Demand-Driven Biomass Feed Simulation

A feed system running at 50 m³/h discharges into a boiler drawing 0.42 m³/h — and needs no large buffer between them. Under demand-driven pull control the two rates never have to match: each light sensor calls for material, the valve delivers, the sensor covers and the call drops out. A faster feed simply runs less often. This mimic is for working that interface through with an extraction supplier at integration design, rather than arguing it by email.

Demand-driven biomass feed — mimic

150 kW plant · single shift · 1 t dust + 2 t offcuts per week
Ranheat Engineering
Mon 08:00
Shift running
Extraction — saws, CNC, sanders, planers
Chipper stopped
Offcut store
6.0 m³
0%
0.00 m³
Silo
12.0 m³
0%
0.00 m³
Transfer screw drop
152 mm × 0.5 m
Light sensor — 2/3
0%
0.000 m³
Stoker drop box
152 mm × 0.5 m
Light sensor — 2/3
0%
0.000 m³
Combustion
150 kW rated
150
KW OUT
0.42 m³/h
Vessels drawn to volume. Silo and offcut store share one scale; the two drops share another, about 390× larger — at the silo’s scale a 9-litre drop would be a third of a pixel. The strip below is the only part drawn at one scale throughout.

Rotary valve duty — rolling hour

to scale — each mark is one valve run
−60 min−45−30−15now
SatisfiedDemand contact to EDS panel
Running & healthyBoiler status contact

Set on the extraction supplier’s panel. The outfeeder and its rotary valve run together, so this is also when the valve turns — which is why the duty below is measured on it.

At zero the screw sits on the sensor’s switch point, topping the box up by a hair and dropping straight out again — so the start count below is a symptom rather than a measurement. Wind this up and it collapses to something a contactor would survive.

Valve run, per hour0.0 s
Valve duty cycle0.00%
Valve starts per hour0
Screw starts per hour0
Void in screwnone
Longest possible pulse—
In transfer screw0.000 m³
In stoker screw0.000 m³
Reaching the fire100%
Time to starve, drops—
Fuel in silo0 h
Week: made / burnt0.0 / 0.0 m³
Feed : draw ratio298 : 1
Check the motor → Open the thermal model with this duty
Feed rate and boiler draw never need to match. Each light gate closes its demand contact, material is delivered, the gate covers and the contact opens — a faster feed simply runs less often. A start delay is absorbed for as long as the buffer below it can cover the fire, and no longer: the 9-litre drop is 193 seconds of fire at 40% load but only 77 at full load, and the drop box at its sensor is 129 and 51. Both sensors behave the same way in this respect — the stoker leg is the transfer leg again, one stage downstream. One light sensor sits two thirds up each drop. Running the valve the moment it uncovers keeps the drop charged and puts no void in the screw; delaying it trades a void for fewer, longer valve runs. The drop holds only 0.009 m³, so the valve delivers in short frequent pulses rather than one long run — the hourly total is the same either way. Both legs are conveyors, and both behave the same way: material picked up at the inlet reaches the discharge one transit later, and a gap in the feed travels down with the material rather than closing up. The stoker screw is the one that meters the fire — it runs whenever the boiler fires, its speed following the load, so at 40% load it turns at 40% speed and takes two and a half times as long to deliver. That lag is what the drop box is for: when the box runs down, the fire keeps burning on what is already in the flights, and the transfer screw — three times faster than the stoker — refills it well before the gap ever arrives. The silo holds roughly a day of full-load fuel; the offcut store and the chipper inhibit are the same level-driven pull, further upstream.

What this is, and is not

  • An interface teaching aid for integration design — not a commissioning document and not a quotation.
  • Modelled on a 150 kW joinery installation. Bulk densities, chute geometry and shift pattern are assumptions until confirmed on site.
  • Three figures were cross-checked against the real plant and agree: the 30 s hourly valve run, the 15.8-litre screw standing charge, and the invariance of hourly run time to feed rate.
  • The start delay is the extraction supplier’s parameter to set. It trades valve starts against a void in the screw flights — see the rotary valve thermal duty tool for what those starts cost the motor.
  • A delay is only free while it is shorter than the buffer beneath it can cover at the current firing rate. Set it against the worst case, which is full load, not the load the plant usually sits at.