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Excellence in measurements

Combustion and Flue Gas Analysis

December 2006

Combustion & Flue Gas Analysis

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Excellence in measurements

SummaryCombustion Theory Fuels Combustion with Methane / Natural Gas Combustion in practice Flue Gases Boilers Loss & Efficiency Regulations

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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 ) FuelDecember 2006

GasCombustion & Flue Gas Analysis 3

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FuelsFuels compositionMost 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

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Calorific PowerThe 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

Diesel oil Benzine for car LPG Natural gas Coke oven gas Blast furnace gas

10200 10500 11000 8300 4300 900

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CombustionComplete 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

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CombustionCombustion 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+ 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 combustionDecember 2006

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CombustionPractical Combustion ( Excess of Air 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

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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 : 537C Explosive limits : 5%-15% Calorific Power inferior: 8500 kcal/m3 Calorific Power superior: 9400 kcal/m3

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Combustion of MethaneTheoretical 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 + HeatDecember 2006

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Combustion of MethaneTheoretical with AirAir : 21% O2 + 79% N2

1 m3 CH4 + ( 2 m3 O2 + 7,52 m3 N2 ) 1 m3 CO2 + 2 m3 H2O + 7,52 m3 N2 +HeatDecember 2006

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Combustion of MethaneTheoretical with Air1 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 ).Combustion & Flue Gas Analysis 12

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Combustion of MethanePractical Excess of Air1m3 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)Combustion & Flue Gas Analysis 13

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Combustion of MethanePractical 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.Combustion & Flue Gas Analysis 14

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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 Methane/Natural gas LPG Oil % CO2 max 11.7 13.9 15.7

Methane is the fuel that produces less quantity of CO2.December 2006

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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.16

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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!

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Combustion in

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