Abstract:
A system of selective catalytic reduction units for reducing nitrogen oxides by operating a plurality of parallel combustion units in parallel with a spare selective catalytic reduction reactor is disclosed. A primary selective catalytic reactor with spent catalyst is isolated for maintenance while the flue gas is diverted to the spare selective catalytic reduction reactor while continuing to discharge flue gas essentially free of nitrogen oxides from the spare selective catalytic reduction reactor. The spare selective catalytic reduction reactor can include a spare induced draft fan to provide an alternate means for operation and maintenance.

Description:
BACKGROUND OF INVENTION 
     The present invention is directed to the continuous operation of a plurality of combustion units using a selective catalytic reduction (SCR) system that reduces NO x  in the flue gases, and more particularly to a system wherein a spare SCR reactor is installed in parallel with the primary SCR units so that the spare unit can be utilized while any one or more of the primary SCR units are placed out of service for maintenance. 
     Selective catalytic reduction (SCR) reactors convert nitrogen oxides (NO x ), present in flue gases from combustion sources, into a harmless by-product of nitrogen and water. A single SCR reactor is commonly installed for each combustion source for the reduction of NO x . Such combustion sources include but are not limited to refinery heaters, industrial furnaces or boilers. 
     The performance of an SCR unit is limited by the effectiveness of the catalyst placed within the reactor. Unfortunately, the effectiveness of the catalyst diminishes over time due to catalyst inactivation from sulfur gases and/or other impurities in the exhaust gases. Catalyst suppliers are reluctant to guarantee that a catalyst will last more than 3 or 4 years, even if operating at ideal conditions. Ideal conditions for optimal performance of the catalyst include: 1) operating the catalyst within its specified temperature range; 2) utilizing combustion products with low sulfur compounds such as SO 2  and SO 3 ; and 3) no particulate matter present in the exhaust gases. These ideal conditions are difficult, if not impossible, to meet in industrial installations, chemical plants and petroleum refineries. Since these ideal conditions are not met, the life of the catalyst will be cut short and thus require replacement more often than every three or four years. To replace the catalyst, the industrial plants must shut down the facility or exceed desirable NO x  emission levels. Either alternative can be costly to the plant in terms of both time and money. 
     Conventionally, each heater will have its own ducting, SCR unit, fan, emission monitoring system, and stack. Under this arrangement, the probability of catalyst failure to occur within four years is high, thus resulting in an unnecessary shutdown of the plant. Such shutdowns often result in a delay of 2 to 3 days before the plant can resume its normal operations. A single shutdown can burden operations with millions of dollars worth of lost opportunity. 
     To extend the operational life of the SCR unit and attempt to avoid the unnecessary delays, plant designers have simply added additional catalyst to the SCR unit. The increased amount is usually 25% or more of catalyst to the unit for every extra year of life needed for the unit. This technique is not desirable because the additional catalyst burdens other aspects of the operation, such as: 1) the size of the fan, 2) the size of the SCR reactor, and 3) the additional cost of the catalyst. Adding the additional catalyst to the reactor results in an increase in the pressure drop of the flue gases. In order to maintain a constant pressure of flue gas, the plant operator is required to install a larger fan. Also, a larger reactor is required to handle the increased volume of catalyst. The addition of larger fans and larger reactors generally results in higher capital costs. 
     Another prior art approach has been to replace all of the catalyst in all of the associated heaters in the section of the plant when one of the heaters has catalyst inactivity. This logic is based on the fact that if one SCR unit has stopped performing, then it is likely the other units are near the end of the useful catalyst life, and the operators do not want to experience another plant shutdown within a few months due to problems with the SCR of a different heater. This replacement technique is inefficient because the catalyst discarded from the functioning SCR units can have substantial remaining viability. 
     The present spare SCR invention provides a solution that allows industrial plants to continuously operate and replace the spent catalyst in an SCR unit, while simultaneously reducing the need to obtain government waivers for exceeding allowable NO x  emissions. 
     SUMMARY OF INVENTION 
     The present invention is directed toward the continuous operation of an industrial selective catalytic reduction (SCR) system. The present invention allows for the replacement of catalyst within an SCR reactor while not disrupting the operations of a heater or other combustion process. 
