No abstract

Reduction of both atomizing steam and particulate emissions were investigated in a 350 MWe utility boiler. A residual fuel oil was dispersed as a fine mist into the furnace with sixteen atomizers of internal turbulent chamber type. The existing atomizers were replaced by Y-jet type atomizers. To do this, full scale prototypes were designed and tested in a cold model rig using mineral oil as the fuel and compressed air as the atomizing medium. The oil droplet size distribution was measured from a single port of each prototype by using a Malvern particle sizer. The prototype to be tested in the power station was selected based on the smallest oil droplets produced along with lower compressed air consumption. In the power station, the burners were modified to install the new design of Y-jet atomizers. Field tests were conducted at 50, 75 and 100% load. Atomizing steam was measured, as well as particulate emissions and the furnace exist flue gas temperature. With the Y-jet atomizers, the atomizing steam was reduced 55% with respect to the original atomizers; the unburned carbon particles were reduced by a maximum of 50%, the furnace exit gas temperature was similar between the two type of atomizers and no side effects were observed in the boiler.

Two older boilers were burning low grade heavy fuel oil (number 6) and emitting large amounts of unburned carbon particles. Owing to the short life remaining of the units and economic constrains, it was not possible to change to a better fuel or install new burners. To contribute to the solution of this problem, an experimental program was carried out by emulsifying water in the fuel oil. Tests were performed in a scale furnace (0.35MWth) and the emulsions that produced the best results were assessed in the two boilers, 28 and 34 MWe capacity with Y-jet atomizer type. The system to prepare the emulsion was very simple: water was added into the oil before the fuel oil pump, no chemical products were added and a static mixed was used to improve the water size distribution, which 90% ranged from 1 to 9 micron. In the pilot furnace the emulsions were prepared with 5 and 10% water and atomized with compressed air. Particle reductions of 43 and 67% were obtained compared with the net heavy fuel oil. In the boilers, the emulsions were prepared with the same amount of water, and were atomized with saturated steam. In the 28 MWe boiler, a similar particle reduction was obtained to that of the scale furnace. However, in the 34 MWe boiler there was no particle abatement. By using a commercial fluid dynamic computer code, it was found that the combustion air transferred heat to the steam raising its temperature. Thus, in the mixing chamber of the Y-jet atomizers, the steam was superheated and destroyed the water droplets of the emulsion. Compressed air and saturated steam as atomizing medium of the emulsions had similar effect on the unburned particle reduction. However, the effectiveness of the emulsions may be affected by the steam. Care should be taken to avoid the use of steam with a temperature higher than the saturated water temperature.

A study was carried out to find out the cause of premature plugging of air heaters of a 350 MWe oil fired boiler. The unit burnt a heavy fuel oil number 6, with both high levels of sulfur (3.75%) and asphaltenes (16.2%), as well as high viscosity (555 SSF at 50°C) and API gravity of 11.2. Particle concentration at the furnace exit and at the stack were measured, also flue gas analyses were performed at the same sites. In the furnace were employed water cooled probes of six meters in length which allowed traversing 70% of its width. In addition, the oil droplet size distribution from an atomizer was measured with a Malver Particle Sizer. Cold condition using simulating fluids were taken in this analysis. Also, the unburned carbon particles size distribution, both from the furnace exit and from the stack, was performed with a particle Malver Sizer. The atomizer produced large oil drops, 5.7% by volume larger than 300 micron size, which were considered as promoters of unburned carbon. The concentration of carbon particles in the stack was 60% of that of the furnace exit. Furthermore, the particles from the stack were of smaller size (95% <150 μm) than those of the furnace (89% <150 μm). Deposition of carbon particles in the internal component of the boiler, mainly in the air heaters, was the cause of this finding. To solve the premature plugging of the air heaters of this oil fired boiler, the atomizers should be modified to reduce at a minimum level the oil drops larger than 200 micron size.

The role of different levels of nitrogen in natural gas on the emissions of NOx was investigated in a furnace. Nitrogen was added to the natural gas in concentrations of 4, 7 and 11% by volume. In addition, a trial was carried out as a baseline with 0.17% N2. The thermal load of the furnace was 90 kW and was kept constant in all four trials. Flue gas temperatures were measured with a small suction pyrometer, as well as incident heat flux with an ellipsoidal radiometer. In addition, flue gas composition was conducted at the stack of the experimental furnace. NOx emissions decreased as nitrogen was added to natural gas. This was due to the reduction of flue gas temperature near the burner zone where the thermal mechanism can play a significant role. With 11% nitrogen in natural gas, the flue gas temperature and the incident heat flux near the burner were the lowest, but the highest at the furnace exit. Therefore, care should be taken to avoid damages in gas turbines or furnaces while burning natural gas with high nitrogen content.