Air to fuel ratio control

Boiler House Division Air

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Air to fuel ratio controlFICCI – June 2006 - Bangalore


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The Next Hour and a half

• Boiler Efficiency• Combustion basics• Traditional control systems• Oxygen trim control• Oxygen measurement• Ensuring optimum efficiency• Boiler Efficiency improvement packages


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Cost of operation–Oil fuels


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Cost of operation-Solid fuel


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Operating costs

• Capacity Oil Gas Coal• 1 TPH 110L 60L 35L• 5 TPH 690L 300L 172L• 10 TPH 1350L 600L 345L

• Cost of fuel Rs 22/kg Rs 9/Nm3 Rs 2/kg• Hours of operation 8000 8000 8000


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Estimated savings

• Capacity Oil Gas Coal• 1 TPH 5L 2.2L 1.5L• 5 TPH 25L 11L 7.5L• 10 TPH 50L 22L 15L

• Improvement 80 to 83 79 to 82 70 to 73• Cost of fuel Rs 22/kg Rs 9/Nm3 Rs 2/kg• Hours of operation 8000 8000 8000


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Boiler Efficiency

• Boiler efficiency depends on both, the heat generation and heat utilization process.

• Heat generation covers the combustion process itself

• Heat utilization coves heat transfer from combustion to water and other operational losses like radiation and blowdown.


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Boiler Efficiency-Losses

• Heat generation– Stack loss– Enthalpy loss

• Heat Utilization– Radiation loss– Blowdown loss


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Losses – Typical values


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Boiler Efficiency-Methods

• Direct Efficiency• In-Direct Efficiency

– BS– ASME– IS

• Energy balance• S:F


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Controllable losses

• Stack loss– Can be easily controlled– One of the chief contributes to total boiler

losses• Blowdown loss

– Automatic control helps• Other losses: Enthalpy, Radiation, Ash


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Combustion and efficiency• Combustion is the burning of a fuel with Air leading to

release of energy. It is the process by which the Chemical energy contained in the fuel is converted into Heat energy.

• All conventional fossil fuels whether Solid, Liquid or gaseous contain basically carbon and/or Hydrogen which invariably react with the oxygen in the air forming carbon dioxide, carbon monoxide or water vapor.

• The heat energy released as a result of combustion can be utilized for heating purposes or for generation of steam in a boiler.


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Heat generation process

• In fossil fuels there are only three elements of interest: carbon, hydrogen & sulfur.

• During combustion each reacts with oxygen to release heat:

• C + 02 CO2 + Heat• H2 + ½ O2 H20 + Heat• S + O2 SO2 + Heat• Pure carbon, hydrogen and sulfur are rarely used as fuels.

Instead, common fuels are made up of chemical compounds containing these elements.


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Heat generation process

• CnHn + O2 + N2 CO2 + H2O + N2 + Heat(Air)

• From the above equation it can be seen that hydrocarbon burns completely to produce water, CO2 & heat. This kind of complete burning is known as stoichiometric combustion.

• The heat released when the fuel burns completely is known as heat of combustion.

• Nitrogen doesn’t play a role in combustion and appears in the output as it is.


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Excess Air

• The Minimum amount of air required for the complete combustion of a fuel is known as “theoretical air “.

• In boilers, one always needs to supply more air than what is required by stoichiometric calculations . The extra air, that is needed for complete combustion, taking into realities of combustion, over and above the stoichiometric air is known as “ Excess air “.

• The fuel rich mixtures, or mixtures with stoichiometric or less than stoichiometric air give incomplete combustion that results in some quantity of undesirable carbon monoxide in the exhaust gases and also some loss of heat energy.


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Excess Air• Too little excess air is inefficient

because it permits unburned fuel, in the form of combustibles, to escape up the stack. But too much excess air is also inefficient because it enters the burner at ambient temperature and leaves the stack hot, thus stealing useful heat from the process.

• “Maximum combustion efficiency is achieved when the correct amount of excess air is supplied so that sum of both unburned fuel loss and flue gas heat loss is minimized”.

CO 2

O2

CO+H 2

Theoreticaloptimumpoint

Real worldoptimumpoint

% Excess Air0 3010- 20

% Flue gas concentration

Real World : Combustibles appeareven when excess air is supplied.


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Traditional control systems

• All burners operate with more air than required.

• Often, the most immediate way of improving efficiency and reducing emissions

• Reduction of oxygen by 1 % typically will increase efficiency by 0.5 %.


