Solid recovered fuel gasification in sliding bed reactor

Energy 278 (2023) 127830 Available online 15 May 2023 0360-5442/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Solid recovered fuel gasification in sliding bed reactor J. ˇ Cespiva a , * , J. Skˇ rínský a , J. Vereˇ s a , M. Wnukowski b , J. Serenˇ cíˇ sov´ a a , T. Ochodek a a Energy Research Centre, Centre for Energy and Environmental Technologies, VSB – Technical University of Ostrava, Ostrava, 708 00, Czech Republic b Department of Energy Conversion Engineering, Faculty of Mechanical and Power Engineering, Wrocław University of Science and Technology, Wrocław, 50-370, Poland ARTICLE INFO Handling Editor: Petar Sabev Varbanov Keywords: Waste management Gasification Solid recovered fuel Circular economy ABSTRACT This study examines a solid recovered fuel gasification process in the context of a regional, clean energy supply from an affordable source. The examination of this approach was performed under various equivalence ratios and load regimes. A unique cross/updraft gasification reactor with a fixed (sliding) bed over the circular grate with tangential gasification media intake was utilised. This technology is a perspective in waste-to-energy and waste-to-materials production. The investigated parameters included producer gas quality and purity, overall conversion efficiency and char material yield. It was found that a low material load is very beneficial in terms of gas purity and conversion efficiency, reaching up to 93%. Also, the formation of tar compounds was measured as low as 0.7 g/m 3 . However, when the equivalence ratio parameter was 0.14, the gas’s lower heating value was only 2.4 MJ/m 3 . Also, a lower heating value equal to 5.0 MJ/m 3 was reached in a low-efficiency regime (48%) when the fuel load was more significant (48.5 kg/h), and the equivalence ratio was only 0.04. The low tar content suggests a very clean process. Also, the material valorisation in the form of char is beneficial as this carbon-rich material no longer has a waste character and can be utilised in many fields. 1. Introduction The generation of waste materials is omnipresent worldwide. Every political scene deals with this problem differently, and sometimes the solutions tend to ignore the energy content of waste materials. Such content is usually interesting and can be utilised in order to decrease the extraction and utilisation of fossil resources. By 2050, 3.4 billion tonnes of waste material will be generated [1]. Nowadays, our population produces over 2.01 billion tonnes of waste, out of which less than two-thirds are managed in accordance with environmental protection schemes, while the rest harms it in some way. Of these environmentally hazardous materials, around 17% are formed from paper, 12% from plastics and 2% from wood. The rate of globally recycled or energetically utilised waste materials is only around 30.4% out of the total materials suitable for such treatments, causing an evitable gap in the circular economy of waste management [2]. The rapidly increasing energy demand and its subsequent uncertainty also put pressure on society. The utilisation of compounds with the heating value contained in waste materials could help with solving both of the issues: critical waste cumulation and sustainable energy demand. Several waste utilisation processes in terms of energy conversion principles exist. A major role is played by combustion and cocombustion in order to harvest thermal energy. This waste-to-energy principle is advanced, safe, efficient and feasible [3], yet not widely spread due to the challenge of fossil fuel incineration plants [4]. Besides combustion, other principles, such as fixed bed [5] or fluidised bed gasification [6], slow [7], flash [8] or fluidised pyrolysis [9], torrefaction [10] or hydrothermal carbonisation [11], could be applied for the utilisation of waste materials. Successful pyrolysis of municipal solid waste (MSW) derived fuel, containing a reasonable amount of plastic and paper material, was demonstrated in various scales of fixed bed reactors [12] and fluidised bed reactors [9] applied for sorted waste collection or mining the landfills [13]. Products of this process include pyrolysis oil, pyrolysis gas and solid char [14]. On the other hand, the principle of gasification has no liquid products as it is focused chiefly on synthetic gas production. The scheme of decomposition of any calorific material in gasification units is described by a general mathematical equation determining the chemical composition of organic content of the fuel into gasification products [15]: C x H y O z → aCO 2 +bH 2 O +cCH 4 +dCO +eH 2 +fC 2 +tar +char (1) Gasification of waste was previously studied in an attempt to find an alternative waste energy storage approach. The advantages of synthetic gas production include relatively clean combustion in local equipment, easy transport and good thermodynamic properties. It may also be used * Corresponding author. E-mail address: [email protected] (J. ˇ Cespiva). Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy https://doi.org/10.1016/j.energy.2023.127830 Received 28 October 2022; Received in revised form 11 April 2023; Accepted 13 May 2023 Energy 278 (2023) 127830 2 in applications for liquid fuel production, so-called waste-to-liquid (WtL) [16]. Another benefit of waste material gasification is its valorisation in the form of solid carbon-rich char [17], which is generated as a by-product of synthetic gas production. For its high specific surface area, this material has broad possibilities of usability, for instance, as soil amendment [18] or as a sorbent for the purification of air [19] or water [20]. The acquisition of gasification is multidisciplinary, however, most of the attempts of such applications failed due to technical obstructions emerging from wrong technological complexes implication or difficult economic feasibility, not only in the case of waste material utilisation. Therefore, such approaches require deeper studies before their full utilisation in the commercial scale. The waste material containing unrecyclable (in the objective of technical or economic feasibility) particles of non-biological substances, mainly from commercial production, such as plastics, paper, cardboard, textiles or wood, is called Solid Recover Fuel (SRF). The character of this product fuel is very variable in its content and physical-chemical properties. SRF is mainly produced from non-hazardous industrial wastes of very stable composition, but several fractions of sorted MSW can also be included in the production [21]. This fuel can be processed in different particle sizes, moisture and shape (fluff, pellets, briquettes). The heating value and the content of volatile compounds of SRF material range drastically and must be defined individually. Usually, this material is certified and used as a fuel, for instance, in centralised power plants or cement production facilities. The utilisation of such material follows the strict conditions of quality check and applied emission limits for individual pollutants (in case there are any) [22]. Despite its good properties, SRF is not commercially used through gasification processes, but combusted instead in order to generate electricity and heat. Gas production application remain laboratory or pilot-scaled. This fact is not due to the lack of the source material, however. The cumulation of SRF is ever-growing. In this paper, research on SRF gasification and on broadening of its utilisation possibilities is deepened from the technical variation point of view, where the behaviour of the thermochemical reactions and their ultimate effect on mass balance and producer gas quality is looked at through different environment scenarios. Gas and char quality/quantity and polluting agent’s characteristics were monitored and analysed, as well as efficiency parameters of the process were determined in order to support evaluation of this unique and, so far, not spread approach and to promote its future development towards the less carbon-dependent society. In this study, a gasification reactor with a cross/updraft working principle with a sliding bed over a circular grate was used. 2. Materials and methods Many variable approaches exist in terms of gasification (numerous reactor designs and working principles) of solid waste-based fuel (numerous material sources and declared quality). In order to determine the most suitable conditions, SRF from local sources was applied in this study. The choice of converting technology was based on previous studies and the hypothesis of sufficient properties, low demand on fuel adjustment, low sensitivity to load fluctuations and good feedstock variation acceptance of the gasification reactor. 2.1. Feedstock In this study, a specific SRF material was utilised. This mixture of industrial (unrecyclable plastics, wood, paper and textile) and MSW (unrecyclable plastics and paper) wastes was produced in the form of fluff with a particle size of 1–25 mm. This fuel material was generated from waste materials in the region of Ostrava, Czech Republic. The lower heating value (LHV) of this certified SRF is19.1 MJ/kg. Its declared sulphur and chlorine contents are below 0.9 and 0.5% wt., respectively. The specific content of individual components is roughly defined as 40% wt. Wood, 30% wt. Plastics, 15% wt. Paper. 