Lignocellulosic biomass pyrolysis mechanism: A state-of-the-art review

Wood heat treatment has increased significantly in the last few years and is still growing as an industrial process to improve some wood properties. The first studies on heat treatment investigated mainly equilibrium moisture, dimensional stability, durability and mechanical properties. Mass loss, wettability, wood color, and chemical transformations have been subsequently extensively studied, while recent works focus on quality control, modeling, and study the reasons for the improvements. This review explains the recent interest on the heat treatment of wood and synthesizes the major publications on this subject on wood properties, chemical changes, wood uses, and quality control.

https://bioresources.cnr.ncsu.edu/BioRes_04/BioRes_04_1_0370_Esteves_P_Wood_Mod_Heat_Treatment_Rev_367.pdf

Wood modification by heat treatment: a review.

Trees, and their derivative products, have been used by societies around the world for thousands of years. Contemporary construction of tall buildings from timber, in whole or in part, suggests a growing interest in the potential for building with wood at a scale not previously attainable. As wood is the only significant building material that is grown, we have a natural inclination that building in wood is good for the environment. But under what conditions is this really the case? The environmental benefits of using timber are not straightforward; although it is a natural product, a large amount of energy is used to dry and process it. Much of this can come from the biomass of the tree itself, but that requires investment in plant, which is not always possible in an industry that is widely distributed among many small producers. And what should we build with wood? Are skyscrapers in timber a good use of this natural resource, or are there other aspects of civil and structural engineering, or large-scale infrastructure, that would be a better use of wood? Here, we consider a holistic picture ranging in scale from the science of the cell wall to the engineering and global policies that could maximise forestry and timber construction as a boon to both people and the planet.

The wood from the trees: The use of timber in construction

Let X(t) be the number of tumor cells at time t, and Pr{X(t) = n} = pn(t) is the density of X. A “birth”, i.e., an increase of one of the total population of cancer cells, can occur either by mutation of a normal cell caused by the action of the carcinogen, consisting of randomly (Poisson) distributed hits, or by reproduction of existing cancer cells. A death of a tumor cell occurs as an additive combination of non-immunological and immunological elements. Once a tumor is initiated by carcinogenic action, it undergoes a birth and death process with infinitesimal birth rate linear and infinitesimal death rate composed of a linear and a nonlinear term, the former due to non-immunological deaths, the latter to immunological feedback. The death rate per tumor cell due to immunological response is proportional to the total number of antigen-producing (tumor) cells; thus, the total death rate is quadratic. Although this assumes a very simple mechanism for the action of immunological feedback, it is nevertheless a first step.

The Mathematical Model

Characterisation of thermally modified hard- and softwoods by 13C CPMAS NMR

Thermal modification of wood produces a wood material with many interesting properties, such as enhanced dimensional stability, lower equilibrium moisture content and increased biological durability. Changes in the chemical structure of pine (Pinus sylvestris) caused by thermal treatment were investigated by studying various components of wood using 13 C CPMAS NMR spectroscopy. Electron spin resonance (ESR) spectroscopy on the same set of samples was used to study the formation and stability of free radicals formed during the treatment. The most remarkable changes revealed by solid state NMR were the increase in relative crystallinity of cellulose and destruction and deacetylation of hemicelluloses. Changes in the lignin fraction were mostly registered as diminishment in the methoxyl content, although the intensity of the aromatic region increased relative to the carbohydrate fraction during the treatment. Increase in the intensities of the ESR signals from thermally treated wood samples proves the formation of stable free radicals. In addition, radical formation is believed to take part in condensation reactions leading to crosslinks within the lignin and possibly between lignin and other wood components. Both of the methods used indicate that the changes are most remarkable when the treatment temperature is over 200°C.

