure.
Typically during retrofits, the wind box is modeled and, if necessary, modified for uniform air distribution.
Gas- and oil-fired utility burners are usually sized for the available draft loss typically in the range from 4 to 6 in. W.C. (10–15 mbar). Sizing burners for lower
differential air pressure is helpful for reducing the NOx emissions and creating more luminous flames, but may also result in longer flames, impingement problems, as well as additional difficulties with achieving the desired quality of the air distribution.
3.7.1.1 NOx Reduction Techniques in Wall-Fired Utility Boilers
Air staging and FGR are the main NOx reduction tech- niques used in wall-fired utility boilers. The selection of the means of NOx reduction must be considered in combination with an evaluation of its effect on the boiler operation in addition to NOx reduction. The air-staging techniques tend to increase the luminosity of the flame and may reduce the mostly unwanted shift in the heat distribution toward the superheater when converting boilers to fire gas. The addition of FGR may increase this shift toward the superheater. FGR introduced through the burners for the purpose of NOx control has a strong effect on the thermal NOx and a strong effect on the heat transfer in the furnace and superheater. By com- parison, FGR introduced through the boiler hopper has a relatively small effect on the effluent NOx, but practi- cally the same effect on the superheater. The quantita- tive evaluation of the effects, checking adequacy of the available attemperating flows, etc., often requires cre- ation of complex CFD models of the boiler.
Air staging using low-NOx utility burners can be done to a larger extent than in package and small field- erected boilers. This is due to the substantially higher residence time in utility furnaces, higher combustion air temperatures, and more intense mixing taking place in utility furnaces. The techniques are effective for both oil- and gas-fired boilers.
The leading modern technique of air staging when firing heavy oil was originally developed by a series of Electric Power Research Institute (EPRI) funded proj- ects in the early 1990s. The technology is called REACH.
The first early reports on REACH are summarized in Ref. [22]. The essence of the refined REACH technology resulting from those studies is the creation of a two- lobed pattern of fuel injection by the atomizer in con- junction with a simple venturi-type burner equipped with a refined medium-size spinner. The exact pattern of fuel injection and the atomizer components are tailored to the specifics of the application. For heavy oil firing, a typical level of NOx reduction with air-staged burners is ∼40%. Similar staging techniques were developed for firing gas with 25%–50% effective NOx reduction from the original uncontrolled levels (see Figure 3.33).
When firing large arrays of burners, the effects of air staging can also be achieved or enhanced with uneven fuel distribution between the burner levels with some levels firing fuel rich and other levels firing fuel lean. Obviously
0.9 0.8 0.7 0.6 0.5 NOx, Ib/MBtu 0.4
0.3 0.2 0.1
00 20 40 60 80 100 120 140 160 180
Gas reach + IFGR Gas reach + 3 BOOS Gas reach + 2 BOOS Gas reach
Original burner
Load, MW (a)
0.5 0.4 0.3 0.2 0.1
00 10 20 30 40 50 60
0.06 NOx guarantee
NOx, Ib/MBtu LNB + IFGR + STEAMLNB + BOOS + IFGRLNB + BOOSLNBPre-retrofit
Boiler generation, MW (b)
Uncontrolled
Staged NOx ranges adjusted
for 0.3% nitrogen content in #6 oil 700
600 500 400 300 200 100
0150 200
NOx, ppm, 3% O2 dry
250 300 350 400
(c) BZHR (kBtu/ft2–h)
Figure 3.33
Examples of NOx reduction with air staging and FGR in utility boilers when firing natural gas (various boilers). (a) and (b) illustrate NOx reduction firing natural gas; (c) illustrates NOx reduction when firing #6 oil in different boilers.
such shifts can be done only up to the extent of maintain- ing combustion stability at each burner. A field study on fuel biasing performed on a 250 MWe (850 × 106 Btu/h) boiler with 16 burners arranged in four rows23 illustrates the effectiveness of fuel biasing when firing natural gas.
With 13% higher than average gas flow to the lower level and similar reduction in the gas flow to the upper level of burners, the NOx was reduced by 30% in comparison with the unbiased operation (see Figure 3.34). The biasing also helped to reduce attemperation in the superheater so the boiler could operate up to its full capacity.
For stronger staging effects, the boiler may also oper- ate with the fuel to some burners cutoff, but without the burner isolation on the air side. These relatively simple techniques are called fuel biasing and burners out of service (BOOS). The effectiveness of BOOS on NOx reduction is very boiler dependent. The optimal pattern of burners with shutoff fuel is usually estab- lished experimentally. The optimum pattern may also depend on the boiler firing rate and availability of the burners and the BOOS effect on the temperature at the superheater. The BOOS technique can be applied in combination with staged burners. The effectiveness of the described NOx reduction techniques is illustrated by Figures 3.33 and 3.34.
The cumulative NOx reduction from the uncon- trolled level will always be less than the sum of reduc- tions of each technique applied separately as illustrated by Figure 3.33. The NOx emissions data shown on Figure 3.33c are shown in relation to the burner zone heat release rate (BZHR). The parameter is defined as the total heat input coming into the furnace from fuel relative to the surface area of a parallelepiped match- ing the boiler cross-sectional dimensions and having a height from the slopes of the hopper to 4 ft (1.2 m) above the upper level of firing burners. The BZHR at
high-fire operation is a boiler design parameter to which many different boiler emission data are correlated with reasonable statistical accuracy.
