When Physical Chemistry Meets Circular Economy To Solve Environmental Issues: How The ReScA Project Aims At Using Waste Pyrolysis Products To Improve And Rejuvenate Bitumens Part 2

4. Preliminary Results: RDF as a Case Study

In this section, preliminary results on the characterization of bio-oil and char produced through RDF pyrolysis, selected as a case study, at different final temperatures, and their feasible use as rheological modifiers, rejuvenating agents, and antioxidant agents are reported.

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The RDF used as the first feedstock for pyrolysis testing was provided by Calabra Maceri S.p.A. (Rende, CS, Italy).

The RDF was pyrolyzed in a tubular lab-scale quartz reactor under fast pyrolysis conditions (heating rate 30 ◦C/min) at three different final temperatures (550 ◦C, 650 ◦C, 750 ◦C). The final temperatures have been selected based on the TG profiles reported in Figure 3.

Figure 4 highlights the chemical composition of bio-oils collected after each pyrolysis test. As can be seen, the chemical composition of the bio-oil depends on the pyrolysis final temperature. It is interesting to note that the presence of specific compounds (for example, fluorene and 1-nonadecene) is scarcely influenced by the pyrolysis temperature,  while other compounds (benzoic acid, for instance) are characterized by amounts that can vary as a consequence of a temperature change. This aspect is related to the occurrence of secondary reactions at high temperatures, leading to the decomposition of more reactive compounds [52].

When using char as a bitumen filler in asphalt preparation, the relevant char features to take into account are the composition, the surface chemistry, and the textural and morphological properties. These chemical-physical and morphological characteristics greatly affect the chemical interactions between the char particles (acting as a filler) and the macromolecules forming the bitumen. These influence the stability and/or the mechanical properties of the asphalt [27,68]. By acting on pyrolysis conditions, the possibility of influencing the chemical composition of the char can be achieved. For this reason, it becomes advisable to optimize the whole process by varying the pyrolysis conditions in such a way that the relative quantities of the desired pyrolysis products and their compositions match as much as possible those of the additives for asphalts.

Figure 5 evidences the influence of the final temperature on the chemical composition of char obtained from RDF fast pyrolysis. The results for the final temperatures of 550 ◦C, 650 ◦C, and 750 ◦C are reported.

To demonstrate the feasibility of the usage of the liquid and solid products from  pyrolysis tests as bitumen additives, bio-oil, and char produced by the pyrolysis of a wood-based RDF supplied by Calabria Maceri S.p.A. (Rende, CS, Italy), have been tested at two  different temperatures (550 ◦C and 750 ◦C):

• neat bio-oil (P-Oil) obtained from a pyrolysis test at 750 ◦C (temperature test allowing  for the highest yield of the liquid fraction); 

• neat char, (P-C1) obtained from a pyrolysis test at 550 ◦C (temperature test allowing  for the highest yield of the solid fraction); 

• a 50:50 w/w mixture of P-Oil and P-C1 (P-C2).

The modified bitumen samples were prepared by adding 2 wt.% of the three additives to different aliquots of a neat bitumen. The bitumen used was a 50/70 penetration grade bitumen kindly supplied by Polyglass SpA (Ponte di Piave, TV, Veneto, Italy) and derived from a crude oil originating from Saudi Arabia, asphaltene content 32.4 wt.%.

Time cure tests to evaluate how the additive addition can change the mechanical properties of a given bitumen were performed. In particular, the effectiveness of their use as a rheological modifier, as a rejuvenating agent, and as an antioxidant agent was evaluated. Rheology time cure tests were performed with a temperature ramp at a constant heating rate of 1 ◦C/min (tan δ = G00 /G0 ) [69] under the regime of a small amplitude oscillatory shear at a frequency of 1Hz using a dynamic stress-controlled rheometer (SR5, Rheometric Scientific, Piscataway, NJ, USA) equipped with a parallel plate geometry (gap 2 mm,  diameter 25 mm), and the temperature was controlled by a Peltier element (uncertainty ±0.1 ◦C). These conditions are those generally adopted for accurate studies on bitumen mechanical properties [70,71].

The time cure test results are reported in Figure 6.

P-Oil can lower the transition temperature of the neat bitumen. This is an expected result because as far as it is known, only an oily compound has a softening ability, so bio-oil can be considered a bituminous fluxing agent. Conversely, P-C1 and P-C2,  showing a moderate modifying action, might be used as bituminous conglomerates fillers.

All the additive formulations were also tested as anti-aging agents. In Table 1, the transition temperatures of bitumens modified by P-Oil, P-C1, and P-C2, before and after the aging procedure, are reported.

The rheological analysis showed that P-Oil is the only additive that can be used as a  bituminous antioxidant. Its tan δ tends to resist the hardening induced by oxidation.

