3.1 Pre-Reduction-Smelting Reduction
According to the practical operation conditions, some assumptions are made to calculate the carbon rate of “Pre-reduction-Smelting reduction” process.
- The feed materials are Fe2O3 and C at 25°C, and the product is liquid Fe at 1550°C.
- Pre-reduced iron ore is hot charged to smelting vessel.
- Oxygen used in smelting vessel is pure O2.
- The exit gas of pre-reducer and FeOa in pre-reducer are in chemical equilibrium.
- The heat requirement for the operation of pre-reduction is assumed to be readily available from the sensible heat of hot exit gas from smelting vessel (or others).
- The variables are degree of pre-reduction of Fe2O3 and the percentage of CO2 in exit gas from smelting vessel. The degree of pre-reduction is defined as “Degree of Pre-reduction = [(Oxygen removed from iron oxides)/(Total original oxygen in iron oxides)] × 100%”.
If the chemical energy of gaseous product, CO, from smelting vessel partially post-combust (generate CO2) to supply heat to smelting vessel, there are two disadvantage effects: (1) increases the oxygen potential and result in re-oxidation of hot metal in smelting vessel at high temperature. (2) The reducing potential of CO/CO2 mixture gas is lower in pre-reduction. Therefore, the exit gas from smelting vessel cannot have a too high CO2 content, usually <20%.
For better understanding, the calculation procedures for “Pre-reduction-Smelting reduction” process, an example (Point M in Figure 3) is given below. For Point M, the parameters include, (1) the temperature of pre-reduction is 800°C, (2) the composition of exit gas from smelting vessel is 90% of CO and 10% of CO2, (3) and the degree of pre-reduction is 49%. Forty-nine percent of degree means FeO1.5 is pre-reduced to FeO0.76. So, in order to produce 1 mol Fe, 0.5 mol Fe2O3 is pre-reduced to 0.76 mol FeO and 0.24 mol Fe at 800°C in pre-reduction process, and these hot 0.76 mol FeO and 0.24 mol Fe are feed into smelting vessel. In smelting vessel, the calculation of carbon rate is listed in Table 1.
Table 1. Carbon rate in smelting reduction
|Heat required||0.76FeO + 0.68C = 0.76Fe + 0.62CO + 0.07CO2||ΔH1 = 106 008 J|
|1 mol Fe is heated from 298 to 1823 K||ΔH2 = 70 991 J|
|0.62 mol CO is heated from 298 to 1823 K||ΔH3 = 30 692 J|
|0.07 mol CO2 is heated from 298 to 1823 K||ΔH4 = 5410 J|
|Heat supply||0.76 mol FeO liberates heat from 1073 to 298 K||ΔH5 = 32 953 J|
|0.24 mol Fe liberates heat from 1073 to 298 K||ΔH6 = 6411 J|
|C + 0.55O2 = 0.9CO + 0.1CO2||ΔH7 = 1 38 828 J||ΔHSup.1 = ΔH7–ΔH8–ΔH9 = 86 031 J mol C−1 (heat supply by combustion of 1 mol carbon: C + 0.55O2 = 0.9CO + 0.1CO2)|
|0.9 mol CO is heated from 298 to 1823 K||ΔH8 = 44 843 J|
|0.1 mol CO2 is heated from 298 to 1823 K||ΔH9 = 7954 J|
|C (As fuel) = [(ΔH1 + ΔH2 + ΔH3 + ΔH4) − (ΔH5 + ΔH6)]/ΔHSup.1 = ΔHReq.1/ΔHSup.1 = 2.02 mol|
|C (total) = C (As fuel) + C (As reductant) = 2.02 + 0.68 = 2.70 mol C mol Fe−1 (578 kg C tHM−1).|
The hot exit CO/CO2 gas from smelting vessel is the reductant and heat source for pre-reduction. Then check if the reducing potential and heat of the gas is sufficient for pre-reduction. In pre-reduction process, 0.5 mol Fe2O3 is pre-reduced to 0.76 mol FeO and 0.24 mol Fe, which consumes 0.74 mol CO to generate 0.74 mol CO2. The mass balance, reducing potential check, heat check and chemical reaction equations are listed in Table 2.
