EN3004 Air Pollution Control Engineering Lecture 12 PDF

EN3004 Air Pollution Control Engineering Week 12 Oxides of Sulfur– Characteristic and Control Oxides of Nitrogen – Characteristic & Control Tuti Lim School of Civil and Environmental Engineering Nanyang Technological Univ...

EN3004 Air Pollution Control Engineering Week 12 Oxides of Sulfur– Characteristic and Control Oxides of Nitrogen – Characteristic & Control Tuti Lim School of Civil and Environmental Engineering Nanyang Technological University 1  Announcement: 2nd quiz NEXT week  09:30 AM on 15 November 2024  25 MCQs in 50 minutes  Covered week 8-12  Calculator needed Teaching feedbacks online (10 Nov 2024 onward)  Outline (I) Oxides of Sulfur– Characteristic and Control ⚫ Sulfur compounds ⚫ Source control: fuel substitution, desulfurization & modification ⚫ Tailpipe control (II) Oxides of Nitrogen – Characteristic and Control ⚫ Nitrogen compounds – oxidation and reduction ⚫ Similarity and differences with SOx ⚫ Formation of NOx (thermal NOx, prompt NOx, fuel NOx) ⚫ How to control? Source, process & tailpipe controls PM & VOC control by physical process (no change in chemical nature except oxidation for VOCs) but SOx & NOx control by chemical process. 3  Forms of Sulphur Oxides Sulphur monoxide (SO) Sulphur tetroxide (SO4) Sulphur dioxide (SO2) – 98% Sulphur sesquioxide (S2O3) Sulphur trioxide (SO3) – 2% Sulphur heptoxide (S2O7) SOx refers to the mixture of SO2 and SO3. S: 16th most abundant element on earth’s (260 ppm), mostly as gypsum (CaSO4.2H2O) which is slightly soluble in water. 4  SO2 and SO3 Sulfur dioxide (SO2) at high temperatures can oxidize to form sulfur trioxide(SO3). Sulfur dioxide is a colorless gas. Its moderate solubility in water and aqueous liquids is the basis for using wet scrubbers as one effective control technique for this gas. SO2 to SO3 ratio is about 50:1 5  Sources of SOx ⚫ Anthropogenic sources ✓ Combustion of sulphur-containing fuels (coal, oil) – about 2/3 of all SO2 comes from electric power plants S +O2 SO2 ✓ Other industrial processes: e.g., smelting process (chalcopyrite, CuFeS2); petrochemical process, etc. 2CuFeS2 + 5O2 2Cu + 2FeO + 4SO2 (smelting) ⚫ Natural sources ✓ Natural decay of organic matter ✓ Volcanoes 6  Impacts of SO2 Emissions ⚫ Health impacts: respiratory diseases and heart disease ⚫ Environmental impacts: ✓ Acid rain phenomenon 2SO2 + 2H2O + O2 2H2SO4 ✓ Reduction in visibility (sulphurous smog) ✓ Injury to vegetation ✓ Metal corrosion ✓ Attack on building materials CaCO3 + H2SO4 CaSO4 + CO2 + H2O partially water soluble limestone and can be washed away 7  Acid Rain Industrial activities release SOx and NOx into the atmosphere. The following reactions explain the formation of these acids. NO + 0.5O2 ⇔ NO2 3NO2 + H2O → 2HNO3 + NO SO2 + H2O → H2SO3 2SO2 + O2 → 2SO3 H2SO3 + 0.5O2 → H2SO4 SO3 + H2O → H2SO4 CO2 + H2O → H2CO3 These acids are absorbed by water droplets or particles in the atmosphere and then come down with the rain. These acids reduce the pH in the rainfall which is known as acid rain (wet precipitation). They can also come down with snow or hail. 8  Normal pH of rainwater = 5.6 resulting from water droplets (pH = 7) in equilibrium with CO2 in the atmosphere pH is lowered due to transformation of SO2 and NOx to acids pH 2-4 has been reported for rainwater in Scandinavian countries, Canada, etc. Acidification of natural waters cause destruction to aqua life and reproduction Aquatic Environment Leaching of nutrients from soil (leads to poor crop productivity) Crops Solution is to control at source. Forestation Infrastructure Wildlife Human Health How to quantify the impact of acid rain to ground pH? Emission amount of SOx/NOx Amount of the emission transferred to the ground by rain Amount of hydrogen ion (H+) Affected area and volume of acid rain Increase of hydrogen ion concentration [H+] Total hydrogen ion concentration 2 SO2 + O2 2SO3 pH=-log [H+] SO3 + H2O H2SO4 H2SO4 2H+ +SO4 2- 10  Control of SO2 Emissions (I) Source control ✓ Fuel substitution S13 Prevention ✓ Fuel desulphurization