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Air Pollution Control Systems for Boiler and Incinerators.Unique control problems_7

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Nội dung Text: Air Pollution Control Systems for Boiler and Incinerators.Unique control problems_7

  1. Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com 5-815-1/AFR 19-6 TM 10-5
  2. TM 5-815-1/AFR 19-6 Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com with a solution of sodium carbonate or sodium hydrox- scrubber under controlled reactor conditions. ide to produce a solution of dissolved sodium sulfur The principal advantages of the dual alkali salts. The solution is then oxidized to produce a neutral system are: solution of sodium sulfate. Because it is a throwaway (a) Scaling problems associated with direct process, the cost of chemicals make it an unattractive calcium-based scrubbing processes are SOx removal process when burning high sulfur fuels significantly reduced. (greater than 1 percent). (b) A less expensive calcium base can be t. Dual alkali sodium scrubbing. used. (1) The dual alkali SOX removal system is a (c) Due to high solubility and concentration regenerative process designed for disposal of of active chemicals, lower liquid volumes wastes in a solid/slurry form. As shown in can be used thereby lowering equipment figure 10-6, the process consists of three costs. basic steps; gas scrubbing, a reactor system, (d) Slurries are eliminated from the and solids dewatering. The scrubbing system absorption loop, thereby reducing utilizes a sodium hydroxide and sodium plugging and erosion problems. sulfite solution. Upon absorption of SO2 in (e) A sludge waste, rather than a liquid waste, the scrubber, a solution of sodium bisulfite is produced for disposal. and sodium sulfite is produced. The scrubber (f) High SO2 removal efficiency (90% or effluent containing the dissolved sodium salts more). is reacted outside the scrubber with lime or u. Absorption of SO2. limestone to produce a precipitate of calcium (1) Activated carbon has been used as an absor- salts containing calcium sulfate. The bent for flue-gas desulfurization. Activated precipitate slurry from the reactor system is carbon affects a catalytic oxidation of 502 to dewatered and the solids are deposed of in a SO3, the latter having a critical temperature of landfill. The liquid fraction containing 425 degrees Fahrenheit. This allows absorp- soluable salts is recirculated to the absorber. tion to take place at operating temperatures. Double alkali systems can achieve efficiencies The carbon is subsequently regenerated in a of 90 - 95% and close to 100% reagent separate reactor to yield a waste which is used utilization. in the production of high grade sulfuric acid, (2) This system is designed to overcome the and the regenerated absorbent. There are inherent difficulties of direct calcium slurry serious problems involved in the regeneration scrubbing. All precipitation occurs outside the of the absorbent, including carbon losses due 10-6
  3. Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com 5-815-1/AFR 19-6 TM to attrition, chemical decomposition, serious subsequently store it as a sulphate in the pores corrosion problems, and danger of of the zeolite. combustion of the reactivated carbon. v. Cost of flue-gas desulfurization. The actual (2) Zeolites are a class of highly structured alumi- capital and operating costs for any specific installation num silicate compounds. Because of the reg- are a function of a number of factors quite specific to ular pore size of zeolites, molecules of less the plant and include: than a certain critical size may be — Plant size, age, configuration, and locations, incorporated into the structure, while those — Sulfur content of the fuel and emission greater are excluded. It is often possible to control requirements, specify a certain zeolite for the separation of — Local construction costs, plant labor costs, a particular material. Zeolites possesses and cost for chemicals, water, waste disposal, properties of attrition resistance, temperature etc., stability, inertness to regeneration techniques, — Type of FGD system and required equipment, and uniform pore size which make them ideal — Whether simultaneous particulate emission absorbents. However, they lack the ability to reduction is required. catalyze the oxidation of SO2 to SO3 and thus 10-3. Procedure to minimize SOX emission cannot desulfurize flue-gases at normal operating temperatures. Promising research is a. Efficiency requirement. The SOx removal effi- under way on the development of a zeolite ciency necessary for any given installation is dependent material that will absorb SO2 at flue-gas upon the strictest regulation governing that installation. temperatures by oxidation of SO3 and Given a certain required efficiency, a choice can be 10-7
  4. TM 5-815-1/AFR 19-6 Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com 10-8
