Desulfurization thermal-chemical process

Review of Process Alternatives, Superior Graphite began in 1968 to investigate possible alternatives for desulfurization of petroleum cokes. The methods considered included various chemical treatments and direct thermal purification processes. The chemical treatment methods included hydrodesulfurization and reactions with various alkali metal compounds. Fine grinding of the coke appeared to be required and reaction conditions generally involved high pressure. 

Sulfur dioxide is generated in flue gas as a result of combustion of fossil fuel in, for example, thermal plants. Although the sulfur dioxide content in the flue gas is usually small, namely below about 0.1-0.4 vol.% [86], the volume of the produced gas is so large that considerable amounts of sulfur dioxide contaminate the atmosphere, and therefore proper desulfurization becomes important. Here, absorption of SO2 by the sodium method is considered as part of the combined chemical/biological process shown in Fig. 9.21. The process includes SO2 removal, NaHCCh recovery, and elemental sulfur production [70]. 

The chemical methods reported at the time therefore appeared complex and unlikely to achieve at a reasonable cost the low levels of product sulfur required. Alternatively, thermal desulfurization at about 2500C was known to yield a product meeting sulfur specifications as made evident by the conventional process of graphitizing shaped mixtures of petroleum coke and coal tar pitch. In addition to simplicity, thermal purification offered a significant advantage in not requiring fine grinding of the coke. 

To avoid major modifications of the power plant, the optimal position of a CO2 capture process seems to be in between the flue gas desulfurization unit (FGD) and the stack. Particulate matter as well as SO and NO concentrations are reduced to a large extent, which lowers the requirements regarding chemical resistance and reduces the risk of plugging. To quantify the flue gas condition subsequent to the FGD unit, a model of a state of the art hard coal fired power plant was developed and implemented in Aspen Plus . The considered power plant features a thermal duty of 1210 MW and a net thermal efficiency of 45% (based on lower heating value of the feedstock). 

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