March 22, 2018 | Author: Sabari Gireaswaran | Category: Furnace, Combustion, Boiler, N Ox, Coal
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Optimizing boiler and coal mill performance Presented By: Richard Storm Innovative Combustion Technologies, Inc. 2367 Lakeside Drive, Suite A-1 Birmingham, Alabama 35244 (205) 453-0236 I 2011 Conference

Symptoms of a Boiler Needing Combustion Optimization STEAM AND STEAM TEMPERATURE CONTROLS • High de-superheating spray flows • Higher or lower steam temperatures than design

BOILER DRUM LEVEL • Uneven furnace heat release can contribute to nonuniform steam generation in the waterwall circuits, resulting in varied steam by weight in the furnace circuitry, and sometimes tube failures or steam purity problems

FOULING AND SLAGGING • Furnace exit S.H. inlet slagging • Fouling of the convention pass and/or the air heater baskets • Burner eyebrows and waterwall slagging HIGH GAS TEMPERATURES •Flue gas temperature at the furnace exit is greater than 2,150°F (1177°C) peak •Stratified flue gas temperatures •Economizer gas outlet temperature greater than 750°F (399 °C Respectively) •Overhead tube metals in the superheater and the reheater

OXYGEN AND AIR: •Stratified oxygen at the furnace or boiler exit •Air heater leakage greater than 10% •Combustion air distribution to the burners exceeds ±10% •Air in-leakage through the ash hoppers •Air in-leakage through the nose arch, penthouse or convection pass areas

FLYASH •Flyash unburned carbon (LOI) greater than 5% for bituminous coals and greater than 0.5% for subbituminous coals •Electrostatic precipitator performance reduced due to ash conductivity or high carbon content

PULVERIZER AND BURNER LINES FUEL DISTRIBUTION: •Fuel Imbalances •Primary airflow for the Air/Fuel ratio is not correct •Poor fineness (Less than 75% passing 75 micron & >0.3% not passing 300 micron •Fuel temperatures less than 135°F (57°C) •Pulverizer rejects high •Mechanical tolerances are out of specification and the burners are not within ±1/4”

FANS AND DAMPERS: • I.D. fan capacity inadequate • I.D. and F.D. fan clearances are not optimum • Damper, register, and fan control louvers are not timed from 0-100% on the operating drive or hand control I 2011 Conference

The Definition of Optimum Combustion is at least these factors: • • • • • • • • • • • • • • •

Completed combustion within the furnace (no Secondary combustion at the Superheater) Acceptable NOX Acceptable CO Fly ash unburned carbon satisfactorily Full load capability and meet all environmental and fuel quality requirements De-superheating spray water flows minimal Design Steam temperature attained No reducing atmosphere in the lower furnace causing waterwall wastage Primary airflow is optimized No furnace slagging No convection pass fouling Minimal Pop corn ash Burn lowest quality (least expensive) fuel with no adverse consequence Flames stable and satisfy flame scanners Satisfactorily low LOI so that ESP performs satisfactorily for minimum opacity Blue Highlighted parameters are environmentally driven factors Highlighted in Green are heat rate factors of “optimum combustion” I 2011 Conference

Common boiler tests to optimize combustion and boiler reliability I 2011 Conference

13 Prerequisites For Optimum Combustion (Ensures Proper and Optimum “inputs”) Fuel lines balanced in fuel flow to 10% or better Fuel lines balanced by “Dirty Air” test to 5% or better. Fuel line minimum velocities shall be 3,300 fpm. Fuel line fineness >75% passing a 75 Micron screen. 300 Micron particles 50% are Related to the Pulverizers) High furnace exit gas temperatures contribute to overheated metals, slagging, excessive sootblower operation, production of popcorn ash, fouling of SCR’s and APH’s

High furnace exit gas temperatures contribute to high de-superheating spray water flows that are significant steam turbine cycle heat-rate penalties.

