WO1997025138A1

WO1997025138A1 - Chemical sorbent and methods of making and using same

Info

Publication number: WO1997025138A1
Application number: PCT/US1997/000048
Authority: WO
Inventors: Liang-Shih Fan; Abhijit Ghosh-Dastidar; Suhas Mahuli
Original assignee: The Ohio State University Research Foundation
Priority date: 1996-01-10
Filing date: 1997-01-08
Publication date: 1997-07-17
Also published as: US5779464A; CN1213327A

Abstract

A novel CaCO3 sorbent with its physical properties tailored for high SO2 removal is produced in accordance with the present invention. The surface area, porosity and the distribution of pore sizes of the sorbent have been designed and controlled to give rise to the optimum internal structure necessary for high SO2 removal. The CaCO3 sorbent possesses 60 m2/g surface area and 0.18 cc/g pore volume and exhibits extraordinary reactivity towards SO¿2? (70 % utilization in 530 milliseconds at 1080 °C and 3900 ppm SO2 concentration) when compared with other carbonate and hydrate sorbents in our laboratory reactor system. The sorbent may be produced by precipitation in a slurry bubble column reactor system and preferably contains about 2 % by weight of a surfactant which is added during its preparation and improves the properties of CaCO3 in the course of precipitation. The high utilization of this tailored CaCO3 sorbent makes it a very strong potential sorbent for dry sorbent injection technology.

Description

CHEMICAL SORBENT AND METHODS OF MAKING AND USING SAME

Technical Field

The present invention is in the field of chemical sorbents.

Background

In Furnace Sorbent Injection (FSI) Technology, calcium-based (i.e., calcium,

hydroxide, Ca(OH)₂ or calcium carbonate, CaCO₃) sorbent powders are injected into the

upper-furnace region of a coal-fired boiler to control SO₂ emission. Emission of SO₂ has

long been associated with severe environmental problems such as acid rain. In spite of

being cost-effective and easily retrofittable in the existing utility plants, the FSI process

fails to be competitive with other more expensive SO₂ control technologies due to its poor

SO₂ removal efficiency. Only a maximum of 60-70% of SO₂ capture (at Ca/S = 2) could

be achieved by this technique with various pure and chemically promoted calcium-based

sorbents; falling far short of the 90% to 95% sulfur removal goal, as mandated by the Clean Air Act of 1990.

Upon injection into the furnace, the sorbent removes SO₂ by a two-step reaction

process. First, the sorbent decomposes into CaO (by a calcination reaction):

Ca(OH)₂ (solid) <3 CaO (solid) + H₂O (gas) (la)

CaCO₃ (solid) <3 CaO (solid) + CO₂ (gas) (lb)

CaO then reacts with SO₂ in presence of oxygen to produce CaSO₄, (by a sulfation

reaction):

CaO (solid) + SO₂ (gas) + 1/2 O₂ (gas) O CaSO₄ (solid) (2). Another important phenomenon influencing the SO₂ removal is the sintering of the

CaO. Sintering of CaO reduces the available surface area and porosity for the ensuing

sulfation reaction; and therefore the rate at which CaO sinters greatly determines the rate of

sulfation.

The performance of a particular calcium-based sorbent is influenced by a number of

factors, all of which play a crucial role in determining the overall reactivity of the solid.

Particle size is an important parameter because large particle size induces transport

limitations for both the calcination and sulfation reactions. However, below 5 μm, particle

size ceases to be a determining factor in the overall sulfur capture, as resistance to heat and

mass transfer become insignificant below such a small size. See Milne et al., High-

Temperature, Short-Time Sulfation of Calcium-Based Sorbents, Ind. Eng. Chem. Res. , Vol.

29, No. 11, pp. 2201-2214 (1990), hereby incorporated herein by reference. Previous work

of Borgwardt, Sintering of Nascent Calcium Oxide, Chemical Engineering Science, Vol.

