Method for operating a coal-fired power plant

A method for operating a coal-fired power plant with a dry-firing furnace includes the steps of supplying coal as a fuel and adding a Ca-containing substance to the coal. The amount of Ca-containing substance is adjusted such that such that after addition of a CaO contents of at least 0.3% is obtained. The coal with the added Ca-containing substance is burned in the dry-firing furnace whereby a flue gas is produced. The flue gas is guided through a NO.sub.x removal device, an air preheater, a particle separator, and a desulfurization device.

BACKGROUND OF THE INVENTION 
The invention relates to a method for operating a coal-fired power plant by 
using a dry-firing furnace, wherein the flue gas stream is guided through 
an NO.sub.x removal device, an air preheater, a dust separator, and a 
desulfurization device. 
Flue gas streams of combustion devices must be scrubbed according to the 
requirements of environmental protection and the service life of the power 
plant of undesired components which are detrimental to the environment 
and/or the parts of the power plant guiding the flue gas stream. 
Flue gas streams contain in addition to solid particles, which can be 
relatively easily filtered, detrimental admixed components which in gas 
streams of a sufficiently high temperature are gaseous. These detrimental 
admixed components in combustion devices which are using fossil fuels, 
are, for example, acid-forming gases such as SO.sub.2, NO.sub.x, SO.sub.3, 
and hydrogen halides. 
For the separation of individual gases from the flue gas stream a plurality 
of methods operating according to different physical-chemical principles 
at different gas temperatures is known. In general, a cooling of the flue 
gas stream between the furnace and the flue gas smoke stack takes place. 
Due to energy considerations, for example, the efficiency of the power 
plant, and with regard to environmental considerations, it is in general 
desired that the temperature of the flue gas stream at the end of the 
path, i.e., at the smoke stack, be low. 
This leads to the fact that at some locations of the flue gas path the dew 
point temperature at least of some of the acid-forming gas components is 
reached and that condensation of acids at the inner parts of the power 
plant takes place. This condensation of acids in the flue gas path of 
power plants, especially of sulfuric acid and hydrogen halides, results in 
a great stress of the power plant parts. The parts must therefore be 
protected with high expenditures, for example, by enameling and coating. 
For example, in conventional power plants the cooling of the flue gas 
stream below the dew point of sulfuric acid and hydrogen halide takes 
place within a heat exchanger that is used for preheating the combustion 
air guided into the furnace. In inexpensive regenerative heat exchangers 
the inner parts must therefore be coated with enamel, must be cleaned 
frequently, and exchanged often whereby in addition to the maintenance 
expenditures for servicing the heat exchanger further expenses are 
incurred due to the required downtime of the combustion device. 
Applicant has described a method for removing undesired gaseous components 
in German Patent Application P 41 13 793.0-43 in which at one location of 
the flue gas path, where the flue gas has a temperature above the dew 
point temperature of the undesired gaseous component, alkaline earth 
oxides or hydroxides are introduced. The flue gas stream is then lowered 
to a temperature below the dew point temperature of the undesired 
component whereby at least one part of the undesired component in 
conjunction with the introduced alkaline earth oxides and/or hydroxides 
are transformed into a solid material. 
In experiments after start-up of modern NO.sub.x removal devices in 
coal-fired power plants it has been found that the limited service life of 
the power plant parts downstream of the furnace, especially of the air 
preheater, the flue gas channels, the electrostatic filter, the forced 
draft channels, the desulfurization device, and the gas preheating device, 
has substantially two causes: 
a) High flue gas temperatures during full load of 390.degree. C., in 
conjunction with a high conversion rate of SO.sub.2 /SO.sub.3 (at 
390.degree. C. approximately 3%, at 350.degree. C. at most 1.1%). 
b) The low flue gas temperature of less than 320.degree. C. at partial 
load, in conjunction with an oversalting of the catalyst that is part of 
the NO.sub.x removal device. 
To date a high conversion rate of up to 3% also resulted in an increase of 
the acid dew point of 90.degree. C. to approximately 125.degree. C. In the 
air preheater contamination were present that could no longer be scrubbed 
with the blowers. This resulted in deposits within the downstream parts of 
the flue gas path, for example, in the electrostatic filter, the flue gas 
channels, in the forced draft channels, and in the desulfurization device 
due to the higher dew point as well as an increased corrosion load. 
