Plasma pyrolysis and plasma gasification, like incineration, are options for recovering value from waste by thermal treatment. Both pyrolysis and plasma gasification convert feedstocks/wastes into energy by heating the waste under controlled conditions. Whereas incineration converts the input waste into a combusted flue-gas that can then be used to recover thermal energy (usually in the form of steam) and ash, pyrolysis and plasma gasification deliberately limit the conversion so that combustion does not take place directly. Instead, they convert the waste into potentially valuable intermediates that can be further processed for materials recycling or energy recovery. Pyrolysis and plasma gasification offer more scope for recovering products from waste than incineration.
The last blog discussed plasma gasification, however more information on plasma pyrolysis is warranted.
Plasma pyrolysis takes place in reactors where oxygen is either absent or only present in very low concentrations. Pure pyrolysis is rarely used by itself, but followed by a downstream combustion or plasma gasification stage that converts pyrolysis tars into end-products that are more re-usable, such as the synthetic gas or “syngas.”
Plasma pyrolysis and plasma gasification are often used interchangeable because both produce an energy-rich product gas, however they are somewhat different. Some plasma companies convert the tars to carbon monoxide and hydrogen in a secondary cracking reactor (main reason for this two stage is tighter control over the syngas/flue gas production), while in other configurations, the plasma gasification (or partial combustion for some) reactions take place in the very same reactor, thus further blurring the boundaries between the two reactions. PEAT’s Plasma Thermal Destruction and Recovery systems are designed whereabouts plasma pyrolysis and plasma gasification occur in the very same reactor.
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 plasma torches (sometimes referred to as plasma-assisted gasification) to maintain the desired plasma gasification temperatures in the reactor.
Additionally, 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 gasification.
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.)