Some companies focus on processing municipal solid waste (MSW) via plasma-arc plasma gasification. In this application, MSW is generally shredded, then fed into the plasma-arc plasma gasification reactor with very limited and calculated volumes of oxygen or air. The syngas produced is cleaned and then used to make electricity. The vitrified product is still produced by the plasma-arc plasma gasification process. As mentioned previously, this is a glass-like byproduct, known as vitrified slag, does not leach out. This material has more uses than ash from conventional gasification, including as insulation material, flooring tiles and garden blocks. A recent article cited that plasma-arc plasma gasification technology can generate 816 kWh per ton of MSW.
In the last blog entry it was mentioned that 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. This entry looks to discuss in slightly more in depth the differences between these types.
In the updraft (sometimes referred to as “counter current”), air/oxidant is injected from the bottom and the material enters at the top. Following gravity, the material is dried then reduced to char (pyrolyis) and finally any ungasified solid remnant is burned. This type has a high energy efficiency because of the heat exchange between the rising gas and descending material. The main issue is the high concentration of oils and/or tars in the syngas, which must be cleaned prior to any utilization, which can decrease the overall efficiency. Updraft gasifier usage is generally focused to direct heating applications as little to no gas cleaning is necessary. PEAT’s plasma-arc plasma gasification process is configured similarly in that the feedstock material is fed from the top, however it also has similarities to a fluidized bed (see below) in that the vitrified material in PEAT’s plasma-arc plasma gasification is maintained in a somewhat molten form.
In a downdraft gasifier (or co current), the gas is drawn out from below through a combustion zone. The material and oxidant flow in the same direction. Compared to the updraft configuration, the gas tends to be cleaner with fewer tars – the reason being in a downdraft configuration the tars (product from any pyrolysis phase) have to pass through a higher-temperature oxidization zone. The key I in a downdraft design is temperature maintenance and if the feedstocks vary in composition and moisture content, this can be difficult to achieve. The gas must rapidly cooled prior to be used.
Finally, a fluidized bed, which is more common in large plants, has a hot bed of sand located at the bottom of the gasifier. Gas clean-up is key as many times tar and ash can be found in the syngas. Waste is typically shredded or pulverized prior to being fed from the top.
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-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.)
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.