S14 ✓ Fuel modification Modifying the existing fuels to cleaner fuels (eg, converting coal into syngas or methane) S16 (II) SO2 removal from waste gases (Tailpipe control) ✓ SO2 removal from rich waste gases (SO2 concentration > 4 %) - absorbing SO2 to make H2SO4 Recovery ✓ SO2 removal from lean waste gases (SO2 concentration < 4 %) - Flue gas desulphurization (FGD) S20 Elimination 11  Fuel Substitution Fossil Fuel Emission Levels - Pounds per Billion Btu of Energy Input Pollutant Natural Gas Oil Coal Carbon Dioxide 117,000 164,000 208,000 Carbon Monoxide 40 33 208 Nitrogen Oxides 92 448 457 Sulfur Dioxide 1 1,122 2,591 Particulates 7 84 2,744 Mercury 0.000 0.007 0.016 Source: EIA - Natural Gas Issues and Trends 1998 S12 S47 12  Fuel Desulphurization Sulfur Removal from Refinery Fuels (Gasoline, Kerosene, Diesel) Hydrodesulfurisation (HDS) or hydrofining (HF) operating at high temperature and pressure in the presence of catalysts (Co/Ni with Mo/W) where the sulfur is converted to H2S. R-S + H2 250-380°C, 30-100 atm R + H2S where R is hydrocarbon Treatment required 13  S12 CLAUS PROCESS H2S S Catalytic Step Thermal Step 200 - 350ºC 10H2S + 5O2 → 2H2S + SO2+7S + 8H2O ⚫ Catalytic Step 1000-1400ºC The remaining H2S is reacted with the SO2 at lower Partial oxidation of H2S (⅓) temperatures (~200-350°C) over a catalyst to convert to H2S + 1.5Osulfur: 2 SO2 + H2O 2H2S + SO22H==> 2S + SO2 3S + 2H3S 2 O+ 2H2O Difficult to achieve complete conversion to sulfur. 2 or 3 stages are used, with sulfur being removed between the stages to achieve high conversion → 99.8%. 14  SO2 Removal from Rich Waste Gases - SO2 concentration > 4 %, e.g. smelting - The copper ore smelted is principally chalcopyrite (CuFeS2). Concentrated SO2 in the emissions stream. (2-40% SO2) 420⁰C water 4 bed catalyst packed particle w/ intercoolers scrubber removal 425⁰C smelter SO2 +0.5O2 SO3 Exothermic sulfuric acid worth $$$! SO3 +H2O H2SO4 Conversion : ~98% (single absorption) up to 99.7% for double absorption15  Example (1) One of the largest copper smelters in the United States (Kennecott, at Salt Lake City) produces 3108 kg of copper per year. The copper ore smelted is principally chalcopyrite (CuFeS2). (i) If all the sulphur were emitted to the atmosphere as SO2, how much would be emitted? (ii)If all the sulphur in the ore were converted to sulphuric acid, how much sulphuric acid per year would the smelter produce? Molecular weight: SO2: 64; H2SO4: 98; CuFeS2: 183 Atomic weight: Cu: 63 16  Solution: Smelting reaction: 2CuFeS2 + 5O2 → 2Cu + 2FeO + 4SO2 with 2 mols of Cu produced, 4 mols of SO2 are generated. Production of Cu: AW(Cu)=63 g/mol 3108 kg/yr = 3108 103 (g/yr)/63 g/mol = 4.8 109 mol/yr Generation of SO2: MW (SO2) =64g/mol 4.8109 (4/2) = 9.6 109 mol/yr = 9.6 109 mol/yr  64 g/mol = 6.1 108 kg/yr This is 3.2% of the total US SO2 emissions! 17  SO2 to H2SO4 reaction: SO2 + ½ O2 Vanadium catalyst SO3, SO3 + H2O H2SO4 1 mol of SO2 produces 1 mol of H2SO4 Generation of SO2: 9.6 109 mol/yr Production of H2SO4: MW (H2SO4) =98g/mol 9.6109 mol/yr98 g/mol = 9.4108 kg/yr 2% of the total US H2SO4 production! 1000 kg H2SO4 = $82 in 1999 (79 in 2021, 231 in 2022), sulphuric acid is worth around 77 million US dollars ($74.2M, $217.1M)! S12 18 SO2 Removal from Lean Waste Gases - Flue gas desulphurization (FGD) ✓ Limestone wet scrubbing ✓ Lime wet scrubbing ✓ Dual Alkali processes ✓ Lime-spray drying ? What are the differences of the four processes? 19  Working Principle of Scrubber : Absorption  Absorption processes use the solubility of sulfur dioxide in aqueous solutions to remove it from the gas stream.  Once sulfur dioxide has dissolved in solution to form sulfurous acid (H2SO3), it reacts with oxidizers to form inorganic sulfites (SO--3 ) and sulfates (SO--4 ).  This process prevents the dissolved sulfur dioxide from diffusing out of solution and being re-emitted. 