RESEARCH ARTICLE

 

Co-combustion of tyre waste char and biomass blends using thermogravimetric-mass spectrometric analysis

 

 

Cleopatra T. Dube^I; Hope Baloyi^II

^ICentre for Rubber Science and Technology, Department of Chemistry, Nelson Mandela University Gqeberha, South Africa
^IIDepartment of Chemical and Materials Engineering, University of South Africa, Johannesburg, South Africa

Correspondence

 

 

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ABSTRACT

Tyre waste chars from pyrolysis are a potential fuel source due to their attractive energy properties and high carbon content. However, the high concentration of sulfur in tyre char and the release of carbon dioxide (CO₂) when burned prevent its use as a fuel. In this study Scenedesmus microalgae and torrefied pine wood waste were blended with tyre waste chars. Blending was done at varying biomass ratios. The aim was to evaluate the effects of biomass additions on tyre char combustion characteristics and gas emission resultants using the thermogravimetry-mass spectrometry method. Results show that the combination of biomass with tyre char significantly affects the physicochemical properties of the tyre char Biomass enhances the combustion characteristics of tyre char Ternary blends with optimised proportions of microalgae led to higher burnout temperatures and prolonged combustion phases. Torrefied wood-tyre char blends at a 90:10 ratio showed higher rates of CO₂ evolution, but adding torrefied wood at 15-20 wt% and increasing microalgae from 4 to 11 wt% reduced CO₂ emissions. The experimental results show that tyre char could be a potential combustion co-fuel with biomass feedstock and suitable for clean combustion applications. The study emphasises the critical role of incorporating clean, renewable biomass sources in mitigating gas emissions during the combustion of tyre waste chars, thereby promoting the sustainable utilisation of carbon resources. Additionally the integration of biomass to enhance the usability of tyre waste chars presents a promising opportunity to improve the economic viability of the waste tyre pyrolysis process.
SIGNIFICANCE:
Tyre char is a waste by-product from the pyrolysis of waste tyres. It possesses good energy properties and can be used as a fuel, but its combustion is undesirable due to the generation of carbon dioxide during combustion. The blending of tyre char with biomass material influences the physicochemical properties of tyre char and can have a consequent effect on the combustion characteristics of these chars. The addition of microalgae and torrefied wood at various blending ratios had a significant influence on the reduction of CO₂ emission patterns in the tyre char-biomass blends. Tyre char-biomass blends can be considered for cleaner combustion processes.

Keywords: microalgae, tyre char, ternary blends, CO₂ emission, cleaner combustion

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Introduction

The by-product of waste tyre pyrolysis is known as tyre pyrolysis char and presents an inescapable environmental challenge due to its detrimental impact on both health and the environment if left unattended after the tyre pyrolysis process.¹ Consequently there has been a growing need to explore viable methods for repurposing these pyrolytic chars, promoting environmental sustainability and enhancing the economic viability of the waste tyre pyrolysis process.^1,2 The focus of many studies has been on recycling and upgrading waste tyres to create products that can replace commercially available carbon black.^1,2 In addition, researchers have explored thermal treatments for tyre chars to remove impurities and enhance their physicochemical properties, thereby elevating their potential as substitutes for carbon black in high-value applications.¹

Tyre chars are also considered a viable option for energy production as they can serve as a solid fuel source for heat and energy generation in various industries, including cement factories.^3-5 The potential utilisation of tyre pyrolysis char for fuel purposes is attributed to its high calorific value and carbon content, as well as its chemical composition that is comparable to coal chars.^3,6 However, the practical use of tyre pyrolysis chars as a combustion fuel has been hindered by a combustion process characterised by extended residence times at high temperatures and the generation of large amounts of carbon dioxide (CO₂).^6,7

Additionally the tyre chars contain significant levels of retained sulfur (2 wt%) and heavy metals that could be released during combustion.^1,7,8 An effective strategy to overcome the limitations that prevent the utilisation of tyre chars as a fuel feedstock is the blending and co-firing of tyre char with environmentally friendly and carbon-neutral bio-renewable resources, such as aquatic and woody biomass. This approach offers several advantages, including the elimination of the need for extensive treatment or purification processes, such as leaching or thermal treatment, to produce activated carbons from tyre char.^1,7,8 Aquatic biomass, including microalgae biomass, has been successfully used in the blending and co-combustion of solid fuels such as coal.⁹ Microalgae offer an attractive feature in fuel blending by reducing the ash and sulfur content by enhancing the combustion characteristics of the fuel mixture.^10-12

