Combustion Characteristics Evaluation of Styrene-Butadiene Rubber (SBR) Pyrolysis Products

Combustion characteristics evaluation of styrene-butadiene rubber (SBR) pyrolysis products provides essential data on heating value, ignition behavior, flame propagation, and exhaust emissions for the pyrolysis oil, non-condensable gas, and solid char recovered from waste rubber. This technical article describes the sample types, evaluation parameters, standardized testing methods, and industrial applications that underpin fuel quality assessment, combustion safety design, and environmental compliance. The resulting data support waste tire valorization, alternative fuel development, and the engineering of safe, efficient pyrolysis and energy recovery systems.

Technical Overview

Styrene-butadiene rubber (SBR) is the highest-volume synthetic elastomer in the world, used predominantly in tire treads, conveyor belts, footwear soles, hoses, and molded rubber goods. Because vulcanized SBR forms a cross-linked network that cannot be remelted or re-dissolved, end-of-life articles are increasingly treated by pyrolysis, a thermochemical conversion process in which the rubber is heated to approximately 400–700 °C under an oxygen-free atmosphere. The polymer backbone cleaves into smaller hydrocarbon fragments that leave the reactor as vapor and are subsequently separated by condensation into a liquid pyrolysis oil and a non-condensable pyrolysis gas, while the original carbon black filler, inorganic additives, and pyrolytic carbon remain as a solid residue known as recovered carbon black (rCB).

The commercial value of each fraction depends directly on its combustion behavior. The oil and gas are candidate fuels for boilers, kilns, dryers, and engines, while the char can be co-fired with coal or upgraded for material reuse. Combustion characteristics evaluation therefore measures the calorific value, ignition and flammability parameters, burning rate, heat release dynamics, and emission profile of each product under controlled laboratory conditions. Because SBR is vulcanized with sulfur and compounded with zinc oxide and aromatic extender oils, the evaluation must also capture sulfur-related emissions such as SO2 and H2S, as well as the high aromaticity of the oil, which strongly influences soot formation and PAH release during burning.

Typical mass yields from SBR-rich scrap such as whole waste tires are approximately 38–45 wt% pyrolysis oil, 30–38 wt% solid char, and 12–25 wt% non-condensable gas, with the exact distribution governed by reactor temperature, heating rate, residence time, and pressure. A systematic evaluation program converts these raw outputs into certified, specification-compliant fuel and material streams.

Testing Samples

Representative, homogenized, and fully documented samples are the foundation of a valid evaluation. Each product fraction is sampled according to its physical state and preserved to prevent loss of light components or oxidation prior to testing.

SBR Pyrolysis Oil (SBPO)

The oil is a dark brown to black liquid with a strong aromatic odor, boiling across a wide range from light naphtha-type components to heavy tars. Its hydrocarbon composition reflects the styrene-butadiene copolymer structure: high concentrations of styrene, ethylbenzene, toluene, xylenes, and DL-limonene, together with alkenes, dienes, and heavier polycyclic aromatic hydrocarbons (PAHs). Higher heating values typically fall between 38 and 44 MJ/kg, comparable to conventional petroleum fuel oils, but the low flash point, high aromaticity, and sulfur content of roughly 0.5–1.5 wt% demand careful combustion evaluation.

SBR Pyrolysis Gas

The non-condensable gas consists mainly of hydrogen, methane, carbon monoxide, carbon dioxide, and C2–C4 hydrocarbons, with trace hydrogen sulfide and mercaptans originating from vulcanization sulfur. The gas usually exhibits a heating value of roughly 25–40 MJ per normal cubic meter and is most often consumed on-site to supply the endothermic heat demand of the pyrolysis reactor itself, which makes flame stability, laminar burning velocity, and flammability limits key evaluation targets.

Pyrolysis Char and Recovered Carbon Black

The solid fraction combines the carbon black originally compounded into the rubber with pyrolytic carbon deposited on the filler surface, along with ash from zinc oxide, silica, and other inorganic additives. With a higher heating value commonly in the range of 25–32 MJ/kg, the char must be evaluated both as a solid fuel and as a dust explosion hazard, since milling and pneumatic handling generate fine, readily ignitable particles.

Blended and Conditioned Fuel Samples

Evaluation programs frequently extend to derived samples, including SBPO blended with diesel or heavy fuel oil at defined ratios, char–coal or char–biomass co-combustion mixes, and pelletized char. Testing these conditioned samples establishes whether the pyrolysis products meet the specifications of the intended end-use combustion equipment.

