AbstractNitrogen Oxides (NOx) are known to be among the pollutants resulting from fossil fuel combustion with particularly harmful effects on the environment and human health. Increased concern for the effects of NOx and other pollutants has prompted the European Community (EC) to draw up legislation on the limitation of emissions with which industrial plants have to comply.
In the context of this requirement the subject of this work is the investigation of NOx reduction and control methodologies at Aberthaw "B" Power Station (unit 7), a coal-fired 500 MWe power plant owned by National Power PLC. Since retrofitting the latest NOx control technology is not a viable option at Aberthaw, the specific objective of this project was to optimise the combustion process with respect to NOx emissions using existing plant and control systems. In order to meet this objective, applicable NOx control parameters had to be identified and a programme of plant tests had to be designed, carried out and evaluated. To complement these plant tests, a physical 3D model was designed and applied to analyse the prevailing flow patterns under some of the test conditions investigated with the real process.
Although it was not possible to relate the experiences gained from the modeling work as closely as it was envisaged with those gained from the plant experiments, the modeling was effective in showing two characteristic flow patterns (symmetric and asymmetric flow), which could be distinguished for all experimental conditions that were investigated. The potential low NOx operating conditions were identified as: lower boiler load, reduced excess air, air staging (through damper settings), burner biasing, burners out of service (BOOS) in the furnace middle chamber, BOOS across the furnace chambers, and a combination of these techniques. Additionally, the application of oil burners in service as a NOx control methodology (which cannot be considered at Aberthaw as the known 'reburning' effect) was discovered as an interesting NOx control methodology. Amongst the investigated techniques, only the effect of lower boiler load and burner biasing could not be confirmed in the experiments. All other operating conditions showed to different extents reducing effects on the NOx emissions. The optimal NOx control methodology turned out to be a combination of oil burners in service with the reduction of excess air and air staging, yielding minimum NOx levels between 385 and 450 ppm (at 6% 0 2, dry). The particular value of this novel technique is further stressed by the low carbon in ash and CO levels, which it ensures. Based on the experiences, a concise set of operational guidelines for the power plant operators at Aberthaw was summarised in order to facilitate a permanent low NOx operation (according to the best of current knowledge), which does not affect the boiler efficiency. Further refinements of the operational conditions might, however, be possible on the basis of the extended research programme proposed at the end of this work.
|Date of Award||Apr 1997|