Fly ash recycling to reduce toxic gaseous emissions

A process for removing toxic compounds from gaseous emissions of municipal solid waste combustion systems is disclosed in which fly ash produced by the incineration process is used to adsorb toxins, and is recycled to a combustor, which destroys the adsorbed toxins and regenerates the fly ash for further toxin recovery.

FIELD OF THE INVENTION 
The present invention relates to a process for reducing toxic gas emissions 
from municipal solid waste combustors. 
BACKGROUND OF THE INVENTION 
Recent reports indicate that U.S. cities and towns generate more than 
410,000 tons of waste each day. Until recently, most of this municipal 
solid waste was disposed at landfill dumping sites. However, in more 
recent years, the number of available landfills has grown smaller as more 
and more such sites become filled or are closed for violating state and 
federal environmental laws and for contaminating groundwater supplies. 
In an effort to curb the increased environmental pressures caused by 
minicipal solid waste, recent efforts have focused on resource recovery, 
namely the incineration of such wastes and recovery of the heat produced 
thereby. This recovered heat is used to produce steam for heating 
buildings, running air conditioners, or powering the turbine of an 
electrical generator. 
A serious problem associated with municipal solid waste incinerators is the 
emission of dioxins and furans during the combustion process. These 
artificial organic compounds, such as polychlorinated dibenzo-dioxins, and 
polychlorinated dibenzo-furans, are suspected of causing a wide range of 
illnesses, from cancer to birth defects. Scrubbers and baghouses have been 
used to reduce these emissions with some success, but have not completely 
eliminated the problem. Accordingly, a significant advance in the art 
would be realized if a more effective method for reducing toxic gaseous 
emissions from municipal solid waste incinerators were devised. 
SUMMARY OF THE INVENTION 
In order to reduce the toxic emissions from municipal solid waste 
incinerators, a new process has been developed in which fly ash produced 
in the combustion process is passed through a post combustion section 
having sufficiently cool temperatures to permit adsorption of toxic 
compounds on the fly ash. At least a portion of this fly ash is then 
recycled into the combustor, where sufficiently high temperatures destroy 
toxic compounds adsorbed on the fly ash by breaking such compounds down 
into harmless constituent elements and other compounds. 
Other details, objects and advantages of the invention will become apparent 
as the following description of the presently preferred embodiments and 
presently preferred methods of practicing the invention proceeds.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates a typical arrangement for the incineration of municipal 
solid waste. As shown, municipal solid waste, typically unprocessed, is 
fed into an inlet 10, which conveys the waste to a combustor 11. Gas, 
generally combustion air, is injected along the length of the combustor 11 
through gas injection ports 12. The temperature in the combustor is at 
least 1800.degree. F., and may be as high as 2200.degree. F. The combustor 
11 is preferably of the rotary kiln O'Connor type, having water-cooled, 
perforated walls. 
The combustor has three phases, a driving phase, in which much of the 
moisture in the municipal solid waste is removed, a combustion phase, in 
which combustion occurs, and a mass/volume reduction phase, in which over 
80% reduction of the mass and volume of the municipal solid waste material 
occurs. The products of the combustion process, i.e. hot flue gases and 
ashes, leave the combustor at the combustor exit 13 and pass into the 
radiant boiler 14. 
Once in the radiant boiler, the heavier ash particles 15 fall into the 
lower boiler section 16, and are carried out of the process. The finer ash 
particles 17, referred to herein as fly ash, are carried up by the flue 
gas stream into the post combustion sections 18, 19 and 20. Section 18 is 
typically the superheater and the convective boiler sections. Section 19 
is typically a combustion air preheater or economizer, and section 20 is 
typically a dry gas scrubber, a bag filter and/or electrostatic 
precipitator. 
Toxic compounds such as dioxins and furans are adsorbed on the fly ash, 
both by physical adsorption and chemisorption, in these post combustion 
sections, provided the temperature in these sections is sufficiently cool, 
generally below 752.degree. F.. More toxin per unit weight of fly ash is 
adsorbed on the finer fly ash particles, as these have a greater surface 
area per unit weight than the larger particles. 
It has been determined that once these toxic compounds are adsorbed on the 
fly ash, the compounds are extremely resistant to leaching from the ash, 
absent other chemical reactions. For example, it has been estimated that 
99% of the dioxin adsorbed onto fly ash will be retained by the fly ash 
6.5 million years later. However, if certain chemicals are present, the 
toxic compounds that are adsorbed can be caused to leach from the ash. 
After the fly ash passes through the post combustion sections and toxic 
compounds are adsorbed by the ash particles, at least a portion of ash is 
recycled to the combustor, and the remaining non-recycled portion is 
purged from the system. This recycling and purging may be accomplished by 
a number of commercially available mass transfer means, but pneumatic 
transport lines or mechanical transport systems have proven effective. The 
system illustrated in FIG. 1 uses transport gas 21 to convey the ash 
collected from sections 18, 19 and 20 through an ash transport line 22. 
This conveyed ash is preferably fed to a cyclone separator 23, which 
separates the transport gas and feeds the ash to the combustor 11. The 
cyclone separator preferably has a gas transport line 24 which allows the 
outgoing gas from the cyclone to be recycled to the post combustion 
sections. 
Once in the combustor, the extremely high combustor temperatures, greater 
than 1800.degree. F., are effective in desorbing and destroying toxic 
compounds adhering to the recycled fly ash by breaking such compounds down 
into harmless constituent elements and other compounds, such as CO.sub.2 
and water. Other elements, such as chlorine, are removed by a scrubber 
system. Once the fly ash particles are cleansed of adherent toxic 
compounds in this way, the recycled fly ash may make another pass through 
the system to adsorb yet more toxic compounds, which are again recycled to 
the combustor for destruction. Except for the portion of fly ash which is 
purged from the system, the fly ash theoretically can make an infinite 
number of passes through the system, adsorbing toxic compounds, carrying 
them to the combustor for destruction and regeneration of the adsorbing 
capability of the fly ash, and passing through the colder sections of the 
system for further adsorption of toxic compounds. 
The ratio of recycled to purged fly ash and the feed rate of recycled ash 
may be optimized to meet the requirements of a particular system and the 
degree of toxic removal required. Although the purged fly ash can be 
withdrawn at any point in the system, it is preferable to withdraw the ash 
from the process at a point where the removed ash is relatively free of 
toxins. In this way, ideally more toxic compounds are destroyed in the 
combustor and less are purged from the system adsorbed to the fly ash. For 
this reason it is preferred to withdraw the purge stream of the fly ash 
from the hottest portion of the combustor, where the likelihood that toxic 
compounds have not been adsorbed by, or have been desorbed from, the fly 
ash and destroyed is greatest. 
Although the invention has been described in detail in the foregoing for 
the purpose of illustration, it is to be understood that such detail is 
solely for that purpose and that variations can be made therein by those 
skilled in the art without departing from the spirit and scope of the 
invention as described by the claims.