44, No. 1, pp. 53, 60 (1989), hereby incorporated herein by reference, has reported that the

rate of sintering depends on the type of initial sorbent (carbonate or hydroxide) and also on

the foreign ion concentration or impurities in the parent solid. Borgwardt observed that

CaO derived from carbonate (c-CaO) sinters slower than hydroxide-derived CaO (h-CaO),

and also that the sintering rate is accelerated with increasing concentration of foreign ions.

The foregoing suggests that if adequately small particles are used to eliminate

transport resistance during the reactions, calcium carbonate particles should yield higher

sulfur capture than the hydroxide particles because of a relatively slow sintering of its

oxide. However, in both laboratory and pilot-scale studies (See Milne et al., 1990; and Bortz and Flament, Recent IFRF Fundamental and Pilot Scale Studies on the Direct

Sorbent Injection Process, proceedings: First Joint Symposium on Dry S0₂ and Simultaneous S0₂ - N0_X Control Technologies, Vol. 1, EPA-600/9-85/020a, pp. 17-1 - 17-

22 (1985), hereby incorporated herein by reference, hydroxides have consistently shown higher sulfur capture than the carbonates, which can be explained by a comparison of the surface area, pore volume, and pore size distribution of the parent sorbents. Carbonate

powders are usually non-porous and possess extremely low surface area, with a typical

value of less than 2 to 3 m²/gm. The calcium oxide it produces consequently exhibits a pore size distribution consisting of predominantly smaller pores (< 50 A° in radius), since most of them grow from very small pores or cracks inside the carbonate particle.

Based on experimental observations, it is widely accepted that there exists an

optimum pore size range (typically about 100 to 200 A° diameter) for sulfur capture which provides sufficient surface area for the sulfation reaction to occur without causing rapid pore filling and pore-mouth plugging that can lead to a premature reaction termination in

the smaller pores. As a result, the overall reactivity of low-surface area, low-porosity carbonates is believed to result from such predominantly smaller pores in their calcines.

On the other hand, the typical calcium hydroxide sorbents possess an initial surface area of

about 12 to 18 m²/gm and a corresponding well-defined initial pore structure, which on

calcination produces a number of pores in the optimum size range. It is to be noted that for pores larger than 200 A°, surface area to pore volume ratio progressively diminishes, and in spite of an excellent porosity value, the CaO may suffer from relatively lower surface area

for the sulfation reaction.

The sulfur capture ability of a sorbent is greatly influenced by the surface area of its calcined product CaO; the higher the surface area, the greater is the reaction rate between SO₂ and CaO. However, CaO reactivity is lost not only by losing surface area due to the

sulfation reaction, but also by grain coalescence due to high temperature- induced sintering. The general understanding of the research community to date was that to achieve higher

sorbent conversion, a high CaO nascent surface area along with a slower sintering rate is

necessary. Various work including our previous research concentrated on producing

modified calcium hydroxide powders, which resulted in the synthesis of calcium

lignosulfonate-modified hydroxide powder with high initial surface area of greater than 60

m /gm. However, this powder showed only a marginal improvement in sorbent conversion

in 500 ms (38% compared to 32% for pure hydroxide). See Kirchgessner, D. A., &

Jozewicz, W., Enhancement of Reactivity in Surfactant-Modified Sorbents for

Sulfurdioxide Control, Ind. Eng. Chem. Res., 28(4), pp. 413-418 (1989), hereby

incoφorated herein by reference.

The discussion of the previous section comparing calcium hydroxide and carbonate

sorbents indicates that hydrates show superior reactivity in laboratory and pilot-scale studies

because of their more open initial structure, which shifts the pore sizes of the h-CaO to a

relatively favorable distribution. Nevertheless, a faster sintering in h-CaO prevents it from

realizing a very high sorbent utilization, typically yielding 32 to 38% conversion in 1 to 2

seconds.

It is therefore an object of the present invention to produce an improved sorbent in

the form ofa calcium carbonate solid with an open initial pore structure, which can then result in an optimum pore size distribution upon decomposition. The slower sintering of its

calcined product would provide adequate contact area for SO₂ capture for a longer time and

yield a very high sorbent utilization before the rate-retarding phenomena such as pore filling

and pore-mouth plugging drastically slow the reaction rate.

