Reduction and admixture method in incineration unit for reduction of contaminants

The present invention relates to a process for the mixture of and/or for the achievement of and/or the facilitation of the reduction of undesired compounds, such as soot, hydrocarbons and the oxides of nitrogen and, if the fuel contains other substances, such as sulphur and chlorine, and metal compounds such as mercury and arsenic, for bringing about a reduction in emissions of acidifying oxides of sulphur and hydrogen chloride, as well as mercury and arsenic compounds in an incineration unit. In accordance with the invention, this is achieved by a special mixture and mixing process, in which suitable reducing agents for nitrogen compounds and/or for other undesired substances, for example oxides of sulphur, are introduced into the incineration unit in direct association with the combustion zone in conjunction with the supply of gas. This can take place in conjunction with the actual incineration process, or separately from it. The invention also proposes that the utilization of the mixture and mixing process in conjunction with the supply of air or some other suitable gas and/or gas-born powder or aerosol compound in itself should lead to the formation of low levels of the oxides of nitrogen, but that it does permit the utilization of other, processes either alone or in combination with this process. The invention also relates to an apparatus for the execution of the process.

BACKGROUND OF THE INVENTION 
(1). Field of the Invention 
The present invention relates to a process for mixing and a mixture 
contained within an incineration unit intended to bring about the 
reduction of contaminant such as soot, hydrocarbons, and oxides of 
nitrogen. Additionally, if the fuel contains other substances such as 
sulphur, chlorine, and metal compounds such as mercury and/or arsenic, the 
present invention brings about a reduction in emissions of acidifying 
oxides of sulphur and hydrogen chloride, as well as mercury and arsenic 
compounds. 
(2). Description of the Prior Art 
Combustion causes the emission of undesirable compounds. This is true of 
the combustion of wood, peat and fossil fuels, such as oil, gas and coal, 
and of cellulose waste and household refuse along with other compounds. 
The emission of undesirable compounds can be reduced by either more 
efficient combustion or the addition of reducing agents. 
Previously disclosed methods for the reduction of undesired substances are 
over fire air, rotating over fire air, reburning, flue-gas recirculation, 
addition of ammonia substance(s), addition of lime products, and the 
addition of sodium carbonates. 
The problems associated with the use of such previously disclosed methods 
are uneven conditions during combustion, poor turbulence--mixing, and 
difficulty in achieving reliable mixture. 
The use of these previously disclosed methods results in poor efficiency 
and the generation of undesired compounds as by-products. 
The prior art includes the over fire air (OFA) method. The OFA method is 
based on the supply of a proportion of combustion air through separate air 
ducts to the combustion process situated after the combustion zone. The 
following are the advantages of the OFA method: causes 
fuel-rich/sub-stoichiometric combustion to take place in the hearth and 
counteracts the oxidation of the nitrogen present in the air and in the 
fuel; and causes a lowering of the maximum temperature in the core zones 
of the flame, resulting in lower NO.sub.x production. 
The following disadvantages are associated with the OFA mixture of "cold" 
combustion air: inability of the "cold" combustion air to take part in 
combustion; inability to obtain a final product of combustion; and 
inability to keep emissions of, amongst other things, dust, soot and CO at 
a low level. 
In view of this, the OFA method has only been applied to large incineration 
units which are run under low load conditions. 
The prior art in respect of the reduction of nitrogen in incineration units 
also includes the rotating over fire air (ROFA) method. The following are 
the advantages of the ROFA method: NO.sub.x reduction through an increased 
degree of rotation and turbulence in the flame of the unit; and increased 
efficiency from a lower flue-gas temperature and the necessary lower 
excess air. 
Additionally, the prior art includes the recycling of the flue-gas into the 
primary combustion zone and the flame. This method has been found in 
certain cases to produce a good reduction in NO.sub.x. The NO.sub.x is 
reduced due to a reduced oxygen content and an associated reduction in the 
intensity of combustion. 
The following are the disadvantages associated with flue-gas recycling: the 
possibility of condensation associated with corrosion, toxicity in the 
event of leakage, increased flue-gas flow and a resulting reduction in 
incineration capacity, higher electricity consumption, and reduced 
efficiency due to an increase in flue-gas temperature. 
Another prior art method, known as secondary combustion or reburning, 
involves the supply of additional fuel at the end of the flame, in 
conjunction with which previously formed NO.sub.x is reduced. The problem 
with reburning is obtaining a reliable, controlled mixture in the flame. 
A further previously disclosed method is known as thermal NOx reduction 
(SNCR). The thermal NOx reduction method, for example, involves the 
DeNO.sub.x and NO.sub.x OUT processes. The DeNOx and NO.sub.x OUT 
processes involve the addition of chemicals such as ammonia (NH.sub.3) and 
urea ((NH.sub.2).sub.2 CO). 
The previously disclosed effects of this method include the reduction of 
NO.sub.x in the range of temperatures between approximately 900.degree. 
and 1000.degree. C. It is claimed that this temperature range is capable 
of being extended by the use of special auxiliary chemicals. 
