Abstract:
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 4 ) 2  SO 4 . The (NH 4 ) 2  SO 4  is collected and can be sold as a commercial product.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the use of ammonia (NH 3 ) for the desulfurization [removal of sulfur dioxide (SO 2 )] of gases resulting from the combustion of sulfur containing hydrocarbons which are commonly called flue gases. The product of the reaction of NH 3  with SO 2  predominantly is ammonium sulfate (NH 4 ) 2  SO 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 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 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 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 3  for SO 2  removal from flue gases meets these requirements because they would result in the formation of (NH 4 ) 2  SO 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 3  for the desulfurization of flue gases. 
     NH 3  is used in combination with catalysts for the Selective Catalytic Reduction (SCR) of nitrogen oxides (NO x ) However in Request For Proposal (RFP) by the Department of Energy (DOE) [No. DE-RP22-89PC89801] it was stated: &#34;Depending on the lifetime of an SCR catalyst, annualized control costs (for SCR reduction of NO x  with NH 3 ) are likely to be thousands of dollars per ton of NO x  reduced from a high sulfur coal.&#34; The RFP further states: &#34;Commercially available combustion modification techniques (e.g., certain low-NO x  burners) and flue gas treatment processes (e.g. selective catalytic reduction) and selective noncatalytic reduction processes will not qualify&#34; (as a technique applicable to this proposal). 
     The statements on the inapplicability of SCR removal of NO x  with NH 3  is based on a report from the Electric Power Research Institute (EPRI) EPRI CS-3606, &#34;Selective Catalytic Reduction for Coal-Fired Power Plants: Feasibility and Economics&#34;, Oct. 1984. This work documented the research effort by EPRI on the catalytic reduction of NO x  with NH 3 . The operating range of the catalyst was specified by the manufacturer to be 580° F. to 750° 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 x  to nitrogen (N 2 ). The process was less than satisfactory because of the incomplete utilization of the NH 3  used. Furthermore, the investigators concluded that there was a conversion of 1.4% of the SO 2  by catalytic oxidation to SO 3 . The unreacted NH 3  and SO 3  may have reacted to the fly ash. EPRI has reported the formation of compounds such as: NH 4  Al(SO 4 ) 2 , NH 4  Al(SO 4 ) 2  ×12 H 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° F. (161.1° C.). (Al 2  O 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 2  may be removed with NH 3  without the utilization of the catalyst for the conversion of SO 2  to SO 3  . However, use of a catalyst to convert SO 2  to SO 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 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 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 2  and other oxides of sulfur resulting from the combustion of other hydrocarbons containing sulfur. 
     Applicants&#39; invention to provide a process whereby sufficient NH 3  is added to the flue gases containing SO 2  (from which a significant portion of the fly ash has been removed) for sufficient SO 2  to react with the ammonia to form (NH 4 ) 2  SO 4  to meet present and future requirements for SO 2  removal from steam boilers and the like. The (NH 4 ) 2  SO 4  formed by the reaction of the SO 2 , NH 3 , H 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 3 . Applicants control the temperature at which the NH 3  is added to the flue gases to prevent the precipitation of (NH 4 ) 2  SO 4  in the duct work or on the surfaces of heat exchangers of the boiler to prevent the accumulation of (NH 4 ) 2  SO 4  in a manner which will interfere with the efficient and reliable operation of the boiler. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the relationship between the amount of NH 3  in equilibrium with 345 ppm SO 2  (after 90% removal of SO 2  with NH 3 ) as a function of temperature. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the invention may be described with the following equations: 
     
         SO.sub.2 (g)+1/2O.sub.2 (g)=SO.sub.3                       (1) 
    
     Reaction (1) indicates that there always some SO 3  in If it is found desirable to increase the amount of SO 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 2  to SO 3  to increasethe amount of SO 3  in the flue gases. However, when the SO 3  forms a compound with other elements such as described in equation (2) the reaction (1) proceeds further with the formation of more SO 3  from theremaining SO 2 . In this system described by equations (1) and (2) the SO 2  is ultimately removed as (NH 4 ) 2  SO 4  according to equation (3): 
     
         SO.sub.3 (g)+H.sub.2 O(g)+2NH.sub.3 (g)=(NH.sub.4).sub.2 SO.sub.4 (s)(2) 
    
     
         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 4 ) 2  SO 4  formed is a function of the temperature at which the reaction occurs and the amount of NH 3  added to the flue gas stream. 
     Assuming the addition of enough NH 3  to react with 90% of the SO 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 excessof 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° F. to 620.0° F. have been made, and these results have been plotted in FIG. 1. The data shows that at 620.0° F., 2904.6 ppm of NH.sub.3 are necessary to be in equilibrium with 345 ppmof SO.sub.2. When the temperature is reduced to 440.6° 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 airpreheaters (675° F. to 331° F.) utilized in the EPRI experiments which accounts for the precipitation of the ammonia and sulfuroxide containing material in the air preheaters. The analysis of some of 42% of the compounds found in the deposits in the air preheats reported byEPRI include: NH.sub.4 Al(SO.sub.4).sub.2 and NH.sub.4 Al(SO.sub.4).sub.2 ×12 H.sub.2 O. These analyses of the materials found in the preheaters are not surprising considering the possibility of the particlesof 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 materialand the material being nucleated is a minimum. Applicants further provides that heterogeneous nuclei whose planar disregistry is minimal such as solid particles of (NH 4 ) 2  SO 4  can be utilized to acceleratethe precipitation of the ammonium sulfate particles according to the reaction described in equation (3). The use of (NH 4 ) 2  SO 4  in the previous sentence does not preclude the use of other heterogeneous nuclei whose planar disregistry with respect to (NH 4 ) 2  SO 4  is minimal. 
     Since all of the reactants shown in equation (3) are gases, the rate of reaction for the formation of (NH 4 ) 2  SO 4  should be rapid. This is in sharp contrast to the reactions for removal of sulfur from fluegases which are either (1) between solids and gases [SO 2  (gas) and CaO(solid)] where the limiting rate of reaction may be the diffusion of the SO 2  into the crystals of CaO or (2) the case where the CaO is in a slurry the SO 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 2  into the CaO particles in the slurry. All of these reactions which require the diffusion of a gas into a solid are veryslow compared to the reaction between intimately mixed gas species. The fact that the NH 3  containing compounds precipitated in the short timenecessary for the flue gases to traverse the air preheaters attests to the speed of the reaction of NH 3  and SO 2  to form (NH 4 ) 2  SO 4 . 
     If (NH 4 ) 2  SO 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 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° F. less than 5 ppm of NH 3  is required to be in equilibrium with 345 ppm SO 2  after 90% SO 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 extractedfrom the flue gas stream as soon after the SO 2  of the flue gas has completely reacted with the NH 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 3  and SO 2 , which are mainly (NH 4 ) 2  SO 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 4 ) 2  SO 4  to form asolution which may interfere with the extraction of the (NH 4 ) 2  SO 4  from the duct work. Otherwise the (NH 4 ) 2  SO 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, itis 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.