Process for treating product of radiation treatment of ammonia-added effluent gas

A solid product which comprises ammonium sulfate and ammonium nitrate and which contains a sulfamic acid compound is produced when an effluent gas having ammonia added thereto is irradiated with a radiation to thereby desulfurize and denitrate the effluent gas comprising SO.sub.x, NO.sub.x and CO, but the CO gas content being less than ten times the concentration of SO.sub.x. It is possible to remove the noxious sulfamic acid compound from the above-described product while preventing decomposition of the ammonium nitrate in the product which is useful as a fertilizer ingredient by heating the product at a temperature of from 130.degree. C. to 250.degree. C., particularly from 180.degree. C. to 250.degree. C.

Background of the Invention 
The present invention relates to a process for treating a solid product 
which is produced when a combustion gas having ammonia added thereto is 
irradiated with a radiation to thereby desulfurize and denitrate the 
combustion gas, the product consisting essentially of ammonium sulfate, 
ammonium nitrate and a composite thereof and containing a sulfamic acid 
compound. More particularly, the present invention pertains to a process 
for removing the noxious sulfamic acid compound from the above-described 
product a heat treatment while preventing decomposition of the ammonium 
nitrate in the by-product which is useful as a fertilizer ingredient. 
It has heretofore been known that, when a combustion gas which contains 
sulfur oxides (SO.sub.x) and/or nitrogen oxides (NO.sub.x) and which has 
ammonia added thereto is irradiated with a radiation, a solid product 
consisting essentially of (NH.sub.4).sub.2 SO.sub.4, NH.sub.4 HO.sub.3, 
(HN.sub.4).sub.2 SO.sub.4.2NH.sub.4 NO.sub.3 and (NH.sub.4).sub.2 
SO.sub.4.3NH.sub.4 NO.sub.3 is obtained and that, heating this by-product 
at 100 to 350.degree. C., ammonium nitrate (NH.sub.4 NO.sub.3) which is 
hygroscopic is decomposed into N.sub.2 O, N.sub.2, O.sub.2 and H.sub.2 O 
and also relatively pure ammonium sulfate [(NH.sub.4).sub.2 SO.sub.4 ] is 
produced which allows it to be used as a fertilizer [see the specification 
of Japanese Patent Public Disclosure (Laid-Open) No. 52-105565 
(105565/1977)]. 
The above-described process still has some room for improvement in regard 
to the disadvantage that ammonium nitrate, which is useful as a fertilizer 
ingredient, is decomposed. The prior art process is usually carried out by 
adding an amount of ammonia which is 0.25 to 2 times the amount of sulfur 
oxides and nitrogen oxides in terms of molar ratio and then irradiating 
the mixture with a radiation of 0.3 to 3.0 Mrad. 
The above-described process (hereinafter referred to as "EBA process") 
wherein ammonia is added to a combustion gas and the mixture is irradiated 
with radiation to thereby remove sulfur oxides and nitrogen oxides from 
the exhaust gas has been practiced in experimental plants and plants close 
to practical use since 1971, and it was not questioned before the middle 
of 1985 whether or not the product of the EBA process contains a sulfamic 
acid compound. 
However, analysis of the product produced in an experiment recently carried 
out has revealed that the by-product contains several % of a sulfamic acid 
compound. 
The present inventors conducted various studies about the cause of the 
formation of this sulfamic acid compound. Although various causes for the 
formation of the sulfamic acid compound were expected to be found, when 
the present inventors examined the composition of the effluent gas which 
has heretofore been used in the EBA process and the composition of the gas 
used in the experimental process carried out at this time, it was found 
that substantially no CO gas was contained in the gas used in the 
experimental process, whereas a large amount (5000 ppm or more) of CO gas 
was contained in the effluent gas conventionally used. The inventors filed 
an application with regard to preventing the formation of a sulfamic acid 
compound by means of adding CO gas as Japanese Patent Application No. 
61-249889 (249889/1986). 
Then, CO gas was added to the gas used in the experimental process and the 
gas mixture was subjected to the EBA process. As a result, it was found 
that no sulfamic acid compound is produced when the CO gas content is ten 
times or more the concentration of sulfur oxides. 
The content of sulfamic acid compounds in a fertilizer is regulated so as 
to be 0.01% or less with respect to 1% of the nitrogen contained in the 
fertilizer under the Fertilizer Control Act because sulfamic acid 
compounds are harmful to plants. Under these circumstances, since the 
nitrogen content in the product produced when an effluent gas containing 
no CO gas is treated in accordance with the EBA process is usually about 
20%, it is necessary if this product is to be used as a fertilizer to 
reduce the content of the sulfamic acid compound in the product to 0.2% or 
less of the whole weight of the product. 
