The city of St. Cloud, Minn., straddles the Mississippi River near the center of the state a little more than 65 miles north of the twin cities of Minneapolis and St. Paul. This college town with a population of more than 65,000 is home to the state’s third-largest public university, which stands between the downtown district and the Beaver Islands, a popular local recreation area.
The St. Cloud Wastewater Treatment Facility is located in south St. Cloud and also services other nearby Minnesota communities, including St. Augusta, St. Joseph, Sartell, Sauk Rapids and Waite Park. This award-winning operation has been recognized at both the state and national levels for its innovation and sustainability initiatives, which include biofuel and solar co-generation for electric power.
The city’s forward-thinking staff began looking for sustainable green energy solutions. After planning and initiating a series of projects over several years, the site began producing renewable energy with a 20-kilowatt (kW) rooftop solar array, a 220-kW solar array and biofuel electrical generation.
Energy projections for the facility stated that 80% of its energy demand would come from renewable sources. Biofuels would be responsible for meeting much of the facility’s electric power demand.
Treatment to Biofuel Processes
The plant operates with a standard five-step wastewater treatment process for the area’s domestic and industrial wastewater, which includes three treatment phases, clarification and solids processing. After the first three treatments and clarification, the liquids portion is treated with ultraviolet light for disinfection prior to discharge. The remaining solids are dewatered and sent to the anaerobic digester for further treatment. This treatment process includes digester heating over a 15-day period, which produces liquids, gases and solids.
All biofuel produced is scrubbed of hydrogen sulfide gas H2S with a biofilter and conditioned for moisture and siloxane removal. The conditioned biofuel gas is fed to a 630-kW combined heat and power engine and generator system. This generator system produces enough electricity to support 80% of plant operations on average. Heat produced and captured by the generator system heat exchanger, 2 million British thermal units per hour, is used directly for process and facility heating needs year-round.
At 100% efficiency, biofuel energy generation has the potential to generate 5.4 million kilowatt hours (kWh) of electricity annually. In 2016, the facility purchased 5.2 million kWh of electricity to treat more than 9.8 million gal of influent flow per day. Depending on daily conditions such as solar energy potential, there will be days the facility will operate on 100% renewable energy.
Gas Flow Measurement
Accurate, reliable digester gas flow measurement over varying flow rates is essential to the success of wastewater biofuels co-generation for energy production. Digester gas is a combination of methane (CH4) and carbon dioxide (CO2), with a small percentage of other trace gases.
The gas composition can vary with the process and temperature (e.g., seasonally with hot summers and cold or freezing winters), but a typical average is 65% (±5%) CH4 and 35% (±5%) CO2. Digester gas also is a wet and dirty gas, typically containing entrained H2S, which can be present in any condensation on pipe walls and instrumentation.
The CH4 gas is potentially explosive and combustible, and can result in flowmeter installation conditions that require hazardous gas approval ratings. Any excess gas is flared for safe disposal as waste when it is more efficient to make use of digester gas to reduce energy costs.
Accurate, consistent flow measurement is essential to report digester gas production or for plant process control. Local, state and federal authorities require gas production data for regulatory reporting purposes to ensure environmental compliance for greenhouse gas reporting.
There are a number of key criteria to consider when specifying a flowmeter for digester gas measurement:
- Accurate and repeatable measurement;
- Low maintenance with no moving parts to clog or foul;
- Simple threaded insertion for easy installation and periodic maintenance;
- Wide turndown for accurate low and high flow rate measurement;
- Approved for Class 1, Division 1 (Zone 1) hazardous environments;
- Factory calibrated for digester gas compositions;
- Direct mass flow measurement; and
- Temperature compensated flow measurement for accuracy in changing process gas temperatures.
In St. Cloud, Minn., one flow meter was installed on the wet gas pre bio-filter line and the other was installed on the engine side. Each flow meter was set to a different gas composition.
