Nitrogen Oxides Abatement

Regulations

Regulations concerning limitation of atmospheric emission of NOx from industrial sources vary from region to region. Under the 1990 Amendments to the Clean Air Act, the EPA has undertaken a review of its current national standards, and has tightened the standards, particularly as pertains to non-attainment areas. The EPA standard for new nitric acid manufacturing plants is 3 pounds NOx per ton of nitric acid produced. This means that new plants must reduce NOx emission levels from 1500-3000 ppm to 200 ppm. The requirements for existing plants are complicated by controversy over methods for measuring ambient NOx levels, as well as the wide variations in state and regional requirements alluded to earlier. Typically, existing nitric acid plants are allowed to produce 5.5 pounds of NOx per ton of nitric acid produced. Standards for other industries are being promulgated.

Nitrogen Oxide Sources

Combustion of fuels:

At high temperatures, oxygen and nitrogen present in air combine to form nitrogen oxides. Typical flue gas samples contain 100-1500 ppm of nitrogen oxides.

Nitric acid manufacture:

Waste gases which cannot be economically recovered in the final absorber usually contain 2-3% nitrogen oxides based on weight of acid produced.

Metal finishing operations:

Many metal surface treatment operations which use nitrates, nitrites, or nitric acid which evolve nitrogen oxides. Examples of such operations include bright dipping, phosphatizing, desmutting, and pickling of stainless steel.

Chemical processes:

Many processes where nitric acid, nitrates, or nitrites are used as reagents evolve nitrogen oxides. Examples include manufacture of explosives, plastics, dyes, etc.

High temperature processes:

Processes where materials are made at high temperatures such as glass manufacturing, electric furnaces, and cement kilns evolve nitrogen oxides.

Processes for Nitrogen Oxide Removal

There are several methods for controlling NOx emissions. Gas scrubbing is one of the most common forms of NOx treatment, with sodium hydroxide being the conventional scrubbing medium. However, the absorbed NOx is converted to nitrite and nitrate which may present wastewater disposal problems. Scrubbing solutions containing hydrogen peroxide are also effective at removing NOx, and can afford benefits not available with NaOH. For example, hydrogen peroxide adds no contaminants to the scrubbing solution and so allows commercial products to be recovered from the process, e.g., nitric acid. In its simplest application, H2O2 (0.5-1 wt.%) and nitric acid (35-45 wt.%) are used to scrub both nitric oxide (NO) and nitrogen dioxide (NO2) — the chief components of NOx from many industrial sources. The reactions are rapid at moderate temperatures (30-80 deg-C), with about 1.7 and 0.37 lbs hydrogen peroxide required per lb NO and NO2, respectively. The chemistry controlling the process is outlined below:

There are several other processes which also use hydrogen peroxide to remove NOx. The Kanto Denka process (1) employs a scrubbing solution containing 0.2% hydrogen peroxide and 10% nitric acid while the Nikon process uses a 10% sodium hydroxide solution containing 3.5% hydrogen peroxide. A fourth process, the Ozawa process, scrubs NOx by spraying a hydrogen peroxide solution into the exhaust gas stream. The liquid is then separated from the gas stream, and the nitric acid formed is neutralized with potassium hydroxide. The excess potassium nitrate is crystallized out, and the solution reused after recharging with hydrogen. In addition to the methods cited above in which NOx is oxidized to nitric acid or nitrate salts, a series of Japanese patents describe processes and equipment for reducing NOx to nitrogen using hydrogen peroxide and ammonia (3, 4, 5, 6). The processes are efficient but must be carried out at higher temperatures than the processes cited earlier.

A second approach to controlling NOx emissions involves its elimination at the source. This typically involves adding H2O2 directly to HNO3 contained in e.g., metal pickling baths, where the H2O2 reacts instantaneously with HNO as it is formed, thereby eliminating its decomposition to NO and NO2. In this way, the nitric acid is regenerated in-situ without the expense of scrubbers. Compared to urea, which is also used for in-situ control, the hydrogen peroxide process affords true nitric acid recovery and does not degrade the quality of the finished product.

Download our NOx Technical Bulletin (pdf)

Download our NOx Eliminate Visual Emissions Bulletin (pdf)

References

1. Japanese Patent 4815766 to Kanto Denka, Kogyo Co., Ltd. ‘N-Oxide Removal From Exhaust Gas,” February 18, 1973.
2. Japanese Patent 8083-069 to T. Ozawa, “Absorbing Nitrogen Oxides From Exhaust Gases,” November 6, 1973.
3. Japanese Patent 78-144457, to Kokai, “Method and Equipment for Removing NOx in Exhaust Gas,” December 15, 1978.
4. Japanese Patent 78-146262, to Kokai, ” NOx Removal Equipment,” December 20, 1978.
5. Japanese Patent 78-146968, to Kokai, ” NOx Decomposition Method in Boiler Burned Gas,” December 21, 1978.
6. Federal Register, Vol. 42, 160, August 18, 1977.

—FMC Technical Data, Pollution Control Release No. 119