Investigation of the effects of hydrothermal carbonisation process parameters on the produced hydrochar properties and its applications

This study investigated the hydrothermal carbonisation (HTC) of poultry litter (PL); specifically how HTC parameters such as treatment temperature, residence time and initial pH affect the yields (HY), chemical and fuel properties of the hydrochar (HC) produced as well as other potential applications of the HC. The work is divided into three sections. Firstly it investigated the effects of HTC parameters on the HY, chemical and fuel properties, of HC produced at the natural PL pH of 8.8, treatment temperature ranging from 150 to 300 °C with residence times of 5 to 480 minutes. The impact of initial pH was investigated using HCs prepared at 250 °C for 120 minutes in the presence of acetic acid (CH3COOH) at pH 9, 7 and 4 or in the presence of sulfuric acid (H2SO4) at initial pH of 7, 4 and 2. The HCs produced were characterized for ultimate, proximate and fibre analyses as well as for energy content, and the effects of treatment conditions on the HY and composition of HC were evaluated. The results revealed that the treatment temperature and initial pH had a more significant impact than residence time on the HY, chemical composition and energy contents of the HCs. The HY decreased and ash content increased with increasing treatment temperature and residence time, this coincided with a higher carbon (C) content up to 56.9 % and HHV of 25.1 MJ kg-1 . The C content and HHV of the HC increased with acidity while the ash content decreased, however, the change in HY were influenced by the type of acid used. The lowest ash content and the highest HY were observed for prepared HC using H2SO4 at pH of 2. Secondly we examined the effects of HTC parameters on the mineral nutrient profile and phosphorous (P) speciation of the HCs. For this study a standard measurement and testing procedure (SMT) was adopted and all the elements measured were quantified and related to the P fractions. The results indicated that treatment temperature and initial pH can significantly influence the speciation of P and other nutrients, while the effects of residence time were apparent predominantly at low treatment temperature. The results confirmed that HTC can significantly reduce the solubility of most of the measured elements, providing a slow-release fertilizer. The majority of the mineral nutrients remained in the HC however, the acids particularly H2SO4 provided a good approach for nutrient extraction. Thirdly the effect of using H2SO4 modified HC for the adsorption of hexavalent chromium Cr(VI) and methylene blue (MB) from aqueous solution was investigated and compared with the unmodified HC. The Cr(VI) and MB adsorption conditions were optimized and it was found that solution pH was dominant factor. Under optimal conditions, the maximum Cr(VI) removal efficiencies were 76.7 % and 32.9 % for acid modified and unmodified HC, respectively, while the maximum MB removal efficiency measured almost 100 % in 90 minutes for both HCs. The results revealed that the predominant adsorption mechanisms were the electrostatic attraction and chemisorption but ion exchange could also be occurring for Cr(VI) removal by unmodified HC.