How does ammonia escape occur?

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How does ammonia escape occur? Ammonia escape results from the combined effect of various factors, with the main reasons including: 1. Uneven ammonia spraying (improper distribution): This is one of the most common causes. If the ammonia injected into the flue does not mix sufficiently and evenly with the nitrogen oxides in the flue gas, it will result in excessively high or low ammonia concentrations in certain areas. In areas with too high a concentration, ammonia cannot be completely consumed by the catalyst in that area, and thus it escapes directly through the catalyst layer. In areas with too low a concentration, the efficiency of nitrogen oxide removal in those areas decreases. An unreasonable design of the ammonia spraying grid, clogged nozzles, and poor flow field distribution can all lead to uneven ammonia spraying. 2. Decline in catalyst performance (deactivation or clogging): Catalyst poisoning/deactivation: Alkali metals (K, Na), alkaline earth metals (Ca), heavy metals (As, Pb, Zn), phosphorus, sulfur, and other substances present in flue gases can cover the active sites of the catalyst or alter its chemical structure, resulting in a decline in the catalyst’s activity. This prevents the catalyst from effectively catalyzing the reaction between ammonia and nitrogen oxides, leading to the escape of unreacted ammonia. Catalyst clogging/dusting: Fly ash in the flue gas deposits on the surface or within the pores of the catalyst, physically covering the active sites and reducing the contact area between the reactant gases (NH3, NOx) and the catalyst. This lowers the reaction efficiency and increases the risk of ammonia escape. Catalyst aging/sintering: When operating at high temperatures for an extended period, the catalyst gradually ages; the active components sinter and aggregate, resulting in a decrease in specific surface area and a reduction in activity. Catalyst wear: Flue gas erosion causes mechanical wear of the catalyst, resulting in a reduction in its effective active material. 3. Inappropriate reaction temperature: Too low a temperature – When the flue gas temperature is below the catalyst’s optimal operating temperature range, its activity decreases significantly, the reaction rate slows down, and a large amount of ammonia fails to participate in the reaction in time, resulting in increased emissions. At the same time, ammonium bisulfate, a by-product, is more easily formed and deposited at low temperatures. Excessively high temperature: Although catalyst activity may be high at elevated temperatures, temperatures that are too high can exacerbate catalyst sintering and deactivation. Additionally, side reactions such as the decomposition of ammonia itself or its over-oxidation with oxygen may occur. This, in turn, reduces the efficiency of ammonia in denitrification and may also indirectly make it difficult to control ammonia slip. 4. Improper control of the ammonia-to-nitrogen ratio: The key control parameter in an SCR system is the ratio of the amount of ammonia injected to the total amount of nitrogen oxides in the flue gas. If the nitrogen oxide concentration set or measured by the control system is low (due to inaccurate instruments), or if the control algorithm responds slowly, the actual amount of ammonia sprayed will exceed the theoretical amount required for the chemical reaction (i.e., the molar ratio of ammonia to nitrogen > 1), which inevitably results in excess ammonia escaping as it fails to react. Even if the average molar ratio is appropriate, transient fluctuations may lead to short-term excessive ammonia injection. 5. Influence of flue gas components: Sulfur dioxide: SO₂ is oxidized to SO₃ under the action of a catalyst. SO₃ reacts with the escaping ammonia and water vapor in the flue gas to form viscous ammonium bisulfate. NH₃ + SO₃ + H₂O → NH₄HSO₄. Ammonium bisulfate is not only a pollutant in its own right, but it also deposits on equipment downstream of the catalysts (such as air preheaters), causing blockages and corrosion. Although SO₃ primarily affects the final form and hazards of the escaped ammonia, its formation also consumes some of the ammonia. 5. Influence of flue gas components: Sulfur dioxide: SO₂ is oxidized to SO₃ under the action of a catalyst. SO₃ reacts with the escaping ammonia and water vapor in the flue gas to form viscous ammonium bisulfate. NH₃ + SO₃ + H₂O → NH₄HSO₄. Ammonium bisulfate is not only a pollutant in its own right, but it also deposits on equipment downstream of the catalysts (such as air preheaters), causing blockages and corrosion. Although SO₃ primarily affects the final form and hazards of the escaped ammonia, its formation also consumes some of the ammonia. Other components: The moisture and oxygen content in the flue gas, among others, also affect the reaction equilibrium and rate. 6. Flue gas flow rate and residence time: If the flue gas flow rate is too high, or if the catalyst layer is designed to be too thin, the flue gas (containing ammonia) spends too little time within the catalyst bed. As a result, ammonia and nitrogen oxides do not have enough time to react properly before passing through the catalyst layer, leading to emissions. 7. System design and installation issues: Improper flue design (such as the presence of dead corners and vortices), low efficiency of mixers, and poor sealing during the installation of catalyst modules, which can lead to short circuits in the flue gas flow, all affect the mixing effect and reaction efficiency, thereby increasing ammonia emissions. 8. Load fluctuations and start-up/shutdown phases: During periods of rapid changes in the unit’s load or during start-up and shutdown, flue gas flow rates, temperatures, and nitrogen oxide concentrations experience significant fluctuations. In such situations, it may be difficult for the control system to accurately monitor and adjust the amount of ammonia injected, which can lead to excessive ammonia injection or uneven mixing, resulting in temporary increases in ammonia emissions. In summary, the root cause of ammonia escape is that the amount of ammonia injected locally or in total exceeds what the catalyst can effectively handle under specific operating conditions. The key factors leading to this outcome are: uneven mixing, catalyst deactivation, temperature mismatch, and excessive control. Ammonia escape not only leads to waste of resources (an increase in ammonia consumption), but more seriously, it causes blockages and corrosion in downstream equipment such as air preheaters and dust collectors, results in visible \"blue/yellow smoke\" pollution, and generates fine particulate matter. Therefore, strictly controlling ammonia escape is key to ensuring the efficient, safe, and environmentally friendly operation of the SCR system. Control measures typically include optimizing the design of the ammonia injection grid, improving the flow field and mixing, enhancing catalyst management and replacement, precisely controlling the molar ratio of ammonia to nitrogen, and maintaining an appropriate reaction temperature.