| OCR Text |
Show adjusting the pullback of the gas tubes relative to the water jacket. This ability to alter flame shape is also required for the oxygen-pulverised-fuel flame and this is achieved by moving the central pulverised fuel transport tube forward or backwards. This gives the desired control over flame shape, enabling very similar flame shapes to be obtained with a wide variety of fuels. Altering the transport air pressure (and hence flow rate) enables the flame to be lengthened or shortened, giving a range of flame lengths from around 1.Sm to Sm. The system as installed can be fired at any rate over the range 60 to 600 kg per hour of coal and SO to 1100 cubic metres per hour of oxygen. In addition, when co-firing, natural gas may also be used in the range of 20 to S50 cubic metres per hour. The system typically runs in a low fire/high fire mode with preset firing rates. These rates are 250 kg per hour coal, 450 cubic metres per hour oxygen for low fire (2 Mw heat input) and 550 kg per hour coal, 875 cubic metres per hour oxygen for high fire (4.43 Mw heat input). The flame dimensions on low fire are approximately 2.Sm long by 400mm diameter whilst on high fire the dimensions change to around 3.75m by 600mm diameter. Combustion efficiency in a well mixed oxygen-fuel flame fired stoichometrically will be very close to 100% whilst an air-fuel flame can rarely be fired in a stoichometric condition. Typically in an air-fuel burner, excess air is required to achieve complete combustion and this leads to reduced overall furnace efficiency. A typical furnace efficiency measured as (total heat to product) (total heat input) for an air-fuel fired furnace would be 25%. Oxygen-fuel burners used to assist melting in a conventional fired furnace would achieve efficiencies of 60-70% whilst if used as the sole source of heat input efficiencies of 40-50% would be expected. Combustion intensity and heat flux are extremely difficult figures to quantify as they depend on the most predominant mode of heat transfer for the product, furnace pressure and actual firing rate. If radiated heat is the more predominant then, due to the higher flame temperature of the oxygen-fuel flame and the fact that heat transfer is proportional to the fourth power of temperature, an oxygen-fuel flame will be highly beneficial. If convection or conduction are more important then the reduction in waste gas volumes and, consequently, the reduction in waste gas velocities will allow the hot gases to remain in contact with the charge longer when firing with oxygen-fuel than when firing with air-fuel. The benefits will not be, however, as dramatic. Furnace pressure can rarely be maintained significantly above atmospheric pressure and due to physical constraints on waste gas handling equipment, maximum firing rates and, hence, combustion intensity can be restricted when firing with air-fuel - a restriction which is lifted when oxygen-fuel firing. Page 5 |
