Treating the smoke billowing from a power plant is a cleaning job of breathtaking proportions. A typical 500 megawatt fossil fuel facility emits 1.3 million cubic feet of exhaust gas every minute. Over the course of a single day, that plant discharges into the skies 300 to 400 tons of sulfur dioxide and oxides of nitrogen (NOx).
Power plant emissions are a major cause of changes in the chemistry of the atmosphere. Sulfur dioxide and NOx are the chemical precursors of acid rain and urban smog. And, carbon dioxide is a greenhouse gas that ultimately might change global climate.
Neutralizing this leviathan load of emissions is akin to embarking on a mythical battle to douse a dragon's breath. Applied Science's Flue Gas Chemistry Group leader Shih-Ger (Ted) Chang has grappled with this scientific challenge for a decade. Over that time, his scientific machinations have produced five patents related to flue gas chemistry.
Chang believes a new technology he is developing will provide the most practical means yet to reduce NOx and sulfur dioxide. He and his colleagues have invented an elegant, cost-effective process that allows power plant scrubbers, designed to remove sulfur dioxide only, also to capture nitrogen oxides. What is more, the process transforms these pollutants into valuable commercial chemicals.
Over recent years, as it has become evident that the atmosphere cannot continue to absorb increasing loads of pollutants without consequence, a number of technologies have been devised to reduce both NOx and sulfur dioxide emissions. The technology developed by Chang and his research team of David Liu, David Littlejohn, and visiting Chinese scientists D.X. Shen and Y.Z. Wang has multiple advantages.
The LBL process can remove better than 90 percent of both sulfur dioxide and NOx. Too, it creates a stream of commercial chemicals that can be sold to offset the cost of the pollution control equipment. Very little solid waste is created.
Most other technologies transform the costly reagents used to strip out gaseous pollutants into solid waste. Disposing of this sludge is an expensive, land-intensive proposition. A 500 megawatt plant burning medium-sulfur coal over 30 years that is equipped with a conventional limestone scrubber system requires 200 acres for waste disposal ponds and dikes.
Aside from performance, however, the LBL technology has a corollary financial advantage. The Bechtel Corp., a leader in the international business of emissions controls, is advising Chang on commercial and regulatory considerations, and has performed a preliminary economic analysis. Bechtel says compared to existing high quality treatment systems, the LBL process provides improved performance at a lower cost. Cost is the overriding factor affecting the success of emissions treatment technology.
Up until now, dilution has been the solution in the case of most power plants. Around the world and even in the U.S., most power plants do not significantly treat exhaust gases before they are vented into the atmosphere. Some advanced plants are equipped with flue gas desulfurization systems. Only a very few plants include equipment to eliminate nitrogen oxides.
About 85 percent of existing desulfurization systems consist of calcium-based scrubbers that typically react boiler exhaust gases with a wet limestone and water solution, removing sulfur dioxide before the exhaust is discharged from smokestacks.
The LBL process augments this conventional, proven scrubber technology. It allows existing and future scrubbers to perform the double duty of also removing NOx.
Yellow phosphorus is fed into the scrubbing system along with the aqueous calcium-based slurry. Oxides of nitrogen consequently are removed along with the sulfur dioxide. Elegant and effective, the process may be the breakthrough that Chang and scientists worldwide have pursued for years.
Yellow phosphorus, a novel scrubbing reagent conceived by Chang, triggers a chain of reactions within the scrubber that transforms insoluble gases into soluble compounds. Some 95 percent of the oxides of nitrogen in the exhaust stream are nitric oxide, which is highly insoluble.
Initially, phosphorus reacts with oxygen to produce ozone. Ozone, in turn, reacts with nitric oxide to produce nitrogen dioxide. The resulting nitrogen dioxide then can react with residual nitric oxide to form soluble compounds which are subsequently reduced to form ammonium salts. Phosphorus is oxidized to form phosphoric acid.
Most of the treatment byproducts consist of phosphoric acid and gypsum, but also ammonium phosphate. Gypsum is a building material. Phosphoric acid and ammonium phosphate are used to make fertilizers. Phosphoric acid also is used in metal treatment, refractories, catalysts, fuel cells, foods, and beverages. One day, the phosphoric acid in Coca Cola could be manufactured by power plants.
In the U.S., about 50 percent of NOx emissions and 80 percent of sulfur dioxide emissions result from the burning of fuel in power plants, homes, and offices. By reducing emissions of these compounds, LBL's scrubber technology attacks both acid rain and tropospheric ozone, a primary constituent of urban smog.
Acid rain stresses and kills forests, stunts crops, creates sterile lakes, and damages buildings, causing billions of dollars in harm to the environment. It forms when sulfur dioxide and oxides of nitrogen are emitted into the troposphere and are chemically converted into sulfuric and nitric acids.
Nitric oxide, if emitted from a power plant, is oxidized to nitrogen dioxide, giving urban smog its characteristic brown hue, and inducing the formation of ozone. More than half of America's metropolitan areas have ozone levels that exceed federal health standards. Itching eyes, coughing, chest pains, and shortness of breath are the immediate health effects. Still inconclusive evidence indicates that chronic exposure to the ozone levels common in American cities may cause irreversible lung damage and also damage the immune system. Additionally, ozone causes serious injury to the foliage of plants. In Maine's Acadia National Park, for instance, two thirds of the white pines have been damaged by ozone.
Ozone is formed through a complex series of photochemical events that involve hydrocarbons and nitrogen oxides. Efforts to control ozone have focused on reducing hydrocarbons, but new findings are causing that strategy to be reconsidered.
