HYL III: Status And Trends

Transcription

1 HYL III: Status And Trends by Raúl Quintero, President HYL Technology Division, Hylsa, S.A. de C.V. presented at the Gorham/Intertech Conference on Iron & Steel Scrap, Scrap Substitutes and Direct Steel Making Atlanta, Georgia March 21-23, 1995 HYL Technology Division Hylsa, S.A. de C.V. Ave. Munich San Nicolás de los Garza, N.L. México

2 Introduction HYL III technology is characterized by its wide flexibility for adapting to special needs, depending on available reducing gases, energy use and meltshop requirements. Use of spent gases from direct ironmaking processes, coal gasification, energy optimization in DR plants and technology developments aimed to improve EAF productivity have been the objective of HYL R&D and engineering efforts over the past years. One of the newest trends in steelmaking is the application of direct ironmaking technologies for production of molten iron and steel. These technologies are based on the direct use of non-treated, non-coking coal as a reducing agent. Among these technologies, most of which are currently at various stages of development and industrial application, such as AISI, DIOS, etc., the Corex process is presently working at industrial scale. However, due to the low level of post-combustion taking place in the melter gasifier, a significant amount of energy, as spent gas, has to be released from the process. This export energy is so high that if no further and adequate use is considered, the ironmaking process itself is no longer economically feasible. Among the various options for use of the export energy (i.e., power generation, heating, synthesis gas generation, direct reduced iron production), the most economically attractive application is hot DRI production. In this regard, the direct reduction (DR) technology which best complies with the requirements of spent gas treatment and heating as well as direct feed of hot DRI from DR facilities to EAF/BOF, is the HYL III DR process. Feasibility of the HYL III process scheme, based on the use of Corex off-gas and gases from coal gasification, has been confirmed during recent tests being carried out in the HYL demonstration plant. To improve the competitiveness of the DR-EAF route in the steelmaking industry, HYL has developed technological improvements oriented to minimizing the overall energy consumption of the DR-EAF processes. In the DR plant the target has been achieved by energy optimization through the zero kwh scheme. In the EAF, reduction of power requirements and productivity increase can be obtained by the HYTEMP iron and high carbon DRI. HYL III: Status and Trends (3/95) 2

3 HYL III Direct Reduction Process General process description HYL III is a process designed for the direct reduction of iron ores by use of H 2 and CO reducing gases. As presented in figure 1, there are three process schemes available for producing: DRI: Cold DRI, which is commonly used in adjacent meltshops close to the DR facilities. It can also be shipped and exported, provided procedures are taken to avoid reoxidation. HBI: DRI which is hot discharged and briquetted, most commonly used for overseas merchant and export. HYTEMP iron: Hot discharged DRI, pneumatically transported from the DR plant to the meltshop for its direct feed in the EAF/BOF. Water Figure 1 HYL III Process Scheme Nat. Gas H2O Iron Ore CO2 H2O Nat. gas Alternate reducing gases: Coal gasification Coke oven gas Hydrocarbon gasification Corex off-gases Partially spent gases from other DR plant DRI HBI HYTEMP Iron EAF For producing any of these three product forms, the HYL III plant comprises two main process sections: the reducing gas generation and the reduction sections. HYL III: Status and Trends (3/95) 3

4 Both sections are independent from an operational point of view. This feature offers important flexibility for adapting to different reducing gas sources. Typically, the reducing gas generation section consists of a conventional natural gas-steam reformer to produce the H 2 + CO required as make-up for the reduction process. However, alternate sources of reducing gases can be used instead of reformed gas. Among these are: - Gases from coal gasification processes - Coke oven gas - Gases from hydrocarbon gasification - Partially spent gases from another DR plant - Corex off-gases The reduction section consists of the DR reactor, the reduction circuit and, for the particular case of the production of cold DRI, the cooling circuit. Reducing gases are made-up of a mixture of make-up and recycling gases. The basic components of the reduction circuit, aside from the reactor, are: a gas heater to increase the reducing gases temperature up to 925 C; a scrubbing unit for dedusting, cooling and H 2 O elimination from top gases; the recycle gas compressor and the CO 2 removal unit. Here CO 2 is selectively eliminated from the system for a more efficient reuse of the recycle gas. Use of Corex off-gas HYL III - Corex Off-gas process scheme As presented in figure 2, the Corex off-gas is mixed with the reducing recycle gas and the combined gases stream is passed through a CO 2 removal unit to adequate the gas composition for the reduction process. The partially decarbonated gas is preheated in a direct gas heater and fed to the reactor. After reduction of iron ores in the DR reactor, top exhaust gas is passed through a scrubbing unit for dust removal and cooling. The gas is then recycled to the CO 2 removal unit by the compressor. In the case of hot product discharge, there is the flexibility to obtain part of the product as HBI and to send hot DRI pneumatically to the meltshop by the HYTEMP System, simultaneously. Specific requirements of Corex off-gas per tonne (t) of DRI depend basically first, on the N 2 content and secondly on the process selected for CO 2 removal. The higher the amount of gases purged from the DR plant, the higher the specific Corex off- gas make-up required. HYL III: Status and Trends (3/95) 4

