Post-Exam II Air Pollution
Explain the electron transfer in the reduction of nitric oxide, NO using ammonia, NH3
- Oxidation states of nitrogen are +4 for NO2, +2 for NO, zero for N2 and -3 for NH3 - Oxidation/reduction transfers electrons: To reduce NO to N2, two electrons must be transferred to each atom. To reduce NO2 to N2, four electrons must be transferred to each atom, to oxidize NH3 to N2, three electrons must be transferred from each N atom. Reactions 11-5. Reactions: Competitive Reactions for NH3: .
Describe the predictions of equilibrium thermodynamics on the relative concentrations of nitric oxide and nitrogen dioxide in an oxygen-nitrogen system at various temperatures. Explain why equilibrium predictions are not adequate to describe NOx concentrations in a polluted atmosphere or in a combustion unit.
-1/2N2 + 1/2O2 =NO -NO + 1/2O2 =NO2 -At combustion temperatures, most of the NOx should be present as NO -At ambient temperatures, NOx will be very low and can only occur as NO2 -If the reactions were very fast and equilibrium was achieved, most NO produced at elevated temperatures would be reduced at ambient temperatures to N2 with very small amounts of NO2 -Equilibrium modeling shows that: because the combustion gases are cooled from peak temperatures, little NOx should be emitted and there should be essentially no NO and very little NO2 at ambient temperatures. -Equilibrium predictions will not be adequate in a polluted atmosphere or in a combustion unit because the kinetics will be different. Equilibrium NO levels at high temperatures are higher than observed and both NO and NO2 have been observed at parts per million level in ambient air.
Describe combustion control methods used for the control of oxides of nitrogen in a stationary combustion unit including the following: control of excess air, air preheater temperature, burner configuration, flue-gas recirculation, and staged combustion. For each method, explain the mechanism that limits the formation of oxides of nitrogen.
-Control of excess air: high levels of NOx occur at 20-30% excess air hi temp and hi oxygen +Burning with insufficient air produces low NOx due to lack of oxygen +Burning with very high excess air forms relatively low NOx due to low temperatures -Air preheater temperature: tangential-fired burners +Puts the burners in the corners with flame parallel to walls to quickly cool the gases +However, this can raise the temperature of combustion air to a level where NOx forms more readily. By reducing air preheat, the combustion temperature is lowered and NOx formation is suppressed. This can lower efficiency, but can limit NOx generation -Flue-gas recirculation: +flue gas is relatively inert and cooler than flame, recirculating it back to the boiler lowers the combustion temperature due to dilution +lowers combustion temperatures due to dilution = less NOx production -Staged Combustion: 2 stage combustion +Main combustion is run with insufficient air; poor combustion but little NOx produced +Heat is removed from flame +Excess air added to complete combustion -Reburn Technology: +Fraction of the fuel enters through reburn burners that produce a reducing zone +Overfire air completes the combustion +Natural gas reburn is often used with coal in main burner +main combustion run with insufficient air for poor combustion so little NOx is produced, then excess air is added to complete the combustion
Describe processes used in coal cleaning. Indicate what forms of sulfur are removed in the process
-Done to limit sulfur and other impurities in coal to upgrade value -Crushed coal is mixed with magnetite sand in water -The magnetite mixture forms a medium denser than water that will settle out ash and middlings with the higher specific gravity, and produce clean coal. -Magnets are used to recover the magnetite SO2-sulfate
Discuss the role of fuel nitrogen in the formation of oxides of nitrogen in a stationary combustion unit
-From coal and oil which have nitrogen bound in organic molecules; C-N bonds are much weaker than N-N bonds and thus break easier (vs. thermal NOx) -Fuel is oxidized rapidly -Higher N fuel burned at higher NOx emissions -Oxidation of fuel nitrogen is not sensitive to temperature -In additon to production of thermal NOx, up to 80% of fuel nitrogen is converted to NOx -Factors affecting NOx formation: +Fuel type- NOx is higher for coal than oil or natural gas +Air-fuel ratio- NOx emissions peak at 20-30% excess air +Temperature of inlet air- hi inlet temps increase combustion temps (thermal NOx) +Peak combustion temp- high peak temp increase thermal NOx +Rate of combustion zone cooling- rapid cooling leads to less thermal NOx +Burner configuration- concentrating the burners increases peak temps & thermal NOx
Describe what is meant by the terms "rate-limiting reaction" and "frozen kinetics" for nitric oxide formation
-Rate-limiting reaction: the slowest step in a reaction mechanism will determine the rate of the whole thing; for NO, the rate limiting step is O + N2 due to high activation energy needed -Frozen kinetics: At high temps, NO forms rapidly if there is sufficient O. In the hottest zone of the flame, there is a "super equilibrium" level of O atoms. NO increases quickly. However as gases cool rapidly when leaving the hot flame zone, the NO concentration is "frozen" because its removal depends upon the reverse reaction. They remain NO atoms because when temps cool, activation energy and abundance of O atoms decrease. The shift from NO to N2 is limited by kinetics, so the gas leaves with a significant amount of NO.
