Ammonia slip is now a mandatory indicator - can your denitrification system still cope?
If your plant is still running its denitrification system on the "inject a little extra for safety" approach, it is time to redo the math.
In March 2026, the Ministry of Ecology and Environment released the draft "Emission Standard of Air Pollutants for Thermal Power Plants and Boilers" for comment, bringing ammonia slip into nationally mandated air-pollutant control for the first time, with a limit of 8 mg/m³. Ammonia slip is therefore no longer an internal reference figure that a plant can treat loosely - online monitoring data will now serve as a legal red line for enforcement. The cement industry is tightening in parallel: from April 1, 2026, kiln-stack ammonia emissions are subject to the same limit. In the past, many companies treated ammonia-slip exceedances as "no big deal" - the fines were mild and rectification was a formality. The situation is now completely different. An exceedance not only brings environmental penalties, it also directly affects environmental-tax surcharges and the company's performance rating, and that rating in turn determines your priority for production cuts during heavy-pollution weather emergencies. For continuous-production plants, the losses from forced curtailment far exceed the fines.
Your "legacy system" most likely cannot cope 8mg/m³
Start with a fact that is easy to overlook: 8mg/m³ is the unified control limit, but the technical specification for the SCR process demands 2.5mg/m³. The "Technical Specification for Flue Gas Denitrification Engineering of Thermal Power Plants" (HJ 562-2010) explicitly stipulates that SCR ammonia slip be kept below 2.5mg/m³, with an ideal operating window of only 0.3~0.8mg/m³.
Most existing denitrification systems were simply not designed for precise control. The classic problem of a traditional ammonia-injection system is single-point sampling combined with uniform injection. Across the duct cross-section, NOx concentration can deviate by as much as ±50%, yet the system adjusts injection for the entire cross-section from a single reading near the probe. The result: persistently high ammonia slip in the over-injected zones, and fluctuating, non-compliant NOx emissions in the under-injected zones.
Operators are forced into a "better too much than too little" strategy to keep NOx compliant. Ammonia-solution consumption climbs - and ammonia slip climbs with it. This is not an operator-skill problem; it is an inherent flaw of the system architecture.
What does an ammonia-slip exceedance really cost?
Many people focus only on how much extra they spend on ammonia solution, but the biggest cost of ammonia slip shows up in the equipment.
Escaped ammonia reacts with SO₃ in the flue gas to form ammonium bisulfate (ABS) - an extremely sticky, highly corrosive substance that deposits directly on the air-preheater heat-exchange surfaces, causing blockage, rising pressure drop and surging fan power consumption. In severe cases one side of the air preheater plugs with crystallized deposits, the induced-draft fan stalls and fights for air, the furnace goes positive-pressure, and the unit is forced to reduce load or shut down unplanned. For a large thermal power plant, a single shutdown for cleaning and repair can cost on the order of millions of RMB.
Even more insidious is catalyst loss. ABS and fly ash keep plugging the catalyst micropores and catalyst activity decays rapidly. Once denitrification efficiency falls, the only option is to inject even more ammonia to hold NOx - so ammonia slip climbs further. More injection, more exceedance, more damage: an irreversible vicious cycle.
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There is one more easily overlooked hidden cost: once an ammonia-slip exceedance is recorded by online monitoring, the knock-on effects - environmental-tax surcharges and rating downgrades - keep unfolding over the following months. A downgraded rating means you may be among the first required to curtail production during heavy-pollution episodes, a loss that is hard to put a number on.
The core logic of precision ammonia injection is not complicated: let each zone's injection rate follow that zone's actual NOx concentration.
Concretely, the technical route has three steps:
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Step 1: full-cross-section, real-time sensing. Install multi-point grid flue-gas measurement devices at the SCR reactor inlet and outlet to capture real-time NOx and O₂ concentration profiles across the whole cross-section - an end to the blind-men-and-the-elephant guesswork of single-point sampling.
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Step 2: independent zone control. Retrofit the plant-wide injection header into independently controlled zones, each with its own control valve, so that high-concentration zones receive more ammonia and low-concentration zones receive less.
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Step 3: intelligent closed-loop control. Upgrade the denitrification control algorithms to closed-loop, profile-based regulation: the system anticipates NOx trends and adjusts injection before a fluctuation occurs, rather than remedying things after an exceedance.
Three things to do now
First, check whether your online monitoring equipment is truly up to the task. Ammonia slip levels are extremely low - SCR outlets typically run at only 0.3~0.8mg/m³ in the ideal range, so the analyzer's detection limit and interference rejection are critical. If your existing equipment is still an old chemical-absorption system or uses insufficiently accurate sensors, the reliability of the data itself is questionable. Our precision-injection rapid ammonia analyzers LT-D2000,LT-D2000N are unaffected by high temperature, dust and moisture, sample and analyze rapidly in real time, and truly measure the same source at the same time! →Precision-injection rapid analyzer - newly launched, stronger than ever
Second, carry out a full-cross-section concentration profile test. Many plants only know that the outlet NOx "average" is compliant, without knowing how large the cross-section deviation is. If the deviation exceeds 15%, injection uniformity has an obvious problem, and the payoff from a precision-injection retrofit will be immediate.
Third, bring ammonia slip into daily operating-performance assessment. Ammonia-slip data used to be "for reference only"; now it is enforcement evidence. Operator-team KPIs must change accordingly - assessing only NOx compliance is no longer enough; ammonia-slip control must be assessed at the same time.
Closing remarks:The shift of ammonia slip from an optional indicator to a legal red line is, in essence, forcing denitrification systems to move from crude operation to fine-grained management. Companies that completed precision-injection retrofits early have already secured a triple win - "compliance without fines + 20% lower ammonia use + no air-preheater plugging". For those still sitting on the fence, the question is no longer whether to retrofit, but when, and how far, in order to keep pace with tightening standards.