Bergman

Torrefaction for entrained-flow gasification of biomass
P.C.A. Bergman, A.R. Boersma, J.H.A. Kiel, M.J. Prins, K.J. Ptasinski, F.J.J.G. Janssen, ECN, Netherlands, ECN-C--05-067,
http://www.ecn.nl/docs/library/report/2005/c05067.pdf

Abstract
A major technical obstacle in entrained-flow gasification of biomass concerns the size reduction and the subsequent pneumatic transport of the biomass powder. The fibrous structure of fresh biomass makes it very difficult and costly to reduce its particle size down to below 500 microns.

Torrefaction, a thermal treatment of biomass in the temperature range of 200 °C to 300 °C in absence of oxygen, is capable of enhance the size reduction characteristics. Torrefaction partially destructs the fibrous structure of biomass and is therefore an interesting pre-treatment option for this application. However, the influence of the torrefaction process conditions and used biomass on the grindability and pneumatic transport properties of biomass is nowhere quantified, as well as the responsible decomposition mechanisms and its kinetics. This work quantifies these relations with the aim to produce design data for entrained-flow gasificationbased bioenergy conversion chains.

An experimental torrefaction programme has been conducted in multiple reactors on both
laboratory and bench scale, varying from 5 cc to 20 l batch wise capacity. In these facilities, intensive parametric testing has been carried out to provide insight in the thermochemical decomposition mechanisms during torrefaction. Subsequently, the produced torrefied biomass has been used to effectuate size reduction and fluidisation experiments to investigate its grindability and fluidisation behaviour. The size reduction experiments have been carried out with a cutting mill, while a cold-flow bubbling fluidised bed has been used to characterise the fluidisation behaviour of (torrefied) biomass.

This work reveals the influence of the torrefaction temperature and residence time, feed particle size, and type of biomass on the main characteristics of product quality, size reduction (power consumption and capacity of a grinding device), and fluidisation (Geldart classification as a function of particle properties). Furthermore, possible mechanisms responsible for the observed changes in grindability and fluidisation behaviour are proposed. Torrefaction leads to a very substantial improvement of the grindability and fluidisation behaviour. It therefore provides a solution to the problems concerned with entrained-flow gasification related to size reduction of
biomass and the subsequent pneumatic transport of the powder.

Conclusions
Torrefaction
In general, the conducted experiments reveal that torrefaction can be applied with a high biomass to solid energy yield ranging from 95-100% within the lower temperature region (<250°C) to 83-95% within the higher temperature region (250-270 °C). At a temperature exceeding 270 °C, the energy yield drops further, but can still be limited when the reaction time is kept short. The corresponding mass yields are consistently lower and range from 90-100% within the lower temperature region and 80% to 90% in the higher temperature range (up to 270 °C). From the analysis of the volatiles, it is concluded that the increased production of water and CO2 with increased temperature mainly explains the interrelation between both yields. The production of these non-combustibles leave the energy yield unaffected while the mass yield is decreased.

The observed order of reactivity of the examined biomass types is larch << willow/beech