Self-cleaning filters from SPX are being utilised in a biomass processing application for power generation which requires no interruption to flow during cleaning operations

Self-cleaning filters are designed primarily to provide efficient filtration for liquids including those of high viscosity requiring continuous filtration without interruption to the flow during the cleaning process.

This eliminates any operator contact with the process liquid. SPX Airpel self-cleaning filters use high quality stainless steel elements that are highly effective in removing particulates as small as 25 microns (mµ) from fluids including petrochemicals and water.

In the UK, Airpel filtration technology is being taken up by Refgas. Its CHP (combined heat and power) advanced gasification facilities generate renewable energy using waste products, alternatively referred to as RDF (refuse derived fuel) as a feedstock in volumes as low as 5,000 tonnes per annum. Biological feedstocks used include timber and wood waste, refuse-derived fuel, compost residues, animal manure and sewage cake. The specifications for the feedstock include particle size of less than 100mm, a low moisture content and contaminant on non-biological material of less than 30%.

The gasification of biological feedstock developed by Refgas is a cooking process and once the fuel is ignited, air is drawn down through the oven to keep the fuel constantly burning. So long as fuel is available, it is a highly efficient, self-sustaining combustion process. The gas produced from the downdraft cooking process is drawn away before it can combust and passes on through a network of cyclone separators and two stages of wet scrubbing to remove particulates.

Each modular gasification system employs a recirculatory water scrubbing process for cooling the gas down from 600ºC to 40º and cleaning it to make it suitable for safe burning in a gas engine. It is essential that the recirculated water is continuously filtered to remove particulates that may be present in the gas as these can foul the spray nozzles in the water scrubber.

The water for the wet scrubbing is held in a tank on the unit and circulated around the system in what is in effect a closed-loop. At various stages the water is filtered through membranes in order to remove large particulates and can be discharged to drains. The first wet scrubbing stage is very much a quench cool that serves the purpose of removing moisture from the feedstock and condensate from the gas produced by the gas cooling.

This also knocks out any particulates that have come through the cyclones. Because the water is constantly being recirculated, it is necessary to filter it before it passes through the scrubber nozzles.

In-line filtering

In order to preserve the cleanliness of the water used for the scrubbing operations, in-line filtering provides the best solution and the equipment selected by Refgas for its requirements is the Airpel S-200S self-cleaning in-line filter. Located in the water recirculation line, the filter operates by the liquid entering the body and flows through the filter element from the outside to the inside.

Particulates are collected on the external surface of the wire wedge or perforated filter element and removed without interruption of the liquid flow by the rotation of the filter against a stationary cleaning blade. All debris accumulated during rotation is deposited into the bottom of the filter body from where it is periodically discharged either manually or automatically through a drain plug/valve.

The Airpel filter range comprises three forms of blade type filter and a fabricated unit, offering a choice of manual, motorised, fully automatic electric and pneumatic operation. Automated self-cleaning enables standalone, low maintenance liquid processing, so it is ideal for operators of Refgas gasification systems who require 24/7 operation to meet their power generation requirements. In earlier versions of its RG650 gasification system, Refgas employed basket filters but found that emptying these was a messy operation and not popular with end-users.

The first Airpel self-cleaning filters to be specified for the latest systems are manually operated, enabling operators to open the drain plug/valve to a discharge line at various intervals. However, it is planned to move across to the automated filter containing a differential pressure switch which Airpel supplies wired and pre-tested for site connection both to the power source and dump valve.

Using this version means that the filter is completely standalone and discharges waste matter as and when necessary. For the plant operator, the self-cleaning filter provides a safe, clean and efficient method of removing and disposing of particulates.