Ph.D. candidate Inha Tsurkan

SHEI “National Mining University”, Dnipropetrovsk

Ways to reduce sintering plants’ costs due to

energy efficiency improvement with special emphasis on emissions

Iron and steel industry is one of the most intensive material and energy consuming productions. More than a half of the incoming raw materials are converted into gas wastes, solid wastes or by-products after processing. Due to the adverse ecological situation worldwide the creation of "clean energy technologies" with zero-emission and zero-material-waste, on the one hand, and top efficiency in energy consumption, on the other hand, becomes the issue of primary importance.

A typical integrated mill is comprised of a coke oven, sintering plant, blast furnace (BF) and a basic oxygen furnace (BOF), which are necessary for full metallurgical cycle of steelmaking. Firstly, most of the iron ore is agglomerated to obtain sinter or pellets. Then the agglomerates are charged, together with coke and coal, into a blast furnace which produces hot metal (pig iron). Most of the carbon in the hot metal is removed in the basic oxygen steelmaking (BOS) plant, producing the crude liquid steel. At last, the liquid steel could be cast into semi-finished products.

For integrated mills production process is contingently divided into three distinct processes: the production of steel, the production of iron, and the production of sinter. Thus, it is possible to more accurately calculate the amount of harmful emissions and energy efficiency at every stage of production taking into account the features of technological processes. As for the number of pollutants, the emission from sintering plants is much higher than the general volume of emission from integrated mills.

Sintered ore, produced from fine iron ore, iron bearing recycled materials, coke breeze and fluxes are used in the blast furnace as raw material. The sinter feed is deposited as a bed and the coke in the upper layer is ignited. As the sinter bed moves forward, air is drawn through the bed to maintain combustion of the coke.

A sintering system plays a key role at the plants with a full iron and steel production cycle by using residues which otherwise would be disposed. Slag from steel production, dust out of filters of various furnace gas purification systems (including those relating to the sinter plant), and various ferrous materials from the processing residues are recycled in the sintering plant.

Sintering technologies have the potential for heat recovery increase, emissions reduction, electric efficiency, fuel substitution transportation, and energy and fuel storage.

There are two main ways of energy efficiency improvement for sintering plant: managerial and physical. This paper is focused on intensification of the making processes and addition of physical elements to the plant. The list given below describes enhancing technologies from the perspective of energy consumption and emissions.

1) The increase of energy conversion efficiency. Installation of boilers and power plants near the site to generate steam and electricity, which are mainly heated with by-product gaseous fuel, produced from their coke plant, blast furnaces and BOS plants.

2) Waste gas and heat recovery.

2.1) The hot exhaust gas from the sinter bed can be returned to it as combustion air, thus reducing energy consumption through savings in coke use.

2.2) The energy from hot sintered ore is recovered at the end of the sinter bed using a sintered ore cooling system, where air is heated up and can later be used to generate steam.

3) Optimized sinter-pellet ratio. The CO₂ emission related to pellet production is lower than to sinter production. However, only very few Blast Furnaces operate on high pellet concentrations. The purpose is to achieve a sinter-pellet ratio of at least 50/50 for each Blast Furnace in order to reduce CO₂ emissions and increase energy savings. The consumption of coke and coal per tonne of hot metal is not affected by this change.

4) Top gas recovery turbine (TGRT). The top gas from the Blast Furnace has an over-pressure which can be used to produce additional electricity using a TGRT. Although the over-pressure is low, the large gas volumes mean that the energy recovery is still economically feasible.

5) Stove waste gas and heat recovery (WGHR). WGHR system lets the thermal energy of waste gas, discharged from the Hot Blast Stoves, is partially recovered in external heat exchangers and typically used to pre-heat the BF-gas and/or combustion air. This reduces or even eliminates the consumption of enrichment gas.

6) Pulverized coal injection (PCI) improvement. PCI leads to the cost saving due to lower coke rates. Replacing coke by pulverized coal injection does not save energy in the blast furnace itself, but rather in the coke making process. Although most major sites have been equipped with PCI systems, their average injection rate is 130 kg/t hot metal. Some experts believe in higher cost-effectiveness when an injection rate is about of 230 kg/t hot metal.

The special issue is the potential re-use of solid sinter plant emissions as separate chemical elements. Volatile dust emissions can arise during handling and transportation of the raw materials and of the cooled sinter as well as during maintenance and unplanned interruptions of the cyclones or filters. It is important that due to the strong thermal convection in the sinter hall volatile emissions are likely to get through leakages in the roof particularly at the end of the sinter belt. At a sinter plant emissions can be “direct” (stack emissions) and “indirect” (volatile emissions).

According to CORINAIR, all standard gaseous compounds except for ammonia are emitted by sinter plants during the combustion and industrial processes: SO_х, NO_x, heavy metals, polycyclic organic material etc.

SO_х is mostly originated from sulphur contained in coke used as fuel. Actual emissions depend on the basicity of the mixture. For example, when magnesium oxide (MgO) dominates, almost all of the sulphur content is converted into SO₂. The major part of the total SO₂ emission is generated in the sinter belt hot part.

NO_x is mainly emitted as NO due to rapid downcooling of the flue gases. NO_x emissions originate from nitrogen which is contained in coke (about 80 %) and iron ore (about 20 %).

Heavy metals are presented in dust emissions. For example, raw materials usually contain zinc (Zn), lead (Pb), cadmium (Cd), and arsenic (As).

Polycyclic organic material, for example, can be formed from chlorine and oily additives.

As for the other elements, they depend on raw material treatment, such as emissions of hydrochloric acid as a result of seawater moistening.

Heavy metals can be removed by passing through the dry gas cleaning facilities (for example, Electrostatic Precipitators) which reduce dust load in off-gas of a typical plant from 3,000 mg/m3 to about 50-150 mg/m3. Accumulated fine particles are used during the sinter return cycle.

To improve steam recovery and increase total fuel savings all of the contaminants (SO_x, NO_x, dioxins, and dust) are processed, absorbed, after that decomposed, and finally collected as non-toxic by-products. NO_x is decomposed to nitrogen, water and oxygen by ammonia. Dioxins are collected or absorbed in activated coke and decomposed at 400°C with no-oxygen. Dust is collected in activated coke.

With the combination of all these methods, it can be achieved costs cut down on 12-24%. Moreover, due to agglomeration of industries such “useful wastes” can be used not only in iron and steel industry, but also in such continuous processes as cement, aluminum production, metal casting, glass industry and others. Synergies among these large energy consumers will lead to environmental, energy-saving and commercially effective cooperation.

References

1) Dinis C. M., 2010, Modeling and Simulation of Processes from an Iron Ore Sintering Plants, ISBN: 978-953-307-125-1

2) Jose A.M., Nicolas P., 2013, The potential for improvements in energy efficiency and CO₂ emissions in the EU27 iron and steel industry under different payback periods, Journal of Cleaner Production №52, p.71-83

3) Villar A., Arribas J.J., Parrondo J., 2012, Waste-to-energy technologies in continuous process industries, Springer №14, p.29-33

4) EMEP/EEA air pollutant emission inventory guidebook - 2013, http://www.eea.europa.eu