1. WHY IS AMMONIA NEEDED?
Nitrogen oxides (NOx) emissions from power plants are regulated and usually require the plants to put in NOx reduction technology. One of the important methods for removing NOx derived from the burning of fossil fuels embodies the conversion to inert nitrogen gas and water vapor by reduction processes such as Selective Catalytic Reduction (SCR) or Selective Non-Catalytic Reduction (SNCR). Two main reactants have achieved commercial acceptance for this purpose: ammonia and urea. For the SCR process ammonia gas is generally injected in the process exhaust gas in a 500° to 6500F temperature zone up stream of a catalyst bed. However, ammonia presents substantial environmental and operating hazards and risks because of its high volatility and noxious nature.
Safety Issues with Ammonia?
Numerous accidents that have resulted in deaths have occurred in the transport and handling of ammonia (Noroozi, 1993). Additionally, local authorities have placed restrictions on its use in many locations. Urea, on the other hand, is a stable, non-volatile, environmentally benign material that is safely transported, stored and handled without such risk. This is the material, which is stored and transported for use with the U2A® system.
- Urea poses less risk in the workplace
- Using urea, the permitting process is easier
- Anhydrous and aqueous ammonia (>20%) are classified as regulated toxic substances by the EPA (40 CFR Parts 9 and 68)
- Anhydrous ammonia is a "highly hazardous chemical" under OSHA 29 CFR Part 1910
- Many local governmental authorities have imposed use restrictions
- Ammonia is a highly volatile and noxious material
Here is why:
| Physiological responses: | ppm in air |
|---|---|
| Least detectable odor | 53 |
| Maximum concentration - prolonged exposure | 100 |
| Least amount causing immediate throat irritation | 408 |
| Least amount causing coughing | 1720 |
| Dangerous for even short exposure | 2500-4500 |
| Immediately likely to be fatal | >10,000 |
How It Works
In the U2A® process, dry urea is dissolved to form an aqueous solution, which is fed to an in-line reactor at a rate to produce the required ammonia. Heat is applied to carry out the generation under controlled conditions to maintain a constant ammonia gas supply pressure. The process produces a gaseous mixture of ammonia, carbon dioxide, and water. The process requires no storage of ammonia except for the small amount in the reactor in which it is at an active concentration of less than 2%.
The process is automated, easy to control and far safer than standard ammonia supply systems. The only chemicals required are urea and water.
