Valorization of Forest Biomass Through Pyrolysis: A Study on the Energy Potential of Wood Tars

Abstract

Forest biomass is a renewable source of environmentally friendly material—wood. However, wood processing generates large amounts of by-products, including branches. These byproducts are often used as firewood; however, they can be used much more effectively. In this study, the pyrolysis of two woods, namely birch and pine, was proposed. The liquid products of pyrolysis were studied by FTIR spectroscopy, and the heating value of these products was evaluated. In order to find the optimal pyrolysis temperature from the point of view of the calorific value of the product, the process was carried out at four temperatures: 450, 500, 550, and 600 °C. The liquid product yielded three fractions, from which two were analyzed, namely the dense tar fraction and light liquid fraction. FTIR analysis results clearly demonstrated that samples from different fractions differ from one another, yet the results within the same fraction are remarkably similar. The tar fraction was characterized with a higher gross calorific value between 42 to 50 MJ/kg, while the liquid fraction gross calorific value was between 29 and 39 MJ/kg; in general, pine wood yielded products with higher calorific values. The pyrolysis of small wood industry by-products is an interesting method of utilization, yielding not only a liquid product with good calorific properties, but also a solid product, namely biochar, which may be used in carbon storage or used as a soil amendment.

1. Introduction

Thanks to the renewable character of forest biomass, products derived from it are considered environmentally friendly. However, the processing of trees into usable goods generates significant quantities of by-products, including branches, bark, and sawmill residues. Biomass, including wood, is now known as a carbon-zero source of energy. In general, biomass accounts for 14% of energy consumption, but in the countries with growing economies, this number rises to 38% [1]. Currently, one of the most common methods of utilizing biomass and biomass by-products is through combustion [2]. This method is simple and straightforward; however, it raises some environmental concerns. Is straightforward burning truly the most sustainable option?

In 2019, biomass was the number one source for heat production in Europe (97% of production), and in the Americas and Asia (100% of renewable heat production). The same report shows that the supply of liquid biofuels from biomass increased by almost 10 times between 2000 and 2020, from 0.44 to 4.06 EJ, respectively. Also, the production of wood charcoal is increasing on almost every continent; only in Asia has production decreased since 2015. In Europe, the production of charcoal has more than doubled, from 0.30 to 0.64 mln tons [3]. This increase in wood charcoal production means that liquid by-product production is also growing, by-products which, after careful investigation, may become another important biofuel. Additionally, this type of fuel is forest-derived. Forests are huge global storage sites of CO₂; in Europe alone, forest biomass annually sequestered 155 mln tons of net carbon (eq. 568 mln tons of CO₂) in 2022, equivalent to 10% of Europe’s greenhouse gas emission [4]. This numbers looks good, but when forest biomass is combusted, either for household purposes or for industry, sequestered carbon dioxide is released back to the atmosphere, which diminishes the storage effect. Nowadays, we should work on the valorization of forest biomass in a way which extends the CO₂ sequestration of trees. There are several methods of producing energy from biomass. The simplest seems to be combustion—burning the wood. Other methods include biogas production by inorganic combustion, the Fischer–Tropsch method, which produces liquid product by catalytic chemical method, or pyrolysis, which also yields liquid products [5]. The production of energy from biomass is always connected with the release of CO₂, but it is possible to minimize this emission. One way of minimizing the release of carbon dioxide is to use processes which lead to the production of products rich in solid carbon. Combustion, when conducted correctly, yields only energy and inorganic ash, while other processes typically yield three products, gas, liquid and solid. In the case of pyrolysis, three types are obtained: biochar as a solid product, wood tar as a liquid product, and gas [6]. Biomass wood tar was used for centuries as a wood-protecting agent, in medicine, and as a magical ingredient in folklore cults. Recent studies show that wood tar can be added to conventional polymeric materials to obtain films with antimicrobial properties [7,8,9]. In addition, liquid pine wood pyrolysis products were used as an additive to films obtained on the matrix of carboxymethylcellulose, and their bactericidal and bacteriostatic effects against human pathogens were demonstrated [10]. In turn, tar made from birch bark has been used as a liquid preparation to protect crops against canola (Brassica napus L.) pathogens supported by microorganisms [11]. The importance of wood tar decreased with the coming of the industrial era, which led to the replacement of wood tar with fossil-based chemicals [12]. Currently, wood tar is mostly a by-product of biochar production; however, it can be a main product in the production of liquid fuels, and biochar is also a by-product with the capacity for CO₂ storage [13]. Previous studies describe wood tar as a product with a higher heating value of 21.3 MJ kg⁻¹ [14]. Moreover, a study of Fagernas et al. indicated the need for a detailed study on the heating value of liquid products of wood pyrolysis [15].

