Comparative evaluation of dry and wet carbonization of agro industrial wastes for the production of soil improver

This study examined the influence of process variables on the hydrochar production from wet biomasses and compared the agronomic value of hydrochars with that of the biochars produced by dry carbonization. Response surface methodology was used to investigate the effect of each three process variables (temperature, biomass: water ratio and reaction time) in hydrothermal carbonization (HTC) as well as their combined interactive effect on the mass yield and stable-C content of hydrochars. The results showed that the temperature and reaction time were significant factors affecting the stable-C content in HTC of the orange pomace, whereas the effect of all process variables is evident on stable-C content in HTC of grape pomace. For orange pomace, the effect of interaction between temperature and reaction time on mass yield was significant. However, in case of grape pomace, there existed no interaction effect between process variables on stable-C and mass yield. Findings from this work showed that char properties related to soil amendment are essentially determined by the type of carbonization process. Compared to the biochars produced by dry carbonization, the hydrochars had higher CEC and lower EC but more acidic pH values and lower stable-C content. Although chars have almost similar nitrogen content, the amount of water soluble nitrogen compounds in biochars were found to be significantly higher compared to the hydrochars. This result suggests that N-containing structures in the biomasses during HTC process were probably condensed to form N-heterocyclic aromatic structures, while much of nitrogen in biochars was present as available nitrate and amine N.