The higher the moisture content, less energy per kilogram is released during combustion. As a result, the energy generatioin cost is higher and the storage system has to be larger.

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Ash Content - contamination

The non combustible content of woody biomass is referred to as ash. It can be a part of the wood source, e.g. deposited within the biomass during plant growth or contaminant such as dirt, sand or even heavy metals. It is usually expressed as a dry basis or in other words the percentage of mass that won’t be a part of the combustion process, assuming a moisture content of 0%.

The main problems with ash content are not directly related with the energy production but with emissions and maintenance.

Problems may occur when, for example, highly contaminated ash melts forming lungs of slag, which may block grates and cause erosion or jamming of ash augers. More ash means more maintenance for cleaning, which may rise the cost of keeping the system running.

Another issue may result from the emissions present with highly contaminated ash. The higher production of NOx, PM10, and heavy metal may cause environmental problems.

Its presence in the fuel is indicated in a similar way to how it is done with moisture content. CEN/TS 14961:2005 also provides specification about how those contents may be categorised, as shown below:

Ash content wood chip

Ash Content (dry basis)
A0.7	Ash content ≤ 0.7%
A1.5	Ash content ≤ 1.5%
A3.0	Ash content ≤ 3.0%
A6.0	Ash content ≤ 6.0%
A10.0	Ash content ≤ 10.0%

Ash content wood pellet

Ash Content (dry basis)
A0.7	Ash content ≤ 0.7%
A1.5	Ash content ≤ 1.5%
A3.0	Ash content ≤ 3.0%
A6.0	Ash content ≤ 6.0%
A6.0+	Ash content ≥6.0%

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Forms of wood fuel

Having a better understanding of what the main characteristics of a wood fuels are, the forms at which they usually are utilized may be introduced. Wood logs, wood chips, pellets, straw and even liquid fuels produced from wood and bales of compressed forest residues are examples of it. The feasibility tool was designed to work with wood pellets and wood chips, usually the kinds of fuel utilized with systems from 50 kW upto approximately 1 MW. These two types of fuel are described bellow:

Wood Chips

Whole trees, branchwood and coppice products may be mechanically shredded by a chipping machine in order to create wood chips. These wood sources may come from forest thinning, arboricultural trimming, energy cropping or even residues from activities such as furniture creation.

Given the variety of sources from which the chips may be created, the range of moisture content (from now on referred as MC) may be wide. A dry wood would be around the 20% of MC, with a reasonable value around the 30% and in case of green wood, recently extracted and chipped, being able to reach 60% of MC, what in most cases would request drying before being used in a biomass boiler or even stored.

Another important factor, especially when sizing a biomass system, is defining the density of utilized fuel. Knowing this value, adding information about moisture content and calorific value, one may determine total energy available per kilogram of wood. Just like with calorific value, knowing the kind of wood utilized (soft or hard) the dry bulk density (when wood is considered completely dry) may be defined: 195 kg/m3 for softwood and 260 kg/m3 for hardwood [2].

One last important attribute involves dimensions of a chip. Different boilers may have different maximum size of chips which they are be able to burn. Once again working with ranges, the following were defined by the CET/TS 14961:

 

Wood Pellets

Saw dust or wood shavings may be compressed in order to create what is known as wood pellets. This compression process allows wood pellets, in an overall view, to be a more concentrated form of fuel. In this way, smaller storage site needs to be built or refill can be less frequent. On the other hand, it requires more energy to be created, thus rising its price (which is usually higher than of wood chip).

Compared with wood chips, wood pellets are usually drier, with moisture content rarely above 20% and produce less ash. This makes storage and maintenance of the system simpler. Being more practical to handle, usually containing less contamination and having a possibility to be stored in hoppers, pellets are a very flexible fuel, which can be used for small residential heating (around 9kW) as well as big power generation (for example Longannet, a power station in Scotland with capacity of 2.4 MW, where wood pellet are mixed with coal before the burning process).

Just like with woodchip, is important to know the dimensions of the pellet and compare with the capacity of the boiler installed. CET/TS gives the following specification:

Since each wood pellet producer may use a different kind of wood and compress a wood pellet in a different way, there is neither a standard calorific value nor fuel density. This information must be provided by the supplier.

[2] Food and agricultural organization of the United Nations, (2004). UBET Unified Bio-energy Terminology.

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