323 Hot Gas Filters

McDermott Corporation, Lynchburg, Virginia, engineers use this same process technology to produce CFCC flanged closed-end porous tubes. These tubes perform as filters used to remove solids from gases. The solids may be either the desired product or a contaminant or a catalyst to be recovered and recycled. Their removal protects downstream equipment from erosion. Filters must withstand chemical corrodants, high-speed hard-particle impingement, long-term strength retention, pulse stress, vibration, fatigue, temperature, pressure, and high mechanical and thermal stresses. They must maintain low pressure drops and high flow rates to sustain production rates. Filter materials must not contaminate the product stream (Table 3.4).

Metals and monolithic ceramics have been the materials of choice. Metals tend to corrode and have temperature limitations. They also require cooling the gas stream prior to filtering, thereby decreasing efficiency, increasing costs and complexity by requiring gas dilution air scrubbers or heat exchangers. Monolithic

TABLE 3.4 CFCC Candle Filter Material Typical Properties

Construction: Nextel™ Ceramic Fibers in a silicon carbide matrix Continuous use temperature: 1204°C (2200°F) Maximum short-term temperature: 1315°C (2400°F) Coefficient of thermal expansion: 4.6 x 10-6/°C (2.5 x 10-6/°F)

ceramics are limited due to susceptibility to thermal stresses, mechanical shock, and damage during installation.

One specific application is filtering coal ash from coal gas. Coal gasification plants generate electricity from gas created by heating coal. The gas is fed, as fuel, to a turbine engine. The engine turns an electric generator.

Coal gasification plants are of interest since they offer certain advantages over traditional coal or natural-gas-fired utility plants. Gasification plants emit less carbon dioxide and oxides of nitrogen and cost less to operate.

The turbines require particle-free gas fuel for safety, cost, and meeting clean air regulations. Coal ash is filtered from the coal gas stream through the use of candle filters. These filters are porous, hollow tubes 1.5 m (59 in.) long. They are ganged into arrays as shown in Fig. 3.8.

Hot coal gas is pumped onto and through the filter leaving the coal ash on the outside. The clean gas passes through the open end of the tube and into the turbine. The coal ash accumulates until the system is occasionally back-pulsed, every 15 min at 90 psi gage, to drop the ash into collectors. The ash buildup can become so great that it bridges between tubes and creates a surprisingly high mechanical stress. The back-pulse is also a major physical shock on the filters.

Power plants in Karhula, Finland, and Wilsonville, Alabama, are evaluating CFCC hot-gas filtration systems. The Karhula plant has operated CFCC filters successfully for 580 h. The Wilsonville plant is operated by Southern Company Services. Its CFCC filters continue to operate successfully beyond 3000 h at 850°C (1800°F). The CFCC filters are resisting the corrosive coal ash, high temperatures, and both thermal and physical shocks.

CFCCs - Tough, Lightweight Gas Filters

FIGURE 3.8 Individual filters on left are ganged into assembly shown on right. Porosity is built into CFCC cylinder to filter fine solid particles from coal-derived gas prior to burning in turbine engine. Engine energy is converted to electricity. These filters are also useful for filtering other solids from other gas streams.

FIGURE 3.8 Individual filters on left are ganged into assembly shown on right. Porosity is built into CFCC cylinder to filter fine solid particles from coal-derived gas prior to burning in turbine engine. Engine energy is converted to electricity. These filters are also useful for filtering other solids from other gas streams.

Within other gaseous streams there is often a need to remove particulates before further processing. Conventional filtration materials and techniques either are inefficient or insufficiently rugged to survive in the process environment. Energy efficiency and throughput can be accomplished with CFCC filters that withstand both corrosive and high-temperature environments and mechanical and thermal stress. The proposed filtration concept cleans itself via back-pulsing, with apparent indefinite life.

A candle filter system offers two advantages over electrostatic systems that are coupled with liquid scrubbers. It eliminates the need to cool the stream saving energy, capital, and maintenance. Second, the filtered stream is available for heat recovery.

McDermott Corporation CFCC filters have a density of 0.8 g/cm3. The combined effect of lower weight and thinner walls enables a smaller, simpler supporting structure. CFCC hot-gas filters offer thermal stability, strength, resistance to thermal shock, resistance to fatigue, corrosion, erosion, and general inertness. Anticipated benefits include reduced downtime, increased throughput, reduced energy consumption, longer life, reduced emissions, increased product yield, increased efficiency, and reduced cost.

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