Emerging trends in improving Blast Furnace (BF) performance of ironmaking – More environment-friendly and cost effective proposition:

Emerging trends in improving Blast Furnace (BF) performance of ironmaking – More environment-friendly and cost effective proposition:

Blast furnace (BF) technology is the central to the crude steel industry and is continually undergoing refinements to improve productivity and reduce operating costs. Continuous improvements in productivity, coke consumption and fuel use within steel works have been driven by competition in world steel market.

The major raw materials generally used in blast furnace technology are high quality of iron ore (lumps, sintered or pallets), coal (ranging from coke to coking coal) as fuel, limestone and dolomite as fluxing materials.

A. At present, the real challenge in ironmaking industry by blast furnace lies to ensure that each process meets the emission limits prescribed by law and for that purpose each process is to examine for (a) use of energy, (b) use of material, (iii) waste generation for the entire life cycle of the project. These factors are believed to be root cause of environmental degradation. Some the processes which can economize on energy, process waste and material inputs are:

(i) High temperature hot blast technology for Blast Furnaces for lower energy consumption.

(ii) Cast House Slag Granulation Technology for utilizing Blast Furnace slag.

(iii) Ceramic welding technology for increasing life and reducing energy loss in coke oven.

(iv) Bell-less top technology for BF for increasing productivity and reducing coke rate.

(v) Continuous casting technology for reducing energy consumption and process waste in steel casting.

(vi) Water treatment technologies for economical water management.

B. As mentioned, coal / coke is an important raw material, which constitutes about 42% of the cost of sales and 58% of the raw material cost of an Integrated Steel Plant. The conventional blast furnace route of iron making needs prime coking coal. In order to reduce the cost of fuel (coke) and to use more of coking or non-coking coal replacing most of coke, many of the emergent technologies have been developed and tried. Many of these coals, extractable through lowcost mining, can be injected directly through the tuyeres of blast furnaces, substituting good quality coke in a very cost-effective manner.

C. In one example, during the implementation of improved method of coal injection to blast furnace, this inferior coal replaces almost same weight of blast furnace grade coke, which is produced from almost one and one-half unit of prime coking coal (at more than double the cost), after carbonization for several hours at substantial processing cost. The coke ovens are very capital intensive and pollute the atmosphere. Thus, coal injection not only reduces the operating cost, but also saves capital expenditure substantially, while maintaining greener and cleaner environment. More important and helpful to the blast furnace (BF) operator is that it facilitates operational stability and optimization by providing endothermic heat to control the combustion. There are two modes of coal injection:

(a) The pulverized coal injection (PCI) and

(b) The granular coal injection (GCI).

Most of the newly constructed blast furnaces generally have installed PCI, a very common technology available everywhere. However, the existing blast furnaces with no injection facilities can go for GCI, which is less energy intensive and more environment friendly. The savings using appropriate variety of low cost non-coking coal would be of the order of about US$ 4 to 6 million per annum per blast furnace of 1 Mtpa capacity.

Experts opine that, for coal injection, GCI is emerging as a more cost effective and energy efficient technology.

D. The advantage of GCI over PCI is given here:

(i) Reduction in coal preparation costs due to low energy consumption (GCI: 20kWh/t, PCI: 32kWh/t);

(ii) Easier to handle in pneumatic conveying systems since granular coal is less sticking to the conveying pipe.

(iii) System availability is more;

(iv) Granular coal’s coarseness delays gas evolution and temperature rise associated with coal combustion in the raceway. Therefore, it is favorable compared to PCI because of less likely generation of high temperature and gas flows at the furnace walls, which results in high heat losses, more refractory wear and poor utilization of reducing gases;

(v) Granular injection system is superior while using low volatile coal to avoid line plugging and other related problems. Thus, the use of granular coal may increase the range of coals available for blast furnace injection.

(vi) There is a significant economic advantage to using granular coal over pulverized coal, since not only is less grinding equipment required resulting in capital savings, but operating costs are also reduced as approximately 60% less grinding energy is required for granular coal. Capital including infrastructure costs for GCI are lower than those for PCI. (PCI ~ US$ 43,000 whereas GCI ~ US$ 37,500 per daily ton of injected coal).