     The spare SCR unit is operated by diverting gases from one combustion unit into the spare SCR unit with the use of dampers in the diversion ducts and the insertion of isolation blind plates at appropriate locations. The spare SCR can utilize the fan, the ammonia distribution system, and the emissions monitoring system of the existing combustion unit. The gas is then re-introduced into the same stack. An alternate embodiment eliminates the return ducting and dampers but requires an additional fan, an emissions monitoring system and stack for the spare SCR unit. 
     Once installed, the spare SCR system can be operated in a variety of ways to solve different problems that may arise in an industrial setting. For instance, when one of the main SCR units loses catalytic activity and NO x  emissions rise to an unacceptable level, the spare SCR unit can be operated without shutting down the heater, until the main SCR is serviced for catalyst or fan replacement. Additionally, where one heater in the SCR system is performing below the environmental requirements, or where another heater in the plant is performing poorly, the spare SCR can work in conjunction with the poor-performing SCR thereby meeting the environmental requirements until such time as the heater can be shut down to change the catalyst. 
     The spare SCR can also remain empty while the need for the unit remains low. This allows plant operators with the means to regulate the amount of catalyst stored as on-site inventory, for example, where there are a number of spare SCR units installed at the plant site, each installed spare serving as a spare for a plurality of on-line SCR units. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a simplified schematic of one embodiment of a continuous selective catalytic reduction system with return ducting to each fan and stack. 
     FIG. 2 is a simplified schematic of an alternative embodiment of a continuous selective catalytic reduction system wherein the spare SCR unit has its own fan and stack. 
    
    
     DETAILED DESCRIPTION 
     Broadly, the invention provides continuous operations of a heater or other combustion unit when the catalyst is spent by installing one spare SCR reactor in parallel as a common unit to serve several parallel SCR units. Thus, the spare unit can be operated while any one or more of the SCR units are having problems operating. 
     In accordance with one embodiment of the invention illustrated schematically in FIG. 1 with three heaters  10 A ,  10 B, and  10 C, which are approximately the same size. The system could also be used with 2 combustion sources or 4, 5, 6, 7, or 8 or more combustion sources, but three are illustrated for exemplary purposes only. The flue gas quantities and temperatures from all three are generally assumed to be within similarly narrow operating ranges. A properly functioning heating system conventionally allows for the exhaust gases to pass through a respective convection section  16 A ,  16 B and  16 C, then through an ammonia injection grid  18 A ,  18 B and  18 C, then up through an isolation blind plate  20 A,  20 B and  20 C, into an SCR unit  22 A,  22 B and  22 C, past a stack damper  28 A,  28 B and  28 C, then past an emission sampling system  30 A ,  30 B and  30 C, and finally through a fan  32 A ,  32 B and  32 C, in a manner well known in the art . The stack damper  28 A ,  28 B and  28 C is preferably located between the respective SCR  22 A  22 B and  22 C and the emissions sampling system  30 A ,  3 OB and  30 C, and is normally open during operation of the primary SCR. 
     The ammonia injection grids  18 A ,  18 B and  18 C conventionally distribute ammonia evenly into the flue gas. As is known in the art, the injected ammonia reacts with the NO x  in the SCR reactors  22 A ,  228 , and  22 C and converts the compound into nitrogen and water. The SCR reactors  22 A ,  22 B, and  22 C are conventionally filled with catalyst that can be either high temperature catalyst, low temperature catalyst or moderate temperature catalyst, with a corresponding level of cooling of the flue gas achieved in the convection section  16 A ,  16 B, and  16 C. 
