Understanding Ash

For operations struggling with ash and slagging, the solution can often be found in the management of potential contamination sources. It's important to understand differences between tree species, and varying inclusion rates of bark, dirt and dust.
By Chris Wiberg | May 26, 2020

When it comes to biomass fuels, it really doesn’t matter which market you serve, ash content is always a very important parameter. For residential pellet-burning appliances, very low ash is generally desired, as the amount of ash directly relates to how often a stove needs to be cleaned. For power markets, higher amounts of ash are manageable, but how much ash can be managed is dependent on the type of combustion system, as well as the mineral composition of the ash, which is related to slagging. While the concept of ash seems simple, there are a lot of factors that determine how much ash will be present and how it will affect a combustion system. 

When freshly harvested, a tree contains minerals such as potassium, sodium and calcium, which are brought up into the tree through the root system. These minerals are non-combustible so when the wood is burned these minerals remain in the form of ash. For softwoods, the ash content of clean fiber will generally range from about 0.15% to 0.35%, and for hardwoods it will range from about 0.35% to 0.55%. Mineral composition of the ash will vary, but for clean wood fiber, the mineral composition is generally such that there will not be a slagging problem.

Now let’s add bark. The simple nature of bark is that it is exposed to the atmosphere during the life of the tree, so it is free to pick up particulates from the air. Additionally, when the tree is felled, it can pick up dirt and debris from the forest floor, so the inclusion of bark in a fuel mix can and often does significantly increase the amount of ash contained within the fuel. When tested by itself, coarse bark generally contains several percent ash. However, when blended with the tree (e.g. a whole tree chip), if the materials are kept reasonably clean during the harvest, ash content will generally be in the 1 to 2% range. Mineral composition of bark ash is highly varied and largely dependent on the prevailing soil types of the region. Slagging may be associated with bark, especially in regions with silicon-based soils.

Next, let’s add dirt. Dirt is a major contaminant, as it is almost all ash. To the fullest extent possible, dirt should be kept out of the fuel mix. This is not always easy if the materials used include land clearing debris. Materials with poor dirt control often range from 2% to 6% ash and can even be over 10% ash. These materials are also highly likely to have slagging issues.

Finally, let’s add airborne particulate matter. Let’s say, for example, that your fiber source is expected to be very clean (no bark, no dirt), but your ash content is tracking higher than the baseline levels mentioned above. Chances are, an elevated ash content is the result of airborne dirt/dust particles finding their way into your fiber. A good example is to picture a clean fiber pile located close to a gravel road. As a car passes, there is a cloud of dust that wafts into the air. That dust can easily settle on your fiber, whether it is under cover or in the open air. This mechanism of dust accumulation is very apparent with agricultural residues. Typically, clean ag residues range from about 1% to 3% ash, however it is very common for ash to be 4% to 6%, or even higher.

A few words about slagging. It no doubt seems odd that a fuel can have a very high ash content, but still not have a slagging problem. Conversely, materials can sometimes have lower amounts of ash (even below 1%) and have a slagging problem. This is because slagging is not necessarily caused by ash, but rather the mineral composition of the ash, along with combustion temperature. Calcium is a common mineral component of ash, which has very favorable ash fusion characteristics (melts at very high temperatures). When calcium comprises a high percentage of the ash there are generally no problems with slagging. Conversely, silica has the opposite effect—complex silicates generally melt at low temperatures—and is easily present when dirt and airborne dusts find their way into wood fuels. In most instances, slagging problems are the result of silica contamination.

Finally, remember that the end user’s experience with ash is also a function of the efficiency of the combustion system. The relative percentages I have described above are representative of ideal conditions in the laboratory. When combustion systems are operated in less-than-optimal conditions (e.g., not enough air flow), the fuel may not completely burn. Dark, or even black, ash is likely the result of incomplete combustion, which can greatly increase ash, facilitate slagging, etc. 

In summary, if you are struggling with ash, either the overall amount or via the slagging of a combustion system, it is likely the solution can be found in the management of your sources of contamination. 

Author: Chris Wiberg
Manager, Biomass Energy Laboratory