Grate Bars With Air Holes: Will They Fit, and Do They Raise Efficiency?

A common request from operators running a biomass stepped (reciprocating) grate is to swap the plain grate bars for bars with air holes, hoping for better combustion and a longer life. The question that follows is always the same four-part one: will they fit the existing bar seats, how many pieces are needed, will efficiency rise, and what does it cost. The answers depend far more on geometry and air supply than on the tonnage of the boiler.

1. What the holes in a grate bar actually do

Air holes (also written as combustion holes or ventilation holes) are part of the primary-air path. Air enters the wind box, passes the bar seat and the gaps between bars, and then either escapes at the leading edge or through the holes straight into the fuel bed. Changing the plain bar for a holed bar therefore does not just add openings: it changes where the air leaves the bar and at what velocity.

Two consequences matter in practice. First, the opening ratio decides the exit velocity for a given air flow. A small total hole area gives a few high-velocity jets that penetrate a thick bed but also lift fine particles and erode the hole rims; a large opening gives gentler, more diffuse air but weak penetration. Hole pattern, hole diameter and hole depth all change the result, so two bars that look alike can behave quite differently. Second, holes remove metal from the hottest face and add stress raisers at every rim, so the bar runs hotter locally and is more sensitive to thermal cycling. A holed bar is not automatically a better bar.

2. Fitting a holed bar to an existing grate: three checks

Geometry of the bar and its seat. Length, width, height, the pitch set by the engagement lugs, the support face on the beam or cross beam, the pin holes and the side-seal faces all have to match. As an example, the bars quoted for a 15 t/h biomass stepped grate were 390 × 55 × 50 mm at about 5 kg each, 540 pieces for the set. A holed plate of the same length but a wider face changes the pitch: fewer bars cover the same metre of grate width, the end bar no longer meets the side seal, and the seal faces no longer close.

Kinematics. Moving and fixed bars slide against each other. A taller section, a different overlap or a hole rim sitting on the sliding surface changes the clearance over the stroke and the bar can catch, lift or jam.

Air path. Holes only work if they sit above a pressurised wind-box compartment. Holes placed over a compartment divider, over a dead plate or over the side seal feed nothing, and air then short-circuits through the remaining open area, which makes the distribution worse than before.

If any of the three fails, a holed bar is not a drop-in replacement. The realistic options are to have the holed pattern cast onto the original outline and pitch, or to change the whole grate assembly including the seats and seals. Both need a drawing, not a photograph.

3. Never mix holed and plain bars for cost saving

Mixing the two types on one grate creates four problems at once. Flow resistance differs, so air takes the easy path and the plain-bar zones run starved while the holed zones over-aerate. Section height differs, so the bed depth and the stoking step vary from row to row. Elongation differs, because the perforated face runs hotter, and the resulting growth mismatch shows up as jams and loose pins. Wear rate differs, so the mixed grate ages unevenly and the replacement log becomes meaningless. Zone-by-zone upgrading is acceptable only when the outline, pitch, height and seal faces are identical and the air path is adjusted to suit.

4. How many pieces will fit: count from pitch, not from tonnage

The count is fixed by geometry:

• bars across the width = usable grate width ÷ pitch, rounded down, leaving the side-seal allowance;
• rows along the grate = effective grate length ÷ bar length, allowing for the overlap at each row;
• total = bars across × rows.

Working backwards from the set above: 540 pieces × 0.390 m × 0.055 m gives about 11.6 m² of gross face area, or roughly 10.6 m² of effective area after the overlap. A 15 t/h biomass boiler is about 10.5 MW, so the effective grate heat release works out near 1.0 MW/m², which is a normal order of magnitude for a biomass stepped grate. That is why the 540-piece figure is credible for this duty, and also why it cannot be transferred to another boiler without the length and width. A wider bar with a longer pitch will need fewer pieces for the same area, but each piece is heavier and the rows are fewer, so the total weight of the set barely falls.

For any accurate count, the useful inputs are: effective grate length and width (from the drawing, or measured during a shutdown), the pitch measured between bar centres, the inclination, the stroke, and the number of zones.

5. Will efficiency actually improve?

Only under three conditions: the current limitation is air at the bed rather than fuel or draught, the hole pattern matches the wind-box compartments, and the fan has pressure headroom. When all three hold, the gain appears as better oxygen supply in the main combustion zone, lower CO, and better burnout of the char from straw-like fuel. When one is missing, the same holes can make things worse: fines drop through, local velocity raises fly-ash carryover and unburned carbon in the fly ash, and hot spots accelerate slagging.

For a unit whose symptoms are a life under one year and incomplete combustion of straw, the holes address the air side only. The two dominant causes in that duty are the ash chemistry of straw and the surface temperature of the bars, which are covered in which fuels sinter and slag grate bars and in choosing a grade by service temperature. A holed bar is not a substitute for fixing those.

6. Cost comparison

Per piece, a holed bar costs more: the pattern needs a core or additional machining, the reject rate on thin sections is higher, and the piece is often heavier. Per square metre of grate the gap narrows, because fewer, larger pieces can cover the same area. For the project as a whole the deciding items are usually outside the bars themselves: whether the seats and seals must change, whether the wind box needs repartitioning, and how many days of outage the change costs.

7. What to send for a firm answer

We supply moving and fixed bars, side seals and the matching parts for biomass reciprocating grates, dispatched from a partner factory, with dimensions and material checked against a drawing or against measured samples. Send the five items above and the fit, quantity and cost questions can be answered together.