Unfortunately, in spite of an abundance of calcium carbonate in nature, all the known

carbonate sorbents are typically non-porous and fail to exhibit the desired structural properties for high SO₂ capture. It is therefore an object of the present invention to produce such a

sorbent by chemical or physical methods.

Commercially, calcium carbonate powders are prepared for many industrial uses such

as fillers and extenders in the paper, paints and plastics manufacturing industries. In the paper

industry for example, the typical usage is to increase opacity and brightness of paper while

increasing its bulk. For such applications, the properties of carbonate that are crucial and are

optimized by manufacturers include fineness of particle size, uniformity of product size

distribution, optical properties (opacity, refractive index, etc.) and product purity. However,

the properties desired in CaCO₃ for SO₂ removal application are quite different.

Although the advantages ofthe present invention are described with respect to its use

in the removal of sulfur dioxide, the use ofthe sorbent ofthe present invention is not limited

to that use, as reflected in the following disclosure. The disclosure of the present invention

and its use may make apparent other advantages and the solution to other problems in the

chemical sorbent field. Summary ofthe Invention

The present invention includes a calcium carbonate sorbent, and methods of its manufacture and use.

Fundamentally, the present invention is directed to the development of a CaCO₃

sorbent for SO₂ removal (although other uses are anticipated), and hinges on optimizing two

properties very crucial for high SO₂ capture: (i) high surface area (S), and (ii) high surface area to pore volume (P) ratio, i.e., S/P.

The latter criterion has been developed from a fundamental understanding of internal

pore structure and evolution of pore size distribution. A brief discussion of the conception

and application of these criteria follows. Consider two parent CaCO₃ powders, Carb, and Carb₂, ofthe same high surface area

but different pore volumes, such that,

S, = S₂, but P, < P₂ (3)

Thus, Carb i possesses a higher S/P ratio than Carb₂. Let r, and r₂ be the average pore radii of

these powders. Since most ofthe surface area is internal,

S = 2 π r n (4) P = π r² n (5)

where n is the total length ofthe pore system.

From equations (3), (4) and (5), it can be seen that,

r, < r₂ (6)

Thus, parent Carb_! possesses pores of smaller average radii. Now consider a single pore of

radius r on the inner surface of which, calcination takes place. Due to the formation of a

lower volume product, CaO, there is loss of mass from the inner surface of he pore leading to

increase in its inner radius by Δr. During calcination reaction, the rate of decomposition is directly proportional to the available surface area inside the pore.

Rate of calcination = k, [(r + Δr)² - r²] α Surface area = k₂ r, (7)

where ki and k₂ are constants.

Hence,

Δr = (k₂/ 2k,) = constant (8)

so,

Sc_ao/S_car_b = 2π (r+Δr)/2πr = 1 + Δr/r (9).

Since, Δr is independent of pore size, as r decreases, S_CaQ increases, for a given S^^

CaO of higher surface area is generated. Thus, a parent carbonate with smaller r (hence larger

S/P) would generate a higher surface area CaO upon calcination. A high surface area to pore volume ratio is also crucial from the perspective of

generating pores ofthe optimum size range. A high S/P implies that most of the pores in the

parent carbonate would be less than 100 A⁰ in diameter. Upon calcination, these pores would

increase in diameter and would most likely fall under the optimum size range of about 100 to

200 A⁰, which allows sufficient surface area for sulfation reaction without the pores becoming

plugged or filled. On the contrary, if surface area to pore volume ratio is small, most of the

pores in the parent carbonate might already be within the approximate 100 to about 200 A⁰

range, and once calcined, pore diameters for a large number of pores shift to greater than 200

A⁰, which do not provide adequate surface area for the sulfation reaction.

With this theoretical basis, calcium carbonate sorbents possessing the above-outlined

properties have been successfully generated using a wet-precipitation technique, i.e., by

reacting calcium hydroxide powder with CO₂ gas in aqueous solution, preferably in the

presence of trace quantities of surfactant. The preparation and testing of the sorbent, its

properties, and its SO₂ reactivity experiments and results are described below.

