Patent Application
20060029531
The invention relates to a process to reduce emissions of mercury from coal fired furnaces and other devices that burn fuels containing mercury.
Mercury is identified as a hazardous air pollutant and is the most toxic volatile metal in the atmosphere. Elemental mercury vapor can be widely dispersed from emission sources. Other forms of mercury pollutants include organic and inorganic compounds that accumulate in plants and animals. Mercury is a constituent part of coal mineral matter. Its emission from coal-fired power plants is suspected to be a major anthropogenic source of environmental mercury. Consequently, substantial effort has been made to develop devices and methods that will remove mercury from flue gas before the flue gas is released into the atmosphere.
Mercury is emitted in power plant flue gases because the elemental form is almost completely insoluble in water and flue gas desulfurization (FGD) scrubbing solutions. As such, the elemental mercury is either emitted as a vaporous gas, Hg(v), which is very difficult to separate or filter, or adsorbed onto flyash particulates and sorbents. If the mercury is oxidized it is Hg2+, which readily dissolves in water and FGD scrubbing solutions. Moreover, the oxidized form of mercury dissolved in aqueous scrubbing solutions is retained in wastewater streams and on suspended solids. Those streams are collected at very high efficiency with routine handling procedures. Any oxidized or elemental mercury bound to particulates is also removed with very high efficiency in electrostatic precipitators, baghouse filters, or cyclones. So almost all the mercury emitted from coal-fired power stations leaves the smokestack as elemental mercury vapor.
Consequently, most of the techniques which have been proposed for removing mercury from flue gas involve some action to prompt the formation of mercuric chloride, HgCl2 also called mercury chloride, and thereby minimize the amount of elemental mercury vapor. Perhaps the most obvious technique is to inject chlorine or a chlorine compound into the mercury containing flue gas. U.S. Pat. No. 6,447,740 describes a method for removing mercury from flue gas in which chlorine is injected into the flue gas. In a somewhat similar method disclosed by Ocher in United States Published Application No. U.S. 2004/0161771 a molecular halogen, such as chlorine gas, or molecular halogen precursor, such as calcium hypochlorite solution, is injected into the flue gas. However, chlorine is so corrosive to metals that furnace operators are reluctant to add chlorine in any form to a combustion system for controlling mercury emissions.
Another technique uses activated carbon and other fine particulates to absorb mercury. In this method carbon is impregnated with a halogen species, such as chlorides, iodine and/or sulphides. Unfortunately, the use of activated carbon requires extremely high carbon to mercury ratios. For that reason, collection by the use of activated carbon is very expensive.
Lanier et al. in published United States Patent Application No. U.S. 2004/0134396 observe that mercury emissions from flue gas containing fly ash that contains unburned carbon are lower than mercury emissions from flue gas containing fly ash that contains no unburned carbon. They attribute this result to a reaction between mercury and char that results in mercury being bound on the surface of the char. Therefore, they teach several techniques for controlling the operation of a coal burning furnace to increase the amount of unburned carbon in the fly ash and thereby reduce mercury emission. But, the highest amount of mercury capture reported by Lanier et al. was 43.7%.
Consequently, there continues to be a need for a method of removing mercury from flue gas that does not introduce corrosive materials into the combustion system and removes most, if not all, of the mercury from the flue gas.
We provide a method for removing mercury from flue gas produced by combustion devices burning coal and other fuels, such as municipal waste, that contain mercury and chlorine. In these devices the fuel is burned in a combustion zone in which the temperature exceeds 2600° F. Combustion produces flue gas containing fly ash that is directed through a first temperature zone in which the temperatures range from 1750° F. to 2100° F., through a second temperature zone in which the temperatures range from 900° F. to 1450° F., through a particle removal device, and through a wet scrubber.