10% wt. Textiles and 5% wt. Other components, such as non-ferrous metals or ceramics. The producer of this material is OZO Ostrava a. s. Waste collection and management company. For the purpose of this study and for safety reasons concerning transportation and proper utilisation, the SRF material was mixed with plain softwood pellets (Mayer-Melnhof Holz, Germany) at the mass rate of 3:2 to form a final fuel mixture. The wood pellets of similar chemical composition (except for lower ash and higher oxygen content) also have a pretty comparable value of LHV equal to 17.0 MJ/kg. The average size of the individual pellets was 15 ×0.5 mm. The LHV and elemental composition of the final fuel in its raw state are evident in Table 1 ([m 3 ] and all other values to be defined for T =293.15 K and p =101 325 Pa in the whole article). 2.2. Technology The gasification process was carried out using a sliding bed cross/ updraft reactor with 200 kW power input. This technology works in an autothermal gasification/combustion regime with air as gasification media. The fuel is decomposed over a circular grate with tangential air intake, causing a vortex within the gasification zone. The producer gas is withdrawn in the upper part of the reactor. After thermochemical conversion, the residual material, char, in this case, falls down the grate spontaneously or as a consequence of a manual oscillation and is then taken away into a collection vessel. The process temperature is strictly dependent on the condition and amount of the fuel and oxidising media. The reactor vacuum and producer gas extraction are realised by a Abbreviations λ Equivalence ratio CGE Cold gas efficiency HGE Hot gas efficiency PM Particulate matter LHV Lower heating value MSW Municipal solid waste SRF Solid recovered fuel TC Tar compounds WtL Waste-to-liquid Table 1 Fuel mixture parameters. Parameter Symbol Unit Value Standard Deviation [%] Particle size D p [mm] 0.5–25 EN ISO 17827-2 – Bulk density ρ b [kg/ m 3 ] 215 ISO 567:2021 – Lower heating value LHV r [MJ/ kg] 18.2 EN 18125 – Volatile matter V r [% wt.] 77.3 EN ISO 22167 1.47 Water W r [% wt.] 7.1 EN ISO 181234-2 0.24 Ash A r [% wt.] 6.2 EN ISO 18122 0.56 Carbon C r [% wt.] 48.2 EN ISO 16948 2.24 Oxygen O r [% wt.] 33.6 EN ISO 16993 – Hydrogen H r [% wt.] 6.2 EN ISO 16948 0.19 Nitrogen N r [% wt.] 1.0 EN ISO 16948 0.10 Total sulphur S r [% wt.] 0.1 EN ISO 16994 0.01 J. ˇ Cespiva et al. Energy 278 (2023) 127830 3 suction fan situated at the tail of the downstream technology. Between the reactor and the suction fan, a pair of condensers and a cyclone barrier are situated in order to purify the producer gas from tar compounds (TC), particulate matter (PM) and other polluting agents [23]. A cross-section of the reactor is seen in Fig. 1. As a consequence of the heating value of the fuel, the process temperature was held between 935.8 and 1005.4 K average values, while the relative pressure ranged from −0.1 to −0.3 kPa to the atmospheric. The temperature indicator is situated in close vicinity to the hearth of the reactor while the pressure indicator is above in the freeboard area. The values of the fuel flow, airflow and equivalence ratio for each experiment are summarised in Table 2. 2.3. Sampling track The analyses of the raw producer gas obtained in this study were realised through a sampling probe situated immediately at the reactor outlet. Following the methodology described in Ref. [23], the attached sampling track consisted of five impinger bottles with 100 ml of propane-2-ol immersed in a cold-water environment (4 ◦C). The polluting tar compounds remained trapped within propane-2-ol and were subjected to subsequent analyses. The first round consisted of weight difference analysis immediately after the sampling. This analysis provides a comparative overview of the individual processes and a quick evaluation of the producer gas quality, however, the evaporation of lighter tar compounds and propane-2-ol may have caused an error. The condensing water may have affected the weight analysis, and thus, this must be considered only a comparable tool and a quick preview of the gas character. After the sampling track, the gas was led into the gas composition analyser GAS 3000p (Pollutek Gas Analysis, Belgium), capable of CO, H 2 , CH 4 CO 2 , O 2 and C n H m detection using dual beam infrared, thermal conductivity and electrochemical detectors with 0.01% precision. 2.4. Efficiencies determination Determination of the efficiency is a tool for sufficient comparison of the gasification processes across the designs of the reactors, fuel characteristics and operating conditions. The value of the hot gas efficiency (HGE) signifies the efficiency of the fuel-to-gas energy conversion, including the sensible heat of the producer gas, while the cold gas efficiency (CGE) excludes this value. These efficiencies are being determined as [24]: HGE = ˙ Vg• ρ g•(ΔT•cp+∑LHVCO,H2,CH4) ˙mf•LHVf [%](2) CGE = ˙ Vg• ρ g•LHVg ˙mf•LHVf [%](3) Where LHV signifies the lower heating value in [J/kg] ([J/m 3 ]), ΔT signifies a difference between 20 ◦C normal conditions and the actual temperature of the producer gas in [K], and c p is the specific heat capacity at a constant pressure of the gas in [J/kg•K]. V g is a volumetric flow of the producer gas in [m 3 /h], while ρ g is its density at relevant conditions [kg/m 3 ]. ˙ m f is a fuel mass flow in [kg/h]. 