(81a) BIOMASS Upgrading for the PRODUCTION of Biofuels FROM Biowastes

Keywords: Renewable Energy, Biomass, Torrefaction, Mild pyrolysis, Weight loss kinetics, Energy densification, Oxygen removal

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Biomass resources play an important role in the energy transition. However, the use of raw biomass sources as fuel has various problems, such as high moisture content, low calorific value, hydrophilic nature and its tenacious and fibrous structure. The use of biomass is also subjected to limitation of land, water and competition with food production. The agricultural production of biomass is relatively land intensive and involves high logistics costs due to low energy density of biomass. For biomass based systems a key challenge is to develop efficient conversion technology which can also compete with fossil fuels.

Torrefaction is a technology which can improve biomass properties and therefore offers some solutions to above problems. Torrefaction is a thermal conversion pre-treatment technology of biomass resources to produce a high quality solid biofuel which can be applied in other energy conversion technologies such as combustion and gasification. Torrefaction operates in the temperature range of 200 - 300°C at atmospheric conditions in the absence of oxygen. In this temperature range the biomass decomposes and during this decomposition a gas is produced. Torrefied biomass has higher energy density, improved grindability and hydrophic nature.

Precise control over the process conditions temperature and residence time during biomass and large wood particles torrefaction provides a better reliability of the product quality. The key factor in torrefaction is the heat transfer phenomenon into and in the (large) wood particle. Reactivity of and volatile product formation from the biomass depend on the temperature at which torrefaction take place. The reactivity rate of the constituents of biomass hemicellulose, cellulose and lignin differ with temperature, residence time and matrix structure of the biomass material. Heat transfer limitations result in a temperature profile which leads to reactivity rates variance and heterogeneous material characteristics in the biomass particle itself. Endo and exothermal biomass decomposition reactions can enlarge the effect of the temperature profile and so influence the product quality, but also the process design and process operation regarding temperature set point.

In this work a causality model is derived that shows the causal effects between heating up the reactor temperature, biomass particle temperature and the reactivity of the biomass. The reactivity of small biomass particles and its relation with large biomass is important to describe the quality of the torrefied product. At powder scale biomass is torrefied using TGA to determine the reactivity model. Also cylindrical beech wood particles are torrefied in a fixed bed reactor setup under inert (argon) atmospheric conditions. Several thermocouples are used to determine the internal temperature profile.

It is shown that a causality model is a good model to describe the reactivity of biomass torrefaction. Also it is shown that biomass torrefaction is an exothermal process which can lead to temperatures in the biomass (wood) particle itself that are much higher than the reactor setpoint. This exothermal effect is quantified.

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