May-2021

Monitoring gas emissions

Analysis of gases supports cleaner air initiatives in hydrocarbon processing. Increasingly, plant operators are becoming highly sensitive to their contribution towards greenhouse gas emissions.

Matt Halsey & Sangwon Park
Servomex

Article Summary

This awareness has been driven by ever more stringent environmental regulations and international action to reduce the impact on climate, such as the 2016 Paris Agreement. To support their efforts to reduce emissions and operate in an ecologically responsible way, many plants are looking towards gas analysis systems to provide the solution.

A combination of solutions for combustion efficiency, gas clean-up, and emissions monitoring supports plants in their goals, not only ensuring that air remains clean, but optimising processes for reduced fuel consumption and higher yields in hydrocarbon processing (see Figure 1).

Effective combustion control
Combustion is an integral part of many hydrocarbon processing applications, with no realistic alternatives available to create the extremely high temperatures required. The combustion reaction mixes fuel with oxygen (from air) in a fired heater, delivering heat energy that can be transferred elsewhere in the process. This reaction typically requires a significant amount of fuel, creates potential safety hazards, and generates harmful emissions.

The most efficient reaction is one where the ratio between air and fuel is optimised. Prior to the development of gas analyser technology, fired heaters were typically run in high excess air conditions. This was inefficient and increased the level of fuel consumption, but avoided the creation of unsafe conditions that could lead to an explosion.

An excess of oxygen (O2) also combines with nitrogen and sulphur in the fuel to produce unwanted emissions such as oxides of nitrogen (NOx) and sulphur (SOx). Accurate gas analysis of oxygen and combustibles such as carbon monoxide (CO) has provided a way to better balance the air-to-fuel ratio and control the combustion reaction.

Controlling combustion produces a number of benefits, particularly for plants looking to meet environmental standards requirements. Fuel consumption is reduced, resulting in fewer emissions, a reduction in NOx, SOx and CO, and a decrease in the greenhouse gas carbon dioxide (CO2).

Zirconia based sensing technology is long established as a solution for O2 monitoring in combustion, and delivers reliable, accurate results with a fast response to changing conditions. It has the advantage that a combustibles sensor can be added easily, and at modest cost, to provide an all-in-one combustion control solution, such as in Servomex’s Servotough FluegasExact 2700 combustion analyser.

More recently, tunable diode laser (TDL) technology has been introduced for this application, providing even faster measurement, particularly for carbon monoxide. It also gives an average measurement across the measurement path, rather than the result at a single point. However, since TDL sensing is highly specific to the gas being measured, separate analysers are required for O2 and CO.

Servomex has the Servotough Laser 3 Plus Combustion TDL analyser for this application, and this can be configured to measure either O2 or CO. It can also be configured for a joint measurement of CO and CH4, providing a rapid-response measurement for safety in natural gas fired heaters and boilers.

Additionally, it is important to note that gas analysis is used in many applications to support greater process efficiency. The more efficient the process reaction is, the fewer harmful emissions are likely to be generated, so this also plays its part in cleaner air.

For example, one of the largest air emissions sources in a refinery is the fluid catalytic cracking (FCC) unit, which requires multiple gas measurements across the process.

In a typical FCC unit, a process control oxygen measurement is required in the regenerator off-gas, where low O2 will cause incomplete combustion (and, therefore, removal) of the catalyst coke and excess O2 can reduce catalyst life.

Measurement of CO and CO2 in the same off-gas helps calculate catalyst coke formation, enabling catalyst regeneration efficiency to be determined. Excess O2 and CO levels require monitoring in the regenerator flue gas, while ammonia slip is measured at the selective catalytic reduction (SCR) outlet to control the NOx removal process.

Each point of this process benefits from the application of an appropriate technology: the off-gas measurements for O2 and NH3 slip benefit from the use of TDL open-path measurements which reduce issues with catalyst particulates experienced by in-situ or simple extractive systems. 

Close-coupled extractive systems, such as the FluegasExact 2700, are reliable and cost-effective for making O2 and combustibles (COe) flue gas measurements, while the Servotough SpectraExact 2500 is suited to off-gas CO and CO2 measurements.

Cleaning process gases
The second phase of Servomex’s clean air strategy is to tackle gas cleaning, that is the removal of harmful substances from process gases that might otherwise be emitted by the plant.

Typical applications within this area include ammonia slip treatment and flue gas desulphurisation.

Ammonia slip
To suppress the harmful emissions of NOx from combustion, ammonia or urea is used, either in a SCR or selective non-catalytic reduction (SNCR) process. Both methods require accurate ammonia dosing to reduce NOx levels. If insufficient NH3 is used, then NOx emissions are not sufficiently suppressed, while too much NH3 can lead to the eventual formation of ammonium bisulphate.

Ammonium bisulphate is a white powder that can plug the catalyst in SCR processes, causing equipment damage and reducing the value of the fly ash by-product, so it is vital that plants manage NOx removal processes efficiently, controlling the level of ammonia slip to 2-3 ppm ammonia.

Ammonia can be monitored by extractive sampling, but this is difficult, since the sample must be kept above 290°C to prevent the formation of ammonium bisulphate and sulphuric acid.

Inlet NOx concentration, fuel composition, and catalyst performance can also affect the measurement, while infrared based extractive systems may also be impacted by signal interferences from gases formed by the process, and by high levels of dust.

A more effective solution is a TDL analyser — such as the Servotough Laser 3 Plus Environmental installed directly into the process ducts (see Figure 2). This provides a signal that is averaged across the duct, for a more accurate NH3 reading despite uneven flow conditions.

Flue gas desulphurisation
The purpose of a flue gas desulphurisation (FGD) system is to remove sulphur compounds (SOx, principally SO2) from exhaust gases. It is a process usually utilised by fossil fuelled power plants and operators in other SOx-emitting processes.