Accordingly, the sorbent of the present invention comprises a calcium carbonate

powder comprising powder particles having an average surface area above about 40

m²/gram; with the powder particles having pores of an average pore volume, and the ratio ofthe average surface area to the average pore volume is in the range of from about 200 to

about 350 m²/cubic centimeter.

Preferably, the sorbent of the present invention is such that the average surface area is in the range above about 60 m /gram. It is also preferred that the average pore volume is

in the range of above about 0.18 cubic centimeters per gram ofthe powder. In the preferred

embodiment the sorbent additionally comprises at least one surfactant, preferably present at

a concentration of at least about 2% by weight. The preferred surfactant(s) are those described as sodium salts of carboxyl acids, such as those commercially available under the trademark Dispex®, particularly Dispex® N40V.

Although they might be produced by other methods, it is preferred that the sorbents ofthe present invention be produced by precipitation from a solution of calcium hydroxide.

The present invention also includes a process for making a calcium carbonate

sorbent which includes the steps of: (a) preparing a liquid suspension of calcium (II) ion;

(b) subjecting that liquid suspension to a flow of carbon dioxide gas for sufficient time and

at sufficient temperature so as to form calcium carbonate, wherein the concentration of the

liquid suspension of calcium (II) ion in the suspension of step (a) is preferably at least

about 4sat; (the subscript "sat" indicating the concentration of a saturated solution at standard temperature and pressure, with the preceding number indicated a multiplying factor), and wherein the flow rate of said flow of carbon dioxide gas is preferably at least

about 0.48 liters/minute. Most preferably, the concentration of the liquid suspension of

calcium (II) ion in the suspension of step (a) is at least about lόsat. It is also most preferred that the flow rate ofthe flow of carbon dioxide gas is at least about 2.4 liters/minute.

It is preferred that the process additionally comprises the addition of at least one

surfactant, such as those mentioned above, during step (b).

The present invention also includes a process of removing sulfur dioxide from a gaseous flow. In general terms, the process ofthe present invention involves the steps of:

exposing the gaseous flow to a sorbent of the present invention as described in its many basic and preferred embodiments herein, for sufficient time and at sufficient temperature so

as to bind the sulfur dioxide to said sorbent. The process of the present invention may be carried out using known techniques and parameters for using such sorbents as is known in

the an. The present invention also includes a process of removing heavy metal(s) from a

sample. Fundamentally, the process of the present invention involves the steps of:

exposing a sample containing the heavy metal(s) to a sorbent of the present invention as

described in its many basic and preferred embodiments herein, for sufficient time and at

sufficient temperature so as to bind the heavy metal(s) to the sorbent. The process of the

present invention may be carried out using known techniques and parameters for using

such sorbents as is known in the art. The sample may be, and generally will be in a

gaseous or liquid form, and the sorbent may be brought in contact with the sample through

techniques known in the art.

One of the objects of the present invention is to develop a calcium-based sorbent

with optimum structural properties (surface area, pore volume, pore size distribution) to

enhance its capability for capturing sulfur dioxide from hot flue gas. Currently, a

maximum sorbent conversion of 30 to 40% (at Ca/S = 2) is achieved with all the known

calcium-based sorbents (hydroxide or carbonate) within the available solid residence time

of 1 to 2 seconds in the favorable reaction temperature window of 800 to about 1200°C.

The sulfur dioxide and/or heavy metals adsorbed by the sorbents of the present

invention may be released, and the sorbents regenerated, through techniques known in the

art. Brief Description ofthe Drawings

Figure 1 is a schematic of a slurry bubble reactor used to make the sorbents in

accordance with one embodiment ofthe present invention;

Figure 2 is a chart showing the relationship between the surface area of the particles of a calcium carbonate sorbent produced in accordance with one embodiment of the present invention, to the flow rate of carbon dioxide at various concentrations of the slurry from which the sorbent is precipitated;

Figure 3 is a chart showing the relationship between the surface area ofthe particles

ofa calcium carbonate sorbent produced in accordance with one embodiment ofthe present

invention, to the presence of a surfactant at different concentrations of the slurry from which the sorbent is precipitated;

Figure 4 is a scanning electron microscope micrograph ofa CaCO₃ sorbent sample

produced in accordance with one embodiment ofthe present invention;

Figure 5 is a chart showing the particle size distribution ofa CaCO₃ sorbent sample

produced in accordance with one embodiment ofthe present invention; and

Figure 6 is a chart comparing the performance, measured in terms of extent of sulfation, of a CaCO₃ sorbent sample produced in accordance with one embodiment of the

present invention, to other sorbents ofthe prior art.