In the face of pressing challenges related to the depletion of energy resources, it has become increasingly important to explore and evaluate all potential solutions. Despite this urgency, the scientific literature on the use of wood tars as a fuel source remains limited. This article seeks to address this gap and to contribute to the understanding of this underexplored area.

The primary objective of this study was to investigate the impact of the pyrolysis process temperature on the energy properties of wood tar and to estimate the possibilities of its wider use as an energy source.

2. Materials and Methods

2.1. Materials

2.1.1. Raw Material—Wood

The raw material for the pyrolysis was debarked pine and birch wood, purchased at the local market. Prior to the pyrolysis, the wood was cut into pieces with dimensions of 7 × 3 × 3 cm.

2.1.2. Material Used in the Tests—Wood Tar

A total of 200 g of wood, prepared as described before, was placed in the laboratory pyrolytic oven (retort), and the oven was then closed with sealed, airtight doors. The pyrolytic oven was then heated up with the temperature increasing at a rate of 3 °C/min, starting from room temperature. The process was conducted until the desired temperatures were achieved, namely 450, 500, 550, and 600 °C. The vapor–gas mixture generated as a product of thermal decomposition was directed from the oven to the cooling system, where part of it underwent condensation, forming the liquid product of the pyrolysis process (wood tar). Non-condensable gases, on the other hand, were burned in a mixture with natural gas methane. After the pyrolysis process was completed, charcoal (biochar) remained in the retort. The efficiency of the pyrolysis products is described in

Table 1. Efficiency of pyrolysis products (P and B represent pine and birch wood, respectively).

	Pyrolysis Efficiency [%]
	Pine Product				Birch Product
	P450	P500	P550	P600	B450	B500	B550	B600
Liquid (wood tar)	34	33	56	45	53	54	54	54
Biochar	28	18	22	20	31	28	28	27
Gases	38	49	22	35	16	18	18	19

.

The obtained liquid product were placed in a separatory funnel where they were divided into three fractions over time. The fractions were subjected to further analysis. However, the top fraction was present in a small volume and was composed mainly of water; therefore, it was not used in the thermal analysis. The other two fractions, namely tar (t) and liquid (l), from each temperature (450, 500, 550, and 600 °C) from each raw material (pine and birch) were subjected to FT-IR and calorimetric analysis. The samples are described in

Table 2. Description of the samples (pine (P) and birch (B) woods) used in the experiments.

Material, Temperature, and Product	Symbol	Material, Temperature, and Product	Symbol
Pine wood 450 °C, tar	P450t	Birch wood 450 °C, tar	B450t
Pine wood 450 °C, liquid	P450l	Birch wood 450 °C, liquid	B450l
Pine wood 500 °C, tar	P500t	Birch wood 500 °C, tar	B500t
Pine wood 500 °C, liquid	P500l	Birch wood 500 °C, liquid	B500l
Pine wood 550 °C, tar	P550t	Birch wood 550 °C, tar	B550t
Pine wood 550 °C, liquid	P550l	Birch wood 550 °C, liquid	B550l
Pine wood 600 °C, tar	P600t	Birch wood 600 °C, tar	B600t
Pine wood 600 °C, liquid	P600l	Birch wood 600 °C, liquid	B600l

.

