What Your Offtake Tolerates: A Specification Guide for Syngas Purification

"Clean gas" is not a specification.

A gas cleaning system has no inherent purity target. The target is set downstream, by the most sensitive thing on the other end of the pipe — a combustion chamber, a synthesis catalyst, a membrane, a fuel cell. Change the offtake and the same untreated gas is either perfectly acceptable or instantly fatal.

This is the part of waste-to-energy that gets discussed least, because it is a specification conversation rather than a technology conversation. It is also where the money is decided.

FOUR OFFTAKES, FOUR TOLERANCE LEVELS

Syngas leaves a plant in one of four main directions. Each tolerates a different amount of everything the gasifier sends with it.

GAS ENGINE / CHP

• Tar: 10–50 mg/Nm³
• Sulfur (H₂S / COS): tens of ppm
• Particulate: under 10 mg/Nm³
• Also sensitive to: alkali metals, ammonia

METHANOL SYNTHESIS

• Tar: essentially zero
• Sulfur: ppb to sub-ppm
• Particulate: filter grade
• Also sensitive to: H₂/CO ratio, CO₂

FISCHER-TROPSCH LIQUIDS

• Tar: essentially zero
• Sulfur: under 0.1 ppm
• Particulate: filter grade
• Also sensitive to: H₂/CO near 2, CO₂

HYDROGEN (PSA / MEMBRANE)

• Tar: none
• Sulfur: ppb
• Particulate: none
• Also sensitive to: CO in the ppm-to-ppb range

These are indicative orders of magnitude of the kind used in project specification. The binding limit is always the number written into the equipment or catalyst vendor's guarantee.

The pattern matters more than the digits: combustion forgives in parts per million. Synthesis catalysts forgive in parts per billion. Fuel cells forgive almost nothing.

THE RULE THAT CATCHES PEOPLE

A specification is not an average. It is the single least tolerant component in the chain.

Put a catalytic converter after the engine and you no longer have an engine spec — you have the converter's spec, which is stricter. Add heat recovery with a finned tube bundle and fouling behaviour starts to matter more than combustion chemistry. Pair a methanol loop with a water-gas shift to correct the H₂/CO ratio and the shift catalyst brings its own sulfur limit upstream of everything you were originally worried about.

Most expensive purity decisions in this industry come from discovering the sensitive component late. The gas meets the spec it was designed against and still destroys the equipment it actually feeds.

SPECIFY FROM THE MOLECULE OUTWARD

The practical consequence is that a plant should be specified from the offtake backwards.

1. Pick the offtake. Not the aspiration — the real buyer or the real application.
2. Read its tolerance limits and subtract a margin.
3. That gives the contaminant budget — the maximum mass of tar, sulfur, chlorine, ammonia and particulate per normal cubic metre.
4. The budget decides which cleaning stages exist and how hard each one has to work.
5. Only then does equipment selection begin.

Run it the other way round and you get the common failure: a cleaning train designed to a round number, followed by a search for a buyer who can live with the result. That search usually ends at combustion, because combustion is what is left when the gas is not clean enough for anything more valuable.

PURITY IS A PRICE TAG, NOT A COMPLIANCE COST

The same carbon can leave a site as electricity, as methanol, as diesel-range liquid, or as hydrogen. Those are four products with four prices, and the only thing separating them is the specification the gas meets.

Every contaminant removed converts a molecule into a more valuable molecule — or into a useless one, if the cleaning is aimed at the wrong target. Gas cleaning is a revenue decision.

WHERE SYNGAPURE FITS

SyngaPure's three cleaning functions each own one contaminant family. Catalytic tar reforming handles tar. Sorptive binding handles sulfur, chlorine and ammonia. Filtration handles particulate below 10 mg/Nm³. Because the stages are mapped to contaminant families rather than to a fixed output specification, changing the offtake moves the configuration with it instead of forcing a redesign.

Development is backed by the BMWK go-inno programme, with research partnerships active on catalyst and sorbent work.

WHAT ARE YOU DESIGNING FOR?

If your specification is written around a specific offtake — engine, methanol, FT, hydrogen — we would like to see it. If you are carrying an inherited spec that nobody can trace back to a buyer, that is worth a conversation too.

Talk to us at zero-x.co