Encyclopedia of Environmental Science and Engineering, Volume I and II

NITROGEN OXIDES REDUCTION 767

principle behind the catalytic converter is that the exhaust

gases are passed through a catalyst bed which effectively

lowers the temperatures at which CO and hydrocarbons are

oxidized and at which NO x is reduced.^18 Typical catalysts

include alumina, platinum, or metal oxides.

Diesel engines Control measures for diesel engines are

similar to those for spark ignition engines. However, because

diesel engines generally operate at fuel lean conditions, cata-

lytic reduction of NOx is not feasible. Therefore, NO x emis-

sions are reduced from diesel engines through combustion

based control measures. These methods include exhaust

gas recirculation and injection timing retardation. As with

the spark ignition engine, EGR reduces NO x emissions by

lowering the peak combustion temperature through dilution

with cool combustion gases. Likewise, retarding the injec-

tion timing results in a delay of the heat release in the cycle,

thereby lowering the peak flame temperature. As noted

above, both of these techniques result in reduced NO x emis-

sions, but at the potential cost of fuel economy and engine

performance.

CONCLUSION

As our knowledge of NO x advances and as political

climates change, NO x emission regulations will inevitably

change, prompting new or modified control technologies to

be developed. Although sometimes viewed as cumbersome

and costly, regulations are the key to providing a healthy

environment for generations to come.

REFERENCES

1. Stern, A.C., Air Pollution, Volume 1, Air Pollution and Its Effects, 2nd Ed.
   Academic Press, New York (1968).


2. US Department of Health and Human Services, Centers for Disease
   Control, and National Institute for Occupational Safety and Health.
   “NIOSH Pocket Guide to Chemical Hazards” (1990).


3. Ross, R.D., Air Pollution and Industry, Van Nostrand Reinhold Com-
   pany, New York (1972).


4. Abrahamson, D.E., The Challenge of Global Warming, Island Press,
   Washington, D.C. (1989).


5. Howard, R. and Perley, M., Acid Rain, The North American Forecast,
   Anansi Press, Toronto (1980).


6. Gallagher, John, and Foxen, Robert J., “Clean Air: Stricter Standards,
   New Technology and Better Planning,” Engineering News Record, 232,
   E71–E73 (1994).


7. Clean Air Act, 42 U.S.C.A., pp 7401–7671q.


8. USEPA. “National Primary and Secondary Ambient Air Quality Stan-
   dards,” 40 CFR Part 50.11 (1992).


9. Santire, Stanley, “Regs Serve Up Alphabet Soup,” Pollution Engineer-
   ing, 25, 86–87 (1993).


10. USEPA. “New Source Performance Standards,” 40 CFR Part 60 (1995).


11. Kuehn, Steven E., “Retrofit Control Technology Reducing NO x Emis-
    sions,” Power Engineering, 23–31 (1994).


12. NYSDEC. “New York Air Pollution Control Regulations,” NYCRR,
    Title 6, Part 200 (1993).


13. NYSDEC. “New York Ambient Air Quality Standards,” NYCRR, Title 6,
    Part 256 and 257 (1989).

Condenser

Water

Electricity

Exhaust gas

Steam turbine & generator

Steam Heat exchanger

Ash

Gas

cleaning

Gas turbine & generator

Electricity

Coal &

limestone

Air

Combustor

Exhaust steam

FIGURE 10 Schematic of pressurized fluid bed combustor.^32

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