A boiler that runs well on one solid fuel does not automatically run well on another. A 90mm white coal briquette is larger, denser and more regular than much of the coal and loose biomass that feeders and grates were set up for, so a few adjustments usually make the difference between a smooth trial and a frustrating one. This guide gives operators and plant managers qualitative guidance on feeding, air and ash. It deliberately avoids fixed numbers, because the right settings depend on your equipment, and the boiler manufacturer's instructions always come first.
How Briquette Size and Density Shape Feeding
BBI's main briquette product is 90 mm in diameter. Ask for the product dimensions and any available bulk-density data, then check the narrowest point of the feed path against the boiler and feeder manufacturer's limits. A feeder designed for a smaller fuel may need an engineering review before using briquettes.
Before the first delivery, ask your engineer to confirm that a 90mm piece can pass the narrowest point of the feed path without wedging, and that the feeder's speed range lets you trim the rate gently rather than only at its extremes. If the feeder was built for a smaller fuel, discuss any modification with the manufacturer rather than improvising.
Hopper Design and Bridging
Bridging happens when briquettes arch over the outlet and stop flowing, leaving a void below. It is a geometry problem more often than a fuel problem. Steep, smooth hopper walls and a generous outlet encourage flow, while flat walls, sharp corners and a narrow throat encourage arching.
Practical Ways to Reduce Bridging
- Keep fines and broken pieces out of the hopper, since they pack around whole briquettes and lock them in place.
- Avoid filling the hopper in a way that compacts the load, such as dropping fuel from a large height.
- Keep the fuel dry, because damp briquettes crumble and add fines.
- Ask the manufacturer about wall liners, vibrators or agitators if bridging persists.
Log each bridging event during a trial. Occasional events are an operating nuisance, while frequent ones are a design finding to raise with the boiler maker.
Grate Type and Air Distribution
Grate type matters because it decides how fuel moves through the furnace and how air reaches it. A fixed horizontal grate, a travelling or chain grate and a reciprocating grate each behave differently with a large, dense briquette. Our briquettes suit manual-fed kilns and traditional grate boilers, and for automated boilers the feeder and grate combination should be checked with the manufacturer first.
The aim is an even bed. Air should pass uniformly through the grate openings, with no blocked zones or bypass gaps, because briquettes in a thick bed burn from the outside inward and need air along their length. Uneven air shows up as hot spots, unburnt cores in the ash and, with some feedstocks, clinker. Before a trial, inspect the grate for worn or blocked bars and for gaps at the edges, and confirm that damper and fan controls move freely.
Primary and Secondary Air
Primary air comes up through the grate and supports burning of the fuel bed. Secondary air is added above the bed to burn the gases released from the fuel. Biomass releases a large share of its energy as volatile gases, so the balance between the two matters.
Too little overall air leaves smoke and unburnt gas. Too much cools the furnace and wastes heat up the stack. Too much primary air can lift fines and cool the bed, while too little leaves a sluggish fire. Operators should adjust one air setting at a time, wait for the furnace to respond, and record the change. Use your analyser readings and the manufacturer's guidance as the reference, not a number from a different plant.
Bed Depth
Bed depth is the thickness of burning fuel on the grate. A bed that is too thin leaves open areas where air bypasses the fuel. A bed that is too deep restricts airflow, slows burning and encourages clinker and incomplete burnout. The right depth depends on your grate, your load and the briquette arrangement, and it is found by observation. Start from the manufacturer's guidance and adjust in small steps while watching flame colour, ash appearance and steam or heat output.
Start-Up and Load Changes
A cold start with briquettes takes patience. Establish a stable ignition bed first, and add briquettes gradually rather than loading the full amount at once. Allow the furnace to come up in line with the manufacturer's start-up procedure.
Load changes need a similar approach. Because each briquette holds substantial heat, sudden increases in feed can overshoot pressure or temperature, and sudden cuts can leave a heavy bed burning with too little demand. Change feed and air together in small steps, and give the system time to settle. Our guide on white coal in thermic fluid heaters covers the extra care needed where outlet temperature control is critical.
Ash Removal
Our briquettes have ash below 10 percent, typically 2 to 8 percent depending on feedstock. This is BBI's own specification, confirmed by the batch lab report. Indian coal ash is generally 25 to 45 percent, so the ash stream is different in quantity and character, and your existing ash handling should be reviewed. Watch for these signs.
- Fused lumps on the grate, which suggest local overheating or uneven air.
- Ash that is dark or contains unburnt fuel, which suggests poor burnout.
- Fine, light ash carried into the flue passes, which suggests too much air or velocity.
Set your cleaning routine from what you observe in the trial, not from a schedule copied from another fuel. Our article on ash handling and boiler planning goes deeper.
What to Watch During a Trial
| Observation | Likely area to check | First response |
|---|---|---|
| Fuel stops flowing from hopper | Throat size, wall angle, fines | Clear safely, screen fines, log the event |
| Smoky or lazy flame | Air level, fuel moisture, bed depth | Check storage, then adjust one air setting |
| Hot spots or fused ash | Grate blockage, uneven air | Inspect grate, review air distribution |
| Unburnt fuel in ash | Bed too deep, short burnout, air | Trim feed and air in small steps |
| Load not held | Feeder range, fuel supply, air | Check feeder response and hopper level |
Run a trial long enough to cover your normal range of load, and keep a shift log of fuel fed, output, air settings, ash and operator comments. Our 25 MT trial checklist provides a full logging plan.
When to Call the Boiler Maker
Operators can adjust air, feed rate and bed depth within the limits of the manufacturer's instructions. Call the boiler maker or a qualified engineer when the problem calls for a change to the equipment or its safe operating envelope. Typical examples are repeated bridging that adjustments do not solve, a feeder that cannot pass the briquette size, persistent clinker in one location, damaged grate bars, uneven air that dampers cannot correct, or any question about pressure, temperature or safety limits. Never override safety interlocks to make a fuel work. For wider conversion questions, see our guide to retrofitting a boiler from coal to biomass.
Next Step
Our team can share the batch lab report and discuss your feeder and grate set-up before you order. See the specification on our biomass briquettes page and compare fuel cost with our calculator.
Contact our team with your boiler type, feeder and grate details to plan a 25 MT trial.
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About 50.0 MT/month of 90mm Biomass Briquettes (White Coal) matches 50 MT of Indian Coal. Enter both prices for a cost comparison.
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