reduction of no x and so x from coal combustion ezra bar-ziv department of mechanical engineering...
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Reduction of NOReduction of NOxx and SO and SOxx from from
Coal CombustionCoal Combustion
Ezra Bar-ZivEzra Bar-Ziv
Department of Mechanical Engineering
and
Institutes for Applied Research
Ben-Gurion University of the Negev
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Can Coal Combustion be Clean?Can Coal Combustion be Clean?
• Increase in coal combustion, will double by 2030• Severe environmental impact, include:
1. High level of CO2
2. Particulate emission (soot, small fly ash)
3. SOx, NOx
4. Volatile metals
5. PAH
6. Fly ash
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Coal Combustion in Utility BoilersCoal Combustion in Utility Boilers
• Coal combustion in utility boilers is strongly coupled with boiler geometry, flow conditions, local stoichiometries, and temperature
• Two phase flow complicates even further coal combustion in utility boilers
• Impossible to predict behavior unless system behavior and characteristics is well known
• Further complications due to changes in boiler walls
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Raw CoalRaw Coal
• Mineral matrix
• Carbonaceous infrastructure
• Volatile matters
• Moisture
Above depend strongly on: coal age, Above depend strongly on: coal age, source, packing conditions, etc.source, packing conditions, etc.
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Combustion of Pulverized CoalCombustion of Pulverized Coal
Coal particle devolatilization + highly
porous char particle (1)
Volatiles+O2 CO2, H2O, CO, NOx, SOx, etc. (2)
Highly porous Char particle +O2 CO2, CO, NOx,
SOx, etc. (3)
Involves: heat & mass transfer and gas- Involves: heat & mass transfer and gas-
phase and heterogeneous reactionsphase and heterogeneous reactions
heat
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Combustion of charCombustion of char
• Chars are highly porous
• Mechanism for combustion through adsorption-desorption: reacting sites
• Reacting sites responsible for -N reaction as well
• Reacting sites depend on parent coal and carbon structure within char
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In this Presentation: Emphasis In this Presentation: Emphasis on NOon NOxx & SO & SOxx Control Control
1. Introduction: various pollutant emissions
2. Effect of various emissions and control
3. Fate of Fuel-Nitrogen (N)
4. Fate of volatile-N & volatile-S
5. Fate of char-N & char-S
6. Conversion to N2
7. New concepts
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Impact of EmissionsImpact of Emissions
• CO2: the green house effectthe green house effect, imagine when third world (4/5 of world population) will start to approach Western consumption of fuels
• NOx and SOx: major hazard to vegetation by being
acid rainacid rain precursors
• Soot and PAH are generally carcinogeniccarcinogenic
• Fly ash: if above 5% carbon content - gain
if bellow - loss
• Small particulate: lung diseaseslung diseases
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COCO22 Reduction Reduction
• In general: increase conversion efficiency from heat to electricity
• Combined cycle
• High pressure combustion
• Pinpoint heat release to certain zones
• Solution: better boilers based on CFD simulations
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SOSOxx Reduction Reduction
• No benign gaseous sulfur species, hence chemistry will not help
• SOx must be cleaned up post combustion
• Sorbent injection
• Scrubbing
• Low sulfur coal
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Poly-Aromatic Hydrocarbons Poly-Aromatic Hydrocarbons (PAH) and Soot(PAH) and Soot
• Small poly-aromatic molecules
• PAH precursors to soot
• Produced and terminates in gas phase
• PAH adsorbs in soot and fly ash
• Can control concentration if mechanism known -- control chemistry
• Modeling
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NONOxx Reduction Reduction
