COAL-FIRED POWER PLANTS:NITROGEN OXIDE CAPTURE

NITROGEN OXIDE CAPTURE

As with sulfur dioxide, it is possible to remove nitrogen oxide after it has been formed in the flue gas of a power plant. The process involves the injection of either ammonia gas or urea into the flue-gas stream. Either chemical reacts with the nitrogen oxide present, converting the oxides into nitrogen and water.

If the ammonia or urea is injected into the hot flue-gas stream where the temperature is between 870 oC and 1200 oC, the reaction will occur spontaneously. This is called selective noncatalytic reduction (SNCR). At lower temperatures, the reaction is too slow, so a special metal catalyst is necessary to stimulate the reaction process. Where a catalyst is used, the process is called selective catalytic reduction (SCR).

SNCR will remove between 35% and 60% of the nitrogen oxide from the flue-gas stream. However, if care is not taken it can lead to contamination of fly ash with ammonia and to ammonia slip—the release of excess ammonia into the atmosphere. Nevertheless, it has been utilized at power plants in several parts of the world.

More widely used than SNCR is SCR. SCR units are commonly found on gas turbine power stations but may also be fitted to a coal-fired power plant where low nitrogen oxide combustion strategies do not reduce the emissions levels to below the regulatory limits (Figure 3.12). Typical flue-gas temperatures in an SCR unit are 340–380 oC. The hot gases pass over a solid catalyst that is normally made from a vanadium-titanium-based material or from a zeo- lite. The system is generally capable of removing 70–90% of the nitrogen oxide emissions from a flue-gas stream. Ammonia slip, although still possible, tends to be less of a problem.

There are two major drawbacks to SCR. First, it can only be used with low sulfur coals (up to 1.5% of sulfur), and second, it is expensive. The catalyst requires changing every three to five years. In addition, sulfur in coal entering the SCR can be converted into trioxide (SO3), which becomes highly corrosive on contact with water when it forms sulfuric acid. This has to be carefully con- trolled or it can be extremely damaging both in the power plant and in the atmosphere.

Power Generation Technologies-0200

It is also possible to remove nitrogen oxide using a wet scrubbing system. The reagent sprayed into the flue-gas stream in this case is a solution sodium hydroxide, although hydrogen peroxide has also been used. The reaction that takes place leads to the formation of nitrites and nitrates that must be isolated for disposal. Wet scrubbing is not normally used in power stations for nitrogen oxide removal.

Incoming search terms:

Related posts:

Testing and Commissioning of Protective Relays and Instrument Transformers:Protective Relays and Cla...
Electrical Power System Grounding and Ground Resistance Measurements:Selection of Grounding Method
Engine mechanics:Lubrication system
COAL-FIRED POWER PLANTS
COAL-FIRED POWER PLANTS:BOILER TECHNOLOGY
NATURAL GAS–FIRED GAS TURBINES AND COMBINED CYCLE POWER PLANTS:ADVANCED GAS TURBINE CYCLES
HYDROPOWER:CATEGORIES OF HYDROPOWER PLANT
Thermal Loading of Several Multilevel Converter Topologies for 10 MW Wind Turbines Under Low Voltage...
The Current Situation and Perspectives on the Use of Wind Energy for Electricity Generation:Denmark
Probabilistic Modeling and Statistical Characteristics of Aggregate Wind Power:Aggregate Wind Power ...
Conversion Efficiency Improvement in GaAs Solar Cells:FDTD Software for the Simulation of Nanostruct...
Emerging SMES Technology into Energy Storage Systems and Smart Grid Applications:SMES Application To...
Multilevel Converters for Step-Up-Transformer-Less Direct Integration of Renewable Generation Units ...
Frequency Control and Inertial Response Schemes for the Future Power Networks:System Frequency Respo...
Wind Farm Protection:Wind Farm and Conventional Generation Protection

Leave a comment

Your email address will not be published. Required fields are marked *