It is therefore an object of the invention to improve the aforementioned 
method such that the service life of the power plant parts downstream of 
the furnace within the flue gas path can be increased without detrimental 
side effects and that deposits and corrosion stress can be reduced. 
SUMMARY OF THE INVENTION 
This object is solved according to the present invention by using as a fuel 
a CaO-containing coal dust and by adjusting the CaO contents of the fuel 
in a range of between 0.3% and a maximum amount at which the softening 
point of the combustion particles is considerably reduced. 
The method for operating a coal-fired power plant with a dry-firing furnace 
according to the present invention is primarily characterized by the steps 
of: 
supplying coal as a fuel; 
adding a Ca-containing substance to the coal; 
adjusting the amount of Ca-containing substance such that after addition a 
CaO contents of at least 0.3% is obtained; 
burning the coal with the added Ca-containing substance in the dry-firing 
furnace whereby a flue gas is produced; and 
guiding the flue gas through a NO.sub.x removal device, an air preheater, a 
particle separator, and a desulfurization device. 
The step of adding a Ca-containing substance preferably includes the step 
of continuously mixing CaO into the coal for adjusting the CaO contents to 
0.4 to 0.6%. 
The CaO contents is advantageously adjusted to greater than 0.6% and, 
preferably, a substance for increasing a softening point of combustion 
particles is added to the dry-firing furnace. The substance for increasing 
the softening point of the combustion particles is bauxite. 
The method preferably further comprises the steps of transporting coal to a 
bin (bunker); during transporting performing the step of adding the 
Ca-containing substance to the coal; conveying the coal with the added 
Ca-containing substance from the bin to a grinding device; grinding the 
coal with the added Ca-containing substance to produce Ca-containing coal 
dust; and supplying the Ca-containing coal dust to the dry-firing furnace. 
Advantageously, the method further comprises the step of separating 
suspended and dissolved compounds as a by-product from cooling water to be 
used for a cooling tower of the power plant and using the by-product as 
the Ca-containing substance. 
The method further comprises the steps of sedimenting the suspended and 
dissolved compounds to form a sediment; thickening the sediment; and 
removing water from the sediment to form the by-product. 
The step of transporting the coal to a bin preferably includes the step of 
guiding the coal over a conveyor-type scale for weighing. The step of 
adjusting an amount of Ca-containing substance includes the step of 
determining the amount of Ca-containing substance based on the weight of 
the coal. 
Expediently, the step of adding a Ca-containing substance to the coal 
includes the step of feeding the Ca-containing substance into a grinding 
device for the coal. 
The step of adding a Ca-containing substance to the coal includes the step 
of feeding the Ca-containing substance to the coal upstream of a grinding 
device for the coal. 
The inventive method preferably also includes the step of binding arsenic 
with CaO to form calcium arsenate. It further comprises the step of 
reducing the conversion rate SO.sub.2 /SO.sub.3 in a catalyst of the power 
plant. 
Due to the adjustment of a high CaO, respectively, Ca contents in the fuel 
of the dry-firing furnace a substantial SO.sub.3 reduction upstream of the 
air preheater is possible. This SO.sub.3 reduction according to 
experiments performed by the applicant is based on two mechanisms: 
1. Reaction of SO.sub.3 and CaO to CaSO.sub.4 ; 
2. Conversion damping within the catalyst due to the increased alkalinity 
(increased CaO contents). 
For example, it has been shown in practice that by increasing the CaO 
contents from 0.2% to 0.3% a lowering of the SO.sub.3 contents in the flue 
gas upstream of the air preheater of about 60% can be reached. By 
correspondingly adjusting the contents of CaO in the fuel already upstream 
of the furnace the SO.sub.3 binding is therefore controllable. 
Correspondingly, fluctuations of the acid dew point therefore no longer 
have a strong effect or have no effect on deposits and corrosion within 
the power plant parts downstream of the NO.sub.x removal device. 
