The term nitrogen oxide typically refers to any binary compound of oxygen and nitrogen, or
to a mixture of such compounds:
* Nitric oxide (NO), nitrogen(II) oxide
* Nitrogen dioxide (NO2), nitrogen(IV) oxide
* Nitrous oxide (N2O), nitrogen (I) oxide
* Dinitrogen trioxide (N2O3), nitrogen(II, IV) oxide
* Dinitrogen tetroxide (N2O4), nitrogen(IV) oxide
* Dinitrogen pentoxide (N2O5), nitrogen(V) oxide
Chemical reactions that produce nitrogen oxides often produce several different
compounds, the proportions of which depend on the specific reaction and conditions. For
this reason, secondary[clarify] production of N2O is undesirable, as NO and NO2 — which
are extremely toxic — are liable to be produced as well.
NOx
NOx is a generic term for mono-nitrogen oxides (NO and NO2). These oxides are
produced during combustion, especially combustion at high temperatures.
At ambient temperatures, the oxygen and nitrogen gases in air will not react with each
other. In an internal combustion engine, combustion of a mixture of air and fuel produces
combustion temperatures high enough to drive endothermic reactions between
atmospheric nitrogen and oxygen in the flame, yielding various oxides of nitrogen. In areas
of high motor vehicle traffic, such as in large cities, the amount of nitrogen oxides emitted
into the atmosphere can be quite significant.
In the presence of excess oxygen (O2), nitric oxide (NO) will be converted to nitrogen
dioxide (NO2), with the time required dependent on the concentration in air as shown
below:
NO concentration in air
(ppm)
Time required for half NO
to be oxidized to NO2 (min)
20,000 0.175
10,000 0.35
1,000 3.5
100 35
10 350
1 3500
When NOx and volatile organic compounds (VOCs) react in the presence of sunlight, they
form photochemical smog, a significant form of air pollution, especially in the summer.
Children, people with lung diseases such as asthma, and people who work or exercise
outside are susceptible to adverse effects of smog such as damage to lung tissue and
reduction in lung function.[2]
Mono-nitrogen oxides eventually form nitric acid when dissolved in atmospheric moisture,
forming a component of acid rain. The following chemical reaction occurs when nitrogen
dioxide reacts with water:
2NO2 + H2O → HNO2 + HNO3
(nitrogen dioxide + water → nitrous acid + nitric acid).
Nitrous acid then decomposes as follows:
3HNO2 → HNO3 + 2NO + H2O
(nitrous acid → nitric acid + nitric oxide + water),
where nitric oxide will oxidize to form nitrogen dioxide that again reacts with water,
ultimately forming nitric acid:
4NO + 3O2 + 2H2O → 4HNO3 (nitric oxide + oxygen + water → nitric acid).
Mono-nitrogen oxides are also involved in tropospheric production of ozone.[3]
NOx should not be confused with NOS, a term used to refer to nitrous oxide (N2O) in the
context of its use as a power booster for internal combustion engines.
Definition of NOx and NOy in atmospheric chemistry
In atmospheric chemistry the term NOx is used to mean the total concentration of NO plus
NO2. During daylight NO and NO2 are in equilibrium with the ratio NO/NO2 determined by
the intensity of sunshine (which converts NO2 to NO) and the concentration of ozone
(which reacts with NO to give back NO2). NO and NO2 are also central to the formation of
tropospheric ozone. This definition excludes other oxides of nitrogen such as nitrous oxide
(N2O). NOy (reactive odd nitrogen) is defined as the sum of NOx plus the compounds
produced from the oxidation of NOx which include nitric acid and peroxyacetyl nitrate. In
this context nitrous oxide and ammonia are not considered as reactive nitrogen
compounds.
Industrial sources of NOx
The three primary sources of NOx in combustion processes:
* thermal NOx
* fuel NOx
* prompt NOx
Thermal NOx formation, which is highly temperature dependent, is recognized as the most
relevant source when combusting natural gas. Fuel NOx tends to dominate during the
combustion of fuels, such as coal, which have a significant nitrogen content, particularly
when burned in combustors designed to minimise thermal NOx. The contribution of prompt
NOx is normally considered negligible. A fourth source, called feed NOx is associated with
the combustion of nitrogen present in the feed material of cement rotary kilns, at between
300° and 800°C, where it is also a minor contributor.