The effect of pyrolysis-derived additives on the oxidized bitumen (aging simulated by the standard procedure of RTFOT, according to the standard protocol ASTM D2872) is shown by the mechanical spectra (results of time cure tests) reported in Figure 7.

As widely demonstrated in the literature, to understand the real regenerative capacity of an additive, it is necessary to make a preliminary rheological analysis. Since the tan δ trend is similar to that for virgin bitumen, it is possible to claim that this additive might act as a rejuvenating agent [68].

According to the results in Figure 6, P-Oil and P-C1 seemed to have the ability to rejuvenate the aged bitumen since both showed an intermediate rheological behavior between the virgin and aged samples. On the other hand, P-C2 had a profile almost similar to that of the aged bitumen, so the possibility of using it as a bituminous regenerator is excluded.

These preliminary tests demonstrated that the pyrolysis products act in different ways when integrated into the formulation of bitumen, and they can also effectively act as rejuvenating agents.

This work is expected to have important impacts, in both technological and social fields, with beneficial effects on the economy, as detailed in the next section.

5. Expected Impacts in Technological, Social, and Economic Fields

The approach proposed by the ReScA project is expected to impact the quality of people’s lives, fostering technological, economic, and social development.

The focus on the pyrolysis process foresees related advantages: the production of energy vectors, smaller dimensions of treatment plants and their cleaning sections, with consequent lower investment costs, and in general, greater global efficiencies, as well as operating flexibility and reduced greenhouse gas emissions.

In the vision of the ReScA project, RDF is not used to produce energy (“quaternary recovery”, according to the European waste hierarchy introduced by the waste directive (Dir. 2008/98/EC) and recently amended in the Circular Economy Package of 4/7/2018),  but as a starting point for added-value materials recovery (“tertiary recovery”) to be used for the production of asphalts.

From the technological point of view, the use of char to enhance bitumen is a promising strategy to exploit the use of carbonaceous nanoparticles as modifiers since, at present,  their application, despite the advantageous results achieved on fullerenes, nanotubes,  and graphene-related materials [25,26], is limited due to the high production costs. The availability of carbonaceous particles at a low cost and with high performance as bitumen enhancers will explode studies in this sector. It has been predicted that the bitumen modified with char will have a greater resistance to cracking and rutting phenomena occurring at both high and low temperatures. This greater resistance to thermal fluctuations would undoubtedly offer greater safety for motorists and a drastic reduction in road maintenance activities. Moreover, it can be stated that the use of char as a modifier for asphalt, in addition to giving better mechanical performance and an increase in shelf life, can also lead to significant advantages in the regeneration phase. Indeed, the use of bio-oil for the regenerative purposes of aged asphalt could reasonably be effective in establishing synergistic effects with the char already present in the improved asphalt. The hydrocarbon molecules present as a fraction of the bio-oil are chemically similar to the carbonaceous particles of char, offering an enhanced rejuvenating effect, thanks to adsorption and chemical interaction phenomena. This would represent a breakthrough in the use of multi-functional and multi-effect additives, providing safer, longer-lasting, easy-to-regenerate roads with reduced maintenance and production costs. A study conducted in 2008 [72] estimated that the energy consumption reduction would be about 23% if the asphalt was reused for the construction of new road pavements. This result is by those obtained by a project financed by the European Community [73] and highlights the environmental advantages of the reuse of exhaust asphalts (a lower release of heavy metals and polycyclic aromatic hydrocarbons (PAHs)).

The recent work of Moins et al. [19] indeed, demonstrated through LCA studies that concerning the total economic and environmental impact of the asphalt industry:

- bitumen production is the main hotspot and accounts for 12% to 41% of the  environmental impact and 10% to 39% of the economic impact; 

- the virgin aggregates supply has an economic impact from 5% to 16%; 

- the transport of raw materials contributes between 10% and 24% to the environmental  impact and between 6% to 14% to the economic impact; 

- plant operation activities have an economic impact from 12% to 24%; 

- the energy use in asphalt mixture production has an environmental impact ranging between 11% and 24%.

The approach suggested by ReScA pointed towards a reduction in the consumption of bitumen for the production of any new asphalts and would boost the strategies for the regeneration of aged asphalts, thus limiting exhaust asphalt landfilling and new asphalt production. All this, therefore, would lead to a reduced and rationalized use of petroleum materials and derivatives, as well as aggregates and sands, constituent elements of asphalt extracted from natural resources, achieving economic returns, resource preservation, and landscape and environmental protection.

