WO2014077979A1

WO2014077979A1 - Activated carbon from boiler ash residue

Info

Publication number: WO2014077979A1
Application number: PCT/US2013/063988
Authority: WO
Inventors: Yinzhi Zhang; Seyed B. Ghorishi
Original assignee: Albemarle Corporation
Priority date: 2012-11-13
Filing date: 2013-10-09
Publication date: 2014-05-22

Abstract

This invention provides methods for selecting and preparing boiler ash residue from wood waste boilers of pulp and paper mills for mercury adsorption. The selection and preparation are such that the boiler ash residue has a carbon content of about 70% or more, and a BET surface area of about 300 m²/g or more or a butane adsorption of about 130 mg/g or more. When the boiler ash residue has a BET surface area of less than about 300 m²/g or a butane adsorption of less than about 130 mg/g, the boiler ash residue is subjected to activation with steam or carbon dioxide, to increase the BET surface area to about 300 m²/g or more or the butane adsorption to about 130 mg/g or more. Also provided are methods for brominating boiler ash residue, and methods for reducing mercury emissions, which methods employ brominated boiler ash residue.

Description

ACTIVATED CARBON FROM BOILER ASH RESIDUE

TECHNICAL FIELD

[0001] This invention relates to preparation of activated carbon from boiler ash residue, bromination of the activated carbon so produced, and to reduction of mercury emissions from combustion gas streams.

BACKGROUND

[0002] About 4 million tons of boiler ash residue (BAR) are generated annually from pulp and paper mills in the United States. Of the total ash generated, 35% is used in beneficial applications, about 10% in land applications and about 25% in other beneficial applications; the remainder of the ash (65%) is disposed of as waste. Soil amendment, road construction, and fertilizer are among the beneficial uses of wood fly ash. Another potential application of the wood fly ash is as an adsorbent for various air and/or water pollutants and odorants due to its high unburned carbon content, which ranges from 10% to 90%.

[0003] Activated carbon injection has been used to control mercury emission in coal combustion flue gas. A majority of activated carbons used for mercury emission control are coal based, including anthracite, bituminous, lignite. There are also wood-based and coconut shell-based activated carbons. Production of activated carbon is costly. Activated carbon production processes include several heat-treatment steps that are both energy intensive and time consuming.

[0004] Brominated activated carbons, formed by treating activated carbon with either a bromide salt solution or B¾ gas, are known for mercury removal, and also capture and hold mercury. Low levels of bromination have been observed to increase the mercury- removal performance of activated carbon sorbents; see in this regard U.S. Pat. No. 6,953,494.

SUMMARY OF THE INVENTION

[0005] This invention provides a low-cost activated carbon from boiler ash residue from wood waste boilers of pulp and paper mills. Advantageously, the material that forms the boiler ash residue from waste wood boilers of pulp and paper mills has undergone a heat treatment process in the boiler to decrease waste volume at the pulp or paper mill, and to produce energy for the pulp or paper mill. As a result of the heat treatment process, the boiler ash residue contains considerable amounts of carbon with a porous structure. In addition, boiler ash residue from waste wood boilers of pulp and paper mills has a low volatile content.

[0006] This invention also provides methods for preparing sorbents which can be used to reduce mercury emissions from combustion gas streams. Since the sorbents of this invention are particulates, they can be removed from the combustion gas stream by the same mechanism employed to remove other particulates present in the combustion gas stream.

[0007] An embodiment of this invention is a method for selecting and preparing boiler ash residue from wood waste boilers of pulp and paper mills for mercury adsorption. The selection and preparation are such that the boiler ash residue has a carbon content of about 70% or more, and a Brunauer-Emmett- Teller (BET) surface area of about 300 m²/g or more or a butane adsorption of about 130 mg/g or more, as determined by the butane adsorption method described below. When the boiler ash residue has a BET surface area of less than about 300 m²/g or a butane adsorption of less than about 130 mg/g, the boiler ash residue is subjected to activation with steam or carbon dioxide, to increase the BET surface area to about 300 m²/g or more or to increase the butane adsorption to about 130 mg/g or more.

[0008] Another embodiment of this invention is a method of preparing a brominated sorbent. The method comprises contacting boiler ash residue from pulp and paper mills and a bromine source which is (i) elemental bromine and/or hydrogen bromide or (ii) a solid bromine source.