First, we control the combustion process to generate a flue gas comprising fly ash containing at least 0.25% unburned carbon, and preferably at least 5.0% unburned carbon. Any of the several techniques for enhancing unburned carbon content in fly ash disclosed in published United States Patent Application No. U.S. 2004/0134396 could be used. Typically, the control would involve an adjustment to the operation of one or more burners in the furnace.
Next, we rapidly cool the flue gas from a temperature within the range of 1450° F. to 900° F. to a temperature below 900° F. at a rate of at least 1000° F. per second. This step will enhance the formation of mercury chloride, both in the flue gas and on the surfaces of the unburned carbon in the fly ash. Whereas the amount of mercury chloride is enhanced, the amount of elemental mercury vapor is reduced in inverse proportion. Once the mercury chloride and mercury bound to particles in the flyash are recovered in conventional exhaust system components, there is less elemental mercury vapor to be emitted from the smokestack.
Other objects and advantages of the present method will become apparent from the description of certain present preferred embodiments thereof which are illustrated by the accompanying drawings.
In the inventive process we oxidize mercury with chlorine to HgCl2, HgCl, HgO and other species, but we believe that the HgCl2 is the predominant oxidized specie. We believe that HCl is released from the burning coal, and subsequently partially decomposes into atomic (Cl) and molecular (Cl2) chlorine that oxidize mercury in the gas phase. We believe that HCl also chlorinates sites on the surfaces of unburned carbon and some of the minerals in flyash, and that these chlorinated sites also oxidize elemental mercury into mercuric chloride, HgCl, which subsequently leaves the surface and oxidizes to HgCl2. The Cl and Cl2 concentrations are dependent upon the HCl concentration, the OH concentration, and the temperature as well as several other species. The reaction pathway to mercuric chloride in the gas phase is said by Widmer to be:
Hg+Cl+M=HgCl
HgCl+Cl2=HgCl2+Cl
The reaction pathway to mercuric chloride on the particle surfaces is said by Niksa to be:
HCl+S-Open=S—Cl+Cl
S—Cl+Hg=HgCl
HgCl+Cl2=HgCl2+Cl
S-Open denotes an unoccupied site and S—Cl denotes a chlorinated site on the particle surface. The surface is chlorinated by HCl, the most abundant Cl-species in coal-derived exhausts, and the large storage capacity of carbon for chlorine ensures that a source of chlorine will be present to oxidize mercury over a broad temperature range. Chlorinated sites partially oxidize mercury into HgCl, which then leaves the surface. In the gas phase, the HgCl is completely oxidized to HgCl2 by Cl2, as indicated above, or the HgCl may be decomposed by OH into elemental mercury vapor. Provided there is a relative abundance of Cl2, mercury oxidation on particle surfaces converts elemental mercury vapor into mercuric chloride.
The chlorine species for both reaction paths come from chlorides in the coal. All coal contains some chlorine but the concentration may be from 0.05 to 1.0% in UK coals. U.S. coals have lower chlorine content and are usually less than 0.3%. Powder River Basin coals typically have chlorine concentrations of 0.03%. We have observed that the mercury emissions will decrease with increasing chlorine in the coal.
To calculate the species concentrations at various temperatures we have used the CHEMKIN36 software library and a detailed kinetic mechanism for coal combustion flue gas reactions, comprised of 51 species and 289 reaction steps. The calculation of the concentration of Cl as a function of chlorine in the coal is shown in
Referring to
Faster quenching also enhances the rate of mercury oxidation on particle surfaces, even though this so-called “heterogeneous reaction mechanism” occurs in a lower temperature window than Cl-atom activation. Faster quenching increases the concentration of Cl-atoms, as previously illustrated in
Referring to
Operation of the boiler 12 requires a supply of fuel to be burned, such as a coal supply 22. The coal supply 22 supplies coal at a predetermined rate to a pulverizer 24, which grinds the coal to a small size sufficient for burning. The pulverizer 24 receives a primary flow of air from a primary air source 26. Only one pulverizer 24 is shown, but many are required for a large boiler, and each pulverizer 24 may supply coal to many burners 20. A stream of primary air and coal is carried out of the pulverizer 24 through line 28. The primary stream of air and coal in line 28 is fed to the burner 20, which burns the fuel/air mixture in a combustion zone 30 in which the temperature exceeds 1700K (2,600° F.).
