The Combustion Process - Combustion Optimization

Gas analysis techniques

Terms for gas analysis techniques
 Concentration
The term concentration describes the amount of a substance, expressed as mass, volume, or number of particles in a unit volume of a solid, liquid, or gaseous substance e.g. alcohol in beer or oxygen in air.
Different units are in use to describe concentration in gases:
Mass concentration
Concentration expressed in terms of mass of substance per unit volume [g substance/m3 gas volume]
¢ Volume concentration
Concentration expressed in terms of gaseous volume of substance per unit volume [cm3 substance/m3 gas volume]
Part concentration
Concentration expressed as number of particles of substance per a certain number of particles

In flue gas analysis both the terms mass concentration and part concentration are common and used in parallel. The mass unit is gram (and mg, g, see table 15) and the most popular expression for part concentration is ppm (parts per million). "ppm" means "x number of parts in a million parts". ppm is usually used for low concentrations; larger concentrations are expressed in "percent" (%), see table 16.
Consequently the concentration of a gaseous pollutant is expressed
 either using (or mg or g etc.) with reference to a definite gas volume, usually
cubic metres (m3), e.g. 200 mg/m3 or using ppm without any reference, e.g. 140 ppm

Please note
Because of the variation of a gas volume with temperature and pressure changes it is necessary to use one of the following alternatives for describing a concentration value:
 additional specification of gas temperature and pressure values existing during measurement
or conversion of the measured concentration value into the corresponding value at standard zero conditions, see the following chapter. After conversion, the volume is expressed as standard volume (standard cubic meter, Nm3 or m3N).

Standard zero conditions of a gas

The volume of a gas depends on its actual temperature and pressure. To achieve comparable results a standard zero volume has been defined: A gas has its standard zero volume at a pressure of 1013 mbar (hPa) and a temperature of 273 K (corresponding to 0°C).

Conversion of concentration values

Conversion of a measured value to standard conditions
The conversion of an actual measuring value (status 1) to standard conditions (status 2) is performed using the formula

with the following expressions:

Example
The value "200mg/m3" at conditions of 35 °C and 920 hPa
results, after conversion into standard conditions, in the value "248,4 mg/Nm3".

Conversion of ppm to mass concentration [mg/m3]

ppm ( parts per million) is a very common concentration unit as expression for a relation of particles in a gas volume. Simultaneously the unit mass concentration is used.
A concentration value in [ppm] can be converted to the corresponding value expressed as mass concentration [mg/Nm3] using the standard density of the gas as a factor. For that the dilution of the gas by air (excess air, specially added air or
false air from leaks) must be considered by using the oxygen concentration as reference. All measured values must be in reference to a certain oxygen content ("reference O2"). Only concentration values with identical oxygen reference values
are comparable to each other! Therefore, in official regulations, limit concentration values of pollutants are always specified together with a certain oxygen reference value.
The actual oxygen concentration value is also required for the conversion calculation as measure for the actually existing gas dilution level.
 

Conversion formulas for CO, NOx , and SO2

The factor (1,25 etc.) applied in the above formulas corresponds to the standard density in mg/m3 of the gas concerned. For that please note the following comments:
 for SO2 standard density values are reported in the literature between 2,86 and 2,93; the difference is caused by the difference between calculated and measured values;
 for NOx the standard density value of NO2 (2,05) is used, because only NO2 is a stabile compound and NO will react very fast with oxygen to NO2  for H2S (formula not shown) the factor is 1,52.
For conversion calculations without reference to the oxygen concentration the above formulas are simplified to (shown only for CO)

For conversion calculations without reference to the oxygen concentration the above formulas are simplified to (shown only for CO)

