- SCR DeNOx systems require anhydrous ammonia at a minimum purity of 99.5 percent by weight conforming to IS 1588.
- The stoichiometric molar ratio of NH3 to NOx is 1:1; operating NSR is typically controlled between 0.95 and 1.05.
- Ammonia slip must be kept below 3 to 5 parts per million at the SCR outlet to comply with CPCB emission standards.
- Vaporiser sizing must include a 20 to 25 percent safety margin above maximum calculated ammonia consumption.
- Storage vessels require PESO approval under SMPV Rules and must be sited within a bunded containment area.
- MoEFCC NOx emission limits effective from 2015 effectively mandate DeNOx systems at large Indian power plants.
- What Is SCR DeNOx and Why Anhydrous Ammonia Is the Preferred Reagent
- The Chemistry of SCR: NOx Reduction Reactions Explained
- Anhydrous Ammonia Specification for SCR Service
- Dosing System Design: Flow Measurement, Control and Distribution
- Vaporiser Selection and Sizing
- Ammonia Storage Requirements for SCR Installations
- SCR Catalyst Types and Operating Conditions
- Indian Regulatory Framework: MoEFCC NOx Standards and PESO Requirements
- Selecting the Right System Configuration for Your Application
- Operational Best Practices and Maintenance Schedule
- Industries Using SCR DeNOx Systems in India
- Frequently Asked Questions
Nitrogen oxides (NOx) are among the most regulated pollutants in combustion flue gases worldwide, and the Selective Catalytic Reduction (SCR) process is the most effective commercially proven technology for their removal. At the heart of every SCR DeNOx installation is a controlled supply of ammonia, the reducing agent that reacts with NOx over a catalyst to produce harmless nitrogen gas and water vapour. For large fixed installations such as coal-fired power plants, industrial boilers and cement kilns, anhydrous ammonia is the reagent of choice due to its low cost per unit of NOx removed, its well-established handling technology and its compatibility with high-throughput dosing systems.
At Jaysons Chemical Industries, we supply anhydrous ammonia to SCR installations across India and provide technical support on specification, storage design and regulatory compliance. This guide covers every technical aspect of anhydrous ammonia use in SCR DeNOx systems, from chemistry to dosing control to PESO-compliant storage.
1. What Is SCR DeNOx and Why Anhydrous Ammonia Is the Preferred Reagent
Selective Catalytic Reduction is a post-combustion flue gas treatment process that removes nitrogen oxides by reacting them with ammonia over a solid catalyst at elevated temperatures. The term “selective” refers to the fact that the reaction preferentially targets NOx rather than other flue gas components such as oxygen and carbon dioxide. The process achieves NOx removal efficiencies of 80 to 95 percent under properly designed and maintained conditions.
Three ammonia-based reagents are used in SCR systems worldwide: anhydrous ammonia, aqueous ammonia solution (typically 19 to 29 percent concentration) and urea solution. Anhydrous ammonia offers the highest ammonia content per unit mass at virtually 100 percent NH3, the lowest reagent cost per kilogram of NOx removed, and the simplest dosing chemistry because no decomposition step is required before the ammonia enters the SCR reactor. Its main handling challenge is that it is stored under pressure as a liquefied gas, requiring PESO-compliant pressure vessel storage and comprehensive safety systems.
For a 500 MW coal-fired power plant operating at full load with an inlet NOx concentration of 800 mg per normal cubic metre and a target removal efficiency of 85 percent, the daily anhydrous ammonia consumption is approximately 8 to 12 tonnes. This scale of consumption makes the reagent cost advantage of anhydrous ammonia over urea solution significant: typically 15 to 25 percent lower on a cost-per-tonne-NOx-removed basis.
2. The Chemistry of SCR: NOx Reduction Reactions Explained
The primary chemical reactions in the SCR process involve the reduction of nitric oxide (NO) and nitrogen dioxide (NO2), which together constitute the bulk of NOx in combustion flue gas, with ammonia as the reducing agent over a vanadium-titanium dioxide (V2O5-TiO2) catalyst.
The standard SCR reaction for NO reduction is: 4NH3 + 4NO + O2 → 4N2 + 6H2O. This reaction produces only nitrogen and water as products and is the predominant reaction at typical SCR operating temperatures between 300 and 420 degrees Celsius. The fast SCR reaction, which occurs when the inlet NOx contains a significant proportion of NO2, is: 2NH3 + NO + NO2 → 2N2 + 3H2O. This reaction proceeds at a significantly faster rate than the standard SCR reaction and is exploited in advanced catalyst systems to achieve higher NOx conversion at lower temperatures.
