Reduction of energy consumption and enhancement of hydrogen yield from E-waste via plasma gasification process with waste heat recovery system

Abstract

Electronic waste (e-waste) poses environmental issues and risks to health, but it holds polycarbonate, epoxy, and metal oxide, which have the potential for hydrogen production via the gasification process. This research involves the proper recycling of e-waste to an effective feedstock for hydrogen production via a plasma gasification process featuring a waste heat recovery system. During the plasma gasification process, the operating temperature is varied from 1500 to 3000 °C with an interval of 500 °C without a catalyst. The optimum plasma gasification temperature involves different concentrations of calcium oxide (CaO) catalyst for enriching the hydrogen yield. Effects of plasma gasification processing with a waste heat recovery system on energy consumption reduction, syngas yield, reactor stability, and carbon conversion efficiency are studied. The system featuring higher plasma gasification temperature (3000 °C) found reduced energy consumption (48.9%) and enhanced hydrogen yield of 66.7 mol/kg while reducing CH₄ and CO₂ emissions to 9.8 mol/kg and 10.5 mol/kg, respectively. The incorporation of a heat recovery system improved energy utilization efficiency to 71.5% and increased CCE (carbon conversion efficiency) from 69.8% to 91.6%. Additionally, CaO catalyst addition (15 wt%) further optimized gasification performance, leading to a hydrogen yield of 72.6 mol/kg while reducing CH₄ and CO₂ emissions to 8.1 mol/kg and 7.8 mol/kg, respectively. These findings highlight plasma gasification as a viable and sustainable technology for hydrogen production from e-waste, offering significant environmental and energy efficiency benefits.

Introduction

Electronic waste (e-waste) is a growing environmental concern, but it also holds potential as a valuable feedstock for hydrogen production¹. Due to its high content of metals and plastics, e-waste can be effectively processed using thermochemical/electrochemical techniques to recover syngas components, with hydrogen being a key target². The e-waste feedstock was processed through pyrolysis and gasification of plastic components to generate syngas, which can be further refined for hydrogen³. Moreover, metal-based e-waste, such as circuit boards, can aid in catalytic processes to enhance hydrogen yield⁴. Furthermore, using e-waste for hydrogen production reduces landfill burden and supports sustainable energy transitions with minimal environmental pollution. This approach aligns with circular economy principles by converting waste into clean fuel⁵. The challenges include material variability, processing complexity, and emissions control. Advancing efficient conversion technologies can make e-waste a viable source of green hydrogen⁶. Compared to other processing methods, plasma gasification is an advanced waste-to-energy technology that converts e-waste into syngas using plasma torches operating at extremely high temperatures (3000–10000 °C)⁷. The intense heat breaks down organic materials into hydrogen-rich syngas while melting inorganic residues into vitrified slag⁸. This process minimizes hazardous emissions, reduces landfill waste, and enables hydrogen recovery for clean energy⁹. The produced syngas can be purified and used for electricity generation or hydrogen extraction. Despite its efficiency, plasma gasification requires high energy input and advanced infrastructure for economic feasibility¹⁰. Hence, plasma gasification produces less waste than conventional gasification by completely breaking down e-waste and converting inorganic residues into inert slag¹¹. It is more efficient, generating high-quality syngas with minimal emissions.

The plasma gasification method has a 21.22% higher hydrogen production capacity, but the combined plasma gasification with a synthetic chemical looping process offers better exergy efficiency and financial returns¹². A novel plasma gasification process with ionic liquid-based CO₂ capture is proposed for hydrogen production from medical waste, and the surgical masks produce twice as much hydrogen as others, with steam as the best gasification agent¹³. The process achieves a high exergy efficiency of about 78.25% and provides a sustainable solution for medical waste treatment. Erdogan et al.¹⁴ investigated the plasma gasification process of medical waste, with an updraft plasma gasifier used. The effect of equivalence ratios on syngas production was evaluated. The 0.1 equivalence ratios showed the maximum hydrogen production with increased carbon conversion efficiency. Moreover, the waste plastic was involved in the gasification process for hydrogen production. While increasing the waste exergy ratios leads to superior hydrogen production with higher sustainability behaviour¹⁵. Moreover, the catalyst featured gasification process recorded high gasification efficiency with increased hydrogen and decreased carbon formation¹⁶. Moreover, the economic analysis showed that plasma gasification has the highest hydrogen production cost. Similarly, the plasma gasification using algae for hydrogen production from enteromorpha feedstock shows the highest efficiency of about 74.46% exergy¹⁷. The switch waste was used as a feedstock for hydrogen production through the carbon dioxide plasma gasification process under high torch power.

