Sustainable Management of Nitrile Butadiene Rubber Waste Through Pyrolysis

Abstract

Many industries use synthetic rubber for various industrial purposes; thus, disposal of synthetic waste rubber into the environment causes huge environmental problems. A feasibility study to convert synthetic Nitrile Butadiene Rubber (NBR) waste into valuable products using batch-type slow pyrolysis will benefit small- to medium-scale industries in their waste handling. The main objective of this research was to find sustainable waste management and energy solutions for synthetic NBR waste using pyrolysis technology. This study employed a two-phase experimental approach where the first five preliminary experiments were conducted to assess the thermal decomposition behavior of NBR waste. In the second phase, three experiments were conducted to quantify the yields of pyrolysis products: pyrolysis oil, solid char, and syngas. The results reveal that the weight distribution of pyrolysis oil, solid char, and syngas was 39.00%, 42.91%, and 18.09%, respectively. Moreover, density, heating value, flash point, and viscosity were analyzed to investigate the quality of the pyrolysis oil, where it was found to be 972.50 kg/m³, 42.50 MJ/kg, 36.50 °C, and 25.44 cP, respectively. Analysis of the syngas composition revealed that carbon monoxide, carbon dioxide, methane, hydrogen, oxygen, and C_nH_m were 14.14%, 5.04%, 18.69%, 11.08%, 0.02%, and 10.00%, respectively. The heating value of syngas was 3873.70 Kcal/m³. The carbon footprint analysis indicated that a significant reduction is possible in greenhouse gas emissions compared to conventional incineration. Additionally, a cost benefit analysis highlighted the superior cost effectiveness of pyrolysis over incineration of NBR waste. These findings underscore the potential of pyrolysis as an effective tool for sustainable waste management and energy recovery solutions.

1. Introduction

Waste rubber, being non-reusable and non-degradable, poses significant environmental challenges when exposed directly to the environment. Industries, increasingly opting for synthetic rubber like Nitrile Butadiene Rubber (NBR) over natural rubber, contribute to the mounting volume of rubber waste [1]. Traditional disposal methods, such as landfilling or incineration of synthetic rubber, are unsustainable and exacerbate environmental issues by occupying valuable landfill space and causing long-term impacts [2]. Some industries repurpose a portion of their generated waste rubber as raw material for other products, while the remainder is disposed of, often by paying waste disposal companies. These companies typically incorporate the rubber waste into energy-generating processes, such as incineration, alongside other waste to produce thermal energy, although this approach is environmentally unfriendly and far from sustainable.

Thermochemical conversion refers to the processes that use heat and chemical reactions to convert carbon-based materials into energy, fuel, or value-added products. Thermochemical conversion includes combustion, pyrolysis, and gasification. Thermochemical conversion processes offer significant advantages in terms of sustainability and efficiency. For example, co-combusting waste rubber with coal lowers the ignition temperature and reduces NOx emissions, improving energy efficiency and reducing pollutants [3]. Gasification using pre-treated fly ash as a catalyst enhances the production of hydrogen-enriched syngas, offering a sustainable method for hydrogen production while recycling waste [4]. Additionally, catalytic reactions using Fe-modified zeolite catalysts increase the yield of valuable chemicals like BTEX (benzene, toluene, ethylbenzene, xylene), optimizing petrochemical production [5]. These processes showcase how thermochemical conversion can reduce emissions, improve energy efficiency, and produce valuable byproducts, making it a promising approach for both environmental sustainability and industrial innovation.

Synthetic waste rubber contains significant energy potential, surpassing other waste types. Pyrolysis technology offers a method to extract this energy, particularly beneficial for NBR waste rubber. Through pyrolysis, waste is converted into bio-oil, facilitating renewable energy production and valuable chemical acquisition, while minimizing environmental harm [6]. Pyrolysis involves thermochemical decomposition at 400 °C–900 °C without oxygen, yielding various products [7]. These include pyrolysis oil, producer gas, and char, which are usable for energy applications. Pyrolysis serves as the initial step in combustion and gasification processes. Industries employ pyrolysis in waste-to-energy projects, utilizing the resulting products for energy generation, thereby addressing environmental concerns while harnessing resource potential [8].

Pyrolysis oil, also known as bio-oil, possesses versatile applications within factory settings; its utility is contingent upon quality, composition, and operational requirements [9,10]. Primarily, it serves as a valuable resource for heat and energy generation, substituting traditional fossil fuels like natural gas or diesel in industrial boilers to produce steam and heat vital for various factory processes [11,12]. Direct combustion of pyrolysis oil efficiently provides thermal energy for heating applications and electricity generation through internal combustion engines or turbines, meeting factory power needs [6,13].

