Choice of Technological Regimes of a Blast Furnace Operation with Injection of Hot Reducing Gases

Injection rate of fossil fuels is limited because of drop in the flame temperature in the raceway and problems in the deadman region and the cohesive zone. The next step for obtaining a considerable coke saving, a better operation in the deadman as an well as increase in blast furnace productivity and minimizing the environmental impact due to a decrease in carbon dioxide emmision would be injection by tuyeres of hot reducing gases (HRG) which are produced by low grade coal gasification or top gas regenerating. Use of HRG in combination with high pulverized coal inyection PCI rate and oxigen enrichment in the blast could allow to keep and to increase the competitiveness of the blast furnace process. Calculations using a mathematical model show that the HRG injection in combination with pulverized coal (PC) and enriching blast with oxigen can provide an increase in PC rate up to 300-400 kg/tHM and a rise in the furnace productivity by 40-50 %. Blast furnace operation with full oxigen blast (100 % of process oxigen with the exception for the hot blast) is possible when HRG is injected.


Introduction
One of the largest global challenges in this age is the preservation of the environment.The world-wide energy consumption is approx.9.8 *10 18 J per year, the CO 2 -emissions causing the greenhouse effect and coming out of human activities are approx.860 Mt/a / 1 /.Regarding the agreement of Kyoto/ 2 / the steel industry must find a compromise between environmental compatibility, economy, saving of resources and supply guarantee.The steel industriy is responsible for approx.5 % of the world-wide energy consumption/ 3 / (in Germany about 10%) and therefore in the future has to deal more and more with the preservation of resources as well as the lowering of CO 2 -emissions.

Steel work and rolling mill 14%
Blast furnace and sinter plant 72% Power plant and others 14% Currently approx.95 % of the pig iron is made by blast furnace process, which in spite of modern technologies contributes considerably to the high energy consumption and environmental impact.
The blast furnace, sinter plant and coke oven plant consume 70-75% of the entire energy consumption of an integrated steelwork (Figure 1).In consideration of the linked energy, this value is in the range of 11-12 GJ /t HM /4/ The main part of the required fuels is covered by metallurgical coke, which makes up 40-50% of the total fuel / 5 /.By production of 1 t coke is released about 4150 kg of CO 2 ./ 6/ Apart from the ecological consequences, coke causes a majority of production costs of the pig iron.But despite all ecological and economical problems resulting from the coke use, the blast furnace route remains the most efficient way to produce crude steel.Only about 5% of the primary metal is manufactured by alternative steelmaking processes such as direct-and smelting-reduction methods / 7 /.Thus the lowering of coke consumption and total energy consumption at the blast furnace process is of large importance not only from aspects of environmental protection, but also from economic regard.
Previous Development: Decreasing the coke rate has been a priority throughout the entire history of the blast furnace.Operating improvements have been remarkable over the years.The total fuel rate for example in all German blast furnaces was decreased from 800 kg/tHM in the 1960s to below 470 kg/tHM in 1999.As it is shown in Figure 2, the coke rate was decreased to 340 kg.The mean coke rate of all European blast furnaces in 1998 was 364 kg/tHM; its minimum value was 286 kg/tHM / 8 /.Present burden and coke quality have reached at European blast furnaces such a high level that the reserves for coke saving by means of improvement of their preparation are almost exhausted.According to the statistical study, only about 10% of coke rate variation are explained by coke and ore burden properties /8/.
Partial replacement of coke by other fuels has been within the last two decades the main way of coke saving.Auxiliary fuels as natural gases (NG), oil, pulverized coal (PC) and occasionally coke oven gases and organic wastes are injected via the tuyeres.Coke consumption of about 286-320 kg/tHM have been achieved at some blast furnaces by the PC injection of 170-200 kg/tHM / 11 / 12 / 13 /.
Consumption of fossil fuels injected via the tuyeres is limited by their endothermic effect and by the oxidizing potential of the raceway which has to be able to provide a gasification of injectants within the raceway.Incomplete conversion of injected fuels leads to char generation and causes drop in the gas permeability, dirtying of the dead man and finally decrease in the furnace productivity and increase in the coke rate.In next section, measures for increasing fossil auxiliary fuel efficiency are listed.Further ways for approach to the theoretical minimum of coke rate and for decrease in total energy consumption should be: Recently, a process scheme for HRG injection based on the coupling of COREXprocess and blast furnace was suggested / 25 /.In this technology the COREX-Export gas after the removing of CO 2 , is heated up to 400°C and then injected into the blast furnace (Fig. 4).In this paper limit conditions of blast furnace technology with fossil fuel injection are summarized and calculation results of technological regimes with HRG injection using a balance mathematical model are presented.Ways for realization of this technology are also proposed.