20  First SO2 Removal Process with River Water Cleaner gas limestone scrubber Settling tank Water in Thames River is naturally alkaline Battersea London Power plant 1933~1940 21 Limestone Wet Scrubbing Most widely used process for removing SO2 Limestone (CaCO3) is injected into scrubber with water to form a slurry Efficiency is at 80-95% Large amount of solid waste produced and CO2 generated 2 CaCO3 + H2O + 2SO2 → 2 CaSO3 + 2CO2 + H2O CaSO3 can be further oxidized to CaSO4 22  Possible operational problems for limestone wet scrubbing ⚫ Corrosion - Due to corrosive compound in flue gas (e.g. Chlorides) ⚫ Solids deposition, scaling and plugging - CaSO4 loves to deposit, so scaling and plugging is a big problem ⚫ Poor reagent utilization - CaSO3 and CaSO4 could precipitate on the limestone surface to block the contact between the limestone with SO2 ⚫ Poor solid-liquid separation - CaSO3 tends to form fine crystals which are difficult to be separated from liquid 23  Lime Wet Scrubbing Similar to limestone scrubbing except limestone (CaCO3) is replaced by lime (CaO). Ca(OH)2 is more chemically reactive. Lime scrubbing offers better utilization of the reagent. The operation is more flexible. The major disadvantage is the high cost of lime compared to limestone. (1) CaO + H2O → Ca(OH)2 hydrate (hold tank) (2) SO2 + H2O H2SO3 dissolve H2SO3 + Ca(OH)2 → CaSO3.2H2O } (Scrubber) (3) CaSO3.2 H2O + 0.5O2 →CaSO4.2H2O further oxidize  Dual Alkali Processes Lime and Limestone scrubbing lead to deposits inside scrubber → scaling and plugging problems The Dual Alkali system uses two reagents to remove the SO2. Na2CO3/NaHCO3 inside a scrubber Lime/limestone in a reaction tank 25  Reaction inside regeneration tank Na2SO3 + CaCO3 + 0.5O2 + 2H2O → Clean gas CaSO4.2H2O + Na2CO3 Water containing Na2CO3/NaHCO3 Na2CO3 Scrubber Flue gas Thickener Scrubbed water CaSO4.2H2O containing Na2SO3 Regeneration discharge which is soluble in water Reaction inside scrubber Tank The scrubbed water is regenerated with Na2CO3 + SO2 → Na2SO3 + CO2 limestone , CaSO4 are precipitated, and Na2CO3 can be recycled. 26  Lime-Spray Drying Lime wet scrubbing Lime –spray drying CaO +H2O CaO+H2O Atomizer Flue gas Flue gas (SO2) (SO CaO 2) 2 → CaSO3 + SO Process is same as lime scrubbing except the water evaporates before the droplets reach the bottom of the scrubber Liquid was dispersed as droplets by a high pressure gas-atomizing nozzle 27 ✓ SO2 is absorbed by the a fine slurry mist of lime ✓ The flue gas is hot and the liquid-to-gas ratio is maintained such that the spray dries before it reaches the bottom of the chamber ✓ The dry solids are carried out with the gas, and collected in fabric filter (baghouse) or ESP Why dry?? ✓ 1) Waste product is dry and is easier to handle and dispose of ✓ 2) Fewer corrosion and scaling difficulties 28 SO2 Removal from Lean Waste Gases - Flue gas desulphurization (FGD) ✓ Limestone wet scrubbing Ca(OH)2 is more chemically reactive than limestone. ✓ Lime wet scrubbing Na2SO3 is soluble in water , ✓ Dual Alkali processes so no deposits are formed in the scrubber. ✓ Lime-spray drying ? No corrosion and scaling, no What are the differences of wet waste product. the four processes? Coal Combustion and Acid Rain (Britannica.com) 29 (II) Oxides of Nitrogen – Characteristic and Control ⚫ Nitrogen compounds – oxidation and reduction ⚫ Similarity and differences with SOx ⚫ Formation of NOx (thermal NOx, prompt NOx, fuel NOx) ⚫ How to control? Source, process & tailpipe controls 30  Nitrogen 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. (c.f. S is 16th most abundant element @ ~260 ppm) Reaction Reaction with NH4 + Reduction Elemental Oxidation Oxidation form with water or other First step Second step cations Nitrate NH3 N2 NO NO2 HNO3 particles sulfate H2S S SO2 SO3 H2SO4 particles 31  Forms of Nitrogen Oxides Nitric oxide (NO) Nitrogen dioxide (NO2) Nitrous oxide (N2O) } present in atmosphere in significant amounts Nitrogen trioxide (N2O3) Nitrogen pentoxide (N2O5) NOx refers to the mixture of NO and NO2 ~ 90 to 95% of the nitrogen oxides generated in combustion processes are in the form of nitric oxide (NO) in the atmosphere, NO NO2. 32  Major Nitrogen Oxide Pollutants ⚫ NO – colourless;