There are different solutions for measuring the constituents within flue gases. As with most measurement protocols, especially within the industrial sector, the determination of the gases within a flue, alongside the temperature of the local environment, are performed using sensors. In this article, we look at some of the sensing methods for flue gases.
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What is Flue Gas?
Flue gas, simply put, is the gas from industrial processes and automotive exhausts that exits into the atmosphere by a flue - which is a duct, pipe or chimney opening that enables the gases to travel from the source to the local atmosphere.
Flue gases come in many different forms, and from many various sources, but the most common output is from industrial plants and power plants. In most cases, the flue gas is the exhaust gas from combustion processes within the plant. So, given that many different processes give off various gases, the constituents within the flue gas will vary from plant to plant.
However, because most flue gases are a product of combustion, there are certain gases which will always be present, and these include nitrogen, carbon dioxide, oxygen, and water vapor. Some of the other common constituents include carbon monoxide, hydrogen fluoride, sulfur oxides, ammonia, and various volatile organic compounds (VOCs); as well as solid particles arising from incomplete combustion processes.
The Sensor Types and Characteristics Measured
There are many different reasons why someone would want to measure flue gas, and there are individual sensors and multi-capable sensor systems that can measure a wide range of gaseous concentrations and temperatures. Nowadays, with issues surrounding global warming and climate change, environmental regulatory agencies have stipulated that all plants need to continuously monitor the flue gases to check that the concentration of greenhouse gases and pollutants being released into the atmosphere are low (and within regulations).
Many sensors are designed to measure a range of gas concentrations, and while the emphasis is on checking that the concentrations are not too high, the sensors used can measure in the parts per million (ppm) range.
Temperature is also an important parameter to check, as it ensures that the flue is not overheating and any components between the combustion source and the exit point of the flue are not at risk of becoming damaged. This is often done using a thermocouple.
Aside from temperature, the other important parameter to check is the concentration of gases within the flue. This is to ensure that the greenhouse gas emissions are not too high and that large concentrations of toxic gases are not being released into the local atmosphere. While there are many ways of measuring the different gases, electrochemical sensors are primarily used to measure oxygen, carbon monoxide, nitric oxide, and nitrogen dioxide concentrations. Whereas infrared sensors are used to measure the concentration of carbon dioxide, sulfur dioxide, and carbon monoxide.
How Sensors are Implemented to Measure Flue Gas
With a phenomenal amount of flue gases on the market today, most flue gas sensors/analyzers fall into two distinct categories. These are continuous emission monitoring (CEM) systems and portable flue gas analyzers.
Continuous Emission Monitoring (CEM) Systems
Continuous emission monitoring systems is a broad term for any sensor system that continuously monitors the flue. But, a more accurate description of these instruments is those which conform to certain standards set out by government environmental departments. Because they are designed to perform a continuous analysis, the equipment is installed permanently and are of a higher cost than portable analyzers.
There are benefits of using these systems, if the budget affords, especially if the flue is in a country that is strictly regulated. If non-CEM systems are used, then they can be deemed non-compliant with the current standards set out by the relevant body, but mobile CEM systems are available for those who need to provide compliant measurements to satisfy the environmental bodies but don’t wish to install the equipment permanently.
Portable Flue Gas Analyzers
As expected, portable flue gas analyzers are not a permanent fixture in the flue, nor are they a system that performs continuous monitoring. Rather, they are used to do spot checks at given time points at defined locations within the flue.
These are much cheaper, as the equipment is a lot smaller and doesn’t need to be permanently integrated. But as mentioned above, they can be deemed non-compliant as they are not monitoring the flue continuously - but as stated above, there are ways around it. On the other hand, portable flue analyzers are much easier to use and a lot more versatile than CEM systems.
Sources and Further Reading