Effect of fossil and artificial auxiliary fuels on the blast furnace process
Injecting fossil reducing agents influences strongly the heat exchange, the gas permeability and the slag regime in the blast furnace.In numerous works both changes of the blast furnace operating condition as well as the conversion processes in the raceway have been investigated.From this reason only some aspects of the change in the raceway conditions, in the gas permeability and liquid product drainage as well as special features of HRG injecting are regarded here.

Flame temperature and oxidizing potential of the raceway
By injecting auxiliary fuels, flame temperature is reduced since the bosh gas volume rises more strongly than the quantity of heat generated by fuel gasification and carried by the hot blast.The amount of heat generated by combustion of auxiliary fuels decreases in comparison to the heat amount released by the coke combustion because of their pyrolysis and lower heat of incomplete combustion.
The difference in the decrease of the flame temperature for various fuels depends on the C/H ratio.Heat of decomposition increases with the drop of the C/H ratio in fuel.
Therefore the less C/H ratio, the less heat is released in incomplete combustion (Table 1).
where: T: flame temperature, °C T n : temperature of burden and gases in the reserve zone of heat exchange (idle zone), °C r d : direct reduction rate, (-) K : coke rate, kg/t HM V: bosh gas volume, m 3 /t coke A = 1 -0.9 / W b W b : water equivalent of burden, kJ/t HM.
The necessary flame temperature as it follows from Eq. (1) depends on furnace operation conditions.E.g. a change in direct reduction rate, gas volume or coke rate requires correction in the flame temperature value.The PCI affects the necessary flame temperature not only because the drop in direct reduction rate but also due to the radiation of the coal particles within the raceway /34/ 36 /.
The oxidizing potential of the raceway at constant oxygen concentration in blast depends on the rate of injected fuels and can be maintained or changed controlling their ratio, according to the equation / 37 /: where: S 1 and S 2 : injecting rate of gaseous and liquid / solid respectively, m 3  Injection of HRG does not require additional oxidizing potential of the raceway.

Gas permeability and drainage of liquid products
The lower the coke rate, the more difficult to maintain the gas permeability in the cohesive zone and in lower part of the furnace as well as the drainage of the melted products.The minimum coke rate which maintains the drainage of the liquid products in a counter flow corresponds to the critical voidage of 0,23-0,24 m 3 /m 3 / 39 /.
High coke qualitiy is necessary to limit the contamination of the furnace especially in the area of the dead man as well as to guarantee the necessary hydraulic and gas dynamic conditions in the hearth.
The following measures are necessary for the preservation of the gas permeability at the substantial decrease of coke rate / 40 /: the reverse V-profile of the cohesive zone the peak of the cohesive zone should be shifted possibly far upwards to maintain a sufficient number of coke windows and therefore the sufficient gas permeability in the cohesive zone (Figure 5) during a retention of the coke layer thickness the ore layer thickness must be decreased.
These measures permit to enable the gas permeability of furnace during the coke replacement up to 40-50%.Injecting the HRG could improve the conditions in the hearth, since it practically forms neither soot nor char or ash.The coke characteristics in the hearth can likewise improve.
Beyond that the use of HRG has a substantially smaller influence on the slag regime (physical characteristics, composition, quantity) as pulverized coal, oil or plastics.

