Combustion method

Details Combustion method Combustion method

1997-02-13

1998-02-10

Lazarus, Ira S.

Furnaces

Refuse incinerator

Refuse suspended in or supported by a fluid medium

110214, 110345, F22B 102

Patent

active

057157642

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a combustion method, and more specifically a method for combustion of solid fuels in a fluidised bed combustor (FB combustor).
There are two reasons for the rapid increase of fluidised bed combustion (FBC) in combustors. First, many different types of fuels, which are difficult to burn in other combustors, can be processed in FB combustors. Precisely the liberty of choice in respect of fuels in general, not only the possibility of using fuels which are difficult to burn, is an important advantage of fluidised bed combustion. The second reason, which has become increasingly important, is the possibility of achieving, during combustion, a low emission of nitric oxides and the possibility of removing sulphur in a simple manner by using limestone as bed material.
It is well known that in combustion of coal and other sulphurous solid fuels in FB combustors it is possible to affect the contents in the flue gases of noxious emissions of NOx (i.e. both NO and NO.sub.2) and sulphur oxides (SO.sub.2 and SO.sub.3).
Since a number of years ago it has also been found that nitrous oxide (laughing gas) promotes the greenhouse effect as well as the reduction of the ozone layer in the stratosphere, extensive research into precisely this type of emissions has been carried out in recent years. Several "Emissions of nitrous oxide (N.sub.2 O) emissions from fluidized bed boilers", 10th International Conference on Fluidized Bed Combustion (ed. Manaker), ASME, San Fransisco, 1989; Mjornell et al "Emissions control with additives in CFB coal combustion", 11th International Conference on Fluidized Bed Combustion, ASME, Montreal, 1991; .ANG.mand et al "N.sub.2 O from circulating fluidized bed boilers--present status", LNETI/EPA/IFP European Workshop on N.sub.2 O Emissions, Lisbon 1990; and EPA Workshop on N.sub.2 O emissions from combustion (eds Lanier and Robinson), EPA-600/86-035, 1986! have demonstrated N.sub.2 O emissions from fluidised bed combustion in the order of 20-150 mg MJ.sup.-1 (40-250 ppm at 6% O.sub.2). Bo Leckner and Lennart Gustavsson have shown in an article entitled "Reduction of N.sub.2 O by gas injection in CFB boilers" in Journal of the Institute of Energy, September 1991, 64, 176-182, that it is possible to reduce the emissions of nitrous oxide during combustion in a circulating fluidised bed (CFB combustion) by effecting in the cyclone, after separation of the circulating bed particles, afterburning in the cyclone by means of a gas burner mounted therein for combustion of a separately supplied combustible gas, usually methane. In the experiments carried out it was found that considerable reductions of the emissions of nitrous oxide could be achieved without significant increases of the NO emissions, at the same time as a reduction of the CO emissions could be achieved when combustible gas was supplied for this afterburning.
A further example of a similar technique for reducing the emissions of nitrous oxide is described in the published European Patent Application EP-A-0,569,183, according to which afterburning of the flue gases is also carried out after the cyclone, which is used for separating the bed particles in a CFB combustor (i.e. a combustor operating with a circulating fluidised bed). In the method according to this publication, the combustor operates under reducing conditions in the fluidised bed, thus leaving a sufficient amount of combustible material in the flue gases, such that it should be possible to achieve the desired afterburning when oxygen-containing gases are added to the separated flue gases. Secondary air is supplied to the combustion chamber above the fluidised bed, but substoichiometric conditions are still maintained in the entire combustion chamber. An NOx-cleaning agent is added to the separated flue gases, which are then used for superheating of generated vapour in a subsequent superheater.
The published European Patent Application EP-A-0,571,234 discloses a two-stage combustion process in an FB combustor,

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