Surprisingly, a further substantial disadvantage can be avoided by using 
coal dust with a strong CaO contents: the relatively high CaO contents 
binds arsenic traces (binding of arsenic with CaO to form calcium 
arsenate) so that the catalyst service life is prolonged. 
According to a preferred embodiment of the invention it is suggested that 
CaO is continuously mixed into the coal dust before the coal reaches the 
furnace and that the CaO contents is adjusted to a range of between 0.4 to 
0.6%. The adjustment of the CaO contents within this range is relatively 
uncritical. It is therefore possible to use coal with greatly varying CaO 
contents and to adjust the desired CaO contents by preferably continuously 
liming the fuel upstream of the furnace. 
In certain cases it may be expedient to adjust the lime contents above the 
critical limit of 0.6%. In order to avoid falling below a predetermined 
softening point of the combustion particles, according to another 
embodiment of the invention a substance, for example bauxite (Al.sub.2 
O.sub.3), may be added to the dry-firing device which substance increases 
the softening point of the combustion particles. 
For a conventional fuel loading of modern coal-fired power plants (in which 
the coal is transported to a bunker, conveyed from the bunker to a 
grinder, ground and added to the furnaces of the dry-firing device), the 
addition of a Ca-containing substance to the coal is preferably performed 
during the transport of the coal to the bunker. In a preferred embodiment 
of the invention it is suggested in this context that the Ca-containing 
substance is a by-product resulting from the water treatment device (KZA) 
for treating additional water for the cooling tower. This KZA by-product 
is sedimented, thickened, and dewatered before it is added to the fuel. 
During the transport to the bunker the coal may be guided over a conveyor 
type scale and the value measured at the conveyer-type scale may be used 
as a parameter for the addition of the Ca-containing substance. The 
adjustment of the CaO contents in the coal dust (fuel) to between 0.4 and 
0.6% furthermore makes it possible to use the air preheater as an acid 
trap. 
Known measures for increasing the service life of the involved parts, for 
example, the replacement of the air preheater sheet metal by an enameled 
ones on the cold side, are favorably supported by the inventive method. 
The inventively ensured high calcium oxide contents supports the acid 
formation within the air preheater and its use as an acid trap. The flue 
dust formed is mostly alkaline. The SO.sub.3 contents downstream of the 
desulfurization device is substantially reduced with the inventive method. 
This is also true for the corrosion damage within the flue gas channels 
leading to the smoke stack.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The described method is based on the following power plant processes: the 
coal is transported from a coal storage 1 via a conveying device 2, 
schematically represented as a dash-dotted block 2, to a coal bunker 3 and 
from there to the corresponding coal grinder 4. The coal is ground in the 
coal grinder 4, dried with hot air and guided as fuel to the furnace 
arrangement 5 of a dry-firing combustion device 6. The heat resulting from 
the combustion of the coal in the dry-firing combustion device is 
transferred via a pipe system to the steam generator 7. The water 
streaming through the pipes 8 is evaporated under pressure and guided 
subsequently in the form of heating steam to the turbine 9. The energy 
present at the turbine is then transformed into electric energy. At the 
low pressure side of the turbine a cooling system, which as a whole is 
indicated by reference numeral 11, performs the condensation of the steam. 
The condensate is returned via a boiler water feed pump into the steam 
generator 7. 
The flue gases from the combustion process within the steam generator 7 are 
scrubbed of NO.sub.x in the NO.sub.x removal device 20 and are cooled in 
the air preheater 21 by preheating the combustion air. Subsequently, a 
dust separation takes place within the electrostatic filter 22 and the 
flue gas freed of dust is guided with a flue gas blower 23 into the 
desulfurization device 24. The scrubbed gas exiting from the 
desulfurization device 24 is then released into the atmosphere via the 
smoke stack 25. The cooling takes place in a closed cooling circuit 11. In 
the main cooling circuit water losses are unpreventable. On the one hand, 
a considerable evaporation takes place via the cooling tower as a function 
of the output of the power plant block. On the other hand, losses occur 
due to wash water, spray water for condensate cooling, and finally due to 
removal of deposits within the cooling tower bottom. These losses must be 
compensated for whereby high quality requirements are placed onto the 
water to be added to the closed cooling water circuit in order to avoid 
boiler scale deposits as well as a contamination within the condensator 
area and in the inserts of the cooling towers, respectively, to at least 
minimize such effects. 