2.5. Impurities analyses An essential consideration in terms of producer gas quality is its purity, affected by PM, sulphur and nitrogen oxides, chloride and fluoride compounds, and, especially, TC (hydrocarbon substances with molecular weight higher than benzene). The content of PM was investigated through the same sampling track as the producer gas. There, PM remained trapped on a glass filter paper placed in a filter holder, to be analysed subsequently. Quantitative analysis was performed as weight difference on the filter. PM was also examined via ultimate and proximate analyses to detect the content of entrained energy. This was done using CHNS628 (Leco, USA) thermogravimetric analyser (determination of C r , H r , N r ), CHNS628S (Leco, USA) analyser (determination of S r ), VF110 (Memmert, Germany) electric furnace (determination of W r ) and LAC LEO 5/11 (LAC, Czech Republic) ash determination unit (determination of A r ). Similarly to PM, also TC, considered one of the chief drawbacks and obstructions in gasification processes in general, were analysed with the use of gas chromatography techniques (GC). 7820-A chromatograph and A5977B MSD electron ionisation mass spectrometer (both Agilent, USA) were used in the initial step of the tar components analysis. Samples were injected with the use of an autosampler in 2 μ l amount, and an HP5MS column (30 m ×0.32 mm x 0.25 mm; Agilent J&W) was used. The temperature profile was set to 50 ◦C (5 min) and then gradually increased by 10 ◦C per minute up to 200 ◦C (20 min). The MS scanning range was m/z 10–450 with 1.7 scans per second frequency. The mass spectra results were subsequently compared to the NIST-14 MS library (with a minimum match factor of 90%) for the purpose of qualitative analysis. However, the majority of the detected compounds was confirmed by standards that were used for the quantitative analysis. Due to the lack of standards, a quantitative analysis of two compounds was done with the use of GC-FID (HP6890). As the response of the FID detector is proportional to the number of carbon atoms in a molecule, the amount of these two compounds was determined by comparison with quantitative results for toluene. In the case of similar aromatic compounds, this approach provides a relatively low error [25]. The applied GC-FID method was very similar to the one applied of GC/MS: 2 μ l samples, HP-5 column, identical gas velocity. The main difference was the extension of the isothermal profile (up to 8 min) so that all the quantified compounds emerged in the column at the same temperature. Other polluting agents, such as nitrogen and sulphur oxides or Fig. 1. Schematic cross-section of the autothermal, cross/updraft gasification reactor with circular grate [23]. TIC – temperature indicator controller, PIC – pressure indicator controller, FIC – flow indicator controller. Table 2 Parameters of the gasification process. Parameter Symbol Unit Experiment 1 2 3 4 Temperature T [K] 938.8 941.9 1005.4 935.8 Relative pressure p [kPa] −0.1 −0.2 −0.3 −0.1 Fuel flow ˙ m f [kg/h] 48.5 48.6 50.3 14.2 Airflow Q a [m 3 /h] 9.0 14.8 18.3 9.8 Equivalence ratio λ [−] 0.04 0.06 0.08 0.14 J. ˇ Cespiva et al. Energy 278 (2023) 127830 4 volatile organic compounds (VOC), were analysed on-site at the outlet from the sampling track. The measuring devices consisted of a multisensor portable device MultiRAE Lite (MultiRAE Systems, USA). The calibrated and replaceable detectors included SO 2 and NO 2 sensors with a 0–20 ppm range and 0.1 ppm resolution, NH 3 and H 2 S sensors with a 0–100 ppm range and 1 and 0.1 ppm resolution, respectively. Other broad-range sensors included NO and VOC sensors with 0–250 and 0–1000 ppm range and 0.5 and 1 ppm resolution, respectively. The measured values were memorised after stabilisation of the sensors (around 1 min). 3. Results and discussion The gasification process was performed in four individual rounds with various material and oxidiser flows and/or different process temperatures. The experiments were carried out with a very low equivalence ratio λ in the range of 0.04–0.14. This range is relatively lower compared to the literature concerning updraft experiments [26], or updraft mathematical [27] models where the λ parameter is usually between 0.2 and 0.35 [28]. However, the structure of the grate area and different gasification principles allow good working conditions with such low gasification media income. Tangential oxidiser income allows good residency for a sufficient conversion. The results had shown a nonlinear dependency of the gas efficiencies on other gasification parameters. 