Granular-bed Filtration Assisted by Filter Cake Formation: Advanced Design and Experimental Verification

Abstract

Particulate matter removal at high temperatures has gained a great attention in the last years due to the vast growing of power production plants around the world. A reliable and robust gas cleaning method is inherently necessary to facilitate efficient fuel to energy conversion while protecting all the downstream equipments in a plant. In accordance with the current demand, a detail study has been conducted on panel bed filter which exhibits the suitable characteristic for high temperature applications.

The thesis focuses on four main parts related to the  design evaluation, experimental verification and development of new apparatus and method for regeneration  in a panel bed filter. The four main parts are as following;

1. Testing of an advanced new louver design, “filter tray” at room and elevated temperature

2. Development of a new apparatus and method  for puff-back cleaning

3. Testing of “filter tray” with the new puff-back apparatus and

4. Industrial scale panel bed filter testing in a commercial combustion plant.

A very first laboratory scale panel bed filter with filter tray louvers was built and tested at room temperature. Tests were performed mainly at higher filtration velocities (7 – 31cm/s) with sintered bauxite spherical particles (mean diameter of 710 m) as filtration medium with standardized test dust (mean diameter of 9 m). The filter demonstrated excellent results at filtration velocity of 30 cm/s however unstable residual pressure drop profile and high dust penetration were recorded, suggesting the depth of the granular beds are too shallow.

A second laboratory scale panel-bed filter with filter tray louvers with deeper granular beds, i.e. 40 mm was built. The gas-entry surfaces are much larger than the first test rig. Tests were performed from low to high filtration velocities with sintered bauxite spherical particles (mean diameter of 470 & 710 m) and olivine sand particles (mean diameter of 545 m) as filtration medium and standardized test dust (mean diameter of 4 & 9 m) as test dust at room temperature and at 120°C. The filter illustrated excellent results however failure in the conventional puff-back method to supply sufficient sharp pressure pulse led to improper removal of filter cake. Accordingly, there is a need for an improved means of supplying puff-back gas to the tall wide panel beds.

A novel high speed valve, a new method and apparatus for creating a longitudinally disposed sharp puff of gas for renewal of gas-entry surfaces was built and tested independently. The quick opening valve consists of a standard cylinder with air outlet holes, a rod with nylon plugs and an actuator. The valve was built in the laboratory; several adjustments were made along the test until the time for emptying of the pressure in the tank was less than 50 ms. It was then integrated with the second test rig, referred to as third  test rig. Tests were performed at room temperature with the aid of high-speed camera to obtain visualization of the movement of the granular beds during puff-back with and without the presence of filter cake.

Filtration test were performed at elevated temperature of 120°C. Tests were performed from low to high filtration velocities with sintered bauxite spherical particles (mean diameter of 470 µm) and olivine sand particles (mean diameter of 545 m) as filtration medium and standardized test dust (mean diameter of 4 m). The results and video file analysis revealed that the new puff-back method is capable of removing the entire filter cake, resulting in stable residual pressure profile and very low dust penetration compared to the conventional puff-back.

An industrial scale panel bed filter testing is the fourth test rig. The field test was performed with a single filter module of louver design L10-56 at a 450-kW biomass combustion plant at Bjertnæs Sag AS in Jevnaker in Norway. The test was performed with a slip stream from the main stream of exhaust gas before emitting to the chimney. The test was performed at superficial velocity between 5.4 - 6.9 cm/s with total flow rate of 290 - 370 m 3/h at 120°C with inlet dust concentration between 1.4 to 1.8 g/m3. Olivine sand particles with mean diameter of 545 m were used as filtration medium. The results showed filter cakes are being formed on the gas-entry surfaces and the penetration was lower than the allowed limit during the beginning of the test. Nonetheless, due to some instability caused probably by some slight variation in the louver geometry led to higher emission towards the end of the test and hence further testing was interrupted.