CHLORIDE PRE-SCRUBBER
Think Outside the Box
It is time to do a little creative thinking when it comes to coal-fired power generation. It is possible that a coal-fired power plant could make more money than it does now, create a green image and even achieve a net reduction in fine particulate. In other words, the fine particulate released by the plant is less than the fine particulate reduced by plant by-products.
Calcium chloride effectively reduces road dust and a big source of fine particulate is road dust. But calcium chloride is too expensive for widespread application. So if power plants supply calcium chloride for road dust applications, the reduction in particulate emissions could be greater than the amount emitted from the stack.
By-product calcium chloride is only one of the benefits of biomass gasification (with some added PVC) for reburn combined with chloride pre-scrubbing before the FGD system. Here are all the benefits.
It is amazing that we have been talking about this concept for many months and no one is rushing ahead to run mercury tests on the pre-scrubbers that are operating on coal-fired boilers. On the other hand, Babcock claims that their double loop scrubber is more effective for mercury control because of the low pH in the first loop. ALSTOM has published papers showing more than 80 percent removal of mercury in the chloride pre-scrubber and an additional 12 percent in the downstream SO2 scrubber. This is on a waste incinerator. But why is it not just as applicable to coal-firing? Why consider buying HCl to add to the coal, when you can recirculate as much as you want? The reduction in PM is new. So we are addressing it in detail below. Concerns about chloride corrosion in the boiler have been raised, so we are addressing them below as well.
Removal of trace elements from flue gas by wet FGD systems has been studied in the Netherlands where only pulverized coal-fired dry-bottom boilers equipped with high-efficiency electrostatic precipitators (ESP) and FGD using a wet lime/limestone-gypsum process with pre-scrubber are used. Mostly bituminous coals imported from the U.S. and Australia are fired. In one study the Hg concentration upstream of FGD was 3.4 µg/m3 and downstream was 1.0 µg/m3. The relative distribution of mercury between bottom ash, collected flyash and flyash in the flue gas, and the vapor phase was about 10 percent on fine particles and about 90 percent in vapor phase. Flue gas contained 87 percent of the mercury found in the coal and up to 70 percent of that was removed by the wet FGD. About 60 percent of mercury removal takes place in the pre-scrubber and about 40 percent in the main scrubber. So the Dutch have found that the pre-scrubber makes a substantial contribution to the overall mercury reduction.
Chloride Pre-scrubber Can Reduce PM
States cannot afford to buy calcium chloride to reduce the dust created on unpaved roads. If the power plants provided this product at zero cost, states could reduce PM emissions from the largest source. In Idaho 50 percent of the PM10 comes from unpaved roads and 30 percent of the PM2.5 comes from this source.
Calcium chloride absorbs water vapor from the air and liquid water from the road bed. At 77°F and 75 percent humidity, for example, it absorbs more than twice its weight in water. In addition, calcium chloride solutions attract more moisture to the road than they give up in evaporation. The road remains dense and compact under almost any level of traffic because calcium chloride keeps materials on the road by keeping moisture in the road, even under a burning sun on a sweltering day. Calcium chloride is generally sprayed as a 35 percent solution using a tank truck with a rear-mounted distribution bar that spreads the liquid evenly over the road. One pass will cover an 8-to 12-foot-wide road. Two passes are needed on roads 16-to 18-feet wide. As soon as calcium chloride enters a road, it's attracted to negatively charged soil particles, such as clays, which help resist leaching. Calcium chloride may move deeper into the base during wet weather but will rise toward the surface during dry spells. An unpaved road stabilized with calcium chloride retains a smooth dustless surface. The moisture retained keeps the surface plastic enough so fines can migrate into gaps formed between aggregates under the varying pressure of car and truck traffic. In short, calcium chloride does the following:
Iowa estimates the yearly cost for dust control on unpaved roads at $2-3 million per county, therefore it is too expensive. So the state allows individuals to apply calcium chloride for small stretches adjacent to their property.
Chloride Boiler Corrosion Can be Minimized
Chloride corrosion problems can be avoided by several methods. One is to limit the chlorides to 0.3 percent of the coal. The U.S. burns one billion tons of coal. So 0.3 percent would be three million tons of chloride. Many British studies have correlated superheater/reheater corrosion in pulverized coal boilers with the total chlorine (Cl) content in coals, which has led many U.S. boiler manufacturers to set their maximum recommended Cl level at 0.25 percent to 0.3 percent. However, Cl-related boiler corrosion has not been reported by U.S. utilities burning high-Cl Illinois coals. Other factors, such as alkali metals, sulfur, or boiler parameters, have been studied for their effect on corrosion rate. A study was conducted to: 1) measure the rate of corrosion caused by two high-Cl coals (British and Illinois) and one low-Cl Illinois coal under identical combustion conditions for a duration which will give reliable comparisons, 2) determine the concentration and occurrence of chlorine, sodium, sulfur, and potassium compounds in the gas and solid phases produced during combustion, and 3) define the nature of Cl in coals and its behavior during combustion.
