Abstract
Formation mechanisms of the fly ash and bottom ash during
circulating fluidised bed combustion of a bituminous coal
and two solid biomass fuels were studied experimentally.
The biomass fuels were forest residue and willow (Salix).
The experiments were carried out at full-scale boilers.
As a result, the main ash formation mechanisms are
presented for coal and biomass combustion.
Ash formation was studied using the methods of
traditional ash sampling and aerosol technology. By
traditional ash sampling methods, samples of ash were
collected from different ash streams of the boiler. The
collection streams were the fuel feed, sorbent or bed
material feed, boiler bottom ash, and the fly ash
collected in the electrostatic precipitator. The methods
of aerosol technology enabled sampling of the particles
directly from the flue gas flows. In this way, the size
distributions of the particles were determined. In
addition, the elemental contents of the particles were
analysed with different methods to obtain the
concentrations of the different ash compounds in the flue
gas. The aerosol instrumentation included the following
devices: low-pressure impactors, cyclones, a differential
mobility analyser combined with a condensation nucleus
counter and an electrical low-pressure impactor. The
morphology of the particles was analysed using scanning
electron microscopy.
In coal combustion, limestone sorbent was used to capture
SO2 and no additional bed material was utilised. The
bottom ash consisted mainly of coal minerals and
unfragmented sorbent particles. The coal minerals
agglomerated during combustion forming bottom ash
particles with a wide range of compositions. Sorbent
particles captured SO2 and were mainly present as CaSO4
and CaO. During biomass combustion, quartz sand was fed
into the furnace to maintain an adequate bed inventory.
No sorbent was used. The adhesion of ash to the sand
particles and the subsequent growth of bed particles
formed the bottom ash. The ash compounds adhered to the
sand particles by two mechanisms: i) adhesion of the
non-volatile ash compounds as particles on the sand
surface, and ii) diffusion of the volatile ash compounds
into the quartz sand and subsequent chemical reaction.
The ash particles formed a sticky layer on the sand
particles which in high temperatures or when grown
sufficiently thick might have caused agglomeration and
deposition problems in the bed as well as in the upper
parts of the furnace and in the cyclone loop.
The fly ash size distributions were bimodal with all
fuels. The fine mode was formed in the submicron size
range by volatilisation and subsequent nucleation of the
volatilised species. These particles then grew by
condensation. The mass of the fine particle mode
consisted 0.3 % of the total mass of the fly ash
particles during coal combustion, 2 % during forest
residue combustion, and 8 % during willow combustion. The
major fraction of the mass in the fine mode consisted of
HCl with coal, KCl with forest residue, and K2SO4 with
willow. The coarse fly ash particles, so-called residual
ash, were formed mainly from the ash compounds that did
not volatilise during combustion. The coal minerals
formed agglomerates with a few minerals in one particle.
In addition, a major proportion of the limestone sorbent
fragmented and escaped the furnace as fly ash. The coarse
fly ash particles from biomass combustion were large
agglomerates comprised of mainly submicron-sized primary
particles up to several thousand in number.
The agglomerate shape of the particles was found to
affect the gas-to-particle conversion of the volatilised
species. Consequently, the resulting concentration size
distributions were different from those for spherical
particles. Condensation of the volatilised species
resulted in the enrichment of the condensed species in
the fine particles whereas the gas-to-particle conversion
by a chemical surface reaction resulted in a
concentration of the volatilised species that did not
depend on the particle size.
Original language | English |
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Qualification | Doctor Degree |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 19 Mar 1999 |
Place of Publication | Espoo |
Publisher | |
Print ISBNs | 951-38-5356-X |
Electronic ISBNs | 951-38-5365-9 |
Publication status | Published - 1999 |
MoE publication type | G5 Doctoral dissertation (article) |
Keywords
- fluidized beds
- fluidized bed combustion
- ashes
- fly ash
- bottom ash
- coal combustion
- biomass