It has been discussed previously within this forum that plasma-arc, plasma gasification systems, such as PEAT’s plasma thermal destruction and recovery (PTDR) systems, have the ability to process a wider range of feedstocks that other thermal treatment technologies largely because the heat source is independent of the waste being processed. This means that plasma-arc plasma gasification systems can process industrial hazardous waste feedstocks with very low calorific values (i.e. high moisture and inorganics). Further, plasma systems can co-process a variety of waste streams simultaneously.

With this said when a feedstock has a higher moisture and/or inorganic material content, the required plasma heating power from the plasma arc increases. Simply put, waste streams with 30% moisture content vs. ones with 10% moisture require more plasma-arc power (i.e. more electricity) to ensure a complete destruction of the waste feedstock.

For example, here is a representative composition for medical waste treatment (infectious red bag) in Asia, reflecting a moisture content of 18%.

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Based on modeling exercises and experience, the plasma-arc power required in a PTDR-100 system is 80 kWe (based on 75% electrical-to-thermal efficiency). This plasma-arc power is required to vaporize the moisture required for medical waste treatment.

Now, here is a representative composition for medical waste treatment from the United States, reflecting a moisture content of 8.4% (we have been told the differences between US and Asia are related to how the medical waste is ultimately sorted).

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Based on modeling exercises and experience, the required plasma-arc power required in a PTDR-100 system is under 5 kWe (based on 75% electrical-to-thermal efficiency).

The medical waste treatment streams are largely the same except for the moisture content; however the required plasma-arc plasma gasification power is reduced by over 93%!

An important reminder, plasma gasification is a thermal chemical conversion process designed to optimize the conversion of waste into the syngas. The chemical reactions take place under oxygen starved conditions. The ratio of oxygen molecules to carbon molecules can be less than one in a plasma gasification reactor (sometimes a stoichiometric amount of oxygen to achieve pyrolysis).

The following simplified chemical conversion formulas describe some of the thermo-chemical processes that are typically occurring in gasification.

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Some of the waste undergoes partial oxidation by precisely controlling the amount of oxygen fed into the plasma-arc reactor (see first reaction above). The heat released in the above exothermic reactions provides additional thermal energy for the primary gasification reaction (endothermic formulas above) to proceed very rapidly.

At higher temperatures (around 3,600°F) the endothermic reactions are typically favored as such in a PTDR reactor only equations 1-3 are seen.

Industry may have to find a replacement for a key ingredient in all types of construction products – fly ash residue created by burning coal. It is used in wallboard, siding, concrete and other construction products. The EPA is currently trying to decide whether to regulate coal ash as a hazardous or a solid waste under RCRA – the Resource Conservation and Recovery Act. It floated both options in a proposal last year and according to the Environmental Compliance Alert, seems to favor classifying coal ash as a lightly regulated solid waste so it could be recycled and reused.

Whatever option is chosen, the fly ash residue can be treated in a PTDR, plasma arc system where it would be transformed into an inert, reusable vitrified matrix. This 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 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 processing incinerator fly ash in a PTDR plasma arc waste-to-energy system is presented in the below table.

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

Further, 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.

Coal Combustion Residuals, often referred to as coal ash, are currently considered exempt wastes under an amendment to RCRA. They are residues from coal combustion in power plants and captured by pollution control technologies, like scrubbers. Potential environmental concerns from coal ash range from landfills leaching into the ground water to structural failures of impoundments, like that which occurred at the Tennessee Valley Authority’s plant in Kingston, Tennessee. The spill flooded more than 300 acres of land with CCRs and flowed into the Emory and Clinch rivers.

Finally, it is worth noting that the Environmental Integrity Project, Stockholm Environmental Institute-U.S. and Earthjustice says the EPA has “wildly exaggerated” the economic benefits of regulating the ash as a solid waste. They claim EPA overstates the benefits by 20 times. Its economic value is $1.15 billion a year, not $20 billion, they say. Whatever the number, the vitrified matrix from plasma arc treatment can be reused and recycled in a number of beneficial manners.