DRI – The EAF Energy Source of the Future?
Sara Hornby Anderson
Product Manager – Steelmaking/Melting
Midrex Direct Reduction Corporation
For a nominal increase in DRI plant operating costs of $0.11/Tonne, an increase of 0.52%C will save 34.6 kWh or $2.12 /Tonne(liquid steel)in the EAF shop. If the DRI is charged directly to the EAF at 700oC, additional energy savings of $4.63/Tonne(liquid steel)can be realized. These figures do not take into account the value of productivity increases available.
For years integrated Direct Reduced Iron/EAF mills felt the optimal DRI carbon level from a MIDREX® Direct Reduction Process was 1.5%C to 1.8%C (and that for HBI 1.0% to 1.3%). In excess of this, the economical and technical benefits of high %C DRI to the EAF shop were offset by decreased metallic yield at both the DRI and EAF plants. Whilst these conclusions were site specific, and will remain so, this paper will demonstrate that current, new DRI production technology can offer high %C DRI, increased DRI plant productivity and metallic yield, with financial and technical benefits in the melt shop that far outweigh the minimal additional production cost.
Benefits, and capitalization thereof, of high %C DRI and the use of HOTLINK TM 3 in a DRI/EAF facility will be discussed.
Historical DRI Use
During direct reduction, oxygen is removed from iron ore (mostly comprising iron and compound oxides) reducing it to metallic iron and the more stable oxides – silica (SiO2), alumina (Al2O3), lime (CaO), magnesia (MgO) – and phosphorus (P). As the oxygen is removed, the concentration of the feed materials’ other constituents increase about 50%; for example, if the feed material contains 2% SiO2the resulting DRI will contain about 3% SiO2.
There is no “generic DRI”. Properties and end composition are strongly influenced by the feed materials’ content of SiO2, Al2O3, other oxides and to a major extent, the total iron -Fe -direct reduction process used and the process operating parameters – metallic iron (Fe)and carbon. Therefore, the properties will be site specific.
There are three significant issues for EAF steelmakers to consider when melting DRI:
The direct reduction efficiency -ñ both the inherent process efficiency and the operating efficiency – is defined by metallization:
Different processes economically achieve different upper limits of metallization. Operation at, or below, this upper limit is subject to the operator’s control. Table I represents some direct reduced iron compositions produced by processes using different reductants and feed materials.
As gangue content in DRI increases, it displaces other constituents. Since iron is the major constituent, the iron content is reduced the most ñ almost percent for percent. The amount of gangue, and the specific composition, contained in the DRI, will affect both electric power input requirements and metallic yield – the higher the gangue, the more energy is required for melting and the lower is the iron recovery per metric Tonne (T) of DRI charged.
EAFs operate with a “V” ratio of 2 to 3, where the “V” ratio is defined by the ratio of basic to acid components:
The DRI product “V” ratio can vary significantly (Table I) impacting the lime (or dolomitic lime) additions made in the EAF to increase the DRI “V” ratio to the acceptable EAF level. Lower “V” ratio materials require more lime, hence more energy to melt. As lime additions affect slag fluidity, viscosity and desulfurization capabilities, the %DRI charged to the EAF increases the importance of this factor. Figure 1 illustrates the difference in lime requirements for two DRI compositions from Table I – Plant 1 and Plant 3 ñ whose materials have DRI “V” ratios of 0.6 and 0.1, respectively. The significant difference in lime requirement stresses the need for EAF shops to know the exact chemistry of the DRI material they are using.
Historically, carbon flexibility has been an inherent part of the MIDREX natural gas-based direct reduction process. Plants have been able to generate whatever carbon content the meltshop operators wanted (up to 3.5% in DRI). If %C increases in the DRI, iron content correspondingly decreases. Similarly, if %Fe decreases so will % metallization (Equation 1). Decreasing metallization affects energy consumption and steel yield negatively ñ a drop from 94.5% to 92% metallization increases EAF energy requirements by 20 kWh/T (Figure 2) and relative steel yield drops from 1.03 to 1.0 (Figure 3).
