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Blast Furnace Productivity and the Influencing Parameters

• satyendra
• June 23, 2013
• 9 Comments
• BF coke, BF productivity, blast furnace, Burden distribution, CRI, CSR, Hot metal, RAFT, sinter,

Blast Furnace Productivity and the Influencing Parameters

Blast furnace (BF) ironmaking is the most viable means of producing hot metal (HM) mainly because of its well established and proven performance, flexible raw material usage, and high thermal energy conservation capability. It is the most dependable process of iron making. There are no definitive dates available for the inception of BF ironmaking. However, important process designs and re-engineering began to be implemented in the ironmaking furnaces in Europe as far back as the 14th century. Since then continuous developments are taking place in BF ironmaking technology to make it more productive and economical.

BF has undergone tremendous modifications and development to increase production and improve the overall efficiency. Both technological development and scientific research have driven the BF ironmaking technology to reach optimum operation conditions. The technology has become more matured and the BF ironmaking process is now a highly developed process operating close to the thermodynamic limits of efficiency.  Even the development of alternative iron smelting processes is getting stiff competition from BF technology.

The BF is essentially a counter-current moving bed furnace with solids (iron bearing burden, coke and flux), and later molten liquids, travelling down the shaft. Pulverized coal and oxygen (O2) enriched hot air blast is injected at the tuyere level near its base. The reducing gases which are formed by the various reactions taking place move up the furnace shaft, reducing the iron bearing materials charged at the top of the furnace.

BF process consists of a multivariate system which is subjected to a large number of inter-influencing variables affecting the performance of the BF. It is necessary to isolate the inter-influence of the variables to understand the role played the each variable on the performance of the BF. The performance of the BF is determined by several parameters out of which productivity is the major one.

The BF was created in a long way of experiences through which tremendous modifications and improvements were conducted to reach the present status. Intensive work was done to increase the productivity of the BF. The working volume of the BF has increased from less than 100 cum to more than 5000 cum. A furnace of this size produces around 10,000 tons of hot metal (tHM) a day to 13,000 tHM a day with annual production of more than 4 million tons of HM. It has been reported that BFs with an internal volume in the range of 3000 cum to 5000 cum seems optimal for BF performance. This means the BF performance is more correlated to specific productivity which measures the efficiency which is normally expressed in tons per day per cubic meter (t/d /cum) of working volume. In some countries, in place of working volume, useful volume is considered. Several blast furnaces are operating around 2.5 t/d/cum of specific productivity.

The furnace size is only one variable which has influence on the improvements in the productivity of the BF. There are many other parameters which influence the BF productivity. Developing of charged burden, furnace design, injection technologies, and process control helps in the improvement of BF productivity. For example the BF bell les top charging system was developed to sustain a good distribution of feeding materials inside the furnace and consequently improve the gas flow and the production rate.

The BF productivity is the quotient between possible gas throughput per unit of time and required specific gas generation for one ton of HM. Hence an increase in the productivity on the one hand requires an increase in the gas throughput, which implies improvement in the furnace permeability and on the other hand a reduction in the specific gas requirement, which means a reduction in the specific consumption of reducing agent.

There are several factors which influence the productivity of a blast furnace. Major amongst them are described below. However, it is to be noted that the levels indicated are when the factors are taken in isolation. These influences are not additive since BF operation is an integrated operation and different parameters interact with  each other within the BF with some parameters have reinforcing effect while other parameters can have weakening effect.

Raw materials

Besides fuels and reductant like BF coke, nut coke and pulverized coal, the BF needs for the production of hot metal (HM) (i) iron bearing raw materials like sinter, pellet, and calibrated lump ore also known as sized iron ore, (ii) fluxing materials like lime stone, dolomite, and quartzite, and (iii) miscellaneous materials (also known as ‘additives’) like manganese ore, and titani-ferrous iron ore etc.

In the iron bearing materials, the higher iron (Fe) content in these materials means that lower gangue material is going inside the furnace which needs to be fluxed for slag formation. Hence higher Fe content helps in the reduction of slag volume and improves the BF productivity. With every 1 % increase in the Fe content in the iron bearing material mix being charged in the BF, the productivity improvement is around 2.4 % when the Fe content in the charge mix is upto 50 %, around 2 % when the Fe content in the charge mix is in the range of 50 % to 55 %, and around 1.7 % when Fe content in the charge mix is in the range of 55 % to 60 %.  In case some scrap is charged in the BF then the effect on the BF productivity is 0.6 % increase for every 10 kg/tHM of Fe input in the form of scrap.

