Furnaces – Process – Treating fuel constituent or combustion product
Patent
1996-10-24
1999-06-29
Bennett, Henry
Furnaces
Process
Treating fuel constituent or combustion product
110229, 110245, 110348, F23J 1100
Patent
active
059153113
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a process for the thermal treatment of waste material with production of thermal energy in accordance with the preamble of claim 1.
Degasifying as a thermal process for energy production from waste, also termed pyrolysis, low-temperature carbonization or coking, is known (cf. in this context the specialist journal Mull und Abfall 12/1978 or Swiss Patent Application No. 01 510/94-8, A 10364 CH). In all processes based on degasifying, the waste is heated in the absence of oxygen by direct or indirect supply of heat. During this heating the organic compounds in the waste become unstable; the volatile constituents escape, and the non-volatile constituents are converted into coke.
The low-temperature carbonization gases produced in the degasifying have a high heating value. When these low-temperature carbonization gases are directly burnt in conventional afterburning chambers with oxygen or oxygen-enriched air, very high temperatures of above 2000.degree. C. result, which are difficult to control.
The object underlying the present invention is to create a process of the type mentioned at the outset which enables control of the temperature profile in the afterburning.
This object is achieved according to the invention by the features specified in the characterizing part of claim 1.
DE-A 33 07 848 discloses reburning and cleaning metallurgical process off-gases containing combustible constituents in a circulating fluidized bed, the process off-gases and oxygen-containing gases being introduced separately into the fluidized-bed reactor and being reburnt and simultaneously cleaned therein in the presence of solids containing gas-cleaning agents. The process off-gases used have a low heating value.
WO-A-93/18341 discloses burning homogeneous fuels such as coal, oil or petroleum coke in two separate stages. The combustion proceeds in these two stages with supply of oxygen. In order to burn solids which are not burnt in the first stage, i.e. carbon and gases, an oxygen excess is used in the second stage.
The process of the invention relates to the pyrolysis of waste, in particular refuse, in which, as mentioned above, very high temperatures result in the afterburning with oxygen; by means of the afterburning according to the invention in a circulating fluidized bed, optimum and uniform reaction conditions are created for the afterburning, since a very homogeneous temperature distribution is achieved. At the same time, a highly efficient cooling of the hot carbonization gases is achieved. The gas-solids flow present in the fluidized bed gives a very good heat transfer, which leads to a diminution of the heat-transfer surfaces and thus also of the boiler size. The reduction in the amount of flue gas achieved by the afterburning with oxygen also causes a decrease in the size of the fluidized-bed reactor and the downstream equipment, an increase in boiler efficiency, a reduction in expenditure for gas cleaning and a reduced risk of corrosion of the heat-transfer surfaces.
A problem in the thermal treatment of waste is the formation of nitrogen oxides. For reasons of environmental protection, these cannot be freely released into the surroundings. A number of processes have previously been disclosed, for example the SNCR process (Selective Noncatalytic Reduction Process), see U.S. Pat. No. 3,970,739, in which nitrogen oxides in flue gases are reduced to nitrogen by spraying-in an ammonia solution, or other suitable reducing agents, in the presence of the oxygen present in any case. The ammonia for this purpose is customarily introduced into the flue gas stream at a suitable point. The flue gas temperature at this point of introduction plays an important role. It must lie between 700.degree. and 1100.degree. C. If the flue gas temperature is too low, a great ammonia excess is required. The unreacted ammonia in the flue gas is termed slip and represents environmental pollution. If the temperature is too high, some of the ammonia burns. In both cases, the amount of ammonia needed is unnecessarily high.
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Muller Patrick
Ruegg Hans
Bennett Henry
Tinker Susanne C.
Von Roll Umwelttechnik AG
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