Optimizing Biogas Production Efficiency With Humidity Measurements

A s the world transitions into a greener, cleaner and more sustainable place for future generations, biogas, or biomethane, is a key contributor to the sustainable energy industry. Produced from a variety of organic feedstocks, this renewable and reliable source of energy can reduce greenhouse gas emissions and dependence on fossil-based energy sources, but its production is subject to many variables—notably, the impact of humidity.

From methane volume calculations to the frequency at which carbon filters need to be changed, understanding the levels of humidity is crucial to ensure maximum efficiency and profitability at every stage of the biogas production process. Fortunately, advanced continuous in-line humidity measurement solutions offer producers the insight to optimize biogas production and profitability while helping build a more sustainable future.

Impacting the overall moisture content of the biogas production process, high humidity can affect biogas generation in several ways, including the equipment reliability, greater risk of water pass-through, accurate volume calculations and maintenance frequency. These factors combine to destabilize biogas production and lead to fluctuations that reduce overall biogas yield. Consequently, the gas must be treated before being utilized for electricity and renewable natural gas (RNG) production, whether removing moisture, hydrogen sulfide, siloxanes and other volatile organic compounds. Most countries have strict regulations to ensure the quality of delivered RNG, in order to protect both infrastructure and the environment.

Humidity invariably impacts different aspects of biogas production, so monitoring and managing humidity levels is essential to optimize production efficiency.

Heat Exchange Dryers and Humidity
The easiest way to remove excess humidity is to cool the gas to force condensation, followed by reheating the gas. When gases are removed from reactors, they naturally cool, but this level of cooling is not enough. To further remove humidity, plant operators use a heat exchanger to cool the gas down to the same temperature as in the coldest part of the heat exchanger, dropping the dew point to the same temperature.

In practice, the gas comes out fairly quickly, and the dew point is often higher than the cooler set point, as water vapor does not generally reach the set point uniformly and can be unstable. As a result, measuring the dew point after cooling is the only reliable way to know the true moisture content. Therefore, to optimize and ensure the performance of heat exchange dryers, it is important to monitor and control the humidity levels of the gas while in the process with sensors.

Active Carbon Filtration Efficiency and Humidity
Research shows a strong link between relative humidity of gas and filtration efficiency. High humidity can cause moisture to condense on the active carbon filter media and affect its absorption capacity, underscoring the vital need for drying. Too low humidity, on the other hand, significantly drops filtration efficiency, as carbon’s structure changes when too dry, especially in ambient temperatures. The best results appear in a range between 50% and 70% relative humidity.

When the cost of one activated carbon refill is typically in the $2,000 to $4,000-plus range, depending on filter type and size, improper humidity management can cut the lifetime of carbon filters in half. Worse yet, if humidity is not optimal and hydrogen sulfide and other impurities pass through, the cost of damage to biogas engines and biomethane upgrading units can be even more severe.

Humidity significantly impacts the efficiency of active carbon filtration, so monitoring and controlling humidity inside the process is crucial when implementing active carbon filtration techniques for gas purification.

CHP Engines and Humidity
Combined-heat-and-power (CHP) engines are sensitive to humidity. One important consideration is that dry gas contains more energy per volume than wet gas. As such, reducing humidity in a 1-MW CHP engine by one volume percent can increase the heat content of the gas by one percent.

High humidity levels not only cause moisture to condense in the engine’s air intake system, leading to a decrease in engine performance, but they can also increase the likelihood of corrosion and wear on the engine components. Condensate in fuel lines or the engine is also against most engine manufacturer specifications.

If water enters an engine, moisture can very quickly put the machine out of action—and when the loss of production for just a medium-sized engine costs roughly $5,000 per day, that result is untenable. Even if the engine doesn’t stall due to humidity, the noncondensing humidity levels still impact the engine’s lubrication, leading to increased maintenance needs.
From active carbon filtration to CHP engines, humidity monitoring costs significantly less than the combined costs of increased carbon filter replacement and oil exchange intervals, so including humidity measurement solutions in biogas plant instrumentation is best practice.

Biogas Upgrading Membranes and Humidity
Biogas presents a better value if it is upgraded to biomethane and sold as a replacement for fossil gas. The gas membranes often used to separate out the methane are operated at elevated pressures (above 10 bar), and gas is circulated in a cascade of upgrading membrane units.

As the pressure increases, so does the dew point, which can cause water vapor to condense on the surface of the membrane, decreasing production efficiency. If the dew point of raw biogas is too high before compression, condensation will occur after compression, even at high temperatures. Given that information, maintaining dew point at or below 0 degrees Celsius is important for the trouble-free operation of upgrading plants, and measuring this in-situ can provide valuable insight.

Biogas upgrading systems are more sensitive than CHP engines, and the value of the gas produced is higher than that of the electricity produced. Hence, the financial impact of lost production is even more significant.

To mitigate the impact of humidity on biogas upgrading membrane performance, maintaining and verifying appropriate humidity levels in the biogas stream is vital.

Leveraging Humidity Measurements
One effective solution to optimize biogas production and mitigate the impact of humidity is through in-situ, optical humidity measurements. As discussed, measuring humidity in the biogas production process has three main benefits.

First, reliable humidity data helps provide a comprehensive picture of biogas quality to calculate methane volume. Second, continuous in-line monitoring delivers accurate humidity readings directly from the gas line so that decision makers can adjust water removal accordingly, reducing CHP engine maintenance and downtime. Finally, humidity process control reduces how often to change the active carbon filters.

Moving toward a greener and more sustainable future will require innovation, collaboration and a commitment to finding new and better ways to harness the power of renewable energy. Understanding the humidity levels in the biogas production process—and being able to control them—empowers operators to optimize biogas production, save a significant amount of money annually, and ultimately build a more sustainable world.

Author: Antti Heikkilä
Product Manager, Vaisala
Vaisala.com

PRINTED IN ISSUE 2, 2023 OF BIOMASS MAGAZINE