Safety in the Plant
Precise biogas flow measurement
Scott Rouse reveals how to overcome the challenges of changing gas composition.
H
arnessing biogas from landfills is a quickly growing source of renewable energy. The Environmental Protection Agency (EPA) estimates that there are approximately 6,000 landfills in the USA, contributing an estimated 650 billion cubic feet of methane per year. Landfill gas, containing mostly CH4
and CO2 , is
produced by wet organic waste, decomposing under anaerobic conditions in a landfill that is covered and mechanically compressed by the weight of the material that is deposited from above. This material prevents oxygen exposure thus allowing anaerobic microbes to thrive. As seen in Fig. 1, this gas builds up and is slowly released into the atmosphere. The flow measurement challenge here is the fact that
the composition of biogas varies depending upon the source. Most flow meters are calibrated for one specific gas mix composition; they cannot provide accurate readings if the composition changes without being sent back to the factory for recalibration.
How to measure biogas Since the biogas composition is critical to its energy- producing value, facilities need to assess the best flow meter measurement technology to manage the compositional changes. Many companies with varying technologies are interested in measuring the biogas as it leaves the landfill or digester tank, but this is a challenging application for many reasons. Firstly,
Fig. 1. Biogas collection and use.
varying gas compositions make accurate measurements difficult because most meters are calibrated for one gas or mixture; when the composition changes, the flow measurements are no longer accurate and the meter must be recalibrated. Secondly, low pressure makes differential pressure
devices such as orifice plates unsuitable since they require a fairly large differential pressure to operate. Finally, biogas is often very dirty with a high
moisture and particulate content, which can clog up devices such as annubars and orifice plates, and gum up turbine meters and similar instruments that have moving parts. Traditionally, thermal mass flow meters have
been the instrument of choice. They offer reasonable accuracy for the price (2.0% of reading) and use a convenient insertion design that eliminates pressure drop. They also have no moving parts and can measure both high and low flows with a 100:1 turndown. Although such meters do many things well, one
thing they cannot do is account for changes in biogas composition. These flow meters must be calibrated for a specific biogas mix and rapidly lose accuracy if gas composition changes, which means the instrument must be sent back to the factory to recalibrate for the changing gas composition – wasting time, resources and money. One way to account for variable composition is the use of a continuous real-time sampling system integrated with a flow meter.
44
www.engineerlive.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68