Additionally, pyrolysis oil acts as a feedstock for chemical production, potentially yielding methanol, hydrogen, or synthetic fuels like gasoline, diesel, or aviation fuel through further refining processes [14,15]. Its role extends to industrial heating requirements, including heat treatment processes and steam generation in boilers, where precise temperature control is essential [16,17]. Furthermore, pyrolysis oil can be co-fired with conventional fuels, reducing overall fossil fuel consumption and greenhouse gas emissions, thereby contributing to sustainable industrial practices [18,19]. Pyrolysis oil functions as a hydrocarbon source, facilitating the production of lubricating oils and greases crucial for the maintenance of machinery and vehicles [20]. Additionally, its potential transcends practical applications and extends to research and development initiatives, offering opportunities to explore its diverse applications across multiple industrial sectors [21,22]. However, ensuring that the quality and composition of pyrolysis oil align with the intended purpose is imperative [23]. Adhering to safety, environmental, and regulatory standards is essential when integrating pyrolysis oil into various factory processes, guaranteeing responsible and sustainable utilization [24,25].

Previous studies using thermogravimetric analysis (TGA) demonstrate that different rubber types decompose at distinct temperature ranges, following the following order: natural rubber (NR), styrene–butadiene rubber (SBR), nitrile butadiene rubber (NBR), and butadiene rubber (BR) [26]. The pyrolysis of NR starts at 326 °C, peaks at 375 °C, and completes at 455 °C, while SBR begins at 286 °C, peaks at 452 °C, and finishes at 491 °C. BR decomposition starts at 374 °C, peaks at 483 °C, and ends at 497 °C [27]. The pyrolysis process is divided into four stages: below 320 °C (decomposition of plasticizers and moisture evaporation), 320 °C–400 °C (NR decomposition), 400 °C–520 °C (SBR, NBR, and BR decomposition), and above 520 °C (minimal mass reduction) [26]. These insights are essential for optimizing pyrolysis processes, tailoring energy input, and improving resource recovery through precise temperature control [28].

In this study, the feedstock material (waste rubber) that is required for the pyrolysis experiment was obtained from a reputed glove manufacturer in Katunayake, Sri Lanka. Part of the rubber waste generated during the production of gloves is converted into various products at the company’s rubber reprocessing center, whereas the other part was sold to INSEE Eco-Cycle (Pvt.) Ltd., Sri Lanka for disposal by means of incineration. This study aims to investigate how pyrolysis can effectively convert NBR waste into valuable byproducts such as oils, gasses, and carbon black, while reducing the environmental impact associated with conventional disposal methods like incineration. By assessing the energy recovery potential, emissions reduction, and resource recovery from NBR, this study seeks to provide a cleaner, more efficient alternative for recycling waste rubber and contributing to the circular economy. Ultimately, this research aims to establish pyrolysis as a viable, eco-friendly solution for the rubber industry, offering both environmental and economic benefits.

2. Materials and Methods

2.1. Sample Collection and Preparation

In this experiment, a lab-scale batch-type fixed-bed pyrolysis unit was used where the rubber was heated to obtain the pyrolysis product. Figure 1 illustrates the summary flow chart of the methodology. The raw material (waste NBR) used for the batch pyrolysis process was collected from a reputed glove manufacturer in Katunayake, Sri Lanka. The pyrolysis unit was built at the site, while the waste rubber was dried using the rotary dryer (manufactured by New Royal (Pvt.) Ltd., Sri Lanka) to keep the moisture content below 10%. Proximate analysis and ultimate analysis results of the raw NBR waste sample are indicated in

Table 1. Proximate and elemental analysis of NBR waste.

	Proximate Analysis (wt.%)			Ultimate Analysis (wt.%)
Sample	Carbon (Fixed)	Ash	Volatile	C	H	N	S
Natural Rubber	-	0	100.00	80.60	9.90	7.60	0
Nitrile Butadiene Rubber	-	0	100.00	85.80	11.40	1.00	0
Styrene Butadiene Rubber	-	0	100.00	85.80	10.90	0.30	0

.

2.2. Carbon Footprint Assessment of NBR Waste Removal

The carbon footprint of NBR waste removal was assessed by quantifying greenhouse gas (GHG) emissions across three key scopes: Scope 1 (direct emissions), Scope 2 (indirect emissions from electricity consumption), and Scope 3 (indirect emissions from transportation). The analysis boundary encompassed the transportation of NBR waste to the incineration facility and the incineration process itself.