4.2 Gas analysis
4.2.1 Terms and application areas Gas Analysis
Gas analysis includes the detection of gases and vapors in connection with control of chemical and metallurgical processes, control of environment and in the field of safety control. A whole class of analytical instruments is available using physical or physico-chemical detecting methods. Although analyzers are considered to be electrical instruments some knowledge of chemistry is required to understand their operating principle and to obtain correct and accurate measuring results.
Process analysis
Process analysis, in contrast to laboratory analysis, includes all continuous measuring methods for real-time determination of physical and chemical properties and concentrations in process streams. The equipment is defined as Process Analytical Instrumentation (PAI) or process analyzers. Because of the harsh environment of many processes, and the use of untrained personnel, the measurement systems must meet higher standards of automation, ruggedness, and ease of operation than do laboratory instruments. The most common applications of process analysis are those in plants of chemical and petrochemical industries, energy generation, metals and minerals, food, pulp and paper, cement and ceramics, but to some extend also in research and development. In the case of gaseous process streams (combustion or flue gases, process gases, also air) the term process gas analysis is used. Measuring points with continuously operating sampling devices, including the analyzers are located at various spots in a plant (at the burner or boiler, at the stack, at the cement kiln, close to the electrostatic filter etc.) in partially very rough ambient and operating conditions. The difference to laboratory analysis, is where the analyzers are installed in a laboratory at the best operating conditions and where the samples are taken discontinuously from the process and brought to the laboratory. Laboratory analyzers are normally designed for sophisticated measurements and require trained operators.

The results of process gas analysis are utilised for process control by e.g. controlling raw material and additives supply and by optimizing the process steps,
 protection of persons and plants by controlling the plant atmosphere for explosive or toxic gas mixtures, product quality control by monitoring the production process steps and controlling the final product specification and environmental protection by monitoring the exhaust gases for compliance with the standard limits specified for the various pollutants.

Analysers
Heart of any analyzer is a species-specific sensor or sensor system. Its functionality is based on a definite physical or physico-chemical principle such as absorption, adsorption, transmission, ionization, conductivity, magnetism etc. The sensors react on variations of the property in question of the sample with a corresponding variation of their property (increased light absorption or reduced electrical conductivity for instance) where from a measuring signal is obtained.
Regarding their design analysers may be classified as follows:

Portable analysers for mobile application of one analyzer at different measuring locations for short time measurements
analyzers for fixed installations at definite locations of a plant for continuous measurements over months and years

In-situ analysers, which are in-stalled and operated directly in the process stream
 

Extractive analysers, which are installed in a rack or an analyzer house (container) outside the process stream and which use a sample probe and a sample gas conditioning unit for sample gas supply.

Application areas
Process gas analyzers have different major application areas:
1. Combustion optimisation
Optimization of combustion processes with the objectives to save energy, to protect the plant (service life extension) and to minimize emissions.
2. Process control
Control of a specified gas composition to monitor a manufacturing process for obtaining certain product specifications and qualities.
3. Emission control
Control of gas cleaning plants for correct operation and monitoring the pollutant concentrations of flue gases for their compliance with the limit value specifications.

Sensors
The term sensor generally describes all kind of devices that measure a physical quantity or the change in a physical quantity such as temperature, pressure, gas concentrations etc. A sensor consists of the actual sensing element (elemental sensor) and a transmitter. The sensing element must have a feature (e.g. conductivity, light absorption) that changes with variations of the measuring component. This "reaction" of the sensing element is then transformed by the
transmitter into a measuring signal.
The most common groups of sensors are
Sensors for temperature measurement
Sensors for pressure measurements
Sensors for flow and level measurements

 Sensors for measurements of concentrations and material properties (Analysers)

Portable analyzers
Portable analyzers in process applications are a great challenge to the instrumentation manufacturer. The harsh measuring environment, the required accuracy and reliability of the measuring values together with small dimensions and
low weight of the analyzer form a profile of requirements, which could not be fulfilled until a few years ago. The development of the testo 350 and testo 360 gas analyzers, however, has established a new standard that have been confirmed by official certifications: both analyzers are mobile instruments with the qualification of performing continuous measurements over longer time periods!

testo 350 and testo 360 are portable extractive gas analyzers, which are, because of their specific features (i.a. sample gas cooler, automatic calibration, automatic sample probe cleaning) also qualified for stationary installations and continuous measurements over several weeks. With that they open new fields of applications and offer very universal and cost effective instrumentation solutions to the user. Electrochemical sensors are preferably used that, because of their small dimensions and low power requirements, are very well suited for portable analyzers. The measuring principle of these sensors, however, requires extensive know how and experience of the manufacturer during development and production to meet the strong specifications of process applications.

