The RDF/SRF torrefaction: An effect of temperature on characterization of the product - Carbonized Refuse Derived Fuel

Techno-environmental analysis to valorize the secondary energy resources from refuse-derived fuel-based waste to energy plant

The present study quantifies the environmental and sustainability impacts associated with municipal solid waste management (MSWM) in India which plays a vital environmental issue in recent times. The upsurge in population has resulted in massive waste generation, leading to a concerning rise in the level of greenhouse gas (GHG) emissions. Therefore, the sustainable management of MSW has been discussed and highlights the conversion of MSW into refuse-derived fuel (RDF) to identify its potential for generating electricity in waste-to-energy (WtE) plants. The life cycle assessment (LCA) study has been done to identify and compare the environmental impacts associated with different scenarios (SC) as SC1: landfilling without energy recovery, SC2: open burning and SC3: processing of RDF in WtE plant by considering the nine impact categories from the inventory data obtained over a period of 12 consecutive months (Jan 2021-Jan 2022). The results exhibited that the global warming potential caused by emissions of GHG are in the order of SC1 (1188 kg CO2 eq) > SC2 (752 kg CO2 eq) > SC3 (332 kg CO2 eq), respectively from 1 t of MSW. It is concluded that the WtE plant can help in the reduction of environmental issues, strengthening the capacity of electricity generation and improving the aesthetic view of the city which is socially acceptable as well. Thus, WtE technology can help in achieving sustainable development goal 12 to regenerate the sustainable secondary resources for the twenty-first century and minimize global climate change.

Municipal solid waste management instruments that influence the use of the refuse as fuel in developing countries: A critical review

Landfills are the destination of most of the refuse generated whereas composting, material recycling, and Waste-to-Energy (WtE) technologies are not commonly employed in developing countries. However, the destination for energy purposes could be supplied with this refuse, improving the viability of energy use. Thus, this article raises some questions to identify aspects that could encourage its use as refuse-derived fuel (RDF) in these countries. Among them, does environmental education affect the municipal solid waste (MSW) source separation with emphasis on a destination? Can selective collection and extended producer responsibility (EPR) affect the MSW for energy recovery? Is there competition between the recycling market and the energy market for RDF? A systematic review of the literature was conducted to gather data and provide answers to such questions. This enabled to observe that EPR, selective collect expansion and source separation influence the quantity and quality of waste sent for energy use. Both internal and external factors impact on source separation. Additionally, there is evidence to support that despite several studies showing their technical, economic, environmental and social viability, the methods of energy usage of the refuse still need to improve their deployment in developing countries. In addition to identifying the main research gaps to be filled in future studies, the article also identified the instruments of MSW management that are to be applied in developing countries to divert recyclable and organic waste from landfill.

Emission Characteristics of Polychlorinated Dibenzo-p-Dioxins/Dibenzofurans (PCDD/DFs) in Commercial Bio-SRF and SRF Incineration Plants

Incineration plants using solid refused fuel (SRF) should control their air pollution materials to minimize environmental impact. This study evaluated the emission of polychlorinated dibenzo-p-dioxin/dibenzofurans (PCDD/DFs) congener patterns in seven commercial incineration plants in Korea using SRF and biomass SRF (bio-SRF). We examined the reduction rate differences of PCDD/DFs, depending on the air pollutant control device. All seven incineration plants sufficiently managed their dioxin emissions. However, both SRF and bio-SRF incineration plants showed active chlorination reactions and resulted in a large amount of highly chlorinated dioxins. The average dioxin concentration was 0.02 ng international toxic equivalency quantity (I-TEQ)/Sm3. Ratios of 1,2,3,4,6,7,8-HpCDF and 1,2,3,7,8-PeCDF were high in the waste heat boilers of both SRF and bio-SRF incineration plants. The octachlorinated dibenzofuran (OCDF) ratio was only high in the SRF incineration plants. Octachlorodibenzo-p-dioxin (OCDD) and OCDF exhibited high dioxin ratios. SRF incineration plants had a low ratio of OCDF to 1,2,3,4,6,7,8-HpCDF. In addition, the reduction rate of PCDD/DFs was substantially high after treatment with the air pollutant control device.

