Download Combustion and Flue Gas Analysis

Excellence in measurements
Combustion
and
Flue Gas Analysis
December 2006
Combustion & Flue Gas Analysis
1
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Summary
Combustion Theory
Fuels
Combustion with Methane / Natural Gas
Combustion in practice
Flue Gases
Boilers
Loss & Efficiency
Regulations
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Combustion & Flue Gas Analysis
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Combustion
Combustion or burning is a chemical process, an exothermic
reaction between a substance (the fuel) and a gas (the
oxidizer), usually O2, to release thermal energy (heat),
electromagnetic energy (light), mechanical energy (noise)
and electrical energy( free ions and electrons ).
In a complete combustion reaction, a compound reacts with
an oxidizing element, and the products are compounds of
each element in the fuel with the oxidizing element.
For example:
CH4 + 2 O2 → CO2 + 2 H2O + Heat ( +light/noise/ions )
Fuel
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Fuels
Fuels composition
Most fuels are mixtures of chemical compounds called
hydrocarbons
( combinations of hydrogen H2 and carbon C ).
Fuels are available as gaseous, liquid and solid.
Solid fuels
Solid natural fuels include Coal, Peat, Lignite and Wood.
Solid artificial fuel is Coke derived from Coal.
High contents of Sulphur and Ash.
Liquid Fuels
Liquid fuels are processed at refineries from Petroleum.
Light, medium and Heavy Fuel Oil, Gasoline and Kerosene are the
most common used.
Gaseous Fuels
Natural gas is a gaseous natural fossil fuel consisting primarily of
methane. It is found in oil fields and natural gas fields.
Town gas is manufactured from Coal ( half calorific value of Natural
gas ).
LPG ( Liquid Propane Gas ) is manufactured from Petroleum and
usually supplied in pressurized steel bottles ( cooking is a typical
application ).
Gaseous fuels include also Coke oven gas and Blast furnace gas.
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Combustion & Flue Gas Analysis
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Calorific Power
The principal characteristic of a fuel is his power calorific. This represents the
amount of heat developed in the reaction of combustion in conditions predefined
standard. Generally is measured in kcal/kg for the solid and liquid, while for the
gases is expressed with kcal/m3. In many fuels, that contain hydrogen, has
distinguished a superior calorific power (that it includes the heat of condensation of
the water vapor that shape in the combustion) and a inferior calorific power (than it
does not consider such heat).
Inferior calorific power of some fuel (p.c.i.)
Fuel p.c.i. (kcal/kg - kcal/m3)
Firewood to burn
2500 - 4500
Peat
3000 – 4500
Firewood coal
7500
Lignite
4000 - 6200
Coke
7000
Fuel oil
9800
December 2006
Diesel oil
Benzine for car
LPG
Natural gas
Coke oven gas
Blast furnace gas
Combustion & Flue Gas Analysis
10200
10500
11000
8300
4300
900
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Combustion
Complete combustion
In complete combustion, the reactant will burn in oxygen,
producing a limited number of products. When a hydrocarbon
burns in oxygen, the reaction will only yield carbon dioxide and
water. When elements such as carbon, nitrogen, sulfur, and iron
are burned, they will yield the most common oxides. Carbon will
yield carbon dioxide. Nitrogen will yield nitrogen dioxide. Sulfur
will yield sulfur dioxide. Iron will yield iron(III) oxide. Complete
combustion is generally impossible to achieve unless the
reaction occurs where conditions are carefully controlled (e.g. in
a lab environment).
Fuel + Oxygen → Heat + Water + Carbon dioxide.
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Combustion & Flue Gas Analysis
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Combustion
Combustion Stoichiometry ( Theoretical )
If sufficient oxygen is available, a hydrocarbon fuel can be completely oxidized,
the carbon is converted to carbon dioxide (CO2) and the hydrogen is converted
to water (H2O).
During combustion, each element reacts with Oxygen to release heat :
C + O2 -> CO2 + Heat
H2+ ½ O2 -> H20 + Heat
Pure Oxygen is rarely available so Air is mainly used for combustion.
It contains 21 percent of Oxygen O2 and 79 percent of Nitrogen N2.
