Valorization of Pre- and Post-Consumer Textile Waste: A Review of Energy Recovery Potential

Waste refers to any substance or object that is discarded, considered worthless, defective, or of no further use following its primary purpose [7], or which the holder discards, intends to discard, or is required to discard [8]. For a long time, waste was seen merely as something to get rid of, piling up in landfills and harming the environment. But today, by the contribution of the principles of environmental science and the circular economy, these discarded materials are increasingly viewed as potential resources. When properly managed and redirected, waste can retain significant value, presenting an opportunity for reuse, recycling, or even transforming it into new materials or energy. Waste comes in various types, each with distinct characteristics and examples as presented in Figure 2. Municipal Solid Waste (MSW) includes everyday materials like food scraps, paper, plastics, and post-consumer textiles. Industrial Waste consists of by-products from manufacturing processes, such as fabric scraps and chemical waste. Hazardous Waste is particularly dangerous, including medical waste and toxic chemicals that require special handling. Sewage Sludge includes the residual material left after municipal wastewater treatment and textile effluent treatment plant sludge [9]. Biomass Waste includes organic materials like crop residues, wood, and food waste. Plastic Waste is primarily made up of packaging materials and synthetic textiles. Refuse-Derived Fuel (RDF) is created from processed plastics, paper, and textiles (both pre- and post-consumer waste). Lastly, Post-Consumer Textile Waste refers to discarded clothes and garments, while Pre-Consumer Textile Waste includes waste generated during textile production, such as cutting scraps and factory waste. These waste types not only contribute to pollution but also hold the potential to be managed in ways that can help reduce environmental harm while recovering valuable resources. [10]

Waste holds considerable untapped energy potential that can be unlocked through a variety of thermal, biological, and advanced conversion processes. The following outlines each major process in detail.

Incineration and co-combustion involve feeding textile sludge and textile waste into a high-temperature furnace operating at 750–1100 °C under a full oxygen supply for complete combustion, generating heat and electricity [14]. Figure 4 presents a schematic diagram of the incineration process, showing the flow of materials and energy through the system. Due to the low gross calorific value (~7.63 MJ/kg) and high ash content of textile sludge, co-combustion with textile waste (e.g., cotton, polyester, nylon, and blends) is commonly applied to enhance combustion stability, burnout performance, and overall energy efficiency [15].

The combustion process proceeds through distinct stages influenced by the composition of textile materials, as summarized in Table 3.

The gaseous emissions from co-combustion mainly consist of H₂O and CO₂, with smaller amounts of CO, SO₂, and NOₓ. Proper feedstock selection and blending can reduce harmful emissions such as SO₂. Thermogravimetric analysis further simplifies the combustion behavior into two broader thermal stages, presented in Table 4.

Incineration outputs include heat energy, combustible gases, and solid char. Heat energy can be used for steam generation, electricity production, and industrial heating. The gases (H₂O, CO₂, CO, and traces of CH₄) can be recovered for heat utilization or hydrogen production. The solid char is carbon-rich and microporous, and can be used as fuel, activated carbon, or an adsorbent for water and air purification [15]. The process offers key advantages such as converting non-recyclable textile sludge into energy, reducing landfill volume, handling diverse fiber types, and enabling recovery of valuable chemicals like caprolactam from nylon waste. However, challenges include high capital cost for furnaces and flue gas systems, need for pre-treatment, variability in gas and ash composition, strict emission controls for SO₂, NOₓ, and particulates, and additional environmental compliance requirements for ash and flue gas management [15].

Plasma gasification of textile sludge employs a DC non-transferred plasma torch operating under a controlled, limited-oxygen environment. The plasma arc generated by the torch reaches extremely high localized temperatures ranging from approximately 3,000 to 14,000 °C, enabling the rapid decomposition and gasification of organic materials [16]. Prior to treatment, the sludge is pre-dried to reduce moisture content and minimize energy losses and is pelletized with PVC binders to improve feedstock stability during processing. Although the plasma arc generates extremely high temperatures, the reactor operating temperature is typically maintained above 600 °C, which is sufficient to sustain efficient thermochemical gasification reactions. Therefore, the reported temperature range of 3,000–14,000 °C refers to the localized plasma arc temperature, whereas the value of above 600 °C represents the reactor operating temperature within the gasification chamber.

The outputs of plasma gasification include a gas fraction (H₂O, CO₂, CO, and minor hydrocarbons), a solid residue composed of char and minerals (e.g., calcite), recoverable thermal energy from exothermic reactions, and altered particles with reduced heavy metal content for safer disposal. Pre-drying improves energy efficiency, and the correlation coefficient of 0.929 indicates effective energy transfer during the process. The simplicity of the single-chamber reactor used in plasma gasification is a notable advantage over multi-stage gasifiers [16].

