Method of protecting incinerator surfaces

A method of preventing corrosion of incinerators designed to burn sewage sisting essentially of from about 90 to 98 percent water and from about 2 to 10 percent waste solids by increasing the fusion temperature range of the ash product above the operating temperature of the interior surfaces of the incinerator. The sewage is mixed with additive materials selected from the group SiO.sub.2, CaO, Al.sub.2 O.sub.3, MgO wherein the CaO, Al.sub.2 O.sub.3 and MgO are selected from within the range of 10 to 30 percent of the weight of the waste solids and the SiO.sub.2 is selected from within the range of 25 to 30 percent of the weight of the waste solids. Thereafter, the sewage-additive mixture is injected or otherwise dispersed into the combustion zone of an incinerator such that the waste solids and additive particles remain in contact therein for sufficient time for the additives to chemically combine with sewage solids and form combustion products having fusion temperatures ranges above the operating temperatures of the incinerator surfaces.

BACKGROUND OF THE INVENTION 
This invention relates generally to the prevention of corrosion of 
refractory and metal surfaces. More particularly, this invention has 
special application to incinerators and a method of inhibiting corrosion 
of incinerator elements by introducing additives to sewage and waste 
products burned therein. 
Incineration of sewage and other waste materials including combustible 
chemicals and oils is used in many places where other forms of disposal 
are precluded. Generally, incineration of wastes allows the advantageous 
use of relatively compact, lightweight treatment units and greatly reduces 
the volume and weight of the end products. Thus, incinerators find 
particular application in ships and marine vessels. As opposed to land 
based incinerators, which are normally designed to burn a different 
character of waste products and often contain heavy ceramic refractory 
linings, shipboard incinerators tend to be smaller, usually have metal 
lined refractory surfaces to effect weight reduction as exemplified by 
U.S. Pat. No. 3,861,330 and are often operated at higher liner or wall 
temperatures than such land based incinerators. 
Waste products commonly disposed of in shipboard incinerators include 
sewage mixed with oily residues, seawater and various other chemical 
compounds. Normally, the combustion of such products produces ash products 
which if they undergo partial or complete fusion in the combustion zones 
become sufficiently soft, plastic or molten enough to adhere to the 
internal surfaces of the incinerators. Such an occurrence produces results 
which tend to adversely affect the efficient performance of incinerators. 
For example, the accumulation of molten ash or slag on heat exchange 
surfaces tends to reduce the heat absorbing ability thereof and continued 
accumulation of molten combustion products often clogs various flow paths, 
thereby reducing the flow and perhaps raising the temperature of the 
combustion products. Importantly, it was found that combustion products in 
a molten or liquid state react much more rapidly with interior surfaces of 
incinerators than when in the solid state and, as a consequence, promote 
more rapid corrosion. Accordingly, it was desirable to cause the 
combustion products to exist in a solid phase at the incinerator 
interface, thereby reducing the reaction rate with the incinerator 
surfaces without reducing the operating temperatures of the incinerators. 
This is particularly critical with incinerators designed to burn shipboard 
wastes, which typically contain sewage having about a 95% water content 
wherein a minimum operating temperature is normally required to evaporate 
the water content and burn the waste solids in a relatively short period 
of time. The chemical structure of the compounds found in the solid waste 
or fecal matter is unknown in general because of the variable and complex 
nature of sewage. 
The teachings of the following references are primarily concerned with 
additives for decreasing the deleterious effect of impurities in the fuel 
oils: British Pat. Nos. 496,692; 728,812; 878,723; Canadian Pat. Nos. 
