Ammonia for the desulfurization of sulfur containing gases

A process for the reduction of sulfur oxides from flue gases is provided in which ammonia is added to the flue gas to precipitate out (NH.sub.4).sub.2 SO.sub.4. The (NH.sub.4).sub.2 SO.sub.4 is collected and can be sold as a commercial product.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the use of ammonia (NH.sub.3) for the 
desulfurization [removal of sulfur dioxide (SO.sub.2)] of gases resulting 
from the combustion of sulfur containing hydrocarbons which are commonly 
called flue gases. The product of the reaction of NH.sub.3 with SO.sub.2 
predominantly is ammonium sulfate (NH.sub.4).sub.2 SO.sub.4 which is 
widely used as a source of nitrogen in materials such as fertilizers. 
2. Description of the Prior Art 
Processes for the desulfurization of gases containing SO.sub.2 currently 
being evaluated to achieve the degree of desulfurization of flue gases 
proposed recently by the President of the United States are based on the 
use of calcium oxides and combinations of calcium oxide and oxides of the 
alkaline earth elements. These calcium oxide based and calcium oxide, 
alkaline earth oxides mixtures cannot be regenerated and must be discarded 
into landfills. As a result of more stringent enforcement of the 
regulations regarding landfills by the Environmental Protection Agency, 
(EPA) the number of landfills in the United States has decreased from 
14,000 to 6000 in the last several years. It is estimated that there will 
be a further decrease of 33% in the number of landfills in the next 
several years. As a result the price of placing a ton of waste material 
into a landfill has increased from about $6 when 14000 landfills were in 
operation to four or five times that amount at present. When the number of 
landfills has been further reduced, the price of placing a ton of material 
in a landfill will increase further. It is estimated by the Wall Street 
Journal that 15 states will have no landfills available in 10 years. 
If the recommendations of the President with regard to reduction of the 
components of acid rain are approved by the Congress, the large increase 
in partially sulfated calcium oxide sorbents resulting from SO.sub.2 
removal will occur at approximately the same time as the acute shortage of 
landfill sites. Therefore, there is a need for a method for reducing the 
SO.sub.2 emissions from power plants that is based either on the use of 
regenerable sorbents or the use of a process that creates a sulfate 
material that is an item of commerce. The use of NH.sub.3 for SO.sub.2 
removal from flue gases meets these requirements because they would result 
in the formation of (NH.sub.4).sub.2 SO.sub.4 which is one of the most 
widely used chemicals known. 
There are no research projects being funded in the current phase of the 
Clean Coal Technology Demonstration Program of the Department of Energy 
related to the use of NH.sub.3 for the desulfurization of flue gases. 
NH.sub.3 is used in combination with catalysts for the Selective Catalytic 
Reduction (SCR) of nitrogen oxides (NO.sub.x) However in Request For 
Proposal (RFP) by the Department of Energy (DOE) [No. DE-RP22-89PC89801] 
it was stated: "Depending on the lifetime of an SCR catalyst, annualized 
control costs (for SCR reduction of NO.sub.x with NH.sub.3) are likely to 
be thousands of dollars per ton of NO.sub.x reduced from a high sulfur 
coal." The RFP further states: "Commercially available combustion 
modification techniques (e.g., certain low-NO.sub.x burners) and flue gas 
treatment processes (e.g. selective catalytic reduction) and selective 
noncatalytic reduction processes will not qualify" (as a technique 
applicable to this proposal). 
The statements on the inapplicability of SCR removal of NO.sub.x with 
NH.sub.3 is based on a report from the Electric Power Research Institute 
(EPRI) EPRI CS-3606, "Selective Catalytic Reduction for Coal-Fired Power 
Plants: Feasibility and Economics", Oct. 1984. This work documented the 
research effort by EPRI on the catalytic reduction of NO.sub.x with 
NH.sub.3. The operating range of the catalyst was specified by the 
manufacturer to be 580.degree. F. to 750.degree. F. This required that the 
catalyst be placed in operation between the economizer and air preheaters 
of the boiler. The investigators showed that the catalyst did result in 
the reduction of NO.sub.x to nitrogen (N.sub.2). The process was less than 
satisfactory because of the incomplete utilization of the NH.sub.3 used. 
Furthermore, the investigators concluded that there was a conversion of 
1.4% of the SO.sub.2 by catalytic oxidation to SO.sub.3. The unreacted 
NH.sub.3 and SO.sub.3 may have reacted to the fly ash. EPRI has reported 
the formation of compounds such as: NH.sub.4 Al(SO.sub.4).sub.2, NH.sub.4 
Al(SO.sub.4).sub.2 .times.12 H.sub.2 O which account for over 42% of the 
deposits found in the air preheaters which were designed to have an exit 
temperature of 331.degree. F. (161.1.degree. C.). (Al.sub.2 O.sub.3 
constituted 25% of ash in the coal used in this trial.) These precipitates 
increased the pressure drop in the air preheaters to a level that 
interfercd with the efficient operation of the boiler. 
