With a capacity of 1400MW, the As Pontes power station operated by Endesa in the Spanish province of A Coruña is the country's largest coal-fired power station. It burns vast amounts of coal to produce steam for its turbine generators.

As with any coal-fired power station, an excess of oxygen (air) above the stoichiometric quantity is required to achieve complete combustion. Insufficient oxygen results in more carbon monoxide being produced.
However, it is more common for too much air to be supplied, causing a significant reduction in combustion efficiency due to an increased loss of heat to the atmosphere, plus there is more oxygen available to combine with nitrogen and sulphur dioxide to produce undesirable emissions nitrogen oxides and sulphur trioxide.

Varying air-fuel ratio

To further complicate matters, the optimum ir-fuel ratio continually varies as a result of changes in the loading conditions, fuel composition and particle size, aging of the equipment and other process conditions.
Endesa was previously monitoring the level of oxygen in the post-combustion gases, but this does not give as accurate a picture as measuring oxygen and combustibles together at a single point. These analysers had also suffered as a result of the high levels of dust and sulphur present.
The company therefore decided to install a trial 2700 analyser from Servomex that would monitor both oxygen and combustibles and allow continuous fine-tuning of the combustion process.
This analyser was fitted downstream of the economiser, so had to operate with a sample temperature in the region of 500°C.
To cope with the extremely dusty conditions, a special double filter was specified, and the blowback facility on the analyser was used on a regular basis to maintain its performance.

Sensor technology

Two different technologies are used within the 2700 sensor head. Oxygen is measured using a zirconium oxide sensor, giving high accuracy and a fast response; carbon monoxide levels are monitored using a thick film calorimeter that gives a precision of ±25ppm, with sufficient sensitivity and speed of response to enable the combustion process to be continuously ine-tuned.
Both of these technologies are highly reliable, non-depleting, and can operate for extended periods without requiring attention. According to Servomex, thanks to the inherent reliability and auto-calibration facility, the analyser required no servicing whatsoever throughout the one-year trial.
Endesa was initially attracted to the 2700 analyser's ability to monitor oxygen and carbon monoxide concurrently, but reliability and ost-of-ownership are also important. As a result, the company is purchasing four analysers for use in relatively inaccessible locations -- taking advantage of their ease of installation, commissioning, operation, plus minimal requirements for servicing.
Whereas the excess oxygen was previously measured at 3.8percent, using the 2700 analyser has enabled this figure to be reduced to 2.7percent, with a corresponding reduction in carbon monoxide emissions and an increase in combustion efficiency -- and, therefore, profitability.

Oxygen levels

With the oxygen levels given on a typical application of this type, a fuel saving of about 0.75percent can be expected, so users find that the improved efficiency easily pays for the cost of the instrumentation upgrade.
As with the trial analyser, these instruments are being supplied through Instrumentación Analítica, the Servomex distributor in Madrid.

For more information about dual oxygen and carbon monoxide analysers and to discuss particular applications, contact Servomex on +44 1892 652181 or -mail info@servomex.com