Lab-Scale Study of Temperature and Duration Effects on Carbonized Solid Fuels Properties Produced from Municipal Solid Waste Components

In work, data from carbonization of the eight main municipal solid waste components (carton, fabric, kitchen waste, paper, plastic, rubber, paper/aluminum/polyethylene (PAP/AL/PE) composite packaging pack, wood) carbonized at 300–500 °C for 20–60 min were used to build regression models to predict the biochar properties (proximate and ultimate analysis) for particular components. These models were then combined in general models that predict the properties of char made from mixed waste components depending on pyrolysis temperature, residence time, and share of municipal solid waste components. Next, the general models were compared with experimental data (two mixtures made from the above-mentioned components carbonized at the same conditions). The comparison showed that most of the proposed general models had a determination coefficient (R²) over 0.6, and the best prediction was found for the prediction of biochar mass yield (R² = 0.9). All models were implemented into a spreadsheet to provide a simple tool to determine the potential of carbonization of municipal solid waste/refuse solid fuel based on a local mix of major components.

Biochars from Post-Production Biomass and Waste from Wood Management: Analysis of Carbonization Products

Waste biomass can be used as an alternative source of energy. However, such use requires prior treatment of the material. This paper describes the physicochemical characteristics of biochar obtained by the thermochemical decomposition of six types of agricultural waste biomass: residues from the production of flavored spirits (a pulp of lime, grapefruit and lemon), beetroot pulp, apple pomace, brewer’s spent grain, bark and municipal solid waste (bark, sawdust, off-cuts and wood chips). The biomass conversion process was studied under conditions of limited oxygen access in a reactor. The temperature was raised from 450 to 850 °C over 30 min, followed by a residence time of 60 min. The solid products were characterized in terms of their elemental compositions, mass, energy yield and ash content. The gaseous products from pyrolysis of the biomass were also analyzed and their compositions were characterized by GCMS (Gas Chromatography–Mass Spectrometry). The carbonization process increased the carbon content by, on average, 1.7 times, from an average percentage of 46.09% ± 3.65% for biomass to an average percentage of 74.72% ± 5.36% for biochars. After carbonization, the biochars were found to have a net calorific value of between 27 and 32 MJ/kg, which is comparable or even higher than good-quality coal (eco pea coal 24–26 MJ/kg). The net calorific values show that the volatile products can also be considered as a valuable source of energy.

Impact of torrefaction on biomass properties depending on temperature and operation time

Torrefaction is an effective way to upgrade biomass for producing fuels. The experimental results of torrefaction for three materials, oak wood, mixed wood mainly from deciduous trees and sewage sludge are presented. The comparison between three materials is performed to evaluate the influence of temperature and time on torrefaction operation. The influence of the operating temperature and time was analysed in order to determine optimal operation parameters for the newly developed process which has been patented. Properties, such as heating value, mass loss, chemical compositions, energy yield and enhancement factor were investigated. The results show that from an energy point of view the optimal operation time for oak and mixed wood is around 1.2 h at 260 °C. The torrefaction of sewage sludge is energetically unjustified. The highest carbon loss is shown for mixed wood, following by sewage sludge and oak wood. Torrefaction severity index was established based on the, most severe conditions. Torrefaction severity index could be applied as an indicator for prediction of torrefaction efficiency of chosen material.

