Sulfur Dioxide Air Emission Recovery and Reuse in China
Robert Kenson, PhD
1126 Cardinal Drive
West Chester, PA 19382
Recovery of sulfur dioxide air emissions and reuse as industrial or agricultural materials has been demonstrated in both North America and Europe. Numerous commercial, demonstration and pilot installations have proven their technical and economic viability. The Peoples Republic of China (PRC) has expressed interest in these recovery/reuse technologies but has few such systems installed. At present, the majority of such recovery/reuse systems in the PRC are for sulfur dioxide emissions from non-ferrous smelters. In this application hundreds of thousands of metric tons/year sulfur dioxide emissions have been eliminated and sulfuric acid has been produced for use in the smelter.
Several power & steam boilers in the PRC have installed technologies to recover/reuse sulfur dioxide emissions by converting them to ammonium sulfate fertilizer or gypsum useful in producing cement, sulfuric acid or wallboard. Some information on the degree of success of these installations has been forthcoming.
A new and emerging sulfur dioxide recovery/reuse application is that from cement kiln exhaust gas. Gypsum (hydrated calcium sulfate) is produced that can be blended into the raw materials for making cement to improve the quality. No installations in the PRC have yet been reported.
In 1997-1999 two PRC copper smelters installed large capacity sulfuric acid plants. The feed gases for these sulfuric acid plants were high sulfur dioxide (10+ %) content exhausts from copper flash furnace and copper converter processes at the smelters.
In order to accomplish this, the exhaust gases were first cleaned by wet scrubbers to remove large to medium size dust particles and fumes. Subsequently, a wet electrostatic precipitator removed finer particles, fumes and aerosols to complete the gas cleaning. Gas cleaning prevents deactivation of the catalyst that subsequently oxidizes the sulfur dioxide to sulfur trioxide in the sulfuric acid plant. The sulfur trioxide thus formed is then hydrated to form sulfuric acid which is used in the copper ore leaching process at the smelter.
In 2001 a lead smelter in the PRC will install a similar gas cleaning system and large sulfuric acid plant using sinter machine exhaust air, containing only 3-4% sulfur dioxide as a feed gas. A newly developed sulfuric acid process that can tolerate this more dilute sulfur dioxide feed gas will be installed. The lead smelter will use the sulfuric acid thus produced in the leaching of lead containing ores at the smelter.
Sulfur dioxide emission reductions in the PRC from the installation of the above mentioned gas cleaning and sulfuric acid manufacturing processes total about 500,000 metric tons/year. Substantial smelter operating cost savings are also realized by avoiding significant external purchases of sulfuric acid.
Power & Steam Boilers
One flue gas desulfurization (FGD) system installed on a boiler in the PRC reacts dry ammonia with the sulfur dioxide emissions in the presence of an electron beam to produce dry ammonium sulfate for use as fertilizer. The use of dry ammonia is unique as conventional FGD systems would normally employ an ammonia in water solution as a neutralizing agent.
Using dry ammonia as the sulfur dioxide neutralizing agent avoids the high capital and operating costs of a large capacity water evaporator. An evaporator would be required if a water solution of ammonia were used as the neutralizing agent for the sulfur oxide emissions.
Several conventional FGD systems have been installed on boilers in the PRC to control sulfur oxide emissions using low cost lime or limestone slurry as the sulfur dioxide neutralizing agent. The intent is to produce gypsum by air oxidation of the calcium sulfite/calcium bisulfite solution resulting from the neutralization reaction. The resulting insoluble gypsum, if sufficiently pure, can be subsequently dewatered and used in the production of cement, sulfuric acid or wallboard.
Several cement kilns in the United States and Europe have installed FGD systems based upon wet scrubbing using limestone bearing rock that is processed in the cement kiln. It is made into a water slurry and used to neutralize sulfur dioxide emissions from high sulfur fuel (coal or petroleum coke with 6% or greater by weight sulfur content) burned to generate heat in the cement kiln.
The insoluble gypsum resulting from air oxidation of the calcium sulfite/calcium bisulfite solution produced by the neutralization reaction is dewatered. The dewatered gypsum can then be recycled into the cement kiln by blending it with the crushed rock and other ingredients to improve the quality of the cement produced by the kiln.
Performance data on an FGD system installed in a European cement kiln has recently been published in a trade magazine and at a technical conference. As previously mentioned, there are presently no FGD systems installed at cement kilns in the PRC.
Kenson, R. E., 1999, “Clean Metal Smelter Gas To Prevent SO2 Air Pollution” Chinese Sulfuric Acid Assn. Conference On Air Pollution”, Tongling, PRC (article available from the author).
Ellison, W., 1999, “Worldwide Progress In Ammonia FGD Application”, EPRI-DOE-EPA Combine Utility Air Pollution Control Symposium, Vol. 1, 6-1 to 6-14, Palo Alto, CA, EPRI
Boarder, D., 1998, “Environmental Investment At Castle Cement”, World Cement, December, 41-45, Farnham, England, Palladian Publications