Method of conditioning flue gas

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The conditioners useful in the present invention are finely divided phosphate salts (e.
g.
, diammonium phosphate, (NH.
sub.
4).
sub.
2 HPO.
sub.
4 ; monoammonium phosphate, NH.
sub.
4 H.
sub.
2 PO.
sub.
4 ; disodium phosphate, Na.
sub.
2 HPO.
sub.
4 ; monosodium phosphate NaH.
sub.
2 PO.
sub.
4 ; trisodium phosphate, Na.
sub.
3 PO.
sub.
4, and mixtures thereof.
The conditioner may be utilized either in dry form (for example, as a powder of finely divided particles) or preferably as an aqueous solution.
The amount of conditioner to be injected into the gas stream varies according to the amount of solids entrained in the gas stream and the degree of improvement needed in the electrostatic precipitator efficiency, for example, in order to meet a maximum allowable emissions requirement of a local, state or federal regulatory body.
Generally for conditioning the fly ash in a coal-burning utility boiler, sufficient conditioner is injected into the gas stream to provide 24-1200, and preferably the quite low values of 60-480 grams of the conditioner agent (e.
g.
, diammonium phosphate) per metric ton of coal burned to form the gas.
Since the weight of flue gas is dependent on the weight of coal burned, another way of expressing this value is about 2.
3-115, and preferably 5.
8-46, parts by weight of conditioner per million parts by weight of flue gas, and in particular this would be an appropriate way to designate conditioner amount when the gas was not a product of coal combustion.
Generally conditioner levels below this range do not appreciably improve the collection characteristics of the particles, while any conditioner levels in excess of the specified range not only increase the cost of conditioning unnecessarily, but also increase the possibility of blockage of the preheater or other heat exchanger downstream of the point of injection.
The quantity of conditioner determined according to the foregoing criteria is preferably added in the form of an atomized aqueous solution, preferably a 20-40% by weight solution, depending upon the solubility limits of the specific salt used