What are the various factors which Top gas temperature (TGT) in a BF depends upon ? What are the disadvantages of running a BF at very low TGT ? ? What is the optimum range of TGT for a smooth running of a BF ?
The TGT primarily depends on the thermal flow ratio in the shaft, i.e., the ratio between the heat capacity of solids and gas, m_s*c_s/(m_g*c_g). If this increases, TGT decreases. TGT is also affected by the initial burden temperature and moisture, as any moisture will be driven off in the top layers. In colder climate, where the burden may have a temperature below 0"C if taken from the yard, a rule-of-thumb is not to go (much) below 100"C as H2O in the gas may then condensate on the burden in the top layers. In the worst case this may lead to a buildup of water, which shortens the effective height of the BF, delaying the preheating of the burden. The thermal flow ratio strongly depends on the level of oxygen enrichment of the blast. If the O2 level is high, the N2 level is correspondingly low, so there is less gas that can give off heat to the solids. Usually, a BF may have a maximum thermal flow ratio (maybe 0.85?) above which the operation becomes unstable or at least difficult to master.
Sir, Very well explained the concept of TFR by you. Instead of TGT, a TFR is a better representation of the furnace stability.
According to the experience of operation of BF’s of Ukrainian Iron and Steel Works, the TGT has a direct dependence on the sinter temperature (sinter charging is practiced with a temperature of more than 400 °C), and also has an inverse dependence on the raceway adiabatic flame temperature (thermal regime tuyere zones). The optimum temperature regime is determined mainly by its rational distribution over the radii and circumferences of the charge surface.
Although TFR seems to be a better index for monitoring of a BF stability, but the usefulness of TGT can't be under-estimated as TGT has to be above a safe limit for smooth flow of Top Gas to Gas Cleaning Plant and for maintaining healthiness of uptakes, downcomers and Dust catcher/Axial Cyclone.
May please comment.
Moreover, i would like if someone can explain the concept of TFR in bit detail and its optimum value for stable operation of BF.
“Thermal flow ratio” is a measure of the degree of heat exchange during the counter-current gas-solid movement in BF and is defined as the ratio of heat capacity of descending burden to the heat capacity of ascending gases. The concept of thermal flow ratio is pioneered by Kitaev et al. in late 1960s for the heat-transfer in counter-current reactor.
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