Upgrading Wastewater Biogas to Clean Energy

Byproduct Can Make Vehicle Fuel, Natural Gas, Electricity and Heat

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Posted: Wednesday, March 23, 2016 10:41 am

Wastewater management has come a long way since the time untreated sewage was simply discharged into rivers and canals.

Today, modern municipal wastewater treatment plants trend towards using anaerobic digesters to stabilize and break down biosolids and typically flare biogas created during the process.

Anaerobic digesters can, however, create clean energy and provide additional value to wastewater treatment when the biogas is utilized instead of flared. Biogas is composed primarily of methane and carbon dioxide and is produced during the naturally occurring anaerobic digestion process as organics break down in the absence of oxygen. This gas mixture can be converted to electricity and heat when combusted in a combined heat and power unit (CHP), upgraded to renewable natural gas (RNG) for grid injection or used as a transportation fuel in the forms of compressed natural gas (CNG) or liquefied natural gas (LNG).

Except in instances where biogas production is extremely low, flaring often wastes a beneficial resource and potential revenue stream.

Advantages and Options for Biogas as an Energy Source

Pumping, treating, disinfecting and processing biosolids at municipal wastewater treatment plants (WWTPs) is an energy-intensive process; U.S. WWTPs consumed approximately 30.2 billion kWh of electricity annually, as of 2013, according to the Water Research Foundation. With increased attention on community energy efficiency, greenhouse gas reductions, and the decarbonization of energy sources, WWTPs can head towards net neutral energy consumption by using their biogas as a source of energy.

Electricity and heat produced from the combustion of biogas may be used onsite to offset operating costs and, with net metering, excess generation can be sold to the grid for additional revenue. This process provides consistent, on-demand electricity.

Producing electricity and heat from a CHP was traditionally considered the best use for biogas, however, the advent of economically-viable biogas upgrading technologies and low electricity prices opens up alternative energy end-use options.

Biogas upgrading increases the methane concentration by removing carbon dioxide and other minor constituents including: hydrogen, nitrogen, oxygen, hydrogen sulfide, ammonia, hydrocarbons, siloxanes, and water. In the end, this process creates RNG—also known as biomethane. Upgrading biogas to RNG increases the calorific value (amount of energy released when a known volume of gas is combusted completely), and thus raises the energy potential. Whereas biogas from anaerobic digestion’s calorific value is typically around 23 MJ/Nm (or 617 BTU/ft3), the calorific value of RNG is usually approximately 39 MJ/Nm (or 1,047 BTU/ft), according to a Swedish Gas Center report published in 2012.

RNG can be used as a natural gas replacement and transportation fuel due to this elevation in energy potential. By ramping up RNG production locally, strides are made towards energy independence and carbon intensity reduction of our fuel supply. Decarbonization of fuel consumption can also reduce health related risks, hospitalizations, medical expenses and even the number of deaths associated with poor air quality.

Furthermore, greater financial value can typically be derived due to this higher energy potential.

One of the world’s largest wastewater treatment plants will soon be taking advantage of biogas upgrading benefits by converting the resource to vehicle fuel using Carbotech Pressure Swing Adsorption (PSA) Technology.

“The biogas upgrading plant was installed in late 2015 in the outskirts of Stockholm. The wastewater of approximately 1 million citizens is treated, converted to biogas and upgraded in our BUP3000 to renewable natural gas,” said Volker Eichenlaub, the CEO of Carbotech. “The RNG is compressed to CNG, and used locally as a vehicle fuel, saving over 130,000t CO2 per year. Our BUP replaces a water scrubber because of its high methane yield and overall low upgrading cost.”

Biogas Upgrading Technologies

Several technologies exist to upgrade biogas to natural gas quality. Common technologies include: Pressure Swing Adsorption, Water Scrubbing, Chemical Scrubbing, Cryogenic and Membrane Separation. In deciding amongst biogas upgrading systems, several factors should be considered:

• Quality requirements for pipeline injection or as a vehicle fuel

• Minimum heating value to be achieved

• Electricity consumption for upgrading process

• Heat requirement for upgrading process;

• Process water requirements and related treatment

• Process chemical requirements and related disposal

• Available biogas flow

• Environmental impact from methane off-gas

• Investment cost

• Operation and maintenance cost

While advancements are continuously being made to each of the upgrading technologies there are advantages and disadvantages to each system type. (See Table 1)

This article was produced by BioFerm Energy Systems based in Madison, Wis., in cooperation with the Viessmann Group.

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