Furnaces – With fuel treatment means – Preliminary refuse treatment means
Patent
1990-03-15
1992-04-14
Bennett, Henry
Furnaces
With fuel treatment means
Preliminary refuse treatment means
110246, 432107, 432112, B23K 302
Patent
active
051037432
DESCRIPTION:
BRIEF SUMMARY
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 causticizing processes in the pulp industry and mainly consisting of CaCo.sub.3, is regenerated for combustion. CaCO.sub.3 decomposes in combustion according to the reaction CaCO.sub.3 - - - CaO+CO.sub.2.
Theoretically, CaCO.sub.3 decomposes at a temperature as low as about 900.degree. C., but in order to reach decomposition rates high enough, it is combusted at about 1100.degree. C. and in short rotary kilns at about 1300.degree. 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 is normally about 1200.degree.-1400.degree. C.
Heavy fuel oil or in some cases natural gas is used in heating up the lime kiln. In modern lime kilns, equipped with coolers for burnt lime, heat consumption is about 1800 Mcal 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 is recovered 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, however, proved to be a very successful solution because Si and Al of the fuels are taken with lime into the chemical circulation of pulp manufacturing, where they cause problems.
In accordance with the Finnish patent publication FI 72542, it is 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 is 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 is 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 circulation is 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 U.S. Pat. No. 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 e
REFERENCES:
patent: 3787292 (1974-01-01), Keappler
patent: 4376343 (1983-03-01), White et al.
A. Ahlstrom Corporation
Bennett Henry
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