It result showed that the higher gasification troch lead to a higher hydrogen syngas, and reduced carbon monoxide¹⁸. However, the selection of the optimum gasification process and feedstock provided a better syngas formation and carbon conversion efficiency^19,20. Su et al. investigated the hydrogen from the food waste via a supercritical water gasification system, and a 450 °C gasification temperature non-catalyst system was found to have a marginal enhancement in hydrogen syngas²¹. Moreover, a minimum of three trials were repeated to find a statistical significance of the hydrogen yield from the waste feedstock, and the average was computed and considered not more than 5%^22,23. Venkatesh et al.²⁴ studied the greywater to hydrogen, in which biomass concentration was activated by using a cerium oxide, which was used as a feedstock for hydrogen production via a gasification route associated with a potassium hydroxide catalyst. The cerium oxide activated the biomass growth, and potential for quality feedstock for increased hydrogen production. Moreover, the higher gasification temperature (500 °C) and pressure (25 bar) with potassium hydroxide catalyst resulted in a higher hydrogen yield and gasification efficiency. Moreover, an increased gasification temperature with CaO catalyst leads to a higher hydrogen yield, reduced Tar formation, and a higher carbon conversion efficiency^25,26. However, the catalyst featured gasification process showed a better gasification performance, improved hydrogen gas, reduced tar formation, and a higher carbon conversion efficiency, which was better than the conventional gasification process without a catalyst^27,28. Dang et al.²⁹ experimentally studied the hydrogen production from sorption via a nickel-calcium oxide catalyst featuring steam reforming, and the results showed a higher hydrogen yield and better catalyst stability to 10 repeated cycles.

Furthermore, the nickel and calcium oxide catalyst featured gasification system found to have an improved syngas yield, and reduce the tar formation, which result an improved gasification efficiency^30–32. An optimized catalyst wt% and its best combinations of nickel and calcium oxide provided a rich hydrogen and reduced carbon formation³³. The waste grocery bags were involved in the catalyst conversion associated with thermal cracking with varied process parameters, and the optimum process parameters were identified via response surface methodology. The operating temperature, heat rate, and time to process were the dominant factors for fuel oil production³⁴. Furthermore, the higher frequency of microwave plasma gasification processed with 18 kg/h polypropylene waste feedstock resulted in increased syngas formation and reduced ash formation³⁵, which was economy³⁶ and sustainable pathway for waste management³⁷.

According to the summary Table 1 and past literature, the waste-to-energy technique is a sustainable, economic, and potential for hydrogen energy production from the gasification process, exhibiting better syngas formation and variation in carbon conversion due to breaking down organic materials and reducing inorganic residues to inert slag. Compared to conventional gasification, plasma gasification produces cleaner syngas with minimal emissions and less waste. It also enhances hydrogen production capacity, but integrating it with synthetic chemical looping improves efficiency and economic returns. Furthermore, the waste to hydrogen energy via plasma gasification technique was found to have a better syngas gas yield, and excessive heat (thermal loss) limits the overall hydrogen yield and carbon conversion efficiency. The objectives of the study are to address the above research gap and extract the hydrogen from e-waste via a plasma gasification process with varied gasification temperatures of 1500 °C, 2000 °C, 2500 °C, and 3000 °C without a catalyst. The optimum performance gasification temperature involves the catalyst process with the support of 5, 10, and 15 wt% of calcium oxide (CaO). To enhance the overall performance and reduce energy consumption, the waste heat recovery system is adopted. The effect of gasification temperature without and with catalyst actions on energy reduction, syngas, reactor stability, and carbon conversion efficiency behaviour is studied, and its optimum results are compared with past literature to highlight the present research novelty. The novel study focuses on the effective utilization of electronic waste (e-waste) for hydrogen production via a plasma gasification process, and a waste heat recovery setup was adopted to enhance the overall performance. This is achieved by enhancing carbon conversion efficiency through a plasma gasification process combined with a heat recovery system.

Table 1.

Summary of the past research related to the present study.

Details of the study	Advantage and drawbacks	Reference
E-waste to hydrogen energy via gasification process	It found a marginal enhancement in hydrogen, variation in carbon conversion, which was used as an alternative fuel for an internal combustion engine	2
E-Waste-to-energy production via plasma gasification process	It led to an enhanced hydrogen yield and higher gasification efficiency. However, increased thermal loss limits the overall performance	9
Medical waste to hydrogen energy via plasma gasification process	An increased microwave heating of plasma gasification showed an improved hydrogen yield, and increased equivalence ratios limit the hydrogen yield.	14
Algae Biomass to hydrogen energy via plasma gasification	This technique was found to be economic and to have a superior exergy efficiency. However, it found a higher acid level, which leads to variation in energy production	17
Reviewed the various techniques for waste-to-energy	Economic and plasma gasification techniques provide a higher energy conversion. But, without a heat recovery system limits the overall gasification process and limits the gasification efficiency	20

Materials and methods

Feedstock

Figure 1 presents the flow process layout for e-waste to feedstock preparation. E-waste feedstock contains various metals, including copper, aluminium, silver, and gold and plastics like polyvinyl chloride, polyethylene, and polycarbonates, and electronic components, which are collected from academic computer and electrical laboratories’ waste at M/S. Saveetha University, Chennai, Tamil Nadu. It contained printed circuit boards, wires, connectors, capacitors, resistors, and plastic casings.

