Techno-economic analysis of fluidized bed combustion of a mixed fuel from sewage and paper mill sludge

Citation: Carsky, M.; Solcova, O.; Soukup, K.; Kralik, T.; Vavrova, K.; Janota, L.; Vitek, M.; Honus, S.; Jadlovec, M.; Wimmerova, L. Techno-Economic Analysis of Fluidized Bed Combustion of a Mixed Fuel from Sewage and Paper Mill Sludge. Energies 2022,15, 8964. https://doi.org/10.3390/en15238964 Academic Editor: Marcin D˛ebowski Received: 5 November 2022 Accepted: 25 November 2022 Published: 27 November 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). energies Article Techno-Economic Analysis of Fluidized Bed Combustion of a Mixed Fuel from Sewage and Paper Mill Sludge Milan Carsky 1,2 , Olga Solcova 1,* , Karel Soukup 1, Tomas Kralik 3, Kamila Vavrova 4, Lukas Janota 3, Miroslav Vitek 3, Stanislav Honus 2, Marek Jadlovec 2and Lenka Wimmerova 5 1Institute of Chemical Process Fundamentals of the CAS, 165 00 Praha, Czech Republic 2Faculty of Mechanical Engineering, Department of Energy, VSB-Technical University of Ostrava, 708 33 Ostrava, Czech Republic 3Faculty of Electrical Engineering, CTU Prague, 166 27 Prague, Czech Republic 4 Silva Taroucy Research Institute for Landscape and Ornamental Gardening, 252 43 Pr˚uhonice, Czech Republic 5 Faculty of Environmental Sciences, Czech University of Life Sciences Prague, 165 00 Prague, Czech Republic *Correspondence: [email protected]; Tel.: +420-220-390-279 Abstract: The treatment and disposal of sewage sludge is one of the most important and critical issues of wastewater treatment plants. One option for sludge liquidation is the production of fuel in the form of pellets from mixed sewage and paper mill sludge. This study presents the results of the combustion of pelletized fuels, namely sewage and paper mill sludge, and their 2:1 and 4:1 blends in a fluidized bed combustor. The flue gas was analysed after reaching a steady state at bed temperatures of 700–800 ◦ C. Commonly used flue gas cleaning is still necessary, especially for SO 2 ; therefore, it is worth mentioning that the addition of paper mill sludge reduced the mercury concentration in the flue gas to limits acceptable in most EU countries. The analysis of ash after combustion showed that magnesium, potassium, calcium, chromium, copper, zinc, arsenic, and lead remained mostly in the ash after combustion, while all cadmium from all fuels used was transferred into the flue gas together with a substantial part of chlorine and mercury. The pellets containing both sewage and paper mill sludge can be used as an environmentally friendly alternative fuel for fluidised bed combustion. The levelized cost of this alternative fuel is at the same current price level as lignite. Keywords: pelletizing; sewage sludge; paper mill sludge; combustion; fluidized bed; environmental assessment; economic evaluation 1. Introduction The treatment and disposal of sewage sludge from wastewater treatment plants is one of the most essential wastewater treatment and management issues. World volumes of sewage sludge and paper mill waste grow every year. Countries of the European Union produce about 10 million tonnes of dry matter of sewage sludge annually [ 1 , 2 ]. In 2021, paper production reached 90.1 million tonnes in the EU [ 3 ], and 4.3–40 kg of dry matter of paper mill sludge is generated for every tonne of paper production [ 4 ]. Sewage sludge often contains pollutants harmful to human health (heavy metals, toxic substances, drug residues, harmful metabolites, hormones, pathogenic organisms) [ 5 ]. However, regarding sewage sludge, the main concern in the European Union is the content of heavy metals (Cd, Cu, Hg, Ni, Pb, and Zn). The major use of sewage sludge is associated with agriculture, and to less extent, with power and thermal generation. A promising and frequently tested method of the liquidation of sewage sludge is its combustion and co-combustion with other fuels. Sewage sludge calorific values range from approximately 6 MJ/kg to 16 MJ/kg, depending on the water content and the level of fermentation [ 6 ]. Sewage sludge combustion and co-combustion with other fuels is a topic