Method of removing sulfur dioxide from a flue gas stream

Abstract

A method of removing SOx from a flue gas stream including SOx includes providing a source of trona and injecting the trona as a dry sorbent into the flue gas stream. The temperature of the flue gas is at an elevated temperature greater than 400° F., preferably between about 600° F. and about 900° F. The trona is maintained in contact with the flue gas for a time sufficient to react a portion of the trona with a portion of the SOx to reduce the concentration of the SO2 and/or SO3 in the flue gas stream.

Description

BACKGROUND

The present invention relates to the purification of gases, and more particularly to a method of purifying flue gases which contain noxious gases such as SO₂.

Dry sorbent injection (DSI) has been used with a variety of sorbents to remove SO_x and other gases from flue gas. However, DSI has typically been done in the past at temperatures much lower than 400° F. because equipment material, such as baghouse media, cannot withstand higher temperatures. Additionally, many sorbent materials sinter or melt at temperatures near or greater than 400° F., which makes them less effective at removing gases. The reactions products of many sorbent materials also adhere to equipment and ducts at higher temperatures, which requires frequent cleaning of the process equipment. To operate at these lower temperatures, the combustion gases must often be cooled before the sorbent was injected. This is an undesirable extra process step.

Thus, there is a need for a sorbent injection method that is effective at removing SO_x gases at elevated temperatures.

SUMMARY

In one aspect, a method of removing SO₂ from a flue gas stream including SO₂ is provided. The method includes providing a source of trona and injecting the trona into the flue gas stream. The temperature of the flue gas is between about 600° F. and about 900° F. The trona is maintained in contact with the flue gas for a time sufficient to react a portion of the trona with a portion of the SO₂ to reduce the concentration of the SO₂ in the flue gas stream.

In another aspect, a system for the removal of SO₂ from a flue gas stream including SO₂ is provided. The system includes a source of trona and a flue gas stream. The system also includes an injector for injecting the trona into the flue gas stream. The temperature of the flue gas is between about 600° F. and about 900° F. The system also includes an area for maintaining the trona in contact with the flue gas for a time sufficient to react a portion of the trona with a portion of the SO₂ to reduce the concentration of the SO₂ in the flue gas stream.

The foregoing paragraphs have been provided by way of general introduction, and are not intended to limit the scope of the following claims. The presently preferred embodiments, together with further advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of one embodiment of a flue gas desulfurization system.

FIG. 2 is a graph showing the % SO₂ removal as a function of normalized stochiometric ratio (NSR) for trona and sodium bicarbonate.

FIG. 3 is a graph showing the % SO₂ removal as a function of flue gas temperature for one embodiment of a flue gas desulfurization system.

FIG. 4 shows a perforated plate of an electrostatic precipitator after operation in one embodiment of a flue gas desulfurization system using trona.

FIG. 5 shows a perforated plate of an electrostatic precipitator after operation in one embodiment of a flue gas desulfurization system using sodium bicarbonate.

DETAILED DESCRIPTION

The invention is described with reference to the drawings in which like elements are referred to by like numerals. The relationship and functioning of the various elements of this invention are better understood by the following detailed description. However, the embodiments of this invention as described below are by way of example only, and the invention is not limited to the embodiments illustrated in the drawings.

Dry sorbent injection (DSI) has been used as a low cost alternative to a spray dry or wet scrubbing system for the removal of SO₂. In the DSI process, the sorbent is stored and injected dry into the flue duct where it reacts with the acid gas. The present invention provides a method of removing SO₂ from a flue gas stream comprising SO₂, preferably by injecting a sorbent such as trona into a flue gas stream to react with SO₂. Trona is a mineral that contains about 85-95% sodium sesquicarbonate (Na₂CO₃·NaHCO₃·2H₂O). A vast deposit of mineral trona is found in southwestern Wyoming near Green River. As used herein, the term “trona” includes other sources of sodium sesquicarbonate. The term “flue gas” includes the exhaust gas from any sort of combustion process (including coal, oil, natural gas, glass raw material, etc.). Flue gas typically includes SO₂ along with other acid gases such as HCl, SO₃, and NO_x.

A schematic of the process is shown in FIG. 1. The furnace or combustor 10 is fed with a fuel source 12, such as coal, and with air 14 to burn the fuel source 12. From the combustor 10, the combustion gases are conducted to a heat exchanger or air heater 40. The outlet of the heat exchanger or air heater 40 is connected to a particulate collection device 50. The particulate collection device 50 removes particles made during the combustion process, such as fly ash, from the flue gas before it is conducted to the gas stack 60 for venting. The particulate collection device 50 may be an electrostatic precipitator (ESP). Other types of particulate collection devices, such as a baghouse, may also be used for solids removal. The baghouse contains filters for separating particles made during the combustion process from the flue gas. Because of the relatively small particle size used in the process, the trona may act as a precoat on baghouse filter media.

