Method of controlling deactivation of denitrating catalyst

The deactivation of a denitrating catalyst that is caused by exhaust gas dust in a boiler, a furnace or the like which employs a fossil fuel, particularly pulverized coal, can be controlled with excellent results by adding to a fuel at a mill installed in a coal fuel line or at a point upstream of the mill at least one iron compound in a small amount in the form of an aqueous solution, or a powder or water slurry containing coal particles, or in case of employing pulverized coal or heavy oil as a fuel by adding a mixture of an iron compound, a vanadium compound and a tungsten compound, said mixture being in the form of powder, a water slurry or an oil slurry of powder, or an aqueous solution.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of controlling the deactivation 
of a denitrating catalyst resulting from an exhaust gas dust in a boiler, 
a furnace or the like which employs a fossil fuel such as heavy oil, 
pulverized coal, COM, CWM, etc. 
2. Description of the Prior Art 
As environmental pollution grows worse, boilers, furnaces and the like 
which employ fossil fuels such as coal and petroleum suffer from the 
imposition of particularly strict environmental regulations these days. In 
regard to fuels, particularly coal and petroleum, those which have a 
relatively high content of N matter or S matter are relatively low in cost 
and are therefore in general use. Under these circumstances, the discharge 
of nitrogen oxides has become a serious world-wide problem. A particularly 
serious problem is the effect of nitrogen oxides on acid rain and other 
similar undesirable phenomena. 
Examples of measures designed to reduce the generation of nitrogen oxides 
NOx from fossil fuels include: (1) improvements in burning techniques, for 
example, low oxygen combustion, two-stage combustion, exhaust gas 
recirculation combustion, and low NOx burner, (2) selection of fuel types 
(selection of fuels having a low content of N matter); and (3) development 
of exhaust gas denitration techniques. Among these measures, (3) is 
considered to be the most practical approach. 
Examples of exhaust gas denitration techniques include: the catalytic 
reduction method in which NOx is reduced into N.sub.2 at 300.degree. C. to 
400.degree. C. by means of a reducing gas such as ammonia in the presence 
of a catalyst; the catalytic decomposition method in which NOx is 
decomposed at 700.degree. C. to 800.degree. C. in the presence of a 
catalyst; and the absorption method in which NOx is absorbed into active 
carbon. Among them the catalytic reduction method which consists of a 
relatively simple process and utilizes ammonia is widely used and is 
regarded as being the most reliable. This invention relates to this dry 
ammonia catalytic reduction method. 
The principle of this method is that NOx is reduced into N.sub.2 and 
H.sub.2 O generally by adding NH.sub.3 to the exhaust gas (300.degree. C. 
to 400.degree. C.) from a boiler outlet and then by passing the resultant 
mixture through a catalyst layer (for example, V.sub.2 O.sub.2, Fe.sub.2 
O.sub.3, WO.sub.3, etc.) in a reaction vessel. This process is simple and 
suitable for treating a large volume of exhaust gas. The reaction formulae 
of this process are shown as follows: 
EQU 4NO+4NH.sub.3 +O.sub.2 .fwdarw.N.sub.2 +6H.sub.2 O 
EQU 2NO.sub.2 +4NH.sub.3 +O.sub.2 .fwdarw., 3N.sub.2 +6H.sub.2 O 
Another method is the non-catalytic reduction method which consists of 
injecting ammonia into a high temperature area of about 800.degree. C to 
1100.degree. C and effecting denitration in the absence of a catalyst. 
However, this method is hardly even used. 
This invention relates to the above-mentioned dry ammonia catalytic 
reduction method. The most serious problem of this method is deactivation 
of the catalyst employed, which causes a reduced denitration efficiency 
and thus leads to the need for an increased amount of ammonia to be added. 
However, increasing the amount of ammonia added leads to an increased 
amount of unreacted ammonia passing through the denitrize, and this 
unreacted ammonia reacts with the SO.sub.2 and SO.sub.3 present in a gas 
to produce NH.sub.4 HSO.sub.4 which has a low melting point of 147.degree. 
C. Adhesion of this low-melting point substance to the elements of a 
downstream air heater (AH) causes clogging of the elements and a rising 
draft, which may in turn result in an unexpected shut-down. In order to 
prevent such a problem, inspection and repair of the catalyst becomes very 
important. 
Causes of catalyst deactivation may be considered as follows: 
(1) alkali metals such as Na, K, and alkaline earth metals such as Ca, Mg, 
Ba react with SO.sub.3 and the like to produce sulfates, which enter the 
catalyst receptacle to cause clogging of the catalyst; 
(2) the surface of the catalyst may become coated with ash (particularly, 
Si, Al, unburnt matter, etc.) that is present in an exhaust gas, resulting 
in a reduction in the surface area of the catalyst; 
(3) the catalyst may be poisoned and deactivated by sulfur oxides such as 
SO.sub.2 ; and 
(4) a decrease in the amount of catalyst compounds (wear of catalyst 
components by dust and eluation of catalyst components by water). 
In order to solve these problems, the catalyst is water-washed to remove 
any adhering matter after a boiler shut-down. If the catalyst function can 
be restored without stopping the boiler operation, great financial 
advantage will be obtained. Under these circumstances, a method of adding 
an iron compound powder just before and after the position of a denitrizer 
by using a sootblower is employed. The iron compounds added include 
Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Fe(OH).sub.2, Fe(OH).sub.3, 
FeCO.sub.3, FeOOH, etc. 
