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Show that yield the economy of scale and reliable 90 percent reduction afforded by SCR. Thus, in a nutshell, we see the low NOx technology mix rather naturally resolving itself into a pretty simple formula - in addition to fuel-switching to eliminate the firing of nitrogenous fuel oils, a lot of ultra low NOx burners and a few large selective catalytic reduction flue gas treatment plants. SCR Evolution In the 1970s, there emerged quite a trade in junkets to view japanese installations of selective catalytic reduction flue gas treatment plants. Naturally, we learned great deal and filed away a great many observations that constitute an important element of the experience that we apply in our judgements today. But the bottom line of the japanese junkets, really, is that while SCR can cost significantly more than combustion modification per pound of NOx removed, selective catalytic reduction flue gas treatment plants work just fine , at least for stationary power generation . Keep in mind, however, that steam doesn 't burn, a point to which we will return in a moment. Early in the evolution of SCR technology, with the certainty that SCR is expensive there came also wide spread fear of frequent catalyst changes at costs upwards of a million dollars in large systems, the inevitability of catalyst activity degradation or bed plugging by ammonium sulfate deposition , operational difficulties and maintenance breakdowns, and doubt about reduction claims of 90 percent. In the japanese junkets of the late 1970s, no foundation was found for any of these fears . But then again , the Japanese systems hadn 't been operating all that long. So some eight years later we returned, perhaps to find that all those systems that had apparently been operating so well in 1979 had by then fallen apart. It turned out they were still doing just fine, thank you, so much so that some operators didn 't even know they had SCRs. Of course, that is the pragmatic dream of an operationally " good" system - one that runs all the time, achieving its process objective virtually without operator attention and nearly maintenance free . Except for one plant, nobody had changed catalyst, nor had any plans to change catalyst, nor had any indication of catalyst degradation despite regular tests. The one plant that had renewed catalyst was one that, upon start up, had the misfortune to purchase and store a halfcharge of fresh catalyst in order to "be prepared." When the plant operators got tired of management's questions about utilization of the stored catalyst, they simply arbitrarily changed it out, disposing of the old catalyst and taking care not to order any more for the store house . As to performance degradation or plugging due to ammonium sulfate deposition, in conference after conference, no problems had been experienced anywhere. Then we visited a huge power station that had a monster boiler in peaking service firing sulfurous fuel oil. If anybody was going to have problems, they had to be the ones. Upon start up and shut down, the operators confirmed, the flue gas dropped below the sulfate condensation temperature . Nevertheless, they neither turned off the ammonia during these periods nor utilized the 500tblowers installed on the catalyst bed to clean off the sulfate deposits. They simply waited for the flue gas temperature to come up again, whereupon the deposits disappeared. Although the sulfate deposition "problem" seems to be self-correcting, it remains prudent to stop injecting ammonia at low flue gas temperatures and, with the cooperation of the South Coast Air Quality Management District. we have built this procedure into our permits. In the Japanese stationary power generation experience, the only serious problem associated with selective catalytic reduction flue gas treatment plants is cost, exacerbated for retrofits chiefly by the lack of space in tight existing plants, and the necessity of major surgery on the existing flue gas system to find the right reaction temperature in the neighborhood of 700·F. However, steam doesn 't burn and in the refinery we heat hydrocarbon liquids and vapors that do . Thus, it is useful to keep in mind that tube ruptures and subsequent fires do not threaten SCRs on utility boilers. An alternative to major surgery to provide access to the right SCR reaction temperature window, indigenous to many japanese stationary power installations that have extensive waste 4 |
