European Bio-Energy Projects

BIO-AEROSOLS
Objectives
The project focused on the solution of
problems related to aerosols and fly ashes
formed during fixed-bed biomass
combustion, namely particulate emissions
and deposit formation. The main aims of
the project were to investigate the
characteristics and the behaviour of
aerosols by considering different biomass
fuels (wood chips, bark, and waste wood),
as well as to identify mechanisms
governing deposit formation in furnaces
and boilers. Based on this knowledge,
technologies able to reduce these
problems should be developed. Efficient
aerosol precipitators should be enhanced
by setting up an aerosol database for filter
manufacturers, accompanied by technoeconomic
recommendations, and by
optimising the rotational particle separator
which is an innovative dust precipitator for
small-scale applications. Additives to
influence aerosol formation and growth
should be investigated. Finally, the project
also aimed to evaluate health risks caused
by aerosol emissions from biomass
combustion.
Aerosols and fly ashes
in biomass combustion –
new data, modelling
approaches and results
Problems addressed
In general, particulates formed during the
combustion of solid biomass can be divided
into two fractions, the aerosols (particles formed
from condensable vapours by gas to particle
conversion) and coarse fly ashes (ash particles
entrained from the fuel bed with the flue gas).
Before BIO-AEROSOLS was initiated, very little
information about the formation mechanisms
and characteristics of these two fly ash fractions
during fixed-bed combustion of woody biofuels
was available. Therefore, the objectives of the
project were defined as above.
To reach the project objectives, extensive test
runs were performed at biomass combustion
units, as well as mathematical modelling of the
processes involved in aerosol, fly ash and deposit
formation and behaviour. In total, five test runs
were carried out at a pilot-scale combustion unit
(440 kWth) and a large-scale CHP plant (40 MWth),
comprising fuel, aerosol, fly ash, ash and deposit
sampling with subsequent wet chemical and
SEM/EDX-analyses of the samples being taken.
From these tests, a huge amount of high-quality
measurement and analyses data was collected
concerning relevant characteristics of aerosols
and fly ashes formed during fixed-bed combustion
of woody biofuels. In Figure 1 and Figure 2
examples of particle size distribution, shape
and chemical composition of aerosols and fly
ashes formed during the combustion of woody
biofuels are presented. These new data (shapes,
concentrations, particle size distributions and
chemical compositions of aerosols and fly ashes)
have been summarised in an aerosol and fly ash
102
database. Furthermore, a huge amount of new
data concerning the characterisation of furnace
and boiler tube deposits, such as build-up rates,
structures and chemical compositions, resulted
from the project and have also been summarised
in a database.
Results
The results of the test runs clearly indicated
that the chemical composition of the fuel is the
main parameter influencing aerosol formation,
while plant operation parameters (excess air
ratio, furnace temperature, etc.) do not have a
significant influence.
Based on the data and experiences gained from
the test runs, existing models able to predict
aerosol and deposit formation as well as deposit
melting behaviour have been improved, and new
models for the prediction of the behaviour of
aerosols and fly ashes in fixed-bed biomass
combustion units developed. Three different
aerosol formation processes for different types
of woody biomass (chemically untreated wood,
bark, and waste wood), depending mainly on
the chemical composition of the fuels used,
were identified during these investigations.
Consequently, the knowledge about these
processes was increased substantially and, in
future, the furnace and boiler designs as well as
process control strategies of the industrial
partners taking part in this project, will be
adjusted according to the project results in order
to reduce deposit formation as well as particulate
emissions.