Pyrolysis: what it is and how it works

Pyrolysis involves breaking down - cracking - waste molecules at temperatures ranging from 300 to 900°C in an oxygen-free environment. Without oxygen, combustion cannot occur, meaning this is not incineration: the waste does not burn; instead, its molecules crack and thermally decompose. 

For plastic waste, the operating temperature range is between 400 and 550 °C depending on the waste and processes. 

This process yields three valuable streams: gases (such as CO, H₂, CH₄, and CO_2), oils, or pyrolysis liquor with different compositions, and a solid fraction or carbon/ char. The specific ratio depends, as mentioned, on the conditions of pressure, temperature, and contact time, as well as type of waste, technology, etc. In general, higher temperatures and longer contact times at atmospheric pressure leads to higher yields of gaseous and liquid fractions, however, above a certain temperature, secondary cracking of the products leads to greater gas production.  

The absence of oxygen reduces emissions while enhancing output value. Thus, the ultimate goal is not merely waste elimination or energy recovery, but turning waste into high-value materials within the circular plastics lifecycle.

Through this process, pyrolysis transforms municipal, industrial, and other hard-to-manage waste that would otherwise overwhelm landfills into recyclable materials, opening up significant recovery opportunities while improving overall plastics management. 

This technology is applicable across a broad spectrum of plastic waste, showing high tolerance for contamination and mixing; nevertheless, specific waste streams benefit most from this management solution. These are:  

• Plastic recycling: plastics unsuitable for mechanical recycling, the most complex to process, are converted through pyrolysis into reusable oils and chemical components. This reduces reliance on primary virgin raw materials and promotes a continuous reuse model. For example PE, PP and PS along with additives, pigments and contaminants.  
• End-of-life tires (ELTs): an abandoned tire can take up to 1,000 years to decompose, releasing pollutants into the environment. Nationally, over 300,000 metric tons are generated annually, much of which lacks a clear recovery pathway. Through pyrolysis, Repsol transforms the plastic and rubber components of tires into pyrolytic oil and recovered carbon black (r-Carbon Black). 

Additionally, although energy generation is not the primary objective, combustible gases produced during thermal decomposition can be used directly as an alternative energy source. They are especially useful in industries that require gas for process heating in furnaces or material drying. 

In the specific case of pyrolyzing organic waste, such as MSW, agricultural, forestry, and livestock waste, the process yields a solid by-product known as biochar. This carbon material is incorporated into the ground to enhance soil quality, boosting its water and nutrient retention capacity. The properties of this biochar depend on the process conditions, feedstocks, and impurities present.

The true potential of this technology is realized when waste-derived pyrolysis oil is integrated into Repsol's industrial facilities. This oil is incorporated into the petrochemical process as an alternative feedstock, replacing non-renewable fossil resources. The result is polymers that retain the exact same properties, functionality, and top quality as those traditionally produced from virgin feedstocks. 

Since 2015, Repsol has been using this type of oil continuously at an industrial scale at its Puertollano complex, positioning the company as a national pioneer in its utilization. This technical effort allowed Repsol to become the first Spanish company to obtain the ISCC PLUS certification for the production of circular polymers in 2020. Currently, the industrial complexes in Tarragona, Puertollano, and Sines hold this rigorous certification, which is based on the mass balance approach. 

This technical capability extends beyond polyolefins, which are used across a wide range of plastic applications (from industrial packaging to everyday products such as toys). Repsol also produces circular styrene from pyrolysis oil extracted from used tires.

All circular polymers produced by Repsol, with certified recycled and/or renewable content are marketed under the Repsol Reciclex® trademark. This range covers three lines of innovation designed to meet different industrial needs: 

• Mechanical recycling. Incorporates recycled plastics into new products, achieving formulations with up to 85% recycled material. It offers over 40 different grades with virgin-like technical properties for non-food applications, certified by Recyclass, achieving carbon footprint reductions of up to 40% compared to traditional virgin plastic production.
• Chemical recycling. It includes circular polymers obtained from pyrolysis oil. They are fully suitable for sensitive applications such as food contact, cosmetics and medical applications. Highlights include the Repsol Reciclex® circular polyol, one of the first non-fossil polyols developed in the Iberian Peninsula, which enables the prodcution of flexible foams for mattresses and furniture, reducing the carbon footprint by 20% to 30% compared to its fossil-based equivalent. 
• Bio-circular solutions. Bio-based polymers, with ISCC PLUS certification, made from renewable waste. They have a negative carbon footprint, as they remove more CO₂ from the atmosphere than is generated across their supply chain according to the cradle-to-gate methodology (covering all stages from raw material extraction and processing to the final product manufacture).

