Sulfur retention

In AFBC units, heat is removed from the flue gas by a convection-pass tube bank. The particulates leaving the boiler with the flue gas consist of unreacted and spent sorbent, unburned carbon, and ash. Multiclones after the convection pass remove much of the particulate matter and recvcle it to the combustor, increasing the in-furnace residence time an improving combustion efficiency and sulfur retention performance. Bubbling PFBC units do not have convection-pass tube banks and do not recycle solids to the boiler. 

In the reactions with phosphonio-a-methoxycarbonyl-alkanides, the products of type 261 derived from 1,3-cycloaddition can rearrange to the tautomeric lif-pyrazolo-triazole (87MI2). The reaction of 3-diazopyra-zoles and 3-diazoindazole with acyl-substituted phosphonium ylides led to pyrazolo-triazine and indazolo-triazine derivatives 266 instead of the expected triazole compounds (8IJHC675). In this case, the ylides, which can exist as phosphonium enolates, possess nucleophilic and electrophilic centers in a /8-relationship, giving [7 + 2] or [11 -I- 2]cycloaddition reactions. With dimethylsulfonio-a-aroyl-methanides, very complex, temperature-dependent mixtures were obtained, in some cases with sulfur retention (87MI3). 

Remark of the translator More recent experience does not agree with this explanation of the promoting action of alkali metal oxides. On the one hand the sulfur retention by alkalinized iron is only weak on the other hand, the promoter action by alkali oxides is also observed in the sulfur free system. 

Experimental data were obtained on the carbonaceous residue (char), and sulfur distribution was calculated for the solid and gaseous products from the pyrolysis of model compounds. Sharp differences were observed in the quantity of char and the sulfur distribution for the different substances studied. The quantity of volatile matter varied from 21 to 43%. The sulfur retained by the char varied from 21 to 74% of the total present in the compound pyrolyzed (see Table I). The raw data show a possible relationship between the volatile matter and sulfur retention which indicates that as volatile matter decreases, sulfur retention generally increases (Table I). Neither structural features nor the molecular size of the various model compounds appear to have a significant relationship to sulfur distribution. 

Values of Pt calculated from Equation 2 also appear in Table I, and the relationship between Pt and experimental values of sulfur retained is presented graphically in Figure 3. The data indicate that sulfur retention by the char increases as the value of Pt decreases. 

Values of Pt were therefore calculated from Equation 1 wherein Ph,s = Pt for blended materials. The calculated values can be seen in Table II, and the graphical relationship between Pt and sulfur retained in the char can be seen in Figure 3. It is again apparent that sulfur retention increases as Pt decreases. The curve obtained from the data on the model compounds and blended materials can be used to predict the sulfur retention from the analytical data of the material pyrolyzed. 

If one assigns, after Yao (38), the area occupied by a surface sulfate group as 30 A2, and if the sulfur content and the BET area are known, it is possible to estimate whether, indeed, sulfur retention is kept below one monolayer. Table VI shows a compilation of sulfur retention, recalculated as number of monolayers. The buildup of sulfur at a given temperature takes place during relatively short exposures to the exhaust, typically of the order of a few thousand miles of vehicle operation. When the temperature is changed, the extent of retention will change. Thus, a certain pelleted catalyst (42) accumulated in 3000 miles about 0.5% of sulfur,... 

Variations in the amount of ash arise from the retention of sulfur that originates from the pyrite. Sulfur in ash is usually determined as sulfate (ASTM D-1757 ASTM D-5016), and the method may give abnormally high amounts of sulfur. This is due to the sulfur retention from pyrite (and marcasite). If the forms of sulfur in coal are known (ASTM D-2492), the amount of pyrite retention can be estimated (see also ASTM D-3174, Note 2). Nevertheless, sulfur retention will give rise to anomalous results. 