619,117; 714,846; and U.S. Pat. Nos. 2,800,172; 2,844,112. Designed to 
clean up the fuel oils, the additives are normally introduced directly 
into the fuel such that reactions with the undesirable impurties in the 
fuel can occur during the combustion process. In contrast, the process of 
the present invention relates to the incineration of shipboard wastes 
consisting essentially of about 95% water and about 5% solids of 
chemically complex constituents, wherein the additives are intended to 
react therewith. In view of the limited reaction times in the present 
process, as short as one second with some incinerators, it can be 
appreciated that proper combustion of the waste solids requires controlled 
dispersion of the waste mixture into the combustion zone so that the 
additives remain in contact with the waste solids, wherein reactions 
between the additives and the impurities in the fuel are of negligible 
importance. 
Additionally, the following references generally relate to fluidized bed 
processes wherein the chemical reaction time is often relatively long as 
compared with many incineration processes; U.S. Pat. Nos. 3,881,430; 
3,888,193; 3,888,194; 3,921,543 and 4,060,041. In view such relatively 
long contact times, additives are commonly introduced into the fluidized 
bed materials to prevent such bed materials from sticking together and 
sintering, to remove noxious and toxic gasses by reaction therewith and to 
act as reaction catalysts; but such additives are not primarily intended 
to inhibit or prevent corrosion of the walls of the reaction chamber. As 
aforementioned, the process of the present invention is designed for the 
combustion of wastes having a high water content wherein the reaction 
period is relatively brief. 
Examples of incinerator structures are generally disclosed in U.S. Pat. 
Nos. 3,805,714; 3,861,330; 3,892,190 and 4,002,147. In the incineration of 
aqueous waste containing organic material and other solids, it is 
preferred that such solid waste material is finely dispersed and suspended 
in a liquid slurry prior to introduction into the combustion zone. Common 
means for introducing the liquid waste slurry into the combustion zone 
includes conventional conduits and liquid lines with pressure sources 
connected thereto such that the waste slurry is directed through injection 
nozzles of various design. Upon introduction into the combustion zone, the 
sudden expansion of the waste slurry into a larger volume produces a 
relatively large effective reaction surface for rapid evaporation of the 
water and combustion of the waste solids. The turbulence created by the 
injection of the waste slurry into the combustion zones and the 
directional flow pattern of gasses in the combustion zones promotes rapid 
incineration of the waste solids. This is particularly relevant in view of 
the fact that the reaction and retention periods of waste products in the 
combustion zones of some incinerators is on the order of a few seconds. 
Customarily, the operating temperatures of the walls of the incinerators 
are lower than the flame or combustion temperatures occurring in the 
combustion zones of the incinerators. 
SUMMARY OF THE INVENTION 
Accordingly, in view of the aforementioned combustion conditions, it is a 
general object of the present invention to prevent corrosion of 
incinerator surfaces by introducing additives into the aqueous wastes to 
prevent or inhibit the decomposition ash product of the waste material 
from forming a liquid phase adjacent the incinerator surfaces. 
A further object of the present invention is to introduce sufficient 
additive to suitable increase the fusion temperature range of 
substantially all of the decomposition product of the waste materials yet 
limit such additions within proportional ranges such that surplus 
additives are not counterproductive to the chemical combustion reactions 
during the relatively short reaction times. 
These and other objects of this invention are met by mixing one or more 
additive materials selected from the group SiO.sub.2, CaO, Al.sub.2 
O.sub.3, MgO with sewage and dispersing the sewage-additive mixture into a 
combustion zone of an incinerator such that the sewage solids and additive 
particles are maintained in contact. The sewage solids and additive 
particles are retained in contact within the combustion zone of a 
sufficient time for the additives to chemically combine with the waste 
solids so that the ash product formed by the combustion of the waste 
solids and additive particles has a range of fusion temperatures above the 
operating temperature of the incinerator surfaces.

DETAILED DESCRIPTION OF THE INVENTION 
Incinerators currently being employed by the U.S. Navy for burning 
shipboard waste products include ceramic-lined multipurpose incinerators 
and metal-lined incinerators of the type shown by U.S. Pat. No. 3,861,330. 
