OCR Text |
Show Finally, heat balances were undertaken for the kiln using oil and gas, table 5. This shows an increase in fuel consumption of less than 2% compared with oil firing, however, the feed also contained more moisture so this contributed to much of the increase in energy consumption. The kiln did not suffer from any loss of production as a resul t of 10 fan limitations. Burner Fuel Specific Fuel Kiln lIe~t Oistribution Consumption (million Btu/ ston) Drying Feed Calcination Exit Gases Product Shpll Loc;s~ <,; MMBtu/h , MMAtll/h \ MMBtu/h ~ MMAtll/h OJ; MMntll /h ~. Original Oil 3.29 16.04 9 83.94 46 38.56 21 22.18 12 21.04 12 FCT Gas 3.35 19.45 11 79.84 45 35.83 20 20.13 1'2 20.47 12 Table 5 Summary of Kiln Heat Balance Overall, the conversion was successful, no loss of production occurred, and the increase in fuel consumption was minimoized and should be reduced as operators gain experience. 3.3 Design of a Flash Ca1ciner Many of the products calcined in rotary kilns can also be processed in flash calciners, which are extensively used in the cement and alumina industry. Flash calcination is not new, having been invented by Martin (18) in the 1920's, but it was not until the 1960's onwards that the technology saw widespread application. Flash calciners, figure 19, offer greatly reduced NOx emissions compared wi th rotary kilns, because the combustion takes place a t much lower temperatures as a resul t of superior hea t transfer. The sys tern also offers potentially lower energy consumption because the external shell losses are much lower than for rotary kilns owing to the smaller surface area and the ability to install thicker refractory, with better insulation characteristics. Unfortunately, this potential for an improved fuel consumption has often failed to materialise owing to an extremely poor combustion performance, with large amounts of unburnt fuel emitted to atmosphere in the form of carbon monoxide and even unburnt hydrocarbons. The authors have witnessed as much as 3-5% carbon monoxide which negates any potential fuel saving and greatly reduces the environmental benefits which accrue from the lower NOx emissions. The principle cause of the inadequate combustion performance has been a poor understanding of the interaction between the calciner aerodynamics, the fuel injection and burn out, and the dust loading and residence time. FCT has undertaken modelling projects to assist with the design of flash calciners for several companies. Adelaide Brighton Cement decided to upgrade their existing energy efficient preheater kiln rated at 1985 ston/day to 4520 ston/day by the installation of a precalciner (flash calciner), new product cooler and other plant upgrading, figure 20 (22). Installation of the precalciner would allow an increase in the proportion of cement produced by the energy efficient dry process and reduce the NOx emissions of the dry process |