Detailed Description ofthe Preferred Embodiment

In accordance with the foregoing summary of the invention, the following presents

a detailed description of the prefened embodiment, which is presently considered to be the

best mode of the invention as applied to the production and use of a calcium carbonate

sorbent for the removal of sulfur dioxide from a gaseous waste stream. Sorbent Preparation and Characterization

A calcium carbonate powder sorbent may be produced by a wet precipitation

technique in a slurry bubble column reactor system. The process entails reacting calcium

hydroxide (Ca(OH)₂) powder with CO₂ gas in aqueous solution, preferably in presence of

trace quantities of surfactant. A 15 inch long plexiglas column of about 2 inches O.D. may be used as the reactor. The column is fitted with a ceramic filter at the bottom for uniform gas distribution. The Ca(OH)₂ concentration in the solution (e.g., 25.6 gms. in 1 liter), the

amount ofthe surfactant (e.g. Dispex® N40V as 2 % by wt. of calcium hydroxide), and the

CO2 flow rate (5 SCFH at STP) may be established as optimum reaction conditions to

tailor the specific properties, namely, surface area, pore volume, and pore size distribution

of the precipitated CaCO₃. The CaCO₃ powder may be filtered and dried overnight in a

vacuum oven (at 75°C), and then ground to produce fine particles.

The powder was then tested in a high-temperature, entrained-flow reactor to

examine its effectiveness in capturing SO₂. The sorbent shows a 70% conversion to

calcium sulfate (CaSO₄,) in only 530 ms at a reaction temperature of 1080°C. This is

nearly twice the conversion obtained with standard calcium-based sorbent, currently

employed as the solid for high-temperature sorbent injection process to control SO₂

emission.

A schematic of the laboratory-scale column 1 is shown in Figure 1. An aqueous

suspension of calcium hydroxide 2 is used as the slurry medium, and CO_2> supplied through

rotameter 3 is used to bring about the carbonation reaction. Typical dimensions of the

apparatus shown in Figure 1 include a capacity ofthe column of 1 liter, with a diameter of 6.6

cm and a height of 38 cm. A sintered glass plate 4 fused to the bottom of the column may

serve as the gas distributor. The CO₂ is bubbled through the suspension for a sufficient time

to lead to the formation and precipitation of CaCO₃; typically 5 to 25 minutes for the

concentrations used herein. This can be readily ascertained by the change in the appearance

of the slurry, i.e. by a thicker, more viscous appearance in the slurry. The carbonate suspension may then be filtered and dried in a vacuum oven for a period of time (typically at

least 6 hours under vacuum at 70 degrees Centigrade for about 25 to 50 grams of sorbent; this

may be scaled up using industrial drying equipment, such fluidized bed dryers). The various parameters which affect the carbonate formation and its physical properties are the Ca(OH)₂

concentration, the CO₂ flow rate, the time of carbonation and the presence of any additives.

Various parametric studies were conducted in order to identify the optimum

conditions for preparing CaCO₃ with properties outlined earlier. These parametric tests

included varying the Ca(OH)₂ concentration from saturation, 1.6 g/lit (at 1^ to 25.6 g/lit (at

16_sat), and the CO₂ flow rate from 0.24 1pm to 2.4 1pm. Figure 2 shows the variation of

CaCO₃ surface area with CO₂ flow rate at specific concentrations. As can be seen, at

saturation conditions or slightly above saturation (up to 2_SU), high CO₂ flow rates (greater

than 0.48 1pm) are not favorable for obtaining good surface area. On the contrary, at higher

slurry concentrations of 4^ and above, there exists an optimum CO₂ flow rate which gives a

maxima in the surface area ofthe final precipitated product.