  5. Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com 5-815-1/AFR 19-6 TM made among the different reduction techniques. This (3) Local market demand for recovered sulfur, section shows how a rational basis can be utilized to (4) Plant design limitations and site charac- determine the best method. teristics, b. Boiler modification. This technique is useful in (5) Local cost and availability of chemicals, util- reducing SOx emissions by 0 to 6% depending upon ities, fuels, etc., the boiler. For industrial boilers operating above 20% (6) Added energy costs due to process pumps, excess-air the use of proper control equipment or low reheaters, booster fans, etc. excess-air combustion will usually reduce emissions by 10-4. Sample problems. 4 to 5%. If the operating engineer is not familiar with boiler optimization methods, consultants should be uti- The following problems have been provided to lized. illustrate how to determine the maximum fuel sulfur c. Fuel substitution. This method can be used for content allowable to limit SO emission to any almost any percent reduction necessary. Availability particular level. and cost of the fuel are the major factors to be consid- a. Approximately 90 to 97 percent of fuel sulfur is ered. Fuels can be blended to produce the desired sul- oxidized to sulfur dioxide (SO2) during combustion. fur input. Care must be taken, however, so that the ash This means that for every lb of sulfur in the fuel, produced by the blending does not adversely affect the approximately 2 lbs of sulfur oxides will appear in the boiler by lowering the ash fusion temperature or caus- stack gases. (The atomic weight of oxygen is ½ that of ing increased fouling in the convection banks. sulfur.) Since most of the sulfur oxides are in the form d. Flue-gas desulfurization. Various systems are of SO2, emissions regulations are defined in these units. available for flue-gas desulfurization. Some of these To estimate maximum probable SO2 emissions, the fol- systems have demonstrated long term reliability of lowing equation applies: operation with high SOx removal efficiency. Lime/lime- stone injection and scrubbing systems have been most frequently used. It must be recognized that each boiler control situation must be accommodated in the overall system design if the most appropriate system is to be b. Assume a fuel-oil burning boiler must limit emis- installed. The selection and design of such a control sions to .35 lbs/MMBtu. What is the maximum allowa- system should include the following considerations: ble sulfur content if No.6 Residual fuel-oil is to be (1) Local SO2 and particulate emission require- used? ments, both present and future, (1) From table 10-3, Typical Analysis of Fuel-Oil (2) Local liquid and solid waste disposal regula- Types, an average heating value of 18,300 tions, 10-9
  6. TM 5-815-1/AFR 19-6 Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com Btu/lb for No.6 residual fuel has been assumed. Maximum allowable sulfur content is determined as: d. Assume a boiler installation burns No.4 fuel-oil with a heating value of 19,000 Btu/lb. What is the maximum fuel sulfur content allowable to limit SOx emissions to .8 lbs/MMBtu? (2) Table 10-3 shows that No.5 and No.6 fuel oils have fuel sulfur contents in excess of .32%. If No.4 fuel oil is chosen, a fuel with less than .32% sulfur may be available. e. Assume a coal burning boiler must limit SOx c. Assume a fuel-oil burning boiler must limit SOx emissions to 1 lb/MMBtu. If sub-bituminous coal with emission to .65 lbs/MMBtu. If No.6 residual fuel oil is a heating value of 12,000 to 12,500 Btu/lb (see table to be used, can SOx emission limits be met? 10-4) is to be used what is the maximum allowable (1) From table 10-3, the minimum sulfur content fuel sulfur content? in No.6 fuel oil is .7%. If .7% sulfur fuel can be purchased, the heating value of the fuel must be: f. Since coal of this low sulfur content is not avail- (2) Since the heating value of No. 6 fuel oil is able, what SOx removal efficiency would be required generally between 17,410 and 18,990 Btu/lb, burning 1% sulfur coal? SOx emission limits cannot be met using this fuel. If we assume a No.6 fuel-oil with one percent sulfur and a heating value of 18,600 Btu/lb is used the percent SOx removal effi- ciency that will be required is determined as: 10-10
  7. Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com 5-815-1/AFR 19-6 TM 10-11
  8. TM 5-815-1/AFR 19-6 Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com 10-12
  9. Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com 5-815-1/AFR 19-6 TM CHAPTER 11 NITROGEN OXIDES (NOx) CONTROL AND REDUCTION TECHNIQUES 11-1. Formation of nitrogen oxides. tions produce more NOx. The more bulk mixing of fuel and air in the primary combustion zone, the more tur- a. Nitrogen oxides (NOx). All fossil fuel burning bulence is created. Flame color is an index of flame processes produce NOx. The principle oxides formed turbulence. Yellow hazy flames have low turbulence, are nitric oxide (NO) which represents 90-95 percent whereas, blue flames with good definition are consid- (%) of the NOx formed and nitrogen dioxide (NO2) ered highly turbulent. which represents most of the remaining nitrogen c. Burner number. The number of burners and their oxides. spacing are important in NOx emission. Interaction b. NOx formation. Nitrogen oxides are formed