A complete burning, with nothing but Carbon Dioxide, Water, and Nitrogen as
the end products is known as the stoichiometric combustion.
The stoichiometric air/fuel ratio refers to the proportion of air and fuel present
during a theoretical combustion.
The heat released when the fuel burns completely is known as the heat of
combustion
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Combustion & Flue Gas Analysis
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Combustion
Practical Combustion ( Excess of Air – λ Lambda )
Due to fluctuations in fuel flow and the lack of perfect mixing
between fuel and air in the combustion zone, excess air is required
to achieve more complete combustion of the fuel.
Without this extra air, the
formation of partial products of
combustion such as carbon
monoxide and soot may occur.
However, supplying too much
excess air will decrease
combustion efficiency and a
balance between too much air
and not enough air must be
maintained.
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Combustion & Flue Gas Analysis
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Fuels : Methane ( Natural Gas )
Discovered by Alessandro Volta in 1778
The simplest hydrocarbon, methane, is a gas with
a chemical formula of CH4.
Pure methane is odorless, but when used
commercially is usually mixed with small
quantities of odorants, strongly-smelling sulfur
compounds to enable the detection of leaks.
Autoignition Temperature : 537°C
Explosive limits : 5%-15%
Calorific Power inferior: 8500 kcal/m3
Calorific Power superior: 9400 kcal/m3
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Combustion of Methane
Theoretical with pure O2
CH4 + O2 => CO2 + H2O + Heat
CH4 + 2 O2 => CO2 + 2 H2O + Heat
1 m3 CH4 + 2 m3 O2
=> 1 m3 CO2 + 2 m3 H2O + Heat
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Combustion & Flue Gas Analysis
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Combustion of Methane
Theoretical with Air
Air : 21% O2 + 79% N2
1 m3 CH4 + ( 2 m3 O2 + 7,52 m3 N2 )
⇒
1 m3 CO2 + 2 m3 H2O + 7,52 m3 N2
+Heat
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Combustion & Flue Gas Analysis
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Combustion of Methane
Theoretical with Air
1 m3 CH4 + ( 2 m3 O2 + 7,52 m3 N2 )
⇒ 1 m3 CO2 + 2 m3 H2O + 7,52 m3 N2 +Heat
•
For a complete burning of 1 m3 of Methane
you need 9.52 m3 ( 2+7,52 ) of air ( Stoichiometric ).
•
It develops 10.52 m3 ( 1+2+7,52 ) of wet flue gases.
•
It develops 8.52 m3 ( 10.52 less 2 H20 ) of dry flue gases.
•
1 m3 of Carbon Dioxide CO2 is generated each 1 m3 of Methane.
On dry flue gas contents is 11.7% ( 1 m3 1/ 8.52 m3).
•
Oxygen is not present in flue gases ( Stoichiometric ).
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Combustion & Flue Gas Analysis
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Combustion of Methane
Practical – Excess of Air
1m3 CH4 + (2 m3 O2 + 7,52 m3 N2) + (1 m3 O2 + 3,76 m3N2)
Theoretical Air
Excess of Air
=> 1 m3 CO2 + 2 m3 H2O + 1 m3 O2 + 11,28 m3 N2 +Heat
•
You use for burning 1 m3 of Methane 14.28 m3 ( 2+1+3,76+7,52 ) of air.
•
It develops 15.28 m3 ( 1+2+1+11,28 ) of wet flue gases.
•
It develops 13.28 m3 ( 15.28 – less 2 H20 ) of dry flue gases.
•
1 m3 of Carbon Dioxide CO2 is generated each 1 m3 of Methane. On
dry flue gas contents is 7.5% ( 1 m3 / 13.28 m3).
•
Oxygen is 7.5% ( 1 m3 / 13.28 m3)
December 2006
Combustion & Flue Gas Analysis
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Combustion of Methane
Practical – Excess of Air
•
•
•
•
•
•
Theoretically you use 9.52 m3 ( 2+7,52 ) of air ( Stoichiometric ).
Practically you use 14.28 m3 ( 2+1+3,76+7,52 ) of air.