Coal pulverizer spillage from pulverizer throats that are too large Flyash Carbon losses

Bottom ash carbon content High primary airflows contribute to unnecessarily high dry gas losses and also poor fuel distribution and poor coal fineness.

Non optimum primary airflow measurement and control ; Excessive NOX levels I 2011 Conference

Burner Belt Performance is never Optimal with less than perfect Pulverizer Performance Consequences of Non-Optimum Burner Belt Performance: ● ● ● ● ● ● ● ●

High spray water flows to S.H. and R.H. Tube metal over heating and reliability problems Slagging and Fouling Higher NOx “Popcorn” Ash SCR Fouling APH Fouling Elevated economizer outlet gas temperature

The “inputs” must be Optimal No control of air and fuel after it enters the boiler I 2011 Conference

Optimizing Mill and Burner Performance 1. Evaluate Coal Factors that influence mill capacity (Raw Coal Size, HGI, Moisture, HHV, Fineness, Hp/Ton) 2. Fuel Loading & Feed Rate Control 3. Clean Air Balance within + 2% 4. Dirty air flow Balance within + 5% 5. Measured Primary air Hot “K” Factor calibrations +2-3% (measured vs. actual) 6. Mill temperature Control, Damper Control and Responsiveness to Load Control 7. Air-Fuel Ratio /fuel ratios are required for Optimum Flame Lengths and Carbon Burnout 8. Total air flow Measurement / Control Optimized ; Balance of Mass Air & Fuel Flow 9. Fuel line fineness and distribution testing by air/fuel ratio sampling & ensuring optimum fineness levels of >75% thru 200 mesh (75 micron) & 99.7% thru 50 Mesh (300 Micron) 10. Fuel line balancing through classifier changes or fuel line distribution modifications to achieve +10% 11. “Blueprinting” of tolerances, mechanical settings and control settings I 2011 Conference


Poor Pulverizer Performance Increases FEGT by Delaying Combustion Increased Slagging and Lower Performance Excessive de-superheating water sprays, for both S.H. and R.H.

Tube spacing permits slag bridging between the tube assemblies, when the ash is soft, sticky and/or molten

Sticky plastic slag deposits on pendants. Slag temperature at or above ash softening temperature

Reducing areas w/fuel stratifications and excessive CO levels Molten slag on the furnace wall

Good /uniform mixing in the burner zone. Burner mechanical tolerances, fineness, fuel/air balance and PA flow proper and precise

Air inlet & outlet flue gases higher than design

Poor fuel fineness and distribution aggravates high center of combustion

9 I 2011 Conference

Importance of Fineness  Higher fineness levels always promote more even distribution of fuel between a mill’s separate burner lines.  Better distribution promotes better combustion, inherently lower NOx emissions and lower fly ash L.O.I. or carbon content.  Better than ±10% fuel balance is not achieved until better than 70% passing 200 Mesh (75 micron) is achieved. I 2011 Conference

Major Factors Related to NO Production X

Fixed Factors: – Boiler Design – Fuel Factors – Burner Configuration/Design Variable Factors: – Pulverizer Performance – Fuel Line Balancing – Combustion Air Balancing and Proportioning – Air In-Leakage I 2011 Conference

Sources of NO – Low NO Pulverized Fuel Firing X


Thermal NOX • •

Accounts for approximately 20 - 30% of NOX produced Formed from the nitrogen in the combustion air (>2800°F)

Fuel NOX • •

Accounts for approximately 70%-80% of NOX produced Formed from the nitrogen in the fuel I 2011 Conference

Understanding the Effects of Coal Fineness on NOX and/or Slagging •

Higher coal fineness will promote less slagging

When combustion is completed lower in the furnace cavity, the water walls have increased time to absorb heat released from the fuel

Increased heat release in the lower furnace results in a higher proportion of heat absorbed by the waterwalls

Higher heat absorption by the waterwalls relates to a reduction in furnace exit gas temperature