It should be noted that many plants advocate that the theoretical carbon required to neutralize the FeO contained in the DRI is 0.215%C for every 1%Fe as FeO from:
So, for a typical DRI composition with 93% total iron and 93% metallization, theoretical “neutralizing” carbon is 1.4% from:
Any carbon in excess of that needed to reduce the FeO is available for combustion with oxygen (O2) in the EAF and will reduce electrical energy requirements according to the following equations:
DRI Value in Use (VIU) predictions have been postulated by numerous authors. As a whole, VIU requires better quantification and knowledge of steelmill cost accounting to allow a viable comparison with other metallic sources. For example, DRI charge includes the carbon content (for both neutralization of the DRI FeO and the excess which will contribute to chemical energy and/or foamy slag) and is often compared with a scrap charge excluding the carbon additions required for steelmaking process. Also, a better definition of efficiency of the injected and charged carbon versus DRI combined carbon is needed. Inevitably, the value of carbon and DRI will be extremely site specific. Nowadays, increased DRI plant metallic yield accompanying the high carbon levels (see Figures 11 and 12) will change assumptions and increase EAF yield (charge to liquid steel) and productivity on a per unit time basis.
If one considers ore-to-steel versus scrap-to-steel in terms of melt energy alone, obviously the latter would win hands down (theoretical melt energy is 500kWh/T and 360kWh/T, respectively). Site-specific advantages of DRI can be lack of local scrap coupled with ore and NG abundance or just, simply, there is a need for a high quality charge material. Some of the other advantages and disadvantages are shown in Table II.
Technology Changes Impact DRI Production
Over the last 30 years, steel demand has increasingly pushed the direct reduction plants to become more efficient and productive (Table III). To satisfy the steelmills’ needs, the major challenge has been to increase the volume and number of sources of reducing gases (CO and H2), and the rate at which they are consumed in the shaft furnace. Improving uniformity of solid/gas contact and increasing the operating (gas) temperatures has increased reducing gas volume by 25% and productivity of the shaft furnace by 36.8% (Table IV).
Figures 4, 5, 6, 7 and 8 show the evolution of the DR flowsheet and Table IV shows the specific operational changes related to the original practice (base case) and five (5) different scenarios which have impacted the DR furnace productivity and product quality – higher reducing gas temperature, increased use of lump ore and DRI oxide coatings (CaO or CaO/MgO ) and oxygen use.
Though reducing gas quality is lower with oxygen injection, the higher gas temperature enhances the reduction kinetics with the net impact of an increase in productivity. OXY+ MIDREX ís new partial oxidation technology, scheduled to start up Sept., 2000, increases production and product quality with lower reducing gas temperature than O2enrichment Alone.
The first two improvements represent increased operating costs while the latter three options entail capital cost for equipment and related systems as well as marginal operating costs. This is more so for OXY+ which entails installation of a burner after the reformer.
Increased %C in DRI
DRI plants have always been able to generate the %C needed by the meltshop. In the early days, the alternate flowsheet (Figure 9), with reducing gas temperatures of 740oC to 760oC, promoted reasonable carbon additions by putting CO rich gas in the temperature zones thereby promoting carbon by:
But, effective cooling, and maintenance of process control, was an issue with the Alternate Flowsheet. Thus, the Standard Flowsheet (Figure 10) was adopted, allowing for better cooling and up flow control (through the bed) but lacking CO rich gas.
A need for steelmills to decrease operating costs and increase productivity (to remain competitive worldwide) has created an interest in higher %C DRI. It is expected to significantly improve carbon combustion efficiency to >90% (because the carbon is combined with the DRI) as compared to the 25% to 75% efficiency of injected carbon which may not penetrate the slag and either burn or be sucked to the off-gas system (or both). Site-specific cost and availability of NG and O2 will determine the economic feasibility of increasing the DRI %C and melting it.
The importance of having a sufficient oxygen supply and off-gas system capacity in the melt shop, as well as oxygen tools to capitalize on the additional energy from the high %C DRI, should not be overlooked. It is imperative that the steelmaker has the wherewithal to decrease the carbon in the bath and post combust the CO generated quickly enough to avert excessive carbon boils, CO burning in the off-gas ductwork, or high melt out carbon levels. The latter will lengthen heat times and negate the productivity available from the higher carbon DRI. The off-gas system must be large enough and have sufficient cooling water to compensate for the increased productivity.