Side Reactions and Their Consequences
At temperatures below 280 degrees Celsius, the SCR reaction competes with undesirable side reactions that produce ammonium sulphate and ammonium bisulphate on the catalyst surface. These deposits progressively block catalyst pores, reduce the active surface area and eventually cause irreversible deactivation. Sulphur in the fuel is the precursor to sulphur trioxide (SO3) which drives ammonium bisulphate formation. Minimum catalyst operating temperature must be maintained at all load conditions to prevent sulphate fouling.
Key Reaction Data: The standard SCR reaction is highly exothermic at elevated temperatures but is essentially thermoneutral at typical SCR operating conditions of 300 to 400 degrees Celsius. The catalyst does not provide the energy for the reaction but rather lowers the activation energy barrier, enabling the reaction to proceed at temperatures far below those required for thermal (non-catalytic) NOx reduction.
3. Anhydrous Ammonia Specification for SCR Service
The purity of anhydrous ammonia used in SCR systems is not merely a commercial consideration. Impurities in the ammonia reagent can poison SCR catalysts, accelerate corrosion of dosing system components and contribute to secondary pollutant formation in the flue gas. The specification for SCR-grade anhydrous ammonia must address each of these risks.
| Parameter | Minimum Specification | Preferred Specification | Effect of Exceedance |
|---|---|---|---|
| Ammonia purity (% w/w) | 99.5 minimum | 99.8 minimum | Increased impurity burden on catalyst and piping |
| Water content (% w/w) | Below 0.2 | Below 0.1 | Corrosion of carbon steel components; catalyst activity reduction |
| Oil content (ppm w/w) | Below 5 | Below 2 | Catalyst pore blocking; reduced NOx conversion efficiency |
| Sulphur compounds (ppm w/w) | Below 1 | Below 0.5 | Catalyst poisoning; accelerated ammonium bisulphate formation |
| Iron content (ppm w/w) | Below 1 | Below 0.5 | Catalyst active site deactivation |
| Non-condensable gases (%) | Below 0.1 | Below 0.05 | Dosing flow measurement errors; vaporiser inefficiency |
The relevant Indian standard for anhydrous ammonia quality is IS 1588. Most industrial-grade anhydrous ammonia produced in India and supplied by established manufacturers including Jaysons Chemical Industries meets or exceeds the 99.8 percent purity specification. However, purchasers should always request a Certificate of Analysis (CoA) with each delivery and verify that the CoA covers all parameters listed in the SCR system design specification.
4. Dosing System Design: Flow Measurement, Control and Distribution
The ammonia dosing system is the interface between the storage vessel and the SCR reactor. Its function is to deliver a precise, continuously adjustable flow of ammonia vapour to the injection grid in the flue gas duct upstream of the SCR catalyst. The accuracy and responsiveness of the dosing system directly determines the SCR system’s ability to maintain the target outlet NOx concentration while keeping ammonia slip within regulatory limits.
Liquid Ammonia Transfer and Flow Measurement
Liquid ammonia is withdrawn from the storage vessel through the liquid valve at the base of the vessel and transported to the vaporiser via insulated carbon steel pipework. A mass flow meter, typically a Coriolis-type meter, measures the liquid ammonia flow rate with accuracy of plus or minus 0.5 percent. The Coriolis meter signal is the primary control input for the dosing control system. Turbine or rotary meters should not be used for liquid ammonia metering because ammonia flash vaporisation across the meter internals causes measurement errors and accelerated wear.
Ammonia Distribution Grid
Uniform distribution of ammonia across the cross-section of the flue gas duct upstream of the catalyst is critical for achieving the design NOx removal efficiency. Non-uniform ammonia distribution results in regions of local excess (causing high local ammonia slip) and regions of local deficit (causing high local NOx breakthrough). The injection grid is typically designed using computational fluid dynamics (CFD) analysis to account for the specific flow profile of the installation. Grid materials must be austenitic stainless steel or carbon steel with appropriate ammonia compatibility ratings.
