Ironmaking by Blast Furnace and Carbon di Oxide Emissions
Ironmaking by Blast Furnace and Carbon di Oxide Emissions
It is widely recognised that carbon di-oxide (CO2) in the atmosphere is the main component influencing global warming through the green-house effect. Since 1896 the concentration of CO2 in the atmosphere has increased by 25 %. The iron and steel industry is known as an energy intensive industry and as a significant emitter of CO2. Hence, climate change is identified by the iron and steel industry as a major environmental challenge. Long before the findings of the Inter-governmental Panel on Climate Change in 2007, major producers of iron and steel recognized that long term solutions are needed to tackle the CO2 emissions from the iron and steel industry. Therefore, the iron and steel industry has been highly proactive in improving energy consumption and reducing greenhouse gas (GHG) emissions.
In the present environment of the climate change, within the iron and steel industry, there is a constant drive to reduce energy costs, reduce emissions and ensure maximum waste energy re-use. In the traditional processes for producing iron and steel, emission of CO2 is inevitable, especially for the blast furnace (BF) process, which requires carbon (C) as a fuel and reducing agent to convert iron oxide to the metallic state, and hence is the main process for the generation of CO2 in an integrated iron and steel plant. Climate policy is in fact, an important driver for further development of the ironmaking technology by BF.
Critically, amongst the challenges facing the BF operation is decarbonization. Significant steps have been made by the iron and steel industry to increase the thermal efficiency of the BF operation, but ultimately there is a hard limit in decarbonization, associated with the need for C as a chemical reductant. Since the 1950s, significant R&D (research and development) efforts have been carried out to make more efficient the BF ironmaking technology. These R&D efforts include (i) improved coke and sinter quality, (ii) oxygen (O2) enrichment,(iii) injection of other reductants like pulverized coal and natural gas, (iv) burden distribution, and (v) measurement technologies and so on. In the 1950s the reductant rate was around 1000 kilograms per ton of hot metal (kg/tHM), and since then it has been reduced by a factor of 2 because of the R&D efforts and the implementation of the outcomes of the R&D efforts.
The reducing agent consumption at the conventional BF is nowadays with around 500 kg/tHM approaching only 5 % above the lowest possible thermodynamic values under classical BF operation. The BF process is now a highly developed process operating close to thermodynamic limits of efficiency. There are no obvious major enhancements which are expected fundamentally to reduce its C demand or significantly improve its thermal efficiency, but, since the BF is the predominant emission generator, efforts to mitigate the environmental impact of the industry have, of necessity, are to be focused on the BF ironmaking process.
Breakthrough ironmaking technologies are required for further significant reduction of C consumption or CO2 emission. Several technologies have been proposed for further reduction of the fossil C usage and the reduction of the CO2 emission in the BF process itself. These include (i) recycling of CO from the BF top gas, (ii) usage of biomass, (iii) substitution of CO by H2 as a reducing agent, (iv) usage of C-lean direct reduced iron (DRI), hot briquetted iron (HBI), or low reduced iron (LRI), (v) Use of C composite materials, (vi) usage of C-lean electrical energy, and (vii) CO2 capture and storage (CCS) etc. However, the needed approach is to be to propose incremental improvements that offer steps to reduce emissions or to produce more from the potential which exists within the current process.
It is inevitable, that when considering such technologies a number of cross-cutting themes around economics and overall CO2 emissions need to be considered. For example, the use of CO2 and process gases as chemical feed stocks can require additional purchase of fuels for the reheating furnaces, which can impact on integrated works costs, steel quality and total CO2 emissions. Any solution to be taken further for consideration needs to have the potential to achieve a multi-component optimization of these individual aspects.
The key challenges facing future BF operation are therefore (i) reducing capital and operating expenditures significantly to generate a sustainable return on capital expenditure throughout the economic cycle, and (ii) reducing effective CO2 emissions to a point even below that determined from chemical thermodynamics of the conventional coke based process. For facing these challenges, it is essential to identify a number of technology opportunities. These are described below.
Top gas recycling and carbon capture technologies
The reduction of input C is limited by the reduction equilibrium of gas in the BF. The decrease of input C can be achieved by lowering the direct reduction ratio (an endothermic reaction) by strengthening gas reduction inside the BF through decarbonization and recirculation of top gas by injection into the furnace. Typical flow sheet of blast furnace with top gas recycling (TGR) is shown in Fig 1.
