| Description |
Many industrial processes rely on heating that is currently achieved through combustion of fossil fuels. The industrial sector generates approximately 23 percent of the greenhouse gas emissions in the US. As infrastructure associated with production and distribution of hydrogen continues to expand, replacement of carbon containing fossil fuels with non-carbon hydrogen for process heating in industrial plants is becoming an interesting option for reductions of greenhouse gases in the industrial sector. Hydrogen has several characteristics that create challenges when considering replacement of natural gas or other hydrocarbon fuels. The high adiabatic flame temperature and high flame speed of hydrogen-air combustion can lead to significant increases in NOx emissions, as well as large impacts to the thermal profile. The high flame speed limits the practical ability to rely on lean pre-mixing to mitigate high NOx emissions, as has been successfully implemented in low NOx natural gas burners. However, the wide flammability limits of hydrogen make fuel and air staging, along with induced flue gas recirculation, attractive options for limiting NOx emissions. Both 1-D process modeling and 3-D computational fluid dynamics (CFD) modeling are useful tools which will be heavily utilized for assessment of NOx emissions and thermal performance as industrial boilers and furnaces convert to higher concentrations of hydrogen in fuels. Simulations of idealized reactor networks with well verified detailed chemical kinetic mechanisms effectively show how fuel and air staging, as well as gas recirculation, can impact the peak flame temperature to reduce NOx emissions. These calculations can be used to approximate the degree of staging and/or the amounts of gas recirculation that must be achieved to target specific NOx emission limits. These estimates can be further refined by incorporation of realistic descriptions of fuel and air mixing, coupled with finite rate chemical kinetics within CFD simulations for assessment of NOx emissions and heat flux distributions. This paper will show results of both idealized reactor network calculations and 3-D CFD simulations to quantify impacts of hydrogen air combustion compared to natural gas combustion in a full-scale industrial furnace. |