2.2. Methods

2.2.1. FTIR Analysis

Fourier transform infrared (FT-IR) spectroscopy was used to identify chemical differences between the analyzed samples. This technique enables the detection of characteristic absorption bands corresponding to the vibrations of chemical bonds in the studied materials, such as pitch, tar, and other organic compounds.

The analysis was performed using an FT-IR spectrometer manufactured by Bruker Optics GmbH (Göttingen, Germany), equipped with an attenuated total reflectance (ATR) accessory with a diamond crystal. Thanks to the ATR technique, no additional sample preparation (e.g., KBr pellet formation) was required. The samples were placed directly onto the diamond crystal surface and pressed to ensure maximum contact.

Number of scans: 32 scans per sample—this enhances the signal-to-noise ratio and improves spectral precision.

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Wavenumber range: 3700 cm⁻¹–700 cm⁻¹—covers the vibrational regions of key functional groups.

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Resolution: 4 cm⁻¹—provides sufficient spectral resolution for detailed peak analysis.

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Background correction: performed before each measurement to eliminate environmental interference (e.g., water vapor and carbon dioxide absorption).

The obtained spectra were analyzed with reference to the literature on FTIR studies of pitch [16,17,18,19], tar, and organic compounds, as well as the data available on the Sigma-Aldrich website (www.sigmaaldrich.com, accessed 10 December 2024).

To process and interpret the acquired spectra, Spectragryph v1.2.16.1 software was used. This specialized tool allows for visualization, spectral correction, and comparative analysis of FT-IR data.

Using Spectragryph, v1.2.15.1 the following data processing steps were performed:

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Spectral normalization to eliminate differences in sample thickness.

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Baseline correction to minimize background interference from atmospheric absorption.

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Smoothing algorithms to reduce noise and enhance peak clarity.

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Differential analysis to highlight spectral differences between samples.

2.2.2. Moisture Content, Elemental Composition, and Ash Content

Moisture content was determined using a drying and weighing method at a temperature of 105 ± 2 °C. Determination of the content of carbon (C), nitrogen (N), and hydrogen (H) was performed using the procedures described in ISO 16948:2015 [20]) using the elementary analyzer Flash 1112 of (Thermo Electron Corporation, Waltham, MA, USA). The ash content in the samples was determined according to EN ISO 18122:2016 [21]. Samples were roasted at a temperature of 550 ± 10 °C.

2.2.3. Calorific Value

The heat of combustion was determined according to EN ISO 18125:2017 [22]. The samples were burnt in a calorimetric bomb filled with oxygen under a pressure of 3.0 ± 0.2 MPa using the IKA C6000 calorimeter (Staufen, Germany). The heat of combustion was calculated using the Regnault–Pfaundler method, considering the temperature increases and the thermal capacity of the calorimeter. Calculations of the calorific value were performed according to EN ISO 18125:2017. The sulfur (S) content was determined using the ICS-1100 ionic chromatograph (Thermo Scientific, Waltham, MA, USA) according to EN ISO 16994:2016 [23].