Fig 1 Typical flow sheet of blast furnace with top gas recycling
Any solution for de-carbonization of the BF route requires some element of C capture. For achieving substantial CO2 reduction (greater than 50 %), the application of CCS technology is necessary, though there is general consensus in the industry that reductions greater than 80 % are not possible. One encouraging variation on C capture is top gas recycling in the process of ironmaking by the BF process. It is the most promising technology which can significantly reduce the CO2 emission and consists of recycling of CO and H2 from the gas leaving the BF from the top.
TGR technology is mainly based on lowering the usage of fossil C (coke) by re-usage of the reducing agents (CO and H2) after the removal of the CO2 from the top gas. This leads to lower energy requirements. The main technologies of the TGR-BF are (i) Scrubbing of CO2 from the top gas and injection of the balance reducing top gas components CO and H2 in the BF shaft and hearth tuyeres, (ii) lower fossil C input due to lower coke rates, (iii) usage of pure O2 instead of hot air blast at the hearth tuyere i.e. removal of nitrogen (N2) from the process, and (iv) recovery of pure CO2 from the top gas for underground storage.
Most C capture schemes are generally associated with storage, but utilization can also be considered. This connection between C capture and utilization highlights an important area of research which is currently of interest is around process integration. Compared with aspects such as collection, transport and storage, the area of process integration by retrofitting of an existing BF with a C capture system, has received little consideration.
It is to be expected that for the majority of the sites where BFs are operating, C capture is to be commissioned alongside BFs which have operated for many decades. There is the potential for a significant level of process interference associated with aspects such as gas quality, pressure, operational protocols and the relative optimization of both the BF and C utilization plant. Retrofitting and subsequent operation are needed to be achieved without compromising either operational efficiency, or product quality, of the existing assets.
In this area of process integration, the advanced process simulation and modeling techniques are to be deployed to optimize the combination of an integrated BF and C capture system. In this respect a combination of thermo-fluid modeling, with process kinetics and through process economic modeling, aligned to an understanding of the key ironmaking process parameters are required. Given such a focus, the application of C capture to existing BF operations can be realized.
The important environmental challenge for the BF process is the use of C as a chemical reductant. This has a hard thermodynamic limit, below which further C reduction is not possible without a significant process change. One such process change is a partial switch from C to hydrogen (H2) as the reductant. Examples of reducing agents with high H2 content are waste plastics (CnHm) or natural gas (with main component CH4). H2 is already used in direct reduction processes for the production of DRI and so there is a basic understanding of the mechanisms and chemical thermodynamics, but there is an opportunity for further process research and innovation around the extent to which the balance between H2 reduction and C reduction can be shifted within the furnace.
Use of waste plastics (WP) to promote H2 reduction in BF is done through injection of WP in the BF. WP is injected as a solid through the tuyeres in a similar way to pulverized coal (PC). Normally it is done as a co-injection of WP and coal into the BF. The combustion energy of WP is generally at least as high as that of PC normally injected, and their higher ratio of H2 to C means less CO2 is produced within the BF from the combustion and iron ore reduction processes. Also, there is lower energy consumption since H2 is a more favourable reducing agent than C. Injection of WP increases the bosh gas H2 concentration. Since the chemical reaction rate of H2 reduction is higher than that of CO, the extent of Boudouard reaction reduces as bosh gas H2 increases. CO2 and H2O are present in the upper part of the BF due to the reduction of iron oxides.
For promoting H2 reduction in the blast furnace, another method is being investigated through the COURSE50 Project in Japan, work on which has begun in 2008.This project is an attempt to reduce CO2 emissions by further developing the technique of injecting reducing gas into the BF shaft, in combination with H2 amplification by reforming coke oven gas. The H2 reduction technology proposed by this project consists of H2 increase by (i) gas reforming of coke oven gas, (ii) H2 ore reduction technology, and (iii) coke making technology for H2 reduction blast furnace. In this project, the reducing gas is injected into the BF shaft. From the momentum balance of two gases, it has been found that the penetration area of shaft injection gas is proportional to the injection gas rate and iron ore reduction is promoted by H2. However, since H2 reduction is an endothermic reaction, special attention is needed for the maintenance of the temperature at the furnace top.
