(628d) Modeling of An Auger Reactor for Biomass Torrefaction

The use of biomass as a renewable alternative to fossil fuels has spiked some interest in the recent years. In addition to its renewable nature, biomass has the advantage of being carbon-neutral since the carbon dioxide released by burning is already part of the carbon cycle [1]. Nonetheless, to make the large-scale use of biomass economically viable, biomass needs to be preprocessed in order to increase its energy density and to make its chemical and physical characteristics suitable for gasification or co-firing. Bio-coal (or bio-char) is a product of biomass preprocessing that is seen as a carbon-neutral and renewable alternative to coal.

A process that is used to produce biocoal is torrefaction (slow pyrolysis). Torrefaction is a process that is known to have very complex kinetics, with the rates of reactions typically driven by the rates of heat and mass transfer into the particles. Many parameters affect the torrefaction process, such as the temperature in the reactor, the heating rate and the particle size and composition of the feedstock. In torrefaction, wood is heated to 200-300°C at very slow rates in an inert environment, until hemicellulose and some of the lignin in the wood are devolatilized. In this work, an auger reactor is used for biomass torrefaction. 

An auger reactor consists of a stationary vessel and a rotating screw inside the stationary vessel.  The rotating auger translates the solid particle biomass ``reactant" through the reactor.  The auger reactor is especially useful when dealing with particulate flow, since it eliminates the need for a carrier gas [2]. In addition, the system can be easily adapted to deal with different particle sizes and wood compositions, by adjusting the speed of rotation of the screw. Large operating condition changes can cause the formation of tar condensates in the reactor vessel, which are very viscous and can clog the reactor. Understanding the effect that each of these parameters has in the torrefaction process is essential to the optimal operation of the reactor.

In this work we consider a model of the auger reactor for the purpose of dynamical analysis, economic optimization and control of the torrefaction process. In particular, we develop the auger reactor’s dynamics model.The model describes the heat and mass transfer characteristics of the distributed parameter system and leads to the identification of spatially distributed dynamics that is responsible for the onset of space and time varying hot spot along the reactor. Simulations of the reactor model are used to carry out a factorial experimental design, in which we evaluated the effect of different operating conditions in the energy content and the fraction ofthe biochar produced.

References

[1] Tumurulu et al., A review on biomass torrefaction process and product properties for energy applications, Industrial Biotechnology, Oct. 2011, pp. 384-401

[2] Brown, J.N., Development of a lab-scale auger reactor for biomass fast pyrolysis and process optimization using response surface methodology, 2009, Graduate Theses and Dissertations, paper 10996