     The present invention includes the installation of a spare SCR unit  34  of appropriate size in parallel to the other main SCRs  22 A ,  228  and  22 C. The spare SCR unit  34  is preferably located at the same height as the primary units and in reasonably close proximity to minimize the amount of ducting between units. Where one or more of the SCR units  22 A ,  22 B and  22 C is larger than the other(s), the SCR  34  is preferably essentially the same size as the largest SCR  22 A ,  22 B and  22 C. If there is an appreciable amount of ducting between the spare SCR unit  34  and the primary units  22 A,  22 B and  22 C, then the fans  32 A ,  32 B and  32 C may need to be slightly oversized to handle any additional pressure drop. Alternatively, the SCR  34  only needs to be sized sufficiently to function for a short term as a replacement or stand-by for the largest SCR unit  22 A ,  22 B and  22 C for the period of time necessary to service the primary SCR unit  22 A ,  22 B and  22 C in need thereof, and could thus be substantially smaller. The spare SCR unit  34  can be installed at the same time as the primary SCR  22 A ,  22 B and  22 C are installed, or it can be installed as a retrofit application. 
     A respective bypass supply duct  36 A ,  36 B, and  36 C is installed from each tee  38 A,  38 B, and  38 C disposed between ammonia injection grid  18 A ,  18 B, and  18 C and isolation blind plate  20 A ,  20 B, and  20 C. Each bypass duct  36 A ,  36 B, and  36 Cis in fluid communication with the spare SCR  34 , for example, by means of an inlet manifold  39 . The use of such a manifold  39  can be beneficial in diverting a portion of the flue gas from an underperforming SCR unit  22 A,  22 B, and  22 C to one or both of the remaining units. For example, if unit  22 A is not performing adequately, a portion of the flue gas can be diverted to SCR units  22 B and/or  22 C by reducing the speed of the fan  32 A and increasing the speed of the fans  32 B and/or  32 C. 
     A damper  40 A ,  40 B, and  4 OC is installed in each respective bypass pipe  36 A  36 B, and  36 C between the tee  20 A ,  20 B, and  20 C and the manifold  39  for isolating flow to the spare SCR unit  34 . An exit manifold  41  and respective return ducts  42 A ,  42 B, and  42 C allow for the treated exhaust to be reintroduced into the original heating system above the respective stack damper  28 A ,  28 B, and  28 C but upstream from the emissions sampling system  30 A ,  30 B, and  30 C and the fan. A damper  44 A ,  44 B, and  44 C is installed in each respective return duct  42 A ,  42 B, and  42 C for isolating the spare SCR unit  34 . 
     An example of the operation of the system occurs according to the following scenario. Assume that the primary SCR  22 A begins to develop an operating problem, such as, for example, its catalyst has lost activity and is not sufficiently reducing NO x  to required levels so that excessive NO x  bleeds through the SCR  22 A , and the catalyst needs to be replaced. The plant operator diverts the exhaust gases into the spare SCR unit  34  by initially opening dampers  40 A and  44 A leaving dampers  40 B,  40 C,  44 B and  44 C closed. Next, the plant operator closes the stack damper  28 A and inserts the isolation blind plate  20 A . The catalyst in the main SCR unit  22 A should be allowed to cool, and then the plant operator may then replace the spent catalyst with new catalyst using conventional catalyst replacement methodology and equipment. During this entire procedure, heaters  10 A ,  10 B, and  10 C can continue to operate. It should be noted that during the replacement of the catalyst the tee  38 A and the ducting downstream from the damper  28 A are under negative pressure minimizing the risk to maintenance personnel from exposure to hot gases. Once the new catalyst is introduced, the operator can bring SCR unit  22 A back on line by opening the stack damper  28 A and withdrawing the isolation blind plate  20 A, followed by closing the dampers  40 A and  44 A. 
     An alternate embodiment of the system is disclosed in FIG.  2 . The alternate embodiment provides a fan  60  and emission sampling system  62  associated with the spare SCR unit  34 , and eliminates the return ducting  42 A ,  42 B, and  42 C of the FIG. 1 embodiment. This has the advantage of eliminating ducting and dampers, but the added expense of the additional fan, emission sampling system, fan and stack. This system allows the operation of the spare SCR unit  34  for repair or replacement of a fan  32 A ,  32 B, and  32 C or other equipment, and can be particularly advantageous where the ductwork is difficult to install or the amount of ductwork is cost prohibitive. Therefore, having a dedicated spare fan  60  on the spare SCR  34  makes the system considerably more reliable and provides for longer operation between heater shutdowns.