Coal dust pyrolysis can turn a waste fines stream into electricity, but the headline output depends on one decision that is easy to overlook: how the turbine exhaust is cooled. In an outline proposal for a southern African site, about 26 dry tonnes a day of coal dust supported roughly 1.2 MW of gross electricity and about 1 MW of net export, with cooling choice swinging the plant’s own consumption between about 0.2 and 0.4 MW.
The challenge: fines that are too small to sell and too useful to waste
Coal fines and dust are costly to handle and often sold at a discount, if at all. They are, however, rich in energy. This feedstock was assumed at 28 MJ/kg on a dry basis, which is a high figure for a fuel. At 26 dry tonnes a day, the average thermal input comes to about 7 MW, enough to support a small steam power plant.
The proposal was an outline to a developer, not a finished design. It was explicit about that, and it set out the steps a client would take before spending real money.
The approach: pyrolysis, oxidation and a steam turbine
The process chain was:
- Feed: a hopper and a compactor screw feed the fuel into the system and seal it from outside air.
- Pyrolysis and gasification: a multi-train pyrolyser heats the fuel without air to produce gas, and a gasifier converts the remaining carbon char.
- Thermal oxidation: the gas is burned at high temperature in an oxidiser, with a small liquefied petroleum gas burner for start-up.
- Heat recovery: a boiler captures about 85% of the energy in the feedstock as high-pressure superheated steam.
- Power: a steam turbine drives a 400 V, three-phase, 50 Hz generator, with controls for export to the local network.
The equipment was containerised where possible, so that final assembly on site could be kept short. An exhaust-gas clean-up system sits between the boiler and the stack.
Why cooling matters so much
A steam turbine needs somewhere to reject heat. That choice affects the fan and pump power the plant uses itself, called the parasitic load.
| Cooling option | Effect on the plant’s own power use |
|---|---|
| River or large lake water | Lowest. The load can fall well below 0.2 MW. |
| Cooling tower | An intermediate solution, assumed in the quotation |
| Closed-circuit air-blast cooler | Highest. Fans alone can take about 0.4 MW, depending on ambient design temperature. |
In a 1.2 MW plant, a 0.4 MW load removes a third of the output. On a dry inland site with no river, the air-cooled option would cut net export from roughly 1 MW to about 0.8 MW. Water availability therefore belongs in the first site visit.
Moisture and calorific value
Moisture is the other number that moves output. The design assumed up to 20% moisture on a wet basis. If the fuel is wetter than that, throughput does not fall, but the net energy available for power does. Any proposal like this should state the moisture limit, and it should be checked against real samples.
A staged route to a decision
The proposal separated the work into stages so that the client could stop early:
- Front-end feasibility study (FEFS): confirms the fuel analysis, quantities, electricity output, capital cost and delivery time. The capital cost is given as a budget number that will not move by more than 10% for the given scope.
- Front-end engineering design (FEED): a heat and mass balance, process flow diagram, piping and instrumentation diagram, general arrangement and equipment specifications.
- Detailed design, manufacture, installation and commissioning.
The overall programme was about 52 working weeks:
| Stage | Duration |
|---|---|
| Feasibility study | 2 weeks |
| FEED design | 2 weeks, plus 1 week for client approval |
| Detailed design | 8 weeks, plus 1 week for approval |
| Manufacture and procurement | 24 weeks |
| Installation | 10 weeks |
| Commissioning and performance test | 4 weeks |
Long-lead items, notably the steam turbine, must be ordered once the feasibility study is done if that programme is to hold. Approvals from third parties are not in the 52 weeks and must be added to a real project plan.
What is excluded
Like most equipment-supply proposals, this one left out import duty and shipping, civil work, site access, the grid connection with its transformer and meter, operating staff, permits and consumables. Training was planned as a two-month course for shift engineers, followed by an operations engineer on site five days a week for three months, then on call.
What to consider for your own project
- Test the fuel. Calorific value, moisture, ash and chlorine determine the design.
- Check water availability. It sets the cooling method and the net export.
- Know the grid. Export needs a transformer and a meter, and these were outside the quoted scope.
- Allow for approvals. They fall outside the supplier’s programme.
FAQ
What is the difference between gross and net output?
Gross is what the generator produces. Net is what remains after the plant’s own fans, pumps and controls are supplied.
Can pyrolysis plants process coal dust?
The proposal was designed around coal dust at 28 MJ/kg dry, with a feed of about 26 dry tonnes a day.
How long does a project like this take?
About 12 months from start to commercial operation, excluding third-party approvals.
Why is a feasibility study done first?
It confirms fuel, quantities, output, cost and delivery before large commitments, and fixes the budget price within 10%.
Considering a fuel-to-power project?
Send us your fuel analysis and we will outline a plant for it.