The present invention relates to a method and apparatus for drying solid material with waste heat generated in processes in which materials are treated at a high temperature in a rotary kiln the jacket of which is hot and wherefrom hot gases are discharged in addition to the treated material.

The present invention is especially suitable for utilization of waste heat of lime kilns in the pulp industry. The method of the invention can, however, utilize waste heat from other burning processes as well, such as lime burning, in which materials are heat treated at a high temperature in a rotary kiln and in which hot exhaust gases are produced.

Lime kilns are long, rotary drum combustors in which lime mud, separated in connection with the cauεticizing processes in the pulp industry and mainly consisting of CaCO.,, is regenerated for combustion. CaCO, decomposes in combustion according to the reaction CaC0 ₃ CaO + CO.-

Theoretically, CaCO., decomposes at a temperature as low as about 900 °C, but in order to reach decomposition rates high enough, it is combusted at about 1100 C and in short rotary kilns at about 1300°C. Lime mud is fed cold at a dry solids content of about 60 - 65 % from the lime mud filter into the rotary kiln. Hot flue gases produced at the opposite end of the kiln are conducted against the lime mud flow. In long kilns, the combustion time may be about 4 hours, whereby the evaporation of the water contained in the lime mud, production of lime mud grains, heating and combustion take place slowly yielding a good result. The temperature of the outlet lime iε normally about 1200-1400 °C.

Heavy fuel oil or in some cases natural gas iε used in heating up the lime kiln. In modern lime kilns, equipped with

coolers for burnt lime, heat consumption iε about 1800 Meal per burnt lime ton the dry solids content of lime mud being - 60 % in combustion. Due to high energy costs, there are. continuous efforts to improve the energy economy of the pulp industry. Lime kilns are equipped with flue gas scrubbers in order to recover the waste heat and to separate dust from flue gases. The heat content of the outcoming burnt lime iε re¬ covered in the preheaters of secondary air for the kiln.

Furthermore, cheaper solid fuels, such as bark, wood waste and peat, have been tested in heating of the lime kiln.

Direct combustion of solid fuels in lime kilns has not, how¬ ever, proved to be a very successful solution because Si and Al of the fuels are taken with lime into the chemical circu¬ lation of pulp manufacturing, where they cause problems.

In accordance with the Finnish patent publication FI 72542, it iε also known to gasify solid fuel with a separate gasifier or with a gasifier built in connection with the lime kiln, whereby clean gas is produced for the heating of the lime kiln. The ashes are already separated in the gasifier and are not taken into the chemical circulation.

When fuel oil is replaced by, for example, fuel gas made of bark, the bark mostly has to be dried first. This requires additional energy and separate plants. If too wet bark iε gasified, the lime will not be totally calcined in the lime kiln. If bark is too wet, oil can be used as a support fuel in combustion, but energy losses will consequently increase.

In modern lime kilns, the heat iε normally recovered from the process by preheating the secondary air by bringing it into contact with the outcoming lime. Air, however, causes dusting of lime mud in the kiln. Fine dust also flows outside the lime kiln and therefrom with air into the kiln again. Dust cir-

culation iε thus formed around the lime kiln, which reduces the capacity of the lime kiln.

In order to reach a more efficient heat economy, it is known, as disclosed in US publication 4 626 202, to burn for example lime in a rotary kiln, in which calcination takes place in several cylindrical and parallel calcination spaces disposed around a cylindrical combustion area parallel to said spaces. This solution is, however, extremely complicated and expensive and it cannot be applied to existing lime kilns.

It iε also known, as disclosed in patent publications DE 2 747 457 and FI 60609, to recover radiation heat from the shell surface of the rotary kiln with water-cooled heat recovery means. Utilization of heat has then been attempted in some other process.

The object of the present invention is to improve the heat economy of the rotary kilns without the above-mentioned drawbacks. Another object of the invention is to enable the use of a larger variety of fuelε in gaεification in connection with the rotary kiln.

A further object of the invention is to accomplish heat recovery means which can be applied to existing rotary kilns without great changes.

The invention iε characterized in that solids are dried by simultaneously utilizing both radiation and/or convection heat of- the hot jacket of the rotary kiln and heat of hot gases discharging from the hot rotary kiln. It has been calculated that loεseε of the radiation heat, for example in a lime kiln, are 10 % of the total heat capacity.

The method of the invention can be realized with an apparatus characterized in that it comprises a rotary kiln and a jacket¬ like construction surrounding at least part cf the

jacket of the kiln, which together form a drying space pro¬ vided with an inlet for wet solids and hot gases as well as an outlet for dry solids and cooled gases.

The present invention is especially suitable for drying of wet bark, sawdust, peat, wood waste or other biowaste. With the method according to the invention it is also possible to dry, for example, wet coal or sludge cakes. The method of the present invention can also be employed in drying other ma¬ terials than the actual fuel. Lime mud, for example, can be dried by means of this method whereby the actual lime kiln can be shortened as heat energy is not needed for evaporation of water. Usually solid materialε, which are not too wet, can be dried with the method of the present invention. The water con¬ tent of the bark to be dried can be as much as 70 %, but the lime mud can become too sticky when it containε less than 60 % water.

As the water content of solid materials and the amount of waste heat of the rotary kiln vary, it iε poεεible to εecure steady drying with an extra combustor by means of which a sufficient amount of hot flue gaseε are produced in addition to the hot gaεeε discharging from the furnace.

An embodiment of the invention is described more in detail below with reference to the accompanying drawing which iε a schematic illustration of the lime mud combusting equipment.

