Furnaces – Process – Treating fuel constituent or combustion product
Patent
1996-10-24
1999-03-09
Bennett, Henry
Furnaces
Process
Treating fuel constituent or combustion product
110245, 110216, 422147, F23J 1100
Patent
active
058786775
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a process for cooling and cleaning flue gases.
The function of waste treatment is now passed on to refuse incineration plants. The wastes are substantially mineralized, utilizing the thermal energy contained in the waste. Two primary problems are posed by this: one is the most complete recovery possible of energy from the hot flue gases, the other is flue gas cleaning, i.e. removal of environmental problem substances from the hot flue gases.
From the multiplicity of substances contained in the waste, there are formed in the incineration a large number of pollutants. The most important gaseous pollutants in flue gas to be mentioned are: hydrochloric acid HCl (and Cl.sub.2) from the incineration of PVC-containing wastes, and sulphur dioxide SO.sub.2 from the incineration of rubber, sewage sludge, paper and dyestuffs. In the incineration of such materials, in contrast to other fuels, such as coal for example, very large amounts of HCl are formed.
In most countries, strict emission limit values are now prescribed by the legislator for the emission of these pollutants, i.e, for example, in Switzerland by the Clean Air Ordinance (LRV), in Germany by the Federal Air Pollution Control Ordinance (BImSchV) and in the EC by the Directives on Incineration of Municipal Refuse. Thus, the 17th BImSchV in Germany prescribes a clean gas limit value of 10 mg/m.sup.3 (S.T.P.) for HCl (based on dry flue gas containing 11% O.sub.2).
A typical refuse incineration plant is made up as follows: the waste materials are incinerated in a furnace plant having an afterburning chamber. The flue gases produced in the incineration flow through the waste-heat boiler downstream of the furnace for heat recovery and then enter the downstream flue gas cleaning devices, the pollutants being separated off at low temperatures. "Wet processes" are in this case most frequently employed for flue gas cleaning, i.e. treatment of the flue gases in a single- or multistage wet scrubber. "Dry processes" are also sometimes employed, in which the gaseous pollutants are sorbed on fine-grained, solid sorbents. These solids are brought into contact with the flue gas stream in a suitable manner for this purpose.
This typical structure of a refuse incineration plant is accompanied by some disadvantages. Since the flue gas cleaning is only performed downstream of the boiler end, the very aggressive pollutants flow through the boiler, where they lead to serious corrosion problems. The highest corrosion rates occur in this case on the hottest heat-transfer surfaces, i.e. on the superheater heating surfaces. In this case there are two principal mechanisms: one is the direct high-temperature corrosion of the heat-transfer surfaces by corrosive substances in the flue gas, in particular by HCl; the other is the deposit formation on the heat-transfer surfaces by furnace fly ash containing sticky, corrosive substances, with heavy corrosion under these deposits. HCl again plays a dominant role here. These serious corrosion phenomena on hot heat-transfer surfaces restrict the steam temperatures attainable and thus, if the steam is used for power generation, the electrical efficiency of the incineration plant. In addition, they lead to periodic shutdowns of the plant and complex boiler overhauls at great expense to remove the deposits on the heat-transfer surfaces.
Arranging dry cleaning processes downstream of the boiler has the further disadvantage that at the low temperatures prevailing there, for reasons of reaction kinetics, sufficient removal of SO.sub.2 can only be achieved using great excesses of sorbent.
The dry removal of SO.sub.2 and HCl is known. Sorbents which are suitable are the substances contained in natural limestone, calcium carbonate CaCO.sub.3 and dolomite CaMg(CO.sub.3).sub.2, and also calcium oxide CaO and calcium hydroxide Ca(OH).sub.2 and the analogous alkaline earth metal compounds (for example MgO, Mg(OH).sub.2 etc.). CaSO.sub.4 and CaCl.sub.2 are formed as products of the removal. Typical concentration of the pollutants i
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Muller Patrick
Ruegg Hans
Bennett Henry
Tinker Susanne C.
Von Roll Umelttechnik AG
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