Experimental and numerical analysis of the biomass innovativ solar pyrolysis process

Paper present the experimental and numerical analysis of biomass photopyrolysis process. The experimental tests is performed on the solar pyrolysis installation, designed in Institute of Thermal Technology, Gliwice. It consist of the copper reactor powered by artificial light simulating sun. The paper shows the result of the solar pyrolysis of wood. The yield of the main fraction as a function of the process temperature is presented. Additionally the gas composition is determined. The numerical model is prepared in the Ansys Fluent 18.2 software, which allow at the same time for capturing geometry of the real system and easy change of input data. The results indicate that both the product yields (liquid, solid and gaseous) and gas components shares are strongly influenced by pyrolysis parameters and feedstock composition.


Introduction
The European Environment Agency (EEA) has estimated that around 235 Mtoe/year of biomass could be made available in the European Union (EU) by 2020 without harming the environment [1]. Agriculture (95 Mtoe), waste (100 Mtoe) and wood industry (30 Mtoe) are the key suppliers [2]. Due to this fact, in EU countries, biomass is currently one of the main renewable energy sources used to heat and electricity production and for transportation purposes [3]. Moreover, its use is rapidly increasing. Nevertheless, there are a number of restrictions related to the production of biomass, in particular legal solutions concerning the environmental protection and the principles of biodiversity crops. Additionally, the low energy density biomass is distributed in a wide range of remote areas. Therefore, for energy purposes locally available waste products from agriculture, agriculturalfood industry, spatial and other biodegradable waste, like the sludge should be used [4]. The annual biomass energy potential in Poland is equal to [5]: -More than 20 million Mg waste straw, -Approx. 4 million Mg of waste wood, -Approx. 6 million Mg of sewage sludge. Thermal methods of the waste biomass utilization are gaining importance for many years. The main reason for this fact are the EU requirements because the aim of the European Commission (EC) is to increase the share of the renewable energy sources in overall energy consumption to 32% by 2030 [6,7]. The thermal processes include combustion, cocombustion, gasification and pyrolysis [8]. The most common is direct combustion of biomass material. The biggest problems with biomass-fired plants are in handling and pre-processing the fuel. This is the case with both small grate-fired plants and large pulverized coal power plants. Drying the biomass before combustion improves the overall process efficiency, but may not be economically motivated in many cases. Exhaust systems are used to vent combustion byproducts to the environment. Emission controls might include a cyclone or multi-cyclone, a baghouse, or an electrostatic precipitator. The primary function of all of the equipment listed is particulate matter control, and is listed in order of increasing capital cost and effectiveness. Cyclones and multi-cyclones can be used as pre-collectors to remove larger particles upstream of a baghouse (fabric filter) or electrostatic precipitator. In addition, emission controls for unburned hydrocarbons, nitrogen oxides, and sulfur might be required, depending on fuel properties and the Law regulations. Despite the observed ecological, economic and social benefits, the use of biomass creates many technical problems: (1) a wide range of humidity causing difficulty in combustion stabilization, (2) biomass fuels have ash that is more alkaline in nature, which may exaggerate the fouling problems, (3) low density makes it difficult for transportation and storage, (4) high volatile content causing combustion process to be rapid and difficult to control, (5) the relatively low the lower heating value (6) heterogeneity and diversity of composition, including the content of chlorines lead to process during which hydrogen chloride, dioxins and furans are formed [9]. These problems cause that co-combustion of biomass with fossil fuels (mainly hard coal) gained importance. This process can be advantageous with regard to substitution of fossil fuels, reducing fuel cost and emissions of NOx and CO2 minimizing waste and reduce soil and water pollution and increasing boiler efficiency.

