Management of Lignocellulosic Waste towards Energy Recovery by Pyrolysis in the Framework of Circular Economy Strategy

: The article presents the possibilities of effective management of lignocellulosic waste by including it in the circular economy. The pyrolysis process was chosen as the thermal conversion method. This approach, due to a high ﬂexibility of the obtained products, better quality of the solid residue (char), and the lower emission of pollutants into the atmosphere, e.g., SO 2 and NO x , is a competitive solution compared to combustion process. Wood waste from alder and pine were analyzed. As part of laboratory tests, the elementary composition was determined, i.e., C, H, N, S, and O. The pyrolysis process was carried out at a temperature of 600 ◦ C on an experimental stand for the conversion of solid fuels in a stationary bed. For the obtained data, using the Ansys Chemkin-Pro calculation tool, the detailed chemical composition of gaseous products of the pyrolysis process was modeled for a varying temperature range and residence time in the reactor. The studies have shown that for certain process conditions it is possible to obtain a high caloriﬁc value of pyrolytic gas, up to 25 MJ/m 3 .


Introduction
Progressive urbanization, digitization, and industrialization lead to a high demand for energy sources [1]. Today, the major source of energy are still fossil fuels such as coal, petroleum, and natural gas [2]. However, due to increasing awareness of resources depletion and environmental issues such as air pollution (mainly by CO 2 , NO x , SO x , and particulate matter emissions) being responsible for global warming and climate change, acid rain, etc., more sustainable energy sources are sought after [3]. So, there is a need to modify the recent trends in energy management [4]. Thus, a number of activities, policies, and legislation were implemented to replace conventional fuels and improve energy generation technologies in accordance to the principles of the sustainable development of society [5][6][7]. Due to the above, more and more attention is paid to the production of energy from biomass which is regarded as a promising, clean and renewable fuel [8].
However, converting raw biomass directly into energy is not very profitable due to the fact that it is characterized by a high moisture and oxygen content, low calorific value and highly variable composition and properties [9]. Worth noting is the fact that biomass can be thermochemically transformed into different types of fuels, such as biochar, bio-oil, and gas, during such processes as torrefaction [10], pyrolysis [11], gasification [12] and hydrothermal liquefaction ( Figure 1) [13,14]. Furthermore, pyrolysis and gasification seem to be, at the moment, the most reasonable and cost-effective methods for converting biomass The rational utilization of these products makes the pyrolysis process a serious candidate to be included in the chain of waste management activities consistent with the idea of a circular economy [44,45]. The definitions of circular economy presented in the world literature take into account the evolution of its creation and implementation. This idea appeared in the 1960s, which partly refers to the concept presented in the publication by Barry Commoner, The Closing Circle, from the early 1970s, which details the relationship between the level of technological development, the ecosystem, and the economy. The role of circularity in six important areas was also indicated: energy resources (including electrochemical and water), metals (including coking coal), non-metallic raw materials, forest biomass, agricultural resources, and flora and fauna resources [46]. Many research works appeared in the late 1970s after the publication of the Club of Rome Reports. They emphasized the need for changes in the world economy due to the depletion of natural resources. It is worth noting that already in the 1980s, the circular economy concept was described as a closed-loop economy by Stahel and Reday, who emphasized not only the necessity of recycling in the economy, but also the reuse and regeneration of products. Circular economy was defined as a development strategy in 2011, which maximizes resource efficiency and minimizes waste production in the context of sustainable economic and social development. The idea behind the circular economy is that the used product does not become a waste, but a raw material for further production. It is a move from the current approach "take-produce-consume-dispose" to a material re-use model that ultimately aims to eliminate the end-of-life stage of a product. The circular economy is, therefore, an economy model in which all products, materials, and raw materials should be used for as long as possible. As a result, many countries started to treat circular economy not as a concept, but as a model, strategy, and even economic system, adjusting legislation and economic instruments to facilitate its implementation [46,47]. The definitions of this concept emphasize the distinction between the specificity of circular cconomy and sustainable development, and at the same time indicate that in this strategy it is important to search for economic solutions that are profitable and beneficial for the natural environment, which create not only new business models, but most of all innovative technological solutions and new materials. In this context, it is worth noting that the new circular economy action plan for a cleaner and more competitive Europe, and the European Commission (2020) encourages the stimulation of the circular economy by implementing several economic instruments such as landfill and waste incineration taxes. This attracts the attention for other available waste-to-energy technologies, such as gasification or pyrolysis [48]. The approach of the lignocellulosic waste management proposed in the present article may be helpful in the initial assessment of the profitability and direction of their energy use. A type of biomass also determines the characteristics of the pyrolysis end products [53]. The research performed by Pang et al. [32] showed that depending on the biomass composition, different percentage proportions of products can be obtained. Biomass with high proportion of hemicellulose would result in products being rich in bio-gas, while cellulose-dominant biomass would yield products rich in bio-oil. In turn, biomass with high lignin content would lead to products rich in bio-char. Moreover, the economic evaluation and the sensitivity analysis showed that lignocellulosic components influence the economic efficiency of biomass pyrolysis [32].
The article presents a comprehensive approach to biomass pyrolysis, including not only laboratory experiments, but also computer simulations. Taking into account the complexity of the chemical phenomena occurring during the thermal conversion of lignocellulosic waste, conducting the laboratory tests alone does not allow for a thorough analysis and optimization of the process. Due to the use of computer simulations in the article, the  [19,[49][50][51][52].
A type of biomass also determines the characteristics of the pyrolysis end products [53]. The research performed by Pang et al. [32] showed that depending on the biomass composition, different percentage proportions of products can be obtained. Biomass with high proportion of hemicellulose would result in products being rich in bio-gas, while cellulosedominant biomass would yield products rich in bio-oil. In turn, biomass with high lignin content would lead to products rich in bio-char. Moreover, the economic evaluation and the sensitivity analysis showed that lignocellulosic components influence the economic efficiency of biomass pyrolysis [32].
The article presents a comprehensive approach to biomass pyrolysis, including not only laboratory experiments, but also computer simulations. Taking into account the complexity of the chemical phenomena occurring during the thermal conversion of lignocellulosic waste, conducting the laboratory tests alone does not allow for a thorough analysis and optimization of the process. Due to the use of computer simulations in the article, the presented research results not only supplement the knowledge obtained so far in the above-mentioned subject, but also provide a new knowledge in the field of detailed chemical analysis of gaseous pyrolysis products. The complexity of the pyrolysis process, which hinders its mathematical description, makes it difficult to understand and predict the ecological effects of this process, which is the basis for the development of competitive technological solutions aimed not only at minimizing the negative environmental effects, but above all at reducing the financial outlays. Moreover, the literature on the subject repeatedly emphasizes that modeling of chemical kinetics makes a significant contribution to the chemical analysis of complex phenomena occurring during the process of thermal biomass conversion. Due to the above, the results of the computer simulations presented in the article are the starting point for deliberations on the detailed chemical composition of the pyrolytic gas formed, and thus its quality, decisive for energy use. The high calorific value of pyrolysis gas may contribute to lower consumption of conventional gaseous fuels, e.g., natural gas, which will bring tangible economic benefits.

