Oxycombustion with preheated reactants for low calorific value fuels: A solution to improve flame stability and energy efficiency
Résumé
In the global energy scenario, the demand will continue to grow by around a third by 2040, with the industrial sector accounting for around half of the overall increase, particularly for energy-intensive industrial sectors, such as steel and cement. Carbon emissions also will continue to rise, signaling the need for a comprehensive set of actions to develop, commercialize and adopt emerging energy-efficiency and CO2 emissions-reduction technologies. The Low Calorific Value (LCV) fuels generated in any industrial processes are today widely seen as a potential energy source. In steelmaking process, LCV fuels with CO/H2/CH4/CO2/N2 mixtures are typically used using air and blended either with natural gas or with a gas with higher calorific value such as the coke oven gas (COG). Having air to burn this type of gas limits the thermal efficiency due to the amount of heat wasted in the nitrogen, can provide high NOx emissions and, is not an economical viable alternative for Carbon Capture Use and Storage (CCUS) solution in the light of the very low concentration of CO2 in the flue gases. In addition to this and compare to typical natural gas, their use has several issues due to different combustion characteristics, such as flame velocity, adiabatic temperature, and flammability limits, which significantly influence their performance as fuel. Therefore, in order to guarantee the flame stability and allow a reliable operation, using LCV fuel without blending with any fuel with higher calorific value, the present work proposes the combination of technologies, such as oxycombustion and preheating of the reactants. This will also satisfy the actual need for increasing energy efficiency reducing the environmental impact in terms of pollutant emissions such as CO2 and NOx. Three different combined tasks at different scales were carried out. At laboratory scale, CORIA developed the understanding for preheated LCV/O2 combustion process using thermochemical calculations. This, together with a critical Damkohler number allowed calculating burner dimensions. The detailed flame characteristics were investigated through experimental tests using imaging OH* chemiluminescence imaging. Semi-industrial pilot scale were also performed at Air Liquide PIC, with the burner scaled up to 180 kW using the constant velocity and the geometrical similarity criteria. These tests were run applying similar operational conditions as laboratory scale. Finally, CMI used engineering simulation tools to evaluate the feasibility of this technology in radiant tube burners considering heat transfer, the presence of hot spots and flame length, as well as, potential CO2 emissions. The results show that significant enhancement of oxyfuel flame properties can be obtained thanks to the preheating of the fuel and oxygen. The analysis of flame stability diagrams as function of reactant velocities, thermal power, oxygen distribution and preheating temperatures points out the limits of flame stability and the stable combustion regimes achieved at the different operational conditions. Further analysis of the results shows that a critical convection velocity in the fuel-oxygen mixing layer, controls transitions between the various types of flames. This is quantified from the measurements, and matches with the theoretical prediction. At pilot scale, the flames show similar structures as obtained at laboratory scale, validating the physical phenomena that controls the limit of oxyfuel flames stability and, demonstrating the benefit of preheated reactants and oxyfuel combustion in the stabilization of LCV flames. Regarding the emissions, very low levels of pollutant emissions such as CO and NOx are achieved in most cases, depending on the oxygen distribution. Particularly NOx reached limits up to 32 mg/MJ at 3% excess oxygen, which is much lower than typical values of 100 mg/MJ from air-natural gas burners, showing a very high potential to reach the standards levels of NOx required for environmental regulations.
Simulations for reheating furnaces having a comparison between air and oxycombustion with preheated reactants show an efficiency increment of 8 points. For CO2 emissions, calculations for a particular case of a reheating furnace producing 50000 Ton/year of steel coil, the CO2 avoided is up to 62% due to the efficient use of the energy sources available, which includes avoiding flaring the blast furnace gas (BFG).
Domaines
Milieux fluides et réactifs
Origine : Fichiers produits par l'(les) auteur(s)
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