Hot gas filter

The SO2 in the gases is converted to SO3 by the catalytic converter which is then absorbed in a stream of sulfuric acid of optimum concentration and temperature. 

Old plants had a three-pass converter which generally could not achieve an overall conversion of more than about 90-93%. This caused a lot of atmospheric pollution due to the escape of unconverted SO2 as well as higher cost of production due to a greater consumption of sulfur. Besides, the energy efficiency was also low because little effort was made to recover the heat of the reaction  

Later designs were based on a four-pass converter with inter-pass cooling of the gases. This was able to increase the overall conversion to 96.0-97.5% in the single contact single absorption process. Heat recovery equipment was installed between catalyst passes to improve the energy efficiency. 

Modem designs of tlie DCDA process with five catalyst passes) further improved the overall conversion to 99.0-99.5% with better heat recovery too. 

The higher investment required for these plants was offset by reduced cost of production by operating the plants at higher percentage SO2 to the converter from the current level of 7.5-8.0% to 9.5-11.0%. 

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In addition to dense monolithic ceramics, porous silicon nitrides are gaining more importance in technological applications [24], Some porous silicon nitrides with high specific surface area have already been applied as catalysis supports, hot gas filters and biomaterials [25], There is an emerging tendency to facilitate silicon nitride as biomaterial, because of specific mechanical properties that are important for medical applications [25], Moreover, in a recent study it was shown that silicon nitride is a non-toxic, biocompatible ceramic which has the ability to propagate human bone cells in vitro [25], Bioglass and silicon nitride composites have already been realized to combine... 

The synthesis gas leaving the gasifier contains entrained particles of char and ash. Particulate removal is performed using cyclone separators and ceramic candle type hot gas filters. The coal gas is primarily comprised of H2, CO, C02, and H20. Since there is less than 0.1 mol% CH, reforming of the syngas is not necessary. However, in order to maximize hydrogen production, shift reactors will be needed to convert the carbon monoxide to hydrogen. 

Heidenreich S and Scheibner B. Hot gas filtration with ceramic filters Experiences and new developments. Filtr. Sep. 2002 May 22-25. Heidenreich S and Wolters C. Hot gas filter contributes to IGCC power plant s reliable operation. Filtr. Sep. 2004 June 22-25. Larbot A, Bertrand M, Marre S, and Prouzet E. Performances of ceramic filters for air purification. Sep. Purif. Technol. 2003 32 81-85. DeFriend KA and Barron AR. A simple approach to hierarchical ceramic ultrafiltration membranes. J. Membr. Sci. 2003 212 29-38. Endo Y, Chen D-R, and Pui DYH. Collection efficiency of sintered ceramic filters made of submicron spheres. Filtr. Sep. 2002 March 43-47. Sakol D and Konieczny K. Application of coagulation and conventional filtration in raw water pre-treatment before microfiltration membranes. Desalination 2004 162 61-73. 

After the cyclone, the gas produced flows to a gas cooler and a hot gas filter. The gas cooler is of a fire tube design and cools the gas to a temperature of 350 - 400 C. After cooling the gas enters the candle filter vessel where the particulate clean-up occurs. Ash is discharged from the candle filter, as well as from the bottom of the gasifier, and is in the meantime cooled and depressurised. 

The diagram below (Figure 4) shows a typical operating curve for the hot gas filters and clearly indicates that no continuous increase in pressure drop is taking place. 

Figure 4 Pressure drop in a hot gas filter cleaned by nitrogen pulsing...

Originally a hot gas filter of the ceramic type was installed. This filter consisted of ceramic filter candles arranged in six groups with separate back-pulsing. 

The ceramic filter showed good filtration efficiency, with stable pressure drop. However, after more than 1200 hours of trouble free operation suddenly two ceramic candles broke. The difficulty to detect a relatively small failure in a hot gas filter was then noticed in practice even though no serious damage was caused. 

To protect the gas turbine in case of a hot gas filter break-down a metallic police filter has been installed down-stream of the main filter. Rather soon after this installation was completed another failure of the main filter occurred. From figure 8 can be seen that the pressure drop over the police filter increases rapidly, whereas it is hard to spot the broken candle from the pressure drop across the main filter. 

In the figure 8 the upper curve represents the police filter pressure drop, whereas the lower curve represents the pressure drop across the main hot gas filter. 

During the summer 1998 it was decided to install metal filter candles instead of the ceramic candles in the main hot gas filter. The metal filter candles are installed in the original filter vessels but with a new tube sheet and back-pulsing arrangement. 