The dosing of the chemical in this case takes place after the actual 
combustion zone. The result of this dosing is greatly dependent on the 
reliable and controlled mixture of the chemicals. Furthermore, the 
stratified flow in the flame chamber has a marked effect on the 
efficiency, consumption of chemicals, generation of undesirable substances 
as by-products, and on the achievable degree of reduction. 
The principal object of the present invention is to create the necessary 
conditions for the reduction of environmentally harmful substances 
resulting from the products of combustion that are present in existing 
incineration units without the need for major and costly conversions. 
The aforementioned object is achieved by the method and apparatus of the 
present invention in which combustion air, recirculated flue-gases, 
residual fuel, and injected reducing agents are all introduced into the 
incineration unit in conjunction with the OFA or ROFA combustion 
processes, through a plurality of asymmetrically positioned ducts. In so 
doing all, the reducing agents and gases introduced to the incinerator are 
rotated in the combustion zone. 
In accordance with the invention, an asymmetrical arrangement of delivery 
ducts along the length of the incineration unit is provided at 
successively increasing relative spacings along the combustion zone, 
thereby decreasing gas stratification by transforming parallel flow into 
rotational flow. 
The invention is based on the principle of introducing a proportion of the 
combustion air into the incinerator during the actual incineration process 
through asymmetrically positioned ducts placed at one or more "levels" of 
the hearth. The asymmetrical positioning of the ducts, in conjunction with 
their configured design and air velocity, induces rotation of the 
flue-gases over the entire cross-section of the hearth, thereby increasing 
the degree of rotation and turbulence directly upstream of the point of 
entry of the combustion gases into the convection section of the boiler. 
This provides for more complete combustion, allowing the opportunity for 
mixing in of recirculated flue-gases (0-100%). This further results in a 
better fuel mixture, better mixing and better rotation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In accordance with the present invention, one or more separate NO.sub.x 
reduction techniques are simultaneously used. A very good result is 
achieved in this way. In addition to the reduction of NO.sub.x, rotating 
over fire air in itself leads to more effective combustion, resulting in 
better efficiency. Introduction of chemicals, such as ammonia substances, 
via the rotating over fire air, flue-gas or similar methods, produce more 
effective mixtures, lower chemical consumption and a broader area of 
application, due to the temperature-reducing effect of the method through 
the increased absorption of heat in the hearth. 
The method in accordance with the present invention for bringing about the 
reduction of the oxides of nitrogen in an incineration unit, 1, 1.sup.1, 
1.sup.2, 1.sup.3, 1.sup.4 . . . 1.sup.N to which air 2 or some other gas 
is supplied, initially takes place by the introduction of reducing agents 
3 which exhibit a nitrogen reducing function into the combustion zone 4, 
4.sup.1, 4.sup.2, 4.sup.3, 4.sup.4 . . . 4.sup.N of the incineration unit 
in question. Reducing agents 3 for nitrogen may also be introduced at a 
later stage in conjunction with the introduction of air 2 or some other 
gas during the actual combustion process. It is also possible to introduce 
reducing agents 3 which bring about the reduction of sulphur or the 
reduction of some other substance into the combustion zone 4-4.sup.N or at 
a later stage. 
The reducing agent 3 is either introduced with the combustion air 2, the 
recirculated flue-gas 5, or with the residual fuel 6, so that the desired 
penetration and impulse for rotation and mixing are achieved. Preferably 
agents 3 are introduced with the air. 
The introduction of the reducing agent 3 is performed so that rotation is 
achieved in the combustion zone 4-4.sup.N, preferably by its introduction 
through asymmetrically located ducts on opposite sides 7-8 and 9-10 of the 
combustion zone 4-4.sup.N. The addition of asymmetrically located ducts on 
opposite sides of the combustion zone 4-4.sup.N causes the deflection of 
and turbulence in the mixture of the combustion air 2, recirculated flue 
gas 5, and residual fuel 6, resulting in effective mixing through 
rotation. 
The introduction of gas in the form of at least air 2 preferably takes 
place from one or more ducts 11. The ducts 11 are positioned in 
preferably, successively increasing relative spacings, which act at a 
successively greater distance A, B, C, along the combustion zone 
4-4.sup.N. The ducts 11 are preferably of a previously disclosed kind. The 
ducts 11 are positioned on essentially opposite sides 7-8; 9-10 of the 
incineration plant 1-1.sup.N. As shown in FIG. 8, in one embodiment, the 
ducts may be positioned on two, three or four walls of an incinerator 1 
having a circular cross-section. 
In order to utilize the invention to its fullest extent, a reducing agent 3 
is added in the form of a suitable chemical which brings about the desired 
reduction of the oxides of nitrogen, or a reduction in sulphur, or 
performs some other reduction function. The reducing agent 3 may consist 
of one of the following: ammonia (NH.sub.2), urea ((NH.sub.2).sub.2 CO), 
lime products, sodium carbonates and active carbon. The reducing agent may 
be supplied in one or more of the following forms: liquid, spray or 
powder. 