Table 2. Mass balance, reducing potential check and heat check in pre-reduction
| ||Input to pre-reduction|| ||Output from pre-reduction|
|Mass balance||0.5 mol Fe2O3 at 298 K||0.76 mol FeO at 1073 K|
| ||0.24 mol Fe at 1073 K|
|2.43 mol CO at 1823 K (90% of exit gas from smelting vessel)||1.69 mol CO at 1073 K|
|0.27 mol CO2 at 1823 K (10% of exit gas from smelting vessel)||1.01 mol CO2 at 1073 K|
|Reducing potential Check||FeO + CO = Fe + CO2 K1073K = 0.60|
|For the output gas from pre-reduction, (%CO2)/(%CO) = 1.01 mol/1.69 mol = 0.60 = K1073K. So, the reducing potential of the output gas from pre-reduction is sufficient for the metallic iron to stable exist.|
|But if (%CO2)/(%CO) > K1073K, extra carbon (or CO) is required to sustain the reducing potential, until (%CO2)/(%CO) ≤ K1073K. That's why the curve of carbon rate increases with the increase of pre-reduction degree at later stage (Fig. 3).|
|Heat check||Heat required||0.76 mol FeO is heated from 298 to 1073 K||ΔH10 = 32 952 J|
|0.24 mol Fe is heated from 298 to 1073 K||ΔH11 = 6411 J|
|1.69 mol CO is heated from 298 to 1073 K||ΔH12 = 40 702 J|
|1.01 mol CO2 is heated from 298 to 1073 K||ΔH13 = 37 311 J|
|0.38Fe2O3 + 0.38CO = 0.76FeO + 0.38CO2||ΔH14 = 3603 J|
|0.12Fe2O3 + 0.36CO = 0.24Fe + 0.36CO2||ΔH15 = −3315 J|
|ΔHReq.2 = ΔH10 + ΔH11 + ΔH12 + ΔH13 + ΔH14 + ΔH15 = 1 17 665 J|
|Heat supply||2.43 mol CO liberates heat from 1823 to 298 K||ΔH16 = 1 21 075 J|
|0.27 mol CO2 liberates heat from 1823 to 298 K||ΔH17 = 21 475 J|
|ΔHSup.2 = ΔH16 + ΔH17 = 1 42 550 (from the hot exit gas of smelting vessel)|
|ΔHSup.2 > ΔHReq.2, so the sensible heat from the hot exit gas of smelting vessel is sufficient for pre-reduction process, no extra carbon in pre-reduction is required.|
|Therefore, the total adiabatic carbon rate for Point M C (in smelting) + C (in pre-reduction) = 578 + 0 = 578 kg tHM−1.|
From Tables 1 and 21 and 2, the calculated total adiabatic carbon rate of “Pre-reduction-Smelting reduction” process is the sum of that used as reductant and that used as fuel. So the carbon rate is a function of pre-reduction degree (mixtures of hematite, magnetite, wustite, and metallic iron), exit gas composition, and pre-reduction temperature. The calculations about carbon rate at different conditions are similar with Point M, and are shown in Figure 3. In the figure, the percentage of CO2 is 0, 10, and 20% respectively, and the temperature of pre-reduction is 800 and 1000°C, respectively.
The exit CO/CO2 mixture gas from smelting vessel is by-product, but it is the input as reductant and heat source in pre-reduction reactor. So it serves as coupling link of these two reactors. If the CO/CO2 gas from smelting vessel is the source of pre-reduction, there are contradictory requirements in smelting vessel and pre-reduction. (1) When the degree of pre-reduction is lower, the work load in smelting vessel is higher, and result that the carbon rate in smelting process is higher. (2) On the other hand, in order to lower the work load in smelting vessel, the iron ore should have higher degree of pre-reduction, i.e. more CO/CO2 mixture gas of higher reducing potential is needed in pre-reduction. However, the smelting vessel which is charged with ore of higher degree of pre-reduction will produce less CO gas, instead of more CO to satisfy the increased need in pre-reduction. In order to sustain the pre-reduction operation, other extra carbon in addition to CO from smelting vessel may be used to supply reducing potential and heat. Therefore, the total adiabatic carbon rate first decreases and then increases with the increase of degree of pre-reduction (Figure 3).