widely elaborated on in the literature, contrary to the combustion or co-combustion of paper mill sludge [ 7 – 12 ]. Raw sewage sludge can be burned with other fuel, usually coal [ 13 – 24 ], or it can be dried to improve its calorific value Energies 2022,15, 8964. https://doi.org/10.3390/en15238964 https://www.mdpi.com/journal/energies Energies 2022,15, 8964 2 of 13 for mono-combustion [ 16 , 25 – 28 ]. However, some of the works used thermogravimetric analysis (TGA) only [7–9,20,29]. On average, sewage sludge contains 26 g of phosphorus/kg dry matter, which can be recovered from the ash after sludge combustion. Thermochemical modification by alkaline carbonate with the doping of magnesia minerals is a possible way to fix and recover phosphorous in the ash [ 30 ]. As expected, the combustion temperature, steam, and oxygen concentration affected the retention of Zn, Mn, and Cr in the ash after sewage sludge combustion [ 31 ]. The migration behaviours of As, Se, and Pb during the co-combustion of sewage sludge with coal was investigated in circulating fluidized-bed (CFB) boiler units with a capacity between 150–350 MW and two pulverized coal boiler (PC) units with a capacity of 350 MW and 600 MW. In the wet flue gas desulphurisation unit, the proportions of As, Se, and Pb in gypsum are higher than those of fly ash and bottom slag [ 32 ]. Karasek [ 33 ] investigated the behaviour of heavy metals and their compounds during the sewage sludge incineration process. A comprehensive analysis of heavy metals in all products of a standard flue gas treatment and in the flue gas itself from sewage sludge combustion was presented in the study. Leckner et al. [ 18 ] used laboratory and pilot plant circulating fluidized bed boilers for the co-combustion of sewage sludge together with coal or wood. Their results from a CFB plant showed that neither EU nor German emission limits were exceeded for the sludge fraction of less than 25%, except for the chlorine emission. However, that could be reduced by a flue gas treatment. Moreover, a considerable reduction of nitrogen oxide was achieved despite large quantities of nitrogen in the sewage sludge with only a few percent of the nitrogen converted to NO or N 2 O. Sulphur dioxide that formed during the combustion of sulphur, which may also be present in sewage sludge, can be captured by the conventional method of limestone addition. Combustion experiments of sewage sludge with rice husk briquettes were conducted with a Fenton (a solution of hydrogen peroxide with ferrous iron) CaO conditioner [ 34 ]. The results showed that the NO x emissions of conditioned sludge combustion were reduced approximately 1.3 times compared to that of the sludge alone with a rice husk mixing ratio of 43.8%, the Fenton/CaO conditioner dosage of 220 mg/g, and the temperature of 829 ◦ C. Complete combustion using fluidized bed technology can be achieved with 20–50% excess air. This is about half the amount of air used for multiple hearth furnaces. The fluidized bed technology is therefore a promising way to combust fuels with a low heating value because of the maximized thermal efficiency, minimized char, and emissions control. A relatively low and uniform process temperature together with low excess air within the bed reduces the formation of NO x . The emissions of CO in flue gas are low. Additions of limestone into the bed and/or ammonia into the freeboard initiate desulphurization and denitrification processes [ 35 – 38 ]. However, fluidized bed combustion emerged as an advantageous method for the treatment of other hazardous wastes as well [39–41]. Caputo et al. [ 12 ] estimated savings of EUR 15–20 million for the combustion of paper mill sludge during an estimated plant life of 15 years, with a pay-back period of about four years. This was based on their feasibility analysis and significant savings; compared to the landfill option, the waste-to-energy plant was built in 1999. Folguearas et al. [ 17 ] investigated the fluidized bed combustion of five different fuels (sewage sludge samples, bituminous coal, and sludge–coal blends). They found that the addition