Sulfur dioxide, from burning fuels removal

Removal of sulfur dioxide from a gas stream. If a fuel that contains sulfur is burned, the product gas contains sulfur dioxide. If the gas is released directly into the atmosphere, the SO2 combines with atmospheric oxygen to form sulfur trioxide. The SO3 in turn combines with water vapor in the atmosphere to form sulfuric acid (H7SO4), which eventually precipitates as acid rain. To prevent this occurrence, the combustion product gas is contacted with a liquid solution in an absorption or scrubbing process. The SO2 dissolves in the solvent and the clean gas that remains is released to the atmosphere. 

Sulfur dioxide is formed whenever sulfur-containing fuels are combusted in air. Sulfur dioxide can lead to the formation of acid rain and is a controlled pollutant in most countries. U.S. 5,196,176, assigned to Paques B.V, describes a biological process for removing sulfur dioxide from a vent gas and converting it to elemental sulfur. Estimate the cost (in /kWh) of using the Paques process to treat the flue gas from a 1000 MW power station that burns Illinois Number 6 coal in pressurized fluidized bed combustors. 

The goal of beneficiation is to remove as much sulfur from a fuel as possible before it is ever burned. When burned, fuel with lower sulfur content will produce less sulfur dioxide. Beneficiation is usually accomplished by a physical process that separates one form of sulfur, pyritic sulfur, from coal. Pyritic sulfur consists of sulfur minerals (primarily sulfides) that are not chemically bonded to coal in any way. The name is taken from the most common form of mineral sulfur usually found in coal, pyrite, or iron sulfide (FeS2). 

Chemists have found ways to reduce sulfur dioxide emissions from fossil fuels. They now produce Tow-sulfur versions of petrol and diesel. In power stations that burn fossil fuels, the sulfur is removed from the fuel before burning. Alternatively sulfur dioxide can be removed from the waste gases before they are released to the atmosphere through tall chimneys. 

This can be controlled to a certain extent by the use of low nitrogen fuels or by modifying the combustion conditions. Low excess air addition, flue gas recirculation, low NOX burners or water/steam injection can provide 50-60% reductions in NOX concentration. The scale of the operation is enormous and a typical US power plant burning 9500 tons of selected coal per day will have to remove about 135,000 tons of sulfur dioxide (95% conversion) and 45,000 tons of NOX per year. During 2000 about 50% of US power was generated from coal but by 2020 60% is expected to come from natural gas. ... 

One way to reduce the quantity of SO2 released into the environment is to remove sulfur from coal and oil before these fuels are burned. Although difficult and expensive, several methods have been developed. Powdered limestone (CaC03), for example, can be injected into the furnace of a power plant, where it decomposes into lime (CaO) and carbon dioxide ... 

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