pri- between closely spaced burners, especially in the center marily in the high temperature zone of a furnace where of a multiple burner installation, increases flame sufficient concentrations of nitrogen and oxygen are temperature at these locations. The tighter spacing present. Fuel nitrogen and nitrogen contained in the lowers the ability to radiate to cooling surfaces, and combustion air both play a role in the formation of greater is the tendency toward increased NOx emis- NOx. The largest percentage of NOx formed is a result sions. of the high temperature fixation reaction of d. Excess air. A level of excess air greatly exceeding atmospheric nitrogen and oxygen in the primary the theoretical excess air requirement is the major combustion zone. cause of high NOx emissions in conventional boilers. c. NOx concentration. The concentration of NOx Negotiable quantities of thermally formed NOx are found in stack gas is dependent upon the time, tem- generated in fluidized bed boilers. perature, and concentration history of the combustion e. Combustion temperature. NOx formation is gas as it moves through the furnace. NOx concentration dependent upon peak combustion temperature, with will increase with temperature, the availability of oxy- higher temperatures producing higher NOx emissions. gen, and the time the oxygen and nitrogen simul- f. Firing and quenching rates. A high heat release taneously are exposed to peak flame temperatures. rate (firing rate) is associated with higher peak tem- peratures and increased NOx emissions. A high rate of 11-2. Factors affecting NOx emissions thermal quenching, (the efficient removal of the heat a. Furnace design and firing type. The size and released in combustion) tends to lower peak tem- design of boiler furnaces have a major effect on NOx peratures and contribute to reduced NOx emissions. emissions. As furnace size and heat release rates g. Mass transportation and mixing. The con- increase, NOx emissions increase. This results from a centration of nitrogen and oxygen in the combustion lower furnace surface-to-volume ratio which leads to zone affects NOx formation. Any means of decreasing a higher furnace temperature and less rapid terminal the concentration such as dilution by exhaust gases, quenching of the combustion process. Boilers generate slow diffusion of fuel and air; or alternate fuel- different amounts of NOx according to the type of rich/fuel- lean burner operation will reduce NOx firming. Units employing less rapid and intense burning formation. These methods are also effective in from incomplete mixing of fuel and combustion gases reducing peak flame temperatures. generate lower levels of NOx emissions. Tangentially h. Fuel type. Fuel type affects NOx formation both fired units generate the least NOx because they operate through the theoretical flame temperature reached, and on low levels of excess air, and because bulk misting through the rate of radiative heat transfer. For most and burning of the fuel takes place in a large portion of combustion installations, coal-fired furnaces have the the furnace. Since the entire furnace acts as a burner; highest level of NOx emissions and gas-fired precise proportioning of fuel/air at each of the individ- installations have the lowest levels of NOx emissions. ual fuel admission points is not required. A large i. Fuel nitrogen. The importance of chemically amount of internal recirculation of bulk gas, coupled bound fuel nitrogen in NOx formation varies with the with slower mixing of fuel and air, provides a combus- temperature level of the combustion processes. Fuel tion system which is inherently low in NOx production nitrogen is important at low temperature combustion, for all fuel types. but its contribution is nearly negligible as higher flame b. Burner design and configuration. Burners oper- temperatures are reached, because atmospheric nitro- ating under highly turbulent and intense flame condi- 11-1
  10. TM 5-815-1/AFR 19-6 Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com gen contributes more to NOx formation at higher tem- peratures. 11-3. NOx reduction techniques a. Fuel selection. Reduction of NOx emissions may be accomplished by changing to a fuel which decreases the combustion excess air requirements, peak flame temperatures, and nitrogen content of the fuel. These changes decrease the concentration of oxygen and (3) Changing from a higher to a lower NOx nitrogen in the flame envelope and the rate of the NOx producing fuel is not usually an economical formation reaction. method of reducing NOx emissions because (1) The specific boiler manufacturer should be additional fuel costs and equipment capital consulted to determine if a fuel conversion costs will result. For additional information can be performed without adverse effects. on fuel substitution, see paragraph 10-3. In The general NOx reduction capability of doing so, it should be noted that changing initiating a change in fuel can be seen from coal to oil or gas firing is not in comparatively in table 11-1. accordance with present AR 420-49. (2) A consideration when comtemplating a b. Load reduction. Load reduction is an effective change in fuel type is that NOx emission technique for reducing NOx emissions. Load reduction regulations are usually based on fuel type. has the effect of decreasing the heat release rate and Switching to a cleaner fuel may result in the reducing furnace temperature. A lowering of furnace temperature decreases the rate of NOx formation. necessity of conforming to a more strict (1) NOx reduction by load reduction is illustrated emission standard. in figure 11-1. As shown, a greater reduction 11-2
