co-combustion of straw and miscanthus with brown coal in a
pulverized combustion system. The system was a roof-fired,
U-shaped combustion chamber, designed for a thermal input
between 0.5 and 1 MW. The biomass was premixed with the
coal and via a fan mill the mixture was transported into the
burner with the help of recirculated flue gas. The results
showed that co-combustion of straw (up to 30% of the
total thermal input) with brown coal was possible and without
serious difficulties. The co-combustion of miscanthus
led to low burn-out, high amounts of char in the ash hopper
and high CO emissions when the miscanthus contributed
more than 25% to the total thermal input and when excess
air ratios of less than 1.1 were used. CO emissions for all
flames with straw and miscanthus up to 25% of the thermal
input were comparable to the ones obtained with the monocombustion
of brown coal. A stable ignition and burn-out
was achieved when the biomass particles were small in size
and sufficiently dry, whereas higher moisture contents in the
biomass retarded the combustion. Increasing the biomass
input reduced the furnace temperature peaks and led to
longer flames. Similar results were obtained by Kicherer
et al. [77] who investigated the effect of co-firing of bituminous
coal with straws on the combustion and emission
characteristics in a 500 kWth pulverized coal plant. The
straw samples were prepared in two sizes, i.e. coarse particles
with average diameters greater than 6 mm and fine
particles with average diameters less than 0.75 mm. The
burn-out efficiency decreased slightly from 99.5 to 99.3%
as the portion of coarse straw was increased from zero to
40% of the thermal input, whereas for fine straws, an
increase in burn-out efficiency to 99.9% was recorded.
The high reactivity and high volatile matter contents
combined with the large surface area of the straw improved
the combustion behaviour of the coal.