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2024 (English)In: Fuel, ISSN 0016-2361, E-ISSN 1873-7153, Vol. 378, article id 132959Article in journal (Refereed) Published
Abstract [en]
Industrial thermal plasma torches can heat a gas up to 5000–20,000 K, i.e., well above the temperature needed to replace the heat generated from the combustion of traditional fossil fuels (e.g., coal, oil, and natural gas) in large-scale process industry furnaces producing construction materials (e.g., iron, steel, lime, and cement). However, there is a risk for significant NOx emissions when air or N2 are used as plasma-forming gas since the temperature somewhere in the furnace always will be higher compared to the threshold NOx formation temperature of ∼1800 K. Torch NOx forms inside the high temperature region of the plasma torch (>5000 K) when air is used as gas. Process NOx forms instead when the hot gas (when air or nitrogen is used as plasma forming gas) from the plasma torch mixes with process air downstream the torch. By analysing the complex chemistry of both the torch- and process NOx formation with thermodynamic equilibrium and one-dimensional chemical kinetic calculations it was shown that adding H2 to the plasma-forming N2 gas significantly reduces the NOx emissions with more than 90 %. Verifying experiments with air, pure N2, and mixtures of H2 and N2 as plasma-forming gas were performed in a laboratory scale insulated laboratory furnace with different pre-heating temperatures of process air (293, 673, and 1073 K) which the plasma gas mixes with downstream the torch. Depending on the pre-heating temperature the NOx emissions were between 12,000–14,000 mg NO2/MJfuel when air was used as plasma forming gas. Substantial NOx emission reduction occurs both when N2 replaces air, where the NOx emissions was in the span of 8000–11,500 mg NO2/MJfuel and furthermore when H2 was mixed into the N2 gas stream. For the highest degree of H2 mixing (28.6 vol-%), the NOx emissions were between 450–1700 mg NO2/MJfuel depending on the pre-heat temperature of the process air, i.e., a reduction of 88–96 % and 85–94 %, respectively when air or N2 was used as plasma forming gas. The measured NOx emissions are then of the same order of magnitude as would be expected from the combustion of traditional fuels (coal, oil, biomass and pure H2). Finally, by analysing the aerodynamics in an axisymmetric furnace with an experimentally validated computational fluid dynamics (CFD) model using reduced chemistry for the NOx formation (19 species and 70 reactions), further guidelines into the process of NOx reduction from thermal plasma torches are given.
Place, publisher, year, edition, pages
Elsevier Ltd, 2024
Keywords
Catalytic cracking; Coal combustion; Coal industry; Coal oil mixtures; Lignite; Melt spinning; Metal casting; Metal castings; Natural gas wells; Oil shale; Petroleum tar; Pipelines; Plasma welding; Preheating; Putty; Steelmaking furnaces; Wire drawing; Down-stream; Nitrogen oxide emissions; NO 2; NO x; NO x emission; Plasma forming gas; Pre-heating; Process industries; Thermal plasma; Thermal plasma torch; Plasma torches
National Category
Materials Engineering
Identifiers
urn:nbn:se:ri:diva-74989 (URN)10.1016/j.fuel.2024.132959 (DOI)2-s2.0-85202556500 (Scopus ID)
Note
This work was conducted as part of the Hydrogen Breakthrough Ironmaking Technology (HYBRIT) research project RP1. The Swedish Energy Agency is acknowledged for financial support of HYRIT. HYBRIT is a joint initiative of SSAB, LKAB, and Vattenfall with the aim of developing the world’s first fossil-fuel-free ore-based steelmaking route. Sofia Nordqvist (LKAB/HYBRIT), Christian Fredriksson (LKAB), and Fredrik Normann (LKAB/Chalmers Technical University) are also acknowledged for their constructive suggestions
2024-09-092024-09-092024-09-09Bibliographically approved