A closed-loop NOx control system adjusting ammonia flow based on continuous stack NOx monitoring reduces ammonia consumption by 5 to 12 percent compared to open-loop systems operating at fixed stoichiometric ratios. The payback period for the additional instrumentation cost is typically under 18 months at full plant load for a 200 MW unit.
High-Purity Anhydrous Ammonia for SCR Applications
Jaysons Chemical Industries supplies 99.8 percent purity anhydrous ammonia in bulk tankers and tonners to power plants, cement kilns and industrial boilers across India. Certificate of Analysis provided with every delivery.
5. Vaporiser Selection and Sizing
The vaporiser converts liquid ammonia from the storage vessel into ammonia vapour suitable for injection into the flue gas duct. Vaporiser selection depends on the required vapour flow rate, the available heating medium and the operating flexibility required.
Electric Immersion Vaporisers
Electric vaporisers use electric resistance heaters immersed in a water bath, which in turn heats the liquid ammonia through the vaporiser tube wall. They are compact, require no steam supply and offer precise temperature control. They are preferred for installations with a maximum ammonia throughput below 100 kilograms per hour. The heating element must be correctly sized to prevent local overheating of the ammonia, which would cause premature decomposition of trace organic impurities and increase the catalyst fouling rate.
Steam-Heated Shell and Tube Vaporisers
For larger SCR installations, shell and tube vaporisers heated by low-pressure steam at 3 to 5 bar are the standard design. Liquid ammonia passes through the tube side and steam condensate heats the shell side. The design must incorporate a superheat section to ensure the vaporiser outlet is fully in the vapour phase under all operating conditions, including minimum load and cold start. A vaporiser outlet temperature of 20 to 30 degrees Celsius above the saturation temperature at the outlet pressure confirms adequate superheating.
| Vaporiser Type | Typical Throughput | Heating Medium | Best Application | Key Design Consideration |
|---|---|---|---|---|
| Electric water bath | Up to 100 kg/hr | Electric resistance | Small to medium SCR, no steam available | Element temperature limit; water bath level control |
| Steam shell and tube | 100 to 2,000 kg/hr | Low-pressure steam 3 to 5 bar | Large power plant SCR | Superheat confirmation; steam trap sizing |
| Hot water shell and tube | 50 to 500 kg/hr | Hot water 80 to 100 deg C | Waste heat recovery integration | Water loop pump sizing; fouling factor |
| Ambient air vaporiser | Up to 50 kg/hr | Ambient air (finned tubes) | Standby or emergency use only | Frost formation in high humidity; capacity reduction in cold weather |
6. Ammonia Storage Requirements for SCR Installations
Ammonia storage for SCR installations must balance operational continuity (sufficient inventory to sustain plant operation through supply disruptions) against the regulatory and safety implications of maintaining a large ammonia inventory on site. The MSIHC Rules 1989 threshold for anhydrous ammonia is 150 tonnes, above which a full safety report and emergency planning notification is required. Most power plant SCR installations are designed with storage inventories below this threshold to avoid the additional regulatory burden.
Vessel Design and Siting
Horizontal pressure vessels to IS 2825, designed for 17.5 bar (gauge) working pressure and ambient temperature, are the standard storage solution for SCR applications in the 5 to 100 tonne range. Vessels must be installed within a bunded area sized to contain 110 percent of the largest vessel volume in the storage installation. The bund must be constructed from reinforced concrete or compacted earth lined with an impermeable membrane, and must have a valved drain connection to a neutralisation sump.
The storage area must be located at a minimum distance of 15 metres from the plant boundary, 30 metres from occupied buildings, and 10 metres from ignition sources and roads. These distances may be reduced in accordance with a formal quantitative risk assessment submitted to and accepted by PESO and the local factory inspectorate. Fixed ammonia gas detectors must be installed around the vessel perimeter at grade level, with alarms set at 25 parts per million and emergency shutdown initiated at 50 parts per million.
7. SCR Catalyst Types and Operating Conditions
The SCR catalyst is the functional core of the DeNOx system. Catalyst selection determines the operating temperature window, the susceptibility to poisoning by flue gas impurities, and the catalyst replacement interval. Understanding catalyst requirements helps in specifying the correct ammonia dosing parameters.
Vanadium-Titanium Dioxide Catalysts
Vanadium pentoxide on a titanium dioxide support (V2O5-TiO2) is the standard SCR catalyst for coal-fired power plants and large industrial boilers. It operates in the temperature window of 300 to 420 degrees Celsius and achieves NOx removal efficiencies of 80 to 95 percent. The catalyst is typically formed into honeycomb modules with a cell pitch of 6 to 10 millimetres, allowing flue gas to pass through without excessive pressure drop even when carrying fly ash particles.
