Polysius
Burn what
you get.
prepol® SC – Step CombustorBurn what you get.
An add-on for the prepol® precalciner: a simple combustion grate on which waste burns for more than 1,000 seconds at high temperature – instead of the up to 7 seconds an ordinary calciner offers. That opens a new dimension in burning, a new level of fuel-cost savings, and a practical path to replacing coal, gas and oil entirely.
Key figures strip
| Residence time on the grate | more than 1,000 seconds |
| Max. fuel edge length | 300 mm |
| Possible substitution of coal, gas & oil | 100 % |
| Moving parts in the hot zone | none |
| Proven in operation | six years, several step combustors in service |
Overview – Waste Management in the Cement Industry
The cement industry can be a huge lever for a greener future and for the waste challenges ahead. With the prepol® SC, coal, gas and oil as primary energy sources can be replaced completely by alternative fuels — a transformation journey rather than a single step, from grey to green.
ADVANTAGES AT A GLANCE
Incineration of large amounts of very coarse and wet alternative fuels of up to 300 mm edge length
Patented air blast transport — no moving parts inside the hot zone
Highest fuel flexibility thanks to the fuel separation effect on the grate
Constant agitation of the fuel for a superior oxygen supply and the fewest CO peaks
Complete and safe operation with lowest emissions through the most suitable combustion conditions
Maximum reliability — six years of operation with lowest maintenance costs and several step combustors in service
Small in size, huge in combustion potential
The step-shaped grate connects to any kind of precalciner loop and is little more than a tube-like extension. Because the fuel lies on the grate instead of following the gas stream, the same residence time needs a far smaller volume than extending the calciner loop — and the lower weight means less civil reinforcement. Connected to the tertiary air channel, the hot air delivers exactly what complete combustion requires: heat for drying and ignition, oxygen for a safe and complete burnout.
Residence Time — The Decisive Difference
A conventional calciner loop gives a fuel particle a few seconds to dry, pyrolyse and burn out. The step combustor holds it for minutes — which is why non-flyable, wet and coarse waste becomes usable at all.
| System | Residence time | Note |
| Ordinary calciner loop | up to 7 s | Fuel entrained in the gas stream |
| prepol® SC-S (short) | approx. 150 – 300 s | Ignition table plus first three steps |
| prepol® SC (full grate) | more than 1,000 s | Full step grate, fuel lies on the steps |
Even the shortest version still offers more than fifty times the combustion time of an existing calciner loop.
Working Principle — One Step in Preparation, Several Steps in Combustion
The refractory steps of the grate are equipped with nozzles connected to air-blast cannons. Each blast agitates the fuel, turns it over and conveys it progressively along the grate — so a very high throughput per unit of grate area is achieved.
| 01 — Defined feeding | Two screw conveyors push the fuel onto the first step. The cold fuel itself protects the conveyors from direct contact with the fire. |
| 02 — Ignition table | The fuel piles up on a platform-like first step, drying, pyrolysing and igniting over several minutes. The heap seals against false air, prevents hot gas from flowing back into the conveyors, and buffers fluctuations in feed rate and calorific value. |
| 03 — Agitate & convey | Air blasts swirl the fuel up, bring it into contact with fresh oxygen and move it step by step along the grate — active conveyance throughout the whole retention time. |
| 04 — Separate & release | Light, already-converted char is blown out and entrained in the calciner gas stream, while heavier particles stay on the grate for further thermal processing. |
Main Components
Every mechanically moved component sits outside the combustion chamber, where it is easy to reach. The parts inside are refractory and static.
| Screw feeding system | Two screw conveyors deliver the fuel in a defined way onto the grate. Specially optimized for the rough conditions of fuel handling — and outside the thermal and chemical load of the combustion space. |
| Ignition table | The platform-like first step where fuel piles up, dries, pyrolyses and ignites. Acts as a seal against false air and prevents hot gas from flowing back into the conveyors. |
| Refractory step grate | Several static, non-moving refractory steps — the feature that gives the system its name. Width and length can be adjusted to the given boundary conditions; for complex retrofits the grate can be shortened. |
| Air-blast cannons & nozzles | Patented air-blast transport agitates and conveys the fuel. Proven mechanical components from the cement industry, mounted outside the hot zone. |
| Tertiary air connection | Delivers heat for drying and ignition plus the oxygen for a safe, complete burnout. The SC-S variant is integrated into the precalciner and needs no separate tertiary air or meal connection. |
| Airlock slide valves | Pneumatically operated valves isolate the combustor completely from the conveyors during a stoppage, and prevent fuel ignition in the conveyors in an overpressure situation. |
Fuel Preparation — Shift the Effort from the Shredder to the Grate
With direct calciner feeding, refuse-derived fuel only works if it has passed a multistage mechanical processing plant. The step combustor changes what your preparation line actually has to achieve.