Another promising feedstock for co-combustion with waste tyre char is torrefied biomass. Torrefaction significantly reduces the moisture content in biomass materials, whether they are of woody or herbaceous origin. This reduction in moisture content limits biological activity (such as rotting), leading to improved shelf life and higher energy density As a result, torrefied biomass exhibits higher calorific values compared to raw biomass.^13,14 The co-processing of waste tyre chars with renewable biomass offers a promising solution to address environmental and energy challenges. This innovative approach not only reduces the reliance on fossil fuels but also mitigates the waste disposal issues associated with tyre pyrolysis by-products, specifically waste tyre char The integration of these materials can significantly enhance waste tyre management, improve the economic viability of tyre utilisation and tap into the energy potential of biomass sources.

In several studies, various researchers investigated the combustion characteristics of solid fuels and biomass combinations using thermo-gravimetric analysis. Xinjie et al.¹⁵ found that adding biomass to high-volatile bituminous coal increased reactivity during combustion, especially compared to low-volatile anthracite coal. Magida et al.¹¹ explored coal-microalgae blends and observed that higher microalgae ratios lowered ignition and burnout temperatures, resulting in reduced emissions of CO₂, NO₂, SO₂ gases and oxides of nitrogen (NO_x. Ye et al.⁹ noted that microalgae enhanced coal combustion by providing additional heat. Ejesieme et al.¹² observed interactions among different fuels, with coal-microalgae and coal-charcoal blends showing synergistic behaviour Mundike et al.found that blending plant char with coal decreased burnout temperatures and improved coal combustion. Jayaraman et al.¹⁷ studied coal-biomass blends and reported increased reactivity and lower burnout temperatures with higher biomass ratios. In a recent study by Qiao et al.⁶ that focused on the combustion characteristics of tyre chars and changes in combustion products in comparison to various coal types, it was found that tyre char can be a potential substitute fuel for coal coke and semicoke because of its reactivity and high calorific values. Slyusarasky et al.analysed the co-combustion characteristics of tyre char/peat and tyre char/pine sawdust. Their findings showed that biomass additions to maximum loadings (15 wt%) could enhance the reactivity of the chars and significantly decrease the ignition delay times at heating medium temperature in a combustion chamber Also, the use of biomass as an additive could substantially reduce the emission of sulfur dioxide. The literature findings highlight the positive impact of biomass on solid fuel combustion and emphasise the need for further research on the blending effects of biomass on tyre pyrolysis char

To our knowledge, no prior study has investigated the co-effects of aquatic and ligneous biomass on the combustion characteristics of tyre waste chars. This study explores the use of microalgal and torrefied biomass as additives to tyre chars to form fuel blends. In this study Scenedesmus microalgae and torrefied pine wood waste were blended with tyre waste chars. Blending was done at varying biomass ratios. Thus, the objectives were to (1) prepare different formulations (fuel blends) of char and biomass and characterise these and the raw samples for their physicochemical properties, to study the effects of biomass ratio on the physicochemical properties of tyre waste char and (2) carry out the combustion tests on the raw and blended samples using a thermogravimetry-mass spectrometry to study the combustion behaviour of the blended samples. The aim of this study was therefore to evaluate the effects of biomass additions on tyre char combustion characteristics and gas emission resultants using the thermogravimetry-mass spectrometry method. The high sulfur content in tyre char and the release of CO₂ during combustion limit its viability as a fuel source. Co-combustion with clean, renewable biomass sources is intended to enhance the combustion characteristics of the char while contributing to the reduction of gas emissions.