Lime stone and dolomite when charged directly in the BF get calcined inside the BF. This calcination reaction needs heats which result into increase in the specific fuel consumption. If these fluxes are charged through sinter or pellets then the calcination reaction takes place outside the BF and the BF working volume is more effectively used by the iron bearing materials. This in turn improves the BF productivity. For every 10 kg/tHM reduction in the consumption of raw limestone increases the BF productivity by 0.5 %. In case of dolomite, the increase in the BF productivity is 0.4 % for every 10 kg/tHM.

For achieving higher productivity in a BF, it is essential that burden materials provides high permeability and homogeneity in the BF. Hence the charging of the undersize of the burden material is to be controlled for improvement in the BF productivity. For every reduction of the content of the less than 5 mm in the iron containing charge improves the productivity by 1 %.

Further the burden materials are to have high reducibility to promote short retention time. Burden materials are also to have low content of tramp elements such as zinc, lead and alkalis to avoid process disturbances. Blast furnace productivity greatly depends on the quality of sinter. Sinter is to have optimum grain distribution, high strength, high reducibility, high porosity, softening temperatures greater than 1250 deg C, constant FeO content in the range of 7 % to 8 % and constant basicity.

Control of burden distribution plays an important role in the improvement of the productivity of the blast furnace. The burden distribution control ensures a stable burden descent, adjusts the flow of gasses in the wall (this avoids high heat loads without generating inactive zone) and helps in achieving a good solid gas contact. Increased uniformity of distribution of ore loads on the radius of the furnace top for two bell charging device improves BF productivity by 2 % and same in case of bell-less charging device improves BF productivity by 3 %. Replacement a two bell charging device with bell-less charging device improves BF productivity by 4 %.

Fuel / reducing agents

Two types of fuels / reducing agents are used in the BF. These are metallurgical coke (BF coke) which is charged from the top and pulverized coal / natural gas / coke oven gas / oil / coal tar which are injected at the tuyere level.

BF coke influences the productivity of BF in many ways. High ash content in the coke results into charging of the furnace with more slag forming materials. These materials are to be fluxed to form slag. This increases the slag volume. Every 1 % reduction of ash content in coke improves the BF productivity by 1.3 %.

Other properties of the BF coke which affects the productivity are CSR (coke strength after reaction), CRI (coke reactivity index), and Micum indexes (M 40, M 25, or I 40 and M 10 or I 10). These parameters affect the permeability in the furnace shaft and the mechanical strength of the coke at the tuyere level. M 40 represents crushability of the coke and M 10 wearability. Higher values of CSR and M 40 and lower values of CRI and M 10 result in improvement in the BF productivity. Every 1 % increase in the percent of M 25 increases BF productivity by 0.6 % and every 1 % increase in the percent of CSR increases BF productivity by 0.7 %. In case of M 10 value, every 1 % decrease increases the BF productivity by 2.8 %. Fig 1 shows the influence of BF coke properties on BF productivity.

Fig 1 Influence of BF coke properties on BF productivity

Sulphur content of the BF coke has also got its affect on the BF productivity. Reduction of sulphur content in the coke for every 0.1 % increases the BF productivity in the range of 0.18 % to 0.71 %. The increase is 0.18 % at 0.05 % sulphur level in HM, 0.22 % at 0.04 % sulphur level in HM, 0.27 % at 0.03 % sulphur level in HM, 0.38 % at 0.02 % sulphur level in HM and 0.71 % at 0.01 % sulphur in HM.

The size of the coke charged in the BF also has influence on the BF productivity. Every 1 % reduction in the content of plus 80 mm coke fraction increases BF productivity by 0.2 %, while every 1 % reduction in the content of minus 25 mm coke fraction increases BF productivity by 1 %.

Pulverized coal / natural gas / coke oven gas /oil / coal tar injected at the tuyere level are normally affect the specific flow of the gas causing a reduction in the top temperature and an increase in the adiabatic temperature (RAFT) in the tuyeres. These effects are compensated by the injection of substitute fuel. The injection of auxiliary fuel does not have any effect on the BF productivity but since it is accompanied with the injection of oxygen, there is productivity increase due to injected oxygen.

Hot air blast and oxygen enrichment

Increasing in the temperature of the hot air blast for every 10 deg C increases the BF productivity in the range of 800 deg C to 900 deg C by 0.5 %, in the range of 900 deg C to 1000 deg C by 0.4 %.

