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

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

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

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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)

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

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

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

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

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

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

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)

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

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)

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

Transformation of Coal-S Transformation of Coal-S

Coal-S (CS, S2, S, SH) SO SO2 SO3

-SO4O2, M

COS, CS2 H2S

char

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

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

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

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

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

Models for Sulfur Capture by Models for Sulfur Capture by Sorbents/Sorbents/continuescontinues

SO2

O2CO2

CaCO3

CaOCaSO4

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)

Summary: Summary: NONOxx and SO and SOxx Reduction Reduction

• NOx can be reduced during combustion, with right conditions

• SOx should be reduced pre-combustion