Zeolite Catalysts
Copper and iron-exchanged zeolite catalysts are used in diesel engine SCR applications operating at higher temperatures between 400 and 600 degrees Celsius. They are also used in applications where sulphur levels in the flue gas are very low, as zeolites can be irreversibly deactivated by sulphur trioxide even at concentrations as low as 1 part per million. Zeolite catalysts are not typically used in coal-fired applications due to their sulphur sensitivity.
Catalyst activity degrades over time due to thermal sintering, poisoning by arsenic and alkali metals from the coal ash, and physical erosion by fly ash particles. Most power plant SCR catalysts are designed for a minimum service life of 16,000 hours before the first regeneration or replacement. Maintaining the ammonia dosing within the design NSR range is one of the most effective ways to preserve catalyst activity and extend service life.
8. Indian Regulatory Framework: MoEFCC NOx Standards and PESO Requirements
The regulatory driver for SCR installation in India is the Ministry of Environment, Forest and Climate Change (MoEFCC) notification of December 2015, which set significantly more stringent emission limits for coal-fired thermal power plants. The notification established NOx emission limits of 100 mg per normal cubic metre for new plants commissioned after January 2017 and 300 mg per normal cubic metre for existing plants in critically polluted areas. Achieving these limits from a typical uncontrolled NOx level of 600 to 1,200 mg per normal cubic metre requires SCR or SNCR installation at most large plants.
For the ammonia storage and handling side of the SCR system, PESO compliance under the SMPV (Unfired) Rules 1981 is mandatory for all pressure vessels. The Gas Cylinders Rules 2016 apply to any ammonia cylinders or tonners used as intermediate storage or for start-up supply. The MSIHC Rules 1989 apply to facilities storing above the threshold quantities. Additionally, state pollution control board consent to operate must be updated to reflect the SCR system modification.
9. Selecting the Right System Configuration for Your Application
The choice between anhydrous ammonia, aqueous ammonia solution and urea as the SCR reagent is the first and most consequential decision in SCR system design. For large fixed installations above 50 MW thermal input, anhydrous ammonia is almost universally the preferred choice on cost and operational grounds. For smaller installations, mobile applications or sites where the political or regulatory difficulty of anhydrous ammonia storage is prohibitive, urea solution is the practical alternative.
Within anhydrous ammonia-based SCR systems, the choice of vaporiser type, storage vessel configuration and dosing control strategy depends on the specific flue gas flow rate, NOx inlet concentration, load variation profile and available utilities. A properly conducted front-end engineering design (FEED) study, supported by CFD modelling of the ammonia injection grid, is essential for new SCR installations above 100 MW to confirm the design before detailed engineering begins.
10. Operational Best Practices and Maintenance Schedule
SCR system performance degrades over time primarily due to catalyst deactivation and ammonia injection system fouling. A structured maintenance programme addresses both mechanisms and maintains system performance within regulatory compliance limits throughout the catalyst service life.
Daily and Weekly Operational Checks
The following checks should be performed daily: verify ammonia storage vessel pressure and level are within expected range for the ambient temperature; confirm ammonia flow rate matches the control system setpoint; check outlet NOx and ammonia slip monitor readings against the previous day trend; verify all fixed ammonia detectors show zero or background level. Weekly checks include inspection of vaporiser outlet temperature and comparison with design curves, and valve position verification for all emergency shutdown valves.
Annual Maintenance Items
Annual maintenance activities include full internal inspection of the vaporiser, cleaning and calibration of all flow meters and pressure transmitters, testing of all ammonia emergency shutdown valves under simulated trip conditions, inspection and recertification of all pressure safety valves on the storage vessel and vaporiser circuit, and a catalyst pressure drop measurement to assess the degree of ash plugging.
11. Industries Using SCR DeNOx Systems in India
SCR DeNOx technology is deployed across a growing range of industries in India as environmental regulations tighten. Understanding the full spectrum of SCR users helps identify the appropriate system scale and ammonia supply arrangement.
Related Reading
Key Takeaways
- SCR DeNOx systems use anhydrous ammonia at 99.5 to 99.8 percent purity as the NOx-reducing reagent; impurities cause catalyst poisoning and dosing system corrosion.
