A vitrified matrix or slag is the primary solid byproduct of plasma arc waste-to-energy processing. The vitrified matrix from plasma arc processing contains the mineral matter associated with the feed materials in a vitrified form – a hard, glassy-like substance. The amount of matrix produced is a function of how much non-combustible mineral matter is present in the feedstock.

This matrix is the result of operating temperatures above the melting temperature of the mineral matter. Under these conditions in the plasma arc reactor, non-volatile metals and metal oxides bind together in molten form until it is cooled via natural heat loss or via a pool of water, where it would fracture and granulate.

The compressive strength of a slag sample generated from fly ash from coal-fired power plant as well as some sodium carbonate (fluxing agent) was 480 kg/cm², while its average mortar strength was tested at 169 kg/cm. A report issued by Geo Engineering Services of Baroda, India, who was testing the slag for its physical suitability for use as coarse grained material in the sub-base layers of pavement, indicated that the “slag can be effectively used in the lean concrete (up to M10) as a sand replacement” Slag from plasma arc treatment is typically used in construction or road asphalt.

The vitrified matrix or slag generated by plasma arc treatment is primarily made up of silicon dioxide (SiO₂), aluminum oxide (Al₂O₃) and calcium oxide (CaO). Toxicity Characteristic Leaching Procedure (TCLP) tests are designed to determine the mobility of both organic and inorganic analytes present in the slag. The results from various plasma arc waste-to-energy process testing is presented in the below table.

It is not unreasonable to presume that vitrified slag from the plasma arc treatment of hazardous waste will have far superior leaching characteristics that incinerator bottom ash in comparable leaching tests.

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As mentioned earlier in this blog, PEAT advancing a project for a client in China to treat refinery sludge and petrochemical waste. The system construction is finished and the system is currently en route to China. This plasma gasification system is expected to be commissioned during the later part of July.

Unlike incineration, where combustion gases are treated at atmospheric pressure, the volume of syngas that is generated in a plasma gasification system is significantly reduced. The smaller flue gas volumes also bring benefits in terms of the scale of downstream air pollution control equipment.

Here is a representative syngas composition, composition and heat value for processing 500 lbs/hr of petroleum sludge and fluxing agents with the following chemical composition (Carbon: 30.85%, Hydrogen: 3.18%, Chlorine: 0.65%, Oxygen: 0.41%, Sulfur 0.31%, water 23.82%, metals 0.17%, inorganics 40.54%) in a plasma thermal destruction and recovery system.

The syngas generated from a plasma gasification system could be converted into a variety of different fuels such as ethanol, natural gas (methane) and hydrogen or can be used to generate electricity directly similar to gasification. Some companies are trialing systems for upgrading syngas from their plasma gasification processes to be used as a chemical feedstock for products such as hydrogen recovery. However, the commercial status of such projects indicates that these are still in the development stage.