Coke making process and its environmental impacts:

Coke making process and its environmental impacts:

Coke is a solid carbon fuel and carbon source used to melt and reduce iron ore. Coke and coke by-products, including coke oven gas, are produced by the pyrolysis (heating in the absence of air) of suitable grades of coal. The process also includes the processing of coke oven gas to remove tar, ammonia (usually recovered as ammonium sulfate), phenol, naphthalene, light oil, and sulfur before the gas is used as fuel for heating the ovens.

A. Coke making process: In the coke-making process, bituminous coal is fed (usually after processing operations to control the size and quality of the feed) into a series of ovens, which are sealed and heated at high temperatures in the absence of oxygen, typically in cycles lasting 14 to 36 hours. Volatile compounds that are driven off the coal are collected and processed to recover combustible gases and other by-products. The solid carbon remaining in the oven is coke. It is taken to the quench tower, where it is cooled with a water spray or by circulating an inert gas (nitrogen), a process known as dry quenching. The coke is screened and sent to a blast furnace or to storage. Coke oven gas is cooled, and by-products are recovered. Flushing liquor, formed from the cooling of coke oven gas, and liquor from primary coolers contain tar and are sent to a tar decanter. An electrostatic precipitator is used to remove more tar from coke oven gas. The tar is then sent to storage. Ammonia liquor is also separated from the tar decanter and sent to wastewater treatment after ammonia recovery. Coke oven gas is further cooled in a final cooler. Naphthalene is removed in the separator on the final cooler. Light oil is then removed from the coke oven gas and is fractionated to recover benzene, toluene, and xylene. During the coke quenching, handling, and screening operation, coke breeze is produced. It is either reused on site (e.g., in the sinter plant) or sold off site as a by-product.

B. Pollution during coke making process:

The coke oven is a major source of fugitive air emissions. The coking process emits particulate matter (PM); volatile organic compounds (VOCs); polynuclear aromatic hydrocarbons (PAHs); methane, at approximately 100 grams per metric ton (g/t) of coke; ammonia; carbon monoxide; hydrogen sulfide (50–80 g/t of coke from pushing operations); hydrogen cyanide; and sulfur oxides, SOx (releasing 30% of sulfur in the feed). Significant amount of VOCs may also be released in by- product recovery operations. For every ton of coke produced, approximately 0.7 to 7.4 kilograms (kg) of PM, 2.9 kg of SOx (ranging from 0.2 to 6.5 kg), 1.4 kg of nitrogen oxides (NOx), 0.1 kg of ammonia, and 3 kg of VOCs (including 2 kg of benzene) may be released into the atmosphere if there is no vapor recovery system. Coal-handling operations may account for about 10% of the particulate load. Coal charging, coke pushing, and quenching are major sources of dust emissions.

Wastewater is generated at an average rate ranging from 0.3 to 4 cubic meters (m3) per ton of coke processed. Major wastewater streams are generated from the cooling of the coke oven gas and the processing of ammonia, tar, naphthalene, phenol, and light oil. Process wastewater may contain 10 milligrams per liter (mg/l) of benzene, 1,000 mg/l of biochemical oxygen demand (BOD) (4 kg/t of coke), 1,500–6,000 mg/l of chemical oxygen demand (COD), 200 mg/l of total suspended solids, and 150–2,000 mg/l of phenols (0.3–12 kg/t of coke). Wastewaters also contain PAHs at significant concentrations (up to 30 mg/ l), ammonia (0.1–2 kg nitrogen/t of coke), and cyanides (0.1–0.6 kg/t of coke). Coke production facilities generate process solid wastes other than coke breeze (which averages 1 kg/t of product). Most of the solid wastes contain hazardous components such as benzene and PAHs. Waste streams of concern include residues from coal tar recovery (typically 0.1 kg/t of coke), the tar decanter (0.2 kg/t of coke), tar storage (0.4 kg/t of coke), light oil processing (0.2 kg/t of coke), wastewater treatment (0.1 kg/t of coke), naphthalene collection and recovery (0.02 kg/t of coke), tar distillation (0.01 kg/t of coke), and sludges from biological treatment of wastewaters.

