Transcript Slide 1
Control of Nitrogen Oxides Forms of nitrogen • Nitrogen forms different oxides. • NO and NO2 are principal air pollution interests (NOx). • N2O • N2O3 • N2O5 • N2O4 • N2O2 NOX and SOX: similarities NOX SOx Causes acid rain HNO3 H2SO4 Contribute to PM10 and PM 2.5 + + Are they regulated pollutants? Yes Yes Respiratory irritants? + + Major sources are combustion + + Emissions of Nitrogen oxides Relatively higher contribution Pros and cons NOX SOx Acid rain HNO3 H2SO4 Contribution to PM10 and PM 2.5 + + Are they regulated pollutants? Yes Yes Respiratory irritantion + + Major sources are combustion + + Motor vehicles are major emitters + - Removal of Sulfur from fuels solves SOX problem yes Removal of nitrogen from fuels solves NOX problem No Manipulation of time, temperature and oxygen content reduce formation of oxides Yes No Low solubility salt formation is a solution for pollutant removal No Yes Absorption is a proper technique for pollution prevention No Yes Concentration (ppm) Reactions of Nitrogen oxides • NO+HC+O2+sunlight • • • • • • NO2 +O3 NO2 +h O+NO (2) O+O2+M O3+M (3) NO+O3 NO2+O2 ozone is consumed In the presence of VOC VOC+2NO+O2H2O+RCHO+2NO2 (1) NO is converted to NO2 without consuming ozone NO and NO2 equilibrium • N2+O2 2NO • They are reversible reactions NO K O2 N 2 2 •N2+O22NO •NO+1/2O2NO2 Increase with T Decrease with T Conclusions • If the only mechanism is the chemical equilibrium, we should have less than a ppb of NO and NO2. However concentrations of NO and NO2 exceed this values in urban atmospheres… So equilibrium does not explain alone the observed concentrations. • The equilibrium concentration of NO increase rapidly with temperature. • At low temperatures equilibrium concentration of NO2 is much higher than that of NO. • Flames and lightning strikes are major sources of NO Thermal, Prompt and Fuel NOx • Thermal NOx: forms by heating of N2 and O2 in flames • Prompt NOx: N2, O2 plus some hydrocarbon species in fuel. • Fuel NOx: Conversion of nitrogen in fuel into NOx Contribution of Thermal, Prompt and Fuel NOx THERMAL NO (Zeldovich mechanism) •N2+O2 2NO • • • • • N22N O2 2O H2O H + OH O+ N2 NO+N N+ O2 NO+O NO NOeq 1 exp(t ) 1 exp(t ) 2NO eq k b O2 2 1 Reaction Rate is fast. Equilibrium is reached in about 0.3 s. Equilibrium conc. of NO is higher Equilibrum conc of NO is low. Reaction rate is slow. Even at 30 th second it does not reach equilbrium Remarks: Zeldovich mechanism • In order to decrease NO • Reduce T • Reduce O2 At high temperature flames, Zeldovich mechanism predictions are high in accuracy. • At lower temperatures it predicts much lower NO concentrations. In automobile engines and large coal fired furnaces, thermal NOX forms. Prompt NO Forms due to carbon bearing radicals from the fuels CH+ N2HCN +N N+O2NO+O NO in low temperature flames are prompt NO and weekly depend on T. Prompt NOx formation cannot be prevented in spite of the temperature and oxygen amount adjustment. FUEL NO • Fuels contain little nitrogen. • NO due to fuel nitrogen depend on NO/O2 ratio. • Lowering O2 lowers the fraction of N converted to NO Control of NOx emissions • Modify the process • Post flame treatment Combustion Modification NO increase by • Increase in T • Increased time at high T • High oxygen content at high T • Reduction of air nitrogen is a way but expensive (instead of air, pure oxygen can e used) and not practical Overfire air, Staging (Two-stage or off-stochiometric combustion) • The oxygen amount is reduced in the first flame zone by using fuel-rich mix (results in reduction of NOx). Unburnt fuel exists. • Air is supplied again by forming a second combustion zone. Thus, the unburnt fuel in the first stage burns and the CO formed in the first combusiton is oxidized to CO2. • In the second combusiton zone, the flame temperature is low since the amount of fuel is quite low. • Hence, NOX formation is minimized in both the first (less O2) and the second combusiton zone (low T). (Low-Excess Air Firing) • Reducing the amount of excess air causes less NOx formation since the amount of oxygen in the flame zone also decreses. • But CO emissions may increase. • It is achieved with very low investments but it requires very careful operation and maintenance. • Efficiency around% 0-25, and in advanced systems %15-55 Flue Gas Recirculation • Part of the flue gas is recycled back to the combustion air. • Therefore, the oxygen in the combustion air is diluted (reduced O2) • The nitrogen present in the recycled air also serves as a heat sink and reduces the flame temperature (reduced T). Flue Gas Recirculation Reducing the air preheat • In many industries, the temperature of the flue gas is used for the pre-heating of the combustion air. But this causes the increasing of the flame temperature. • (Unheated air has a higher capacity for absorbing the heat released during combustion) • Reducing the amount of pre-heating reduces the NOx formation by lowering the flame temperature. Reducing the firing rate • Reducing both the air and the fuel amount would not change the theoretical flame temperature. • But since there is heat loss from the walls and similar effects in the combustion chamber, reducing the fuel and air amount reduces the flame temperature. Water/steam injection • The injection of water or steam into the combustion chamber creates a heat sink and reduces the flame temperature. • This measure can achieve NOx reductions reaching 50% in systems burning natural gas. • But, the reducing medium created by the breakdown of steam to hydrogen and oxygen may create a more serious problem. Burners out of Service (BOOS) • In