Low-NOx thermal plasma torches: A renewable heat source for the electrified process industry

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 largescale process industry furnaces producing construction materials (e.g., iron, steel, lime, and cement). However, there is a risk for significant NO x emissions when air or N 2 are used as plasma-forming gas since the temperature somewhere in the furnace always will be higher compared to the threshold NO x formation temperature of ~1800 K. Torch NO x forms inside the high temperature region of the plasma torch (>5000 K) when air is used as gas. Process NO x 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 torchand process NO x formation with thermodynamic equilibrium and one-dimensional chemical kinetic calculations it was shown that adding H 2 to the plasma-forming N 2 gas significantly reduces the NO x emissions with more than 90 %. Verifying experiments with air, pure N 2 , and mixtures of H 2 and N 2 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 preheating temperature the NO x emissions were between 12,000-14,000 mg NO 2 /MJ fuel when air was used as plasma forming gas. Substantial NO x emission reduction occurs both when N 2 replaces air, where the NO x emissions was in the span of 8000-11,500 mg NO 2 /MJ fuel and furthermore when H 2 was mixed into the N 2 gas stream. For the highest degree of H 2 mixing (28.6 vol-%), the NO x emissions were between 450-1700 mg NO 2 / MJ fuel depending on the pre-heat temperature of the process air, i.e., a reduction of 88-96 % and 85-94 %, respectively when air or N 2 was used as plasma forming gas. The measured NO x emissions are then of the same order of magnitude as would be expected from the combustion of traditional fuels (coal, oil, biomass and pure H 2 ). Finally, by analysing the aerodynamics in an axisymmetric furnace with an experimentally validated computational fluid dynamics (CFD) model using reduced chemistry for the NO x formation (19 species and 70 reactions), further guidelines into the process of NO x reduction from thermal plasma torches are given.