Deposit control for a black liquor recovery boiler

Abstract

Disclosed is a process for reducing slag in a black liquor recovery boiler, the process comprising: injecting and burning black liquor in a boiler by contacting it with primary air and secondary air; introducing a slag-reducing chemical into the gases above the injection locations through interlaced, tangential or concentric secondary, tertiary, and/or quarternary air ports.

Claims

1. A process for reducing deposits in a black liquor recovery boiler, the process comprising: injecting and burning black liquor in a boiler by injecting it into the boiler and into contact with primary air and secondary air before collecting on a char bed in the boiler near the bottom; introducing sprays of deposit-reducing chemicals as droplets from a nozzle positioned within a duct into the gases above the injection locations for the black liquor through interlaced, tangential or concentric secondary, tertiary, and/or quarternary air ports to increase the momentum of the droplets, and adjusting the flow from at least half of the ducts to achieve plug flow and thereby help shield the injected chemical from dispersion before sufficient penetration and permits the air to carry the chemical from 60 to 95% of a distance across the boiler from the point of introduction of the sprays, whereby the flow from at least half of the ducts within the air ports is plug flow.

2. A process according to claim 1 wherein the deposit-reducing chemicals are injected into the gases above the injection locations for the black liquor through interlaced secondary air ports.

3. A process according to claim 2 wherein the secondary air comprises from 30 to 50 percent of the air supplied for combustion.

4. A process according to claim 1 wherein the deposit-reducing chemicals are injected into the gases above the injection locations for the black liquor through interlaced tertiary air ports.

5. A process according to claim 4 wherein the tertiary air comprises from 0 to 50 percent of the air supplied for combustion.

6. A process according to claim 1 wherein the deposit-reducing chemicals are injected into the gases above the injection locations for the black liquor through interlaced quaternary air ports.

7. A process according to claim 2 wherein the quaternary air comprises from 20 to 50 percent of the air supplied for combustion.

8. A process according to claim 1 wherein the deposit-reducing chemicals includes a member selected from the group consisting of magnesium oxide, magnesium hydroxide, magnesium carbonate, manganese oxide, manganese hydroxide aluminum oxide and aluminum hydroxide.

9. A process according to claim 1 wherein there are from 4 to 8 secondary, tertiary or quaternary ducts of an approximate dimension of from 4″ by 4″, to 18″ by 18″, and a horizontal length of from 2 feet to 12 feet.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood and its advantages will become more apparent when the following detailed description is read in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is a schematic view of one embodiment showing a black liquor recovery boiler with interlaced tertiary and quaternary air ports through which deposit-reducing chemical is introduced.

(3) FIG. 2 is a schematic view of one arrangement of injectors for deposit-reducing chemicals in an air port according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(4) Reference will first be made to FIG. 1, which is a schematic view of one embodiment showing a black liquor recovery boiler with interlaced tertiary and quaternary air ports through which deposit-reducing chemical can be introduced. FIG. 1 shows a black liquor recovery boiler 10 having primary air ports 12, secondary air ports 14, tertiary air ports 16 and quaternary air ports 18. The boiler has four vertical walls 19, and the air ports are positioned on opposite vertical walls. Black liquor is heated until flowable and introduced into the combustion chamber 20 through nozzles/liquor guns 21 positioned above secondary air ports 14 but below the tertiary air ports 16. Importantly, the secondary and tertiary air ports 16 are arranged in what is known as interlaced fashion, with each port on one vertical wall being laterally offset from ports on the opposing vertical wall such that the air from each port is able to move the maximum distance across the boiler without direct impingement by air from the other side. In other arrangements, the air ports can be in tangential or concentric configurations to promote mixing of the chemicals with combustion gases after introduction. This embodiment conveys air in high volume to the air ports by means of a manifold 22 to carry sprays of deposit-reducing chemicals/additives from nozzles 30 (best seen in FIG. 2) positioned in the tertiary air ports and direct them across the cross section of the boiler to achieve complete mixing of the deposit-reducing chemicals in a section above nozzles used to introduce the black liquor and primary combustion air. The momentum of the spray droplets is greatly increased from direct injection with the tertiary air flow.