Table 5 presents the elemental composition of plasma-treated textile sludge obtained from SEM–EDX analysis, where the dominance of calcium and oxygen suggests the formation of calcite (CaCO₃), while magnesium and silicon indicate minor mineral phases.

Conventional gasification is a thermochemical conversion process in which carbon-based feedstocks such as biomass, coal, or municipal solid waste are partially oxidized at high temperatures (800–1200 °C) under limited oxygen supply to produce synthesis gas (syngas) [14], [17]. Figure 5 illustrates the schematic presentation of the gasification process, highlighting the key components involved in syngas production. The produced syngas mainly consists of hydrogen (H₂), carbon monoxide (CO), and methane (CH₄), which can be used for electricity generation, heat production, or as a feedstock for chemical synthesis (Table 6). Compared to incineration, gasification provides higher energy efficiency and lower emissions.

The environmental performance of waste-to-energy (WtE) technologies depends on conversion efficiency, emission characteristics, energy recovery potential, and the management of by-products. Each technology presents distinct environmental benefits and challenges that should be considered alongside its technical and economic performance.

Incineration is an established WtE technology capable of reducing waste volume by more than 80% while recovering thermal energy for electricity and steam generation However, the complete combustion of textile waste generates greenhouse gases (primarily CO₂), nitrogen oxides (NOₓ), sulfur oxides (SO₂), particulate matter, and trace organic pollutants. Therefore, modern incineration facilities require advanced flue gas cleaning systems to comply with increasingly stringent environmental regulations [15].

Pyrolysis generally exhibits a lower environmental footprint than direct combustion because the process occurs in an oxygen-deficient atmosphere, reducing direct atmospheric emissions. In addition to producing bio-oil and syngas as renewable energy carriers, pyrolysis generates biochar that can potentially be utilized for carbon sequestration, activated carbon production, or adsorption applications. Nevertheless, condensable organic compounds and process gases require appropriate treatment to prevent secondary environmental pollution [11], [13].

Conventional gasification provides relatively higher energy efficiency than incineration while producing cleaner synthesis gas that can be purified before utilization, thereby reducing overall pollutant emissions. Despite these advantages, tar formation, wastewater generated during gas cleaning, and solid residues remain important environmental challenges that require appropriate treatment and disposal [14], [17].

Plasma gasification demonstrates excellent environmental performance due to its extremely high operating temperatures, which effectively decompose hazardous organic compounds and immobilize heavy metals within a vitrified inert slag. Consequently, the technology significantly reduces hazardous residues requiring disposal. However, the process is highly electricity-intensive, and its overall environmental benefit depends on the carbon intensity of the electricity used to operate the plasma torches [11], [13].

Overall, WtE technologies can substantially reduce landfill disposal while recovering valuable energy from textile waste. Nevertheless, the sustainability of each technology depends on balancing energy recovery efficiency, emission control, resource recovery, and life-cycle environmental impacts. Appropriate feedstock selection, efficient pollution-control systems, and optimized operating conditions remain essential for minimizing environmental impacts while maximizing resource recovery [14], [17].

The Higher Heating Value (HHV), also known as the Gross Calorific Value (GCV), is a fundamental metric in thermodynamics and energy science that represents the total amount of heat energy released when a specified quantity of fuel undergoes complete combustion, with all products of combustion including water vapor cooled back to the initial pre-combustion temperature, allowing the latent heat of vaporization of water to be fully recovered. This stands in contrast to the Lower Heating Value (LHV), which excludes this latent heat by assuming that water vapor exits the system without condensing. A true representation of the actual energy content of a fuel is its HHV, as it accounts for the maximum potential thermal energy obtainable from complete combustion [18]. The calorific value, expressed in kcal/kg or kJ/kg, is experimentally determined using a bomb calorimeter (ASTM D 5865-85), in which the heat generated at a constant temperature of 25 °C from the combustion of a dry sample is measured [19]. Where direct measurement is not feasible, empirical models derived from proximate or ultimate analysis accounting for the mass fractions of carbon, hydrogen, oxygen, nitrogen, and sulfur are widely employed to estimate HHV with acceptable accuracy. Accurate HHV data is vital for energy policy, industrial processes, and cost-benefit analysis in energy-intensive sectors [18], making it an indispensable parameter across both conventional and renewable energy systems.

