NOx ABATEMENT CATALYSTS
Overview
NOx ABATEMENT CATALYSTS
The environmental regulations adopted in most countries, require that the nitrogen oxides content of the tail gas, exhausted to the atmosphere from nitric acid plants or caprolactame plants, is kept below 50 ppmv or even less. To meet those strict limits, many different techniques have been proposed, but catalytic reduction is the most widely adopted.
The ESPINDESA technology for NOx abatement is based on the use of its own, or partner, selective catalyst.
The heat evolved in the reaction increases the tail gas temperature by5 to 25º C depending on the NOx content of the tail gas.
Ammonia reacts selectively with nitrogen oxides in the presence of oxygen. The chemical reactions which occur in the presence of an adequate catalyst are:
6NO + 4NH3 ······> 5N2 + 6H2O
6NO2 + 8NH3 ······> 7N2 + 12H2O NO + NO2 + 2NH3 ······> 2N2 + 3H2O 4NO + O2 + 4NH3 ······> 4N2 + 6H2O
The heat evolved in the reaction increases the tail gas temperature by5 to 25º C depending on the NOx content of the tail gas.
Temperatures below 180° C must be avoided due to the risk of formation of ammonium nitrate-nitrite. Anyway, if due toa miss operation those salts deposit over the catalyst, ESPINDESA has developed procedures for regeneration «in situ» which restore the activity of the catalyst in few hours.
The operating pressure can range from atmospheric to 10 bar and the location of the catalyst can be either upstream or downstream the tail gas expansion turbine.
Process Description
Gaseous ammonia is mixed with the tail gas in a Static Mixer. If the temperature of gas is lower than the optimum for the operation of the catalyst, the gas is preheated in a gas exchanger with the hot gas leaving the Reactor.A start up heater uses to be provided in those cases.
The mixture of ammonia and tail gas passes through the Reactor where the NOx reduction to nitrogen takes place.
The purified tail gas returns to the Nitric Acid Unit. Critical parameters are recorded and backed with alarms. An interlock contact is provided to close the ammonia feed in the event of a shutdown of the nitric acid plant.
OPERATING REQUIREMENTS
Ammonia consumption
The ammonia consumption depends of the NOx content in the tail gas but never is higher than 2-3% over the stoichiometric requirements.
Effluents
NOx less than 50 ppm
NH3 less than 5 ppm
Process Flow Diagram