Reduction in furnace exit gas temperature resulting from completion of combustion in the lower furnace with higher fineness results in lower slagging propensities and lower NOx. I 2011 Conference

Burner Stoichiometry differs when fuel is not balanced and NOX is higher Very High NOX

Very High NOX

1.3 1.2

Lack of Excess Air to these burners will yield secondary combustion

Stoichiometry 1.17

1.1 1 1.10 0.9 0.8 0.7

0.90 0.87 0.76 0.72

Fuel Lean

Fuel Rich

Fuel Lean

Fuel Rich I 2011 Conference14

Effect of Mill Air Flow on NOx, 500 Mw Wall Fired Boiler NOX reduced by: •

Optimized mill air flow

Air flow was high due to oversized vane wheels – coal spillage with proper air flow.

Improvement in coal fineness.

Decreasing Mill Air Flow (Primary Air Flow) I 2011 Conference15

Mill Fineness Improves NOx in at least (3) ways 1. Better Fuel Bound Nitrogen Release (Majority of NOx is fuel derived). 2. Better Fuel Distribution. 3. Permits lower Furnace Excess O2 without complications. Poor Fineness

Good Fineness









Nozzle N










“Release of Fuel Bound Nitrogen in the De-Volatilization Zone” I 2011 Conference

Proper and Optimum Boiler Air Flow Management is Essential to Achieving Lowest NOX without upper or lower furnace slagging, 725 Mw boiler firing subbituminous coal

Secondary air flow measured to ensure uniform and proper total air to fuel ratio between burner elevations

Pulverizer air flow measured within ±3%; Critical for best NOX, slagging and exit gas temperature I 2011 Conference

Precise measurement & management of all airflow inputs to the boiler is ideal I 2011 Conference

Proper O2 in the right places is needed because combustion must be completed and carbon to CO2 in ~1 to 1.5 seconds at full load I 2011 Conference

The Importance of Fuel Preparation & Furnace Residence Time

Heating & Minor De-volatilization Ignition Major De-volatilization Carbon Burnout






1.00 I 2011 Conference

Common Mistake: Low or No CO at Economizer = Optimized Air and Efficiency Can be false if there are large furnace imbalances or boiler setting air ingress; CO can continue to burn into the convection pass.

1.0% O2 8,000 PPM CO

1.9% O2 500-1200 PPM CO

0.5% O2 > 30,000 PPM CO

3% O2 150 PPM CO I 2011 Conference

Difference Between Complete and Incomplete Combustion Products of Complete Combustion

O (CO2)



14,540 Btu


Products of Incomplete Combustion





4,350 Btu I 2011 Conference

The Inter-Relationship of Combustion and Tube Reliability Incomplete combustion at the furnace exit results in a hazy furnace, stack opacity, and measurably high CO. It also contributes to boiler exit flue gas temperatures being too high, and therefore, can contribute to super-heater tube overheating, super-heater, and boiler generating bank tube plugging.

Due to Non Optimal Lower Furnace Heat Absorption Resulting in Upper Furnace Short or Long Term Failures` I 2011 Conference

Typical Outage Opportunities * Thoroughly inspect and repair all ductwork and expansion joints

Inspect tubes for corrosion or wear, check for any problems with alignment bars and tube shields.

Refurbish burners to design dimensions, Dampers and/or tilts synchronized

Rebuild pulverizer grinding elements

Air-in leakage inspections and repairs

* Verify damper strokes (all dampers to be verified from inside ducts)

Optimize air heater seals, basket cleanliness, check and repair sector plates and all moving parts