With this portfolio, we have set an industrial target that, by 2030, 10% of our polyolefin production will incorporate recycled or renewable content.

Repsol's strategy is not limited to using existing technology: it is firmly committed to its continuous development. 

Research efforts are consolidated through projects such as Plastics2Olefins, a consortium coordinated by Repsol comprising eleven European partners. Funded by the European Union's Horizon Europe program, the project is developing a high-temperature pyrolysis technology that will directly allow the production of circular olefins. By integrating renewable electricity, CO₂ emissions are estimated to decrease by approximately 70%. The pilot plant currently operates at the Repsol Tech Lab with a capacity of 10 kg/h, and plans are underway to scale the technology to a 1 t/h demonstration plant that will be installed at the Puertollano Industrial Complex. 

This is further supported by key strategic alliances. Repsol has collaborated with the French Institute of Petroleum (IFPEN) and Axens to develop and patent the RewindTM Mix process, designed to improve the purification of pyrolysis oil from plastics, of which Repsol is co-owner of five patent families. Likewise, through Repsol Venturing, it supports the startup GreenVal Technologies in scaling up end-of-life tire pyrolysis, by providing technical support to its pilot plant in Zaragoza and ensuring the resulting pyrolytic oil is integrated into Repsol's industrial complexes to manufacture new materials.

Industrial-scale deployment of pyrolysis is supported by an increasingly stringent European and Spanish regulatory framework in terms of recycled content and waste management. 

• PPWR (EU Regulation 2025/40). It establishes obligations for the minimum recycled plastic content in packaging and regulates design for recyclability. It represents a step up in ambition and provides momentum for technologies such as pyrolysis, which enable these percentages to be met. 
• End-of-Life Vehicles (ELV) Regulation. It regulates end-of-life vehicle management and introduces recycled plastic targets for manufacturing new vehicles, including chemically recycled materials. 
• Law 7/2022 on Waste and Contaminated Soils. Spanish regulations establish a clear hierarchy in waste management: first prevent, then reuse, recycle, recover energy and, only as a last resort, dispose of it in a controlled landfill.

Law 7/2022, of April 8, on waste and contaminated soils for a circular economy frames the Spanish approach and places material recovery - to which pyrolysis contributes - ahead of energy recovery or landfill.

The deployment of pyrolysis is part of a broader ecosystem: waste recovery Waste recovery means giving a new technical or material utility to what was previously considered a waste, the inefficient management of which could generate problems for society and the environment. 

The numbers show that there is substantial room for operational improvement. In 2021, Spain generated nearly 138 million metric tons of waste, including plastic waste, of which barely 15% was recycled. Mature technologies such as pyrolysis represent a solid industrial response to this large-scale challenge. OECD reports on plastics (Global Plastics Outlook) confirm this trend on a global scale.

A plastic bag that is not suitable for conventional mechanical recycling or a tire that would take nearly a millennium to degrade now has a radically different destination than a landfill. When the technology and industrial rigor exist to apply it, environmental challenges can be transformed into competitive resources. 

Pyrolysis thus joins the suit of technological solutions applicable to plastic waste management. Chemical recycling, alongside mechanical recycling, demonstrate that - with the right technology and necessary investment - materials can increase their recyclability and value within a consolidated circular economy. 

What was discarded a few years ago for lack of alternatives, now becomes - thanks to circularity and continuous innovation - the feedstock for food packaging, an automotive part, or the key polymer for a medical package. Chemical recycling, and specifically pyrolysis, does not replace prevention, reuse or mechanical recycling: it expands the options available for complex, hard-to-manage waste streams.