Measurement methods in combustion processesfluid.wme.pwr.wroc.pl/~spalanie/dydaktyka/... ·...

COMBUSTION AND FUELS

MEASUREMENT METHODS IN COMBUSTION PROCESSES


Major aim of combustion diagnostics

1. Cognitive (knowledge extension on combustion

processes)

2. Practical

a) improvement of combustion effectiveness in

furnaces

b) abatement of pollutant emissions

c) improvement of control of combustion

processes


Subjects of combustion diagnostics

1. Flames

a) flame zone

b) after-flame zone

2. Combustion gases (composition, pollutants)

3. Solid furnace’s residues

a) fly ash, bottom ash (content, slagging)

b) properties (sizing, toxicity, solubility,

unburned coal)


Measured parameters of combustion processes

1. Temperature (in flame, of combustion gases, its

pulsations)

2. Content of flue gases (radicals, stable molecules,

solid particles)

3. Radiation

4. Flow velocities (turbulence)

Measurements made in a point and distributions


FLAME DIAGNOSTICS


TEMPERATURE MEASUREMENTS

IN FLAME


Flame characteristic

1. Flame is highly reactive environment, often far from

equilibrium

2. Flames are characterized by high temperatures and

great gradients of temperature

3. Accuracy of measurement of the temperature in flame

depends not only on the selected method of

measurement, but also on: the type of flame, sort of

fuel, turbulence and radiation of flame.


Methods of temperature measurements in flame

Method of measurement

Max. of

temperature

Selectivity Error

Required correction

Flame affection

K mK

or %

ThermocoupleResistance thermometer GasdynamicsPirometrySpectrum analysisAbsorption of X radiationInterferometer

30003000

3500---

-

10–4

10–3

10–5

5⋅10–3

5⋅10–3

5⋅10–3

5⋅10–4

31

3%55

3%

1%

radiationradiation

accelerationnonequilibrium effectnonequilibrium effectmolecular mass

molecular mass

catalyticcatalytic

extinquish----


Types of thermocouples used in flames

Metal ofthermocouple

Maximum of measured temperature

ErrorMaximum of

signal

K K mV

W/W-50MoTa/MoIr-20Re/Re-30IrIr/Re-30IrRh/Rh-8RePt-20Rh/Pt-40RhPt-6Rh/Pt-30RhRh/Pt-8RePt/Pt-10RhPt/RhIr/Ir-40Rh

3300290029002700230022002150215021002100

–

50504035–1010–31515

8.019.511.015.07.45.013.518.019.030.0

–


Errors associated with temperature

measurement in flame

A thermocouple introduced inserted into flame shows

the temperature different from flame temperature

because of:

• catalytic effect,

• aerodynamic effect,

• heat conduction by thermocouple’s wires,

• radiation of a thermocouple.


Correction of error resulted from radiation:

method of two thermocouples

Scheme of double thermocouple: 1 – thermocouples Pt-RhPt,

2 – thermocouple measuring the temperature of free ends of

thermocouple 1,

d1, d2 – diameters of thermocouple’s joins

)(

)(1

42

41

5,0

1

2

212

4ot

4ot

pl

TT

TT

d

d

TTTT

−−

−

−+= −


Optical methods of

temperature measurements

in flames


Method of reversed spectral lines (of sodium)

1 – reference lamp,2 – lens,3 – screen, 4 – flame,5 – spectroscope, 6 – solution of NaCl, 7 – electric arc,8 – NaCl filter, 9 – mixer, 10 – burner.

Methods of temperature measurement based on

radiation measurement – Planck’s formula

1

5

2

12 −

−= kThc

ehc

L λλ λ

For gray body ε<1, ε = const.

Hλ = εLλ


Single colour pyrometer

Manufacturer:

Raytec


Two colour pyrometer – principle of

temperature measurement

BE

EA

T+

=

2

1

pl

ln1

λ

λ

For gray bodyε<1, ε = const.

Hλ = εLλ


Two colour pyrometerSchem

e o

f optics


MEASUREMENTS OF SPECIES

CONCENTRATIONS IN FLAMES


Types of reactants in flames

1. Stable: long lifetime, easy to separate and analyse(molecules of fuel, oxidizer and products of oxidation, e.g. O2,

N2, CH4, CO2 H2O,..,). Their concentrations are large, from a

few to several tens percents of volume.