Describe the reactions that produce ozone in the stratosphere. Discuss the significance of stratospheric ozone to our environment. Describe why CFCs may destroy stratospheric ozone
-Wavelength<242nm: O2+hv --> 2O(3P) O(3P) is O triplet P=ground state of atomic O O(3P) + O2 + M --> O3 + M' (collision deactivation) -Stratospheric ozone absorbes UV radiation, protecting us from high-energy photons -CFCs or freons, are very stable compounds in the lower atmosphere, but in the stratosphere, UV light can remove chlorine atom, wich will destroy the ozone, results in an "ozone hole" - CFC + hv --> Cl+ Cl + O3 --> ClO* + O2 ClO* + O --> Cl + O2
Describe the following pollution control methods for spark-ignition, gasoline engines and explain the mechanism that lowers emissions in each case
During the power stroke, gases blow by the compression rings into the crankcase. Ventilated crankcase allows the emission of theses gases. (Blowby Emissions) The crankcase is sealed and fumes are routed back to the intake manifold. A positive crankcase ventilation valve (PCV) controls flow and prevents blowback. (Positive Crankcase Ventilation Valve) Evaporative controls for a fuel system include sealed cap on fuel tank, carbon adsorption system for recovery of gasoline vapors, regeneration of the carbon by fresh air pulled through the carbon bed and bled into intake manifold. Hot soak solved with fuel injection. <--dunno if this is needed? Exhaust Gas Recirculation: as with stationary combustors, a portion of the exhaust can be recirculated to the intake. This lowers the average temp of combustion and requires an exhaust gas recirculation (EGR) valve to control the flow of the exhaust gases. Three-Way Catalytic Converter- Current automobiles use three-way catalyst to simultaneously oxidize CO and hydrocarbons and reduce NO. Requires careful control of A/F ratio by computer Gasoline is a mixture of hydrocarbons and is changed with seasons and geography (higher VP in colder weather) -PCV valve: Positive Crankcase Ventilation valve; the crankcase is sealed instead of ventilated, so the fumes are routed back to the intake manifold; it controls flow and prevents blowback -Carbon Canister: a carbon adsorption system for recovery of gasoline vapors, the carbon is regenerated by fresh air pulled through the carbon bed and bled into intake manifold
Describe post-combustion methods to decrease the oxides of nitrogen concentration including selective noncatalytic reduction and selective catalytic reduction.
Selective Catalytic Reduction (SCR) - catalysts are used to reduce NO with NH3 at relatively low temperatures 175-290C - Platinum and palladium, metal oxides, zeolites at higher temperatures - catalyst bed is placed upstream or downstream of air preheater, beds fouled with particulate matter, can achieve 60-90% NOx control Selective Non-Catalytic Reduction - NH3 used to reduce NO at 930-1090 C without catalyst - operation at higher temperatures cause NH3 to oxide to NO, operation at lower temperatures encourages formation of ammonium sulfate - aqueous urea solutions also used in noncatalytic reduction
Describe various types of flue gas desulfurization technologies in use including spray dry scrubbing, and wet lime or limestone scrubbing. Discuss their relative advantages and disadvantages.
Spray dry scrubbing: -Lime slurry or sodium carbonate solution is sprayed into fluegas -SO2 is absorbed by wet droplets and adsorbed by solids -Solids continue to absorb SO2 in baghouse -Can achieve 70%+ efficiency Limestone scrubbing (wet): -Most popular process, can achieve 90%+ efficiency -Limestone slurry is sprayed in with flue gas, limestone is recycled -Waste will be a mixture of calcium sulfite hemihydrates; calcium sulfate dehydrate (easier to dewater); limestone; fly ash -Throwaway: ADV—simple, proven technology; DIS—slurry or solid waste disposal -Lime, limestone, limestone/flyash, sodium carbonate in wet scrubbing -Lime, sodium carbonate, sodium bicarbonate in spray dry scrubbing -Saleable Product: ADV—product sales to offset costs; DIS—higher costs; commodity products subject to market fluctuations -Can recover CaSO4 and H2SO4 depending on what reagent is used
Describe various types of furnaces or boilers used in coal combustion including stoker, pulverized coal, fluidized bed, and integrated gasification combined cycle (IGCC).
Stoker Combustion: -Underfeed -Chain grate: lumps of coal conveyed on a chain conveyor, combustion on the chain grate stoker with underfire and overfire air; produces bottom ash and flyash -Spreader stoker: pea-sized coal is flung across bed; suspension burning and burning on chain grate; more rapid changes in burning rate Pulverized Coal Combustion -Pulverized coal 80%<200mesh; 100%<50mesh; -Blown into boiler, suspension burning -Relatively high flyash emissions -High combustions per volume -Saturated steam is made in boiler, heat is removed from the flue gas to recover energy Fluidized Bed Combustion: -Bed made of Coal, limestone, and gravel form bed, fluidized by air -Steam generation within bed lowers combustion temperature and lowers NOx -Recycle of coal and limestone Integrated Gasification combined Cycle: - is a technology that uses a gasifier to turn coal and other carbon based fuels into gas-synthesis gas - Then removes impurities from the syngas before it is combusted
Describe the reactions that produce ozone in the lower atmosphere. Tell which reaction in the principal photochemical reaction in the formation of ozone.