 

Experimental

Origin and preparation of samples

A fine granular tyre char sample (250 μm) was obtained from a South African company A representative sample of the char was pulverised on a Keegor vertical spindle pulveriser to a particle size of sub-250 μmand sieved to obtain a sub-150 μmsample that was stored in an airtight container Pinewood waste (purchased from a local store in Gqeberha, South Africa) was torrefied in a fixed bed reactor at 320 °C, at atmospheric pressure, in an inert environment at a constant flow rate of 400 mL/min. A representative sample of torrefied wood was prepared in the same manner as the tyre char A microalgae Scenedesmus sp. sample was obtained from the Institute of Chemical Technology (InnoVenton). The microalgae were cultivated in the integrated vertical column photo-bioreactor-raceway cultivation system. The microalgae were then harvested and concentrated through sedimentation as reported by Gaqa and Watts¹⁰. The biomass was harvested by concentrating the microalgae slurry in the growth medium using natural settling in a settling pond overnight and returning the top layer of the growth medium to the cultivation system. Thereafter, the concentration of microalgae (on a dry weight basis) was determined as the mass of dry microalgae in grams per gram of slurry harvested.

Microalgae slurry was used for the blending experiments with tyre char and torrefied wood. However, for analysis purposes, the microalgae slurry was lyophilised in a Vacutec freeze drier connected to a 2XZ-2 rotary vane vacuum pump for at least 72 h until the sample had reached 25 °C. The freeze-dried sample was then crushed into flakes using a mortar and pestle and stored in an airtight container A total of 14 mixtures, including four duplicates, were prepared using a D-optimal mixture design. The lower and upper limits for the mixture design were as follows: tyre char 55-90 wt%; torrefied wood 10-30 wt% and microalgae 0-15 wt%. These limit ranges were selected to ensure that the tyre char was the main feedstock in the blend and to ascertain that only the minimum required amounts of torrefied wood and microalgae were used, to be able to study their effects on the combustion characteristics of the tyre char. The tyre char and torrefied wood blends were homogenised using a mixing rod and continuously stirred in a container, whereas the mixtures of tyre char, torrefied wood and microalgae slurry were stirred continuously at 100 rpm using a Heidolph RZR 2041 stirrer and centrifuged using HEMLE Z-383 (Lasec SA) at 4500 rpm, for 15 min to ensure that the microalgae slurry was well mixed with the tyre char and torrefied wood. The temary mixtures were oven dried ovemight at 105 °C. The dried samples were then ground with a mortar and pestle. All mixtures were screened and sieved to a sub-150 μmparticle size and stored in an airtight container.

Characterisation of parent fuels and char-bio mass blends

All samples were analysed for physical and chemical properties. The proximate analysis was conducted according to the ASTM 7582 standard methods (using an ELTRA Thermostep macro TGA). The ELTRA Thermostep TGA has an integrated programmable furnace, weighing balance and heating program. It performs an automated proximate analysis that determines the weight percentages (wt%) of the volatile matter content at 900±20 °C for 7 min and the ash content at 750 °C for 2 h, which is preceded by the moisture analysis at 105±2 °C for 1 h. The fixed carbon is then calculated by difference. For each analysis, 1 g of sample was weighed and analysed. The ultimate analysis was conducted to determine the percentages of weight (wt%) of carbon (C), hydrogen (H), nitrogen (N) and sulfur (S) according to ASTM D3176 using the Vario EL Cube. The elemental content was determined by combusting 5 mg of the sample at 1150 °C. The calorific values of the raw and mixed samples were measured with a LECO AC 600 bomb calorimeter using the TruSpeed® method (ASTM D5865). The PANalytical Epsilon 3 X-ray fluorescence instrument was used for the identification and quantification of inorganic elements present in all samples (5 g of each sample). Micromeritics Instrument Corporation (Tristar II 3020) analyser was used to determine the surface areas of the parent samples using adsorption.

Thermogravimetric analyses

Thermogravimetric analysis of the samples was performed using the TGA 5500 Discovery series coupled with a bench-top quadruple mass spectrometer About 5 mg of the powdered sample was contained in platinum plates and subjected to combustion under air The sample was heated at 10 °C/min from ambient to 650 °C, and the airflow rate was maintained at 50 mL/min.