2. Unstable: short lifetime, difficult to separate (undergo

termination on the wall of a probe. They are radicals (O, H,

OH, CH3 ...) and ions. Their concentration in flame is low

10-6 ÷ 10-15 %).

3. Excited molecules (e.g. CO2*...)


Sampling of species in flames

Sampling in flames can be made in two ways:

1. Izokinetic sampling (subsonic), to prevent

separation of particles (mainly solids)

2. Ultrasonic sampling with freezing of a sample.

Isokinetic probe for suction of components in gaseous flame


Supersonic „freezing” probe for suction of gas

components in gaseous flame

Filter made of

sintered bronze

Quartz

tube


Gas chromatography

1. Gas chromatography is a method of separation and

detection of chemical compounds in gaseous mixtures.

2. Separation of components of a mixture occurs on the

boundary of phases:

a. solid (fixed),

b. gaseous (gas carrying a sample)

3. A sample carried by gas through a column (steel pipe

filled with an adsorbent) undergoes adsorption and

desorption.


Scheme of chromatograph

1 – container with carrying gas, 2 – flow rate controller, 3 – samples injector, 4 – thermostatic chromatographic column, 5 – temperature controller, 6 – detector,

7 – amplifier, 8 - recorder, 9 – integrator


Record from chromatograph of

hydrocarbons mixture

ProgramedTemperature


Mass spectroscopy

1. Mass spectroscopy is a method for determination of very

small concentrations of components, e.g. radicals.

2. The identification of a compound and its concentration

is based on the ratio of mass m and charge of an ionised

molecule e.

3. Before the detection the sample has to be ionsed (by

the electrons beam) under low pressure (<10-5 Pa) .

4. The ionised sample is send into the detector (e.g.

magnetic detector), which separates ions depending on

the ratio m/e.


Scheme of the mass-spectroscope

1 – sample injector, 2 – ionization chamber, 3 – inlet slot, 4 – screen,

5 – outlet slot, 6 – channel, 7 – detector, 8 – recorder


Spectroscopy1. The identification of chemical compounds is based on the

analysis of the spectrum of emission and absorption of

electromagnetic radiation of a sample.

2. In the visible and ultraviolet range emission and absorption of

radiation results in the change of electron states. And, in the

range of infrared radiation, it is the result of the change of

oscillatory-vibration energy of a molecule.

3. The spectrum of radiation is composed of atomic lines,

molecular bands and the continuous spectrum.

4. Radiation of chemical elements and compounds is concentrated

in characteristic lines and bands and is used for their detection.

5. A source of excitation (emission) is flame temperature and

chemical reactions.


EPR spectroscopy

Spectroscopy of electron resonance (EPR) is used for

the detection of spines in paramagnetic free radicals,

therefore it has found application in the detection

and determination of the concentration of radicals in

flames.


Raman spectroscopy

1. The development of lasers as the monochromatic sources of

light has given a basis for the development of many non-

invasive methods of investigations in combustion chemistry.

2. The identification of a chemical compound is based on the

dispersion of weak radiation emitted by a molecule and

induced by laser radiation.

3. The result of a laser excitation is Rayleigh dispersion and

Raman, Stocks and unti-stocks dispersion

4. During flame investigation the CARS is often used – together

with the use of compact anti-Stocks Raman dispersion.


Scheme of Raman spectrometer

1 – laser, 2 – primary optics, 3 – mirror, 4 – cuvette, 5 – depolarizer,

6 – monochromator, 7 – diffraction mesh, 8 – interference filter,

9 –photomultiplier, 10 – amplifier, 11 – recorder


LIF (Laser Induced- Fluorescence)

spectroscopy

Scheme of LIF spectroscopy


PLIF (Planar Laser-Induced Fluorescence) spectroscopy

1. PLIF is an optical method of diagnosis for visualization and

measurement processes in flow.

2. It can be applied for the determination of space

distribution of concentrations, temperature, velocity and

pressure in gas.

3. The main components of PLIF area: laser, optics,

fluorescence medium and detection system.

4. Light excites the medium, which radiate (fluorescence).

This signal is received by the detector and is used for the

determination of different properties of medium.


Fluorescence


Example of PLIF application


Measurement of ions and electrons in flame

1. The problem concerns the measurement of ions and

electrons in flame and their spatial distribution.