Introduction
The European Environment Agency (EEA) has estimated that around 235 Mtoe/year of biomass could be made available in the European Union (EU) by 2020 without harming the environment [1]. Agriculture (95 Mtoe), waste (100 Mtoe) and wood industry (30 Mtoe) are the key suppliers [2]. Due to this fact, in EU countries, biomass is currently one of the main renewable energy sources used to heat and electricity production and for transportation purposes [3]. Moreover, its use is rapidly increasing. Nevertheless, there are a number of restrictions related to the production of biomass, in particular legal solutions concerning the environmental protection and the principles of biodiversity crops. Additionally, the low energy density biomass is distributed in a wide range of remote areas. Therefore, for energy purposes locally available waste products from agriculture, agriculturalfood industry, spatial and other biodegradable waste, like the sludge should be used [4]. The annual biomass energy potential in Poland is equal to [5]: -More than 20 million Mg waste straw, -Approx. 4 million Mg of waste wood, -Approx. 6 million Mg of sewage sludge. Thermal methods of the waste biomass utilization are gaining importance for many years. The main reason for this fact are the EU requirements because the aim of the European Commission (EC) is to increase the share of the renewable energy sources in overall energy consumption to 32% by 2030 [6, 7]. The thermal processes include combustion, cocombustion, gasification and pyrolysis [8]. The most common is direct combustion of biomass material. The biggest problems with biomass-fired plants are in handling and pre-processing the fuel. This is the case with both small grate-fired plants and large pulverized coal power plants. Drying the biomass before combustion improves the overall process efficiency, but may not be economically motivated in many cases. Exhaust systems are used to vent combustion byproducts to the environment. Emission controls might include a cyclone or multi-cyclone, a baghouse, or an electrostatic precipitator. The primary function of all of the equipment listed is particulate matter control, and is listed in order of increasing capital cost and effectiveness. Cyclones and multi-cyclones can be used as pre-collectors to remove larger particles upstream of a baghouse (fabric filter) or electrostatic precipitator. In addition, emission controls for unburned hydrocarbons, nitrogen oxides, and sulfur might be required, depending on fuel properties and the Law regulations. Despite the observed ecological, economic and social benefits, the use of biomass creates many technical problems: (1) a wide range of humidity causing difficulty in combustion stabilization, (2) biomass fuels have ash that is more alkaline in nature, which may exaggerate the fouling problems, (3) low density makes it difficult for transportation and storage, (4) high volatile content causing combustion process to be rapid and difficult to control, (5) the relatively low the lower heating value (6) heterogeneity and diversity of composition, including the content of chlorines lead to process during which hydrogen chloride, dioxins and furans are formed [9]. These problems cause that co-combustion of biomass with fossil fuels (mainly hard coal) gained importance. This process can be advantageous with regard to substitution of fossil fuels, reducing fuel cost and emissions of NOx and CO2 minimizing waste and reduce soil and water pollution and increasing boiler efficiency.

Experimental and numerical analysis of the biomass innovative solar pyrolysis process
Sebastian Werle 1,* , Szymon Sobek 1 , Zuzanna Kaczor 1 , Łukasz Ziółkowski 1 , Zbigniew Buliński 1 and Mariusz Dudziak 2 Abstract. Paper present the experimental and numerical analysis of biomass photopyrolysis process. The experimental tests is performed on the solar pyrolysis installation, designed in Institute of Thermal Technology, Gliwice. It consist of the copper reactor powered by artificial light simulating sun. The paper shows the result of the solar pyrolysis of wood. The yield of the main fraction as a function of the process temperature is presented. Additionally the gas composition is determined. The numerical model is prepared in the Ansys Fluent 18.2 software, which allow at the same time for capturing geometry of the real system and easy change of input data. The results indicate that both the product yields (liquid, solid and gaseous) and gas components shares are strongly influenced by pyrolysis parameters and feedstock composition.

