Gas Monitoring For Clean Air

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Bengt Löfstedt discusses gas concentration monitoring in flue gas treatment applications

There are several flue gas treatment methods that can be applied to reduce pollutant emissions to air from combustion-based power generation. Gas concentration monitors are often used to control and protect the reduction processes. This article describes some common reduction methods and highlights a suitable gas monitoring technique.

Combustion processes can give rise to a variety of pollutants that are bad for the environment and therefore often regulated when it comes to allowed emissions to air. The types of pollutants can vary depending on fuel and incineration process. However, some types of pollutants are essentially generic and seen in most combustion-based power generation facilities. This applies to dust, sulfur dioxide (SO2) and some other acidifying gases, and nitrogen oxides (NOX). Similar techniques are used to reduce the emissions of these types of pollutants.

There are also other types of pollutants that may be of concern in specific types of combustion processes. By example, mercury and bromides might be emitted when incinerating hazardous waste.

Emissions Reduction Methods

A generic flue gas treatment process consists of three different subprocesses. First, there is usually a filter used to reduce the dust levels. Then comes a scrubber which removes acidifying gases such as SO2 (therefore often referred to as a “DeSOx” stage) and further reduces the dust levels, and finally there is a selective catalytic reduction (SCR, the “DeNOx” stage) stage, which reduces the NOX levels.

Filters for dust reduction can be cyclones, fabric collectors, and/or electrostatic filters. There are also some different types of scrubbers. In a dry scrubbing process, a solid substance such as lime is added to the flue gas. The lime reacts with the gases and forms solid particles that can be separated in subsequent dust filters. In a wet scrubber, the flue gas is sprayed with water or a neutralising agent. Water-soluble pollutants are then removed from the flue gas and the wastewater is neutralised. Semi-dry processes can also be applied.

In the SCR process, NOX is forced to react with ammonia (NH3) to form nitrogen and water. The reaction is facilitated by using a catalyst that consists of a ceramic carrier covered with rare metal oxides typically based on vanadium, molybdenum or tungsten.

In some cases, other pollutants can also be caught by either of these treatment processes. Sometimes more specialised methods must be applied, for example the injection of activated carbon to reduce mercury emissions.

Monitoring Needs

The SCR process is an example of where there are needs for gas concentration monitoring. The NOX levels before and after the SCR unit are usually measured to give control signals to the NH3 injection. The NH3 level after the SCR unit can also be measured to reveal slip and give further feedback to the injection. To protect the catalyst, the levels of SO2 and other acidifying gases prior to the SCR unit are often also monitored and the signals are used to control the preceding injection of the scrubber agent, and as a last resort to apply an SCR bypass to avoid damage to the catalyst.

In either case, the needs require instruments operating in hot and aggressive gas mixtures, and with very short response times to facilitate the control loops. This can be readily achieved with instruments measuring directly in the flue gas (“in-situ”) where no sampling lines can delay the analysis, and notably with laser diode-based units providing high-precision results with time resolution down to a few seconds or better.

Operating Principle Of A Laser-Diode Based Gas Monitor

An example of such an instrument is the Opsis LD500 gas monitor. The central unit contains a narrow-band laser whose wavelength is scanned across an absorption line of the molecule to be monitored. The laser light is led via an optical fibre to an emitter situated on the duct. The laser light is directed through the flue gases inside the duct and then picked up by a sensor inside a receiver. Data from the sensor is sent back via another optical fibre to the central unit, where the signal is analysed, and the gas concentration can be calculated and fed to the control system.

Each molecule type requires its own laser, but the central unit can be equipped with up to four laser heads, allowing concurrent monitoring of multiple gases. Light multiplexing enables a single unit to monitor the same gas type at several locations. Further, the optical fibres can be made very long, giving additional flexibility to the monitoring system. All in all, this forms a very cost-efficient solution to the needs of an SCR monitor

Bengt Löfstedt is with Opsis

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