Ammonia slip: what it is and how to prevent it in NOx abatement

Ammonia slip is a recognised operational consideration in industrial emissions control, particularly in Energy From Waste (EfW) facilities using ammonia-based NOx abatement systems.

During the combustion of waste, nitrogen oxides (NOx) are formed and must be controlled before flue gases are released into the atmosphere. Selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR) systems use ammonia or urea to convert nitrogen oxides into nitrogen and water.

However, not all the ammonia introduced into the process necessarily reacts with the available NOx. Any remaining unreacted ammonia that passes through the emissions treatment system is known as ammonia slip.

Controlling ammonia slip is important for maintaining emissions performance, reducing unwanted secondary effects and managing the consumption of aqueous ammonia.

What is ammonia slip?

Ammonia slip occurs when ammonia injected into a flue gas treatment system does not fully react with nitrogen oxides.

In an effective NOx abatement process, ammonia acts as a reducing agent. It reacts with nitrogen oxides to produce nitrogen and water vapour.

The simplified reaction with nitric oxide is:

4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O

When the reaction is incomplete, some ammonia remains in the flue gas and can pass through to downstream treatment equipment or the stack.

Ammonia slip is generally measured as an ammonia concentration in the flue gas, commonly expressed in mg/Nm³ or ppm, depending on the monitoring and reporting requirements.

For EfW operators, managing this concentration is an important part of emissions control and environmental permit compliance.

Why does ammonia slip occur?

There are several reasons why ammonia may not react completely with nitrogen oxides.

The effectiveness of the reaction depends on the relationship between ammonia dosing, NOx concentration, temperature, mixing and the time available for the reaction to take place.

Incorrect ammonia dosing

One of the main causes of ammonia slip is introducing more ammonia than the available NOx can react with.

The amount of ammonia required changes according to combustion conditions and the concentration of nitrogen oxides in the flue gas.

If dosing does not respond appropriately to these changes, excess ammonia can remain unreacted.

Accurate dosing control is therefore essential to balancing NOx reduction against ammonia consumption and slip.

Incorrect operating temperature

Temperature has a significant influence on ammonia-based NOx reduction.

In SNCR systems, the reaction takes place within a specific high-temperature operating window. If the temperature is too low, the reaction may not proceed effectively, leaving ammonia unreacted.

If temperatures are too high, unwanted competing reactions can occur, reducing the effectiveness of the process.

In SCR systems, the catalyst allows the reaction to take place at lower temperatures, but performance still depends on operating within the catalyst’s specified temperature range.

Poor reagent distribution and mixing

Ammonia needs to be distributed effectively throughout the flue gas stream.

If injection nozzles or lances do not provide appropriate coverage, some areas may receive too much ammonia while others receive too little.

This uneven distribution can reduce overall NOx removal efficiency and increase the amount of unreacted ammonia.

Insufficient reaction time

The ammonia and nitrogen oxides need sufficient contact time under suitable conditions to complete the reaction.

Flue gas flow rates, injection positioning and the design of the abatement equipment all influence the available reaction time.

If the conditions do not allow sufficient contact, ammonia slip may increase.

Ammonia slip in SNCR systems

Selective non-catalytic reduction (SNCR) is widely used in industrial combustion processes, including Energy From Waste facilities.

The system injects ammonia or urea into the furnace at a suitable temperature, allowing the reagent to react with nitrogen oxides without requiring a catalyst.

SNCR typically operates within a temperature window of approximately 850°C to 1,100°C, although the optimum range depends on system design and reagent selection.

The main operational challenge is maintaining the correct relationship between temperature, NOx concentration and ammonia dosing.

Changes in waste composition, combustion conditions and plant load can affect the conditions within the furnace.

If ammonia is injected outside the optimum temperature zone or distributed unevenly, the reaction may be incomplete.

For this reason, SNCR systems require careful process monitoring and appropriate control of reagent injection.

