by Nathan Long

1
Muskingum River Plant Emissions Reduction Programs

• by Nathan Long

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Precipitators

• Ohio Environmental Protection Agency (EPA)
  dictates opacity must be below 20. Other states
  may have different regulations.
• Units 1-4
• Each unit has a Joy/Western precipitator.
• Consists of 16 fields powered by 8
  transformer/rectifiers.
• Neundorfer voltage and rapper controls.
• Opacity averages between 5-12
• Unit 5
• Research Cottrell two boxes.
• Each box has 30 rectified fields.
• Neundorfer voltage and rapper controls
• Opacity averages approximately 5.

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Precipitator Problems

• Units 1-4
• Casing leaks allows moisture to enter
  precipitator creating high levels of sparking
  thus driving power levels down.
• Transformer/rectifiers failing more frequently
  than in the past.
• Reaching end of life?
• Neundorfer controls aggresively ramp voltage back
  up following spark quenching which may be
  shortening life of transformer/rectifiers.
• Electrodes were failing causing grounds.
  Resolved by installing a heavier gauge electrode
  wire.
• Precipitators are undersized so any upsets often
  cause load curtailments to stay under 20
  opacity.
• Unit 5
• Precipitators are oversized. No real problems.

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Sulfur Trioxide (SO3) and Ammonia (NH3) Flue Gas
Conditioning System

• Used on units 1-4 (subcritical units)
• Decreases flyash resistivity to improve
  collection of ash in precipitators.
• Advantageous when sulfur content of coal drops
  below 3.5 lbs/Mbtu.
• SO3 system
• Molten sulfur stored in 100 ton tank (approx. 60
  day capacity) at 143 deg. C (290 deg. F).
• Sulfur pumped through steam jacketed lines to
  sulfur burners.
• Sulfur gas passes through vanadium pentoxide
  catalyst which accelerates the natural process
  of the chemical reaction to ensure at least 95
  of the sulfur dioxide is converted to sulfure
  trioxide.
• 15 kw (20 HP), 3600 rpm process air blower 15
  cm/min (530 scfm)
• 2.2 kw (3 HP), 3600 rpm purge blower keeps piping
  and nozzles clear when system is out of service.
• 6 injection probes distribute the SO3 into the
  duct upstream of precipitator.

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Sulfur Trioxide (SO3) and Ammonia (NH3) Flue Gas
Conditioning System

• NH3 System
• Used with SO3 system to enhance collectibility
  of the ash.
• Anhydrous ammonia stored as a pressurized liquid
  in a bulk storage tank.
• Tank equipped with 2 10 kw heaters which
  vaporizes the ammonia.
• Ammonia vapor passes through a regulating valve
  (1-5 ppm) before being injected through 6 probes.

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Sulfur Trioxide (SO3) and Ammonia (NH3) Problems

• Sulfur Trioxide
• Must maintain proper insulation on steam jacketed
  sulfur lines or sulfur will solidfy. Experienced
  this on startups, had to heat lines with torches
  to get sulfur to flow. Improved insulation took
  care of the problem.
• The sulfur metering pumps are a submersible gear
  type pump with very close tolerances. The pumps
  wear and eventually will not pump at which time
  they have to be replaced and sent out for
  refurbishment. They cannot be repaired on site.
• When the system is shut down, the purge air
  blower must be in service or flue gas will come
  back into the lines and plug the probes.
• The probes are bulky and heavy and must be
  removed to be unplugged.

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Sulfur Trioxide (SO3) and Ammonia (NH3) Problems

• Ammonia (NH3)
• Care must be taken not to over inject or it will
  foul the precipitator.
• Ammonia detection systems and alarms must be
  maintained around the storage tank and injection
  skids to alert personnel of any leaks.