  11. Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com 5-815-1/AFR 19-6 TM in NO2 is attainable burning gas fuels because (2) The successful application of LEA firing to they contain only a small amount of fuel- any unit requires a combustion control system bound nitrogen. Fuel-bound nitrogen to regulate and monitor the exact conversion does not appear to be affected by proportioning of fuel and air. For pulverized furnace temperatures, which accounts for the coal fired boilers, this may mean the lower NOx reductions obtained with coal and additional expense of installing uniform oil firing. Some units such as tangentially distribution systems for the coal and air fired boilers show as much as 25 percent mixture. decrease in NOx emissions with a 25 percent (3) Low excess air firing is a desirable method of load reduction while burning pulverized coal. reducing NOx emission because it can also (2) Although no capital costs are involved in load improve boiler efficiency by reducing the reduction, it is sometimes undesirable to amount of heat lost up the stack. Con- reduce load because it may reduce steam sequently, a reduction in fuel combustion will cycle efficiency. sometimes accompany LEA firing. c. Low excess air firing (LEA). In order to complete d. Low excess air firing with load reduction. NOx the combustion of a fuel, a certain amount of excess air emissions may be reduced by implementing a load is necessary beyond the stoichiometric requirements. reduction while operating under low excess air condi- The more efficient the burners are in misting, the tions (table 11-2). This combined technique may be smaller will be the excess air requirement. A minimum desirable in an installation where NOx emissions are amount of excess air is needed in any system to limit extremely high because of poor air distribution and the the production of smoke or unburned combustibles; resultant inefficient operation of combustible equip- but larger amounts may be needed to maintain steam ment. A load reduction may permit more accurate con- temperature to prevent refractory damage; to complete trol of the combustion equipment and allow reduction combustion when air supply between burners is unbal- of excess air requirements to a minimum value. NOx anced; and to compensate for instrument lag between reduction achieved by simultaneous implementation of operational changes. Practical minimums of excess air load reduction and LEA firing is slightly less than the are 7 percent for natural gas, 3 to 15 percent for oil combined estimated NOx reduction achieved by sepa- firing, and 18 to 25 percent for coal firing. rate implementation. (1) Since an increase in the amount of oxygen e. Two-stage combustion. The application of delayed and nitrogen in a combustion process will fuel and air mixing in combustion boilers is referred to increase the formation and concentration of as two stage combustion. Two-stage combustion can NOx, low excess air operation is the first and be of two forms. Normally it entails operating burners most important technique that should be fuel-rich (supplying only 90 to 95 percent of utilized to reduce NOx emissions. A 50 stoichiometric combustion air) at the burner throat, and percent reduction in excess air can usualy admitting the additional air needed to complete reduce NOx emissions from 15 to 40 percent, combustion through ports (referred to as NO ports) depending upon the level of excess air located above and below the burner. There are no ports normally applied. Average NOx reductions to direct streams of combustion air into the burner corresponding to a 50 percent reduction in flame further out from the burner wall thus allowing a excess air for each of the three fuels in gradual burning of all fuel. Another form of two-stage different boiler types are shown in table 11-2. combustion is off-stoichiometric firing. This technique Reductions in NOx emission sup to 62 percent involves firing some burners fuel-rich and others air- have been reported on a pulverized coal fired rich (high percentage of excess air), or air only, and is boiler when excess air is decreased from a usually applied to boilers having three or more burner level of 22 percent to a level of 5 percent. levels. Off-stoichiometric firing is accomplished by staggering the air-rich and fuel-rich burners in each of the burner levels. Various burner configuration tests have shown that it is generally more effective to operate most of the elevated burners air-rich or air only. Off-stoichiometric firing in pulverized coal fired boilers usually consists of using the upper burners on air only while operating the lower levels of burners fuel-rich. This technique is called overfire air operation. (1) Two-stage combustion is effective in reducing NOx emissions because: it lowers the concentration of oxygen and nitrogen in the primary combustion zone by fuel-rich firing; it lowers the attainable peak flame temperature by allowing for gradual 11-3