Impact of Calcium Oxide on Hygienization and Self-Heating Prevention of Biologically Contaminated Polymer Materials

During the storage of spent polymer materials derived from municipal solid waste, which contain biodegradable impurities, an intense growth of microorganisms takes place. The aerobic metabolism of microorganisms may cause these materials to combust spontaneously and to become a real epidemiological risk for humans. The aim of the research is to determine the optimal addition of calcium oxide (CaO), which effectively reduces the number of selected microorganism groups populating the analyzed materials, in which spent polymers represent a significant fraction: refuse-derived fuel (RDF) and an undersize fraction of municipal solid waste (UFMSW). The main novelty of the experiments is to assess the benefits of using the commonly available and cheap filler (CaO), to hygienize the material and to reduce the fire hazard arising from its storage. During the mixing of spent polymer materials with pulverized CaO (mass shares: 1, 2, and 5% CaO), temperature changes were monitored using thermography. Moisture content (MC), pH, respiration activity (AT4) and bacterial count were determined before and after the experiment. During the addition of CaO (especially when the content was at 5%) to the UFMSW, higher maximum temperatures were obtained than in the case of RDF analyses, which may be the result of a high percentage of the biodegradable fraction and higher MC of the UFMSW. In all cases the waste temperature did not increase again after 3 min. CaO used in the experiment effectively limited the number of microorganisms. The addition of 5% of CaO has showed the strongest antimicrobial properties, and it can be recommended for hygienization of the analyzed materials and for the reduction of the risk of self-heating during their storage in windrows.

Waste to Energy: Solid Fuel Production from Biogas Plant Digestate and Sewage Sludge by Torrefaction-Process Kinetics, Fuel Properties, and Energy Balance

Sustainable solutions are needed to manage increased energy demand and waste generation. Renewable energy production from abundant sewage sludge (SS) and digestate (D) from biogas is feasible. Concerns about feedstock contamination (heavy metals, pharmaceuticals, antibiotics, and antibiotic-resistant bacteria) in SS and D limits the use (e.g., agricultural) of these carbon-rich resources. Low temperature thermal conversion that results in carbonized solid fuel (CSF) has been proposed as sustainable waste utilization. The aim of the research was to investigate the feasibility of CSF production from SS and D via torrefaction. The CSF was produced at 200~300 °C (interval of 20 °C) for 20~60 min (interval 20 min). The torrefaction kinetics and CSF fuel properties were determined. Next, the differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) of SS and D torrefaction were used to build models of energy demand for torrefaction. Finally, the evaluation of the energy balance of CSF production from SS and D was completed. The results showed that torrefaction improved the D-derived CSF’s higher heating value (HHV) up to 11% (p < 0.05), whereas no significant HHV changes for SS were observed. The torrefied D had the highest HHV of 20 MJ∙kg−1 under 300 °C and 30 min, (the curve fitted value from the measured time periods) compared to HHV = 18 MJ∙kg−1 for unprocessed D. The torrefied SS had the highest HHV = 14.8 MJ∙kg−1 under 200 °C and 20 min, compared to HHV 14.6 MJ∙kg−1 for raw SS. An unwanted result of the torrefaction was an increase in ash content in CSF, up to 40% and 22% for SS and D, respectively. The developed model showed that the torrefaction of dry SS and D could be energetically self-sufficient. Generating CSF with the highest HHV requires raw feedstock containing ~15.4 and 45.9 MJ∙kg−1 for SS and D, respectively (assuming that part of feedstock is a source of energy for the process). The results suggest that there is a potential to convert biogas D to CSF to provide renewable fuel for, e.g., plants currently fed/co-fed with municipal solid waste.

Low-Temperature Pyrolysis of Municipal Solid Waste Components and Refuse-Derived Fuel—Process Efficiency and Fuel Properties of Carbonized Solid Fuel

