Mechanism of NO x Formation from Nitrogen in the Combustion of the Coals Used in Sintering Process
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UCTION
IRON ore sinter is considered the main iron-bearing burden of the blast furnace (BF) ironmaking, which accounts for more than 75 pct of all the raw materials in BF.[1–3] Iron ore sintering process consumes about 10 pct of the total energy in an integrated steel plant of which 75-80 pct is supplied by the combustion of solid fuels, such as coke breeze and anthracite.[4,5] After the ignition of the sintering process, coal/coke particles are exposed to a combustion atmosphere, where N present in the fuel reacts with O2 to form NOx (98 pct represented by NO).[6] NOx formed in the sintering process is called fuel-NOx since it is generated from the
LEONARDO TOMAS DA ROCHA and SUNG-MO JUNG are with the Graduate Institute of Ferrous Technology (GIFT), Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea. Contact e-mail: [email protected] HYOJUNG KIM is with the POSCO, Pohang, Korea. CHANGMIN LEE is with the POSCO, Kwangyang, Korea. Manuscript submitted February 23, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS B
burning fuel-N present in coal/coke. In addition, there are two possible ways to form NOx: thermal-NOx by the reaction between N2 and O2 gases present in the air and prompt NOx in terms of the decomposition of nitrogen compounds in the combustion zone.[3,7,8] Among them, fuel-NOx represents the majority of total NOx formed in the sintering process.[8–15] Furthermore, the combustion of solid fuels in sintering bed materials is also the main source of CO2 and SOx. Nitrogen oxides (NOx) are one of the major air pollutants in the world, and very harmful to human health, directly related to photochemical smog and acid rain.[9,10,13,16] Iron ore sintering process represents more than 40 pct of the total NOx emissions from steel industries, which accounts for more than 10 pct of the total NOx emission among all kinds of industry.[3,6,12,14,17–21] Nowadays, steelmaking companies are facing bigger challenges for environmental protection due to change in gas emission regulation, especially in NOx case.[7,13,18,19] As an example, South Korea and China determined to reduce NOx emission by national law. This global concern about NOx emission is the driving force for the reduction of NOx in the sintering
process. In addition, the shortage in the supply of anthracite containing low nitrogen will lead to the use of the coals containing high nitrogen, which results in the emission of more NOx.[10–12,14] The previous studies have shown that sintering parameters such as temperature, oxygen content, heating rate, fuel type and additional materials had a notable influence on the nitrogen conversion ratio to NOx which decreased with increasing temperature— NOx reduction by CO (g) and C (s) were predominant at high temperatures,[22,23] but the opposite trend was verified by other studies.[6,24] NOx increased when oxygen content increased—more O2 was available to react with fuel-N.[23,25] Some studies showed that CaO could enhance the conversion ratio of nitrogen providing a non-desirable effect.[6,26–3
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