[0009] Other embodiments of this invention include methods of reducing mercury emissions, which methods employ the brominated sorbents just described.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Fig. 1 shows a comparison of the mercury removal performance of Darco^® Hg- LH and an inventive sample in Example 1.

[0011] Fig. 2 shows a comparison of the mercury removal performance of Darco^® Hg- LH and an inventive sample in Example 2.

[0012] Fig. 3 shows a comparison of the mercury removal performance of Darco^® Hg- LH and an inventive sample in Example 3. [0013] These and other embodiments and features of this invention will be still further apparent from the ensuing description, drawings, and appended claims.

FURTHER DETAILED DESCRIPTION OF THE INVENTION

[0014] As used throughout this document, the term "boiler ash residue", sometimes abbreviated BAR, refers to the ash residue produced in wood waste boilers of pulp and paper mills. The terms "brominated sorbent" and "brominated sorbents" as used throughout this document refer to the brominated boiler ash residue sorbents of this invention, unless otherwise noted.

[0015] Throughout this document, the term "particulates" refers to small particles (generally about 45 μιη or less in diameter) suspended in the gas stream. The term "gas stream", as used throughout this document, refers to a quantity of gas that is moving in a direction. As used throughout this document, the phrase "combustion gas" refers to the gas (mixture) resulting from combustion. Flue gas is a type of combustion gas. In this connection, the term "stream" as used in "combustion gas stream" refers to a quantity of combustion gas that is moving in a direction. The terms "brominated sorbent" and "brominated sorbents" as used throughout this document refer to brominated sorbents formed from boiler ash residue, unless otherwise noted.

[0016] As mentioned above, one of the methods provided by this invention is a method for selecting and preparing boiler ash residue from wood waste boilers of pulp and paper mills for mercury adsorption; the boiler ash residue selected and prepared has a carbon content of about 70% or more, and a BET surface area of about 300 m²/g or more or a butane adsorption of about 130 mg/g or more. When the boiler ash residue has a BET surface area of less than about 300 m²/g or a butane adsorption of less than about 130 mg/g, the boiler ash residue is subjected to activation with steam or carbon dioxide, to increase the BET surface area to about 300 m²/g or more or the butane adsorption to about 130 mg/g or more.

[0017] Selection of boiler ash residue having a carbon content of about 70% or more is accomplished by analyzing the boiler ash residue, usually with Standard Test Method for Total Ash Content of Activated Carbon (ASTM D2866), and using only the boiler ash residue that has a carbon content of about 70% or more. Carbon contents that can be used in this invention range from about 70% to about 99%; a preferred carbon content is about 80% or more, and a preferred carbon content range is about 80% to about 95%. A more preferred carbon content range is about 85% to about 95%.

[0018] Activation steps increase the surface area and the butane adsorption. Generally, the activation is with steam or carbon dioxide. Steam activation is preferred.

[0019] Preferably, the surface area of the boiler ash residue is about 400 m²/g or more, and more preferably about 500 m²/g or more. For butane adsorption, a preferred butane adsorption for the boiler ash residue is about 140 mg/g or more, and more preferably about 150 mg/g or more.

[0020] The brominated sorbents of this invention are formed by treating boiler ash residue with an effective amount of a bromine source for a sufficient time to increase the ability of the sorbent to absorb mercury and/or mercury-containing compounds. To form a brominated sorbent, boiler ash residue and the bromine source are contacted, forming a brominated sorbent. The bromine source is (i) elemental bromine and/or hydrogen bromide, which are normally gaseous or in aqueous solution, or (ii) a solid bromine source. Elemental bromine is a preferred bromine source. Contacting boiler ash residue and a bromine source significantly increases the ability of the brominated sorbent formed thereby to absorb mercury and mercury-containing compounds.

[0021] Although the size of the particles during bromination is not critical, preferred average particle sizes for the boiler ash residue are in the range of about 1 μιη to about 50 μιη, more preferably in the range of about 3 μιη to about 20 μιη. If the particles are larger than desired, their size can be reduced by conventional methods, such as grinding or milling. When the bromine source is a solid bromine source, the reduction in particle size of the substrate can occur in the presence of the bromine source.

[0022] For the bromination, if the boiler ash residue begins at ambient temperature, preferably it is preheated, usually to a temperature of above about 100°C. One purpose of such preheating is to drive off any physically-adsorbed moisture from the boiler ash residue which blocks the pores. The boiler ash residue can be used without drying, if desired. Preferably, such heating is performed before bromination.