New technologies to valorize refuse-derived fuels (RDFs) will be required in the near future due to emerging trends of (1) the cement industry’s demands for high-quality alternative fuels and (2) the decreasing calorific value of the fuels derived from municipal solid waste (MSW) and currently used in cement/incineration plants. Low-temperature pyrolysis can increase the calorific value of processed material, leading to the production of value-added carbonized solid fuel (CSF). This dataset summarizes the key properties of MSW-derived CSF. Pyrolysis experiments were completed using eight types of organic waste and their two RDF mixtures. Organic waste represented common morphological groups of MSW, i.e., cartons, fabrics, kitchen waste, paper, plastic, rubber, PAP/AL/PE composite packaging (multi-material packaging also known as Tetra Pak cartons), and wood. The pyrolysis was conducted at temperatures ranging from 300 to 500 °C (20 °C intervals), with a retention (process) time of 20 to 60 min (20 min intervals). The mass yield, energy densification ratio, and energy yield were determined to characterize the pyrolysis process efficiency. The raw materials and produced CSF were tested with proximate analyses (moisture content, organic matter content, ash content, and combustible part content) and with ultimate analyses (elemental composition C, H, N, S) and high heating value (HHV). Additionally, differential scanning calorimetry (DSC) and thermogravimetric analyses (TGA) of the pyrolysis process were performed. The dataset documents the changes in fuel properties of RDF resulting from low-temperature pyrolysis as a function of the pyrolysis conditions and feedstock type. The greatest HHV improvements were observed for fabrics (up to 65%), PAP/AL/PE composite packaging (up to 56%), and wood (up to 46%).

Waste-to-Carbon: Is the Torrefied Sewage Sludge with High Ash Content a Better Fuel or Fertilizer?

Sewage sludge (SS) recycling is an important part of the proposed ‘circular economy’ concept. SS can be valorized via torrefaction (also known as ‘low-temperature pyrolysis’ or ‘roasting’). SS can, therefore, be considered a low-quality fuel or a source of nutrients essential for plant growth. Biochar produced by torrefaction of SS is a form of carbonized fuel or fertilizer. In this research, for the first time, we tested the feasibility of torrefaction of SS with high ash content for either fuel or organic fertilizer production. The research was conducted in 18 variants (six torrefaction temperatures between 200~300 °C, and three process residence times of 20, 40, 60 min) in 5 repetitions. Fuel and fertilizer properties and multiple regression analysis of produced biochar were conducted. The higher heating value (HHV) of raw SS was 21.2 MJ·kg⁻¹. Produced biochar was characterized by HHV up to 12.85 MJ·kg⁻¹ and lower H/C and O/C molar ratio. Therefore, torrefaction of SS with high ash content should not be considered as a method for improving the fuel properties. Instead, the production of fertilizer appears to be favorable. The torrefaction increased C, N, Mg, Ca, K, Na concentration in relation to raw SS. No significant (p < 0.05) influence of the increase of temperature and residence time on the increase of biogenic elements in biochar was found, however the highest biogenic element content, were found in biochar produced for 60 min, under the temperature ranging from 200 to 240 °C. Obtained biochars met the Polish regulatory criteria for mineral-organic fertilizer. Therefore SS torrefaction may be considered a feasible waste recycling technology. The calculation of torrefaction energy and the mass balance shows energy demand <2.5 GJ∙Mg⁻¹ w.m., and the expected mass yield of the product, organic fertilizer, is ~178 kg∙Mg⁻¹ w.m of SS. Further investigation should consider the scaling-up of the SS torrefaction process, with the application of other types of SSs.

Parameters Affecting RDF-Based Pellet Quality

Increasing production of waste has compelled the development of modern technologies for waste management. Certain fractions of municipal solid wastes are not suitable for recycling and must be utilised in other ways. Materials such as refuse-derived fuel (RDF) fractions are used as fuel in cement or CHP (combined heat and power) plants. The low bulk density leads to many problems pertaining to transportation and storage. In the case of biomass, these problems cause reduction in pelletisation. This paper therefore presents a comprehensive study on RDF pellet production, which is divided into three major areas. The first describes laboratory-scale tests and provides information on key factors that affect pellet quality (e.g., density and durability). Based on this, the second part presents a design of modified RDF dies to form RDF pellets, which are then tested via a semi-professional line test. The results show that RDF fraction can be compacted to form pellets using conventional devices. Given that temperature plays a key role, a special die must be used, and this ensures that the produced pellets exhibit high durability and bulk density, similar to biomass pellets.

