biogas system diagram
After scrubbing, biogas is fed to a combined heat and power (CHP) engine/generator system.

By Steve Craig

A small municipal wastewater treatment plant, with responsibility for its local city and surrounding rural areas, decided several years ago to convert its operations to a more sustainable green electric power model. Over more than a decade, in phases, the plant’s engineers set a series of goals to convert its operation from relying solely on power from its local electric utility to a mix of biofuel co-gen, solar and conventional power.

The plant’s reusable solar energy rooftop arrays and biofuel co-generation system produce more than 200 kW of green energy.

The biofuels system is tasked with meeting much of the treatment plant’s energy requirements. Over three-fourths of these are now being met with sustainable green energy, thanks to co-gen electric power fueled by waste gas, that in the past would have been flared into the atmosphere.

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A conventional wastewater treatment process has been in place for years to treat the area’s residential and industrial wastewater. Treatment phases include the typical solids processing, clarification and disinfection prior to discharge. After the initial phases, the solids are dewatered and sent to an anaerobic digester. The digester process includes heating the tank over a two week time frame, with the end result being gases, liquids and solids.

The resulting biofuel is scrubbed to remove H₂S gas and is conditioned for moisture and siloxane removal. Biogas is fed to a combined heat and power (CHP) engine/generator system. This system generates enough electricity to support on average 75%, or more, of the plant’s operational needs. The heat captured by the generator system’s heat exchanger is used for process and facility heating needs, which is especially important in the winter.

At full capacity, the biofuel co-gen energy production system can generate millions kW’s of electricity each year. The combined biofuel co-gen and solar power energy generating capacity is capable of supplying all the plant’s daily energy needs, depending on weather conditions that affect solar energy generation.

Measuring gas flow

Measuring digester gas flow accurately over variable flow rates is critical to the success of wastewater biofuel co-gen energy production. Digester tank gas is a combination of methane (CH₄) and carbon dioxide (CO₂), with a small percentage of other trace gases.

The actual composition of digester gases can vary with the process and ambient temperature conditions (e.g., seasonally with hot summers and cold or freezing winters), but a typical average is 65% (±5%) CH4 and 35% (±5%) CO₂. This dirty wet gas typically contains particles of hydrogen sulfide (H₂S), which can be present in condensation on the pipe walls and instrumentation.

Digested CH₄ gas can be explosive and combustible, and it can result in flow meter installation conditions that require hazardous approval ratings. When there is more gas production available than can be consumed for electric power generation purposes, this waste gas is flared for safe disposal.

Repeatable, accurate gas flow measurement is essential to support digester gas production or for plant process control. Many government 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 flow meter 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.

Temperature compensated flow measurement for accuracy in changing process gas temperatures.

The new biofuel gas system

In an earlier phase of the overall project, the plant team first specified four thermal mass flow meters for its original digester gas system. These were primarily required for environmental monitoring, with one meter placed on the plant’s first digester, one on the plant’s second digester, one on the flare gas header and one on the flare gas burner.

The ST51 flowmeter by Fluid Components International.

Several years later, after the first four meters performed well, the plant contacted Fluid Components International (FCI) to discuss the next phase of the project’s needs. After consultation with the plant engineers on the next co-gen capability phase of the project, the FCI applications group recommended that the company’s ST51 Thermal Mass Flow Meter be installed at the site to support the co-gen process.

Thermal dispersion sensing technology provides direct mass flow measurement, without the need for additional components or equipment. It places two thermowell protected platinum resistance temperature detectors (RTD)in the process stream. One RTD is heated, while the other senses actual process temperature. The temperature difference between these sensors generates a voltage output, which is proportional to the media cooling effect. This can be used to calculate the mass flow rate.

Thermal dispersion diagram
Thermal dispersion sensing technology uses two temperature detectors, placed in the process stream.

With their direct mass flow sensor technology, the thermal flow meters also include built-in temperature compensation to ensure repeatable and reliable measurement for process temperature changes over ±30°F. This automatic temperature compensation technology adjusts automatically to changes in seasonal temperatures, for extremely dependable measurement of the gas flow rate and totalized flow.

Biofuel generation units

When the facility added its full biofuel co-gen power capability, the ST98 flow meters had performed well for almost a decade and were considered a natural choice. An additional flow meter was placed on the wet gas pre bio-filter line, and a second meter was placed on the co-gen engine side. The actual gas composition and percentages was different for each of the two flow meters. The wet gas meter measured a combination of CH₄, CO₂ and H₂O. The co-gen meter was calibrated for CH₄, CO₂ and O₂.

To process the wet gas, it was first heated and then chilled to near freezing for moisture removal. The plant’s control system monitors both gas production and co-gen engine gas usage. The two flow meters are connected to the control system with their 4-20 mA outputs. Readings from the meters for the wet and dry side of the process are compared by the process control system. If the two meters operate outside of a pre-set differential, they are then taken out of service for cleaning and calibration checks.

The new insertion style thermal mass flow meters were installed at a 45-degree angle pointing up. Due to the crowded equipment conditions at this installation, there was not enough pipe straight-run to create a uniform gas flow profile in the pipe, which could have affected meter accuracy. Both meters were, therefore, installed with vortab insertion panel (VIP) flow conditioners to ensure measurement accuracy.

The plant’s new meters were installed with a ball valve and packing gland assembly kit, which simplifies inspection and maintenance as necessary. The meters are easily removed from process line without shutting down the process or interrupting service.

Steve Craig is with Fluid Components International (FCI). This article will appear in ES&E Magazine’s December 2018 issue.

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