Torrefaction of beechwood: A parametric study including heat of reaction and grindability
Highlights
► Beechwood chips are torrefied at lab scale with different parameters. ► Mass loss of the dry solid is a good indicator for the degree of torrefaction. ► Heat consumption of the reactor is measured to determine the heat of reaction. ► The heat of reaction ranges from −199 J/g (exothermic) to 148 J/g (endothermic). ► Grindability in terms of HGI is moderate or easy for solid mass loss >30%.
Introduction
Biomass is a source of renewable energy, which can be converted to heat and power in line with demand. This is an advantage over photovoltaic and wind power making biomass an important pillar in the energy supply today and in the foreseeable future. However, the energetic usage of biomass is challenging, since transport over large distances is often not viable due to its low heating value, the grindability is not sufficient for milling with existing equipment, e.g. in coal-fired power plants and, furthermore, storage is difficult due to micro organisms causing decay. To overcome these challenges torrefaction has been proposed as a process for upgrading biomass by thermal treatment [1], [2], [3]. Torrefaction is a kind of mild pyrolysis, typically utilizing temperatures ranging from 200 to 300 °C and residence times in the magnitude of a few minutes to about one hour. To avoid combustion, torrefaction has to take place in the absence of oxygen. Ciolkosz and Wallace [4] published a review paper in 2011 summarizing most of the available literature about torrefaction. Numerous publications have already addressed the change in chemical composition and model development to predict decomposition rates. Nevertheless, literature dealing with the following two important aspects of torrefaction is quite rare:
- 1.
Grindability in units the consumers need.
- 2.
Energy demand and heat of reaction during torrefaction.
The objective of this work is to give an overview of the influence of torrefaction temperature and residence time on important fuel parameters like chemical composition, heating value and grindability. Additionally, the energy demand of the torrefaction reactor is measured to determine the heat of reaction, which is an important parameter for the control of large scale torrefaction plants.
Section snippets
Experimental setup
Beechwood chips without bark have been used as raw material for the torrefaction experiments since they can be purchased with relatively constant quality except for the moisture content. The latter varied between 5% and 15%, hence the feedstock has been dried at 105 °C until the weight kept constant and then remoistened to the desired moisture content. The thickness of these chips is about 2 mm, the particle size determined via sieve analysis is given in Table 1.
Torrefaction experiments were
Torrefaction
A parameter variation has been conducted to expand the data for torrefaction behaviour of woody biomass. Reproducibility of tests with the utilised reactor is high as has been proven in previous tests [5], hence no repetitions were performed. To keep the number of tests small, a reference has been defined and starting from this, the parameters temperature, residence time and moisture content of the feed have been varied. Since several authors (e.g. Arcate [6], Bergman et al. [1] and Pach et al.
Torrefaction
Englisch [13] stated that the degree of relative volatiles reduction, defined asis a good indicator for the degree of torrefaction. In particular, for the comparison of different biomasses this is probably a reasonable approach. In this work beechwood is investigated exclusively and therefore the normalized mass loss of the solid is used as degree of torrefaction. Mass loss measurement is possible as torrefaction was performed batchwise; in a continuously
Conclusion
A continuously working torrefaction reactor was used to conduct a parametric study of beechwood torrefaction. Besides ultimate and proximate analysis of the products, grindability tests have been performed and additionally the heat of reaction was determined. Heating values are in line with literature data and are almost monotonously increasing with the mass loss of the solid (dry ash-free basis). Energy yield is monotonously decreasing on this basis, therefore the mass loss of the solid seems
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