The mayor and council of Lebanon, Tennessee, aimed to improve the environment and solve the city’s waste disposal issues with an Aries Clean Energy waste-to-energy plant that was completed and went online in 2016. When it was commissioned, this was the only plant of its kind in the United States, according to the company. It was and still is the world’s largest downdraft gasification plant, Aries Clean Energy says.
The city was awarded federal Qualified Energy Conservation Bonds that cover 70% of the interest for the city’s bond issuance and a $250,000 matching funds grant from the Tennessee Department of Environment and Conservation’s Clean Tennessee Energy Grant program.
The footprint of the plant less than an acre, which means it can be expanded or replicated easily. In that space, the gasifier has a 64 ton-per-day capacity. Using Aries Clean Energy technology, the syngas produced is combusted in an industrial thermal oxidizer. That thermal energy is transferred to heat water, and the hot water drives three ORC generators with an average output of 420 Kw. Feedstock is locally sourced and prepped by a company that ensures that wood and tires are cut to 1-inch to 3-inch size. Sludge is blended on site. The target moisture content of the total blend is 30%. The blend will yield 8% biochar that is 70% carbon and recyclable.
Previously, local companies were paying to have wood waste hauled to landfills. A process was developed so they could pay tipping fees to the city to take the wood instead of paying a hauler to take it to the landfill. This lowered their costs and provided feedstock for the downdraft gasification plant.
The county officials decided that several hundred tons of scrap tires that were previously hauled away could be sent to the gasification blend at considerable savings. The local company Rockwood Recycling became the vendor coordinating the waste stream pickup, storage, and preparation for the plant’s biomass feedstock.
The plant is located on the site of the city wastewater treatment facility and the power produced is utilized “behind the meter” to offset power previously purchased. The wastewater treatment facility was also able to retire a power- hungry digestor unit since the sludge is being sent directly to the gasification unit, according to Aries Clean Energy.
Each year, the plant is expected to eliminate 2,500 tons of carbon emissions, keep 8,000 tons of wood and sludge out of the landfill, convert 36,000 scrap rubber tires into energy, and generate 1.8 million kilowatt-hours of electricity. The plant will also produce a leftover product called biochar, another possible revenue stream. In addition, the plant will support growth at Rockwood Recycling, which is driving the public-private partnership formed to collect and process local waste into feedstock.