However, this method has the following problems: 
(1) since ordinary iron compound powders have large particle diameters, 
their activity is low, and a small specific surface area requires the 
addition of a large amount of powder; 
(2) the use of an iron compound powder having a small particle diameter 
(about 0.1 .mu.m) increases the cost considerably, and since the particles 
are small, they are readily blown off rearward by means of the gas stream 
or the pressure from the sootblower, and therefore the proportion of 
particles adhering to the catalyst inside the denitrizer is uneconomically 
small; 
(3) a powder surface with sharp angles causes erosion of the catalyst under 
the pressure applied by the sootblower, resulting in accelerated 
deactivation; and 
(4) since the position where an iron compound powder is added is just 
before or near the denitrizer and the temperature (300.degree. C. to 
400.degree. C.) thereat is therefore lower than the temperature 
(600.degree. C. or higher) at which the iron compound gains activity, most 
of the iron compound which is charged in large amounts does not function 
as a catalyst, resulting in extensive waste. 
On the other hand, if an iron compound powder is added to a gas atmosphere 
with a temperature of 600.degree. C. or higher, a large amount of iron 
compound may be deposited on the heating surfaces of various devices which 
are disposed on the downstream side, such as a superheater (SH), a 
reheater (RH), a feedwater heater or economizer (ECO), etc., resulting 
undesirably in a rise in the exhaust gas temperature and an increase in 
the draft in the furnace. 
Although iron compounds are inexpensive, they are readily poisoned and 
deactivated by SOx, and therefore employment of an iron compound alone 
limits any possible extension of the life of the catalyst. For this 
reason, methods have heretofore been proposed wherein an oxide of a heavy 
metal such as Ti, V, W or the like is employed as an active ingredient as 
well as an iron compound and is injected into the denitrizer using an 
ammonia injection nozzle or the like. These oxides of heavy metals are 
added in the form of an aqueous solution of an ammonia compound. 
These methods, however, suffer from the following disadvantages: 
(1) Since the denitrizer and structures in its vicinity are generally 
formed from structural carbon steel SS and the temperature near the 
denitrizer is about 300.degree. C. to 400.degree. C., addition of the 
above-described oxidizing water-soluble substance causes corrosion of the 
steel material. 
(2) Since the position where the ammonia compound aqueous solution is 
injected is ahead of the position of the denitrizer, the injected solution 
cannot effectively be dispersed into the exhaust gas. Therefore, if there 
are a plurality of catalyst layers, the ammonia compound solution cannot 
be uniformly attached thereto, i.e., an excessive amount of the solution 
may adhere to the first layer, or the catalyst may partially be coated 
with the injected solution in excessively large amounts due to the action 
of a gas drift. Accordingly, in order to obtain effective results it is 
necessary to charge a large amount of the ammonia compound aqueous 
solution, i.e., 500 to 600 ppm or more. 
(3) Most alkali metals in coal, such as K, Na and Mg, adhere to the 
catalyst layer in the form of sulfates. Therefore, if an additive in the 
form of an aqueous solution is injected ahead of the position of the 
denitrizer, water and steam wet the catalyst layer together with such 
sulfates and dust, and this leads to an increase in the amount of alkali 
sulfates, which are even more soluble in water, resulting in an increase 
in the amount of substance poisoned. 
(4) The temperature at the position where the oxide of a heavy metal is 
added is about 300.degree. C.. to 400.degree. C.., which is much lower 
than the temperature (about 600.degree. C.. to 700.degree. C..) at which 
the oxide gains activity. Accordingly, in order to obtain adequate 
activity a large amount of the oxide must be charged. However, the 
addition of a large amount of the above-described oxidizing substance 
increases the rate of oxidation, i.e., SO.sub.2 .fwdarw.SO.sub.3, so that 
SO.sub.3 increases by a large margin and corrosion due to H.sub.2 SO.sub.4 
is accelerated. 
Thus, the addition of a large amount of these heavy metal substances ahead 
of the position of the denitrizer involves many problems. 
It has heretofore been considered that vanadium compounds act as a strong 
oxidizing catalyst, have a low melting point and produce low-melting 
compounds such as n.Na.sub.2 O.mV.sub.2 O.sub.5 to corrode tubes in 
boilers and the like, and therefore they have been excluded from the group 
of substances which may be employed as additives for the abovedescribed 
purposes. On the other hand, tungsten oxides are known as oxidizing 
catalysts having a high melting point which act so as to cover the 
low-melting property of vanadium. However, these compounds have not been 
positively added to fuel. 
If these substances are added in excessive amount, the rate of oxidation, 
i.e., SO.sub.2 .fwdarw.SO.sub.3, increases, and this leads to corrosion of 
boilers, furnaces and the like and causes an increase in the amount of 
slag on the heating surfaces. Therefore, the effect and side effects of 
the addition of such substances are greatly affected by the particle 
diameter and amount of iron compound charged and those of the vanadium and 
tungsten compounds added thereto. Accordingly, it is very important to 
select optimal particle diameters and amounts of these substances. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a method of 
treating a catalyst in order to control the rate of deactivation thereof 
and extend its life by adding in a coal mill or at a point upstream of the 
mill a small amount of an iron compound in the form of an aqueous solution 
or a powder or water slurry containing coal particles capable of passing 
through a 100-mesh screen so as to cause the iron compound to adhere 
strongly to the coal particles, the catalytic activity of the iron's 
oxidizing action increasing due to the high temperature of a furnace and a 
reducing atmosphere, and the activated catalyst then being caused to 
adhere to a catalyst present in a downstream denitrizer. 
