Recovery boiler

Details Recovery boiler Recovery boiler

1997-11-06

2001-02-13

Lazarus, Ira S. (Department: 3743)

Furnaces

Refuse incinerator

For liquid refuse

C110S206000, C110S214000, C110S297000, C110S234000, C431S010000, C162S031000

Reexamination Certificate

active

06186080

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a recovery boiler for recovering a soda component etc. from pulp spent liquor etc.
2. Description of the Prior Art
In a pulp spent liquor generated in a paper making process, there are contained, in a large quantity, a portion of organic matter of wood materials and a soda component added in the process of cooking. The spent liquor (hereinafter referred to as “black liquor”) is once condensed and then burnt in a recovery boiler comprising a furnace. A main object is to recover the soda component so that the soda component in the black liquor is recovered as a sodium carbonate and a sodium sulfide in a molten state.
FIG. 8
is a schematic view showing a recovery boiler in the prior art. A black liquor
42
a
is ejected from a plurality of black liquor burners
42
into a recovery boiler
41
. On the other hand, a combustion air
40
is fed thereinto from a primary air nozzle
45
a,
a secondary air nozzle
45
b
and a tertiary air nozzle
45
c
via a fan
43
and air dampers
44
a,
44
b
and
44
c.
The black liquor
42
a
is burned on a char bed
46
formed at a lower portion of the boiler.
In the recovery boiler
41
, the black liquor
42
a,
containing comparatively coarse particle sizes, is ejected from the black liquor burners
42
to a mid portion of the furnace, as shown by dotted line arrows in FIG.
8
. The liquor
42
a
falls while it is being dried by a furnace combustion gas and is then accumulated on a furnace floor so as to form the char bed
46
to be burned.
With the enhancement of evaporator performance to condense the water content from spent liquor, it is a recent tendency that a solid concentration in the black liquor
42
a,
which had so far been 60%, has been increased to 80%. The result is that boiler combustion efficiency has been increased, and also the concentration of the black liquor itself is enhanced.
Also, a sufficient quantity of the primary air, the secondary air and the tertiary air is being fed to the surroundings of the char bed
46
and the degree of combustion of the black liquor
42
a
on the char bed
46
is enhanced, which results in a combustion state causing a sharp rise of nitrogen oxides (hereinafter referred to as “NO
x
”), an object of pollution control regulations. Thus in order to operate the recovery boiler in accordance with pollution control regulations, it is indispensable to reduce the quantity of NO
x
discharged from the furnace outlet.
One method for reducing the NO
x
considered is to generate a combustion zone of a reduction atmospheric field in which the air ratio in the surroundings of the char bed is 0.8 or less and to feed additional air from an upper portion of the furnace. In this case, however, a quantity of all or any of the primary air, the secondary air and the tertiary air is necessarily reduced, the flow velocity of air fed into the furnace is lowered, and the air quantity distribution in the furnace becomes irregular, so that there occurs a non-uniform combustion, a carry-over of unburnt char, a deformation of the char bed, etc. This makes the holding of stable combustion difficult.
SUMMARY OF THE INVENTION
In view of the problems in the prior art, it is an object of the present invention to provide a recovery boiler which is able to effect a NO
x
reduction securely.
In order to attain the object, the present invention provides a recovery boiler comprising a burner for ejecting a black liquor into a furnace and a combustion air supply system, wherein the combustion air supply system consists of a main air supply for feeding air so as to form a reduction atmospheric field where an air ratio in the surroundings of a char bed formed on a furnace bottom is 0.8 or less. A first additional air nozzle is disposed downstream of the main air supply for feeding air so as to form a reduction atmospheric field where an air ratio is 1.0 or less and unburnt components exist. A second additional air nozzle is disposed downstream of the first additional air nozzle for feeding a shortage of air so as to form a combustion zone where combustion completes. The recovery boiler further comprises a means for feeding a recirculated gas or an inert gas together with a combustion air and/or along a furnace side wall around the char bed. There are thereby sequentially formed a combustion zone of the reduction atmospheric field of an air ratio of 0.8 or less formed by the main air supply, a combustion zone of the reduction atmospheric field of an air ratio of 1.0 or less formed by the first additional air nozzle and a combustion zone for completing the combustion formed by the second additional air nozzle. A combustion with a reduced quantity of NO
x
generation is thereby attained.
Moreover, a recirculated gas or an inert gas is fed along the furnace side wall around the char bed so as to form a pneumatic curtain. The furnace side wall including side wall pipings is thereby prevented from being corroded by sulfide generated from a sulfur component in the black liquor at the combustion zone of the reduction atmospheric field.
Also, the present invention provides a recovery boiler comprising a burner for ejecting a black liquor into a furnace and a combustion air supply system, wherein the combustion air supply system consists of a main air supply for feeding air so as to form an air ratio in the surroundings of a char bed formed on a furnace bottom of 0.8 or less and an additional air nozzle disposed downstream of the main air supply for feeding a shortage of air. The main air supply consists of a primary air nozzle for feeding air toward between the char bed and the furnace bottom, a secondary air nozzle for feeding air toward an inclined side face of the char bed and a tertiary air nozzle for feeding air downwardly toward a furnace side from an upper portion of the char bed and directed in a direction generating a swirling force from a furnace side wall or a furnace corner. The char bed is thereby prevented from coming nearer to the furnace side wall by the primary air nozzle so that the char bed configuration becomes stabilized. The air distribution in the furnace is homogenized by the secondary air nozzle and the unburnt char which is liable to be carried over is suppressed so as not to be carried over to the furnace upper portion. A stable combustion is thus attained.
Also, the present invention provides a recovery boiler as mentioned immediately above, wherein the primary air nozzle feeds air at an air flow velocity of 30 m/s or more and the secondary air nozzle at an air flow velocity of 50 m/s or more, each with an air quantity of 40% or less of an entire combustion air, and the tertiary air nozzle feeds a shortage of air. The stabilization of the char bed configuration, homogenization of the air distribution in the furnace and carry-over of the unburnt char are thereby attained further accurately and securely and a stable combustion is further accelerated.
Also, the present invention provides a recovery boiler comprising a burner for ejecting a black liquor into a furnace and a combustion air supply system, wherein the combustion air supply system consists of a main air supply for feeding air so as to form a reduction atmospheric field where an air ratio in the surroundings of a char bed formed on a furnace bottom is 0.8 or less. A first additional air nozzle is disposed downstream of the main air nozzle for feeding air so as to form a reduction atmospheric field where an air ratio is 1.0 or less and unburnt components exist. A second additional air nozzle is disposed downstream of the first additional air nozzle for feeding a shortage of air so as to form a combustion zone where combustion completes. The combustion air supplied from the main air supply thereby forms the reduction atmospheric field of an air ratio of 0.8 or less, and even if the additional combustion air is added downstream thereof from the first additional air nozzle, the reduction atmospheric field is maintained with its air ratio of 1.0 or less, and then the shortage

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