Title : Sustainable hydrothermal processing of bio-wastes
Abstract:
Biofuels seem to be a promising substitute for fossil fuels and a strategic objective to curb greenhouse gas emissions and mitigate global warming. Biofuels are considered carbon-neutral as they are produced from renewable feedstocks such as lignocellulosic biomass, municipal solid wastes, sewage sludge and other organic resources. Cattle manure is a waste generated in surplus amounts from the feedlot that has significant environmental concerns. Animal manure is one of such biogenic wastes that is available in surplus amounts as faecal waste matter from poultry and livestock farming. Owing to odour, pests, as well as CH4 and N2O emissions during composting, animal manure necessitates effective management and valorization approach. Pyrolysis and gasification of animal manure can yield energy products (bio-oil, biochar and syngas), and reduce manure volume, recover inherent nutrients, as well as reduce odour, pests and pollution. Supercritical water gasification is an attractive thermochemical technology for effectively converting lignocellulosic biomass and other wastes to synthesis gas. The thermo-physical properties of water change beyond its critical temperature (≥ 374°C) and critical pressure (≥ 22.1 MPa), making it behave as a homogeneous fluid and green solvent.
This study is focused on identifying the potential of horse manure as a novel feedstock for biofuel production through hydrothermal gasification. Horse manure was gasified in supercritical water to examine the impacts of temperature (400-600°C), pressure (23-25 MPa), feed concentration (1:5 and 1:10 biomass-to-water ratio) and residence time (15-45 min). Horse manure and resulting biochar were characterized through carbon-hydrogen-nitrogen-sulphur-oxygen (CHNSO), inductively coupled plasma-mass spectrometry (ICP-MS), thermogravimetric and differential thermogravimetric analysis (TGA-DTA), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy and scanning electron microscopy (SEM).
Supercritical water gasification was conducted in a stainless steel tubular batch reactor pressurized by inert gas nitrogen. The individual gas yield, total gas yields, carbon gasification efficiency, hydrogen selectivity and lower heating value of the gas products were determined. The effects of alkali catalysts such as NaOH, Na2CO3 and K2CO3 at concentrations ranging from 1 to 2 wt% were investigated to maximize hydrogen yields. In contrast to 1:5 biomass-to-water ratio of feed concentration, 1:10 biomass-to-water feed concentration at 600°C and 45 min gave higher yields of hydrogen (2.54 mmol/g) and total gases (9.9 mmol/g) with a carbon conversion efficiency of 20.3%. This was due to enhanced water-gas shift reaction that is favoured at high temperatures, longer reaction time and lower feed concentration. Supercritical water gasification of horse manure with 2 wt% Na2CO3 at 600°C and 1:10 biomass-to-water ratio of feed concentration for 45 min revealed maximum hydrogen yields (5.31 mmol/g), total gas yields (20.8 mmol/g) with greater carbon conversion efficiency (42.8%) and enhanced lower heating value of gas products (2920 kJ/Nm3). The manure-derived biochar generated at temperatures greater than 500°C demonstrated higher thermal stability (weight loss < 34%) and larger carbon content (> 70 wt%) suggesting their application in enhancing soil fertility and carbon sequestration. The results propose that supercritical water gasification could be an effective remediation technology for horse manure to generate hydrogen-rich synthesis gas.