The technology changes outlined above permit higher carbon DRI to be produced without sacrificing DRI plant productivity. The higher reducing gas temperature predominantly increases productivity. Higher metallization is an option in lieu of some of this productivity. Carbon from CO, decreases as a result of the higher burden temperatures so increased NG cracking and/or in-situ reforming is promoted using the additional heat available from these higher reduction furnace temperatures. When NG cracks, it effectively produces additional reducing gas and, thus, not only is carbon available but more hydrogen is generated, increasing the plant efficiency and productivity.
In the five scenarios above, production of carbon in the DRI is affected as follows:
• Lump Ore ñ allowed increased reducing gas temperature which increased production but decreased carbon
• Iron oxide coating ñ CaO or CaO/MgO coating is used to coat the ore to prevent the ore from sticking in the shaft at the elevated reducing gas temperatures
•O2injection ñ NG enrichment has traditionally been < 5% but with oxygen can be higher. The O2additions to the reducing gas increase the gas temperature, hence the reduction reaction rate, and excess heat over that consumed by the reactions cracks the NG and/or promotes in-situ reforming. NG additions to the cooling gas and transition zone are other options (Figure 11). • OXY+ - increases the available CO, H2, sensible heat and plant productivity. It maintains reducing gas quality and consistency at a lower gas temperature than O2injection (O2 injection with reducing gas temperature of 1000oC is equivalent to OXY+ at 950oC. •O2injection and OXY+ - couples the benefits of both, allowing higher reducing temperatures and productivity
The use of O2achieves similar results to increasing the reformer capacity, though for significantly less capital, as seen from the example in Table V.
The big question is ñ How much carbon is needed? This remains a controversial subject. Traditionally the range of carbon in DRI has been 1.5% to 1.8% and that of HBI has been 1.0% to 1.3%. As previously mentioned, many mills advocate the need for 1.4%C in the DRI to balance the FeO contained therein (Equation 4). Thus, the steel industry has tended to advocate 1.8%C in DRI as optimal for this reason and because in the past:
• this produced the lowest liquid steel cost
• levels in excess of 1.8%C in the DRI were found to be uneconomical – “additional carbon in the DRI does not fulfill any technical need and is not offset by any cost savings in the meltshop”.
• increasing carbon decreased iron and lowered ultimate iron yield
• the EAF shop lacked sufficient O2to blow down excess carbon in the bath
In 1999, the average %C in the 25.2 million Tonnes of DRI produced in MIDREX’s 49 worldwide modules (25 plants in 16 countries) was 1.77% (with a spread of 0.49% to 2.23%C), metallization ranged from 92.71% to 96.06% respectively with the minimum and maximum being 91.29% (1.83%C) and 96.31% (1.76%C). This has increased from the 1.63% average reported in 1995 and does not include IMEXSA’s current operating practice of between 2.5%C and 3.1%C.
The economic analysis shows the benefits, efficiencies and cost savings potential for high %C and hot charged DRI. Basic data have been taken from DRI carbon trials conducted at IMEXSA coupled with predicted benefits of HOTLINK DRI. The latter data have been compared to hot charging results from Essar. The author has assigned generic cost data. The magnitude of the potential productivity increase and cost savings should direct industry to reconsider their future needs, depending upon their capability to provide appropriate oxygen and/or the oxygen tools in the meltshop and sufficient off gas system capacity.
The current sustainable DRI carbon level demonstrated is 3.1% without any major changes to the DRI plant. Where to next? In excess of 4.0%C? Figures 11 to 16 below summarize and predict plant operating data and the impact of actions required to increase %C and productivity on plant performance (it should be noted that the actual productivity and carbon data achieved at IMEXSA fall on these curves). The following assumptions were made when generating the curves:
Metallization was kept constant at 93%; the price for electricity, NG and O2are $0.035/kWh, $2.50/mscf NG and $0.045/Nm O2(from 1.218 kWh/Nm O2 for an owner operated plant). Table VI shows increasing DRI carbon from 1.8% to 3.5%C increases the DRI plant productivity by 10.9%, metallic iron production by 8.7% and total iron processed by 9% for an additional incremental cost of $0.18/T. For example, if one assumes DRI production at 1.8%C to be 203 T/hour with 86.5% metallic iron and 93% total iron, total iron produced would be 188.9T/hour. When the DRI carbon is increased to 3.5%C, the metallic iron and total iron drop to 85% and 91.4%, respectively, BUT, due to the increased productivity of 225 T/hour, total iron production would actually increase to 205.9 T/hour, for a net increase of 16 T/hour of Fe units available to the EAF shop.