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Continuous Emissions Monitoring System (CEMS)

• All five units equipped with a redundant
  continuous emissions monitoring system.
• Monitors
• Opacity
• Sulfur Dioxide (SO2)
• Nitrous Oxide (NOx)
• Carbon Dioxide (CO2)
• Flue gas sample is diluted with air 3001 to
  extend life of instrumentation and make the
  system more reliable.
• Monitors analyze the gas sample and send raw data
  to a polling computer which calculates mass
  emissions rates.
• The CEMS technician performs daily checks of the
  data before sending on to AEP, Columbus. The
  data is submitted to the Ohio Environmental
  Protection Agency on a quarterly basis.
• Preventive maintenance is performed on the
  equipment on a quarterly basis.
• Monitors automatically calibrate daily.

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Nox Reduction Goals

• Units 1- 2 Maintain NOx Emission Rate under 779
  kg/kcal (0.44 lb/mbtu)
• Units 3-4 Maintain Nox Emission Rate under 466
  kg/kcal (0.308 lb/mbtu)
• Unit 5 Maintain NOx Emission Rate under 101
  kg/kcal (0.067 lb/mbtu) with SCR

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Terminology

• Theoretical air is the minimum air required for
  complete combustion of the fuel, resulting in
  stoichiometric combustion.
• Stoichiometry (also called stoichiometric ratio)
  is a term relating the actual air to the
  theoretical minimum air required to complete
  combustion.

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Terminology

• Excess air is the amount of air supplied for
  combustion in "excess" of that theoretically
  required for complete combustion.
• This additional air is required because of
  imperfections associated with the combustion
  process and the practical limitation of providing
  the Three T's of Combustion.

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Overfire Air
__________________________________________________
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How OFA System Reduces NOx U12

• Fuel-rich combustion zone
• The reducing environment helps control fuel and
  thermal NOx
• The lower peak flame temperature helps further
  reduce the formation of thermal NOx
• Overfire air zone
• Thermal NOx formation is limited due to lower gas
  and flame temperatures

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How OFA System Reduces NOx U34

• Fuel-rich zone
• Withholding air causes the cyclones to be
  operated fuel-rich. As a result, CO is produced
  within the cyclone. In the lower furnace, this
  CO aggressively reduces already formed NOx to N2
  in its attempt to form CO2.
• The lower peak flame temperature provides an
  additional incremental reduction in NOx.
• Overfire air zone
• Thermal NOx formation is limited due to lower gas
  and flame temperatures.

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Thermal NOx Mechanism
NOx
O2, temp.
Atmospheric Nitrogen
Critical Temperature
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Non SCR NOx Reduction Problems

• Reducing atmosphere caused
• Accelerated tube wastage in primary furnace
• Had to find acceptable stoichiometric setting to
  limit wastage
• Severe erosion/loss of refractory on furnace
  floor. Large amounts of molten iron pooled on
  the furnace floor at a higher frequency.
  Suffered numerous tap throughs.
• Started with silicon carbide refractory.
  Experimented with other forms of refractory.
  High alumina, 5 chrome refractory seems to hold
  up best.
• Severe pluggage in convection pass. Added 6
  electric sootblowers to keep area clean.

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Selective Catalytic Reduction (SCR)

• Began operation in May, 2005.
• Operates from May 1 through September 30.
• Will be required to operate year round beginning
  in 2009.
• Therefore, it was decided not to install bypass
  ducts and dampers.
• Two reactor boxes (north and south)
• Sized to hold four layers of catalyst. Initial
  operation is with two layers. Third layer is to
  be installed in Spring of 2007. Fourth layer is
  for future use, perhaps for mercury removal.
• Catalyst supplied by Hitachi America Ltd.
• Plate type, titanium dioxide (TiO2) carrier
  impregnated with tungsten trioxide (WO3) and
  vanadium pentoxide (V2O5).
• 72 catalyst modules per layer (9 wide X 8 deep
  grid).
• Initial guarantee is for 16,000 hours of 90 NOx
  removal before third layer is added.