5 Figure 2 HYL III - Corex Off-Gas Process Scheme Additives/ore Corex offgases Iron Ore Reduction Shaft Scrubber CO2 H2O DR Reactor Coal Melter Gasifier Oxygen Slag Hot metal Pneumatic Transport System hot DRI Carrier gas Meltshop HBI Most suitable DR technology for using Corex Off-gas When comparing the basic HYL III scheme and the one required for Corex off-gas use, the following main aspects can be easily noticed: General process scheme No major changes or innovations are required in the basic process scheme. The reduction section is incorporated as it is in typical HYL III plants. CO 2 removal use in DR plant environment The Corex off-gases have to be conditioned through CO 2 removal before being used in the DR reactor. The CO 2 removal unit is an intrinsic system of the HYL III scheme. Direct heating of reducing gases The type of gas heater required for this application is also a part of the HYL III process scheme. Heating this type of gases involves a thorough knowledge and experience of phenomena associated with gas characteristics, materials behavior, etc. HYL III: Status and Trends (3/95) 5

6 HYTEMP iron use Potential incorporation of the HYTEMP System for use of hot DRI to the EAF/BOF leads to important economic benefits related to improvements of DRI/hot metal ratio, power savings and productivity increase. HYTEMP iron presents a unique option as an additional product for the Corex off-gas / HYL III scheme. High operating pressure The high operating pressure of the HYL III process -about 5 bars- is an important parameter for this particular application. Since CO 2 partial pressure (CO 2 concentration x gas pressure) is the driving force for CO 2 removal, elimination efficiency of CO 2 is improved by the high operating pressure of the HYL III process. By properly selecting the CO 2 removal system, reducing gas heating and proper energy use, an overall optimized process scheme can be achieved, in terms of both: - plant productivity and - energy consumption Thus, it is possible to optimize the use of Corex off-gas in the HYL III process since: - There is no surplus gas to be exported from the DR plant -for N 2 < 4%. All tail gas is used as fuel in the gas heater. - All thermal energy requirements for CO 2 removal (via steam) are covered in- plant by taking advantage of process waste energy. - A totally energy-balanced scheme is achieved while maximizing the amount of DRI being produced from available Corex spent gas. These figures, presented in table 1 are based on a typical HYTEMP iron of 92% metallization and carbon of 0.8%, being discharged at 700 C. HYL III: Status and Trends (3/95) 6

7 Table 1 DR Plant Productivity and Consumption Figures Item unit/tonne DRI Remarks Productivity: 1.2 ton DRI/ton HM as minimum Corex off-gas: 1250 Nm3 ( ~2.35 gcal) depends on the N 2 concentration Electricity: 170 kwh depends on the CO 2 removal system Water makeup: 1.4 m3 depends on the CO 2 removal system Steam: 0 ton depends on the CO 2 removal system (steam req d ~ 0.35 is generated in-plant) Labor: 0.25 m-h Maintenance: $US3.50 Energy optimization in the HYL III process As shown in figure 3, in the typical HYL III process scheme the sensible heat of hot reformed gas and flue gases from the reformer is used mainly for steam generation. Steam requirements for the plant are for two end-users: steam for reforming and exhaust steam for the CO 2 absorption system in the reduction circuit. The amount and pressure of the steam produced have been specified in order to attain an optimum thermal and mechanical balance of the plant. In the improved energy scheme, the steam is produced at high pressure (63 kg/cm 2 A), in order to take maximum advantage of the steam enthalpy for electricity generation in a single high-efficiency turbogenerator, before being used for reforming and in the CO 2 stripper reboiler. In this way the total electric power requirements can be generated in-plant without depending on external power sources. The capacity of the turbogenerator is about 90 kwh/ton DRI for cold discharge or 105 kwh/ton HBI for hot discharge, sufficient to comply with the total electricity needs of the plant. This self-sufficient electric power scheme, without the need for additional equipment or major modifications to the plant, is exclusive to the HYL III process. It offers important advantages related to reductions of investment and operating costs, and improvements in plant availability, particularly in areas where the reliability of electricity supply is low. HYL III: Status and Trends (3/95) 7