multi-burner furnaces, feeding of the fuel to some burners may be stopped and the fuel is distributed to other burners. But the air is distributed to all burners. • This achieves the previously mentioned staged firing (The oxygen is reduced in the first combustion zone, the flame temperature is low in the second combustion zone because of the small amount of fuel). Reburn • In order to create a second combustion zone after the primary flame zone, extra hydrocarbon is added to the outer part of the primary flame zone. • The hydrocarbon radicals formed in this operation react with the NOx. • In order to complete the combustion, overfire air is added after this second combustion zone. • Research has shown that NOx reductions of 5877% percent can be achieved with this technique in coal-fired plants. Low-NOx Burners • It is, principally, an aplication of the previously mentioned techniques (staged firing and recombustion) at the burner with a certain burner design. • There are two approaches: staging of the air or staging of the fuel Low-NOx Burners (Staging of air) The same principle as in staged air combustion technique Low-NOx Burners (Staging of fuel) • In this design, contrary to the previous one, air/fuel ratio is high in the primary flame zone. Therefore there is high NOx and high flame temperature. • In the second combustion zone, the remaining fuel is introduced. Since the oxygen is low and the temperature is high in this case, NOx is converted back to oxygen an nitrogen because of kinetic reasons. Low-NOx Burners (Staging of fuel) Since the flame will be physically longer, the hitting of the flame on furnace walls may cause problems like corrosion (aşınma ) Flue gas treatment Selective Noncatalytic Reduction (SNCR) The principle is the reduction of NOx to nitrogen and water by using NH2-X (mostly amonnia, NH3) or chemicals like urea. 30-50 % NOx reduction is achieved Selective Noncatalytic Reduction (SNCR) • It is critical to operate at the mentioned temperature range. • In lower temperatures, amonnia is oxidized and causes NOx formation! Selective Noncatalytic Reduction (SNCR) Selective Catalytic Reduction (SCR) • By the use of a catalyst bed together with ammonia, reduction is enhanced and also the reaction efficiency is increased at lower temperatures. •70-90 % NOx reduction is achieved •NO2 removal is also achieved Selective Catalytic Reduction (SCR) Selective Catalytic Reduction (SCR) • Some amount of amonnia may escape from the catalyst bed and be emitted from the stack. NH3 is among hazardous air pollutants. • Catalysts which are effective and work at lower temperatures are more expensive. • In the case of sulfur-containing fuel, there is the problem of SO2→SO3 conversion. Some special catalysts may be needed to prevent this conversion. • If particulate emissions are also high, catalyst fouling is possible because of dust loading. Some special measures/designs to reduce this effect are needed. Low-temperature oxidation followed by absorption • NOx, is oxidized to N2O5. • The solubility of N2O5in water is higher. Removal efficiencies reaching 99% have been observed Low-temperature oxidation followed by absorption • Ozone is used as the oxidizing agent. • N2O5, forms nitric acid in the wet scrubber column. • Caustic (NaOH) is used for the neutralization of the nitric acid. • If there is also SO2 problem, coastic has a dual benefit. Low-temperature oxidation followed by absorption • While reducing the gas emissions, the nitrate amount in the wastewater may be increased. Absorption • If no ozone oxidation is done, NO/NO2 molar ratio should be 1:1. Then strong aqueous alkaline solutions like NaOH and MgOH can be used • Neutralization can be done by using H2SO4, too. • NO + NO2 +2H2SO4 → 2NOHSO4 + H20 (needs elevated temperatures since H2O drives the reaction to the left otherwise) Catalytic absorption • A single catalyst achieves both NOx and CO removal. • First; NO, CO and unburnt HC, are oxidized to NO2 and CO2 • NO2 is absorbed in the catalyst coated with potassium carbonate. NOx emissions as low as 2 ppm can be observed when this technique is used together with other NOx control techniques Catalytic absorption • When potassium carbonate is consumed, it is regenerated with dilute hydrogen and carbon dioxide. • During this process, the absorbed nitrogen is released as molecular nitrogen, N2. Since the regeneration must be done in oxygen-free medium, the gas flow in the column under regeneration is stopped. Catalytic absorption • There are not problems like the storage or emission of ammonia and similar chemicals Corona-Induced Plasma • A non-thermal plasma may be created with Corona discharge. • Te plasma creates radicals which oxidize NO to NO2 and N2O5. • Since these NOx species are soluble, as mentioned earlier, they can be stripped by absorption. • A classical electrostatic precipitator can be used for creating the Corona discharge. Heating and Cooling times • N2+O2 2NO • The concentration of NO smostly depend on heating and cooling rates of flames Ultra Low-NOx Burners • Apart from staging of air/fuel, obtaining flue gas recycling inside the furnace can create extra staging effect according to the principle explianed earlier. • This recycle effect is achieved by the feeding of the gas or the liquid with high pressure. Ultra Low-NOx Burners • This design can be enhanced by the additon of inserts (barriers which will increae the contact time) efficient combustion can be achieved with lower oxygen amount and lower temperature. • Since the combustion efficiency is increaed, HC and CO emissions resulting from low oxygen can be reduced.