Atmospheric hydrocarbons either are released by man -- motor vehicles are a chief source -- or are emitted naturally by vegetation. Recent studies show that in the U.S., from 62 to 77 percent of the hydrocarbons are natural with peak outputs of vegetative hydrocarbons on hot summer days that coincide with peaks in ozone concentrations.
Attempts to diminish ozone levels by reducing auto hydrocarbon emissions may be futile. Instead, it may be necessary to limit the oxides of nitrogen being discharged into the atmosphere.
Currently, LBL and Bechtel are jointly testing the new technology at LBL with a laboratory bench scale scrubber with a 20 cubic feet per minute flue gas flow rate. The test apparatus does not include a fuel-burning boiler that generates flue gas. This would turn the laboratory into an oven. Rather, scientists synthesize the boiler emissions. To accomplish this, liquid nitrogen is passed through an evaporator which acts as a heat exchanger, transforming liquid nitrogen into nitrogen gas. Then this stream of nitrogen is mixed with compressed air, simulating the flue gas of a coal-fired power plant which has four-to-five percent oxygen. Next, appropriate quantities of sulfur dioxide, nitrogen oxides, and carbon dioxide are added to create a simulated coal-fired power plant flue gas. The simulated flue gas is heated to 150 degrees C., the temperature at which it typically enters the pollution control system.
At this point, the flue gas is mixed with a spray of aqueous emulsion of yellow phosphorus in a prescrubber, and the alchemy from pollution to commercial solution begins. Yellow phosphorus reacts with oxygen to produce ozone, which then oxidizes nitric oxide to the more soluble nitrogen dioxide. Nitrogen dioxide also may react with the remaining nitric oxide to form dinitrogen trioxide, which also is very soluble in aqueous solutions.
The absorber is a state-of-the-art Chiyoda system designed by a Japanese company (but the LBL process also works with a conventional absorber). The Chiyoda improves on the conventional absorber in that the flue gas is allowed to bubble through the aqueous slurry. Because of the high velocity of the flue gas, the solution froths like spilled beer, creating additional surface area with which the sulfur dioxide, nitrogen dioxide, and dinitrogen trioxide in the flue gas can make contact and subsequently dissolve.
As treated flue gas and the limestone are fed into the absorber, the scrubbing continues. Nitrogen dioxide and dinitrogen trioxide are reduced by sulfur dioxide in solution to form ammonium ions, and sulfur dioxide is oxidized to form sulfate ions, which precipitate out of the solution as gypsum.
During bench-scale tests, Chang analyzed a remaining white plume of gas being emitted subsequent to scrubbing and learned that it was phosphorus pentoxide (P4O10), an aerosol used during past wars to create smoke screens. Familiar with the compound, Chang realized that if absorbed with an aqueous phosphoric acid solution, the aerosol could be transformed into more phosphoric acid. He modified the test apparatus so the absorption could take place in a separate absorber. The process acts as a concentrator of phosphoric acid, further improving the economics of the technology.
Chang's breakthrough is timely. Air quality regulations around the world are tightening, requiring decreased sulfur dioxide emissions and what in many cases is the first mandatory reduction in nitrogen oxides emissions.
The U.S., along with 24 other nations, has signed a protocol that calls for freezing NOx emissions at 1987 levels by 1995. Other nations have decided the status quo is unacceptable. German and Japanese emissions standards are more stringent than those in the U.S. Additionally, 12 European nations have agreed to cut NOx emissions 30 percent over the next 10 years.
Several classes of technology are available for high efficiency reduction of sulfur dioxide and NOx.
Many older plants were expected to be candidates for "repowering." In a repowered plant, a refurbished combustion system is engineered to minimize emissions, averting treatment of the flue gas. Atmospheric fluidized-bed combustion is perhaps the most promising of these processes. In this system, a turbulent bed of pulverized coal and limestone is injected into the boiler, suspended by jets of air. Combustion subsequently takes place at lower and more even temperatures, limiting the formation of NOx, and allowing limestone to capture the sulfur dioxides. But these systems suffer in comparison to the LBL process. They remove only 50 percent of NOx and generate huge loads of solid waste. Too, tests indicate they also may generate nitrous oxide, a greenhouse gas.
To date, the best commercial system for high levels of NOx treatment has been selective catalytic reduction. Using catalysts to reduce NOx by ammonia at high temperature, it is capable of removing up to 90 percent of NOx. The technology produces no solid waste and is the technology of choice in Japan and Europe. But the process is expensive. Selective catalytic reduction consumes valuable ammonia and requires periodic replacement of catalysts. Additionally, a separate system to scrub for sulfur dioxide is necessary.
A third option under consideration is retrofitting existing plants with scrubbers. Only about 190 of some 1,350 U.S. coal-fired power plants currently are equipped with flue gas desulfurization scrubbers. LBL's combined treatment system makes it possible to remove nitrogen oxides and sulfur dioxide in the same scrubber. Of the necessary reagents, limestone is inexpensive, and the cost of phosphorus is partially offset by its use to produce phosphoric acid, a more valuable chemical feedstock.
Bechtel's economic analysis documents the savings possible from LBL's system. In plants with existing scrubbers, Bechtel says phosphorus treatment is substantially cheaper than the best available alternative technology. In plants without scrubbers, the process likewise appears to be the cheapest way to provide high quality emissions treatment.
Working with Bechtel, Chang and colleagues are continuing to fine-tune their emissions treatment system. Field tests, necessary to prove the technology and gain industry acceptance, are planned for the future.
As governments grapple with deteriorating air quality and dictate acceptable levels of emissions, they and industry will decide the usefulness of LBL's high quality treatment process. Tougher air quality standards are on the horizon in many countries. Come the day that nations decide to clean the skies above says Chang, affordable technology will be available to do the job.