Scheme of the co-pyrolysis bench-scale plant.
Biomass fast pyrolysis for bio-oil production has been regarded as one of the most promising processes for reducing the current dependence on fossil fuels. Bio-oil, produced with a yield in the 60–75 wt% range, has several applications as fuel, source of value-added products or for the production of hydrocarbons and hydrogen through catalytic processes. These upgrading processes can be undertaken in new centralized facilities (bio-refineries) or in existing refineries, once bio-oil has been transported from delocalized pyrolysis units that operate in the areas where biomass is available. The pyrolysis process has been extensively studied and has accomplished a substantial technological development stage, since several types of reactors have been implemented, such as bubbling and circulating fluidized beds, spouted beds, ablative, auger or rotating cone reactors. However, the oxygenated nature of the bio-oil involves serious drawbacks for its full-scale utilization and has become an important bottleneck for its valorisation. Thus, bio-oil involves storage and transportation problems due to corrosion and ageing reactions caused by its high reactivity. Furthermore, its poor miscibility with hydrocarbons hinders the blending with oil fractions, and the high-molecular-weight compounds in the bio-oil (pyrolytic lignin) cause the blocking of process equipment and produce a fast catalyst deactivation in conversion units.
The production of a stable bio-oil with improved features that may be directly incorporated into oil refineries is vital in order to progress towards a bio-based energy scenario. In this way, several processes have been proposed for reducing the oxygen content in the bio-oil, with the most studied ones being catalytic cracking and hydrodeoxygenation. Alternatively, co-pyrolysis of biomass with other carbon-containing wastes, such as plastics or tyres, has also been regarded as an interesting option. Although the liquid fractions (oils) produced separately in the pyrolysis of plastic wastes, tyres and biomass are not miscible (or have limited miscibility), it has been reported that the synergetic effects are due to radical interactions that occur during co-pyrolysis allow obtaining a stable oil. In this sense, co-pyrolysis of biomass and tyres is a promising process for obtaining an appropriate oil for its integration in oil refineries. Furthermore, co-pyrolysis is an attractive route for large scale scrap tyre management due to its simplicity and effectiveness. In a scenario where the development of sustainable waste tyre valorisation routes is fundamental, a reduction of waste whose inadequate disposal leads to serious environmental consequences can be achieved.
Scrap tyre fast pyrolysis yields an oil fraction in the 30–60 wt% range and has been performed in similar reactors as those developed for biomass pyrolysis, such as fixed, fluidized and spouted beds or screw and rotary kiln reactors. Tyres pyrolysis oil (TPO) has a high carbon content (around 85 wt%) and calorific value (42 MJ kg1), which are similar to those of gasoil or light fuel oil. However, the high content of sulphur and aromatic components of TPO make unviable its direct utilization as fuel and imply the need for an upgrading process, with hydro desulphurisation, hydrodearomatisation, and hydrocracking being the most interesting pathways for its integration into refinery streams. Consequently, the oil produced in the co-pyrolysis of scrap tyres and biomass could also be integrated into current oil refineries by means of hydroprocessing treatments for the production of high-quality fuels.
Furthermore, they have been conducted in a discontinuous regime and with low heating rates, thereby hindering the extrapolation of their results to real industrial conditions. Moreover, according to recent literature reviews about co-pyrolysis of biomass and other consumer society wastes, such as plastics, under slow pyrolysis conditions, not all the compounds degrade at the same temperature range. Thus, the interactions and synergies that may occur between the pyrolysis products are not the same as in the case of fast pyrolysis, in which the degradation of biomass and tyres takes place simultaneously. Thus, MartÃnez et al. (2014) conducted co-pyrolysis in a continuous 5 kg h1 auger reactor, observing significant synergistic effects in the oil, which had a single stable phase. Also operated in a continuous regime in the fast co-pyrolysis of bamboo sawdust and waste tyres, using a bubbling fluidized bed.