The conditions of 16_sat concentration and 2.4 1pm CO₂ flow rate are seen to give

nearly 35 m²/g surface area. The CaCO₃ properties were further improved by adding a small

quantity of dispersing agent or surfactant during the precipitation process. Ionic surfactant

additives are known to act as dispersing agents in aqueous systems leading to reduced

agglomeration of crystallites. The additive used may be an anionic surfactant such as

Dispex® (commercially available from Allied Colloids Inc. of Suffolk, Virginia), which is a

sodium salt of a polycarboxylic acid. Dispex® is a low molecular weight, water soluble

surfactant and is obtained as a mobile liquid with 40% solids content. Although the invention is not limited to any theory of its operation, the surfactant is believed to produce a stabilizing

and dispersing action by ionizing in water to produce a sodium cation together with a

polyanion. This polyanion is believed to adsorb irreversibly onto the particle surface causing

the particle to become negatively charged. Adjacent particles are thought then to repel one another to maintain a state of dispersion. The concentration of Dispex® in the Ca(OH)₂

slurry was about 2.0 wt% w.r.t. Ca(OH)₂.

Figure 3 shows how the presence of Dispex® leads to an increase in surface area of

the carbonate. The sorbent produced under conditions of 16_sat concentration with 2.4 1pm

CO₂ flow rate in the presence of 2 wt% Dispex® is clearly an improved product with

properties much superior to the products described in the prior art. Hence, all the further

characterization and testing were performed with this sorbent possessing 60 m /g surface area

and a pore volume of 0.18 cc/g (henceforth referred to as the "60pal 8" sorbent).

Scanning Electron Microscope (SEM) studies, X-ray diffraction (XRD) studies,

primary particle size distribution studies and pore size distribution studies were conducted

with the 60pal8 sorbent. Figure 4 shows the highly porous and granular nature of this

carbonate powder. The powder is observed to possess a calcitic crystalline structure from

XRD studies. From an analysis of the primary particle size distribution (using the

Micrometrics Model 5100 Sedigraph analyzer) shown in Figure 5, the mass median size (d₅₀)

is about 0.9 μm. Reactivity Testing

The 60pal8 sorbent is tested for its SO₂ removal ability in a reactor system. The reactor system is a drop-tube entrained flow reactor system equipped with a water-cooled

powder injection and collection probes, on-line residence time measurement, and in situ

particle sizing. A detailed description of the reactor system and its operation are given in

Raghunathan et al. Raghunathan ., A. Ghosh-Dastidar, and L.-S. Fan, A Technique for the Study of Ultrafast Gas-Solid Reactions for Residence Times less than 100 ms, Rev. Sci.

Instrum., 63(11), 5469 (1992) and Ghosh-Dastidar et al. Ghosh-Dastidar, A., Mahuli, S.,

Agnihotri, R., and Fan, L.-S., Ultrafast Calcination and Sintering of Ca(OH)2 powder: Experimental & Modeling, Chem. Eng. Sci., Vol. 50, No. 13, pp. 2029-2040, (1995), both

of which are incoφorated herein by reference. The test runs are conducted under a SO₂

concentration of 3900 ppm which is representative of the SO₂ concentration in flue gas. The

remaining components are 5.45% O₂ and the balance N₂. Experimental runs were conducted

at a temperature of 1080° C and for a solids residence time of 530 milliseconds. In order to

compare the reactivity of the 60pal8 sorbent with other commonly used sorbents, tests are

also conducted with a commercially available limestone (LC, Linwood CaCO₃), a calcium

hydroxide (LH, Linwood Ca(OH)₂) and a modified hydrate (MH, 1.5% lignomodified

Linwood hydrate). The modified hydrate was first developed at U.S. EPA (Kirchgessner and

Jozewicz, 1989) in order to improve the reactivity of the pure hydrate. In their experiments,

calcium lignosulfonate, an anionic surfactant was added to the water of hydration which was observed to increase utilization of the hydrate. The optimum lignosulfonate concentration

was determined to be 1.5% which resulted in a 20% increase in utilization over the pure

hydrate. In the present testing, the modified hydrate (MH) was prepared following a similar

procedure detailed by Kirchgessner and Jozewicz (1989). Further, all the sorbents are tested

under the same conditions of temperature, gas composition and residence lime in order to do

comparative analysis. Moreover, for all the sorbents, particles were classified in situ in a post

reactor cyclone assembly. A mean size (aerodynamic) of 3.9 μm is analyzed for sorbent

conversion using sulfate analysis in an Ion Chromatography (IC) system.