It is another object of the present invention to provide a method of 
controlling the deactivation of a denitrating catalyst that is caused by 
an exhaust gas dust in a boiler, a furnace or the like which employ a 
fossil fuel wherein at least one of the oxidizing catalysts which are not 
readily poisoned by SOx such as vanadium and tungsten compounds is added 
in a very small amount to a relatively small amount of an iron compound 
having a small particle diameter to reduce the rate at which the catalyst 
is poisoned by SOx and increase the catalytic activity of the iron's 
oxidizing action, thereby controlling the deactivation of the catalyst 
inside a denitrizer and extending the life thereof, and thus obtaining 
industrially excellent effects and suppressing the adverse effect on the 
boiler, furnace or the like due to the addition of the oxidizing catalyst. 
Other objects and advantages of the present invention will become apparent 
to those skilled in the art from the following description and disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In a method of reducing the NOx present in an exhaust gas by using a 
denitrating catalyst in a boiler, a furnace or the like which employs a 
pulverized coal as a fuel, deactivation of a denitrating catalyst can be 
effectively prevented by adding to a mill installed in a fuel line or at a 
point upstream of the mill an iron compound having an extremely small 
particle diameter in an amount in the range of 5 to 2000 ppm relative to 
the amount of fuel, by further pulverizing the added iron compound 
together with coal agglomerates in the mill to ensure even and strong 
adhesion of the iron compound to the surface of coal particles, by 
conducting the iron compound into an exhaust gas dust together with alkali 
metal oxides such as Na.sub.2 O, K.sub.2 O, etc., alkaline earth metal 
oxides such as CaO, MgO, BaO, etc., or unburnt carbon, and then by 
ensuring adhesion of the iron compound to a catalyst layer in a 
denitrating reactor. Also, because the iron compound can be activated by 
reduction in a high temperature region, Fe.sub.2 O.sub.3 or Fe.sub.3 
O.sub.4 is produced as an oxidizing catalyst on the surface of fly ash. 
Coal usually contains 2 to 20% by weight of iron compounds, most of which 
are present as FeS.sub.2, FeCO.sub.3 or the like. These compounds lie 
buried in the coal particles and stay in them after burning, so that most 
of these compounds may not show adsorptive action toward NOx as catalysts. 
When iron compounds are burnt together with coal particles, catalyst 
poisoning substances in coal such as CaO, Na.sub.2 O, sulfur-containing 
matter, and the like fall as clinker to the bottom of a furnace to some 
extent, resulting in an extension of the life of the catalyst. 
Water-soluble iron salts such as ferrous sulfate, iron acetate, iron 
chlorides (FeCl.sub.3, FeCl.sub.4), iron hydroxides (Fe(OH).sub.2, 
Fe(OH).sub.3) and the like and aqueous solutions thereof are effective as 
iron compounds for this invention. Powders such as Fe.sub.2 O.sub.3, 
Fe.sub.3 O.sub.4, FeO, FeOOH, Fe(OH).sub.3, Fe(OH).sub.2 and the like and 
water slurries thereof are also effective as iron compounds provided that 
their particle diameter is smaller than 100 mesh pass. It is a matter of 
course that the smaller the particle diameter, the higher, the activity 
and the smaller the amount added. 
Further, in a method of reducing the NOx present in an exhaust gas by using 
a denitrating catalyst in a boiler, a furnace or the like which employs a 
fossil fuel, deactivation of a denitrating catalyst can be effectively 
prevented by adding a small amount of the above-described additive due to 
the fact that the additive adheres to a catalyst layer in a denitrating 
reactor, together with alkali metal oxides such as Na.sub.2 O, K.sub.2 O, 
etc., alkaline earth metal oxides such as CaO, MgO, BaO, etc., unburnt 
carbon or exhaust gas dust, without any fear of the additive being 
poisoned by SOx in the exhaust gas, thus providing advantages of great 
economic value. In addition, since the additive has a small particle 
diameter and is added in a small amount, there is substantially no adverse 
effect such as corrosion of the boiler, furnace or the like. 
It is possible to add at least one compound selected from the following 
three different kinds of compound, that is, iron compounds in an amount of 
5 to 200 ppm (in terms of Fe.sub.2 O.sub.3), vanadium compounds in an 
amount of 3 to 50 ppm (in terms of V.sub.2 O.sub.5), and tungsten 
compounds in an amount of 1 to 15 ppm (in terms of WO.sub.3). Within the 
above-described ranges, these compounds can be added without any adverse 
effect on the combustor and the effect of addition of the compounds is 
great. However, if the amount of each of these compounds is less than the 
lower limit, no effect can be expected. 