 

Results and discussion

Physicochemical properties of the parent fuels and char blends

The results of the proximate and ultimate analysis of the parent fuels and the blended char-biomass samples are shown in Table 1. The results show the average values and uncertainty (+/-) values at a 95% confidence interval. The fixed carbon and oxygen were calculated by difference. The proximate analysis results revealed that tyre char is characterised by a low volatile matter content. However, the addition of high volatile biomass contributed to the marked increase in the volatile matter content of the resulting mixtures (ranging from 13.9 wt% to 24.5 wt%) as biomass loadings increased. Furthermore, the addition of high-energy biomass resulted in a marginal improvement in the calorific values of the blends. Ultimate analysis results show that the tyre char exhibited the highest sulfur content at 3.5 wt%. This elevated sulfur content can be attributed to the presence of thermally stable sulfur-containing compounds, such as zinc sulfide (ZnS) and iron sulfide (FeS), which persist in the char after the pyrolysis process.⁸ The combustion of materials with high sulfur content is associated with the release of various sulfur compounds, making the fuel undesirable.¹⁹ In particular, the incorporation of biomass in the blends appears to reduce the sulfur content (to up to 1.2 wt%), as evidenced by the lower sulfur levels in the temary blends compared to the tyre char. It is worth highlighting that the biomass of microalgae contains a substantial amount of hydrogen, specifically 8.7 wt%. Consequently, char-biomass blends that exhibit significant microalgae concentrations (8 wt% and 15 wt%) exhibit a notable enhancement in hydrogen content compared to the original tyre chars. This increase in hydrogen content has the potential to improve the combustibility of the blended fuel material. X-ray fluorescence results of the composite materials indicated that the binary mixtures and certain ternary mixtures exhibited significant concentrations of calcium, approaching the levels found in the original tyre char. Calcium plays a catalytic role in the combustion process.²⁰ At elevated temperatures, calcium can be released as volatile emissions and calcium oxide serves to sequester sulfur, resulting in reduced sulfur emissions during combustion.²¹ The highest amounts of silicon were detected in ternary blends (blends 1 and 2); these blends contain high amounts of biomass. Silicon, recognised for its thermal stability during combustion, is essential for minimising the accumulation of ash deposits.²² Zinc was found to be the predominant metal present in tyre char and maintained relatively high levels, even after the addition of biomass, except for the ternary blends (blends 1 and 2), where a notable decrease in zinc was observed. Typically tyre chars generated via tyre pyrolysis tend to preserve significant levels of zinc, which exist in the compounds of zinc sulfide.⁶ The Brunauer-Emmett-Teller surface area measurements showed that tyre char was found to be much larger than that of the microalgae biomass and the torrefied wood samples (Figure 1). The surface area measurements were as follows: 49.7 m²/g (tyre char), 1.6 m²/g (torrefied wood) and 21.3 m²/g (microalgae). No further analysis was done on the blended samples. Therefore, it is not expected that any addition of biomass material within the lower and upper limits of the set could have any impact on the surface area of the tyre char.

Thermogravimetric analysis

Thermogravimetric mass loss and derivative thermogravimetric curves of the individual fuels are shown in Figure 2. The combustion characteristics (the initial decomposition temperature, derivative thermogravimetric maximum of each peak, as well as the char ignition and bum-out temperatures) of the raw and blended samples are listed in Table 2. Figure 2 shows the thermal events associated with microalgae combustion. These events exhibit distinct characteristics. The first significant event is the volatile combustion event between 150 °C and 350 °C. During this phase, inherent lipids, carbohydrates and proteins in microalgae are decomposed, and the production of char occurs.^23,24 Another thermal event occurs between 450 °C and 550 °C, attributed to the breakdown of lipids. This phase is marked by the disintegration of hydrocarbon fatty acids.²⁵ The reactivity of the primary combustion events in microalgae diminishes with increasing temperature because of the depletion of compounds with low thermal stability such as volatile components, at lower temperatures.²⁴

The decomposition of torrefied wood reveals distinctive temperature peaks at 323 °C and 466 °C. The first peak (at 323 °C) is considered a precursor to the main peak, while the second peak (at 466 °C) indicates a diverse range of compounds, consistent with the high content of volatile

matter in torrefied wood.²⁶ The first thermal event involves simultaneous ignition and decomposition of cellulose, along with ignition of lignin as elucidated by López-González et al.Cellulose decomposition Is completed around 400 °C, leading to the main decomposition of Iignin and fixed carbon in the second event. The burnout temperature at 518 °C Is a common feature of torrefied wood combustion, attributed to the high reactivity of biomass. This increased the reactivity with rising combustion temperature related to elevated volatile matter content while influencing ignition behaviour and enhancing particle surface reactivity.²⁸