Figure 2 shows the results of the pyrolysis experiments conducted with the Spanish lignite at 750-960°C at residence times of 0.52-0.72 sec. It is seen that under the pyrolysis conditions used, 60 - 70% of the sulfur in this coal appears in the gaseous products as H2S, COS, and CS2. As in the previous sulfur study (1), the principal sulfur gaseous product at all temperatures is H2S, with some CS2 formed at T >840°C. The CS2 is apparently formed at the expense of the H2S, by any of several reactions H2S may react with the carbon of the coal and/or the methane evolved in the pyrolysis of the coal to form CS2- A small amount of COS is detected at all temperatures trace amounts of SO2 are also detected. Moreover, the total sulfur yield appears to reach a maximum about 900°C. The decrease in sulfur volatilization as pyrolysis temperature is increased above 900°C is attributed to sulfur retention in the char due to the reaction of H2S with coke or char to form more stable thiophenic structures (2). GC/MS analysis of the tars (diluted to 10 ml) from the pyrolysis at 750 and 850°C did not reveal any sulfur-containing structures. Tars from the pyrolysis at 900 and 950°C, however, contain dibenzothiophene. 

The decrease in molar volume associated with the conversion of MgC03 to MgO Increases the porosity of the stone. Although the MgO is relatively inert toward reaction with S02, the increased porosity of the stone provides a greater surface area of CaC03 for the direct reaction with S02 described by Eq. (3). There is an optimun temperature for maximum sulfur retention in the range of 800-850 C which is more pronounced for limestone than it is for dolomite (15,19-21). 

As the optimun temperature is exceeded, S02 reacts with CaO before 1t can diffuse into the core of the sorbent particle and pores become blocked to further sorbent utilization. No such effect is observed at elevated pressures, where sulfur retention increases for temperatures at least to 900 C (15). 

It (1 ) deals with the derivation of relations giving the particle size distribution in the bed, overflow, and carryover streams and their respective weights. This theory will be extended to include the effects of particle growth or shrinkage (Z>1 or Z<1). For typical combustion of char containing sulfur followed by sulfur dioxide absorption by limestone, relations will be derived to determine the extent of sulfur retention. The reaction, carryover, and overflow rates will be evaluated with particular attention to their dependence on Z. 

The development of mathematical models to describe the thermochemical process occurring in a fluidized bed involves setting up the material and energy balance equations. The total process is represented in terms of a set of independent equations which are solved simultaneously to obtain such quantities as combustion efficiency, sulfur retention, oxygen utilization, oxygen and sulfur dioxide concentration profiles in the bed, etc. 

Furthermore, the dolomite requirement is directly proportional to d /dt. Consequently, is altered in magnitude which is directly proportional to the changes made in k3(To) and r. The changes in the limestone requirement are also related to the residence time of the limestone in the bed. If the rate of reaction k3(To) is increased, less reaction time is needed to achieve the same degree of sulfur retention. Shorter residence times are obtained by increasing the limestone feed rate for the same bed volume. Thus, f will increase with an increase and d /dt. Alternately, if for the same volumetric feed rate d /dt is increased, an improved sulfur retention will result. 

These results suggest that if the feed size of limestone is kept fixed, an increase in the limestone feed rate will result in the reduction of sulfur absorption efficiency- These results also emphasize that if the same sulfur retention is to be obtained when the size of the limestone particles is decreased the feed rate must be increased. However, for the same feed rate of limestone, a decrease in the size of limestone particles results in an increased sulfur retention. This may be explained on the basis of an increase in the overall surface area per unit volume of the bed when the average diameter of the particles decreases. It may be noted from Figure 9 that regardless of the limestone particle size, if sufficient residence time is allowed for limestone particles in the bed, it is possible to obtain sufficiently high sulfur retention. 

Sulfur retention is more prevalent in low-rank coal ashes, due to the generally higher calcium contents. Some of the slags from these viscosity studies retained up to 2% SO3 (Table II), even after being heated above 2600 F (1427 C). 

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