Activity data were collected from operational records at a reputed glove manufacturer in Katunayake, Sri Lanka. This included diesel fuel consumption for transportation (0.286 L per kilometer), daily Liquid Petroleum Gas (LPG) use in rotary dryers (128.48 kg per day), and electricity usage by machinery such as dryers, crushers, and conveyers. Machine efficiencies, ranging from 70% to 92%, were factored into the calculations. Emission factors were obtained from reliable sources, including the United States Environmental Protection Agency (US EPA) and Intergovernmental Panel on Climate Change (IPCC) guidelines. Key factors included 2.74 kg CO₂ per liter of diesel, 0.71 kg CO₂ per kWh of electricity, and 1.98 kg CO₂ per kilogram of NBR during complete combustion.

GHG emissions were calculated using Equation (1):

$$\mathrm{G}\mathrm{H}\mathrm{G}\mathrm{E}\mathrm{m}\mathrm{i}\mathrm{s}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}\left(\mathrm{C}\mathrm{O}2\mathrm{e}\mathrm{q}\mathrm{i}\mathrm{n}\mathrm{k}\mathrm{g}\right)=\mathrm{A}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}\mathrm{D}\mathrm{a}\mathrm{t}\mathrm{a}\left(\mathrm{A}\right)\times \mathrm{E}\mathrm{m}\mathrm{i}\mathrm{s}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{F}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{o}\mathrm{r}\left(\mathrm{E}\mathrm{F}\right)$$

(1)

Scope 1 emissions included direct contributions from NBR combustion and LPG use. Scope 2 emissions accounted for electricity consumption in rotary dryers, crushers, and conveyers. Scope 3 emissions were derived from transportation over a 105 km distance using rigid vehicles carrying 6000 kg of NBR waste per shipment.

Several assumptions were made to simplify the calculations. It was assumed that NBR undergoes complete combustion without partial oxidation, the machinery operated within the reported efficiency ranges, and all shipments were uniformly sized. Validation of the results was performed by cross-referencing the calculated emissions with industry reports and similar studies, ensuring consistency and accuracy in the findings.

2.3. Pyrolysis Trial No. 1–5

The initial weight of the NBR waste was measured, and its moisture content was analyzed using a MAC Moisture Analyzer (manufactured by Radwag in Radom, Poland). The first five trials were performed in a reactor inside the muffle furnace (manufactured by Thermo Fisher Scientific in Germany). These trials were performed to investigate the optimum reaction conditions such as temperature, reaction time, and moisture content for efficient conversion. NBR waste was inserted into the reactor and sealed well using nuts and bolts. However, an asbestos gasket sealant sheet or high-heat-resistant silicone sealant was not used to seal the reactor. After the respective time, the reactor was taken out and allowed to cool, and the weight loss of the rubber sample was measured.

Table 2. Reaction parameters used for trial No. 1–5.

Trial No.	Trial 1	Trial 2	Trial 3	Trial 4	Trial 5
Initial weight (g)	10	10	10	10	10
Initial Moisture content (wt%)	7.24	7.55	7.59	7.76	7.86
Temperature (°C)	250	300	400	500	500
Reaction time (min.)	20	20	20	20	60
Weight loss (g)	0.00	1.24	2.13	1.96	1.18

summarizes the parameters of the first five experiments.

In Trial 5, the reaction time was increased to 60 min to compare the effect of the residence time. In trial 5, the weight loss reduced to 1.18 g. This lower weight loss compared to Trial 4 may indicate secondary reactions, such as char stabilization or repolymerization of volatile compounds into the solid phase [29].

2.4. Pyrolysis Trial No. 6–8

The weight of the NBR waste was measured, and its moisture content was analyzed using a Moisture Analyzer. Unlike in trial 1–5, trials 6, 7, and 8 were performed in a laboratory-scale galvanized steel reactor (750 mL) placed on an LPG furnace. The schematic diagram of the laboratory-scale pyrolysis reactor setup with measurement points is illustrated in Figure 2.

About 400 g of the sample was inserted into the reactor and was subjected to pyrolysis at 350–420 °C without inert gas and any catalysis under atmospheric pressure. The temperature was measured using a TiS55+ Thermal Imaging Camera (manufactured by Fluke Corporation in Shenzhen, China). After completion of the process (60 min), the reactor was cooled down in an atmosphere of neutral gas to prevent char oxidation. The oil fraction and charcoal-solid residues were collected into the beaker and weighed, while the volatile matter was calculated using the difference. Trial 7 and 8 were performed, and the parameters and observations are indicated in

Table 3. Reaction parameters used for trial No. 6–8.