8 Figure 3 HYL III Energy Recovery Scheme Turbogenerator fuel H2O Iron ore DR Reactor CO2 Heater Nat. gas Reformer water H2O fuel DRI Nat. gas This energy scheme presents total energy consumption figures on the order of 2.50 Gcal + 0 kwh/ton DRI or 2.60 Gcal + 0 kwh/ton HBI (table 2). Among the benefits of this scheme are: Decrease of operating costs by about $6-8/ton DRI as compared to a typical DR process based on a minimum thermal energy scheme. A more reliable plant operation because it does not depend on external electric power supply. Decrease of investment cost. External power facilities are not needed, particularly in the case of merchant DR plants oriented to HBI production for export. The energy scheme described above has been incorporated in the Grasim HYL III plant, in Maharashtra, India. This design allows the DR plant to be self-sufficient in electric energy requirements, and to export electricity to other areas of the industrial complex. It can be noticed from table 2 that due to the optimal reuse of reductants from the exhaust reducing gas, only 55-60% of total thermal energy is required as natural gas for reforming and reactor injection; the balance can be supplied by any suitable fuel available. HYL III: Status and Trends (3/95) 8

9 Table 2 Expected Energy Consumption of the HYL III DR Plant Self-sufficient Electric Power Scheme DRI HBI 1. Thermal Energy (Gcal/ton) Natural Gas - Reforming Reactor Injection Total natural gas Fuel Total Thermal Energy Electric Energy (kwh/ton) 0 0 HYTEMP iron and high-carbon DRI HYTEMP system description Hot DRI, or HYTEMP iron, discharged at temperatures ranging from 650 to 700 C, with metallization up to 95% and controlled carbon of 1.2 to > 4.0%, is fed directly to the pneumatic transport system and sent to the EAF feeding bins in the meltshop. In this way, the DRI s heat is capitalized on in the EAF. As presented in figure 4, the DR plant is designed for hot discharge of DRI. According to the particular requirements of steelmaking installations, an off-line discharge for DRI cooling can be used when the EAF is not in operation, or a briquetting machine can be installed if part of the production is being sent for export. The hot DRI is discharged from the rotary valve directly to the pneumatic transport piping loop. Transport is carried out at the same rate as the DR reactor production rate. Process gas or inert gas is utilized as transport gas make-up, at the same pressure and similar temperature as prevails at the DR reactor discharge. Process description for high-carbon DRI Combustion chambers at the reactor inlet in HYL I (fixed bed) process based plants have historically been utilized to boost the reducing gas temperature coming from low temperature heaters. Partial combustion made it possible to overcome the old heaters constraints for achieving reduction temperatures above 900 C. This concept has recently been incorporated in the HYL III process scheme to attain high carbon content in the DRI. Normal figures of carbon content in the HYL III DRI range from 1.2 up to 2.4%. To obtain carbon levels above 2.4% and up to >4.0%, there is a modified process scheme, which is shown in figure 4. HYL III: Status and Trends (3/95) 9

10 Figure 4 Process Scheme for HYTEMP iron and High-carbon DRI H2O Iron Ore CO2 Combustion chamber Nat. gas O2 Nat. gas DRI HBI HYTEMP Iron Carbon in the DRI, mostly as iron carbide (Fe 3 C), is derived from CH 4 and/or CO. Carburization based on CH 4 is endothermic while carburization from CO is exothermic. The main potential carburization reactions can be summarized as follows: 3Fe + CH 4 > Fe 3 C + 2H 2 Endothermic 3Fe + 2CO > Fe 3 C + CO 2 Exothermic 3Fe + CO + H 2 > Fe 3 C + H 2 O Exothermic The dominant path of these reactions is a result of operating temperature, pressure and concentration of the reducing gas compounds (H 2, CO, CO 2, H 2 O and CH 4 ). To promote higher DRI carburization, larger amounts of natural gas (CH 4 ) are injected into the reactor. However, due to the endothermic nature of the carburization reaction through CH 4, the higher the amount of natural gas fed to the reactor, the lower the temperature of the reducing gases in the reduction zone. A low temperature of reducing gases may affect the level of metallization and/or productivity. The partial combustion of natural gas provides additional energy and improves the carburizing quality of the gases, which are required for the carburization of the metallic iron. Oxygen is used in order to keep a low level of inerts in the reducing gas stream. Partial oxidation of natural gas also provides an increase of reducing EAF HYL III: Status and Trends (3/95) 10