Limit for fossil auxiliary fuels and fundamental advantages of HRG
The highest rate of NG injection of 155 kg/tHM was achieved in USA (coke rate was 310 kg/tHM), in Russia and Ukraine its rate at some BFs was 150-170 m 3 /tHM /5/ 42 /.
The higher NG rate leads to local supercooling of the hearth, an increment of slag viscosity, incompleteness of NG combustion with char generation, and worsening of melting products drainage.
The injection rate of coke oven gas (COG) (which is only injected occasionally in some blast furnaces because its free resources at an integrated plant are usually It is necessary to clearly understand that all efforts in the direction of increasing the PC rate could shift the achieved limit of PCI to a higher level but not eliminate the limitation on fossil fuel injection generally.
Hot reduction gases have following fundamental advantages in comparison with the fossil auxiliary fuels: they enable to introduce a higher quantity of carbon monoxide and hydrogen, as fossil fuels the quantity of hot reduction gas in combination with enriching blast with oxygen could be increased up to a nitrogen-free-process, because no (or almost none) heat is necessary for the splitting of the hydrocarbons and in the raceway no processes of combustion do take place the relative share of CO and H 2 increases because of reduction of the nitrogen quantity the furnace productivity increases, because in the fraction of time, more carbon converses and more raw material gets melted the mixing of the gas stream with the hot blast is improved.This leads to the increase of CO -and H 2 -utilization rates the lower part of the blast furnace remains "cleaner" and the coke characteristics under the cohesive zone can be maintained.
The advantages of the HRG mentioned above refer on the injection into the hearth.
When injecting into the shaft or belly, in the lower part of the blast furnace with reduced gas permeability the gas volume would not rise, so that an increase of the furnace productivity is possible; beyond that the physical heat of the HRG is entered at that point, where the temperature is low.
The decisive argument agaist the HRG injection into the shaft consisits of enormous difficulty to provide the necessary gas distribution in the shaft, especially in the cohesive zone.The distribution of the HRG all-over the furnace radius and the gaspenetrating up to the center will be one of the main problems of this method of HRG injection.
Further disadvantages of the HRG injection into the shaft consisit of the costly and complex constructional modifications; the shaft is more stressed by the additional tuyere rank.

Mathematical Model
The calculations represented in the following were carried out using a mathematical model of the Donetsk State University of Technology / 47 /.This is a total balance model that does not require any input parameters to be assumed (e.g., the " Four Fluid Model" /26/ needs raceway geometry, softening temperature of the burden, distribution of burden-and coke size all-over the furnace radius, coke size in deadman and other parameters of the inner state as input parameters).
The model calculates the coke rate, the blast volume, parameters of the inner state (bosh gas volume, flame temperature, direct reduction rate, heat generated and absorbed), and output parameters (slag volume, relative productivity, top gas composition and temperature, etc.).The model was developed on the base of a complex method of Prof. A.N.Ramm /26/.This method based on the interrelations of material and heat balances equations.Its characteristic feature is following: a system of equations of material balance of different input components according to a target hot metal chemical composition is formed; to this system one equation of heat balance is added which determines the correlation between coke rate and remaining components.The coke rate is introduced as unknown value in all equations of the material balance, rates of iron bearing and flux components in the heat balance equation.
The main steps of calculation are:

Calculation conditions
The injection of hot reducing gases with various parameters into the blast furnace The burden of BF-1 consists of 59.1% sinters, 34.3% pellets and 6.6% lump ore; the S-content in the coke makes up 0.5%.At the BF-2 the sinter/pellets ratio in the burden makes up 2:1.In Tables 2 and 3 the burden and coke parameters for both blast furnaces as well as PC parameters for BF-2 are given.

