The grate edge: care point number one in a biomass boiler

Lay the inspection reports of several biomass boilers side by side and the same line appears again and again: grate edge reworked. It is the place that comes up in almost every overhaul, regardless of manufacturer and year of construction.

There is a reason for that, and it does not lie with the material.

Three loads within one centimetre

• Movement. The grate works — it pushes, it expands, it vibrates. The lining beside it stands still. At the boundary between the two, a relative movement occurs in every operating hour.
• Thermal shock. Above the grate lies the fuel bed, below it the primary air. The transition area sees both and changes accordingly fast.
• Fuel. Biomass brings alkalis with it which penetrate the surface at operating temperature and alter the material at the face. Together with thermal shock it is precisely this altered layer that spalls off.

Why the joint there is not a building defect

The obvious reaction is to seal the gap between grate and lining tight. That holds exactly until the first expansion. After that the material is not only loose again but cracked — and the crack runs on into the surface where there was nothing before.

The transition needs a defined, yielding joint. It is a wear part and belongs in every overhaul record, not as a defect but as an item.

What belongs in every overhaul

• Check and renew the joint filling over its full length, not only at the visible damaged spot.
• Assess the first twenty to thirty centimetres of lining beside the grate separately — that is where it is decided whether the next campaign holds.
• Do not patch broken-out areas but take them back to a sound substrate. A repair on alkali-soaked material comes away together with that substrate.
• Compare the condition with the fuel quality actually fired. A change of biomass alters the damage pattern within one campaign.

What operations contribute

The primary air distribution determines where the fuel bed ignites and how far the hot zone reaches towards the edge. Shifting the air distribution often makes operational sense and takes immediate effect at the grate edge.

That is why the same note pays off here as with the burner: record changes to firing control with a date. At the next inspection that is the difference between an explanation and a guess.

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