As Dr. John Stubbles said, to achieve any more significant improvements in power on time/EAF productivity, the future challenge will be to input more energy to the EAF (electrical and chemical) in a shorter period of time. One solution would be hot charging high chemical energy materials ñ liquid iron or hot DRI.
HOTLINK, based on tried and tested technology, has the primary goal of direct charging hot DRI (HDRI at >700oC) from the shaft furnace to an adjacent EAF. As HBI plants routinely gravity feed HDRI at these temperatures directly to the briquetting machines, this technology will be easily implemented. The hot DRI discharge (outside and above the meltshop) feeds directly to a surge bin from which it is directly gravity fed to the EAF(s). To account for the disparity in the annual DRI and EAF operating times (8000 hours and 7200 hours, respectively), and any unforeseen EAF stoppages, the plant design allows for immediate switching to a DRI cooler and cold discharge storage bins. Options for cold DRI use include: direct charging DRI or a mixture of DRI and HDRI to the EAF, returning a 10% (maximum) charge of DRI to the shaft furnace to be reheated (as no reduction is required, shaft furnace productivity will increase by nearly the % of DRI returned) or merchant sales.
Close coupling of the shaft furnace and the EAF will allow the steelmaker to capitalize on the HDRI’s sensible heat thus increasing productivity and decreasing significantly the electrical power requirements. Predicted benefits are 20 kWh/100oC/T of DRI or 140 kWh/T at 700oC, with 0.004kg/T electrode savings. For 100% HDRI charge, this represents a $7/T saving for kWh and electrodes alone. Provided the EAF shop has the O2tools and ancillary equipment (for example off gas capacity) to handle it, the HDRI plant can adjust HDRI %C to their needs, increasing productivity and reducing operating costs further.
Current trials at Essar, India, with 600oC HDRI being container transported to the EAF and roof charged, indicate these predictions are well within reason. Essar has achieved savings of 124 to 145 kWh/T of HDRI reducing their overall power from 588 to 470.4 kWh/T steel with 135T of HDRI charged (71.2% of charge; total HDRI and HBI charged is nominally 82% depending upon scrap prices). The HDRI trials have been so successful, Essar will charge 75% HDRI by year end and will convert their system to be as fully automatic as possible (possibly to include pneumatic transport). Electrode savings of 0.03 kg/T have been realized over the eight (8) month test period and power on time savings of 2.7 minutes and 5.4 minutes (on a cold charge time of 72 minutes) for 35 T and 90 T HDRI charges, respectively. Metallization has increased 0.85% and FeO in the HDRI has dropped 1.43% from that of the HBI.
HOTLINK has been included in proposals in the US, Mozambique, Australia and Trinidad with two feasibility studies recommending HOTLINK’s inclusion in new plant contracts.
Economics for a 1.1 mm Tonne/year, 60% HDRI/40% HM#2 scrap operation on the US Gulf Coast were recently presented showing an HDRI cost of $97.60/T for an average metallic charge cost of $87.20/T or $98.53/T and a total cost of $134.78/T liquid steel to the caster.
Other DRI operational costs quoted recently are: HDRI projected cost at MIS $75/T; ISPAT’s worldwide cold DRI cost $80/T. Capital would be an additional $15-$25/T.
Figure 17 compares DRI/EAF steelmaking versus other steelmaking routes and the total carbon emissions in the form of CO2/T steel(including excess energy converted to electricity). NG based DRI with the EAF is the most efficient process after scrap based EAF melting. As the DRI option is based on 100% DRI, and few operations achieve this rate, emissions will be better than stated.
Further, while CO2emissions/unit time will increase in the EAF, the overall electrical requirements in the EAF will decrease due to the sensible and chemical energy contained in the hot, high %C DRI. Savings of 30,100,000 kWh/year in an EAF shop will reduce electric power industry CO2generation by 3,170 T/year and that of SOx and NOxby 22.7 T/year and 8.3 T/year, respectively. For the following example, this would mean the hot, high %C DRI use would reduce CO2generation by 68,509.91 T/year.