of sludge up to 10 wt% did not affect coal reactivity. For the 50 wt% blends, the reactivity depended on the temperature of combustion. At a temperature of combustion below 350 ◦ C, the blend reactivity was close to that of sludge, whereas for the combustion temperature above 350 ◦ C, it was close to that of coal. The kinetic process was successfully explained by the first order reaction mechanism related to Arrhenius law. Otero et al. [ 13 ] investigated the fluidized bed combustion of three different sludge samples and sludge–coal blends. The combustion parameters were measured by thermogravimetry. Some additives, e.g., coal or various forms of biomass, improved both parameters of the pelletizing (dewatering, pressure, temperature) [ 6 ] and combustion processes [ 42 ], respectively. In general, biomass of various origins [ 43 – 45 ] may be either incinerated as waste or used as alternative fuel. Energies 2022,15, 8964 3 of 13 In the former case, the waste biomass must usually be co-combusted with other fuels. The critical emissions from the combustion of waste biomass are heavy metals, organic pollutants, chlorinated and fluorinated compounds, SO 2 , NO x , and CO (see, e.g., [ 46 ]). The combustion of sewage sludge is well elaborated on in the literature, and studies on the combustion of a paper mill sludge can be found there as well; however, no research has been conducted on the fluidized bed combustion of paper mill–sewage sludge mixtures and a flue gas analysis. This paper presents the results of a pilot plant fluidized bed combustion of pellets of sewage sludge, mixed paper mill sludge, and mixtures of 2:1 and 4:1 of sewage and mixed paper mill sludge to judge the potential use as an alternative fuel to coal. 2. Materials and Methods Fuel pellets of sewage sludge, mixed paper mill sludge, and 2:1 and 4:1 blends of sewage and mixed paper mill sludge, delivered by ENVISAN-GEM, Czech Republic, were used for a pilot plant fluidized bed combustion. Both sewage sludge and paper mill sludge of 80% moisture content were sun dried. The product obtained was free of pathogens. Unlike the sewage sludge, it was necessary to crush the fibrous paper mill sludge after drying. Material densities were determined as a ratio of masses of ten pellets and a sum of their calculated volumes, and the bulk density was determined from a mass of pellets in a one-litre beaker. The higher and lower heating values and the moisture content were obtained from the Engineering Test Institute, Public Enterprise, Brno, Czech Republic [47,48]. For the properties of fuel pellets, see Table 1. Table 1. Fuel properties (as delivered by ENVISAN-GEM). Fuel Material Density Bulk Density Higher Heating Value [47] Lower Heating Value [47] Moisture Content [48] [kg/m3] [kg/m3][MJ/kg] [MJ/kg] [wt.%] Sewage sludge 1439 ±7 716 ±16 9.83 8.10 19.3 ±2.5 SPB 4:1 10.87 9.32 SPB 2:1 11.13 9.49 Paper mill sludge 14.0 12.6 Notes: SPB = blend of sewage and paper mill sludge. Shape of particles: pellets, mean length = 12.5 mm, mean diameter = 6 mm. Sand (size 0.9–2 mm, 1.44 mm mean size, density 2600 kg/m 3 ) was chosen as a bed inert material. The minimum fluidization velocity of 0.764 m/s at ambient temperature was determined experimentally by a standard method of plotting a superficial velocity vs. bed pressure drop. The minimum fluidization velocities of 10, 15, and 20 wt.% mixture of sand and fuel pellets were determined in the same way to be 0.88–1.04 m/s. It has been observed that, at too-low fluidization velocities or with no replacement of bed particles, the bed may agglomerate. The pilot plant fluidized bed combustor used for the tests is shown in Figure 1. The combustor had a circular cross-sectional area of an inner diameter of 140 mm. A fan equipped with a frequency controller (SIEMENS 6SL3210-1NE21-0UG1 Germany) delivered the air, for which the flowrate was measured by a mass flowmeter. The duration of all tests was 60 min after reaching a steady state. The fluidizing air was preheated to the temperature T 0 equal, on average, to 390 ◦ C, and its flowrate was kept constant at 31.67 Nm 3 /h. The fluidized bed