  12. TM 5-815-1/AFR 19-6 Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com combustion of all the fuel; and it reduces the mixing accompanying the increased amount of time the fuel and air mixture is combustion air/ gas volume. Gas recirculation exposed to higher temperatures. does not significantly reduce plant thermal (2) The application of some form of two stage efficiency but it can influence boiler combustion implemented with overall low operation. Radiation heat transfer is reduced excess air operation is presently the most in the furnace because of lower gas effective method of reducing NOx emissions temperatures, and convective beat transfer is in utility boilers. Average NOx reductions for increased because of greater gas flow. this combustion modification technique in (2) The extent of the applicability of this utility boilers are listed in table 11-3. modification remains to be investigated. The However, it should be noted that this quantity of gas necessary to achieve the technique is not usually adaptable to small desired effect in different installations is industrial boilers where only one level of important and can influence the feasibility of burners is provided. the application. Implementing flue-gas recirculation means providing duct work and recycle fans for diverting a portion of the exhaust flue-gas back to the combustion air windbox. It also requires enlarging the windbox and adding control dampers and instrumentation to automatically vary flue-gas recirculation as required for operating conditions and loads. h. Steam or water injection. Steam and water injec- tion has been used to decrease flame temperatures and reduce NOx emissions. Water injection is preferred over steam because of its greater ability to reduce tem- perature. In gas and coal fired units equipped with standby oil firing with steam atomization, the atomizer offers a simple means for injection. Other installations f. Reduced preheat temperature. NOx emissions are require special equipment and a study to determine the influenced by the effective peak temperature of the proper point and degree of atomization. The use of combustion process. Any modifications that lower water or steam injection may entail some undesirable peak temperature will lower NOx emissions. Lower air operating conditions, such as decreased efficiency and preheat temperature has been demonstrated to be a increased corrosion. A NOx reduction rate of up to 10 factor in controlling NOx emissions. However, reduced percent is possible before boiler efficiency is reduced preheat temperature is not a practical approach to NOx to uneconomic levels. If the use of water injection reduction because air preheat can only be varied in a requires installation of an injection pump and attendant narrow range without upsetting the thermal balance of piping, it is usually not a cost-effective means of the boiler. Elimination of air preheat might be expected reducing NOx emissions. to increase particulate emissions when burning coal or oil. Preheated air is also a necessary part of the coal 11-4. Post combustion Systems for NOx pulverizer operation on coal fired units. Jn view of he reduction. penalties of reduced boiler efficiency and other disad- vantages, reduced preheat is not a preferred means of a. Selective catalytic reduction (SCR) of NOx is lowering NOx emissions. based on the preference of ammonia to react with NO, g. Flue-gas recirculation. This technique is used to rather than with other flue-gas constitutents. Ammonia lower primary combustion temperature by recirculating is injected so that it will mix with flue-gas between the part of the exhaust gases back into the boiler com- economizer and the air heater. Reaction then occurs as bustion air manifold. This dilution not only decreases this mix passes through a catalyst bed. Problems peak combustion flame temperatures but also requiring resolution include impact of ammonia on decreases the concentration of oxygen available for downstream equipment, catalyst life, flue-gas NOx formation. NOx reductions of 20 to 50 percent monitoring, ammonia availability, and spent-catalyst have been obtained on oil-fired utility boilers but as yet disposal. have not been demonstrated on coal-fired units. It is b. Selective noncatalytic reduction (SNR) Ammonia estimated that flue gas recirculation has a potential of is injected into the flue-gas duct where the temperature decreasing NOx emissions by 40 percent in coal-fired favors the reaction of ammonia with NOx in the flue- units. gas. The narrow temperature band which favors the (1) Flue gas recirculation has also produced a reaction and the difficulty of controlling the tem- reduction on CO concentrations from normal perature are the main drawbacks of this method. operation because of increased fuel-air 11-4
