Nov-2019

Laser based analysis for thermal incinerators

An analyser design based on a tunable diode laser aims for more reliable and cost-effective combustion measurement in sulphur removal operations

STEPHEN FIRTH
Servomex

Article Summary

Sulphur removal facilities are located at the majority of oil and gas processing sites throughout the world. The units may use a natural draft or forced draft design, with oxidising temperatures varying between 650°C and 1000°C (1200-1800°F), and a residence time of 0.6-1.0 seconds.

Combustion control is important for process efficiency, emissions control, and safety. This requires the optimisation of the air-to-fuel ratio. Conditions of excess air lead to cooler burning, which reduces combustion efficiency significantly, due to an increased loss of heat to the atmosphere, and increases costly fuel consumption. Also, the excess oxygen available will combine with nitrogen and sulphur to produce unwanted emissions.

However, low-oxygen, fuel-rich conditions have the potential to cause a dangerous explosive mixture, so it is necessary to find an effective balance where the process is as efficient as possible while remaining safe.

Accurately measuring oxygen is essential to combustion efficiency, and carbon monoxide (CO) measurement has increased in importance with the recognition that fuel-rich (high CO) conditions are a source of explosions.

However, the presence of high levels of sulphur compounds in the gas stream makes sulphur recovery unit (SRU) combustion applications highly corrosive. This presents serious challenges for any gas analysers used for combustion control and efficiency.

In the Middle East region, existing analyser installations tend to be either zirconia or extractive solutions to monitor combustion control. While these have been very successful, the zirconia sensor can be attacked by sulphur compounds, while sample tubes can be clogged by the corrosive effects of sulphuric acid (see Figure 1).

Zirconia sensors are constructed from ceramic zirconium oxide, stabilised with an oxide of yttrium or calcium to form a lattice structure. This cell has a conductive coating which serves as electrodes on both sides of the lattice.

At process temperatures above 700°C (1292°F), the openings in the lattice allow O2 ions to pass through. When a sample gas is introduced on one side of the lattice, the rate of passage of the O2 ions is determined by temperature and the difference in the O2 partial pressures of the sample gas and the reference gas (usually air) on the other side of the lattice.

The passage of O2 ions through the lattice produces a voltage across the electrodes, the magnitude of which is a logarithmic function of the ratio of the O2 partial pressures of the sample and reference gas. Since the partial pressure of the reference gas is predetermined, the voltage produced by the cell indicates the oxygen content of the sample gas.

Zirconia based oxygen analysers exhibit excellent response characteristics to changes in O2 content at ppm levels, and the same sensor can be used to measure 100% O2.

However, constant exposure to sulphur compounds will, over time, affect the measurements delivered by a zirconia cell.

The corrosive conditions mean that both zirconia and extractive solutions require high levels of maintenance and frequent recalibrations, leading many SRU operators to look for an alternative that delivers the same accuracy while requiring less support and upkeep.

A tunable diode laser approach
Following successful field trials, several customers in the Middle East have switched to a tunable diode laser (TDL) arrangement for all their SRU thermal oxidiser applications, specifying the Servotough Laser 3 Plus combustion analyser optimised for O2.

This is a compact gas monitor that can be installed in situ, measuring across the stack, using TDL absorption spectroscopy for strong performance.

TDL is a single-line, monochromatic spectroscopy technique that is highly specific to the gas being measured, producing a stable, continuous and rapid measurement while avoiding cross-interference from other gases.

A typical system consists of a TDL light source, transmitting optics, an optically accessible medium, receiving optics and detector (see Figure 2). Signal information is held in the gas absorption line shape, which is obtained by scanning the laser wavelength over the specific absorption line.

This causes a reduction of the measured signal intensity, which is detected by a photodiode and used to determine the gas concentration.

Servomex TDL technology was developed by the company’s research and development team. It uses a second harmonic detection (2f) modulation technique that enables greater accuracy, sensitivity, and reliability of measurement, particularly in low parts-per-million measurements where gas molecule absorption lines tend to be close to other possible interfering components.

The key benefit of using TDL in the SRU application is that the laser has no contact with the sample, so there is no corrosion of the sensor, which significantly reduces maintenance and eliminates the need for cell replacement.

The high stability of laser measurement means that calibration is required less frequently than with a zirconia cell. These significant, cost-saving advantages have led to the supply of Servomex’s TDL systems throughout the Middle East, the US, Singapore, and Europe.

Greater stability
In the Laser 3 Plus, TDL sensing is supported by a line lock cuvette system, which ensures the analyser remains fixed on the target gas.

TDL analysers from the older generation of instruments were set to measure a specific gas, in the same way as newer models. However, if none of the given gas was present, the analyser would drift to measure an incorrect, nearby absorption line instead.

For example, an analyser set up to measure CO might begin to measure an adjacent water line instead, delivering an incorrect reading. To solve this problem, the Laser 3 Plus uses a line lock system comprised of a beam splitter, cuvette filled with the target gas, and a secondary detector.

The filled cuvette means that the secondary detector always has a known target gas to sense, and can   lock onto the centre of the absorption peak. This, in turn, keeps the main detector locked into position, delivering an accurate measurement of the gas in the process, even if the measurement falls to zero.