Biomass hydrothermal carbonisation for sustainable engineering

Hydrothermal carbonisation (HTC) could form the basis for rendering human faecal wastes safe whilst at the same time generating a carbon-rich material (hydrochar) and providing prospects for the recovery of energy. The work presented here has an objective of the search for optimal conditions for the HTC conversion of human faecal waste. Primary sewage sludge (PSS) and synthetic faeces (SF), of various moisture contents, were used as feedstocks to investigate the kinetics of decomposition of solids during HTC over a range of reaction times and temperatures. Decomposition was found to follow first-order kinetics, and the corresponding activation energies were obtained. Temperature was of primary importance to influence solid decomposition. Higher temperatures resulted in higher solids conversion to hydrochar. The energy contents of the hydrochars from PSS carbonised at 140 200oC for 4 h ranged from 21.5 to 23.1 MJ kg 1. Moisture content was found to affect the HTC process and feedstocks, with higher initial moisture contents resulted in lower hydrochar yields. The effect of reaction conditions on the characteristics of the hydrochar, liquid and gas products from HTC of faecal material, and the conditions leading to optimal hydrochar characteristics were investigated using a Response Surface Methodology (RSM). Models were developed here which could aid in the identification of reaction conditions to tailor such products for specific end uses. The results showed that the amount of carbon retained in hydrochars decreased as temperature and time increased, with carbon retentions of 64 77% at 140 and 160oC, and 50 62% at 180 and 200oC. Increasing temperature and reaction time increased the energy content of the hydrochar from 17 19 MJ kg 1 but reduced its energy yield from 88 to 68%. HTC at 200oC for 240 min resulted in hydrochars suitable for fuel, while carbonation at 160oC for 60 min produced hydrochars appropriate for carbon storage when applied to the soil. Theoretical estimates of methane yields resulting from subsequent anaerobic digestion (AD) of the liquid by-products are presented, with the highest yields obtained following carbonisation at 180oC for 30 min. In general, HTC at 180oC for 60 min and 200oC for 30 min resulted in hydrochars having optimal characteristics, and also for obtaining optimal methane yields. Maillard reaction products were identified in the liquid fractions following carbonisations at the higher temperatures. It was also found that the TOC, COD and BOD of the liquid products following HTC increased as the reaction temperature and time were increased and that these would require further treatment before being discharged. The results indicated that the gaseous phase following HTC contained carbon dioxide, nitrogen dioxide, nitric oxide, ammonia, and hydrogen sulphide indicating that additional treatment would be required before discharge to the atmosphere. In order to identify the optimum conditions leading to greater filterability of slurry resulted from HTC, the effects of reaction temperature and time on the filterability of PSS and SF slurries were investigated and optimised using RSM. It was shown that filterability improved as the reaction temperature and time at which the solids were carbonised was increased, with the best filtration results being achieved at the highest temperature (200°C) and longest treatment time (240 min) employed here. The specific cake resistance to filtration of the carbonised slurries was found to vary between 5.43 x 1012 and 2.05 x 1010 m kg 1 for cold filtration of PSS, 1.11 x 1012 and 3.49 x 1010 m kg 1 for cold filtration of SF, and 3.01 x 1012 and 3.86 x 1010 m kg 1 for hot filtration of SF, and decreased with increasing reaction temperature and time for carbonisation. There was no significant difference in specific resistance between cold and hot filtration of SF. The RSM models employed here were found to yield predictions that were close to the experimental results obtained, and should prove useful in designing and optimising HTC filtration systems for generating solids for a wide variety of end uses. Mass and energy balances of a semi-continuous HTC of faecal waste at 200oC and a reaction time of 30 min were conducted and based on recovering steam from the process as well energy from the solid fuel (hydrochar) and methane from digestion of the liquid by-product. The effect of the feedstock solids content and the quantity of feed on the mass and energy balances were investigated. Preheating the feed to 100oC using heat recovered from the process was found to significantly reduce the energy input to the reactor by about 59%, and decreased the heat loss from the reactor by between 50 60%. For feedstocks containing 15 25% solids (for all feed rates), energy recycled from the flashing off of steam and combustion of the hydrochar would be sufficient for preheating the feed, operating the reactor and drying the wet hydrochar without the need for any external sources of energy. Alternatively, for a feedstock containing 25% solids for all feed rates, energy recycled for the flashing off of steam and combustion of the methane provides sufficient energy to operate the entire process with an excess energy of about 19 21%, which could be used for other purposes.