3.1. LHV and efficiency The depression of the efficiencies (HGE and CGE) seems to be around λ =0.08, however, the fuel flow parameter has a reasonable effect on the efficiency course. In experimental round no. 4, when only 14.2 kg/h were used and λ =0.14 (the highest), the CGE was equal to 68%, and HGE was as high as 93%, as shown in Table 2. The obtained values are difficult to compare with other studies because they are strongly dependent on the fuel characteristics, reactor design and its parameters and principles. Chiemchaisri et al. [29] obtained in their work CGE =66% in a small-scale downdraft gasification application, when landfill-mined SRF was used as fuel. However, the LHV was relatively low – showing only 1.6 MJ/m 3 . Also the study of Sobolewski et al. [30] described a downdraft reactor. This application reached 62% of the CGE for SRF/wood biomass mixture (3:2 – similar to this study). The LHV was better in this case: 3.05 MJ/m 3 . Both these studies presented a production of the gas with LHV value much lower, compared to the hereby presented study (up to 5.0 MJ/m 3 ). An experimental downdraft reactor facility capable of plastic SRF gasification (SRF/wood =1:4) was presented by Vonk et al. [31]. In two similar experiments with λ =0.23 and 0.28, CGE =35 and 39% were obtained. Technical issues, such as bridging or clogging, were observed in both cases. However, the LHV was quite promising: 5.1 and 4.1 MJ/m 3 , respectively, being a function of relatively good CO (12.9 and 13.7% vol.), H 2 (14.9 and 10.2% vol.) and CH 4 (3.1 and 2.3% vol.) content. These results are quite similar to those from this study, however, the input SRF/wood rate was different in this case. The compared studies utilised slightly different input materials in terms of quality and composition. The mentioned technical issues have not occurred in the case of this study. The values of the producer gas composition and its LHV obtained in this study are summarised in Table 3. Other studies on the utilisation of SRF in fixed bed gasifiers are difficult to find as most applications work with fluidised bed technologies. Arena and Gregorio [32] investigated SRF gasification in a circulating fluidised bed gasifier of 400 kW and up to 70 kg/h fuel consumption. The experiments were performed in temperatures from 849 to 932 ◦C and λ from 0.256 to 0.332. The best-obtained results were CGE =61% and LHV =5.6 MJ/m 3 . Both values are quite promising and higher than the results obtained in this study. However, the technology suffered significant formation of TC (up to 73 g/m 3 ), which is typical for fluidised applications and much more than the value obtained in this study (see Table 4 below). The LHV of the fuel was 18.6–21.3 MJ/m 3 . Dunnu et al. [33] measured in their experiment LHV of SRF producer gas to be equal to 4.3 MJ/m 3 in T =973 ◦C and λ =0.2. Nevertheless, fluidised bed applications are not optimal for comparison with the fixed bed ones because of quite different process principles and outputs. Han et al. [24] studied the SRF gasification in a lab-scale bubbling fluidised bed rector varying the λ parameter at constant gas velocity. They obtained the highest value of CGE at 800 ◦C, equal to roughly 50%. At the same time, the LHV was at its maximum of 5.23 MJ/m 3 . The equivalence ratio was relatively higher than in the hereby presented study (0.25), however, tar content was also higher, approx. Equal to 5 g/m 3 . In the graph in Fig. 2 below, the dependency of both efficiencies on the equivalence ratio is evident. However, as was mentioned above, the efficiency parameters are not only influenced by the fuel/air rate but also by the total mass flow within the reactor. The highest efficiencies were reached in the case of the experimental round no. 4, where only 14.2 kg/h were fed into the reactor, whereas in all other cases, the fuel consumption was significantly bigger. It is reasonable to consider this factor more important than, for instance, gasification temperature. As seen in the graph in Fig. 3, the gas composition does not differ as much in cases of λ =0.06 (T =941.9 K) and λ =0.08 (T =1005.4 K) as it does in the case of low fuel flow (λ =0.14 and T =935.8 K). This finding indicates the operating conditions and adjustment possibilities towards better energy conversion and functionality of the technology. The higher conversion efficiency is also important from economic and environmental points of view. The lower LHV, in this case, is relative and is affected by significantly lower media load in the reactor, thus, lower material consumption. The efficiency parameters are connected to the amount of the energy converted. Nevertheless, it is necessary to point out the different LHVs of the producer gas. This value is not directly proportional to the conversion efficiency, and a choice must be made by the technology operator between higher conversion efficiency