The thermal decomposition of the tyre char is mainly characterised by two distinct peaks within the combustion region of the char (500-600 °C), which are associated with the combustion of fixed carbon. The high ignition temperature of 546 °C was anticipated because components of the tyre with lower thermal stability were released as volatiles during the pyrolysis process. The first peak, marked by a higher mass loss, can be attributed to the separate combustion of the tyre char and chemisorbed complexes such as carboxylic, quinonic and/or phenolic groups found on the surface of carbon blacks. The second peak represents the combustion of graphite-like material.²⁹

Thermogravimetric mass loss and derivative thermogravimetric curves of blended char-biomass samples are shown in Figure 3. In Figure 3a, the differential thermogravimetric profiles of binary mixtures containing 10 wt% of torrefied wood exhibit a distinct pattern that resembles the combustion behaviour of tyre char. In contrast, for binary blends comprising 30 wt%. For torrefied biomass, the differential thermogravimetric profiles exhibit two discernible peaks and elevated rates of mass loss. As the proportion of torrefied wood increases, the ignition temperature decreases, mainly because of the early release of volatile components at lower temperatures. This lowered ignition temperature facilitates the initiation of combustion processes.

 

 

The initial peak corresponds to the decomposition of torrefied wood, while the second, more pronounced peak, occurring above 500 °C, represents the combustion of tyre char. In particular, the mass loss rate during the second peak in binary blends with 30 wt%. The torrefied wood loading was comparatively lower than that of tyre char alone. This finding is attributed to the reduced content of fixed carbon in these blends within the temperature range of 550-650 °C. In general, an increase in the torrefied biomass ratio leads to enhanced reactivity of the blends. In Figure 3b, the impact of the addition of microalgae was inconspicuous at low microalgae ratios. Ternary blends containing 15-30 wt% torrefied wood and 4-11 wt% microalgae closely resemble the combustion behaviour of binary blends with 30 wt% torrefied wood (Figure 3a).

In particular, the blend labelled TC70-TW30-IVI0 showed a delayed initial decomposition temperature, an indistinct bridge preceding the combustion of char, the maximum mass loss rate during the combustion phase of the char and a lower ignition temperature of the char compared to other blends. An increase in the microalgae ratio results in a further rightward shift of the char combustion peaks. This phenomenon can be attributed to the elevated ash content of the microalgae and the gas-phase products of volatile combustion, which retard the ignition of tyre char in the blends.³⁰ Similarly the burn-out temperature increases with an elevation in the microalgae ratio, indicating an extended combustion duration.

In Figure 3c, tyre char and blends consisting of 10-30 wt% torrefied wood, with a maximum microalgae content of 15 wt%, are depicted. The decomposition temperature of the fuel blends with 15 wt% microalgae and 20-30 wt% of torrefied wood loading is significantly lower compared to other char blends. The primary combustion event in these fuel mixtures shifts to the right, resulting in an approximately 10 °C increase in the burnout temperature. The combustion peaks are broader and shorter for these blends, indicating increased material diversity and enhanced reactivity compared to the combustion of tyre char alone. A similar observation was reported by Xinjie et al.¹⁵ who found that increasing the biomass ratio increased the reactivity of the tri-fuel blend. Similarly Jayaraman et al.¹⁷ noted that high biomass proportions (25-75 wt%) enhanced the reactivity of blended samples during combustion. The results of this study regarding biomass addition are consistent with the findings of Slyusarsky et al.¹⁸, who demonstrated similar effects during the thermal decomposition of tyre char with peat and tyre char with pine sawdust mixtures.