Measuring principles used in Testo analyzers
A variety of measuring principles is utilized to determine the concentration of different gases in a gas mixture. Table 21 gives an overview with highlights of which principles are used in Testo analysers.

Electrochemical (potentiometric) sensors
Sensors suitable for the determination of oxygen as well as of CO, SO2 and NOx work according to the electrochemical principle of ion selective potentiometry. The sensors are filled, specifically for their measuring task, with an aqueous electrolytic solution. Two or three electrodes (again task-specific) are placed in the solution with an electrical field applied to them. The sensors are sealed to the outside with a gas-permeable membrane, see figures 10 and 11.
Detailed design and operating principle differ depending on the gas component to be measured as shown in the following two examples:
Example 1: Sensor for oxygen (2 electrodes)
The flue gas resp. the oxygen molecules pass through the membrane and reach the sensor cathode. Because of the material composition of the cathode a chemical reaction takes place resulting in the release of OH--ions (ions = charged particles) from the cathode. The free ions migrate through the liquid electrolyte to the anode of the sensor thus generating an electric current in the external electrical circuit that is proportional to the oxygen concentration.

This current creates a voltage drop across the resistor R, which is the measuring signal and used for further electronic processing. The integral resistor is a thermistor with negative temperature coefficient, NTC. It serves to compensate for temperature influences and thus ensures thermally stable sensor performance.
Oxygen sensors have a service life of around 3 years.

Example 2: Sensors for CO, SO2, and NOx (three electrode sensor)
To determine gases such as CO, SO2 or NOx, three-electrode sensors are used.
The operating principle is explained in the following using the CO sensor as example.

The carbon monoxide (CO) molecules pass through the gas-permeable membrane to the sensing electrode of the sensor. There a chemical reaction takes place that results in the formation of H+-ions migrating from the sensing electrode to the counter electrode. At the counter electrode a second chemical reaction occurs with the aid of oxygen delivered from fresh air. This second reaction causes a current to flow in the external circuit. The current can be evaluated as measure of the carbon monoxide concentration. The reference electrode is used to stabilize the sensor signal.
This type of sensor has a service life of around 2 years.

Infrared absorption (IR)
Infrared radiation (IR, wavelength range is some m) is absorbed by gases such as CO, CO2, SO2, or NO at a wavelength that is specific for the gas. As IR radiation passes through a measuring cell filled with the gas to be analyzed an increase
in the gas concentration causes an corresponding increase in IR absorption and thus a decrease in the radiation intensity received by the IR detector. To ensure selectivity for just one gas component two different principles are used:
The dispersive principle uses radiation, which has been dispersed by a prism or grating before entering the measuring cell. Only two wavelength (or narrow wavelength ranges) of the entire spectrum are used, one that the component in
question absorbs and one that not, as a reference. The ratio of absorption at these two wavelength is a measure for the gas concentration. The non dispersive (NDIR) principle uses a broad band of radiation from a lamp or glower without dispersion before entering the measuring cell. When a component that does absorb IR radiation is present in the sample gas the radiation intensity reaching the detector will be reduced at the specific part of the spectrum. Two methods are in use for detecting the level of absorption:
 a gas detector filled specifically with the gas of interest will, from the broadband radiation leaving the measuring cell, detect only the component of interest resp. its variations caused by the varying concentration of the component
in the sample gas passing through the measuring cell. Radiation absorption affects the gas pressure in the detector cell; these pressure variations are used to generate a detector signal as measure of the concentration of the gas of interest. A solid state IR detector is used together with a narrow-band interference filter that allows from the broadband spectrum of IR radiation only the radiation range of the component of interest (e.g. CO2) to pass through. As the IR radiation passes through the measuring cell a variation in CO2 concentration will cause a proportional variation in the IR absorption and thus in the detector

Ultraviolet absorption (UV)
Certain gas components, e.g. SO2 or NO, absorb also ultraviolet (UV) radiation.
For these components the UV method competes with the IR method offering the major advantage, that water does not cause any interference effect in the UV absorption range. In contrast to IR, however, UV light sources are more expensive
and therefore the UV method is used preferably for particular applications.

