Getting the measure of a gas that won't keep still

Mark Allinson, a process-flow specialist at ABB, explains how thermal-mass meters can accurately monitor variations in the generation of biogas

The main focus in wastewater treatment is on achieving optimal purification, while providing for economical energy exploitation.

Sludge putrefaction produces methane gas, which can be used for power generation. The quantity of the gas produced is an important indicator of proper operation of the putrefaction basin and the processes in it.

At a wastewater treatment plant in northern England, thermal-mass meters monitor the biogas produced by four digesters, which have been converted from floating-roof to fixed-roof units.

The gas, which is predominantly composed of methane, is used to power three boilers. The long-term aim is to introduce combined heat and power (CHP) to the site.

Accurate information of the gas quantities allows for optimised process engineering and efficient use of the gas produced as an energy source. Due to low pre-pressure, varying gas quantities and a pollution component, which, in parts, is quite important, this necessary measurement is a challenging task.

The meters were chosen because of their ability to provide accurate readings at low gas flows and variable pressures. The amount of gas and the pressure vary, depending on the time of year, and variations in the sludge entering the process. Consequently, the yield is not always the same, which means the end user needs an accurate way to monitor the flow.

Thermal mass meters deliver an accuracy of about 1% over a turndown ratio of about 150:1. This means they are far more accurate than most flow measurement techniques for the low flows often found in flare-gas applications. For example, a typical orifice plate or venturi meter can only manage a turndown of between 3 and 5:1.

Insertion devices, such as thermal-mass meters, are also more economical in larger-diameter pipes than instruments that have to be sized to fit a particular diameter. For example, although they are more common for measuring liquid flows, some manufacturers claim their Coriolis meters can be used for gases. But, because they have to be sized to effectively form part of the pipework, they are likely to be several times more expensive than thermal-mass meters in flare-gas applications.

In addition, because thermal-mass flow meters take measurements using two small probes on the end of an insert, they form only a minor obstruction to the surrounding flow. This means that correctly sized thermal-mass flow meters offer an extremely small pressure drop of between one and two millibars. Instead, vortex meters drop the pressure by between 50 and 100 millibars, while the drop across an orifice plate can be even higher.

There has never been more pressure on industry to provide accurate emissions data. Against this background, thermal mass meters offer accuracy for biogas measurement.

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