Applicants have determined by thermodynamic calculations that SO.sub.2 may 
be removed with NH.sub.3 without the utilization of the catalyst for the 
conversion of SO.sub.2 to SO.sub.3 . However, use of a catalyst to convert 
SO.sub.2 to SO.sub.3 may be accelerated by the use of a catalyst. 
SUMMARY OF THE INVENTION 
The description of the invention is based on the removal of SO.sub.2, one 
of the sulfur oxides created by the combustion of coal which is a sulfur 
containing mixture of carbon and hydrocarbons. The use of coal as the 
source of hydrocarbon, and SO.sub.2 as the sulfur oxide to be removed from 
the products of combustion (flue gas), does not preclude the use of this 
invention for the removal of SO.sub.2 and other oxides of sulfur resulting 
from the combustion of other hydrocarbons containing sulfur. 
Applicants' invention to provide a process whereby sufficient NH.sub.3 is 
added to the flue gases containing SO.sub.2 (from which a significant 
portion of the fly ash has been removed) for sufficient SO.sub.2 to react 
with the ammonia to form (NH.sub.4).sub.2 SO.sub.4 to meet present and 
future requirements for SO.sub.2 removal from steam boilers and the like. 
The (NH.sub.4).sub.2 SO.sub.4 formed by the reaction of the SO.sub.2, 
NH.sub.3, H.sub.2 O and oxygen in the flue gas would be of sufficient 
purity to be suitable for use in fertilizer as a source of NH.sub.3. 
Applicants control the temperature at which the NH.sub.3 is added to the 
flue gases to prevent the precipitation of (NH.sub.4).sub.2 SO.sub.4 in 
the duct work or on the surfaces of heat exchangers of the boiler to 
prevent the accumulation of (NH.sub.4).sub.2 SO.sub.4 in a manner which 
will interfere with the efficient and reliable operation of the boiler.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments of the invention may be described with the 
following equations: 
EQU SO.sub.2 (g)+1/2O.sub.2 (g)=SO.sub.3 (1) 
Reaction (1) indicates that there always some SO.sub.3 in If it is found 
desirable to increase the amount of SO.sub.3, the stack gases may be 
exposed to a catalyst such as vanadium pentoxide or other catalysts known 
to those skilled in the art of converting SO.sub.2 to SO.sub.3 to increase 
the amount of SO.sub.3 in the flue gases. However, when the SO.sub.3 forms 
a compound with other elements such as described in equation (2) the 
reaction (1) proceeds further with the formation of more SO.sub.3 from the 
remaining SO.sub.2. In this system described by equations (1) and (2) the 
SO.sub.2 is ultimately removed as (NH.sub.4).sub.2 SO.sub.4 according to 
equation (3): 
EQU SO.sub.3 (g)+H.sub.2 O(g)+2NH.sub.3 (g)=(NH.sub.4).sub.2 SO.sub.4 (s)(2) 
EQU SO.sub.2 (g)+1/2O.sub.2 (g)+H.sub.2 O(g)+2NH.sub.3 (g) =(NH.sub.4).sub.2 
SO.sub.4 (s) (3) 
The amount of (NH.sub.4).sub.2 SO.sub.4 formed is a function of the 
temperature at which the reaction occurs and the amount of NH.sub.3 added 
to the flue gas stream. 
Assuming the addition of enough NH.sub.3 to react with 90% of the SO.sub.2 
in a typical fuel gas whose composition is: 
______________________________________ 
CO.sub.2 
13.21% 
H.sub.2 O 
9.21% 
N.sub.2 
73.48% 
SO.sub.2 
0.3450% 
O.sub.2 
3.75% 
NO 0.075% 
N.sub.2 O 
0.0025% 
______________________________________ 
it is possible to compute the amount of NH.sub.3 required in excess 
of the stoichiometric amount to achieve 90% SO.sub.2 reduction (to 345 ppm 
SO.sub.2) at any temperature. Calculations over a range of temperatures 
from 440.6.degree. F. to 620.0.degree. F. have been made, and these 
results have been plotted in FIG. 1. The data shows that at 620.0.degree. 
