Advanced thermal recycling system

An advanced thermal recycling system (or an ATR system) is derivative of energy-from-waste technology. An ATR system transforms municipal solid waste (MSW) into electricity or steam for district heating or industrial customers. The combustion bottom ash and the combustion fly ash, along with the air pollution control system fly ash, are treated to produce products that can be beneficially reused. Specifically, ATR systems consist of the following:

• Solid waste combustion, boiler and combustion control system, energy recovery and air pollution control equipment;
• Combustion bottom ash and fly ash treatment systems that produce commercially reusable products; and
• An optional pre-processing system to recover recyclable materials contained in the MSW delivered to the facility before the MSW enters the thermal processing area of the facility.

Application

Germany

As a result of the growing need to manage waste throughout greater Hamburg, the first commercially operating ATR facility – the Müllverwertung Rugenberger Damm in Hamburg, Germany – was commissioned in 1999. The German Green Party has endorsed the specific features of this facility in its "Concept 2020" initiative to cease all landfilling of waste by 2020 as an essential part of an integrated waste management system achieving the highest standards in the energy-from-waste industry. No landfilling of unprocessed waste has been allowed in Germany since 2005.

Description

Overhead refuse cranes are used to hold approximately five tons of garbage each. The waste is then mixed in the bunker to create a homogeneous mixture to ensure that the bottom ash byproduct has good combustion and low carbon content. These cranes then deliver the mixed waste into the feeding hopper, which leads down onto stoker grates. These grates control the rate at which the waste travels through the boiler. The heat ignites the trash as it moves along the forward feeding grates until only the byproduct bottom ash remains at the end of the grate. Each combustion line feeds a boiler that operates above 1,560 °F (850 °C) for two seconds. The temperature in the combustion zone is measured through acoustic monitoring. A computer controls the temperature, the grate speed, the amount of air used, and all other aspects of the process that enable complete combustion and minimization of emissions.

Maintaining the furnace's high temperature is essential to rid the waste and the resulting combustion gases of complex organic compounds such as dioxins and furans. To prevent the reformulation of pollutants, fly ash is separated from the flue gas downstream of the superheaters to reduce the fly ash content, which could act as a catalyst in the critical reformulation temperature range of 600 to 400 °F (316 to 204 °C). At the exit of the boiler, the flue gas is cooled down to a level of 340 °F (171 °C).

As the waste is combusted, heat is released in the boiler. This heat produces high-pressure, high-temperature steam, which generates electrical energy when passed through a turbine generator. The electricity is fed into the public power grid or sold directly to a customer. The steam can also be exported directly for use in district heating or industrial processes.

Each unit has an independent air pollution control system. Flue gas cleaning begins in the boiler, where oxides of nitrogen are reduced by injecting ammonia water into the combustion chamber. Lightly loaded absorbents (activated carbon from the second bag house) are injected into the flue gas downstream of the first bag house to separate any contaminants that have reformed (such as organic compounds), any condensed heavy metals, salts and other gaseous contaminants, as well as residue fly ash.

The first baghouse makes it possible to produce reusable by-products such as hydrochloric acid and gypsum from the consecutive air pollution control process steps. Acid gases are removed from the flue gases by passing through a two-stage scrubber to remove acid components, especially halogen compounds such as hydrochloric acid and hydrofluoric acid. A counter-flow neutral scrubber follows, using a lime slurry to remove sulphur oxides. The pollutant gases are either dissolved in water droplets (acids) or bound as calcium salts and thereby removed from the flue gas. A second baghouse acts as a polishing filter to capture any remaining aerosols, organic compounds and heavy metals, which thereby are reduced to levels usually below detection.

Following combustion, the material left consists of the non-combustible components of the waste and the inert materials produced during combustion. This is known as bottom ash. The bottom ash is washed to eliminate soluble salts. Iron scrap and non-ferrous metals such as aluminium, copper and brass are separated and sold in secondary metals markets. The bottom ash is then screened, crushed and sold for use as a construction material.

Gypsum is created when the oxides of sulphur (SO2 and SO3) are separated by the single stage scrubber. It is purified, then sold to the construction industry.

The acid scrubbing process in the flue gas treatment system also produces a raw hydrochloric acid at a concentration of 10% to 12%. The acid is distilled (rectified) to yield commercial-grade (30% concentration) hydrochloric acid.

Fly ash, separated in the boiler and baghouses and constituting up to 5% by weight of the combusted MSW, is treated to recover metals and minerals for reuse, resulting in an overall ATR process landfill diversion rate of approximately 98.5%.

Advantages

ATR systems offer flue gas cleanup and emissions performance that meet or exceed the strictest clean air laws in the United States and the European Union. Emissions from an ATR plant contain fewer metals, dioxins and conventional pollutants than other energy-from-waste approaches.

ATR systems increase a facility’s "diversion rate" – the percentage of materials coming into plants that are destroyed or converted to energy and useful byproducts. Most energy-from-waste plants in the US, after incineration and recovery of some metals, combine both bottom ash and fly ash (20-25% of MSW input by weight) and send it to a landfill. ATR systems, by contrast, landfill less than 2 percent of a plant’s MSW input; all remaining MSW is converted into either energy or usable products.