WO1992000136A1

WO1992000136A1 - Simultaneously reducing dioxins and controlling hydrochloric acid emissions from solid waste incinerators

Info

Publication number: WO1992000136A1
Application number: PCT/US1991/004473
Authority: WO
Inventors: Laszlo Takacs
Original assignee: Occidental Chemical Corporation
Priority date: 1990-06-25
Filing date: 1991-06-20
Publication date: 1992-01-09
Also published as: AU8105691A

Abstract

Disclosed is a method and apparatus for reducing the formation of dioxins in flue gas from municipal solid waste incinerators and of removal hydrochloric acid from the flue gas. Optionally, the concentration of NOx can also be reduced and SOx can be removed by injecting ammonia through injection ports (17, 24, 29). Ammonia is injected into the flue gas in an amount at least sufficient to react stoichiometrically with the amount of hydrochloric acid present in the flue gas. If the control of NOx and SOx is also desired, then additional amounts of ammonia are injected with those gases and by use of a controller (38). Ammonium chloride and ammonium sulfite/sulfate precipitates from the flue gas when the temperature falls below about 180 °C, and can be collected on a moving grate (2).

Description

" SIMULTANEOUSLY REDUCING..DIOXINS AND CONTROLLING HYDROCHLORIC ACID EMISSIONS FROM SOLID WASTE INCINERATORS"

Background of Invention This invention relates to a method and apparatus for reducing the formation of dioxins in the flue gas from municipal solid waste incinerators, for removing hydrochloric acid from the flue gas, and optionally also for the removal of nitrogen oxides and sulfur oxides. In particular, it relates to the injection of ammonia into the flue gas, in an amount at least sufficient to react stoichiometrically with the amount of hydrochloric acid present in the flue gas, and to the subsequent cooling of the flue gas to about 180°C or less to precipitate ammonium chloride.

The term "dioxin" refers to polychlorinated dibenzo-para-dioxins, (PCDD) a family of compounds consisting of two benzene rings joined by two oxygen atoms and having from 1 to 8 chlorine atoms attached to the two rings. The dioxin of most concern is 2,3,7,8-tetrachlorodibenzodioxin (2,3,7,8-TCDD) . The term "furan" refers to polychlorinated dibenzo-para-furans (PCDF) , which are similar to dioxin except that only a single oxygen joins the two benzene rings. The term "dioxins," as used herein, shall mean either dioxin or furan or both.

Dioxins, particularly 2,3,7,8-TCDD, have been shown to be extremely carcinogenic to certain animals. As a result, there are federal, state, and local regulations that limit their

SUBSTITUTESHEET emission into the environment, even in levels of parts per billion (ppb) .

Municipal solid waste (MSW) consists of household garbage and garbage from various commercial facilities that is collected by municipalities. The quantity of MSW is approaching the critical point, primarily due to larger quantities of waste generated and to fewer landfill sites. While recycling and reduction of waste at its source are useful, they can eliminate only a small percentage of the total amount of waste. As a result, more and more of the waste must be burned in MSW incinerators. These MSW incinerators greatly reduce the volume of waste to a small amount of ash, but the incineration itself produces dangerous and undesirable substances. In addition to producing dioxins in the flue gas, these incinerators also produce hydrochloric acid, sulfur oxides (S0₂ and S0_) , and various nitric oxides (NO A.) , particularly nitric oxide, NO,_(_•.

The removal of dioxins and hydrochloric acid from the flue gas currently is accomplished by spraying the cooled flue gas with lime to form calcium chloride contaminated with dioxins. While the lime spray apparently does remove the dioxins from the flue gas, it does not destroy the dioxins and the disposal of dioxins-containing calcium chloride presents another environmental problem. Until now, the successful prevention of the formation of dioxins in the flue gas of a MSW incinerator has not been accomplished.

