Abstract
In this study, a series of seven residual biomass feedstock was treated by hydrothermal carbonization (HTC) at temperatures of 180 °C, 210 °C, 240 °C, and 270 °C and residence times of 0.5 h, 2 h, and 4 h. The processed samples were analyzed with focus on properties that are relevant for the combustion of a fuel. Temperature was found to have the highest impact on fuel properties. HTC has a positive effect on the energy density of the material, increasing lower heating values typically by 10–15% at 180 °C and 47–59% at 270 °C. At the same time, mass yield was decreasing for increasing treatment temperature. The hydrothermal treatment was found to have a profound impact on the inorganic composition of the fuels, lowering significantly the alkali metal and chlorine content while increasing silicon and phosphorous concentrations in the ash. These transformations lead to improvements in ash melting temperatures and in molar S/Cl ratio, an indicator commonly used to assess the risk of high-temperature corrosion in biomass combustion. HTC is also expected to have a positive impact on fine particle emissions upon combustion due to lowered concentrations of elements responsible for aerosol formation after HTC treatment. On the other hand, HTC leads to higher nitrogen contents in the fuel, thereby potentially increasing the risk for higher NOx emissions upon combustion of HTC-treated fuels. Overall, HTC clearly shows a positive effect on combustion properties, but the effects are fuel specific and especially interesting for biogenic waste that originates from lignocellulosic material. Applying the criteria of this study, the fuel properties of sewage sludge could not significantly be improved. For feedstock like this, the advantage of utilizing HTC as treatment lies in improved dewatering, storage, and feedstock logistics.
Highlights
Moving towards a more sustainable society has become one of the most important challenges for the twenty-first century
The lower heating value (LHV) on dry basis is increasing significantly for all treatment temperatures above 180 °C where hydrolysis of hemicellulose (180 °C). An exception to this is sewage sludge where the LHV is slightly decreased by hydrothermal carbonization (HTC)
The reason for this is that upon HTC treatment of sewage sludge, the ash content is increased from 21.8 wt.% to 55.8 wt.% when comparing the starting material with sewage sludge treated at 270 °C
Summary
Moving towards a more sustainable society has become one of the most important challenges for the twenty-first century. A major part of the worldwide greenhouse gas emissions originates from the energy production from fossil fuels. Renewable energy carrier offers the possibility to reduce net greenhouse gas emissions by substituting fossil. Firing of biomass in power boilers can lead to high-temperature corrosion of superheaters due formation of deposits by alkali chloride condensation and subsequent reaction of the superheater metal with chlorine (Cl) [4, 7]. To prevent high-temperature corrosion, alkali chloride formation needs to be suppressed. A possible process for this is the binding of alkali with sulfur (S) in the form of alkali sulfate
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