11 gases for reduction (as CO and H 2 ). Since a significant amount of natural gas is injected directly to the reactor, the reformed gas make-up for this case is lower when compared to a typical process scheme. This process scheme can be adapted to DR plants designed for either cold and hot DRI production with no adverse effect on plant productivity. Typical analysis of normal and high carbon DRI, obtained from high quality iron ores, are summarized in table 3. Table 3 Typical DRI characteristics Typical DRI High carbon DRI Item (% weight) (% weight) Metallization Total iron Metallic iron FeO Carbon Gangue Fe3C The high-carbon cold DRI is stable enough to be transported inland safely. Highcarbon cold DRI can be produced by merchant plants supplying the product to various domestic EAF based mini-mills. To some extent, this product can also be used for export, mainly for regional distribution, decreasing significantly the operating cost of merchant DR plants while producing a more valuable metallic charge for the EAF. Current projects under execution In the past years, the R&D department has done extensive work at the pilot plant facilities 24 tonnes per day reactor, using partial combustion of natural gas at the reactor inlet for increasing DRI carbon content. The experimental results are presently being extrapolated to industrial scale to give the required flexibility to DR plants in order to obtain a wide range of carbon in the DRI. Since this process scheme requires lower make-up of reducing gas, additional advantages are related to increase of production for existing DR facilities. Plants currently operating with this process scheme are: The Hylsa 2M5 and 3M5 DR plants in Monterrey, Mexico. Combustion chambers have been installed to achieve an increase of 6-8% DRI production with carbon content of >3%. This process scheme went into operation at the end of Feb Net savings in the meltshop by utilizing HYL III: Status and Trends (3/95) 11

12 this medium carbon DRI are about $1.9 per tonne of liquid steel. Future incorporation of the HYTEMP system to the 3M5 plant will allow delivery of hot high-carbon DRI directly to the new meltshop, which will allow further reductions in EAF operating costs. Usiba DR plant in Brazil has been converted to the HYL III process. Incorporation of the combustion chamber has allowed the plant capacity to increase from 240 to 320,000 tonnes/year of DRI with carbon levels of about 2.5%, while taking advantage of the existing low temperature heaters, thus decreasing significantly the project capital cost. The plant started up on Dec. 25 and the performance test was successfully achieved at the beginning of Feb Overall benefits of high-carbon and HYTEMP iron in the EAF The combined effects of high temperature and high carbon content of DRI have a positive influence on the productivity of the EAF, arising from the corresponding decrease of the electric energy required to melt the charge. The sensible heat of the DRI results in a lower electric energy consumption in the furnace, increasing productivity and reducing related operating costs, such as electrodes, refractories and fluxes. Additionally, the high carbon content (mostly as iron carbide) plays a significant role in providing energy to the system in a clean and easy manner, without graphite additions to the bath. This also allows the use of increased amounts of oxygen, which has the positive effect of increasing productivity. The amount of oxygen consumption in the EAF using different charges of DRI at various carbon levels is presented in figure 5. This information is used as basis for the estimation of electricity consumption and productivity in the EAF. Experimental results have been extrapolated for an EAF of 100 t/heat. Figure 6 shows the decrease in the amount of electrical energy needed for melting in the EAF according to the carbon content in DRI: at 15% and 60% of DRI as metallic charge, and two levels of DRI temperature: ambient and 600 C. Figures are calculated for DRI of 92% metallization and use of 3.7 kg of graphite. Figure 7 depicts the productivity increase of the EAF, in terms of the corresponding power on time - that is, the time required for melting and refining - depending on DRI temperature, DRI carbon and metallic charge composition. HYL III: Status and Trends (3/95) 12

13 Figure 5 EAF Oxygen Consumption using different charges of DRI with different Carbon levels % DRI 30% DRI 45% DRI 60% DRI % Carbon in DRI Figure 6 Effect of DRI Temperature and Carbon Content on Electricity Consumption % DRI 2.2% C 15% DRI 4.0% C 60% DRI 2.2% C 60% DRI 4.0% C Oxygen consumption Nm 3 O2/tls 13 13/ DRI Temperature, C HYL III: Status and Trends (3/95) 13

14 Figure 7 Effect of Temperature and Carbon on Melting and Refining Time % DRI 2.2% C 15% DRI 4.0% C 60% DRI 2.2% C 60% DRI 4.0% C Temperature, C Conclusions When combined with Corex off-gas as a source of reducing gas, the HYL III DR plant offers high productivity using available spent gas, and benefits in steel production by using HYTEMP iron together with hot metal in EAF/BOF based steel mills. The HYL III plant scheme allows zero electric power consumption with total thermal energy requirements of about 2.50 Gcal/ton of product. This has been achieved by the efficient utilization of the steam required for final users, with the adequate specification of the steam thermodynamic path. This scheme presents advantages in terms of the DR plant investment and operating costs, and offers a unique position for DR plants to be located far from electric power distribution facilities. Carbon levels ranging from 1.2 to >4.0% can be achieved in the HYL III process for both DRI or hot DRI (HYTEMP iron). Benefits of this process scheme are related to important improvements of EAF operating costs and productivity. HYTEMP iron and high carbon DRI (mostly as iron carbide) play a significant role in providing energy to the EAF by decreasing electricity consumption and allowing utilization of oxygen, thus leading to the positive effect of increased productivity. HYL III: Status and Trends (3/95) 14