Smell of Success: Implementing an Active Landfill Gas System
Up until January 2015, the Lebanon Regional Solid Waste Facility operated without an active landfill gas collection and control system (GCCS). Located in Lebanon, New Hampshire, the site has been operated by the city of Lebanon Department of Public Works since the 1960s. The site currently includes a recycling and transfer center, a closed construction and demolition landfill, a closed unlined landfill, and an active lined landfill. The site takes in over 50,000 tons of solid waste per year. For decades, the only form of landfill gas (LFG) control that existed at the site was a passive system that consisted of a series of vents that emitted LFG to the atmosphere, and to the surrounding city of Lebanon and its citizens. The site is located less than a mile away from a business area, and because of this passive venting system, odors have been an issue for the citizens of Lebanon and their visitors.
In an effort to reduce odors and move the city closer to producing energy from the site, the city’s engineering consultant, Sanborn, Head & Associates Inc. helped the city develop an active GCCS system. The active GCCS consists of vertical gas extraction wells, gas collection trenches, condensate management features, gas conveyance pipe, connections to existing vents, and a blower/utility flare station. The active GCCS was developed in three distinct stages.
Stage 1: Lined Landfill
Stage 1 was the first step in the right direction for the city. As waste in a landfill ages, the LFG production rate declines. For this reason, the LFG produced by the newer waste in the active landfill was viewed as a higher priority than the LFG being emitted from the older unlined landfill. The major component of Stage 1 was the installation of 12 vertical gas extraction wells to a maximum depth of 80 feet into the waste mass of the lined landfill. The vertical gas extraction wells consist of 8-inch diameter perforated PVC pipe encased in crushed stone. The wells are fitted with a wellhead, consisting of a valve and monitoring points that can measure and regulate the applied vacuum as well as other LFG parameters (e.g., gas composition and temperature). Typically, the effective radius of influence of a vertical gas extraction well is about 100 feet.
Vertical wells are an effective GCCS component for extracting LFG, and are typically installed in areas that are at or near final grade. In areas of the lined landfill that were not final grade, LFG collection trenches were installed. The collection trenches consist of 6-inch diameter perforated high-density polyethylene (HDPE) pipe that is encased in crushed stone. The difference between the vertical wells and LFG collection trenches is that the trenches are installed almost horizontally, a minimum slope is provided to manage liquids (condensate) generated when the collected LFG is cooled. Because the trenches are laid out in a horizontal manner, they are more likely to be subject to differential settlement as the surrounding waste settles. The LFG collection trenches have an elliptical zone of influence estimated to be 100 feet horizontally and 40 feet vertically.
To increase the collection potential of the GCCS, the 24 existing passive LFG vents were connected to the gas conveyance pipe network. By connecting the vents to a vacuum source, the potential for fugitive LFG emissions, and hence odors, was reduced. Condensate management features were installed at low points throughout the system in order to capture the liquid and discharge it to the leachate collection system of the lined landfill.
Stage 2: Unlined Landfill
The second stage of the active GCCS focused on the site’s unlined landfill and its 18 passive LFG vents. Similar to the passive vents located on the lined landfill, each of these passive gas vents were modified and connected to the primary gas conveyance pipe (GCCS header). A series of wellheads allow for the control of vacuum at each vent location. Wellheads can be adjusted to control the amount of vacuum applied to a GCCS component. Operators do not want to apply more vacuum than necessary; otherwise, too much air can be pulled into the system. Through a series of testing ports located on the wellheads, operators can adjust the vacuum, and optimize the performance of the GCCS.
Stage 3: Looking Forward
Stage 3 of the active GCCS has yet to be constructed. Stage 3 focuses on operations and odor reduction, and will consist of expanding the GCCS to collect LFG going forward as the site continues to accept waste. Stage 3 will consist of installing additional LFG collection trenches installed at succeeding levels in the active filling area. Once the site achieves its permitted final grades, Stage 3 also will include about 16 more vertical gas extraction wells.
Following construction of Stages 1 and 2 of the GCCS and the receipt of an air permit to operate the LFG utility flare, the city activated the GCCS in January 2015. The startup phase faced some initial operational challenges, which were further hindered by bitter cold weather. The startup issues were resolved through excellent cooperation between the city, its contractors and Sanborn Head. At present, the city is extracting and flaring approximately 400 cubic feet of LFG per minute from the two landfills. The general public has been very supportive of this project, and local businesses and their customers have been pleased with the odor control measures and improved air quality.
The next step in the city’s GCCS endeavors is to install a landfill-gas-to-energy (LFGTE) engine. LFGTE engines have the ability to take LFG and convert it into electricity. With the anticipated LFG flow, the city hopes to power the site’s facilities independent from the electrical grid. As new waste is accepted at the active landfill, LFG flow is expected to continue to increase and generate additional electricity. The city intends to set up a LFGTE plant capable of producing approximately 1 MW of electricity. The physical “brick and mortar” component of a LFGTE plant is not difficult to design, and there are hundreds of similar facilities in the U.S. and abroad. The problems associated with LFGTE projects stem from the financial side of the operation. Appropriately structuring ownership and identifying positive revenues from a project of this size can be challenging. Currently, the city is looking into the alternatives including selling the LFG to a developer that would own and operate the LFGTE facility, or financing the project themselves and net metering the energy produced at the landfill to offset its own load. Other possible revenue sources include carbon credits associated with the methane destroyed in the flare or the LFGTE power plant.
In many ways, this project was a great success for the city of Lebanon. Not only does the city have an active GCCS that improves air quality and has the potential to serve as an additional source of revenue, but local businesses and the citizens of Lebanon are beginning to realize how important an asset this landfill is to the city. In times where public resistance against landfill projects is commonplace, it is a breath of fresh air to complete a landfill-related project with public backing. These types of facilities are not the “dumps” people think they are; they are highly-engineered facilities that are designed with considerations geared toward protecting human health and the environment.
Contact: Edward A. Galvin
Senior Project Engineer, Sanborn, Head & Associates