Lambda = Volume (Practical Air / Theoretical Air) =14.28/9.52= 1.5
Excess of Air = ( Lambda – 1 ) * 100 = ( 1,5 – 1 ) * 100 = 50%
Excess of Air measured from O2 ( 7.5% )
= %O2 measured * 100 / ( 20.9 - %O2 measured ) x Coeff KL= 50%
To little excess of air is inefficient because it permits unburned fuel, in
the form of combustibles, to escape up the stack. But too much excess
of air is also inefficient because it enters the burners 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 of air is supplied so that the sum of both
unburned fuel loss and flue gas heat loss is minimized”.
December 2006
Combustion & Flue Gas Analysis
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Carbon Dioxide – CO2
•
The carbon dioxide concentration in the flue gas gives and clear
indication of the quality ( efficiency ) of the burner.
Is the proportion of CO2 is as high as possible with a small excess air,
the flue gas losses are at their lowest.
The maximum CO2 concentration on flue gas depends only on carbon
content of the fuel burned.
Fuel
% CO2 max
Methane/Natural gas
11.7
LPG
13.9
Oil
15.7
Methane is the fuel that produces less quantity of CO2.
December 2006
Combustion & Flue Gas Analysis
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Combustion in practice
• To obtain the most efficient combustion you need a
slight excess of air
• Flue gas volume will be more than theoretical
combustion ( stoichiometric ).
• Carbon dioxide will be less than maximum
achievable ( CO2 max )
• Oxygen will be always present in flue gas.
December 2006
Combustion & Flue Gas Analysis
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Combustion in practice
Reduce as much as you can the Excess of Air to reach the
maximum level of Carbon Dioxide CO2
Pay attention to Carbon Monoxide CO level!
December 2006
Combustion & Flue Gas Analysis
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Combustion in practice
Carbon Monoxide is the result of incomplete
combustion.
This could mean a a deficiency of air at the
burner.
December 2006
Combustion & Flue Gas Analysis
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Carbon Monoxide - CO
Carbon monoxide is a colorless, odorless,
tasteless, flammable and highly toxic gas. It is a
major product of the incomplete combustion of
carbon. It is called the “Silent Killer”.
Concentration
9 ppm
35 ppm
200 ppm
800 ppm
3200 ppm
December 2006
Effects
The maximum allowable concentration for short
term exposure in a living ambient ( ASHRAE )
The maximum allowable concentration for
continuous exposure in any eight hour period.
According to US federal law
The maximum allowable concentration for any
time. According to OSHA.
Headaches, fatigue, nausea after 2-3 hours
Nausea and convulsion within 45 minutes. Death
in 2-3 hours.
Headaches and nausea within 5-10 minutes.
Death within 30 minutes.
Combustion & Flue Gas Analysis
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Flue Gas contents
Water Vapor (H2O)
Nitrogen (N2) Typical contents 75-80%
Carbon Dioxide (CO2) Typical contents 7-15%
Carbon Dioxide and Hydrogen (CO, H2) due to incomplete
combustion. Typical contents 50-150 ppm.
Oxygen (O2) due to excess of air. Typical contents 2-8%.
Nitrogen Oxides NOX (NO + NO2) due to N2 and O2
combination at high temperatures. Typical contents
<100ppm.
Sulphur Dioxide (SO2) due to S2 presence in solid/oil fuels.
Typical contents <200ppm.
Uncombusted Hydrocarbons and Ashes
December 2006
Combustion & Flue Gas Analysis
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Boilers
Wall-hang type :
The body of the boiler
is fitted on the wall
20-30 KW
Floor-installation type :
The boiler is fitted on the
support or on the floor.
30-100 KW
Heating + Hot Water or Hot Water only
• Instantaneous supply type
The main heat exchanger or heat exchanger for hot water inside the boiler
body supplies hot water.
• Storage Tank type
Hot water is stored in the separate storage tank and is supplied when
necessary.
December 2006
Combustion & Flue Gas Analysis
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Boilers
Sealed Chamber
Energy Efficiency
Boiler
(92/42 European Directive)
Classification ( Stars )
European standards (UNI EN 297
and UN 483) classify
boilers in 5 classes according to
their NOx emissions.
