Method of producing lime in a rotary kiln

Biomass residue is added at the cold end of a rotary calcination kiln to generate heat in situ thereby diminishing substantially the amount of fossil fuel normally fired at the hot end of the kiln.

BACKGROUND OF THE INVENTION 
This invention relates to an improved process for calcining calcium 
carbonate (CaCO.sub.3) in a rotary kiln to obtain calcium oxide (CaO). 
In the manufacture of paper by the Kraft and other alkaline pulping 
processes wood chips are cooked with a solution of sodium hydroxide, 
sodium sulphide, and other chemicals depending on the grade and kinds of 
pulp desired and the spent liquor after it has been separated from the 
pulp is processed to recover values therein. At some stage of the process 
the spent materials being treated are causticized with lime usually with 
resultant recipitation of calcium carbonate. 
In order to improve the economics of the process the calcium carbonate is 
recovered and is subsequently calcined in a kiln to produce calcium oxide 
which may then be reused to causticize additional spent liquor. Naturally 
occurring limestone is also calcined to produce calcium oxide for a 
variety of known commercial uses. 
There are two types of calcination kilns in general use. In the vertical 
shaft kiln coarse lime rock is mixed with solid fuel or, layered 
alternately and calcined to produce lime. Fine sized stone and fuel cannot 
be used since they would create too much resistance to the upward flow of 
combustion and reaction gas (CO.sub.2). The relative position of the 
limestone and fuel does not change during the descent of the charge in the 
kiln thus permitting a variety of solid fuels. Coal as well as wood has 
been used as fuel in the vertical shaft kilns. The vertical kiln is not 
suitable for the calcination of lime mud produced during causticization of 
spent materials in the pulping process due to the fineness of the mud. 
The rotary kiln is especially suited for calcining finely powdered 
CaCO.sub.3 such as lime mud from the causticizing process. Hitherto the 
fuel used to supply the heat required to convert the mud to CaO has been 
exclusively natural gas, oil or powdered coal fired from the hot product 
discharge end of the rotary kiln. 
U.S. Pat. No. 1,323,294 to R. W. Lesley relates to the use of low grade 
coal, oil shale or lignite in a cement calcination process employing a 
rotary kiln. The low grade material is gasified externally in separate 
retorts to obtain fuel gas and chemical values. The evolved gases are used 
to supplement or reduce the fuel conventionally fired from the product 
discharge end of the kiln. The gasification residue is mixed with the raw 
cement clinker. The fine carbonaceous part of the residue which has a 
heating value is burnt during calcination. 
U.S. Pat. No. 4,022,629 to Garrett et al is directed to the manufacture of 
cement clinker in a rotary kiln. Combustion of a residual fuel is effected 
directly in the bed of the kiln to reduce the normal fuel requirement. 
Representative types of residual fuels are coke, coal, char, coal 
gasification residue or mixtures thereof. The chosen fuel must have a high 
heat value (over 10,000 BTU/lb), a maximum particle size of less than one 
quarter of an inch, (preferably between 8 and 325 U.S. screen mesh size) 
and a high ash and sulphur content. 
Neither the ash nor the sulphur content of the above fuels can be tolerated 
in a kraft mill lime kiln. 
I have discovered biomass material to be an inexpensive yet suitable fuel 
in the calcination of lime mud originating from the causticizing 
operations of an alkaline wood-pulping process such as kraft or soda. 
The biomass material may comprise wood residues incidental to the wood chip 
production at a pulp manufacturing facility, or other woody residues, hog 
fuel, bark, sludges, agricultural residues and the like, provided it does 
not contain large amounts of sulfur or other elements which would 
adversely affect the use of the calcium oxide in the causticizing 
operation for which it is intended. 
The use of a low grade biomass fuel when combusted directly in the bed of a 
rotary kiln provides a substantial portion of the required process heat 
thereby reducing the need for more expensive natural gas, oil or powdered 
coal fired in the hot produce discharge end of the kiln. 
It is a primary object of the invention to provide an economical method of 
calcining lime mud or lime stone in a rotary kiln using a low grade 
material as the fuel to provide a substantial portion of the heat required 
for calcination. A second object is to provide a low grade, relatively 
sulphur-free, fuel material which will not affect the normal operation of 
the kiln. Another object is to recover the lime product from the kiln and 
utilize it to causticize a wood pulping liquor. A further object is to 
recover the heat values of any char and/or volatile organic material 
escaping with the combustion gases from the kiln for further combustion in 
the kiln. Other objects will be apparent to those skilled in the art. 
SUMMARY OF THE INVENTION 
According to this invention there is provided in a method for calcining 
lime mud or lime stone in a rotary kiln fired at the hot end by burning 
fossil fuel, an improvement which comprises adding to the lime mud or lime 
stone at the cold end an amount of biomass residue which is combusted in 
the kiln to generate heat in situ, said amount being sufficient for 
effecting calcination and effective enough to substantially diminish the 
amount of fossil fuel normally fired. 
DETAILED DISCUSSION 
In the process of the present invention coarse, low cost residues in the 
form of wood residues, hog fuel, bark, sludges, agriculture residues, 
etc., are fed directly into the cold end of the rotary kiln with the lime 
mud. After thorough preheating of the kiln with fossil fuel at the hot 
end, stable combustion of the coarse biomass residue can be maintained 
throughout the calcination zone. Thusly maintained, the amount of fossil 
fuel can then be substantially diminished to effect fuel economy. 
Coarse woody residues up to several inches in size, having moisture 
contents of up to 60% and a low heat value of about 3,500 to 8,000 BTU/lb 
depending on the actual moisture content can be burned despite the 
inhomogenous mixture with fine lime mud, to provide a substantial portion 
of the fuel requirements. A 95+% conversion into CaO can be maintained 
producing a product of acceptable quality. 
Preparation of the biomass residue prior to burning may be practiced 
optionally. Such preparation may include any one of the following steps or 
combinations thereof: (a) separation of the fines; (b) comminution to an 
appropriate size; and (c) pressing or drying to a desirable moisture 
content. 
In attaining one objective of this invention, namely near-complete 
replacement of fossil fuel, a combination of direct in-situ combustion of 
biomass residues charged with lime into the cold end of the kiln, and hot 
end combustion of woody fines (from screening and/or any other source) 
introduced with the primary air into the existing oil or natural gas flame 
in a mixed fuel burner is used. Alternatively, two separate burners can be 
used, one for the fossil fuel and the other for the biomass fines. 
Any unburnt fines, tars and volatile components of the biomass residue in 
the kiln, carried over with the flue gas, is first directed to a scrubber 
and then routed to the lime mud filter where it is mixed with the 
dewatered lime mud for recycle burning in the kiln.