2. Most often the concentration of ions and electrons in

flame is measured using Langmuira’s probe.

3. Ions concentration can be measured using mass

spectroscopy.


Langmuir’s probe

1 – electrode,

2 – isolation,

3 – screen

1. Langmuir’s probe is collected of two electrodes: the first

of a small and the second of a large surface area.

2. The current depends on the flow of ions or electrons

between the surfaces of electrodes.

3. If the electrode of the small area is positive, then the

current is proportional to the concentration of electrons.

4. If the electrode of a small area is negative, then the

current is proportional to the concentration of ions.


FLAME DETECTORS


Main tasks of flame detectors

a) improvement of operation safety of burners, combustion

chambers, inner combustion engines,

b) detection of incorrect operation (e.g. burner)

c) application in control systems of burners (flame

supervision systems)

d) fulfil standards’ requirements (safety standards, insurance

requirements)


Types of flame detectors

a) optical (UV – ultraviolet, IR – infrared, visible

range)

b) electric (using flame ionization)

c) temperature (thermocouple, pyrometers)

Example of a flame control system


Optical flame detectors

Radiation spectrum of different fuels

Criteria of flame detectors selection

detectors


Criteria of flame detectors selection


UV sensors

Types of ultraviolet (UV) flame detectors:

a) UV vacuum tube detectors

b) UV semiconductor detectors


Radiation of gaseous flame

99%

infraredvisible

lightUV

max 1/3 of

flame length

<1% of

radiation

<10% of

radiation

90% of

radiation

1%


infraredvisible

light

UV detection

range

wavelength (nm)

inte

nsity

ofra

dia

tion

Radiation passing trough the atmosphere

UV detection of hydrocarbon flame


Vacuum UV detector tubes

Quartz tube with two electrodes

is filled with gas. UV radiation

activates cathode plate which

produces electrons initiating gas

ionisation.

Voltage between the electrodes

is in the range 75-200 V.

Types of disturbances of UV detectors:

- ignition spark,- welding arc,- halogen light,- hot furnace walls (>1200 oC)

electrodes quartz tube

UV flame detector

tube


UV flame semiconductor detector


UV hydrocarbon flame sensor - semiconductor

(nitrate)


Design of semiconductor UV sensors

(nitrates)


Examples of UV flame sensors from Siemens

UV flame detector

(Lamtec F 200 K Ex Compact Flame Detector)

Example of specification of optical sensors

(Lamtec F 200 K Ex Compact Flame Detector)


Optical sensors in the range of visible and

infrared spectrum

1. Visible and infrared radiation is 99% of the whole

spectrum of flame.

2. Not the whole visible and infrared part of spectrum

(> 400 nm) can be used for flame detection – a hot

furnace strongly radiates and forms a background.

3. In this range of radiation the optical sensor must be

able to identify variable signals from the flame.

4. The source of a variable component of radiation is

flame flickering.


Comparison of photodetector (PbS resistor) signals

from solid body and flame radiation


FLAME SCANNERS


Flame scanners

1. Flame scanners are called flame detectors (in the visible

and IR range); they are equipped with the identification

system of a radiation component characteristic of a

superintendent flame.

2. The flame scanner is composed of optics (lens), IR sensor,

te signal amplifier and the frequency selector.

3. The typical optical flame sensors in the visible and IR

range are photoresistors (e.g. PbS) and photodiodes (e.g.

InGaAs).


Frequency selection of of flame flickering


Scheme of optic and electronic device

of selection flame signals

Position of scanner in a burner

Wrong position of scanner

Proper position of scanner

Two scanners control two flames

Amplitude- frequency characteristic of flame flickering

Wg. mat. firmy AC System sp. z o.o.

low amplitudehigh frequency

high amplitudelow frequency

low flickering frequency

high flickering frequency

amplitude(dB)

amplitude(dB) measurement

points

approximation line

FAST FOURIER TRANSFORM

time (s) frequency (Hz)


Application of optics fibres in scanners


Infrared flame scanner (Fireye)


Principle of ionization flame detectors

Flame is weakly ionized medium,

therefore voltage causes electric

current between electrodes.