Two specific metal temperatures, which are commonly adopted in the superheater of U.S. (1,100°F) and the UK (1,200°F) boilers, and two metal alloys, 304SS and 310SS, were examined during long-term 1,000-hour combustion corrosion tests. For the most commonly used 304SS, the rate of corrosion increased significantly with an increase in the tube wall temperature from 1,100°F to 1,200°F for all three coals. The rate of corrosion for 310SS, which had the greater chromium and nickel content among the two alloys and is of interest to boiler manufacturers as a test material for boiler construction, did not follow the trend observed with 304SS for both Illinois coals. The results showed no evidence of a correlation between coal Cl content and rate of corrosion, but showed a correlation between the rate of corrosion and the type of metal and the metal temperature used. The results suggested that the different history in corrosivity experienced by the superheater/reheater of UK and U.S. utility boilers burning high-Cl coal could be a result of the difference in the boiler temperatures used in the UK and U.S. boilers. Also, other factors, such as potentially volatile alkali metals or different alloy compositions of the utility boiler, could change the corrosion behavior of a coal. The results also suggested that high-Cl Illinois coals, like low-Cl coals, could be successfully used if other coal properties or combustion conditions could be controlled.
Bill Ellison Says Water Balance a Concern with Chloride Pre-scrubbers
McIlvaine reviewed the chloride pre-scrubber option with Bill Ellison. Bill has extensive experience with chloride pre-scrubbers - most of it painful. Chloride pre–scrubbers were installed in a GEESI FGD at the Newton, Illinois plant. One of the reasons was to allow the potential for using high chlorine Illinois coals. Some Illinois coal is as high as 0.6% sulfur. Alloy steel construction was utilized in the pre-scrubber and high rates of corrosion took place. As a result the utility switched to using the limestone slurry in the pre scrubber as well as the main scrubber.
Ellison warns that you have to address three problems: Corrosion, cost, and water balances. He agrees with McIlvaine that materials such as reinforced concrete with Stebbins Semplate would solve the corrosion problem. He also agrees that the capital cost should be less than $15/kW. Relative to the water balance problem, he explains that if you have a pre-scrubber it will saturate the flue gas. This means that no further evaporation will take place in the limestone scrubber. This results in more wastewater to treat.
Further discussion resulted in a positive rather than negative aspect of the water balance question. The FGD wastewater can be used for makeup with the HCl from the chloride scrubber if in fact the end product is going to be calcium chloride.
As Bill points out there is considerable negative experience with chloride pre-scrubbers in the past. The main reason is the wrong materials of construction. But equally important is the fact that there was no interest in removing mercury or making calcium chloride. Now these two benefits make the addition of chloride pre scrubbers very attractive.
How efficient is the chloride pre-scrubber in capturing mercury? There is uncertainty about the ability of chloride pre-scrubbers to remove mercury. Some waste incinerators are reporting very high mercury removal in the chloride pre-scrubbers. Hans Gutberlet published a paper in VGB Kraftwerkstechnik # 72 in 1992 saying that only 35 percent of the mercury was removed in pre-scrubbers in coal-fired plants operated by VKR but the combination pre-scrubber and limestone scrubber resulted in almost total HgCl capture. HgCl represented 90-95 percent of total mercury. The percentage of oxidized mercury is due to the SCR. Investigations on three dry bottom furnaces and one slag tap furnace revealed that while metallic mercury was 40 to 60 percent of the total upstream of the SCR it was only 2-12 percent of the total downstream. Oxidation did not occur in tail-end SCR systems. This was due to the lack of HCl. Investigation was undertaken as to why the mercury removal was not higher in the pre-scrubbers. The conclusion was that the recycled HCl with the mercury led to re-emission of the mercury. As a result, in experiments thioacetamide and sodium sulfide were added in the pre-scrubber to precipitate out the mercury. This led to substantial increases in mercury capture of 15 percent to 35 percent.
So the conclusion is that chloride pre-scrubbers, in combination with limestone scrubbers, will remove nearly all the mercury chloride. It is possible to recycle some of the HCl from the pre- scrubber back to the incoming coal to ensure that little if any elemental mercury exits the furnace. Precipitation of the mercury from the recycling HCl will prevent re-emission. Therefore, mercury can be removed without buying any chemicals and there is a salable end product.
