Deep Analysis: Know About The Parameters That Lead To Coking Of Pellet Fuel

 Regarding the science of biomass combustion, nothing is more complicated than ash fusion (coking) parameters. When the coking characteristics are good, the ash is still ash, and the only challenge is that the ash is not excessively removed from the combustion system by accumulation. On the contrary, when the ash fusion properties are not favorable, something strange happens - the ash clumps together and needs to be broken or even chiseled out of the ash tray. Later, it can form a brick that looks like a piece of molten glass or even a honeycomb. When it accumulates in an industrial burner, this state of ash is called coking or slagging. Whatever you call it, whatever it looks like, it's a relatively simple thing to do, because it's just a function of the melting point.

First, let's first determine that "clean" ash (free of dirt, rock, unburned carbon, etc.) is primarily a combination of inorganic oxides. When biomass is burned, organic matter (basically all carbon, hydrogen, nitrogen and oxygen) is released, while inorganic minerals remain in the oxidized form, which we consider ash. Through detection, biomass ash is mainly composed of calcium, silicon dioxide, aluminum, magnesium, potassium, manganese, sodium, iron, phosphorus and other mineral oxide forms. Each of these oxidized minerals exists as a solid and, like any other solid, has a melting point. The range of melting points of the various mineral oxides present can vary widely, with the total melting point of ash occurring at high temperatures as a function of all mineral components and chemical interactions. As a result, ash usually melts within a certain temperature range, not a specific temperature. The range can range from a few degrees to 50 or even 100 degrees Celsius. This is why when you see the ash melt test results, it is reported as a temperature range (for example, deformation temperature =1310 °C, hemisphere temperature =1330 °C, flow temperature =1350 °C). In this case, the ash melts at 40 degrees Celsius.

Deformation temperature (DT) is considered to be a key parameter in ash melting tests, as this is the temperature at which the ash first begins to melt and become "sticky". Sticky ash will accumulate on almost all the surfaces in the combustion system, resulting in an insulating effect, resulting in an increase in the temperature of the entire combustion system. Higher temperatures lead to more melting. This process continues until the ash becomes fluid and essentially slags. Interestingly, the properties of slag can tell you something. If the ash is lumpy, it can still be broken by hand. If you find real glass, the ash has completely melted. A piece of coking usually falls somewhere in between.The key to preventing ash fusion (coking) is to keep the temperature of the combustion system below the DT of the ash. Since most biomass combustion systems operate at 1200 degrees Celsius or below, fuel is usually evaluated by verifying DT above this temperature. Fortunately, for "clean" wood (no bark, sand, dirt or other debris), coking is usually not a problem. The fusion of ash and woody biomass is almost always associated with some form of raw material. The same cannot be said for other forms of biomass (nut shells, agricultural grasses, energy crops, etc.). These materials often have high ash content, increasing the chances of low DT.That is, high ash content alone is not a good predictor of ash fusion (coking) problems with a particular form of biomass. The nature of the mineral composition of ash is the contributing factor. For example, if the calcium content of ash is high, the melting temperature of ash is usually high. Ash melting problems are more likely if silica levels are high, but not always. The interesting thing about silica is that if it were in the form of silica, the actual melting temperature would be very high (1710 degrees Celsius). However, like carbon, silica has four active electrons that can bond with other minerals, often resulting in complex silicates with low melting points. For this reason, when we see coking problems, 90 percent are related to silica. There are other minerals that can be problematic when temperatures rise.There are many other factors that can complicate coking. Combustion systems can be oxygen-rich or oxygen-poor, varying the melting point conditions. Biomass can be contaminated with non-obvious materials, such as fertilizers and salt, often because of the use of an unclean transport system. Contaminants usually vary intermittently, so testing the next batch of fuel won't necessarily help you figure out what's causing the coking problems associated with the previous batch. All in all, if you understand the above principles, you should have a better chance of determining how to deal with the problem of particle coking.