METHOD AND DEVICE FOR PURIFICATION OF GAS

Abstract: Method and apparatus for circulation of sand for a sand filter for cleaning of gases, in which a number of filter modules arranged in a filter housing (1) is supplied with sand from a number of sand silos (2, 3). Sand and dust are periodically blown off the filter modules down in a {bottom funnel} in the filter housing (1) and subsequently led by a rotary valve (4) to a separator (5) where the sand is separated from the dust. The sand falls down into a sand conveyor (6) which transports the sand back to the sand silos (2, 3) in a sand loop (7) when the sand conveyor (6) is full. The sand is weighed in the sand conveyor (6) before it is led by the sand loop (7) to the silos (2, 3). This step is repeated until the bottom cone in the filter housing is empty. Net sand flow in the loop is calculated and sand refilled to or drained from the sand silos (2, 3) accordingly.

Description: Method and device for purification of gass The present invention concerns a method and an apparatus for circulation of sand in a sand filter for cleaning of gases according to the introductory part of Claim 1 and Claim 4, respectively. Several techniques for removal of particles from smoke gases are known, e.g. cyclones, bag filters and electrostatic filters. A concept known from e.g. US patent No. 4,004,897, called Panel Bed Filter or sand filter, is an alternative to the more conventional methods for particle removal. Shortly explained, the concept is based on the utilisation of sand and other granulated material for the filter medium. This means that the filter can withstand high temperatures and that unburned material and glowing particles that are often included in the gas, do not constitute any problem. The cleaning of the gas takes place by leading the gas towards a sand module with deposition of the particles on the surface of the sand module in the form of a filter cake. When the filter cake has reached a thickness corresponding to a limit value of resistance across the filter, the filter is cleaned by pausing the circulation of gas and ejecting a short pressure pulse in the opposite direction through the filter. The filter cake is thereby blown off together with some amounts of sand. This mixture is transported to a separator where sand is separated from the dust and returned to the filter module. Thus, the sand is circulated in a closed loop. A disadvantage with this known circulation loop for sand filters is that there is no control of the amount of sand in the loop, and there is consequently no possibility of performing a controlled refill or removal should the sand level become to low, respectively to high. There is an object with the present invention to provide a method and an apparatus of the kind mentioned above, in which it is easy to maintain control of the amount of sand in the system, and with means for performing a controlled adjustment of the sand level. This object is reached with a method and an apparatus with features as described in the characterising part of Claim 1 and Claim 4, respectively. Further features are disclosed in the dependent claims. In the following, the invention shall be explained in further detail with reference to an example of an embodiment of the invention as disclosed by the enclosed drawing, providing a schematic view of a circulation loop for sand according to the invention. The drawing shows a filter housing 1 equipped with a number of filter modules (not shown). The filter modules are in a preferred embodiment arranged in two concentrical circles within the filter housing 1 in such a way that addition of sand can be performed by forces of gravity from adjacent sand silos 2, 3. A number of silos 2 are placed around the periphery of the filter housing 1, and a silo 3 is placed in the centre of filter housing 1. The filter modules have a bottom cone (not shown) which captures sand and dust that are blown off from the filter housing. This material is led by a rotary valve 4 to a separator 5 where sand and dust are separated. The dust is removed by a dust discharge, whereas the sand falls down into a sand conveyor 6. The sand conveyor 6 pneumatically transports the sand back to the sand silos 2, 3 by a sand loop 7 when the sand conveyor 6 is full. A sand return tank 8 is arranged to collect the sand from the sand silos 2, 3, when these are full. Transport of sand from the silos 2, 3 to the sand return tank takes place by a sand return loop 9. A sand filling reservoir 10 is arranged to add further sand to the sand conveyor 6 when necessary. A pressure air supply 11 gives high pressure air to by a air pressure loop 12 to the separator 5, the sand conveyor 6 and the sand loop 7. The sand return tank 8 and the sand conveyor 6 is equipped with weighing cells, so that it is possible at any time to keep control with how much sand the sand silos 2, 3 can receive. The weighing cells are connected to a calculating unit (not shown) for handling of input data and controlling of the total sand loop. When the silos 2, 3 are full the sand will be passed into an overflow outlet. When the sand conveyor has completed the transportation of a weighted amount of sand, the weight of the sand in the sand return tank are registered before it is dropped into the sand conveyor again. The process of discharging sand and dust from the filter continues until there is nothing more left in the bottom cone. By registering the sand flow in the total loop, it is possible to determine whether the sand volume in the system is increasing or decreasing, thus enabling refilling or draining of sand, accordingly. The invention is described herein with reference to a circular filter housing, but it is evident that other geometrical shapes, such as rectangular shapes will fall within the scope of the invention.

Claims: Claims: 1. Method for circulation of sand for a sand filter for cleaning of gases, in which a number of filter modules arranged in a filter housing (1) is supplied with sand from a number of sand silos (2, 3), and sand and dust being blown off the filter modules down into a bottom cone in the filter housing (1) and subsequently led by a rotary valve to a separator (5) where sand is separated from dust and the sand falls down into a sand conveyor (6) which transports the sand back to the sand silos (2, 3) in a sand loop (7) when the sand conveyor (6) is full, characterised in comprising the following steps: a) weighing of the sand in the sand conveyor (6) before it is led by the sand loop (7) to the silos (2, 3), b) repeating step a) until the bottom cone is empty, c) calculating net sand flow in the loop and refill sand to - or drain sand from the sand silos (2, 3). 2. Method according to claim 1, characterised in that sand that are drained according to step c) are supplied to a sand return tank (8) by a sand return loop (9) where the sand is weighted before it is dropped into the sand conveyor (6) as and when required. 3. Method according to claims 1-2, characterised in that sand to be refilled according to step c) is picked up from a sand filling reservoir (10). 4. Apparatus for circulation of sand for a sand filter for cleaning of gases, in which a number of filter modules is arranged in a filter housing (1) for being supplied with sand from a number of sand silos (2, 3), where a bottom cone is arranged in the filter housing for temporarily keeping sand and dust being blown off the filter modules and where a rotary valve (4) is arranged to feed sand and dust to a separator (5) where sand and dust are separated, where after the sand falls into a sand conveyor (6) which through a sand loop (7) transports the sand back to the sand silos (2, 3) when the sand conveyor (6) is full, characterised in that the sand conveyor (6) is equipped with a weighing cell provided to weigh the sand before it is led by the sand loop (7) to the silos (2, 3). 5. Apparatus according to claim 4, characterised in that the sand return tank (8) is equipped with a weighing cell provided to weigh the sand before it is dropped into the sand conveyor (6) as and when required. 6. Apparatus according to claim 4-5, characterised in that a sand filling reservoir (10) is provided for supplying sand to the sand conveyor.

Inventors: NOS PER O, SOENJU OTTO K, NOS, PER, O., SOENJU, OTTO, K.
Applicants: NOS PER O, NYFOTEK AS, SOENJU OTTO K, NOS, PER, O., SOENJU, OTTO, K.
Application year: 1998
Main classification: B01D46
IPC codes: B01D46