Exhaust Gas Analysis

Transcription

1 Exhaust Gas Analysis The analyzers have (in situ) two versions: Cross-stack and analyzers with a measuring probe (extractive measurement). Cross-stack - Measurement across section of the channel; this produces results with high statistical representation. - Determination of gas concentrations in real time; this means very fast measurement, results reflect the current state of the process. Fig. 1. Cross-stack method The Advantages of In-situ: - Direct and immediate measurement in processes. - Non-contact measurement; therefore suitable for use with aggressive or corrosive gases. - Very short response times. - Very low maintenance requirements. Probe technique: - For applications with high dust concentrations. - Also suitable for difficult measurement tasks such as overpressure, wet gases or very high gas concentrations. Applications:

2 - Power stations and district heating stations. - Refuse combustion plants. - Domestic furnaces, cracking lime and cement kilns. - Quench, sintering, smelting and tempering furnaces. In-situ Measurement: - Analyzers for direct gas analysis are available as complete devices systems. Gas components such as CO, CO 2, SO 2, NO, NO 2, O 2, NH 3, and H 2 O are measured in situ i.e. directly in the measurement location. The following measuring principles are applied: - UV spectrometry - IR filter correlation - Electrochemical methods, e.g. zirconium dioxide. Extractive Measurement: - With extractive gas analysis, the analyzers measure numerous gas components, for example SO 2, NO, NO 2, CO, CO 2, O 2, HCI, NH 3, H 2 O, mercury, hydrocarbons, and total carbon. - The following measuring principles are applied: Chemiluminescence Photometry: - Universal measurement principle based on the absorption of electromagnetic radiation (NIR, VIS, UV). Using various measurement methods (NDIR, interference filter and gas filter correlation, atomic absorption etc.), numerous gas components can be detected specifically. Flame Ionization Detection (FID): - Principle for determination of total hydrocarbon in gases e.g. for monitoring of explosion limits. Paramagnetic Principle: - Sensitive measurement method for detection of oxygen concentrations. Electrochemical Principle: - Cost-effective measurement method for detection of oxygen concentrations. Thermal Conductivity Measurement: - Approved method for detection of binary or quasi-binary gas mixtures for example hydrogen in carbon dioxide. The analyzers can be extended to form a complete analysis system with numerous external system components, for example:

3 - Gas sampling probes- from a simple probe to the cooled sampling system which are adapted to the conditions at the measuring point. - Sample gas lines, heated and unheated. - Gas coolers, also low-temperature coolers, with different materials. - Pumps, filters, precipitators, valves etc. This enables the analyzers to be adapted specifically to the measurement task. 1- heated filter; 2- heated gas line; 3- cooler; 4- sample pump; 5- filter(s); 6- flow meter; 7- analysers Fig. 2. Presample system Chemiluminescence Analyzer Principle of Operation: The chemiluminescence detection method is based on the principle that nitric oxide (NO) reacts with ozone (O 3 ) to produce nitrogen dioxide (NO 2 ), 10 % electronically excited nitrogen dioxide (NO 2 * ), and oxygen. Following the NO-O 3 reaction, the NO 2 * molecules immediately revert to NO 2. This process emits photons that produce a light emission directly proportional to the NO concentration in the ambient air sample. The detection method is the same for each parameter, but the analytical process differs slightly. For NO detection, the sample gas and the ozone are introduced directly into the reaction chamber for analysis. To determine NO 2 (NO+NO 2 ) concentration, the sample is first routed through the converter where the NO 2 is converted to NO, and is then routed to the reaction chamber for analysis. NO 2 analysis is achieved by an electronic subtraction circuit that automatically cycles the analyzer between the NO and NO 2 modes and then automatically determines the difference for a direct NO 2 output. The intensity of the resulting light emission is then measured by a photomultiplier tube and associated electronics.

4 1- air or oxygen; 2- ozone generator; 3- reaction camber; 4- photo multiplier; 5- recorder; 6- power supply; 7- converter and NO x detection; 8- filter; 9- sample; 10- NO detection Fig. 3. NO/NO 2 /NO x analyser NDIR Analyser Analyzers utilize a wavelength reference mode of operation. Two analysis channels for analyses for two distinct gas constituents may be incorporated into a single analyzer unit. Broadband emission from two IR-sources passes through a chopper wheel and optical filters, and enters the gas analysis cells. Light transmitting the cells is focused onto detectors. Their preamplified output signals are sent to electronics. The spectral absoprtivites of the gas constituents CO, CO 2, and CH 4 are shown in Fig. 4. Fig. 4. spectral absorptiveness of HC, CO and CO 2

5 1- electronics section; 2- capacitance system; 3- sample cell; 4- reference cell; 5- constant light; 6- interrupter blade (chopper wheel); 7- detector Fig. 5. NDIR analyser FID Analyser (Flame Ionization for Measurement of Hydrocarbon) Principle: An oxygen/hydrogen flame is virtually ion free if the burning mixture contains only pure hydrogen, oxygen, and inert gases. If hydrocarbon is introduced, the ion flux is increased, and only a minute amount o hydrocarbon need be present to produce the effect. Moreover, under appropriate conditions the ion yield is proportional to the amount of hydrocarbon introduced into the flame. This characteristic of the hydrogen flame is now utilized in the flame-ionization measurement of hydrocarbon. Fig. 6. Total Hydrocarbon analyser (FID)

6 1- air; 2- H 2 ; 3- sample; 4- carbon ions; 5- flame; Fig 7. Flame ionisation detection O 2 Analyser Two nitrogen-filled (N 2 is nonparamagnetic) quartz spheres are arranged in a dumbbell configuration and suspended free to rotate on a thin, platinum ribbon in a cell. A small mirror that reflects the light beam to a photodetector is mounted on this ribbon. A strong permanent magnet, specially formed to produce a highly inhomogeneous magnetic field inside the analysis cell, is mounted outside the cell. If oxygen molecules enter the cell in the sample-gas stream, their paragmagnetism will cause them to be drawn toward the region of greatest magnetic field strength. The O 2 -molecules thus exert differing forces on the two quartz spheres. This produces a torque acting on the sphere arrangement, and the suspended dumbbell, along with the mirror mounted on its suspension ribbon, will be angularly rotated away from their equilibrium position. The mirror will then deflect an incident light beam onto the photodetector, resulting in the generation of an electronic signal. The signal current is amplified and fed back to a conducting coil at the dumbbell, forcing the suspended spheres back to their equilibrium position. The current required to generate the restoring torque required to return the dumbbell to its equilibrium position is a direct measure of O 2 -concentrations in the gas mixture.

7 Fig. 8. O 2 analyser Oxygen sensor An oxygen sensor, or lambda sensor, is an electronic device that measures the proportion of oxygen (O 2 ) in the gas or liquid being analyzed. The original sensing element is made with a thimble-shaped zirconium ceramic coated on both the exhaust and reference sides with a thing layer of platinum and comes in both heated and unheated forms. The most common application is to measure the exhaust gas concentration of oxygen for internal combustion engines in automobiles and other vehicles. Device to measure the partial pressure of oxygen in their breathing gas. 1- air; 2- exhaust gas; 3- sensor; 4- electrolyte (zirconium dioxide); 5- exhaust side; 6- reference side; 7- span Fig. 9. Electrochemical method

8 Literatures: ROSEMOUNT: Measuring Methods of Gas Analyzers, BECKMAN: Air Quality Monitoring Instrumentation, Bulletin 4199, SICK MAIHAK: Analysers and Process Instrumentation,