A Guide to Combustion Analysis

Resource Description

Introduction

Combustion occurs when fuel, most generally a fossil fuel, reacts with the oxygen in the air to produce heat. The heat generated by the burning of a fossil fuel is used in the operation of equipment such as boilers, furnaces, kilns and engines. Along with heat, CO₂ (carbon dioxide) and H₂O (water) are created as byproducts of the exothermic reaction.

CH4 + 2O2 => CO2 + 2H2O Reactants => Products + Heat

By monitoring and regulating some of the gases in the stack or exhaust, it is easy to improve combustion efficiency, which conserves fuel and lowers expenses. Combustion efficiency is the calculation of how effectively the combustion process runs. To achieve the highest levels of combustion efficiency, complete combustion should take place. Complete combustion occurs when all of the energy in the fuel being burned is extracted and none of the Carbon and Hydrogen compounds are left unburned. Complete combustion will occur when the proper amounts of fuel and air (fuel/air ratio) are mixed for the correct amount of time under the appropriate conditions of turbulence and temperature.

Although theoretically stoichiometric combustion provides the perfect fuel to air ratio, which thus lowers losses and extracts all of the energy from the fuel; in reality, stoichiometric combustion is unattainable due to many varying factors. Heat losses are inevitable thus making 100% efficiency impossible.

In practice, in order to achieve complete combustion, it is necessary to increase the amounts of air to the combustion process to ensure the burning of all of the fuel. The amount of air that must be added to make certain all energy is retrieved is known as excess air.

In most combustion processes, some additional chemicals are formed during the combustion reactions. Some of the products created such as CO (carbon monoxide), NO (nitric oxide), NO₂ (nitrogen dioxide), SO₂ (sulfur dioxide), soot, and ash should be minimized and accurately measured. The EPA has set specific standards and regulations for emissions of some of these products, as they are harmful to the environment.

Combustion analysis is a vital step to properly operate and control any combustion process in order to obtain the highest combustion efficiency with the lowest emissions of pollutants. Combustion analysis is part of a process intended to improve fuel economy, reduce undesirable exhaust emissions and improve the safety of fuel burning equipment. Combustion analysis begins with the measurement of flue gas concentrations and gas temperature, and may include the measurement of draft pressure and soot level.

To measure gas concentration, a probe is inserted into the exhaust flue and a gas sample drawn out. Exhaust gas temperature is measured using a thermocouple positioned to measure the highest exhaust gas temperature. Soot is measured from a gas sample drawn off the exhaust flue. Draft is the differential pressure between the inside and outside of the exhaust flue. Once these measurements are made, the data is interpreted using calculated combustion parameters such as combustion efficiency and excess air.

Measurement Tools

Portable Electronic Instruments

In recent years, electronic Combustion Analyzer have been developed to analyze combustion routinely for tune-ups, maintenance and emissions monitoring. These instruments are extractive. They remove a sample from the stack or flue with a vacuum pump and then analyze the sample using electrochemical gas sensors. Thermocouples are used for stack and combustion air temperature measurements, and a pressure transducer is used for the draft measurement. An on-board computer performs the common combustion calculations, eliminating the need to use tables or perform tedious calculations. Electronic instruments show the results of boiler adjustments in real-time and give more accurate information to help ensure that a system has been tuned properly. Continuous Emission Monitors Continuous emission monitors, or CEMS, are a class of electronic instruments designed to measure exhaust stack gases and temperature continuously. CEMs are sometimes used for combustion control, but typically are used for monitoring pollutant gas emissions as required by government regulations. CEMs can use both extractive and in-situ (sensors in the stack) sampling methods, and employ a variety of electronic sensor technologies for gas detection. CEMs are used most often on larger installations or when required by regulatory agencies.

What do combustion analyzers measure and why?

• Oxygen (%O₂ ): primarily determines complete combustion.

• Carbon Dioxide (%CO₂ ): the maximized value relative to efficiency and “ideal combustion”, but does not necessarily indicate complete combustion.

• Carbon Monoxide (CO): lethal gas formed from incomplete combustion.

• Excess Air: needed to show complete combustion, but high values indicate high heat stack loss – poor efficiency.

• Efficiency: represents the total amount of heat available from the fuel minus the losses from the gasses going up the stack.

• Temperature (ambient and stack): needed to determine efficiency.

• Draft pressure: confirms the proper venting of combustibles out of the flue, an appliance has 5 minutes to prove draft under ANSI standards.

• Pressure: confirmation or setting of gas pressures.

• Nitrogen oxides (NO_X): principally nitric oxide (NO) and nitrogen dioxide (NO₂), are pollutant gases that contribute to the formation of acid rain, ozone and smog.

• Sulfur Dioxide (SO₂): Airborne sulfuric acid is a pollutant in fog, smog, acid rain and snow, ending up in the soil and ground water. Sulfur dioxide itself is corrosive and harmful to the environment.

Combustion Analyzer Brands

• Bacharach
• Dwyer Instruments
• E-Instruments
• Fieldpiece
• Testo
• TPI
• UEi
• Wohler

Why use portable combustion analyzers?

Measuring the exhaust gases allows you to tune your equipment for combustion efficiency, decreasing fuel consumption and thus decreasing emissions and for the sake of safety improvement.

What type of analyzer is right for the application?

Analyzers range from small handheld units for basic tuning to larger multi-gas analyzers for compliance-level emissions testing. Determining which type of analyzer best fits your needs depends on the fuel type and size of your equipment, along with the application (e.g. spot-checking, installation, production line, etc.)

Taking a Snapshot Start the analyzer in fresh air. Insert probe into gas stream and wait 2 minutes. Observe the readings on the display. Remove sample line and allow analyzer to purge with fresh air until O2 readings are above 20.0% and other readings are below 15 ppm before powering off. NOTE: Always follow a manufacturer's specifications!