Alternative carbon bearing materials
Alternative C bearing materials are C composite agglomerates (CCA) or C iron composites (CIC). These are agglomerates of carbonaceous material and iron oxide mixture and are a kind of formed coke containing metallic iron. The carbonaceous material can be coke fines, coal, charcoal, C rich in-plant fines, biomass, waste plastics, etc., while the iron oxide can be low-grade iron ores, iron rich in-plant fines, etc. C composite materials due to the catalytic effect of the iron particles have remarkably high reactivity with CO2 gas in comparison with the metallurgical coke. Normally C composite materials react with CO2 gas from a temperature around 150 deg C lower than the metallurgical coke.
The ore reduction reaction is promoted by C composite materials because of (i) the higher reactivity of these materials, and (ii) the fact that solution loss reaction of these materials begins from a lower temperature. Utilization of such agglomerates not only help in mitigating CO2 emission but also help in coke and energy saving. The close distance between iron and C in such agglomerates improves the reaction kinetics significantly. The other benefits which can be visualized upon utilization of such agglomerates are (i) possibility of using iron and/or C rich in-plant fines, (ii) lower gasification temperature due to the coupling effect between the gasification reaction and iron oxide (wustite) reduction, and (iii) less dependency on CO2 and energy intensive ore preparation processes.
The method of producing C composite materials consists of crushing, blending, and briquetting inexpensive iron bearing materials and non-coking or slightly-coking coal, followed by heating and carbonization in a shaft furnace. The strength of these materials is an important property for BF feed, and the strength on the same level as metallurgical coke can be achieved, even from low quality raw materials, by the densification effect of briquetting and comparatively high accuracy temperature control in the shaft furnace.
The C bearing materials can also be introduced to the BF process through several ways. In the sintering process, biomass or WP can partially substitute coke breeze. In-plant fines can be used as source of both C and iron. In coke-making, attempts have been made to add biomass, as well as WP to the coking coal blend. Alternative carbon bearing materials can either be charged to the BF from the top along with burden materials as lumps or the C rich in-plant fines or biomass can be injected to the BF through the tuyeres.
Flue gas recycling within BF gas stoves
A new technology known as ‘flue gas recycling’ (FGR) is under development for the hot blast stoves. This technology involves conversion of the stoves, from air-fuel to oxy-fuel combustion increasing the CO2 percent of the flue gas. The flame temperature generated is going to be moderated by waste gas recirculation to the stove burners. Schematic comparison of conventional air-fuel stove operations and enhanced oxy-fuel operations employing the flue gas recycle is shown in Fig 2.
Fig 2 Schematic comparison of conventional air-fuel stove operations and enhanced oxy-fuel operations employing the flue gas recycle
FGR operation of stoves can be on the basis of constant mass or constant volume flow of the combustion products. Constant mass flow ensures that convective heat transfer is unchanged relative to the conventional air-fuel operations, and the recycle of hot flue gas reduces the combustion energy requirement of the stoves. The constant volume flow option arises because of the increased density of the combustion products, when flue gas is recycled. In this mode, heat recovery can be combined with increased burner gassing rates and this converts to higher hot blast temperatures and a potential for lower consumption of coke in the BF.
Considering the potential for C capture, the CO2 content of the flue gas is essentially doubled by comparison to conventional heating practices for the stoves. In mass terms the flue gas contains 0.8 tons of CO2/ton of hot metal (HM), which is more than one third of current specific emission levels. Generation of the O2 required to facilitate this reduces the C capture benefits marginally by virtue of the power consumed to operate the air separation plant. This reduces the net emission reduction potential by around 6 %.
Flue gas recycling in the stoves eliminates the use of both air and coke oven gas in the combustion process. Hence, generation of sulphur oxides and nitrous oxides are reduced substantially. The specific objectives of this new technology under development include (i) confirmation of a CO2 content of 40 % to 50 % in the modified flue gas, (ii) verification of waste heat recovery and enhanced thermal efficiency of the stoves, and (iii) confirmation that the new operating conditions maintain or increase the temperature of the hot blast delivered to the BF and hence avoids negative impacts on the BF operation.