Reduction of NO x and SO x from Coal Combustion Ezra Bar-Ziv Department of Mechanical Engineering and Institutes for Applied Research Ben-Gurion University of the Negev

Can Coal Combustion be Clean? Increase in coal combustion, will double by 2030 Severe environmental impact, include: 1. High level of CO 2 2. Particulate emission (soot, small fly ash) 3. SO x, NO x 4. Volatile metals 5. PAH 6. Fly ash

Coal 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 Coal Mineral matrix Carbonaceous infrastructure Volatile matters Moisture Above depend strongly on: coal age, source, packing conditions, etc.

Combustion of Pulverized Coal Coal particle devolatilization + highly porous char particle (1) Volatiles+O 2  CO 2, H 2 O, CO, NO x, SO x, etc. (2) Highly porous Char particle +O 2  CO 2, CO, NO x, SO x, etc. (3) Involves: heat & mass transfer and gas- phase and heterogeneous reactions

Combustion 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 on NO x & SO x 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 N 2 7. New concepts

Impact of Emissions the green house effectCO 2 : the green house effect, imagine when third world (4/5 of world population) will start to approach Western consumption of fuels acid rainNO x and SO x : major hazard to vegetation by being acid rain precursors carcinogenicSoot and PAH are generally carcinogenic Fly ash: if above 5% carbon content - gain if bellow - loss lung diseasesSmall particulate: lung diseases

CO 2 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

SO x Reduction No benign gaseous sulfur species, hence chemistry will not help SO x must be cleaned up post combustion Sorbent injection Scrubbing Low sulfur coal

Poly-Aromatic Hydrocarbons (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

NO x Reduction Can be converted to benign gas N 2 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) 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-N Nitrogen contained in coal -- coal-N (1) Coal-N HCN + Volatile-N (2) Volatile-N HCN + NH 3 (3) Volatile + O 2  NO x + … (4) Char-N + O 2  NO x + … (5) HCN + O 2  NO x + … (6) HCN + NO x  N 2 + … (7) HCN + Char  N 2 + … (8)

Effect of: Nitrogen & Volatile Content 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 Size Indirect effect of particle size on conversion to NO x 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 Temperature Formation of NO x from coal depends weakly on temperature due to competing effects: increase of generation of NO x and N 2 with temperature

Effect of Stoichiometry Strong effect of stoichiometry on NO x formation Monotonic decrease of NO x with fuel/O 2 ratio Extreme importance of volatile-N/O 2 ratio to NO x formation -- same as for fuel

Fate of Fuel-N Nitrogen contained in coal -- coal-N (1) Coal-N HCN + Volatile-N (2) Volatile-N HCN + NH 3 (3) Volatile + O 2  NO x + … (4) Char-N + O 2  NO x + … (5) HCN + O 2  NO x + … (6) HCN + NO x  N 2 + … (7) HCN + Char  N 2 + … (8)

Conclusion 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-N Most of NO x emission arises from volatile-N Rate of release of NO x seems kinetically controlled, indicative to gas-phase reaction Release of NO x follows devolatilization rate There are still many contradictions, arising from coal rank (type), variability (probably due to catalysts in coal)

Fate of Char-N The two main products of char-N oxidation are: NO and N 2 O Occur via homogeneous formation/destruction HCN heterogeneous formation/destruction of HCN

How is NO x Formed? Heterogeneous through adsorption of O 2 that interacts with -N site then desorption via thermal process to NO or N 2 O Heterogeneous reactions are very sensitive to evolution of porous structure Indications that at high temperature, heterogeneous reaction is controlled by diffusion

Fate of Fuel-N Nitrogen contained in coal -- coal-N (1) Coal-N HCN + Volatile-N (2) Volatile-N HCN + NH 3 (3) Volatile + O 2  NO x + … (4) Char-N + O 2  NO x + … (5) HCN + O 2  NO x + … (6) HCN + NO x  N 2 + … (7) HCN + Char  N 2 + … (8)

How is NO x Formed? Indication that for heterogeneous reactions NO is generated at reacting sites and N 2 O is produced within pores Strong correlations between reacting sites and formation of NO, N 2 O, HCN (formed always at surface) Homogeneous through oxidation of HCN Still homogeneous pathways are likely to be strongly involved in NO, N 2 O formation

