Influence of fuel pre-treatments on ash-forming elements and implications on corrosion

The demand for increased use of CO2-neutral biomass fuels has led to an increased use of difficult fuels such as straw, agrofuels, and biomass waste streams. However, these types of fuels have resulted in limited steam temperatures in power boilers and thus limited efficiencies. To challenge these limitations one alternative is to pre-treat fuels in such a way that the harmful elements either is removed from the fuels or stabilized so they are not able to form corrosive compounds. Torrefaction, steam explosion and hydrothermal processes are examples of fuel up-grading treatments.
In this paper, six fuel samples have been studied:
- Original spruce bark
- Torrefied spruce bark
- Pre-washed spruce bark followed by torrefaction
- Original wheat straw
- Torrefied wheat straw
- Pre-washed wheat straw followed by torrefaction
The fuels have been chemically fractionated by treating them consecutively with three different solvents (H2O, ammonium acetate, 1 M HCl). The solubilities of the elements were determined showing that the pre-treatments did not significantly change the expected behaviour of the ash-forming elements.
The fuel qualities were improved mainly due to lower chlorine contents. In the case of bark, torrefaction was enough to improve the quality while in the case with wheat straw also the pre-washing step was needed.