Tropospheric Ozone Formation NO2 + hv → NO + O(3P) O(3P) + O2 + M → O3 +M' NO + O3 → NO2 + O2 Formation of Hydroxyl Radical O3 + hv → O2 + O(1D) O(1D) + H2O → 2OH⋅
Describe various strategies to control ozone formation in urban areas.
➔ VOC's and NOx are both required to produce ozone in lower atmosphere ◆ need to determine which is the limiting reactant ➔ western US = arid climate and low biogenic carbon emissions, needs to control VOC emissions ➔ eastern US = humid climate and high biogenic carbon emissions, needs to control NOx emissions
Describe methods for the control of emissions from diesel engines.
➔ diesel engines already have significantly lower fuel usage and emissions ◆ w/o controls have less CO, hydrocarbon, and NOx emissions than gasoline engines ◆ heavier and less volatile than gasoline ◆ higher density and higher heating value than gasoline ◆ BUT have higher PM emissions (poses health threat) ➔ ultra-low sulfur diesel: limits sulfur to 15 ppmm, allows use of clean technologies (like catalytic converters) ➔ non-road diesel fuel: limits sulfur ➔ diesel oxidizing catalysts: used to oxidize hydrocarbons, CO, and PM ◆ replaces conventional muffler, minimal maintenance, higher efficiencies ◆ low emissions need ultra-low sulfur diesel ➔ particulate filters: trap soot and other particles, ash may need to be removed periodically, need ultra-low sulfur diesel ◆ passive:: uses catalysts to oxidize the soot on the filter ◆ active:: heats filter to oxidize soot ➔ NOx controls: ◆ exhaust gas recirculation after cooling achieves 25-40% efficiency ◆ lean NOx reduction catalyst with oxidizing catalys = 5-40% efficiency ◆ selective catalytic reduction catalysts with reducing agent (urea or diesel fuel) and high excess oxygen = 75% efficiency
Describe the role of formaldehyde in ozone formation. Describe the general role of hydrocarbons in photochemical smog
➔ formaldehyde = HCHO ◆ Methane is converted to formaldehyde, then oxidized to carbon monoxide, and then to carbon dioxide ◆ Four HO2• radicals are formed. The role of nitrogen oxides is to form back the OH• species. nitric oxide (NO), nitrogen dioxide (NO2) and ozone (O3) are, depending on the amount of light, in equilibrium. The result is a net production of four ozone molecules. Two extra OH• molecules are generated and, the original NO molecules have reformed ➔ smog must have NOx, hydrocarbons, and sunlight to form ◆ hydrocarbons are produced by burning fossil fuels ◆ NO and hydrocarbons produced by fossil fuel burning. NO reacts with tropospheric ozone or a hydrocarbon radical to produce NO2. NO2 absorbs solar energy to create NO. NO reacts to form tropospheric ozone, which feeds back into the NOx system. Atomic oxygen can also react with hydroxyl radicals, OH, and ozone to form the reactive hydrocarbon radicals utilized in the NOx system. These radicals also react to form other smog components.
Describe the use of oxygenated fuels and reformulated gasoline to control pollution emission in gasoline engines
➔ oxygenated fuels ◆ contain ethanol or MTBE ◆ oxygen content decreases CO emissions ➔ reformulated gasoline ◆ less aromatic and alkene content ◆ lower vapor pressure and is oxygenated ◆ decreases both hydrocarbons and CO
Describe sources of hydrocarbon, carbon monoxide, and oxides of nitrogen formation in spark-ignition, gasoline engines. For those processes occurring within the combustion chamber, describe the emission of pollutants as a function of the air-to-fuel ratio. Explain the mechanism that causes the emissions to change with the air-to-fuel ratio.
➔ problem:: there is NO A/F ratio that can reduce all three emissions ➔ hydrocarbon ◆ source: HC emissions decrease with increasing A/F ratio ● become constant for A/F greater than stoichiometric because of quench layer emissions next to the combustion chamber ◆ tailpipe = 62%, crankcase = 20%, fuel tank = 9%, hot soak = 9% ➔ CO source: ◆ CO emissions decrease with increasing A/F ratio ● because of excess O2 ● very low for A/F ratios higher the stoichiometric ◆ tailpipe = 100% of emissions ➔ NOx source: ◆ highest at A/F = 16 (which is slightly higher than stoichiometric ratio) ● because of high combustion temperatures and excess oxygen ◆ tailpipe = 100% of emissions
Describe how the atmosphere absorbs certain wavelengths of radiation. Indicate the role of stratospheric ozone in absorbing solar ultraviolet radiation
➔ there are species in the atmosphere that can absorb radiation with 300-700 nm wavelength ➔ due to the many different gases and particles in the atmosphere, it absorbs and transmits many different wavelengths of electromagnetic radiation ➔ layer of ozone absorbs UV light and protects us from the high-energy photons
