Fueling anaerobic digestion
Wastewater treatment facilities looking outside of the box to meet energy needs
Many wastewater treatment plants are beginning to use anaerobic digesters to produce biogas that can be used in their combined heat and power plants to generate electricity and heat. In addition, the plants are selling the biogas into the gas grid.
The growing interest in biogas has led to more attention paid to measuring biogas flow and composition, because proper gas engine operations depends upon use of biogas with the right methane (CH4) content.
New ultrasonic technology is being developed to provide the kind of reliable and accurate flow measurement needed to advance this important strategic energy source.
Biogas as a fossil fuel alternative
Over the past decade, North American wastewater treatment operators have been taking a page out of their European counterparts’ playbook by adding a spate of anaerobic digestion facilities to produce biogas, which can be can be recovered, treated and used to generate energy in place of traditional fossil fuels.
Anaerobic digestion systems are used in a variety of settings, including wastewater treatment, food waste processing and agricultural (manure) processing. During anaerobic digestion, bacteria break down wastewater, food waste, or manure in an oxygen-free environment, producing biogas, which typically contains 60 to 70 per cent methane, 30 to 40 per cent carbon dioxide and trace amounts of other gases.
The effluent remaining after controlled anaerobic decomposition is low in odour, rich in nutrients and can be recycled. Wastewater treatment plants, in particular, are finding that adding anaerobic digestion benefits both the plant and community. The plant gets to use the gas produced for power, reducing overall operating costs and ultimately reducing charges to the customer. Plus, they can earn money by feeding any extra gas back into the utility.
The sludge left over after digestion is recyclable. So rather than having to pay to incinerate the material, it can be used for deep well injection or as a fertilizer. In North America, the fertilizer has been mainly used for golf courses or for growing animal feed; in the rest of the world, the sludge left over is widely used as fertilizer for food intended for human consumption.
Currently, less than one-third of larger wastewater treatment plants use anaerobic digestion, while many continue to incinerate residual material at considerable expense. The key reason why more do not adopt anaerobic digestion is the capital expense of adding digesters.
Another factor adding to the slow adoption is that today’s lower natural gas prices result in a very long return on investment 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 is definitely in favor of wastewater treatment plant biogas recovery.
Market demand is expected to increase, most likely leading to increased future gas prices. As gas prices stabilize, projections indicate a growth in anaerobic digestion at wastewater treatment plants. In addition, there is likely to be a reduction in the capital expense of adding anaerobic digesters to wastewater treatment plants as demand grows.
Flow measurement technology for biogas applications
One of the key aspects of biogas use at wastewater treatment plants is gas-flow measurement at a facility’s 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 can only be guaranteed 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 is also used by combined heat plant operators to know how much energy they have available. If they are low, they might need to bring some gas in from the grid. If they’re high, they want to sell the excess back to the grid. Reliable measurements are crucial for these biogas deliveries to gas grid operators. Demanding measuring parameters associated with biogas applications have definitely created a measurement challenge.
Several technologies are available, including thermal mass, mechanical, vortex and ultrasonic technologies.
Thermal mass flowmeter technology has historically been used for gas measurements. With this technology, 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, one knows exactly how heat will transfer from one to the other. This tried-and-true method has been used in thousands of installations. The problem with using thermal mass flow metering for biogas is that biogas is wet; when water is introduced, it skews the results, creating the possibility of large measurement errors.
Mechanical meters are not really suited for the types of measurement needed for biogas. 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 off mechanical metering. This measurement is pressure sensitive, and with mechanical instrumentation, there is substantial pressure loss.
Vortex metering has been more successful. However, operators must be careful when picking the frequency, especially if the vortex device uses a membrane — water soaking the membrane shortens the life of the vortex measurement device. Using 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 users 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 operators 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.
Ultrasonic measurement technology, meanwhile, uses the time transit differential method to guarantee flow measurement with a high degree of long term stability, regardless of the gas composition. Ultrasonic measurement is particularly well suited to biogas applications because it guarantees a full transit without loss of pressure or any other negative effect on the flow, and it can cover a wide measuring range. With no pressure drop, ultrasonic measurement devices have quite a leg up on other measurement technology used for biogas.
Ultrasonic measurement technology is rapidly gaining in industry acceptance. In the U.S., the American Petroleum Institute has been accepting ultrasonic measurement for liquid custody transfer for nearly 20 years. The American Gas Association (AGA) has also been rewriting its standards around ultrasonic technology in recent years. AGA is especially interested in ultrasonic technology’s diagnostics capabilities, which allow meters to troubleshoot themselves and alert users if anything is wrong, including liquid content in the gas being measured.
Research and development to improve biogas metering
KROHNE has conducted significant research and development initiatives to advance its biogas metering capabilities by taking what it has learned from gas measurement technology in other applications, including 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 issue, and drift), researchers conducted a three-year R&D process to adapt existing ultrasonic meters for use in biogas. The result was the OPTISONIC 7300 ultrasonic meter, which can measure gas content, flow and temperature at the same time.
The gas content is obtained from the velocity of sound through the product; the 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 five pounds per square inch. 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 per cent carbon dioxide at low pressure, making it unique in the biogas industry. Wastewater treatment decisions are costdriven, so using one meter for several measurements is preferable to using 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 at wastewater treatment plants in North America. Yet they still have 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.
About the author: Richard Lowrie is water and wastewater industry manager for KROHNE.
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