Gases removal from stack

Metzinger, J., Hudgins, R. R., Silveston, P. L., Gangwal, S. K., Application of a periodically operated trickle bed to sulfur removal from stack gas. Chem. Eng. Sci. 47, 3723-3727... 

The size of fly ash particles is a very important factor in determining their removal from stack gas and their ability to enter the body through the respiratory tract. Although only a small percentage of the total fly ash mass is in the smaller size fraction of around 0.1 pm size, it includes the vast majority of the total number of particles and particle surface area. Submicrometer particles probably result from a volatilization—condensation process during combustion, as reflected in a higher concentration of more volatile elements such as As, Sb, Hg, and Zn. In addition to their being relatively much more respirable and potentially toxic, the very small particles are the most difficult to remove by electrostatic precipitators and bag houses (see Chapter 8, Section 8.4). 

Suggest a series of reactions by which sulfur dioxide can be removed from stack gas and converted to a commercially valuable product that ranks high in annual production of inorganic chemicals. 

Purer quicklime is obtained from vertical kilns. In vertical kilns limestone is fed into the top of the kiln while fuel is introduced at the bottom. Either fuel oil or natural gas is used to cook the limestone. Fuel oil usually contains enough sulfur to generate SO2 and SO3 that are trapped in the quicklime, in much the way these gases are removed from stack gases in powerplants. Natural gas is the highest grade fuel and produces high quality lime. In my opinion it is the only lime that should be used as a raw material in chemical processes where it is to become a part of a consumer product. 

The oxidant preheater, positioned in the convective section and designed to preheat the oxygen-enriched air for the MHD combustor to 922 K, is located after the finishing superheat and reheat sections. Seed is removed from the stack gas by electrostatic precipitation before the gas is emitted to the atmosphere. The recovered seed is recycled by use of the formate process. Alkali carbonates ate separated from potassium sulfate before conversion of potassium sulfate to potassium formate. Sodium carbonate and potassium carbonate are further separated to avoid buildup of sodium in the system by recycling of seed. The slag and fly-ash removed from the HRSR system is assumed to contain 15—17% of potassium as K2O, dissolved in ash and not recoverable. 

Baghouses are preferred over venturi scrubbers for controlling particulate matter emissions from loading and pushing operations because of the higher removal efficiencies. ESPs are effective for final tar removal from coke oven gas. Stack air emissions should be monitored continuously for particulate matter. Alternatively, opacity measurements of stack gases can suffice. Fugitive emissions should be monitored annually for VOCs. 

The solubilities of the various gases in [BMIM][PFg] suggests that this IL should be an excellent candidate for a wide variety of industrially important gas separations. There is also the possibility of performing higher-temperature gas separations, thanks to the high thermal stability of the ILs. For supported liquid membranes this would require the use of ceramic or metallic membranes rather than polymeric ones. Both water vapor and CO2 should be removed easily from natural gas since the ratios of Henry s law constants at 25 °C are -9950 and 32, respectively. It should be possible to scrub CO2 from stack gases composed of N2 and O2. Since we know of no measurements of H2S, SO, or NO solubility in [BMIM][PFg], we do not loiow if it would be possible to remove these contaminants as well. Nonetheless, there appears to be ample opportunity for use of ILs for gas separations on the basis of the widely varying gas solubilities measured thus far. 

Assuming plug flow of both phases in the trickle bed, a volumetric mass transfer coefficient, kL a, was calculated from the measurements. The same plug flow model was then used to estimate bed depth necessary for 95% S02 removal from the simulated stack gas. Conversion to sulfuric acid was handled in the same way, by calculating an apparent first-order rate constant and then estimating conversion to acid at the bed depth needed for 95% S02 removal. Pressure drop was predicted for this bed depth by multiplying... 

While the use of low-sulfur fuels is one mechanism to reduce sulfur dioxide emission, alternatively most approaches focus on scrubbing or ridding the emissions in smoke stacks of sulfur dioxide gas. A number of different types of scrubbers, i.e., sulfur dioxide removal systems, are available for industry. One system sprays the flue gas into a liquid solution of sodium hydroxide. The hydroxide combines with SO2 and O2 to form the corresponding sulfate which can be removed from the aqueous solution ... 

The steam requirement for simple absorption/stripping with 90% removal of S02 from stack gas containing 3000 ppm S02 at 55°C was estimated to be about 40 kg/kg SOj. 

Selection of Oxides. At Amoco, previous studies in the literature on SO2 removal from flue gas have been used to guide the selection of oxides for the UltraCat process but they have been of limited direct usefulness. This was true because of the peculiar requirements of the UltraCat process of high adsorption temperature, low regeneration temperature, and non-interference with the cracking reactions. The previous literature studies generally assumed that SO2 would be adsorbed at temperatures close to a stack gas temperature of 600 E, and desorb at either the same temperature or higher. The conditions of these studies was set. 

There is no shortage of SO2 stack gas removal processes. The issues involved, however, range from the economics to the technical feasibility. Both are likely to be more suitable by 1990. From a list of 65 processes (29) ranging from on-stream to developmental,... 

It is also doubtful that the industry will be in a position for many years to come to undertake sulfur removal from residual fuels solely to improve product quality. A number of consumer industries demand low sulfur fuel oils, but these special requirements can at present be met more appropriately by selection of crude rather than by adoption of desulfurization processes. In general industrial use, it is corrosion and atmospheric pollution that are the main disadvantages of high sulfur content. But there is no sign yet of the development of a cheap desulfurization process, the cost of which can be substantially offset by the gain in efficiency resulting from permissible lower stack temperatures or by the elimination of flue gas scrubbing equipment previously necessary for reduction of sulfur dioxide content. 

---