As shown in the drawing unprocessed woody residue, referred to as hog fuel 
in the drawing, is conveyed to a fuel preparation unit 10. In general, the 
hog fuel requires little or no preparation in terms of comminution or 
pre-drying. Hog fuel of very inhomogeneous size distribution and high 
moisture content can be introduced into the cold feed end of the kiln and 
burned in-situ with the lime stone. However, in order to obtain a more 
stable combustion, it is often advantageous to separate the hog fuel fines 
before feeding the coarse particles into the cold end of kiln 14 and under 
certain circumstances, it is advantageous to reduce the moisture of the 
hog fuel. Comminution may also be required if size distribution of the 
wood waste is too inhomogeneous and presents difficulties in feeding and 
transporting the wood waste particles in the kiln. Hence the hog fuel 
preparation 10 may include all or any combination of (a) screening or any 
other suitable means to separate the fines, (b) comminution, and (c) 
moisture reduction. The prepared hog fuel is then passed into a storage 
bin 12 from which it is fed into the cold end of kiln 14 by means of a 
suitable feeder 13 which may be the pusher type, the screw type, or any 
other suitable form. 
In attaining near complete replacement of fossil fuel, the hog fuel fines, 
separated in the hog fuel preparation 10, can be supplemented with chip 
screenings or sawdust and directly injected from the hot product end of 
kiln 14 along with the primary air into an existing oil or natural gas 
flame in a mixed fuel burner, or, alternatively, may be burned in a 
separate burner directed into the hot end of kiln 14, or both may be used. 
The product lime as well as ash and any other inert inorganic material 
introduced with the hog fuel are discharged from the kiln 14 at the hot 
product end thereof. In order to prevent an accumulation of ash and inerts 
in the chemicals recovery cycle of the pulping process, the lime must be 
separated from ash and inerts. This separation is accomplished in an ash 
separation unit 15. The product lime is discharged from the kiln 
substantially in the form of pellets of 1/8 inch to 1/2 inch in diameter 
and can be different in size and density from the ash and inorganic 
inerts. Hence the product lime may be separated from the ash by using a 
screen, cyclone or centrifuge. Any other suitable form of physical 
separation may be used to accomplish this. 
After the ash separation step, make-up lime is added to the product lime 
and passed to a slaker 16. Some residual ash and inerts are removed with 
the grit from the slaker 16. Green liquor is added to the slaker 16 and 
passed to a causticizer 17 in which the green liquor is causticized to a 
white liquor. The while liquor is then passed to clarifier 18, where the 
lime mud, largely free of ash and inerts, is separated from the white 
liquor. The lime is then conveyed to a lime mud washer 19. The lime mud, 
after washing, is fed to a filter 20 which yields a final mud product of 
about 70 percent solids. Volatile organic decomposition products or 
unburned char contained in the flue gas issuing from the lime kiln 14 can 
be recycled into the lime kiln 14 by passing the bleed of the lime kiln 
scrubber 21 into lime mud filter unit 20. The filtrate from lime mud 
filter 20 may be diluted with wash water and utilized in the lime mud 
washer 19. 
In order to illustrate the invention described in the present disclosure, a 
few tests were carried out with a pilot scale lime kiln (16 inches i.d., 
18 feet long). The tests in which wood fed from the cold end of the kiln 
was used to supplement natural gas are summarized in the Table below. 
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Natural Rot'l 
Run Stone Wood Air Gas Percent Speed 
No. kg/h kg/h m.sup.3 /min 
m.sup.3 /min 
Calcination 
rpm 
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1 50 7 1.642 0.159 92.5 1.5 
2 50 0 1.642 0.170 98.2 1.5 
3 10 34 1.076 0.057 98.8 3.0 
4 10 0 0.991 0.099 99.7 3.0 
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In all tests, dry calcium carbonate stone, 0.1 to 0.3 cm in size, was used. 
In Run 1, screened chips fines were used, while in Run 3, chips were used 
as the wood supply. In both cases, the wood was fed as is, i.e. 
non-pre-dried and with a moisture content of 45-50% on wet basis. It can 
be seen that in Run 1 and Run 3 approximately 6.5% and 42.5% respectively, 
of natural gas was replaced by wood. 
Although this invention has been described in its preferred form and 
preferred practice with a certain degree of particularity, it is 
understood that the present disclosure of the preferred form and preferred 
practice has been made only be way of example and that numerous changes in 
the details of the combination and arrangement of parts and steps may be 
resorted to without departing from the spirit and the scope of the 
invention as hereinafter claimed.