This problem exists in all smelting reduction process. In order to satisfy the reducing agent and heat requirement (carbon rate) both in pre-reduction and in smelting reduction, it could be worked out a compromise on mass balance and heat balance between smelting vessel and pre-reduction. For the illustration of such contradictory requirement, a minimum value of carbon rate (the compromise degree of pre-reduction) at a certain pre-reduction temperature and certain composition of exit gas from smelting vessel is shown in Figure 3 (point with lowest carbon rate in each curve). More or less than this degree of pre-reduction, the carbon rate will increase. From Figure 3, one can conclude that basically the total adiabatic carbon rate of “Pre-reduction-Smelting reduction” is more than 600 kg tHM−1.
3.2 Direct Reduction-Melting
Same as before, according to the practical operation conditions, some assumptions are made to calculate the carbon rate of “Direct Reduction-Melting” process.
- The feed materials are Fe2O3 and C at 25°C, and the product is liquid Fe at 1550°C.
- DRI is hot charged to melting converter.
- Oxygen used in “oxidation compartment” is air (20% O2 and 80% N2). In some oxygen-enriched cases, the percentage of O2 is 25% and 30%.
- Oxygen used in melting converter is pure O2.
- Without re-oxidation of nascent iron in direct reduction (because paired straight hearth (PSH) furnace can resolve the problem of re-oxidation).
- The variable is the temperature of direct reduction.
For better understanding the calculation procedures for “Direct reduction-Melting” process, an example (Point N in Figure 5) is given below. For Point N, the parameters include, (1) direct reduction temperature is 1200°C (1473 K), (2) Air is pre-heated at 450°C (723 K), the heat for pre-heating comes from the hot CO gas from melting, (3) and no oxygen-enriched. The calculations of carbon rate in direct reduction and in melting are listed in Tables 3 and 43 and 4, respectively.
Table 3. Carbon rate in direct reduction
|Reduction compartment||0.5Fe2O3 + 1.5C = Fe + 1.5CO||ΔH18 = 2 44 865 J|
|1 mol Fe is heated from 298 to 1473 K||ΔH19 = 42 796 J|
|Oxidation compartment||1.5CO + 0.75O2 = 1.5CO2 (1.5 mol CO comes from reduction compartment)||ΔH20 = −4 24 470 J|
|0.75 mol O2 liberates heat from 723 to 298 K||ΔH21 = −9982 J|
|3 mol N2 liberates heat from 723 to 298 K||ΔH22 = −38 307 J|
|1.5 mol CO2 is heated from 298 to 1473 K||ΔH23 = 88 478 J|
|3 mol N2 is heated from 298 to 1473 K||ΔH24 = 1 11 546 J|
|ΔHReq.3 = ΔH18 + ΔH19 + ΔH20 + ΔH21 + ΔH22 + ΔH23 + ΔH24 = 14 926 J|
|C + O2 = CO2||ΔH25 = 393 510 J||ΔHSup.3 = ΔH25 + ΔH26 + ΔH27–ΔH28–ΔH29 = 250182 J (Heat supply by complete combustion of 1 mol carbon: C + O2 = CO2)|
|1 mol O2 liberates heat from 723 to 298 K||ΔH26 = 13309 J|
|4 mol N2 liberates heat from 723 to 298 K||ΔH27 = 51076 J|
|1 mol CO2 is heated from 298 to 1473 K||ΔH28 = 58985 J|
|4 mol N2 is heated from 298 to 1473 K||ΔH29 = 148728 J|
|So. C (As fuel) = ΔHReq.3/ΔHSup.3 = 0.06 mol|
|C (In DR) = C (As reductant in reduction compartment) + C (As fuel in oxidation compartment) = 1.5 + 0.06 = 1.56 mol C mol Fe−1|
Table 4. Carbon rate in melting
|1 mol Fe is heated from 1473 to 1823 K||ΔHReq.4 = 28195 J|
|C + 0.5O2 = CO||ΔH30 = 110530 J||ΔHSup.4 = ΔH30 − ΔH31 = 60705 J (heat supply by incomplete combustion of 1 mol carbon: C + 0.5O2 = CO)|
|1 mol CO is heated from 298 to 1823 K||ΔH31 = 49825 J|
