Abstract:
A process and apparatus for simultaneously removing NO X  and SO X  from the exhaust of a furnace includes an injection of limestone into a region of the furnace having a temperature of about 2,000°-2,400° F., and an injection of ammonia into a region in the furnace having a temperature of about 1,600°-2,000° F. The limestone absorbs at least some of the SO X  and the ammonia absorbs at least some of the NO X . The exhaust from the furnace which contains particulate and gases, is supplied to a dry scrubber where further reactions take place between unused ammonia and SO X , and calcium sorbent and SO X . Sorbent and ammonia regeneration can also be utilized to further improve the efficiency of the system.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates in general to furnace and post combustion emission control technology, and in particular to a new and useful process of simultaneously reducing both SO X  and NO X . 
     Selective non-catalytic reduction (SNCR) is known for controlling NO X  by injecting ammonia in the furnace downstream of the combustion zone. 
     Limestone injection dry scrubbing (LIDS) is also known whereby SO X  is reduced by injecting limestone or other sorbent in the furnace downstream of the combustion zone and by injecting a calcium-based sorbent into a dry scrubber system attached to the outlet of the furnace system. To date, these two techniques have never been combined nor have the advantages of their combination been described or suggested. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a process for the simultaneous removal of NO X  and SO X  from the exhaust of a furnace having a combustion region, a first injection region at a temperature of 2,000°-2,400° F. and a second injection region at a temperature of 1,600°-2,000° F., the process comprising the steps of injecting a calcium based sorbent into the first injection region in an amount sufficient to absorb at least some SO X  generated in the combustion region, injecting ammonia into the second injection region in an amount sufficient to react with and reduce by at least 50% the NO X  generated in the combustion region to produce an exhaust containing gas and particulate material, supplying the exhaust to a dry scrubber where unreacted ammonia in the exhaust reacts with unabsorbed SO X , and supplying an output from the dry scrubber to a particulate collector for separating particulate from gas. 
     A further object of the present invention is to recycle a portion of the particulate to a slurry tank where unused calcium containing absorbent is mixed with water and returned to the dry scrubber to remove more of the unabsorbed SO X . 
     A still further object of the invention is to add water to the particulate removed from the particulate collector to regenerate ammonia, and return the generated ammonia to the dry scrubber or furnace. 
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawing: 
     FIG. 1 is a schematic diagram showing a system used to practice the process of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The process of the present invention provides a potentially low-cost, efficient method of simultaneous NO X  /SO X  removal that also improves the efficiency of the boiler heat cycle. Such a low-cost, low risk, efficient NO X  /SO X  system may be attractive to utilities which must meet the pollution control standards passed in the Clean Air Act of Nov. 1990. 
     The process involves combining the technologies of selective non-catalytic reduction (SNCR) and limestone injection dry scrubbing (LIDS). The result is a new and superior process that solves the problems of the individual technologies through unexpected interactions. The process should be capable of &lt;50% NO X  reduction and 95% SO 2  reduction at a furnace NH 3  /NO X  molar ratio near one and a furnace Ca/S molar ratio between 1-1.5. Boiler heat cycle efficiency may also be improved by as much as 1.5%. 
     A process schematic is shown in FIG. 1. The major overall chemical reactions are listed in Table 1. Referring to this figure and the table, a brief description of a stand-alone SNCR and LIDS process is given, followed by a description of the combined process. 
