Furnaces – Process – Treating fuel constituent or combustion product
Reexamination Certificate
2001-12-06
2004-10-05
Ciric, Ljiljana (Department: 3753)
Furnaces
Process
Treating fuel constituent or combustion product
C110S188000, C110S238000, C110S214000, C431S010000, C162S030100
Reexamination Certificate
active
06799526
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of combustion of kraft black liquor and to chemical recovery in kraft recovery boilers.
RELATED ART
The kraft black liquor recovery boiler is a critical component in the production of paper pulp. Two functions are associated to the recovery boiler: the combustion of the organic materials contained in the black liquor for the production of heat and steam, and the conversion of the inorganic chemicals of the black liquor into a smelt which consists mainly of sodium carbonate (Na
2
CO
3
), sodium sulfide (Na
2
S), and a small amount of sodium sulfate (Na
2
SO
4
). In further steps of the pulping process, the smelt is converted into cooking liquor, the chemical used to transform wood chips into pulp. This transformation in turn produces black liquor that must be disposed of and recycled in the recovery boiler. One of the most important functions of the recovery boiler is to convert the sodium and sulfur content of the black liquor into sodium sulfide. The efficiency of this conversion is expressed as the reduction efficiency, defined as the ratio of sodium sulfide (Na
2
S) in the smelt to the total of sodium sulfide and sodium sulfate (Na
2
SO
4
) in the smelt. Operation with the highest reduction efficiency is desirable.
Because recovery of the chemicals contained in the black liquor is so important in the pulping process, the recovery boiler is often the bottleneck in increasing the pulping capacity of a mill. Insufficient capacity to burn and recover chemicals from black liquor yields a deficiency in cooking liquor, that can force mills to slow down production, and/or ship black liquor to other mills that have excess recovery capacity, and/or to buy make up chemicals to compensate for the lack of cooking liquor. All of these activities are detrimental to the cost of the pulp produced in the mill.
Adding additional recovery boiler capacity can be obtained by installing a new recovery boiler, or another means of burning black liquor, such as a gasifier, or a fluidized bed. These solutions are avoided when possible, because they are associated with high capital costs, and represent an additional unit to operate and maintain. Preferred solutions are solutions that retrofit an existing recovery boiler and add black liquor combustion capacity without excessive capital costs or time required to install.
In recovery boilers, black liquor combustion occurs by in-flight burning, and char bed burning. During in-flight burning, the water from the black liquor droplets is evaporated—drying stage—, organic materials are volatilized and burn—volatilization stage—, and the solid residue known as char, burns—char burning stage—. The unburned residue falls onto the char bed located at the bottom of the furnace, where combustion and chemical recovery are completed. To sustain combustion, air is injected at various heights in the furnace: primary air is the lowest level of air injection, situated at the lower level of the char bed; some boilers have a secondary air level is positioned above the top of the char bed. Both the primary and the secondary air levels are located below the level of the liquor guns used to spray coarsely atomized black liquor into the furnace. Modern recovery boilers have a third level of air injection which is located above the level of the liquor guns (tertiary air). Some very large boilers have a fourth air injection level (quaternary air) above the liquor gun elevation.
One important limitation to the increase of throughput of a recovery boiler is fouling of the convection heating surfaces. When attempting to increase the black liquor feedrate, the quantity of combustion air required must be increased, which results in an increased volume of combustion products, increased vertical gas velocity, and additional entrainment of liquor, char, and/or smelt particles (carryover) would occur. This problem is worsened when the combustion chamber height is relatively small. Fouling of the upper sections of the boiler by the carryover material can result in the plugging of the flue gas passages, and will eventually cause a boiler shut down. Boiler shut downs and start-ups are complicated operations that should be avoided as much as possible. In addition, during boiler shut down, a temporary solution for black liquor disposal and cooking chemical make up must be found, which is often a cost penalty.
Another difficulty of increasing boiler throughput is the need to supply additional combustion air and handle more flue gas. Fan capacity limitations can be found, either on the combustion air supply side, the exhaust side, or both, that will prevent combustion of additional black liquor in an existing furnace.
Solutions to the previous limitations can be found by minimizing the excess air (excess air is the volume of air over and above the volume of air needed to complete combustion) for combustion. However, lowering the excess air may result in increased pollutant emissions, such as carbon monoxide (CO) and sulfur compounds (TRS). In older boilers with two levels of air, where the tangential injection of combustion air does not provide good penetration of the air into the center of the furnace, adding a third level is a convenient solution to reduce the excess combustion air because three level air injection gives a more efficient distribution and mixing of the air in the different areas of the boiler. Two level air furnaces that are overloaded have also shown not to be as efficient at completely oxidizing CO and sulfur compounds and thus yield higher pollutant emissions. For this reason, and the obligation of operating with a large excess air to compensate for the poor mixing of the combustion air with the combustibles, the boilers with two levels of air are progressively retrofitted by more efficient three level air systems. On three level air systems, retrofitting the boiler with high velocity air nozzles such as described in US Pat. No. 4,940,004 in the secondary and tertiary levels of the furnace can improve mixing and allow operation with even less excess air.
Reducing the excess air has also a positive effect on the thermal efficiency of the boiler, defined as the ratio of energy in the steam produced in the boiler over the amount of energy in the black liquor.
Oxygen injection has been proposed to further reduce the air and the flue gas volumes in recovery boilers. In a boiler operated with an oxidant which oxygen concentration exceeds that of the air, more black liquor can be burned with a constant volume of air and flue gas. For example, in a review article entitled “Increasing Recovery Boiler Throughput” by T. M. Grace, published in the November 1984 issue of Tappi Journal, given as a reference, the author cites oxygen enrichment as a means of reducing the volume of air and flue gas for a given heat release. In U.S. Pat. No. 4,823,710, an oxygen injection method is described where combustion is improved by introducing an oxygen containing gas, preferably with an oxygen concentration higher than air, from at least one location remote from the boiler sidewalls. U.S. Pat. No. 4,857,282 discloses a method to use oxygen enrichment in the primary and secondary air system of the boiler. However, the '282 patent does not disclose or suggest oxygen enriched air injection at the secondary or tertiary air levels without injection at the primary air level.
SUMMARY OF THE INVENTION
In accordance with the present invention, methods are presented to increase the throughput of recovery boilers equipped with at least two levels of injection of air without increasing the carryover of inorganic materials in the recovery boiler in order to prevent plugging of the convective sections of the boiler. Another advantage of the methods of the invention is lowering the emissions of gaseous pollutants from the recovery boiler. Another advantage of the methods of the invention is to improve the furnace control and stability of operation, and to eliminate or reduce the need for an auxiliary fuel to sustain the combustion of low hea
Duchateau Eric L.
Dye Edward C.
Philippe Louis C.
Scheeff David R.
Verloop Arie
American Air Liquide Inc.
Ciric Ljiljana
Haynes Elwood L.
Russell Linda K.
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