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What leads to variations

• Air temperature• Fuel temperature• Fuel pressure• Moisture in fuel• Loading pattern• Changing calorific value of fuel• Use of multiple fuels


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Effect of Air temperature

Air temperature deg C4.510

26.737.848.9

- Excess air (%)- 25.5- 20.2- 15.0- 9.6- 1.1


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Linkage Control

• Fixed setting of fuel and air

• No compensation for variation

• Typical of Oil and Gas fired boilers

• Gear back lash and deadband


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Parallel control

• One step above the jack shaft control

• Settings fixed for each point of fuel and air

• Settings can be changed easily


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Cross Limiting Control

• Based on feedback of actual fuel and air flow

• A better system to have• Involves more

instrumentation• Cannot cater to fuel

composition changes


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Oxygen trim control

• Control of air as per combustion requirements

• Sounds good• Complicated to implement• Needs study before implementation


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What is Oxygen trim control?

• Control of EXCESS air in the stack of the boiler

• Done by sensing oxygen percentage in the stack

• On-Line measurement of CO not necessary• Done by independent modulation of air

damper or VFD.


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Before trim controlOxygen level versus firing rate

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Firing rate (%)

Oxy

gen

leve

l (%

)

OXYLEVELBNR2A

Trend


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After trim controlOxygen level versus firing rate

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Firing rate (%)

Oxy

gen

(%)

OXYLEVELBNR1APoly. (OXYLEVELBNR1A)


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Basic system - Oil / Gas fired boilers

Motor Burner

Boiler

Controller

PTDamper

Plunger

Servo

EffiMax4000

ModulationON /OFF

TS

OL

BlowerVFD

Oil CirculationM t


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Basic system - Solid fuels

BlowerVFD EffiMax

4000

TT

O2

FT

DYNO DRIVE

PT

Primary Air

Fuel

FURNACE

ID FAN

TT


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Pre-Implementation Checks

• Observe the boiler operation for 1-2 hours.• Check if it is modulating continuously or

only in high low mode.• Observe the average load on the boiler -

Preferably it should be above 50 % load.• Check what the fuel/air modulation

mechanism is - Servo motor based, etc.


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• Check the Oxygen values at high fire, low fire and mid firing conditions.

• These values should be between 3-8% for oil fired boilers, 2-5 for gas fired boilers and 5 to 12% for FBC boilers.

• Also check the CO values. Typically these should be below 200 ppm.


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• Ask the boiler operator to tune the burner and try reducing oxygen as much as possible with CO being below 200 ppm.

• The pay back of the system and the improvement in efficiency will depend on the higher oxygen measured earlier and later.


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Implementation pre-requisites

• Need to ensure that there is a provision for installing an additional feed back mechanism for damper position feed back.


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Some considerations

• Simple PID cannot work because of the large dead-time involved– Dead time compensation technique is used

Time

Step change in damper

Response of Oxygen

Dead time


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Dead time compensation

• Basically holds the output of the PID controller till the dead time is over

• Effectively makes the controller wait till the response is fully over

Measure O2

Move damperWait for O2 change


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Some more considerations

• Air damper has to respond immediately, without waiting for the dead time to be over, when the firing of the boiler changes with a change in load.

• While moving, it has to replicate the curve of Oil-Air relationship of that particular burner


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Some more considerations

• The damper has to be moved to a particular position, normally fully open, during the purging time of the burner


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One final consideration

• The air damper can be either– servo motor controlled, which requires one

current output– power cylinder controller, which requires an

I/P converter and an analog signal output– VFD controlled, which requires an analog

signal


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Features of our trim control loop

• Accepts inputs from– Oxygen analyzer– Burner On/Off– Burner firing position

• Has a characterizer to replicate the response of a mechanical link

• Tracking / non-tracking set point

• Has – a bump-less A/M

station– Dead time

compensation

• Displays– Oxygen value (P & S)– Damper opening (%)

• Gives outputs to– Damper actuator


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Trim control for solids

• Additionally it has furnace pressure control also.

• The trim output is interlocked with the furnace pressure such that if the furnace pressure increases, the trim output and the boiler pressure control are reduced.

• It should also have bed temperature interlock.

Back to main Menu?


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l Oxygen measurement using zirconia technology is todays industry standard and is accepted as a costeffective and reliable measuring instrument.

Oxygen measurement


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A process gas (A) with unknown oxygen (O2)-concentration flows over a measuring probe, which is sealed against the process gas with a heated zirconia cell (B)

At high temperatures a voltage V is generated between the two surfaces of thecell, which, at constant cell temperature, depends only on the ratio of the oxygen concentrations (partial pressures) in A and C.

With air (oxygen content constant 20,95%) as reference gas the measured voltage is a direct measure for the oxygen concentration in the process gas A, as long as...

The reference gas air (C) with its known and constant O2 - concentrationcontacts the cell from the inside.

VBC

A

The Zirconium measuring principle (is very simple


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As long as

the seal between process and reference gas is absolutely and perfectly gas tight and therefore any influence to the measuring results are eliminated for ever !