(5) In similar arrangements, deposit-reducing chemical can be introduced through the secondary air ports 14 or quaternary air ports 18. In each case described, a similar effect is achieved using the air flow from the noted ports as a driver to provide excellent mixing and distribution within the combustion chamber 20 in advance of the bull nose 15.

(6) FIG. 2 is schematic view of one embodiment of the invention wherein a two-phase deposit-reducing chemical nozzle 30 is shown positioned in a tertiary air port 16. The injectors are preferably two phase injectors and utilize air supplied via line 32 to atomize an aqueous slurry of the slag-reducing chemical supplied via line 34. Other nozzle arrangements that permit a high degree of independent penetration and mixing with the hot combustion gases in the combustion chamber 20 can be utilized also. Shown is a nozzle 30 positioned in a rectangular duct 36, which is positioned in each port 16, preferably centrally and set close to flush with the exit of the duct. For a boiler designed to burn 1,000-2,000 tons per day of black liquor, there will typically be from 4 to 8 tertiary ducts of an approximate dimension of from 4″ by 4″, to 18″ by 18″, and a horizontal length of from 2 feet to 12 feet. They are preferably spaced laterally along the furnace wall to achieve good air distribution. In embodiments, the flow from at least half of the ducts 32 will be plug flow, which will help shield the injected chemical from too-rapid dispersion before sufficient penetration and permits the air to carry the chemical well into the boiler. To achieve plug flow of air from duct 36 it will be understood that the flow rates can be adjusted so that the hydraulic diameter does not exceed a calculated mean velocity. And, during the design phase, the hydraulic diameter of the velocity of air from the duct must be sufficient to project the deposit reducing chemical from the nozzles from 60 to 95%, e.g., at from 60 to 90%, of the distance across the furnace from the point of injection.

(7) The spray of black liquor from each of nozzles 21 positioned below the tertiary air ports enters the combustion chamber 20 of the boiler 10 at the correct temperature and droplet size to permit best utilization. Typical temperatures of the black liquor will be from 300° F. to 400° F., and droplets will be in the range of from 0.5 mm to 5 mm following impingement onto the splash plates of the injectors. The sprays from the injectors penetrate the vertical boiler walls above a char bed 36 and desirably above primary and secondary air ports, 12 and 14, respectively. The injectors typically spray the black liquor from opposite walls with droplet velocity and momentum sufficient that a majority reach beyond the midpoint of boiler but none reach the opposite wall by the time the droplets fall to the char bed 36.

(8) The elevated temperatures in the combustion chamber 12, cause some volatiles to be removed in the fall of the black liquor to the char bed 36 and some carbonization is effected, but the main burning of the black liquor occurs under reducing conditions in the char pile. Primary air is introduced at the approximate elevation of the char pile and supplies about 40 percent or less of the stoichiometric oxygen. Secondary air is introduced below the black liquor guns and adds another 30 to 50 percent of the needed air. Above the black liquor injectors are ports for additional air (e.g., from 30 to 50 percent), typically tertiary air and sometimes quaternary air. These additional air ports are essential to supply sufficient air to obtain maximum combustion without unduly cooling combustion gases which are required to heat steam to produce utilizable energy. Quaternary air may comprise 20 to 50 percent of the needed air.

