Definition and Properties
In order to produce wood chips, woody biomass, with the intention of being burned afterwards, must be chopped. The chipping process is described in more detail on the pre-treatment page.
Because of the chipping process, wood chips are a relatively uniform fuel, which is able to flow and can be fed to a boiler automatically. The average dimension of a wood chip is from 16 to 45mm. For wood chips there are standard sizes, which are defined in the standard mentioned below. The size specification refers to the cross-sections of the holes in a round hole sieve. Because of the high surface area-to-volume ratio, they can be burned efficiently.
Compared to log wood, wood chips have a lower bulk energy density, which leads to a larger space requirement during transport and storage.
The quality of the wood chips depends on the used raw material and the chipper. With respect to the raw material, wood chips can be divided into the following groups:
- Forest chips (produced from logs, whole trees, logging residues, or stumps)
- Wood residue chips (produced from untreated wood residues, recycled wood, offcuts)
- Sawing residue chips (produced from sawmill residues)
- Short rotation coppice chips (produced from energy crops)
Standards and Quality
The European Standard DIN EN ISO 17225-4:2014-09: Solid biofuels – Fuel specification and classes – Part 4: Graded wood chips, defines four different quality classes for wood chips (A1, A2, B1, B2) and three different grain size fractions (P16S, P31S, P45S). The quality classes A1 and A2 are intended for use by private households and the classes B1 and B2 are usually used by the industry. For plants larger than 1MW, own quality agreements are defined.The grain size fractions indicate the maximum fine portion, the permissible coarse portion, the maximum particle length and the maximum cross sectional area of the particles. The use of this standard is not mandatory, but voluntary.
The following points also must be taken into consideration for good fuel quality: absence of splitters and dust, maximum moisture content, maximum ash content. Wood chips from freshly felled trees have a moisture content of up to 70%. Large boilers over 400kW are able to burn such moist wood chips, but smaller boilers need a moisture content below 30%. The ash content increases with increasing contamination of the fuel, thus contamination with soil and dust should be avoided during the chipping process, transport and storage.
Application in wood chip-fired boilers
There are several different technologies for the combustion of wood chips. Wood chip-fired boilers are usually operating in a high performance area (for example underfeed stokers are available from 10kW to 2.5MW and grate furnaces are available up to 60MW). Therefore wood chip-fired boilers are often used in industrial applications. However, also farmers and/or forest owners operate these boilers either in communities or alone.
The fuel (wood chips) can be fed automatically into the combustion chamber. The requirements on fuel quality vary much between large scale and small scale boilers . This is because industrial plants are different in structure and use other combustion technologies. These are usually grate-firing systems which are less vulnerable to disorders than smaller plants.