PA, FD, ID Fan clearances and damper/inlet vane checks I 2011 Conference

Root Cause of Upper Furnace Slagging FEGT Exceeds Ash Fusion Temperature  Slagging will occur if furnace exit gas temperatures (F.E.G.T.) exceeds the fusion temperature of ash from the coal being fired.  F.E.G.T. should be 100°F to 150°F below the ash softening temperature.  Higher fineness, proper primary (pulverizer) airflow and good fuel balance reduce FEGT by allowing combustion to complete lower in the furnace. combustion in completed lower in the furnace, the waterwalls absorb a higher amount of total heat release by the fuel and FEGT is reduced.  Reducing ash fusion temperatures are always lower than those in an oxidizing atmosphere. Ash melts into a slag due to lower melting points caused by a reducing atmosphere allowing slag to be formed at lower temperatures. I 2011 Conference

Typical FEGT Measurement Location Controlling Furnace Exit Conditions, one if not the most important factor to controlling slagging, optimum steam temperatures & combustion efficiency. I 2011 Conference

FEGT is controlled by the amount of heat absorbed by the water walls, FEGT is lowered when: 1. Wall blowers are blown 2. Burner tilts down on tangentially fired units 1. Combustion is completed faster • Better fineness • Good fuel & air Balance

2. Better mixing in the lower furnace, more uniform: • O2 • Temperature • Slag deposition I 2011 Conference

Burner Tilts in upward orientation on tangentially fired units reduces furnace residence time I 2011 Conference

Changing FEGT with Burner Tilts




Burner Tilts (+) UP

Burner Tilts (0) Horizontal

Burner Tilts (-) Down

Low Retention Time Low W.W. Heat Absorption Higest FEGT

Moderate Retention Time Moderate W.W. Heat Absorption Lower FEGT

Higest Retention Time Highest W.W. Heat Absorption Lowest FEGT I 2011 Conference

Relationship between FEGT and Furnace Heat Release Rate

Delayed combustion = reduces waterwall absorption = high FEGT (same effect as smaller furnace)

Air Heater

To Precip/ Bag House Air Inlet I 2011 Conference

Ash Fusion Temperatures




FT I 2011 Conference

The Furnace Exit should be Oxidizing because: 1. 2.

Reducing Ash Fusion Temperatures are always lower Low or No O2 increases Furnace Exit Gas Temperature I 2011 Conference

Low O2 at the Furnace Exit also causes slagging Ash “chemistry changes and ash fusion (melts) at lower temperature FEGT is higher because there is insufficient excess oxygen to complete combustion in the lower furnace I 2011 Conference

Poor coarse coal fineness (>300 Micron particles) can impact on the lower furnace slope causing heavy slagging in the lower furnace I 2011 Conference

SLAGGING ZONE Tube spacing becomes more restrictive as the heat transfer process changes from “Radiant” in the furnace to “Convective” heat transfer in the back pass


Slagging / Fouling I 2011 Conference

Use good walk downs and/or permanent cameras to identify slag before it becomes a problem in the SH, the plant can then shift from a “normal” to “aggressive” sootblowing/cleaning mode of operation to manage or remove the clinker online. Take Action rather than waiting for a forced outage.

20 Tips to help Prevent Slagging

Good control of the furnace exit conditions to minimize or stop slagging. (Proper & uniform O2 and Temperature) Uniform furnace exit conditions across the furnace (°F/O2) = uniform slag deposition. (Uniform slag is more easily managed. Active monitoring of the FEGT is KEY. Operators need to be aware of FEGT to optimize their cleaning strategies and make adjustments. Trust but verify optical, acoustic and calculated FEGT. High Velocity Thermocouple Testing is the Gold standard of FEGT measurement – HVT measures bulk and discrete point temperatures. Don’t overuse OFA – NOx can be “too good” – the benefits of over-staging will be short lived Practice “preventative” not “reactive” soot blowing by cleaning water walls, reducing FEGT and Slagging conditions. Keeping the walls clean and lowering furnace temperatures can also reduce NOx, sometimes as much as 15%. Know your coal before it enters the furnace (Operator awareness) Control the coal quality issues that you have control of, “Plant” coal quality control starts in the coal yard. Raw coal sizing, moisture (coal pile management), coal drying (mill outlet temperature) and fineness. Optimize lower furnace fuel & air interactions to maximize water wall heat absorption. Pulverizer performance is critical to preventing lower furnace slag/clinkers. Avoid the “splat” factor. I 2011 Conference

