Wood chips and biomass: why storage is the hard part

A biomass boiler is a steady machine fed by an unsteady material. Wood chips arrive from different forests, different chippers and different weather, and what reaches the boiler is the average of all of it plus whatever the store has done to it in the meantime. The trouble is rarely combustion. It is a store that will not release fuel on a cold morning, a screw wrapped in slivers, or a pile that has started to warm up on its own. The storage system, not the boiler, is the design problem.

What is actually being stored?

A range, not a material. Species, chipper type and blade condition set the nominal size; the oversize slivers and the fines come with it whatever the specification says. Bark, needles and soil arrive with whole-tree chips. Moisture follows the season and the supplier's yard. The figures below are the kind of thing to expect; the values that matter are measured on the fuel this plant actually receives, across a year, and the note on bulk density, moisture and flowability explains how.

Bulk density

Indicatively 0.2–0.35 t/m³ for wood chips as delivered — light enough that volume, not mass, sizes everything. Pellets are denser; sawdust and bark vary widely.

Moisture

Commonly 30–55 % wet basis for fresh forest chips, lower for dried material; it changes with the season and drives bulk density, flow and self-heating.

Size

A nominal chip size with a tail of oversize slivers, which interlock, and a fines fraction, which fills the voids and holds water.

Compressibility

High. Chips under the weight of a pile compact and gain strength, so a full store flows worse than a half-full one.

Angle of repose

Steep, and steeper when wet or fibrous; a chip pile stands almost as it was tipped.

Biological activity

Present in any moist wood; it produces heat and gases in large piles.

Winter

Wet chips freeze into blocks that no hopper outlet was drawn for.

Why do wood chips bridge and rathole?

The mechanics are those in the pillar note on bridging, ratholing and mass flow, with three amplifiers. Slivers interlock into a mat that can span an outlet far wider than any single particle. Compaction under load gives loose chips real cohesive strength, and the strength keeps rising with time at rest. And moisture — as water, as ice, or as the binding between fines and fibres — adds cohesion on top. A conical hopper with a modest outlet, which would empty granular material without complaint, is close to the worst geometry for chips: an arch forms across the cone, and if it is broken a rathole forms above the outlet and the rest of the contents stays where it is.

• Outlets need to be long slots rather than small circles, with the feeder drawing along their full length.
• Walls should be vertical or nearly so; chips do not slide on inclined walls, they pack against them.
• Live capacity of a funnel-flow chip silo can be a small fraction of its volume; the level gauge reads full while the boiler starves.

Which storage system fits which fuel?

None of these is universally right. A plant burning screened chips from one supplier can run a silo and sweep auger for years; the same silo on whole-tree chips with bark and slivers will bridge in the first winter. We size the reclaim for the worst fuel the plant will accept, not the best fuel the contract describes.

How is biomass conveyed once it is moving?

Screening comes first. An oversize screen before any silo or enclosed hopper removes the slivers that would interlock in the store and jam the feeders, and a magnet takes out tramp metal before it reaches a screw or a grate. After that the conveyor follows the route. Shaftless screws handle chips well because there is no centre shaft for fibres to wrap around and no hanger bearing to block; belt conveyors suit long runs and inclines within what the chips will hold; drag chain conveyors give an enclosed, dust-tight route where the fuel is dry. All of them need the speed monitoring, plugged-chute detection and pull-cords described in the note on conveyor safety switches and flow aids.

Pellets and sawdust are a separate case. They store more easily but produce fine, dry, combustible dust, and any enclosed handling system for them needs a dust explosion assessment before the equipment is chosen. That assessment is outside this note; its consequences for zoning, venting, earthing and housekeeping are not optional.

What to check on an existing biomass store

• Fuel actually received — size, moisture and fines over a year, against what the store was drawn for.
• Screening — whether an oversize screen exists, whether it is bypassed, and where the slivers end up.
• Dead zones — compare the indicated level with what the store delivers before the boiler alarms; the gap is stagnant fuel.
• Bridging record — where it happens, when (season, fuel, time at rest) and what the operators do about it.
• Reclaim wear — screw flights and trough liners, walking-floor slats and seals, sweep auger teeth and drive.
• Temperature — whether the pile is monitored, how, and what the trend has been.
• Freezing points — outlets, transfers and the last metres before the boiler where wet fuel meets cold air.
• Dust — deposits on ledges and equipment where the fuel is dry; ventilation in enclosed stores.

A store that behaves is a combination of geometry, reclaim and fuel discipline, and it is nearly always cheaper to fix than to fight. Our storage and flow control work covers reclaim systems, live-bottom hoppers and the feed to the boiler; the biomass duties in the Flow Finder list the equipment types involved.