[0023] Elemental bromine (Br₂) and/or hydrogen bromide (HBr) are normally used in gaseous form or in the form of aqueous solutions. For the aqueous solutions, concentrations of the bromine source are generally about 0.1 wt% or more, usually in the range of about 0.1 wt% to about 10 wt%, and preferably in the range of about 0.5 wt% to about 5 wt%. Preferably, the bromine source is in gaseous form when brought into contact with the boiler ash residue. Elemental bromine is a preferred bromine source for bromination of boiler ash residue. Mixtures of elemental bromine and hydrogen bromide can be employed; usually, such mixtures are in the same form (e.g., both are gases or both are in aqueous solution).

[0024] When the bromine source is gaseous B¾ and/or gaseous HBr, use of undiluted Br₂(g) and/or HBr(g) is preferred, although a carrier gas can be used to transport the Br₂(g) and/or HBr(g). Typical carrier gases are inert, and include nitrogen and argon; air can also be employed as a carrier gas.

[0025] In one production method, Br₂(g) and/or HBr(g) can be injected into a sealed processing vessel containing the boiler ash residue with only a modest, temporary rise in vessel pressure, which pressure subsides as the gas species become incorporated into the boiler ash residue, with or without agitation of the vessel and/or its contents. When the gas contacts the boiler ash residue, it is usually quickly adsorbed. In some embodiments, this contacting occurs at an elevated temperature, with the boiler ash residue at least as hot as the bromine-containing gas; preferably the boiler ash residue is at a temperature at about 60°C or above during the contacting at elevated temperatures. Suitable temperatures during the contacting are in the range from ambient temperature to about 200 °C; preferred temperatures during the contacting are in the range of about 60°C to about 150°C.

[0026] Suitable solid bromine sources include bromide salts, such as alkali metal bromides (e.g. , lithium bromide, sodium bromide, potassium bromide, cesium bromide), alkaline earth bromides (e.g., magnesium bromide, calcium bromide, barium bromide), zinc bromide, ammonium bromide, and the like, as well as mixtures of two or more solid bromine sources. Preferred solid bromide sources include lithium bromide, sodium bromide, and calcium bromide, especially calcium bromide. The solid bromide salts can be hydrated bromide salts. Anhydrous bromide salts can be used, although this is not believed to be necessary. Solid bromide salts that are finer, i.e. , have smaller particle sizes, are preferred.

[0027] Standard impregnation or dry blending techniques are normally used to contact the carbonaceous substrate and the solid bromine source. Another method is to grind or mill the solids while mixing them together. The equipment used to contact carbonaceous substrates with the solid bromine sources can be, for example, a stationary mixer, a rotating drum, a transport reactor, or any other contactor suitable for blending solid ingredients. Any equipment or method that quickly and evenly distributes the solid bromine source to intimately contact the carbonaceous substrate is acceptable.

[0028] After the contacting of the bromine source and the boiler ash residue, an additional step, removal of any weakly-held bromine species from the brominated sorbent, is optionally performed. This can be accomplished by various methods, including applying vacuum to the vessel holding the brominated sorbent, purging the vessel with air or an inert gas, and/or heating the brominated sorbent to a temperature above the temperature at which the bromination was conducted. Preferably, this is accomplished by heating the brominated sorbent, normally to one or more temperatures of about 60°C or more, preferably in the range of about 60°C to about 200°C, more preferably in the range of about 100°C to about 150°C.

[0029] Treatment of boiler ash residue with the bromine source is generally conducted such that the sorbent has about 0.1 wt to about 20 wt bromine, based on the weight of the brominated sorbent after contact with the bromine source. Preferably the brominated sorbent has about 0.5 wt to about 15 wt bromine, more preferably about 3 wt to about 12 wt bromine based on the weight of the brominated sorbent. Amounts of bromine greater than 20 wt can be incorporated into the boiler ash residue if desired. However, as the amount of bromine in the sorbent increases, there is a greater possibility that some of the bromine may evolve from the sorbent under some circumstances.

[0030] To achieve the desired amount of bromine in the brominated sorbent, an amount of the bromine source that contains the appropriate amount of bromine is combined with the boiler ash residue. For example, to form a brominated sorbent having 5 wt bromine, the weight of the bromine from the bromine source and the weight of the boiler ash residue are added together; when the amount of bromine from the bromine source is 5% of the total weight, a brominated sorbent having about 5 wt bromine is formed, since all of the bromine in the bromine source is usually incorporated into the brominated sorbent.