The raw water introduced via the line 12 is mixed in a mixing and 
distributing chamber with iron chloride solution in order to flocculate 
suspended solid particles. In a flocculating device lime suspension and 
contact sludge is added to the water and intensively mixed. The calcium 
carbonate formed from the lime suspension and hydrogen carbonate is 
deposited as a solid material. Together with the iron hydroxide it forms 
large flocculate particles that settle within the calm zone 13 of the 
flocculating device. The treated water then flows via a treated water 
chamber to the cooling tower bottom. 
The by-product of interest in the inventive process, which has been 
flocculated and settled and consists of calcium carbonate, sediments 
(solid particles) of the raw water, and iron hydroxide, is conveyed with 
conveying scrapers into pockets and is guided via a buffer container 15 to 
a thickening device 16. Here a predewatering of the sludge takes place. 
With high pressure pumps that are not represented in the drawing the 
thickened sludge is then forced through chamber filter presses 17 and 
dewatered. The filter cake falls into a bunker 18 and is then available 
for further use or disposal. 
Up till this point the described process for operating a power plant is 
conventional. As mentioned above, the conventional realization of the 
methods of operating power plants resulted in high conversion rates in the 
NO.sub.x removal device 20 in addition to an increase of the acid dew 
point at full load at the exit of the air preheater 21 to approximately 
125.degree.. This resulted in the fact that the uncritical air preheater 
could not be used to the desired extent as an acid trap for binding 
SO.sub.3. The aggressive flue gas components, especially SO.sub.3, caused 
in the downstream parts, i.e., electrostatic filter 22, flue gas channels 
and blower as well as within the desulfurization device 24 in deposits and 
a considerable corrosion stress. 
Inventively, it is suggested to provide already during fuel preparation in 
the area of the coal conveying device 2 the coal with a higher Ca, 
respectively, CaO contents so that the aforementioned disadvantages of 
conventional power plant processes with dry-firing combustion devices are 
eliminated or at least substantially reduced. For this purpose, upstream 
of the coal bunker 3 the by-product of the water treatment device for 
additional water for the cooling towers, that is in the form of the filter 
cake of the filter presses 17 and is stored in the bunker 18, is added. 
This by-product is comprised to approximately 80 to 90% (dry) of calcium 
carbonate (CaCO.sub.3) corresponding to a CaO contents of 45 to 55%. The 
addition of the KZA by-product takes place via the pump 19 in a continuous 
manner. The KZA by-product is added to the coal at the location 30 on the 
conveyor belt conveying the coal from the bunker. As a parameter for the 
adjustment of the CaO-containing KZA by-product the conveyor-type scale 31 
is used in the described embodiment. The ratio can be adjusted via a 
suitable regulator 32 as a function of the CaO contents of the coal taken 
from the coal storage 1. The continuous liming at the location 30 upstream 
of the coal bunker 3 has the advantage that the CaO contents for the 
entire contents of the coal bunker 3 can be adjusted in a uniform manner 
and independent of the partial or full load operation of the power plant 
and the amount of coal removed from the bunker 3. A special advantage of 
the described inventive process lies in the fact that the Ca-containing 
substance is produced directly within the process of the power plant so 
that no additional and likely expensive materials must be used. It has 
been shown in experiments that for the described type of cooling and the 
use of coal low in Ca as a fuel substantially exactly such an amount of 
the KZA by-product is produced in the KZA process as is needed for 
increasing the amount of CaO of the fuel at the location 30 (balanced mass 
flow). 
Of course, it is also possible to use any other suitable Ca-containing 
substance in connection with cooling systems or as an alternative for 
addition to the fuel upstream of the furnace device 5. The addition may 
also take place in the area of the coal grinder 4 or downstream thereof in 
the fuel-air mixture. For the given conditions of the described process of 
operating a power plant, the connection of the KZA by-product production 
to the coal-conveying device 2 is especially favorable. 
The present invention is, of course, in no way restricted to the specific 
disclosure of the specification and drawings, but also encompasses any 
modifications within the scope of the appended claims.