Materials
The material selected for the purpose of the present research was lignocellulosic biomass in the form of alder and pine chips, for which the content of carbon, hydrogen, nitrogen, and sulfur was determined using the Vario Macro Cube CHNS analyzer. The oxygen content, being present in small amounts during the pyrolysis process, was determined by the direct method in a reducing atmosphere (N 2 /H 2 95%/5%) at the temperature of 1120 • C. Oxygen was determined using the automatic elemental analyzer by alementar analsensysteme GmbH. Determination of the calorific value for the analyzed biomass was performed using the calorimetric method in the calorimetric bomb under constant volume conditions. The calorific value of the tested materials was determined based on the rate of the temperature rise, the calorimeter heat capacity, and the mass of the samples. The obtained value was corrected by the heat value resulting from the combustion of auxiliary substances, i.e., the ignition wire. The results of the tests performed are summarized in Table 1. presented research results not only supplement the knowledge obtained so far in the above-mentioned subject, but also provide a new knowledge in the field of detailed chemical analysis of gaseous pyrolysis products. The complexity of the pyrolysis process, which hinders its mathematical description, makes it difficult to understand and predict the ecological effects of this process, which is the basis for the development of competitive technological solutions aimed not only at minimizing the negative environmental effects, but above all at reducing the financial outlays. Moreover, the literature on the subject repeatedly emphasizes that modeling of chemical kinetics makes a significant contribution to the chemical analysis of complex phenomena occurring during the process of thermal biomass conversion. Due to the above, the results of the computer simulations presented in the article are the starting point for deliberations on the detailed chemical composition of the pyrolytic gas formed, and thus its quality, decisive for energy use. The high calorific value of pyrolysis gas may contribute to lower consumption of conventional gaseous fuels, e.g., natural gas, which will bring tangible economic benefits.

