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List and values of wood fuel parameters - Part 3

 

If ash is exposed in the boiler to over 1200 C0 temperature, this causes the melting of ash which has several negative influences (e.g. sintering of ash, -> bed sand material would not work as supposed to). The typical ash melting points are shown in the table below.


 Wood chips
Clean bark   
 Softening point
 1200  1340-1405 (°C)
 Melting point
 1250  1650  
 Complete melting
 1275 1650   

 


 


 

In general, however, the ash content of wood based biomass is very low (< 2%) and highest in bark (2-5%), which collects sand and other impurities.

Altogether ash should not be a major issue for a properly designed biomass plant.

COFORD is currently funding research to develop a marketable fertiliser from wood ash. This could eliminate the disposal costs for wood ash and create another advantage over other fuels.

Volatiles

Wood and other types of biomass contain approx. 80% volatiles (in percentage of dry matter). This means that the component part of wood will give up 80% of its weight in the form of gases, while the remaining part will be turned into charcoal. This is one reason why a sack of charcoal seems light compared to the visual volume. The charcoal has more or less kept the original volume of the green wood, but has lost 80% of its weight.

The high content of volatiles means that the combustion air should generally be introduced above the fuel bed (secondary air), where the gases are burnt, and not under the fuel bed (primary air).

Structural Elements of Wood

The structural elements (ultimate analysis) of the organic portion of wood are carbon (45 - 50 percent), oxygen (40 - 45 percent), hydrogen (4.5- 6 percent) and nitrogen (0.3 - 3.5 percent). The distinct advantage of woody biomass over fossil fuels is the small amount of sulphur. The ultimate analysis of some tree species show that carbon and hydrogen contents are rather uniform among species. Bark has a higher percentage of carbon and hydrogen than wood. This is most visibly the case with birch and alder.

In the proximate analysis the amount of volatiles is 65- 95 percent, fixed carbon 17 -25 percent and ash content 0.08- 2.3 percent. Please note that the information of the properties of wood fuels has been collected from several different sources. The most comprehensive data of wood fuel properties was available from ECN laboratories from Netherlands.

Copyright COFORD 2006.