Furnaces – Process – Supplying fluid
Reexamination Certificate
2000-07-03
2001-08-28
Bennett, Henry (Department: 3749)
Furnaces
Process
Supplying fluid
C110S213000, C110S238000, C110S297000, C431S010000
Reexamination Certificate
active
06279495
ABSTRACT:
FIELD OF THE INVENTION
This invention is directed to a method and an apparatus for optimizing the combustion air system in power boilers. More particularly, the present invention pertains to a method and an apparatus for optimizing the combustion air system of a chemical recovery boiler in order to improve its efficiency.
The invention presented herein pertains to a method and an apparatus for optimizing the combustion air system in chemical recovery boilers found in pulp and paper mills employing a Kraff pulping process.
BACKGROUND OF THE INVENTION
In the pulp and paper industry, recovery boilers are used to burn spent liquor from the Kraft pulping process. The concentrated black liquor is burnt in the Kraft recovery boiler to regenerate sodium sulfide and sodium carbonate which is, in turn, converted to sodium hydroxide in a recausticizing plant. The produced white liquor, containing sodium sulfide and sodium hydroxide, is used in pulping wood. Organic matter that is dissolved in the pulping process is destroyed during combustion in the recovery boiler and the heat of combustion is recovered as steam. The steam is used to provide the mill's heat and energy requirements and/or to generate electricity for other uses.
The current practice for introducing combustion air into Kraft recovery boilers involves injection of the air at two or more elevations in the furnace cavity. Moving upward from the bottom or floor of the furnace, the air injection ports at succeeding elevations are referred to as primary, secondary, tertiary and quaternary air. All chemical recovery boilers have at least two levels of air injection. Newer boilers tend to have more levels. At each level, air is injected through port openings found on at least two opposing walls of the boiler. The port openings, which form the air jets, are usually rectangular. Dampers are typically provided to control the effective size of a port opening and the air pressure upstream of the port.
A need exists for a method and an apparatus for optimizing the combustion air system in chemical recovery boilers to improve boiler capacity and combustion uniformity. A need also exists for reducing particulate and gaseous emissions that are generated when the boiler is operating inefficiently.
Thus, an object of the present invention is to provide an optimized, combustion air supply system for a power or recovery boiler that can provide these improvements and eliminate or, substantially reduce operating problems. It is a further object of the present invention to provide an optimized combustion air system for older boiler installations, using existing combustion air ports to improve boiler efficiency and avoid costly boiler modifications. In another aspect of this invention, the optimized combustion air supply system would also be applicable to power boilers having two, three or more levels of combustion air, and can thus be applied in new and previously rebuilt recovery boilers. In yet another aspect, the optimized combustion air supply system would be especially applicable in a Kraft recovery boiler.
DESCRIPTION OF RELATED ART
Conventional combustion air systems in power boilers suffer from many deficiencies. Due to a lack of jet momentum, and because of interferences between colliding jet streams, as described by Adams et al in U.S. Pat. No. 5,305,698, and in other publications, conventional air systems invariably create a high velocity, upward flowing core somewhere in the furnace cross-section. This fast, upward flowing core is initiated at the primary air level and contains a high concentration of combustibles having a deficiency of oxygen for complete combustion. It can occupy a large percentage of the furnace cross-sectional area and can induce undesirable, recirculating flow patterns in the boiler as described in U.S. Pat. No. 5,305,698. This “chimney” causes an unnecessary carryover of liquor droplets and dry liquor particles, hindering gas mixing and delaying their combustion to higher levels in the furnace. Furthermore, portions of the char bed below the high velocity chimney are starved of liquor droplets and oxygen, thereby resulting in a generation of excessive, odorous, total reduced sulfur (TRS) gas, while the hotter regions of the bed generate excess sodium fume, contributing to plugging problems in the upper furnace.
Liquor droplets are sprayed from a higher elevation in the boiler, usually from at least one liquor nozzle or “gun” on each wall. These droplets dry and then begin to swell due to gas evolution from pyrolysis of the organic materials. As the dry liquor particles swell to a volume 20 to 30 times that of the original liquor droplet, their density decreases and they are readily entrained in this upward flowing core or “chimney” of air. In boilers with two levels of air entry below the liquor sprayers or guns, the introduction of secondary air from all four furnace walls is said to reinforce the detrimental updraft core phenomenon or “chimney effect”, according to the teachings of U.S. Pat. Nos. 5,121,700; and 5,305,698.
Blackwell et al., U.S. Pat. No. 5,305,698 have shown that air introduced above the liquor level to break the strong upward flow of gas has little influence on the recirculating flow patterns and can seldom break the upward flowing gas core.
One of the major operational problems in Kraft recovery boilers is the formation of fireside deposits on the pendent heat transfer surfaces in the upper part of the boiler. The most troublesome deposits occur in the superheaters and the first part of the boiler bank. These deposits are formed mainly by particles that originate from the entrainment of some of the liquor spray particles in the upward flowing gases. The mass of a particle that can be entrained in a flowing gas varies proportionally to the sixth power of the gas velocity. From a conceptual point of view, it is, therefore, very important to minimize gas velocity extremes.
Deposit growth on the pendent heat transfer banks in the upper furnace is controlled using sootblowers. The sootblower lance tube (3 to 6inches in diameter) remains outside the boiler when not in service and is automatically inserted into the boiler and traversed across the boiler while being rotated. Sootblower length is usually about half the furnace width with sootblowers on opposite walls thus providing full width coverage between convection section heat transfer banks. High pressure, superheated steam is generally used as the blowing medium. Our studies have shown that sootblowers are completely ineffective in removing deposits caused by mechanical carryover of liquor particles. The best way to reduce carryover and furnace plugging is to reduce peak gas velocities in the lower boiler and the size of the upward flowing gas core.
A number of modifications to the Kraft recovery boiler combustion air system have been proposed and patented in attempts to overcome many of the deficiencies outlined above. Simonen, U.S. Pat. No. 5,022,331, proposed a secondary air system in which the hydraulic diameters of air ports in each of the four furnace walls increased from the corners of the furnace to the centers of each furnace wall. With this design, the degree of penetration of the respective air jets is said to increase as one moved from the corner to the center of each wall and the penetration of the air jets would be maintained constant under different loading conditions. Such an air system is very difficult and expensive to fabricate even when the larger ports are formed from the close grouping of smaller ports. Consequently, this proposal has never, to our knowledge, been implemented on a full-scale Kraft recovery boiler.
Svensk, U.S. Pat. No. 5,450,803, proposed a secondary air system in which the air was supplied to the furnace in such a way that the gas was forced to rotate in a plane substantially perpendicular to the longitudinal axis of the furnace. The so-called Rotafire™ air system counter-acts the so-called “chimney effect”, but throws injected liquor outward against the walls of the boiler. Liquor can thus be depo
Karidio Ibrahim
Uloth Victor
Weiss Klaus
Bennett Henry
Pulp and Paper Research Institute of Canada
Rinehart K. B.
Swabey Ogilvy Renault
LandOfFree
Method and apparatus for optimizing the combustion air... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for optimizing the combustion air..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for optimizing the combustion air... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2544233