Heating – Processes of heating or heater operation – Including passing – treating or conveying gas into or through...
Reexamination Certificate
2003-08-05
2004-08-10
Wilson, Gregory (Department: 3749)
Heating
Processes of heating or heater operation
Including passing, treating or conveying gas into or through...
C432S058000
Reexamination Certificate
active
06773259
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to cement kilns and more particularly a system for feeding waste derived fuel to a calciner kiln in a continuous manner during the manufacturing of cement.
Cement kilns are massive cylindrical structures, lined with refractory brick, into which a lime-bearing material and fuel such as coal or gas is fed. There is a range of different kiln designs but all rely on the same basic processes—raw feed material passing through the kiln is heated to very high temperatures by the burning of fuel and is transformed chemically and physically into a grey pebble-like material called clinker. Clinker is ground to produce cement. The three broad processes of cement manufacturing are: (1) raw milling, the preparation of the kiln feed material; (2) calcining and burning or clinkering, the material conversion process that takes place within the cement kiln and associated equipment; and (3) finish milling, the grinding of clinker to produce cement with the addition of gypsum.
The main raw material for the manufacture of clinker is a lime-bearing material. Ideally, this material is mined or dredged from a location near the plant to minimize transport and handling costs. Limestone is the most common material used, although other calcareous material (calcium carbonate) can be used. A large percentage, by weight, of the lime-bearing material is lost as carbon dioxide in the manufacturing process. Raw materials are combined to form a raw mix, and the proportions are governed by the chemical properties of the materials to be used and the desired properties of the clinker that is produced. The raw materials for a dry process kiln are mixed to produce an aerated dry raw material.
There are four stages to the process of the burning of the raw mix in a calciner kiln to form clinker; evaporation and preheating, calcining, clinkering and cooling. Evaporation and preheating remove moisture and raise the temperature of the raw mix preparatory to calcining. Calcining takes place at 800-900 C (gas temperature) and breaks the calcium carbonate down into calcium oxide and carbon dioxide which is evolved in the process. Clinkering completes the calcination stage and fuses the calcined raw mix into hard nodules resembling small grey pebbles. Kiln temperatures in the burning zone range from 1350-1450 C (material temperature). Retention times in the calciner range from 4 to 6 seconds. Retention times in the kiln is approximately 20 minutes.
Clinkering is critical to the quality of cement and requires accurate control of the energy input. Insufficient heat will cause the clinker to be underburnt, containing unconverted lime, and reducing the hydration (setting and hardening) properties of the resulting cement. Excess heat will shorten the life of the refractory bricks lining the kiln, may damage the kiln shell and diminish product reactivity. The high temperatures required for burning of the raw mix mean that the process is energy intensive. Electrical energy is required for the raw milling and mixing, burning, and finish milling. The largest energy demand is for fuel for burning of the raw mix. This has been the focus of technological developments, and the need to reduce energy in the cement manufacturing process remains a problem.
Rotary kilns are large steel tubes typically over 50 meters in length and up to 7 meters in diameter. They are slightly inclined to the horizontal and are slowly rotated at about 1 to 4 revolutions per minute. Raw mix is fed into the kiln at the back (the upper end of the kiln), and gravity and the rotation of the kiln allows the mix to flow down the kiln at a uniform rate through the burning zone. Clinker is formed in the burning zone and flows out of the front of the kiln (the lower end of the kiln) where it is cooled by blowing air through the clinker bed, and the heated air is used for combustion. The primary fuel is introduced and burnt at the front of the kiln. The flame is drawn up the kiln to the burning zone where the heat intensity is highest and fusion of chemicals in the raw mix takes place. Hot combustion gases continue to flow up the kiln and exit from the back end. Opportunities for recovering heat from kiln exhaust gases have made incremental improvements in lowering the energy demand of the kiln.
Suspension preheaters comprising a series of vertically aligned cyclones attached to the back end of the kiln have been advanced to reduce energy requirements in calciner kilns. Hot kiln exhaust gases flow up through the cyclones, and the raw mix is fed in at the top of the cyclones. As the raw mix travels down through the cyclones, it is heated by the exhaust gases, preheating the mix and initiating calcination.
The cement manufacturing process is carried out in wet kilns or calciner kilns using a rotary kiln. The calciner kiln process is carried out in a kiln that is commonly referred to as a calciner kiln. Both the wet kiln and the calciner kiln have utilized waste materials in order to reduce energy demands. In the wet kiln process, both liquid and solid waste materials have been introduced in various ways at the burner end of the kiln. Typically the liquid hazardous waste is injected into the burner by way of a nozzle. The hazardous solid waste materials are typically pneumatically fed into the burner end of the kiln. In the calciner kiln, liquid waste fuels are burned by introducing the liquid waste fuel mainly by injecting the fuel through a nozzle into the calciner or the kiln. One process for burning solid waste material is the hot disk process where solid waste fuel is placed on a rotating disk and combusted, and the ashes are removed while the disk rotates. The air passing through the hot disk is drawn into the calciner for heat and further combustion. Another process is a gasifier process, which is basically a pyrolysis process, where the solid waste material is roasted to evaporate the volatile ingredients which are then combusted in the calciner. There is also a batch process using calciner kilns where packages or containers of solid waste fuel are delivered to the feed shelf and combusted in the kiln. The batch process is generally not cost efficient because the waste material has to be processed into the containers which adds an additional cost to the process and the containers are combusted in a intermittent manner rather than a continuous manner.
U.S. Pat. No. 6,470,812 B1 discloses a method and apparatus for recovering energy from solid wastes by burning in the tertiary air duct of a calciner kiln. The patent includes a good discussion of prior patented inventions relating to reducing energy demands of cement kilns by burning solid wastes.
Another recent patent, U.S. Pat. No. 6,345,981 B1 discloses a ram feed for feeding solid wastes directly into the riser duct of a calciner kiln. U.S. Pat. No. 5,555,823 discloses an auger feed for feeding solid wastes directly into the calcining zone of a preheater kiln. U.S. Pat. No. 5,349,910 discloses a decomposition chamber for incinerating solid wastes using heat from preheated or calcined raw meal.
While advances have been made in reducing the energy demands and burning solid waste materials in calciner kilns, considerable attention still needs to be given to improving complete burnout of solid waste fuels with minimal residuals and recovering more of the heat of combustion.
Accordingly, an object of the present invention is to provide a system for feeding solid waste derived fuel (SWDF) material to a calciner in a consistent manner to provide kiln stability and more efficient operation.
Another object of the invention is to provide a system and method for feeding (SWDF) material to the calciner of a calciner kiln wherein the feeding of the material is continuous and, therefore, consistently provided to the combustion process.
Another object of the present invention is to provide a more stable calciner kiln process wherein (SWDF) materials are metered and delivered in consistent blends in a continuous manner during the calcination and kiln process.
Another object of the
Bech Claus
Murray, Jr. Charles R.
Flint Cort
Giant Cement Holding Inc.
McNair Law Firm PA
Wilson Gregory
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