A waste management company in Southeast Asia wrote to us last year. Their RDF line was running well — the shredder hit its rated capacity every shift, the conveyor never stopped. But their fuel kept failing the buyer’s test. The net calorific value was too low, and the chlorine was over the limit.
They asked us to quote a bigger RDF shredder.
The honest answer was that a bigger shredder would not have changed either number. The machine was doing its job. The problem was what the line was asking it to do — and what it could not do no matter how big it was.
A resposta curta
An RDF shredder does not create calorific value. The calorific value arrives in the trucks, in the plastic and paper and textiles. What the shredder does is open the mixed waste up — physically separate what is tangled together, expose surfaces, and cut to a size the rest of the line can work with.
That matters more than it sounds, because in an RDF plant the money is made in the separation, not the shredding. Wind sorting, magnetic separation, eddy current separation — none of them work well on unopened bales and tangled film. The shredder decides whether they can work at all.
An RDF shredder is the size-reduction stage of a refuse-derived fuel line. Its real function is to open and expose mixed waste so downstream separation can pull out the inert, wet and high-chlorine fractions, and to cut the combustible fraction to a particle size the buyer’s combustion process accepts. Cut too coarse and separation fails; cut too fine and the separator loses its discrimination and dust becomes a hazard. The target is a window, not a minimum.
First, get the word right: RDF versus SRF
Most buyers use RDF and SRF interchangeably. The buyer of your fuel almost certainly does not.
FTR — refuse-derived fuel — is the broad term for waste processed to burn. SRF — solid recovered fuel — is the classified, specified subset that meets a documented standard. Under ISO 21640:2021 (which extends the earlier European EN 15359), untreated municipal solid waste cannot be called SRF at all. It can only be feedstock for a plant that produces it.
If your buyer’s contract says SRF Class 3, that is a legal-ish commitment with numbers behind it, and the numbers are worth knowing:
| Class | NCV, mean, MJ/kg (as received) | Chlorine, mean, % (dry) | Mercury, median mg/MJ | Mercury, 80th percentile mg/MJ |
|---|---|---|---|---|
| 1 | ≥ 25 | ≤ 0.2 | ≤ 0.02 | ≤ 0.04 |
| 2 | ≥ 20 | ≤ 0.6 | ≤ 0.03 | ≤ 0.06 |
| 3 | ≥ 15 | ≤ 1.0 | ≤ 0.05 | ≤ 0.10 |
| 4 | ≥ 10 | ≤ 1.5 | ≤ 0.10 | ≤ 0.20 |
| 5 | ≥ 3 | ≤ 3.0 | ≤ 0.15 | ≤ 0.30 |
Cement plants generally buy Class 1 or 2 for their main burners; coal-fired boilers typically accept fuel above about 11 MJ/kg. The specification also requires particle size — reported as d95, the size below which 95% of the material falls — plus ash, moisture and a list of heavy metals.
Notice what is not in the table: throughput. Nothing in the standard cares how many tons per hour your shredder did.
What the shredder actually does on the line
A typical RDF/SRF line runs in this order:
- Coarse shredding — open bags and bales, break bulky items, get everything moving. Often a twin-shaft machine, slow and torque-heavy, because the feed is unsorted and nobody knows what is inside.
- Separação — magnets for ferrous metal, eddy currents for aluminium, wind or ballistic separators for the light/heavy split, hand-picking for the rest.
- Fine shredding — cut the combustible fraction to the buyer’s particle window. Often a single-shaft machine with a screen, because the output size has to be controlled.
- Drying and densification — optional, depending on moisture and how the fuel ships.
The shredder appears twice, and the two appearances want different things.
The coarse stage wants to open, not to cut small. Its output feeds separators, and separators work best on pieces big enough to be recognised as one thing or another. Shred too fine here and a piece of film and a piece of wet paper come out the same size and weight — and the wind separator has nothing left to work with.
The fine stage wants a controlled top size. Its output feeds a burner or a contract, and it is judged by d95 and consistency, not by how fast it ran.
Buying one machine to do both is where most line-level mistakes happen. If you want the general version of this argument — that “shredder” is one name for machines bought for opposite reasons — the film recycling process walks through a similar split on the plastics side, and what a shredder is and how it differs from a crusher sets out the mechanism.
Particle size is a window, and both edges cost money
The intuitive move is: the fuel burns, so finer must be better. It is not, and the two failure modes come from opposite ends.
Too coarse:
– The buyer’s d95 fails and the load is rejected.
– Large pieces burn incompletely — higher ash carryover, unstable combustion.
Too fine:
– The wind separator stops discriminating. Light plastics and heavy wet paper in the same size range behave the same aerodynamically. This is the failure that quietly caps your calorific value.
– Dust. RDF dust is combustible, and fine dust in the right concentration is an explosion risk in a way coarse material is not. Fine cutting also raises the energy per ton — the same trade-off documented in why speed is the wrong spec on wood.
