Annex 31 Complementary information to source category 3b Biomass Power Plants

Overview of recent revisions

Two new classes were added to category 3b Biomass power plants, i.e. straw fired boilers as class 3 and boilers fired with bagasse, rice husk, etc. as class 4. Straw-fired boilers need to be adapted to this fuel with regard to ash properties (slagging) and combustion conditions. Due to the chlorine content of straw, PCDD/PCDF emissions are expected to be higher than clean wood. As for boiler fired with various types of herbaceous biomass such as rice husk or bagasse, these are especially used in Asian countries, along with a wide range of agricultural residues. Nevertheless, information on PCDD/PCDF emissions from this source is still scarce.

Derivation of emission factors

Release to Air

Releases to air are the predominant vector for biomass combustion. The default emission factor for class 1 was derived from average values for straw combustion reported for the UK ranging between 17 and 54 μg TEQ/t. Data reported from Austria ranges between 2–500 μg TEQ/TJ. Based on an average heating value of about 8–11 MJ/kg a default emission factor of 500 μg TEQ/TJ was chosen as a representative value even though values as high as 5,000 μg TEQ/TJ could be found. Class 2 was derived from mean values reported between 4.7 (Belgian study) and 5.4 (UK study) μg TEQ/t of wood burned. Based on an average heating value of 12–15 MJ/kg, a default emission factor of about 350 ng TEQ/GJ can be calculated. (LUA 1997, IFEU 1998, Environment Canada 1999). The factor of class 3 characterizing emissions into the air is based on Schleicher (2002). As no reference for emissions into the air is available for class 4, the factor derived for straw is proposed as a first expert estimate.

Release to Water

No release to water is expected. However, in cases where wet scrubbers are installed and effluents are generated, this release vector needs to be highlighted. Presently, no numeric value can be provided to estimate this release.

Release to Land

No release to land is expected unless untreated residue is directly placed onto or mixed with soil.

Release in Products

The process has no product; thus no release to product occurs.

Release in Residues

PCDD/PCDF in the ash residue can be assumed to be present. However, very limited detailed data regarding the amounts could be found. Based on a German study, typical concentrations range from 30–3,000 ng TEQ/kg for bottom ash and 30–23,300 ng TEQ/kg for fly ash. Due to the large overlap in values reported for bottom ash and fly ash, no further differentiation was deemed necessary. Thus, an average value of 3,000 ng TEQ/kg based on an ash content of 0.5% was chosen as a default emission factor. New emission factors are proposed for residues in class 3 derived from Nielsen (2003). The class 4 emission factor for releases through residues is based on Choong Kwet Yive (2008).