Torrefaction of beechwood: A parametric study including heat of reaction and grindability

doi:10.1016/j.fuel.2012.06.112

Fuel

Volume 104, February 2013, Pages 607-613

https://doi.org/10.1016/j.fuel.2012.06.112 Get rights and content

Abstract

Torrefaction describes a thermal treatment of biomass in order to obtain a solid product with more or less coal-like properties. Therefore a mild pyrolysis in an oxygen-free atmosphere is applied. In this work beechwood was used as feedstock because of its consistent quality in commercially available chips. A parametric study was conducted with a continuously working, indirectly heated reactor having a throughput on the order of 1 kg_product/h, wherein each parameter was varied independently. The reactor temperature ranged from 270 to 300 °C, residence time from 20 to 60 min, and feed moisture content from 0% to 20%. Ultimate and proximate analysis of the products as well as grindability tests have been performed. Additionally, the integral heat of reaction was determined for each test run by measurement of the heat consumption of the reactor. Mass loss of the dry solid is shown to be a good indicator of the degree of torrefaction and ultimate as well as proximate analysis show good agreement with existing literature data. Grindability in terms of HGI depends on the degree of torrefaction and reaches from difficult to grind to very easy to grind. In terms of beater wheel mills used for combined drying and milling in lignite fired power plants, grindability is poor and in general worse than that of Rhenish lignite. The heat of reaction is found to be close to the border between endo- and exothermic with a trend to more exothermic behaviour for increasing degree of torrefaction. Despite the uncertainty of the heat capacity of wood and torrefied wood above a temperature of 420 K, the determined heat of reaction is found to be in the range of −199 J/g (exothermic) to 148 J/g (endothermic) for all tests with spans of about ±70 J/g for each single test run. Besides severe torrefaction, caused by a high reactor temperature, an increased residence time is found to cause a remarkable exothermic heat of reaction.

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 as $Torrefaction = 100 - \frac{V_{dry,torr}}{V_{dry,raw}} \cdot 100,$ is 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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The production of high-value syngas from bio-wastes through thermal conversion is a promising way to achieve carbon neutrality. In order to reduce the cost of the process, molten salt thermal treatment has been used for bio-wastes syngas production. Moreover, the removal of oxygen and water induced by torrefaction can effectively enhance the energy density of biomass, and the effect of these changes on the quality of syngas was widely investigated. This study proposed to torrefy bio-wastes and then produce syngas in molten NaNO₃-NaNO₂-KNO₃ at 300 ℃. Results showed that the evaluation indices of syngas were improved, while the cold gas efficiency reached 44.02 % for beech wood torrefied at 280 ℃, and the syngas quality is equivalent to the products obtained by gasification of bio-wastes. The gas yield of the torrefied bio-wastes was increased by 8.08–45.23 % compared to the raw bio-wastes. Na⁺ and K⁺ in the molten salt catalyzed the cracking of propyl chains and ring opening of aromatic rings in lignin, resulting in a prominent enhancement in CO yield. Meanwhile, torrefaction could change the solid structure, leading to a delay in gas production, and torrefaction could also increase graphitization degree of char (C–C/C = C up to 50.72 % for SB-280), thus preventing the reaction proceeding. This study provided a new approach to enhance the content and quality of syngas from bio-wastes at low temperatures.
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Dependence on inert gas such as nitrogen during torrefaction is not favourable due to the burden in terms of operational costs. Therefore, various alternatives have been introduced to reduce or eliminate the dependence on inert gas. The present article seeks to review the various alternatives to inert torrefaction for viable solid fuel production. The details on non-inert torrefaction, including experimental setup, effect of various essential parameters, and major findings, are reviewed and discussed. The primary trends in the torrefaction field were extracted from previous studies, were compiled and presented in several tables and graphs. The review findings showed that temperature, residence time, gas type, gas velocity, heating rate, and particle size can affect the performance, physicochemical, and combustion properties of torrefied biomass. The degree of the effectiveness of each parameter also depends on the biomass type and shape. Based on the van Krevelen diagram, the various alternative techniques can produce competitive torrefied solid products with viable fuel properties compared to commercial coals and products torrefied under inert torrefaction conditions. Overall, it is expected that the number of alternative techniques can be implemented on a larger scale after certain modifications are performed.
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Catalytic torrefaction using potassium carbonate (K₂CO₃) impregnation is a pretreatment method demonstrated to catalyze wood powder's thermal degradation for energy use. In this study, beech wood boards were impregnated with K₂CO₃, with the aim to scale up from the studies on wood powder found in the literature. The beech boards were impregnated with five different concentrations and torrefied at 300°C for four durations (5–60 min). The impregnation procedure was successful with a linear increase of K content in wood from 0.103 wt% for raw to 0.207 wt% for the 0.012 M sample. The weight loss during torrefaction increased with the increasing potassium (K) content in wood, reaching a maximum increase of 27.17% between 0.012 M and washed (no K₂CO₃) after 30 min. For the longest duration, the extent of the catalytic action of K decreased, similar to what is observed in wood powder. After 60 min torrefaction, potassium increased the torrefaction severity index by up to 10% and the higher heating value (HHV) by up to 55%. Potassium efficiently increased the fixed carbon and decreased the volatile matter to values comparable to coal by catalyzing the devolatilization during torrefaction. The atomic H/C and O/C ratios shifted to similar ratios as coal. The energy yield (EY) was above 80% for the shorter durations but dropped drastically at 30 and 60 min torrefaction. The prediction of the solid yield (SY), energy yield (EY), and enhancement factor of the HHV (EF) through an artificial neural network was robust with a fit quality R² $\geq$ 0.999. The proposed method for catalytic torrefaction on wood boards was efficient and could be used prior to grinding and transportation for bioenergy production. This process could decrease the production costs of biomass fuel to compete with fossil fuels.
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Torrefaction of beechwood: A parametric study including heat of reaction and grindability

Abstract

Highlights

Introduction

Section snippets

Experimental setup

Torrefaction

Torrefaction

Conclusion

Fuel

Fuel Process Technol

J Anal Appl Pyrol

Fuel

A review of torrefaction for bioenergy feedstock production

Biofuels Bioprod Biorefin

New process for torrefied wood manufacturing

Bioenergy Update