300-Million HCl Produced But Only $24 Million Merchant Market
There are sixty-nine sites producing HCl with a combined annual capacity in excess of 4.62 million tons. However, only fifty-four sites ship product into the merchant market. Most manufactured HCl is captive capacity. More than 90 percent of the HCl is by-product production as a result of aliphatic and aromatic hydrocarbon chlorinations, the phosgenation of amines for isocyanates and halogenations for making chlorofluorocarbons. The balance of the HCl comes from intentional manufacturing: burning chlorine with hydrogen; the reaction of sodium chloride with sulfuric acid (Mannheim Process); or by reacting sodium chloride, sulfur dioxide, oxygen and water (Hargreaves Process). These last two processes find use at only one plant each. The market is made up of both anhydrous and aqueous product.
The growth in demand for HCl between1996 and 2001 was 1.5 percent per year; future growth is projected at 1.1 percent per year through 2005. Over the last five years the price has reached $138/ton but has moved both lower and higher than the current price of $72/ton.
Uses, excluding HCl consumed in EDC production, include organic chemicals, 29 percent; inorganic chemicals, 12 percent; food processing, 11 percent; brine treatment, 10 percent; steel pickling, 8 percent; oil well acidulation, 4 percent; swimming pools, 2 percent; miscellaneous, including semiconductors, catalyst for chemical syntheses, mineral processing and regeneration of ion-exchange resins used for water treatment, 24 percent.
Chemical Production with Biomass Gasification and Reburn Could Be a Bigger Winner
Recent papers on mercury control have proposed the addition of chlorides to the coal in order to oxidize mercury for capture in the scrubber. Tests at a Minnesota power plant indicated that a special and expensive chloride compound was more effective than the cheap alternative. Hundreds of millions of dollars spent for these chlorides would still be less costly than the activated carbon which is the present popular choice.
A dealer of scrap plastic pellets has been selling as much non-PVC plastic as he can produce to Illinois Power. It is a cheap high Btu fuel. The dealer has large quantities of PVC scrap, but Illinois Power has been hesitant to add this type of plastic because of the chloride content. But if the power plant gasifies PVC waste along with biomass, it can add the optimum amount of chlorine to the boiler to oxidize mercury. So by adding the biomass gasifier you reduce NOx, obtain credit for reducing greenhouse gas emissions and also eliminate the need to buy chlorides for mercury removal.
The amount of biomass needed for gasification into a reburn fuel is only a percentage of the total heat input at the power plant. So in quantities of 5-10 percent of total Btu, there would be enough biomass to supply coal-fired boilers throughout the country.
The idea of chemical production can be extended. Municipal incinerators in Europe utilize a two stage scrubbing system. The first stage captures the chloride and recirculates it. The bleed stream is industrial grade hydrochloric acid. Since this scrubber is an efficient mercury capture device, an additive such as the Degussa TMT 15 is used to remove the mercury from the HCl. Merchant acid sales in the U.S. match the emissions of HCl to the atmosphere from power plants. If power plants sold this acid instead of emitting it and supplanted HCl production plants, they would be credited with the energy savings and environmental benefits associated with HCl production. This broader role for coal fired power plants would make them greener and more popular with environmentalists.
HCl is the Most Valuable Coal Byproduct
HCl sells for $200/ton and calcium chloride sells for as much as $400/ton. HCl is therefore the most valuable byproduct. Some Illinois coals have more than 0.4 percent chlorine. A 2,000 MW plant would generate 20,000 tons/yr of HCl worth $2 million. The Monroe plant of Detroit Edison is now emitting 9.3 million pounds of HCl per year. So the amounts are not insignificant. You can make HCl by simply using a pre-scrubber before your SO2 scrubber. This will result in major mercury reduction. So this option has to be very attractive. It is easy to remove the mercury from the HCl and then to make calcium chloride. So you have concentrated mercury into a matrix that is just a few hundred pounds per year and at the same time have a salable product. You will also reduce chloride corrosion in the SO2 scrubber and chloride contamination of gypsum.
Carbon Slurry in Pre-scrubber
We still have not had anyone give us a good reason why chloride pre-scrubbers are not a viable route to mercury reduction. We learned at Power-Gen Europe that you can even recirculate a carbon slurry in your HCl scrubber. This would seem to provide very high mercury removal.
80 to 90 Percent Mercury Removal in Pre-scrubber
We believe a real winner would be the chloride pre-scrubber. It can be used with any coal type. It eliminates chloride corrosion in the SO2 scrubber and reduces the cost of wastewater treatment and gypsum washing. ALSTOM reports on one waste incinerator in Germany where the chloride pre- scrubber removes 80-90 percent of the mercury and in tandem with the downstream SO2 scrubber removal is as high as 95 percent. So this is a proven technology with lots of installations in Europe.
John Tarabocchia of Degussa updated us on the German experience.
“Most of Degussa's experience with TMT for mercury removal is with garbage
incinerators. However, we also have many coal-fired power plants using TMT as
well. The traditional application of TMT has been in treating the blowdown
stream in a wastewater treatment system.