     An SNCR system controls NO X  and involves injecting ammonia (NH 3 ) or any ammonia precursor at 14, into the upper region (12) of a furnace (10). This produces the reaction of equation (1) in Table 1. The optimum temperature for NO X  reduction is about 1,800° F. Injection at higher temperatures causes ammonia to decompose to NO X , which is undesirable since NO X  reduction is the purpose of SNCR. Injection at lower temperatures increases ammonia slip. Ammonia slip is undesirable in SNCR processes because it has been shown to lead to ammonia bisulfate (NH 4  HSO 4 ) formation (equation 4). Ammonium bisulfate is very corrosive and is known to condense at temperatures below 
     
                       TABLE 1______________________________________IMPORTANT CHEMICAL REACTIONS______________________________________Furnace (desirable) - 1,600°-2,200° F. ##STR1##                     (1) ##STR2##                     (2) ##STR3##                     (3)Air Heater - &lt;350° F. ##STR4##                     (4) ##STR5##                     (5)Dry Scrubber (desirable) - &lt;300° F. ##STR6##                     (6)or . . . ##STR7##                     (7) ##STR8##                     (8)Baghouse (desirable) - ˜140° F.See equations 6, 7 and 8.Ammonia Regeneration (desirable) - ambientIn an alkaline solution: ##STR9##                     (9) ##STR10##                    (10)______________________________________ 350° F., as found in most air heaters (17). The formation of ammonium bisulfate can be controlled by reducing the SO.sub.3 concentration, or by having a high excess of ammonia. A high excess of ammonia favors ammonium sulfate ([NH.sub.4 ].sub.2 SO.sub.4) formation (equation 5), which does not lead to air heater fouling. Other detrimental effects of ammonia slip on the SNCR process are that it has been shown to lead to odor problems and a white plume at the stack. 
    
     LIDS is an SO 2  control technology that involves furnace limestone (CaCO 3 ) injection at (16) followed by dry scrubbing at (18). SO 2  removal occurs at both stages for greater total efficiency (equations 2, 3, and 8). The optimum temperature for limestone injection is about 2,200° F. in the upper region (20) of furnace (10). Injection at higher temperatures causes dead burning, which decreases sorbent reactivity. Injection at lower temperatures inhibits calcination which also reduces sorbent reactivity. One of the main features of LIDS is that a portion of the unreacted sorbent leaving the furnace can be slurried in a tank (28) and recycled to the dry scrubber by a stream (22) to remove more SO 2 . Additional SO 2  removal occurs in the particulate control device (24), especially if a baghouse is used. 
     The combined process, hereafter referred to as A +  -LIDS, begins with dry limestone injection into the upper furnace at (16) and at a Ca/S stoichiometric ratio of about 1-1.5. Excess calcium in the furnace absorbs SO 3 , as well as SO 2  (Equations 2 and 3), which prevents ammonium bisulfate formation in the air heater and lowers the acid dew point. Unreacted calcium passes through the system to the particulate collector (24) where a portion is recycled at (26) to make slurry in tank (28) for the dry scrubber (18). Additional SO 2  removal occurs in the dry scrubber and particulate collector to increase removal efficiency and sorbent utilization (Equation 8). 
     Furnace limestone injection is closely followed by the addition of excess ammonia to control NO X  at (14) (Equation 1). The best temperature for ammonia injection in the A +  -LIDS process will probably be slightly lower than the optimum temperature for an SNCR process to prevent decomposition to NO X . Excess ammonia in the furnace increases NO X  removal and inhibits ammonium bisulfate formation by favoring ammonium sulfate ([NH 4  ] 2  SO 4 ) formation (Equation 5). 
     Unreacted ammonia passes through the system to the dry scrubber (18), or similar system, and it is here that the greatest advantage of combining the two technologies is realized. Tests have shown that ammonia reacts quantitatively with SO 2  to increase the overall removal efficiency (Equations 6 and 7). The reaction has been shown to produce extremely high ammonia utilization, near 100%, as long as some SO 2  remains. Therefore, it should be possible to obtain high levels of SO 2  removal, with virtually no ammonia emission at the stack. 
     There is also data that indicates that ammonia can be recovered from the baghouse ash by mixing the ash in an ammonia regeneration chamber (30) with a small quantity of water at (32). In an alkaline environment, calcium displaces the ammonia in ammonium salts releasing ammonia gas (Equations 9 and 10). The system could recycle this ammonia at (34) to the scrubber or at (36) to the furnace to further improve sorbent utilization. 
     In the following, the problems encountered with SNCR and LIDS and how they are solved by combining the technologies are disclosed. Other non-obvious advantages are also included. 
     SNCR--NO X  REMOVAL 
     The combustion of coal is known to produce oxides of nitrogen that have been identified as precursors to acid rain. Utilities must control NO X  emissions and are penalized for not meeting ever tighter NO X  emission limits. 
