More Details on the Gasification Process – Part 1

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The main aspect of gasification, whether it is plasma-arc plasma gasification or “traditional” gasification is to raise carbon–rich materials or waste to a high temperature in an oxygen–deficient reactor, where the materials break down thermochemically versus combustion.

This process is more efficient than incineration, has a significantly lower environmental footprint, while the syngas can be transformed into a number of end products (liquid fuels, power, chemicals, etc.).

The feedstocks for traditional gasification processes range from coal, the organic components of municipal waste and biomass while the range is even greater for plasma-arc plasma gasification processes, which can handle just about any waste stream with the exception of radioactive materials.

Due to the fact that gasification occurs pre-combustion (assuming the syngas would be burned to generate electricity), it supports easier carbon capture than incineration where the chemistry can be more complex.

Plasma-arc plasma gasification is just one type of gasification. Other common forms include (1) updraft, (2) downdraft, (3) fixed bed and (4) fluidized bed. The first two are quick similar with exception of the gas flow. More on the differences in the next posting.

Plasma Gasification Chemistry

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Plasma-arc plasma gasification is a phrase heard often when discussion hazardous waste treatment or waste-to-energy technologies, however this entry looks to give a closer look as to what plasma-arc plasma gasification is and its associated reactions. Plasma-arc 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-arc 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 plasma-arc plasma 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-arc plasma gasification reactor (see first reaction above). The heat released in the above exothermic reactions provide additional thermal energy for the primary plasma-arc plasma gasification reaction (endothermic formulas above) to proceed very rapidly.

At higher temperatures (around 3,600°F) the endothermic reactions are typically favored. Some plasma-arc companies (not PEAT however) introduce supplementary fuels such as coal, petroleum coke or even other hot gases generated by plasma torches (sometimes referred to as plasma-assisted gasification) to maintain the desired plasma-arc plasma gasification temperatures in the reactor.

Additionally, plasma-arc plasma gasification currently appears to be the option being promoted most widely for larger scale waste-to-energy applications mainly because of its ability to produce the syngas from which energy can be recovered in high efficiency recovery units so offsetting the high energy requirements of plasma-arc plasma gasification.

The reducing atmosphere within the plasma-arc 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.)

Smaller physical footprint for plasma gasification systems

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Plasma arc plasma gasification reactors do not require moving grates and the smaller volume of gases generated means that the plasma arc plasma gasification reactors can accommodate the required minimum residence times in a smaller volume. (Residence time – sometimes referred to as removal time – is the average amount of time that a particle spends in a particular system, which is important in hazardous waste or industrial waste processing to ensure that any toxic nature in the feedstock was exposed to the high temperatures generated within the plasma arc plasma gasification for a certain period of time).

Additionally, the smaller gas production and reducing environment (condition in which oxidation is prevented) within the plasma arc plasma gasification reactor does facilitate smaller sized air pollution control systems as a smaller volume of gas is required to be cleaned as compared to a combustion process.

China Gets Plasma Thermal Waste Destruction System

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PEAT International has successfully commissioned a Plasma Thermal Destruction and Recovery (PTDR) systems in Shanghai, China. The system was designed to deconstruct medical waste and oil refinery sludge.

PEAT International, Inc. (PEAT), a plasma-thermal waste destruction company, has installed a PTDR system in Shanghai, China. The 60 kg/hr system was specifically designed for the treatment of medical waste and oil refinery sludge for Abada Plasma Technology Holdings, Ltd.

“This is end-stage technology and sets the standard for clean hazardous waste remediation. Only with plasma can you achieve temperatures high enough for waste destruction in a single-staged process,” said Joseph Rosin, PEAT International chairman. “It’s a 21st century solution that addresses three important needs: significant volume reduction, full pollution control and competitive pricing. We are currently preparing for other projects already under contract.”

PEAT’s PTDR “single stage” plasma-thermal process transforms hazardous waste through molecular dissociation at 1,500°C (2,732°F) into recoverable, non-toxic end-products, synthetic gas and heat (sources for energy recovery), metals, and a vitrified glass matrix. Emissions are below the most stringent environmental standards.

PTDR systems are in operation in California, Taiwan, and China. For more information and to watch a video of operations, please click here.

Plasma Arc Gasification and Its Recyclable End-products and Energy Consumption

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There are a number of perceptions within the marketplace regarding plasma arc treatment and plasma arc gasification processes. Two weeks ago, this blog addressed the smaller physical footprint with regards to plasma arc gasification waste-to-energy systems.

This week we discuss the idea/claim regarding the plasma arc technology’s ability to generate significant useable recyclable end-products and energy with no residual waste.

Certainly this depends on the waste feedstock; however it is worth noting that if metals and glass are processed simultaneously in a plasma arc system, additional processing would be required to separate out these products for any re-use potential. The re-use of the vitrified slag product generated from to plasma arc systems has been demonstrated commercially in France and Japan.

Depending on the feedstock and moisture content and plasma utilization (combustion vs. plasma gasification/ plasma pyrolysis), to plasma arc gasification waste-to-energy systems can require significant amounts of energy to operate as such the net energy claims made by some within the industry may be overly optimistic in theory and largely unproven at this point in time in commercial operations. Criticizing the electrical loads associated with processing municipal solid waste in a plasma gasification waste-to-energy system may be valid as the primary goal would be net energy production, however when it comes to industrial, hazardous and universal waste treatment, the primary goal is destruction efficiency and thus the electrical consumption/generation should be considered secondary. Energy balances associated with to plasma-arc gasification waste-to-energy systems should to be reviewed on a project-by-project basis, rather than at a macro level within the industry.

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