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Analysis Of Fuel Calorific Value Of Wood Residue Of Wood Pellet Machine

Date: 08/26/2020 08:29:43 From: wood-pellet-plant.com Clicks:

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I. Heating Value

When evaluating the properties of a combustible material with respect to its use as a fuel, the heating value, expressed in this document as gross calorific values or higher heating values, is one of the most important factors, which indicates the amount of thermal energy which may be obtained by combusting one mass unit of the material.

Analysis Of Fuel Calorific Value Of Wood Residue
Analysis Of Fuel Calorific Value Of Wood Residue

The heating value of wood depends very much on the species and the part of the tree being used and varies between 17 to 23 MJ/kg of bone dry wood; generally softwoods have higher caloric values than hardwoods, with an average value of 21 MJ/kg BD for resinous woods and 19.8 MJ/kg BD for other woods being used. In fact, there is very little variation in the heating values of the wood substance itself, being some 19 MJ/kg BD, as it is, in fact, the variation in resin content, with a calorific value of 40 MJ/kg BD, which accounts for the differences in values between the species. It is for this reason that bark, with a high gum and resin content, tends to have a higher value than wood.

However, although the fuel value may be fairly consistent in bone dry wood, the heating value depends on several factors, namely moisture content, particle size, type and efficiency of combustion equipment being used and the level of its operation and maintenance. Hence, in order to put the heating values of various wood residues into perspective one must take into consideration the heat content per unit of waste according to its moisture content, together with the efficiency of the energy conversion process which, as indicated in Table 9, provides a comparative analysis to be made with other alternative fuels.

Table 1. The effect of moisture content on the net heating value of wood compared to that of other fuels

Fuel As fired Gross calorific value Typical burner efficiency Useable Net heating value
(MJ/kg) (%) (MJ/kg)
Wood at 0% m.c 19.8 80 15.8
10% m.c 17.8 78 13.9
20% m.c. 15.9 76 12.1
30% m.c. 14.5 74 10.7
40% m.c. 12.0 72 8.6
50% m.c. 10.0 67 6.7
Anthracite 31.4 83 26.1
Lignite 26.7 80 21.4
Heavy fuel oil 42.6 82.5 35.1
Light fuel oil 43.5 82.5 35.9
Butane 49.3 79.0 38.9
Propane 50.0 78.7 39.4

II. Effect Of Moisture Content And Particle Size On Heat Values

Wood at the time of logging generally has a moisture content of approximately 50 to 55 percent, although the amount varies according to species, age and the portion of the tree from which it originated, i.e. branches, trunk, etc. Further fluctuations from the mean are influenced according to the season it is cut and the manner in which it is transported to the mill site and stored; logs that are floated down stream, wet-debarked or left in conditioning ponds could have moisture contents as high as 65 to 70 percent, whereas that which is road-hauled and dry-debarked would be in order of 45 to 50 percent m.c. Spring and summer storage can bring about a moisture loss of 10 to 25 percent.

The moisture content of the manufacturing residues depend very much on at what stage of the process they are extracted and whether there has been any drying of the product before that stage. For instance, sanding dust from plywood or particleboard manufacture is taken from the plant after the driers and hot presses, where its moisture content could be as low as ten percent or less, as indicated in Table 2.

Table 2. Range of characteristics of typical wood residues

Residues Size Moisture content Ash & dirt content
(mm) (%) (%)
Sanderdust - 1 2 - 10 0.1 - 0.5
Shavings 1 - 12 10 - 20 0.1 - 1.0
Sawdust 1 - 10 25 - 40 0.5 - 2.0
Bark (hogged) 1 - 100 25 - 75 1.0 - 2.0
Log-yard clean-up up to 100 40 - 60 5.0 - 50
Forest residuals needles to stumps 30 - 60 3.0 - 20

As mentioned previously, moisture content is a major determinant in the heating value of wood waste, which, from 19.8 MJ/kg at 0 percent m.c. drops to 10 MJ/kg at 50 percent m.c., as can be seen by refering to Figure 12. Although wood may be burnt at 55 percent m.c., and up to 58 percent m.c. with careful operator attention and boiler tuning, it is always better to aim for a moisture content of 50 percent or lower in order to achieve satisfactory and substained operation. When the moisture content rises to 60 percent, burning of the wood residues become difficult as its heating value drops dramatically, to the extent where, at approximately 68 percent m.c., "furnace blackout" occurs, being the point at which combustion can no longer be sustained, unless a supplementary fuel is used to maintain combustion.

A high moisture content not only lowers the as-fired heat value of wood waste, but seriously affects the overall combustion efficiency due to the large amount of energy needed to heat considerable quantities of excess air and to vapourise the moisture in the waste, which, together with the moisture formed by the combustion process itself is subsequently lost up the stack as latent heat. Hence, it stands to reason that wood waste at ten percent m.c., with an as-fired heat value of 17.8 MJ/kg and a combustion efficiency of some 78 percent is preferable to green wood at 50 percent m.c. with an as-fired heat value of 10 MJ/kg and 67 percent combustion efficiency.

The effect of wood residue moisture content on combustion efficiency

Figure 1. The effect of wood residue moisture content on combustion efficiency

The size and form of the wood particle is also critical in both the handling characteristics and burning efficiency of residues and plays a major role in their combustibility and the selection and operation of processing and combustion plant. Whereas fine sanderdust and wood shavings may be burnt in suspension, larger sized wood-waste, in the form of large chips, coarsely hogged waste and slabs need a longer dwell time to burn which is generally undertaken on grates.

Hence, all steps taken to reduce the moisture content and size of the residues to a minimum, pays dividends in energy generation. The provision of prepared storage, suitably protected against the elements, the use of flue gases to dry the fuel etc., all contribute towards maintaining low residual moisture and optimum combustion efficiency.

0.5-0.8 T/h Wood Waste Pellet Production Line In Bulgaria

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