SUMMARY OF THE INVENTION 
Accordingly, it is a primary object of the present invention to provide a 
process for removing a sulfamic acid compound from a product which is 
produced when an effluent gas is subjected to the EBA process and, at the 
same time, efficiently collecting a fertilizer containing no sulfamic acid 
compound. 
To this end, the present invention provides a process wherein a product 
which is produced when an effluent gas having ammonia added thereto is 
irradiated with radiation to thereby desulfurize and denitrate the 
effluent gas is heated at a temperature of from 130 to 250.degree. C., 
particularly from 180 to 250.degree. C., thereby removing a sulfamic acid 
compound from the product. 
The present inventors found that a relatively small amount of a sulfamic 
acid compound is contained in a solid product which is produced when a 
combustion gas containing a relatively small amount of CO gas is treated 
by the EBA process, and made exhaustive studies in order to find a process 
for removing the sulfamic acid compound from the product. As a result, the 
present inventors have found that it is possible to remove the sulfamic 
acid compound by heating the product at a temperature of from 130 to 
250.degree. C., particularly from 180 to 250.degree. C., for a relatively 
short period of time. The present invention has been accomplished on the 
basis of this finding. 
The present inventors have also found that the sulfamic acid compound 
contained in the product is decomposed through the reaction of 1 mole of 
said compound with 1 mole of ammonium nitrate contained in the product 
and, within the above-described temperature range, no ammonium nitrate is 
unnecessarily decomposed while the sulfamic acid compound is being 
decomposed.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention will be described hereinunder in detail. 
In the case where the EBA process is applied to an effluent gas, in the 
present invention the amount of ammonia which is to be added to the 
effluent gas is preferably selected so that the ammonia content in the 
resulting mixture is 0.25 to 2.0 times the respective contents of sulfur 
oxides and nitrogen oxides in terms of molar ratio, and an adequate 
radiation dose is 0.3 to 3.0 Mrad, in the same way as in the ordinary EBA 
process. 
Examples of kinds of radiations which may be employed in the present 
invention include alpha-ray, beta-ray, gamma-ray, X-rays, electron beam, 
neutron rays, etc. Among these forms of radiation, electron beam may 
conveniently be used. 
The temperature of the effluent gas when irradiated with radiation may be 
within the range from 50 to 150.degree. C. Usually, the gas is irradiated 
at a temperature of from 50 to 90.degree. C. because in this temperature 
range the rate at which sulfur oxides and nitrogen oxides are removed is 
maximized. 
It is preferable to mix ammonia with the effluent gas before the 
irradiation, but the mixing may also be carried out during the 
irradiation. 
It is conjectured from various kinds of basic experimental data and the 
like that the mechanism of formation of ammonium sulfate, ammonium nitrate 
and the sulfamic acid compound by the EBA process proceeds from the action 
of OH, O and HO.sub.2, radicals produced by the irradiation. The formation 
of ammonium sulfate, ammonium nitrate and the sulfamic acid compound may 
be expressed as follows: 
##STR1## 
It is understood that the formation of the sulfamic acid compound (NH.sub.2 
SO.sub.3 NH.sub.4) is suppressed by the presence of CO. 
It will be understood from the above-described formulae that the contents 
of ammonium nitrate (NH.sub.4 NO.sub.3) and sulfamic acid compound 
(NH.sub.2 SO.sub.3 NH.sub.4) in the product depend on the concentrations 
of sulfur oxides and nitrogen oxides in the treated gas and when the 
nitrogen oxide concentration is particularly low, the molar ratio of 
ammonium nitrate with respect to the sulfamic acid compound may be 1 or 
less. In such a case, it is necessary to add ammonium nitrate so that the 
product is in the presence of a molar amount of ammoniumnitrate which is 
sufficient to reduce the amount of the sulfonic acid compound to a 
predetermined level, e.g. 0.2% or less of the whole weight of the product. 
The product obtained by the EBA process is heated, according to the present 
invention, at a temperature of from 130 to 250.degree. C., preferably from 
180 to 250.degree. C. to thereby decompose the sulfamic acid compound 
contained in the product. 
The heating time depends on the heating temperature. Adequate heating times 
are as follows: about 30 minutes at 180.degree. C.; about 15 minutes at 
200.degree. C.; and about 10 minutes at 250.degree. C. In the case where 
the heating temperature exceeds 250.degree. C., ammonium nitrate is 
unnecessarily decomposed, that is, ammonium nitrate is unfavorably 
decomposed more than an amount equimolar with the sulfamic acid compound. 
