[JulianMorrison]

Dry the algae, press it into bricks, put it down a mine.

Charcoal is another option, including putting that into soil as a supplement for agriculture.

[dredmorbius]

Any processing which requires specific processing and handling is ... likely to scale poorly. The quantities involved are simply enormous.

Human CO2 emissions are about 43 billion tonnes annually. Let's say we're hoping to just take a bite out of that rather than compensate the whole thing, so we'll aim for a bit over 10% with 5 billion tonnes of stored CO2, to make the maths easier.

Let's say we're making and storing hay bales as a way of sequestering carbon.

A hay bale 16" * 18" * 36" weighs about 50 lb. (40 * 45 * 90 cm and 22 kg respectively). That's what you'll see as a traditional human-handled hay bale.

I'm not positive of the precise chemical composition, but I'll assume a 20% moisture content (by mass), and the remainder consisting of cellulose (C6 H10 O5). That leaves about a 35% net carbon content.

Note that carbon itself is only 27% of the mass of CO2, with most of the molecular weight being oxygen, so to sequester 1 billion tonnes of CO2, we need only bury 260 million tonnes of pure carbon.

We want to sequester five billion tonnes CO2 here, so 1.3 billion tonnes of carbon, or in the form of hay, 3.7 billion tonnes of hay bales. Let's round again, for convenience, and call it 4 billion tonnes of hay.

One tonne is roughly 40 bales.

One billion tonnes is 40 billion bales.

Four billion tonnes is 160 billion bales.

If we stack these to a height of 10m (33 feet), roughly 20 bales high, each tower weighs a half tonne. We need 8 billion of those stacks. Simply taking squares, that's a stack roughly 90,000 bales on a side, and 20 bales tall. (89,442.719 bales on a side if you want to be precise, I'm ... not.)

90k * 40 cm is 36 km (22 mi).

90k * 90 cm is 81 km (50 mi).

You'd need to bale, stack, and rack that much hay in a year. And repeat it every year.

And you're only accounting for 10% of annual human carbon emissions.

A process which captures, converts, and buries the carbon on its own would ... likely be preferable.

Note as well: large stacks of hay have a strong tendency to self-ignite through metabolic action. Or by other mechanisms in places such as Gävle.