Pyrolysis profile of a rectangular kiln – natural scientific investigation of a traditional charcoal production process

doi:10.1016/j.jaap.2019.104757

Journal of Analytical and Applied Pyrolysis

Volume 146, March 2020, 104757

https://doi.org/10.1016/j.jaap.2019.104757 Get rights and content

Highlights

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First time a temperature profile of traditional rectangular kiln is presented.
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Infrared spectroscopy can be used to assess pyrolysis temperature out of charcoal pieces.
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Prediction models for elemental analyses based on the same spectra are presented.

Abstract

Rectangular kilns are a traditional technique in charcoal production in the second half of Modern Period in Middle Europe. Only in Austria there is still an active tradition that is of broader interest in an archaeological context. One of these kilns in Rohr im Gebirge, Lower Austria, was equipped with thermocouples to record pyrolysis temperature during the whole process. The removal of ready charcoal was paralleled by sampling, in total 103 charcoal samples were collected. Based on the infrared (FTIR) spectral pattern the pyrolysis temperature was predicted. It is the first reconstruction of the temperature profile of the whole kiln. Elemental analyses were performed and prediction models for carbon (R² = 0.93), hydrogen (R² = 0.96) and oxygen (R² = 0.98.) content based on FTIR spectra established. Therefore, FTIR spectroscopy rolls the degree of pyrolysis and elemental analyses into one single analytics. Further analyses by means of NMR spectroscopy, X-ray diffraction and He-pycnometry complete the picture.

Graphical abstract

Introduction

Charcoal production has been a major energy source for melting processes till the 19^th century, when fossil stone coal became available even in rural areas [1]. In Austria, three historical methods are reported: charcoal pits, circular and rectangular kilns [2]. Since the 18^th century, rectangular kilns became popular in Sweden [3]. The technique has been transferred to Austria in the second half of the 18^th century [4] and due to any reason it became the dominant one.

Rectangular kilns are constructed with logs of 3 m laying perpendicular to the main axis of the kiln. The stack measures about 15 m (length) and about 2.5–3 m (height). The height at the starting front is lower (∼ 2 m) increasing continuously towards the middle of the length. The technique of charcoal production in rectangular kilns is reported by a film available at The Austrian Media Library with an associated description in German [5]. Charcoal production became UNESCO Intangible Cultural Heritage of Austria in 2011.

Natural scientific work about the charred material produced in kilns, however, is scarce. Anthracological studies provided insights into historic wood use for charcoal production [6,7]. Only little is known about the exact pyrolysis conditions in this traditional process. Material characteristics are mainly discussed in connection with its calorific value. A lot of knowledge about charcoal production is handed over by oral tradition of the charcoal burners. In the context of archaeological work, but also of biochar usage further interest arose to describe material characteristics in a broader sense. FTIR spectroscopy and thermal analysis have been used to detect the factors “age” and the factor “pyrolysis temperature” in recent, historic and prehistoric charcoal [8].

This paper focuses on the historic technique of rectangular kilns. The pyrolysis process is described by a temperature profile and material characteristics of the charcoal produced. Additionally prediction models for elemental contents in charcoals by means of FTIR spectroscopy are provided. Analytical discussions base on a prior publication on charcoal analytics [9].

Section snippets

Material

The kiln was constructed on an old kiln place. The basement consists of clay partly covered with a pyrolyzed layer – probably solid remains of accumulated pyrolysis oil mixed with pulverized charcoal. The wood was placed on a wooden grate of thin logs resulting in a small hollow space between the ground and the material (until the pyrolysis reaches the lowest parts). On the top of the logs brushwood of spruce ensured a separation of wood and the covering material. This material consists of a

Mass balance of the kiln

The input wood consisted of approximately 60 % spruce (Picea abies) and 40 % beech (Fagus sylvatica) with a gross density of 430 kg/m³ and 680 kg/m³ respectively [12]. The calculated volume of the kiln was 106 m³, to account for cavities between the stems this number was multiplied with a factor of 0.6. So approximately 64 m³ of solid wood with a weight of 34 t were pyrolysed. The finished charcoal was filled in 599 bags with a volume of 0.06 m³ and 61 bags with a volume of 0.03 m³. The mass of

Conclusions

The paper reveals a concise picture of what happens in a traditional rectangular kiln from an analytical point of view. Temperature profile and material characteristics are described mainly by means of FTIR spectroscopy. The temperature profile was confirmed by measurements with thermocouples. The temperatures vary from 400 to 700 °C. Especially in the front part, where the kiln is ignited, temperatures reach higher levels than in the middle part. With the knowledge of the temperature profile

Author statement

JTi developed idea, experimental setup, writing of the draft, supervision of CPr, corresponding author CPr was responsible for on-site temperature measurements, FTIR measurements, and data evaluation CPf was responsible for on-site temperature measurements, supervision of CPr JTh were responsible for elemental analyses FO, KW were responsible for XRD MP was responsible for NMR analyses ES had a major input in project concept, FTIR interpretation and the draft of the paper, supervision of CPr

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Special thanks to Peter and Gertrud Wieser for their friendly support at the kiln. They took care of the temperature sensors and allowed us taking samples. Thanks for the background information about the traditional heritage and for the excellent meals. We thank Marion Sumetzberger-Hasinger for a part of the elemental analyses. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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