Atmospheric Boiler
December 2006
Condensing Boiler
Combustion & Flue Gas Analysis
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Boilers
Boiler Type A
Boiler Type B
Boiler Type B
It takes combustion air
from the indoors and
vents exhaust gas in
surrounding ambient.
It takes combustion air
from the indoors and
vents exhaust gas
through the exhaust stack
It takes combustion-use
air from additional strack
from outside and vents
exhaust gas through the
exhaust stack ( dual
stack ).
December 2006
Combustion & Flue Gas Analysis
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Flue Gas Analysis
Combustion analysis is part of a process intended to :
SAFETY ( Improve safety of fuel burning equipments )
ENERGY SAVING ( Improve fuel economy )
POLLUTION (Reduce undesiderable exhaust emissions)
For these reasons combustion analysis is a must and
Flue Gas Analyser is a fundamental tool for plumbers.
December 2006
Combustion & Flue Gas Analysis
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Analysis Methods
During combustion with an excess of air λ=1.1 it develops 11.5 m3 of flue
gases ( for each m3 of burned gas) as :
(CO2) 1.0m3 + (O2) 0.2m3 + (N2) 8.3m3 + (H2O) 2.0m3 = 11.5 m3
Analysis on Dry basis
If you remove all water contents from flue gases, condensating, the analyzer
will measure Oxygen as O2 = 0.2 : 9.5 = 2.1%.
This is a measurement on “dry basis” as we refer to Oxygen contents to the
volume of dry flu gases (9.5 m3) with excess of air λ=1.1
Analysis on Wet basis
If we don’t remove all water contents the analyzer will
measure Oxygen as O2 = 0.2 : 11.5 = 1.7%
This is a measurement on “wet basis” as we refer to Oxygen contents to the
volume of dry flu gases (11.5 m3) with excess of air λ=1.1
Flue gas analyzers use electrochemical sensors that need dry gas to measure.
For this reason all measurements are obtained on dry basis.
December 2006
Combustion & Flue Gas Analysis
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Units of measurements
The gas concentration is measured in ppm.
Ppm means part per millions.
100 ppm is equivalent to 0.01%
1000 ppm is equivalent to 0.1%
10000 ppm is equivalent to 1%
Pollutants can be measured in
mg/Nm3 ( milligrams per cubic meter )
this is mass refer to a volume in normal condition ( 0°C 1013 mBar ). Ppm is
converted in this unit with a coefficient different for each gas.
Example : CO mg/Nm3 = CO ppm x 1,25
mg/kWh ( milligrams per kilowatt-hour of energy )
the conversion from ppm to energy-related unit will use coefficient different for
each fuel.
Example : CO mg/kWh = CO ppm x 1,074
December 2006
Combustion & Flue Gas Analysis
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CO and Pollutants referred to O2
To avoid dilution of pollutants during inspections the CO and
other toxic gases has to be measured referred to Oxygen.
This is required by regulation.
Example :
CO measured 100 ppm and Oxygen measured 6% .
If O2 reference is set by law to 3%.
CO ref O2 = CO x ( 20.9 – O2 reference )/( 20.9 – O2 measured)
CO ref O2 = 100 x ( 20.9 – 3 ) / ( 20.9 – 6 ) = 120 ppm
If O2 reference is set to 0% usually the CO ref O2 is also called
CO undiluted.
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Combustion & Flue Gas Analysis
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Loss and Efficiency
One of the first task of flue gas analysis is
Energy saving. The regulations require that
all heating generators have to be measured
as Efficiency.
Useful efficiency is the ratio between the
heat transferred to water ( usefull output )
and the heat generated at the burner ( gross
heat input )
December 2006
Combustion & Flue Gas Analysis
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Example
Boiler :
20.000 kcal/hour ( Gross Heat input )
10 liter/minutes. with DT=30°C
Water use 300kCal/minutes that is equivalent
to 18.000 kCal/hour ( useful output )
The useful efficiency will be 90% ( 18.000 / 20.000 ).
1kCal is the heat quantity necessary to grow 1°C in 1 liter of water.