Magnetic resonance studies of thermally modified wood

Scots pine planks were heat‐treated under steam in a temperature range of 100–240°C at every 20°C. Changes in chemical structure of the wood samples were examined with UV resonance Raman (UVRR) and Fourier transform infrared (FT‐IR) spectroscopies. Prior to the FT‐IR and UVRR analysis, the heartwood part of wood samples were ground and extracted with acetone. Both the extracts and the extracted samples were analyzed. In addition, Klason lignin contents of the samples were determined. The FT‐IR and UVRR spectroscopy were powerful techniques to monitor chemical changes in the heat‐treated wood samples. Results confirmed most of what determined previously by other research groups using different techniques. The UVRR and FT‐IR spectra of acetone extracts showed that lignin became partly acetone soluble after a heat treatment of 180°C or greater. Increased levels of free phenolic hydroxyl groups were detected in lignin, probably due to cleavages of β‐O‐aryl ether interunit linkages. The amount of extr...

Thermal Modifications in Softwood Studied by FT‐IR and UV Resonance Raman Spectroscopies

Scots pine battens were heat-treated at 100–240o C under saturated steam. Cross-sections of heat-treated battens were analysed using ATR and reflection FTIR microscopies. A typical absorption band of fats and waxes at 1740 cm-1 was detected on the sapwood edges in the temperature range of 100–160o C, indicating that fats and waxes moved along the axial parenchyma cells to the surface of the sapwood during the heat treatment. At the elevated temperatures (above 180° C) fats and waxes disappeared from the sapwood surface and were no longer detected with FTIR spectroscopy. Resin acids were detected at temperatures between 100 and 180o C in the middle of the battens. IR spectra of these spots showed a characteristic absorption band of resin acids at 1697 cm-1. At 200o C resin acids were not detected in the middle of the battens; however, resin acids were detected at distances of 500 and 600 mm from the midpoint of the battens and on the edges of battens. At temperatures above 200° C, resin acids had disappeared from the wood.

The effects of a heat treatment on the behaviour of extractives in softwood studied by FTIR spectroscopic methods

Scots pine samples, heat-treated (225°C under steam) and reference (kiln-dried), were exposed to natural weathering for 7 years in Espoo, Finland. The weathered and unweathered samples were examined with FTIR, UV resonance Raman, and 13C CPMAS NMR spectroscopies. The spectroscopic results revealed that the lignin contents of the weathered heat-treated and especially of the weathered reference softwood samples diminished significantly. The surface of the weathered heat-treated sample was still rich in aromatic and conjugated carbonyl structures, whereas the surface of the reference sample was enriched with cellulose. These results indicated that weathering products of lignin were leached out with water from the reference sample, whereas in the heat-treated wood they were largely unleachable. The structure of the heat-treated wood was modified and degradation products did not leach out as easily as in the case of the reference sample. The weathering also resulted in a decreased content of amorphous polysaccharides of the reference sample, whereas the changes in the polysaccharide contents between weathered and unweathered heat-treated samples were not as dramatic because the amorphous carbohydrates were already degraded in the heat treatment. The results indicated that heat-treated wood is more resistant to natural weathering than untreated wood. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2128–2134, 2004

Heat‐treated softwood exposed to weathering

Heat treatment reduces hygroscopicity and accompanying dimensional changes in wood. Prior to coating, pine and spruce boards were heat treated at 225°C for six hours under steam, in order to achieve dimensional stability and durability of wood substrate. The panels were coated surface finishes which are commonly used on exterior cladding, joinery and fences in Finland. Performance of the coated heat‐treated and untreated panels was monitored during five years’ outdoor exposure. Without coating the heat‐treated wood is not weather resistant. The original dark brown colour of the uncoated heat‐treated wood panels was not stable when exposed to weather, turning grey. Cracking of the heat‐treated wood without coating was at the same level as that of the untreated wood despite the lower moisture content of the heat‐treated wood. The unpigmented or low build stains and oils did not prevent cracking of the heat‐treated wood. Weather resistance of the heat‐treated wood was improved by the water‐ or solvent‐borne paints. Wood heat treated by means of this process is comparable to untreated wood as a substrate for coatings and no alterations in coating recommendations are needed when considering coating of heat‐treated wood.

Saila Jämsä

Papers

Magnetic resonance studies of thermally modified wood

Thermal Modifications in Softwood Studied by FT‐IR and UV Resonance Raman Spectroscopies

The effects of a heat treatment on the behaviour of extractives in softwood studied by FTIR spectroscopic methods

Heat‐treated softwood exposed to weathering

Long‐term natural weathering of coated ThermoWood