Fig. 1.

Procedure for e-waste preparation.

The total amount of e-waste collected was about 15 kg. The metal-to-plastic ratio is followed by 1:1. The e-waste was used for hydrogen production through plasma gasification due to its high carbon content, which makes it a suitable feedstock for syngas generation¹⁸. Table 2 shows the properties of e-waste with impact on H₂ generation¹⁹.

Table 2.

Details the content impacts on hydrogen production¹⁹.

Property	Values	Impact on Hydrogen Production
Carbon (%)	27.8	Provides a carbon source for syngas formation (CO + H₂).
Hydrogen (%)	7.9	Directly contributes to hydrogen-rich syngas.
Oxygen (%)	-	Promotes partial oxidation/ enhances the gas
Sulfur (%)	-	Trace impurity and influences gas cleaning
Nitrogen (%)	-	It contributes to NOx formation at higher temperature processing
Volatile Matter	65.3	Increases gas yield, enhancing hydrogen production.
Fixed Carbon	15.7	Affects char formation and influences syngas quality.
Ash content	18.2	Affects the reactive portion available for gasification
Higher heating value (HHV in MJ/kg)	21.4	Potential for higher hydrogen production
Moisture content	-	Required for drying

Initially, the e-waste was washed with demineralized water to remove surface contaminants such as dust, dirt, and soluble impurities. After the washing process, the cleaned e-waste was stored in the hopper to ensure a consistent feedstock supply for further processing. It was then transferred to the shredder (dual-shaft), where it was broken down into fine particles to enhance uniformity by a Double Shaft Electronic Waste Shredder with a capacity of 400 kg/h. The shredding process increased the surface area of the e-waste, facilitating better thermal decomposition and syngas generation during plasma gasification. After the shredding and milling process was done with a planetary ball mill, the e-waste feedstock was reduced to an average particle size of approximately 50 nm, as confirmed via Scanning Electron Microscope (SEM) and used in the plasma gasification reactor. It helps to increases in surface area, and support increases in mass and heat transfer, which leads to accelerating thermal decomposition and kinetic reaction during the plasma gasification.

Catalyst preparations

The calcium oxide (CaO) catalyst was prepared and integrated with e-waste feedstock to enhance the gasification efficiency. Initially, the calcium carbonate (CaCO₃) is selected as the precursor and subjected to calcination at a temperature of 900 °C with a heating rate of 10 °C/min for 3 h in a static air or nitrogen atmosphere, resulting in the decomposition of CaCO₃ into highly reactive CaO. To further enhance catalytic activity, metal impregnation is performed using transition metal nitrates such as nickel nitrate (Ni(NO₃)₂·6 H₂O), ferric nitrate (Fe(NO₃)₃·9 H₂O), with metal loadings of 5–10 wt%. The impregnation is carried out in deionized water/ ethanol, followed by continuous stirring using a magnetic stirrer (IKA C-MAG HS hotplate stirrers) for 12 h to ensure uniform dispersion. The mixture is then dried at 110 °C for 12 h and subjected to a second calcination at 850 °C for 3 h at a heating rate of 10 °C/min to activate the catalyst and improve its surface area and porosity. According to the earlier literature [29,33], the above-mentioned operating parameters were fixed, which meets the thermo-chemical requirement, and catalyst activity. The final catalyst was mixed with the different volume fractions (5, 10, and 15 wt%) with the e-waste feedstock. While more than 15 wt% of CaO catalyst found saturation of active sites, reduced interaction between the reactant gases and plasma, and reduced surface area leads to minimizing the overall hydrogen yield [31,33].

Plasma gasification

The actual setup of the thermal plasma gasification systems is shown in Fig. 2. The plasma thermal gasification setup was installed, and its experiments were conducted at M/S. Saveetha University, Chennai, and Tamil Nadu. The process was composed of two sub-systems: a thermal plasma gasification system, which converts the e-waste into syngas, and a waste heat recovery system. The thermal plasma gasification system was configured with a multi-layer refractory system combining by high purity of alumina and zirconia layer (hot face) and a mullite layer acting as an insulation layer, combined with a ceramic-fiber backing plate featuring a steel shell, which ensures a higher thermal stability and limits the corrosion against the slag-induced. During the operation, the gasification temperature varies from 1500 to 3000 °C with an interval of 500 °C, controlled feed rate of 15 kg/h, significantly below the maximum capacity (300 kg/day), to ensure optimal residence time and efficient gasification.

Fig. 2.

Actual setup of the plasma gasification.