Influence of HRG-parameters
The influence of HRG temperature and composition on blast furnace process has been investigated for the conditions of the BF-2 (hot blast temperature 1180°C and PCI rate 160 kg/t HM).Four cases with the same injection rates of PCI and HRG (160 kg/t HM and 150 m³/t HM respectively) were examined and compared with the basic case.A constant flame temperature of 2150°C was maintained by controlling the oxygen concentration in the blast.
The main results of the calculation are shown in

Only HRG injection and co-Injection of HRG and PC
In Table 5  Injection of HRG accompanying with enriching blast with process oxygen and PCI provides decrease in total energy consumption (by 450 MJ/tHM or 4.6% when injecting 500-550 kg/tHM reducing agents).Total energy loss was dropped in this case by more than 400 MJ/tHM or 50% (Fig. 6).In Table 6 the results of similar calculations for conditions of BF-1 are shown.The HRG parameters are the same as for calculations discussed above (Table 5).Cases 1-3 represent blast furnace operating parameters at only HRG (without PCI).At HRG-injection rates of 150, 300 and 600 m³/t HM the coke consumption decreases by 44, 76 and 124 kg/tHM respectively.An essential decrease of direct reduction rate (relatively by 20 and 43 % during injection of 300 and 600 m³ HRG/tHM) allows to save still more coke.However the HRG must have thereby a temperature of more than 1000°C.Injection of 150 m³ HRG/tHM doesn't require perceptible changes in the combined blast parameters.At 300 m³ HRG/t HM the blast moisture was reduced down to the atmospheric value and a blast volume to less than 800 m³/tHM; the oxygen enrichment rises up to 27%.At 600 m³ HRG/tHM the blast volume decreases to approx.330 m³/t HM and the O 2 -enrichment makes up 60 %.The furnace productivity rises in cases 1-3 by 3.5, 13 and 34% respectively.
Cases 4-5 illustrate the operating parameters at co-injection of PC (150 kg/t HM) and HRG (150 and 300 m³/t HM).Besides a considerable coke saving of 160 (32%) and 180 kg/t HM (37%) these technological regimes provide an increase in furnace productivity of 24 and 36% respectively.The effectiveness of heat use increases in case 5 by 3.3%; the heat loss decreases from 5.5% in the basis case to 4.5%.
Case 6 represents a special technological regime, the so-called "Oxy-Coal-Process".
Since in this process 100 % O 2 is injected i.e. no hot blast is used, neither cowpers are necessary nor other costs for the heating of blast arise.This regime enables coke saving of 182 kg/t HM and increase in furnace productivity by 52 %.Extra economic benefits could be derived by utilization of the top gas with a high calorific value which contains no nitrogen.
Total energy consumption decreases by 55-80 MJ/tHM for every 100 m 3 /tHM of HRG.The higher value corresponds to the cases with PC co-injection (Fig. 7, e).
Change in the value of energy loss is similar.E.g., external energy losses make up 20 and 28 MJ/tHM per 100 m 3 /tHM of HRG for the cases without and with coinjection of PC respectively (Fig. 7, f).
Further study of blast furnace operation with the injection of preheated top gas after CO 2 removal (HRG parameters are the same as in Table 4, case 4) has been carried out under BF-2 conditions for the wide range of oxygen enrichment of blast and PCI rate.Hot metal temperature has been kept on a constant level by maintaining the necessary value of flame temperature.Results are presented in Fig 8.
Maximum coke saving of about 370 kg/tHM or more than 70% has been achieved when injecting over 400 m 3 /tHM HRG, 300 kg/tHM PC with blast consisting almost completely of cold process oxygen (80-90%O 2 ).Decrease in direct reduction rate by 2.5 times is an important positive factor for the hearth operation, decrement of carbon consumption to direct reduction, improvement of heat of liquid products, operational conditions and hot metal desulphurization.Injection of HRG with high hydrogen content and a few percents of methane ensures the best operating results from the coke saving and productivity points of view.Such a gas can be generated e.g. by steam-conversion of natural gas or coke-oven gas.
Effective use of HRG when injecting into the blast furnace hearth can be reached under following conditions: • gas temperature should be on the level of the hot blast temperature, e.g.1000-1250°C; this requirement is important when injecting high amounts of HRG • a minimum content of oxidizing agents in the HRG (usually less than 3-5% CO 2 + H 2 O).Every one percent extra can cause an increase in coke consumption up to 3 % • a constant gas composition.Variations of the chemical analysis (particularly CO and CO 2 ) can disturb the process and lead to an excessive consumption of coke • economic manufacture of the HRG