Introduction
The European Environment Agency (EEA) has estimated that around 235 Mtoe/year of biomass could be made available in the European Union (EU) by 2020 without harming the environment [1]. Agriculture (95 Mtoe), waste (100 Mtoe) and wood industry (30 Mtoe) are the key suppliers [2]. Due to this fact, in EU countries, biomass is currently one of the main renewable energy sources used to heat and electricity production and for transportation purposes [3]. Moreover, its use is rapidly increasing. Nevertheless, there are a number of restrictions related to the production of biomass, in particular legal solutions concerning the environmental protection and the principles of biodiversity crops. Additionally, the low energy density biomass is distributed in a wide range of remote areas. Therefore, for energy purposes locally available waste products from agriculture, agriculturalfood industry, spatial and other biodegradable waste, like the sludge should be used [4]. The annual biomass energy potential in Poland is equal to [5]: -More than 20 million Mg waste straw, -Approx. 4 million Mg of waste wood, -Approx. 6 million Mg of sewage sludge. Thermal methods of the waste biomass utilization are gaining importance for many years. The main reason for this fact are the EU requirements because the aim of the European Commission (EC) is to increase the share of the renewable energy sources in overall energy consumption to 32% by 2030 [6, 7]. The thermal processes include combustion, cocombustion, gasification and pyrolysis [8]. The most common is direct combustion of biomass material. The biggest problems with biomass-fired plants are in handling and pre-processing the fuel. This is the case with both small grate-fired plants and large pulverized coal power plants. Drying the biomass before combustion improves the overall process efficiency, but may not be economically motivated in many cases. Exhaust systems are used to vent combustion byproducts to the environment. Emission controls might include a cyclone or multi-cyclone, a baghouse, or an electrostatic precipitator. The primary function of all of the equipment listed is particulate matter control, and is listed in order of increasing capital cost and effectiveness. Cyclones and multi-cyclones can be used as pre-collectors to remove larger particles upstream of a baghouse (fabric filter) or electrostatic precipitator. In addition, emission controls for unburned hydrocarbons, nitrogen oxides, and sulfur might be required, depending on fuel properties and the Law regulations. Despite the observed ecological, economic and social benefits, the use of biomass creates many technical problems: (1) a wide range of humidity causing difficulty in combustion stabilization, (2) biomass fuels have ash that is more alkaline in nature, which may exaggerate the fouling problems, (3) low density makes it difficult for transportation and storage, (4) high volatile content causing combustion process to be rapid and difficult to control, (5) the relatively low the lower heating value (6) heterogeneity and diversity of composition, including the content of chlorines lead to process during which hydrogen chloride, dioxins and furans are formed [9]. These problems cause that co-combustion of biomass with fossil fuels (mainly hard coal) gained importance. This process can be advantageous with regard to substitution of fossil fuels, reducing fuel cost and emissions of NOx and CO2 minimizing waste and reduce soil and water pollution and increasing boiler efficiency.