Materials
The material selected for the purpose of the present research was lignocellulosic biomass in the form of alder and pine chips, for which the content of carbon, hydrogen, nitrogen, and sulfur was determined using the Vario Macro Cube CHNS analyzer. The oxygen content, being present in small amounts during the pyrolysis process, was determined by the direct method in a reducing atmosphere (N2/H2 95%/5%) at the temperature of 1120 °C. Oxygen was determined using the automatic elemental analyzer by alementar analsensysteme GmbH. Determination of the calorific value for the analyzed biomass was performed using the calorimetric method in the calorimetric bomb under constant volume conditions. The calorific value of the tested materials was determined based on the rate of the temperature rise, the calorimeter heat capacity, and the mass of the samples. The obtained value was corrected by the heat value resulting from the combustion of auxiliary substances, i.e., the ignition wire. The results of the tests performed are summarized in Table 1. presented research results not only supplement the knowledge obtained so far in the above-mentioned subject, but also provide a new knowledge in the field of detailed chemical analysis of gaseous pyrolysis products. The complexity of the pyrolysis process, which hinders its mathematical description, makes it difficult to understand and predict the ecological effects of this process, which is the basis for the development of competitive technological solutions aimed not only at minimizing the negative environmental effects, but above all at reducing the financial outlays. Moreover, the literature on the subject repeatedly emphasizes that modeling of chemical kinetics makes a significant contribution to the chemical analysis of complex phenomena occurring during the process of thermal biomass conversion. Due to the above, the results of the computer simulations presented in the article are the starting point for deliberations on the detailed chemical composition of the pyrolytic gas formed, and thus its quality, decisive for energy use. The high calorific value of pyrolysis gas may contribute to lower consumption of conventional gaseous fuels, e.g., natural gas, which will bring tangible economic benefits.

Materials
The material selected for the purpose of the present research was lignocellulosic biomass in the form of alder and pine chips, for which the content of carbon, hydrogen, nitrogen, and sulfur was determined using the Vario Macro Cube CHNS analyzer. The oxygen content, being present in small amounts during the pyrolysis process, was determined by the direct method in a reducing atmosphere (N2/H2 95%/5%) at the temperature of 1120 °C. Oxygen was determined using the automatic elemental analyzer by alementar analsensysteme GmbH. Determination of the calorific value for the analyzed biomass was performed using the calorimetric method in the calorimetric bomb under constant volume conditions. The calorific value of the tested materials was determined based on the rate of the temperature rise, the calorimeter heat capacity, and the mass of the samples. The obtained value was corrected by the heat value resulting from the combustion of auxiliary substances, i.e., the ignition wire. The results of the tests performed are summarized in Table 1.

Experimenatal Stand
The lignocellulosic biomass pyrolysis process was carried out on the experimental stand for solid fuel conversion in a stationary bed, described in detail in [54]. The main element of the stand was an electric furnace with a steel retort, in which the biomass conversion process took place together with the liquid and gas phase collection system. The diagram of the stand is shown in Figure 2.