Ammonia slip in catalytic SCR systems

Selective catalytic reduction (SCR) uses a catalyst to promote the reaction between ammonia and nitrogen oxides.

The catalyst enables NOx reduction at lower temperatures than SNCR, with the operating range depending on catalyst type and system configuration.

Ammonia is introduced upstream of the catalyst, where it mixes with the flue gas before passing through the catalytic material.

SCR can achieve high NOx removal efficiencies, but ammonia slip remains possible.

Potential causes include:

• Excessive ammonia dosing
• Poor ammonia distribution
• Uneven gas flow through the catalyst
• Catalyst ageing or deterioration
• Operating outside the catalyst’s specified temperature range
• Changes in flue gas composition

Catalyst condition is particularly important. Over time, catalyst activity can decline, affecting the system’s ability to achieve the required NOx reduction.

Routine inspection and performance monitoring help operators identify deterioration and assess whether maintenance or replacement is required.

What are the consequences of ammonia slip?

Ammonia slip is not simply an indication that a NOx abatement system is using too much reagent. It can also affect downstream equipment, emissions performance and operational costs.

Environmental emissions

Unreacted ammonia can pass through the treatment system and be released into the atmosphere.

Ammonia emissions are regulated within the wider framework of industrial emissions control, and EfW facilities must operate within the limits specified by their environmental permits.

Formation of ammonium salts

Ammonia can react with acidic components in flue gas, including sulphuric acid and hydrochloric acid, to form ammonium salts.

Depending on the conditions and the composition of the flue gas, these compounds can contribute to deposits within downstream equipment.

Potential consequences include fouling, blockages and increased maintenance requirements.

Increased operating costs

Excessive ammonia dosing increases reagent consumption without necessarily delivering additional NOx reduction.

For facilities using substantial quantities of aqueous ammonia, this can represent a significant avoidable operating cost.

Improving dosing efficiency can therefore support both emissions management and chemical procurement efficiency.

Downstream equipment performance

Ammonium salt deposits can affect heat exchangers, ductwork and other components within the flue gas treatment system.

Managing ammonia slip helps reduce the potential for these issues and supports reliable plant operation.

How can ammonia slip be prevented?

Reducing ammonia slip requires a combination of process control, equipment performance and effective monitoring.

There is no single adjustment that will suit every plant. The appropriate approach depends on the abatement technology, furnace design, flue gas characteristics and environmental permit requirements.

1. Optimise ammonia dosing

Ammonia dosing should be matched to the NOx concentration and operating conditions of the plant.

Automated control systems can use emissions monitoring and process data to adjust reagent injection in response to changing conditions.

The objective is to introduce sufficient ammonia to achieve the required NOx reduction without unnecessary excess.

2. Maintain the correct temperature window

Temperature is particularly important in SNCR systems.

Operators need to ensure that ammonia is injected into the appropriate temperature zone and that the process remains within the operating parameters established for the equipment.

For SCR systems, maintaining the catalyst within its specified operating range is equally important.

3. Improve ammonia distribution

Injection equipment should provide appropriate ammonia distribution across the flue gas stream.

The number and positioning of injection lances, nozzle condition and gas flow characteristics all influence mixing.

Plant operators should work with the system manufacturer or emissions control specialist to assess distribution where ammonia slip indicates a potential problem.

4. Monitor NOx and ammonia emissions

Continuous emissions monitoring and process data provide valuable information about abatement performance.

Monitoring NOx and ammonia concentrations together helps operators understand whether changes in reagent dosing are improving emissions performance or increasing ammonia slip.

Temperature, flue gas flow and other relevant process measurements should also be considered.

5. Maintain abatement equipment

Injection nozzles, dosing pumps, pipework and control systems require appropriate inspection and maintenance.

For SCR installations, catalyst condition should also be assessed in accordance with the manufacturer’s recommendations.

Poorly maintained equipment can affect reagent delivery, mixing and reaction efficiency.

