June 2001, Vol. 19 (2) SCR AND SNCR PROCESSES INCREASE RISK OF AIR PREHEATER FOULING

The selective catalytic reduction

(SCR) process for NOx control involves injection of ammonia into the flue gas where it reacts with NOx in the presence of a catalyst to form molecular nitrogen and water. Not all of the injected ammonia participates in NOx reduction reactions. . . some reaches the air preheater as ammonia slip, where it reacts with SO3 to form ammonium sulfate and bisulfate compounds. These compounds can precipitate onto air preheater basket surfaces and cause problems of fouling and plugging. The available evidence suggests that bisulfate fouling will be a serious problem for some U.S. boilers. Researchers at the Energy Research Center have been developing the tools needed to Ammonium bisulfate deposits at ESP inlet in a coal-fired boiler with ammonia determine how best to limit air injection for NOx control. preheater fouling due to ammonium compounds. burned a 2.9 percent sulfur U.S. can lower the SO3 concentration in According to Richard Conn, coal without air heater fouling, but in the flue gas and thereby reduce the “Utilities in Japan and Western this case ammonia slip had to be potential for ammonium bisulfate Europe have had considerable maintained at very low levels.” formation. experience with SCR with coal-fired Conn continues, “A recent Of equal importance to SO3, is boilers and for the most part, they technical review by EPRI showed the level of ammonia slip. German have not experienced problems due there is very little SCR experience in experiences have shown that the to air preheater fouling. However, power stations firing coals deposition rate increased by a factor their SCR installations are on containing greater than 1.5 percent of four as the ammonia slip boilers which normally fire coal of sulfur. Of the issues most critical to increased from two to four ppm.” one percent or less sulfur content SCR application in the U.S., coal A major problem associated and the low incidence of fouling is sulfur content is probably the most with ammonium bisulfate formation probably due to the correspondingly significant. Almost 50 percent of is its impact on air preheater design low flue gas SO3 content at the air the coal fired east of the Mississippi and operation. The formation preheater inlet. In fact, in one case, contains two percent or more of temperature of the ammonium a boiler with SCR in Europe fired sulfur. Also significant is the fact bisulfate is such that it will U.S. Appalachian coal with two that Eastern U.S. coals contain precipitate out in the intermediate percent sulfur and this resulted in relatively low alkali levels. This and cold end baskets of the air excessive air preheater fouling eliminates the possibility that an preheater. Deposits, formed in the within only a week. A Danish unit alkali constituent such as calcium cold-intermediate layer, are hard to

Page 1 remove with sootblowers. This • modifying the basket design to there additives or changes in deposited ammonium bisulfate a notched flat surface instead of sootblowing practice or on-line water degrades heat transfer performance a double undulating surface, wash strategies which can be of the air preheater, acceler-ates • installing twin primary and helpful in controlling bisulfate corrosion of air heater surfaces, and secondary air heaters to allow fouling? Are there improved ways of traps fly ash, which can lead to on-line cleaning, controlling the ammonia injection excessive air preheater pressure • placing high pressure, high rate to minimize ammonia slip?” drop and ultimately cause a plant temperature steam sootblowers Sarunac continues, “Our group at shutdown. Ammonium bisulfate at both inlet and outlet of the air the Energy Research Center has deposition forms on heat transfer heater, had considerable experience in surfaces where the temperatures fall • installing enamel baskets to dealing with air preheater within 300-430°F. Experience in provide additional corrosion performance issues and fouling Japan shows that the fouling was resistance, problems. For example, we’ve noticeably higher when this • installing spring nozzle banks at developed modeling techniques for temperature band extended across the inlet of the coal basket to accurately calculating air and gas two layers of the air preheater. permit on-line water washing. In flow rates through the air preheater. It should be noted that addition, installing two- speed We’ve also developed software for ammonium bisulfate formation is not motors to slow the basket computing basket metal limited to SCR systems. This can rotation and allow more effective temperatures as a function of also occur with selective non- washing. basket design, unit load,

catalytic reduction technology Air preheater modifications such economizer O2 and other key (SNCR), which also involves as those described above can be operating factors. These codes can expensive. Some design studies be used to obtain insight into which Sarunac adds, “...The end goal is have shown that the cost of these portions of the air preheater are to develop the means to be able modifications are 25 percent of the susceptible to fouling due either to to minimize problems of bisulfate cost of the SCR catalyst. For a 500 formation of ammonium bisulfate or fouling through design changes, MW power plant, the cost of air condensation of sulfuric acid. operational changes and/or preheater modifications is We’ve also developed techniques for maintenance practice.” approximately $6 million. calculation of acid deposition rate Replacement of the air preheater on a cold metal surface. The rates injection of ammonia into the flue can cost as much as $25 million for depend on the local metal

gas. Ammonia slip can occur with a 500 MW power plant. temperature, concentration of SO3 in SNCR processes and, here too, this Conn adds, “Given the potential the flue gas and concentration of

can lead to the formation of impact on the air preheater and the H2O vapor in the inlet air to the ammonium sulfate compounds and expense associated with making preheater. We’ve carried out air preheater fouling. modifications to the air preheater to laboratory experiments, field tests One possible solution is to minimize the impact of fouling, we and theoretical analyses. The modify the air preheaters to make believe that it is important to find out techniques we developed previously them less susceptible to corrosion more about the mechanisms of for predicting rate of fouling are and plugging due to bisulfate bisulfate formation. We also need geared toward sulfuric acid formation. Indeed, extensive air information on the rates at which deposition. We plan to use those preheater modifications are currently bisulfate fouling occurs and plant as a starting point for investigating underway at some U.S. power design and operational rates of bisulfate formation. We are plants which are installing SCRs. characteristics that affect bisulfate also beginning to work on This typically involves either formation.” development of a technique for complete replacement of the air Nenad Sarunac adds, “We’ve calculation of the rate of ammonia preheater or extensive preheater developed a research program which slip out of an SCR and to calculate

upgrades. Some of the more we think will provide answers to the the SO3 concentration leaving an significant modifications to an air key questions. They are: What air SCR reactor.” preheater include: preheater basket design changes Sarunac adds, “We are • combining the cold and are most cost effective in minimizing interested in forming a consortium of intermediate baskets of the air bisulfate fouling? How should a sponsors to enable us to complete preheater into one continuous boiler and SCR system be operated this work on air preheater fouling. basket, to minimize ammonia slip? Are The end goal would be to develop

Page 2 the means to be able to minimize problems of bisulfate fouling through design changes, operational changes and/or maintenance practice.” €

For more information on APH fouling or fly ash ammonia removal, please contact John Sale at (610) 758-4545 or at [email protected].

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