Review of Related Literature

CHAPTER II
REVIEW OF RELATED LITERATURE

Coal Ash Coal ash is a collective term for the residues left during the combustion of coal. Depending on the source and makeup of the coal being burned, the composition of the coal ash vary considerably, but all includes substantial amounts of both amorphous and crystalline Silicon dioxide (SiO2) and Calcium oxide (CaO), both being endemic ingredients in many coal-bearing rock strata. Coal ash is made up of two types of combustion by-products: bottom ash and fly ash. Bottom ash is a coarse, granular, incombustible by-product that is collected from the bottom of the combustion chamber. Bottom ash is coarser portion of the coal ash, with grain sizes spanning from fine sand to fine gravel. Fly ash, on the other hand, is the finest of coal ash particles. It is called "fly" ash because it is transported from the combustion chamber by exhaust gases. Fly ash is the fine powder formed from the mineral matter in coal, consisting of the noncombustible matter in coal plus a small amount of carbon that remains from incomplete combustion. Fly ash is generally captured by electrostatic precipitators or other particle filtration equipments before the flue gases reach the chimneys.

Flotation Flotation is one of the many methods in mineral separation. It could be used for separation of phases for instance to remove solid particles or oil drops from water. More frequently flotation is used for separation of particles having different hydrophobicities. Hydrophobicity is a feature of material characterizing its ability to be wetted with a liquid in the presence of a gas phase. Solids, which can be easily wetted with water, are called hydrophilic while solids with limited affinity for wetting are called hydrophobic. As a result of hydrophobicity, particles adhere to the gas bubble forming a particle-air aggregate which is lighter than water, and travels upwards to the surface of water. The hydrophilic particles do not adhere to the bubbles and fall down to the bottom of the flotation tank. In order for the flotation take place, two essential reagents are added: the froth-enhancing reagent known as frother and the hydrophobicity-enhancing reagent known as a collector. The role of frother is to keep the floating particles in the most upper layer called froth for easy removal of floating particles from the flotation system. The collector selectively renders hydrophobicity into a particle surface by creating a thin hydrophobic film around the particle.

Desulfurization

Sodium sulfite is commonly produced by reacting soda ash(Sodium carbonate) with sulfur dioxide in an aqueous medium. Sulfur dioxide-containing gas is passed through an aqueous solution of Sodium carbonate to form a solution of Sodium bisulfite, which is then neutralized, as by addition of excess Sodium carbonate to form an anhydrous Sodium sulfite.
Formation of Sodium bisulfate:
〖Na〗_2 〖CO〗_3+〖2SO〗_2+H_2 O→〖2NaHSO〗_3+CO_2
Neutralization by excess soda ash: 〖2NaHSO〗_3+〖Na〗_2 〖CO〗_3→〖Na〗_2 〖SO〗_3+H_2 O+CO_2

For producing anhydrous Sodium sulfite, it is only critical that the reaction between the sodium carbonate and the sulfur dioxide be initiated in an aqueous medium, provided the process is initiated in a saturated solution of sodium sulfite. Furthermore, the temperature of the liquid reaction medium must be maintained within 50° and 75° C.

Sodium sulfite

Anhydrous Sodium sulfite is a white powder or crystalline solid with no odor but a slightly salty taste. It is an essential chemical in the pulp and paper industry. Just over half of all the sodium sulfite made in the United States is used by the pulp and paper industry. The compound acts as a pulping agent for wood, rags, and straw. A pulping agent is a substance that breaks down raw materials and converts them into the pulp from which paper is made. Sodium sulfite is also used to remove excess chlorine used to bleach wood pulp and other raw materials needed in the production of paper.
The second largest application of sodium sulfite is in water and wastewater treatment plants, where it is used to react with and neutralize excess chlorine used in the water and wastewater treatment processes. The third most important application of sodium sulfite is in photography. The compound is used in the developing process, and it acts as a preservative for the final picture produced. Sodium sulfite is still used as a food preservative also, although the conditions under which it can be added are somewhat limited. It is also widely used in the wine-making industry for the control of bacteria involved in the wine-making process.

CO2 Capture

The process integrates a physical solvent, SELEXOLtm (utilizing di-methyl ethers of polyethylene glycol - DEPG), for removing CO2 to within pipeline specification. The maximum benefit of this process is achieved when CO2 is sold into a CO2 pipeline for use as an agent for enhanced oil recovery or when the CO2 is injected back into the earth for sequestering.

Physical solvents are an attractive option for treating produced gas streams with high acid gas partial pressures. The SELEXOLtm process absorbs CO2 from the produced gas at relatively high pressures (950-1,200 psig) and is regenerated in a series of flashes to lower pressure. Successive flashes during the SELEXOLtm regeneration process release primarily CO2 vapor, with the final regeneration flash near atmospheric pressure. CO2 vapor that is removed at the various flash pressures is compressed from relatively low pressures to the CO2 pipeline delivery pressure through compression.

Carbon dioxide

Humans use carbon dioxide in many different ways. The most familiar example is its use in soft drinks and beer, to make them fizzy. Carbon dioxide released by baking powder or yeast makes cake batter rise. Some fire extinguishers use carbon dioxide because it is denser than air. Carbon dioxide can blanket a fire, because of its heaviness. It prevents oxygen from getting to the fire and as a result, the burning material is deprived of the oxygen it needs to continue burning. Carbon dioxide is also used in a technology called supercritical fluid extraction that is used to decaffeinate coffee. The solid form of carbon dioxide, commonly known as Dry Ice, is used in theatres to create stage fogs and make things like "magic potions" bubble.