Hydrothermal Torrefaction of Agricultural and Woody Biomass

Abstract

As a non-edible renewable resource, biomass is an attractive fuel source of energy and a potential feed stock for further conversions that can be greenhouse gas emission neutral. However its application in energy industry is limited due to its properties such as low heating value, high moisture content, low bulk density, and high ash content. A thermal pretreatment process referred as “hydrothermal-torrefaction” has been proved to enhance biomass properties for efficient combustion. In recent years interest in torrefaction of biomass has increased among both wood pellet manufacturers and biomass energy producers. In addition to low levels of EPA criteria pollutants, torrefied biomass has substantially higher energy values, higher bulk density, hydrophobic nature, and increased friability for easier grinding. To date, no common process exists for hydrothermal-torrefaction of biomass. Most agree that the process involves heating biomass in hot compressed water to temperature range of 200-260°C, under high pressure(~4MPa) in the absence of oxygen. In addition to temperature, time spent under elevated temperature is a factor but no consensus exists regarding how long it takes to torrefy. It is suggested that hydrothermally torrefied biomass(Hydrochar) improves fuel properties and has potential applications in existing coal-fired boilers. Moreover, highly porous surface structure with improved adsorption capacity of Hydrochar makes it versatile applicative in industries such as agriculture, waste water treatment, Carbon Capture and Storage (CCS) etc. During this series of tests, the effect of time and temperature on the energy densification of hydrothermally torrefied biomass is measured. Different combinations of residence time (5-30mins) and temperature (200-260?C) was used to determine the effect on physical and chemical properties of biomass.