Experimenatal Stand
The lignocellulosic biomass pyrolysis process was carried out on the experimental stand for solid fuel conversion in a stationary bed, described in detail in [54]. The main element of the stand was an electric furnace with a steel retort, in which the biomass conversion process took place together with the liquid and gas phase collection system. The diagram of the stand is shown in Figure 2. In each pyrolysis run, the analyzed waste (100 g) was placed in a steel retort and closed tightly. Nitrogen was supplied at the bottom of the retort for about 15 min at a constant flow of 500 cm 3 /h, which allowed obtaining chemically neutral conditions of the process. After 15 min, the nitrogen flow was reduced to 100 cm 3 /h and the heating of the retort was started. The pyrolysis process was performed at a temperature of 600 °C. In all the experiments conducted, the heating rate was kept constant at 10 °C/min. After reaching the set temperature, the retort with the sample was kept at the final temperature for 5 min, then immediately removed from the furnace, and then cooled to an ambient temperature. Keeping the sample in the given temperature conditions was related to the time of heat penetration and its normalization inside the retort.

Computer Calculations
The purpose of employing the computer calculations was to determine the detailed chemical composition of pyrolytic gas for variable process conditions, i.e., temperature and residence time, and then to calculate its calorific value. The proposed approach showed the energy potential of gaseous pyrolysis products and, as a consequence, made it possible to indicate an effective direction for the management of lignocellulosic waste.
The pyrolysis process of waste was modeled using the widely distributed ANSYS CHEMKIN-PRO software [55][56][57][58]. A detailed chemical mechanism dedicated to biomass conversion, developed by the CRECK Modeling Group, was implemented for the calculations. The mechanism involved 137 compounds and 4533 chemical reactions. The calculations were made for the process temperature variable in the range of 600-900 °C and the residence time from 1-15 min. The process was carried out under the assumption that the In each pyrolysis run, the analyzed waste (100 g) was placed in a steel retort and closed tightly. Nitrogen was supplied at the bottom of the retort for about 15 min at a constant flow of 500 cm 3 /h, which allowed obtaining chemically neutral conditions of the process. After 15 min, the nitrogen flow was reduced to 100 cm 3 /h and the heating of the retort was started. The pyrolysis process was performed at a temperature of 600 • C. In all the experiments conducted, the heating rate was kept constant at 10 • C/min. After reaching the set temperature, the retort with the sample was kept at the final temperature for 5 min, then immediately removed from the furnace, and then cooled to an ambient temperature. Keeping the sample in the given temperature conditions was related to the time of heat penetration and its normalization inside the retort.

Computer Calculations
The purpose of employing the computer calculations was to determine the detailed chemical composition of pyrolytic gas for variable process conditions, i.e., temperature and residence time, and then to calculate its calorific value. The proposed approach showed the energy potential of gaseous pyrolysis products and, as a consequence, made it possible to indicate an effective direction for the management of lignocellulosic waste.
The pyrolysis process of waste was modeled using the widely distributed ANSYS CHEMKIN-PRO software [55][56][57][58]. A detailed chemical mechanism dedicated to biomass conversion, developed by the CRECK Modeling Group, was implemented for the calculations. The mechanism involved 137 compounds and 4533 chemical reactions. The calculations were made for the process temperature variable in the range of 600-900 • C and the residence time from 1-15 min. The process was carried out under the assumption that the reactants were perfectly mixed with the PSR Reactor (perfectly stirred reactor). Based on the determined gas composition, the calorific value was calculated, indicating the most favorable variant. The diagram of the performed calculations is shown in Figure 3.
reactants were perfectly mixed with the PSR Reactor (perfectly stirred reactor). Based on the determined gas composition, the calorific value was calculated, indicating the most favorable variant. The diagram of the performed calculations is shown in Figure 3.

Experimental Results
The Table 2 summarizes the properties of the pyrolysis products obtained from the tested lignocellulosic wastes.

Experimental Results
The Table 2 summarizes the properties of the pyrolysis products obtained from the tested lignocellulosic wastes.

