Control of Sulfur Oxides

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Presentation transcript:

Control of Sulfur Oxides CHAPTER 11 Control of Sulfur Oxides 18:52

Control of Sulfur Oxides Unlike PM and VOCs, control of sulfur oxides are largely chemical rather than physical (cyclones, ESPs, filters, vapor capture, condensation, adsorption). Sulfur and nitrogen oxides have many sources and they are respiratory irritants, principal causes of acid rain. 18:52

Elementary oxidation and reduction of S and N 18:52

An Overwiew of Sulfur problem S is the sixteenth most abundant element in the earth crust (260 ppm) Sulfur in the earth crust exist mostly in the form of sulfates CaSO4.2H2O: gypsum dihydrate Chemically inert, slightly water soluble, nontoxic 18:52

Sulfur in the fuel Coal (0.5 to 3 %) Oil Natural gas Peat Wood (<0.1 %) Other organic matter S+O2 SO2 H2SO4 Acid rain (in fuel) (in the atmosphere) 18:52

Fuel oil 4 (kalorifer yakıtı) 1.5 % Fuel Oil 5 2.0-2.8 % Fuel Oil 6 S content Fuel Oil 3 1.0 % Fuel oil 4 (kalorifer yakıtı) 1.5 % Fuel Oil 5 2.0-2.8 % Fuel Oil 6 2.8-3.5 % Unleaded gasoline 95 octane 10 mg/kg www.opet.com.tr 18:52

Sulfur in the environment as a result of human activities 18:52

Control of S compounds Removal of reduced S compounds from petroleum and natural gas Removal of SO2 from Rich waste gases Removal of SO2 from lean waste gases Alternatives to Burn and then scrub Flue gas desulfurization Forced oxidation limestone wet scrubbers Other approaches Wet systems Dry systems Wet-Dry systems regenerative systems 18:52

Basis of SO2 control CaCO3 +SO2+0.5 O2 CaSO4 +CO2 limestone Gypsum Forms of sulfur in the fuels Natural gas H2S Oil (liquid petroleum) S is bonded to hydrocarbons Coal Chemically bound S or pyritic (crystals of iron pyrite, FeS2 18:52

Mining as a source of SO2 Processing of sulfur bearing ores To obtain cupper from ore, high temperature smelting reaction occur CuFeS2 +5/2 O2 Cu +FeO +2SO2 Chalcopyrite, CuFeS2 principal copper ore 18:52

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Which sectors contribute more? Coal combustion is major contributor of SO2 emissions 18:52

Absorption (Scrubbing) The removal of reduced S compounds from petroleum and natural gas streams Absorption (Scrubbing) Solute gas out Gas liquid separator Alkali scrubbing solution Weak alkaline 18:52

Natural gas containing 1%H2S 18:52

H2S, SO2, SO3, NO2, HCl, CO2 are acid gases If absorption is used for removal of H2S fom a gas stream using water, the required flow rate is 148 kg/s too much H2S, SO2, SO3, NO2, HCl, CO2 are acid gases H2S(gas)  H2S (dissolved in water) H+ +HS- Solubility of H2S increase If we add OH- into the scrubbing soln 18:52

OR Another solvent in which H2S dissolve more readily can be used such as Monoethanolamine (liquid flow rate=6.1 kg/s) Dithanolamine Triethanolamine Potassium carbonate 18:52

Uses and limitations of Absorbers and strippers in APC SO2 can also be removed by absorption using alkali solns BUT If there are other gases present such as NO and NO2….. 3NO2 +H2O 2HNO3 +NO (NO2 is an acid gas but the rxn rate is slow) NO is not an acid gas 18:52

Sulfur Removal from Hydrocarbons Once H2S has been separated from the other components of the gas H2S +1/2 O2 S+H2O H2S +3/2 O2 SO2+H2O Claus Process Sold for use for the H2SO4 production Further oxidation yields SO2 Sulfur in hyrocarbons can be reduced to H2S but it is not economical,H2 is expensive, Oxidation is more feasible, air is free. Sulfur in hydrocarbon mixtures is converted to SO2 during combustion H2S concentration in natural gas should not exceed 4 ppm. 18:52

S in fuel for use in vehicles S in fuels normally converted to SO2 during combustion. Large oil burning facilities have equipment to capture SO2 but vehicles do not. Reduction in fuel S content is a major way for reducing SO2 emissions, OR catalyst (hydrocarbon containing S) +H2 hydrocarbon+H2S Further removal by absorption 18:52

Removal of SO2 from rich waste gases During smelting SO2 emissions may result from 2 to 40 %. Sulfuric acid production is an economical solution SO2 +1/2 O2 SO3 SO3 +1/2 H2O H2SO4 catalyst Exothermic reaction catalyst catalyst catalyst catalyst Exothermic reaction 18:52

App. 98% of SO2 is converted to H2SO4 in a sulfuric acid plant catalyst SO2 +1/2 O2 SO3 Exothermic reaction 18:52

Single and double absorption sulfuric acid plants 18:52 Phosphate fertilizer plants use sulfuric acid to produce phosphoric acid.