Waste to Carbon: Estimating the Energy Demand for Production of Carbonized Refuse-Derived Fuel

We have been advancing the concept of carbonized refuse-derived fuel (CRDF) by refuse-derived fuel (RDF) torrefaction as improved recycling to synergistically address the world’s energy demand. The RDF is a combustible fraction of municipal solid waste (MSW). Many municipalities recover RDF for co-firing with conventional fuels. Torrefaction can further enhance fuel properties and valorize RDF. Energy demand for torrefaction is one of the key unknowns needed for scaling up CRDF production. To address this need, a pioneering model for optimizing site-specific energy demand for torrefaction of mixed RDF materials was developed. First, thermogravimetric and differential scanning calorimetry analyses were used to establish thermal properties for eight common RDF materials. Then, the model using the %RDF mix, empirical thermal properties, and torrefaction temperature was developed. The model results for individual RDF components fitted well (R2 ≥ 0.98) with experimental torrefaction data. Finally, the model was used to find an optimized RDF site-specific mixture with the lowest energy demand. The developed model could be a basis for estimating a net energy potential from the torrefaction of mixed RDF. Improved models could be useful to make plant-specific decisions to optimize RDF production based on the energy demand that depends on highly variable types of MSW and RDF streams.

A Review of Biochar Properties and Their Utilization in Crop Agriculture and Livestock Production

When it comes to the use of biochar in agriculture, the majority of research conducted in the last decade has focused on its application as a soil amendment and for soil remediation. This treatment improves soil quality, increases crops yields, and sequestrates atmospheric carbon to the soil. Another widely studied aspect connecting biochar with agriculture is the composting processes of various agricultural waste with the addition of biochar. Obtaining the material via the pyrolysis of agricultural waste, including animal manure, has also been investigated. However, given the remarkable properties of biochar, its application potential could be utilized in other areas not yet thoroughly investigated. This review paper summarizes the last decade of research on biochar and its use in crop agriculture and livestock production. Knowledge gaps are highlighted, such as using biochar for the mitigation of odorous emissions from animal manure and by feeding the biochar to animals.

Proof-of-Concept of Spent Mushrooms Compost Torrefaction—Studying the Process Kinetics and the Influence of Temperature and Duration on the Calorific Value of the Produced Biocoal

Poland, being the 3rd largest and growing producer of mushrooms in the world, generates almost 25% of the total European production. The generation rate of waste mushroom spent compost (MSC) amounts to 5 kg per 1 kg of mushrooms produced. We proposed the MSC treatment via torrefaction for the production of solid fuel—biocoal. In this research, we examined the MSC torrefaction kinetics using thermogravimetric analyses (TGA) and we tested the influence of torrefaction temperature within the range from 200 to 300 °C and treatment time lasting from 20 to 60 min on the resulting biocoal’s (fuel) properties. The estimated value of the torrefaction activation energy of MSC was 22.3 kJ mol−1. The highest calorific value = 17.9 MJ kg−1 d.m. was found for 280 °C (60 min torrefaction time). A significant (p < 0.05) influence of torrefaction temperature on HHV increase within the same group of torrefaction duration, i.e., 20, 40, or 60 min, was observed. The torrefaction duration significantly (p < 0.05) increased the HHV for 220 °C and decreased HHV for 300 °C. The highest mass yield (98.5%) was found for 220 °C (60 min), while the highest energy yield was found for 280 °C (60 min). In addition, estimations of the biocoal recirculation rate to maintain the heat self-sufficiency of MSC torrefaction were made. The net quantity of biocoal (torrefied MSC; 65.3% moisture content) and the 280 °C (60 min) torrefaction variant was used. The initial mass and energy balance showed that MSC torrefaction might be feasible and self-sufficient for heat when ~43.6% of produced biocoal is recirculated to supply the heat for torrefaction. Thus, we have shown a concept for an alternative utilization of abundant biowaste (MSC). This research provides a basis for alternative use of an abundant biowaste and can help charting improved, sustainable mushroom production.

