the preferred embodiments of the invention may be described with the following equations : 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 ): 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 fig1 . 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 fig1 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 .