The large scale production of H2SO4 from smelter gases is only economical if there is a nearyby market (fertilizer plant) Production of sulfuric acid is not economical when the SO2 concentration is low. 18:52

Removal of SO2 from Lean waste gases Major source of SO2 in developing countries are coal and oil burning facilities (power plants) Typical SO2 content of the exhaust gas is 0.1% FGD(flue gas desulfurization) is widely used. Major drawbacks of absorption especially used in power generation High amount of water flow Waste water stream contains high amount of SO2 Aqueous phase SO2 reacts with dissolved O2 in river water consuming O2 needed for fishes. Oxidizing SO2 to SO42- before discharging is a solution. Or NaOH can be used in the scrubbing solution 2NaOH+SO2+1/2 O2 Na2SO4 +H2O (Not economical, NaOH is expensive) If we regenarate SO2 it is also not economical High flow rate causes much more pressure drop, you need power to reduce it, not economical 18:52

18:52

Three plausible arrangement for scrubbing gas with a liquid bubbler Spray chamber.Pressure drop is small, contact area is small Packed column 18:52 Pressure drop

2NaOH+SO2+1/2 O2 Na2SO4 +H2O (Not economical) Na2SO4 needs to be very pure for selling it to a detergent manusfactuting plant or paper making. Na2SO4 produced always contaminated with fly ash. Usually ESP follows absorption equipment. Market is not large Na2SO4 is soluble that you can not send it to a landfill. CO2 Problem 18:52

2 NaOH +CO2 Na2CO3 +H2O CO2 (gas)  CO2(dissolved); +H2O H2CO3 H+ +HCO3- SO2 (gas)  SO2(dissolved); +H2O H2SO3 H+ +HSO3- pH between 4-6 is needed to precipitate Na2SO4 Even pH adjustment still it is problematic pH adjustment Amount of alkali neededn is high, costly Waste product, Na2SO4 18:52

Forced-oxidation Limestone Wet Scrubbers CaCO3 +SO2+0.5 O2 CaSO4 +CO2 18:52 hydroclone

18:52

Problems with limestone scrubbers Corrosion: Chlorides in the exhaust gases are corrosive Solids deposition, scaling and plugging: precipitatipn of calcium salts on solid surfaces Entrainment separator plugging: Spray nozzles do not ptoduce uniform drops, some small drops might be carried out by the gas and result corrosion Poor reagent utilization: Sulfates and sulfites can precipitate on the limestone particles reducing absorption efficiency of SOx Poor solid-liquid separation: CaSO3.0.5H2O, tootpaste like structure formation, retain water. 18:52

Solutions corrosion:Chloride content of the circulating liquid is controlled. Solids deposition, scaling and plugging,Enough gypsum is kept in the ciculating slurry to prevent supersaturation. Entrainment separator plugging: chevron type Poor reagent utilization: larger tanks, oxidation air 18:52

Other approaches Other wet systems Dry systems Wet dry systems Regenarative systems Tomorrow’s limestone control devices 18:52

Other wet systems Ca(OH)2 (quicklime) is an alternative of CaCO3 in thwrowaway processes. CaO is oxidized in the ozidation tank to Ca(OH)2. Ca(OH)2 is more reactive than limestone. Prepartion of CaO needs extra process. 18:52

18:52

Double alkali throwaway systems They were the early systems designed because of solid deposition, scaling and plugging problems due to use of CaCO3 Na2CO3+SO2 Na2SO3+CO2 in the scrubber (liquid and gas) Na2SO3+CaCO3+1/2 O2+2H2O CaSO4.2H2O+Na2CO3 Small amount Out of the scrubber Na2CO3 is regenerated 18:52

Dry systems CaO+SO2 CaSO3 Excess alkaline reagent (CaO, CaCO3 But more reactive reagent such as Na2CO3 can be used to reduce amount of alkaline Dry alkaline particles are injected 18:52

Wet dry systems Slurry of basic reagents were introduced via atomizer.10-50u drops Water evaporates, particlees formed 18:52

Other alternatives Change to a lower sulfur content fuel Remove sulfur from fuel Solvent refined coal Modify the combustion process (Fluidized bed combustion) 18:52

Alternatives to “Burn and then scrub” Change to a lower sulfur content fuel Remove sulfur from the fuel (coal cleaning and solvent refined coal) Modify the combustion process Do not burn at all 18:52

Modify the combustion process Fludized bed combustion 18:52

SUMMARY Major sources of SO2 are combustion of S containing fuels and smelting metal sulfide ores Major control strategy of SO2: conversion to CaSO4. 2H2O and return to ground (landfill) For fuels containing reduced sulfur: claus process or H2SO4 production Metal sulfide ores containing high amounts of sulfur: H2SO4 and marketing For large power plants: CaSO4. 2H2O and transfer landfill or a wallboard plant 18:52