6. Conclusions and Future Perspectives 

The simultaneous coupling and closing of waste pyrolysis and asphalt cycles have been proposed. In this approach, the solid (char) and liquid (bio-oil) residues of waste pyrolysis can be used as added-value ingredients to (i) produce improved asphalts, with increased performances for motorists’ safety and with an increased life-cycle, and (ii) regenerate exhaust asphalt. In this way, a virtuous mechanism where urban wastes are no longer disposed to landfills has been individuated. In addition, the asphalt's prolonged life cycle and the possibility to regenerate asphalt with pyrolysis-derived oil will reduce waste, slowing down landfilling. Of course, the pyrolysis conditions (temperature, temperature ramp,  duration of thermal treatment) are all factors that can be tuned to optimize the pyrolysis process to obtain residues with ad-hoc characteristics for asphalt technology. The benefit of this approach is also to be seen from a circular economy perspective. Sustainable development is pursued through research and innovation and the improvement of infrastructures. To increase the knowledge on process development, scientists, policymakers, and entrepreneurs must work together to develop new and innovative approaches to waste reuse that address both safety and sustainability.

The overall perspective of the ReScA project is to contribute to the recovery of added-value materials through the valorization of wastes and their exploitation as additives for improved bitumens and asphalt production. The main goal of the ReScA project is to pursue both process sustainability and environment protection by taking these into account for all levels of the production cycle, namely from the limitation of waste disposal in the environment to the development of new protocols for bitumens and asphalt production and management.

The choice of pyrolysis, an extremely promising and flexible thermochemical conversion technology (but not yet consolidated on a global scale) will allow the verification of its potential and applicability in terms of sustainability and the framework of the circular economy paradigm.

The proposed idea integrates the urban waste transformation process with that of  asphalt production, leading to:

- the promotion of cleaner technologies for the use of urban wastes; 

- the production of road materials with improved properties from the reuse of  urban wastes; 

- alternative applications of pyrolysis products (liquids and solids) outside of the  fuel and chemicals industries; 

- the reduction of the costs for the construction of safer and longer-lasting road pavements and those related to maintenance activities; 

- the integration of systems and processes; 

- the optimization of low-cost processes by operating on the parameters involved; 

- energy saving and environmental protection (LCA analysis indicates that the chemical recycling of plastic wastes through pyrolysis has a climate change impact 42% lower than the energy recovery option) [10].

These aspects are by the community policies dealing with the circular economy approach, the Sustainable Development Goals pillars, and the Kyoto Protocol,  since they pursue the security of energy supply, sustainable use of urban solid wastes, the reduction of gaseous emissions, landscape, and environmental protection, and the limited consumption of resources. The recovery and reuse of wastes are strongly encouraged since wastes are considered a source of new functional materials.

The use of char to enhance bitumen properties, exploiting the char composition and characteristics that are very close to those of the carbonaceous nanoparticles currently used for this purpose (fullerenes, nanotubes, and graphenes) [26], is a very promising strategy,  first of all, because a fine modulation of char morphological and functional characteristics (granulometry and porosity, to name a few) can be obtained by operating on the pyrolysis process parameters. These interesting potentials would make char an excellent candidate to replace fullerenes, nanotubes, and graphenes, which, despite being recently considered as very valid additives for bitumen due to their high performance-enhancing abilities [26],  currently have a very limited application due to their high production costs [25].

To conclude, one aspect of complex systems physics has to be considered: it is known that often, different additives yield an overall effect that is not the sum of the two single effects, but the result of synergistic effects [74]. Therefore, the simultaneous use of char and bio-oil can enlarge the scenario of beneficial effects in bitumens. For these reasons, the ReScA project will foster future developments in the use of multi-functional and multi-effect additives, a quite novel field in bitumen and asphalt technology. The benefits achievable through safer, longer-lasting roads, with reduced maintenance and production costs, and with an overall reduction of wastes to be disposed into landfills, would be indisputable.

Author Contributions: P.C. (Paolino Caputo)—investigation; P.C. (Pietro Calandra)—writing and editing, supervision, funding acquisition; V.L.—investigation; A.L.P.—methodology; A.-M.P.—investigation; A.A.A.—writing; L.M.—investigation; B.T.—conceptualization; M.L.L.—methodology; M.A.—  writing and editing, funding acquisition; V.G.—investigation, writing, and editing; G.R.—investigation,  writing, and editing; C.O.R.—funding acquisition, methodology. All authors have read and agreed to the published version of the manuscript.

Funding: This research was funded by (i) Fondo per la crescita sostenibile—Sportello Fabbrica  intelligente, PON I&C 2014-2020, Progetto n.F/190182/00/X44, CUP:B21B19000680008 COR:1460220,  and by (ii) @CNR Project ReScA, “Recupero degli scarti da pirolisi di rifiuti urbani per potenziare e  ripristinare asfalti,” decision of Administration Council dated 21 Dicembre 2021.

Acknowledgments: Financial support from the CNR-RA Romania bilateral project 2020–2022 (proposal  n. 4657/2019) is acknowledged: it permitted fruitful discussions. The help of Renata Migliaccio (CNRSTEMS) and Massimo Urciuolo (CNR-STEMS) in performing pyrolysis tests is kindly acknowledged.

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