Economic Analysis of Hot, High Carbon DRI Benefits
This section will look at the operational and economic impact of melting high %C DRI in the EAF and postulate the additional impact of charging hot, high %C DRI.
Rather than use totally predictive numbers, this analysis is based on the initial results gained from increasing %C in the DRI from 2.08% to 2.6%C at Ispat Mexicana, S. A. de C. V. (IMEXSA). The results and cost data generated from generic costs assigned by the author, are shown in Table VII.
The analysis below is based on a constant 95% metallization and the following parameters:
KWh/Te for 2.08%C DRI
Excess carbon combustion efficiency
Yield – charge to liquid steel
Power on time
Tap to tap time for 2.08%C DRI
Annual operating time
New tap to tap time for 2.6%C DRI
267 T (Scrap 15 T, DRI 252 T)
66 min/heat @ 2.08%C
3.9 million T from 3 EAFs
If a combustion efficiency of 95% is assumed for the excess carbon in the DRI, and an energy balance is performed for the increased or decreased factors listed in Table VII, to achieve the resultant 5% increase in productivity IMEXSA realized (9 T/h/EAF), the actual energy saving must be 29.25 kWh/T from:
Combustion efficiency of the coke replaced was 54.6% to satisfy this balance. Coke replacement represents a saving of $0.171/T liquid steel alone or $666,900/year.
Liquid steel production would increase from 180.2 T/hour/EAF to 189.2 T/hour/EAF.
No attempt has been made to quantify the benefits of lower residuals, N2and H2 resulting from DRI use and additional carbon flushing.
Hot Charging 2.6%C DRI
For 2.6%C DRI charged cold to the EAF:
Tap to tap time is 76 min
Power on time is 62 min
Turnaround time is 14 min
Production is 189.2 T/hour
Power requirement is 551.3 kWh/T
If instead of the charge ratio of DRI to scrap above being charged cold, the 94.4% DRI is charged at 700oC and the 5.6% scrap charged at 25oC, the reduction in kWh/T would be:
700oC x 20 kWh/100oC x 94.4% DRI = 132.2 kWh/T (Essar gained 124 to 145 kWh/T at 600oC)
New total power requirement would be 551.3 ñ 132.2 kWh/T = 419.1 kWh/T
Tap to tap time decrease
New tap to tap time
New production rate
= 14.4 min
= 61.6 min
= 35.8 T/hr/EAF
= 225.1 T/hour/EAF
= 4,862,160 T/year for 2.6%C, hot DRI
Just considering kWh/T savings alone, this would save $4.63/T liquid steel, from hot charging 2.6%C DRI for a total savings of $6.75/T liquid steel moving from 2.08% cold DRI to hot, 2.6%C DRI. This does not include productivity increase dollars.
According to the additional savings seen by Essar, this savings is on the conservative side. Essar reported additional benefits gained from hot charging DRI at 600oC (they have only just commenced the hot charging practice and it is yet to be optimized):
Power on time
124 to 145 kWh/T HDRI charged
– 0.03 kg/T
– 0.06 min
decreased from 8.26% HBI to 6.83% in HDRI
increased from 93.0% to 93.9%
Original predictions of benefits from high %C DRI and hot DRI are being confirmed Industrially.
Increasing DRI carbon from 2.08% to 2.6% and combined with hot (700oC) charging indicates a savings of $6.75/T liquid steel, for kWh alone. This does not account for the 25% increase in productivity achieved in this example. Additional benefits of lower nitrogen and hydrogen contents in the liquid steel will be available from more efficient carbon reactions in the EAF. The economics of hot, high carbon DRI charging make this option a favorable alternative to scrap steelmaking for green field steelmills.
It should be emphasized that, due to the diversity of equipment, steel grades, operating practices, and local economic conditions, the cost (and therefore economic benefits) of producing hot, high %C DRI will be site specific, as will be the meltshop benefits.
The ability of a steelmill to capitalize on the potential energy savings will depend upon their ancillary equipment and the availability of O2.
Higher carbon levels are certainly within the capabilities of any Midrex DR Plant with minimum improvements. Further industrial demonstrations are required to optimize the EAF and DRI systems to derive the greatest benefits from both the high carbon and hot charging operations.
A more in depth analysis of current EAF operating practices, efficiencies and post combustion will facilitate their use of high %C DRI in the future.