It was observed that deterioration of shipboard incinerators, which 
commonly operate with wall temperatures of about 1600.degree. F. and 
perhaps within a temperature range of from about 1400.degree. F. to 
1700.degree. F., was due to the corrosive environment produced by 
combustion of human excrement, saltwater and lasser amounts of other 
wastes. Softening point studies of the ash formed by the decomposition of 
human waste were initiated to determine the feasibility of introducing 
additives to the waste solids to produce combustion products having a 
stable, solid ash. Although scientific examination of the ash constituents 
of wastes commonly incinerated aboard ships revealed the presence of 
aluminum, calcium, chlorine, magnesium, phosphorous, silicon, sodium, and 
sulfur, specific identification of particular compounds was not possible 
in view of the fact that the ash aggregate is formed by complex chemical 
reactions during the decomposition of the human waste in the flame. 
However, it was deduced that the type of compounds which appeared to be 
present in the ash included: (1) phosphates, most likely complex 
calcium-base compounds; (2) chlorides based on potassium and sodium 
compounds; and (3) sulfates based on potassium and sodium compounds, 
wherein the structure and composition of the compounds is unknown. 
Thereafter, the effects were determined of mixing various additives, such 
as SiO.sub.2, Al.sub.2 O.sub.3, CaO, MgO, CaCO.sub.3 and combinations 
thereof with actual waste products burned within a combustion flame at 
temperatures of about 2400.degree. F. in a shipboard incinerator of the 
general type disclosed in U.S. Pat. No. 3,861,330. For example, additions 
of SiO.sub.2 were made in 5% increments from 5% to 30% of the weight of 
the waste solids and the effects of some additions, such as MgO, CaO plus 
MgO and CaCO.sub.3, were observed at temperatures above 1900.degree. F. 
More specifically, for example, with additions of CaO at 5% and 10% of the 
weight of the waste solids it was observed that addition the 10% 
proportion resulted in retention of essentially all of the major elemental 
constituents except chlorine at temperatures up to 2150.degree. F. and 
that sulfur containing compounds were converted into nonvolatile compounds 
at temperatures between 1700.degree. F. to 1900.degree. F. Additional 
experiments were made by mixing the additives in weighted proportions with 
synthetic urine which included the following known constituents: 
______________________________________ 
Synthetic Urine 
______________________________________ 
urea 3.5(grams) 
(NH.sub.4).sub. 2 SO.sub.4 
3.67 
MgCl.sub.2 . 6H.sub.2 O 
1.67 
NaH.sub.2 PO.sub.4 . 1H.sub.2 O 
4.9 
K.sub.2 SO.sub.4 4.5 
CaCl.sub.2 1.11 
NaCl 9.11 
______________________________________ 
With respect to the various additives used in the experimental tests, the 
following results were observed: 
1. Additions of SiO.sub.2, Al.sub.2 O.sub.3, and MgO all increased the 
softening and melting points within the range of additions investigated, 
additive weight from 5% to 30% of the weight of the waste solids. While 5% 
additions of CaO, MgO and Al.sub.2 O.sub.3 provided measurable increase in 
softening point, 10% additions appear to be the minimum amount necessary 
for significant fusion temperature increases of the ash product; 
2. Calcium oxide, CaO, was the most efficient additive in increasing the 
softening point of various ash products to 2200.degree. F. for a 10% 
addition; 
3. Silica additions had little or no effect on ash softening points for 10% 
to 20% additions, but substantially increased the softening points of ash 
products of the shipboard wastes for an additive range of from 25% to 30% 
of the weight of the waste solids; 
4. Additions of 15% SiO.sub.2 in combination with 5% and 10% Al.sub.2 
O.sub.3 and CaO were no more effective than Al.sub.2 O.sub.3 and CaO added 
alone; and 
5. Additions of 5% and 10% proportions of Al.sub.2 O.sub.3 and MgO were 
more effective than similar proportions of SiO.sub.2, but were less 
effective than CaO and CaCO.sub.3 in increasing the ash softening points. 
