Furnaces – Process
Reexamination Certificate
1998-08-13
2001-11-06
Lazarus, Ira S. (Department: 3743)
Furnaces
Process
C110S347000, C110S346000, C110S229000, C110S266000, C048S092000, C048S198200, C048SDIG001, C048SDIG002, C048SDIG004, C266S236000, C266S900000, C422S184100
Reexamination Certificate
active
06311629
ABSTRACT:
SUMMARY OF INVENTION
It may be that in the future, waste with organic admixtures will no longer be disposed of in landfills. Therefore, to an increasing extent, waste is being subjected to thermal disposal in the form of burning (refuse incineration) or gasification.
For refuse incineration there are technically sophisticated processes which, with maximum efficiency for the generation of thermal and electrical energy, produce environmentally neutral by-products. This requires incineration parameters that ensure the production of slags that are highly resistant to having the heavy metals contained therein being leached out by water. It is also necessary to ensure that dust, nitrogen oxides, and dioxins/furans are extensively scrubbed out of the flue gases. Filter dust and processing water that are produced also need to be converted into environmentally neutral products at a reasonable cost. As a result, the technical costs involved in the environmentally neutral incineration of waste products are so high that only units with large throughput rates are able to work economically on waste.
Large throughput rates in turn require a large area from which to draw raw material to supply the requisite amount of waste. This causes the cost of shipping the waste from the point of origin to the incineration facility to become a non-negligible part of the overall costs.
As an alternative to incineration, waste can also be gasified with oxygen. Compared to incineration, gasification has a number of advantages:
a) Unlike incineration, gasification operates with an oxygen deficit. The main components in the gasification flue gas are therefore H
2
, CO, and CH
4
. Thus, the gasification flue gas can be used as a fuel gas. Sulfur turns into H
2
S, which is comparatively easier to remove from the combustion gas than removing SO
2
from the flue gas of incineration. Also, no NO
x
is produced.
b) As a rule, gasification takes place at a higher temperature than incineration. This ensures that organic pollutants are more efficiently destroyed, the dioxin-furan problem is reliably solved, and heavy metals can be bonded into the slag to form compounds that cannot be eluted.
c) The amount of fuel gas used in incineration processes, relative to a standard state, is approximately one-tenth of the volume amount of flue gas generated by incineration. When gasification is carried out under pressure, the volume flow of the fuel gas amounts to even less than one percent of the volume flow of flue gas. This means that the equipment required to scrub the flue gas is kept relatively small, especially in comparison to incineration processes.
In making a cost comparison between incineration and gasification, the oxygen costs for gasification are a drawback.
Technically, gasification is performed in a fixed-bed pressure gasifier. This gasifier is combined with a slag reactor and is characterized by a relatively low oxygen demand. See, e.g., Thomé-Kozmiensky: Reaktoren zur thermischen Abfallbehandlung, EF-Verlag für Energie und Umwelttechnik GmbH, Berlin, 1993. Such a gasifier has the disadvantage, however, that large pieces of coal have to be added in order to create a support structure for the waste. In addition, the thermodynamically inherently favorable counter-current type of flow of waste and gasification gas builds up a pyrolysis zone in the gasifier shaft, so that the flue gas discharged from the fixed bed gasifier contains typical admixtures of a pyrolysis gas (pyrolysis oils, tars) that require expensive gas scrubbing.
The gasification of waste in an entrained bed is known as the Noell-KRC process. Here, the scrubbing of the gas is comparatively simple because, except for methane, the gas does not contain any hydrocarbons. Entrained-bed gasification requires, however, that the waste be ground up to a grain size of <0.5 mm.
In the Noell-KRC process, the actual entrained-bed gasifier is preceded by a pyrolysis drum. In this pyrolysis drum, waste that is only coarsely ground is converted into a pyrolysis gas, as well as an easily grindable pyrolysis coke. The pyrolysis gas and ground pyrolysis coke are then further decomposed in the entrained-bed gasifier. This upstream pyrolysis stage, the subsequent compression of the pyrolysis gas to the pressure of the entrained-bed gasifier, and the equipment required for cooling, grinding, intermediate storage, and proportioning of the pyrolysis coke are highly cost-intensive. An advantage of slag bath gasifiers is that such upstream pyrolysis drums are not necessary.
In the Thermoselect process, a pyrolysis stage also precedes gasification. In this case, the costs for preparing the waste for gasification are very low because the waste is pressed into the horizontal pyrolysis pit without any special pre-treatment.
To be sure, however, the gasification process can be run only at normal pressure because the pyrolysis pit does not ensure reliable sealing of the gas space. This makes the equipment required for gas scrubbing comparatively large and expensive.
In addition, the pyrolysis process in the pyrolysis pit is very incomplete, so that waste with sometimes very large dimensions drops into the gasification chamber uncontrolled and floats there on the slag. This makes the operation of the gasifier very irregular, leading to large variations in the amount and composition of the gasification flue gas and/or a highly variable oxygen demand. These large variations in the gasification flue gas make it difficult to use the gas. It is also difficult to compensate for the fluctuating oxygen demand.
An object of the invention is, therefore, to provide a process and a device for the gasification of waste that allow economical operation even at relatively low throughput rates.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
On the process side, these objects are accomplished according to the invention by virtue of the fact that gasification takes place in a single stage in a gasifier with a liquid, rotating slag bath.
This makes it possible to have smaller, decentralized plants, thereby reducing the costs for transporting waste.
The process according to the invention is characterized by a single-stage gasification process by which feed waste material is converted into a usable fuel gas and a slag granulate that can be disposed of in a landfill. Expensive pre-treatment of the feed material is not required.
The feed material can be fed into the gasifier with grain sizes of up to 40 mm, so that only coarse crushing of the waste is required in advance. The mixture is divided by screening, for example, into the following fractions:
d=0-5 mm
d=5-40 mm
d>40 mm.
The overflow from the screen is fed to a mill and then recycled to the screening machine.
A magnetic separator can be installed upstream of the gasifier to remove iron components.
The liquid slag bath, located in the gasification zone, performs a number of functions. Mineral components and heavy metals in the feed material are melted down and adsorbed. At the same time the slag bath acts as a heat buffer and reaction mediator and thus ensures an intensive exchange of heat and material.
An important function is the reliable ignition and, optionally, re-ignition of the burners, to provide the desired reaction temperature.
According to a preferred embodiment of the invention, excess slag is removed by a slag drain along with the cracked gas that accumulates during gasification. The slag drain protrudes above the slag bath, and the slag flows down into it through a lateral discharge opening.
The slag bath is preferably caused to rotate by tangentially feeding in the gasification medium and/or at least a portion of the waste. It is advantageous for at least a portion of the waste to be fed to the gasifier in the form of pellets via at least one solid burner, with recycled cracked gas as a carrier gas. In this case waste with a diameter of up to 5 mm is fed into the gasifier above the slag bath, and a jet o
Halang Sven
Marschner Siegmar
Ciric Ljiljana V.
Lazarus Ira S.
Linde-KCA-Dresden-GmbH
Millen White Zelano & Branigan
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