Examples of iron compounds which may be effectively employed in the present 
invention include water-soluble ferrous salts such as organic acid ferrous 
salt, ferrous sulfate, ferrous acetate, ferrous chloride and iron 
hydroxide, or an aqueous solution, a water slurry and an oil slurry of 
these ferrous salts; and Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, FeO, FeO.OH, 
Fe(OH).sub.3 and Fe(OH).sub.2 in the form of powder, a water slurry or an 
oil slurry. 
Examples of vanadium compounds include water-soluble vanadium compounds 
such as organic acid vanadium salt, ammonium metavanadate (NH.sub.4 
VO.sub.3), vanadium sulfate (VOSO.sub.4), sodium vanadates (NaVO.sub.3, 
Na.sub.3 VO.sub.4), or an aqueous solution of these compounds, and 
vanadium pentaoxide (V.sub.2 O.sub.5), ferrous vanadate or the like in the 
form of powder and a water slurry. 
Examples of tungsten compounds include water-soluble tungstates such as 
organic acid tungsten salt, ammonium tungstate [(NH.sub.4).sub.2 W.sub.4 
O.sub.18 ] and sodium tungstate (Na.sub.2 WO.sub.4), or an aqueous 
solution of these tungstates, and tungsten oxides (WO.sub.3, WO.sub.2), 
tungsten carbide (WC), iron tungstate [Fe(WO.sub.4).sub.3 ] or the like in 
the form of powder or a water slurry. It is a matter of course that as the 
particle diameter of these compounds decreases, the activity becomes 
stronger, and the amount of compound that needs to be added decreases. 
As to the powder, the average particle diameter is preferably selected so 
as to be 5 .mu.m or less. The smaller the particle diameter, the larger 
the specific surface area, and the stronger the activity. Therefore, the 
amount of powder added may be reduced. An average particle diameter in 
excess of 5 .mu.m requires that a large amount of powder be added. As a 
result, an excessive amount of powder may adhere to the heating surfaces 
to lower the heat absorbing capacity, and this leads to a rise in the 
exhaust gas temperature, resulting in economic losses or trouble. 
In the case of a water slurry or an oil slurry, the average particle 
diameter is preferably selected to be 2 to 3 .mu.m or less. An average 
particle diameter in excess of 2 to 3 .mu.m deteriorates the stability of 
the product, so that, even if an excellent surface active agent is used, 
particles are, undesirably, readily precipitated. 
Accordingly, in either case, it is essential to grind the material employed 
using a mill such as a sand mill so that the average particle diameter is 
minimized. 
In the present invention, a catalyst which is formed by coating an iron 
oxide, a vanadium oxide and a tungsten oxide onto a TiO.sub.2 carrier is 
employed. Since the present invention enables a fresh active catalyst to 
be supplied at all times, the life of the denitrating catalyst can be 
extended irrespective of its composition. The carrier of the catalyst is 
not necessarily limited to TiO.sub.2 and other substances may also be 
employed such as Al.sub.2 O.sub.3 provided that the substance employed is 
not harmful. 
The present invention will be described hereinunder in detail with 
reference t o t he accompanying drawings. FIGS. 1(A), I(B), 1(C) and I(D) 
show the way in which an additive adheres to a denitrating catalyst in 
comparison with the case where no additive is employed. 
Referring first to FIG. 1(A), which shows the case where no additive is 
employed, dust particles attached to the catalyst may reduce the surface 
area of the catalyst and therefore lower the activity. 
FIG. 1(B) shows ultrafine iron oxide particles (about 0.1 .mu.m) blown off 
together with steam ahead of the position of the denitrizer by the use of 
a sootblower. Because of this sootblowing, the proportion of small iron 
oxide particles being discharged to the outside may be much larger than 
the amount of particles adhering to the catalyst, which is uneconomical. 
In addition, ultrafine iron oxide particles are very costly. 
FIG. 1(C) shows the case where an additive is added to fuel before the 
position of a mill and the catalyst is in its fresh state. Since dust 
particles which have small iron particles attached to their surfaces may 
adhere to the surface of the catalyst, there is no fear of the surface 
area of the catalyst being reduced, and since an iron compound, a vanadium 
compound and a tungsten compound which are active are supplied at all 
times, lowering of the activity of the catalyst is prevented and the 
activity is improved instead. 
FIG. 1(D) shows the case where an additive is added to fuel after the 
catalyst has been used for a given period of time. Iron, vanadium and 
tungsten compound particles (mainly in the form of oxides) which are 
attached to dust particles may adhere to the dust particles which have 
already been attached to the surface of the catalyst before the additive 
was used, thus preventing lowering of the activity. 
In FIG. 2, reference numeral 1 denotes a bunker for temporarily storing 
coal, 2 is a coal feeder which weighs the coal delivered from the bunker 
and feeds a fixed amount of coal, 3 is a mill which pulverizes the coal to 
a particle size of less than 100 mesh preferably less than 200 mesh, 4 is 
a blower which uses air to convey the pulverized coal toward a burner 7, 6 
is a storage tank containing an additive, and 5 is a pump which injects 
the additive. This is a constant-delivery pump which is capable of feeding 
a fixed amount of additive for a given amount of fuel. The injection point 
is located at an inlet of the mill 3, where the additive is blended with 
the pulverized coal. It is to be noted that the mill inlet is the most 
suitable point for injection because the iron compound is strongly pressed 
against the surface of coal particles by a roller in the mill. This leads 
to an increase in the oxidizing catalytic function of the iron carried on 
carbon. When there are a plurality of similar mills, it is effective to 
add an equal amount of additive at a point upstream of each mill. 