     Injecting ammonia, or any ammonia precursor, into the furnace at about 1,800° F. has been shown to reduce NO X  emissions by 50% or greater. However, SNCR is faced with several problems including ammonium bisulfate formation, which fouls air heaters, and ammonia slip, which causes odor problems and white plumes. By combining SNCR with LIDS, the problems with SNCR can be eliminated, as described below, and NO X  reduction efficiency can be increased by injecting higher levels of ammonia. 
     SNCR--AIR HEATER FOULING CAUSED BY AMMONIUM BISULFATE FORMATION AND CONDENSATION 
     Ammonium bisulfate is known to form during the SNCR process below 350° F. if the relative ratio of NH 3  to SO 3  is near or below one (Equation 4). If this ratio can be maintained above one; that is, by increasing the ammonium concentration or by decreasing the SO 3  concentration, the kinetics favor the formation of ammonium sulfate (Equation 5). Ammonium sulfate does not foul air heater surfaces. 
     Injecting excess ammonia in the furnace is an integral part of A +  -LIDS because ammonia is needed later in the process for SO 2  removal. The non-obvious feature of injecting excess ammonia at 1,800° F. is that it reduces the likelihood of bisulfate formation while increasing NO X  removal in the furnace. NO X  reductions in excess of 50% are expected for this technology. The likelihood of ammonium bisulfate formation is further decreased because the calcium based sorbent injected in the furnace will absorb most of the SO 3 . 
     SNCR--Ammonia Utilization and Slip 
     Ammonia slip is a great concern for utilities considering SNCR because of odor problems, white plume formation, and the threat of bisulfate formation. The current procedure is to operate SNCR systems at NH 3  /NO X  ratios below one to prevent slip, or to inject at temperatures above the optimum so that excess ammonia decomposes to NO X . Both methods reduce system efficiency and limit the practical NO X  reduction capability to around 50%. 
     Combining SNCR with LIDS turns one of SNCR&#39;s greatest disadvantages into a necessary advantage. A +  -LIDS requires ammonia at the scrubbing step, thereby allowing excess ammonia injection in the furnace at temperatures near the optimum. Excess ammonia in the furnace increases NO X  reduction and ammonia utilization and reduces the likelihood of bisulfate formation. 
     SNCR--Complicated Injection System 
     Current SNCR injection systems consist of combinations of complicated, multi-level, high energy injection nozzles and metering systems designed to inject precise amounts of various concentrations of ammonia solutions, containing enhancers, at appropriate stages in the boiler, according to load, in order to prevent ammonia slip and maximize NO X  reduction in the short residence times available. These systems are expensive and require a great deal of fine tuning. 
     Injecting excess ammonia in the furnace is an integral part of A +  -LIDS because ammonia is needed later in the process for SO 2  removal. This simplifies the ammonia injection system because it is easier to inject excess ammonia than it is to inject precise amounts. Higher ammonia flow rates also lead to higher jet momentum that increases jet penetration and flue gas mixing. The projected results are increased NO X  removal and ammonia utilization at shorter residence times. 
     A typical control scheme can be based on maximizing calcium utilization and using only enough ammonia to maintain high levels of SO 2  removal. Several factors dictate this type of control scheme. First, ammonia is the more expensive of the two reagents and should, therefore, be used sparingly. Secondly, because calcium utilization is typically below 60%, it is important to operate the system at conditions that maximize calcium utilization (i.e., low scrubber approach temperature, high slurry solids, etc.). Finally, because ammonia utilization will always be near 100%, it is best to use as little as possible. This type of control scheme ensures the lowest operating cost for reagents. It could be implemented by operating all systems at conditions known to produce maximum calcium utilization and then controlling the ammonia flow to the furnace to maintain 95% SO 2  removal. An alternative would be to monitor for ammonia at the stack and adjust the feed rate accordingly. 
     LIDS--SO 2  Removal 
     The combustion of coal is known to produce oxides of sulfur that have been identified as precursors to acid rain. Utilities must control SO 2  emissions and are penalized for not meeting ever tighter SO 2  emission limits. 