(9) The combustion gases rising through the boiler contain ash formers (carryover and fume) and unburned char which are desirably recovered as solids in an electrostatic precipitator or other solids recovery equipment, e.g., generally shown as 48. Unfortunately, the ash from burning the black liquor will often contain components that maintain it as an adhesive mass until in passes beyond a bull nose 15 at the top of the combustion chamber 20 and into contact with an array of heat exchangers 40, such as those that form the screen tubes, super heater 42, the boiler bank 44 (or reheat) and the economizer 46 prior to exiting the combustor via stack 50. Deposit control chemicals and processes are known, but it is always a challenge to introduce them in a manner effective for treatment of deposits in black liquor recovery boilers. This problem has existed since the first such boilers were made and there have been only a few successes, and none which have universal effectiveness.

(10) The art has endeavored to solve the slagging problem by the introduction of various chemicals, such as magnesium oxide or hydroxide. Magnesium hydroxide has the ability to survive the hot environment of the furnace and react with the deposit-forming compounds, raising their ash fusion temperature and thereby modifying the texture and friability of the resulting deposits.

(11) While all effective deposit-reducing chemicals are included, such as, without limitation magnesium oxide, magnesium hydroxide, magnesium carbonate, manganese oxide, manganese hydroxide, aluminum oxide and aluminum hydroxide, magnesium hydroxide is the chemical of choice for many black liquor recovery boilers and will be used in this description as exemplary. The magnesium hydroxide reagent can be prepared in any effective manner, e.g., from brines containing calcium and other salts, usually from underground brine pools or seawater. Dolomitic lime is mixed with these brines to form calcium chloride solution, and magnesium hydroxide which is precipitated and filtered out of the solution. This form of magnesium hydroxide can be mixed with water, with or without stabilizers, to concentrations suitable for storage and handling, e.g., from 25 to 65% solids by weight. For use in the process, it is diluted as determined by computational fluid dynamics (CFD) to within the range of from 0.1 to 10%, more narrowly from 1 to 5%. When it contacts the hot gases in the combustor, it is believed reduced to submicron and/or nano-sized particles, e.g., under 200 nanometers and preferably below about 100 nanometers. Median particle sizes of from 50 to about 150 nanometers are useful ranges for the process of the invention. Other forms of MgO can also be employed where necessary or desired, e.g., “light burn” or “caustic” can be employed where it is available in the desired particle size range.

(12) To best achieve these effects, the invention will preferably take advantage of CFD to project initial flow rates and select initial reagent introduction rates, reagent introduction location(s), reagent concentration, reagent droplet size and reagent droplet momentum. CFD is a well understood science, and it is utilized with full benefit in this case, where it is desired to supply a minimum amount of chemical for maximum effect.

(13) The following examples are presented to further explain and illustrate the invention and are not to be taken as limiting in any regard. Unless otherwise indicated, all parts and percentages are by weight.

EXAMPLE 1

(14) This example illustrates the effect of introducing Mg(OH).sub.2 (magnesium hydroxide) into a furnace burning 2,000 tons of black liquor per day.

(15) The magnesium hydroxide was fed as a slurry at 2 pounds of 60 weight % slurry per ton of black liquor consumed. Density of the magnesium hydroxide slurry was approximately 12.7 pounds/gallon. Therefore, the feed rate was about 315 gallons per day for the Mg(OH).sub.2 slurry.

(16) We have seen that the invention provides at least the following advantages: (1) tertiary air protects the nozzles used to introduce the slag-reducing chemicals from the temperatures that exist in the area above the main combustion in the lower part of the furnace, (2) extremely good mixing is achieved and (3) high utilization of deposit-reducing chemicals is achieved due to the good mixing and the ability of deposit-reducing chemical to mix with slag formers by reaching the bull nose of the boiler in the zone just preceding the heat exchangers.

(17) The above description is for the purpose of teaching the person of ordinary skill in the art how to practice the invention. It is not intended to detail all of those obvious modifications and variations, which will become apparent to the skilled worker upon reading the description. It is intended, however, that all such obvious modifications and variations be included within the scope of the invention which is defined by the following claims. The claims are meant to cover the claimed components and steps in any sequence that is effective to meet the objectives there intended, unless the context specifically indicates the contrary.