Gas from waste digesters contains high levels of methane, CO2, and H2S, and little to no oxygen present. The Control Equipment Digester Gas Monitor checks for all these gases on a cyclic basis. A powerful air aspirator pulls a sample from up to 30 metres away. Since digester gas contains high humidity and high H2S, both of which can cause damage to sampling systems, the Digester Gas Monitor is designed to handle these with no damage to the sampling system or sensors. The sample is filtered through a series of dirt, dust, and moisture stopping filters, and these filters are automatically blown back and cleaned out with fresh air at the end of each cycle.
Measurement time is just 4 minutes, taken at periodic cycles. Cycle time is selectable, with settings for cycle time of 1 hour, 4 hours, 8 hours, 24 hours or 7 days. A cycle can also be initiated anytime by pressing a button. In between cycles the sensors and sampling system are flushed with fresh air so as to minimise corrosion caused by the high H2S content. The system is fault tolerant minimising the possibility of expensive repairs or downtime.
System integrity is maintained at all times using a flow fail monitoring device which provides a fail alarm and relay if there is ever a problem with the flow system, such as a blockage, or if the air aspirator compressed air supply is removed or interrupted.
The system is housed in a wall mounting NEMA 4X enclosure. The gas readings can be viewed through the enclosure door clear window, and also flowmeters and filters are easily visible for confirmation of correct flows and operation. The Gas Digester Monitor contains adjustable alarm levels for each gas, and also programmable alarm relays. In addition, the unit provides 4-20mA signals for each gas concentration for connection to an external DCS, PLC or other site control system. A Modbus output is also available.