Process for Removing SO.sub.2 and Fly Ash From Flue Gas

This invention is directed to a process for enhancing the removal of SO.sub.2 and fly ash from flue gases and to such a process wherein the ratio of SO.sub.2 sorbent to SO.sub.2 may be reduced resulting in a substantial saving in the cost of sorbent for plant operation.

It is known that the technique of particle charging prior to fabric filtration results in a pressure drop reduction as particle charging prior to fabric filtration produces a charged particle bed on the filter surface which is more open and less densely packed than when non-charged particles are collected. It is also known that SO.sub.2 may be removed from flue gases by certain sorbents such as sodium bicarbonate and Nahcolite.

Examples of prior art are:

Schuff ----4,290,786;

Schuff ----4,220,478;

Johnstone ----2,924,294;

Calvert ----4,169,714.

The present invention is based on a process for removing SO.sub.2 and fly ash from flue gases by directing a particulate SO.sub.2 sorbent, such as sodium bicarbonate or Nahcolite, and flue gas containing suspended fly ash and sulfur dioxide through an electrostatic charger and thereafter directing the charged sorbent and flue gas through a bag filter to separate the fly ash and formed sodium sulfate from the flue gas.

The drawing is a flow diagram of apparatus for carrying out the process of the invention.

Referring to the drawing directional arrow 10 represents the flow of flue gas containing suspended fly ash and sulfur dioxide into an electrostatic charger 12 of conventional design. Also directed into the electrostatic charger is particulate SO.sub.2 sorbent preferably sodium bicarbonate or its naturally occurring form, such as Nahcolite or Trona, from a storage bin 14.

The combined flue gas and SO.sub.2 sorbent are then directed to the bag filter 16 where the electrostatically charged sorbent creates a bed of loosely packed particles. Due to reduced resistance to gas flow, this bed of sorbent can be allowed to increase in depth, resulting in enhanced flue gas contact and enhanced conversion of the SO.sub.2 to sodium sulfate. The discharge from the bag filter 16 is fly ash and sodium sulfate as at 18 and clean gas as at 20. Nahcolite (NaHCO.sub.3) is most often considered as the SO.sub.2 sorbent of choice, and Na.sub.2 /S ratio of 1.2 or higher is generally considered to be needed for 90% SO.sub.2 removal. Following the teachings of the present invention reduces the sorbent to SO.sub.2 ratio such that Na.sub.2 /S ratio can be lowered to 1.0.

Considering a 500 MW power plant using 12,000 BTU/pound, 2% sulfur coal and operating 7,000 hours/year a Na.sub.2 /S ratio reduction from 1.2 to 1.0 and assuming a Nahcolite cost of $75.00 per ton would result in a yearly saving of approximated $2.4 million. This can be compared to the estimated installation cost of the electrostatic charger of 0.5 million. Preferably the sorbent particles are in the size range of from about 1 micron to about 100 microns.

The electrostatic charger makes use of high voltage corona discharge, such as is utilized in electrostatic precipitators.