Introduction
The European Environment Agency (EEA) has estimated that around 235 Mtoe/year of biomass could be made available in the European Union (EU) by 2020 without harming the environment [1]. Agriculture (95 Mtoe), waste (100 Mtoe) and wood industry (30 Mtoe) are the key suppliers [2]. Due to this fact, in EU countries, biomass is currently one of the main renewable energy sources used to heat and electricity production and for transportation purposes [3]. Moreover, its use is rapidly increasing. Nevertheless, there are a number of restrictions related to the production of biomass, in particular legal solutions concerning the environmental protection and the principles of biodiversity crops. Additionally, the low energy density biomass is distributed in a wide range of remote areas. Therefore, for energy purposes locally available waste products from agriculture, agriculturalfood industry, spatial and other biodegradable waste, like the sludge should be used [4]. The annual biomass energy potential in Poland is equal to [5]: -More than 20 million Mg waste straw, -Approx. 4 million Mg of waste wood, -Approx. 6 million Mg of sewage sludge. Thermal methods of the waste biomass utilization are gaining importance for many years. The main reason for this fact are the EU requirements because the aim of the European Commission (EC) is to increase the share of the renewable energy sources in overall energy consumption to 32% by 2030 [6, 7]. The thermal processes include combustion, cocombustion, gasification and pyrolysis [8]. The most common is direct combustion of biomass material. The biggest problems with biomass-fired plants are in handling and pre-processing the fuel. This is the case with both small grate-fired plants and large pulverized coal power plants. Drying the biomass before combustion improves the overall process efficiency, but may not be economically motivated in many cases. Exhaust systems are used to vent combustion byproducts to the environment. Emission controls might include a cyclone or multi-cyclone, a baghouse, or an electrostatic precipitator. The primary function of all of the equipment listed is particulate matter control, and is listed in order of increasing capital cost and effectiveness. Cyclones and multi-cyclones can be used as pre-collectors to remove larger particles upstream of a baghouse (fabric filter) or electrostatic precipitator. In addition, emission controls for unburned hydrocarbons, nitrogen oxides, and sulfur might be required, depending on fuel properties and the Law regulations. Despite the observed ecological, economic and social benefits, the use of biomass creates many technical problems: (1) a wide range of humidity causing difficulty in combustion stabilization, (2) biomass fuels have ash that is more alkaline in nature, which may exaggerate the fouling problems, (3) low density makes it difficult for transportation and storage, (4) high volatile content causing combustion process to be rapid and difficult to control, (5) the relatively low the lower heating value (6) heterogeneity and diversity of composition, including the content of chlorines lead to process during which hydrogen chloride, dioxins and furans are formed [9]. These problems cause that co-combustion of biomass with fossil fuels (mainly hard coal) gained importance. This process can be advantageous with regard to substitution of fossil fuels, reducing fuel cost and emissions of NOx and CO2 minimizing waste and reduce soil and water pollution and increasing boiler efficiency. However, attention must be taken to increase deposit formation in the boiler and limitations in ash use due to compositions in biomass, especially alkali metals, which may disable the use of ash in building materials. Due to undesired changes of ash compositions, the share of biomass is usually limited to approximately 20% of the fuel input [10]. Additionally, there are other barriers of co-combustion biomass with coal. These barriers include: (1) biomass procurement practices to obtain low-cost fuels in a long term reliable manner; the impact of co-combustion on ash composition and ability to sell fly ash; (2) the trade-off between the impact of biomass on emissions and fuel cost [11]. In case of the Polish market the new Act of the renewable energy sources should be also emphasized [12]. Based on this document, co-financial assistance of the co-combustion installation will be limited. All presented facts cause that new pioneer and innovative thermal solutions for biomass conversion are needed. An example of such technology is pyrolysis. Biomass pyrolysis is defined as a thermal degradation of the biopolymers present in the organic matter under an inert oxygen-free atmosphere [13]. Three products are always produced (solid, liquid and gaseous), but the proportions can be varied over a wide range by adjustment of the process parameters [14]. Pyrolysis is a process carried out at a lower temperature in comparison to combustion or co-combustion [15]. Thanks to it, the formation of toxic substances is limited and the complex gas cleaning system is not necessary. There are many other important advantages of pyrolysis in comparison to combustion, as follows: (1) Pyrolysis can be performed at relatively small scale and at remote locations which enhance energy density of the biomass resource and reduce transport and handling costs; (2) Due to lower temperature of the process corrosion problem is reduced the maintenance costs of the installation are lower; (3) Due to the lower temperature of the process, recovery of the selected elements (mainly non-ferrous metals) from solid products is possible; (4) Due to the endothermic nature of the pyrolysis, control of the process is easier in comparison to combustion; (5) Pyrolysis is characterized by high level of the fuel flexibility; (6) The pyrolysis products can be stored and later used for energy purposes [16]. Despite its advantages, pyrolysis has the disadvantage of requiring an external energy input to reach the operating temperature [17]. This external energy input is generally derived from a non-renewable source that has a negative impact on the environment. A possible solution to this problem is to use thermo-solar energy to heat the reactor to achieve conditions appropriate for solar pyrolysis initialization. In such process the concentrated solar radiation supplies high temperature heat for biomass pyrolysis reaction [18]. Then biomass and solar energy can be converted into transportable and dispatchable solar fuel [19]. Solar processes has the potential to produce higher calorific value products with lower CO2 emission compared with conventional process [20,21]. The biomass energy is upgraded through solar energy providing pyrolysis reaction enthalpy transferred into products. Solar pyrolysis is an endothermic process of converting a biomass in an inert atmosphere in which the required heat is provided by concentrated solar energy. The direct solar radiation is concentrated and redirected to the pyrolytic reactor and the biomass to reach pyrolytic temperatures. Paper present experimental and numerical analysis of wood photopyrolysis process. The yield of the main fraction as a function of the process temperature is presented. Additionally the gas composition is determined. The numerical model is prepared in the Ansys Fluent 18.2 software, which allow at the same time for capturing geometry of the real system and easy change of input data. The results indicate that both the product yields (liquid, solid and gaseous) and gas components shares are strongly influenced by pyrolysis parameters and feedstock composition.  Figure 1 presents the scheme and the photo of the main elements of the solar installation and Figure 2 shows the view of the xenon arc lamp.  The heart of the laboratory station was artificial sun, the high-power xenon arc lamp. The 1.6 kW Sciencetech© lamp produces stable heat flux in a form of concentrated radiation, with spectral characteristics close to the natural sunlight. Lamp radiation is directed onto reactor surface, containing biomass samples. Reactor is a copper block with four ducted channels, with 169 mm of total length. Volatiles released during pyrolysis are moved out from reaction zone with flow of inert gas (nitrogen with 1.5 l/min flow rate) and directed to liquid fraction (biooil) condenser. Bio-oil condenser consists of a set of water-cooled laboratory condensers with assigned Dreshl scrubbers in order to trap condensed bio-oil and pass non-condensable gases to ABB© gas analyser through Bronkhorst® El-Flow Prestige mass-flow meter.  Table 2 presents the experimental procedure. The total mass of wood feed was 25 grams distributed in four reactor channels in linear form wrapped in copper net. The pyrolysis process was realized in a wide range of temperatures, from approximately 500 to 800 °C. Six Ktype thermocouples with the outer diameter 0.5 mm were located in precisely drilled holes in 3 pellets from 2 channels. Control of the process temperature was executed by the variable output power (80-90%) of the lamp controlled by the power supply of the lamp. After being loaded with wood pellets, the reactor was exposed to xenon lamp radiation for 90 min duration time.