December 2006
Combustion & Flue Gas Analysis
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Loss
Loss for radiation, wall, opening and conveyor are negligible on modern
boiler.
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Combustion & Flue Gas Analysis
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Combustion Efficiency
Efficiency = 100 – Qs Stack Flue Loss
Sensible heat is the amount of energy in the form of heat that is
required to grow temperature of water.
Latent heat is the amount of energy in the form of heat that is required
for water to undergo a change "change of state".
December 2006
Combustion & Flue Gas Analysis
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Stack Flue Loss
Qs = k * (Tg - Ta) / CO2
Qs
k
Tg
Ta
CO2
Stack Flue Loss
Factor related to fuel
Flue Gas Temperature
Supplied Combustion Air Temperature
% of Carbon Dioxide
Regulation UNI 10389 provide similar formula using Oxygen to calculated CO2
and pertinent factors related to different fuels
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Combustion & Flue Gas Analysis
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Italian Regulation
1990 - Law 10
Energy saving legislation
1993 - D.P.R. 412
Legislation on efficiency control and reduction of fuel
consumption. Minimum Efficiency for boilers.
1994 - UNI 10389
Technical regulation on flue gas analyzers, how to perform
analysis and maximum limit definition for CO and Smoke.
2000 - D.P.R. 551
Legislation upgrade of D.P.R. 412
2005 - D.L 192
Execution of European directive 2002/91/CE
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Combustion & Flue Gas Analysis
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DIRECTIVE 2002/91/EC OF THE
EUROPEAN PARLIAMENT
Energy performance of buildings
( Active from January 2006 )
Inspection of boilers (Article 8)
Member state compliance with this part of the Directive is either through a system
of regular inspections or through the provision of advice leading to an outcome
similar to that of a regular inspection system:
Regular inspection of boilers rated 20 kiloWatt to 100 kiloWatt using nonrenewable liquid or solid fuels. over 100 kiloWatt are to be inspected at least every
two years, although the period for gas boilers may be extended to four years.
For all boilers over 20 kiloWatt and over 15 years old, a one-off inspection of the
whole heating system is to be conducted.
This should cover boiler efficiency and sizing compared to the requirements of the
building; advice should then include suggestions for replacement, improvements
to the heating system and possible alternative solutions, or advice on
improvement, replacement or alternative solutions.
December 2006
Combustion & Flue Gas Analysis
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UNI 10389 ( 1994 )
• Define technical specification of
flue gas analyzers (O2+CO)
• Three measurements every 2 minutes
• Formula to be used for Efficiency Calculation with
factors for the different fuels
• Stack Draft measurement
• CO undiluted referred to 0% O2 ( max 1000 ppm )
• CO2 calculation
• Smoke measurement for Oil boiler
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Combustion & Flue Gas Analysis
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EN50379
According to the CENELEC, all national directives not compliant
with EN 50379-2 will expire as of March 1, 2007 and will be replaced
by EN 50379.
The norm consists of three parts.
Part 1 describes the general requirements and test procedures.
Part 2 defines the requirements for devices used in statutory
inspections and assessments. This means that inspections and
measurements required by law may only be made with EN 50379-2
certified devices.
Part 3 describes the requirements for devices in non-regulated
areas in the maintenance of gas-fuelled heating facilities. This
means that measurement results produced by devices tested
according to part 3 have no legal relevance at all but may be used to
set up boilers and determine maintenance intervals. Therefore, the
user must carefully check the certification of a device.
After March 1, 2007, statutory maintenance may only be performed
with EN 50379-2 certified devices
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Combustion & Flue Gas Analysis
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Excellence in measurements
How to perform an analysis
•
The boiler has to work at
maximum power in stable condition.
•
Insert probe into stack at height
of 2 times diameter and in the middle of tube.
•
For Type C boiler use remote combustion air probe
to be inserter in the aspiration stack.
•
Perform Draft measurement
•
Select the right Fuel on instrument to obtain the right
factor for calculation.
•
Perform a Flue Gas Analysis ( more time if required
by legislation ).
•
Print the report.
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Combustion & Flue Gas Analysis
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