For remote sites, a small pyrolysis plant is best paired with an Organic Rankine Cycle (ORC) generator rather than a gas engine, because syngas from mixed waste is variable and tends to foul engines. In a white paper for a US defence research programme, Organics proposed a containerised plant that treats up to 5 tonnes of unsorted waste a day and produces about 50 kWe, using the syngas as its own fuel once running.
The challenge: waste disposal at a remote site
The programme was looking for small-scale waste-to-energy for remote locations. The requirements, as the proposal understood them, were ease of use, simple operation and low emissions. The plant had to be robust enough for the most junior personnel to run, with minimal downtime for maintenance.
Incineration, the usual approach, needs a constant supply of fossil fuel to burn the waste. Pyrolysis works differently. Once the plant is up to temperature, the waste produces a synthesis gas, and that gas becomes the fuel that keeps the process going and can export heat or gas.
The approach: pre-treatment, pyrolysis, oxidation, ORC
1. Prepare the feed
The pyrolyser needs waste that is no larger than 30 mm and at 15% moisture or less. A shredder and a thermal dryer at the front end deliver that. Waste enters through a hopper, goes directly to the shredder, and then to the dryer. The feed height was limited to no more than about 3 feet, to make loading easy.
2. Pyrolyse and gasify
The pyrolyser is a single tube with a screw. Heat is applied to the outside of the tube, so the waste inside is heated without oxygen. The screw moves the waste along the tube, and at the end the carbon char falls into a gasifier chamber. There, a combination of heat, char and steam converts the char to ash and produces a syngas of mainly carbon monoxide and hydrogen. This mixes with the volatile organic gases driven off in the pyrolyser to form the fuel gas.
3. Oxidise the gas
The fuel gas goes to a fully contained thermal oxidiser, where it burns under controlled conditions. The hot exhaust gas has two uses: it heats the outside of the pyrolyser, and it drives the ORC generator through a thermal oil circuit and an economiser. The exhaust then goes to a gas clean-up system. The proposal stated that the syngas is heated to 1,000°C to prevent the formation of tars, dioxins and furans.
4. Generate with an ORC
The ORC turns the recovered heat into about 50 kWe. The unit is housed with the rest of the plant in two 40 ft ISO containers, with a design throughput of 5 tonnes a day.
5. Start-up and control
The plant is heated to temperature in roughly 30 to 60 minutes using containerised gas. Once pyrolysis starts and syngas is made, the supply gas is shut off. The control system allows autonomous operation with minimal operator input.
Why it works: the choice of ORC
The proposal explained its choice of generator in some detail. A gas engine would give a higher power-to-heat ratio. But syngas from mixed waste varies in composition and risks tarring the engine, which then blocks and needs cleaning very regularly. The proposal’s authors noted many commercial waste-to-energy projects that have failed for this reason.
An ORC is a closed loop. It has fewer moving parts and produces no emissions of its own, since the heat is transferred through a thermal oil circuit. It is safer and more reliable than a gas or steam-driven generator, which carry greater maintenance and health and safety demands. For a remote site with few skilled operators, that is the better trade.
Emissions
To meet state and federal air emissions requirements, a continuous emissions monitoring system (CEMS) was included. The plant has a simple traffic-light indicator: green for compliance, yellow for borderline and red for non-compliance. In the UK, the equivalent rules for thermal treatment sit within the Environment Agency’s permitting regime; see the gov.uk environmental permit check.
Risks the proposal flagged
- Technical. The proposal rated technical risk as minimal because all components were commercially demonstrated. The main caveat was that a thermal process likes steady conditions, whereas the waste input at a remote site is highly random. That could mean more support fuel than expected.
- Cost. The performance testing regime was the most uncertain item, because such activities tend to become protracted. Support fuel costs would depend on usage.
- Schedule. Manufacture and delivery were firm, but approvals were not defined.
Delivery programme
The work was phased, with payments linked to milestone deliverables. It covered contract finalisation, preliminary design (process flow diagram, P&ID, general arrangement, mass and energy balance and draft manual), front end engineering design, manufacture, factory test, transport and a site test programme. A mass and energy balance model would show expected performance at various scales in terms of feedstock and energy output. The overall duration was 22 months from award, with the longest period being manufacture.
What to consider for your own project
- Define the waste first. Density, mass and calorific value decide whether the waste can sustain the heat needed and meet the energy output.
- Pick the generator for the gas you will really get. Variable syngas favours a heat-recovery route over a gas engine.
- Plan for random feed. Allow for support fuel and consider buffering the feed.
- Design for the operators you will have. Simple indicators and autonomous control reduce the training burden.
- Agree acceptance tests early. Performance testing is where time and cost can run on.
For a different pyrolysis application, see our post on questions to ask a bamboo charcoal pyrolysis supplier.
FAQ
What is the difference between pyrolysis and incineration?
Pyrolysis heats waste without oxygen to give char and syngas. Incineration burns waste with oxygen and, in the proposal’s description, typically needs a constant fossil fuel supply.
Why use an ORC instead of a gas engine?
Syngas from mixed waste is variable and can tar a gas engine. An ORC works on recovered heat in a closed loop, with fewer moving parts.
How much waste can a containerised plant treat?
This design handled up to 5 tonnes a day of unsorted solid waste in two 40 ft containers.
How is the char dealt with?
Char from the pyrolyser falls into a gasifier chamber, where heat, char and steam convert it to ash, reducing the residue volume.
Looking at waste to energy for a remote site?
Tell us your waste type and daily tonnage and we will discuss a suitable configuration.