Plasma Gasification Chemistry and More Details on the Gasification Process
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 (âsyngasâ). 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 âCO2 + heat (exothermic) Equation 2 C + H2O (steam) â CO + H2 (endothermic) Equation 3 C + CO2 â 2CO (endothermic) Equation 4 C + 2H2 â CH4 (exothermic) Equation 5 CO + H2O â CO2 + H2 (exothermic) Equation 6 CO + 3H2 â 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 provides 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.)
The main aspect of gasification, whether it is plasma-arc plasma gasification or âtraditionalâ gasification is to raise carbonârich materials or waste to a high temperature in an oxygenâdeficient 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.