It comprised several key components, including a waste feeding system, a thermal plasma furnace with a non-transferred plasma torch, a heat recovery system, and a temperature and pressure control unit. Here, the non-transferred DC arc plasma torch was used with a maximum power rate of 10 kW. The electrode featured a copper anode (water-cooled), a tungsten cathode, and a closed-loop water cooling system. A pressurized ram-type feeder was used to introduce waste into the plasma furnace, and its torch was mounted at 20º. This angled configuration generated cyclonic gas flows, enhancing gas mixing and minimizing fly ash emissions. Hence, the gasifier had an operational voltage of 20 ± 0.5 V and a current of 15 A. The plasma torch maintained an energy efficiency of 70%. A two-stage PSA reactor with a zeolite molecular sieve and activated carbon sites generated 93% pure O₂ (12 N m³/h at 506.5 kPa) for injection into the plasma torch. During the process, the equivalence ratio is followed by 0.2, and the steam injection rate of 1 kg/h per kg of e-waste, which supports an optimal balancing between the steam reforming and partial oxidation. An inert gas of CO₂ was used for bag filter pulsing to eliminate oxygen presence, reducing the risk of syngas ignition. This prevented accidental combustion by displacing air and maintaining a non-reactive environment. Hence, the gasification was followed by the temperature of about 1500 °C, 2000 °C, 2500 °C, and 3000 °C. During the process, the Optris made CSvision R1M optical pyrometer (range from 500 to 3500 °C with spectral range of 0.8 to 1.1 μm and ± 0.3% accuracy) is used to measure the temperature, which has high temperature stability (3500 °C). Totally, 3 pyrometers are positioned at the waste feed inlet (measure the preheat temperature), plasma flame interaction (monitor the gasification temperature), and gas outlet (syngas exit zone).

Performance measures

The energy consumption reduction, recovered heat, and preheat temperature were calculated by Eqs. (1), (2), and (3) [12,25].

1

2

3

Similarly, the gas yield and carbon conversion efficiency (CCE) are calculated by Eqs. (4) and (5) [21]. The Eq. 

4

5

By using Eq. 6 [15], the syngas energy was measured.

6

The overall error of the gasification reactor and measured quantities was estimated by Eq. (6) [22,28].

7

Where u and were the standard uncertainty and accuracy of the measurements, Table 3 shows the uncertainty values of the calculated values.

Table 3.

Uncertainty of the measured quantities.

Parameters	Unit	Uncertainty values
Gas yield	%	2.9
CCE	%	3.1
Energy reduction	%	2.6

Results and discussion

Effect of plasma gasification temperature on gas yield/carbon conversion efficiency/ and energy consumption reduction

Figure 3 shows the effect of gasification temperature on gas yield by using e-waste. As the plasma gasification temperature increases from 1500 °C to 3000 °C, significant changes occur in the gas yield values due to enhanced reaction kinetics, increased energy input, and improved thermal decomposition of complex hydrocarbons.

Fig. 3.

Effect of plasma gasification temperature on gas yield.

At higher temperatures (3000 °C), the cracking of heavy hydrocarbons and tars becomes more efficient, leading to a reduction in CH₄ yield from 21.5 mol/kg to 9.8 mol/kg. This occurs because CH₄, which forms at lower temperatures due to incomplete cracking, undergoes further decomposition into smaller molecules, reducing its presence in the syngas [23,29]. Additionally, the reforming reaction intensifies at elevated temperatures, further consuming methane and increasing H₂ production, represented by the reaction (8, 9) [16].

8

9

Similarly, CO₂ yield decreases from 16.9 mol/kg to 10.5 mol/kg due to the enhanced Boudouard reaction represented in reaction (Eq. 8), which converts CO₂ into CO. However, despite the initial increase in CO production, the reaction is consumed through the water-gas shift reaction, converting CO to additional hydrogen, resulting in an overall reduction of CO yield from 5.2 mol/kg to 2.1 mol/kg. The water-gas shift reaction is mentioned in Eq. 10. The combination of these reactions explains why both CO and CO₂ concentrations drop as the temperature rises [18].

10

The most significant effect of increasing the plasma temperature is the drastic rise in hydrogen yield, which increases from 21.3 mol/kg at 1500 °C to 66.7 mol/kg at 3000 °C. This is due to the complete thermal decomposition of hydrocarbons and intensified hydrogen-producing reactions such as methane reforming, enhanced kinetics, complete thermal cracking, and direct hydrogen release from carbonaceous e-waste feedstock. Moreover, the higher temperatures ensure nearly complete molecular breakdown, maximizing hydrogen production while minimizing unwanted hydrocarbon and CO₂ emissions [24]. This trend highlights the advantage of operating at higher plasma temperatures, as it promotes cleaner syngas with higher hydrogen content, making the process more efficient in syngas production through e-waste feedstock.

Based on Eqs. 1 and 2, the energy reduction %, heat recovery, and preheat temperature are measured, and their details with mass conversion efficiency are presented in Table 4.

Table 4.

Mass conversion efficiency, Energy reduction, heat recovery and preheating temperature of the current investigation.

