Abstract
Corrosive ash species are the single most important
factor limiting the electric efficiency of steam boiler
plants fired with waste or biomass. Chlorine has been
found to have a central role in the chemistry involved as
it reduces the melting temperature of ash, forms
corrosive vapour and gas species in the furnace and
halogenated deposits on boiler heat transfer surfaces. In
this context chlorine has been extensively researched.
At the time of writing this thesis there was hardly any
published data available on the occurrence of bromine
(Br) in the aforementioned context. The objective of this
work was to review the occurrence of bromine in solid
fuels and characterise the behaviour of bromine in
full-scale fluidised bed combustion. The central
questions in the context of ash chemistry were: In which
fuels bromine can be found and in which form, what is the
speciation of bromine in the furnace and fly ash, what
role bromine may have (if any), and finally, which are
the practical implications?
The review on the occurrence of bromine in solid fuels
revealed that in anthropogenic wastes bromine is mainly
found in connection to flame retarded substances. Several
weight percentages of bromine can be found in plastics
treated with brominated flame retardants (BFRs). Bromine
is typically found some 100-200 mg kg-1 in mixed
municipal solid wastes (MSW). Bromine may be enriched in
fuels with high share of plastics, such as solid
recovered fuel (SRF) or refuse derived fuel (RDF). Up to
2000 mg kg-1 was found as a monthly average in SRF,
typical levels being 20-200 mg kg-1. Wastewater sludge
from paper mills may contain bromine 20-100 mg kg-1 due
the use of bromine based biocides. In other fuels bromine
may be found in significant amounts in marine influenced
coal deposits and peat as well as in biomass treated with
brominated pesticides.
In the experimental part SRF, spruce bark and wastewater
sludge from a paper mill were co-fired in a full-scale
bubbling fluidised bed (BFB) boiler, and the collected
fuels, aerosols and waterwall deposits were analysed with
the focus on the fate of bromine. Bromine was mainly
found to form water soluble high vapour pressure alkali
metal halides in the furnace - in the form of KBr(g) and
NaBr(g) as estimated by thermodynamic equilibrium
modelling. A positive correlation was observed between
the halogen (Br, Cl) and the metal (Cu, Pb, and Zn)
concentrations in the lower furnace vapours when measured
next to the waterwalls. Further, bromine was observed
both in the wall deposits and in the corrosion front of
the wa-terwall tubes. This was found to take place even
if the bromine content in the SRF was less than 60 mg
kg-1 in SRF. Laboratory scale oven tests with commercial
boiler steels showed that a bromine containing salt
induced a higher corrosion rate than the corresponding
chloride.
Original language | English |
---|---|
Qualification | Doctor Degree |
Awarding Institution |
|
Supervisors/Advisors |
|
Award date | 20 Dec 2011 |
Place of Publication | Espoo |
Publisher | |
Print ISBNs | 978-951-38-7765-1 (soft back ed.), 978-951-38-7766-8 (PDF) |
Publication status | Published - 2011 |
MoE publication type | G5 Doctoral dissertation (article) |
Keywords
- bromine
- bromide
- combustion
- ash
- corrosion
- aerosol
- vapour