Patent Publication Number: US-2004052712-A1

Title: Method for operating a flue gas purification plant

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention concerns the field of flue gas purification technology. It relates to a method for operating a flue gas purification plant according to the preamble of claim 1.  
       [0003] 2. Discussion of Background  
       [0004] The brand name SCONOx describes a relatively new process for reducing NOx emissions in combustion flue gases of gas turbines, diesel engines and the like. NOx is deposited as potassium nitrite and potassium nitrate on an SCONOx absorber (see U.S. Pat. No. 5,953,911 and the article by L. Czarnecki et al., SCONOx—Ammonia Free NOx Removal Technology for Gas Turbines, Proc. of 2000 Int. Joint Power Generation Conf., Miami Beach, Fla., Jul. 23-26, 2000).  
       [0005] Since it is easy for the SCONOx absorber to be deactivated by SO 2  in the flue gas, another absorber is connected upstream of it—a so-called SCOSOx absorber—which absorbs SO 2  from the flue gas and hence protects the SCONOx absorber. The chemical reactions taking place in the two absorbers are described in detail in the aforementioned article by L. Czarnecki.  
       [0006] As soon as the deposition capacity of at least one of the absorber types has been exhausted (typically after about 20 minutes), the absorbers need to be regenerated. This is done by subdividing the overall absorber into individual chambers, which can be individually disconnected from the flue gas stream by using switchable dampers. For regeneration, selected chambers are disconnected from the flue gas stream while the other chambers remain in the flue gas stream. A regeneration gas, which consists of hydrogen, natural gas or other hydrocarbons and an oxygen-free carrier gas (usually steam), is passed through the disconnected chambers in order to regenerate both the NOx absorbers and the SO 2  absorbers of the chambers in question. However, since the two different absorber types respond differently during the regeneration, they are regenerated separately. This is made possible by an arrangement of feed and discharge lines and valves for the regeneration as reproduced by way of example in FIG. 1.  
       [0007]FIG. 1 shows an absorber chamber  11  of a flue gas purification plant  10 , through which flue gas to be purified is sent from a combustion process. The unpurified flue gas  25  flows into the chamber  11  from the left. The purified flue gas  26  flows out of the chamber  11  again toward the right. The chamber  11  can be disconnected from the flue gas stream for regeneration purposes using two dampers  12  and  13 , which are arranged at the input and the output. In the figure, the dampers  12 ,  13  have already been closed.  
       [0008] In the chamber  11 , a first absorber  14  (SCOSOx) for absorbing SO 2  and a second absorber  15  (SCONOx) for absorbing NOx are arranged successively spaced apart in the flow direction. A feed line  26  for the regeneration gas opens via a first valve  17  (inlet valve) into the intermediate space between the first and second absorbers  14  and  15 . Discharge lines  21  and  24 , in which a second valve  16  and a third valve  19  (outlet valves) are respectively fitted, are connected before the first absorber  14  and after the second absorber  15 , as seen in the flow direction. Within a regeneration phase, the first valve (inlet valve)  17  is opened so that regeneration gas can flow in. The other two valves (outlet valves)  16  and  19  are opened in succession, so that the associated absorbers  14  and  15  can be regenerated successively. The SO 2  absorber  14  is usually regenerated first (valves  16  open; valve  19  closed). The regeneration gas in the feed line  27  is produced from steam  23 , by means of a reformer  20 , and natural gas containing methane delivered through a valve  18 .  
       [0009] In the flue gas purification plant  10 , typically about ten chambers  11  of the type represented in FIG. 1 are connected in parallel, two of which will be in the regeneration phase at any given time. With a regeneration time of 5 minutes per individual regeneration, 25 minutes are required in total to regenerate each of the chambers  11  once (=25-minute cycle time).  
       [0010] While the SCOSOx absorber  14  is being regenerated, flue gas containing oxygen is still present in the section of the absorber chamber  11  which contains the SCONOx absorber  15 . A CFD (computational fluid dynamics) analysis of the flow distribution inside the absorber chamber  11  during the regeneration has now shown that there is a risk that some of this flue gas may be entrained in the injection zone owing to turbulence, so that oxygen could enter the SCOSOx absorber  14 . However, the presence of oxygen can impede the regeneration of the SCOSOx absorber. This may lead to local breakthrough of SO 2  during the absorption phase, and therefore to accelerated deactivation of the SCONOx absorber.  