Figure 6 shows the test results in terms of extent of sulfation of the sorbents. The

extent of sulfation of 60pal8 is nearly 70% compared with the 32%, 27% and 38% for the

LH, LC and MH, respectively. The results obtained with the LH, LC and MH are consistent

with data previously reported in the literature (See Milne et al., (1990); and Kirchgessner &

Jozewicz, (1989)). This serves as a confirmation of the validity and reliability of the results obtained using the sorbent ofthe present invention. The repeatability ofthe results shown by

60pal 8 were confirmed by conducting multiple experiments under identical conditions, and

by observing that the conversion lies within ±5% ofthe average. The true significance ofthe

reactivity shown by the 60pal8 becomes clearer when expressed in terms of utilization under

actual conditions. The experiments here were conducted under conditions differential with

respect to SO₂. If the results are extrapolated to actual operating conditions with the Ca/S

molar ratio of about 2, a sorbent conversion of 70% would lead to a very high SO₂ removal (>

95%).

The sulfur capture ability of a sorbent is greatly influenced by the surface area of its

calcined product CaO. The higher the surface area, the greater is the reaction rate between

SO₂ and CaO. However, CaO reactivity is lost by not only losing surface area due to the

sulfation reaction, but also by grain coalescence due to high-temperature induced sintering.

The general understanding of the research community to date was that to achieve higher

necessary. Earlier work concentrated on producing modified calcium hydroxide powders,

which resulted in the synthesis of calcium lignosulfonate-modified hydroxide powder with

high initial surface area of greater than 60 m /gm. However, this powder showed only a marginal improvement in sorbent conversion in 500 ms (38% compared to 32% for pure

hydroxide).

The experiments with Linwood carbonate (very low surface area carbonate obtained

from Linwood Mining & Minerals Co.) showed that though this powder showed a poor

conversion, its calcined form, CaO, possesses high initial surface area and retains this high

value for a longer time than the calcium hydroxide-derived CaO, by undergoing a slower sintering. In its case, the unfavorable pore size distribution is responsible for poor SO₂

capture rather than a drastic loss of surface area due to rapid sintering.

Thus, it was determined that if a calcium carbonate powder with optimum pore

structure (which on calcination will give rise to CaO again with favorable pore size

distribution) is synthesized and employed for SO₂ capture, that may result in very high

sorbent conversion, and hence considerably high SO₂ capture.

Accordingly, the sorbents ofthe present invention are shown to give demonstrable

advantages in the removal of SO₂, over the sorbents of the prior art. Such advantages may

include:( 1 ) relatively high SO₂ removal (up to 95 % or more); (2) reduced sorbent

requirements (may achieve > 95 % SO₂ removal at Ca/S = 1.5); (3) ability to enhance use

of high-sulfur coal; (4) the FSI process, which itself is low cost retrofittable technology can be made more economically attractive by improving its SO₂ removal performance;

and (5) such highly reactive powder can have potential use in removing other toxic heavy

metals and/or inorganics from hot flue gas or other media.

In view ofthe present disclosure, it will be within the ability of one of ordinary skill

in the art to make modifications to the present invention, such as through insignificant

changes in component materials, their concentrations and/or their physical or chemical

parameters, in the process parameters, etc., to allow one to use the present invention

without departing from the spirit of the present invention as reflected in the appended

claims.

Claims

What is claimed is:

1. A sorbent comprising: a calcium carbonate powder comprising powder particles having an average

surface area above about 30 m /gram; said powder particles having pores of an

average pore volume, and the ratio of said average surface area to said average

pore volume is in the range of at least about 200 m²/cubic centimeter.