Plasma temperature in ⁰C	Feedstock rate in kg/h	Mass conversion efficiency in %	Energy reduction in %	Recovered heat in kJ/h	Preheating temperature in ⁰C
1500	15	60	45.1	16,236	902
2000	15	69.82	53.4	19,224	1068
2500	15	78.58	60.3	21,708	1206
3000	15	83.39	71.5	25,740	1430

According to Table 4, the increase in plasma gasification temperature from 1500 °C to 3000 °C leads to a significant enhancement in energy recovery from 16,236 kJ/h to 25,740 kJ/h due to the availability of sensible heat in the product gas. The recovered heat energy can be effectively used for preheating a feedstock, and improves its preheat temperature from 902 °C to 1430 °C. Moreover, the addition of heat energy can reduce the demand for external energy. It leads to an increase in overall energy reduction (45.1%-71.5%) and enhances the mass conversion efficiency from 60% to 83.39%.

Figure 4 shows that the carbon conversion efficiency (CCE) and energy utilization reduction vary with the plasma gasification. At a lower operating temperature of 1500 °C, the CCE is around 69.8%, while the energy utilization reduction is about 45.1%.

Fig. 4.

CCE and energy consumption reduction Vs gasification temperature.

This lower efficiency occurs due to incomplete carbon syngas conversion, which results in unreacted carbon residues and increased energy consumption to sustain the reaction by e-waste feedstock. As the temperature increases to 3000 °C, the CCE rises significantly to 91.6%, and the energy utilization reduction improves to 71.5%. This improvement is due to the integration of a heat recovery system, which captures and recycles excess thermal energy from the gasification process. In plasma gasification without heat recovery, much of the thermal energy is lost as waste heat, reducing overall gasification efficiency. The heat recovery system enhances energy efficiency by utilizing recovered heat for preheating feedstock, driving endothermic gasification reactions, and reducing external energy input. This leads to a lower net energy demand and ensures that more carbon is efficiently converted into syngas components, particularly H₂. Additionally, the higher temperatures (3000 °C) supported by heat recovery accelerate key reactions such as methane reforming and the water-gas shift reaction, leading to higher hydrogen yields and lower CO₂ emissions [25].

Moreover, by reducing energy consumption, the heat recovery system significantly lowers operating costs and enhances the sustainability of the plasma gasification process using e-waste feedstock. It also helps in stabilizing temperature fluctuations, ensuring a more controlled and optimized reaction environment. Hence, the integration of a heat recovery system improves gasification process efficiency, increases hydrogen yield, and reduces overall energy consumption, making plasma gasification more economically and environmentally viable.

Effect of CaO catalyst featured plasma gasification temperature on gas yield/carbon conversion efficiency/ and energy consumption reduction

From the effect of gasification temperature analysis, the maximum temperature (3000 °C) shows a better result compared to the lower temperature. Hence, the effect of the catalyst on the gasification process was studied at a higher temperature (3000 °C). Figure 5 depicts the effect of the CaO catalyst on gas yield. The gas yield composition of CH₄, CO₂, H₂, and CO is significantly influenced by the addition of CaO as a catalyst in plasma gasification. Without a catalyst, the gasification process produces 9.8 mol/kg of CH₄, 10.5 mol/kg of CO₂, 66.7 mol/kg of H₂, and 2.1 mol/kg of CO. However, as CaO concentration increases from 5 wt% to 15 wt%, there is a clear shift towards higher H₂ production while CH₄, CO₂, and CO yields decrease [26].

Fig. 5.

Effect of CaO catalysts on gas yield at 3000 °C.

The increases in CH₄ yield directly result from CaO’s catalytic activity, which enhances CH₄ reforming reactions. At 5 wt% CaO, CH₄ increases to 11.6 mol/kg, and at 15 wt% CaO, it further reduces to 8.1 mol/kg. This occurs because CaO promotes the cracking of CH₄ into H₂ and CO, which are then involved in secondary reactions. The lower methane content indicates improved gasification efficiency and a higher degree of carbon conversion. Similarly, CaO reduces the CO₂ yield from 15.6 mol/kg to 7.8 mol/kg due to the gas shift reaction and promotes its conversion into hydrogen and carbon monoxide. This reaction helps to remove CO₂ from the gas stream, making the syngas composition cleaner and reducing greenhouse gas emissions [27]. At higher CaO concentrations, more CO₂ is captured, leading to improved carbon utilization efficiency.

The most significant effect of CaO is the substantial increase in H₂ yield, which rises from 60.5 mol/kg without a catalyst to 72.6 mol/kg at 15 wt% CaO. This occurs because CaO enhances hydrogen-producing reactions such as steam reforming and the water-gas shift reaction (CO + H₂O → CO₂ + H₂). These reactions consume CO and produce additional H₂, making syngas more hydrogen-rich and suitable for clean energy applications. Additionally, CO yield decreases from 7.6 mol/kg to 1.7 mol/kg as CO is consumed in hydrogen-generating reactions. The presence of CaO ensures that more CO participates in the reaction, producing extra hydrogen while reducing CO levels [28]. This improves the overall hydrogen-to-carbon monoxide ratio, which is beneficial for fuel cell applications and hydrogen separation processes.