into less valuable gas and higher overall efficiency losses with a production of energetically richer gas (higher share of the combustibles). The best thermochemical parameters of the producer gas were obtained in the experimental round no. 1. Then, LHV =5.0 MJ/m 3 was reached with 41 m 3 /h of producer gas production, generated from 48.5 kg/h of fuel (fuel flow average value). Table 3 Producer gas characteristics. Parameter Symbol Unit Experiment 1 2 3 4 Producer gas flow Q g [m 3 /h] 41 50 48 41 Carbon monoxide CO [% vol.] 15.7 9.3 9.3 5.8 Hydrogen H 2 [% vol.] 0 2.9 3.3 1.8 Methane CH 4 [% vol.] 8.8 3.6 3.5 4.3 Carbon dioxide CO 2 [% vol.] 14.0 13.3 12.6 14.4 Oxygen O 2 [% vol.] 3.5 0.5 0.4 0.9 Other X [% vol.] 58.0 70.4 70.9 72.8 Lower heating value LHV [MJ/m 3 ] 5.0 2.7 2.7 2.4 Cold gas efficiency CGE [%] 42 27 25 68 Hot gas efficiency HGE [%] 48 37 35 93 Table 4 Producer gas impurities. Parameter Symbol Unit Experiment 1 2 3 4 Sampling flow rate Q s [l/min] 1.1 1.1 1.1 1.0 Particulate matter PM [g/m 3 ] 1.7 1.9 5.0 0.5 Tar compounds TC [g/m 3 ] 2.3 2.2 1.9 0.7 J. ˇ Cespiva et al. Energy 278 (2023) 127830 5 The producer gas contained CO =15% vol. In this case. The lack of H 2 was compensated by the rich content of CH 4 =8.8% vol. CO 2 rate as a consequence of necessary combustion reactions was equal to 14% vol. Other components (58.0% vol.) were majorly represented by the atmospheric N 2 . The value of LHV was promising, compared to other literature [33], where such value was reached and even exceeded, however, usually with lower efficiency [31] or increased formation of the polluting agents [32]. Also, a low load regime with a significantly low equivalence ratio parameter can be seen as beneficial from the initial energy consumption point of view. 3.2. Gasification char Another advantage and possibility of gasification technology utilisation on SRF management is the production of char. During the experimental rounds, the yield of this material was between 10 and 20% (mass percentage of the initial fuel, which remained in the charred form). For its high C content, this material has the potential to be used in many industries, including air/flue gas purification, construction or agriculture. The SRF gasification char obtained in this study contained 75.5 ±6.2% wt. Of C d (in the dry state) and 10.3 ±4.8% wt. Of A d in all performed experiments. Especially the carbon content can be considered high, and the quality of such material to be satisfying in terms of further utilisation in applications where porous, carbonaceous materials are used, or energy accumulation in a solid material is required. In the previous study of ˇ Cespiva et al. [19], this material was successfully analysed and utilised as a mercury sorbent from the coal combustion process in fluidised bed boiler flue gas. The obtained data had shown the efficiency of mercury capture reaching up to 62.3% (after steam activation performed), which a value reasonably comparable to the commercially produced coal-based activated carbon materials applied in the industrial conditions. However, in this study, pure SRF mixture was used as a feedstock for the gasification process [19]. Dependent on the grain size its S BET parameter (specific surface determined by Brunauer-Emmett-Teller calculus) was ranging from 316.5 to 328.3 m 2 g −1 for the non-activated samples and from 480.0 to 550.7 m 2 g −1 for the steam-activated ones. The capture of other substances (e.g. heavy metals, polyaromatic hydrocarbons) on the gasification char in polluted environment will be subjected to the following research. Also, the application in so-called adsorption chillers [34] appears to be promising and will be studied in the future. The LCA analysis on the production of this progressive material was also performed [35]. With a nearly absolute lack of water and nitrogen, this char makes a combustible material of good quality with LHV =27 ±2 MJ/kg and relatively low bulk density equal to 250 ±30 kg/m 3 for a good burnout. It also represents a reasonably good material for energy accumulation. Material valorisation through gasification is essential because, in most cases, thermally treated waste is no longer considered a waste material but a char product, which provides new ways of utilisation and may crucially reduce legislation restrictions. 