The rising global demand for energy and the increasing discussion about saving
carbon dioxide in the generation of heat and electrical energy are bringing conventional
power plants, where energy is generated from fossil fuels, into focus. The decision by
politicians to phase out nuclear power and the generation of electricity from coal is
making the supply situation in many parts of Germany more difficult.
Renewable energies such as the use of solar and wind power can only close this gap
to a limited extent without suitable storage solutions due to the fluctuation of the energy
supply over time. In addition, the large wind farms that have the potential to replace
coal-fired power plants are mainly located at the coasts in northern Germany. However,
due to the lack of distribution grid capacity, the energy generated here cannot be
transported to industrial centers such as those in the Ruhr region and southern
Germany.
It therefore makes sense to convert the existing coal-fired plants, which will be deprived
of their operating basis by the planned phase-out of coal-fired power generation, to
biomass.
Due to the large amounts of fuel needed as well as the dust firing technology in use,
not every biomass is suitable. The decisive factors are availability, energy density as
well their usability in a dust firing system. Pellets made of wood and agricultural
residues such as sunflower husks have proven to be promising, due to their energy
density, which is comparable to dry lignite.
Due to the location of coal-fired power plants at the coast or near rivers, which have
already been used to transport coal, the logistics for transporting biomass in the form
of pellets are already in place.
The necessary modifications to the existing power plant units are mainly limited to fuel
preparation. Here, adapted fuel mills are needed to break down the bond of the
biomass fiber in the pellet again.
The use of biomass in conventional pulverized fuel furnaces offers the advantage that
the fuel is considered CO2-neutral, since the amounts released correspond to what the
plants have sequestered from the atmosphere during their growth. In addition, the use
of biomass leads to a significant reduction in the ash mass flows resulting from the
combustion process.
Additional to the advantages, the use of biomasses also entails various disadvantages.
For example, most biomasses contain significantly higher concentrations of phosphor
and alkali metals, which cause damage and higher operating costs in the area of the
furnace and the boiler, due to corrosion, and also have negative effects in the
downstream units of the flue gas cleaning system. This applies in particular to the SCRDeNOx-catalyst, in the following named DeNOx-catalyst, which is installed in most
pulverized fuel furnaces to reduce nitrogen oxides from the combustion process. This
catalyst usually consists of a matrix of titanium dioxide doped with active components
such as vanadium pentoxide. Phosphorus compounds, for example, cause sticky
coating in the pores and thus reduce the catalyst surface. However, this process is
reversible with appropriate measures. In contrast, catalyst deactivation caused by
potassium compounds is not reversible.
For this reason, this work deals with the influence of additives added to the combustion
process to reduce the negative effects on the catalyst. There are various approaches
to this in literature, but in this work the approach of incorporation into alumino-silicate
structures is investigated based on experimental tests.
The work is divided into three parts, starting with the characterization of fuels and
additives on a laboratory scale. In the second part, based on the data from the
investigations carried out in the first part, calculations are performed using FactSageTM
on the release or incorporation behavior of the alkali compounds. The third part of the
work is followed by investigations on an entrained-flow reactor as well as a dust furnace
in order to verify the statements of the second part of the work under real conditions.
In addition to the analyses carried out directly during the test series, such as emission
measurements, ash and catalyst samples were also examined and evaluated
subsequently using a scanning electron microscope.
The investigations show that the use of alumino-silicate-based additives results in rapid
and stable incorporation of the potassium bound in the fuel in the combustion reaction
zone.
In addition, it has been shown that chlorine compounds also present in the fuel lead to
an increased HCl concentration in the combustion waste gas due to the lack of
potassium as a reaction partner. In addition to emission problems, this can lead to
corrosion with metallic materials, if the acid dew point is undershot.
The findings obtained from the test series show that, if aluminosilicate-based additives
are used, the formation of potassium chloride during the thermal conversion of the
biomasses investigated, can be significantly reduced and the potassium can be stably
incorporated into the additive.