Trial No.	Trial 6	Trial 7	Trial 8
Initial NBR weight (g)	400	400	400
Moisture content (wt%)	6.35	6.76	6.78
Temperature (°C)	350–420	350–420	350–420
Reaction time (mins.)	60	60	60
Insulation and Sealant	NB, SS	NB, GS, AS	NB, SS, GW
Observation	S, Char, Oil	NS Tar, Char, Oil	NS Tar, Char, Oil

AS—Asbestos, GS—Gasket Sealant, NB—Nuts and Bolts, SS—Silicon Sealant, GW—Glass Wool, S—Solid, NS—Non-Solid.

. All of the trials (No. 6–8) were performed in the same laboratory conditions under controlled environment conditions.

2.5. Analysis of Synthetic NBR Waste Pyrolysis Products

The pH value, viscosity, density, flashpoint, higher heating value, metal identification, and syngas analysis were conducted to analyze the quality of the pyrolysis products. Trial 7 demonstrated the best bio-oil production and was chosen as the best products for further analysis.

2.6. Reactor Parameter Estimation

The temperature of the reactor was measured using a TiS55 + Thermal imaging camera where it was possible to take temperature measurements from a safe distance at a high accuracy. Figure 3 illustrates the measurement of the reactor temperature using an infrared (IR) thermometer. With the reactors’ dimensions, the IR temperature was assumed to be the internal temperature.

2.7. Analysis of Pyrolysis Oil

The pH value of the pyrolysis oil was measured by using a calibrated pH meter, and an SMV 3001 Automatic Kinetic Viscometer (manufactured by Anton Paar in Graz, Austria) was used to measure the density and viscosity of the pyrolysis oil. Metal identification was conducted using Microwave Plasma-Atomic Emission Spectroscopy (MP-AES), and the flashpoint of the pyrolysis oil was measured by using a PMA 500 flash point tester (manufactured by Anton Paar in Graz, Austria) using ASTM D93 standard [30]. The higher heating value of the pyrolysis oil was estimated using CKIC 5E-C5508 Automatic Calorimeter (manufactured by CKIC in Changsha, China) using ASTM-D5865 standard [31].

2.8. Analysis of Syngas Composition and Content

The Gasboard-3100GP portable infrared syngas analyzer (manufactured by Cubic Instruments in Wuhan, China) was utilized to analyze the syngas composition using Non-Dispersive Infrared Spectroscopy (NDIR), Thermal Conductivity Detector (TCD), and Electron Capture Detector (ECD) methods. It is capable of simultaneously measuring key components such as CO, CO₂, CH₄, and H₂.

2.9. Cost and Revenue Comparison

This study compares the economic and environmental aspects of pyrolysis and incineration for managing NBR waste. Data were collected for a 10-ton batch, focusing on costs, revenue, and environmental impact.

Incineration: Disposal-only method costing Rs. 35,000.00 (USD 97.22) per ton with no revenue.

Pyrolysis: Includes fixed costs (diesel, electricity, and labor) and variable costs (maintenance, forklift operations, and materials), totaling Rs. 376,972.10 (USD 1047.14) per batch. Revenue is generated from pyrolysis oil (5 tons) and char (3.2 tons), yielding Rs. 1,100,600.00 (USD 3057.22).

Cost and revenue calculations were validated using industry data, with currency converted at Rs. 360 = USD 1. Environmental impact was assessed by comparing resource recovery (pyrolysis) with the emissions-only output of incineration. This methodology provides a clear framework for evaluating the economic and sustainability benefits of pyrolysis.

3. Results and Discussion

3.1. Carbon Footprint of NBR Waste Removal

This study quantified the greenhouse gas (GHG) emissions associated with NBR waste removal, focusing on incineration as the primary disposal method. Emissions were analyzed across three scopes: Scope 1 (direct emissions), Scope 2 (indirect emissions from electricity use), and Scope 3 (indirect emissions from transportation). The results are summarized in

Table 4. GHG emissions associated with NBR waste disposal.