The SO₂ removal system includes a source of trona 30. The trona 30 preferably has a mean particle size between about 10 micron and about 40 micron, most preferably between about 24 micron and about 28 micron. The trona is preferably in a dry granular form. A suitable trona source is T-200® trona, which is a mechanically refined trona ore product available from Solvay Chemicals, Green River, Wyo. T-200® trona contains about 97.5% sodium sesquicarbonate and has a mean particle size of about 24-28 micron. The SO₂ removal system may also include a ball mill pulverizer 32, or other type of mill, for decreasing and/or otherwise controlling the trona particle size on site.

The trona is conveyed from the trona source 30 to the injector 20. The trona may be conveyed pneumatically or by any other suitable method. Trona can be easily aerated for pneumatic transfer. Apparatus for injecting the trona or sodium sesquicarbonate is schematically illustrated in FIG. 1. Trona injection apparatus 20 introduces the trona into flue gas duct section 42, which is disposed at a position upstream of the baghouse inlet and upstream of the heat exchanger 40, if a heat exchanger or preheater is present. The trona injection system is preferably designed to maximize contact of the trona with the SO_x in the flue gas stream. Any type of injection apparatus known in the art may be used to introduce the trona into the gas duct. For example, injection can be accomplished directly by a compressed air-driven eductor.

The process requires no slurry equipment or reactor vessel if the trona is stored and injected dry into the flue duct 42 where it reacts with the acid gas. However, the process may also be used with humidification of the flue gas or wet injection of the trona. Additionally, the particulates can be collected wet through an existing wet scrubber vessel should the process be used for trim scrubbing of acid mist.

The temperature of the flue gas varies with the location in the injection system and may also vary somewhat with time during operation. The temperature of the flue gas where the trona is injected is between about 600° F. and about 900° F. The trona is maintained in contact with the flue gas for a time sufficient to react a portion of the trona with a portion of the SO₂ to reduce the concentration of the SO₂ in the flue gas stream. The temperature of the flue gas is preferably greater than about 630° F., and most preferably greater than about 700° F. The temperature of the flue gas is preferably less than about 800° F., and most preferably less than about 750° F. The temperature of the flue gas is most preferably between about 700° F. and about 750° F.

The process may also be varied to control the flue gas temperature. For example, the flue gas temperature upstream of the trona may be adjusted to obtain the desired flue gas temperature where the trona is injected. Additionally, ambient air may be introduced into the flue gas stream and the flue gas temperature monitored where the trona is injected. Other possible methods of controlling the flue gas temperature include using heat exchanges and/or air coolers. The process may also vary the trona injection location or include multiple locations for trona injection.

For the achievement of desulfurization, trona is preferably injected at a rate with respect to the flow rate of the SO₂ to provide a normalized stoichiometric ratio (NSR) of sodium to sulfur of between about 1.0 and 1.5. The NSR is a measure of the amount of reagent injected relative to the amount theoretically required. The NSR expresses the stoichiometric amount of sorbent required to react with all of the acid gas. For example, an NSR of 1.0 would mean that enough material was injected to theoretically yield 100 percent removal of the SO₂ in the inlet flue gas; an NSR of 0.5 would theoretically yield 50 percent SO₂ removal. SO₂ neutralization requires two moles of sodium per one mole of SO₂ present.

Unlike sodium bicarbonate, trona does not melt at elevated temperatures. Rather, sodium sesquicarbonate undergoes rapid calcination of contained sodium bicarbonate to sodium carbonate when heated at or above 275° F. It is believed that the “popcorn like” decomposition creates a large and reactive surface by bringing unreacted sodium carbonate to the particle surface for SO₂ neutralization. The byproduct of the reaction is sodium sulfate and is collected with fly ash. The chemical reaction of the trona with the SO₂ is represented below:2[Na₂CO₃·NaHCO₃·2H₂O]→3Na₂CO₃+5H₂O+CO₂ Na₂CO₃+SO₂→Na₂SO₃+CO₂ Na₂SO₃+1/2O₂→Na₂SO₄The solid reaction products of the trona and the SO₂ (primarily sodium sulfate) and unreacted soda ash may be collected in an electrostatic precipitator, or other particulate collection device. The total desulfurization is preferably at least about 70%, more preferably at least about 80%, and most preferably at least about 90%.

