Flue Gas Analysis Techniques PDF

r5> Chapter 3 - Fuels, Combustion, and Flue Gas Analysis OBJECTIVE 11 Describe typical automatic flue gas analyzers. There are numerous devices used to analyze flue gas. Some measure only the carbon dioxide (COz) content of the sample, while others measure only the oxygen (02) content. Another de...

r5> Chapter 3 - Fuels, Combustion, and Flue Gas Analysis OBJECTIVE 11 Describe typical automatic flue gas analyzers. There are numerous devices used to analyze flue gas. Some measure only the carbon dioxide (COz) content of the sample, while others measure only the oxygen (02) content. Another device may analyze a sample for 0-^ and combustibles such as carbon monoxide (CO) and hydrogen (l-y. Some analyzers draw a sample offlue gas from the boiler flue (or from the stack), analyze it, and require the operator to read and record the results. Other analyzers record the results directly into a data collection system. More sophisticated control systems can incorporate the analyzer reading as a control output that adjusts burner controls. CLASSIFICATION OF ANALYZERS Analyzers can be classified into two groups: in situ analysis, which is performed directly through probes in the stack or flue, and extractive analysis, where gas is drawn out and sent to an analyzer. Wet Versus Dry Flue Gas Samples Dry analysis gives the percent by volume of all the components in a gas except the water vapour. This type of analysis is typical ofextractive gas sample systems. When a sample is extracted from the gas stream, the water vapour normally condenses, and the sample gives a dry analysis. Wet analysis includes water vapour as one of the constituents of the flue gas. This type of analysis is typical of in situ analyzers. When the gas is measured with an in situ analyzer, or when precautions are taken to keep the moisture in the sample from condensing, the sample gives a wet analysis. In Situ Analyzers In situ monitoring is done by inserting an instrument into the stack or vessel without removing any gas from the process. In general, there are two types of in situ monitoring: 1. Cross-stack (also called path monitors): This analysis is performed over the entire stack diameter. For example, a light source is sent across the interior diameter of the stack to a detector. 2. Probe (also called point in situ): This system is performed by inserting a probe, which contains part of a measuring cell or an entire measuring cell, into the stack at a precise point for measurements. 166 3rd Class Edition 3 • Part A2 Fuels, Combustion, and Flue Gas Analysis • Chapter 3 Extractive Analyzers In extractive sampling, a gas is drawn out of a duct or stack by an aspirator or a pump. Then the gas is sent through a sample conditioner before reaching the measuring instrument. Portable analyzers are extractive. In general, there are three types of extractive monitoring: 1. Direct extractive: There is a continuous extraction and transportation of the flue gas from the sampling point, through a conditioner, and to the analyzer. There are two types of the direct extractive method: • Cold dry extractive (dry basis analysis): The gas sample is extracted and conditioned to have all moisture and condensable components removed prior to its analysis. • Hot wet extractive (wet basis analysis): The gas sample is extracted and transferred through heated (above 180°C) sampling lines to stay above dew point. 2. Dilution extractive: The flue gas is extracted, filtered, and diluted with clean, dry air before it is sent to the analyzer. This dilution technique lowers the flue gas dew point and keeps the sample temperature under the ambient temperature to eliminate all condensation issues. 3. Continuous sampling: Known volumes of flue gas are continuously extracted from stacks or ducts through a specific sorbent trap that is positioned in-stack or out-of-stack. A small pump extracts flue gas at a low flow rate from stacks or ducts to send through the sorbent trap. After a period of time, the sorbent trap is tested with a reagent. This method is typical for analyzing volatile organic compounds, such as benzene. ANALYZER TECHNOLOGIES There are several technologies used in flue gas analyzers. Older analyzers (like the Orsat analyzer), though effective, were large, non-portable, and required significant manual operation. Newer electronic analyzers are smaller, faster, and more portable. Many digital analyzers are capable of analyzing several different flue gas components and can give a direct reading to the operator. Some of the primary modern analyzer designs are as follows: a) Optical methods: These include infrared sensors where light of a known wavelength is absorbed by a gas. The amount of absorption is proportional to the number of gas molecules in the path of the light. Other optical technologies include optical scintillation, chemiluminescence, ultraviolet fluorescence, opacity, and light scattering. b) Electrochemical sensors: This design uses two electrodes separated by an electrolyte. One electrode is a reference electrode not exposed to the flue gas. There is an electrochemical reaction between the other electrode and a constituent in the flue gas. This chemical and electrical interaction can be measured, and this small measurement is equated to the concentration of the constituent in the flue gas. An example is the zirconium oxide cell used for 02. c) Paramagnetic sensors: Used for oxygen measurement, this sensor has a dumbbell-shaped glass device which contains nitrogen in the two spheres. This device is suspended inside a magnetic field. When oxygen enters, it affects the magnetic field and causes the dumbbell to rotate. The amount of movement is related to the oxygen content of the gas. 3rd Class Edition 3 - Part A2 167 ,Sr Chapter 3 • Fuels, Combustion, and Flue Gas Analysis Types of Analysis for Gases and Particulates Different methods are used to measure the various gases and particulates found in flue gas. The following describes the common or standard methods used to analyze each gas or particulate: Oxygen A common technology for measuring combustion flue gases is the zirconium oxide fuel cell oxygen analyzer. This analyzer uses in situ probes that are inserted into the stack. Zirconium-oxide sensors are a type of solid-state electrochemical cell. A layer of zirconium oxide is typically heated to between 600°C and 700°C, allowing oxygen ions to pass through it from a higher concentration to a lower concentration. The differential oxygen concentration produces a voltage that is proportional to oxygen concentration. Other ways to detect oxygen include optical (fluorescence), electro chemical, electro-galvanic, and paramagnetic analyzers. Carbon dioxide The most common type of carbon dioxide (€02) analyzer is the non-dispersive infrared (NDIR). An infrared beam passes through the sampling chamber and each gas component in the sample absorbs a particular frequency of infrared light. NDIR is suitable for C02, carbon monoxide (CO), sulfur and nitrogen compounds, methane, and other hydrocarbons. Oxygen, hydrogen, and nitrogen do not absorb infrared radiation, so these compounds are not detected by this type of instrument. Carbon monoxide The desired CO levels in combustion flue gases are typically less than 200 ppm, and infrared spectroscopy is well suited to measuring at these low levels. These can be extractive or in situ. Nitrogen oxides (N0^.) Chemiluminescent analyzers are commonly used for N0^; detection. Sulfur dioxide Ultraviolet fluorescence is the standard method for analyzing SO^. Particulate monitoring There are various types ofparticulate monitors and methods, which include opacity monitors or meters (transmissometer), scintillation or received light modulation, and light scattering. 168 3rd Class Edition 3 • Part A2