• Can be converted to benign gas N2 during combustion
• Need to know right conditions
• Mechanism is essential knowledge
• Experiments were done at various conditions:
1. Gaseous flames
2. Coal and char in gaseous flames
3. Combustion of coal/char in pc reactor
4. Combustion of coal/char in fb reactor
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Fate of Fuel- Nitrogen (N) Fate of Fuel- Nitrogen (N)
Determined by a variety of factors
• coal rank (C/H ratio)
• nitrogen content in fuel
• volatile content
• particle size
• temperature
• local stoichiometry
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Fate of Fuel-NFate of Fuel-N
Nitrogen contained in coal -- coal-N (1)
Coal-N HCN + Volatile-N (2)
Volatile-N HCN + NH3 (3)
Volatile + O2 NOx + … (4)
Char-N + O2 NOx + … (5)
HCN + O2 NOx + … (6)
HCN + NOx N2 + … (7)
HCN + Char N2 + … (8)
heat heat
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Effect of: Nitrogen & Volatile Effect of: Nitrogen & Volatile Content in FuelContent in Fuel
• No correlation was found with nitrogen content in fuel
• No correlation was found to total -N content, but on -N functionality
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Effect of Coal Particle SizeEffect of Coal Particle Size
• Indirect effect of particle size on conversion to NOx
• Size affects strongly both devolatilization and char oxidation, can vary from chemically controlled to diffusion controlled
• Consequent reactions depends strongly on reaction regime via reacting sites
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Effect of TemperatureEffect of Temperature
• Formation of NOx from coal depends weakly on temperature due to competing effects: increase of generation of NOx and N2 with temperature
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Effect of StoichiometryEffect of Stoichiometry
• Strong effect of stoichiometry on NOx formation
• Monotonic decrease of NOx with fuel/O2 ratio
• Extreme importance of volatile-N/O2 ratio to NOx formation -- same as for fuel
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Fate of Fuel-NFate of Fuel-N
Nitrogen contained in coal -- coal-N (1)
Coal-N HCN + Volatile-N (2)
Volatile-N HCN + NH3 (3)
Volatile + O2 NOx + … (4)
Char-N + O2 NOx + … (5)
HCN + O2 NOx + … (6)
HCN + NOx N2 + … (7)
HCN + Char N2 + … (8)
heat heat
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Conclusion for Fuel-N FateConclusion for Fuel-N Fate
• Depends strongly on coal-N fate
• Depends strongly on volatile-N fate
• Stoichiometry
• Coal rank -- reactivity
• Particle size
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Fate of Volatile-NFate of Volatile-N
• Most of NOx emission arises from volatile-N
• Rate of release of NOx seems kinetically controlled, indicative to gas-phase reaction
• Release of NOx follows devolatilization rate
• There are still many contradictions, arising from coal rank (type), variability (probably due to catalysts in coal)
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Fate of Char-NFate of Char-N
• The two main products of char-N oxidation are:
NO and NNO and N22OO
• Occur via
homogeneous formation/destruction
heterogeneous formation/destruction of HCNHCN
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How is NOHow is NOxx Formed? Formed?
• Heterogeneous through adsorption of O2 that interacts with -N site then desorption via thermal process to NO or N2O
• Heterogeneous reactions are very sensitive to evolution of porous structure
• Indications that at high temperature, heterogeneous reaction is controlled by diffusion
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Fate of Fuel-NFate of Fuel-N
Nitrogen contained in coal -- coal-N (1)
Coal-N HCN + Volatile-N (2)
Volatile-N HCN + NH3 (3)
Volatile + O2 NOx + … (4)
Char-N + O2 NOx + … (5)
HCN + O2 NOx + … (6)
HCN + NOx N2 + … (7)
HCN + Char N2 + … (8)
heat heat
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How is NOHow is NOxx Formed? Formed?