|C (in melting) = ΔHReq.4/ΔHSup.4 = 0.46 mol|
|Therefore, the total adiabatic carbon rate for Point N = C (in DR) + C (in melting) = 1.56 + 0.46 = 2.02 molC mol Fe−1 (433 kg tHM−1).|
From Tables 3 and 43 and 4, the calculated total adiabatic carbon rate of “Direct reduction-Melting” process is the sum of that used as reductant and that used as fuel. So the carbon rate is a function of direct reduction temperature, air pre-heated, and oxygen-enriched. The calculations about carbon rate at different conditions are similar with Point N, and are shown in Figures 4 and 54 and 5.
In Figure 4, the carbon rate of direct reduction (not include melting, similar with Table 3) is plotted in a wide range of direct reduction temperature (800–1400°C). In the figure, when the temperature is lower, the carbon rate is a constant and it is the stoichiometric value of 1.5 mol C mol−1 Fe (321 kg tHM−1). The reason is that the amount of carbon as reductant is “1.5 mol C mol Fe−1” and generates 1.5 mol CO. The heat liberated from the post combustion of this 1.5 mol CO (to CO2) exceed the amount of heat required in “reduction compartment”. Therefore, no additional carbon as fuel is required for direct reduction, and the carbon rate in direct reduction process keeps at a constant, that is “1.5 mol C mol Fe−1” or “321 kg tHM−1”. But at higher process temperature, the heat generated from post combustion of CO from “reduction comparment” is not sufficient for heat required in “reduction comparment”, additional post combustion of carbon as fuel is required to supply heat to “reduction comparment” and result that the carbon rate increases with the increase of direct reduction temperature.
The total adiabatic carbon rate for liquid iron production at different conditions is shown in Figure 5 (include direct reduction and melting, similar with Table 4). In case of hot charging DRI to melting converter at 1550°C, when the carbon rate of direct reduction is constant (Figure 4), higher temperature of DRI will result lower work load in melting process. That is why, the total adiabatic carbon rate firstly decreases in Figure 5. Same as before, when the temperature of direct reduction is higher, the heat generated from post combustion of CO is not sufficient for heat required in direct reduction, additional carbon as fuel is needed and result that the total carbon rate increases with the increase of direct reduction temperature (more carbon is required in direct reduction process, more air is required, more heat is required to heat N2 to high temperature, and more energy is wasted).
Both oxygen-enriched and pre-heating air are optimized methods to decrease carbon rate. So the carbon rate of “Direct reduction-Melting” process with oxygen-enriched (left section) and pre-heating air (right section) is plotted in Figure 5. The two top curves in each section are standard and consequent result of the curve in Figure 4 (no oxygen-enriched and no pre-heating). The hot gas (1550°C) from melting converter may be used as the source of pre-heating. From the figure, one can conclude that the oxygen-enriched and pre-heating air can effectively decrease the carbon rate. The total adiabatic carbon rate of “Direct reduction-Melting” process may be lower than 440 kg tHM−1. If both of these two optimized methods are simultaneously taken in direct reduction, the total carbon rate may be further decreased to lower than 400 kg tHM−1.
