Upgrading of refuse derived fuel through torrefaction and carbonization: Evaluation of RDF char fuel properties

Highlights

• Torrefaction and carbonization of industrial RDF was studied at different conditions.

• Carbonization yielded the RDF chars with higher carbon content and heating value.

• Chlorine and ash contents of the RDF chars could be reduced by water leaching.

• Carbonization presented energy efficiencies compatible with autothermal operation.

Abstract

Refuse derived fuel (RDF) is produced in large amounts but its heterogeneous nature and low calorific value reduce the potential for energetic valorization of this material.

In this work, the torrefaction and carbonization of RDF was studied in a temperature range of 200–400 °C and residence times from 15 to 60 min, yielding chars with increased density, high heating value (HHV), carbon content, ash content and fixed carbon. Leaching experiments showed that some inorganic components like calcium or chlorine could be removed from the RDF char, upgrading its fuel quality. For temperatures in the torrefaction range (200 °C and 250 °C), energy yields were higher than 96.3% and process energy efficiencies were higher than 85%, but the HHV of the RDF chars were lower than 19.4 MJ kg⁻¹. In the carbonization range (300 °C to 400 °C) it was possible to obtain RDF chars with HHV in the range of 20.1–26.2 MJ kg⁻¹, with energy yields from 84.5 to 91.7% and process energy efficiencies from 70.8 to 79.2%. The obtained results showed that thermochemical processing can be used to upgrade RDF thus promoting sustainable solutions for its management and valorization.

Introduction

Global increase in energy demand, excessive waste production and resource depletion are three of the main problems that our society currently faces. As the world transitions to more sustainable waste treatment strategies, valorization of solid wastes as fuels becomes an increasingly important topic.

Sustainable resource management demands the reduction of landfill deposition and the enhancement of waste reuse, recycle and energetic valorization. The most widespread methods for energy recovery from waste materials are refuse incineration or raw waste processing to obtain a refuse derived fuel (RDF) adequate for combustion or gasification [1]. RDF is a solid fuel with a more uniform particle size distribution and higher energy density than the untreated solid wastes, such as municipal solid wastes (MSW), regular industrial wastes (RIW) or construction and demolition wastes (CDW) [2,3].

Due to its origin, RDF contains a large diversity of components, from cardboard to textiles and non-recyclable plastics, as well as several unidentifiable materials [4]. This heterogeneity can negatively influence the properties of RDF, decreasing density and calorific value and increasing moisture, ash and chlorine contents, thus limiting their application in thermochemical conversion processes [5]. Fuels with high chlorine and ash contents are known to cause slagging and fouling phenomena, as well as boiler corrosion [6]. High chlorine contents are also associated with problematic emissions such as HCl and PCDD/Fs [7].

Torrefaction or carbonization are thermal conversion technologies that may enhance some of the RDF fuel characteristics, by modifying its composition and heating value. In these processes, organic matter is heated in a non-oxidizing atmosphere, usually at atmospheric pressure, in temperature ranges from 200 °C to 300 °C for torrefaction and from 300 °C to 500 °C for carbonization [8,9]. Both processes include endothermic pyrolytic reactions, such as dehydrogenation, condensation or hydrogen transfer, that extensively modify the raw materials to yield an upgraded solid fuel and gaseous products [10]. The solid product (char or biochar) contains most of the carbonaceous and mineral components of the raw materials, while the gaseous products are mainly composed of water, CO₂, CO and several organic compounds, formed and released during the process [[10], [11], [12]]. Generally, torrefaction biochar presents around 70 wt% of the original mass and can retain up to 90% of the initial energy content from the feedstock, properties that account for their higher energy density [13].

The efficiency of these processes and the characteristics of their final products are affected by several parameters, such as temperature, residence time, heating rate, atmosphere composition, and reactor configuration [14]. In particular, temperature and residence time strongly impact feedstock decomposition and reorganization of its physical structure. According to Prins et al. [15,16], temperature is crucial for the kinetics of the torrefaction reaction, whereas residence time is more important for process characteristics, albeit only for certain temperature ranges. Overall, the characteristics of the final products are more affected by temperature than by residence time. From 200 °C to 300 °C, the water bonded to the raw material is volatilized and polar groups such as hydroxyl and carbonyl groups are eliminated from its structure through dehydration and decarbonylation reactions. In biomass feedstocks, hemicelluloses are extensively volatilized between 250 °C and 260 °C, while cellulose and lignin decompose at temperatures above 300 °C [13,17,18].

For the particular case of RDF torrefaction, the plastic component also has to be considered. Polymers such as polyethylene terephthalate (PET), polypropylene (PP), polystyrene (PS) or polyvinyl chloride (PVC) are some of the most commonly found in RDF [19,20]. When considering blends of plastics and biomass, there are synergistic effects to be taken into account [21,22]. Plastic material pyrolytic decomposition occurs at different temperature ranges depending on the type of polymer, for instance PVC decomposes between 250 °C and 320 °C [23], whereas PET, PP and PS begin their decomposition above 350 °C [24].

Torrefaction has been largely applied to lignocellulosic materials yielding homogeneous biochars with lower moisture content, higher calorific values, lower O/C ratios, higher hydrophobicity and enhanced grindability when compared with the original biomass [9,[25], [26], [27]]. These improved physical-chemical properties make the produced char more appropriate for energy conversion processes. On the other hand, thermal upgrading processes may increase ash concentration in final products, decreasing their fuel quality and calorific value [28]. Nonetheless, RDF chars with higher ash contents still present potential to be used as low-cost adsorbents, activated carbon precursors, or soil ameliorants, depending on the char surface electrochemistry and ion exchange properties [29]. Finally, even when the obtained RDF chars are not adequate for energetic or material valorization, torrefaction will promote an increase in density and homogeneity that is advantageous for the landfill solution, reducing the land use impact and associated emissions [30]. Waste landfilling has into consideration the evaluation of the wastes in function of their organic load and of certain limit-values like metal leachability, which have to be in compliance with the values established by the EU landfill regulation [31]. Hwang et al. [32] characterized various chars produced from different solid wastes in order to assess their fuel recovery and their production as a pretreatment before landfilling. The authors describe that carbonization was responsible for removing a considerable amount of organic matter from the raw waste samples, while suppressing the leaching of certain heavy metals, such as chromium or lead.

Throughout the years, few research groups have investigated, specifically, torrefaction or carbonization of refuse derived fuels, but some examples can be found in the literature namely, evaluation of carbonization as a pre-treatment before landfilling [30], characterization of the waste derived chars [32], determination of kinetic parameters of RDF torrefaction [11], use of carbonized RDF for soil amendment [33], valorization of waste derived char as precursor for activated carbon production [[34], [35], [36]], determination of the effect of torrefaction temperature on the properties of RDF chars [37], evaluation of toxic emissions (heavy metals and organic pollutants) resulting from RDF torrefaction [5] or other applications [38].

The main goal of this work was to test torrefaction and carbonization of RDF produced from non-hazardous industrial wastes, at different temperatures and residence times, in order to define which conditions yield RDF chars with enhanced fuel properties. The energy yield and energy efficiency of the torrefaction and carbonization processes were also evaluated in order to determine their sustainability as RDF pre-treatment technologies.