• Indication that for heterogeneous reactions NO is generated at reacting sites and N2O is produced within pores
• Strong correlations between reacting sites and formation of NO, N2O, HCN (formed always at surface)
• Homogeneous through oxidation of HCN
• Still homogeneous pathways are likely to be strongly involved in NO, N2O formation
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Homogeneous ReactionsHomogeneous Reactions
• If HCN released, oxidation to NO, N2O occurs homogeneously through NCO
• NCO will react with O2 or OH to form NO or N2
• No time for homogeneous reactions to occur within particle, must be outside
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Heterogeneous ReactionsHeterogeneous Reactions
• Some evidence that NO is also formed heterogeneously
• NO can be reduced to N2 or/and N2O either heterogeneously or homogeneously
• Heterogeneous reduction of NO was found to strongly depend on:
CO, surface area, and temperature
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Reduction of NOReduction of NOxx by External by External
AgentsAgents
NH3 + NO N2 + H2O
(HOCN)3 + NO N2 + H2O
(cynuric acid)
N2H4 + NO N2 + H2O
(hydrazin)
CO(NH2)2 + NO N2 + H2O
(urea)
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Reduction of NOReduction of NOxx by External by External
AgentsAgents
NH3
+ OH NH2+H2O + NO N2
(NH3)2CO NH3+HNCO NH2+CO
(HNCO)3
HNCO + OH NCO+H2O +NO N2O
+OH,M,H
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Catalytic Reduction of NOCatalytic Reduction of NOx x
from Flue Gasfrom Flue Gas
Selective Catalytic Reduction:Selective Catalytic Reduction:
NH3 + NO N2 + H2O
Metals:Metals: Pt, Pd
Oxides:Oxides: Ru/Al2O3, Fe2O3/Cr2O3, V2O5/TiO2,
V2O5/MoO3/WO3/Al2O3
Zeolites Zeolites (Al(AlxxSiSiyyOOzz/M)/M)
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Forms of Sulfur in CoalForms of Sulfur in Coal
•Organic-S compoundsOrganic-S compounds (thiophenes, sulfides, thiols)
•Pyritic sulfurPyritic sulfur (FeS)
•SulfatesSulfates (Ca/FeSO4)
Significant chemical changes of sulfur occur during coal devolatilization and combustion
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Transformation of Coal-S Transformation of Coal-S
Coal-S (CS, S2, S, SH) SO SO2 SO3
-SO4O2, M
COS, CS2 H2S
char
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Sulfur Pollutant ReductionSulfur Pollutant Reduction
No benign sulfur gas compounds
Reduction of sulfur pollutantReduction of sulfur pollutant
• Pre-combustion coal cleaning
• In-situ cleaning
• Post-combustion cleaning: Solidification to sulfur salt compounds
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Sulfur Pollutant Reduction: Sulfur Pollutant Reduction: Pre-CombustionPre-Combustion
1. Differences in density removes 30-50% of FeS
2. Leaching by sodium/potassium bases
R-C-SH + NaOH NaS-C-R + H2O
3. Biological cleaning by bacteria or fungi with high affinity to sulfur
By leaching and biological techniques 90% can be removed
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Sulfur Pollutant Reduction: Sulfur Pollutant Reduction: In-Situ CleaningIn-Situ Cleaning
Addition of sorbents: Ca/Mg/Zn/Fe/Ti oxides
In furnace conditionsIn furnace conditions
CaCO3 CaO + CO2
Ca(OH)2 CaO + H2O
2CaO + O2 + 2SO2 2CaSO4
CaO + H2S CaS + H2O
CaO + SO3 CaSO4
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Models for Sulfur Capture by Models for Sulfur Capture by SorbentsSorbents
• Sorbents are porous spheres
• Known properties of porous structure
• Rapid heating of sorbent -- CaCO3, Ca(OH)2
• Decomposition sorbent to oxide & CO2
Diffusion of CO2 through CaO to outer surface
• Mass transfer of CO2 from surface to bulk gas
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Models for Sulfur Capture by Models for Sulfur Capture by Sorbents/Sorbents/continuescontinues
• Diffusion of SO2 and O2 from bulk gas to
surface
• Diffusion of SO2 and O2 from outer surface to
inner pores
• Reaction of SO2 and O2 with CaO to form
CaSO4
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Models for Sulfur Capture by Models for Sulfur Capture by Sorbents/Sorbents/continuescontinues
SO2
O2CO2
CaCO3
CaOCaSO4
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Sulfur Pollutant Reduction: Sulfur Pollutant Reduction: PostcombustionPostcombustion
Lime or limestone scrubbersLime or limestone scrubbers
CaCO3 CaO + CO2
2CaO + O2 + 2SO2 2CaSO4
CaO + SO3 CaSO4 (Gypsum)
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Summary: Summary: NONOxx and SO and SOxx Reduction Reduction
• NOx can be reduced during combustion, with right conditions
• SOx should be reduced pre-combustion