SUBSTITUTESHEET Summary of the Invention I have discovered that the formation of dioxins in the flue gas from a MSW incinerator can be reduced while simultaneously removing the hydrochloric acid from the flue gas by injecting ammonia into the flue gas, where the amount of ammonia is at least sufficient to react stoichiometrically with the amount of hydrochloric acid present in the flue gas, and subsequently cooling the flue gas to about 180°C. Below about 180°C ammonium chloride precipitates from the flue gas, which can be separated from the flue gas and collected with other particulates. Unlike the prior process where lime is sprayed into the flue gas to form calcium chloride contaminated with dioxins, the process of this invention prevents the formation of dioxins rather than permitting dioxins to form, then removing them. I have found that the process of this invention is very effective and prevents the formation of about 90% of dioxins that would otherwise form in the flue gas, while removing about 70 to 90% of the hydrochloric acid in the flue gas. The process has the additional advantage that the injection of ammonia can also be used to reduce the amount of sulfur dioxide in the flue gas, typically by about 70%, and the amount of nitrogen oxides in the flue gas, typically to about 40%.

Description of the Invention

SUBSTITUTE SHEET Figure 1 is a diagrammatic view illustrating a certain presently preferred process and apparatus according to this invention for injecting ammonia into the flue gas of a MSW incinerator.

In Figure 1, municipal solid waste 1 on moving grate 2 is passed into the combustion chamber 3 of boiler 4 where it is burned, forming a flue gas 5. The walls of boiler 4 are lined with water-containing tubes (not shown) for capturing the heat from the boiler for use in generating steam. Flue gas 5 typically has a composition of about 60 to about 1000 ppm hydrochloric acid, about 40 to about 400 ppm sulfur dioxide, and about 50 to about 1000 ppm nitric oxides. In the absence of treatment according to this invention, the flue gas would also typically contain about about 10 nanograms per dry cubic meter

(ng/ 3) to about 2500 ng/ 3 "total" dioxins (i.e., those dioxins that have four (tetra) to eight (octa) chlorine atoms) . While I do not wish to be bound by any theory, it is believed that the dioxins form from reactions between the hydrochloric acid and/or chlorine and the trace amounts of unburned hydrocarbons that are typically present in the flue gas. Ammonia in ammonia storage tank 6 is passed through line 7 to vaporizer 8 then back to tank

6 through line 9 where the gaseous ammonia enters line 10, passes through main control valve 11, then through line 12 to distribution point 13. From there it passes through line 14, regulator valve 15, and line 16 to injection port 17 in "denox"

SUBSTITUTE SHEET zone 18 of boiler 4. The temperature at the point of injection is about 700 to about 1040°C. The amount of ammonia injected through injection port 17 is at least sufficient to react stoichiometrically with the amount of NO present in the flue ^x gas. Nitric oxide is believed to react with ammonia according to the following equations:

6N0₂ + 4NH₃ > 5N₂ + 6H₂0.

N0_χ + NH₃ + 0₂ + (H₂) > N₂ + H₂0

The flue gas passes through superheater 19 which consists of a bank of tubes which capture additional heat from the flue gas for use in generating steam. The flue gas then enters dioxin suppression zone just before economizer 20 (a heat exchanger) . Ammonia from distribution point 13 enters line 21, passes through regulator valve 22, through line 23, and is injected into dioxin suppression zone 20 through injector port 24. The temperature of the flue gas at injector port 24 is about 230 to about 500°C. The amount of ammonia injected at injector port 24 should be at least sufficient to react stoichiometrically with the amount of hydrochloric acid present in the flue gas. However, the amount of ammonia injected at injection port 24 should not exceed the amount required to react stoichiometrically with the amount of hydrochloric acid present in the flue gas plus an additional amount equal to the amount required to react stoichiometrically with the amounts of sulfur dioxide and sulfur trioxide present in the flue gas. While the entire amount of the ammonia needed for

SUBSTITUTE SHEET NO and HCl control can be injected at injection port 17, it is preferable to inject ammonia for NO control at injection port 17 and ammonia for HCl control at injection port 24, as that is more effective in controlling the reduction of dioxins. The hydrochloric acid is believed to react with the ammonia according to the equation HCl + NH_ > NH.C1. The sulfur dioxide is believed to react with the ammonia according to the equation S0_

+ 2NH- + H-0 > (NH₄)₂SO_. The ammonium sulfite may oxidize to some extent to ammonium sulfate ((NH.)_SO.) in the presence of oxygen. The ammonium chloride and ammonium sulfite/sulfate formed remain in a vapor state until the temperature of the flue gas cools below approximately 180*C.

