Answer To: SEN700 Assessment Assessment Item 2: Literature Review Objective: To produce an individual...
Harisankar answered on Apr 27 2021
Over the centuries of human civilization, the demand for energy has always been increasing. The rapid declination of fossil fuels and increasing greenhouse emissions has led to a worldwide interest in a quest for alternatives to cope up with the increasing energy demand. Due to the population bloom, increased production of plastics and organic wastes, particularly from domestic sourcesis a major problem faced by humankind. Efficient treatment or utilization of these are essential for sustainable development.Organic wastes from domestic sources mainly contain carbohydrates, lipids and proteins (1). So this presents a huge opportunity to recover valuable products from these wastes.
Thermochemical conversion processes are the most used method of processing wastes to convert them to energy. Different thermochemical methods of processing wastes are Pyrolysis, Gasification, Combustion and Hydrothermal Liquefaction. Of this, Hydrothermal Liquefaction (HTL) is one of the most promising thermo-chemical conversion technologies that uses water near critical state (250-350 oC and 150-250 bars) to convert biomass to bio-crude, water-soluble aqueous products, bio-char and non-condensable gases (2). Subcritical/supercritical water has gained focus as an environmentally friendly reaction media to break organic matter into smaller molecules (3). Sub- or supercritical water undergo change in properties with respect to density, dielectric constant, viscosity, diffusivity and solvent ability. The decrease in viscosity and dielectric constant of water near its critical point makes it an excellent solvent for non-polar organic molecules. This is the basis for HTL.
The bio-crude from HTL shows superiority over bio-oil from pyrolysis because bio-crude contains lesser amount of moisture and has more carbon + hydrogen to oxygen ratio, hence has a higher HHV than pyrolysis bio-oil. However, one major drawback of HTL bio-crude is its high viscosity. Bio-crude from HTL can show viscosity in the range of 15000 cp at 61 oC which is highly undesirable (4).The characteristics and yield of the products primarily depends on the type of biomass feedstock, particle size of the feedstock and operating conditions such as solvent : biomass ratio, operating temperature, residence time, operating pressure and presence of catalyst; which has been widely investigated with different feedstocks such as municipal waste (5), livestock manure (6), plastics(8), etc.
The two major reactions involved when using sub- or supercritical water are oxidation and hydrolysis. The process of supercritical water oxidation of organic matter has been studied extensively and commercialized. The reaction pathways of HTL of biomass have been found to change with acidic, basic or neutral pH conditions (9). Fundamental carbohydrate research showed that pH plays an important role in the formation of HTL bio-crude (10). Comparative studies of HTL of biomass to bio-crude under different pH conditions were extensively studied by Sudong et. al. The study showed that under acidic and neutral pH conditions, the main component of bio-crude was 5-Hydroxymethylfurfural; while under alkaline conditions, the main components...