C. Pollution Prevention and Control: Pollution prevention in coke making is focused on reducing coke oven emissions and developing coke-less iron & steel-making techniques. The following pollution prevention and control measures should be considered.

1. General -

(a) Use cokeless iron- and steel-making processes, (b) such as the direct reduction process, to eliminate the need to manufacture coke. (c) Use beneficiation (preferably at the coal mine) and blending processes that improve the quality of coal feed to produce coke of desired quality and reduce emissions of sulfur oxides and other pollutants. (d) Use enclosed conveyors and sieves for coal and coke handling. Use sprinklers and plastic emulsions to suppress dust formation. Provide windbreaks where feasible. Store materials in bunkers or warehouses. Reduce drop distances. (e) Use and preheat high-grade coal to reduce coking time, increase throughput, reduce fuel consumption, and minimize thermal shock to refractory bricks.

2. Coke Oven Emissions –

(a) Charging: dust particles from coal charging should be evacuated by the use of jumper-pipe systems and steam injection into the ascension pipe or controlled by fabric filters.

(b) Coking: use large ovens to increase batch size and reduce the number of chargings and pushings, thereby reducing the associated emissions. Reduce fluctuations in coking conditions, including temperature. Clean and seal coke oven openings to minimize emissions. Use mechanical cleaning devices (preferably automatic) for cleaning doors, door frames, and hole lids. Seal lids, using a slurry. Use low-leakage door construction, preferably with gas sealing.

(c) Pushing: emissions from coke pushing can be reduced by maintaining a sufficient coking time, thus avoiding “green push.” Use sheds and enclosed cars, or consider use of traveling hoods. The gases released should be removed and passed through fabric filters.

(d) Quenching: where feasible, use dry instead of wet quenching. Filter all gases extracted from the dry quenching unit. If wet quenching, is used, provide interceptors (baffles) to remove coarse dust. When wastewater is used for quenching, the process transfers pollutants from the wastewater to the air, requiring subsequent removal. Reuse quench water.

(e) Conveying and sieving: enclose potential dust sources, and filter evacuated gases.

3. By-Product Recovery –

(a) Use vapor recovery systems to prevent air emissions from light oil processing, tar processing naphthalene processing, and phenol and ammonia recovery processes.

(b) Segregate process water from cooling water.

(c) Reduce fixed ammonia content in ammonia liquor by using caustic soda and steam stripping.

(d) Recycle all process solid wastes, including tar decanter sludge, to the coke oven.

(e) Recover sulfur from coke oven gas. Recycle Claus tail gas into the coke oven gas system.

Environment-friendly Corex process of iron and steel making:

Environment-friendly Corex process of iron and steel making:

The highlight of the process is it does not require coking coal. The process differs from the conventional blast furnace route; where un-treated non-coking coal can be directly used for ore reduction and melting work, eliminating the need for coking plants. The use of lump ore or pellets also dispenses with the need for sinter plants.

The ability to operate without coke gives this process two environmental advantages over the conventional blast furnace. First, because coke ovens are not needed, all of the problems associated with the generation of benzene and other coal tar byproducts are eliminated. Second, the dust problems associated with blast furnaces are also eliminated because the off-gas is used as fuel. This process along with direct reduction of iron (DRI) process is being implemented in many countries. The off-gas obtained being used to fuel the adjoining DRI plant.

Viewing the process from the coal-route perspective, non-metallurgical coal is directly charged into the melter gasifier. Due to the high temperatures predominating in the dome of the melter gasifier (in excess of 1000 °C), a portion of the hydrocarbons released from the coal during devolatilization are immediately dissociated to carbon monoxide and hydrogen. Undesirable by-products such as tars and phenols, etc. are destroyed and therefore cannot be released to the atmosphere. Combustion with oxygen injected into the melter gasifier results in the generation of a highly efficient reduction gas. Hot metal and slag tapping are carried out as in conventional blast furnace practice. The quality of the hot metal is equivalent to that produced in a blast furnace.

Environmental Aspects: In this system emissions contain only insignificant amounts of NOx, SO2, dust, phenols, sulphides and ammonium. Emission values already exceed by far future European standards. Also, waste-water emissions from are far lower than those in the conventional blast-furnace route. These environmental features are additional key reasons for the attractiveness of the present system.