3. Results and Discussion

3.1. Results of FTIR Analysis

The FTIR spectroscopy method allows us to identify functional groups and molecular structures within the tar, providing insights into its chemical characteristics and other potential applications, particularly as a fuel or chemical feedstock. The results of the tar wood FTIR analysis are shown in Figure 1 and Figure 2. They clearly demonstrate that samples from different fractions differ from one another, yet the results within the same fraction are remarkably similar. All samples exhibit a characteristic peak in their spectrum attributed to carbonyl groups at 1709 cm⁻¹ and a peak at 1642 cm⁻¹ corresponding to the vibrations of an unsaturated C=C bond. These compounds can have a high energy value and be used as fuel components [24]. The presence of phenolic compounds is confirmed by peaks at 1515 cm⁻¹ and 1600 cm⁻¹. Aromatic systems, especially phenolic derivatives, can improve calorific value because they are thermally stable and can participate in exothermic combustion reactions. Due to the presence of oxygen in phenolics and other compounds present in the tested samples, the influence of oxygen in other aspects described in the literature should be considered [25]. The presence of hydrocarbon chains is indicated by peaks at 1456 cm⁻¹ and 1374 cm⁻¹. Stretching vibrations of C–O bonds in various structures are visible within the range of 1300–1000 cm⁻¹. The region between 900 and 700 cm⁻¹ contains various bands associated with C–H bonds in aromatic structures with different degrees of substitution. The FTIR spectra confirm differences in the chemical composition of the analyzed fractions, while maintaining similarity among samples from the same fraction obtained at different temperatures. The most different FTIR spectra were obtained for liquid fractions obtained in 550 and 600 °C, which lack peaks in the range from 3000 to 2750 cm⁻¹. This may suggest the degradation of long hydrocarbon chains, leading to the loss of calorific value. These samples had large peak at 3350 cm⁻¹ characteristic of the OH groups present in alcohols and phenols. As mentioned earlier, the presence of oxygen affects the combustion of fuels [25]. All other samples had two characteristic peaks in this region. The first was at 2925 cm⁻¹, attributed to the asymmetric stretching of C-H bonds in methyl (CH₃) and methylene (CH₂) groups attributed to aliphatic hydrocarbons. These kind of organic structures are typical of paraffins, waxes, oils, and other organic compounds with long carbon chains. The second peak at 2860 cm⁻¹ corresponded to the symmetric stretching of C-H bonds in methyl (CH₃) and methylene (CH₂) groups, while lower vibrational energy was associated with the symmetric vibrations of the aromatic ring. This type of structure is characteristic of saturated fatty acids in lipids and fats. These compounds have a high calorific value, which suggests their potential as fuel. It was also observed that both liquid fractions obtained at 600 °C have no visible peaks in the fingerprint region between 1200 to 500 cm⁻¹.

FTIR analysis gives an insight into the chemical compositions of the tested samples. Interestingly, two peaks were found at 1642 and 1709 cm⁻¹, with both indicating stretching of the C=C bond. Peaks at 1642 cm⁻¹ in alkenes and aromatic rings and at 1709 cm⁻¹ in carbonyl groups may indicate the calorific value of the samples. A higher ratio between them and absorbance at between 1709 cm⁻¹ and 1642 cm⁻¹ (shown in Figure 3) are connected to a higher calorific value. This connection is highly visible in tar obtained from pine wood. The tar fraction of birch tar has similar ratios to pine tar; however, the liquid fractions of birch tar have higher variation in this ratio. In other research, the connection between soot production and aromatic structure was reported. Lefort and Tsang [26] showed that biomass with more high-molecular-weight aromatic structures should exhibit slower combustion and higher flame temperatures, which may lead to greater soot production. This may suggest that liquid samples will demonstrate slower combustion and greater soot production.

3.2. Elemental Composition

The elemental composition of the examined samples is presented in

Table 3. Elemental composition of the liquid (l) and tar (t) samples (birch (B) and pine (P) wood products).