Hence, the addition of CaO significantly enhances syngas quality by increasing hydrogen yield while reducing CH₄ and CO₂ emissions. Moreover, the catalytic effect of CaO with the e-waste feedstock optimizes reaction kinetics, improves carbon conversion efficiency, and lowers unwanted hydrocarbon and CO₂ formation. This makes CaO a highly effective catalyst for plasma gasification, particularly in applications where hydrogen-rich syngas is desirable for energy production. Moreover, the CaO influences the capture of carbon dioxide and shifts the equilibrium towards more hydrogen production. The 15 wt% CaO catalyst featured a plasma gasification system with 3000 °C is found a maximum hydrogen yield of 72.6 mol/kg. It is 102% better than the CeO₂-Ni-CaO catalyst featured system [31]. The mechanism for the CaO catalyst for higher hydrogen is shown in Eq. 11 as follows.

11

While calcium oxide (CaO) is recognized for its ability to capture carbon dioxide (CO₂) through carbonation at moderate temperatures (below 900 °C), this reaction falls short at the extreme temperatures (1500–3000 °C) employed in plasma gasification. Consequently, the remarkable increase in hydrogen yield observed in our study can be attributed to the powerful catalytic effect of CaO on tar reforming and water-gas shift reactions, rather than the transient sorption of CO₂ as calcium carbonate (CaCO₃). Importantly, CaO maintains an active oxide state throughout the process, significantly enhancing gas-phase reactions while minimizing the formation of stable carbonates. This underscores the crucial role of CaO in optimizing hydrogen production in high-temperature environments.

Figure 6 shows the variation of CCE and heat utilization reduction by plasma gasification using a CaO catalyst. The CCE and energy utilization reduction are significantly influenced by the addition of CaO as a catalyst in plasma gasification. Without a catalyst, the CCE is 91.6%, and the energy utilization reduction is 71.5%. However, with the progressive addition of CaO from 5 wt% to 15 wt%, both parameters improve substantially, demonstrating the catalytic role of CaO in enhancing gasification efficiency and reducing energy consumption. At 5 wt% CaO, the CCE decreases to 84.9%, and the energy utilization reduction reaches 56.7%. This improvement occurs because CaO promotes the decomposition of carbonaceous materials, accelerating gasification reactions and reducing residual carbon in the system.

Fig. 6.

Effect of CaO catalysts on CCE and energy utilization reduction.

Additionally, CaO facilitates methane reforming and the water-gas shift reaction, ensuring more efficient carbon conversion into syngas components such as H₂ and CO. The higher reaction efficiency leads to lower unreacted carbon, which directly enhances CCE. As the CaO concentration increases to 10 wt%, the CCE further rises to 90.5%, and the energy utilization reduction improves to 63.5%. This is because a higher CaO concentration provides a greater surface area for catalytic activity, accelerating carbon conversion and reducing energy input requirements. CaO also acts as a CO₂ adsorbent, lowering the presence of CO₂ in the syngas while driving additional reactions that produce hydrogen [28]. This process results in higher energy efficiency and better syngas quality.

At 15 wt% CaO, the CCE reaches its highest value of 93.6%, and the energy utilization reduction peaks at 76.2%. This demonstrates that CaO maximizes carbon conversion efficiency by ensuring nearly complete gasification of the feedstock. The presence of a higher CaO concentration enhances tar cracking, methane reforming, and CO₂ capture, leading to higher hydrogen yields and lower carbon losses. Furthermore, the reduction in energy utilization is due to heat recovery from exothermic reactions and the minimized need for external energy input. The efficient use of thermal energy ensures that the gasification process operates with lower energy losses, making it more sustainable and cost-effective. Hence, the integration of CaO as a catalyst significantly improves CCE and reduces energy consumption, making plasma gasification more efficient and environmentally friendly by using e-waste feedstock. Moreover, the higher CaO catalyst concentrations with the e-waste feedstock facilitate faster reaction kinetics, enhanced syngas quality, and greater gasification efficiency, making it an ideal catalyst for hydrogen production and sustainable applications through waste utilization for hydrogen production [23]. Although the carbon conversion efficiency (CCE) decreases slightly to 84.9% at 5 wt% CaO, this does not indicate process degradation. Instead, the reduction is due to the formation of solid CaCO₃ from CO₂ sorption, which positively influences gas-phase reactions and leads to higher hydrogen production. Therefore, the lower CCE reflects a change in carbon distribution rather than a loss, resulting in improved syngas quality, higher H₂ yield, and a cleaner gas profile.

Moreover, the energy flow diagram (Sankey diagram) for syngas yield for varying plasma gasification temperature and varying CaO catalyst at 3000 °C is shown in Fig. 7a and b, which makes it easy to understand the energy flow from the e-waste to useful syngas, including CH₄, CO₂, H₂, CO, and energy loss.