Calculation Results
The results of the computer simulations are presented in Figures 4

Calculation Results
The results of the computer simulations are presented in Figures 4 and 5.    An important part of the study was to verify the correctness of the model adopted for calculations by comparing the results obtained during the calculations with the results of the experiment (Figures 6 and 7).  An important part of the study was to verify the correctness of the model adopted for calculations by comparing the results obtained during the calculations with the results of the experiment (Figures 6 and 7). An important part of the study was to verify the correctness of the model adopted for calculations by comparing the results obtained during the calculations with the results of the experiment (Figures 6 and 7).  In a further step, four main components of the pyrolysis gas, namely: H2, CO, CO2, and CH4 obtained in laboratory conditions, were compared with of the ones obtained from the computer simulations (Figure 8).  In a further step, four main components of the pyrolysis gas, namely: H 2 , CO, CO 2 , and CH 4 obtained in laboratory conditions, were compared with of the ones obtained from the computer simulations ( Figure 8). In a further step, four main components of the pyrolysis gas, namely: H2, CO, CO2, and CH4 obtained in laboratory conditions, were compared with of the ones obtained from the computer simulations ( Figure 8). Subsequently, the calorific value of pyrolysis gas was calculated for all variants of the performed calculations and compared with the calorific value obtained in the experimental conditions (Figure 9).

Discussion and Future Work
The main product in the pyrolysis process was the liquid fraction-bio-oil, and its share was 56.2% for pine wood and 60.38% for alder wood. The solid (char) and gaseous (pyrolysis gas) fractions were comparable and amounted to approx. 20%. The calorific values of the obtained pyrolysis products were, respectively, char-approx. 33%, bio-oil from 24.81-26.51% and pyrolysis gas from 5.41-7.86%. Similar results were obtained by Manzano et al. [30], who carried out the pyrolysis process for pine and poplar waste. As indicated by numerous studies [59,60], the process temperature and residence time have a significant impact on the yield of pyrolysis products. As demonstrated in their works by Garcia-Perez et al. [61], Fan et al. [62], as well as Özbay et al. [63], with increasing temperature, in the range from 350 °C to 600 °C, the share of the gaseous fraction increases (pyrolysis gas) from 22% to 32% and liquid (bio-oil) from 36% to 40%, while the solid fraction (char) significantly decreases from 42.5% to 28.5%. Yan et al. [64] explained that the increase in pyrolysis gas efficiency and the decrease in oil yield with increasing temperature, is the effect of stronger cracking of carbon bonds at higher temperatures and obtaining lighter hydrocarbons with shorter carbon chains. Moreover, they indicated that with the increase in residence time in the range of 0-15 min, a significant decrease in carbonizate yield is observed in favor of the increase in both the yields of oil and pyrolysis gas [64]. The quality of the obtained products was determined primarily by the elemental composition of the analyzed biomass. As indicated by the obtained test results, the pyrolysis gas contained mainly CO2 (28.54% for the alder wood and 31.4% for the pine wood), CO (22.97% for the alder wood and 28.24% for the pine wood), H2 (5.14% for the alder wood and 7.88% for the pine wood) and lighter hydrocarbons, in particular methane (10.25% for the alder wood and 20.32% for the pine wood) and in small amounts (less than 1%) ethane, ethene, propane, propene, and butane. As indicated by the results, for other types of waste, including municipal waste [65][66][67][68][69][70], the temperature and residence time also have a significant impact on the quality of the obtained products, in particular on the pyrolysis gas as its calorific value increases with increasing process temperature [71].