[0031] The brominated sorbents of this invention typically contain about 0.1 to about 20 wt bromine, preferably about 0.5 wt to about 15 wt bromine, and more preferably about 3 wt to about 12 wt bromine. Greater degrees of bromination generally correlate with greater maximum mercury capacities for a particular sorbent. The optimum level of bromine-containing substance to combine with boiler ash residue varies with the particular situation. [0032] In the practice of the present invention, the reduction of mercury emissions employs a brominated sorbent having about 0.1 wt to about 20 wt bromine therein, based on the total weight of the brominated sorbent. The brominated sorbent is formed from boiler ash residue and (i) elemental bromine and/or hydrogen bromide or (ii) a solid bromine source. As noted above, greater amounts of bromine, e.g. , up to about 30 wt , can be incorporated into the boiler ash residue if desired. However, as the amount of bromine in the brominated sorbent increases, there is a greater possibility that some of the bromine may evolve from the brominated sorbent under some circumstances.

[0033] This invention provides flexible methods that can be applied to a number of combustion gas streams and a wide range of exhaust system equipment configurations. In these methods, i) a brominated sorbent is introduced into the combustion gas stream at one or more points upstream of a particulate collection device; and ii) the brominated sorbent is collected from the combustion gas stream, to reduce mercury emissions from an exhaust system which comprises at least a combustion gas stream and a particulate collection device. Generally, the brominated sorbent can be injected at any point upstream of a particulate collector. The brominated sorbent is introduced into a combustion gas stream, usually by injection, and is carried with the other particulates and gases to a particulate collection device, where the sorbent is collected. Typically, the sorbent is collected along with other particulates present in the combustion gas stream.

[0034] The brominated sorbent may be injected either before the gas is passed through a heat exchanger or air preheater, i.e., on the so-called "hot side" of a combustion gas exhaust system, or after the gas has passed through a heat exchanger or preheater, i.e. , on the "cold side" of a combustion gas exhaust system. The preferred point(s) for injecting the brominated sorbent can vary, depending upon the configuration of the system. When injected, the brominated sorbent contacts a combustion gas stream, intimately mixes with the combustion gas stream, and is separated from the gas stream in a particulate collector, usually along with other particulates present in the combustion gas stream. Operating temperatures on the cold side are generally about 400°F (204°C) or less.

[0035] It is recommended that the brominated sorbent is injected into the particulate- containing gas at a point such that the brominated sorbent is not exposed to temperatures above the ignition temperature of the brominated sorbent, since such temperatures destroy the adsorptive properties of the brominated sorbent. At or above the ignition temperature, the brominated sorbent tends to decompose or ignite. The ignition temperature depends on the thermal properties of the sorbent, and varies with the particular composition of the brominated sorbent.

[0036] Within these parameters, it is recommended that the brominated sorbent be injected to maximize both the residence time of the sorbent in the system and the best distribution of the sorbent in the system, in order to provide the greatest opportunity for contact of the brominated sorbent and the mercury and/or mercury-containing compounds. Due to the wide variation in plant configurations, the optimum injection point(s) will vary from plant to plant.

[0037] The brominated sorbents are typically injected at a rate of about 0.5 to about 15 lb/MMacf (8xl0^~6 to 240xl0^"6 kg/m³). Preferred injection rates are about 1 to about 10 lb/MMacf (16xl0^~6 to 160xl0^"6 kg/m³); more preferred are injection rates of about 2 to about 5 lb/MMacf (32xl0^~6 to 80xl0^"6 kg/m³), though it is understood that the preferred injection rate varies with the kinetics of reaction for mercury species with the sorbent, the mercury capacity of the sorbent, the relevant mercury emission limit, and the particular system configuration.

[0038] Optionally, other agents, such as conditioning agents, can be injected if needed or desired. Preferably, no agents other than the brominated sorbent are added. It is preferred to practice the invention in the absence of conditioning agents.

[0039] Without wishing to be bound by theory, it is believed that the brominated sorbent comes into contact with mercury and/or mercury-containing compounds, which are then absorbed by the brominated sorbent. The brominated sorbent travels from the injection point with the combustion gas stream and can be collected, along with other particulates, in a particulate collection device placed in the combustion gas stream.