Thermal NOx
Thermal NOx refers to NOx formed through high temperature oxidation of the diatomic
nitrogen found in combustion air. The formation rate is primarily a function of temperature
and the residence time of nitrogen at that temperature. At high temperatures, usually
above 1600°C (2900°F), molecular nitrogen (N2) and oxygen (O2) in the combustion air
disassociate into their atomic states and participate in a series of reactions.
The three principal reactions producing thermal NOx are:
(Extended Zeldovich Mechanism)
* N2 + O → NO + N
* N + O2 → NO + O
* N + OH → NO + H
all 3 reactions are reversible. Zeldovich was the first to suggest the importance of the first
two reactions. The last reaction of atomic Nitrogen with Hydroxyl radical, OH, was added
by Lavoie, Heywood and Keck to the mechanism and makes a significiant contribution to
the formation of thermal NOx.
Fuel NOx
The major source of NOx production from nitrogen-bearing fuels such as certain coals and
oil, is the conversion of fuel bound nitrogen to NOx during combustion. During combustion,
the nitrogen bound in the fuel is released as a free radical and ultimately forms free N2, or
NO. Fuel NOx can contribute as much as 50% of total emissions when combusting oil and
as much as 80% when combusting coal.
Although the complete mechanism is not fully understood, there are two primary paths of
formation. The first involves the oxidation of volatile nitrogen species during the initial
stages of combustion. During the release and prior to the oxidation of the volatiles,
nitrogen reacts to form several intermediaries which are then oxidized into NO. If the
volatiles evolve into a reducing atmosphere, the nitrogen evolved can readily be made to
form nitrogen gas, rather than NOx. The second path involves the combustion of nitrogen
contained in the char matrix during the combustion of the char portion of the fuels. This
reaction occurs much more slowly than the volatile phase. Only around 20% of the char
nitrogen is ultimately emitted as NOx, since much of the NOx that forms during this
process is reduced to nitrogen by the char, which is nearly pure carbon.
It is possible to revert NOx emitted from a diesel engine back into Nitrogen and Water by
using an SCR Selective Catalytic Reduction Unit. This requires the addition of an urea
based liquid re-agent namely AdBlue to initialise the chemical reaction.
Prompt NOx
This third source is attributed to the reaction of atmospheric nitrogen, N2, with radicals
such as C, CH, and CH2 fragments derived from fuel, where this cannot be explained by
either the aforementioned thermal or fuel processes. Occurring in the earliest stage of
combustion, this results in the formation of fixed species of nitrogen such as NH (nitrogen
monohydride), HCN (hydrogen cyanide), H2CN (dihydrogen cyanide) and CN- (cyano
radical) which can oxidize to NO. In fuels that contain nitrogen, the incidence of prompt
NOx is especially minimal and it is generally only of interest for the most exacting emission
targets.
Regulation and emission control technologies
The Kyoto Protocol, ratified by 54 nations in 1997, classifies N2O as a greenhouse gas,
and calls for substantial worldwide reductions in its emission.
As discussed above, atmospheric NOx eventually forms nitric acid, which contributes to
acid rain.NOx emissions are regulated in the United States by the Environmental
Protection Agency, and in the UK by the Department for Environment, Food and Rural
Affairs.
Technologies such as flameless oxidation (FLOX) and staged combustion significantly
reduce thermal NOx in industrial processes. Bowin low NOx technology is a hybrid of
staged-premixed-radiant combustion technology with a major surface combustion
preceded by a minor radiant combustion. In the Bowin burner, air and fuel gas are
premixed at a ratio greater than or equal to the stoichiometric combustion requirement.
Water Injection technology, wherby water is introduced into the combustion chamber, is
also becoming an important means of NOx reduction through increased efficiency in the
overall combustion process. Alternatively, the water (e.g. 10 to 50%) is emulsified into the
fuel oil prior to the injection and combustion. This emulsification can either be made in-line
(unstabilized) just before the injection or as a drop-in fuel with chemical additives for long
term emulsion stability (stabilized). Other technologies, such as selective catalytic
reduction (SCR) and selective non-catalytic reduction (SNCR) reduce post combustion
NOx.
The use of Exhaust gas recirculation and catalytic converters in motor vehicle engines
have significantly reduced emissions.
Biogenic sources
Agricultural fertilization and the use of nitrogen fixing plants also contribute to atmospheric
NOx, by promoting nitrogen fixation by microorganisms.
No comments:
Post a Comment