Patent Grant
7531154
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 SOx 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 SOx gases at elevated temperatures.
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° C. 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.
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 NOx.
A schematic of the process is shown in
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
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+CO2Na2CO3+SO2→Na2SO3+CO2Na2SO3+1/2O2→Na2SO4
The 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 S03. 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+CO2Na2CO3+SO3→Na2SO4+CO2
The trona injection system may also be combined with other SOx removal systems, such as sodium bicarbonate, lime, limestone, etc. in order to enhance performance or remove additional hazardous gases such as HCl, NOx and the like.
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.
Trona was injected into flue gas at a temperature of 750° F. at NSR values of 1.0, 1.2, and 1.4.
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
Trona was injected into flue gas at a NSR of 1.5 in a temperature range of 750° F. to 805° F.
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.
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