Category	Parameter	Value
Scope 1: Direct Emissions	LPG consumption per kg of NBR	0.09 kg
	Emission factor for LPG	2.73 kg CO2eq/kg
	Direct emissions per kg of NBR	0.23 kg CO2eq
Scope 2: Indirect Emissions	Electricity consumption per kg of NBR (all machines)	0.38 kWh
	Emission factor for electricity	0.71 kg CO2eq/kWh
	Indirect emissions per kg of NBR	0.27 kg CO2eq
Scope 3: Transportation	Distance traveled	105 km
	Diesel consumption (rigid vehicle)	0.29 L/km
	Emission factor for diesel	2.74 kg CO2eq/L
	Transportation emissions per kg of NBR	0.01 kg CO2eq
Total Emissions per kg of NBR	Direct + Indirect + Transportation	2.49 kg CO2eq
Total Emissions per Shipment	Shipment size	6000 kg
	Scope 1 emissions	1407.44 kg CO2eq
	Scope 2 emissions	1598.66 kg CO2eq
	Scope 3 emissions	82.28 kg CO2eq
Overall emissions per shipment		3088.38 kg CO2eq

.

This study highlights the significant environmental impact of NBR waste disposal, with incineration contributing 2.49 kg CO₂eq per kilogram, primarily from direct combustion (79%) and energy-intensive processes like rotary drying. While transportation emissions were minimal, inefficiencies in current waste management infrastructure exacerbate the carbon footprint. These findings underscore the urgent need for alternative disposal methods, such as pyrolysis or chemical recycling, which can convert waste into reusable materials while reducing emissions. Energy efficiency upgrades and renewable energy integration can further lower indirect emissions. Policy measures promoting circular economy practices and the development of recyclable or biodegradable NBR alternatives are essential to mitigate the long-term environmental impact of synthetic rubber waste disposal. These strategies align with global sustainability goals and offer a pathway to more responsible waste management practices.

3.2. Pyrolysis Product Distribution and Material Balance

At the end of the waste synthetic rubber pyrolysis process, three main products were obtained. These include pyrolysis oil, syngas, and solid char, as illustrated in Figure 4. The average weighing of pyrolysis oil, syngas, and solid char was performed using trial no. 6–8, and the material balance of the products were found to be 39.00%, 18.09%, and 42.91% as pyrolysis oil, syngas, and solid char, respectively. The NBR waste pyrolysis oil was a dark brown oil, highly viscous, with a sharp and irritating smell which may be due to the high percentage of sulfur. According to Qirong Yang, the pyrolysis of NBR waste finishes at a temperature of 461 °C [27]. However, the maximum temperature that could be obtained during the trial 6–8 was 420 °C because of the insulation provided and the LPG heat source. The formation of this significant amount of solid char could be due to the incomplete pyrolysis as a result of the temperature variation. The differences in material balance among trial 6–8, as illustrated in Figure 5, were due to the different type of substrate waste material used for different trials. Moreover, the increased char content in trial 8 could mainly be attributed to the better insulation measures taken in the reactor, resulting in an efficient conversion process.

3.3. Properties of the Raw NBR Waste Sample

To analyze the quality of the pyrolysis products, various parameters were measured. According to the results, the higher heating value of synthetic NBR waste was 36.81 MJ/kg. To examine the heavy metal content of the waste rubber sample, the MP-AES 4210 method was utilized. It uses a microwave-induced plasma to atomize and excite the sample, producing characteristic emission spectra for each metal present. The analysis results of the composition of heavy metals of different biomass samples and raw NBR waste samples are summarized in

Table 5. Comparison data of NBR waste with other biomass wastes.

	NBR Waste	Wheat Straw	Beech Wood	Sewage Sludge	Paper Sludge
Source	Current Study	[32]	[32]	[33]	[34]
Property	Result (mg/kg) Using Dry Digestion Method MP-AES 4210
Zinc (Zn)	1759.77	-	15	1318	470
Cadmium (Cd)	0	0.2	1	38	<0.4
Copper (Cu)	12.87	0.06	43	330	310
Nickel (Ni)	1.22	0	0	39	-
Lead (Pb)	3.98	0	33	159	160
Manganese (Mn)	1.57	0	73	950	55
Chromium (Cr)	1.25	25	2.5	91	10
Aluminum (Al)	474.33
Silver (Ag)	<1.07
Iron (Fe)	248.56
Magnesium (Mg)	860.1
Calcium (Ca)	5509.78
Sodium (Na)	-
Potassium (K)	-
*HHV of raw NBR waste	36.81
Moisture content	7.65%

*HHV—Higher Heating Value.

. When compared with other commercial biomass samples, NBR waste heavy metal contents are at acceptable levels.