Testing Items

A complete combustion characteristics evaluation covers energy content, ignitability, flame propagation, heat release dynamics, and pollutant formation. The core testing items include:

• Higher heating value (HHV) and lower heating value (LHV) of the oil, gas, and char fractions.
• Flash point (closed cup and open cup) and autoignition temperature of the pyrolysis oil.
• Lower and upper flammability (explosion) limits of the pyrolysis gas and oil vapors.
• Minimum ignition energy, minimum ignition temperature, and dust explosibility parameters (Pmax, Kst) of char dust clouds and layers.
• Limiting oxygen concentration (LOC) for inerting system design.
• Ignition temperature and ignition delay time under oxidative conditions.
• Burning rate, mass loss rate, and burnout temperature.
• Peak heat release rate (pHRR), total heat release (THR), and effective heat of combustion.
• Laminar burning velocity and adiabatic flame temperature of the pyrolysis gas.
• Smoke production rate, total smoke release, and soot yield.
• Flue gas concentrations of CO, CO2, SO2, NOx, H2S, VOCs, PAHs, and particulate matter.
• Ultimate analysis (C, H, N, S, O) and proximate analysis (moisture, volatile matter, fixed carbon, ash).
• Derived indices: ignition index, burnout index, and comprehensive combustion index S.

Testing Methods

Each testing item is executed under recognized international standards, ensuring that results are traceable, reproducible, and acceptable to regulators, certification bodies, and industrial customers.

Bomb Calorimetry for Calorific Value

An oxygen bomb calorimeter operating to ASTM D240 or ISO 1928 measures the gross (higher) heating value of liquid and solid samples, while the calorific value of the gas is determined with a continuous gas calorimeter or calculated from GC-determined composition. The lower heating value is derived from the HHV by correcting for the latent heat of the water vapor formed from hydrogen combustion. Duplicate determinations must agree within tight repeatability limits before a certified value is issued.

Cone Calorimetry for Heat Release Behavior

A cone calorimeter per ISO 5660-1 exposes a 100 mm by 100 mm specimen to radiant fluxes of 25–75 kW/m2 and quantifies the time to ignition, peak and total heat release rates, effective heat of combustion, mass loss rate, smoke production, and CO/CO2 yields by oxygen consumption calorimetry. This method is particularly valuable for comparing the forced-combustion behavior of char, oil-impregnated residues, and rubber crumbs.

Ignition and Flammability Limit Testing

The flash point of SBR pyrolysis oil is measured with a Pensky-Martens closed-cup tester (ASTM D93) or a Cleveland open-cup apparatus (ASTM D92). Autoignition temperature is determined per ASTM E659. Flammability limits of gas and vapor mixtures in air are established in closed vessels according to ASTM E681 or EN 1839, generating the lower and upper flammability limits used for hazardous area classification and ventilation design. The limiting oxygen index (ASTM D2863 / ISO 4589) characterizes the intrinsic flammability of solid rubber-derived samples.

Thermogravimetric and Calorimetric Analysis in Oxidative Atmosphere

Simultaneous thermogravimetric analysis (TGA), derivative thermogravimetry (DTG), and differential scanning calorimetry (DSC) in air at heating rates of 5–20 K/min reveal the oxidative degradation stages of SBR products. From the ignition temperature Ti, the peak mass loss rate (dW/dt)max, the mean mass loss rate (dW/dt)mean, and the burnout temperature Tb, the comprehensive combustion index S = [(dW/dt)max × (dW/dt)mean] / (Ti² × Tb) is calculated; higher S values indicate easier ignition, faster burnout, and more vigorous combustion. These tests also yield apparent activation energies for combustor modeling.

Micro-Scale Combustion Calorimetry

Pyrolysis-combustion flow calorimetry (MCC, ASTM D7309) anaerobically pyrolyzes milligram-scale samples and then combusts the evolved volatiles in excess oxygen, delivering the heat release rate profile, peak heat release capacity, and total heat of combustion. The technique requires only trace quantities of material and is well suited to screening oils and comparing the effects of stabilizers or antioxidants.

Burner and Engine-Based Combustion Trials

Bunsen-type or flat-flame burners measure the laminar burning velocity and flame stability of the pyrolysis gas, while oil-fired furnace trials verify atomization quality, flame length, and heat transfer in practical equipment. Blends of SBPO with diesel are further evaluated in single-cylinder diesel engine test rigs for brake thermal efficiency, in-cylinder pressure, and exhaust emission behavior.