[0040] The following examples are presented for purposes of illustration, and are not intended to impose limitations on the scope of this invention.

[0041] The butane adsorption test is a modified version of the Standard Test Method for Determination of the Butane Working Capacity of Activated Carbon (ASTM D5228), which can only be applied to granular activated carbon. The modified method described herein can be applied to powdered activated carbon as well as to granulated activated carbon. In a typical test, approximately 1 to 2 grams of a pre-dried sample is weighed and put into a modified weighing bottle that has an inlet and an outlet. The sample and weighing bottle are heated and kept at 35 °C during the test, in which n-butane, preheated to 35 °C, is passed across the empty space of the weighing bottle while the sample remains at the bottom of the bottle for 30 minutes, so that the sample contacts and adsorbs n- butane. The difference of the sample weight after contact with n-butane and the sample weight prior to contact with n-butane (in milligrams) divided by the sample weight (in grams) is defined as butane adsorption. Shown in equation form:

Butane adsorption, mg/g = (sample weight after n-butane - sample weight before n- butane), mg

÷ sample weight, g [0042] The brominated sorbents were evaluated for mercury removal in a pilot duct injection system with a simulated flue gas. The pilot scale test system included a natural gas burner unit to generate the hot flue (combustion) gas, an elemental mercury spiking subsystem with elemental mercury permeation tubes, a flue gas spiking subsystem with mass flow controllers for SO₂, NO_x, and HC1, a small sorbent feeder and eductor with compressed air to carry the sorbent to the duct, insulated duct thermocouples, an electrostatic precipitator (ESP) with an effective specific collection area (SCA) of about 500 ft²/Kacfm (27.3 m²/1000m³/h), a safety filter, an orifice plate to measure flow, and a variable-speed induced draft (ID) fan.

[0043] The simulated flue gas contained 17% 0₂, 4% C0₂, and 8% H₂0, with the balance of the gas being N₂. Elemental mercury was introduced into the simulated flue gas from permeation tubes, and S0₂, NO_x, and HC1 were introduced into the simulated flue gas from lecture bottles (gas bottles). In the simulated flue gas, the concentrations of added substances were about 6 μg/Nm³ Hg°, 800 ppm S0₂, 400 ppm NO_x, and 5 ppm HC1. The flue gas temperature at the injection point was about 500°F (260°C), and at the electrostatic precipitator the gas temperature was 360°F (182°C). The flow rate of the flue gas was about 100 acfm (170 m³/h).

[0044] Samples were injected at various rates into the hot gas with a ductwork residence time of about 2 seconds before reaching the electrostatic precipitator. In each test run, a few grams (usually 3 grams) of the sample being evaluated was injected into the simulated flue gas at 500 +10°F (260 + ~2°C); the brominated sorbent remained in-flight in the simulated flue gas for about 2 seconds, and then the brominated sorbent was collected by the electrostatic precipitator.

[0045] Each test run normally lasted for about 20 minutes. Mercury levels in flue gas were continuously monitored with an on-line gas-phase elemental mercury analyzer (Tekran^® Stack Hg CEM Series 3300, Tekran Instruments Corporation). Mercury removal rates were calculated based on the mercury concentrations before (baseline mercury content in the flue gas was about 6 μg/m³) and during injection of the brominated sorbent.

[0046] The average mercury removal rate was the difference of the average baseline Hg concentration before injection and the average Hg concentration during sorbent injection, relative to the baseline Hg concentration, and is expressed as a percentage. One way to calculate the average mercury removal rate is by the following formula:

. _TT , Tavg. Hg cone, (pre-injection)— avg. Hg. cone, (iniection)l _{Λ nn} Avg. Hg removal rate = -—≥—≥ ² ≥—≥ ^ — x 100 average Hg cone, (pre-injection) The equilibrium mercury removal rate was calculated in the same manner as the average mercury removal rate, but replacing the average Hg concentration during the injection period with the steady-state mercury concentration towards the end of the injection period.

EXAMPLE 1

[0047] Boiler ash residue (BAR) was collected from the disposal pond of a paper mill and dried. The ash content of this dried boiler ash residue (BAR A) was measured following the Standard Test Method for Total Ash Content of Activated Carbon (ASTM D2866), and was found to be 13%; the carbon content of BAR A was 87%.