The composition of NBR waste makes it a promising candidate for pyrolysis, balancing high resource recovery potential with lower environmental risks compared to many other waste streams. Effective management of Zn emissions and recovery processes will be critical to fully leveraging its potential, while its low levels of toxic metals position it as a safer alternative to sewage and paper sludge for energy and material recovery. Further research into optimizing pyrolysis conditions for maximum resource recovery and minimal environmental impact is recommended.

3.4. Properties of Solid Char Sample

As discussed previously, the MP-AES 4210 method was utilized to analyze the heavy metal composition of the char sample. It was observed that some metals are lost during the pyrolysis process, as some metals that were initially present in the raw NBR waste sample were not observed in the char sample. Some metals have converted into other forms during the pyrolysis process.

Table 6. Properties of pyrolysis char sample.

Metal Name	Unit	Digestion Method	Result
Zinc (Zn)	mg/kg		14,348
Copper (Cu)	mg/kg		14.19
Nickel (Ni)	mg/kg		7.87
Lead (Pb)	mg/kg		38.28
Manganese (Mn)	mg/kg		34.65
Chromium (Cr)	mg/kg		7.19
Aluminum (Al)	mg/kg	AOAC 990.08 method	3601
Silver (Ag)	mg/kg		<1.5
Ferrous (Fe)	mg/kg	Wet Digestion	859.18
Magnesium (Mg)	mg/kg		1007.55
Calcium (Ca)	mg/kg		5866.47
Sodium (Na)	mg/kg		<176.15
Potassium (K)	mg/kg		200.72

depicts the metal concentration in the char sample.

The pyrolysis process causes the significant redistribution of metals between NBR waste and the resulting char. Non-volatile metals, such as zinc, nickel, and lead, concentrate heavily in the char due to their stability at high temperatures, making the char a potential resource for metal recovery. However, toxic metals like lead and cadmium, though concentrated, require careful handling to mitigate environmental risks. In contrast, volatile metals like aluminum and calcium largely transition into the gas or oil phases, reducing their presence in the char. Some metals, such as copper, partially volatilize during pyrolysis, while others, like silver and sodium, appear in the char, indicating stabilization during the process. These changes enhance the potential utility of the char for industrial applications, but necessitate robust gas-cleaning systems and environmental safeguards to manage volatile and toxic emissions effectively. This redistribution underscores the dual challenges and opportunities of managing pyrolyzed char as both a resource and a material requiring safe disposal or utilization [35].

3.5. Properties of Syngas

The Trial 9 was conducted in the same way as Trial 7. However, the purpose of this experiment was to identify the types of gasses contained in the syngas using the Gasbord-3100GP Portable Infrared Syngas Analyzer and to determine their quantities. According to the literature, long rubber polymer chains break under high temperatures, releasing short-chain gas products. The pyrolysis byproducts will undergo secondary reactions as the temperature rises, resulting in lighter gas with increased hydrogen, methane, and C1–C4 hydrocarbon concentrations. Kaminsky et al. observed a significant increment in syngas production when pyrolysis temperatures exceeded 700 °C [36]. However, in this experiment, the maximum achievable temperature was 420 °C, which resulted in a minimal production of syngas. Syngas composition and its higher heating value is summarized in

Table 7. Syngas composition and higher heating values of NBR waste and different biomass feedstocks.

			Percentage/Value (%)
			Biomass Origin
Gas Type	NBR	Natural Rubber [37]	Oil Palm EFB * [38]	Coconut Shells [39]	Gliricidia Woody Biomass [40]
Carbon monoxide (CO)	14.14	4.9	16.6	21.3	20.3
Carbon dioxide (CO2)	5.04	9.6	19.2	11.8	8.3
Methane (CH4)	18.69	11.9	4.3	1.5	1.7
Hydrogen (H2)	11.08	22.6	5.6	13.5	17.8
CnHm	10	-	-	-	-
HHV of syngas (MJ/m3)	16.21	8.5	5.9	4.9	5.3

* EFB—Empty Fruit Bunch.

.

NBR waste produces syngas with a distinct composition, characterized by high methane content, moderate hydrogen levels, and a high HHV of 16.21 MJ/m³, making it superior to syngas from most biomass sources. These properties position NBR-derived syngas as a highly efficient energy carrier with diverse industrial applications. Future studies should explore optimizing pyrolysis conditions to maximize hydrogen and hydrocarbon yields while minimizing emissions [41].

