CONTROL OF NITROGEN OXIDES

Nitrogen Oxides Problem Overview of the Nitrogen Oxides Problem  Most of the world’s nitrogen is in the atmosphere as an inert gas.  In crustal rocks it is the 34th most abundant element with an abundance of only ≈ 20 ppm.  Although nitrogen forms 8 different oxides, the principle air pollution interest is in the two most common oxides, nitric oxide (NO) and nitrogen dioxide (NO2).  Nitrous oxide (N2O) is beginning to be concerned, however, it is not a common air pollutant, but it may be a significant contributor to global warming and to the possible destruction of the ozone layer.

Principle flow of N in the Environment

Reactions in the Atmosphere  In the atmosphere and in industrial devices NO reacts with O2 to form NO2, a brown gas that is a serious respiratory irritant.  NO and NO2 are often treated together as one problem or as a quasi species, and written NOx.  Most regulations for NOx emissions base all numerical values on the assumption that all of the NO is converted to NO2 .  The principle concern with NOx is the NO2 contribute to the formation of ozone, O3, which is a strong respiratory irritant and one of the principal constituents of urban summer eye- and nose- irritating smog. NO + HC + O2 + sunlight → NO2 + O3

NO and NO2 Equilibrium  Most NO2 derived from human activities are formed in flames.  The most important reactions for producing NO and NO2 in flames are N2 + O2 ↔ 2 NO NO + 0.5O2 ↔ NO2  For any chemical reaction at equilibrium, the Gibbs free energy is at a minimum for the reaction’s temperature and pressure. From that condition it follows that ln K = - ∆G0 RT Where ∆G0 = standard Gibbs free energy change K = equilibrium constant R = the universal gas constant T = absolute temperature (K or oR)

Reaction Equilibrium Constant K = [NO]2 N2 + O2 ↔ 2 NO [O2][N2] NO + 0.5O2 ↔ NO2 K = [NO2] [NO][O2]1/2

Thermal, Prompt, and Fuel NOx Combustion scientists classify the NO2 found in combustion gases as thermal, prompt, and fuel NO2. Thermal NO2 - The most significant; formed by the simple heating of oxygen and nitrogen, either in flame or by some other external heating. (eg: lightning bolt) Prompt - NO2 that form very quickly as a result of the interaction of nitrogen and oxygen with some of the active hydrocarbon species derived from the fuel in the fuel-rich parts of flames. Fuel - NO2 is form by conversion of some of the nitrogen originally present in the fuel to NOx.

Thermal NO At the highest temperatures, the thermal mechanism is the most important of the three ways of making NO. The Zeldovich Kinetics of Thermal NO Formation It assumes that O radicals attack N2 molecules by: O + N2 ↔ NO + N and that N radicals can form NO by: N + O2 ↔ NO + O Various degrees of simplification of this mechanism can be made. Simplified version of the Zeldovich: [NO] = 1 - exp (-αt) [NO]e 1 + exp (-αt) where α = 2 [NO]e . kb [O2]1/2

Heating and Cooling Times We need to explore the temperatures and times in flames to help understand what kinds of flames produce significant thermal NO and what kinds do not. A strong function of how fast the gases heat and cool in the flames can be known by:  How much thermal NO is formed in a flame; as well as  how much is then converted back to N2 and O2.

Square wave Triangle wave **Four possible temperature- time patterns for flames Plausible flame pattern Staged combustion

Prompt NO  During the first part of combustion, the carbon-bearing radicals from the fuel react with nitrogen by: CH + N2 ↔ HCN + N and several similar reactions involving the CH2 and C radicals.  The N thus produced attacks O2 by: N + O2 ↔ NO + O to increase the amount of NO formed.  The HCN partly reacts with O2, producing NO, partly with NO, producing N2 .  From Fig. 1 one would estimate that the production of prompt NO is only weakly dependent on temperature and averages about 30 g/GJ.

Fuel NO  Most gaseous and liquid fuels contain little nitrogen, and contribution of that nitrogen to the total NO in the combustion products is minimal.  Most of the fuel nitrogen is converted in the flame to HCN, which then converts to NH or NH2 .  The NH and NH2 can react with oxygen to produce NO + H2O, or they can react with NO to produce N2 and H2O.  The fraction of the fuel nitrogen that appears as NOx in the exhaust gas is estimated to be typically 20% to 50%, depending on furnace conditions and, to some extent, the chemical nature of the N in the fuel.

Interactions of the three mechanisms of NO formation:

Noncombustion Sources of Nitrogen Oxides The production and utilization of nitric acid lead to emissions of NO and NO2, as do some other industrial and agricultural processes. However, their contribution to the overall NOx problem is generally small due to:  smaller total volume of gases emitted if compared to combustion sources  Most such industrial sources are under fairly strict control

Control of Nitrogen Oxide Emissions Possible approaches to controlling NOx in combustion gases Treat the combustion gas chemically, Modify the combustion processes to prevent the formation of NOx after the flame, to convert the NOx to N2

NO2 Control by Combustion Modification Two-stage combustion or reburning: Involve mixing part of the combustion air with the fuel, burning as much of the fuel as that amount of air will burn, transferring some of the heat from the flames to whatever is being heated, then adding the remaining air and finishing the combustion. First stage: the maximum temperature is lowered because not all the fuel is burned, and the maximum temperature is reached when all the oxygen has been used up, so that there is not enough oxygen to form NO. Second stage: enough of the heat released in the first stage has been removed that the maximum temperature reached - in the presence of excess oxygen - is low enough that the formation of NO is small.

Cont. Reburning: in some schemes a small amount of additional fuel is added for the second stage, often a low-nitrogen-content fuel like methane if the primary fuel has high nitrogen. Advantage: cheap Disadvantage: requires a larger firebox for the same combustion rate (or requires the firing rate of a furnace to be reduced if it is applied to an existing furnace) and that it is difficult to get complete burning of the fuel in the second stage, so that the amount of unburned fuel and/or carbon monoxide in the exhaust gas is increased.

Alternative approach: Design a low-NOx burner

Flow diagram for the whole furnace that uses the low NOx burner:

NO2 Control by Postflame Treatment  Most postflame treatment processes add a reducing agent to the combustion gas stream to take oxygen away from NO.  Almost any gaseous reducing agent can be used, eg: CO, CH4, other HCs, NH3, various derivatives of NH3 .  The reducing agent is normally CO in modern auto engines: 2NO + 2CO platinum-rhodium catalyst N2 + 2CO2 and NH3 (or one of its chemical relatives) in power plants: 6NO + 4NH3 → 5N2 + 6H2O  The reduction of above reaction can be thermal or catalytic.

THE END OF NOx CONTROL