Marine biomass as a feedstock: wet, salty, and worth it
Seaweed is not wood, and it is not a crop. This page explains what macroalgae are made of, why that composition rules out the routes designed for dry land biomass, and what a process has to handle to turn wet marine biomass into fuel.
What macroalgae are
Macroalgae are the seaweeds visible to the naked eye: green, red and brown species that grow in the sea using sunlight, carbon dioxide and the nutrients dissolved in the water. They need no soil, no irrigation and no fertiliser. Sargassum is a brown macroalga. The broad family also includes kelp, farmed for food and alginates, and the red algae behind agar and carrageenan.
What they are made of
A freshly collected seaweed is mostly water, typically eight to nine tenths of its weight. Of the dry matter that remains, the largest share is carbohydrate, in the form of polymers that land plants do not make: alginate, fucoidan and laminarin in brown algae, alongside mannitol and some cellulose. Proteins follow, then a small lipid fraction. Two features set seaweed apart from wood and straw. It contains little or no lignin, the rigid polymer that makes wood hard to break down. And it carries a large mineral fraction, a fifth to two fifths of the dry matter, made of the salts of seawater and of elements the seaweed concentrates from it, including arsenic and other heavy metals. Sand comes with any seaweed collected on a beach.
Land or sea: the resource base
| Terrestrial biomass | Marine biomass |
|---|---|
| Needs arable land, in competition with food and feed | Needs no arable land |
| Needs fresh water and fertiliser | Grows in seawater on dissolved nutrients |
| Yields limited by agriculture and climate | Vast resource; sargassum alone strands by the millions of tonnes |
| Dry or dried before processing | Wet as collected, with salts and minerals |
| Scalability ceiling set by land | Ceiling set by collection and logistics, not by land |
Wet routes and dry routes
Most biomass conversion routes were designed for dry, milled feed: combustion, pyrolysis and gasification all need the water gone first. Evaporating water costs about 2.3 megajoules per kilogram, so drying seaweed that is nine tenths water spends more energy than the seaweed contains. The salts then concentrate in the char and ash, and the chlorides attack ordinary steel. Biological routes such as anaerobic digestion accept wet feed but work slowly, are inhibited by salt and metals, and produce a gas rather than a liquid fuel.
The alternative is to convert the biomass with its water present, under controlled temperature and pressure. That is the family of routes Salacia's process belongs to: thermochemical conversion without a drying step, built for marine feedstocks. Sargassum takes up salts and heavy metals from seawater, and its salt content means chlorides, corrosive to ordinary equipment and unwanted in a fuel. Rather than treating that in extra stages, the first conversion step itself separates these elements from the fuel path while the valuable molecules are transformed, which also helps the energy balance of the process. The large water fraction that conversion produces is a resource of its own: it can be recycled within the process or reused by other industries.
| Route | Accepts wet feed | Main product | What salts and metals do |
|---|---|---|---|
| Combustion, pyrolysis, gasification | No, drying first | Heat, bio-oil or gas | Concentrate in ash and char; chlorides corrode |
| Anaerobic digestion | Yes | Biogas | Inhibit the microbes; stay in the digestate |
| Thermochemical conversion under pressure, water present | Yes | Liquid fuel precursors, aqueous phase, solids | Separated from the fuel path in the first step |
What a process has to handle
- Water: eight to nine tenths of the mass, to be used, not fought.
- Chlorides: corrosion, so materials and conditions are chosen to withstand them.
- Arsenic and other heavy metals: to be kept out of every product, fuel and co-products alike.
- Sand and debris: wear on equipment, removed at collection and conditioning.
- Variability: species mix, season, freshness and salt content change from one delivery to the next, so the feed is standardised before conversion.
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