3.3 Comparison between these two Alternative Ironmaking Processes
The basics of these two routes of hot metal production along idealized conditions, both starting with hematite and carbon at ambient temperature to liquid iron at 1550°C, are summarized in Figures 3 and 53 and 5. Point M in Figure 3 and Point N in Figure 5 are close to the best possible conditions in each case. The operational parameters for Point M and Point N are listed in Table 5. From the table, the difference in total carbon rate based on “Pre-reduction-Smelting reduction” (Point M) and “Direct reduction-Melting” (Point N) is about 145 kg tHM−1. The mass balance and heat balance of these two selected alternative ironmaking processes are listed respectively in Tables 6 and 76 and 7. From the tables, one can conclude that the difference of carbon rate between these two routes is due to the efficient utilization of the element carbon.
Table 5. Operational parameters of Point M and Point N (the best possible conditions in each case)
|Pre-reduction-Smelting reduction (Point M)||Direct reduction-Melting (Point N)|
|Carbon rate||578 kg tHM−1||Carbon rate||433 kg tHM−1|
|Degree of pre-reduction||49%||Temperature of direct reduction||1200°C|
|Feed to smelting vessel||76% FeO + 24% Fe||O2 content in air||20%|
|Exit gas from smelting vessel||90% CO + 10%CO2||Pre-heating air||450°C|
Table 6. Heat and mass balance of selected “Pre-reduction-Smelting reduction” process (Point M)
|Items||T [°C]||mol||ΔH [kJ]||Items||T [°C]||mol||ΔH [kJ]|
|C (fuel)||25||2.02||0|| || || || |
|O2||25||1.11||0|| || || || |
|2.02C + 1.11O2 = 1.82CO + 0.2CO2||−280.43||0.76FeO + 0.68C = 0.76Fe + 0.62CO + 0.07CO2||106.01|
|Total|| || ||−319.79||Total|| || ||319.56|
Table 7. Heat and mass balance of selected “Direct reduction-Melting” process (Point N)
|Items||T [°C]||mol||ΔH [kJ]||Items||T [°C]||mol||ΔH [kJ]|
|C (fuel in DR)||25||0.06||0||N2||1200||3.24||120.47|
|N2 (DR)||450||3.24||−41.37|| || || || |
|C (melting)||25||0.46||0|| || || || |
|O2 (melting)||25||0.23||0|| || || || |
|1.5CO + 0.75O2 = 1.5CO2 (DR)||−424.47||0.5Fe2O3 + 1.5C = Fe + 1.5CO||244.86|
|0.06C + 0.06O2 = 0.06CO2 (DR)||−23.61|| || || || |
|0.46C + 0.23O2 = 0.46CO (melting)||−50.84|| || || || |
|Total|| || ||−551.07||Total|| || ||551.25|
The net heat available by incomplete combustion of C to CO is much lower than its complete combustion to CO2 (the heat liberation of C + 0.5O2 = CO and C + O2 = CO2 is 110.53 and 393.51 kJ mol−1, respectively). This is the main reason for high carbon rate in “Pre-reduction-Smelting reduction” process (Table 6). In all pilot and demonstration plants of smelting reduction process, the reported coal rates are significantly higher than 650 kg tHM−1. By the fundamental analysis of smelting reduction in present work, there is no reason to expect any pleasant surprise in the effort to further lower its total carbon rate than what has been reported.
For “Direct reduction-Melting”, the reduction of iron ore is completed in “reduction compartment”, and the complete combustion of CO could be 100% (of cause, nascent metallic iron should be avoid to be re-oxidized). That is why the total carbon rate of direct reduction is lower (Table 7).
It must be pointed out that there are two key points in the development of a viable direct reduction process of lower carbon rate. (1) The challenge of the process is in the efficient heat transfer from “oxidation compartment” (where it is generated) to “reduction compartment” (where it is consumed by the endothermic reaction and sensible heat of substances). (2) The prevention of re-oxidation of nascent metallic iron by gaseous oxidants generated in “oxidation compartment” should be considered as one of the important tasks. PSH furnace is a considerable innovative direct reduction process to realize the perfect heat transfer and prevention of re-oxidation of DRI. There is not data available on PSH process yet at commercial level, but a project on it is in planning by the Northeastern University, China and its partners. Also, some detailed information about PSH can be viewed in some literatures.[17, 18]