material was heated to temperatures of 700–800 ◦ C with 12 kW electrical heaters. Once the required temperature of the bed was reached, the electrical heaters were switched off. Energies 2022,15, 8964 4 of 13 Energies 2022, 15, x FOR PEER REVIEW 4 of 14 temperature T0 equal, on average, to 390 °C, and its flowrate was kept constant at 31.67 Nm3/h. The fluidized bed material was heated to temperatures of 700–800°C with 12 kW electrical heaters. Once the required temperature of the bed was reached, the electrical heaters were switched off. (A) (B) Figure 1. Scheme ( A ) and photo ( B ) of the experimental unit. Distance of temperature sensors from the grid: T1 120 mm, T2 320 mm, T3 520 mm, T4 720 mm, T5 920 mm, T6 1120 mm, T7 1320 mm, T8 1520 mm, T9 1720 mm, T10 1920mm, and T11 2120 mm. Energies 2022,15, 8964 5 of 13 The fuel was introduced into the fluidized bed by a screw feeder from the fuel bin at a rate of 2–6 kg/h. The flue gas was cooled down in two water coolers, passed through a bag filter, and the discharge fanned to a chimney. There was a provision for continuous measurement and storage of data of temperature by thermocouples delivered by Testo SE & Co. KGaA, Germany, pressure by sensors delivered by Farnell, Germany, and flowrates in different points of the plant. A Gasmet DX4000 portable FTIR gas analyser was used for the analysis of CO 2 , CO, NO x , SO 2 , NH 3 , HCl, CH 4 , and O 2 in the flue gas, and the CVAAS (Cold Vapor Atomic Absorption Spectroscopy) HM-1400 TRX analyser was used for the analysis of all gaseous mercury compounds in the flue gas. The emissions were recorded at a steady state temperature of 700–800 ◦ C. The chemical composition of the ash samples was determined by X-ray Fluorescence with an XEPOS (Spectro, Germany) energy dispersion spectrometer. 3. Economic Evaluation To conduct a correct economic evaluation, it is primarily essential to establish the boundaries of the model under evaluation. Therefore, to be able to directly compare the alternative fuel with its substitutes (in particular lignite), the model boundary was set at the level of the produced alternative fuel, not including transport costs to the final point of use. The start of the evaluation was determined at the primary feedstock output at the point of production. The boundaries of the model respect all costs associated with the production of alternative fuel, i.e., all input costs for the acquisition and commissioning of the required technologies, as well as all fixed and variable costs associated with fuel production. This corresponds to the classical approach in setting LCOE boundaries, as discussed by the authors in [ 43 ]. It was also assumed that the feedstock (waste cellulose and waste sludge) had zero cost. This reflects the current situation where there are costs associated with the disposal of these wastes. Thus, both producers are currently willing to give up this material for free (saving their costs). This is summarised in detail in Figure 2. The processing and utilization of sludges produced by municipal wastewater treatment or other biomass waste treatment comprises a series of processes and can be divided into the following basic stages in terms of economic assessment: • Wastewater treatment, sludge production, and primary sludge dewatering (before the transportation to the processing site or input into the next process), currently implemented at wastewater treatment facilities. •Transportation of dewatered (condensed) sludge. •Sludge drying. •Sludge processing into final fuel (pellets or granules). The economic evaluation was based on the calculation of the levelized costs of energy (LCOE). The LCOE is a well-known standard method for calculating the cost of energy production. The principle of the LCOE calculation is the quantification of all discounted costs over the lifetime of the project per unit of production. In other words, the LCOE represents the cost of production that guarantees the investor a required financial return over the life of the project equal to the specified discount rate. A detailed explanation including all relevant equations is provided