F., 2904.6 ppm of NH.sub.3 are necessary to be in equilibrium with 345 ppm 
of SO.sub.2. When the temperature is reduced to 440.6.degree. F., only 
0.207 ppm of NH.sub.3 is necessary to be in equilibrium with 345 ppm 
SO.sub.2. These two temperatures are within the operating range of the air 
preheaters (675.degree. F. to 331.degree. F.) utilized in the EPRI 
experiments which accounts for the precipitation of the ammonia and sulfur 
oxide containing material in the air preheaters. The analysis of some of 
42% of the compounds found in the deposits in the air preheats reported by 
EPRI include: NH.sub.4 Al(SO.sub.4).sub.2 and NH.sub.4 Al(SO.sub.4).sub.2 
.times.12 H.sub.2 O. These analyses of the materials found in the 
preheaters are not surprising considering the possibility of the particles 
of fly ash in the flue gas acting as heterogeneous nuclei on which the 
(NH.sub.4).sub.2 SO.sub.4 would precipitate. Precipitation of the 
(NH.sub.4).sub.2 SO.sub.4 containing material in the air preheaters 
confirms the validity of the calculations given above. 
It is an established fact that materials used as heterogeneous nuclei are 
most effective when the planar disregistry between the nucleating material 
and the material being nucleated is a minimum. Applicants further provides 
that heterogeneous nuclei whose planar disregistry is minimal such as 
solid particles of (NH.sub.4).sub.2 SO.sub.4 can be utilized to accelerate 
the precipitation of the ammonium sulfate particles according to the 
reaction described in equation (3). The use of (NH.sub.4).sub.2 SO.sub.4 
in the previous sentence does not preclude the use of other heterogeneous 
nuclei whose planar disregistry with respect to (NH.sub.4).sub.2 SO.sub.4 
is minimal. 
Since all of the reactants shown in equation (3) are gases, the rate of 
reaction for the formation of (NH.sub.4).sub.2 SO.sub.4 should be rapid. 
This is in sharp contrast to the reactions for removal of sulfur from flue 
gases which are either (1) between solids and gases [SO.sub.2 (gas) and CaO 
(solid)] where the limiting rate of reaction may be the diffusion of the 
SO.sub.2 into the crystals of CaO or (2) the case where the CaO is in a 
slurry the SO.sub.2 must be absorbed by the water of the slurry and react 
with the suspended CaO where the rate determining reaction may be the 
diffusion of the SO.sub.2 into the CaO particles in the slurry. All of 
these reactions which require the diffusion of a gas into a solid are very 
slow compared to the reaction between intimately mixed gas species. The 
fact that the NH.sub.3 containing compounds precipitated in the short time 
necessary for the flue gases to traverse the air preheaters attests to the 
speed of the reaction of NH.sub.3 and SO.sub.2 to form (NH.sub.4).sub.2 
SO.sub.4. 
If (NH.sub.4).sub.2 SO.sub.4 of sufficient purity for fertilizer use is to 
be produced, at least some of the fly ash must be removed from the flue 
gas stream prior to the addition of the NH.sub.3 into the flue gas. 
Removal may be by venturi scrubbers, fabric filter, electro-static 
precipitators or other means known to those skilled in the art. Since 
analysis of ammonium sulfate particles found in the air preheaters 
indicates that the fly ash may have acted as a heterogeneous nuclei for 
the growth of ammonium sulfate crystals, complete removal of the fly ash 
may not be desirable. 
According to the information contained in FIG. 1, at 500.degree. F. less 
than 5 ppm of NH.sub.3 is required to be in equilibrium with 345 ppm 
SO.sub.2 after 90% SO.sub.2 removal. Therefore, in order to collect as 
much of the valuable ammonium sulfate as possible, the crystals of 
ammonium sulfate, whose size may have been increased by providing 
heterogeneous nuclei to increase their rate of growth, should be extracted 
from the flue gas stream as soon after the SO.sub.2 of the flue gas has 
completely reacted with the NH.sub.3 addition with techniques known to 
those skilled in the art such as fabric filters, venturi filters and 
electro-static precipitators. 
The products of the reaction of NH.sub.3 and SO.sub.2, which are mainly 
(NH.sub.4).sub.2 SO.sub.4, should be removed from the duct work while the 
temperature of the flue gas exceeds its dew point. Otherwise, the 
precipitating water may react with the (NH.sub.4).sub.2 SO.sub.4 to form a 
solution which may interfere with the extraction of the (NH.sub.4).sub.2 
SO.sub.4 from the duct work. Otherwise the (NH.sub.4).sub.2 SO.sub.4 may 
precipitate throughout the duct work of the boiler making it difficult to 
accumulate it for sale. 
While we have described a present preferred embodiment of the invention, it 
is to be distinctly understood that the invention is not limited thereto 
but may be otherwise embodied and practiced within the scope of the 
following claims.