[0048] A portion of BAR A was ground to less than 400 mesh (38 μιη). Ground (powdered) BAR A was brominated with gaseous bromine (Br₂) to obtain a brominated sorbent containing 10 wt% bromine (brominated BAR A). To brominate, ground BAR A was weighed and heated to 120°C; while still hot, the ground BAR A was transferred into a glass well, and a known amount of gaseous bromine was fed into the glass well at the same time, forming brominated BAR A; the contents of the glass well were cooled to room temperature.

[0049] Brominated BAR A was tested using the pilot-scale duct-injection-unit described above to evaluate its performance as a mercury sorbent. In a comparative run, a commercially available mercury sorbent, Darco^® Hg-LH, an impregnated lignite coal- based activated carbon (Norit America), was tested under the same conditions. The mercury removal performance of both samples is summarized in Table 1, and is shown graphically in Fig. 1. EXAMPLE 2

[0050] A portion of unground, unbrominated BAR A from Example 1 was activated by steam in a rotary kiln (inner diameter: 6 inches (15.24 cm); length: 10 feet (3.05 m)) at 840°C for about 15 minutes at a feed rate of 2 kg/hr. The resultant steam-activated boiler ash residue (activated BAR A) was ground to less than 400 mesh (38 μιη), and then brominated as described in Example 1 to obtain a brominated sorbent containing 10 wt bromine (brominated activated BAR A). The brominated activated BAR A was tested using the pilot-scale duct- injection-unit described above to evaluate its performance as a mercury sorbent. Results are summarized in Table 1. Brominated activated BAR A exhibited significantly better mercury removal performance than Darco^® Hg-LH; the mercury removal performance for both brominated activated BAR A and the Darco^® Hg- LH is shown graphically in Fig. 2.

EXAMPLE 3

[0051] Boiler ash residue was collected from another paper mill before the ash pond, so this sample was dry. This boiler ash residue (BAR B) was a mixture of black chips and white powder. Using a 16 mesh (1.18 mm) U.S. Standard Sieve, BAR B was separated into chips and powder. The chip fraction of BAR B was ground to less than 400 mesh (38 μιη), and then brominated as described in Example 1 to obtain a brominated sorbent containing 10 wt bromine (brominated BAR B). Brominated BAR B was tested using the pilot-scale duct-injection-unit described above to evaluate its performance as a mercury sorbent. Results are summarized in Table 1. The mercury removal performance for both brominated BAR B and Darco^® Hg-LH is shown graphically in Fig. 3.

TABLE 1

Darco^® 3.1 lb/MMacf

-- -- -- 60% 75% Hg-LH^* (50xl0^-6 kg/m³)

Comparative run.

[0052] Components referred to by chemical name or formula anywhere in the specification or claims hereof, whether referred to in the singular or plural, are identified as they exist prior to coming into contact with another substance referred to by chemical name or chemical type (e.g. , another component, a solvent, or etc.). It matters not what chemical changes, transformations and/or reactions, if any, take place in the resulting mixture or solution as such changes, transformations, and/or reactions are the natural result of bringing the specified components together under the conditions called for pursuant to this disclosure. Thus the components are identified as ingredients to be brought together in connection with performing a desired operation or in forming a desired composition. Also, even though the claims hereinafter may refer to substances, components and/or ingredients in the present tense ("comprises", "is", etc.), the reference is to the substance, component or ingredient as it existed at the time just before it was first contacted, blended or mixed with one or more other substances, components and/or ingredients in accordance with the present disclosure. The fact that a substance, component or ingredient may have lost its original identity through a chemical reaction or transformation during the course of contacting, blending or mixing operations, if conducted in accordance with this disclosure and with ordinary skill of a chemist, is thus of no practical concern.

[0053] The invention may comprise, consist, or consist essentially of the materials and/or procedures recited herein.

[0054] As used herein, the term "about" modifying the quantity of an ingredient in the compositions of the invention or employed in the methods of the invention refers to variation in the numerical quantity that can occur, for example, through typical measuring and liquid handling procedures used for making concentrates or use solutions in the real world; through inadvertent error in these procedures; through differences in the manufacture, source, or purity of the ingredients employed to make the compositions or carry out the methods; and the like. The term about also encompasses amounts that differ due to different equilibrium conditions for a composition resulting from a particular initial mixture. Whether or not modified by the term "about", the claims include equivalents to the quantities.

[0055] Except as may be expressly otherwise indicated, the article "a" or "an" if and as used herein is not intended to limit, and should not be construed as limiting, the description or a claim to a single element to which the article refers. Rather, the article "a" or "an" if and as used herein is intended to cover one or more such elements, unless the text expressly indicates otherwise.