The Higher Heating Value (HHV) of textile waste is influenced by several physical and chemical characteristics of the feedstock. Among these, moisture content is one of the most important factors, as a significant portion of the released energy is consumed in evaporating water during combustion, thereby reducing the effective energy available. Similarly, high ash content lowers HHV because ash represents the non-combustible inorganic fraction of the material and does not contribute to heat generation. Fiber composition also plays a crucial role, with synthetic fibers such as polyester and polypropylene generally exhibiting higher HHV than natural fibers due to their greater carbon and hydrogen contents. In addition, the presence of dyes, chemical finishes, contaminants, and blended fiber structures may alter combustion behavior and influence the overall calorific value of textile waste. Therefore, accurate characterization of these properties is essential for selecting appropriate waste-to-energy technologies and maximizing energy recovery efficiency.

In the context of Waste-to-Energy (WtE) processes, the significance of HHV becomes especially critical. The heating values of Municipal Solid Waste (MSW) materials play an essential role in the design and operation of energy conversion systems it is vital to possess accurate and valid heating value data for designing, operating, and maintaining a WtE incinerator, as these properties are essential for selecting the appropriate technology, determining the capacity of furnaces and boilers, and designing auxiliary facilities such as flue gas cleanup systems [20]. Beyond design considerations, the HHV directly governs whether combustion within an incinerator can remain self-sustaining: if the combustion process is aimed at waste disposal through incineration, the fuel's heating value must be high enough to sustain combustion without auxiliary fuel input, since the heating value is a strong function of the moisture and ash content of the waste, meaning a maximum tolerable amount of these constituents exists for any given system [18]. Shifting the focus to textile waste specifically, the role of HHV becomes particularly consequential. Textile waste exhibits a higher calorific value than many other waste streams during combustion, and the methane emitted from buried textile can be converted into renewable energy [21], making it a thermally valuable component within mixed MSW streams. Table 8 provides the approximate HHV values for various textile waste types, highlighting their energy potential and suitability for different waste-to-energy processes [22]-[28]. Energy recovery from textile waste via thermochemical processes provides a sustainable management pathway, given the high cellulosic composition of textile waste and its relatively high calorific value [6]. This energy potential, however, is not uniform across all textile types and varies considerably depending on fiber composition, synthetic fibers, owing to their petrochemical origins and elevated carbon content, generally possess a higher calorific value than natural fibers. The high calorific value of polyester wastes makes it an attractive energy feedstock for incineration plants and other combustion processes [18]. These fiber-dependent differences in HHV mean that the energy recovery potential of a textile waste stream is directly tied to its material composition, and accurate characterization of this value is therefore a prerequisite for any informed WtE planning.

Prior to any thermochemical conversion process, elemental analysis and HHV are critical analyses that must be carried out on textile waste, as these serve as indicators of whether a high-quality fuel can be created from the feedstock [6]. Furthermore, the HHV of textile waste is not a fixed property it can be significantly altered by the physical and chemical state of the waste. Moisture content, the presence of dyes and chemical finishes, and blending of fiber types all act to suppress or elevate the effective heating value of a textile feedstock. Traditional disposal methods for textile waste, including incineration and landfilling, pose serious environmental risks due to the release of CO₂, volatile organic compounds, and microplastics [18], underscoring the importance of understanding the HHV not only as an energy metric but also as a parameter that governs the environmental profile of combustion. In this way, the HHV of textile waste functions as a foundational input that determines the feasibility, efficiency, and environmental consequences of integrating textile streams into broader WtE operations.

HHV serves as a key benchmark for selecting waste-to-energy (WtE) pathways by identifying energy-rich and biodegradable wastes. Table 9 summarizes the relationship between different textile waste types and their suitable WtE processes, highlighting how HHV and material characteristics influence process selection. For thermochemical processes like incineration, pyrolysis, and gasification, a higher heating value (HHV) of at least 6–7 MJ/kg is necessary for self-combustion, and ideally over 10–12 MJ/kg for optimal performance. Materials with high HHV, such as polypropylene (PP) and polyethylene (PE) (35 - 48 MJ/kg), are perfect for producing syngas (H₂, CO). On the other hand, moderate-HHV materials like cotton and paper (15–19 MJ/kg) might require blending to work effectively [9], [13]. Pyrolysis can generate char with an HHV of 28–33 MJ/kg and, when heated to 900 °C, produces gas with a hydrogen to carbon monoxide ratio of about 2:1. Gasification, however, operates at higher temperatures, between 800 and 1200 °C [12], [16]. Wastes with very low HHV (less than 10 MJ/kg), such as metals and composites (around 0 MJ/kg), are typically not suitable for energy recovery and are instead sent to landfills [9].

Additionally, some processes, like photobiological conversion, don't rely directly on HHV. Instead, they make use of intermediate products that come from earlier treatment stages, rather than tapping into the original waste's inherent energy content [16].