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Analyse this

05 March 2009

AS Pontes, an Endesa operated coal fired power station in Spain monitors post-combustion levels of oxygen and carbon monoxide, allowing it to continuously fine-tune its process.

It turned to Servomex, gas analysis specialist that recently opened a new facility at its UK home in Crowborough, to supply four Fluegas analysers.

The installation began with a year-long trial at the power station in the Spanish province of A Coruña. With a capacity of 1400MW, the As Pontes power station burns vast amounts of coal to produce steam for its turbine generators. Endesa was previously monitoring the level of oxygen in post-combustion gases, but asked Servomex to measure both oxygen and combustibles and allow continuous fine-tuning of the combustion process.

The Fluegas analyser was fitted downstream of the economiser and was required to operate with a sample temperature of around 500ºC. To cope with the extremely dusty conditions, a double filter was specified, and the blowback facility on the Fluegas was used on a regular basis to maintain the analyser’s performance.

Where the excess oxygen had previously been measured at 3.8 per cent, using the Fluegas enabled this figure to be reduced to 2.7 per cent, with a corresponding reduction in carbon monoxide emissions and an increase in combustion efficiency.

As a consequence, Endesa subsequently installed four Servomex Fluegas analysers to monitor post-combustion levels of oxygen and carbon monoxide and thereby enable the combustion process to be continuously fine-tuned. The products were supplied through Instrumentación Analítica SA, the Servomex distributor in Madrid. As they required installation in relatively inaccessible locations, the Fluegas units were accordingly optimised for easy installation and operate with an auto-calibration facility.

Get your gases right
Insufficient oxygen results in more carbon monoxide being produced unburned fuel in the flue gases and excessive particulates (smoke, measured as opacity). However as opacity is often closely regulated, it is undesirable to emit excessive smoke so plants typically err on the side of introducing excess oxygen to the process. However too much oxygen leads to cooler burning, causing a significant reduction in combustion efficiency due to an increased loss of heat to the atmosphere, while the excess of oxygen available combines with nitrogen and sulphur dioxide to produce undesirable NOx and SO3 (oxides of nitrogen and sulphur) emissions. To further complicate matters, the optimum air-fuel ratio continually varies as a result of changes in the loading conditions, fuel composition and particle size.

Traditional control systems rely on a measurement of the oxygen level in the flue gases, plus a separate measurement of combustibles and/or carbon monoxide. In most cases the oxygen and combustibles are measured at different points in the plant and therefore with separate instruments that inevitably have different response times. In practice, this means that the oxygen level may creep up until the air flow has to be reduced, but the effect of this is not seen until two or three minutes later when the combustibles analyser responds. As the oxygen level will have been reduced too far 'breakthrough' occurs - the point at which unburned fuel in the gas stream rises from insignificant levels to hundreds or thousands of parts per million for a very small change in oxygen. The air flow has to be increased again, and the cycle continues with continuous 'hunting' either side of the ideal control point.


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