Sulfur—Ammonia Cycle

The sulfur—ammonia cycle appears to be a previously unexplored cycle that has been put forward by the Florida Energy Centre in the USA. The novel step in the process is the photocatalytic oxidation of ammonium sulfite, (NH4)2S03, to ammonium sulfate, (NH4)2S04. The latter is then used to split water. 

While the challenging dissociation of sulfuric acid at high temperature remains, there appears to be some significant simplifications in terms of the chemistry of the remainder of the cycle  

On the other hand, the test data available to date suggest that a number of matters remain to be resolved before this cycle can realistically be considered as a serious competitor to the sulfur—iodine and Westinghouse alternatives. Further experimental and engineering development work is required to address the following issues  

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It is often taken for granted that the oxygen content of the air is nearly constant at ca. 20% of the atmospheric volume, that most of the liquid water on the planet is aerobic (i.e. contains O2), and that most water has pH values relatively close to neutral" (close to 7). However, these circumstances are not mere coincidences but are in fact consequences of the interaction of key global biogeochemical cycles. For instance, the pH of rainwater is often determined by the relative amounts of ammonia and sulfuric acid cycled through the atmosphere, a clear example of interaction between the nitrogen and sulfur cycles. 

The most noteworthy multistage element cycles in which bacteria play important roles are the nitrogen and sulfur redox cycles. The fixation of nitrogen is a reductive process that provides organisms with nitrogen in a form usable for the synthesis of amino acids, nucleic acids, and other cell constituents. In essence, the overall conversion to the key intermediate, ammonia, can be represented as ... 

Sulfur-Ammonia photo-thermal hybrid cycle ... 

A flow diagram for the system is shown in Figure 5. Feed gas is dried, and ammonia and sulfur compounds are removed to prevent the irreversible buildup of insoluble salts in the system. Water and soHds formed by trace ammonia and sulfur compounds are removed in the solvent maintenance section (96). The pretreated carbon monoxide feed gas enters the absorber where it is selectively absorbed by a countercurrent flow of solvent to form a carbon monoxide complex with the active copper salt. The carbon monoxide-rich solution flows from the bottom of the absorber to a flash vessel where physically absorbed gas species such as hydrogen, nitrogen, and methane are removed. The solution is then sent to the stripper where the carbon monoxide is released from the complex by heating and pressure reduction to about 0.15 MPa (1.5 atm). The solvent is stripped of residual carbon monoxide, heat-exchanged with the stripper feed, and pumped to the top of the absorber to complete the cycle. 

The addition of ammonia to the variety of acids derivable from either the breakdown of glucose, glycolysis, or of the pentose shunt reaction products, ribose and NADPH, and from the citrate cycle, gives the amino acids (see Table 4.7 and Figure 4.4) Polymerisation of amino acids in cells gives proteins. In some of the amino acids sulfur and selenium can be incorporated easily. We assume NH3 was present. (Note that Se is in a coded amino acid not in Table 4.7.) Some selective metal-binding properties can be seen in Table 4.7, but amino acid carboxylates can bind all. 

The nitrogen cycle is closely connected with the fluxes of hydrogen, sulfur, and other chemicals (Smith et al., 1998 Dimitroulopoulou and Marsh, 1997 Chapin et al., 2002 Rhee et al., 2005 Stevenson and Cole, 1999). Nitrogen and hydrogen react under great pressure and temperature in the presence of a catalyst to make ammonia. The study of correlations between the cycles of these elements is necessary to improve... 

The natural environments—rural, marine and industrial—and some combination of these are of primary concern. Some contaminated atmospheres, such as those containing hydrogen sulfide, ammonia, sulfur dioxide, etc., should be considered rigorously. Relative humidity and its cycling are of major importance for corrosion kinetics and a material s resistance to corrosion. Temperature and pressure are major factors to consider, together with the chemical composition of the medium. 

Unlike the metal snlfate based TCWSCs that employ reaction between sulfuric acid and a metal oxide (e.g. Reaction (108)), the new MSO4-NH3 cycles rely on the Reaction (121). Reaction (121) was first reported in 1955 by Dugger and coworkers who developed a process for the recovery of ammonia from ammonium sulfate in a two-stage reaction as follows [29] ... 

Due to the intrinsic difficulties of sulfuric acid decomposition, we have introduced two new classes of solar driven TCWSCs by modifying the original S-NH3 cycle. They include (I) 12 metal sulfate-ammonia (MSO4-NH3) based TCWSCs and (II) 3 metal pyrosulfate-ammonia (M2S2O7-NH3) based TCWSCs. Our preliminary experimental results of the ammonia released from the ZnO + (NH4)S04 mixtures show the feasibility of these new cycles. More experiments are currently underway to determine the reaction mechanisms and the nature of the reaction intermediates and products formed. These experimental and thermodynamic analyses are expected to lead to development of a highly efficient, solar driven water splitting cycle. 

Hence, by dosing ammonia during the acceptance cycle of the SFGD process, sulfur oxides as well as nitrogen oxides are removed from the flue gas. This variant of the SFGD process has also been applied on an industrial scale in the unit at Showa Yokkaichi refinery in Japan. 

These catalysts have to fulfil new safety requirements as they may be operated in the consumer s home, and also will experience a very different duty cycle than industrial catalysts. Another field that has attracted interest is sulfur tolerant shift catalysts. Catalysts have been developed for high sulfur concentrations in the feed gas since the 1960s, but have not found large-scale application up to the present. The fact is that most ammonia, methanol, and hydrogen plants are based on natural gas or naphtha feedstocks, which have relatively low content of sulfur-containing compounds, and therefore do not require these more expensive catalysts. HDS is typically used to reduce sulfur levels in the feed gas to ppm to ppb levels to protect the steam reforming and downstream catalysts. 

Water is an obvious example of a substance that is global in scope. In this text, specific sources of pollution (stack emissions of sulfur dioxide) and small-scale natural processes (ammonia emissions from a feedlot) are considered only to the extent that they are significant in aggregate at the global scale. Further, the term "cycles" in the title should not imply that only closed, steady-state systems are considered, but should emphasize the importance of understanding where substances come from and what they are turned into. 

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