Production of HRG
At present time gasification of low coal grades and top gas regeneration are the most economic methods for HRG manufacturing.
For coal gasification effective methods should be chosen.Cleaning of top gas from CO 2 is usually accomplished by adding special chemical reagents (e.g., monoethanolamine); this technology is very complicated.The removal of carbon dioxide can also be done in gas scrubbers by solving the top gas in water at high pressure.This technology is used for example in the production of synthetic ammonia.The cowpers can be used for heating up the HRG up to 1000-1300°C.
A technology with recycled top gas after its cleaning from CO 2 could also lead to additional cost saving.Carbon dioxide can be produced in gaseous or solid phase and utilized in the production of food, chemicals, in agriculture 1 , metallurgy, etc.
The recirculation of top gas in the blast furnace without removal of the oxidizers can be used only in relatively low quantities, e.g. to compensate high temperature in the raceway and low gas volume when enriching blast with oxygen.

Blast Furnace Technology
Three technological variants with HRG injection were investigated: A technology with the use of 100% of cold process oxygen requires no hot stoves and causes no costs for the blast heating; it provides a high coke saving and an increase in productivity.In the investigated case these values amounted 182 kg /tHM 1 Increase in CO 2 in air by 2% provides an acceleration in plants growth as twice.This fact could be used for the increase of harvest in hot houses or under glasses.
and 52 % respectively.Additional benefit can be achieved by the use of the top gas as fuel or reductants, since it contains no nitrogen and has a high calorific value.
Nevertheless the optimal value of oxygen concentration in the blast should be determined because the disadvantage of the "oxy-coal process" is the absence of physical heat in the blast.

Tuyere apparatus design
Design of tuyere assembly for high amount of HRG, process oxygen and PC injection in the hearth should provide a complete mixture of PC with oxidizing agent, optimal kinetic energy of streams and reliability and simplicity in exploitation.
Conventional tuyere constructions with lances for auxiliary fuels inserted into the inner cavity of tuyere apparatus through the blowpipe or tuyere body as well as also tuyeres with co-axial, double lances or separate lances for fuel and local oxygen delivery / 52 / 53 cannot simultaneously fulfil two contradictory conditions: from the one hand, prevent ignition of super high amount of coal in high oxygen volume in the tuyere cavity and, from other hand, avoid the dilution of oxidizer with HRG before burning out of coal particles.
Following method and construction of tuyere apparatus can be suggested (Figure 9).
PC with process oxygen and HRG with hot blast and/or additives (e.g., water steam, coke oven gas) are introduced into the hearth separately to improve combustion conditions of coal and to provide more rational use of oxygen.HRG and hot blast or HRG only in the case of "oxy-coal process" are introduced through the tuyere

Conclusions
The results of the work carried out allow us to draw the following conclusions: 1. Injection of the HRG generated outside the blast furnace and simultaneous enriching blast with oxygen should be regarded as a way for further coke saving coke and increase in the furnace productivity beyond the injection of fossil auxiliary fuels.
this technology promotes also decrease of the hearth contamination.
2. The HRG should be heated to about 1000°C and should have a minimal content of CO 2 and H 2 O (< 3-5%) as well as only minimal variations in the chemical analysis.
3. HRG can be manufactured by air-or steam-air -conversion of coal or by top gas recycling.Low grade coals with a high ash content can be used for gasification, whereas rich coals with low ash content should be used for PCI.
4. Co-injection of PC and HRG with simultaneous enriching blast with process oxygen is the most effective technology.The use of HRG, pulverized coal with low ash-content and O 2 -enrichment of the blast up to 80-100% can ensure an increase of the PCI rate up to 300-400 kg/tHM and productivity of 140-150%.This technological regime provides decrease in total energy consumption of 55-80 MJ /t HM for every 100 m³/t HM of HRG.
5. Method and tuyere apparatus design for high amount of HRG, process oxygen and PC injection have been suggested.