Discussion and Future Work
The main product in the pyrolysis process was the liquid fraction-bio-oil, and its share was 56.2% for pine wood and 60.38% for alder wood. The solid (char) and gaseous (pyrolysis gas) fractions were comparable and amounted to approx. 20%. The calorific values of the obtained pyrolysis products were, respectively, char-approx. 33%, bio-oil from 24.81-26.51% and pyrolysis gas from 5.41-7.86%. Similar results were obtained by Manzano et al. [30], who carried out the pyrolysis process for pine and poplar waste. As indicated by numerous studies [59,60], the process temperature and residence time have a significant impact on the yield of pyrolysis products. As demonstrated in their works by Garcia-Perez et al. [61], Fan et al. [62], as well as Özbay et al. [63], with increasing temperature, in the range from 350 • C to 600 • C, the share of the gaseous fraction increases (pyrolysis gas) from 22% to 32% and liquid (bio-oil) from 36% to 40%, while the solid fraction (char) significantly decreases from 42.5% to 28.5%. Yan et al. [64] explained that the increase in pyrolysis gas efficiency and the decrease in oil yield with increasing temperature, is the effect of stronger cracking of carbon bonds at higher temperatures and obtaining lighter hydrocarbons with shorter carbon chains. Moreover, they indicated that with the increase in residence time in the range of 0-15 min, a significant decrease in carbonizate yield is observed in favor of the increase in both the yields of oil and pyrolysis gas [64]. The quality of the obtained products was determined primarily by the elemental composition of the analyzed biomass. As indicated by the obtained test results, the pyrolysis gas contained mainly CO 2 (28.54% for the alder wood and 31.4% for the pine wood), CO (22.97% for the alder wood and 28.24% for the pine wood), H 2 (5.14% for the alder wood and 7.88% for the pine wood) and lighter hydrocarbons, in particular methane (10.25% for the alder wood and  20.32% for the pine wood) and in small amounts (less than 1%) ethane, ethene, propane, propene, and butane. As indicated by the results, for other types of waste, including municipal waste [65][66][67][68][69][70], the temperature and residence time also have a significant impact on the quality of the obtained products, in particular on the pyrolysis gas as its calorific value increases with increasing process temperature [71].
Taking into account the fact that pyrolysis oil obtained from waste, including lignocellulosic ones, are often a mixture of oxidized and unoxidized hydrocarbons [72][73][74][75][76][77], which makes their a direct application difficult [77][78][79]. Hence, the conversion of waste analyzed in the article for parameters that ensure obtaining high calorific pyrolysis gas is fully justified. The chemical composition of bio-oils is determined by many factors, namely [80]:

•
The type of biomass (elemental composition, properties); • The initial preparation of the raw material (fragmentation, particle size and shape, moisture and ash content); • The conditions for the pyrolysis process (temperature, heating rate, residence time, pressure, and gas environment); • Filtration and steam condensation (filter type, condensation method and medium, cooling rate).
An additional argument for a more effective use of pyrolysis gas is the fact that the technologies for storage and use of hydrocarbon gases are well researched and highly developed [81]. Due to the above, in the article, apart from experimental tests, computer simulations were carried out, which allowed the prediction of the composition of pyrolysis gas for a wide range of temperature and residence time in the reactor.
The computer simulations carried out have shown, for both pine wood and alder wood, that with appropriate process conditions it is possible to obtain gaseous products that are valuable in terms of energy (Figures 4 and 5). As the temperature increases, the concentration of CO (from 16% to 32% at 900 • C) and H 2 (from 7% at 600 • C to 20% at 900 • C) increases significantly, and C 2 H 4 and C 6 H 6 rises several times, but only up to the temperature of 700-750 • C, and after exceeding it, it rapidly decreases. The share of carbon dioxide diminishes twice, which has a positive effect on the calorific value of the pyrolysis gas. The opposite situation is observed in Figure 5. With the increase in residence time, the concentration of hydrogen and carbon monoxide decreases several times, while for the remaining compounds it increases over the entire range of the analyzed residence time.
The comparative analysis (Figures 6 and 7) showed a high agreement of the obtained results. The observed differences, in particular for the concentration of CH 4 and CO in the gas obtained from the alder wood pyrolysis, should be explained by the assumption of perfect mixing adopted for calculations and the approximate residence time in the zone of the highest temperatures. Moreover, the calculated composition of the pyrolysis gas relates to the gaseous products at the temperature prevailing in the reactor and not, as in the experiment, to already partially cooled gases. From the point of view of the calculations performed, it was important to estimate the share of higher hydrocarbons, such as C 2 H 4 , C 2 H 6 , and C 3 H 8 , as their content in the calculated gas composition was definitely higher than in the measured one. These compounds, due to the higher boiling point, are much easier to liquefy, which explains their smaller share in the composition of the gas obtained under the conditions of the experiment. Figure 8 shows that the best compatibility, both for pine and alder wood, was obtained at 600 • C and for the residence time of 5 min, i.e., the conditions under which the experiment was conducted, which confirms the correctness of the model adopted for calculations.
The calculations show that the gas obtained from the pine wood pyrolysis carried out at the temperature of 600 • C and during 15 min had the highest calorific value. This is due to the fact that the gas composition has a much higher content of higher hydrocarbons than in other variants. For the remaining modeling cases, comparable values were obtained in the range of 20.61-23.94%. Comparing the modeling results with the calorific value obtained in the experimental conditions, a large disproportion is visible, resulting from the aforementioned differences, in particular the temperature of the analyzed gases. The results obtained as a part of the calculations encourage the energetic use of hot pyrolysis gas for firing e.g., industrial heating chambers [82,83].
As indicated in the article, pyrolysis is one of the most promising technologies for the treatment of various types of waste, including lignocellulosic waste. The conducted experimental studies showed a high yield of pyrolysis liquid, which can be a substitute for petroleum-derived fuels. However, the effective use of bio-oils in the field of liquid fuels requires the knowledge of highly differentiated properties of biomass. Moreover, due to the properties of pyrolysis oils, including the presence of impurities, their direct application is very difficult. On the other hand, technologies for the use and storage of hydrocarbon gases are the opposite, which are now well researched and further improved. Due to the above, the article focuses on the conversion of biomass towards the yield of pyrolysis gas. The computer simulations have shown that it is possible to obtain high-calorific pyrolysis gas.
The authors of the article, based on their own experience and current research in this field, proposed an effective direction for the energy management of pyrolysis gas, namely for firing industrial heating chambers, in particular heating furnaces in the steel industry. In the face of intensive optimization of the production processes and the reduction of incurred financial outlays, an opportunity is perceived to increase the competitiveness of steel plants thanks to the use of pyrolysis gas. At the same time, this method of the energetic use of pyrolysis gas may contribute to lower consumption of natural gas, which will allow for the diversification of fuel and energy sources, and will also bring tangible economic benefits. Thus, such an approach is in full compliance with the idea of a circular economy. Furthermore, the vision of a circular economy has recently gained more and more interest from both governments and business leaders who see it as an attractive alternative to the linear "take, make, throw away" model of the economy. The possibilities of independence of economic development from the supply of primary materials, stimulating innovation, accelerating growth and creating a more stable labor market are particularly important here. The world economy is ready for change and its trend is set, in particular by such countries as the Netherlands, Finland, and Scotland.

