Rapid climate change is leading to an increase of average temperature by 2 °C above pre-industrialized levels. Because of this, significant steps should be taken to reverse this trend [1
]. Developed countries within the European Union are committed to reducing the emission of greenhouse gas (GHG) by 80%–95% compared to levels from 1990. This goal needs to be achieved by 2050 [2
]. In 2018, greenhouse gas emissions increased globally by 1.8% compared to 2017, in which GHG emissions amounted to 60%. The main factors of this change are the increasing gross domestic product (GDP) in G20 countries, CO2
emissions related to energy and the total primary energy supply [3
]. This brings us to the Paris Agreement, which states that the main aim of all G20 countries is to limit the average temperature increase to 1.5 °C. These obligations require firm steps to provide necessary changes in energy management [4
Currently, global energy consumption is focused on a fast and cheap energy supply, which is mainly covered by fossil fuels [5
]. Therefore, the development of Renewable Energy Sources (RES) needs to be focused on finding an energy carrier which will be a substitute for the most desirable properties of fossil fuels. The answer to cover this requirement is the thermochemical conversion of biomass. Here, six technologies which are possibly suitable can be determined depending on their expected final products: pyrolysis, gasification, co-firing, liquefaction, carbonization and combustion [6
In analysing the thermal conversion of biomass, the biomass itself should be defined. According to Dyton and Foust, the definition depends on the final usage of biomass [8
]. This can be limited to not only the organic material formed during the photosynthesis process but also by other sources, such as solid municipal wastes [9
]. The following are categories of biomass: wood and wood wastes, energy crop plants, organic parts contained in municipal solid waste and industrial waste, sewage sludge, plant by-products of food production and manure [13
]. It is important to emphasize that all types of biomass in a raw material form are characterized by high moisture content, hygroscopicity and heterogeneity and a low carbon/oxygen (C/O) ratio, bulk density and calorific value [14
]. Another definition of biomass states that it is an energy source which originates from organic material such as wood, plants, agricultural products and organic wastes as well as their products, by-products and residues [17
According to data published by the Publications Office of the European Union in 2019, biomass is the main renewable energy source in the EU. The share of biomass in all renewable energy sources is almost 60%, and cooling and heating sectors consume 75% of all bioenergy. The main bioenergy consumers within the EU are Germany, France, Italy, Sweden and the UK [18
]. This energy source allows zero or even negative CO2
emissions to be achieved, as well as increasing the share of renewable energy source consumption. However, it needs to be remembered that the energy density of biomass is low, while its moisture content is high in comparison to fossil fuels. Because of this, biomass requires additional treatment to improve its energy properties [17
To achieve the goal of decreasing or even obtaining negative CO2
emissions and producing clean energy, a proper model of biomass conversion needs to be developed. Because of the ultimate properties of biomass (low energy density and high moisture content), it is better to provide the thermochemical conversion of this material using processes such as pyrolysis or gasification than combustion. Pyrolysis occurs under a temperature range from 300 to 700 °C in the presence of a proper agent which does not contain oxygen, such as nitrogen [19
]. Useful products such as syngas, bio-oil and biochar are obtained during biomass decomposition. This process is very complex and includes many simultaneous reactions (e.g., dehydration or carbonization) [20
]. Additionally, researchers are working on novel pyrolysis processes such as solar pyrolysis, which uses solar energy to heat the reactor [22
The biomass life cycle can be presented as a closed cycle (Figure 1
). Once it is converted into bioenergy by thermochemical conversion technologies, it becomes a useful and easy-to-manage energy source. During energy consumption, carbon dioxide is released, which is one of the components of the photosynthesis process; the growth of new biomass closes the cycle.
During the pyrolysis process, three valuable products are produced: gas (syngas), liquid (tar) and solid (char, also called biochar). All of these products contain chemical energy. Moreover, pyrolysis is a safe and suitable process for the conversion of contaminated and toxic biomass due to its high process temperature as well as its isolated reaction process zone [25
The main aim of this study was to investigate a novel approach of syngas production promoting negative carbon emission with simultaneous high-quality biochar production. A wood biomass, represented by a pine sawdust sample (PSD), was processed via the pyrolysis process. The influences of the process temperature and addition of CaO on the chemical and physical properties of solid (char) and gaseous (syngas) phases were studied with a special focus on CO2 capture in order to obtain negative carbon emission. A detailed analysis of the biochars was conducted. Next, the gaseous products were examined taking into account the influence of CaO.
