j osé j aime l óPez j iménez *
2. Los procesos de integración económica en Asia-Pacífico
2.2 Integración regional en Asia-Pacífico
The calculations of the previous section can be extended to include the blast into the furnace. In doing so the output of the furnace can be calculated:
not only the hot metal and slag composition and the reductant rate, but the composition of the top gas as well. Calculation of the top gas composition is done in a stepwise manner in which the balances of the gas components (nitrogen, hydrogen, oxygen, CO and CO₂) and iron and carbon are made.
For the calculations the example of a 10,000 t/d furnace is used. The stepwise approach indicated in Table 6.2.
Input
Top gas Top gas Hot metal Top gas (85%) Hot metal
Table 6.2 Stepwise approach for a simplified mass balance The approach is as follows:
Step 1: nitrogen balance: from the nitrogen balance the total top gas volume is estimated.
Step 2: hydrogen balance: from hydrogen input and a hydrogen utilisation of 40 % the top gas hydrogen can be estimated. In practice hydrogen utilisations of 38–42
% are found.
Step 3: iron and carbon balance: the carbon use per tonne is known from the hot metal chemical composition and coke and coal use per tonne.
Step 4: oxygen balance: the burden composition gives the amount of oxygen per tonne hot metal input at the top, while also the amount of oxygen with the blast is also known per tonne hot metal.
Step 5: the balances can be combined to calculate the top gas composition.
The calculations are based on basic chemical calculations. Starting points for the calculations are, that:
– 12 kilogram of carbon is a defined number of carbon atoms defined as a kilomole.
– Every mole of an element or compound has a certain weight defined by the periodic table of the elements.
62 Chapter VI
– 1 kmole of a gas at atmospheric pressure and 0 °C occupies 22.4 m³ STP. The properties of the various components used for the calculations are indicated in Table 6.3. The present balance is used for educational purposes figures and compositions are rounded numbers. Effects of moisture in pulverised coal and the argon in the blast are neglected.
Atomic weight Molecular weight N2 28 kg/kmole CO 28 kg/kmole O2 32 kg/kmole CO2 44 kg/kmole H2 2 kg/kmole
C 12 kg/kmole
Fe 55.6 kg/kmole Si 28 kg/kmole
Table 6.3 Properties of materials used for mass balance calculations 1 kmol gas (N₂, O₂, etc) = 22.4 m³ STP
1 tonne hot metal contains 945 kg Fe= 945/55.6 = 17.0 kmole
6.4.1 The nitrogen balance
Nitrogen does not react in the blast furnace, so it escapes unchanged via the top gas. At steady state the input equals the output and the top gas volume can be calculated with a nitrogen balance given the nitrogen input and the nitrogen concentration in the top gas. The input data for a simplified model are shown in Table 6.4 and the top gas volume is calculated in Table 6.5.
Blast volume 6500 m³ STP/min
Burden Calculation and Mass Balances 63
Nitrogen from blast (1–0.256)x 6500 4836 m³ STP/min
From coal 16 m³ STP/min
From coke 23 m³ STP/min
Total input 4875 m³ STP/min
Top gas nitrogen 48.5 %
Top gas volume 10051 m³ STP/min
Table 6.5 The nitrogen balance and top gas volume
6.4.2 The hydrogen balance
Moisture in the blast and coal reacts to H₂ and CO according to:
H₂O + C Ⱥ H₂ + CO
All hydrogen in coal and coke are converted to H₂ in the furnace. In the furnace the H₂ is reacting to H₂O; part of the hydrogen is utilised again.
Since the top gas volume is known as well as the hydrogen input, the top gas hydrogen can be calculated, if a utilisation of 40% is assumed. There are ways to check the hydrogen utilisation, but it is beyond the scope of the present exercise.
Table 6.6 shows the input and calculates the top gas hydrogen.
kg/min in m³ STP/min
6.4.3 The iron and carbon balance
Hot metal contains 945 kg Fe per tonne. The balance is taken by carbon (45 kg), silicon, manganese, sulphur, phosphorous, titanium and so on. The precise Fe content of hot metal depends slightly on the thermal state of the furnace and quality of the input. For the balance we use 945 kg Fe/tHM. This amounts to 17 kmole (947/55.6).
The carbon balance is more complicated. The carbon is consumed in front of the tuyeres and is used during the direct reduction reaction (see section 8.2.1).
The carbon leaves the furnace via the top gas and with the iron. The carbon
64 Chapter VI
balance is made per tonne hot metal. Table 6.7 shows the results. The carbon via the top gas is also given in katom per tonne hot metal.
Carbon used In kg/tHM katom/tHM Carbon from coke 261
Carbon from coal 156 Total carbon use 417 Carbon via iron –45
Carbon via top gas 372 31.0
Table 6.7 The Carbon Balance
6.4.4 The oxygen balance
The oxygen balance is even more complicated. Oxygen is brought into the furnace with the blast, with PCI, with moisture and with the burden. It leaves the furnace through the top. The burden with sinter contains not only Fe₂O₃ (O/Fe ratio 1.5) but some Fe₃O₄ (O/Fe ratio 1.33) as well. The O/Fe ratio used here is 1.46, which means that for every atom of Fe there is 1.46 atom O. On a weight basis it means, that for every tonne hot metal, which contains 945 kg Fe atoms there is 397 kg O–atoms. All this oxygen leaves the furnace with the topgas. The balance is given in Table 6.8.
m³ STP /tHM kg O/tHM Katom O/tHM
Input From blast 240 342
From blast moisture 8
From coal 14
From burden 397
Total input 762
Output via top gas 762 47.6
Table 6.8 The Oxygen Balance
6.4.5 Calculation of top gas analysis
The oxygen in the top gas is leaving the furnace in three different states:
– Bound to the hydrogen. The quantity is known since we know how much hydrogen has been converted to process water.
– Bound to carbon as CO.
– Bound to carbon as CO₂.
From the combination of the carbon balance and the oxygen balance we can now derive the top gas utilisation, as shown in Table 6.9.
Burden Calculation and Mass Balances 65
Katom/
tHM Carbon via top gas 31.0 Oxygen via top gas 47.6 Oxygen bound to hydrogen –1.9 Oxygen as CO and CO2 45.7
Oxygen balance: CO+ 2x CO2 45.7 Carbon balance: CO + CO2 31.0
CO2 14.7
CO 16.3
Utilisation CO2/(CO+CO2) 47.3 %
CO2 volume 2283 m³ STP/min CO2% 22.7 %
CO 2539 m³ STP/min CO % 25.3 %
Table 6.9 Calculation of Top Gas Utilisation
The calculations can be used to check the correct input data. More advanced models are available, which take into account the heat balance of the chemical reactions as well (e.g. Rist and Meysson, 1966). The models are useful for analysis, especially questions like “are we producing efficiently?” and for prediction: what if PCI is increased? hot blast temperature is increased? and so on.