The flue gas then passes into duct 25. Additional ammonia can be passed from distribution point 13 through line 26, regulator valve 27, and line 28, where it can be injected into duct 25 through injection port 29. The injection of additional ammonia at injection port 29 is for the purpose of fine-tuning the reduction of hydrochloric acid and sulfur oxides. That is, the quantities of hydrochloric acid and sulfur dioxide in duct 25 can be measured and, if significant quantities have escaped destruction at injection port 24, additional ammonia can be injected to reduce their concentrations. However, the use of injection port 29 is not preferred as injection of ammonia at injection port 24 is usually adequate to control the hydrochloric acid and sulfur dioxide concentrations.

SUBSTITUTESHEET If the temperature of the flue gas, having passed through economizer 20, is higher than the desired temperature of 350°C, then the flue gas can be passed through another heat exchanger, a flue gas cooler 30, which is optional and is used for the purpose of lowering the temperature of the flue gas. The flue gas then passes through duct 31 to particulate controller 32. The particulate controller 32 can be, for example, a scrubber, a dry electrostatic precipitator, a wet electrostatic precipitator, or a bag house. The purpose of particulate controller 32 is to remove the particulates from the flue gas. The particulates pass out hoppers 33 as fly ash which is deposited in landfills. The ammonium chloride and ammonium sulfite/sulfate formed near injection port 24 also solidifies and is collected with the fly ash.

The flue gas then passes through duct 34 where blower 35 forces it through bridge 36 and up stack 37. A continuous emission monitor 38 measures the concentrations of sulfur dioxide, nitric oxides, hydrochloric acid, and ammonia in the flue gas in bridge 36 and controls the amount of ammonia injected through injection ports 18, 24, and 29 by means of regulator valves 15, 22, and 27, respectively, in response to those concentrations.

The following example further illustrates this invention.

Example

SUBSTITUTESHEET The experimental apparatus consisted of a three inch (7.6 - cm) diameter, eight foot (2.44 m) long insulated, stainless steel pipe equipped with various sampling ports. It was attached to the boiler outlet duct, after the superheater and just before the economizer. The flue gas temperature at this point was approximately 454^βC. A jet pump was used to draw a small side-stream of flue gas from the boiler and re-inject it into the boiler. In a separate port, an S-type pitot tube was inserted into the boiler duct to help maintain gas flow through the apparatus at the same velocity as the flue gas. Ammonia was injected at three points across the cross-section of the three inch (7.6 cm) pipe, two feet (0.6 ) upstream from the "dioxin reactor." Dioxin concentrations were measured simultaneously before (Background) and after ammonia injection (after "dioxin reaction") . The "background" sampling apparatus was a modified U.S. Environmental Protection Agency Method 5 sampling train. First, with no ammonia injected, the dioxin formation was simulated in the "dioxin reactor." Next, ammonia was injected to test the suppression of dioxin formation by ammonia in the "dioxin reactor."

For the "dioxin reactor," the first half of an U.S. Environmental Protection Agency Method 5 sampling train was further modified: the cyclone and the flat filter were replaced with two thimble filters in parallel and placed in a temperature controlled oven. The ash particles were trapped by these filters

SUBSTITUTE SHEET so that they were held in continuous contact with the flue gas stream. The remainder (back half) of the "dioxin reactor" was identical to the background dioxin sampling train. The contents of the thimble filters of the "dioxin reactor" were included in the dioxin/furan analysis (just like the contents of the cyclone and flat filter were included in the dioxin/furan analysis of the background train) .

