Integración multilateral en Asia-Pacífico

5.4.1 Oxygen and PCI

At high Pulverised Coal Injection operation about 40% of the reductant is injected via the tuyeres. Therefore, it is important to control the amount of coal per tonne hot metal as accurate as the coke rate is controlled. The feed tanks of the coal injection are weighed continuously and the flow rate of the coal is controlled. It can be done with nitrogen pressure in the feed tanks or a screw or rotating valve dosing system. In order to calculate a proper flow rate of coal (in kg/minute) the hot metal production has to be known. There are several ways to calculate the production. The production rate can be derived from the amount of material charged into the furnace. Short–term corrections can be made by calculating the oxygen consumption per tonne hot metal from the blast

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parameters in a stable period and then calculating the actual production from blast data. Systematic errors and/or the requirement for extra coal can be put in the control model.

The heat requirement of the lower furnace is a special topic when using PCI.

Coal is not only used for producing the reduction gases, but use of coal has an effect on the heat balance in the lower furnace. The heat of the bosh gas has to be sufficient to melt the burden: define the “melting heat” as the heat needed to melt the burden. The heat requirement of the burden is determined by the

“pre–reduction degree”, or how much oxygen has still to be removed from the burden when melting. The removal of this oxygen requires a lot of energy. The

“melting capacity” of the gas is defined as the heat available with the bosh gas at a temperature over 1500 °C. The melting capacity of the gas depends on:

– The quantity of tuyere gas available per tonne hot metal. Especially when using high volatile coal there is a high amount of H₂ in the bosh gas.

– The flame temperature in the raceway.

The flame temperature in itself is determined by coal rate, coal type, blast temperature, blast moisture and oxygen enrichment.

From the above, the oxygen percentage in the blast can be used to balance the heat requirements of the upper and lower furnace. The balance is dependent on the local situation. It depends e.g. on burden and coke quality and coal type used. For the balance there are some technical and technological limitations, which are presented as an example in Figure 5.5. For higher injection rates more oxygen is required. The limitations are given by:

– Too low top gas temperature. If top gas temperature becomes too low it takes too long for the burden to dry and the effective height of the blast furnace shortens.

– Too high flame temperature. If flame temperature becomes too high burden descent can become erratic.

– Too low flame temperature. Low flame temperature will hamper coal gasification and melting of the ore burden.

– Technical limitations to the allowed or available oxygen enrichment.

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Figure 5.5 Limiting factors affecting raceway conditions with Pulverised Coal Injection (RAFT = Raceway Adiabatic Flame Temperature)

The higher the oxygen injection, the higher the productivity of the furnace as shown in Figure 5.5, which is an example based on mass and heat balance of an operating furnace. The highest productivity is reached, with an oxygen level, so that the top gas temperature is at the minimum. The minimum is the level, where all all water of coke, burden and process is eliminated from the furnace, i.e. slightly above 100 °C. From a technological perspective it can be said, that the heat balances over the lower part of the furnace (i.e. from 900 °C to tuyere level) and over the upper part of the furnace (i.e. from top to the 900 °C isotherm) are in balance (Section 8.5).

5.4.2 Effect of additional PCI

The effect of the use of extra coal injection for recovery of a cooling furnace is twofold. By putting extra coal on the furnace the production rate decreases.

Simultaneously, the flame temperature drops. If the chilling furnace has insufficient melting capacity of the gas, extra PCI may worsen the situation. In such a situation the efficiency of the process must be improved, i.e. by a lower production rate and lower blast volume.

This is illustrated in Table 5.1. The table shows that additional coal injection slows down the production rate, because the coke burning rate decreases. It is a typical example; the precise effect depends on coke rate and coal type used.

A furnace recovers from a cold condition by increasing PCI, because it slows down the production rate. If, however, the flame temperature is relatively low, the effect of the drop in flame temperature can be as large as the effect of the decreased production rate.

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Starting Situation Operating parameters

Coke rate 300 kg/tHM

Coal injection rate 200 kg/tHM

Replacement ratio 0.85 kg coal/kg coke

Flame temperature 2,200 °C

Coke and coal consumption in normal operation (as kg standard coke/tHM)

Coke introduced 300

Coal introduced 170

Total coke and coal 470

Consumption to be subtracted to determine burn rates:

Carbon in hot metal –50

Direct reduction –120

Result: total burn rate in front of tuyeres 300

of which coal 170

and thus coke 130

Changed situation if an additional 10 kg/tHM of coal is injected

Total burn rate remains 300

of which coal 178.5

and thus coke 121.5

Production rate decrease (fully determined by coke burn rate) 6.5%

Flame temperature drop 32 °C

Gas melting capacity drop (heat > 1,500 °C) 4.6%

Table 5.1 Effect of additional coal injection

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