2. Materials and Methods
2.1. Material Characteristic
In this study, a wood biomass sample—pine wood in the form of sawdust—was investigated. The biomass originated from Xianyang, Shaanxi, China. The raw material was milled and next processed with 60–100 mesh filters. The proximate and ultimate analyses of raw material are presented in Table 1
. Ultimate analysis was provided according to the following European Standards: moisture content (M): PN-EN ISO 18134-1:2015-11, ash content (A): PN-EN ISO 18122:2016-01, and volatile matter (VM): PN-EN ISO 18123:2016-01. The studied material was characterized by a high volatile matter content (82%) and low ash content (1.3%), which are characteristic for biomass [26
]. The moisture content was in line with typical biomass levels, which varied between 1.25% and around 12% [27
]. The ultimate analysis was conducted using LECO (CHN628). The analysed sample was combusted in pure oxygen at 950 °C in the furnace. The carbon, hydrogen and nitrogen content tests were carried out in accordance with the standards described in the PN-EN 15407:2011. The obtained results showed that, in comparison to other studies, the chosen sample consisted of a typical share of chemical elements for wood biomass. The average concentration of chemical elements based on the literature review was as follows: C (48.9%), H (5.9%), O (44.1%) and N (0.14%) [29
]. The chemical constituents of biomass directly influence its thermochemical qualities and accordingly the efficiency of energy conversion [30
2.2. Experimental Procedure and Methods
The pyrolysis experiment was conducted in a fixed bed vertical reactor. Figure 2
presents a scheme of the test rig used in this study. The experimental reactor consisted of a quartz tube reactor which was electrically and isothermally heated by a tubular electric furnace. The reactor was externally heated by a 1 kW electric furnace, and the furnace dimensions were as follows: 30 mm inner diameter and 250 mm height. The apparatus was equipped a temperature and gas control systems. The reactor was connected to a nitrogen gas bottle and oil collector via pipelines. A flow meter in the pyrolysis processes was used to control the nitrogen flow. Pyrolysis gas was condensed in an ice bath and dried using a drying tube; next, it was collected in a Tedlar gas bag. The obtained pyrolysis gas (syngas) was analysed using a gas analyzer (GC 7900, Techcomp).
The pyrolysis process of pine sawdust was carried out under an inert atmosphere and temperatures of 500, 600 and 700 °C. The nitrogen flow rate was set to 60 mL/min, and the sample mass was 3 g. The weighted sample was placed inside a quartz reaction tube and secured against movement along the tube with quartz wool. Next, nitrogen was injected to ensure that a proper environment was provided before the pyrolysis experiment was started. First, a batch of syngas was collected for 20 min until the process achieved the set temperature; second, the batch was collected at the set temperature for the next 20 min. The produced gas was collected in Tedlar bags and then analysed. Additionally, the impact of calcium oxide (CaO) as a solid sorbent for carbon dioxide capture was investigated. Thus, the pyrolysis process was also carried out in the presence of a CaO solid sorbent layer (in the amount of 3 g) to compare the chemical composition of syngas without and with CaO in the reaction zone. Calcium oxide was pre-treated in the oven to remove the moisture content and to confirm CaO purity. For experiments with sorbent presence, the raw material was placed in the upper part of the quartz reaction tube and secured against movement along the tube with quartz wool; the sorbent was put inside the quartz reaction tube in the bottom part and also secured against movement along the tube with quartz wool. Both layers—the biomass and solid sorbent—were placed in the same heating zone and subjected to the same process temperature during the whole process.
The chemical analysis of obtained chars was provided. The analysis—including carbon, hydrogen and nitrogen concentrations—was carried out using an Elemental Analyser Truespec CHN Leco (CHN628).
Thermal analysis (TA) was applied to study the thermal behaviour of the raw material and chars obtained after the pyrolysis process of pine sawdust. First, the combustion and pyrolysis of raw biomass was carried out using TA. Next, the combustion process of chars obtained during the pyrolysis process was analysed. For the thermogravimetric analysis ((TG) and differential scanning calorimetry (DSC)), the sample was placed in an alumina crucible. A fuel sample of 5 mg was heated from an ambient temperature up to 700 °C at a constant heating rate of 10 °C/min in 60 mL/min flow of air (combustion) and nitrogen (pyrolysis), respectively. TG and DSC curves for each fuel sample were determined. Based on the TG results, the mass changes were determined, whereas DSC allowed us to examine thermal effects (endothermic and exothermic). Additionally, the DTG (differential thermogravimetric ) curves were calculated; DTG is the first derivative of TG.
The morphology and texture of studied samples were investigated using a scanning electron microscope (SEM) ZEISS MultiSEM 505/506. This high-resolution microscope allowed us to investigate the surface topography of fresh and used sorbent as well as raw biomass and after-process chars. The samples were placed on double-sided tape which was coated with a thin gold (Au) film. The specific surface area and porous structure (porosity) were investigated using the Brunauer–Emmett–Teller (BET) method. For this purpose, JW-BK200B (JINGWEIGAOBO Ltd., Beijing, China) was used.
Phase composition analyses, such as the identification of CaCO3 content in the fresh sorbent material (CaO) and after the processes, were performed using X-ray diffraction (XRD, Bruker d8 advance). The device was equipped with a Cu Kα (40 kV, 40 mA, λ =0.154 nm) X-ray source, which allowed us to detect the crystalline phase composition of adsorbents. Tests were conducted in the angular range of 2θ = 6 ÷ 90°, with a step size of 2θ = 0.02° and dwell time of 0.1 s. The phase composition of the analysed samples was determined using the data base PDF-4+ICDD and confirmed with the available literature.
Syngas samples were analysed using a gas chromatograph (GC 7900, Techcomp) equipped with two detectors: a flame ionization detector (FID) and thermal conductivity detector (TCD). The CH4, C2H4, C2H6, C3H6, and C3H8 compounds were analysed using a FID detector, while CO, CO2, H2 and N2 species were determined using TCD.