Experimental procedure
Measurements were carried out for 3 pre-selected lamp power values 80, 85.5 and 90%. During pyrolysis, the temperature and pyrolysis gas composition were recorded using WAGO© PFC 100 programmable logic controller (PLC). After each test products were separated. The volatiles continuously flew out from the reaction zone with the flow of nitrogen, while bio-oil condensed in the water-cooled condenser, passing dry pyrogas to gas analyser. Solid residue remained in the reactor was removed from the reactor and weighted. Liquid phase was collected from scrubbers and extracted with acetone, then weighted. Figure 3 presents final results of the study, listing impact of lamp power on following product yields with dry gas composition. Observed trends in product formation are typical for slow pyrolysis, the higher the final temperature and heating rate, the higher the bio-oil yields with lowering amount of dry gas and char. Char yield is strongly correlated to lignin content of the biomass samples (Table 1), bio-oil to cellulose content, and dry-gas to hemicellulose. Lower yield of bio-oil than hemicellulose content can be explained by cracking of tar components to lighter molecules increasing dry gas yield.

Experimental results
Dry gas quality is typical for slow pyrolysis. Low temperatures favor formation of CO2 and CO, with lower content of CH4 and H2. Finally, the Figure 4 presents indication of gas analyser related to sample temperature for experiment with 80.0% of lamp power. Peaks of CO and CO2 at around 320 °C correspond to decomposition of cellulose, while later CH4 and H2 release can be assigned to slow char formation reactions, with wide peaks at 380 °C and 560 °C respectively.

Numerical investigation
The main aim of the realization this part of the investigation was definition how to model solar pyrolysis process and what phenomenon must be included in such analysis. The most important element of the experimental system is the pyrolytic reactor (Fig. 5), in which four pellet packages are placed in a fixed position which is additionally secured with a copper gauze. The gauze holds the pellets and at the same time allows for free flow of gaseous pyrolysis products out of the reacting bed and for nitrogen flow to provide an inert atmosphere for the process. The reactor is made of copper. Its outer surface is insulated, except the spot in the middle of its upper surface, where the focused xenon lamp rays fall. This light incidence spot will ultimately be covered with a substance of a high, known absorption capacity. Nitrogen supplied to the rector will be split to feed all 4 reactor channels. At the outlet of the channels, nitrogen with a mixture of gaseous products (tar and gas) will get through again in a common collector and flow further to the cooling zone, to separate the tar off the gas, which then goes to the analyser. Such a concept causes that the composition analysis of the process products will average the results for 4 packages of pellets. Based on the the literature review [22 -26] it can be concluded that the model being prepared within this work can definitely be included into the group of large particles pyrolysis models, where heat and mass transport through the reacting bed requires special attention and obligatory including the changes of physical properties. Moreover, due to the fact that many pellets are simultaneously subjected to pyrolysis, being placed at different distance from the heat source, thus in different thermal conditions, it will be necessary to accurately reflect the geometry of all elements of the reactor, to appropriate capture thermal conditions. The model have to include chemical reactions, heat transfer, physical properties change and moisture evaporation, shrinkage.

Conclusions
The paper present both theoretical (introduction to CFD modelling) and practise (experiment) aspects of the solar pyrolysis process. Solar pyrolysis of waste wood can be promising way to utilize waste biomass from wood industry, allowing to produce high-quality, solar enriched, bio-oil and biochar.
Changing the heat transfer conditions is a key element in the mathematical description of the process, especially if the scale of the system is as large as in the presented case. Evaporation of moisture and deposit shrinkage are essential elements of the pyrolysis process and certainly at further stages of the model development would be a valuable complement to it. Within the experimental part of the paper, solar pyrolysis of waste wood study results has been presented. Experiments have been conducted on solar pyrolysis reactor own-designed by authors. Impact of investigated biomass chemical composition on actual products distribution and dry gas composition have been presented and discussed. It was confirmed that during wood solar pyrolysis chemical components cellulose, hemicellulose and lignin favours decomposition course leading towards dry gas, bio-oil and bio-char formation respectively. Product shares and dry pyrolysis gas quality followed results published in the literature with well-known trends of increasing bio-oil yield with increase of final temperature and average heating rate. Xenon-arc lamp provided stable radiant flux and plenty of power. Precise temperature monitor within pellet bed gave insight into samples behaviour during pyrolysis process.