Between those edges sits a window where separation works, combustion works and dust is manageable. The screen in the fine shredder is what holds you inside it — and the window is set by the buyer’s process and your separator’s capability, not by preference.
Chlorine is the one the shredder cannot fix
Here is the part that catches plant owners out.
The chlorine in RDF comes overwhelmingly from PVC — pipe, window profiles, blister pack, film. PVC is everywhere in unsorted municipal waste, and chlorine is the parameter that most often knocks a fuel down a class or disqualifies it: a cement kiln running Class 2 needs the fuel at or under 0.6% chlorine, and unsorted MSW routinely runs well past that.
The uncomfortable fact is that no amount of shredding reduces chlorine. Shredding a PVC bottle into a thousand pieces gives you a thousand small PVC pieces, and now they are spread through the fuel instead of being one pickable item.
What shredding does is expose the chlorine — which makes separation possible. A shredded stream flowing past an optical or density-based separator can have its PVC flagged and pulled. An unshredded stream cannot be sorted at all.
So the accurate statement is:
- Shredding makes chlorine removal possible.
- Separation does the removal.
- If the line has no separation stage for chlorine-bearing material, the chlorine passes straight through, and no shredder — at any price, any size — changes that.
That was the Southeast Asian plant’s real problem. Their line had magnets and an eddy current for metal, which protected the machines, but nothing that could pull chlorine-bearing plastic. The fix was a sorting stage, not a bigger shredder.
The numbers to ask for, in the right order
- Who is buying the fuel, and at what class? Get the ISO 21640 class — or the buyer’s own specification — in writing, including d95.
- What is the feed, really? Composition analysis, moisture, and how much PVC is in it. Guessing at this is guessing at your fuel class.
- What does the separation stage need as input? The coarse shredder’s output size is decided here, not at the machine.
- What moisture will the fuel ship at? Net calorific value moves with moisture; some kilns want 7–12%, some boilers accept far more.
- Then size the shredders. Both of them, each to the window above, with margin for the feed varying month to month.
If you are used to the plastics side of this business, the sequence will feel familiar: downstream requirement first, screen second, machine third — the same logic as what one machine actually puts out per hour, and the same reason metal detection sits upstream of the crusher rather than nowhere at all.
Where the existing RDF conversation fits
We have written about the fuel side of this before, from angles this article deliberately does not repeat:
- What waste can be used for RDF — the feedstock question, material by material.
- RDF fuel vs coal — the substitution economics.
- RDF as an alternative fuel for carbon reduction — the macro picture.
- MSW reduction and resource recovery — where RDF sits in the wider waste stream.
- Textile waste into RDF — one specific feedstock, shredded and converted.
This article is the machine-level view those do not cover: what the shredder is for, what it cannot do, and why the fuel’s grade is decided across the whole line rather than at any single machine.
FAQ
What does an RDF shredder do?
It opens mixed waste and cuts it to a size the rest of the line can work with: coarse shredding feeds the separators, fine shredding sets the particle size the fuel buyer accepts. It does not change the calorific value of the material passing through it — that is decided by composition and by what the separators remove.
What size should RDF be shredded to?
There is no single number. The specification is usually written as d95 and comes from the buyer’s combustion process — cement main burners, coal boilers and gasification plants each want different windows. Coarse and fine are both failures: too coarse rejects the contract, too fine disables the wind separator and creates combustible dust.
Why is my RDF calorific value low even though the shredder runs at capacity?
Because calorific value is decided by composition, not by how fast the line runs. If wet or inert material is staying in the fuel stream, the shredder can be perfect and the fuel still weak. Look at the separation stage and the feed composition first.
Can a shredder remove chlorine from RDF?
No. Shredding only exposes chlorine-bearing material such as PVC so a separator can pull it out. If the line has no chlorine separation stage, the chlorine passes through to the fuel at any shredder size or price.
What is the difference between RDF and SRF?
SRF is the classified subset of RDF that meets ISO 21640 (formerly EN 15359): net calorific value, chlorine and mercury each assigned a class from 1 to 5. Untreated municipal solid waste cannot be called SRF. If a buyer contracts for SRF Class 3, the fuel must be tested and documented to that class.
Do I need one shredder or two on an RDF line?
Usually two, doing different jobs. The coarse stage opens bags and bales for the separators and should not cut too fine. The fine stage cuts the combustible fraction to the buyer’s particle window and is judged on d95 consistency. One machine trying to do both usually does neither well.
Where to go from here
If you are early in the decision, what a shredder is and how it differs from a crusher is the mechanism-level starting point, and what makes a shredder industrial is the duty-rating question you will face the moment you start comparing quotes.
SLECOTECH (Streamline Eco Tech) designs and manufactures industrial shredders, granulators, washing lines, and complete plastic recycling systems. The company is based in Yuyao, Zhejiang, China, and exports to more than 30 countries. If your RDF line is running at capacity and the fuel still will not hold its class, tell us the feed composition and the buyer’s specification — the answer is usually upstream or downstream of the shredder, and we will say so if it is.