In addition, even if the heating temperature is raised higher than 
250.degree. C., the time required to decompose the sulfamic acid compound 
is not shortened correspondingly. Therefore, a heating temperature in 
excess of 250.degree. C. is unfavorable. 
It should be noted that the above-described heating may be carried out in a 
granulating apparatus such as a crusher type granulator or an extrusion 
type granulator to thereby simultaneously effect the decomposition of the 
sulfamic acid compound and the granulation of the remaining fertilizer 
ingredients. 
The following Examples are provided for the purpose of further illustrating 
the present invention but are not to be construed as limiting. 
EXAMPLE 1 
A combustion gas containing sulfur oxides (900 ppm), nitrogen oxides (150 
ppm) and carbon monoxide (120 ppm) and having a temperature of 200.degree. 
C. was cooled down to 70.degree. C. and ammonia gas was added to the gas 
so that the ammonia content reached 1,950 ppm (1 mole equivalent) by 
volume. Then, the mixture was passed through the reactor at a rate of 
about 900 Nm.sup.3 /hour, and the solid product formed through the 
reaction was collected using an electrostatic precipitator. The product 
was obtained at a rate of 5 kg/hour. The contents of sulfur oxides and 
nitrogen oxides in the effluent gas having the product collected therefrom 
were 45 ppm and 18 ppm, respectively. 
It should be noted that the effluent gas in the reactor was irradiated with 
electron beam of 1.8 Mrad. 
The results of analysis of the product thus obtained are shown in Table 1 
below. 
TABLE 1 
______________________________________ 
Analysis of By-Product 
Results of 
analysis 
Compound (formula) (wt. %) 
______________________________________ 
Sulfuric acid ion (SO.sub.4.sup.2-) 
58.5 
Nitric acid ion (NO.sub.3.sup.-) 
6.6 
Ammonium ion (NH.sub.4.sup.+) 
25.8 
Sulfamic acid compound (NH.sub.2 SO.sub.3 H) 
5.0 
Insoluble matter, etc. 
4.1 
Total 100.0 
______________________________________ 
EXAMPLE 2 
The product obtained in Example 1 was heated at each of the following 
various temperatures, that is, 100.degree., 130.degree. C., 150.degree. 
C., 180.degree. C., 200.degree. C., 250.degree. C. and 300.degree. C., to 
obtain the relationship between the heating temperature and the time 
required to decompose the sulfamic acid compound for each of the 
above-described temperatures. The results are shown in FIG. 1. 
The relationship between the concentration of nitric acid ions remaining in 
the by-product and the heating time for each of the heating temperatures, 
that is, 180.degree. C., 200.degree. C., 250.degree. C. and 300.degree. 
C., was also examined. The results are shown in FIG. 2. 
The results shown in FIG. 1 revealed that it is necessary in order to 
decompose the sulfamic acid compound to heat the product at 130.degree. C. 
or more and, in order to put the process into practice on an industrial 
scale, the product must be heated at 180.degree. C. or more. 
It will be understood from the results shown in FIG. 2 that, when the 
product is heated at I80.degree. C., 200.degree. C or 250.degree. C., the 
concentration of the residual nitric acid ions is relatively high, 
whereas, when the heat treatment is carried out at 300.degree. C., 
ammonium nitrate is decomposed at a relatively high rate and almost all 
the nitric acid ions are decomposed in 15 minutes. These facts show that a 
preferable heating temperature is about 250.degree. C. or less, with a 
view to allowing as much as possible of the ammonium nitrate, which is 
useful as a fertilizer, to obtain. 
The results shown in FIGS. 1 and 2 also show that, in any of the three 
cases where the product is heated for 30 minutes at 180.degree. C., for 15 
minutes at 200.degree. C., and for 10 minutes at 250.degree. C., 
respectively, both nitric acid ions and the sulfamic acid compound 
decrease in amounts that are substantially equimolar with each other. 
Next, experiments were carried out to examine the relationship between the 
sulfamic acid compound and ammonium nitrate during the thermal 
decomposition. 
EXAMPLE 3 
A product in which the molar ratio of the ammonium nitrate content with 
respect to the content of the sulfamic acid compound was 0.1 was heated 
for 15 minutes at 200.degree. C. However, there was substantially no 
decrease in the content of the sulfamic acid compound. 