The higher methane (CH₄) and hydrogen (H₂) levels in NBR syngas compared to biomass are primarily due to differences in feedstock composition, thermal behavior, and reaction pathways. NBR’s hydrocarbon-rich and oxygen-deficient structure promotes the formation of CH₄ and H₂ during pyrolysis, while biomass, with its high oxygen content from cellulose, hemicellulose, and lignin, favors the production of CO₂ and CO [42].

Methane in NBR syngas originates from the breakdown of butadiene chains, whereas biomass reactions often involve water–gas shifts and steam reforming, which reduce methane yields. Additionally, the lower oxygen content in NBR minimizes CO₂ production, preserving combustible gasses like CH₄ and H₂. These factors result in NBR syngas being more energy-dense and suitable for industrial energy applications compared to biomass-derived syngas [43].

3.6. Properties of NBR Waste Pyrolysis Oil Sample and Comparison with Diesel and Furnace Oil

The pyrolysis oil sample was analyzed for density, pH, HHV, flash point, and viscosity. Density and viscosity were measured using SVM 3001 (Anton Paar) with ASTM D7042 standard [44] methods. According to the results, the higher heating value of NBR waste pyrolysis oil was 42.50 MJ/kg, whereas it was 36.81 MJ/kg in the raw NBR waste sample, indicating that the NBR waste pyrolysis oil is a better fuel to use in liquid form rather than in solid form. The measured properties of the pyrolysis oil are summarized in

Table 8. Properties of the NBR waste pyrolysis oil.

Description	Value
Density of pyrolysis Oil (kg/m3)	972.50
Higher heating value (HHV) of pyrolysis Oil (MJ/kg)	42.50
Flash point of pyrolysis oil (°C)	36.50
pH value of pyrolysis oil	8.75
Dynamic viscosity of pyrolysis oil at 40 °C (mPs s or cP)	25.44
Kinetic viscosity of pyrolysis oil at 40 °C (mm2/s)	26.16

. The properties of NBR waste pyrolysis oil from the study was then compared with literature data of diesel and furnace oil to understand the suitability of synthetic NBR waste pyrolysis oil as a replacement for furnace oil. The density and viscosity of NBR waste pyrolysis oil were found to be much higher than that of diesel, which indicates that NBR waste pyrolysis oil cannot be used in combustion engines and furnaces unless mixed with diesel. The higher heating value of NBR waste pyrolysis oil is below the value specified for diesel fuel, but can be improved when blended with diesel fuel of a higher heating value.

According to the literature, the flash point of pyrolysis oil is typically lower than 30 °C due to the complexity of its constituent parts and the presence of low-boiling-point chemicals. However, the flash point of the pyrolysis oil in this experiment was 35.5 °C. Since the safety of the product decreases with as the flash point decreases due to its potential for ignition in the natural environment, the pyrolysis oil produced in this experiment could be used as a highly safe fuel when compared with other fuels. Comparison data of the produced bio-oil with commercially available fuels are shown in

Table 9. The comparison of properties of NBR waste pyrolysis oil with diesel and furnace oil.

Parameter	Diesel Fuel		Furnace Oil		NBR Pyrolysis Oil (Current Study)
	Value	Source	Value	Source	Value
Density (kg/m3)	820–860	[45]	970 (max)	[46]	972.50
HHV (MJ/kg)	43–46	[47]	42.71	[46]	42.50
Flash point (°C)	40–65	[45]	66 (min)	[46]	36.50
Viscosity (cp)	1.60–5.50	[48]	-		25.44

.

Pyrolysis oil demonstrates significant potential as an alternative fuel, particularly in industrial applications, due to its comparable energy content to furnace oil. Its high density and viscosity, however, present challenges for direct use in combustion engines and industrial systems, as these properties hinder flow, atomization, and efficient combustion. Additionally, the lower flash point raises safety concerns, especially in storage and handling.

Despite these challenges, pyrolysis oil can be utilized in industrial furnaces and boilers with minimal modifications, offering a sustainable option for energy generation. However, for use in engines, further refinement or blending with conventional fuels like diesel is essential to lower its viscosity, improve flow characteristics, and enhance combustion efficiency. Developing methods to optimize its properties will expand its applicability to transportation and power generation systems, making it a versatile and environmentally friendly alternative to fossil fuels. Future research should focus on upgrading pyrolysis oil to meet the technical and safety requirements of engine applications while maintaining its cost-effectiveness and sustainability.