When the percent of oxygen in the blast is upto 25 %, increasing in the temperature of the hot air blast for every 10 deg C increases the BF productivity in the range of 1,000 deg C to 1,100 deg C by 0.3 %, in the range of 1,100 deg C to 1,200 deg C by 0.28 %, in the range of 1,200 deg C to 1,300 deg C by 0.25 %, and in the range of 1,300 deg C to 1,400 deg C by 0.22 %.

When the percent of oxygen in the blast is between 25 % to 35 %, increasing in the temperature of the hot air blast for every 10 deg C increases the BF productivity in the range of 1,000 deg C to 1,100 deg C by 0.25 %, in the range of 1,100 deg C to 1,200 deg C by 0.2 %, in the range of 1,200 deg C to 1,300 deg C by 0.2 %, and in the range of 1,300 deg C to 1,400 deg C by 0.18 %.

When the percent of oxygen in the blast is between 35 % to 40 %, increasing in the temperature of the hot air blast for every 10 deg C increases the BF productivity in the range of 1,000 deg C to 1,100 deg C by 0.2 %, in the range of 1,100 deg C to 1,200 deg C by 0.18 %, in the range of 1,200 deg C to 1,300 deg C by 0.16 %, and in the range of 1,300 deg C to 1,400 deg C by 0.14 %.

A decrease in humidity of the hot air blast improves the BF productivity. For every 1 gram/cum for air blast volume of 1,500 cum/tHM to1,600 cum/tHM, BF productivity improvement is by 0.14 %, and  for air blast volume of 1,000 cum/tHM to1,00 cum/tHM, BF productivity improvement is 0.06 %.

Enrichment of hot air blast with oxygen improves BF productivity. For every 1 % (absolute) enrichment of the hot air blast with oxygen upto 25 % improves BF productivity by 2 %, from 25 % to 30 %, the improvement in the BF productivity is 1.7 %, from 30 % to 35 %, the improvement in the BF productivity is 1.4 %, and from 35 % to 40 %, the improvement in the BF productivity is 1.6 %.

Increase in the pressure of BF top gases improves the productivity of the BF. With every 10 kPa increase in the pressure of top gases in the BF in the range up to 200 kPa (excess) with a corresponding increase in the mass of the hot air blast the improvement in the BF productivity is 1 %.

For the accelerating of the melting process the differential pressure of gases in the BF is increased. An increase of every 1 % upto the boundary values improves the BF productivity by 0.5 %. The same, but above the boundary values improves the productivity value by 0.3 %.

Hot metal and liquid slag

Tapping practice has an important role to play in achievement of high productivity in a BF. Good tapping practice involves good tap hole length, timely opening of the tapping, control of tapping speed, proper hearth drainage, and closing of tapping after furnace becomes dry. Quality of tap hole mass is very important for good tapping practice.

Decreasing the silicon content in the hot metal has a positive effect on the blast furnace productivity. Decrease in the silicon content is achieved due to better ore-coke ratio and movement of cohesive area downwards. This generates a lower volume for the transfer of silicon to the hot metal. Decrease of silicon content in the hot metal per 0.1 % improves BF productivity by 1.2 %.

Reduction in the content of manganese in the HM has positive effect on the BF productivity. For every 0.1 % reduction of manganese content in the hot metal, the increase in the BF productivity is in the range of 0.22 % to 0.44 % depending upon manganese content of the ore. Higher the content of manganese in the ore lower is the influence.

Reduction of phosphorus content in the HM improves the BF productivity. Every 0.1 % reduction of phosphorus improves the BF productivity by 0. 6 %.

Properties of slag have considerable effect on the productivity of the BF. Lower specific volume of slag of lower viscosity improves the productivity of the BF. A decrease of 10 kg/tHM of the slag contribute to an increase of BF productivity by 0.6 % regardless of its total amount and content of iron in the charge.

Other factors affecting BF productivity

There are several other factors which have influence on the BF productivity. Decrease in the short shut downs of BF by 1% improves BF productivity by 1.5 %. Reduction of BF running on reduced blast by 1 % improves BF productivity by 1 %. The reduction of cases of delays the opening of tapping for every 1%, with the average duration of the delay of 0.5 times the interval between adjacent openings of tapping improves BF productivity by 0.1 %.

Automatic process control improves the furnace productivity since it minimize consumption of reductant, avoids furnace process disturbances such as hanging, slipping, scaffolding, gas channeling etc through an immediate counteraction by the system, stabilizes hot metal and slag parameters etc. the effect of automatic process control on the blast furnace productivity is in the range of 3 % to 5 %.