The initial success of these techniques for a moderate NOx control in 1970s and 1980s prompted changes to the boiler designs with the addition of a row of ports above the upper burner level for injection in the furnace of a portion of the combustion air that otherwise would flow through the burners. The technique was called over- fired air (OFA). NOx control with OFA is more effective than with BOOS, but requires substantial boiler modi- fications as the OFA ports are not part of the original boiler. In some cases, the whole row of upper burners was converted to OFA ports. Typically up to 20% of the total combustion air can be diverted to the OFA ports leaving a uniformly sub-stoichiometric amount of air to the burners. OFA should not be combined with BOOS as it is always more effective to increase the OFA than to shut off fuel to some of the burners.
For maximum effectiveness, the OFA ports need to be properly designed to achieve adequate mixing between the OFA air and the upcoming combustion products and leave sufficient time to complete the fuel oxidation process before the furnace gas gets cooled in the super- heater and the convection pass to temperatures when the oxidation of combustibles—primarily the CO is stopped. OFA is usually controlled separately from the rest of the combustion air. Each port can have an isola- tion damper and resemble a smaller burner without the fuel parts. When properly designed, OFA allows lower excess air operation than with the BOOS. CFD model- ing of the boiler combustion process is often used for designing the OFA system.
Further reduction in NOx emissions can be achieved with FGR injected into the air stream downside of the air heater. This is usually done in lieu of or in parallel with the injection of the FGR through the furnace hop- per. When the boiler is equipped with OFA, the FGR is injected only into the portion of the combustion air going to the burners. The effect of FGR on NOx reduc- tion is very strong in gas-fired boilers operating with highly preheated air. An addition of 10% FGR typically reduces NOx by 50%–55% as illustrated by Figure 3.35.
One of the curves in Figure 3.33a shows another exam- ple of the FGR effect on NOx. The sharp increase in NOx shown by the curve at high fire was due to limited fan capacity of the system causing a substantial drop in the FGR at high fire.
The FGR effect on NOx is much lower when firing #6 oil as a substantial amount of the formed NOx comes from fuel nitrogen. With a high nitrogen content of
∼0.5% to 0.6%, the effect of the FGR may not be signifi- cant if the thermal NOx component is low as illustrated by the test data in Figure 3.36. These old data were taken when firing #6 oil with 0.54% FBN with preheated air to
1.1 1 0.9 0.8 0.7
0.60.6 0.7
Relative NOx
0.8
Ratio of gas flows between lower and upper burners
0.9 1 1.1
Figure 3.34
Effect of fuel biasing on the NOx. 250 MWe CE boiler with 16 wall- fired burner; natural gas firing. (From Lifshits V. and Crovato G., Experience with high efficiency, low emission burners to improve plant operation, Latin America Power’98, Conference Papers, Buenos Aires, Argentina, 1998.)
∼360°F (180°C) in an 8 ft (2.4 m) diameter furnace. The heat input was ∼27 × 106 Btu/h (8 MW). The burner was not a low-NOx design.
The reduction of thermal NOx with FGR when firing
#6 oil is somewhat substantial in boilers with high BZHR not equipped with low-NOx burners. In combination with low-NOx burners and air staging, the use of FGR becomes less effective. The likely explanation of this effect is that lower-temperature less luminous flames with FGR dissipate less heat from the fuel-rich combustion zone. Furthermore, FGR shifts the fuel-rich flammabil- ity limit closer to stoichiometric conditions making it more difficult to apply deep air-staging NOx reduction techniques. When considering the addition of FGR, one also needs to consider that FGR is usually the only NOx reduction technique that may cause an increase in the heat transfer in the superheater. Corrosion issues are also a problem when firing with FGR when applied to fuels like #6 oil containing sulfur.
The combined effects of multiple NOx reduction tech- niques are always less than the simple multiplication of different NOx reduction factors. Applying additional NOx reduction techniques often requires easing or taking away other means of NOx reduction in order to maintain flame stability.
3.7.1.2 Burners for Wall-Fired Utility Boilers
The type of burners used in wall-fired utility furnaces is similar to those used in field-erected boilers. The avail- able round opening between the tightly packed tubes is always all that is available for mounting the burner.
The burners for utilities are built heavy duty with thick, often stainless materials. For reliable operation and light off at any load level, the burners are equipped with high-capacity pilots and flame scanners with good dis- crimination between the monitored burner flame and the flame of adjacent or opposite wall burners. The air isolation dampers are usually part of the burner, except for rare cases when each burner has a dedicated wind box with an air isolation damper being part of the wind box. Other requirements for the wall-fired utility burn- ers stem from the NOx reduction techniques utilized by the system.
The array of burners that operate on oil and gas in large wall-fired utilities and incorporate the necessary fuel-staging techniques includes the already described Delta Power and Variflame burners. Another product for wall-fired utilities is a Dynaswirl™ burner that is a ver- sion of the Variflame burner that permits fitting a larger- capacity burner into the limited opening between the steam-water tubes. The utility versions of these burn- ers are built to withstand high levels of air preheat and thermal radiation from the furnace and usually include air-flow measurement devices added to the venturi passages.
Fuel-staged burners, like QLN, Delta-NOx, or ECOjet burners, are not suitable for wall-fired utility retrofit applications due to the high costs of making additional ports in tube walls for placing staged fuel injectors and likely insufficient benefits over the air-staging techniques.
3.7.2 Combustion Systems for Corner- Fired