If a factory already has a thermal oxidiser, a pyrolysis module can feed it, so the new equipment can be smaller and cheaper than a full waste-to-energy plant. This article describes a containerised pyrolysis unit proposed for a flour-processing site in Asia, designed to destroy residue and reuse the existing oxidiser.

The challenge: a waste stream, and equipment half in place

The site generated a dry, high-calorific residue from flour processing. The system had to handle about 26.7 dry tonnes a day with a calorific value of around 19.5 MJ/kg dry, which is high compared with municipal waste (typically about 18 MJ/kg dry in the other proposals in this series). The site already had a thermal oxidiser, so a design that duplicated it would have wasted money.

The approach: a K100 module that hands over to the existing oxidiser

The proposal was for an Organics K100: a two-tube containerised pyrolysis module sized to fit a 40-foot shipping container, able to process up to 1.1 wet tonnes an hour. The main components were:

• Feed hopper and conveyor.
• Two compactor units, which seal the process against air.
• Two pyrolysis tubes and screws.
• One gasifier and an ash removal system.
• A small thermal oxidiser sized only for the pyrolysis unit’s own heat load.
• A hotbox, a bag filter and two induced-draught fans.

The small oxidiser directs only about 30 kW of heat to drive the pyrolysis tubes. The rest of the heat goes to the existing oxidiser, on the assumption that it operates at negative pressure. If the existing oxidiser cannot be used, the proposal says it would have to be revised. That caveat is the key interface risk to settle with the client early.

Why it works

Heat is shared, not wasted

The pyrolysis tubes are heated indirectly by hot gas from the oxidiser, so the plant is largely self-sustaining once it reaches operating temperature. The process turns the residue into ash and useful heat. Moisture entering the pyrolyser was assumed to be 20% or less, because water uses heat that would otherwise drive the reactions.

Sealed against air

The compactor screws squeeze the waste into a plug that seals the feed point. The gasifier chamber is sealed too. Keeping air out minimises premature combustion and helps the gasification reactions.

Cleaning the flue gas

The exhaust train for this project was more comprehensive than a simple filter:

1. Sodium bicarbonate injection in a venturi removes acid gases (hydrogen chloride and sulphur dioxide, and possibly hydrogen fluoride). The bicarbonate spends at least one second in suspension before reaching the filter.
2. Bag filter captures the reaction products, mostly sodium chloride and sulphite or sulphate salts, and the layer on the bags continues to absorb acid gas.
3. Mercury capture filter removes mercury vapour.
4. Catalytic NOx reduction deals with nitrogen oxides.
5. Feedwater pre-heater recovers remaining heat.

Two induced-draught fans, each able to handle 100% of the duty, pull gas through. They are sized with a 15% flow margin, a 35% pressure margin and a 40°C temperature margin, and silencers are fitted downstream. Bicarbonate arrives in bulk road tankers of around 25 tonnes.

What to consider for your own project

1. Audit the existing oxidiser. Check capacity, temperature, residence time, pressure and condition before relying on it.
2. Know your residue. Calorific value, ash and chlorine decide the flue gas treatment.
3. Keep moisture low. Aim for 20% or less going in.
4. Plan for ash disposal. It is stored in sealed containers for removal off site.
5. Check the permit. In the UK, burning waste needs an environmental permit.

FAQ

Can pyrolysis use an existing oxidiser?

Yes, if it can accept the gas flow and temperature and operate at negative pressure. This must be confirmed on site.

What does the module make?

Ash, and heat that is passed to the oxidiser. Electricity was not part of this scope.

How big is the module?

It is sized to fit a 40-foot shipping container.

Why are the pyrolysis tubes heated indirectly?

So the waste never meets the combustion gases, and the process stays oxygen-starved.

Got a residue stream and a hot oxidiser?

Talk to us about connecting a pyrolysis module to your plant.