Straw Biomass and Grate Bar Life: Potassium, Chlorine, Zone Selection

Boilers that run on wood chips hold their grate bars for several years, and the same boiler moved onto straw loses the set inside a year. The difference is not the boiler and not the grate design. It is the ash chemistry of the fuel, and the way that chemistry interacts with the surface temperature of the bars.

1. What straw puts into the furnace

Straw, wheat straw, rice straw and similar residues carry more potassium, more chlorine and more silicon in their ash than woody fuel does. The ash content itself varies widely with the harvest, the soil and the baling method, so a single figure is misleading, but the element ratios are consistent: potassium and chlorine dominate the ash-forming elements, and the silica content is high. Empty fruit bunch behaves the same way.

Three consequences follow. Potassium and silicon together form silicates with low melting points, so the ash softens well below the temperatures a grate sees in the main combustion zone. Chlorine forms alkali chlorides that are volatile at furnace temperature and condense on cooler metal surfaces. Potassium chloride itself melts at about 770℃, so on a bar running at 900℃ and above the deposit is not a dry powder but a liquid layer. The measurements behind this are summarised in which fuels sinter and slag grate bars.

2. Two chains and a removal step

The failure analysis of used bars from biomass furnaces shows the same three-layer structure every time: an outer deposit layer carrying potassium and chlorine, a porous iron-rich oxide beneath it, and a chromium-depleted substrate with internal oxidation under that.

The first chain is chlorine-driven. Chlorine forms volatile metal chlorides at the oxide–metal interface. Those chlorides are oxidised again a little further out, and the chlorine is released to react once more. The net effect is that metal is transported away and a protective chromium oxide film never closes.

The second chain is fluxing. Potassium reacts with siliceous ash to form potassium silicates, which at grate temperature are molten or semi-molten. A liquid silicate dissolves the oxide film and penetrates the grain boundaries behind it, so even a high-chromium grade loses its protection while the flux is present.

The third step is removal. Weakened scale and deposits are carried off by the flue gas and by the moving fuel bed, fresh metal is exposed, and both chains start again from zero. What looks like steady wear is really a repeated cycle of attack and stripping, and its rate is set by how much liquid phase sits on the bar and how hot the metal under it is.

3. Why surface temperature decides the rate

Both chains need a hot metal surface. The metal temperature of a grate bar is not the furnace temperature: primary air passing through the bar and the bed cools it from below, so a well-fed bar runs a few hundred degrees cooler than the flame above it. That cooling margin disappears in three situations — a blocked air passage, a thin bed with the air going straight past, and a bar sitting under a local hot spot where the neighbouring gap has widened. In each case the metal temperature climbs towards the gas temperature, the deposit on it turns fully liquid, and the attack rate rises steeply.

This is why two boilers with the same fuel, the same grate and the same grade can differ by a factor of two in bar life. It is also why a material upgrade alone does not always fix a straw problem: if the air distribution still leaves dry patches and hot spots, the better grade simply takes longer to fail the same way.

4. Where the grate fails first

The three zones do not fail for the same reason. In the drying zone the bed is wet and the metal is relatively cool, so chloride-rich deposits sit on the surface with water present, and the damage is more chemical than thermal. In the main combustion zone the temperature is highest, the liquid phase is fully developed, and both corrosion chains run at full rate: this is where bars go first, and where the row-to-row difference is largest. In the burnout zone the bed is thin and the char is nearly gone, so the metal sees oxidising gas with little cooling from the bed, and uniform scaling dominates.

A set that is replaced as a whole hides this pattern. Mapping the failed rows onto the zone layout usually shows that a minority of rows account for most of the damage.

5. Zone-by-zone grade selection

Selection should follow the temperature of the position, not the average of the grate. The service temperatures below come from the partner factory’s selection data, and the standard basis matters as much as the figure:

Zone	Duty at the bar	Grade	Service temperature	Standard
Drying zone	Wet bed, chloride deposits, moderate metal temperature	RQTSi5 or RTCr16	850℃ / 900℃	GB/T 9437
Main combustion zone	Highest temperature, liquid slag, both corrosion chains	Cr25Ni20 (310S) heat-resistant steel casting	above 1100℃	GB/T 8492
Burnout zone	Thin bed, oxidising gas, little cooling	RTCr20 or Cr26Ni12	950℃ / lower-temperature duty	per factory specification

Two things are worth stating plainly. Nickel in the austenitic grade buys ductility and thermal-fatigue resistance, which matters because the main zone also cycles; it does not make the alloy immune to chlorine, so the mechanism continues, only more slowly. And a grade is only as good as the casting: shrinkage porosity or a cold shut on the hot face is an initiation site no chemistry can compensate for, which is why a spectrum check per heat and a look at the hot-face finish are part of acceptance rather than extras. Our note on spectral testing of ZG40Cr25Ni20Si2 parts covers what that check should include.

The cost logic follows from the row count. In a typical stepped grate the main combustion zone is a minority of the rows, so restricting the expensive grade to that minority and keeping a cheaper iron in the drying and burnout rows adds far less to the set price than upgrading all three zones, while capturing most of the life gain.

6. What can be done on the fuel and operating side

Material is one lever of four. On fuel, limit the share of high-potassium fuel and blend it with a lower-risk fuel, wash the fuel where the logistics allow, and consider an additive: kaolin raises ash fusion temperature and has been trialled when co-firing empty fruit bunch. On air, keep the primary air distribution even so that no row runs starved, because a starved row is a hot row. On bed, avoid the thin fast bed that operators adopt to keep wet straw alight; it raises the surface temperature of exactly the rows that are already hottest. On operation, avoid long idling at low load with the bed barely covered, and clean slag as soon as it appears rather than letting a hard shell build.

7. What to send us

For a zone-by-zone recommendation the useful inputs are the fuel analysis over a full year rather than a single sample, the grate drawing or the effective length and width, the failed rows with their position on the grate, and the service life achieved with the grade currently in use. With those, the recommendation can name grades per zone and a piece count, instead of quoting one grade for the whole grate.

We supply moving and fixed bars, side seals and matching parts for biomass reciprocating grates, dispatched from a partner factory in the grades above, with dimensions checked against a drawing or a measured sample.