Reference numeral 8 is a boiler, 9 is a superheater (SH) which superheats 
vapor, 10 is an economizer (ECO) which heats feedwater, 11 is an 
ammonia-injecting line for denitrating, 12 is an injection nozzle, 13 is 
an exhaust gas duct, and 14 is a reaction vessel in which the reaction 
between ammonia and NOx takes place. NOx is reduced on a catalytic layer 
that is present in the reaction vessel into N.sub.2 and H.sub.2 O. The 
amount of ammonia to be injected is measured at the inlet and outlet of 
the reaction vessel 14, and is automatically controlled so as not to give 
a lower rate than a predetermined denitration rate. Consequently, when the 
catalyst is deactivated to result in a lowered denitration rate, the 
amount of ammonia to be injected is increased. Reference numeral 15 
denotes an air heater which heats air utilizing the heat of an exhaust 
gas. Once the exhaust gas has left the air heater it is discharged from a 
stack to the atmosphere via an electrostatic precipitator, a desulfurizer, 
and so forth. 
In the case where the fuel is heavy oil, an additive which is in the form 
of an oil slurry or an oil-soluble organic acid salt is supplied to the 
high-pressure area immediately before the position of the burner by means 
of a constant delivery pump. The arrangement of the other section is the 
same as that in the case of a coal combustion boiler. 
A detailed explanation of the present invention will now be given by way of 
Examples and Comparative Examples. 
EXAMPLES 
The denitrating catalyst used was prepared by coating a mixture consisting 
of 30% vanadium oxide, 55% iron oxide, and 15% tungsten oxide onto a TiO, 
carrier. The burning conditions of the boiler and the fuel properties were 
as follows: 
(1) burning conditions: burning was carried out so as to give an excess air 
ratio of 44% O.sub.2. 
(2) fuel: ash 13.5%, volatile matter SO.8%, fixed carbon 5%, nitrogen 
matter 1.3%. 
(3) additive to fuel: .circle.1 no addition, .circle.2 addition of an 
aqueous solution of ferrous sulfate (FeSO.sub.4), .circle.3 addition of 
a water slurry of ferrosoferric oxide (Fe.sub.3 O.sub.4), .circle.4 
addition of a powder of ferrosoferric oxide (Fe.sub.3 O.sub.4). 
EXAMPLE 1 
Table 1 shows the results obtained by charging an aqueous solution of 
ferrous sulfate into a mill installed in a coal fuel line according to the 
method of the present invention. The load of the boiler and the O.sub.2 
ratio at the ECO outlet were set to 175 MW and 4%, respectively. 
TABLE I 
______________________________________ 
Addition of 
No an aqueous solution of 
addition 
ferrous sulfate 
______________________________________ 
Amount of additive 
-- 5 50 2000 
added (ppm) 
(in terms of Fe.sub.2 O.sub.3) 
NOx before denitrizer 
410 400 380 370 
inlet (ppm) 
NOx before denitrizer 
195 185 170 160 
outlet (ppm) 
Reduced amount of 
215 215 210 210 
NOx at denitrizer 
outlet (ppm) 
Denitration rate (%) 
52.4 53.8 55.3 56.8 
Amount of ammonia 
61 56 48 45 
injected (kg/H) 
Leakage of ammonia 
1 or less 
1 or less 
1 or less 
1 or less 
at denitrizer outlet 
(ppm) 
Load (MW) 175 175 175 175 
ECO outlet O.sub.2 (%) 
4.0 3.9 4.0 3.9 
ECO outlet gas 
350 350 350 355 
temperature (.degree.C.) 
______________________________________ 
Note: 
NOx is observed value before converting into O.sub.2 6%. 
An aqueous solution of ferrous sulfate was added to the fuel in amounts of 
5 ppm, 50 ppm and 2000 ppm (in terms of Fe.sub.2 O.sub.3) for comparison 
with the case where none was added. The amount of NOx before the inlet of 
the denitrizer (reaction vessel) (observed value before convorting into 
O.sub.2 6%) was reduced from 410 ppm to 370 ppm, and the amount of NOx 
before the outlet of the denitrizer (observed value before converting into 
O.sub.2 6%) was reduced from 195 ppm to 160 ppm. Consequently, the 
denitration rate increased from 52.4% to 56.8%. Unreacted leakage ammonia 
at the outlet of the denitrizer was 1 ppm or less. This is the value 
obtained by undeactivated catalyst. The amount of ammonia injected 
decreased from 61 kg/H to 45 kg/H corresponding to the reduction of NOx, 
showing that the amount of NOx was definitely reduced at the outlet of the 
cenitrizer. The exhaust gas temperature at the ECo outlet was 350.degree. 
C. with the addition amount of 50 ppm, which is the same temperature as in 
the case where no additive was used. However, when the amount added was 
2000 ppm, the temperature increased to 355.degree. C., showing a 5.degree. 
C. rise in temperature. Any further addition had almost no effect on the 
reduction of NOx. 