The back half of the dioxin reactor sampling train was disconnected during the last few minutes of the experiments and the HCl sampling train, with or without the NH.Cl condenser, and the NH_ sampling train were connected to the back of the filters.

The HCl and the NH_ sampling trains (conforming to appropriate Environmental Protection Agency methods) were either connected to the ports in the three inch (7.6 cm) pipe (off side-stream sample) or were attached to the back of the dioxin reactor sampling train (off reactor sample) . During some of the experiments, an NH.Cl condensation system preceded the HCl sampling train. Without the NH.Cl condensation system, the measurements of ammonia and HCl were used to confirm that the reaction between these two compounds indeed took place, and to establish an HCl ammonia balance.

When the NH.Cl condensation system was employed, it was connected to the back of the "dioxin reactor." It consisted of coiled 1/4 inch (0.6 cm) I.D. polytetrafluoroethylene tubing, providing a gas residence time of approximately 0.25 seconds.

SUBSTITUTESHEET Following the coil were two flat filters in series. The coil-filters assembly was housed in a temperature-controlled oven. The concept was that the condensable, thus collectable, portion of the NH.Cl would be captured in the coil and filter. The non-condensables were captured in the standard HCl sampling train following the condenser system.

The following table gives the percentage of dioxin suppression when ammonia is used.

In the table, tests 2 and 3 were discarded due to anomalous results. Tests B-2, B-4, B-5, and B-12 were to determine the background dioxins; samples were taken before the reactor to determine the dioxin concentration in the flue gas as it left the

SUBSTITUTE SHEET boiler. "Cont. Time" is contact time, "NH STOICH" is the number of moles of NH₃ per mole of HCl, "REAC. TEMP." is reaction temperature, and "DSCM" is dry standard cubic meter of gas. If the dioxin background measured simultaneously with each test was subtracted, an average of 76% of dioxin suppression was measured. If the average dioxin background was subtracted, an average of 94% dioxin suppression was observed.

The following table gives similar results for furan.

PERCENT TOTAL PERCENT SUPPRES. PCDF-AVE SUPPRESS, by NH. BACKGRND by NH- ng/DSCM 101.44 115.58

63 .2 -69.13 168. 1

36. 3 3 .55 96.5

57. 3 -47.24 140.9

52.2 -32 . 60 128.2

-146.51

0.00 The above table shows that the average suppression rate for furan was greater than 52% compared to the simultaneously measured furan background.

SUBSTITUTE SHEET - 12 -

The following table gives the results of ammonia suppression on hydrochloric acid.

TEST NH COND. PERCENT No. STOICH. TEMP. REDUCTION

^•C IN HCl EMISS.

5 1 82 99

6 1 82 98

9 2 82 96 10 2 82 96

7 1 104 98 11 2 104 97

8 1 127 98 12 2 127 91

The above table shows that over 95% of the hydrochloric acid was removed from the flue gas by the use of ammonia.

SUBSTITUTE SHEET

Claims

I CLAIM:

1. A method of reducing the formation of dioxins in the flue gas from a municipal solid waste incinerator and of removing hydrochloric acid from said flue gas comprising

(A) injecting ammonia into said flue gas in an amount at least sufficient to react stoichiometrically with the amount of said hydrochloric acid present in said flue gas;

(B) cooling said flue gas to a temperature below about 180°C to precipitate ammonium chloride therefrom; and

(C) separating said precipitated ammonium chloride from said flue gas.

2. A method according to Claim 1 wherein said precipitated ammonium chloride is separated from said flue gas using a scrubber.

3. A method according to Claim 1 wherein said precipitated ammonium chloride is separated from said flue gas using an electrostatic precipitator.

4. A method according to Claim 1 wherein said precipitated ammonium chloride is separated from said flue gas using a bag house.

5. A method according to Claim 1 where said flue gas contains about 60 to about 1000 ppm hydrochloric acid and trace amounts of hydrocarbons.

SUBSTITUTESHEET - 14 -

6. A method according to Claim 1 wherein said ammonia is injected into said flue gas when the temperature of said flue gas has cooled to about 230 to about 350°C.