Bio-oils were produced m 1995-1998 from various wood species on fast pyrolysis pilot plants, which differed by their construction features NREL (ablative pyrolysis with hot gas filter, poplar wood, 1996), Aston FB (fluidized bed, poplar wood, 1995), IWC, TP 29 (fluidized bed, bamboo, beginning of 1998) BTG (rotating cone, pine wood, 1995), VTT (circulating fluidized bed, pine wood, 1997), IWC, E-filter (electrostatic precipitator, beech wood, end of 1998) and IWC Kl-FIO from different coolers (beech wood, end of 1998). 

Figure 4.26 shows an agglomerating fluidized-bed gasifier developed by KRW. In the KRW process, limestone can be injected with the coal for sulfur capture. The KRW process is employed in a 100 MW IGCC Pinon Pine Plant near Reno, NV. Due to some difficulties in the hot gas filtering section, the plant could not be started up successfully.1... 

The equipment was set up as shown in Figure 2. The hot gas filter, reformer and scrubber/condenser were installed on a side stream taken prior to gas combustion. 

Devitrification of binder at the pore surfaces has been frequently reported for similar and first generation clay bonded hot gas filters after the use in real combustion conditions. In the current study similar devitrification was qualitatively found to increase as function of time and the amount of water vapour in water vapour environment. Further, oxidation of SiC occurred in both materials after 500 h exposure at 850°C atmospheric water vapour. The results of the current study are in accordance with the results of similar filters operated up to 1500 h in pilot plant in varying combustion environments. Our earlier evaluations of the filter material performance have indicated severe oxidation of SiC and this combined with the crystallization of the oxidation layer and cracking due to thermal stresses and mismatch due the phase transformation of Si02 has been pointed as a mechanism to degrade the strength and operation lifetime of SiC-based clay bonded hot gas filters, ... 

After the most severe oxidation exposure of the current study (w), the oxidation layers were rare and only about 100 nm thick for A and 500 nm for B. For material B an estimate for the parabolic oxidation rate constant less than 0.0005 nf/h was calculated from the oxidation scale thickness of 500 nm (Fig.lc). The value is low when compared to parabolic rate constants for SiC in pure atmospheric oxygen at 900°C. However, direct comparison is misleading since in hot gas filters SiC is covered by the binder. This means that water needs time for diffusion through the binder before the oxidation occurs. In addition an initial oxidation layer of unknown thickness may be present. 

For the past 15 years, research and development of CMCs has been sustained because of their potential for military and commercial applications. The applications of interest include (1) aircraft engine components, such as combustors, turbines, compressors and exhaust nozzles (2) ground-based and automotive gas turbine components, such as combustors, first and second stage turbine vanes and blades, and shrouds (3) aerospace engines for missiles and reusable space vehicles and (4) industrial applications, such as heat exchangers, hot gas filters, and radiant burners. 

Composite behavior has also been studied in oxide systems (e.g., oxide fiber-reinforced porous oxide matrix composites with no interfacial coatings). Oxide composites have the attractive features of oxidation resistance, alkali corrosion resistance, low dielectric constants, and potentially low cost. Because of these properties, oxide CMCs could be attractive for hot gas filters, exhaust components of aircraft engines, chemical processing equipment, and long-life, lower temperature components. 

Balanced research of oxide and non-oxide materials on their specific life-limiting characteristics appears to be necessary because neither class of materials can satisfy design and service life requirements for all of the anticipated applications. For example, SiC-based materials have the high thermal conductivities and low thermal expansion coefficients essential for some components, particularly in high performance turbines for which oxides are inadequate. Conversely, in some corrosive environments (e.g., hot gas filters in coal-fired power systems), oxides provide necessary corrosion resistance. 

Nextel 610 AI2O3 950 850 insulation for furnace linings, seals and curtains, high-temperature sleeving, aircraft firewalls, ceramic composites, and hot gas filters reinforcement of <227kg 206/ kg for 227-682 kg 187/ kg for >682kg l,100/kgfor... 

Heidenreich S and WoltersC. Hot gas filter contributes to IGCC power plant s reliable operation. Fr7tr Separat. 2004 41 22-25. 

Use of efficient hot gas filters and sulfur filters to minimize the deposition of ash on the catalyst surfaces. 

It may be noted that the hot gas filters not only minimized deposition of dust particles in the catalyst beds but also increased the heat capacity of the conversion system. In the case of a long plant stoppage, there could be more heat retention in the plant when blinds are inserted in the gas inlet line of the HGF. The unit would also protect the top layer of the first pass catalyst from any soot/unbumt carbon particles during plant start-up (during commissioning or after an annual shutdown.)... 

S02 conversion systems. Hot gas filter (optional—since sulfur filters will be used) and catalytic converters with internal supports for catalyst of the conventional (potassium promoted) and cesium promoted types. 

Insulation materials. Glass/mineral wool, aluminum cladding sheets for gas/ hot air, steam lines, hot water, and sulfur lines, WHR systems, heat exchangers, converters, hot gas filters, etc. 

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