Changing the amount of unreacted ammonia as the operation of a boiler 
proceeds was examined for the case of addition to a mill installed in a 
coal fuel line of 50 ppm of an aqueous solution of ferrous sulfate for 
dust coal in accordance with the present invention, as well the case of 
not adding any, the amount of NOx at the denitrizer inlet being assumed to 
be 450 ppm and the denitration rate 50%. The results are shown in FIG. 3. 
When the catalyst is subjected to deactivation, the amount of ammonia 
injected is increased to maintain the denitration rate at 50%. However, 
when the amount of unreacted ammonia exceeds 5 ppm, replacement of the 
denitrate catalyst is required. As shown in FIG. 3, it took 8 months to 
reach a level of 5 ppm of unreacted ammonia in the case where no additive 
was employed and 17 months in the case where a ferrous sulfate aqueous 
solution is added in an amount of 50 ppm. This indicates that the addition 
of an iron compound extends the life of a denitrate catalyst to more than 
twice its original length. 
When the amount of unreacted ammonias exceeds 5 ppm, the unreacted ammonia 
reacts with SO.sub.3 present in the gas to produce ammonium hydrogen 
sulfate NH.sub.4 HSO.sub.4 which adheres to such apparatus as air heaters 
and causes clogging. 
Thus an aqueous solution of ferrous sulfate is very effective because of 
its extremely small particle diameter of 50 .ANG.. When the iron particles 
are smaller than 100 mesh pass, a water slurry and powder are also 
effective, and the smaller the particle size, the greater the effect. 
EXAMPLE 2 
Table 2 shows the results obtained by pulverizing to 100 mesh pass a water 
slurry consisting of ferrosoferric oxide (10% by weight in terms of 
Fe.sub.2 O.sub.3) and 5% by weight of a surface active agent (for example 
an anionic 
##STR1## 
and then by charging the pulverized water slurry at a point upstream of 
the mill installed in a coal fuel line. The load of the boiler and the 
ratio of O.sub.2 at the ECO outlet were set to 175 MW and 4%, 
respectively. Water slurries of Fe.sub.3 O.sub.4 were prepared by adding 5 
ppm, 50 ppm and 2000 ppm in terms of Fe.sub.2 O.sub.3 to the fuel. 
TABLE 2 
______________________________________ 
No 
addition 
Water slurry of Fe.sub.3 O.sub.4 
______________________________________ 
Amount of additive 
-- 5 50 2000 
added (ppm) 
(in terms of Fe.sub.2 O.sub.3) 
NOx before denitrizer 
410 400 386 379 
inlet (ppm) 
NOx before denitrizer 
195 185 175 167 
outlet (ppm) 
Reduced amount of 
215 214 211 212 
NOx at denitrizer 
outlet (ppm) 
Denitration rate (%) 
52.4 53.5 54.7 55.9 
Amount of ammonia 
61 56 49 45 
injected (kg/H) 
Leakage of ammonia 
1 or less 
1 or less 
1 or less 
1 or less 
at denitrizer outlet 
(ppm) 
Load (MW) 175 175 175 175 
ECO outlet O.sub.2 (%) 
4.0 4.0 4.0 4.1 
ECO outlet gas 
350 351 352 358 
temperature (.degree.C.) 
______________________________________ 
Note: 
NOx is observed value before converting into O.sub.2 6%. 
EXAMPLE 3 
Table 3 shows the results obtained by charging a powder (100 mesh-pass) of 
ferrosoferric oxide (magnetite) at a point upstream of the mill installed 
in a coal fuel line. The load of the boiler and the ratio of O.sub.2 at 
the ECO outlet were set to 175 MW and 4%, respectively. The respective 
amounts of additive were predetermined at 5 ppm, 50 ppm and 2000 ppm. 
TABLE 3 
______________________________________ 
No 
addition 
Powder of Fe.sub.3 O.sub.4 
______________________________________ 
Amount of additive 
-- 5 50 2000 
added (ppm) 
(in terms of Fe.sub.2 O.sub.3) 
NOx before denitrizer 
410 400 388 381 
inlet (ppm) 
NOx before denitrizer 
195 186 177 169 
outlet (ppm) 
Reduced amount of 
215 214 211 212 
NOx at denitrizer 
outlet (ppm) 
Denitration rate (%) 
52.4 53.5 54.4 55.6 
Amount of ammonia 
61 56 48 46 
injected (kg/H) 
Leakage of ammonia 
1 or less 
1 or less 
1 or less 
1 or less 
at denitrizer outlet 
(ppm) 
Load (MW) 175 175 175 175 
ECO outlet O.sub.2 (%) 
4.0 4.0 3.9 4.0 
ECO outlet gas 
350 351 353 362 
temperature (.degree.C.) 
______________________________________ 
Note: 
NOx is observed value before converting into O.sub.2 6%. 
As shown in FIG. 3 which shows the amount of unreacted ammonia which 
changes during continuous operation of a boiler, it took 13 months to 
reach 5 ppm in the case of adding the Fe.sub.3 O.sub.4 water slurry (50 
ppm to fuel in terms of Fe.sub.2 O.sub.3) and 12 months in the case of 
adding the Fe.sub.3 O.sub.4 powder (50 ppm to fuel in terms of Fe.sub.2 
O.sub.3), thus showing that an extended life of an extra 4 to 5 months was 
obtainable in comparison with 8 months in the case where no additive was 
employed. 