     The LIDS process has bee demonstrated in a 1.8 MW pilot facility. Results showed that greater than 90% SO 2  removal is possible with high sulfur coal at a furnace Ca/S ratio of 2, a scrubber approach to saturation temperature (T as ) of 20° F., and using a baghouse for particulate control. Combining LIDS and SNCR should increase SO 2  removal efficiencies to about 95% because of the NH 3  --SO 2  reactions that take place in the scrubber (Equations 6 and 7) and increase calcium utilization to above 60% (Equations 9 and 10). 
     LIDS--Solids Deposition on Scrubber Surfaces 
     The most difficult problem in the design and operation of dry scrubber systems is the control and handling of solids deposition on interior scrubber surfaces. Deposition occurs when water or slurry droplets impact scrubber surfaces before completely evaporating. It is greatly aggravated at the low approach to saturation temperatures needed to achieve high levels of SO 2  removal. There are many causes for deposition including poor inlet gas flow or temperature distribution, recirculation zones, poor atomization, insufficient residence time, direct jet impaction, and jet spray maldistribution. B&amp;W&#39;s initial commercial dry scrubber can be safely operated at 40° F. T as . More recent B&amp;W designs have been operated safely between a 20° and 30° F. T as , but this is perceived as &#34;risky&#34; by utilities. 
     A recent test has shown that ammonia addition ahead of the dry scrubber can be used to maintain 90-95% SO 2  removal efficiency at higher T as  and lower furnace Ca/S ratio. Typical pilot-scale LIDS data have shown that 90% SO 2  removal can be achieved at nominal furnace Ca/S of 2 and a 20° F. T as . Preliminary data with ammonia addition, at a scrubber NH 3  S ratio of 0.4 and a furnace Ca/S ratio of 2, shows that the scrubber can be operated at a 43° F. T as  while maintaining 90% SO 2  removal. Combining SNCR and LIDS should produce similar results, and even higher removals may be obtained if the scrubber design allows safe operation near a 20° F. T as . 
     LIDS--Low Sorbent Utilization 
     Pilot-scale LIDS data has shown that calcium utilization is related to the furnace Ca/S ratio. Tests at a Ca/S ratio of 1.2 yielded 74% SO 2  removal for 61% calcium utilization. A Ca/S ratio of 1.9 yielded 92% removal for 48% utilization, and a Ca/S ratio of 2.4 yielded 97% removal for 42% utilization. Clearly, utilization decreases as the Ca/S ratio increases above one. 
     Recent tests at the University of Tennessee, B&amp;W&#39;s E-SO X  Pilot, and B&amp;W&#39;s Pilots LIDS Facility have shown that ammonia utilization is near 100%. During a short, non-steady state test at the LIDS pilot, results indicated that 90% SO 2  removal was maintained at a nominal furnace Ca/S ratio of 1.0, and a nominal scrubber NH 3  /S ratio of about 0.2. These results suggest that ammonia can be used to maintain high SO 2  removal at more modest Ca/S ratios for better sorbent utilization. Calcium utilization is also increased by the reaction that takes place during ammonia regeneration (Equations 9 and 10). 
     LIDS--Ash Disposal or Alternate Uses 
     LIDS greatly increases the amount of solids loading to the particulate control device and the ash handling and disposal systems. Although the waste material is considered non-hazardous, the large increase necessitates that alternative uses be found for this material. Several ongoing projects are investigating potential alternative uses. 
     Preliminary results have shown that ammonia addition has the potential to reduce the amount of fresh limestone added to the furnace by a factor of two (see above). This greatly reduces the dust loading to the particulate collector and the amount of waste generated by the system. 
     Ammonia reacts in the dry scrubber to produce ammonium sulfite and ammonium bisulfite (the exact mechanism is unclear at this time). These ammonia compounds, along with the calcium and magnesium compounds, are familiar constituents of fertilizer. 
     Finally, there is data that indicates that ammonia can be recovered from the waste product and reused. Research at the University of Tennessee suggests that ammonia gas is released from the waste material when it is mixed with water (Equations 9 and 10). A separate vessel, like a pug mill, could be used to mix the baghouse ash with small quantities of water. The off-gas could be drawn from the vessel and reinjected into the dry scrubber or furnace. The moistened ash could then be more safely handled for disposal or recycled to the slurry tank. Recycling the ammonia further enhances sorbent utilization. 
