Air to fuel ratio control

l o r t n o c oi t a r l e u f o t

Air to f uel ratio control FICC FI CCI I – Ju June ne 20 2006 06 - Ban anga galor lore e

The Next Hour and a half l o r t n o c oi t a r l e u f o t

• Boiler Ef Efficiency • Combustion basics • Tra Traditi ditio onal nal co contro ntroll sy syste stems • Oxygen trim control • Oxygen measurement • Ensu Ensuri ring ng opti optim mum effi effici cien ency cy • Boil Boiler er Effi Effici cien ency cy impr improv ovem emen entt pack packag ages es

The Next Hour and a half l o r t n o c oi t a r l e u f o t

• Boiler Ef Efficiency • Combustion basics • Tra Traditi ditio onal nal co contro ntroll sy syste stems • Oxygen trim control • Oxygen measurement • Ensu Ensuri ring ng opti optim mum effi effici cien ency cy • Boil Boiler er Effi Effici cien ency cy impr improv ovem emen entt pack packag ages es

Cost of operation–Oil fuels l o r t n o c oi t a r l e u f o t

Cost of operation-Solid fuel l o r t n o c oi t a r l e u f o t

Operating costs l o r t n o c oi t a r l e u f o t

• 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

8000

8000

• Hours of operation 8000

Estimated savings l o r t n o c oi t a r l e u f o t

• 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

8000

8000

• Hours of operation 8000

Boiler Efficiency l o r t n o c oi t a r l e u f o t

• 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.

Boiler Efficiency-Losses l o r t n o c oi t a r l e u f o t

• Heat generation – Stack loss – Enthalpy loss

• Heat Utilization – Radiation loss – Blowdown loss

Losses – Typical values l o r t n o c oi t a r l e u f o t

Boiler Efficiency-Methods l o r t n o c oi t a r l e u f o t

• Direct Efficiency • In-Direct Efficiency – BS – ASME – IS

• Energy balance • S:F

Controllable losses l o r t n o c oi t a r l e u f o t

• 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

Combustion and efficiency l o r t n o c oi t a r l e u f o t

• 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.

Heat generation process l o r t n o c oi t a r l e u f o t

• 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.

Heat generation process • l o r t n o c oi t a r l e u f o t

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.

Excess Air l o r t n o c oi t a r l e u f o t

• 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

Excess Air • l o r t n o c oi t a r l e u f o t

•

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 achieved when the correct amount excess air is supplied so that sum both unburned fuel loss and flue heat loss is minimized”.

is of of gas

Real World : Combustibles appear even when excess air is supplied.

% Flue gas concentration CO 2

Theoretical optimum point

- 20

0

Real world optimum point

10

30 % Excess Air

O2 CO+H 2

Traditional control systems l o r t n o c oi t a r l e u f o t

• 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 %.

What leads to variations l o r t n o c oi t a r l e u f o t

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

Effect of Air temperature l o r t n o c oi t a r l e u f o t

Air temperature deg C

- Excess air (%)

4.5

- 25.5

10

- 20.2

26.7

- 15.0

37.8

- 9.6

48.9

- 1.1

Linkage Control l o r t n o c oi t a r l e u f o t

• Fixed setting of fuel and air • No compensation for variation • Typical of Oil and Gas fired boilers • Gear back lash and deadband

Parallel control l o r t n o c oi t a r l e u f o t

• One step above the jack shaft control • Settings fixed for each point of fuel and air • Settings can be changed easily

Cross Limiting Control l o r t n o c oi t a r l e u f o t

• Based on feedback of actual fuel and air flow • A better system to have • Involves more instrumentation • Cannot cater to fuel composition changes

Oxygen trim control l o r t n o c oi t a r l e u f o t

• Control of air as per combustion requirements • Sounds good • Complicated to implement • Needs study before implementation

What is Oxygen trim control? l o r t n o c oi t a r l e u f o t

• 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.

Before trim control Oxygen level versus firing rate 10.0


9.0

8.0

7.0

) 6.0 % ( l e v e l 5.0 n e g y x O 4.0

3.0 OXYLEVELBNR2A Trend

2.0

1.0

0.0 0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

After trim control Oxygen level versus firing rate 10.0 OXYLEVELBNR1A Poly. (OXYLEVELBNR1A)


9.0

8.0

7.0

6.0 ) % ( n e 5.0 g y x O

4.0

3.0

2.0

1.0

0.0 0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

Basic system - Oil / Gas fired boilers PT

Damper


V F D

Blower

Modulation ON /OFF

Controller

Boiler Servo Motor

Oil Circulation

Burner

OL

TS

Plunger

EffiMax 4000

Basic system - Solid fuels PT


O2

FURNACE TT TT

DYNO DRIVE

Fuel

ID FAN Primary Air

V F D

Blower

FT

EffiMax 4000

Pre-Implementation Checks l o r t n o c oi t a r l e u f o t

• 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.


• 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.


• 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.

Implementation pre-requisites l o r t n o c oi t a r l e u f o t

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

Some considerations l o r t n o c oi t a r l e u f o t

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

Dead time Response of Oxygen

Time

Dead time compensation l o r t n o c oi t a r l e u f o t

• 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

Wait for O2 change

Move damper

Some more considerations l o r t n o c oi t a r l e u f o t

• 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

Some more considerations l o r t n o c oi t a r l e u f o t

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

One final consideration l o r t n o c oi t a r l e u f o t

• 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

Features of our trim control loop l o r t n o c oi t a r l e u f o t

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

• Has a characterizer to replicate the response of a mechanical link • Tracking / nontracking set point

• Has – a bump-less A/M station – Dead time compensation

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

• Gives outputs to

Trim control for solids l o r t n o c oi t a r l e u f o t

• 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.