As explained above, according to the present invention, a relatively small 
amount of an iron compound is added to a mill or at a point upstream of 
the mill. After burning, the added iron compound is converted to Fe.sub.2 
O.sub.3 or Fe.sub.3 O.sub.4 while adhering to the surface of dust such as 
to coat catalyst poisoning substances such as alkali metals, and the 
coated substance adheres to the catalyst. Therefore, the period in which 
the catalyst is subjected to deactivation is considerably prolonged. It is 
apparent that because deactivation of a catalyst due to the physical 
erosion of coal dust is inevitable, the replacement of the catalyst will 
always be necessary to some extent. However, the method of the present 
invention provides the catalyst with a markedly extended life and 
represents a significant financial advantage in comparison with 
conventional methods. 
Furthermore, the method of the present invention solves such problems as 
physical erosion due to the use of an iron compound powder, rising drafts 
caused by excessive iron adhesion in a reaction vessel, and high cost due 
to the large amount of additive employed in comparison with a conventional 
method in which a powder of an iron compound is charged just before and 
after the denitrizer. Iron compounds are relatively low in cost and 
produce no unfavorable side effects in the range of 5 to 2000 ppm for dust 
coal. 
In Examples 4-8, the burning conditions of the boiler and the fuel 
properties were as follows: 
(1) burning conditions: burning was automatically carried out so as to give 
an excess air ratio of 4% O.sub.2. 
(2) fuel: mixed coal of domestic coal and imported coal ratio (fixed 
carbon/volatile matter) . . . about 1.8 nitrogen matter 1.3%. 
(3) boiler operation: in the case of FIGS. 4 and 5, the maximum load 
operation mode (only during the gas analysis); in the case of FIGS. 6 and 
7, the normal operation mode. 
EXAMPLE 4 
A mixed aqueous solution of ferrous sulfate, vanadium sulfate and tungstate 
[in amounts of 30 ppm (in terms of Fe.sub.2 O.sub.3), 15 ppm (in terms of 
V.sub.2 O.sub.5) and 5 ppm (in terms of WO.sub.3), respectively] was 
dropped into coal upstream of the mill. 
EXAMPLE 5 
Ferrous sulfate, ammonium metavanadate, tungsten oxide [in amounts of 30 
ppm (in terms of Fe.sub.2 O.sub.3), 15 ppm (in terms of V.sub.2 O.sub.5) 
and 5 ppm (in terms of WO.sub.3), respectively], an anionic surface active 
agent (in an amount of 5% by weight with respect to the overall weight) 
##STR2## 
and water were mixed together and pulverized for several hours in a sand 
mill to obtain a water slurry having an average particle diameter of 
several microns or less, and this water slurry was added to coal. 
EXAMPLE 6 
A powder mixture of ferrosoferric oxide, vanadium pentaoxide and tungsten 
oxide [in amounts of 30 ppm (in terms of Fe.sub.2 O.sub.3), 15 ppm (in 
terms of V.sub.2 O.sub.5) and 5 ppm (in terms of WO.sub.3), respectively], 
and silicon, as well as trichlene (about 1% by weight with respect to the 
overall weight), were pulverized in a ball mill to obtain powder having an 
average particle diameter of 2 to 3 .mu.m or less. The powder was then 
coated with silicon by means of a ribbon blender and added to coal. 
In addition, the following samples were prepared, that is, sample (1) in 
which no additive was used, sample (2) in which a powder of Fe.sub.3 
O.sub.4 (pulverized to about 2 to 3 .mu.m) was added in an amount of 50 
ppm to coal, and sample (3) in which an aqueous solution of ferrous 
sulfate was added in an amount of 50 ppm to coal. 
FIG. 4 shows the results of these Examples 4-6 and samples (1)-(3). More 
specifically, the graph of FIG. 4 shows the conditions of the catalysts 
which had been used for 7 months after replacement. The maximum load was 
520 T/H each. The respective activities of the catalysts may be compared 
with each other on the basis of the relationship between the amount of 
unreacted ammonia and the molar ratio in relation to injected ammonia 
(NH.sub.3 /NOx). In general, as the molar ratio becomes higher, the amount 
of unreacted ammonia increases correspondingly. Therefore, a comparison 
between the effects of the additives at the point where the molar ratio is 
0.5 clearly shows that there are differences in terms of effect: namely, 
1.5 ppm for sample (1) (no addition); 1.1 ppm for sample (2) (powder of 
Fe.sub.3 O.sub.4); 0.3 ppm for sample (3) (aqueous solution of 
FeSO.sub.4); 0.7 ppm for Example 6 (powder of Fe.sub.3 O.sub.4 +V.sub.2 
O.sub.5 +WO.sub.3); 0.2 ppm for Example 5 (water slurry of FeSO.sub.4 
+NH.sub.4 VO.sub.3 +WO.sub.3); and 0.1 ppm for Example 4 (aqueous solution 
of FeSO.sub.4 +VOSO.sub.4 +(NH.sub.4).sub.2 W.sub.4 O.sub.13). It should 
be particularly noted that the smaller the particle diameter of the 
additive component, the greater the effect. 