3.7. Cost–Benefit Analysis of Pyrolysis for NBR Waste Disposal

This analysis compares the economic and environmental implications of pyrolysis versus incineration for managing synthetic NBR waste. A detailed summary of costs, revenues, and outcomes are presented in

Table 10. Cost and revenue comparison of incineration over pyrolysis.

Aspect	Incineration	Pyrolysis
Cost per ton	USD 97.22	USD 104.71
Total cost (10 tons)	USD 972.22	USD 1047.14
Fixed costs	None (disposal only)	USD 449.09
Variable costs	None	USD 598.08
Revenue generated	None (Disposal only)	USD 3057.22
Net outcome	−USD 972.22 (Loss)	+USD 2010.08 (Profit)
Outputs	None (no resource recovery)	Pyrolysis oil (5 tons), char (3.2 tons)
Environmental impact	High CO2 emissions with no recovery	Lower CO2 emissions with recovery
Energy use	Minimal	Diesel: 450 L, Electricity: 373 kWh
Sustainability	Linear process: waste-to-disposal	Circular economy: waste-to-resource
Feasibility	Simple, established infrastructure	Requires specialized equipment

1 USD = Rs. 360 as an approximate exchange rate.

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The generation of NBR waste in industries is a growing concern, with large quantities accumulating due to the extensive use of rubber in glove manufacturing. Even though this study reveals that NBR waste does not contain sulfur through ultimate analysis and syngas analysis, it is recommended, in future research, to effectively manage sulfur during pyrolysis operations to optimize the process to minimize harmful emissions and enhance the quality of pyrolysis products. According to some experiments, co-pyrolysis can be performed to reduce the sulfur content in pyrolysis products [49,50].

Moreover, if these lab-scale operations are upscaled to industrial scale, inert gasses should be utilized for different reactor operations, and further studies are required to investigate the performance of the reactor, such as controlling heat loss to optimize the conversion efficiency. In this current study, it was solely concentrated on the conversion of waste rubber into sustainable byproducts in a batch-type reactor without any carrier gasses.

When considering the marketability of the products, these pyrolysis products are mainly targeted for industries that generates NBR to manage its waste in situ using a field-scale pyrolysis reactor and to utilize the products as valuable energy resources, such as bio-oil and syngas. The syngas produced through this process can be used as an energy source for their industrial heating applications. Further analyses should be carried out to access the marketability potential of pyrolysis products.

4. Conclusions

The carbon footprint analysis demonstrates that pyrolysis significantly reduces the environmental impact of NBR waste management compared to incineration. Incineration emits 2.49 kg CO₂eq per kilogram of NBR waste, contributing to greenhouse gas emissions without generating usable byproducts. In contrast, pyrolysis minimizes direct CO₂ emissions through resource recovery, producing valuable outputs like pyrolysis oil, char, and syngas, which can replace fossil-based products and offset emissions elsewhere.

The pyrolysis of NBR waste has been demonstrated to be a viable and sustainable method for waste management and resource recovery. The low heavy metal content (Cd = 0 mg/kg, Pb = 3.98 mg/kg) and minimal sulfur content categorize NBR waste as a safer feedstock for pyrolysis. The resulting pyrolysis oil exhibits a high heating value of 42.50 MJ/kg, comparable to conventional furnace oil, making it suitable for use as a liquid fuel in industrial furnaces and boilers. However, due to its high viscosity (25.44 cp) and relatively low flash point (36.50 °C), the pyrolysis oil cannot be used directly in combustion engines or industrial applications without further refinement or blending.

The laboratory-scale reactor trials, conducted at a maximum temperature of 420 °C with a reaction time of one hour, produced syngas with a satisfactory combustible composition and a heating value of 16.21 MJ/m³, demonstrating its potential for energy recovery. These trials confirm the technical feasibility of converting NBR waste into valuable pyrolysis products, highlighting its potential as an alternative to conventional fuels.

The analysis highlights the superior cost-effectiveness of pyrolysis over incineration for managing NBR waste. Although pyrolysis has a slightly higher operational cost of USD 1047.14 per 10-ton batch compared to incineration’s USD 972.22, it generates significant revenue of USD 3057.22, resulting in a profit of USD 2010.08 per batch. In contrast, incineration solely incurs a financial loss of USD 972.22, with no revenue or resource recovery.

Overall, the pyrolysis process for NBR waste has shown promise as a sustainable approach for waste management, contributing to resource recovery and the circular economy. Further research is recommended to optimize the process for large-scale applications and improve the usability of pyrolysis oil for diverse industrial purposes.