In the past decade, the U.S. has taken a page out of Europe’s playbook by bringing on-line a spate of anaerobic digestion facilities to produce biogas, which can be recovered, treated, and used to generate energy in place of traditional fossil fuels.

Anaerobic digestion systems are found in settings that include wastewater treatment, food-waste and agricultural processing.

U.S. wastewater treatment plants will use anaerobic digesters to produce biogas for their combined heat and power plants, and, in addition, will sell the biogas into the gas grid.

Interest in biogas means more attention given to measuring biogas flow and composition.

Proper gas-engine operations depend on biogas with the right methane (CH4) content. Recently introduced ultrasonic technology, including KROHNE’s OPTISONIC 7300, deliver reliable and accurate flow measurement for tapping into this important strategic energy source.

What it does

During anaerobic digestion, the bacteria breakdown wastewater, food waste or manure in an oxygen-free environment, producing biogas, which typically contains between 60 to 70 percent methane, 30 to 40 percent carbon dioxide (CO2), and trace amounts of other gases. The effluent remaining, after controlled anaerobic decomposition, is low in odor and rich in nutrients and can be recycled.

Wastewater treatment facilities in particular find addition of anaerobic digestion has benefits for both plant and community. The plant uses the energy the gas makes available, reducing operating costs and, ultimately, customer charges. The larger the plant, the more gas produced, and the savings can be considerable when a plant uses its own gas for operations, not to mention what can be gotten selling the gas back into the utility.

The sludge left over following digestion is recyclable. Rather than pay to incinerate the material, it can be used for deep-well injection or as a fertilizer. In the U.S., thus far the fertilizer is mainly used for golf courses or growing animal feed.

In other parts of the world, sludge left over after treatment facility digestion is widely used as fertilizer for food grown for human consumption. 

Currently, less than one-third of larger facilities use anaerobic digestion and many continue to incinerate residual material. The initial capital expense of adding digesters is the primary inhibiting factor. Another is that today’s lower natural gas prices result in a very long return on investment (ROI) on these systems – twice what it was a few years ago before the steep decline in gas prices. When gas prices are this low, the utility no longer wants to pay for excess gas.

Nonetheless, the long-term trend definitely favors facilities engaging in biogas recovery. Market demand is expected to increase, most probably leading to increased future gas prices. As gas prices stabilize, projections indicate growth in anaerobic digestion. In addition, the capital expense of adding anaerobic digesters will decline as demand grows.

Flow measurement technologies

Flow measurement is key to facility biogas use at a combined heat and power plant. The producer must know the CH4 content of the gas, because smooth and efficient gas engine operations in a combined heat and power plant are guaranteed only if the biogas has the right minimum CH4 content. Since the CH4 content of biogas can vary greatly, plant operators rely on continuous and reliable information about the biogas composition.

Flow measurement also helps plant operators to know how much energy is available. If low, gas might be brought in from the grid. If high, excess can be sold back to the grid. Reliable measurements are crucial for these biogas deliveries to gas grid operators.

Demanding measuring parameters associated with biogas applications are a challenge. Several technologies are available, including thermal mass, mechanical, vortex and ultrasonic.

Thermal mass flow meter technology has historically been used for gas measurements. In this event, two leads are inserted into the gas flow. One generates heat and the other reads the heat transferred to it; because of the properties of gas, how heat will transfer from one to the other is known exactly. This simple tried and true method has been used in thousands upon thousands of installations.

The problem, however, with using thermal mass flow metering for biogas is that the gas is wet; when water is introduced, it skews the results, creating the possibility of large measurement errors.

Mechanical meters aren’t really suited for biogas measurements. They tend to have turndown issues because the flows are so low; in addition, the need to measure gas and water together tends to throw mechanical metering off. This measurement is pressure sensitive, and with mechanical instrumentation, there is substantial pressure loss.

Vortex metering has been more successful. However, one must be careful of how the frequency is picked up if the vortex device uses a membrane; water soaking the membrane shortens the life of the vortex measurement device. Use of all-metal construction on the crystals that pick up the frequency generated can improve the measurement. There is little pressure drop on the vortex meter although one must consider turndown issues.

A more significant issue is that one cannot obtain a CH4 content measurement with a vortex meter. So, while vortex metering works well when the gas content is known and only flow measurement is required, it is less effective with biogas, where you have to measure the CH4 content and other components. Since the vortex meter cannot do this on its own, an additional measuring device must be added. 

This brings us to ultrasonic measurement technology, which uses the time transit differential method to guarantee flow measurement with a high degree of long-term stability, regardless of gas composition. Ultrasonic measurement is particularly well-suited to biogas applications because it guarantees full transit without pressure loss or other negative flow effect and it covers a wide measuring range. With no pressure drop, ultrasonic measurement devices have a leg up on other means of biogas measurement.

Ultrasonic measurement technology is rapidly gaining industry acceptance. The American Petroleum Institute (API) has accepted ultrasonic measurement for liquid custody transfer for nearly 20 years. The American Gas Association (AGA) has been continually rewriting its standards around ultrasonic technology in recent years. They are especially interested in ultrasonic technology’s diagnostics capabilities, allowing the meter to self-monitor and if need be alert users, including as to the liquid content in the gas being measured.

Research and development

KROHNE research and development initiatives advance its biogas metering capabilities by taking what it has learned from gas measurement applications that include process gas flow measurement and gas custody transfer metering.

After finding limitations and problems in the field with differential pressure (DP) transmitters and primary elements (limited range, pressure loss, limited accuracy, maintenance issues, and drift), researchers conducted a 3-year R&D process to adapt existing ultrasonic meters for use in biogas.

The OPTISONIC 7300 ultrasonic meter can measure gas content, flow and temperature at the same time. Gas content is obtained from the velocity of sound through the product; flow is measured with the transit time of the sound; and a built-in temperature sensor provides temperature information. The biogas meter operates in pressures less than 5 pounds per square inch (psi). The mathematical calculations are all done within the device’s electronics unit, providing a reliable and all-in-one measurement.

The meter is constructed of titanium, selected because it has the widest chemical resistance and is suitable for sour gas (with a high hydrogen sulfide (H2S) level). Titanium is also a light material that maximizes the signal level into the gas. Perhaps most importantly, titanium is strong – transducer constructions with optimal acoustic properties can be constructed, since relatively thin walls can be used. With no moving parts, maintenance is reduced compared to mechanical meters. In addition the meter can handle up to 50 percent CO2 at low pressure, making it unique in the biogas industry. 

Closing remarks

As noted, wastewater treatment decisions are cost-driven, so using one meter for several measurements is preferable to having a second device. In addition, there is no need for personnel to conduct preventive maintenance on two separate pieces of equipment, nor is there the need to stock spare parts for two different pieces of equipment.

The new ultrasonic technology is being used successfully in the Midwest and Mid-Atlantic states and in Canada.

The U.S. has some catching up to reach the numbers of anaerobic digesters found in Europe, where there are already more than 10,000 anaerobic digestion plants producing energy. New flow measurement technology will play a key role in extending the benefits of this clean energy source.

Richard Lowrie is water and wastewater industry manager, KROHNEKROHNE is a worldwide technological leader in the development, manufacture and distribution of accurate, reliable and cost-effective measuring instruments for the process industries.