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Selective Catalytic Reduction (SCR)

• Two draft booster fans were installed to overcome
  the new SCR system pressure drop.
• Located between the precipitator and the stack
• Howden Variax constant speed, single stage,
  horizontal, axial flow.
• Utilizes variable pitch blades for flow control.
  Blades are hydraulically controlled.
• Impeller diameter is 3.7 meters (146 inches).
• Operates at 895 rpm.
• Ammonia vapor system
• Urea brought in by truck
• Dissolved in water to a 40 by weight solution in
  a mix tank.
• Heated to 210 deg C (410 deg F) at 28.12 kg/scm
  (400 psig) by steam in a hydrolysis reactor
  (hydrolizer). Steam comes from high pressure
  turbine exhaust.
• Produces ammonia and carbon dioxide.

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Selective Catalytic Reduction (SCR)

• Ammonia vapor system (continued)
• Spent solution (recycle 3 solution) is
  continuosly withdrawn from the hydrolizer, sent
  through an economizer and then to a recycle
  storage tank. It is then used in the mix tank to
  mix batches of urea solution.
• Gaseous ammonia and carbon dioxide leave the
  hyrdolizer vessel and feeds a dilution skid
  upstream of each catalyst box. Each skid has an
  ammonia flow control valve that meters the
  correct amount of ammonia to achieve desired NOx
  reduction.
• The rate of ammonia generation in the hydrolizer
  is controlled to maintain constant manifold
  pressure at the dilution skids.
• When the ammonia vapor gets to the dilution skid,
  it is diluted and mixed with air from dilution
  air fans. It is then injected into the duct
  through a grid of pipes upstream of the catalyst.

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Selective Catalytic Reduction (SCR) Problems

• Large particle ash (LPA)
• Large particles of ash carried through the
  ductwork settles out on the screens above the
  catalyst. As it accumulates it blocks off the
  gas path and finer ash then builds up as well.
• Creates a differential increase through the
  catalyst from 50 kg/sqm (2 inches water) to
  96.5 kg/sqm (3.8 inches of water) at which time
  the unit has to be brought off line for cleaning
  of the catalyst.
• At 96.5 kg/sqm (3.8 inches of water)
  differential, the catalyst chamber is 50
  blocked.
• Creates a problem for the booster fans to
  maintain desired economizer outlet pressure.
• Currently designing a hopper to install under the
  economizer to catch the large ash before it
  reaches the catalyst boxes.
• May incorporate a screen to catch and direct the
  ash to the hopper.
• Plan to install hopper in Spring of 2007.

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Plant Information (PI) Screen
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Flue Gas Desulfurization System

• Work has begun to build a flue gas
  desulfurization system on unit 5. Project has
  been postponed to an in service date of
  12/31/2010.
• Chiyoda design from Japan
• Differs from the conventional spray tower
  scrubber in that it uses a jet bubbling reactor
  which sparges the flue gas into a lime slurry
  bath where 100 of the flue gas reacts with the
  lime slurry before bubbling off the top and
  leaving the reactor to the stack.
• Foundations have been completed for the reactor
  and a new 252 meter (826 foot) high stack.

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Flue Gas Desulfurization System

• Designed to provide sufficient limestone slurry
  to absorb 98 SO2 from the flue gas.
• Allows the burning of 11.3 mgm SO3/kcal (7.5 lb
  SO2/Mbtu) coal
• Redundant 100 ball mill systems capable of
  grinding 40 tons of limestone per hour.
• 895 kw (1200 hp), 4kV motors, horizontal, wet,
  with steel grinding balls.
• SCR booster fans will be replaced with two
  Howden, 50 capacity axial fans, each rated at
  11.2 mw (15,000 hp).
• Gypsum will be produced at 73 tons/hour and
  will be dewatered by one vacuum belt filter
  before being landfilled.

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Conceptual Model for Scrubber
Jet Bubbling Reactor
Typical Spray Tower
Liquid is sprayed to into Gas
Jet Bubbling Layer
Gas is sparged into Liquid
Reservoir
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