Conclusions
The conducted experiments and computer simulations have shown that:

•
The pyrolysis process is one of the most promising technologies for the treatment of various types of waste, including lignocellulosic.

•
The yield and quality of individual fractions depend on the properties of the lignocellulosic waste and the operating conditions of the process, in particular the temperature and residence time.

•
Taking into account the complexity of chemical phenomena occurring during a thermal conversion of waste, including lignocellulosic waste, conducting laboratory tests alone does not allow for a thorough analysis and optimization of the process. Other problems include high costs, technical difficulties related to the collection of samples for testing, limited possibilities of control and measurement equipment, and time-consuming experimental research. • Understanding and anticipating the above-mentioned processes are the key for further development of competitive technological solutions, aimed not only at minimizing negative environmental effects, but above all at reducing financial outlays.

•
The use of the computational tool, which is ANSYS CHEMKIN-PRO software, made it possible to determine the detailed chemical composition of the pyrolysis gas for diversified process conditions, i.e., temperature and residence time.

•
The results obtained during computer simulations made it possible to calculate the approximate calorific value of the pyrolysis gas, ranging from 20.61 to 25.19 MJ/m 3 . The high calorific value of gas encourages its wider use in both the heating and steel industries

•
The computer simulations carried out have shown, for both pine wood and alder wood, that with appropriate process conditions it is possible to obtain gaseous products that are valuable in terms of energy.

•
The main product in the pyrolysis process was the liquid fraction-bio-oil, and its share was 56.2% for the pine wood and 60.38% for the alder wood. In turn, the solid (char) and gaseous (pyrolysis gas) fractions were comparable and amounted to approx. 20%.

•
The conducted experimental studies showed a high yield of pyrolysis liquid, which can be a substitute for petroleum-derived fuels; however, the effective use of bio-oils in the field of liquid fuels requires the knowledge of highly differentiated properties of biomass.

•
The gas from the pyrolysis of the lignocellulosic waste can be co-combusted with the natural gas, which will contribute to its lower consumption. The aforementioned facts show that the pyrolysis gas can be an excellent alternative to conventional fuels, while at the same time contributing to the increasing level of environmental protection and can play an important role of biomass in the circular economy.