2. A sorbent comprising:

a calcium carbonate powder comprising powder particles having an average

surface area above about 40 m /gram; said powder particles having pores of an

average pore volume, and the ratio of said average surface area to said average

pore volume is in the range of from about 200 to about 350 m /cubic centimeter.

3. A sorbent according to claim 2 wherein said average surface area is in the range

above about 60 m /gram.

4. A sorbent according to claim 2 wherein said average pore volume is in the range

of above about 0.18 cubic centimeters per gram of said powder.

5. A sorbent according to claim 2 wherein said sorbent additionally comprises at

least one surfactant.

6. A sorbent according to claim 5 wherein said sorbent comprises at least about 2%

by weight of said at least one surfactant.

7. A sorbent according to claim 5 wherein said at least one surfactant is selected

from the group consisting of a sodium salt of a polycarboxylic acid.

8. A sorbent according to claim 2 wherein said sorbent is produced by precipitation from a solution of calcium hydroxide.

9. A process for making a calcium carbonate sorbent, said process comprising the

steps of:

(a) preparing a liquid suspension of calcium (II) ion; and

(b) subjecting said liquid suspension to a flow of carbon dioxide gas for

sufficient time and at sufficient temperature so as to form calcium

carbonate, wherein the concentration of said liquid suspension of calcium

(II) ion in the suspension of step (a) is at least about 4_sat. and wherein the

flow rate of said flow of carbon dioxide gas is at least about 0.48

liters/minute.

10. A process according to claim 9 wherein said concentration of said liquid

suspension of calcium (II) ion in the suspension of step (a) is at least about 16_sat.

11. A process according to claim 9 wherein the flow rate of said flow of carbon

dioxide gas is at least about 2.4 liters/minute.

12. A process according to claim 9 wherein said process additionally comprises the

addition of at least one surfactant during step (b).

13. A process of removing sulfur dioxide from a gaseous flow, said process

comprising:

exposing said gaseous flow to a sorbent, said sorbent comprising calcium

carbonate powder comprising powder particles having an average surface area

above about 40 m /gram; said powder particles having pores of an average pore

volume, and the ratio of said average surface area to said average pore volume is

in the range of from about 200 to about 350 m²/cubic centimeter, for sufficient

time and at sufficient temperature so as to bind said sulfur dioxide to said sorbent.

14. A process according to claim 13 wherein said average surface area is in the range

above about 60 m /gram.

15. A process according to claim 13 wherein said average pore volume is in the range

of about 0.18 cubic centimeters per gram of said powder.

16. A process according to claim 13 wherein said sorbent additionally comprises at

least one surfactant.

17. A process according to claim 13 wherein said sorbent comprises at least 2% by

weight of said at least one surfactant.

18. A process according to claim 13 wherein said at least one surfactant is selected

from the group consisting of sodium salts ofa polycarboxylic acid and anionic

surfactants .

19. A process according to claim 13 wherein said sorbent is produced by precipitation

from a solution of calcium hydroxide.

20. A process of removing at least one heavy metal from a sample, said process

comprising:

exposing said sample to a sorbent, said sorbent comprising calcium carbonate

powder comprising powder particles having an average surface area above about 40 m /gram; said powder particles having pores of an average pore volume, and

the ratio of said average surface area to said average pore volume is in the range

of from about 200 to about 350 m /cubic centimeter, for sufficient time and at

sufficient temperature so as to bind said at least one heavy metal to said sorbent.

21. A process according to claim 20 wherein said average surface area is in the range

above about 60 m /gram.

22. A process according to claim 20 wherein said average pore volume is in the range

of about 0.18 cubic centimeters per gram of said powder.

23. A process according to claim 20 wherein said sorbent additionally comprises at

least one surfactant.

24. A process according to claim 23 wherein said sorbent comprises at least 2% by

weight of said at least one surfactant.

25. A process according to claim 20 wherein said at least one surfactant is selected from the group consisting of sodium salts ofa polycarboxylic acid and anionic

surfactants.