Then, four different kinds of product in which the molar ratio of the 
ammonium nitrate content with respect to the content of the sulfamic acid 
compound were 0.5, 1.0, 2.0 and 5.0 respectively prepared. These products 
were heated for 15 minutes at 200.degree. C., and the content of the 
sulfamic acid compound in each of the heated products was measured. 
The results are shown in FIG. 3. 
The results shown in FIG. 3 revealed that ammonium nitrate is needed for 
the thermal decomposition of sulfamic acid compound and that 1 mole or 
more of ammonium nitrate is needed per mole of the sulfamic acid compound. 
EXAMPLE 4 
Forty grams of the product obtained in Example 1 was placed in a heating 
apparatus, and the heating apparatus was heated at 200.degree. C. with a 
hot-air drier with helium gas being supplied thereto at a rate of 0.1 
l/minute. The gas coming out of the heating apparatus was collected, and 
the gas generated through thermal decomposition was analyzed. 
The results of the analysis are shown in Table 2. 
Table 2 also shows an analysis of the gas generated when 5 kg of the 
product is decomposed heating, the analysis being based on the data 
obtained from the thermal decomposition of 40 g of the product. 
TABLE 2 
______________________________________ 
Gas generated 
Gas generated 
from 40 g. of 
from 5 kg. of 
by-product 
by-product 
______________________________________ 
Nitrogen (N.sub.2) 
0.562 l 70.25 l 
Oxygen (O.sub.2) 
0.100 l 12.50 l 
Nitrous oxide (N.sub.2 O) 
0.092 l 11.50 l 
Ammonia (NH.sub.3) 
0.029 l 3.63 l 
Nitrogen oxides (NO.sub.x) 
0.017 l 2.12 l 
Total 0.800 l 100.00 l 
______________________________________ 
The results shown in Table 2 show that the gas generated through the 
thermal decomposition of the product consists essentially of nitrogen and 
oxygen and yet contains small amounts of NH.sub.3 gas and NO.sub.x gas. 
It will be understood from Table 2 that, when the product obtained at a 
rate of 5 kg/hour is decomposed by heating, a gas is generated at a rate 
of about 100 l/hour (i.e., about 0.1 m.sup.3 /hour), which is extremely 
low, i.e., about 0.01% with respect to the rate at which the effluent gas 
was treated in example 1, i.e., 900 Nm.sup.3 /hour (the product was 
produced at a rate of 5 kg/hour), and therefore, mixing the decomposition 
gas with the effluent gas to be treated will cause substantially no 
problem from a quantitative point of view. 
By mixing the decomposition gas with the effluent gas to be treated, small 
amounts of NH.sub.3 and NO.sub.x contained in the decomposition gas can be 
collected in the form of ammonium nitrate. 
It is considered from the results shown in Table 2 and so on that the 
sulfamic acid compound contained in the product reacts with ammonium 
nitrate to form nitrogen, oxygen, nitrous oxide, ammonium sulfate, and 
water. 
EXAMPLE 5 
A roll crusher type granulator was heated so that the temperature of the 
machine from its feed opening to its compressing section was 200.degree. 
C., and the dwell time of the feed in the machine was adjusted to 15 
minutes. Thereafter, the by-product obtained in Example 1 was granulated 
this machine. 
Upon analysis of the granulated product, no sulfamic acid compound was 
detected. 
EXAMPLE 6 
The product obtained in Example 1 was granulated using a moistening 
extrusion type granulator. The granulator was heated so that the 
temperature of the machine from its feed opening to its extruding section 
was 200.degree. C. and the dwell time was adjusted to 15 minutes in the 
same way as in Example 5. 
With this machine, three different kinds of product which were respectively 
mixed with 3%, 5% and 10% of water were granulated. Analysis of the 
resulting granulated products revealed that, no sulfamic acid compound was 
to be detected in any of the products. 
The following is a description of the quantity of heat required to heat the 
product obtained at a rate of 5 kg/hour in Example 1 to 180.degree. C. and 
the effects produced when the heat of the effluent gas at 200.degree. C. 
is utilized as a heat source for heating the product. 
The quantity of heat Q (kcal/hour) required to heat the product collected 
at a rate of 5 kg/hour from the effluent gas cooled down to 70.degree. C. 
is obtained from the following equation when the temperature of the 
product is assumed to be 70.degree. C.: 
##EQU1## 
On the other hand, a lowering in temperature (.DELTA.T.degree. C.) of the 
effluent gas caused when the heat for heating the product is collected 
from the effluent gas may be obtained from the following equation: 
EQU 900 Nm.sup.3 /hour.times.1.3 kg/Nm.sup.3 .times.0.25 kcal/kg.degree. 
