Extending Renewable Electricity Generation for Decades at Closed Landfill Sites

How can landfill managers squeeze the maximum possible amount of renewable energy out of the gases their landfills produce over time?

Alain Castro, Ener-Core

Conventional reciprocating engines and gas turbines have problems combusting landfill gas that is below 30-40 percent methane. So, when a landfill becomes inactive, the quality of the emitted landfill gas drops. And as the landfill gas’s methane content goes below 30 percent, the traditional decision is to decommission the power generation equipment, at which time the landfill will stop generating revenues from the sale of energy.

However, what many people don’t realize is that a landfill will typically continue to emit low-quality methane gas for another 65 years after it has become inactive.

The system installed at the Schinnen landfill. Credit: Ener-Core.

The owners and operators of an inactive landfill must comply with laws for how they manage these gas emissions after the power-generation equipment has been decommissioned and removed.

In most cases, the local laws will require the owner of the inactive landfill to continue flaring the gas for as long as there are gases being emitted. But, after a landfill has been closed for many years, the methane content will eventually drop to levels that are 20 percent and lower.

At this stage in the life cycle of a landfill, the low-level methane gas is too weak to burn with a flare system.

However, the landfill must still comply with the laws. And hence, many older landfill sites must use and pay for a supplemental fuel just to destroy the methane.

This is a real operational cost issue, as landfills will continue to emit methane gases for another 65 years after they have been closed. The methane content of these gases is simply too low to be utilized in any useful manner with traditional equipment.

Case Study: Attero

Attero, one of the largest producers of biogas in the Netherlands, was facing this problem at its Schinnen landfill. The original landfill gas-to-electricity project, commissioned in 1995, used three 825-kW engines (with 2.475 MW of total capacity).

How different systems and parameters result in different amounts of kWh generated. Credit: Ener-Core, Inc.

Landfill gas production stages. Credit: Ener-Core. Gas composition test details. Credit: Ener-Core, Inc.

After the landfill was closed, the gas energy density decreased to the point where the engines were struggling to operate.

Attero actively managed the landfill gas collection to optimize the gas quality by separating the low-quality wells from the high-quality ones. This extended the operational life of the engines by several years. But eventually, the engines had to be decommissioned, as they simply could not operate on such low-quality gases.

By 2012, the landfill gas’s calorific value had dropped to the point where only one reciprocating engine would still run at 50-percent power for around three to four days per week. The landfill gas had to be flared for the rest of the week.

GAS COMPOSITION/TEST DETAILS

Attero decided to seek alternatives for reducing its flaring costs — with the requirement that the landfill gas quality was going to be below 30 percent methane (<300 Btu/scf or <12 MJ/Nm3).

The company researched technologies from all over the world and ultimately decided to replace its last struggling reciprocating engine with a 250-kW Power Oxidizer Powerstation (supplied by Ener-Core, Inc. — the author of this article).

There are two distinct advantages of the Power Oxidizer Powerstation:

First, the system is not limited by the physics of the combustion process. It hence has the ability to operate on gases that contain as little as 5 percent methane. This enables Attero to continue generating power (and revenue) from its landfills for many decades after they have been closed and their methane quality levels have fallen.

Second, the system operates with very low NOx (nitrogen oxide) exhaust emissions of below 1 ppm (part per million). In fact, the levels of NOx are likely the lowest of any waste-gas-to-power technology in the world.

Ener-Core’s oxidizer technology (the “Power Oxidizer”) replaces the combustor in a gas turbine.

Combustion is the traditional method used during the last 200 years to release the heat energy from fuel gases. Combustion burns gas to power gas turbines, reciprocating engines, boilers and heaters.

Modern combustors are designed to run on high-quality gases (typically, gases that are close to pipeline specifications).

Hence, the lower-quality gases that exhibit qualities that are far outside the pipeline specifications in terms of energy density or contaminant level are not easily used by any combustion-based technology. This is particularly true in areas where the air-emissions regulations are strict.

The Power Oxidizer allows the combustion of the difficult-to-use industrial waste gases from oil and gas fields, coal mines, landfills, and industrial plants.

As shown in the image on the next page, Ener-Core’s process accelerates the naturally-occurring chemical oxidation reaction that occurs between hydrocarbons and air so that the oxidation reaction takes place in one to two seconds. (The same reaction typically takes 10-20 years if the gases are released to the atmosphere.)

The reaction is contained within a vessel, so a precise amount of heat energy at pressure is released. The gas-turbine prime mover can then harness that pressurized heat energy to turn the generator and feed it back to the facility or utility grid.

This unique capability of productively using the low-energy-density gas (often below 30 percent methane) presented Attero with an opportunity to continue enjoying revenues through the sale of renewable power long after a landfill became inactive.

Power oxidizer and turbine system. Credit: Ener-Core.

It also represented an opportunity for Attero to save on the costs of flaring these gases. The 250-kW Powerstation was delivered to the site in early 2014, with the installation completed by June 2014. The image on page 53 shows the system installed at the Schinnen landfill.

As the landfill methane production from the inactive landfill continues to drop over time, the reciprocating engine’s operation becomes more limited due to the lack of landfill gas above 30 percent methane.

Table 1 shows the expected annual power generation for the smaller 250-kW Powerstation in comparison to the limited intermittent operation of the reciprocating engine operating at 400 kW. The Ener-Core Powerstation technology maximizes the kWh generation from the below-30-percent methane gas, allowing Attero to continue generating renewable electricity from the decaying landfill.

Closed landfills are not the only source of low-energy-density gas. This type of problematic gas with low hydrocarbon content also exists in oil and gas fields all over the world.

In August 2014, Ener-Core completed the pilot phase of a methane emissions reduction project for a major Canadian integrated oil company. The pilot phase involved testing the operation and measuring the exhaust emissions of the methane gas (typically 6-8 percent methane) that is emitted from oil-drilling sites.

Ener-Core was able to demonstrate that its 250-kW Powerstation could generate continuous energy from this low-quality gas that typically gets flared or vented from oil drilling sites. Image 5 shows the low-energy-density gas test results as well as the exhaust emissions. These results were used by the Canadian integrated oil company for the initial project permitting with environmental regulators.

Today, a wide range of industries produce low-quality waste gases that are not suitable for combustion and hence not suitable for power generation using traditional technologies.

These applications include oil and gas drilling processes, oil and gas refining systems, steel mills, coal mines, petrochemical plants, food processing plants and alcohol distilleries.

As the traditional power technologies often cannot operate on the low-quality waste gases that are emitted by these industries, the industries have historically resorted to deploying emissions-destruction equipment such as thermal oxidizers and scrubbers to reduce the emissions of their waste gases.

However, the scrubbing, flaring, and other emissions-destruction processes offer no return on investment for these industries.

Ener-Core believes that the best way to assist these traditional industries in becoming more environmentally-sustainable is to provide them with tools that enable them to productively convert their waste gases into clean power.

In most cases, this clean power can be used and/or sold in a financially profitable manner, thereby making it even more attractive to become environmentally-sustainable.

Alain Castro is the Chief Executive Officer of Ener-Core, Inc.

Editor’s note: The article was adapted from the Power-Gen Europe Paper of the Year in the renewable energy category. Find out who will win Paper of the Year during Power Gen Week by attending Renewable Energy World Conference and Expo, North America, December 8-10, 2015 in Las Vegas, Nevada.

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