SOx removal

Silica from zeolite migrates less readily. In the magnesia-alumina system, spinel, as identified by X-ray diffraction, is inactive for SO2 removal. The effect of temperature on steam stability, oxidative adsorption and reductive desorption of SO2 are described. Five commercial catalyst types are ranked for SOx removal. 

More recently, workers at Unocal (10) have used thermodynamics to select oxides for SOx removal from FCU s and have identified the oxides of 20 elements as likely candidates. This list was narrowed further (11) to several candidates, Ce, Al, Co, Ni and Fe, by matching the decomposition and reduction temperatures of their sulfates to the conditions of the process. 

Platinum, as well as other promotors, can cause problems such as NOx formation or other unfavorable side effects. The rare earths do not have these drawbacks so a good deal of attention has been given to their development for SOx removal. 

An understanding of temperature effects is important to maximize the benefit from SOx removal agents. There are three areas in which temperature effects are particularly important ... 

Steam Stability. Steam stability of SOx removal agents is strongly affected by temperature. We have seen previously that at 1350 F deactivation of cerium/alumina additive, caused by silica poisoning, was influenced by how long the additive was steamed and whether the additive was steamed in the presence or absence of cracking catalyst. These results were extended to other temperatures. 

Calcined LDHs also have application in the reduction of SOx and NOx emissions from the fluid catalytic cracking (FCC) units in oil refineries [194-196], Corma et al. attempted to optimize the performance of mixed oxides produced from MgAl-LDHs as SOx-removal additives to FCC catalysts [194]. Among the oxides studied, that obtained from a MgCuAl-LDH was found to be the most effective at catalysing both the oxidation of S02 to SO2 in the FCC regenerator and the reduction of the sulfates to H2S, which may be recovered,... 

The actual technology involves the formulation of multifunctional cracking catalysts which are composed of different amorphous and crystalline acid functions, and a series of additives for metal passivation, SOx removal, promotors for total combustion, and octane enhancing additives. Among them, zeolite Y is the main component controlling the activity and selectivity of the cracking catalysts. 

When impregnated with Ce(N03)3 solution and calcined this spinel produces a material that has a very poor SOx removal activity (Table If Figure 2) because of a lack of surface area and low pore volume. 

In one application it has been shown that using OEC can increase the efficiency of SOx removal from exhaust gases from a combustion process. One way of removing SOx is to inject a transfer agent into the contaminated gases to collect S03 so that it can be released as H2S in a stripper. Additives are used to convert the S02 in the gases to S03. In a fluid catalytic cracker (FCC), OEC was used to reduce SOx.29... 

J.W. Cook, L.P. Cornell, M. Keyrani, M. Ne)unan and D. Helfritch, Simultaneous particulate NOx, SOx removal from flue-gas by all solid-state eletrochemical technology. Final Report, US DOE Contract, DE-AC22-87PC79856. 

A key research area is the study of the effect of selective catalytic reduction (SCR), selective noncatalytic reduction (SNCR), dry particulate adsorbent injection, and SOx removal systems on the removal efficiency, pressure drop, and lifetime of the baghouses. The use of fabric filters in PC boilers is increasing every year due to the stringent particulate emission standards. Pennsylvania Power and Light was the first utility to install a bag-house in 1973, and up until 2005, more than 129 RGFFs were installed on 28 GW of power plant capacities [64] (Figure 18.10). 

Air pollution control devices (gas cleaning devices) (Chapter 23) found in fossil fuel-fired systems (particularly steam electric power facilities) include particulate removal equipment, sulfur oxide (SOx) removal equipment, and nitrogen oxide (NOx) removal equipment. 

Particulate removal equipment includes electrostatic precipitators, fabric filters, or mechanical particulate collectors, such as cyclones. SOx removal equipment includes sorbent injection technologies and wet and dry scrubbers. Both types of scrubbers result in the formation of calcium sulfate and sulfite as waste products. 

Electrochemical Processes for Gaseous Sulfur Oxides (SO and SOx) Removal... 

Examples of liquid additives currently in use include bismuth and antimony based additives for passivation of nickel contaminants. A number of solid catalytic additives have been developed that are specific for certain functions. Approximately two-thirds of North American units utilize a noble metal promoter to reduce emissions of CO as well as provide beneficial yield effects. During the early to mid-1980 s, SOX removal additives came into use due to tighter environmental restrictions. A ZSM-5 based additive for octane enhancement and light olefin production was developed during the mid-1980 s and is used commercially. Additives have also been proposed as metal traps especially for vanadium passivation. These solid FCC additives have become an increasingly important tool by which refiners meet yield and environmental requirements. 

Each unit is unique in terms of its ability to retain catalyst due to its mechanical design and the operating conditions employed however, a general rule is that the additive should exhibit similar physical properties to the cracking catalyst. This philosophy is particularly true with CO promoters which are added at much less Aan 1 % of the cracking catalyst addition rate. From an economic standpoint, FCC additives are all significantly more expensive than FCC catalyst. CO oxidation promoters are twenty to fifty times more expensive while ZSM-5 based octane additives or SOx removal additives are about five to ten times more expensive than cracking catalyst. As a result, it is critical that the maximum amount of additive be retained in inventory for its useful (active) life. 

The present study also reports introduction of the significant catalytic activity to the pitch-based ACF (S.A.2000m2/g) after the CVD treatment by pyridine. The activity observed in the present study allowed the complete removal of lOOOppm SO2 at room temperature by W/F of 2.5x10" g min ml-i. The ACF modified by pyridine showed higher catalytic activity than the ACF modified by benzene, even though the same amount of carbon was deposited. Pyridine provided at 725°C basic functionality on the surface. Thus, catalytic SOx removal is very much enhanced. 

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