Literature Survey of Wet-Waste and Hydrothermal Carbonization System: A Comparative Evaluation

This thesis describes the process of Hydrothermal carbonization and a comparative assessment on recent work done on the process using different feedstocks such as Raw Apple (RA), Apple chip pomace (ACP), apple juice pomace (AJP), grape pomace (GP), macadamia nut-shell (MNS), Sewage Sludge (SS), Poultry Litter (PL) and Grape Marc (GM). The data obtained from the literature survey was compared with the data obtained by the CCL team on the SWPS gasifier (HTC of poultry litter) to obtain how carbon conversion efficiency, syngas composition, thermal efficiency, char production rate, gas production rate and energy utilization vary with carbonization process conditions, majorly, temperature and time. Hydrothermal carbonization process is an effective, cost efficient and environmental-friendly process, capable of degrading biomass efficiently to obtain energy efficient product. HTC is perfectly suitable for the conversion of wet waste to form hydrochar due to it high conversion efficiency, low operating condition and elimination of pre-drying requirement compared to other processes. Hydrothermal carbonization process is performed in a closed heated reactor in which biomass and water mixture (usually between 20:80 biomass-feedstock ratio) at a temperature ranging between (180- 260 OC), pressure ranging between (2-6 MPa) and residence time between (5 – 480min). The product of hydrothermal carbonization process is majorly solid, refered to as hydrochar (yield usually between 27-80%), liquid and gas (usually between 2-10%). The gas composed mainly of carbon dioxide (sometimes, up to 99%) and carbon monoxide, and trace amount of methane, hydrogen and other gaseous hydrocarbon which are sometimes found at higher temperature of the process. The liquid phase also referred to as process water contains several organic compounds such as acetic acid, aldehydes and alkenes, and aromatics such as furanic and phenolic compounds it serves as the solvent, reactant and heating medium fluid for the process. Hydrochar produced is rich in aromatic carbon and oxygen content, which is present due to dehydration and decarboxylation. Result from the comparison shows that the lowest yield of solid is obtained by using glucose-based biomass such as the fruit waste with high sugar content and poultry liter with high protein content. High Lignin based feedstocks such as wood, grape marc and grape seed are best for when high yield of hydrochar is desired. Comparing with the yield of poultry litter obtained by CCL team, CCL team has the least yield of hydrochar. Temperature and time change have been shown to influence the product of hydrothermal carbonization. Temperature variation changes hydrochar chemical characteristics and its energy content. As temperature increases, the carbon content increase while the oxygen decreases as a result of hydrolysis and dehydration, Volatile matter of carbon content decreases, which signifies the stability of the organic carbon in hydrochar is improved. H/C and O/C ratio decreases with increasing temperature due to dehydration and decarboxylation and gas phase formation. The carbon conversion efficiency for RA, ACP, AJP, GP increases with increasing carbonization time while, efficiency of GM, GS, SS, MIS, MNS and PL decrease with increase in time and temperature. The increase in carbon efficiency is due to their high sugar content. Energy densification ratio increases, Energy yield decreases with increasing time and temperature. Grape seed and Grape marc carbonization have the highest heating value of 29.88MJ/Kg at 250 OC and residence time of 480 minutes. The thermal efficiency of HTC decreases with reaction severity therefore, more energy is being conserved in the hydrochar as the reaction temperature and time increase. The thermal efficiency of GM, MNS and SS is higher than that of RA, AJP, ACP and GP