       SUMMARY OF THE INVENTION  
       [0011] Accordingly, one object of the invention is to provide a novel method for operating, and in particular for regenerating, a combined SCOSOx/SCONOx flue gas purification plant which reduces or fully eliminates the risk of the regeneration of the SCOSOx absorber being compromised by flue gas containing oxygen.  
       [0012] The object is achieved by all the features of claim 1. The central feature of the invention is that the section of the absorber chamber with the absorber to be regenerated later is first purged with a purge gas before the start of the regeneration of the absorber which is regenerated first. In this way, remaining flue gas components which may compromise the regeneration of the absorber are substantially removed from the absorber chamber before the actual regeneration begins.  
       [0013] The regeneration gas is preferably used as the purge gas.  
       [0014] The method is particularly effective if the SCOSOx absorber is regenerated first and the SCONOx absorber is regenerated afterward.  
       [0015] The purging is preferably carried out over a time period of several seconds, in particular between 15 and 30 seconds.  
       [0016] Further refinements of the invention are given in the dependent claims. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0017] A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:  
     [0018]FIG. 1 shows the exemplary layout of an individual chamber with SCONOx and SCOSOx absorbers and regeneration devices, in a flue gas purification plant such as is used in the prior art and is suitable for carrying out the method according to the invention; and  
     [0019]FIG. 2 shows a comparable representation to FIG. 1 of a modified flue gas purification plant which can also be operated with the method according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0020] Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, it is proposed according to the invention that the section of the absorber chamber with the absorber to be regenerated second (generally the SCONOx absorber) be purged before the regeneration of the absorber which is regenerated first (generally the SCOSOx absorber) is begun. In the arrangement of the flue gas purification plant  10  in FIG. 1, this means that after the dampers  12 ,  13  have been closed, the outlet valve  19  is first opened for a predetermined time period in order (with the regeneration gas flowing in through the inlet valve  17 ) to purge the section with the SCONOx absorber  15 . The regeneration gas contains hydrogen and/or hydrogen compounds, for example hydrocarbons such as natural gas or propane. Since higher hydrocarbons can be converted more easily than methane (the main component of natural gas), this may constitute an alternative to natural gas subject to local availability. It is also conceivable to use higher hydrocarbons directly for the regeneration, i.e. without prior reforming into hydrogen. For the regeneration of the SCOSOx absorber  14 , the outlet valve  19  is then closed and the outlet valve  16  is opened, so that the regeneration gas can flow through the SCOSOx absorber  14 . Lastly, the outlet valve  16  is closed and the outlet valve  19  is re-opened, in order to regenerate the SCONOx absorber  15 . A time period of from 15 to 30 seconds for the purging of the SCONOx absorber  15  is generally sufficient to remove the major part of the flue gas in this section of the absorber chamber.  
     [0021] In reality, the effective purging does not begin until the dampers  12 ,  13  have been closed fully. Since the inlet valves  17  are already open at that time when the dampers  12 ,  13  start to close, and it takes about 15 seconds for the dampers to be closed fully, the total opening time of the valves for purging should be between 30 and 45 seconds. Since the regeneration already begins during the purging phase, the purging does not add to the overall duration of the regeneration phase but can be regarded as part of the regeneration of the second absorber (generally the SCONOx absorber).  
     [0022] The purging according to the invention may also be employed with other valve configurations. An example of such a different configuration is reproduced in FIG. 2 and has two inlet valves  17  and  29  and one outlet valve  16 . In this case, the inlet valve  29  (together with the outlet valve  16 ) is opened first in order to flush out the SCOSOx absorber  15 , before the inlet valve  29  is then opened instead of the inlet valve  17  in order to regenerate the SCOSOx absorber  14 . Lastly, the inlet valve  29  is re-opened in order to regenerate the SCONOx absorber  15 .  
     [0023] Generally speaking, the invention can be used for all configurations and operating modes in which regeneration gas needs to be injected into the absorber chamber between an SCOSOx absorber and an SCONOx absorber while the chamber has not yet been filled completely with flue gas.  
     [0024] Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.  
                               LIST OF DESIGNATIONS                                                10   Flue gas purification plant           11   Absorber chamber           12, 13   Damper           14   Absorber (SCOSOx)           15   Absorber (SCONOx)           16, . . . , 19   Valve           20   Reformer           21, 24   Discharge line (regeneration)           22   Natural gas (NG)           23   Steam           25   Flue gas (unpurified)           26   Flue gas (purified)           27, 28   Feed line (regeneration)           29   Valve