Investigations into Hydrothermal Processing of a Wet Lignocellulosic Paunch Waste

With increasing population and related increase in the consumption of beef, the sustainable management of waste generated from the cattle paunch generally refereed as a paunch waste is becoming increasingly challenging. For example, Australia alone produces more than 2 million tonnes of beef, generating 500 million tonnes of wet paunch waste in the local abattoirs annually. Globally, annual paunch waste generation is roughly 18 billion tonnes. Paunch waste carries pathogens such as Escherichia coli, Salmonella spp. and has a high biochemical oxygen demand. Therefore, if not managed efficiently it can cause serious environment and health issues. Paunch waste management options in Australia include composting and landfilling, currently costing $60 per tonne. This work has investigated an upcycling approach where paunch waste is converted into high-value products such as crude-oil and hydrochar using a hydrothermal process. The selection of hydrothermal process is made due to the wet nature of paunch waste which originally contains ~95-97 wt.% water. There exists a very limited literature on the hydrothermal processing of paunch waste. Therefore, a state of the art review was initially conducted on the hydrothermal treatment of known similar wet wastes streams namely sewage sludge, food waste and algal sludge to identify gaps in knowledge and to perform benchmarking of paunch waste with other similar waste. Followed by this, a predictive model was developed for hydrothermal processing of paunch waste where temperature, holding time and solid content were identified as the most critical process parameters. Hydrothermal carbonisation (HTC) experiments were then carried out in a 600 mL Parr reactor under an inert N2 atmosphere, and liquefaction rate of up to 80 % at mild conditions (240 °C) was attained with 35-45 wt.% mixed bio-oil (light and heavy) and 20-30 wt.% hydrochar. Based on the experimental and characterisation data, a unique reaction network was developed and rate constants were predicted using a lumped kinetic model. Analysis of modelling data showed a low activation energy of 13.2 kJ/mol for the conversion of paunch waste to aqueous phase, explaining the high conversion rate obtained. The experimental data were also used in conducting a preliminary techno-economic assessment of HTC process.  The techno-economic assessment component includes process modelling in ASPEN plus V10 and the discounted cash flow analysis using the Nth-Plant financial assumptions. It was identified that higher initial solids content (i.e. 15 wt.%) returned a higher net present value (NPV) and a shorter payback period. The higher initial capital investment and uncertain sale price for hydrochar and crude-oil were identified as the bottlenecks of HTC. The techno-economic analysis concluded with a finding that standalone conventional HTC of paunch waste may not be commercially attractive. Therefore, two new approaches were further studied, 1. Co-HTC and staged-HTC of paunch waste and 2. Production of high-value magnetic hydrochar. Co-HTC and staged-HTC of paunch waste with locally available biosolids produced from wastewater treatment was studied with an aim to enhance the conversion rate of biosolids and demonstrate the production of high-quality biosolids derived hydrochar with a low heavy metal content through synergistic effect between these feedstock. Also, application of such biosolids derived hydrochar for dye removal from wastewater has been demonstrated. In a second approach, an attempt was made to make an energy neutral HTC process by its potential integration with anaerobic digestion for the production of biogas instead of crude-oil and uplifting the quality of hydrochar by converting them into magnetic hydrochar. This approach is expected to eliminate uncertainty around crude-oil quality, sale price and market and increase the sale price of hydrochar. It was demonstrated that such integration can be either net energy generator or neutral with an increase in the hydrochar quality. It was postulated that if iron could be sourced from dissolved air flotation (DAF) cake or ferric sludge, an attractive NPV can be obtained.