| Direct calciner feeding | prepol® SC | |
| Required edge length | 50 – 100 mm | up to 300 mm, including non-flyable fuels |
| Burnout window | approx. 5 seconds in the calciner | more than 1,000 seconds on the grate |
| Preparation effort | Multiple shredding and sorting stages | Only to adjust heating value and control contaminants, inert constituents and harmful substances |
| Fuel requirement | Must be suspendable in the gas stream | Heavy waste fuels usable — the fuel lies on the grate |
The Benefits — Safe, Effective, Simple & Reliable, Flexible
Safe
The thermal process always takes place with an air excess, so dangerous carbonization gases such as CO, H₂ and CₓHₙ cannot form. Forced feeding seals the system off from the environment. In an overpressure situation — for example a failure of the system fans — ignition of the fuel in the conveyors is prevented, and during a stoppage pneumatic airlock slide valves isolate the combustor completely from the conveyors.
Effective
Air cannons actively transport and turn over the fuel. Every blast brings it into contact with fresh oxygen, and every agitation separates the smaller particles so that only the large fraction remains on the step grate. The fuel is processed only as far as necessary until it can enter the calciner as gas-stream-entrainable char — so very large fuel quantities are handled in a small footprint.
Simple & reliable
The combustor consists entirely of mechanical components that have long proven themselves in the cement industry. Screw conveyors and air cannons are optimized for rough fuel-handling conditions, yet they assist fuel conversion without being exposed to the thermal and chemical stresses of the combustion space. The mechanically moved components sit completely outside the combustor, where they can be easily accessed. The simple geometry makes implementation at any existing precalciner straightforward, with the shortest installation periods.
Flexible
The prepol® SC is made for all kinds of fuels and can be adjusted flexibly in size. Thanks to the modular design it integrates easily into existing cement plants; width and length follow the given boundary conditions. For complex retrofits, the grate can be shortened — and even the shortest version still offers more than fifty times the combustion time of an existing calciner loop.
Operation & Maintenance
Because there are no moving parts in the combustion chamber, maintenance concentrates on components you can reach — and on keeping the combustion conditions where they belong. This section is newly written for the landing page and is not on the current live page; please review it closely.
Feeding & airlocks
Keep the fuel heap on the ignition table stable — it is your seal against false air
Verify airlock slide valve function before every planned stoppage
Watch feed-rate fluctuations; the heap buffers them, but not indefinitely
Grate & refractory
Inspect refractory steps and nozzle openings during kiln shutdowns
Check nozzles for blockage by inert constituents in the fuel
Modular design allows section-wise repair rather than full replacement
Combustion conditions
Monitor CO in the calciner as the leading indicator
Match air-blast frequency to the actual fuel fraction, not to the design case
Re-check settings whenever the waste supply changes
Retrofit & Add-on — prepol® SC vs. prepol® SC-S
The prepol® SC was conceived as an add-on. Where space in the preheater tower is tight, or where the available alternative fuel is of higher quality, the short version delivers most of the effect with a minimum of footprint.
| prepol® SC | prepol® SC-S (short) | |
| Retention time | more than 1,000 seconds | approx. 150 – 300 seconds |
| Configuration | Full step grate with ignition table and screw feeding system | Ignition table plus the first three steps of the grate, with screw feeding system |
| Connections required | Connected to the tertiary air channel | Integrated into the precalciner — no separate tertiary air or meal connection |
| Ignition mechanism | Hot tertiary air delivers heat for drying and ignition | Radiation heat pyrolyses the fuel on the grate before gases and size-reduced particles enter the precalciner atmosphere |
| Typical driver | Maximum fuel flexibility, coarse and wet waste, highest substitution ambition | Little available space in the preheater tower, or availability of higher-quality alternative fuels |
| Retrofit effort | Modular, integrates into nearly every existing plant | Even easier to retrofit |
SUPPORTING ARGUMENTS
Less civil work than a loop extension — the small volume and lower weight of the combustion chamber mean less civil construction reinforcement is required
Fits any precalciner — the step-shaped grate connects to any kind of precalciner loop, with the shortest installation periods
Sized to your boundary conditions — width and length are adjusted to the space available; for complex retrofits the grate can be shortened down to the SC-S
A very simple combustion grate, made of several static, non-moving refractory steps” — which is exactly where the name Step Combustor comes from.