Selective Catalytic Reduction is a means of converting nitrous oxides in the exhaust with the help of a catalyst into diatomic nitrogen and water.

A reductant Anhydrous Ammonia (NH3), Aqueous Ammonia (Ammonium Hydroxide) or Urea (Carbamide) solution is added to a stream of exhaust gas and is adsorbed onto a catalyst. Carbon Dioxide (CO2) is a reaction product when urea is used as the reductant.

The chemical equation for the reaction using either anhydrous aqueous ammonia for the process is

4NO + 4NH3 + O2 = 4N2 + 6H2O

2NO2 + 4NH3 + O2 = 3N2 + 6H2O

NO + NO2 + 2NH3 = 2N2 + 3H2O

The reaction for urea instead of anhydrous or aqueous ammonia is

4NO + 2(NH2)2CO + O2 = 4N2 + 4H2O + 2CO2 (in presence of catalyst)

Selective Catalytic Reduction

This exhaust gas after-treatment technology has a NOx abatement capability Of more than 80%. The SCR concept involves injecting a Urea-Water solution into the exhaust gas stream in combination with a special catalyst unit.

The SCR is considered as an additional and independent exhaust treatment system and as such does not interfere with the basic engine design or combustion process.

The process diagram below gives a better understanding of the SCR system wherein the urea interacts with nitrous oxides present in the incoming exhaust gas, in the presence of a catalyst, converting it into free nitrogen and water vapour.

The Maritime Environmental Protection Committee (MEPC) At The IMO has published guidelines for the certification of selective catalytic reduction (SCR) systems, referred to the “SCR Guideline”, namely IMO Resolution MEPC.198(62).

According to their configurations, SCRs can Be Classified into 2 Types- They can be either installed between The Exhaust Gas Manifold & The Turbocharger or between The Turbocharger and The Exhaust Gas Boiler.

1. High-Pressure SCR

In the High-Pressure SCR, the reactor is placed before the turbocharger. A sufficient exhaust gas temperature is to be maintained between 300 to 400 deg Celsius, which might be challenging when the engine is running at low loads and manoeuvring.

Therefore, for two-stroke engines, the most likely location of the SCR unit is before the turbocharger in order to expand the active range of SCR operation. This has little to no effect on the engine combustion process.

It is possible to run high-pressure SCRs on Heavy Fuel Oil.

2. Low-Pressure SCR

In Low-Pressure SCRs, the reactor is placed after the turbine. Pre-heating of the exhaust gas stream may be necessary in order to achieve a sufficient temperature at the reactor inlet for the catalytic reaction. Some power generation may be needed for preheating.

Components of an SCR Dosing Unit

The dosing unit consists of a compact external dosing system having a urea-water solution tank. The tank size depends upon how often the vessel enters NOx Tier III areas and how often the SCR is put in use. Urea Tank capacities range from 4 to 10 cub metres/MW for larger engines.

The urea for marine use is usually dissolved in water having a concentration of 32%-40%. Urea is a non-toxic odourless solution considered safe to transport and store at ambient temperature & pressure. However special caution is required in winter temperatures in order to avoid crystallization.

The dosing handling system provides the reducing agent (urea solution) based on the dosing demand signal provided by the SCR and Engine control and monitoring system.

Vaporizer/ Mixing Unit

The urea from the dosing system is metered and injected into the vaporizer or mixing unit. The injected reducing agent (urea) will vaporise and mix with the incoming exhaust gas.

The mixing unit is in line with the exhaust manifold of the engine and its pipes are designed & constructed after complex flow calculations & intensive testing, to ensure a good mixture of the urea solution & hot exhaust gases. The mixing unit is usually 2 to 6 meters long and 500mm in diameter, however, size may vary as per Engine size.

Injection tubes from the dosing unit penetrate the vaporizer from the bottom, the top of the vaporizer is equipped with an electronic enclosure having a NOx measurement sensor to monitor nitrous oxides in the exhaust gas and Backpressure sensor.

SCR Reactor Chamber

This is where the conversion of NOx in exhaust gas into nitrogen and water takes place in the presence of catalyst material. The SCR reactor contains cassettes of the catalyst substrate material. The substrate elements work in limited temperatures, if exhaust gas temperature is too high, the elements get destroyed.

If the temperature is too low, SCR efficiency is reduced. Catalyst element contains Vanadium Pentoxide (V2O5) which helps the reaction process of converting the urea and exhaust gas into nitrogen and water vapour. The SCR reactor volume is usually 1.5-3 cub metres/MW installed power.

Fuel Oil Quality and SCR technology

The sulphur content in fuel oil and consequent SO2 concentration in the exhaust gas is a critical parameter which has to be observed while operating SCR systems. Urea temperature is to be controlled according to sulphur content in fuel.

A high sulphur content in presence of a low exhaust gas temperature (in case of manoeuvring) will require a higher temperature of urea solution to be injected as a condensation of exhaust gas could result in corrosion and catalyst substrate damage. A lesser content of sulphur in fuel will allow a lesser temperature of urea solution to be injected.

Condensation of water vapour in the presence of sulphur in the exhaust gas during low load operations can cause the formation of solid ammonium bisulphate. Thus, the exhaust inlet temperature is to be kept high enough to avoid condensation of ammonium bisulphate onto catalyst substrate elements.

Condensation would severely affect NOx reduction performance and cause clogging, increasing backpressure due to soot formation in the reactor.

Soot Blowing Unit

To prevent contamination of the reactor elements, a soot blowing system is installed. Soot blowing is done using compressed air of 7 bar.

SCR Control Sensor Unit

NOx sensors measure the NOx concentration before the SCR reactor and the turbocharger.

The reactor chamber also contains outlet NOx sensors and outlet temperature sensors.

Venting System

The venting system vents the SCR reactor when the SCR is bypassed (i.e. when the engine is running in Tier-II mode) to avoid exhaust gas accumulation and soot formation in the reactor. The reactor is vented with Fresh Air during Tier II operation.

The Reactor Sealing Valve is used to seal the reactor during venting when the SCR is not in use.