Can Anybody tell about measuring RAFT (Raceway adiabatic Flame Temperature) in Blast Furnace ?
RAFT is calculated instead of measured. The measured temperature is raceway temperature, i.e. coal/coke combustion temperature with blast.
the method to calculate RAFT could be found in ironmaking text book or papers in web.
RAFT has two specific temperature zones: the average temperature typical for coke gases and coke in circulation in front of the tuyere and the maximum temperature that is a stationary zone at approx. 0.3 - 0.5 m from the front of the tuyere.
The average RAFT can be measured with an infrared thermal camera mounted in the sight of the air nozzle and seeing the spotlight from the wind. The camera must be calibrated correctly because the measured temperature is the effect of the radiation of gases and incandescent coke. This may be between 1450 - 1700 o C. Its continuous measurement can provide information about the thermal status of the blast furnace and signal the occurrence of some process dysfunctions by decreasing this temperature.
The maximum RAFT in the oxidising zone can be measured with a cooled lance inserted through the blow pipe elbow and through the tuyere . Some data: Water velocity in the probe> 10 m / s, thermocouple W – Re reinforced in a Mo pipe through which a small inert gas flow (Ar) the thermocouple distance from the cooled lance is 50 - 70 mm, the diameter of the probe so that its surface is less than 15% of the minimum section of the free cross section of the tuyere, the lance body is made of the Cu pipe and, if possible, a ceramic coating in the plasma jet as the thermal protection is very useful, the water circulation system in the lance is pipe in pipe system , or a longitudinal diaphragm in which the water comes to the top of the probe on the opposite side and returns to the other half of the section, can also be fitted a gas sampling socket with 4 mm copper pipe with walls of 1 mm. Maximum measurement time 7 - 15 min.
It is not possible to measure the blast furnace in low function. The furnace should have an index of use > 1,7 t fo. / m3 x day, or the velocity a hot air blast minimum 160 – 170 m/sec.
The lance must have connections to: a temperature reading device in the thermocouple signal, a CO2 / CO / H2 gas analyzer, a sample filter for the gasses to be passed to the analyzer, a rotameter to indicate and maintain a very low flow rate Ar for the thermocouple protection gas.
Very important :
• For each application this lance must be specially designed for the specific conditions of the furnace to be measured;
• Attention to handling and working safety;
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• Attention to the furnace operating regime: an eventual failure requires stopping the blast furnace. It is recommended to carry out the measurements before a scheduled stop and only after the blast furnace is discharge. For more details please send me your e-mail address.
@ Mircea Hritac Thanks for your in detailed answer. it will be very helpful in my findings
In modern blast furnace practice simple blast alone is not blown through the tuyeres but blast is accompanied by other injectants like steam, oxygen, auxiliary fuels, and fluxes and iron ore/Mn ore fines.
In order to know the effect of these variables on coke rate and productivity, the term RAFT (raceway adiabatic flame temperature) should be understand. The RAFT is the theoretically estimated temperature in raceway zone in front of the tuyeres, assuming that there is no heat loss. The RAFT is controlled by blast parameters like 1) its temperature, 2) oxygen content, 3) humidity content, and 4) fuel injectant rate. First two parameters increase the value of RAFT, and last two parameters decreases the value of RAFT.
The calculation of RAFT is a very useful tool for furnace control. It is known that all inputs through the tuyeres influence the RAFT. The moisture of the air blast causes reaction with carbon: C (s) + H2O (g) = H2 (g) + CO (g), ΔH0298 = 131.4 kJ/mol; which is endothermic, and thus lower RAFT. On the other hand, oxygen enrichment of the air blast increases RAFT, since the volume of gas is the flame is less because of less nitrogen. RAFT should be maintained in between 1900-2100 C.
The heat content (i.e. sensible heat) of the flame, its temperature can be calculated with the help of RAFT.
Heat content of flame = Mass of gas in the flame – specific heat of gas x (RAFT – 298).
Dear S K Dutta Sir,
What will be the effect, if the RAFT temperature is more than 2200 deg C or more ?
That will effect of the coke rate and productivity in general and in particular effect on refractory lining.
In addition to Above, [Si] decreases as the Flame temperature increases because the reduction rate from SiO to [Si] decreases
Dear Mr. Mahesh
I feel higher flame temperature, leads to increase in SiO gas formation. In that case, it will increase in [Si] content in hot metal.
If not, please explain it how reduction rate from SiO to [Si] decreases at higher flame temperture ?
Thank you
Dear Venkateshan
the above said phenomena of lowering reaction rate of SiO to [Si] can be achieved by increasing O/C ratio or thermal flow rate thus lowering the melting down level. This could be done by raising the blast temperature so Reduction of fuel and [Si] can be achieved.
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