Homogeneous Reactions If HCN released, oxidation to NO, N 2 O occurs homogeneously through NCO NCO will react with O 2 or OH to form NO or N 2 No time for homogeneous reactions to occur within particle, must be outside

Heterogeneous Reactions Some evidence that NO is also formed heterogeneously NO can be reduced to N 2 or/and N 2 O either heterogeneously or homogeneously Heterogeneous reduction of NO was found to strongly depend on: CO, surface area, and temperature

Reduction of NO x by External Agents NH 3 + NO  N 2 + H 2 O (HOCN) 3 + NO  N 2 + H 2 O (cynuric acid) N 2 H 4 + NO  N 2 + H 2 O (hydrazin) CO(NH 2 ) 2 + NO  N 2 + H 2 O (urea)

Reduction of NO x by External Agents NH 3 + OH  NH 2 +H 2 O + NO  N 2 (NH 3 ) 2 CO NH 3 +HNCO NH 2 +CO (HNCO) 3 HNCO  + OH NCO+H 2 O  +NO N 2 O +OH,M,H

Catalytic Reduction of NO x from Flue Gas Selective Catalytic Reduction: NH 3 + NO  N 2 + H 2 O Metals: Metals: Pt, Pd Oxides: Oxides: Ru/Al 2 O 3, Fe 2 O 3 /Cr 2 O 3, V 2 O 5 /TiO 2, V 2 O 5 /MoO 3 /WO 3 /Al 2 O 3 Zeolites (Al x Si y O z /M)

Forms of Sulfur in Coal Organic-S compoundsOrganic-S compounds (thiophenes, sulfides, thiols) Pyritic sulfurPyritic sulfur (FeS) SulfatesSulfates (Ca/FeSO 4 ) Significant chemical changes of sulfur occur during coal devolatilization and combustion

Transformation of Coal-S Coal-S  (CS, S 2, S, SH)  SO  SO 2  SO 3 -SO 4 O 2, M COS, CS 2 H2SH2S char

Sulfur Pollutant Reduction No benign sulfur gas compounds Reduction of sulfur pollutant Pre-combustion coal cleaning In-situ cleaning Post-combustion cleaning: Solidification to sulfur salt compounds

Sulfur Pollutant Reduction: Pre-Combustion 1. Differences in density removes 30-50% of FeS 2. Leaching by sodium/potassium bases R-C-SH + NaOH  NaS-C-R + H 2 O 3. Biological cleaning by bacteria or fungi with high affinity to sulfur By leaching and biological techniques 90% can be removed

Sulfur Pollutant Reduction: In-Situ Cleaning Addition of sorbents: Ca/Mg/Zn/Fe/Ti oxides In furnace conditions CaCO 3  CaO + CO 2 Ca(OH) 2  CaO + H 2 O 2CaO + O 2 + 2SO 2  2CaSO 4 CaO + H 2 S  CaS + H 2 O CaO + SO 3  CaSO 4

Models for Sulfur Capture by Sorbents Sorbents are porous spheres Known properties of porous structure Rapid heating of sorbent -- CaCO 3, Ca(OH) 2 Decomposition sorbent to oxide & CO 2 Diffusion of CO 2 through CaO to outer surface Mass transfer of CO 2 from surface to bulk gas

Models for Sulfur Capture by Sorbents/continues Diffusion of SO 2 and O 2 from bulk gas to surface Diffusion of SO 2 and O 2 from outer surface to inner pores Reaction of SO 2 and O 2 with CaO to form CaSO 4

Models for Sulfur Capture by Sorbents/continues SO 2 O2O2 CO 2 CaCO 3 CaO CaSO 4

Sulfur Pollutant Reduction: Postcombustion Lime or limestone scrubbers CaCO 3  CaO + CO 2 2CaO + O 2 + 2SO 2  2CaSO 4 CaO + SO 3  CaSO 4 (Gypsum)

Summary: NO x and SO x Reduction NO x can be reduced during combustion, with right conditions SO x should be reduced pre- combustion