The flue gas cleaning system consists of a wet electrostatic filter, with a capacity of 250 000 m³/h. Photo: Fotograf Cecilia.
The gas cleaning facility deals with all the fire smoke from all our activities. The gases are collected in canopies above the various facilities, or in ceiling extraction points in the large test halls, and drawn through a common evacuation duct, into which water is sprayed, partly to cool the gases and partly to wash out soluble contaminants such as hydrochloric acid.
The resulting saturated gases enter the filter, which is a WESPor® wet electrostatic filter, in which they pass through honeycomb-shaped steel tubes at a low velocity. An electrode, maintained at a high voltage, in the centre of each steel tube charges up particles and droplets, which are then attracted to the surrounding earthed steel surfaces. This is the same principle as that which makes dust collect on the front of a television tube or computer screen, or which makes dry hair stand up when it has been combed. Particles collecting on the inside walls of the steel tubes are washed down by water into a buffer tank beneath the filter. The cleaned gases are then exhausted by two large fans to a silencer and then into the 33 m high chimney.
The water in the buffer tank is constantly circulated and, in principle, does not need to be changed. The particles that it has collected are removed by pumping a constant fraction of the water to a filter which removes the particles. The particles are then discharged to a sludge tank which is emptied at intervals, and from which the sludge is treated as hazardous waste.
The thermal output of many tests can be high, of the order of 1-20 MW, which means that, as the water also cools the gases, the contents of the tank are heated. This energy is recovered, and used for space heating in the premises by passing incoming supply air over a heat exchanger.
The gases from the various fire tests are evacuated from the test areas and pass through
the cooler and electrostatic filter on their way to the chimney. The heat in the gases is
used to warm incoming air.
Combustion conditions in natural fires are generally unfavourable, with the result that large quantities of smoke are formed. In the case of larger fire tests, the dust content of the gases can amount to 1000-2000 mg/m³ at the inlet of the filter plant, which must be reduced to not more than a maximum permitted value of 100 mg/m³ in the chimney, in order to comply with statutory environmental requirements. Measurements have been made of the results of representative types of fires, based on some of the most common materials in our tests, such as diesel fuel, heptane, polystyrene and PVC. The results show that particulate emissions are reduced to considerably less than this permitted value, of the order of 20-50 mg/m³. The quantity of hydrochloric acid in the chimney when dealing with tests of burning PVC is less than 2 mg/m³, and thus far below the permitted limit of 20 mg/m³.
An essential prerequisite for being able to link all the test facilities to a common filter plant is that they must not interfere with each other. The capacities of the various exhausts in the laboratory range from about 400 m³/h in a fume cupboard up to 250 000 m³/h in the large fire testing hall. Many tests require that the gas flow must be constant throughout the test, while others have a requirement for the ventilation to be increased as the fire grows. The pressure in the fire test furnaces must be maintained constant within very narrow limits.
In order to meet these varying requirements, the flue gas cleaning system incorporates an advanced control system. Before starting a test, the personnel responsible for it book the necessary capacity. The system checks that this will not exceed maximum capacity and, when the booking has been accepted, it remains merely for the operator to set the required flow rate at the particular extraction point.
The system then ’soft-starts’ the flow automatically, in order not to disturb flow conditions at other extraction points.
The control system is designed to provide a rapid overview of information.
The test personnel communicate with the control system via a PC in each laboratory, linked to our intranet. The control system uses the same protocols and methods as used for transferring information on the internet/intranets, which means that the system can be accessed from anywhere in the laboratory.
Internationally, there are several fire testing laboratories with gas cleaning equipment, but none which has the same system architecture and type of control system as we have. In terms of expertise and equipment, Fire Research has been among the world’s leading laboratories for many years. With this latest round of investment in additional test facilities, new premises and efficient fluegas cleaning, we are now well placed to meet requirements from customers and society.