Optimizing the Denitrification Process

During combustion - one of the most popular industrial processes across all industry sectors - the chemical energy in fuels, such as natural gas, coal, or oil, is converted into heat by oxidization. The heat is necessary for a broad range of production processes such as production of electricity in coal fired power plants. The flue gas has a range of air pollutants, which are damaging to the environment and people.

Diode laser analyzer LS4000. Ammonia slip measurement for DeNOx process.

With a growing sensibility for environment protection, the decrease of air pollutants has internationally gained the attention of governments. Besides others, nitrogen oxides (NOx), which comprise of nitrogen dioxide (NO₂) and nitrogen oxide (NO), are considered to be a main cause for environmental damage. Hence, its emissions are strictly limited by national and supranational regulations.

So as to comply with the emission restrictions, a variety of measures are applied, which can usually be categorized into primary and secondary measures.

• Primary measures aim to avoid the development of NOx during the combustion process itself
• In contrast, secondary measures remove the pollutant before the flue gas is discharged to the environment

Since primary measures are inclined to have inadequate efficiency levels, often secondary measures such as flue gas treatment systems play a key role for achieving the emission limits.

The part of the flue gas treatment system, in which the abatement of nitrogen oxides occurs is the denitrification unit, in short DeNOx unit (refer to Figure 1).

Figure 1. DeNOx unit as part of flue gas treatment on the example of a coal-fired plant.

Measuring Task – Ammonia Slip Measurement

As illustrated in Figure 1, the exhaust gas from the boiler enters the DeNOx unit where nitrogen oxides are eliminated by a chemical reaction.

For this chemical removal of NO and NO₂, a reducing agent, for instance ammonia (NH₃), is injected into the flue gas in the headspace or upstream of the unit.

The concentration of the injected ammonia is continually regulated based on the NO/NO₂ concentration in the exhaust gas. Even if the standard principle is identical, secondary measures can be split into two types.

A frequently used technology is the selective catalytic reduction (SCR), which utilizes catalysts to quicken the reduction of nitrogen oxides.

In contrast, the alternative selective non catalytic reduction (SNCR) does not utilize catalysts, but necessitates higher process gas temperatures.

Owing to the lower required process temperatures and higher reduction rates of over 95%, SCR is repeatedly the preferred technology.

In case of SCR, the blend of ammonia and flue gas passes the catalysts where the nitrogen oxides are decreased on the SCR catalyst to create elementary nitrogen and water vapor. Downstream to the SCR unit, just very few NOx and NH₃ molecules are present.

The residual NH₃ concentration at the outlet is called ammonia slip, which is continuously monitored (refer to Figure 1).

Why is this Measurement Important to Plant Operators?

Overall, the motivation for the ammonia slip measurement after SCR and SNCR DeNOx units is diverse.

• First of all, the measurement helps plant operators with the trade-off between having adequate ammonia present for the target conversion of NOx and the minimization of the ammonia consumption and ammonia slip simultaneously. This allows to lessen the related operation costs for the reducing agent, but still to be compliant with emission regulations.
• Secondly, the ammonia slip helps to establish the best point in time for catalyst regeneration in SCR units. If there is a trend of increasing ammonia slip at unchanged conditions, this is an indicator that the catalyst activity is declining.

Ammonia slip measurement in DeNOx units.

• Thirdly, it is vital to monitor the ammonia slip since it can have detrimental impacts on the downstream equipment. After the DeNOx outlet, unused ammonia reacts with acidic gas components existing in the flue gas. This results in creation of ammonia salts, of which especially ammonium bisulfate causes problems. As a result of the lower temperature downstream to the DeNOx unit, ammonium bisulfate collects on surfaces of downstream equipment (for example air preheater) causing an increased pressure drop or in worst case plugging of the equipment. Moreover, ammonia bisulfate in combination with water from the flue gas can also result in corrosion of downstream equipment.
• Finally, excess ammonia can contaminate fly ash, which is frequently sold as by-product to the cement sector. If the quality restrictions are exceeded, the ash cannot be sold anymore. This equals a financial loss affecting the total profitability of operation.

To conclude, this measurement enables the operator to

• Enhance efficiency of denitrification process and
• Minimize downstream problems, maintenance work and related costs caused by an excess of ammonia.

Schematic diagram of SCR process.

Applied Denitrification Technology

Both the usual process conditions and the measuring range rely on the applied denitrification technology (i.e. SNCR or SCR). In case of SCR, the measurement range is normally in the low parts per million (ppm) range and the usual conditions for SCR are illustrated in Table 1. For SNCR, the ammonia slip and also the related measuring range are commonly higher. Besides NH₃, an optional H₂O measurement is frequently conducted additionally.

Traditionally Used Technology

Historically, extractive analyzers – frequently with indirect, differential techniques – have been applied for this measuring task. However, ammonia is hard to measure extractive because of its sticky and reactive behavior. It might be lost in the sample handling system or results in plugging of the tubes as a result of the formation of ammonia salts. Since the usual measuring range is a few [ppm], a measurement technology with higher sensitivity is required.

ABB’s Solution

ABB’s LS4000 is an in-situ cross-stack analyzer applying the very selective, optical measuring principle of tunable diode laser absorption spectroscopy (TDLAS). The device has a transmitter unit with a laser light source and a receiver unit with a photo detector, linked via a junction box which includes all interfaces such as analog outputs and power supply.

Highly Selective Measurement is Virtually Cross Interference Free

The optical measuring standard of tunable diode laser absorption spectroscopy (TDLAS) allows a direct, virtually cross-interference free measurement. As a result of the narrow spectral line width of the laser beam and the narrow scan window, just the absorption line of the target measuring component is scanned. Therefore, a high selectivity and accuracy of the measurement is realized, which accomplishes the requirements of the low ppm-level NH₃ measurement.

Fast and Direct

A sample transport and conditioning, like used in extractive systems, is not needed since the LS4000 is straightaway installed at the process (in-situ). This precludes the loss of ammonia in the sample system and minimizes the maintenance efforts for sample handling components. Furthermore, the LS4000 guarantees more representative measurement since local concentration spots (for example owing to inhomogeneity's of catalyst efficiency) can be noticed by measuring across the duct.

Safe, Compact and Easy

Furthermore, LS4000’s compact and lightweight housing makes the LS4000 unresponsive to vibrations and makes the LS4000 easier to handle. Thanks to its long term stability, maintenance efforts for calibration are reduced.

Clean coal power plant.

Diode laser analyzer LS4000.

Table 1. Typical process conditions for NH₃ slip (SCR).

This information has been sourced, reviewed and adapted from materials provided by ABB Measurement & Analytics.

For more information on this source, please visit ABB Measurement & Analytics.