FIG. 5 shows the results of measurement of the amount of unreacted ammonia 
(ppm) carried out every time a predetermined operating time has elapsed by 
using the same catalytic additives. The burning conditions of the boiler 
and the fuel properties were substantially the same as those in the case 
of FIG. 4, i.e., almost the same kind of mixed coal was employed, and 
burning was carried out so as to give an excess air ratio of about 4% 
O.sub.2. Although, the boiler is usually run in the normal operation mode, 
it was run under a maximum load of 520 T/H at the time of measurement for 
analysis. A comparison of the samples in terms of the molar ratio 0.5 
after 12 months had elapsed from the star of the operation shows that the 
amount of unreacted ammonia decreased in the following order: namely, 9 
ppm for sample (1) (no addition); 5 ppm for sample (2) (powder of Fe.sub.3 
O.sub.4); 3 ppm for Example 6 (powder of FeSO.sub.4 +V.sub.2 O.sub.3 + 
WO.sub.3); 2 ppm for sample (3) (aqueous solution of FeSO.sub.4); 1 ppm 
for Example 5 (water sIurry of FeSO.sub.4 +NH.sub.4 VO.sub.3 +WO.sub.3); 
and 0.3 ppm for Example 4 (FeSO.sub.4 +VOSO.sub.4 +(NH.sub.4).sub.2 
W.sub.4 O.sub.13). When the amount of unreacted ammonia exceeds 5 ppm, 
acid ammonium sulfate is rapidly produced in large amounts, resulting in 
AH being clogged. 
Thus, the life of the catalyst can be extended to double that of a catalyst 
which consists of an iron compound powder only, and the present invention 
is thus highly profitable for industrial purposes. Although the additive 
is somewhat costly the industrial merits are so great that the slight rise 
in cost can be ignored. 
Since vanadium and tungsten oxides are strong oxidizing catalyst, there is 
a fear of oxidation from SO.sub.2 to SO.sub.3 proceeding at the same time 
to cause low temperature corrosion. However, in the present invention the 
addition of such oxides is carried out in small amounts, and the 
generation of SO.sub.3 is only about 7 to 9 ppm at the outlet of 
denitrizer, so that substantially no difference is found when comparing 
the case where oxides were added with the case where no additive was 
employed. 
EXAMPLE 7 
FIG. 6 shows data obtained when a powder of Fe.sub.3 O.sub.4, V.sub.2 
O.sub.5 and WO.sub.3 having an average particle diameter of 5 .mu.m was 
added immediately before the mill in the following various mixing ratios 
with respect to fuel: 
(1) 200 ppm; 100 ppm; and 30 ppm 
(2) 200 ppm; 50 ppm; and 15 ppm 
(3) 30 ppm; 15 ppm; and 5 ppm 
(4) 5 ppm; 3 ppm; and 1 ppm 
(5) no addition 
The catalytic compounds employed, fuel properties and the operating 
conditions of the boiler were the same as those employed in Example 6. 
As will be clear from the graph, in the case where no additive was 
employed, the ratio of the activity of the catalyst used to its initial 
activity [ks/ks(0); ks: the constant of the reaction rate of the catalyst; 
ks(0): the constant of the reaction rate of the catalyst in its initial 
state (Nm.sup.3 /m.sup.2 `H.multidot.atm)] fell to 0.97 in 3 months, 
whereas the ratios of the samples (1), (2), (3) and (4) rose to 1.18, 
1.11, 1.06 and 1.02, respectively, in 3 months. However, in the case of 
(1), the amount of SO.sub.3 at the inlet of the denitrizer rapidly 
increased from 5 ppm to 40 ppm 2 months after the addition. As to (2) to 
(4), the amount of SO.sub.3 at the inlet of the denitrizer was 8 to 9 ppm 
or less and therefore involved no problem. 
EXAMPLE 8 
FIG. 7 shows data obtained when a powder of Fe.sub.3 O.sub.4, V.sub.2 
O.sub.5 and WO.sub.3 having an average particle diameter of 5 .mu.m was 
added in the following various mixing ratios: 
(1) Fe.sub.2 O.sub.3 : V.sub.2 O.sub.5 . . . 35 ppm: 15 ppm 
(2) Fe.sub.2 O.sub.3 : WO.sub.3 . . . 35 ppm: 15 ppm 
(3) no addition 
The catalytic compounds employed, fuel properties and the operation 
conditions of the boiler were the same as those in Example 6. 
As will be clear from the graph, in the case where no additive was 
employed, the ratio of the activity of the catalyst used to its initial 
activity [ks/ks(0)]fell to 0.97 in 3 months, whereas the ratios of the 
samples (1) and (2) rose to I.04 and I.02, respectively. Although the 
activity is somewhat weaker than that in the case of the additive 
(Fe.sub.2 O.sub.3 +V.sub.2 O.sub.5 +WO.sub.3) shown in FIG. 6, the 
advantageous effect is clearly revealed. 
As explained above, according to the present invention, a very small amount 
of at least one compound selected from the group consisting of vanadium 
compounds and tungsten compounds is added to a relatively small amount of 
an iron compound to thereby enable the rate at which the catalyst is 
poisoned by SOx to be lower than that in the case where an iron compound 
powder alone is employed. Accordingly, the life of the catalyst is greatly 
extended advantageously. In addition, there is substantially no adverse 
affect on the boiler, furnace or the like. Thus, the present invention 
provides great industrial profitability.