Fig. 7.

Sankey diagram for the energy flow of (a) syngas yield for varying plasma gasification temperature. Sankey diagram for the energy flow of (b) syngas yield for varying CaO catalyst at 3000 °C.

Energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analysis of ash

Figure 8 (a and b) illustrates the EDS and XRD peaks of ash with different elements, which clearly indicate each phase with its mass %.

Fig. 8.

EDS-XRD behaviour of ash (a) EDS, and (b) XRD.

Figure 8 (a) indicates the EDS analysis of e-waste ash, which contained 28.1% of O (oxide) from metal compounds, 17% of silicon (Si) from glass fiber/ceramic, 9.8% of calcium additive from the plastic (Ca), 7.2% of aluminum (Al) from casting compounds in e-waste, 6.2% iron from frames (Fe), 5.1% of copper (Cu) from major phases of electronic circuits, and 1.2% of zinc (Zn) from electronic parts. While the improved oxygen content leads to oxide formation, including SiO₂, Al₂O₃, CaO, and Fe₂O₃, which are typically made by plasma gasification [15], and higher temperature gasification leads to maximum thermal cracking and its ash formation, depending on the feedstock and its processing in gasification [26]. the Moreover, Fig. 8 (b) indicates the XRD peaks of ash, which illustrates the various peaks including SiO₂ (100) (JCPDS: 85–0799) at 21.2 degree, CaO (111) (JCPDS: 37-1497) at 29.4 degree, Fe₂O₃ (110) (JCPDS: 33–0664) at 34.5 degree, Al₂O₃ (113) (JCPDS: 10–0173) at 37.2 degree, Ca₂SiO₄ (112) (JCPDS: 33–0302) at 44.1 degree, and metallic Cu (JCPDS: 04-0836) at 50.2 degree is noted. However, it showed a sharp intensity peak and higher crystallization and phase stabilization, which was formed by a higher temperature of the plasma torch [23,26].

Cold gas efficiency (CGE) behaviour with energy balance

Table 5 illustrates the cold gas efficiency and energy balance sheet for the plasma gasification process of e-waste feedstock with 15 kg/h under varied temperatures.

Table 5.

Energy balance and cold gas efficiency behaviour.

Plasma temperature in ⁰C	Feedstock energy in MJ/h	Syngas energy in MJ/h	Gas yield kg/h	CGE in %
1500	300	130	9.01	43.28
2000	300	155	10.48	51.55
2500	300	173	11.79	57.61
3000	300	180	12.51	60.1

Figure 9 shows the cold gasification efficiency behaviour of the plasma gasification process of e-waste. With respect to Fig. 9, the CGE is gradually increased with improved gasification temperatures. During the 1500 °C gasification process, the CGE shows a 43.28% due to reforming loss, and the gasification temperature is increased by 2000 and 2500 °C, recorded by 51.55 and 57.61% respectively. An increased gasification temperature leads to improved hydrocarbon breakdown, resulting in enhanced hydrogen production. However, the system featuring 3000 °C found a maximum CGE of 60.1%, which is higher than others. Higher temperature provides better methane formation and boosts the hydrogen yield behaviour. However, plasma gasification plays an important role in thermal cracking, and the additional hydrogen produced due to the complete dissociation of the hydrocarbons from the molecule, which is better than the conventional method [2,9].

Fig. 9.

Cold gasification efficiency under varied gasification temperatures.

With the investigational results, the plasma gasification temperature of 3000 °C with 15 wt% CaO catalyst showed optimum hydrogen yield and enhanced carbon conversion efficiency with reduced energy consumption behaviour due to effective utilization of waste heat recovery system and its outcomes are compared with past literature to show the novelty of the present study. These details are shown in Table 6.

Table 6.

Comparative analysis of previous literature.

Details of the research	Hydrogen yield in mol/kg	Carbon conversion efficiency in %	References
E-waste to hydrogen via plasma gasification (3000 °C with 15 wt% CaO catalyst) along with a waste heat recovery system	72.6	93.6	Current research
Electrical switch waste to hydrogen via CO2 plasma gasification (2000 °C without catalyst) process	44.64	-	18
Waste biomass (reed straw/rice husk/corn cob) to hydrogen via steam gasification (600 °C at 25 bar pressure) along with a Ni-based catalyst	59.1	-	30
Biomass to hydrogen via two-stage alkaline thermal gasification (500 °C with 5% CaO catalyst) process	27.36	42.39	33
Polypropylene waste to hydrogen via plasma gasification (1200 °C) with a CO2 reactor	-	82	35