7. A method according to Claim 6 wherein the amount of said ammonia is at least sufficient to react stoichiometrically with the amount of said hydrochloric acid present up to an additional amount sufficient to react stoichiometrically with the sulfur oxides that are present.

8. A method according to Claim 6, wherein NO is also present in said flue gas, including the additional step of injecting ammonia into said flue gas when its temperature is about 700 to about 1040^βC in an amount at least sufficient to react stoichiometrically with the amount of NO present in said flue gas.

9. A method according to Claim 1 wherein all the ammonia is injected into said flue gas when its temperature is about 700 to about 1040"C, and the amount of ammonia injected is about the amount required to react stoichiometrically with the NO _&, HCl, and SO Ji. present in said flue gas.

10. In a MSW incinerator where municipal waste is burned producing a flue gas containing NO and hydrochloric acid, in which dioxins normally form, an improved method of removing said hydrochloric acid from said flue gas and of reducing the formation of said dioxins comprising

SUBSTITUTE SHEET (A) injecting ammonia into said flue gas when its temperature is about 700 to about 1040°C in about an amount required to react stoichiometrically with the amount of NO present in said flue gas;

(B) injecting ammonia into said flue gas when its temperature has cooled to about 230 to about 500°C, in an amount at least sufficient to react stoichiometrically with the amount of said hydrochloric acid present in said flue gas up to an additional amount sufficient to react stoichiometrically with 95% the sulfur oxides that are present, whereby ammonium chloride precipitates from said flue gas when it cools.

11. A method according to Claim 10 wherein said precipitated ammonium chloride is separated from said flue gas using a scrubber.

12. A method according to Claim 10 wherein said precipitated ammonium chloride is separated, from said flue gas using an electrostatic precipitator.

13. A method according to Claim 10 wherein said precipitated ammonium chloride is separated from said flue gas using a bag house.

14. A method according to Claim 10 where said flue gas contains about 40 to about 1000 ppm hydrochloric acid and trace amounts of hydrocarbons.

^ '^ ^

15. In a MSW incinerator having a boiler and a particulate collector, where municipal solid waste is burned and a flue gas falls in temperature as it passes through ducts from said boiler to said particulate collector, an improved dioxin and hydrochloric acid control assembly comprising injector means for injecting ammonia into said flue gas at a point where its temperature has cooled to about 230 to about 350^βC, in an amount at least sufficient to react stoichiometrically with the amount of said hydrochloric acid present in said flue gas, forming ammonium chloride which precipitates from said flue gas and is collected by said particulate collector.

16. An assembly according to Claim 15 wherein the amount of said ammonia that is injected is at least sufficient to react stoichiometrically with the amount of said hydrochloric acid present up to an additional amount sufficient to react stoichiometrically with the sulfur dioxide that is present.

17. An assembly according to Claim 15 wherein said particulate collector is a scrubber.

18. An assembly according to Claim 15 wherein said particulate collector is an electrostatic precipitator.

19. A method according to Claim 15 wherein said particulate collector is a bag house.

SUBSTITUTE SHEET

20. An assembly according to Claim 15 where said flue gas contains about 40 to about 1000 ppm hydrochloric acid and trace amounts of hydrocarbons.

21. An assembly according to Claim 15, where NO is also present in said flue gas, including a second injector means for injecting ammonia into said flue gas when its temperature is about 700 to about 1040°C in an amount at least sufficient to react stoichiometrically with the amount of NO present in said flue gas.

22. An assembly according to Claim 21 wherein the amount of ammonia injected into said flue gas when its temperature is about 700 to about 1040^βC is the amount required to react stoichiometrically with the NO A, HCl, and SO_fi present in said flue gas.

23. An assembly according to Claim 15 including

(A) means for measuring the amount of hydrochloric acid in said flue gas at a point downstream of where said ammonia is injected; and

(B) means for changing the amount of said ammonia that is injected into said flue gas as a function of the amount of said hydrochloric acid that is measured in said flue gas.

SUBSTITUTESHEET