Chemiluminescence Detector (CLD)
This method is mainly used for the determination of low concentrations of NO andNO2. It is based on the feature of NO to react with ozone (O3) resulting in the emission of characteristic light radiation (chemiluminescence). The intensity of the
emitted radiation is proportional to the NO concentration. Such analyzers (CLD, chemiluminescence detectors) are very common for analysis of NO and NO2 in car exhaust gases. They comprise essentially an ozone generator, an enrichment
and reaction chamber, and a photomultiplier detector.

Paramagnetic method
This method is applied particularly for the analysis of oxygen. Oxygen is the only gas with a considerably high value of paramagnetism with the result, that oxygen molecules are attracted by a magnetic field. This effect is used to the determine oxygen concentration in gases. Different analyzer designs are available:
The dumb-bell analyzer
Two balls made of glass and filled with nitrogen are mounted at the ends of a thin bar, forming a dumb-bell, which is suspended in the sample gas stream in a strong inhomogeneous magnetic field. With oxygen present in the sample gas,
the oxygen molecules will be attracted by the magnetic field and get the dumb-bell swinging. This effect is compensated with a pre-stressed spring resulting in a measuring signal.
The differential pressure analyzer
The pressure produced at the contact surface of two gases with more and less magnetic characteristics is used here. Between the sample gas containing more or less paramagnetic oxygen molecules and an oxygen-free reference gas, e.g. nitrogen, a pressure difference is generated resulting in a gas flow that is a measure of the oxygen concentration.

Thermal conductivity method (TC)
This principle (mostly applied for the analysis of H2) uses the different values of thermal conductivity of two gases (sample and reference gas). In the analyzer a Wheatstone bridge circuit is built in the filaments of which act at the same time as source of heat and as detector of temperature changes. The bridge is positioned with one part in the sample gas and with the other part in the reference gas. The different cooling of the filaments due to the different gas compositions result in an electrical signal that is used for further processing.

Catalytic filament method
Oxidation of combustible gases at a heated catalytic filament is used to detect the total concentration of combustibles in a sample gas. The combustibles react with a pre-heated catalytic material (wire or a bead, supported on a platinum coil) and increase, through their combustion heat, the temperature of the wire resulting in a change of its electrical conductivity. This process will only work with sufficient oxygen available in the gas for the combustion.
The change in conductivity is a measure for the content off all combustibles present in the sample gas. These are mainly hydrocarbons (acronym: HC or CxHy) but also CO or H2. Cross sensitivities therefore exist between these components
and must be considered in data evaluation.

Flame ionization detection (FID)
The sample gas together with the hydrocarbons is introduced into a flame (usually hydrogen/air) where the hydrocarbons are ionized. The positively charged ions are collected by a negatively charged collecting electrode generating a current
proportional to the hydrocarbon concentration. The different sensitivity of this principle to different hydrocarbon compounds is taken into account by applying specific response factors during data evaluation.

Solid state electrolyte method
A very common oxygen measuring principle uses a solid state electrolyte sensor (made of zircon oxide ceramics) coated with layers (electrodes) of porous platinum as catalyst on two surface sides. At high temperatures >500 °C oxygen
molecules coming in contact with the platinum electrodes become oxygen ions with great mobility that intend to move through the molecular "holes" of the zirconium cell. As the two electrodes are in contact with gases of different oxygen concentration (sample gas and reference gas, e.g. ambient air) oxygen ions will move through the cell from one electrode to the other and generate a differential voltage across the electrodes. From that, with a known oxygen concentration of the reference gas, the sample gas oxygen content can be determined.
Such analyzers are known as oxygen or zirconium probes. They are capable to measure correctly even under very harsh conditions and are, therefore, very often installed directly in the process flow (in situ). They measure the gas in its original
state including the content of humidity! Because of the dilution effect caused by the humidity content the oxygen values measured by zirconium probes are generally lower than of analyzers that measure dry sample gas.