Ionization flame detector


Flame control using ionization detector


STECHIOMETRY OF

COMBUSTION CONTROL

IN FURNACES


Major objects of combustion control


Simple control of air excess in furnace


Boiler’s lambda sensor

Stoichiometry control by measurement of oxygen

concentration in flue gas using zirconia λ probe

Flue gas channel

Combined control of air excess and CO emission

Air excess control when two types of fuels are burnt


DIAGNOSTICS OF

CORROSION HAZARD IN

PULVERIZED COAL

FURNACES


Methods of corrosion hazard diagnostics

1. Periodic

2. Continuous and quasi-continuous (on-line)


Periodic diagnostics of corrosion hazard

1. Control of water-tube’s thickness losses

2. Measurements of boundary layer content


Periodic control of thickness losses of

evaporator watertubes

a. Supersonic

b. EMAT (Electromagnetic Acoustic Transducer)

c. Infrared (IR)

d. Collecting and testing of watertube’s samples


Supersonic method of measurement of

watertube thickness


Measurement of watertube thickness by EMAT


Watertube sample

1 – fire side, 2 – out side


Distributions of watertube’s thickness losses

1 2 3 4 5 6 7 8 9

szerokość [m]

K3, ściana przednia, 2005r

11

12

13

14

15

16

17

18

19

20

pozi

om [

m]

2

2.2

2.4

2.6

2.8

3

3.2

3.4

3.6

3.8

4

4.2

4.4

4.6

4.8

5

5.2

5.4

1 2 3 4 5 6 7 8 9

�szeroko[ [m]

K3, [ciana prawa, 2005r

1 2 3 4 5 6 7 8 9

�szroko[ [m]

K3, [ciana tylna, 2005r.

1 2 3 4 5 6 7 8 9

�szeroko[ [m]

K3, [ciana lewa, 2005r.

Distributions of oxygen and CO concentrations in boundary layer of furnace walls

1 2 3

4 5 6 7 8

9 10 11 12 13

14 15 16 17 18

19 20 21 22 23

24 25 26 27 28

29 30 31 32 33

-3000 -2000 -1000 0 1000 2000 300012000

12500

13000

13500

14000

14500

15000

15500

16000

16500

17000

17500

18000

18500

19000

19500

20000

20500

21000

21500

22000

22500

23000

SOFA

OFA

SOFA

OFA

0 1 2 3 4 5 6 7 8 9 10 O2[%]

1 2 3

4 5 6 7 8

9 10 11 12 13

14 15 16 17 18

19 20 21 22 23

24 25 26 27 28

29 30 31 32 33

-3000 -2000 -1000 0 1000 2000 300012000

12500

13000

13500

14000

14500

15000

15500

16000

16500

17000

17500

18000

18500

19000

19500

20000

20500

21000

21500

22000

22500

23000

SOFA

OFA0 2000400060008000100001200014000160001800020000

CO [ppm]


Continuous and quasi-continuous methods of

corrosion hazard diagnostics

a. Measurement of electric resistance of watertubes of

evaporator

b. Corrosion probes ENM (Electrochemical Noise Analysis)

c. Corrosion probes based on measurement of resistance

of probe

d. Corrosion probes based on loss of testing tube.


Corrosion probe based on measurement of

probe’s resistance


Corrosion probe based on loss of material

of testing tube


Continuous monitoring of oxygen content near

furnace’s walls

a. Oxygen content in flue gas is measured in the boundary

layer of furnace’s walls.

b. Oxygen content is measured in flue gas using zirconium

sensors.

c. Electric signals from zirconium sensors are converted into

maps of oxygen concentration.

d. Oxygen distributions are converted into maps of corrosion

hazard.


Zirconium sensors

=

2

2ln4 O

O

F

RTV ref

N

CmVF

RT o700at 9,204

=

where


[O2]/voltage transducer with zirconium sensor

zirconium sensor

Oxygen

probe in

furnace

wall

Location of oxygen probes on the furnace walls

poz. 30 900

poz. 19 500

poz. 26 800

poz. 34 500

lewa

poz. 30 900

poz. 19 500

poz. 26 800

poz. 21 800

OFA OFA

tylna

poz. 31 800

poz. 19 500

poz. 26 800

poz. 23 000

prawa

poz. 30 900

poz. 19 500

poz. 26 800

poz. 21 800

poz. 29 700

OFA OFA

przednia

2

3

4

1

2 3

4

55

POMIESZCZENIE NASTAWNI BLOKOWEJ

4

3

2

1

55

4

3

2

11

Maps of corrosion hazard

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