  13. Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com 5-815-1/AFR 19-6 TM d. Procedure. A technical program for implementing c. Copper oxide is used as the acceptor for SO2 a NOx reduction program should proceed with the aid removal, forming copper sulfate. Subsequently both of equipment manufacturers and personnel who have the copper sulfate which was formed and the copper had experience in implementing each of the NOx oxide catalyze the reduction of NO to nitrogen and reduction techniques that may be required in the water by reaction with ammonia. A regeneration step following manner: produces an SO2 rich steam which can be used to man- (1) NOx emission test. A NOx emission test ufacture by-products such as sulfuric acid. should be performed during normal boiler load times to ascertain actual on-site NOx 11-5. Step-by-step NOx reduction method generation. This test should include recording a. Applicability. The application of NOx reduction of normal boiler parameters such as: flame techniques in stationary combustion boilers is not temperature; excess air; boiler loads; flue-gas extensive. (However, NOx reduction techniques have temperatures; and firing rate. These been extensively applied on automobiles.) These tech- parameters can be referred to as normal niques have been confined to large industrial and utility operating parameters during subsequent boilers where they can be more easily implemented changes in operation. where NOx emissions standards apply, and where (2) Reduction capabilities. The desired reduction equipment modifications are more economically justi- in NOx emissions, in order to comply with fied. However some form of NOx control is available standards, should be estimated based on mea- for all fuel-burning boilers without sacrificing unit sured NOx emission data. Specific NOx re- output or operating efficiency. Such controls may duction techniques can then be selected based become more widespread as emission regulations are on desired reductions and reduction capa- broadened to include all fuel-burning boilers. bilities outlined in preceding paragraph 11-3. b. Implementation. The ability to implement a par- (3) Equipment optimization. Any realistic pro- ticular combustion modification technique is dependent gram for NOx reduction should begin with an upon furnace design, size, and the degree of equipment evaluation and overhaul of all combustion operational control. In many cases, the cost of con- related equipment. A general improvement of version to implement a modification such as flue-gas boiler thermal efficiency and combustion effi- recirculation may not be economically justified. There- ciency will reduce the normal level of NOx fore, the practical and economic aspects of boiler emissions. Of major importance are: design and operational modifications must be (a) the cleanliness of all heat transfer surfaces ascertained before implementing a specific reduction (especially those exposed to radiative heat technique. absorption), (1) Temperature reduction through the use of (b) maintaining proper fuel preparation (siz- two stage combustion and flue-gas ing, temperature, viscosity), recirculation is most applicable to high heat (c) insuring control and proper operation of combustion equipment (burners nozzles, release boilers with a multiplicity of burners air registers, fans, preheaters, etc.), such as utility and large industrial boilers. (d) maintaining equal distribution of fuel and (2) Low excess air operation (LEA) coupled with air to all burners. flue-gas recirculation offers the most viable (4) Low excess air operation. Low excess air solution in smaller industrial and commercial operation is the most recommended modific- size boilers. These units are normally ation for reducing NOx emission. Possible designed for lower heat rates (furnace reductions are given in preceding table 11-2. temperature) and generally operate on high How-ever, a control system is needed to levels of excess air (30 to 60%). accurately monitor and correct air and fuel c. Compliance. When it has been ascertained that flow in response to steam demands. Of the NOx emissions must be reduced in order to comply control systems available, a system incorpo- with state and federal codes, a specific program should rating fuel and air metering with stack gas O2 be designed to achieve the results desired. The correction will provide the most accurate program direction should include: control. A system of this nature will generally — an estimate of the NOx reduction desired, pay for itself in fuel savings over a 2 to 3-year — selection of the technique or combination period, and is economically justified on thereof, which will achieve this reduction; industrial boilers rated as low as 40,000 lb of — an economic evaluation of implementing each steam/hr. technique, including equipment costs, and (5) Flue-gas recirculation. Flue-gas recirculation changes in operational costs; is the second most effective NOx reduction — required design changes to equipment technique for boilers where two stage — the effects of each technique upon boiler combustion cannot be applied. Low excess performance and operational safety. 11-5
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