Element [%]	B450l	B450t	B500l	B500t	B550l	B550t	B600l	B600t
Hydrogen	6.7 ± 0.3	7.3 ± 0.1	6.4 ± 0.1	7.3 ± 0.1	6.4 ± 0.1	7.2 ± 0.1	6.6 ± 0.1	7.2 ± 0.1
Carbon	62.9 ± 0.3	69.3 ± 0.2	57.0 ± 0.3	68.7 ± 0.2	58.7 ± 0.6	68.5 ± 4.3	65.1 ± 0.33	68.4 ± 0.4
Nitrogen	0.27 ± 0.04	0.18 ± 0.05	0.27 ± 0.05	0.15 ± 0.01	0.27 ± 0.07	0.14 ± 0.02	0.26 ± 0.05	0.20 ± 0.03
Sulfur	0.02 ± 0.01	0.03 ± 0.01	0.02 ± 0.01	0.03 ± 0.01	0.02 ± 0.01	0.02 ± 0.01	0.02 ± 0.01	0.02 ± 0.01
Element [%]	P450l	P450t	P500l	P500t	P550l	P550t	P600l	P600t
Hydrogen	6.7 ± 0.2	7.3 ± 0.1	6.6 ± 0.1	7.4 ± 0.1	6.6 ± 0.1	7.3 ± 0.1	6.7 ± 0.1	7.2 ± 0.1
Carbon	57.7 ± 0.8	70.2 ± 0.8	59.9 ± 0.5	70.0 ± 0.3	61.5 ± 0.2	70.5 ± 0.3	61.8 ± 0.3	70.4 ± 0.2
Nitrogen	0.25 ± 0.05	0.18 ± 0.03	0.19 ± 0.01	0.11 ± 0.04	0.28 ± 0.07	0.20 ± 0.04	0.26 ± 0.05	0.20 ± 0.04
Sulfur	0.01 ± 0.01	0.03 ± 0.01	0.01 ± 0.01	0.03 ± 0.01	0.01 ± 0.01	0.03 ± 0.01	0.02 ± 0.01	0.02 ± 0.01

.

During the combustion of natural plant oil, chemical compounds containing elementary elements (carbon, hydrogen, nitrogen, and sulfur) are oxidized, resulting in heat generation. Knowledge of the elemental composition of fuels allows an accurate assessment of their calorific value and allows the estimation of the risks associated with the emission of toxic combustion products. The carbon content in the wood tar fractions ranged between 68.4% in B600t and 70.5% in sample P550t, and in the liquid fractions from 57.0% for sample B500l to 65.1% for sample P600l. These results correspond to the values of bio-oil reported in the literature [27,28].

The hydrogen content in the tested fractions ranged from 6.4% to 7.4%. Higher nitrogen content was determined in the liquid fractions (0.19–0.28%). In the wood tar fractions, the content of nitrogen ranged from 0.11% (sample P500t) to 0.20% (sample B600t, P550t and P600t). The obtained values for hydrogen and nitrogen are consistent with those obtained by other authors who studied the wood pyrolysis process [27,29].

To evaluate the quality of biofuels, it is important to know their sulfur content. For the tested liquid fractions, the sulfur content was, on average, 0.019% for birch wood and 0.016% for pine wood, respectively, while for the tar fractions, it was 0.028% for birch wood and 0.027% for pine wood, respectively (Table 3).

3.3. Moisture Content, Ash Content, and Thermal Properties

The ash content of the fuel is of high importance when choosing the appropriate combustion and gas cleaning technologies. Fuels with low ash content are, therefore, preferable. In most of the samples tested, the ash content was below 0.01%, as presented in

Table 4. Thermal properties of liquid (l) and tar (t) samples (birch (B) and pine (P) wood products).