6. Review aqueous ammonia quality and consistency

The quality and concentration of the ammonia solution used in an abatement system are important considerations.

Aqueous ammonia concentration affects how much active ammonia is delivered for a given volume of solution.

If the concentration differs from the expected specification, dosing calculations and reagent consumption may be affected.

Working with a reliable ammonia supplier and maintaining consistent product specifications supports predictable dosing and process management.

The role of aqueous ammonia in Energy From Waste

Energy From Waste facilities commonly use aqueous ammonia as a reducing agent in SNCR and SCR emissions treatment systems.

Aqueous ammonia is ammonia dissolved in water. It provides the active ammonia required for the chemical reaction that converts nitrogen oxides into nitrogen and water.

The concentration selected depends on the plant’s equipment, storage arrangements, dosing system and operating requirements.

Blended Products supplies aqueous ammonia in two established concentrations:

Aquamonia 245 – 24.5% aqueous ammonia

A 24.5% solution for industrial applications, including appropriately designed NOx abatement systems.

Aquamonia 330 – 33% aqueous ammonia

A 33% solution for industrial applications requiring a higher concentration product.

The correct product should be selected according to the plant’s technical specification and handling arrangements. Consistency of concentration and dependable availability are important considerations for facilities operating continuous emissions treatment processes.

Choosing a reliable UK ammonia supplier

For an Energy From Waste facility, ammonia is often a production-critical chemical. A disruption in supply can affect the plant’s ability to maintain the required emissions treatment process.

When reviewing an ammonia supplier, operators and procurement teams should consider more than the delivered price per tonne.

Important factors include:

• Consistent product concentration
• Product quality and documentation
• Reliable bulk delivery
• Suitable storage and transport arrangements
• Supply chain resilience
• Technical understanding
• Responsive customer service

Blended Products is an independent UK supplier of specialist gases and chemicals, with experience in aqueous ammonia supply and bulk chemical logistics.

Our approach focuses on understanding the customer’s application, delivery requirements and expected consumption to establish a dependable supply arrangement.

Frequently asked questions

What is ammonia slip in NOx abatement?

Ammonia slip is the unreacted ammonia that remains in the flue gas after an SNCR or SCR process. It occurs when the ammonia does not fully react with nitrogen oxides before the gases leave the treatment system.

What causes ammonia slip in Energy From Waste plants?

Common causes include excessive ammonia dosing, unsuitable reaction temperatures, poor reagent distribution, insufficient reaction time and deterioration in SCR catalyst performance.

Is ammonia slip worse in SNCR or SCR systems?

Ammonia slip can occur in both systems. It is generally more challenging to control in SNCR installations because the reaction depends on maintaining the correct high-temperature conditions without a catalyst. Actual performance depends on the design and operation of the individual plant.

How can ammonia slip be reduced?

Ammonia slip can be reduced through appropriate reagent dosing, temperature control, effective ammonia distribution, emissions monitoring and regular equipment maintenance.

Does aqueous ammonia cause ammonia slip?

Aqueous ammonia does not inherently cause ammonia slip. It provides the ammonia used in the NOx reduction reaction. Slip occurs when some of that ammonia remains unreacted, usually because of process conditions or dosing inefficiencies.

Can ammonia slip damage plant equipment?

Ammonia can react with acidic compounds in flue gas to form ammonium salts. Depending on operating conditions, these can contribute to deposits, fouling and corrosion-related problems in downstream equipment.

What concentration of aqueous ammonia is used for NOx abatement?

The concentration depends on the plant’s storage, dosing and process requirements. Blended Products supplies Aquamonia 245 at 24.5% and Aquamonia 330 at 33% for appropriate industrial applications.

Where can I find a UK ammonia supplier for Energy From Waste?

Blended Products supplies aqueous ammonia to industrial customers across the UK, including businesses using ammonia for NOx abatement. Our team can discuss product specifications, bulk quantities and delivery requirements.