Gas evolution profiles by mass spectroscopy

The gas evolution profiles of the parent samples are shown in Figure 4a-c. The gases analysed were carbon dioxide (CO₂) (m/e 44), nitrogen dioxide (NO₂) (m/e 46) and sulfur dioxide (SO₂) (m/e 64). Raw fuels are low emitters of NO₂ and SO₂, as highlighted by low counts in the evolution of the respective gases. Despite the low count of NO₂, the mass spectra show a strong NO₂ peak for the torrefied biomass, although the torrefied biomass was found to have lower amounts of nitrogen, as highlighted in Table 1. Changes in the nitrogen contained in torrefied wood may require further investigation to understand the consistent release of NO₂ during its combustion due to the intensity of the peak of NO₂. The emission of CO₂ shows distinct peaks in tyre char and torrefied wood. A further look at the emission of CO₂ during the combustion of binary and ternary char-biomass blends is shown in Figure 5. The mass spectroscopy curves presented in Figure 5a-c exhibit a consistent profile in terms of shape and temperature range, as depicted by the corresponding derivative thermogravimetric curves discussed earlier. This uniformity in the profiles suggests that the evolution of CO₂ occurred at the same peak temperatures and during each thermal event for all the materials examined. These findings align with those reported by Maglda et al.¹¹ Notably, the co-combustion of the binary blends of torrefied biomass and tyre char resulted in significant CO₂ emissions, as expected. The raw materials (tyre char and torrefied wood) contain substantial amounts of elemental carbon, with torrefied biomass containing 76 wt% and tyre char containing 90 wt%. It should be noted that the peak CO₂ observed during torrefied wood combustion exceeds that of tyre char combustion. This suggests that the release of CO₂ in torrefied wood combustion occurs in both the volatile decomposition and char combustion phases. Consequently, it implies that the combustion of tyre chars emits a lesser amount of CO₂ compared to torrefied wood combustion. The combustion of ternary char-biomass blends with 15 wt% and 20 wt% loading of torrefied wood results in lower emission peaks in the 500-600 °C range compared to raw tyre char This Implies a lower release of CO₂ emissions during the combustion of these ternary char-biomass blends with high torrefied wood loading compared to the combustion of tyre char alone. This reduction in the strength of the CO₂ peak as biomass loading increases Is consistent with the findings reported In previous studies.^11,31 However, binary blends containing the least amount of torrefied wood exhibit higher CO₂ emission peaks above 500 °C compared to tyre char. The high evolution of CO₂ in this combustion stage is Influenced by differences in the fixed carbon and ash compositions within the blended samples.¹⁷ Slyusarsky et al.¹⁸ observed that increasing the proportion of pine sawdust in the tyre char/pine sawdust blend from 5 wt% to 15 wt% led to an increase in CO₂ formation, with CO₂ emission peaks exceeding those from the combustion of tyre char alone. In contrast, for tyre char/ peat mixtures, the intensity of CO₂ formation decreased as the biomass (peat) ratio increased.

 

 

 

 

Conclusions

Based on the physicochemical and combustion characteristics of the binary and ternary biends, it is apparent that biomass materiai influences the physicochemical properties of tyre char, consequently impacting the combustion characteristics of these chars, in particular, an increase in the microaigae content within ternary blends resulted in a shift in combustion peaks and burnout temperatures, thereby prolonging the combustion phase as compared to the combustion of tyre char alone. Furthermore, it has been observed that biomass additions in various ratios have a significant effect on the reduction of CO₂ emission patterns.

This study has demonstrated that blending tyre char with biomass, especiaily at increased biomass ratios, enhances the combustion characteristics of tyre char Consequently a mixture of tyre char and biomass has the potential to serve as an environmentally friendly combustion fuel. Therefore, tyre char-biomass blends can be considered for cleaner combustion processes. For future studies, we recommend a techno-economic analysis of tyre char biends to evaluate the economic feasibility of such fuel biends. Additionally future investigations should focus on the large-scale combustion of tyre char-biomass biends as well as explore pelietisation or briquetting methods for agglomerating such char blends.

 

Acknowledgements

We thank Prof. Shanganyane HIangothi for his invaluable support and leadership. Furthermore, we thank the Centre for Rubber Science and Technology and the Institute of Chemical Technology (innoVenton) for their indispensable technical support and assistance.

 

Funding

We are grateful for the financial support received from the South African National Research Foundation (grant UID: 116683).

 

Data availability

The data supporting the results of this study are available upon request to the corresponding author

 

Declarations

We have no competing interests to declare. We have no AI or LLM use to declare.

 

Authors' contributions

C.TD.: Writing - the initial draft, data collection, sample analysis, data analysis, validation. H.B.: Conceptualisation, methodology writing -the initial draft, writing - revisions, student supervision. Both authors read and approved the final manuscript.

 

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Correspondence:
Hope Baloyi
Email:baloyh@unlsa.ac.za

Received: 28 Feb. 2024
Revised: 28 Jan. 2025
Accepted: 28 Feb. 2025
Published: 29 May 2025

 

 

Editors: Michael Inggs, Thywill Dzogbewu
Funding: South African National Research Foundation (UID: 116683)