Plasma Gasification

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Plasma gasification is a phrase heard often when discussion plasma-arc treatment or waste-to-energy technologies, however this entry looks to give a closer look as to what plasma gasification is and its associated reactions. Plasma gasification is a thermal chemical conversion process designed to optimize the conversion of waste into the synthetic gas or (“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.

Equation 1. C (fuel) + O2 –> CO2 + heat (exothermic)
Equation 2. C + H2O (steam) –> CO + H2 (endothermic)
Equation 3. C + CO2 –> 2CO (endothermic)
Equation 4. C + 2H2 –> CH4 (exothermic)
Equation 5. CO + H2O –> CO2 + H2 (exothermic)
Equation 6. CO + 3H2 –> CH4 + H2O (exothermic)

Some of the waste undergoes partial oxidation by precisely controlling the amount of oxygen fed into the plasma reactor (see first reaction above). The heat released in the above exothermic reactions provide 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. Some plasma companies (not PEAT however) introduce supplementary fuels such as coal, petroleum coke or even other hot gases generated by their plasma torches (sometimes referred to as plasma-assisted gasification) to maintain the desired plasma gasification temperatures in the reactor.

The reducing atmosphere within the plasma gasification reactor avoids the formation of oxidized species such as sulfur dioxide (SO2) and nitrogen oxide (NOx). Instead, sulfur and nitrogen (organic-derived) in the feedstock are primarily converted to hydrogen sulfide (H2S) and nitrogen. Finally, typical halogens in the feedstock are converted to inorganic acid halides (HCl, HF, etc.)

Plasma-Arc Overview

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Plasma can be described as an electrically-charged gas where a specific amount of energy is added to separate the molecules into a collection of ions, electrons and charge-neutral gas molecules. Plasma indicates a gas volume with sufficient energy supplied (electromagnetic, electric and/or thermal) so that electrons that normally exist in specific numbers and at distinct energy level orbiting around the nucleus are freed from their orbital bonds. This plasma, with its constituents of individual molecules and electrons acts as a conductor of electricity, the resistance of which converts electrical energy to heat.

Depending on the amount of energy added, the resulting plasma can be characterized as thermal or non-thermal.

Thermal plasma heating technologies were widely developed in the early 1960’s in conjunction with space exploration and military applications programs in the United States (NASA) and the former Soviet Union. In particular, plasma torches were developed to provide an effective method to test the effectiveness and durability of heat shields required for space vehicle re-entry.

Plasma-arc systems have been widely used for destruction of hazardous wastes.

This extreme heat from this temperature breaks down wastes, forming synthesis gas (hydrogen and carbon monoxide) and a rock-like solid byproduct called slag.

The significant difference between plasma-arc systems and other thermal waste processing technologies is that the heat required for waste degradation is generated by the plasma itself and not via combustion of all or part of the waste.

PEAT’s Plasma-arc heating system consists of DC-powered graphite electrodes rather than plasma torches, typically marketed by other companies. There are a number of benefits associated with using DC-powered plasma-arc electrodes.

Minimization of capital costs as plasma-arc graphite electrodes generate plasma-arc directly with exposed anodes and cathodes without requiring an independent torch. Plasma torches are expensive and increase the capital costs associated with overall systems.

Minimization of operational costs as plasma-arc graphite electrodes require no water cooling or any externally-supplied carrier gas (i.e. argon or nitrogen). This increases the electrical to thermal conversion rates (typically seen between 75-80% in PTDR systems). Plasma torches require water cooling, carrier gases and have lower efficiencies as their power output can be as low as 50% of the power input for small torches. This means that one half of the electricity of the plasma torch is dissipated to the cooling water or efficiency of the power supply.

Potential Metal Recovery Through Plasma-Arc Waste Treatment

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In May and June 2011, PEAT conducted an operational campaign on used copper wire (MSW) at its PTDR-100 plasma-arc gasification system located in California. The feedstock included power cables, power supplies, data cables and video cables from computer components. This specific type of high gauge wire is not recycled or processed through a typical mechanical chopping system (too difficult to extract the copper), which means it is typically exported. The objective of this campaign was to demonstrate the feasibility of using the PTDR-100 plasma-arc gasification system for processing used copper wire (class #1 and #2) to gauge its ability to improve yield and/or quality of the reclaimed copper. Further, this testing is also exploring the quantity and quality of syngas generated from the organic portions of the wire using plasma-arc gasification.

During 15 days of operation, almost 820 kilograms (over 1,800 pounds) were processed over a 36 hour span, resulting in an average feed rate of 22.3 kilograms per hour (approximately 50 pounds). 230.40 kilograms of alloy was tapped from the PTDR plasma-arc gasification reactor generating a yield of 28%.

A sample of the alloy tapped from a run on May 19th was sent to an independent lab for compositional analysis (EPA 6010B). The results indicated that the alloy was over 99% copper.

Additional data is currently being compiled.

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