June 2015 | AWE InternationalAWE International | June 201546 47

Case Study

Tuneable diode laser spectrometry (TDLS) from Axetris

Since the 1970s, legislators have regularly passed more and

more stringent environmental laws requiring emissions control

technologies to reduce the impact of emission sources on air

pollution and global warming.

Continuous emission control and monitoringHistorically, the initial focus was on filtering the exhaust air from

emission sources, which was followed by a focus on analysis of

emission components pre- and post-filtering. The instruments used to

control processes in a closed loop could not be used to quantify the

quality of the filtering processes. Continuous emission monitoring

systems (CEMS) are used as the main means to comply with air

emission standards set by the governmental environment authorities.

Facilities like coal or gas power plants or incinerators employ the use

of CEMS to continuously collect, record and report the required

emissions data.

Regulations and standardsThe European Community has published several directives to

harmonise the legislation on industrial emissions, in order to achieve

a high level of protection for the environment and human health. These

include the Large Combustion Plant Directive (LCPD, 2001/80/EC),

Industrial Emissions Directive (IPPC, 2010/75/EU) and the Waste

Incineration Directive (2000/76/EC). Comparable efforts are being

undertaken all over the world. While the intensity of such efforts in the

USA is similar to that in Europe, emerging countries such as China

have started to reduce their emission footprint with greater efforts over

the past years.

“Axetris has chosen the new approach to TDLS and successfully developed a pre-calibrated OEM module carrying out a hot-wet measurement”

The European Standard EN14181 describes the quality assurance

procedures for CEMS in meeting the uncertainty requirements on

measured values given by legislation. EN14181 had far-reaching

consequences for regulators, equipment manufacturers, test houses

and process operators. Three different quality assurance levels (QALs)

are described: QAL1 for verifying the suitability of a system to its

measuring task, QAL2 for the validation of the system following the

installation in a specific plant and QAL3 for verification of control

while operating.

Selective catalytic reductionSelective catalytic reduction (SCR) is a method of converting nitrogen

oxides (NOx) with the aid of a catalyst into nitrogen (N2) and water. The

exhaust gas is mixed with a reducing agent, typically ammonia (NH3)

or urea. One of the major challenges for SCR control is the

determination of the correct NH3 or urea flow under all process

conditions. In general, this flow is controlled with a NOx measurement

instrument. When ammonia is over-injected into the gas stream,

however, high concentrations of ammonia emission (slip) can escape,

which can also lead to degradation of the catalyst. For this reason,

it’s becoming mandatory to continuously measure the ammonia

concentration, maintaining the slip at levels below a few mg/m3.

A Cleaner Environment

Extractive TDLSThe measuring principle can be divided into two major categories:

extractive and in-situ. The advantage of the latter is that the

measurement takes place in the main gas flow. There are several

disadvantages, however, such as the difficulties to keep the windows

clean in an unfiltered air, calibration and the influence of ambient

conditions. Depending on the position on the stack, the maintenance

of such instruments can be hard going.

“the LGD F200-H NH3 module can be used for a certification range of 0 - 10 mg/m3 NH3 and 0 - 30% volume absolute humidity”

Many extractive CEMS capture a sample from the stack, condition the

sample by removing impurities and water, and transport the sample to

an analyser mounted typically in a control room. For components like

ammonia NH3 or HCl that solve easily in water this is not the correct

solution, as an unknown big part of the measure will be washed out

before reaching the instrument.

A much better solution is the use of a system including only heated

components in contact with the air: filter, pipes, pumps and the

instrument itself. The main technologies used in this set-up are fourier

transform infrared (FTIR) spectroscopy, nondispersive infrared (NDIR)

and tunable diode laser absorption spectroscopy (TDLAS). FTIR

interprets a wide IR spectrum and so can detect several gases, but it

is difficult to calibrate, to reduce the cross sensitivities and last but not

least, this technology is rather complicated and expensive.

With the advantages in laser design in the last decade it became possible

to design TDLAS instruments in the near infrared range at reasonably

low costs. These instruments measure one single rotational-vibrational

line somewhere in the range of 700 to 2,500nm. Due to the use of 2f

WMS method and referencing to the detected power level, these

instruments are widely accepted as very stable, calibration free and

without cross sensitivities to other air pollution components.

Axetris has chosen the new approach to TDLS and successfully

developed a pre-calibrated OEM module carrying out a hot-wet

measurement, e.g. ammonia and humidity at 190°C. The technology

increases performance, reliability and cost effectiveness - or simply

enables new opportunities for OEM customers. The LGD F200-H NH3

module can be used for a certification range of 0 - 10 mg/m3 NH3

(lower range) and 0 - 30% volume absolute humidity. n

For further information visit www.axetris.com or contact axetris@axetris.com

Case Study

The Axetris LGD F200 is a fully calibrated TDLS module built for OEM integration, and currently available for gases such as NH3, HCl, CH4, CO2 and H2O

Laboratory lack of fit (linearity) test of two CEMS systems based on the Axetris LGD F200-H NH3Field test comparison with a reference FTIR system (daily average)