3 Pathways to Meet Your Sustainability Goals
Ideally, when a product can no longer be used for its designed purpose, it’s reused or recycled to enable a “circular economy.”
Under this framework, products are either made from renewable feedstocks or designed to be infinitely recyclable. The purpose is two-fold: first, it decreases reliance on finite resources and second, it reduces landfill waste. A subset of this is chemical recycling, where materials are broken down into their simplest molecular constituents and then reformed into virgin feedstock material.
Pyrolysis, also called thermal processing, is the first step in transforming waste materials like plastic back into marketable products. Not only does pyrolysis reduce the need for virgin petroleum-based feedstocks, but it can also be used to make low-carbon alternatives to fossil fuels. This promotes long-term sustainability to help you meet your ESG goals, while also achieving resource efficiency, cost savings, and regulatory compliance.
While the types of pyrolysis may vary, there are three fundamental ways pyrolysis plays into the circular economy: Torrefaction, Pyrolysis-Derived Fuels, and Pyrolysis Chemical Recycling.
Pyrolysis Processing
Pyrolysis starts with solid or liquid feedstock, and ideally takes place in an atmosphere with no oxygen. When the feedstock is heated above 320°C (610°F), molecules begin vibrating with enough energy that carbon-carbon bonds “crack” to produce solids, liquids, and gasses. Cracking can occur with or without a catalyst, though the presence of a catalyst further reduces activation temperature. Proportions of the solid, liquid, and gas products depends on feedstock, temperature, and time. This simple principle is fundamental for maximizing yields of desirable products (Figure 1).
Figure 1: Pyrolysis converts feedstock to solids, liquids, and gases. Longer exposure to high temperatures further converts liquids into solids and gases.
Torrefaction
Torrefaction is a mild form of pyrolysis that converts biomass into a charcoal-like replacement for traditional solid fossil fuels. In this process, biomass is heated to just under 300°C for twenty to thirty minutes, causing a total evaporation of water and partial decomposition of hemicellulose. This yields a dark, brittle, and dry material—torrefied biomass—which can be further processed into pellets or briquettes.
There are several benefits to using torrefied biomass as a drop-in replacement fuel compared to untreated biomass:
- Torrefied biomass has an energy content 50-70% greater than untreated biomass.
- Thermal treatment breaks down long, fibrous components that make grinding biomass difficult. No special equipment is required for grinding torrefied biomass.
- Torrefied biomass is hydrophobic, meaning it can be stored outdoors without taking up moisture. On the other hand, untreated biomass will readily absorb water, lowering its heating value.
- The torrefaction process eliminates any microorganisms that would otherwise cause decomposition in conventional biomass.
The use of torrefaction comes with certain considerations. The thermal breakdown of biomass releases Volatile Organic Compounds (VOC) such as chloromethane, acetonitrile, hexane, and toluene, which cannot be directly released to atmosphere. These vapors require additional thermal treatment to remove pollutants. Fortunately, VOC elimination technology does exist, such as recuperative thermal oxidation. Another consideration for torrefaction is economics, as solid fuels are typically valued comparing their BTU value to coal and torrefied biomass contains 10-20% less energy than coal.
Pyrolysis-Derived Fuels
Increasing pyrolysis temperatures to between 320-900°C yields liquids and gasses that can be further refined into fuels. Generally, there are two types of pyrolysis: fast and slow. Fast pyrolysis is characterized by its high heat transfer rates and small feedstock particle size (< 1mm), with reactors having residence times on the order of seconds. The short residence times in fast pyrolysis favors liquid production.
Slow pyrolysis has larger feedstock particle sizes, and residence times that can last minutes to hours. While slow pyrolysis yields fewer liquids, the liquid products typically contain less oxygen and are more stable. Oxygen is undesirable in pyrolysis oils because it facilitates polymerization. To improve stability and miscibility with conventional transportation fuels, pyrolysis oils should be treated with hydrogen. Other treatments such as oligomerization and isomerization further improve fuel quality, and distillation produces different “cuts” of bio-oil based on boiling point ranges. Advantages of pyrolysis-derived liquid fuels include low lifecycle GHG emissions and low sulfur emissions. On the flip side, pyrolysis oils have high capital costs associated with stabilization and upgrades.
Over extended periods at high temperatures, pyrolysis produces larger quantities of gas. This is because the liquid products continue reacting to form solids and gas (Figure 1). This gas is a mixture of CO, CO2, H2, water, CH4, and C2-C6 components, as well as tar. Pyrolysis gas can be used as combustion fuel with minimal treatment, or it can undergo purification to become Renewable Natural Gas (RNG). Challenges with pyrolysis include high energy requirements and tar plugging.
Pyrolysis Chemical Recycling
Pyrolysis can also be used in plastic recycling to break polymer chains down into their monomer components. In turn, monomers like ethylene and styrene can be re-manufactured into plastic products like polyethylene and polystyrene. Chemically recycled plastics have physical properties that are very similar to those of virgin products. The alternative, mechanical recycling, experiences degradation in the forms of cross-linking and broken polymer chains. Most plastics can undergo two or three rounds of mechanical recycling before they start to degrade in quality. Chemical recycling provides an opportunity to recycle plastics indefinitely.
Chemical recycling is not without its challenges, the first being sorting plastic waste. Most recycled plastics arrive as a mixed stream of polyethylene (PE), polypropylene (PP), polyvinylchloride (PVC), Polystyrene (PS), and polyethylene terephthalate (PET). While PVC is easy to separate by identifying it with hyperspectral cameras, there is limited technology available for sorting the remaining plastics; they all have similar physical properties. In addition, some plastic wastes are composites of multiple plastics--often seen in food and beverage packaging. Other challenges include high energy requirements and capital costs, especially compared to mechanical recycling.
Key Takeaway
Though there are still challenges associated, pyrolysis has considerable potential to enable circular economies. Whether used for solid fuel, transportation fuel, or plastic recycling, there is room for innovative engineering solutions.
Article originally published on the POWER Engineers website.