Torrefaction and carbonization of refuse derived fuel: Char characterization and evaluation of gaseous and liquid emissions

Refuse derived fuel containing non-hazardous industrial wastes was subjected to torrefaction and carbonization in an industrial furnace. The RDF samples were heated at 300 °C and 400 °C, for 30 min, yielding solid products (chars) as well as gases and liquids. Proximate and ultimate composition, mineral composition, chlorine content and high heating value were determined for the original sample and the produced chars. Thermal treatment produced RDF chars with carbon contents of 61.6 and 80.2 wt%, and high heating values of 19.9 and 23.5 MJ/kg, that could be further upgraded by washing with water to reduce ash and chlorine concentrations and improve calorific value. Gas products were composed of carbon dioxide and carbon monoxide with minor amounts of hydrogen. Methane was only detected in the gas produced at 400 °C. The process generated liquid products rich in organic compounds that represent potential in further energy or material recovery.

The Proof-of-the-Concept of Application of Pelletization for Mitigation of Volatile Organic Compounds Emissions from Carbonized Refuse-Derived Fuel

Waste can be effectively reused through the production of carbonized refuse-derived fuel (CRDF) that enables further energy recovery. Developing cleaner production of CRDF requires consideration of practical issues of storage and handling. Thus, it needs to be ensured that CRDF does not pose an excessive risk to humans and the ecosystem. Very few studies indicate a wide variety of volatile organic compounds (VOCs) are present in CRDF, some of which are toxic. During handling, storage, transportation, and use of VOC-rich CRDF, workers and end-users could be exposed to emissions that could pose a health and safety hazard. Our recent study shows that CRDF densification via pelletization can increase the efficiency of storage and transportation. Thus, the following research question was identified: can pelletization mitigate VOCs emissions from CRDF during storage? Preliminary research aiming at the determination of the influence of CRDF pelletization on VOCs emission during storage was completed to address this question. The VOCs emissions from two types of CRDF: ground (loose, torrefied refuse-derived fuel (RDF)) and pelletized, were measured. Pelletization reduced the VOCs emissions potential during the four-day storage by ~86%, in comparison with ground CRDF. Mitigation of VOCs emissions from densified CRDF is feasible, and research is warranted to understand the influence of structural modification on VOCs emission kinetics, and possibilities of scaling up this solution into the practice of cleaner storage and transportation of CRDF.

The Effect of Biochar Addition on the Biogas Production Kinetics from the Anaerobic Digestion of Brewers’ Spent Grain

Biochar (BC) addition is a novel and promising method for biogas yield increase. Brewer’s spent grain (BSG) is an abundant organic waste with a large potential for biogas production. In this research, for the first time, we test the feasibility of increasing biogas yield and rate from BSG digestion by adding BC, which was produced from BSG via torrefaction (low-temperature pyrolysis). Furthermore, we explore the digestion of BSG with the presence BCs produced from BSG via torrefaction (low-temperature pyrolysis). The proposed approach creates two alternative waste-to-energy and waste-to-carbon type utilization pathways for BSG: (1) digestion of BSG waste to produce biogas and (2) torrefaction of BSG to produce BC used for digestion. Torrefaction extended the short utility lifetime of BSG waste turned into BC. BSG was digested in the presence of BC with BC to BSG + BC weight ratio from 0 to 50%. The study was conducted during 21 days under mesophilic conditions in n = 3 trials. The content of dry mass 17.6% in all variants was constant. The kinetics results for pure BSG (0% BC) were: reaction rate constant (k) 1.535 d−1, maximum production of biogas (B0) 92.3 dm3∙kg−1d.o.m. (d.o.m. = dry organic matter), and biogas production rate (r), 103.1 dm3∙kg−1d.o.m.∙d−1. his preliminary research showed that the highest (p < 0.05) r, 227 dm3∙kg−1d.o.m.∙d−1 was due to the 5% BC addition. This production rate was significantly higher (p < 0.05) compared with all other treatments (0, 1, 3, 8, 10, 20, 30, and 50% BC dose). Due to the high variability observed between replicates, no significant differences could be detected between all the assays amended with BC and the variant 0% BC. However, a significant decrease of B0 from 85.1 to 61.0 dm3∙kg−1d.o.m. in variants with the high biochar addition (20–50% BC) was observed in relation to 5% BC (122 dm3∙kg−1d.o.m.), suggesting that BC overdose inhibits biogas production from the BSG + BC mixture. The reaction rate constant (k) was not improved by BC, and the addition of 10% and 20% BC even decreased k relatively to the 0% variant. A significant decrease of k was also observed for the doses of 10%, 20%, and 30% when compared with the 5% BC (1.89 d−1) assays.

