OCR Text |
Show -11- kilns (7) and is now commercially available. The lowest NOx emissions obtained are similar to those achieved by high volatile coal. This new burner utilizes a precessing jet to produce enhanced mixing. It is currently available as a gas only burner and would require complete replacement of the existing burner whilst gas firing. There is the potential for future dual fuel firing but at present this is unproven. 3.5 Effect of Product on HOI Emissions Rotary kilns are used to produce a wide range of products and hence operate over a wide range of temperatures and combustion conditions, from as little as 600°C (lll0oF) to over 1800 0 C (3270 0 F) and from highly oxidising (cement) to reducing conditions (synthetic rutile). The product therefore has a considerable influence on secondary air temperature, flame temperature, fuel heating rates and oxygen concentrations in the flame, and therefore in turn affects both the fuel NOx and thermal NOx formation. This further complicates the prediction of NOx emissions from rotary kilns, compared with boilers. Generally, NOx emissions will increase as both flame temperature and oxygen concentrations in the flame increase. The cement clinkering reaction takes place at approximately 14000 C (2550 0 F) and for the correct cystalline structure, requires oxidising conditions. The highest flame temperatures and in-flame oxygen concentrations therefore occur with large cement kilns, and these conditions were effectively simulated during the CEMFLAM trials. 3.6 Effect of Low HOI Burners on Other Emissions Low NOx kiln burners control flame temperature and oxygen/nitrogen availability in the flame by reducing mixing rates. As a consequence, emissions which are the result of poor fuel/air mixing namely CO and hydrocarbons tend to increase. For example, the burner firing medium volatile coal during the CEMFLAM tests typically produced 400-2000 ppm CO and total unburnts of 2-10% mainly in the form of carbon char. It should be noted however, that this carbon char figure would not be representative of a full size plant, since the residence times for the particles in the CEMFLAM test furnace were only in the order of one tenth of those in a full size kiln, and the particle burnout is largely time |