Transcript No Slide Title

Reduction of NOx and SOx 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 CO2
2. Particulate emission (soot, small fly ash)
3. SOx, NOx
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 heat 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 gasphase 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 NOx & SOx 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 Emissions
• CO2: the 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 rain precursors
• Soot and PAH are generally carcinogenic
• Fly ash: if above 5% carbon content - gain
if bellow - loss
• Small particulate: lung diseases
CO2 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
SOx 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
(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
NOx 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)
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 heat
  HCN + Volatile-N (2)
 
 + NH3
Volatile-N heat
HCN
(3)
Volatile + O2  NOx + …
(4)
Char-N + O2  NOx + …
(5)
HCN + O2  NOx + …
(6)
HCN + NOx  N2 + …
(7)
HCN + Char  N2 + …
(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 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 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 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-N
Nitrogen contained in coal -- coal-N
(1)
Coal-N heat
  HCN + Volatile-N (2)
 
 + NH3
Volatile-N heat
HCN
(3)
Volatile + O2  NOx + …
(4)
Char-N + O2  NOx + …
(5)
HCN + O2  NOx + …
(6)
HCN + NOx  N2 + …
(7)
HCN + Char  N2 + …
(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 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-N
• The two main products of char-N oxidation
are:
NO and N2O
• Occur via
homogeneous formation/destruction
heterogeneous formation/destruction of HCN
How is NOx 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-N
Nitrogen contained in coal -- coal-N
(1)
Coal-N heat
  HCN + Volatile-N (2)
 
 + NH3
Volatile-N heat
HCN
(3)
Volatile + O2  NOx + …
(4)
Char-N + O2  NOx + …
(5)
HCN + O2  NOx + …
(6)
HCN + NOx  N2 + …
(7)
HCN + Char  N2 + …
(8)
How is NOx 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 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 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 NOx by External
Agents
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 NOx by External
Agents
NH3
+
OH

NH2+H2O
+
NO

N2
(NH3)2CO
(HNCO)3
NH3+HNCO
HNCO
 + OH
NCO+H2O
+NO
N2O
NH2+CO
+OH,M,H
Catalytic Reduction of NOx
from Flue Gas
Selective Catalytic Reduction:
NH3 + NO  N2 + H2O
Metals: Pt, Pd
Oxides: Ru/Al2O3, Fe2O3/Cr2O3, V2O5/TiO2,
V2O5/MoO3/WO3/Al2O3
Zeolites (AlxSiyOz/M)
Forms of Sulfur in Coal
•Organic-S compounds (thiophenes,
sulfides, thiols)
•Pyritic sulfur (FeS)
•Sulfates (Ca/FeSO4)
Significant chemical changes of sulfur occur
during coal devolatilization and combustion
Transformation of Coal-S
O 2, M
-SO4
Coal-S  (CS, S2, S, SH)  SO  SO2  SO3
char
COS, CS2
H 2S
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 + 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:
In-Situ Cleaning
Addition of sorbents: Ca/Mg/Zn/Fe/Ti oxides
In 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
Sorbents
• 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
Sorbents/continues
• 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
Sorbents/continues
SO2
O2
CaCO3
CaO
CaSO4
CO2
Sulfur Pollutant Reduction:
Postcombustion
Lime or limestone scrubbers
CaCO3
 CaO + CO2
2CaO + O2 + 2SO2  2CaSO4
CaO + SO3
 CaSO4 (Gypsum)
Summary:
NOx and SOx Reduction
• NOx can be reduced during
combustion, with right conditions
• SOx should be reduced precombustion