Design of an industrial solid waste processing line to produce refuse-derived fuel

Municipal Solid Waste (MSW) from the city of Boa Esperança, Minas Gerais, Brazil, was used to produce refuse-derived fuel (RDF). The MSW contains residues from human society, including product packaging, bottles, batteries, organic waste, fines, textiles, health textiles, plastics, glass, and metals, among others. The following protocol was performed during the conversion of MSW to RDF: (i) the raw MSW was placed in a silo and sent to a primary crusher using a metal conveyor belt, which reduced the particle size to 80 mm; (ii) the biomass was transferred to a selective waste collection platform by a rubber conveyor belt, and the recyclable waste, metals, and glasses were separated manually; (iii) residual metals were removed by a magnetic separator; (iv) the waste was transferred to a secondary crusher which reduced the particle size to 60 mm; (v) the waste passed through an airborne separator to remove materials with high density, such as glass, stones, and organic materials, using a metallic conveyor belt; (vi) the particle size was reduced to 40 mm by a tertiary crusher; (vii) the aluminium was separated from the non-metallic materials (plastic, paper, rubber, etc.) using an eddy current separator; (viii) the particle size was reduced to 25 mm using a quaternary crusher; (ix) the MSW was introduced into a rotary dryer using a metal conveyor belt, where the moisture content was reduced to close to 15 wt%, which required thermal energy equivalent to 186 kWh; (x) the RDF was used in a thermochemical reactor and 4148 kWh of thermal energy was produced. In addition, the MSW and RDF were analysed, and the elemental composition and combustion characteristics were determined. Based on these results, the protocol evaluated was found to be effective in the conversion of MSW to RDF, which can be used as a source of renewable fuel.

Torrefaction of Sewage Sludge: Kinetics and Fuel Properties of Biochars

We propose a ‘Waste to Carbon’ thermal transformation of sewage sludge (SS) via torrefaction to a valuable product (fuel) with a high content of carbon. One important, technological aspect to develop this concept is the determination of activation energy needed for torrefaction. Thus, this research aimed to evaluate the kinetics of SS torrefaction and determine the effects of process temperature on fuel properties of torrefied products (biochars). Torrefaction was performed using high ash content SS at six (200~300 °C) temperatures and 60 min residence (process) time. Mass loss during torrefaction ranged from 10~20%. The resulting activation energy for SS torrefaction was ~12.007 kJ·mol−1. Initial (unprocessed) SS higher heating value (HHV) was 13.5 MJ·kg−1. However, the increase of torrefaction temperature decreased HHV from 13.4 to 3.8 MJ·kg−1. Elemental analysis showed a significant decrease of the H/C ratio that occurred during torrefaction, while the O/C ratio fluctuated with much smaller differences. Although the activation energy was significantly lower compared with lignocellulosic materials, low-temperature SS torrefaction technology could be explored for further SS stabilization and utilization (e.g., dewatering and hygienization).