     LIDS--Degradation of Particulate Collector Performance By Increased Loading and a Larger Amount of Fines 
     As stated above, LIDS greatly increases the dust loading to the particulate control device. Also, ammonia injection alone is known to produce extremely fine fumes of sulfite and sulfate compounds that are difficult to collect. The addition of calcium to absorb SO 3  also lowers ash resistivity making the ash difficult to collect in an electrostatic precipitator (ESP). 
     As previously stated, results have shown that ammonia addition has the potential to reduce the amount of limestone requirement by a factor of two. The same tests have also shown that the fine ammonia compounds can be easily collected in baghouse because they are mixed with larger particulate. The net effect of combining SNCR with LIDS is, therefore, an increase in collection efficiency caused by reduced ash loading. Humidification is also known to make up for SO 3  depletion in ESP&#39;s. Experience has shown that ESP performance can be maintained with low levels of humidification. The dry scrubber in the A +  LIDS process provides sufficient humidification to maintain ESP performance. 
     LIDS--Boiler Efficiency Decrease Caused by Tube Fouling 
     Fouling of boiler tube surfaces can be caused or aggravated by LIDS. Utilities are concerned that the addition of limestone into the upper furnace can cause tube fouling that would result in increased soot blowing and decreased heat cycle efficiency. 
     Recent LIMB testing at the Ohio Edison&#39;s Edgewater Station has shown that tube fouling may be related to grind size. Three limestone sizes were tested: a commercial grind (30 μ median diameter), a fine grind (12 μ), and a special super fine grind (3.5 μ). Results showed that the commercial material actually prevented tube fouling and eliminated the need for soot blowing. The medium grind caused slight fouling, but not higher than normal. The super fine grind caused some fouling, but still less than observed with hydrated lime injection. The respective furnace SO 2  removal efficiencies were about 25%, 35%, 45%. The relative cost ranged from inexpensive for the commercial grade to very expensive for the super fine material. These results suggest that by combining SNCR with LIDS, a high overall level of SO 2  removal could be maintained with commercial grate limestone. This would have the added advantage of a lower cost reagent as well as increasing the heat cycle efficiency and reducing soot blower maintenance costs. However, care must be taken not to choose a limestone grind size that increases tube erosion. Combining LIDS and SNCR is also expected to reduce sorbent usage which will also decrease the potential for fouling. 
     GENERAL--Air heater Fouling and Corrosion by SO 3  Condensation 
     Fouling and corrosion of air heater tubes occurs when the air heater gas temperatures fall below the acid dew point. Current practices dictate that air heater exit gas temperatures remain above about 300° F. to prevent SO 3  condensation. 
     Calcium is known to react with SO 3  at furnace temperatures. Therefore, the A +  -LIDS process has the added benefit of reducing the SO 3  concentrations and eliminating the threat of air heater fouling and corrosion by acid condensation. By lowering the acid dew point, A +  -LIDS will also enable utilities to operate the air heater at a lower exit gas temperature, thereby increasing the efficiency of the boiler heat cycle. An increase of about 1/2% is possible for each 20° F. decrease in air heater exit gas temperature. 
     The A +  -LIDS process has many unexpected and useful features that stem from the integration of two technologies. The advantages gained by combining SNCR and LIDS go far beyond what is possible with the individual technologies and include: 
     1. &gt;90% SO 2  removal; 
     2. 50% NO X  removal with A +  -LIDS (more if combined with low NO X  burners, reburning, etc.); 
     3. Low-cost sorbents (i.e., ammonia and commercial grade limestone); 
     4. No bisulfate fouling of the air heater; 
     5. No SO 3  condensation in the air heater or other duct work; 
     6. Furnace ammonia slip is turned from a disadvantage to an advantage; 
     7. A simplified ammonia injection system; 
     8. The ability to maintain high SO 2  removal at higher scrubber approach temperatures, if necessary; 
     9. High sorbent utilization; 
     10. The possible production of a regeneratable, salable waste product; 
     11. Increased baghouse performance; 
     12. No convective pass tube fouling; 
     13. No need for additional soot blowing and a possible reduction of soot blowing cycles; 
     14. Increased heat cycle efficiency; and 
     15. Relatively easy retrofit. 
     While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.