by Raikar and Adamson [ 49 ]. Table 2summarizes the input data used to calculate the LCOE of the alternative fuel. The economic lifetime of the project was derived from the lifetime of the solar dryer, which is 20 years [ 43 ]. This means a complete renewal of the pelletizing line in the 10th year of operation. The electricity price was taken from long-term contracts and does not reflect the current turbulent times in the electricity markets. Indeed, it can be assumed that the price will stabilise at this level concerning the following few years (as the current panic and nervousness on the commodity markets will be calmed). Energies 2022,15, 8964 6 of 13 Energies 2022, 15, x FOR PEER REVIEW 6 of 14 Figure 2. Boundaries for the economic evaluation. Table 2. Economic inputs [43,44]. Sludge Drying Total volume of imported sludge (sewage + paper) 5410 t/year Cost of sludge import (sewage + paper), producer’s own transport 0.45 EUR/t.km Sludge transport distance 5 km Specific power consumption per kg of evaporated water 0.04 kWhel/kg Total annual electricity consumption 118,335 kWh/year Operator requirement 1 person/year Pellet production Number of shifts per day 1 Number of working days 250 days/year Hours in operation 8 per 1 shift Hourly production capacity of pelletizing line 1015 kg/hour Total hourly electricity consumption 99 kWh/year Economic inputs Investment costs of solar dryer 3533 103 EUR Investment costs of pelletizing line 261 103 EUR Repairs and maintenance 38 103 EUR/year Personnel costs (employees) 33 103 EUR/year Energy and other material costs 129 103 EUR/year Electricity price 0.4 EUR/kWh Discount rate 7 % Long term inflation 2.0 % The economic lifetime of the project was derived from the lifetime of the solar dryer, which is 20 years [43]. This means a complete renewal of the pelletizing line in the 10th year of operation. The electricity price was taken from long-term contracts and does not reflect the current turbulent times in the electricity markets. Indeed, it can be assumed Figure 2. Boundaries for the economic evaluation. Table 2. Economic inputs [43,44]. Sludge Drying Total volume of imported sludge (sewage + paper) 5410 t/year Cost of sludge import (sewage + paper), producer’s own transport 0.45 EUR/t·km Sludge transport distance 5 km Specific power consumption per kg of evaporated water 0.04 kWhel/kg Total annual electricity consumption 118,335 kWh/year Operator requirement 1 person/year Pellet production Number of shifts per day 1 Number of working days 250 days/year Hours in operation 8 per 1 shift Hourly production capacity of pelletizing line 1015 kg/h Total hourly electricity consumption 99 kWh/year Economic inputs Investment costs of solar dryer 3533 103EUR Investment costs of pelletizing line 261 103EUR Repairs and maintenance 38 103EUR/year Personnel costs (employees) 33 103EUR/year Energy and other material costs 129 103EUR/year Electricity price 0.4 EUR/kWh Discount rate 7 % Long term inflation 2.0 % 4. Results and Discussion A typical temperature profile alongside the fluidised bed column is shown in Figure 3. The values of concentration in the following figures and tables were expressed for a dry flue gas at the pressure of 101,325 Pa, at the temperature of 273.15 K, and at the concentration of oxygen of 11%. The concentration of mercury in the flue gas for the combustion of all fuels used in the study is given in Table 3. Energies 2022,15, 8964 7 of 13 Energies 2022, 15, x FOR PEER REVIEW 7 of 14 that the price will stabilise at this level concerning the following few years (as the current panic and nervousness on the commodity markets will be calmed). 