The garbage incinerators are typically equipped with an acid scrubber (to take
out chlorides as HCl) followed by an "alkaline" scrubber operating in the pH
range of 5 to 7. These scrubbers are typically packed towers. One benefit of
having two scrubbers is a cleaner gypsum by-product. Another benefit is better
mercury removal. Most mercury (perhaps 70% to 80%) is typically removed in the
acid scrubber. Oxidized forms of mercury are soluble and form stable chloro
complexes in the scrubbing media. Since SO2 passes through the acid
scrubber, the chemical reduction of Hg (II) ion to elemental mercury by the
sulfite ion is less likely.
The trend at German plants is to spray dry the blowdown so that there is no
wastewater. These plants have decreased the blowdown flow (to lighten the water
load on the spray dryer) and increased the salt concentrations in the scrubbing
media. Naturally, the mercury concentration in the scrubbing media of both
scrubbers increased as well. We believe that this in turn leads to increased
stack emissions. Under this operating scenario, plants with continuous emission
monitors discovered that they could not meet their emission limits. It was found
that TMT directly injected into the scrubbing media of the alkaline scrubber,
decreased the gas phase mercury emissions.”
The conclusion has to be that the cost of mercury removal is likely to be about the same regardless of the coal type. For EPA to micro manage operations and in effect influence the type of coal being burned is astounding. McIlvaine has offered an alternative mercury rule which would protect plants which could not economically remove mercury but is not dependent on unwarranted assumptions about technology
Chloride Pre-scrubber Removes Mercury, Generates Salable Salt and May Reduce Investment Cost of Scrubber
Mercury removal could be the biggest reason to incorporate a chloride pre-scrubber stage in a wet FGD system. The Germans have experienced high mercury removal in the chloride scrubbers used in waste incineration. Take a 300 MW plant for example. Raw mercury emissions are 100 pounds. The SCR/scrubber combination removes 90 pounds. The pre-scrubber removes another five pounds. If the average trading value of these pounds is $10,000, then the pre scrubber would generate $1 million in allowance revenues over a 20 year life.
But the initial investment may even be less with the pre-scrubber. A chloride pre-scrubber does not have to be more than one perforated plate or rod deck with a Chinese hat or mist eliminator deck in between. A separate piping loop would circulate first water and then as HCl is captured it would recirculate an acid around pH 1 or so. A bleed stream would direct the acid for purification and disposal or sale. German waste incinerators sell their hydrochloric acid. One U.S. plant is selling calcium chloride for road salt. This chlorine comes from the gypsum washing.
The advantage would be that the FGD scrubber (on top of the chloride scrubber) and the wet precipitator could be constructed of less expensive materials. As we pointed out in our Fax Alert last week, the price of C276 has increased 300 percent in just six years. If an alloy with half the cost is used for the FGD scrubber and WESP, then the investment in the chloride scrubber may be more than offset. Chlorides are the contaminant which necessitates the use of high nickel alloys.
Another advantage would be in the chloride gypsum wash cycle. Presently plants have a separate wash cycle on the belt filter which removes chlorides. Then this wash water has to be treated prior to disposal.
Avoid Gypsum Contamination with Pre-scrubber
Heizkraftwerk Nord has been using HCl pre-scrubbers to capture mercury for more than 15 years. The gas flow is 300,000 m3/h in two trains. Initially the HCl scrubbers were operated at pH 2 to 2.5 by adding dolomite. But to prevent the reduction of mercury chloride by SO2, the pH was reduced below 2. TMT 15 sold by Degussa is used to precipitate the dissolved mercury.
We talked to John Tarabocchia who says that Degussa is continuing to pursue this application and will be running tests at a German power plant in the next month or two. TMT 15 can be used whether or not there is a pre-scrubber. The reason incinerator plants operate with two scrubbers is so that the gypsum formed in the second scrubber is not contaminated by the heavy metals.
Chloride Additions to Coal Tested on Full Scale Spray Drier and Wet Scrubber Plants
Testing at the Laskin Energy Center and Stanton reveals that there is a lot that is not known about chloride additions to coal in order to oxidize mercury and then capture it in wet scrubbers and spray drier baghouses. The spray drier baghouse at Stanton achieved no mercury removal with a cheap chlorine salt which cost $0.08/lb even though there was some oxidation. The best results were with a salt which achieved 50 percent removal but at a cost of $0.35/lb. The addition rates were 200 lbs/hr. So the yearly cost for a 60 MW boiler would be $490,000 at 7000 hrs/yr. Laskin is a wet particulate scrubber and was only able to achieve 30 percent removal. In all cases, the amount of chloride injected was kept well below the 0.2 percent. High HCl removal was obtained in both the spray drier and scrubber. The message given by Carl Richardson of URS, Ramsay Chang of EPRI and co-authors from Great River, Minnesota Power and Xcel is that there are lots of unknowns about chloride additions. Furthermore, it could be expensive and maybe not very efficient.