C..times..DELTA.T.degree. C. 
=220 kcal/hour 
Hence, .DELTA.T=1.degree. C. 
More specifically, when the product obtained at a rate of 5 kg/hour is 
heated to 180.degree. C. by utilizing the effluent gas at 200.degree. C. 
which is supplied at a rate of 900 Nm.sup.2 /hour, the temperature of the 
effluent gas lowers only to 199.degree. C. which is supplied at a rate of 
900 Nm.sup.2 /hour, the temperature of the effluent gas lowers only to 
199.degree. C., and it is therefore clear that there is substantially no 
problem. 
FIG. 4 shows one example of a method of recirculating the decomposition gas 
so as to be mixed with the effluent gas which is to be treated, the method 
being stated in the appended claim 4. 
Apparatuses for heat-treating the product may roughly be classified into 
two types, that is, a first type of apparatus in which the by-product is 
heated by bringing hot air into direct contact with the product, and a 
second type of apparatus in which the product is indirectly heated by 
utilizing heat transfer. Although either of these two types of heating 
apparatus may, of course, be employed in the present invention, the second 
type of apparatus is illustrated in the figure. 
The product is supplied from a precipitator to a heating apparatus where it 
is heated (180 to 220.degree. C. for 10 to 30 minutes) indirectly by means 
of hot air (200 to 250.degree. C) generated by a burner. Thus, the 
sulfamic acid compound contained in the product is decomposed, and the 
product which now consists essentially of ammonium sulfate and ammonium 
nitrate is discharged. The gas (see Table 2) generated through the heat 
treatment is returned and mixed with the effluent gas which is to be 
treated so that the generated gas is treated again. As described above, 
the rate of generation of the gas is extremely low (about 0.01%) with 
respect to the rate of feed of the effluent gas to be treated and 
therefore mixing of the gas with the effluent gas will cause substantially 
no problem. 
It should be noted that, although in the illustrated example the generated 
gas is returned to the duct provided at the inlet of the cooling tower, 
the gas may be mixed with the treated effluent gas (at the downstream side 
of the precipitator) because the rate of generation of the gas is 
sufficiently low and the contents of NO.sub.x and NH.sub.3 in the 
generated gas are also small. 
FIG. 5 shows one example of a method wherein the effluent gas which is to 
be treated is utilized as a heat source for heating the product, this 
method being recited in the appended claim 6. 
The system in this example differs from the system shown in FIG. 4 in that 
in this system the effluent gas which is by-passed from the inlet of the 
cooling tower is utilized as hot air for heating the product. The 
arrangement of the other parts of the system is substantially the same as 
that in the system shown in FIG. 4. It should be noted that the 
illustrated auxiliary burner is employed when the temperature of the 
effluent gas is so low that it is impossible to obtain desired hot air. 
Although in the illustrated system the by-passed effluent gas is passed 
through a heat exchanger so as to exchange heat with air, the effluent gas 
may, of course, be used directly as hot air. 
EXAMPLE 7 
The product obtained in Example 1 was granulated into various sizes, i.e., 
0.5 mm, 1.0 mm, 3 mm, 5 mm and 10 mm, with no water added thereto. 
Thereafter, the granulated products were heated at the same temperatures 
as those in Example 2. The results of this Example were the same as those 
shown in FIGS. 1 and 2 for each of the different sizes. 
EXAMPLE 8 
The product obtained in Example 1 was mixed with water to prepare two 
different samples containing 20 and 30% by weight of water, respectively. 
Each of the samples was granulated using a pan type granulator to form 
granulated products having various sizes, i.e., 0.5 mm, 1.0 mm, 3 mm, 5 mm 
and 10 mm. These granulated products were heated at various temperatures, 
i.e., 100.degree. C., 120.degree. C., 130.degree. C., 150.degree. C., 
180.degree. C., 200.degree. C. and 250.degree. C. As a result, most of the 
granulated products heated at 130.degree. C. or more burst immediately, 
but those which were treated at 120.degree. C. or less did not burst even 
when they were heated for 10 to 20 minutes. Analysis of these granulated 
products revealed that there was substantially no decrease in the content 
of the sulfamic acid compound but water content decreased to 5-10% by 
weight. Thereafter, the granulated products were heated at various 
temperatures, i.e., 130.degree. C., 150.degree. C., 180.degree. C., 
200.degree. C. and 250.degree. C. The results were substantially the same 
as those shown in FIGS. 1 and 2.