Bio-polypropylene is made in three steps: a renewable feedstock such as sugar crops, wood residues or used cooking oil is converted into propylene by one of several routes, and that bio-propylene is polymerized in ordinary PP plants the same way fossil propylene is.
The routes that have run commercially feed renewable hydrocarbons through existing fossil units alongside fossil feed, so the bio share of each lot is assigned in the books by mass balance; European Bioplastics calls such resin “bio-attributed” rather than biobased. Routes that make propylene from biomass alone, so that all of its carbon is biogenic, have been run at pilot and demonstration scale on ethanol and isopropanol.
Step 1: The Renewable Feedstock
Bio-PP is made from three kinds of renewable feedstock:
- Sugar and starch crops: sugarcane and corn, the crops conventional bio-ethanol is made from.
- Lignocellulosic biomass: crop and forestry residues and municipal organic waste, the non-edible part of plant material.
- Vegetable oils and fats: rapeseed and sunflower oil, used cooking oil, animal fats, and tall oil, a by-product of kraft pulping.
Oils and fats, including waste and residue grades, are the feedstock of the routes that have run commercially.
Step 2: Converting the Feedstock to Bio-Propylene
Renewable feedstock becomes propylene in one of two main ways: oils and fats are hydrotreated into hydrocarbons and fed to existing fossil units alongside fossil feed, or sugars and lignocellulose are first turned into an alcohol that a dedicated chain converts into propylene. The fossil units are a steam cracker or a propane dehydrogenation plant, and the alcohol is ethanol, isopropanol or methanol.
Renewable Hydrocarbons Co-Fed to a Steam Cracker
A steam cracker makes bio-propylene by cracking hydrotreated vegetable or waste oil together with its usual fossil naphtha or gas oil. Vegetable-oil steam cracking was already listed as commercial in a 2015 review.
Because the renewable and fossil feeds are blended, the bio share of a cracker-route lot is attributed by mass balance under ISCC PLUS: the producer can book its certified renewable input to selected output lots, as long as the total does not exceed what went in, less losses. Each delivery then ships with a sustainability declaration stating the booked quantity, issued under the producer’s site certificate, which is valid for 12 months.
Renewable Propane Through Propane Dehydrogenation
A propane dehydrogenation (PDH) unit strips hydrogen from renewable propane to make propylene, exactly as it does with fossil propane. Renewable propane is a by-product of hydrotreating vegetable oils and fats into renewable diesel, jet fuel and gasoline.
Borealis reported starting renewable-PP production on this route in December 2019, feeding Neste’s renewable propane to its PDH plant at Kallo, Belgium, in partial replacement of fossil propane. As on the cracker route, the bio share of each lot is attributed by mass balance under ISCC PLUS.
Bio-Ethanol to Propylene
Bio-ethanol is converted to propylene either through ethylene, in several steps, or directly over a catalyst in one reactor. In the multistep route the ethanol is dehydrated to ethylene, and part of the ethylene is dimerized to butene, which after isomerization is reacted with the rest of the ethylene by metathesis to give propylene.
Commercial production on this route is planned from 2029, when Citroniq’s plant in Nebraska is scheduled to start.
Fermentation to Isopropanol, Then Dehydration
The isopropanol route ferments biomass sugars to isopropanol with an engineered microbe and dehydrates the isopropanol to propylene. Before dehydration, the isopropanol has to be distilled out of a dilute fermentation broth.
Mitsui Chemicals ran the whole chain at demonstration scale in a project with Japan’s Ministry of the Environment. The project’s final report, dated February 2022, named cost and reaction efficiency among the issues still to solve before commercialization.
Gasification to Methanol, Then Methanol-to-Propylene
The gasification route makes methanol from gasified solid biomass, such as wood chips, and then converts the methanol to propylene. The conversion uses either methanol-to-propylene (MTP), built to make mainly propylene, or methanol-to-olefins (MTO), which makes ethylene and propylene together.
The two stages have each run commercially, but separately; combining them to make bio-propylene has been assessed in design studies.
Step 3: Polymerizing Bio-Propylene in Existing PP Plants
Bio-propylene is polymerized in existing PP plants with the same catalysts and conditions as fossil propylene, because it is the same molecule. The polymerization train is the one used in conventional PP manufacturing, and the resin is chemically identical to fossil PP: it comes in the same homopolymer, random and impact copolymer PP grades and goes into the same recycling stream. Being bio-based does not make it biodegradable.
Bio-based carbon in finished PP is measured by radiocarbon analysis, to ASTM D6866 or its equivalent ISO 16620-2, or to EN 16640 in Europe. The test reads what is physically in the sample, so it can verify the bio content of resin from a dedicated route, but on a mass-balance lot it need not agree with the declared share.