Lime mud iε fed from a conduit 2 at the upper part of the rotary kiln into the inclined rotary lime kiln 1. Gas through a duct 3 is fed into the lower part of the rotary kiln 1 from a gaεifyer 4, and burned lime is removed from the kiln 1 through a lime cooler 5 in a duct 6. The rotary kiln receives combustion air from a conduit 7, and the lime cooler receives cooling air from a conduit 8.

As the rotary kiln rotates, the lime mud flows from the upper section of the inclined kiln to the lower εection thereof in countercurrent with the hot flue gases produced at the lower section of the kiln, whereby water evaporates and calcium carbonate burns to lime thereby forming hot flue gases. Flue gases are discharged from the kiln through a duct 9 and taken further to a gas purification means 10, e.g. a dust filter. Therefrom flue gases are conducted further through a duct 11 to a feed channel 12 through which wet bark, or other cor¬ responding solid material, iε fed with, for example, εcrew conveyor 13 from a tank 14 to a dryer 15 through .an inlet 16. The dryer 17 itεelf iε constructed as a jacket, which sur¬ rounds at least part of the shell εurface of the rotary kiln 1. The outer shell surface 18 of the rotary kiln and the inner surface of the jacket-like construction together form a drying space 19, in one end of which iε disposed an inlet 16 for wet solids and hot gases and in the other end of which iε disposed an outlet 20 for the dried solids and cooled gas.

In the embodiment according to the drawing the solid material to be dried iε conveyed in the parallel direction with flue gases, whereby the flue gaseε contribute to the conveyance of the solid material. In some other embodiments, it may be preferable to feed solid material and flue gases in counter¬ current respective to each other in the drying space.

The wet solidε move forward in the drying εpace between the εhell surface and the jacket-like dryer in both axial direc¬ tion and in the direction of the periphery of the kiln. The shell εurface of the rotary kiln iε provided with vanes 22 to regulate the movement of solid material in the desired manner and speed. The vanes can naturally be attached to the jacket of the dryer as well or to both the kiln εhell εurface and the

#__ dryer jacket. The vanes can £>e replaced by various εhovelε or spirals for directing the solid material in the desired direc¬ tion. The rotation of the inclined kiln in itself facilitates the forward movement of the solid material in the drying space.

In the embodiment according to the drawing, the jacket-like construction 17 and the shell surface 18 of the rotary kiln are integrated, whereby the dryer rotates along with the kiln. The inlets and the outlets of the drying space are sealed, which is not shown in the drawing. On the other hand, in some other applications, a solid, self-supporting, non-rotating- dryer can be used, whereby inlets and outlets need not be sealed. This means that the lower section of the dryer jacket could be provided with an outlet for dry material or with a simple sluice gate, through which the drying space could be emptied quickly if needed, for example in case of overheating of the drying εpace.

In the embodiment according to the drawing, the material to be dried is fed through the drying space in the parallel direc¬ tion with the lime mud to be dried. Solid material can also be fed in the oppoεite direction if required.

In accordance with the drawing, moεt of the dried solids are conducted through a channel 24 to a dry material tank 23 under. the rotary kiln. Gases are discharged upwardly through a duct 29 into cyclone separator 25, wherefrom the purified flue gases are discharged and the solids separated therein are conducted to the tank 23. Dry solids are f rther taken through a conduit 26 to a gasifier, where the solids are gasified by means of air 27 entering said gasifier. Ashes and soot 28 are separated from the hot product gas, whereby impuritieε are prevented from entering the lime kiln and further the chemical circulation.

The temperature of flue gases entering the lime kiln in the example could be 650 to 750 C. The temperature of flue gases leaving the lime kiln could be about 300°C. Mere radiation and convection heat iε insufficient for drying the necessary amount of bark in a normal lime kiln. Theoretically, radiation heat could release at most about 40

energy. Thus, heat energy released by flue gases iε also needed. When, for example 20 % of the necessary heat iε re¬ covered in the form of radiation heat, the remaining 80 % of the heat energy needed for drying iε received from flue gaεeε having a temperature of approximately 300 C. Normal retention time for solid material in a dryer could be about 20 minutes in a lime kiln application. Flow of flue gases as well as vanes or other conveyors in the drying space enable, however, the retention time and thus also the drying result to be affected. In case the radiation heat energy and the heat energy of the flue gases are not εufficient for drying the required amount of bark, flue gaεes can be added to the drying circulation by means of an additional combuεtor 30.

In some applications, conventional air-operated lime coolers can be excluded and the heat energy recovered from lime be utilized in drying bark or equivalent.

The amount of heat energy received from the rotary kiln can be affected by controlling the combustion procesε in the kiln, for example, by regulating the amount of air or fuel. Radi¬ ation heat can alεo be increaεed by reducing the kiln re¬ fractories in the places where heat is recovered from the εhell surface. This, however, results in problems with the construction because, with conventional kilns, the extreme limit of endurance approaches at the temperature of about 400°C.

By applying the method according to the invention, it iε poεεible in some caεeε to speed up the processes in rotary kilns and shorten the kilns. Because heat iε recovered from the εhell εurface of the kiln, the conεtruction iε not sus¬ ceptible to damage and higher temperatures favourable to the proceεε can be allowed in the heart of the kiln than with ordinary procesεes in rotary kilns.

The location of the dryer in the longitudinal direction of the kiln iε dependent on the circumstances. For drying solids which are slow to dry and which require a lot of energy, the dryer can be almost as long as the kiln itself. In some appli¬ cations, for example, for increasing the drying effect of an old lime kiln, it can be considered to provide only a part of the kiln with a dryer.

As mentioned earlier, the method according to the invention iε suitable for drying most different wet solids, and it is not intended to be limited to the application described above, which is only a preferred application of the invention.