To assist in the burning, the boiler 12 includes a secondary air duct 32 providing a secondary airflow through overfire air ports to the burner 20. Usually about 20% of the air required for optimum burning conditions is supplied by the primary air source 26. The secondary air duct 32 is used to provide the remaining air. The secondary air duct 32 brings the excess air in from the outside via a fan (not shown) and the air is heated with an air preheater 36 prior to providing the air to the burner 20.
While only three burners 20 are shown in
While we have shown an opposed fired boiler 12 in
The cooling rate of the flue gas as it passes through the economizer is dependent upon the tube configuration and other design aspects of the economizer. Some economizers currently in service can cool flue gas at rates greater than 3000° F. per second (1649° C. per second). Adding fins to the cooling tubes can usually increase the rate of cooling. Therefore, it should be possible to use the present method in many furnaces without substantially modifying the furnace or adding expensive gas cleaning units to the exhaust system. If the economizer in an existing furnace has a slow cooling rate and a higher cooling rate is desired to achieve maximum mercury removal, it can be accomplished by adding fins to the economizer cooling tubes. Alternatively, a heat transfer grid 35 or other structure may be placed in a temperature zone 34 as shown in
The currently unregulated cooling rates of flue gas in the superheaters and economizers of operating coal-fired power stations are partially responsible for broad variations in the extents of mercury oxidation. Reported extents of Hg oxidation span the range of possible values, as seen in the ICR data in
As seen in
The premise that faster gas quenching accelerates Hg oxidation directly connects to several validated observations in lab-scale testing that HgCl2 concentrations at temperatures above 1200° F. are far greater than the equilibrium levels (See: Hall, B.; Schager, P.; Lindqvist, O. Water, Air, and Soil Pollution 1991, vol. 56, pp. 3-14 and Widmer, N. C.; Cole, J. A.; Seeker, W. R.; Gaspar, J. A. Combust. Sci. Tech. 1998, vol. 134, pp. 315-326.). In
The rate of mercury oxidation on particle surfaces is accelerated by the availability of more reactive surface area, which is obtained in practice with higher levels of unburned carbon in flyash. According to the art, higher unburned carbon levels are associated with greater measured values of flyash loss-on-ignition (LOI). The extents of Hg oxidation in Table 1 demonstrate that higher LOI increases the rate of mercury oxidation on particle surfaces. The proportions of oxidized mercury were measured (Gale, T. and Cushing, K.; EPRI-DOE-EPA-A&WMA Combined Utility Air Pollution Control Symposium: The MEGA Symp. 2003, EPRI) for five coals to cover broad ranges of both LOI and the coals' chlorine contents.
As seen in Table 1, all the bituminous coals generated substantially more oxidized mercury than the PRB subbituminous. Bit. #1 and Bit. #2 both had unusually low Cl-levels which were either equal to or below the PRB's. Nevertheless, they generated more oxidized mercury than the PRB because the higher amounts of unburned carbon associated with their greater LOI values accelerated the mercury oxidation rates. In the tests series with Bit. #3, increasing the LOI by adjusting the furnace operating conditions enhanced the production of oxidized mercury. These data indicate that more of the available mercury will oxidize to mercuric chloride in the presence of higher levels of unburned carbon, all else being the same.
Utility boilers burning subbituminous coals like Powder River Basin (PRB) coal often have a high economizer outlet temperature to maintain sufficient primary air temperature for the high moisture PRB subbituminous fuel, which often lowers steam throughput. Consequently, over 90% of the coal-Hg is typically emitted as elemental mercury vapor, due to the combined effects of slow gas quenching and low UBC (0.3% LOI is typical). The inventive method overcomes both obstacles with minimal impact on normal operations to convert most of the coal-Hg into mercuric chloride and mercury bound to particulates.
