Minimizing sulfur emissions

Sulfur leaves the acid plant in the tail gas as SO2, SO3, and acid mist (H2SO4(0). These compounds are either scmbbed or exhausted to the atmosphere. This chapter focuses on methods to optimize acid plant design and operation to minimize sulfur emissions. 

Previous chapters are based on the assumption that equilibrium is achieved for catalytic S02-f 0.502 SO3 oxidation. Equilibrium SO2 oxidation is never quite achieved in an industrial acid plant, resulting in slightly higher than optimum SO2 emissions. 

This chapter examines industrial (nonequUibrium) SO2 oxidation and its effect on tail gas SO2 concentrations. It also discusses methods to decrease acid plant sulfur emissions. 

The SO2 -l- O.5O2 — SO3 reaction kinetics explain why equilibrium is not achieved in each catalyst bed. The kinetics are complex. Many studies have been carried out (Davenport and King, 2006), but the best information resides with catalyst manufacturers such as BASF, Haldor Topspe, and MECS. A literature review, along with industrial data, indicates the following  

The reaction rate slows in the presence of SO3, which is why there is more catalyst in beds 2 and 3 than in bed 1. The last bed of a double contact type plant usually has the most catalyst. 

---

A bleed from the scmbbing system is sent to a sour slurry stripper. The water is then clarified and can be recycled to minimize the volume of effluent to be biotreated and discharged or evaporated. The acid gas from the acid gas removal system and from the sour slurry stripper is fed to a Claus plant, where salable elemental sulfur (qv) is produced. For maximum sulfur recovery and minimal sulfur emissions, the Shell Claus off-gas treating process (SCOT) is used. 

Ethene and propene are produced as bulk feedstocks for the chemical (polymer) industry and therefore their purities are important parameters. In particular, H2S and COS are compounds which may not only cause corrosion problems in processing equipment, but also may have detrimental effects on the catalysts in use. Eurthermore, air pollution regulations issued by, among others, the US Environmental Protection Agency (EPA) require that most of the sulfur gases should be removed in order to minimize Sulfur emissions into the atmosphere. Therefore, these compounds have to be determined to the ppb level. 

The acid gas from the Sul find regenerator must be disposed of in an environmentally acceptable manner. The Claus process offers an effective means for converting nearly all of the sulfur in the acid gas to saleable elemental sulfur. The tail gas from the Claus plant still contains some sulfur compounds. To minimize sulfur emissions from the plant, the Claus tail gas can be fed to a Shell Claus ff-gas Jreating (SCOT) unit where most of this sulfur is recovered and recycled to the Claus plant. With use of the SCOT Process, additional marketable sulfur is recovered within the Claus plant while tail gas sulfur emissions are substantially reduced, to typically less than 250 ppmv. 

Automatic control of this combustion system maximizes acid condensation and minimizes sulfur emission to the environment. 

Before considering treatment of sulfur emissions to atmosphere, their minimization at source should be considered. Sulfur emissions can be minimized at source through ... 

Atmospheric sulfur emissions can be minimized at source by improving the process yields and desulfurization of fuels prior to combustion. The difficulty of fuel desulfurization is solid > liquid > gas. Sulfur can be removed from emissions either as S02 or H2S. Removal of the H2S can be by ... 

Figure 8.2 Two-stage fluidized bed coal-fired boiler that minimizes sulfur dioxide emissions. Adapted from P. F. Fennelly, Am. Sci. 72, 254 (1984).

Process air in sulfur-burning plants is dried by contacting it with 93—98 wt % sulfuric acid in a countercurrent packed tower. Dry process air is used to minimize sulfuric acid mist formation in downstream equipment, thus reducing corrosion problems and stack mist emissions. 

The primary problem with the systems that have been developed to utilize lower feed sulfur concentrations is the reduced efficiency of sulfur recovery. This effect is not so apparent in lost sulfur revenues as it is in the increased size and cost of the downstream tail-gas treating unit required to minimize the sulfur emissions from the overall sulfur-treatment section. 

The recovered sulfur industry exists primarily as a result of the necessity of removing sulfur values from hydrocarbon fuels before combustion so that sulfur emissions to atmosphere are reduced. In the case of sour gas, the principal source of recovered sulfur, the product that results from recovery of the sulfur is clean-burning, non-polluting methane. In the case of refineries handling high sulfur crude the product is low sulfur gasoline and oils. Thus every ton of sulfur recovered is a ton that is not added to the atmosphere. The recovery process itself however, is also the subject of optimization and recent developments in recovery efficiency have further ensured that the environmental impact in the immediate vicinity of these desulfurization facilities will be minimized. 

Concern for the environment has resulted in moves to significantly reduce the noxious components in emissions when fuel oils are burned. Attempts are being made to minimize sulfur dioxide emissions and, as a consequence, a strategy to minimize the sulfur content of fuel oils has been implemented. Although typical diesel fuel oils have, in the past, contained 1 % or more of sulfur (expressed as elemental sulfur) by weight, environmental legislation in the United States has required that sulfur content of diesel fuel be less than 0.05% (11). These levels will be reduced to 15 ppm or less to protect new exhaust catalyst after-treatment devices. In Europe, various jurisdictions have moved to lower sulfur content. In Sweden, for example, taxation of higher sulfur, lower cetane fuels is elevated to reflect to their respective environmental cost (93). 

Gollop, F. M. and Roberts, MJ. (1985). Cost minimizing regulation of sulfur emissions Regional gains in electric power. Review of Economics and Statistics, 67, 81-90. 

To enable them to serve as a replacement fuel in oil-burning installations, COM are usually prepared from a bituminous coal of medium to high volatility and low ash content, since oil burners have only limited capability for ash removal. To minimize ash deposition and fouling problems, it is also desirable that the feed coal be low in moisture and have a moderate-to-high ash fusion temperature (the temperature of initial deformation) (Table 14.10). A low-sulfur content is also required to maintain the low-sulfur emissions of the oil fuels being replaced. 

Figure 26.4 Control system for minimizing sulfuric acid emission to the environment. It operates continuously. A condensation particle counter (top) senses acid droplet concentration in the exit gas (TSI, 2012). It sends an electronic signal to the combustion air flow controller— telling it to increase or decrease air flow rate. This in turn decreases or increases the concentration of nanoparticles in the burner exit gas (represented by mass% smoke in Fig. 26.5) as needed to niinirriize acid droplet-in-exit gas emission. The physical location of the system in the wet sulfuric acid flowsheet is indicated in Fig. 25.1.

---