Figure 1 :
Figure 1: Structure of energy consumption in an integrated steel work / 4 /

Figure 4 :
Figure 4: COREX-Export Gas Injection into the Blast Furnace

Figure 5 :
Figure 5: Desired shape of the cohesive zone at different coke rates a) 500 kg/t HM; b) 250 kg/t HM /40/ limited) is changed from 100 to 250-300 m 3 /tHM /5/11/42/; the coke/COG replacement ratio makes up 0.4-0.45kg/m 3 compare to 0.8-0.85kg/m 3 for NG.PC is the most common auxiliary fuel.Its injection rate of 200-230 kg/tHM with the drop in coke consumption down to 280-300 kg/tHM has been achieved during trial periods at some BFs /11/12/ 43 /.Theoretical, laboratory and pilot investigations as well as the latest industrial experience show that PC rate can be raised up to at least 250 kg/tHM and BF operating with coke/coal ratio = 50/50 (%) could be maintained /5,37/.On the other hand, average PC rate in Europe rarely exceeds 130-150 kg/tHM (in 1999 only IJmuiden 6, the Netherlands operated with PC rate of 204 kg/tHM and coke rate of 316 kg/tHM / 44 /) mainly because of the problems with complete combustion within the raceway, gas permeability in the shaft, dirtying of the deadman and as a result irregular furnace operation and decrease in productivity.Increase in PC rate up to the record level of 266 kg/tHM even under perfect burden and operational conditions at Fukuyama No.3 BF (NKK, Japan) did not result in record low coke rates / 45 /.Total consumption of injected fuel at co-injection of NG, PC and /or oil does also not exceed 180-230 kg/tHM /5/.Considerable success in finding solutions to the above mentioned problems as the provision of the complete or at least high rate of auxiliary fuel utilization, -the provision of a suitable permeability under conditions of a very large decrease of the coke layer thickness and of the thickness of the coke windows in the cohesive zone, -the compensation for the negative changes of heat fluctuation and slag formation processes, -the maintenance of a uniform distribution of injected fuels around the furnace circumference has been recently made.Further investigations and improvements of blast furnace operation with high rate of fossil fuel injection as well using of already developed measures (optimization of burden distribution, blast oxygen enrichment, improvement in delivery of oxidizing agent to the coal jet and their mixing, optimizing of coal grinding, use of catalysts, use of ionized air and injection of a gas-oxygen mixture for intensifying natural gas combustion etc. /5,35/ 46 / could maintain a stable furnace operation at high PCI as well as NG or oil injection and increase the achieved average level of injected fuel rate.

1 .
O 2 -quantity released during reduction 2. useful heat output of physical and chemical conversions of burden, coke and coal ash 3. total quantity of C, H, O and N in 1m³ or 1kg of each injected fuel as well as the enthalpy and calorific value (by burning in the raceway) 4. volume of bosh gas, direct reduction rate and top gas temperature 5. coke and total fuel consumption, as well as blast volume 6. needs of fluxes, slag volume, top gas parameter (volume, composition, in the productivity and intensity of the coke combustion.Furnace productivity is determined with the consideration of material gas permeability /47/: P = P° * (v g /v g ) * (d° * Θ° * γ°/ d * Θ * γ) 1/2 where: d = (L * v b /v c + 1) * (v b /v c + 1) volume, m³ / kg HM γ : average gas density, kg / m² v b , v c : burden and coke volume respectively, m³ / kg HM L : ratio of gas pressure drop in index 0 refers to initial (base) conditions.
hearth has been simulated.Calculations have been carried out for two blast furnaces operating under the different conditions.Blast furnace 1 (BF-1) with a working volume of about 1000 m 3 and 12 tuyeres represents the typical blast furnace with only coke operation without enriching blast with oxygen (21% O 2 ).The hot blast temperature is 1080°C.The produced pig iron contains 0.6% Si and 0.040% S. Blast furnace 2 (BF-2) (working volume of about 3800 m 3 , 40 tuyeres) represents a modern blast furnace with PCI and O 2 -enrichment of the hot blast.The hot blast temperature is in the range 1160°C to 1180°C and the produced pig iron contains about 0.37% Si and 0.029% S.