[0056] This invention is susceptible to considerable variation in its practice. Therefore the foregoing description is not intended to limit, and should not be construed as limiting, the invention to the particular exemplifications presented hereinabove.

Claims

CLAIMS:

1. A method for selecting and preparing boiler ash residue from a wood waste boiler of pulp and paper mills for mercury adsorption such that the boiler ash residue has a carbon content of about 70% or more, and

a BET surface area of about 300 m²/g or more or a butane adsorption of about 130 mg/g or more;

wherein, when the boiler ash residue has a BET surface area of less than about 300 m²/g or a butane adsorption of less than about 130 mg/g, the boiler ash residue is subjected to activation with steam or carbon dioxide, to increase the BET surface area to about 300 m²/g or more or the butane adsorption to about 130 mg/g or more.

2. A method as in Claim 1 wherein the carbon content is about 80% or more.

3. A method as in Claim 1 wherein the activation is with steam.

4. A boiler ash residue selected and prepared as in any of Claims 1-3.

5. A method of preparing a brominated sorbent, which method comprises contacting boiler ash residue prepared as in Claim 1 and a bromine source, which bromine source is (i) elemental bromine and/or hydrogen bromide or (ii) a solid bromine source, to form a brominated sorbent.

6. A method as in Claim 5 wherein said bromine source is elemental bromine.

7. A method as in Claim 5 wherein said bromine source is hydrogen bromide.

8. A method as in any of Claims 6-7 wherein the bromine source is in gaseous form.

9. A method as in Claim 7 wherein the hydrogen bromide is an aqueous solution of hydrogen bromide.

10. A method as in Claim 5 wherein said solid bromine source is at least one alkali metal bromide, at least one alkaline earth bromide, or a mixture thereof.

11. A method as in Claim 10 wherein said solid bromine source is lithium bromide, sodium bromide, and/or calcium bromide.

12. A brominated sorbent prepared as in any of Claims 5-11.

13. A method for reducing mercury emissions from an exhaust system which comprises at least a combustion gas stream and a particulate collection device, which method comprises i) introducing a brominated sorbent prepared as in Claim 5 into said combustion gas stream at one or more points upstream of a particulate collection device; and ii) collecting the brominated sorbent from the combustion gas stream,

with the proviso that the brominated sorbent is not exposed to temperatures above the ignition temperature of the brominated sorbent.

14. A method for reducing mercury emissions from an exhaust system which comprises at least a combustion gas stream and a particulate collection device, which method comprises

i) introducing a brominated sorbent into said combustion gas stream at one or more points upstream of a particulate collection device; and

ii) collecting the brominated sorbent from the combustion gas stream,

wherein said brominated sorbent is formed from boiler ash residue from a wood waste boiler of pulp and paper mills and a bromine source which is (i) elemental bromine and/or hydrogen bromide or (ii) a solid bromine source, with the proviso that the brominated sorbent is not exposed to temperatures above the ignition temperature of the brominated sorbent.

15. A method as in any of Claims 13-14 wherein the brominated sorbent is injected into the combustion gas stream before the gas stream before passes through a heat exchanger.

16. A method as in any of Claims 13-14 wherein the brominated sorbent is injected into the combustion gas stream after the gas stream passes through a heat exchanger.

17. A method as in any of Claims 13-14 wherein the method is carried out in the absence of conditioning agents.

18. A method as in any of Claims 13-14 wherein the brominated sorbent is injected at a rate of about 0.5 to about 15 lb/MMacf.

19. A method as in Claim 14 wherein said bromine source is elemental bromine.

20. A method as in Claim 14 wherein said bromine source is hydrogen bromide.

21. A method as in any of Claims 19-20 wherein the bromine source is in gaseous form.

22. A method as in Claim 20 wherein the hydrogen bromide is an aqueous solution of hydrogen bromide.

23. A method as in Claim 14 wherein said solid bromine source is at least one alkali metal bromide, at least one alkaline earth bromide, or a mixture thereof.

24. A method as in Claim 23 wherein said solid bromine source is lithium bromide, sodium bromide, and/or calcium bromide.

25. A method as in any of Claims 5, 13, or 14 wherein said brominated sorbent has about 0.1 wt to about 20 wt bromine therein, based on the total weight of the brominated sorbent.