Is very simple ,


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T Temperature, is kept constant

With a „leakproof“ fraction line and air as reference gas all values of the Nernst equation except P2 are constant! This means

The voltage output depends only on partial pressure P2 (process gas) and calibration is not required

Reference gas partial pressure P1

Process gas partial pressure P2

The Nernst equation The gas tight fraction line

C Constant offset

K Natural constant

V = log·T · + CP 1

P 2K

V Measured voltage

P1 Partial pressure of reference gas; is constant, if air is used as reference gas and mixture prevented with process gas

Nerst equation and the gas tight fraction line


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Other oxygen measuring methods require a two point calibration, which inpractice has been transferred to the zirconia measuring principle. This is not necessary, as the Nernst equation is a mathematical a linear function and with air as a known reference gas the only paramenter P1 is constant. Therefore calibration is not required!

Any leakage at the cell will cause a migration of process and reference gas that will make regular calibrations necessary.

Only one condition must be fulfilled:The measuring cell must have a totally gas tight seal between the process gas side and the reference gas side.

Calibration ?


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mV VoltageIon migration

Zirconia Electrodes

Seal Heater

Thermocouple

Process gas

ReferenzluftReference gas (air)

Design of the Oxytec Zirconia cell with the gas tight seal


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l Carefully selected high quality materials

Special manufacturing process

Mechanical design

Special cell sealing technology

Production, Test & Quality Control to ISO 9001

Key factors for the reliable gas tight cell


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Ensuring optimum Efficiency

• How do you know correct set-points?• Continuous study and adjustment required.• Look at final performance parameters like

fuel consumption or direct efficiency.• Relate them to operating conditions to find

best operating points.


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Self learning logic

• Builds data base of operating conditions• Is simple to do, but has to be done

continuously• Compares past and present to alter

operating conditions• Better done through computer programs


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Self learning example


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Self learning example


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Effect of boiler loading


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Effect of Oxygen variation


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Boiler Efficiency range of products

• EffiMax 1000 - Online steam to fuel ratio meter with direct efficiency calculations.– Measures Steam flow, Oil/gas flow, Steam

temperature and feed water temperature.– Calculates S:F, Direct efficiency, Steam

pressure, steam and fuel totalization.– Applications - Typically oil / gas fired boilers

below 2-3 TPH capacity.


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• EffiMax 2000 - Indirect Efficiency analyzer with automatic blow down control.– Measures Steam flow, temperature, stack

Oxygen, temperature, ambient temperature, Drum TDS and feed water temperature.

– Calculates Indirect efficiency, indirect S:F, % blowdown loss, steam and blowdown total.

– Application - 3 TPH and above oil, gas, solid fuel fired boilers.


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• EffiMax 3000 - Indirect Efficiency analyzer with ABCO and S:F measurement.– Measures Steam flow, temperature, oil/gas

flow, stack Oxygen, temp., ambient temp., Drum TDS and feed water temperature.

– Calculates Indirect efficiency, direct S:F, % blowdown loss, steam and blowdown total.

– Application - 3 TPH and above oil and gas fired boilers.


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• EffiMax 4000 - Indirect Efficiency analyzer with ABCO and Oxygen trim control.– Measures Steam flow, temp., stack Oxygen,

temp., ambient temp., Drum TDS, feed water temp., damper feedback and boiler on/off.

– Calculates Indirect efficiency, indirect S:F, % blowdown loss, steam and blowdown total.

– Application - 3 TPH and above oil, gas, solid fuel fired boilers.


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EffiMax 2000Touch Screen Based


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EffiMax 2000, the latest Touch Screen based offering

from the EffiMax range of on-line boiler efficiency

analyzers, provides a complete monitoring and data

acquisition solution for boiler performance. The

highlight of this product is the extremely visual Human

Interface and self explanatory mimic of the boiler on the

front display. It also allows for real time / historical

trending on the display.


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lS

ale

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5

Boiler Efficiency Indication (%) in accordance with BS 845 based on indirect efficiency computation.

Stack Loss Indication (%),

Enthalpy Loss (%)

Radiation Loss Indication (%)

Combustion Loss (%)

Steam Flow Indication (kg/h),

Steam to fuel ratio (compensated for Feedwater Temp)

Oxygen Indication (%)


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A simple and cost effective package which monitors the following parameters ON LINE through a extremely visual

human interface and self explanatory touch screen mimic with a diagnostic report generation :-

1. All ealier features/data maintained in Touch screen.

2. The Manager can see the graphics on PC and the operator can see the same on the touch screen.

3. The operator too now has features like- Real time trending. - Customized alarms.

4. All range settings and calibrationis menu driven


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Blowdown loss totalization (kg), Average (kg/h)

Automatic Blowdown control

Steam and F.W. Temperature Indication - deg C

Stack Temperature Indication - deg C

All measured data displayed on a Mimic

Trending, Alarms and Data log.

Proprietary PC based software that provides graphical trending, datalogging, diagnostics, alarms

RS 485/ Modbus output to PC

Features


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lEffiMax 2000 - User Interface


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Documents

Air to fuel ratio control