4. Results and Discussion A typical temperature profile alongside the fluidised bed column is shown in Figure 3. Figure 3. Steady state temperature profile in the fluidized bed reactor at various fuel combustions. The values of concentration in the following figures and tables were expressed for a dry flue gas at the pressure of 101,325 Pa, at the temperature of 273.15 K, and at the concentration of oxygen of 11%. The concentration of mercury in the flue gas for the combustion of all fuels used in the study is given in Table 3. Figure 3. Steady state temperature profile in the fluidized bed reactor at various fuel combustions. Table 3. Average concentration of mercury in the flue gas. Material (Pellets) Average Concentration (µg/m3) Sewage sludge 106.59 ±16.32 SPB 4:1 59.62 ±11.80 SPB 2:1 48.59 ±4.26 Paper mill sludge ≈0 The mercury concentration in the flue gas decreased substantially with the addition of paper mill sludge to the sewage sludge. Such alternative fuel complies with the limits of a flue gas mercury concentration acceptable in most EU countries [50]. The steady concentration of carbon dioxide, carbon monoxide, and sulphur dioxide in the flue gas for the combustion of all fuels used in the study are given in Table 4. The carbon dioxide concentration in the flue gas was not affected by the addition of paper mill sludge. The average concentrations of carbon monoxide in the flue gas of all fuels used in the study were in the range of 34.5–229.3 mg/m 3 , complying with usual worldwide norms, e.g., [ 50 ]. The sulphur content of paper mill sludge was low; therefore, the concentration of SO 2 in the flue gas was negligible. The sulphur content in the sewage sludge may be significant (and fluctuating). Therefore, the SO 2 flue gas concentration of Energies 2022,15, 8964 8 of 13 the other three fuels was high. However, the addition of limestone into the bed initiates the desulphurization process [18,35–38]. Table 4. Average concentrations of carbon dioxide, carbon monoxide, and sulphur dioxide in the flue gas. Material (Pellets) CO2Average Concentration (%) SO2Average Concentration (mg/m3) and (ppm) CO Average Concentration (mg/m3) and (ppm) Sewage sludge 5.3 ±0.5 3307 ±848 1156.9 34.5 ±26.7 27.6 SPB 4:1 5.7 ±0.2 1775 ±220 621.2 106.3 ±49.2 85.1 SPB 2:1 5.0 ±0.1 1608 ±74 562.7 86 ±65.5 68.8 Paper mill sludge 5.4 ±0.6 64.2 ±9.5 22.5 229.3 ±195.2 183.5 Table 5shows the average concentrations of NO x , NH 3 , HCl, and CH 4 in the flue gas for the combustion of the four materials mentioned in Table 1. The concentration of NO x in the flue gas decreased substantially with the addition of paper mill sludge to the sewage sludge to acceptable levels [ 50 ]. However, the addition of ammonia into the freeboard initiates further denitrification [ 35 – 38 ]. The ammonia and methane flue gas concentrations were negligible for all four fuels. The hydrogen chloride concentration in the fumes for the sewage sludge pellets exceeded the acceptable level of 50 mg/m 3 [ 51 ], similarly to the work of Leckner et al. [ 18 ]. The addition of paper mill sludge to the sewage sludge showed a decrease of HCl concentration to acceptable levels. Table 5. Average concentrations in the flue gas. Material (Pellets) NOx(mg/m3) and (ppm) NH3(mg/m3) and (ppm) HCl (mg/m3) and (ppm) CH4(mg/m3) and (ppm) Sewage sludge 576.2 280.8 1.9 2.5 181.4 111.5 5.43 7.6 SPB 4:1 153.7 74.9 0.83 1.1 14.9 9.1 0 SPB 2:1 64.9 31.6 1.74 2.3 60.1 37 0 Paper mill sludge 184.0 89.7 1.2 1.58 19.6 12.1 0 The superficial velocity of fluidization at the combustion of all fuels in the study was about twice the minimum fluidization velocity. Although it was not observed particularly, it might be expected that the possibility of bed agglomeration under identical or similar conditions exists if the process is operated for long enough periods. Therefore, it is recommended to run the process at higher fluidization velocities and to replace the bed, either continuously or periodically. The chemical composition of the ash samples was determined by X-ray fluorescence on the energy dispersion spectrometer XEPOS (Spectro, Germany). The analysis of ash samples compared to the composition of corresponding species in the sewage sludge, SPB 2:1, SPB 4:1, is shown in Table 6. The analysis of the paper mill sludge did not detect any elements shown in Table 6. Furthermore, the combustion of paper mill sludge produced no measurable quantity of ash. The data in Table 6suggest that magnesium, potassium, calcium, chromium, copper, zinc, arsenic, and lead remained mostly in the ash after combustion, while all cadmium from all fuels