3.3. Polluting agents The matter of polluting agents within the producer gas was monitored on PM and TC basis. The first mentioned was captured on the glassfibre filter and examined, as described above. The flow rate of the sampled producer gas was performed at the rate of 1.0–1.1 l/min. The whole sampling procedure took 30 min in the case of each experiment. As seen in Table 4 below, the producer gas PM content ranged from 0.5 g/m 3 in the case of the low fuel load experiment to 5.0 g/m 3 in the case of the experiment, when CH 4 content and conversion efficiency were the lowest and LHV =2.7 MJ/m 3 (experimental round no. 3). Also, in this case, the gasification temperature was the highest, causing more particles to be combusted and entrained subsequently. The values of PM content within the producer gas are quite in accordance with other studies focused on the same problematics in the updraft gasification reactors [31]. At the same time, the fluidised bed applications measure four to ten times greater values [32]. The ultimate and proximate analyses were performed in order to determine the composition of PM. The results had shown 47.5 ±7.2% wt. Of C d content and O d equal to 9.6 ± 2.1% wt. In all experiments. The content of A d was 38.5 ±7.0% wt. Other elements were in the minority. High C d content suggests the potential of secondary utilisation to support waste-free production and a circular economy. Experimental round no. 1 shows the highest value of the TC equal to 2.3 g/m 3 , almost similar to no. 2 (2.2 g/m 3 ) (Table 4 and Fig. 4 below). In the case of experimental round no. 3, which has nearly all parameters’ values, except for the reactor temperature, higher, the content of TC is slightly lower due to thermal cracking at higher temperatures and greater oxidiser intake than in the previous round. The last experiment Fig. 2. Block diagram of the material flow. Fig. 3. Cold gas and hot gas efficiencies vs equivalence ratio. Fig. 4. Producer gas composition vs equivalence ratio. J. ˇ Cespiva et al. Energy 278 (2023) 127830 6 with a significantly lower material load provided considerably pure producer gas, with the TC content as low as 0.7 g/m 3 . Such value would be hardly achievable in typical updraft reactors, however, the crossdraft applications are capable of such achievement in terms of impurities. Also, the state and type of the input fuel have an indisputable effect on the TC formation rate and its character. In this case, the presence of SRF in the SRF/wood mixture has affected the relatively low TC formation within the producer gas, comparable with pure wood gasification in similar conditions and on the same technology complex, presented in Refs. [23,36]. The presented values are in an agreement with previous studies or reasonably better, as demonstrated in this discussion above. Tar compounds individual quantitative analysis provides a solid basement for the tar character evaluation. In the graph in Fig. 5 below, the most dominant compounds, as detected in the GC-MS unit, are apparent. Of those, benzene has the most significant occurrence, however, benzene is often not considered a TC [37]. Toluene, styrene and phenol were also detected with relatively high concentrations. The value of C 8 H 10 signifies either ethylbenzene, m-Xylene or o-Xylene. Due to the nearly identical mass spectrum, it was impossible to distinguish these substances from one another. The presence of specific compounds within all samples is not significantly different from other results achieved in fixed bed technologies (demo-scale [28], open top [38] or similar construction utilising biomass, presented in Refs. [23,36]) (see Fig. 6). Other polluting agents were measured during the sampling of the producer gas. Acidic compounds such as sulphur dioxide and H 2 S were measured in high concentrations (above 20 and 100 ppm, respectively), both above the detection limit of the measuring device. This fact signifies a necessity to purify the producer gas intensely from these compounds in order to protect the downstream technology and pipelines, meet the emission limits of the end-use gas technology or avoid poisoning of the catalyst in the WtL technologies. Nitrous oxides, on the other hand, show relatively acceptable values comparable with other conversion technologies. The value of VOC was measured in high concentration (450 ppm), however, this was probably caused by the upstream cooling track and partial evaporation of propane-2-ol from the impinger bottles. The measured values in Table 5 represent average values from all measurements with a deviation lesser or equal to 10%. 3.4. Economic feasibility The gasification of waste-based materials appears to be beneficial also from the economic point of view. The gas prices resulting from the actual situation in the energy market put alternative approaches to gas production in a more competitive position. In the techno-economic analysis of N´ asner et al. [39] a model was developed to estimate the economic assessment of downdraft gasifier. It was shown that this approach is viable from 120 kW el and that performed CGE is equal to 60% in maximum, which is yet a lower value compared to the hereby presented study. Also, the λ parameter was 2–6 times higher in the study of N´ asner et al. [39], suggesting greater energy demand for sustaining the higher flow of the oxidising media and the producer