Powder Activated Carbon for Flue Gas Treatment
The incineration of wastes such as municipal waste(MSW),hazardous industrial waste, medical waste and sewerage sludge results in the formation of a flue gas containing a range of pollutants .Methods such as scrubbing, bag filters and electrostatic precipitators (ESP) are used to remove most of these pollutants. However these methods cannot remove dioxins and heavy metals such as mercury and cadmium
Activated carbon injection has been identified as the best available control technology (BACT) for the removal of above said range of pollutants. There are a variety of ways to use activated carbon in flue gas treatment. The best application method depends on the existing pollution abatement system.
Boyce standard range of powder activated carbon (PAC) proved to be an efficient solution for this purposes
Dry powdered activated carbon (PAC) is injected into cooled (<200oC) flue gas. The carbon can be injected in numerous locations, including prior to or after acid gas scrubbing. The overall design is similar to the semi wet/semi-dry abatement equipment except the PAC is injected dry. The used PAC will be collected in the particulate matter (PM) collection device and safely disposed of with the ash. Typically, the usage of activated carbon ranges from 50 to 500 mg per Nm3 of flue gas. Carbon usage rate is highly dependent on the flue gas temperature, available contact time, and the particulate matter collection device.
Semi Dry/Semi Wet Abatement System
Powdered activated carbon (PAC) is injected (dry or as a slurry) into the humidified 130 to 160oC flue gas stream. Although the contact time in the gas stream is usually less than a second, the contact time is substantially increased by the residence time of the carbon in the particulate matter (PM) removal equipment. The PAC can typically be disposed of as non-hazardous with the other ash. PAC can be added alone or in conjunction with lime used for acid gas treatment. When mixed with lime, some of the PAC or PAC/lime can be re-circulated. Carbon usage rates range from 50 to 500 mg per Nm3.
For over 20 years, activated carbon has been used to remove mercury and dioxins from water. In a wet scrubber system, granular (GAC) or powdered (PAC) activated carbon is used to remove dioxins from the scrubber liquid. Powdered carbon is added to the re-circulating liquid, and a small slipstream of solution is continuously purged. Alternatively, the re-circulation scrubber solution is continuously pumped through a GAC bed and purified. When the GAC is saturated and no longer performs as required, the spent carbon must be disposed of in an environmentally safe method.
Powdered activated carbons supplied by Boyce, have been consistently meeting emission discharge standards of 0.5 µg/Nm3 mercury and 0.01 ng/Nm3 T. E .dioxins throughout Europe. In the United States, proposed incinerator facilities and existing incinerator facilities face stringent flue gas discharge requirements. Many facilities are currently using Boyce standard range of powdered carbons for mercury, dioxin, and VOC control. Some local governments and state environmental agencies have already established flue gas treatment objectives prior to upcoming federal regulations.
For flue gas treatment, removal of trace levels of mercury and dioxin is required. To accomplish the adsorption of these micro-pollutants, the adsorbent must have sufficient high energy adsorption sites. Boyce coconut-based powdered activated carbons have a high minimum Iodine Number (measurement of available surface area) with up to twice the amount of high energy adsorption sites when compared to other adsorbent carbons. With proper dosing levels, over 95 percent reduction in mercury/dioxin is achievable.