Experimentally it was found that employing additives in excess of 30% of 
the weight of the waste solids did not produce significant improvements in 
the softening points and fusion temperatures of the ash product. At 
proportions higher than 30% of the waste solids it became moe difficult to 
homogeneously mix the additives with the waste solids and such excess 
additives tended to unnecessarily extract heat from the combustion system. 
Accordingly, the method of preventing corrosion of metal-lined incinerators 
which burn sewage and other waste effluent generally comprises the steps 
of mixing additives containing one or more of SiO.sub.2, CaO, Al.sub.2 
O.sub.3 and MgO, in amounts which range from 10 to 30 percent of the 
weight of the waste solids for CaO, Al.sub.2 O.sub.3 and MgO and from 25 
to 30 percent of the weight of waste solids for SiO.sub.2, with sewage 
consisting essentially of from about 90 to 98 percent water and from about 
2 to 10 percent waste solids. The additive is uniformly dispersed within 
the mixture such that when the mixture is injected under pressure into a 
heat or combustion zone and widely dispersed therein, the additive 
particles remain in contact with the waste solids, probably as a result of 
surface tension. The mixture is maintained in the heat or combustion zones 
for sufficient time, normally from about one-half to three seconds, for 
the additives to chemically combine with the waste solids such that the 
additives effect an increased fusion temperature range of the combustion 
products. Such occurrence thereby prevents reaction of the combustion 
products in a liquid state with the liners of incinerators during normal 
operating temperatures. Further, since many incinerators are normally used 
only when there is a demand for sewage disposal, such as when the holding 
tanks are full, the incinerator process is most likely to be of a cyclic 
nature with periodic shutdowns, so that the coalescence and caking of 
molten ash products on the walls of the incinerator would produce more ash 
buildup than if the ash products were in a solid state. 
Normally, sewage and other waste materials to be disposed of in 
incinerators are collected in holding tanks and maintained in a stirred or 
mixed condition therein until the waste materials reach a predetermined 
level. Thereafter, pumps transfer the liquid waste slurry under pressure 
to the incinerator wherein it is commonly injected therein. The 
pressurized fluid causing the sludge to pass from the holding tank to the 
incinerator is preferably compressed air, but steam, gases or other fluids 
may be used which will not impede the flow or eventual combustion of the 
waste materials. Compressed air has the additional advantage of increasing 
the available oxygen in the combustion chamber and serves to promote more 
complete combustion of the waste solids and gases in the combustion zones 
of the incinerators. The pressurized fluid or air also serves to 
automatically clean the orifices of the injection nozzles of waste solids 
and other debris. As a result of the increased velocity of the liquid 
waste slurry as it passes through such restrictive nozzle orifices, an 
atomized mist is dispersed therefrom into the combustion chamber. As 
aforementioned, the sudden expansion of the waste slurry into the 
combustion zone produces a large effective reaction surface for rapid 
evaporation of the water and combustion of the waste solids. The additives 
are normally finely ground to a powdery consistency and mixed with an 
appropriate liquid to form a liquid slurry. The liquid slurry mixture is 
thereafter introduced into the waste conduits transferring the liquid 
waste slurry from the holding tank to the incinerator such that the 
additive particles are in contact with the waste solids and remain so 
after introduction into the combustion chamber. It is within the scope of 
this invention that chemicals may be added to the additives to promote and 
maintain adhesion between the waste solids and the additive particles. In 
view of the turbulent conditions existing within the combustion zones of 
various incinerators, it is unlikely that separate introduction of the 
wastes and additives into the combustion zones and dependency upon 
subsequent collision between the waste solids and additive particles would 
produce similar desired results. It is also unlikely that the desired 
results would be obtained if the additives were introduced into the fuel. 
In this case the additives would first react with the various impurities 
in the fuel and would not be available for further chemical reactions with 
the waste material. 
Obviously many modifications and variations of the present invention are 
possible in light of the above teachings. It is therefore to be understood 
that within the scope of the appended claims, the invention may be 
practiced otherwise than as specifically claimed.