gas. Moreover, the carbonaceous by-product with its potential on the market is not included in the calculation. The economy of the process can be interesting in terms of targeted hydrogen production. Wang et al. [40] presented in their study a promising result of 3.85 € per kilogram of H 2 through an equilibrium model applied to gasification facility construction and 17 years of operation (2500 dry metric tonnes per day). Also this study underlines that the approach of SRF gasification has a reasonable potential in terms of economics and could be beneficial for the operator if handled properly. However, the overall balance would differ from case to case. 4. Conclusions Gasification of solid recovered fuel and softwood mixture was investigated in this study. A unique cross/updraft reactor of pilot-scale with a sliding bed over the circular grate and tangential gasification media intake was used to perform experiments under different conditions. Those included variable equivalence ratio (0.04–0.14), temperature (935.8–1005.4 K) and fuel flow (14.2–50.3 kg/h). The examined parameters included monitoring of the producer gas and its energy value, energy conversion efficiency (for hot and cold gas) and the content of polluting agents. The results had shown an interesting correlation between the producer gas LHV and the conversion efficiency. The best conversion rate was obtained when HGE =93% with relatively low LHV, equal to 2.7 MJ/m 3 . This was influenced by the low content of CO, equal to 5.8% vol. The LHV of 5.0 MJ/m 3 , resulting from CO =15.7% vol. And CH 4 =8.8% vol., seems quite promising. However, this was achieved with only 48% of HGE and 42% of CGE. In these two cases, the value of the equivalence ratio was 0.04 and 0.14, respectively. These results seem to outline two distinct approaches of combined gasification of softwood and SRF: Producing a good quality synthetic gas with worse overall efficiency versus the efficient production of less vital synthetic gas. However, both approaches are in favour of waste material valorisation, representing a Fig. 5. Formation of tar compounds vs equivalence ratio. Fig. 6. An example of the most dominant tar compounds in the sample (experimental round no. 3). Table 5 Polluting compounds within the producer gas (average values from all experiments). Parameter Symbol Unit Value Range Resolution Nitrogen oxide NO [ppm] 95 ±4.8 0–250 0.5 Nitrogen dioxide NO 2 [ppm] 4 ±0.2 0–20 0.1 Ammonia NH 3 [ppm] 46 ±2.3 0–100 1.0 Sulphur dioxide SO 2 [ppm] >20 0–20 0.1 Hydrogen sulphide H 2 S [ppm] >100 0–100 0.1 Volatile organic comp. VOC [ppm] 450 ± 22.5 0–1000 1.0 J. ˇ Cespiva et al. Energy 278 (2023) 127830 7 cheap energy source with local character. During these experiments, the production of high C content (above 70% in all cases) char was observed with the yield of this material ranging from 10 to 20%, providing a promising form of flexible energy accumulation within local energy systems. The hydrogen yield should also be considered in several applications these days. Its highest content (3.3% vol.) was measured when λ =0.08. The problematic of tar compounds is especially interesting. It was confirmed that the presence of treated SRF, especially its plastic compounds, can limit the tar concentration within the producer gas. The qualitative analysis revealed relatively low tar content, especially in the case of low load, when its value was equal to 0.7 g/m 3 . Such value is a function of input material quality, gasifier design and performance. The selected conditions can be varied further in future works to define the even better performance of the technology and to increase the yield of affordable and clean energy for regional purposes. Provided results indicate possible operation of SRF gasification in fixed bed reactors. The energy content within the producer gas was relatively poor, however, the functionality of the technology and process obstructions were not observed due to low concentrations of polluting agents. On the other hand, the yield of the char material must also be considered an advantage and energy and material treatment and storage. Author statement Jakub ˇ Cespiva: Project administration, Conceptualization, Investigation, Formal analysis, Methodology, Writing – original draft, Writing – review & editing; Jan Skˇ rínský: Investigation, Formal analysis; J´ an Vereˇ s: Investigation, Formal analysis; Mateusz Wnukowski: Methodology, Investigation, Formal analysis, Writing – review & editing; Jana Serenˇ cíˇ sov´ a: Investigation, Formal analysis; Tade´ aˇ s Ochodek: Resources, Supervision, Funding acquisition. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Data will be made available on request. Acknowledgements This work was supported in the National Centre for Energy II, reg. no. TN02000025. 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