Comparative Evaluation of Torrefaction and Hydrothermal Carbonization: Effect on Fuel Properties and Combustion Behavior of Agricultural Wastes

This study compares two thermal pretreatment methods for the production of energy-densified solid fuel from agricultural wastes. Olive tree pruning (OP) and vineyard pruning (VP) were subjected to torrefaction and hydrothermal carbonization (HTC) and characterized for fuel applications. Although both processes improved the fuel properties of prunings, biochars have a higher carbonization degree compared to the hydrochars. Ash compositions differ significantly between biochars and hydrocarbons, as well as change with biomass type. The HTC process substantially reduced the fouling potential (AI index <0.1 kg/GJ) due to the reduction of the alkali content, but it was insufficient in decreasing slagging and bed agglomeration tendencies of fuel during combustion. The thermogravimetric analysis displayed that combustion behavior of biochars/hydrochars varied depending on the process type. The ignition temperature (T-i) of biomass increased with an increase of torrefaction temperature from 270 to 346 degrees C for olive pruning and 279 and 353 degrees C for vine pruning, while it varied with respect to the temperature and duration during HTC. The burnout temperature (T-b) of biomass significantly increased after HTC, but it remained almost unchanged after torrefaction. On the other hand, the effect of process type on combustion reactivity varied according to the type of biomass. In summary, the results obtained from this study showed that the effect of pretreatment process type on fuel characteristics of the biomass varies according to the biomass constituents.