Poultry slaughterhouse waste is best split by type. Manure and similar wet organics go to an anaerobic digester for biogas, while carcass remains and dried digestate go to pyrolysis. A proposal for a hot-climate poultry processing site set out this two-route design, with about 60 tonnes a day of manure feeding a digester for roughly 1.3 MW of electricity, and a 1.85 tonnes-per-hour pyrolysis unit taking everything else.

The challenge: waste that does not suit one process

A chicken processing site makes several waste streams. Manure digests well. Feathers, carcass parts and the solids left after digestion do not, and digestate cannot simply be spread without treatment. The owner needed to recover energy and also destroy the residue.

Route one: a continuously mixed reactor

The digester was a continuously mixed anaerobic reactor (CMAR), a stirred tank. Key design figures:

• Feed: 60 tonnes a day of manure at about 25% moisture, diluted with roughly 500 m³ a day of water to a pumpable slurry.
• Retention: about 17 days.
• Tank: one glass-fused-to-steel tank of around 10,000 m³ with a flexible membrane roof that stores about 1,500 m³ of gas. That is roughly 2.5 hours of fuel for a 1 MW engine, enough to smooth out short interruptions.
• Output: about 575 m³/h of biogas at 60% methane, giving an electrical potential of around 1.34 MW in a peak month.

The gas train was a single hydrogen sulphide bioscrubber (about 2,000 ppmv in, 150 ppmv out), a dewatering unit, duty and standby boosters at 300 mbar and a standby flare.

Why a stirred tank rather than a lagoon or UASB

Animal manure has high COD, so a smaller tank suffices, which cuts the footprint. Tank construction is less affected by the rainy season than lagoon earthworks. A UASB was judged unsuitable because suspended solids break up its granular sludge. The trade-off is that a stirred tank is less forgiving of overloading than a lagoon and produces less gas if pushed.

Route two: pyrolysis

Pyrolysis heats material without oxygen so that it breaks down into a fuel gas. The unit was a three-tube module taking up to 1,850 kg/h of wet waste, with the digested solids first dried.

Drying the digestate

• A mechanical dryer takes liquid digestate from about 97% moisture to 65%, removing roughly 555 tonnes of water a day.
• A thermal dryer then takes it from 65% to 25%. Its thermal load was about 1.5 MW.

Drying that much water is the largest hidden load in the whole scheme, which is why the dryer is sized so carefully.

How the pyrolyser operates

1. Compactor screws feed the waste and form a plug that seals out air.
2. Pyrolysis tubes heat the waste indirectly. Above about 250°C organics break down into gas.
3. A gasifier converts the remaining char with steam, leaving ash at about 10% of the feed weight for design purposes.
4. A thermal oxidiser burns the gas at a set point of about 1,000°C. Its hot exhaust heats the pyrolysis tubes, so the process is largely self-sustaining once running.
5. The whole system runs at slight negative pressure, drawn by induced-draught fans, so leaks go in rather than out.

Oxidiser design limits for carbon monoxide and volatile organic compounds were 30 and 7 mg/Nm³ (dry, 11% oxygen), with a stack monitoring system. For comparison with UK practice, see the Environment Agency’s guidance on environmental permits for waste incineration.

Delivery in stages

The package was turnkey and expected to take about 12 months to commission. The proposal split the front end into a feasibility study to confirm waste characterisation, quantities and heat demand, then front-end engineering design (heat and mass balance, process diagrams, general arrangement and data sheets). The budget was to be held within about 10% for the defined scope.

What to consider for your own project

1. Characterise each stream. Separate what digests from what does not, with moisture and calorific value for each.
2. Size the dryer early. Water removal drives energy use and cost.
3. Use the digester gas buffer. A gasholder smooths engine operation and reduces flaring.
4. Plan for ash and effluent. Both need a permitted outlet.
5. Check permitting. Pyrolysis and gasification are regulated as waste treatment in most places.

FAQ

Can chicken manure produce biogas?

Yes. In this design 60 tonnes a day supported around 575 m³/h of biogas and about 1.3 MW of electricity.

Why use pyrolysis as well as digestion?

Because carcass remains and dried digestate cannot be digested, and pyrolysis destroys them while recovering heat.

Why not use a UASB for poultry waste?

Suspended solids damage UASB granules, so a mixed reactor suits this substrate better.

Handling mixed animal-processing waste?

Tell us your waste streams and tonnages and we will suggest a treatment route.