- The stoichiometric molar ratio of NH3 to NOx is 1:1; operating NSR is controlled between 0.95 and 1.05 to balance NOx removal efficiency against ammonia slip.
- Vanadium-TiO2 catalysts operating between 300 and 420 degrees Celsius are the standard for coal-fired applications in India.
- Vaporiser sizing must include a 20 to 25 percent safety margin; steam-heated shell and tube vaporisers are preferred above 100 kg per hour throughput.
- Ammonia storage vessels require PESO approval; most SCR installations are designed below the 150-tonne MSIHC threshold.
- MoEFCC NOx emission limits effective from 2015 require SCR or SNCR at most large coal-fired power plants in India.
- A closed-loop dosing control system based on continuous outlet NOx monitoring reduces ammonia consumption by 5 to 12 percent versus open-loop control.
Frequently Asked Questions
What purity grade of anhydrous ammonia is required for SCR DeNOx systems?
SCR DeNOx systems require anhydrous ammonia with a minimum purity of 99.5 percent by weight, conforming to IS 1588 or equivalent specification. Water content must be below 0.2 percent by weight and oil content below 5 parts per million. Total impurities other than water must remain below 0.1 percent. Industrial-grade anhydrous ammonia at 99.8 percent purity is the preferred specification for all SCR installations.
What is the molar ratio of ammonia to NOx used in SCR systems?
The stoichiometric molar ratio of ammonia to NOx for the standard SCR reaction is 1:1. In practice, SCR systems are designed with a normalised stoichiometric ratio (NSR) of 0.95 to 1.05. Excess ammonia beyond the design NSR causes ammonia slip. Most modern SCR controllers target ammonia slip below 3 parts per million at the catalyst outlet.
Can urea solution be used instead of anhydrous ammonia in SCR systems?
Yes, urea solution (32.5 percent aqueous urea, known as AdBlue or AUS32) is a common alternative in SCR applications, particularly for smaller installations where anhydrous ammonia storage creates safety challenges. Urea requires a decomposition unit to convert it to ammonia before it enters the catalyst. Anhydrous ammonia is preferred for large fixed installations because it eliminates the decomposition step, reduces reagent cost per unit of NOx removed, and simplifies dosing system design.
What temperature range does the SCR catalyst operate in?
Conventional vanadium-titania SCR catalysts operate in the temperature window of 280 to 420 degrees Celsius. Below 280 degrees Celsius, catalytic activity drops sharply and ammonium bisulphate formation accelerates. Above 420 degrees Celsius, catalyst sintering begins to degrade activity. High-temperature zeolite catalysts for diesel SCR operate between 400 and 600 degrees Celsius.
How is the ammonia vaporiser sized for an SCR dosing system?
The vaporiser is sized based on maximum ammonia consumption rate with a 20 to 25 percent safety margin. Electric water bath vaporisers are preferred below 100 kg per hour. Steam-heated shell and tube vaporisers are used above 100 kg per hour. The design must include a superheat section to ensure fully vapour-phase outlet under all operating conditions.
What are the storage requirements for anhydrous ammonia used in SCR systems?
Horizontal pressure vessels rated to 17.5 bar (gauge) and designed to IS 2825 are the standard for SCR ammonia storage. Storage capacity is typically 7 to 15 days of operation at design consumption rate. PESO approval is required for all vessels above 1,000 litres. Vessels must be within a bunded area with fixed ammonia detectors, emergency shut-off valves and a safety shower within 10 seconds travel distance.
What is ammonia slip and how is it controlled in SCR systems?
Ammonia slip is unreacted ammonia in the flue gas downstream of the SCR catalyst. The CPCB specifies a maximum of 5 parts per million for large thermal power plants. Slip is controlled by a closed-loop dosing control system adjusting ammonia flow in response to continuous outlet NOx measurements. Catalyst replacement or regeneration is required when slip cannot be controlled within limits despite correct dosing.
Which Indian regulations require SCR DeNOx systems at power plants?
The MoEFCC notification of December 2015, amended in 2021, sets NOx emission limits of 100 mg per normal cubic metre for new coal-fired power plants and 300 mg per normal cubic metre for older plants in designated areas. These limits effectively require SCR or SNCR DeNOx systems at most large coal-fired power plants in India. Enforcement has intensified significantly since 2023.