Oxygen measurement l o r t n o c oi t a r l e u f o t

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

The Zirconium measuring principle (is very simple l o r t n o c oi t a r l e u f o t

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) The reference gas air (C) with its known and constant O2 - concentration contacts the cell from the inside. At high temperatures a voltage V is generated between the two surfaces of the cell, 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...

A B C

V

Is very simple , l o r t n o c oi t a r l e u f o t

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 !

Nerst equation and the gas tight fraction line The Nernst equation

l P1 o r + C t V = K ·T · log P 2 n o c V o Measured voltage i K t Natural constant a r T l Temperature, is kept constant e P1 u Partial pressure of reference gas; f is constant, if air is used as reference gas o and mixture prevented with process gas t

The gas tight fraction line Reference gas partial pressure P1 Process gas

partial pressure P2

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

Calibration ? Other oxygen measuring methods require a two point calibration, which in practice has been transferred to the zirconia measuring principle. This is l not necessary, as the Nernst equation is a mathematical a linear function o r and t with air as a known reference gas the only paramenter P1 is constant. n Therefore calibration is not required! o c Only one condition must be fulfilled: oi The measuring cell must have a totally gas tight seal between the t a process gas side and the reference gas side. r l Any leakage at the cell will cause a migration of process and reference gas e that will make regular calibrations necessary. u f o t

Design of the Oxytec Zirconia cell with the gas tight seal l o r t n o c oi t a r l e u f o t

Process gas Ion migration

mV Voltage Heater

Seal

Electrodes

Zirconia

Reference Referenzluft gas (air)

Thermocouple

Key factors for the reliable gas tight cell l o r t n o c oi t a r l e u f o t

Carefully selected high quality materials Special cell sealing technology Special manufacturing process Mechanical design Production, Test & Quality Control to ISO 9001

Ensuring optimum Efficiency l o r t n o c oi t a r l e u f o t

• 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.

Self learning logic l o r t n o c oi t a r l e u f o t

• 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

Self learning example l o r t n o c oi t a r l e u f o t

Self learning example l o r t n o c oi t a r l e u f o t

Effect of boiler loading l o r t n o c oi t a r l e u f o t

Effect of Oxygen variation l o r t n o c oi t a r l e u f o t

Boiler Efficiency range of products l o r t n o c oi t a r l e u f o t

• 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.


• EffiMax 2000 - Indire Indirect ct Effici Efficienc ency y analy analyzer zer with automatic blow down control. – Measur Measures es Steam Steam flow flow,, temper temperatu ature, re, stac stack k Oxygen, temperature, ambient temperature, Drum TDS and feed water temperature. – Calcul Calculate atess Indirec Indirectt efficie efficiency ncy,, indirec indirectt S:F, % blowdown loss, steam and blowdown total. – Appl Applic icat atio ion n - 3 TPH TPH and and abo above ve oil, oil, gas, gas, soli solid d fuel fired boilers.


• EffiMax 3000 - Indire Indirect ct Effici Efficienc ency y analy analyzer zer with ABCO and S:F measurement. – Measur Measures es Steam Steam flow, flow, tempe temperat rature ure,, oil/gas oil/gas flow, stack Oxygen, temp., ambient temp., Drum TDS and feed water temperature. – Calcul Calculate atess Indirec Indirectt efficie efficiency ncy,, direct direct S:F, S:F, % blowdown loss, steam and blowdown total. – Appl Applic icat atio ion n - 3 TPH TPH and and abo above ve oil oil and and gas gas fir fired ed boilers.


• EffiMax 4000 - Indire Indirect ct Effici Efficienc ency y analy analyzer zer with ABCO and Oxygen trim control. – Measur Measures es Steam Steam flow flow,, temp., temp., stack stack Oxyg Oxygen, en, temp., ambient temp., Drum TDS, feed water temp., damper feedback and boiler on/off. – Calcul Calculate atess Indirec Indirectt efficie efficiency ncy,, indirec indirectt S:F, % blowdown loss, steam and blowdown total. – Appl Applic icat atio ion n - 3 TPH TPH and and abo above ve oil, oil, gas, gas, soli solid d fuel fired boilers.


EffiMax 2000 Touch Screen Based


EffiMax 2000 , the latest Touch Screen based offering from the EffiMax range of on-line boiler efficiency l o r t n o c oi t a r l e u f o t

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.

Boiler Efficiency Indication (%) in accordance with BS 845 based on indirect efficiency computation. l o r 5 t 0 ‘ n o e c c oi n t e r a r e f l n e o u f C o s t e

Stack Loss Indication (%), Enthalpy Loss (%) Radiation Loss Indication (%) Combustion Loss (%) Steam Flow Indication (kg/h), Steam to fuel ratio (compensated for Feedwater Temp)

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 :- l o r t n o c oi t a r l e u f o t

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 calibration is menu driven

Features Blowdown loss totalization (kg), Average (kg/h) Automatic Blowdown control l o r t n o c oi t a r l e u f o t

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

EffiMax 2000 - User Interface








Air to fuel ratio control

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