used was transferred into the flue gas together with a substantial part of chlorine and mercury. These results are in accord with previously published works [31–33]. In addition to the technological tests, a basic analysis of environmental impacts of the prepared alternative fuels was carried out. The assessment was conducted as a simplified environmental input-output based life cycle assessment (EIO-LCA) [ 52 ] of the selected, but limited, input and output streams. During this short study, material and fuel balances were considered in terms of the input waste used and oxygen consumption needed for the fluidized bed process, together with the output parameters representing the amount Energies 2022,15, 8964 9 of 13 of energy produced, the character of the flue gases, and the post-combustion ashes. Concerning the environmental aspect, the data presented above in Tables 1and 3–6were used together with the material balance of the process. The study was prepared in the open LCA software v1.10.3 (GreenDelta, Berlin, Germany, 2020) using the ecoinvent v.3.8 database and the APOS unit model (Ecoinvent, Curich, Switzerland, 2021). The resulting impacts were assessed by the most common method of the life cycle impact assessment (LCIA), the CML baseline created by the Institute of Environmental Sciences, University of Leiden (the Netherlands) in 2001 [ 53 ], which includes a core group of midpoint impact categories such as climate change, the depletion of abiotic sources, human toxicity, and ecotoxicity [ 54 ]. The CML-IA baseline v.4.4 of January 2015 used was provided within the openLCA LCIA methods as the package v2.1.2 (GreenDelta, Berlin, Germany, 2021) and was compatible with the used ecoinvent v3.8. The functional unit (FU) was set on processing 1 kg of the alternative fuels. The final comparison was converted to the production of 1 MJ heat. Table 6. Analysis of sewage sludge, SPB 2:1, SPB 4:1, and their ash after combustion. Element Sewage Sludge mg/kg Ash (Sewage Sludge) mg/kg SPB 4:1 mg/kg Ash (SPB 4:1) mg/kg SPB 2:1 mg/kg Ash (SPB 2:1) mg/kg Magnesium 4499 7210 4498.8 7980 3131.5 7650 Chlorine 261.63 260 227.1 30 204.7 60 Potassium 2972.5 9570 2554 10,880 2282 9690 Calcium 20,388 82,560 26,340 85,780 30,290 84,110 Chromium 355.85 400 285.46 430 239.71 410 Nickel 142.77 200 115.22 220 97.31 210 Copper 892.44 1390 716.05 1490 601.4 1380 Zinc 1499.4 2990 1202.1 3310 1008.9 2880 Arsenic 7.525 20 6.22 30 5.37 20 Cadmium 4.975 0 4 0 3.37 0 Mercury 1.965 10 1.636 0 1.422 0 Lead 153.23 170 122.73 190 102.91 140 Table 7shows the LCIA results of the assessed fuels. Each selected LCIA category is displayed in the rows, and the project variants are in the columns. The unit is the unit of the LCIA category, as defined in the selected LCIA method. Table 7. LCIA results (CML-IA baseline, v.4.4, 2015) of the pelletized fuels for the production of 1 MJ heat. Indicator Unit Paper Mill Sludge SPB 2:1 SPB 4:1 Sewage Sludge Abiotic depletion (elements) kg Sb eq. 1.61 ×10−84.61 ×10−85.22 ×10−86.75 ×10−8 Abiotic depletion (fossil fuels) MJ 1.88 ×10−15.37 ×10−16.08 ×10−17.87 ×10−1 Acidification kg SO2eq. 3.26 ×10−59.41 ×10−51.07 ×10−41.38 ×10−4 Eutrophication kg PO43−eq. 4.68 ×10−61.34 ×10−51.52 ×10−51.96 ×10−5 Freshwater aquatic ecotoxicity kg 1,4-DB eq. 2.31 ×10−33.14 ×10−23.79 ×10−24.61 ×10−2 Global warming (GWP100a) kg CO2eq. 8.54 ×10−21.42 ×10−11.51 ×10−11.81 ×10−1 Human toxicity kg 1,4-DB eq. 1.67 ×10−36.32 ×10−37.56 ×10−39.84 ×10−3 Marine aquatic ecotoxicity kg 1,4-DB eq. 2.54 ×1001.54 ×1011.81 ×1012.24 ×101 Ozone layer depletion (ODP) kg CFC-11 eq. 2.45 ×10−97.02 ×10−97.95 ×10−91.03 ×10−8 Photochemical oxidation kg C2H4eq. 1.28 ×10−63.69 ×10−64.18 ×10−65.40 ×10−6 Terrestrial ecotoxicity kg 1,4-DB eq. 8.84 ×10−42.86 ×10−33.61 ×10−32.30 ×10−2 The performed basic input-output analysis showed that only the alternative fuel produced from sewage sludge has the most significant impacts on the environment regarding all assessed categories. In the case of both SPB tested, these impacts were generally reduced by 20–30% in average, with lower environmental impacts for the SPB 2:1 fuel, mainly due