Measurement of Exhaust Gas Constituents: Conventional and Modern Methods | Thermodynamics

In this article we will discuss about the measurement of exhaust gas constituents by two methods: 1. Conventional Method and 2. Modern Methods.

The exhaust gas analysis is performed to determine the constituents of various components of gases in the exhaust gas. The analysis is important for determining the air fuel ratio, excess air, combustion products and unburnt components.

There are two methods of measurement of constituents of exhaust gases: 1. Conventional Method 2. Modern Methods.

Method # 1. Conventional Method:

Orsat Analysis:

A typical Orsat apparatus for flue gas analysis is shown in fig. 8-4. The essential parts of the apparatus are:

(i) A measuring burette A, surrounded by water jacket to maintain a constant temperature during the experiment.

(ii) Three absorption pipettes B, C and D which provide for the absorption of CO₂, O₂ and CO respectively. The lower end of each pipette extends almost to the bottom of its respective chemical storage jar. Each of the absorption pipette is connected at its upper end to a capillary tube header and these headers are interconnected by means of a manifold having cocks.

This manifold is connected to the measuring burette. Each of the absorption pipettes is fitted with a number of small glass tubes which give a great amount of surface, wetted by the absorbing reagents, exposed to the gas under analysis.

(iii) A leveling bottle E, connected to the lower end of the measuring burette.

(iv) Breathing chamber to which breathing tubes from absorption pipettes are connected during use. The purpose of this expansion chamber is to provide breathing action and at the same time prevent absorption of oxygen from the atmosphere. Carbon dioxide is absorbed in pipette B, which is filled with caustic potash KOH.

This solution will absorb about twenty times its volume of CO₂. Oxygen is absorbed in pipette C, which contains an alkaline solution of pyrogallic- acid. The solution for absorbing O₂ will absorb only about twice its own volume of O₂.

It will also absorb CO₂ and, therefore, we must be very careful to be certain that all CO₂ is absorbed in pipette B before the exhaust gas sample is passed to pipette C. Carbon monoxide, is absorbed by an acid solution of cupreous chloride in pipette D.

This pipette contains some metallic copper to keep the solution energized. This, solution will absorb CO only to the extent of about its own volume. This solution will also absorb O₂. Therefore, the solutions used in pipettes C and D must be protected from deterioration by absorption of oxygen from atmosphere.

The nitrogen is obtained by difference. In order to get the correct results, the absorption must occur in the order indicated. The reagents should be fresh and kept protected from atmospheric air. All connections between the various parts of the apparatus must be leak proof.

As the sample of flue gas is collected over water, the above analysis is the dry flue gas analysis.

The combustion is seldom complete and some carbon monoxide will usually be present even though excess air is supplied. The oxygen content of the dry products of combustion is a more reliable indicator of excess air than is the carbon dioxide content.

Method # 2. Modern Methods:

The modern methods used for exhaust gas analysis are:

(I) Gas chromatography

(II) Non-destructive Infra Red Analyzer (NDIR)

(III) Flame ionisation detector (FID)

(IV) Smoke meter.

(I) Gas Chromatography:

The analysis at emission product by gas chromatography technique is an extremely versatile method of analyzing the complex chemical mixture based on phenomenon on the preferential absorption separation. The constituent chemical species of gas when passed through the absorbing systems, the separated molecules are then detected by analog sensor as they come out of the chromatographic column.

The analog sensor detects into some different property as the separated molecules. This is the only method by which each component of exhaust sample can be identified and analyzed. However it is very time consuming and sample can be taken in batches.

The arrangements are as shown in fig. 8-5.

(II) Non-Destructive Infra-Red Analyzer (NDIR):

The analyzing system of an excellent NDIR is using a unique micro flow type of detection. The infra-red energy of a particular wave length or frequency is peculiar for a certain gas that gas will absorb a infra-red energy of this wave length and transient infra-red of other wave length.

The absorption board for CO is between 4.5 – 5.0. So energy absorbed at that ware length is the indication of concentration of CO in the exhaust gas. The measurement of CO, CO₂ and HC which have clear infra-red absorption peals can be measured accurately.

However usually the exhaust gas sample to be analyzed contains other species which also absorbed infra-red energy at same frequency. For example an NDIR analyzer is sensitive to n-hexane for detection of HC response equally well to other paraffinic HC but not the acetylenes or aromatics.

The arrangement is shown in fig. 8-6.

(III) Flame Ionization Detector (FID):

It uses FID burner for analysis. The working principle of FID is that H₂ air flame contains a negligible amount of ions. However if ever trace amount of an organic compound such as hydrocarbon are introduced in the flame large, number of ions are produced.

If the polarized voltage is applied across we have burner adjacent collector and ion migration will produce a current proportional to number of ion and thus to HC concentrations present in the flame.

The number of carbon atoms passing through a flame in unit time decides the output of the FID. In other words output of FID is directly proportional to velocity of carbon atoms through flame. This hexane C₆H₁₄ would give the output.

Presence of oxygen in exhaust gas may slightly affect the FID reading but CO, CO₂ water NO₂ and H₂ has no affect on FID reading.

FID analysis is widely used because of the fact that it has very fast process and gives, accurate measurement of HC and exhaust gases concentration as low as ppm can be measured.

(IV) Measurement of Smoke:

The smoke is an amount of carbon in the flue gases which can be visible by eyes, but the % of smoke cannot be assessed. The visual assessment depends on gas velocity and back ground.

To measure of % of smoke exhaust gases or smoke density, two types of smoke meters are used:

(i) Filter darking type

(ii) Light extinction type.

Light extinction type meter can measure both white and black smoke. While the filter darking type meters can give only black smoke reading. The light extinction meter can be used for continuous measurement while filter state conditions.

Following are the most widely used for smoke measuring devices:

(a) Bosch smoke meter

(b) Hartridge smoke meters

(c) PHS smoke meters.

The above devices are basically soot density measuring devices that are meter reading are a function of mass of carbon a given volume of exhaust gas.

Bosch Smoke Meter:

It is a better darkening type a measured. Volume of exhaust gas is allowed to pass through filter paper which is blackened to various degrees depending on amount of carbon present in the exhaust. The density of the soot is measured by determining the amount of light reflected from the sooted paper.