The application of advanced technologies employing combustion/co-firing of coal and
biomass is seen as a promising approach to minimising the environmental impact and
reducing CO2 emissions of heat/power production. The existing uncertainties in the
combustion behaviour of such fuel mixes and the release of alkali metals with other
elements during the combustion (or co-firing) of many bio-fuels are some of the main
issues that are hindering its application. The potential presence of high levels of alkali
chlorides and low levels of sulfates in the deposits formed on heat exchanger can cause
enhanced corrosion and/or limit the heat transfer between the hot combustion gases and
the water/steam system within the process plant.
This work has investigated the detailed gas compositions and deposition characteristics
of the combusted gas streams produced from fossil and biomass fuels pure and/or blend
in a pilot-scale combustors (PF and FBC) at Cranfield University. Combustion gas
analysis were obtained on-line by a high resolution multi-component Fourier Transform
Infra-Red (FTIR) gas analyser and deposits samples were collected from the flue gas
using air-cooled probes with surface temperatures of about 500, 600, 700
o
C and
analysed using SEM-EDX and XRD techniques. Fuels included several biomass fuels
(cereal co-product (CCP) straw, miscanthus (pulverised), oil seed rape straw (against
stored pellets), miscanthus (pellets), willow, fast pyrolysis bio-oil) and two
commercially-used coals (El-cerrejon and Daw Mill). The results of the experimental
studies have been compared with thermodynamic equilibrium predictions.
High combustion efficiency was maintained throughout the range of fuel mixes. The
SO2 and HCl levels were low in pure biomass combustion and increased as the biomass
fraction of the fuel decreased when co-fired with these coals. However, the NOx output
remained stable except for Miscanthus:Daw Mill mixtures and OSR stored pellet
combustion. The deposition flux was highest on the coolest probes for each fuel. The
lowest deposition fluxes were found for the combustion of either fast pyrolysis bio-oil
or coppiced willow. There is evidence of significant differences deposition fluxes
between El-cerrejon coal and Daw Mill coal mixed with CCP and/or miscanthus. The
presence of chlorine was identified in deposits produced from combustion of pure
biomass or high biomass mixes. The lowest levels found here in fast pyrolysis bio-oil
combustion and only detected at higher shares (≥ 80 %) of biomass co-fired with Daw
Mill coal, whereas, mixed biomass with El-cerrejon coal produced Cl in deposits at a
low % biomass share.
The application of thermodynamic equilibrium modelling has been found to be useful
tool for providing a qualitative understanding of elements present and/or control by hot
gas in modern combustion processes
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