The novelty of this work lies in the capability of the conical spouted bed technology for the joint valorisation of materials with different textures and densities (such as pinewood sawdust and waste truck-tyre rubber) by operating in a continuous regime. Furthermore, the addition of waste tyres to the biomass feed in the pyrolysis process would improve significantly the bio-oil properties, and therefore the results obtained are highly encouraging for the full-scale application of the process. The conical spouted bed reactor (CSBR) has been proven to perform satisfactorily in the pyrolysis of separate biomass and tyres, as well as in the co-pyrolysis of biomass and sewage sludge, obtaining high oil yields with suitable properties. The characteristic cyclic particle movement in this reactor allows operating with high mass and heat transfer rates, as well as short gas residence times, which are beneficial for maximizing oil yields, as secondary cracking reactions are hindered. Furthermore, the vigorous solid circulation allows handling irregular and sticky materials with different particle sizes and densities without operational problems, which is especially interesting for co-pyrolysis processes. Indeed, this reactor allows operating in the continuous regime and under conditions (heat and mass transfer rates and gas-solid contact) that may be extrapolated to industrial reactors.
Furthermore, this technology has good prospects for its implementation at a larger scale, as shown in the successful scaling up to a 25 kg h1 pilot plant that performs well in the pyrolysis of wood wastes. This study of biomass/tyre co-pyrolysis involves a step forward on the integration of the CSBR based biomass pyrolysis process in current oil refineries. Pinewood sawdust and the waste rubber from truck tyres have been co-pyrolysed in order to improve the properties of bio-oil for its integration in oil refineries. In addition, an analysis has been conducted of the effect the interactions between these two materials’ pyrolysis reactions have on product yields and properties. Biomass/tyre mixing ratios of 100/0, 75/25, 50/50, 25/75 and 0/100 by weight percentage have been pyrolysed in continuous mode at 500 C in a conical spouted bed reactor, obtaining oil yields in the 55.2–71.6 wt% range. Gaseous, oil and solid fractions have been characterised for the 50/50 biomass/- tyre mixture, paying special attention to the oil fraction by determining its detailed composition, elemental analysis and calorific value. Co-processing enables the stabilization of the liquid, as the co-pyrolysis oil has a stable single-phase, being composed mainly of water, aromatic hydrocarbons and phenols in concentrations of 14.5, 11.1 and 9.7 wt%, respectively. Adding tyre rubber to the biomass in the pyrolysis feed improves the oil’s properties, as a liquid with higher carbon content and lower oxygen and water is obtained, even if sulphur content is also increased.
Conclusions
High oil yields have been obtained in the continuous pyrolysis of biomass and tyres in a CSBR, which is due to the short residence time and high heat transfer rate in this reactor. Tyre addition in the biomass pyrolysis process decreases liquid yield, from 71.6 to 55.2 wt%, for the same biomass + tyre mass feed rate. Synergetic effects between the pyrolysis reaction intermediates from the two raw materials have been observed in both the bench-scale plant runs and the previous thermogravimetric studies, with the main advantage being the stabilization of the liquid, allowing the formation of a single-phase oil that does not show phase separation problems. The co-pyrolysis oil presents improved properties compared to bio-oil, with higher carbon and lower oxygen and water contents; however, sulphur is also increased, which has to be removed for further applications. In this sense, a two-step hydroprocessing route has been proposed as the most feasible refinery integration pathway for the co-pyrolysis liquid, in order to reduce sulphur, oxygenates, aromatics and heavy hydrocarbons. Therefore, the biomass/tyre co-pyrolysis process is regarded as an efficient approach for on the one hand treating waste tyres by means of a sustainable valorisation method, and on the other upgrading the biomass bio-oil and facilitating its integration in current oil refineries. Finally, the properties of the co-pyrolysis char are suitable for the production of high quality activated carbons in order to improve the economic feasibility of the process.
