Combustion And Flue Gas Analy

Excellence in measurements

Combustion and Flue Gas Analysis

December 2006

Combustion & Flue Gas Analysis

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Excellence in measurements

Summary


Combustion Theory




Fuels




Combustion with Methane / Natural Gas




Combustion in practice




Flue Gases




Boilers




Loss & Efficiency




Regulations

December 2006

Combustion & Flue Gas Analysis

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Excellence in measurements

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 December 2006

Gas Combustion & Flue Gas Analysis

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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. December 2006

Combustion & Flue Gas Analysis

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Excellence in measurements

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. December 2006

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 December 2006

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. December 2006

Combustion & Flue Gas Analysis

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Excellence in measurements

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

December 2006

Combustion & Flue Gas Analysis

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Excellence in measurements

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 December 2006

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 December 2006

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 ).

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It develops 10.52 m3 ( 1+2+7,52 ) of wet flue gases.

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It develops 8.52 m3 ( 10.52 less 2 H20 ) of dry flue gases.

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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).

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Oxygen is not present in flue gases ( Stoichiometric ).

December 2006

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.

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It develops 15.28 m3 ( 1+2+1+11,28 ) of wet flue gases.

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It develops 13.28 m3 ( 15.28 – less 2 H20 ) of dry flue gases.

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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).

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Oxygen is 7.5% ( 1 m3 / 13.28 m3)

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Combustion & Flue Gas Analysis

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Excellence in measurements

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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Excellence in measurements

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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Excellence in measurements

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.

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Combustion & Flue Gas Analysis

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Excellence in measurements

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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Excellence in measurements

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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Excellence in measurements

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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Excellence in measurements

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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Excellence in measurements

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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Excellence in measurements

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

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Excellence in measurements

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

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Combustion & Flue Gas Analysis

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Excellence in measurements

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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Excellence in measurements

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

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Excellence in measurements

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.

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Combustion & Flue Gas Analysis

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Excellence in measurements

Loss

Loss for radiation, wall, opening and conveyor are negligible on modern boiler. December 2006

Combustion & Flue Gas Analysis

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Excellence in measurements

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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Excellence in measurements

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 December 2006

Combustion & Flue Gas Analysis

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Excellence in measurements

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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Excellence in measurements

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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Excellence in measurements

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 December 2006

Combustion & Flue Gas Analysis

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Excellence in measurements

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 December 2006

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Excellence in measurements

How to perform an analysis •

The boiler has to work at maximum power in stable condition.

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Insert probe into stack at height of 2 times diameter and in the middle of tube.

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For Type C boiler use remote combustion air probe to be inserter in the aspiration stack.

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Perform Draft measurement

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Select the right Fuel on instrument to obtain the right factor for calculation.

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Perform a Flue Gas Analysis ( more time if required by legislation ).

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Print the report.

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