The significance of the plasma gasification with CaO catalysts (15 wt%) associated with a heat recovery system operated under 3000 °C is found to have a higher hydrogen yield and improved carbon conversion efficiency, which is better than the past literature reported with conventional and plasma gasification processes for waste-to-hydrogen energy. The hydrogen yield of the E-waste to hydrogen via plasma gasification (3000 °C with 15 wt% CaO catalyst) along with a waste heat recovery system is found 62.63%, 22.8%, and 165.3% higher than the electrical switch waste to hydrogen via CO₂ plasma gasification (2000 °C without catalyst) process [18], waste biomass (reed straw/rice husk/corn cob) to hydrogen via steam gasification (600 °C at 25 bar pressure) along with Ni-based catalyst [30], and biomass to hydrogen via two-stage alkaline thermal gasification (500 °C with 5% CaO catalyst) process [33]. However, the significance of the waste heat recovery system could increase the energy reduction and enhance the carbon conversion efficiency, which is 120.8% and 14.1% better than the biomass-to-hydrogen via two-stage alkaline thermal gasification (500 °C with 5% CaO catalyst) process [33] and polypropylene waste to hydrogen via plasma gasification (1200 °C) with a CO₂ reactor [35].

The plasma gasification process of e-waste with 15 wt% CaO catalyst setup and 3000 °C gasification temperature (with heat recovery system) is found to be 72.6 mol/kg and 175%, 102%, 267%, and 60% better than the earlier research executed with Ni-CaO [35], CeO₂-Ni-CaO [36], Ni-CaO [37], and Fe-CaO [37], respectively.

Conclusions

This study highlights the potential of plasma gasification for hydrogen production from e-waste feedstock, emphasizing the impact of temperature, catalyst addition, and heat recovery integration on process efficiency. The present successfully addressed the research gap for the E-waste-to-energy via conventional gasification, found variation in hydrogen production, and reduced carbon conversion by integrating the plasma gasification process associated with calcium oxide (CaO) catalyst and waste heat recovery system. Initially, the research was carried out by varying the gasification temperature (1500–3000 °C on syngas gas yield, carbon conversion efficiency, and energy reduction was evaluated, and an optimum gasification temperature was identified. Catalyst actions (0, 5, 10, and 15 wt%) on syngas, carbon conversion efficiency, and energy reduction were investigated. According to the findings, all important results are summarized below.

• The 15 wt% CaO catalyst featured plasma gasification with a waste heat recovery system, found to have a higher hydrogen yield and reduced carbon dioxide formation, which are 9.29% and 34.6% better than the plasma gasification system operated under 3000 °C without a catalyst. It led to a better E-waste to hydrogen energy conversion as compared to a non-catalyst system.

• Similarly, the plasma gasification process associated with 15 wt% of CaO catalyst exhibited a higher carbon energy conversion and a better energy reduction, which are identified to 93.6%, and 76.2%, and better than the non-catalyst system.

• Hence, the current study establishes plasma gasification of e-waste as a highly efficient and sustainable method for hydrogen production, with optimized conditions enabling superior hydrogen yield, lower emissions, and enhanced energy efficiency. However, the CaO catalyst performance was limited measured and increased loading conditions may influence the stability and limit the life cycle. Future research should develop a multi-functional catalyst with enhanced thermal stability and prevent agglomeration.

Abbreviations

ANNOVA

Analysis of Variance

CaO

Calcium oxide

CaCO₃

Calcium carbonate

CeO₂

Cerium dioxide

CH₄

Methane

CO

Carbon monoxide

CO₂

Carbon dioxide

EDS

Energy dispersive X-ray spectroscopy

Fe

Iron

H₂

Hydrogen

H₂O

Water

NaCl

Sodium chloride

Ni

Nickel

Ni(NO₃)₂.6H₂O

Nickel nitride

Fe(NO₃) ₃.9H₂O

Ferric nitride

O

Oxygen

XRD

X-ray diffraction

XPS

X-ray photoelectron spectroscopy

Symbol

ΔT

Preheat temperature rise in ºC.

CCE

Carbon conversion efficiency in %

CGE

Cold gasification efficiency in %

Esyngas

Syngas energy in MJ/h.

LHVi

Lower heating value in MJ/m3

ni

Volumetric flow rate in m3/h

Author contributions

All the authors contributed to the study’s conception and design. Material preparation, data collection and analysis were carried out by Raga Sudha, Viyat Varun Upadhyay, Chitimireddy Veera Gayathri, Sumeet Kaur, P. S. Ranjit, Jagtej Singh, Ponmurugan Panneerselvam, M. Ramya, M. Murali and Rajkumar Sivanraju. The first manuscript draft was prepared by M. Murali, and subsequently, all the authors contributed to the finalization of the manuscript.

Funding

The authors did not receive support from any organization for the submitted work.

Data availability

All the data required are available within the manuscript.

Declarations

Competing interests

The authors declare no competing interests.

Ethics approval

This is an observational study. Reduction of energy consumption and enhancement of hydrogen yield from E-waste via plasma gasification process with waste heat recovery system: The Research Ethics Committee has confirmed that no ethical approval is required.