Optimal quench rates in the correct flue gas temperature window act to initiate Cl-atom availability which, in turn, increases the availability of Cl2. These chlorine species are highly reactive with Hg and HgCl, and will readily oxidize Hg to HgCl2 by both gas-phase and heterogeneous mechanisms. However, the heterogeneous mechanism is controlled by the ability of the carbon particles to chlorinate, adsorb Hg and release HgCl from the surface of a chlorinated site. This chlorination and subsequent mercury oxidation occurs in a lower temperature window than Cl-atom activation. Thus, the quench rates at both the finishing super heater and economizer control the extent of Hg oxidation and must be optimized, even though Hg truly oxidizes at cooler temperatures. Kinetic simulations of the full process show extents of mercury oxidation above 90% and Hg retention rates over 70%, even for flue gases from subbituminous coals.
We performed actual testing at a 500 MW utility boiler firing PRB coal in January of 2004. The measured mercury emissions documented in these tests are summarized in Table 2.
The tests showed that HgCl2 can be formed in the flue gas at temperatures of 1100-1400° F. in far higher concentrations than the equilibrium values, and these levels could be attained by rapid quenching of the flue gas. Calculated probe quenching rates were 3250° F./sec. The measured percentages of elemental mercury vapor appear in
We also simulated Hg oxidation for the coal properties and operational data for selected furnaces. Simply increasing the quench rate in the superheater and economizer significantly enhances the extent of mercury oxidation from 21 to 54%. Most the enhancement is obtained by accelerating the quench rate by roughly a factor of 2.5. The effect of small changes in LOI is dramaticallyillustrated in
Faster quench rates accelerate mercury oxidation by increasing the concentrations of Cl-atoms and Cl2.
In the gas phase, HgCl is either oxidized by Cl2 into HgCl2 or disintegrated by OH into elemental mercury vapor and HOCl. When Cl is scarce, as in flue gas from low rank coals, the Cl2 concentration determines the outcome of this competition. In the simulations, the steady-state Cl2 concentration increased from 0.5 to 30 to 120 to 310 ppb when super heater quench rates were progressively increased from 916 to 2650 to 3970 to 6620° F./s. This surge in Cl2 shifts the competitive reactions toward HgCl2 production. The effect saturates when all the HgCl generated on unburned carbon is subsequently converted to HgCl2 in the gas phase.
Higher levels of mercury can be removed from the flue gas when the furnace contains unburned carbon in the fly ash and the flue gas is rapidly quenched. The amount of unburned carbon in the flue gas can be increased by changing the operation of one or more burners to make combustion less efficient or by adding additional carbon to the flue gas. The easiest way of doing this is to change the flue air ratio in the combustion zone. Fast quenching can be obtained by the selection or modification of the economizer to increase surface area of the heat transfer tubes. Other ways of achieving these conditions that are known in the art could also be used.
The present method avoids the need to inject chlorine or chlorine components into the combustion system. However, one could make such an injection in addition to controlling the combustion process to provide unburned carbon and rapidly quenching the flue gas. However, such an injection may only be helpful in situations where coal having a very low chlorine content is being burned. If chlorine or chlorine compounds are injected the injection should be made in a zone immediately prior to the zone where rapid cooling of the flue gas occurs. Injections could be made through injectors 11 shown in
We have focused on the formation of HgCl2, but we would expect similar results if another halogen such as bromine or iodine were present and substituted for chlorine in the reactions here described. The rapid cooling will increase the concentration of halogen ions and elemental halogens Similarly, one may choose to inject another halogen or halogen compound in place of chlorine prior to rapidly quenching the flue gas.
Although we have described and illustrated certain present preferred embodiments of our method it is to be distinctly understood that the invention is not limited thereto, but may be variously embodied within the scope of the following claims.