St. Cloud Digester Gas System
The St. Cloud facility team chose four flowmeters for their original digester gas units. At that time, the flowmeters primarily were for environmental monitoring purposes, with one meter installed on Digester 1, one meter on Digester 2, one meter on the flare gas header and one meter on the flare gas burner. The St. Cloud facility team contacted Jasper Eng. and Fluid Components Intl. (FCI) to discuss the facility’s requirements. After reviewing the application, the FCI thermal mass flowmeter was recommended and installed at the site.
The flowmeters have a reliable no-moving-parts design that provides direct mass flow measurement with a single process penetration. Their construction has no orifices to plug or clog in the dirty, wet gas environment, and they have hazardous gas global agency approvals for safety in explosive environments.
The wastewater treatment facility staff recognized the advantages of this flowmeter’s thermal sensing technology design, which reduces cost and installation expenses. Other types of air/gas flow sensors generally require installing separate temperature and pressure sensors, as well as mass flow calculations to provide inferred mass flow measurement.
Thermal dispersion sensing technology provides direct mass flow measurement. It places two thermowell-protected platinum resistance temperature detector (RTD) sensors in the process stream. One RTD is heated while the other senses the process temperature. The temperature difference between these sensors generates a voltage output, which is proportional to the media cooling effect and can be used to calculate the mass flow rate.
With this direct mass flow sensor technology, the meter also includes built-in temperature compensation to ensure repeatable and reliable measurement for process temperature changes over 30°F. This technology compensates automatically to changes in seasonal temperatures, such as cold winters and hot summers.
With no moving parts to plug or foul, the flowmeter delivered future cost savings in maintenance requirements for the digester gas application. This meter provides accurate gas flow measurement for dependable and safe plant operation at a low life-cycle cost.
The St. Cloud facility engineering team was satisfied with meter’s accuracy of ±1% of reading, 0.5% of full scale, with repeatability of ±0.5% of reading. The meters included a NEMA 4X/IP66-rated enclosure and is agency-approved for installation in hazardous gas locations involving combustible biogas and natural gas.
New Biofuel Co-Generation System
When the facility added its full biofuel co-generation power capability, the flowmeters had performed for years and were considered a natural choice. The new system is designed to generate 160 to 180 cu ft of digester gas per minute on average. This meets about two-thirds of the plant’s energy needs at this time.
One flowmeter was installed on the wet gas pre-biofilter line, and the other meter was installed on the engine side. The gas composition was set differently for each of the two flowmeters. On the wet side of the process, the meter was set to measure with a gas calibration of 62% CH4, 37% CO2 and 1% H2O. On the dry side of the process, the meter was calibrated for 60 CH4, 38% CO2 and 2% O2.
In processing the gas from the wet to the dry side, the gas is heated to 170°F and then chilled to 35°F for moisture removal. The process requires a 30% differential in going from the wet to dry side of the gas process. The facility’s Rockwell Control Logix System monitors both gas production and co-generation gas usage. The two flowmeters interface with the system via their 4-20 mA outputs. The meter readings from the wet and dry side of the process are cross-checked for measurement accuracy. If the two meters are outside of a 10% differential, both meters are pulled for cleaning and calibration checks.
The new insertion-style meters were installed at a 45-degree angle pointing up. Vortab Insertion Panel flow conditioners were installed on both meters to ensure accuracy of the short pipe runs.
The new meters included a ball valve and packing glad assembly kit, which simplifies inspection and maintenance. They are easily removed from the process line without shutting down the process.
The new flowmeters have been installed, have been commissioned and have run successfully for several months. All the while, the meters are supporting the St. Cloud facility’s sustainability efforts and power generation goals.
Since the installation of the original thermal flow meters at this facility, FCI has developed three more next-generation thermal flow meters, which are all designed with advanced features to perform in wet dirty biogases in small to large line sizes. These breakthroughs include ultra-reliable, feature-rich electronics, and FCI innovations such as Adaptive Sensing Technology™ (AST) with the industry's most extensive selection of application-matched flow sensors, including FCI's new "wet gas" flow element that resists the effects of moisture in gas flows.