The quantity of the reducing agent 3 required to achieve the desired degree 
of reduction is metered and injected into preferably rotating over fire 
air and/or flue gas via the supply of supply air 2 and/or flue gas 5 
and/or the supply of residual fuel 6. 
An apparatus intended to permit the execution of the method of the present 
invention to bring about the reduction of, for example, oxides of nitrogen 
in an incineration unit 1-1.sup.N, into which air 2 or one or more 
suitable gases is/are introduced, comprises a plurality of ducts 11 
arranged along the length of the incineration unit when viewed along the 
successively increasing distance in the direction away from a combustion 
zone 4-4.sup.N of the kind in question. 
The ducts 11 are preferably arranged to act at mutually separate levels 12, 
13, 14, 15, 16, 17 on the mutually opposing walls 7-8; 9-10 as shown in 
FIG. 1 of an incineration unit 1-1.sup.N and/or are displaced laterally in 
pairs in relation to one another. 
Illustrative of appropriate incineration units for use with the invention 
are a soda recovery boiler in FIG. 1, an oil-gas-powder boiler with a 
top-mounted vertically acting burner in FIG. 2, a solid fuel boiler for 
wood fuels or household refuse in FIG. 3, an oil-gas-powder boiler with 
one or more side-mounted burners in FIG. 4, and a boiler with a 
circulating fluidized bed (CFB boiler) in FIG. 5. The present invention is 
suitable for use with other types and designs of boilers. 
If the reduction obtained by ROFA is insufficient to satisfy external 
requirements or to meet the desired degree of reduction, an appropriate 
level of ammonia (concentrated or diluted) or an ammonia-based 
preparation, such as urea or lime products, or sodium carbonates for 
sulphur reduction, or other suitable chemicals, can be introduced into the 
rotating over fire air in powder, spray or liquid form. 
The mixture of chemical substances is thus also permitted during and in 
conjunction with the actual combustion process. 
The necessary quantity of an ammonia substance, sodium carbonate or other 
suitable chemicals is/are introduced by a control system via the intended 
number of ducts. The process is suitable for all types of installations of 
the following types: grate-fired/solid fuel, liquid gas-fuelled via 
burners, fluidized bed, circulating fluidized bed, soda recovery, engines, 
gas turbines, and afterburning in cyclones, etc. 
The process is applicable to different types of fuels, including but not 
limited to heating oil, fuel oil, natural gas, household refuse, 
bio-fuels, powder fuels, and cellulose waste. The process is also 
applicable to the controlled supply and mixture of reburning fuel. 
The design of the ducts 11 are preferably circular or rectangular in form. 
The chemicals, etc., may be supplied in various ways. The chemicals, etc., 
may be supplied by being mixed into the air 2, flue-gas 5 or equivalent 
(e.g. steam) before they are introduced into the incineration plant 
1-1.sup.N. They may also be supplied by being introduced separately from 
the supply of air or gas to the plant 1-1.sup.N and not mixed with the air 
until it is inside the actual combustion area 4-4.sup.N. The chemicals may 
also be supplied as a combination of the two. 
Combustion air 2 or recirculated flue-gas 5 (carrier gas), a reducing agent 
3 and, where appropriate, residual fuel 6 are introduced for combustion 
via asymmetrically positioned air ducts 11 in the walls 7-10 of the 
boiler. The air ducts 11 are dimensioned with reference to, amongst other 
things, the cross-sectional area of the hearth, so that sufficient 
penetration and impulse for the desired mixture, mixing and rotation are 
achieved. 
The underlying principle of the flow is that air 2 or flue-gas 5 or 
residual fuel 6 are introduced into the hearth in the form of jets via 
ducts 11. The jets are deflected on the opposite side (in the case of 
rectangular section--90 degrees). This deflection is followed by strong 
turbulence resulting in thorough mixture. In addition to this mixture, 
rotation 18 (corkscrew movement) of the entire cross-section is obtained, 
resulting in higher unit volume velocity. The creation of turbulence is a 
physical consequence of the change in direction. 
FIGS. 6-8 depict the turbulence of the air 2 and the reducing agent 3 and 
their mixture in the combustion zone 4-4.sup.N. The recycled flue gas 5 
and the residual fuel 6 are subjected to the same mixture and mixing as 
the introduced air 2. 
An increased area of application, for example for NO.sub.x reduction, is 
thus now achieved with the help of ammonia or some other suitable 
substance due, amongst other things, to the lower flame temperature and 
the larger area at an appropriate temperature, i.e. through the 
displacement of the thermal balance towards the hearth. 
The mixture of NH.sub.3 and (NH.sub.2).sub.2 CO in the flame was not 
previously possible, and these substances were burned up to form more 
NO.sub.x. A lower flame temperature now permits the addition of NH.sub.3 
and (NH.sub.2).sub.2 CO in direct association with and at the final 
combustion stage through effective mixture, mixing and combustion. 
The invention is not restricted to what is described and illustrated above, 
but may be varied within the scope of the Patent Claims without departing 
from the idea of invention.