In one embodiment, the flue gas stream further comprises SO₃. The trona is maintained in contact with the flue gas for a time sufficient to react a portion of the trona with a portion of the SO₃ to reduce the concentration of the SO₃ in the flue gas stream. SO₃ is typically more reactive with the sorbent than SO₂, so the trona would remove the SO₃ first. The chemical reaction of the trona with the SO₃ is represented below:2[Na₂CO₃·NaHCO₃·2H₂O]→3Na₂CO₃+5H₂O+CO₂ Na₂CO₃+SO₃→Na₂SO₄+CO₂

The trona injection system may also be combined with other SO_x removal systems, such as sodium bicarbonate, lime, limestone, etc. in order to enhance performance or remove additional hazardous gases such as HCl, NO_x, and the like.

EXAMPLES

A study was done in a commercial glass plant in Verona, Calif. using a hot side electrostatic precipitator (ESP) and no baghouse. Natural gas was used as a fuel source, and the source of sulfur was from the glass raw materials. The SO₂ concentration in the flue gas was 800 ppm. The trona used was T-200® from Solvay Chemicals. The trona was injected in the duct using a compressed air blower and air lock feeder. Trona flow rates were measured by calibrating the airlock rpm with the trona weight loss in the trona storage bin. Trona feed rates varied from 50 to 211 pounds/hr.

Example 1

Trona was injected into flue gas at a temperature of 750° F. at NSR values of 1.0, 1.2, and 1.4. FIG. 2 shows the % SO₂ removal as a function of normalized stochiometric ratio (NSR) for trona. From these tests it can be seen that trona yielded SO₂ removal rates of around 80% at an NSR of 1.2. FIG. 4 shows a perforated plate of an ESP in the glass plant after operation of the SO₂ removal system for five months using trona. It can be seen that the plate is relatively free of solids buildup.

Example 2

As a comparative example, sodium bicarbonate was injected under the same conditions as Example 1 at an NSR of 1.2. The result is shown in FIG. 2. The % SO₂ removal of 72% was significantly lower than that of the trona at the same temperature and NSR. FIG. 5 shows a perforated plate of an ESP in the glass plant after operation of the SO₂ removal system using sodium bicarbonate. It can be seen that the plate has significant solids buildup.

Example 3

Trona was injected into flue gas at a NSR of 1.5 in a temperature range of 750° F. to 805° F. FIG. 3 shows the % SO₂ removal as a function of flue gas temperature. From these tests it can be seen that trona yielded SO₂ removal rates of up to 91% and was effective over a wide range of elevated temperatures.

From the above experiments it can be seen that trona was more effective than sodium bicarbonate at removing SO₂ from a flue gas stream at elevated temperatures. Thus, the system can use less sorbent material than a sodium bicarbonate system to achieve the same sulfur reduction. Additionally, it can be seen that trona had good performance over a wide range of elevated temperatures. Finally, the SO₂ removal system using trona had much less solids buildup in the perforated plates of the ESP than a system using sodium bicarbonate.

The embodiments described above and shown herein are illustrative and not restrictive. The scope of the invention is indicated by the claims rather than by the foregoing description and attached drawings. The invention may be embodied in other specific forms without departing from the spirit of the invention. Accordingly, these and any other changes which come within the scope of the claims are intended to be embraced therein.

Claims

1. A method of removing SO2 from a flue gas stream comprising SO2 and SO3, the method comprising: providing a source of trona;injecting the trona into the flue gas stream, wherein the temperature of the flue gas is between about 600° F. and about 900° F.; andmaintaining the trona in contact with the flue gas for a time sufficient to react a portion of the trona with a portion of the SO2 and to react a portion of the trona with a portion of the SO3 to reduce the concentration of the SO2 and the SO3 in the flue gas stream.

2. The method of claim 1 wherein the mean particle size of the trona is less than about 40 micron.

3. The method of claim 1 wherein the mean particle size of the trona is between about 10 micron and about 40 micron.

4. The method of claim 1 wherein the mean particle size of the trona is between about 24 micron and about 28 micron.

5. The method of claim 1 wherein the temperature of the flue gas is greater than about 630° F.

6. The method of claim 1 wherein the temperature of the flue gas is greater than about 700° F.

7. The method of claim 1 wherein the temperature of the flue gas is less than about 800° F.

8. The method of claim 1 wherein the temperature of the flue gas is less than about 750° F.

9. The method of claim 1 wherein the temperature of the flue gas is between about 700° F. and about 750° F.

10. The method of claim 1 wherein the trona is injected at a rate with respect to the flow rate of the SO2 to provide a normalized stoichiometric ratio of sodium to sulfur of between about 1.0 and 1.5.

11. The method of claim 1 wherein the trona is injected as a dry material.

12. The method of claim 1 further comprising milling the trona to a desired mean particle size at a location proximate the flue gas stream.

13. The method of claim 1 further comprising collecting a reaction product of the trona and the SO2 in an electrostatic precipitator.