Gas Sensing and Measuring Devices PDF

Unit A-9 • Energy Plant Instrumentation and Controls Objective 6 Describe the types of gas sensing and measuring devices. Chromatograph A chromatograph, Figure 53(a), is used to analyze the components of a gaseous mixture, or of a liquid in vapor form. It operates on a combination of physical and...

Unit A-9 • Energy Plant Instrumentation and Controls Objective 6 Describe the types of gas sensing and measuring devices. Chromatograph A chromatograph, Figure 53(a), is used to analyze the components of a gaseous mixture, or of a liquid in vapor form. It operates on a combination of physical and chemical principles. A physical principle: If a gaseous mixture is forced through a certain material that resists its flow, smaller compounds, or those with lower boiling points, will pass through more quickly than the ones that are larger, or have a higher boiling point. A chemical principle: Compounds in a mixture of gases or vapours having differing types of chemical bonds will be attracted differently to the bonds of the material it passes through. A basic chromatograph consists of a separation column, packed with an absorbent material, and installed in an oven that is maintained at a constant temperature. The column is connected to a regulated supply of inert carrier gas, such as helium or argon, as indicated in Figure 53(a). The gas sample mixes with the carrier gas, and flows through the column to a detector. Each component of the gas mixture is identifiable by how much time elapses between the injection of a sample into the column and the emergence of that component (typically the time at the peak height). See Figure 53(b). Quantitative measurement of each component typically depends on the difference between a value representative of the sample, and a baseline value representative of the carrier gas only. The thermal conductivity detector is commonly used in gas chromatographs to analyze hydrocarbon and flue gas mixtures. It measures the difference between the thermal conductivity of the carrier gas in the reference leg, and of the mixture of the carrier gas and the sample gas in the detector leg. The heat sensitive elements in the detector are often thermistors or semiconductors with rapidly decreasing electrical resistance with increase in temperature. When a greater quantity of a specific gas passes through the detector, a Wheatstone bridge circuit connected to the thermistors becomes increasingly unbalanced. The resulting imbalance is traced on a chart like that shown in Figure 53(b). Peak heights and peak areas above a base line are used to calculate the quantity of a particular gas component in the mixture. Figure 53 – Gas Chromatograph with Thermal Conductivity Detector Temperature Controlled Packing Time (a) 2-42 (b) 4th Class Edition 3 • Part A Ethylene CO2 Ethane Column Base Line Product Peak Sample N2 and O2 Sample Leg CO Reference Leg Sample Injection Pipe Carrier Gas Introduction to Process Measurement • Chapter 2 Figure 54 – Gas Chromatograph CO and CO2 Analyzers CO and CO2 are commonly measured using non-dispersive infrared radiation (NDIR) detection. The NDIR detector (Figure 55) consists of an infrared (IR) light source, an IR detector, and a sample tube for the gas to pass through. A light filter removes wavelengths of infrared that are not used for measuring carbon monoxide or carbon dioxide (CO and CO2 strongly absorb light wave frequencies of 4.26 µm and 4.67 µm respectively). The absorption of these wavelengths is measured by the analyzer, and used to determine the concentration of CO and CO2. The resulting output provides the percent volume of each of the gases being measured. Figure 55 – Non-Dispersive Infrared (NDIR) Detector for CO or CO2 Sample Gas Out Sample Gas In Reflector Visible IR Source (lamp) IR Detector Sample Chamber (light tube) 4th Class Edition 3 • Part A Filter 2-43 Unit A-9 • Energy Plant Instrumentation and Controls Continuous Emissions Monitoring Systems (CEMS) Continuous emission monitoring systems (CEMS) continuously collect, measure, record, and report emission data to comply with jurisdictional environmental standards. Modern CEMS evolved from basic flue gas monitoring systems that measured oxygen, carbon monoxide, and carbon dioxide for combustion control. Industrial facilities can use emissions monitoring to: • Document regulatory compliance with emission limits • Assess and monitor process conditions • Assess and monitor plant efficiency • Inform operating decisions • Determine pollution control device efficiency • Monitor health and safety within the plant Figure 56 shows a typical continuous emission monitoring system. This system measures flue gas flow using an ultrasonic flow measurement device. The flow measurement is passed on to the gas analyzer. The opacity (how clear the gas stream is) is also measured and sent to the gas analyzer. The continuous emission sample point draws a gas sample from the main stream, using a pump. This sample is then sent to the gas conditioning section where the gas sample may be diluted to a certain percentage or dried (and moisture content measured), depending on the type of sample and the type of analyzers being used. The gas analyzer is made of several separate analyzers that measure the different products being emitted. Typical monitored emissions may include oxygen, carbon monoxide, carbon dioxide, sulfur dioxide, nitrogen oxides, hydrogen chloride, mercury, particulate matter, and volatile organic compounds. These measurements are sent to a computer where the data is analyzed and logged. A summary of the results over a measured time span is sent out to the LAN (local area network) for distribution to those that require the measurements. Figure 56 – Continuous Emissions Monitoring System Setup Opacity Measurement LAN Continuous Emissions Sample Point Gas Conditioning Ultrasonic Flow Measurement 2-44 4th Class Edition 3 • Part A Gas Analyzer Data Analysis and Log Computer