Flue Gas and Emission Analysis

Combustion exhaust is analyzed online by Fourier-transform infrared (FTIR) spectroscopy and non-dispersive infrared (NDIR) CO/CO2 analyzers, chemiluminescence NOx analyzers, and ultraviolet fluorescence SO2 analyzers. PAHs and VOCs are sampled and quantified by gas chromatography-mass spectrometry (GC-MS), and particulate matter is collected gravimetrically. These data define the flue gas cleaning train required for regulatory compliance.

Dust Explosion Characterization of Char

Milled pyrolysis char is tested in a 20-liter spherical vessel (ASTM E1226 / EN 14034 series) to determine the maximum explosion pressure Pmax and the deflagration index Kst, while the minimum ignition energy (ASTM E2019), the minimum ignition temperature of dust clouds and layers (ASTM E1491 / E2021), and the limiting oxygen concentration (ASTM E2079) establish the basis for ATEX/IECEx zoning, inerting, and explosion suppression systems.

Applications

The evaluation results directly enable a range of industrial, safety, and regulatory applications:

• Waste tire and rubber valorization: certifying pyrolysis oil, gas, and char as saleable fuels or materials within circular-economy projects.
• Alternative fuel programs: qualifying SBPO as a substitute or blend component for diesel, heavy fuel oil, or kiln fuels in cement plants, lime kilns, and industrial boilers.
• Pyrolysis plant design: sizing gas-fired process burners, designing flare stacks, and specifying safety instrumentation using measured flammability limits and burning velocities.
• Hazard classification and safety engineering: classifying flammable liquids and combustible dusts for storage, transport, hazardous area zoning, and explosion protection.
• Environmental permitting: demonstrating compliance with SO2, NOx, CO, particulate, and PAH emission limits during official trial burns.
• Quality control: monitoring product consistency between production batches and detecting contamination or process drift at an early stage.
• Research and development: validating kinetic models, combustor CFD simulations, and machine-learning predictors of heating value based on ultimate analysis data.

FAQ

What are the main pyrolysis products of styrene-butadiene rubber?

Pyrolysis of SBR yields approximately 38–45 wt% liquid pyrolysis oil rich in aromatic hydrocarbons, 30–38 wt% solid char or recovered carbon black, and 12–25 wt% non-condensable gas composed mainly of hydrogen, methane, and light hydrocarbons, with the exact split depending on process temperature, heating rate, and residence time.

How does the heating value of SBR pyrolysis oil compare with conventional fuels?

The higher heating value of SBR pyrolysis oil typically ranges from 38 to 44 MJ/kg, comparable to petroleum fuel oil and slightly below that of diesel on a mass basis, making it a technically viable substitute fuel provided its flash point, sulfur content, and aromaticity are properly managed.

Why is sulfur dioxide emission a particular concern?

SBR products are vulcanized with sulfur-containing curatives, so the pyrolysis oil and char typically retain 0.5–1.5 wt% sulfur. Upon combustion this converts largely to SO2, which can exceed emission limits without flue gas desulfurization; trace H2S in the pyrolysis gas also requires monitoring and removal before burner use.

Which standards govern the evaluation?

Calorific value is measured to ASTM D240 or ISO 1928, flash point to ASTM D93, autoignition temperature to ASTM E659, flammability limits to ASTM E681 or EN 1839, cone calorimetry to ISO 5660-1, micro-scale combustion calorimetry to ASTM D7309, and dust explosibility to the ASTM E1226, E2019, E1491, and E2079 test protocols.

Can SBR pyrolysis oil be used directly in diesel engines?

Neat SBPO is generally unsuitable for direct engine use because of its low cetane number, high aromaticity, the high viscosity of its heavy fraction, and its sulfur content. Engine trials typically evaluate blends of 10–30 percent SBPO with diesel, often after desulfurization and distillation cut optimization, and the combustion evaluation confirms efficiency, emission, and durability performance.

How should samples be handled before testing?

Pyrolysis oil samples must be collected in sealed, light-protected containers, preferably under a nitrogen blanket and refrigerated, because light ends evaporate readily and oxidation alters the flash point and viscosity. Char samples must be sealed against moisture uptake, and gas samples are collected in gas-tight bags or cylinders using H2S-resistant materials.