NO formation during co-combustion of coal with two thermally treated biomasses
Conference contribution, 2019
The behavior of biomass as a fuel varies a lot. Not only between different sources of raw material, but also depending on if they have been pre-treated, and if so, also depending on the type of treatment. Two types of thermal pre-treatments of woody biomass used for combustion in suspension systems are torrefaction and steam explosion. These two types of pre-treated biomass were investigated in this work with focus on the nitrogen chemistry, and were investigated both experimentally in a 1.5MWth combustion unit and by performing detailed reaction simulations. Three different cases have been investigated. One case with 100% Utah Sufco coal and two cases where 15% of the coal (on a mass basis) has been replaced with either torrefied or steam exploded biomass. Even though only 15% of the coal has been substituted there is a clear difference in the amount of NO formed between the cases. The pure coal had the highest amount of NO formed which was expected due to the higher amount of fuel-bound nitrogen in the coal compared to the biomasses. The fuel analyses indicate that the nitrogen content is the same in the two investigated bio fuels. Despite this fact, the amount of NO formed was when coal was co-fired with torrefied biomass than with steam exploded biomass. The gas composition data from the in-flame measurements show that the concentration of volatile nitrogen species (HCN and NH3) varies between the cases, which is suggested as the reason for the difference in the NO formation. The importance of when and where the nitrogen species are released is also shown in the modelling work, supporting what was observed experimentally.