Figure 6 :
Figure 6: Total energy consumption and energy loss for BF-2 operation conditions

Figure 7 :Figure 8 : 4 . 4 . 1 .
Figure 7: Total energy consumption and energy loss for BF-1 operation conditions 4 -5 m³ HRG from 1 kg coal can be generated by air-or steam-air-conversion of coal.Nitrogen in the reducing gas decreases the coke / HRG replacement ratio.However this can be compensated by increased gas temperature.The use of mini reactor / gasifiers incorporated into the tuyere apparatus allows HRG injection with temperature of 1800-2000°C./18/ Various technologies for coal gasification in the fluidized bed, melting reactor and underground gasification were developed and investigated at the Institute of Ferrous Metallurgy of the Aachen University of Technology / 48 / 49 / 50 / 51 /.
a) the substitute of the pulverized coal (partly and completely) by HRG b) the injection of HRG in addition to the reached level of PCI; the PC rate was kept on the constant level c) the injection of HRG with simultaneous increase of the PCI due to the increased oxidation potential in the raceway.Variant a) is of no interest for the practice because the existing equipment and technology maintains injection of 150-180 kg/tHM PC and the advantages of HRG can not be realized.The consequence is a rise of coke consumption.Variant b) ensures a coke saving of approx.20-25 kg/tHM per each 100 m³ HRG/t HM.Variant c) provides the highest efficiency.Maintaining the flame temperature at a necessary value, which keeps hot metal temperature and silica content, allows extra coke saving.Total fuel rate at HRG injection decreases almost proportional to the drop in the coke rate: the difference corresponds to the heat generation by gasification of CH 4 and heat absorption by decomposition of CO 2 and H 2 O.

Table 1 :
Incomplete combustion heat of carbon in various fuels

Table 2 :
Burden consumption and composition

Table 3 :
Chemical analysis of coke and PC (wt %)

Table 4 .
While the HRGs of cases 1 and 2 are produced by coal gasification, in case 3 a HRG is used, which is generated by steam conversion of natural gas.The reducing gas used in case 4 is top gas, where CO 2 is stripped from the recycled gas.The HRG in case 1 is characterised by a high content of oxidizing agents (CO 2 +H 2 O = 13,5%) as well as a low temperature of 700°C.It is shown, that the physical heat of the HRG can not compensate the lower heat entry with the hot blast (approx.by 430 kJ /kgHM in comparison to the basis case) and the heat consumption for the decomposition of carbon dioxide and water steam (271 kJ /kgHM).For that reason the coke rate has not been changed despite the decrease in the direct reduction rate (r d ) by 8% in comparison to the basis case.The furnace productivity increased by 11 %.

Table 4 :
Influence of parameters of HRG on blast furnace operating parameters the calculation results for the BF-2 operating conditions are shown at simultaneous injection of PC and HRG.The parameters of HRG are equal for all In chapter 2 it was shown, that with a change of parameters of the combined blast, the necessary flame temperature changes as well.The adjustment of flame temperature to the initial level is not necessary.The technological regimes for cases Case 3 shows the co-injection of HRG and "standard level" of PC (150 kg /tHM coal + 150 m 3 /tHM HRG).The direct reduction rate decreases by more than 8% in comparison with the basis case because of the high content of reducing agents.This is the primary cause for the coke saving of approx.35 kg /tHM despite the drop in heat supply.This technological regime is accompanied by lowering of the blast volume by 25 %, the bosh gas volume by 12 % and the top gas temperature by approx.40°C.The hydrogen content in the top gas rises in comparison with the basis case from 3.4 to 6.1%; the nitrogen content decreases from 49.6 to 37.6 % because of the increased quantity of reducing agents and oxygen content in the blast.The calorific value of the top gas rises by 28%.Other operating parameters (slag volume, flux consumption etc.) don't change.The furnace productivity increases by 10 %. m³/tHM and 75% respectively.The top gas has low temperature, contains 60% CO+H 2 and only 8% N 2 .Its calorific value makes up approx.7350 kJ /m³.The high thermal and reducing potential of this gas can be utilized in the blast furnace and/or other aggregates.

Table 5 :
Blast furnace operating results for BF-2

Table 6 :
Blast furnace operating results for BF-1