Injection of pulverized coal (PC) into a blast furnace is the most popular way of lowering the coke rate and thus – of the most economical hot metal production.

However, the drawback of the PC injection is deterioration of the gas permeability of the burden, due to which a redistribution of process gas flows takes place and the chance of the furnace wall work increases.
As a result, the zone of maximum heat loads impact along the furnace height is shifted to the bosh and belly zone.

The problem of tuyeres’ life time extension is directly connected with the thermal load increase in the conditions of the PC injection and with an inefficient heat dissipation system.

Experts of “M Technology” simulated various PC injection modes (up to 170 kg per ton of hot metal).

In the course of the modeling exercise the feed of pulverized coal is simulated, which is mixed with the hot blast and burnt in the raceway.

The following aspects have been determined: maximum thermal loads on the tuyere body, surface temperature distribution and gradients in the tuyere wall metal. Besides, the nature of the cooling water flow and its temperature have been studied.

Fig. 1 Thermal load on the tuyere body (perceived heat flux)

Fig. 2 Temperature distribution over the water surface in the tuyere body (view 1)

Fig. 3. Temperature distribution over the water surface in the tuyere body (view 2)

Fig. 4. Speed distribution over the water surface in the tuyere body (view 1)

Fig. 5. Speed distribution over the water surface in the tuyere body (view 2)

Fig. 6. Water temperature distribution in the tuyere body (longitudinal cross section in XY plane)

Fig. 7. Speed distribution, water flow lines.

Findings and recommendations

One of the methods to increase the air tuyere life time is to switch them to chemically treated water cooling in combination with an increase of the heat dissipation efficiency by increasing pressure and, accordingly, the speed of cooling water in the tuyere channels. Changing to chemically treated water will increase the cooling water enthalpy drop without a risk of scale formation on the inner surface of the tuyere. These measures increase the maximum heat flux from the tuyere metal surface which can be removed without the cooling mode violation.

Besides, according to safety requirements, water pressure in the tuyere must be higher than the furnace gas pressure, in order to avoid accidents, caused by the tuyere stock breaking during tuyere burnouts.

• Calculations of the simple design tuyere thermal state (single-loop type), carried out by specialists of «M TECHNOLOGY» with various pulverized coal quantities lead to an important insight:
Uneven cooling water flow around the tuyre inner surface, vortexes and stagnant pockets lead to an uneven perceived heat flux from the raceway side.
• During normal furnace run, when the designed fractional composition of the burden is preserved and the furnace is running without stock hangings, this tuyere design can withstand the heat load applied including that with the PC injection.
• At the same time, uneven cooling of the tuyere body caused by structural imperfections significantly reduces the tuyere’s resistance to the impact of molten hot metal or slag drops on its surface. Most affected are the tuyere parts in the zone of vortex formation and low cooling fluid velocity. When a drop of molten hot metal or slag gets onto the tuyere surface in the zone of a low cooling water speed a sudden local heating of the spot up to temperatures of about 1200 ° C occurs due to high thermal conductivity of copper. The low cooling water speed in this zone results in sudden film boiling, formation of a steam blanket and a sharp decrease of the perceived heat flux. Combination of these circumstances leads to an almost instant burnout of the tuyere body in this place.

Based on the above mentioned information, the following recommendations can be made to improve the tuyere design and operation reliability:

1. Optimization (modernization) of the tuyere design in order to achieve max. even cooling water speed. Changing to a sequential cooling scheme along the tuyere body (nose – tuyere body). An ideal option is to have separate cooling circuits for the tuyere nose and the body.
2. Cooling water flow rate increase. This option will minimize the risk of burnouts from falling HM drops, film boiling occurrences and a sharp decrease of heat takeoff in this area. This solution will not completely solve the problem of tuyere burnouts, but significantly increase their resistance.