Property	B450l	B450t	B500l	B500t	B550l	B550t	B600l	B600t
Moisture content [%]	87.1 ± 3.2	52.5 ± 2.3	82.2 ± 3.0	53.8 ± 3.5	82.2 ± 3.0	53.9 ± 3.1	72.1 ± 5.1	49.6 ± 2.4
Ash [%]	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Gross calorific value [MJ/kg]	36.4 ± 2.8	45.2 ± 0.7	29.3 ± 3.0	47.2 ± 2.2	39.4 ± 5.8	44.0 ± 0.8	32.3 ± 1.6	41.7 ± 2.4
Net calorific value [MJ/kg] [kWh/kg]	2.4 ± 1.2 0.7 ± 0.3	19.4 ± 1.1 5.4 ± 0.3	2.9 ± 1.0 0.8 ± 0.3	19.7 ± 1.9 5.5 ± 0.5	4.7 ± 1.6 1.3 ± 0.4	18.2 ± 1.4 5.1 ± 0.4	6.8 ± 3.4 1.9 ± 0.9	19.1 ± 1.6 5.3 ± 0.4
Property	P450l	P450t	P500l	P500t	P550l	P550t	P600l	P600t
Moisture content [%]	64.8 ± 4.1	42.7 ± 1.6	60.5 ± 5.5	45.17 ± 1.70	63.0 ± 8.3	49.0 ± 1.9	59.7 ± 16.3	47.1 ± 1.8
Ash [%]	0.04 ± 0.02	0.03 ± 0.01	<0.01	0.06 ± 0.01	<0.01	<0.01	<0.01	<0.01
Gross calorific value [MJ/kg]	30.1 ± 1.1	49.5 ± 1.8	31.7 ± 1.3	47.9 ± 1.2	29.8 ± 2.7	48.3 ± 4.8	30.1 ± 1.6	46.5 ± 1.4
Net calorific value [MJ/kg] [kWh/kg]	8.5 ± 0.9 2.7 ± 0.4	26.4 ± 1.3 7.3 ± 0.4	10.5 ± 1.6 2.9 ± 0.5	24.3 ± 1.0 6.7 ± 0.3	8.9 ± 5.2 2.5 ± 0.4	22.6 ± 2.6 6.3 ± 0.7	10.1 ± 1.2 2.8 ± 0.4	22.6 ± 1.1 6.3 ± 0.3

. This value was only higher for three samples of coniferous wood: 0.03% (sample P450t), 0.04% (sample P450l), and 0.06% (sample P500t).

The most important thermo-physical parameters of the fuel are its heat of combustion (gross calorific value, HHV) and net calorific value (LHV) (Table 4). These values determine the possibility and profitability of the product as fuel. The net calorific value is calculated from the heat of combustion set in a calorimeter bomb, the elemental composition, and the moisture content. The gross calorific value of the studied tar fractions was about 40 MJ/kg, reaching the highest value of 49.5 MJ/kg for sample P450t (Figure 4).

In general, liquid samples had a lower HHV in comparison to tar samples. Pine tar samples from all temperature had a very similar HHV (46.5–49.5 MJ/kg), which was higher that the best birch tar sample 75.3 MJ/kg. The value of the combustion heat corresponds to the HHV value of petroleum oil (40 MJ/kg) [30].

The liquid product of the pyrolysis of alternative biomasses, often called bio-oil, has been previously investigated. The gross calorific value of oil palm trunk and empty fruit shells bio-oils was reported as being between 24.9 to 31.2 MJ/kg with no clear influence of process temperature on calorific value [31]. Another study on palm oil waste pyrolysis oil reported bio-oil heating values from 10 to 15 MJ/kg [32]. Cashew nutshell bio-oil produced at temperatures between 400 to 550 °C exhibited a calorific value up to 40 MJ/kg [33]. Sewage sludge has also undergone valorization through pyrolysis, resulting in a bio-oil with a gross calorific value of 33–36 MJ/kg [34], while another study on sludge pyrolysis oil reported a calorific value of 36 MJ/kg [35]. In another study, the HHV reported for the liquid product from the pyrolysis of agricultural biomass was between 18.7 to 22.5 MJ/kg, while for wood, it was 24.1 MJ/kg [36]. The abovementioned results show that the liquid fraction product of wood pyrolysis has similar thermal properties to many bio-oils. On the other hand, the tar product of wood pyrolysis exhibits much higher gross calorific values, which may indicate that it is a very good source of energy.

4. Conclusions

Pyrolysis is an interesting method of biomass valorization. This process yields three products, of which the liquid was investigated, as this is one of the least studied products. The liquid product divides, over time, into liquid and tar. This product can be used as a source of energy. When comparing the products investigated in this work with the energetic properties of previously investigated bio-oils, the products reported here have good thermal properties, with really good results for the tar fraction, ranging from 42 to 50 MJ/kg. Other potential applications include adhesive use in the palletization of sawdust, during which the products will not only act as adhesives but will also increase the calorific value of the resulting pellets. The results of this work confirm that the pyrolysis of wood waste is a good alternative to simple combustion.