Sulfur removal before combustion

Coal requires initial pulverization, enabling the sulfur to be removed before combustion. This pre-treatment takes place prior to the fuel entering the combustion chamber. Pulverization fractures the carbonaceous material of the coal. The small crystals of inorganic impurities (including sulfides) are detached from the fuel. These can then be separated by the following physical methods. 

Oxydesulfurization. The pulverized fuel is agitated with water in the presence of oxygen. This can be effective in removing inorganic oxidized sulfur as soluble salts, but the process reduces the calorific value of the fuel. 

Flotation. The coal is finely ground down to release the inorganic pyrite particles. These may then be separated physically through density differences (density of pyrite about 5.0gcm , density of coal 1.2-1.5 gcm ). This method is not feasible, however, on the large scale required. 

Bacterial methods for sulfur oxidation have also been considered as a means of sulfur removal. 

All these methods are specific to the precombustion removal of inorganic sulfur and are expensive when applied to the large quantities of coal burned in power stations. The organic sulfur retained within the carbonaceous structure represents an appreciable contribution to acid rain and, therefore, these methods may not provide effective desulfurization. 

---

The methods for controlling sulfur oxides from coal, which, in turn, impact the combustion system design can be classified into three types (1) sulfur removal after combustion, (2) sulfur removal during combustion, and (3) sulfur removal before combustion. 

The plant is designed to satisfy NSPS requirements. NO emission control is obtained by fuel-rich combustion in the MHD burner and final oxidation of the gas by secondary combustion in the bottoming heat recovery plant. Sulfur removal from MHD combustion gases is combined with seed recovery and necessary processing of recovered seed before recycling. 

Much of the sulfur contained in a crude oil remains with the tar, asphalt or coke after refining. As crude prices rise, even this residuum becomes valuable as a feedstock for cracking to make lighter products. This involves deep desulfurization. Whether the coke is burned as a fuel or used for anodes, desulfurization will take place before or after combustion or processing. Sulfur removal from coke plants is a currently feasible process. 

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. 

It is therefore beneficial to pretreat the coal to remove as much sulfur as is practical before combustion, so that the expense of postcombustion desulfurization can be reduced. Combined with the other benefits of coal cleaning, it is evident that precombustion coal treatment is valuable even when it is not sufficient to completely desulfurize the coal by itself. 

In summary, the kinetics of the combustion process is important with regard to sulfur removal. The kinetics must be considered either by modelling or experimentation before a final judgment on desulfurization in a slagging, cyclone combustor can be made. The results of this study show that it is... 

Coal gasification (Section 3) is a relatively new technique for the production of electric energy. During gasification a variety of compounds are formed. These are CO, H2, CO2, HCN, NH3, H2S, COS, HCl, and HF. The coal gas is burnt in the gas turbine. Before combustion takes place all compounds containing sulfur and nitrogen have to be removed in order to avoid formation of SO,t and NO . ... 

There are two ways to minimize the effects of SO2 pollution. The most direct approach is to remove sulfur from fossil fuels before combustion, but this is technologically difficult to accomplish. A cheaper but less efficient way is to remove SO2 as it is formed. For example, in one process powdered limestone is injected into the power plant boiler or furnace along with coal (Figure 17.23). At high temperatures the following decomposition occurs ... 

Nonferrous ores occur mainly in the form of pyrites. The large emission factors associated with nonferrous metal production derive from the fact that sulfur contained in the ores escapes mostly as S02 in spite of control measures. The most significant contribution to S02 emissions from industrial processes lies in the manufacture of sulfuric acid. The conversion of pulp to paper leads to emissions of H2S and organic sulfides, but their magnitude is comparatively small. The combustion of natural gas, which is another important source of energy, causes negligible sulfur emissions so that it is not even listed in Table 10-8. This fuel has a low sulfur content to begin with, and almost all of it is removed before use. 

There are strong incentives to develop processes for removing sulfur from coal before combustion (precombustion cleaning), during combustion, or after combustion (postcombustion cleaning)... 

The coal or oil can be refined to remove the sulfur before combustion. 

A number of processes are being used to remove sulfur and sulfur oxides from fuel before combustion and from stack gas after combustion. Most of these efforts concentrate on coal, since it is the major source of sulfur oxides pollution. Physical separation techniques can be used to remove discrete particles of pyritic sulfur from coal. Chemical methods can also be employed for removal of sulfur from coal. 

Acid rain is best controlled by limiting emissions to the atmosphere of sulfur dioxide and nitrogen oxides, discussed in Sections 8.6 and 8.7. Combustion of coal containing sulfur is a major source of atmospheric sulfur dioxide, the release of which can be limited by stack gas controls, fluidized bed combustion in a sulfur-sequestering medium, and removal of sulfur from coal before combustion... 

In contrast to coal, oil is a mixture of molecular substances, a property that allows the sulfur to be removed by chemical treatment before combustion. 

---