Robert Nachenius

Mild hydrothermal conditioning prior to torrefaction and slow pyrolysis of low-value biomass

The aim of this research was to establish whether hydrothermal conditioning and subsequent thermochemical processing via batch torrefaction or slow pyrolysis may improve the fuel quality of grass residues. A comparison in terms of fuel quality was made of the direct thermochemical processing of the feedstock versus hydrothermal conditioning as a pretreatment prior to thermochemical processing. Hydrothermal conditioning reduced ash content, and particularly nitrogen, potassium and chlorine contents in the biomass. The removal of volatile organic matter associated with thermochemical processes can increase the HHV to levels of volatile bituminous coal. However, slow pyrolysis only increased the HHV of biomass provided a low ash content (<6%) feedstock was used. In conclusion, hydrothermal conditioning can have a highly positive influence on the efficiency of thermochemical processes for upgrading low-value (high-ash) biomass to a higher quality fuel.

Carbonization of biomass

Carbonization is a slow pyrolysis process in which biomass is converted into a highly carbonaceous, charcoal-like material. Typically, carbonization consists of heating the biomass in an oxygen-free or oxygen-limited environment, and reaction conditions are tailored to maximize the production of char. Traditionally, charcoal production is one of the oldest chemical conversion processes known to mankind. Even today, charcoal production continues on a large scale worldwide, and is still making use—in part—of tradition charcoal kilns. Furthermore, there is renewed interest to carbonization, both from a scientific and a commercial point of view, for the production and application of biochar and torrefied biomass. This chapter gives an overview of carbonization, its chemistry, and the physicochemical properties of the resulting products. It also highlights the effects of feedstock properties and process conditions and concludes with an overview of traditional and modern carbonization techniques.

Torrefaction of Biomass in a Continuous Rotating Screw Reactor

This work highlights the results that have been obtained from the continuous torrefaction of pine, in a lab-scale (2.5 kg/h feed) screw conveyor reactor. The temperatures observed in the reactor (as measured on the surface of the conveyor screw) are significantly lower than those measured at the wall of the reactor. Variation in the overall process yields and product qualities are related to variations in operating parameters. Reactor temperature and reactor residence times had the greatest effect on yields in the range of conditions investigated in this work. The overall solid product yield may be used directly to infer product qualities such as calorimetric value and proximate and elemental composition if sufficient data for a given feedstock is available. This makes a detailed knowledge of the thermal history of the biomass unnecessary in terms of quality estimation (if yields are known). Simple tests have been performed to determine the relative grindability of torrefied biomass and the composition of the gases produced during torrefaction are also presented.