The Inter-Relationship of Combustion, Slag and Tube Reliability Non-Optimal furnace cleaning can significantly elevate the furnace exit gas temperature and force heat to the convection pass I 2011 Conference

The “strainer” effect of the boiler tube spacing gets smaller and more restrictive between the furnace and boiler exit Front Front Pendant Pendant Reheater Reheater (9 ½” ctrs.) (9 ½” ctrs.)

Rear Pendant Rear Pendant Reheater Final Reheater Final (4 ¾”(4ctrs.) ¾” ctrs.)

Superheater Superheater Division Panels Division Panels (8’ ctrs.) (8’ ctrs.)

Front Pendant Superheater Final (4 ¾” ctrs.) Front Pendant Superheater Final (4 ¾” ctrs.)

Rear Pendant Rear Pendant Superheater Superheater


Superheater Platen (19 ½” ctrs.) Superheater (19 ½” ctrs.) PreferredHVT HVT Preferred Traverse Plane Traverse Plane





Corner Firing System I 2011 Conference

20 Tips to help Prevent Slagging

Boiler setting air ingress minimized; furnace O2 is not low with normal economizer exit O2. SH/RH Heating surface areas optimized – Good steam temperatures with FEGT at or less than ash softening temperature. Help pendant/platens clean themselves by removing slag anchor points such as certain types of wrapper tubes, alignment lugs and rigid alignment/tie bars to allow some “swinging” of the pendants. Soot blowing technologies have also advanced a long way from a pipe with two holes – Ensure soot blower PM’s are being completed to maximize soot blowing effectiveness. Amount of heat absorbed by the water walls regulates Furnace Exit Gas Temperature. LOOK at the water walls; know what you’re looking for. (Slagging Conditions) Remember the boiler is a heat engine, get the inputs right. Fuel and air need to be in the right places in the right amounts. Air heater is clean & well maintained; a high DP or Leakage doesn’t lower furnace O2 due to fan capacity. Practice prevention of slag rather than managing slag incidents. Listen to your boiler when it tells you it is sick; fevers – high exit gas temperatures or FEGT, hot tubes, vomiting – high spray flows, ash spills, dark bottom ash or fly ash, Shortness of breath – ID and FD fan limitations, high DP’s and low wind box pressures. I 2011 Conference

Typical Slagging conditions on a Tangentially fired boiler I 2011 Conference

Poor Pulverizer Performance Increases FEGT due to Delayed Combustion, Increasing Slagging and Lowering Performance Excessive desuperheating water sprays, for both S.H. and R.H.

Tube spacing permits slag bridging between the tube assemblies, when the ash Is soft, sticky and/or molten

Sticky, plastic slag deposits on pendants. Slag temperature at or above ash softening temperature Reducing areas w/ fuel stratifications and excessive CO levels

Air Heater inlet & outlet flue gases higher than design

Molten slag on the furnace water wall

To Stack From FD Fan

Burner tilts upward or horizontal reduce the furnace mixing and elevate the furnace exit gas temperature

Poor fuel fineness and distribution aggravates high center of combustion

41 I 2011 Conference

Optimized Tangentially fired boiler Tight Penthouse (No Air In-Leakage)

Max. Temperature of 2,150°F and 3% Excess Oxygen At Full Load Conditions 0.5% or Less Oxygen Rise From Furnace to the Economizer Outlet No Air In-Leakage Gas Temperature Leaving Economizer Max. of 775°F Preferred Temperature 750°F Balanced Airflow’s, Balanced Fuelflow, Fuel Fineness >75% Passing 200 Mesh and
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