Wet sludge must be dried before it can be pyrolysed. A small-scale design for a North African client shows how: a recycle dryer takes sludge from 80% to 20% moisture, and a containerised pyrolyser then converts the dried sludge to gas and ash, with a thermal oxidiser burning the gas. This article explains each stage and the design limits.

The challenge: sludge that is mostly water

The design basis was sludge produced at about 10 m³ a day with 80% moisture. That sludge is heavy to move and has little usable energy as received, because most of its mass is water. Taking 10 tonnes of 80% moisture sludge down to 20% moisture removes roughly 7.5 tonnes of water and leaves around 2.5 tonnes of dried material, which is the quantity the pyrolyser would handle. The source documents give slightly different daily tonnages, so any real project needs a mass balance based on measured sludge.

The approach: dry first, then pyrolyse

Stage 1: recycle drying

The optional drying system brings sludge from 80% to 20% moisture. It uses a feed hopper train, a mixing chamber and a heated thermal screw, with a recycle conveyor returning dried material to mix with wet feed. Mixing wet sludge with dried product avoids the sticky, plastic phase that makes sludge hard to dry. Heat is supplied through a hot oil boiler, which can be fired either by the plant’s own exhaust gas or by diesel.

Stage 2: the pyrolysis plant

The dried sludge passes through:

1. A feed hopper and screw compactor, which seals the process from air.
2. A pyrolyser, where indirect heat breaks the organic content into gas.
3. A gasifier, which converts the carbon char to more gas.
4. A thermal oxidiser with a pre-heater burner (LPG), which burns the gas at high temperature.
5. Ash removal and exhaust gas clean-up.

Everything is containerised, which keeps site work to a minimum.

Design data in context

• Calorific value: 12 MJ/kg on a dry basis, within a design range of 8 to 30 MJ/kg.
• Thermal input: about 833 kW.
• Moisture limit: the plant was designed for no more than 20% moisture. Above 40% moisture it is likely to be damaged or to perform badly.
• Ash: up to 40% of the feed by weight.

The 12 MJ/kg figure is a modest calorific value, which is typical of sewage sludge. It shows why the plant relies on a burner for start-up and as a back-up.

Electricity is an option, not the aim

This design was for sludge reduction. If a boiler and steam turbine were added later, the plant could generate electricity, but that was not part of the scope. The primary aim is to cut the volume and hazard of sludge. The thermal oxidiser operates at a temperature that is intended to eliminate toxic organic compounds, including dioxin precursors.

What to consider for your own project

1. Treat drying as part of the system. The dryer accounts for a large part of both capital and running cost.
2. Pick the heat source. Exhaust gas recovery reduces fuel use. Diesel is simpler but costs more to run.
3. Control moisture. Keep feed below the design limit, with monitoring at the hopper.
4. Plan for ash. Ash can be up to 40% of the feed on some sludges.
5. Check regulation. In the UK, thermal sludge treatment needs an environmental permit.

FAQ

Why not pyrolyse wet sludge?

Water absorbs heat that would otherwise break down the organic matter, and high moisture can damage the plant.

What does the plant produce?

Gas that is burned in the oxidiser, and ash. No electricity unless extra equipment is added.

How long does a project take?

The proposal allowed about twelve months from start to commercial operation.

Is the unit containerised?

Yes, to minimise time on site.

Looking to cut sludge volumes?

Tell us about your sludge and we will outline the drying and thermal treatment options.