Furnaces – Process – Treating fuel constituent or combustion product
Patent
1996-10-24
1998-08-25
Bennett, Henry A.
Furnaces
Process
Treating fuel constituent or combustion product
110210, 110216, 110245, F23J 1100
Patent
active
057973369
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a process for the combustion of waste material with production of thermal energy.
Processes and waste combustion plants of this type are known, in which the furnace, e.g. a grate-fired furnace, is operated with an amount of air stoichiometrically required for the complete combustion of the waste materials or even with excess air.
Because of uneven distributions in space of this primary air feed, operating the furnace with a relatively large oxygen excess is almost unavoidable. Only in this manner can a complete burn-up of the waste materials introduced into the furnace be ensured. Thus, e.g. the slag from a grate-fired furnace for waste materials should only contain 3% by weight of volatile substances (measured as loss on ignition at 550.degree. C.).
Some of the substances also leave the furnace unburnt on the flue-gas side. These unburnt gases and solid particles are likewise formed owing to uneven distributions in space of the primary air feed and insufficient flue gas mixing in the furnace chamber and leave the furnace chamber in the form of streams. These substances must be reburnt in an afterburning chamber. It has been customary hitherto to introduce additional combustion air, so-called secondary air, into the afterburning chamber to reinforce the afterburning and, in particular, to improve the cross-mixing of the flue gases.
Because of poor cross-mixing, in order to ensure the degree of burn-up of the flue gases prescribed by law, a relatively long residence time of the flue gases in the afterburning chamber must be ensured and a relatively large amount of secondary air must be admixed. This results in a very large size of the afterburning chamber and an increased size of the downstream apparatuses, such as boilers for heat recovery and gas cleaning devices, since the total volumetric flow rate is increased by the addition of secondary air. This also decreases the boiler efficiency and thus the achievable electrical efficiency of the combustion plant, since an increased flue gas volume also means greater waste-gas heat losses. For the boiler, this likewise results in a very large size, since the heat transfer from the hot flue gas to the cooling surfaces is relatively poor, in particular in the radiant part of the boiler.
A difficult problem in the combustion of waste materials is, in addition, the corrosive flue gases, which lead to corrosion problems in the boiler section. These occur preferentially on the hottest heat-transfer surfaces, i.e. on the superheater heating surfaces. Two principal mechanisms are involved: one is the direct high-temperature corrosion of the heat-transfer surfaces by corrosive substances in the flue gas; the other is the deposit formation on the heat-transfer surfaces by flyash, from the furnace, which contains sticky, corrosive substances, with heavy corrosion under these deposits. These intense 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 combustion 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.
A further problem in the combustion of materials is the formation of nitrogen oxides. For environmental protection reasons, these cannot be freely released into the surroundings. A plurality of processes have already been disclosed, e.g. the SNCR process (selective non-catalytic reduction process), see U.S. Pat. No. 3,970,739, in which nitrogen oxides in the flue gases are reduced to nitrogen by spraying in an ammonia solution or other suitable reducing agent, in the presence of the oxygen which is present in any case. The ammonia is conventionally introduced for this purpose at a suitable point in the flue gas stream. The flue gas temperature at the point of introduction plays an important role. It must be between 700.degree. C. and 1100.degree. C. If the flue gas temperature is too low, a great excess of
REFERENCES:
patent: 4552203 (1985-11-01), Chrysostome et al.
patent: 4932335 (1990-06-01), Bruckner et al.
Muller Patrick
Ruegg Hans
Bennett Henry A.
Tinker Susanne C.
Von Roll Umwelttechnik AG
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