Waste to Carbon: Influence of Structural Modification on VOC Emission Kinetics from Stored Carbonized Refuse-Derived Fuel

The torrefaction of municipal solid waste is one of the solutions related to the Waste to Carbon concept, where high-quality fuel—carbonized refuse-derived fuel (CRDF)—is produced. An identified potential problem is the emission of volatile organic compounds (VOCs) during CRDF storage. Kinetic emission parameters have not yet been determined. It was also shown that CRDF can be pelletized for energy densification and reduced volume during storage and transportation. Thus, our working hypothesis was that structural modification (via pelletization) might mitigate VOC emissions and influence emission kinetics during CRDF storage. Two scenarios of CRDF structural modification on VOC emission kinetics were tested, (i) pelletization and (ii) pelletization with 10% binder addition and compared to ground (loose) CRDF (control). VOC emissions from simulated sealed CRDF storage were measured with headspace solid-phase microextraction and gas chromatography–mass spectrometry. It was found that total VOC emissions from stored CRDF follow the first-order kinetic model for both ground and pelletized material, while individual VOC emissions may deviate from this model. Pelletization significantly decreased (63%~86%) the maximum total VOC emission potential from stored CDRF. Research on improved sustainable CRDF storage is warranted. This could involve VOC emission mechanisms and environmental-risk management.

Quantification of VOC Emissions from Carbonized Refuse-Derived Fuel Using Solid-Phase Microextraction and Gas Chromatography-Mass Spectrometry

In this work, for the first time, the volatile organic compound (VOC) emissions from carbonized refuse-derived fuel (CRDF) were quantified on a laboratory scale. The analyzed CRDF was generated from the torrefaction of municipal waste. Headspace solid-phase microextraction (SPME) and gas chromatography-mass spectrometry (GC-MS) was used to identify 84 VOCs, including many that are toxic, e.g., derivatives of benzene or toluene. The highest emissions were measured for nonanal, octanal, and heptanal. The top 10 most emitted VOCs contributed to almost 65% of the total emissions. The VOC mixture emitted from torrefied CRDF differed from that emitted by other types of pyrolyzed biochars, produced from different types of feedstock, and under different pyrolysis conditions. SPME was a useful technology for surveying VOC emissions. Results provide an initial database of the types and relative quantities of VOCs emitted from CRDF. This data is needed for further development of CRDF technology and comprehensive assessment of environmental impact and practical storage, transport, and potential adoption of CRDF as means of energy and resource recovery from municipal waste.

Waste to Carbon: Densification of Torrefied Refuse-Derived Fuel

In this work, for the first time, the feasibility of obtaining carbonized refuse-derived fuel (CRDF) pelletization from municipal solid waste (MSW) was shown. Production of CRDF by torrefaction of MSW could be the future of recycling technology. The objective was to determine the applied pressure needed to produce CRDF pellets with compressive strength (CS) comparable to conventional biomass pellets. Also, the hypothesis that a binder (water glass (WG)) applied to CRDF as a coating can improve CS was tested. The pelletizing was based on the lab-scale production of CRDF pellets with pressure ranging from 8.5 MPa to 76.2 MPa. The resulting CS pellets increased from 0.06 MPa to 3.44 MPa with applied pelletizing pressure up to the threshold of 50.8 MPa, above which it did not significantly improve (p < 0.05). It was found that the addition of 10% WG to 50.8 MPa CRDF pellets or coating them with WG did not significantly improve the CS (p < 0.05). It was possible to produce durable pellets from CRDF. The CS was comparable to pine pellets. This research advances the concept of energy recovery from MSW, particularly by providing practical information on densification of CRDF originating from the torrefaction of the flammable fraction of MSW–refuse-derived fuel. Modification of CRDF through pelletization is proposed as preparation of lower volume fuel with projected lower costs of its storage and transportation and for a wider adoption of this technology.