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.

As discussed earlier in this blog, a vitrified matrix or slag is the primary solid byproduct of plasma arc waste-to-energy processing. The vitrified matrix from plasma arc processing contains the mineral matter associated with the feed materials in a vitrified form – a hard, glassy-like substance. The amount of matrix produced is a function of how much non-combustible mineral matter is present in the feedstock.

This matrix is the result of operating temperatures within the plasma arc reactor above the melting temperature of the mineral matter. Under these conditions in the plasma arc reactor, non-volatile metals and metal oxides bind together in molten form until it is cooled via natural heat loss or via a pool of water, where it would fracture and granulate.

The compressive strength of a slag sample generated from fly ash from coal-fired power plant as well as some sodium carbonate (fluxing agent) was 480 kg/cm2, while its average mortar strength was tested at 169 kg/cm.

The vitrified matrix or slag generated by plasma arc treatment is primarily made up of silicon dioxide (SiO2), aluminum oxide (Al2O3) and calcium oxide (CaO). Toxicity Characteristic Leaching Procedure (TCLP) tests are designed to determine the mobility of both organic and inorganic analytes present in the slag. The most recent TCLP results conducted in March 2013 on the vitrified matrix from the plasma arc waste-to-energy system located in Shanghai is presented in the below table along with previous results from processing refinery sludge.

Also, here are recent pictures of this system.

http://www.peat.com/ptdr_pictures.html

As you may know from reading this blog, in plasma arc, plasma gasification, waste is broken down at temperatures around 1,500°C. While this form of gasification can be energy intensive, it ensures that the plasma gasification syngas produced is cleaned of residue tar, which makes it a higher value product. Another major benefit of plasma arc, plasma gasification is that it creates little to no emissions. This blog entry attempts to take a quick look at some of the other waste to energy technologies available.

Conventional gasification slightly differs from plasma arc, plasma gasification in that the temperatures inside its primary reactor are lower and the fuel source may be natural gas. However conventional gasification is similar to plasma arc, plasma gasification in that both are thermo-chemical processes in which waste is heated in an oxygen deficient environment to produce syngas which contains hydrogen, carbon monoxide and sometimes methane. This gas can be used as fuel for electricity generation or to produce chemicals or biofuels.

Pyrolysis is sometimes linked to plasma arc, plasma gasification. Pyrolysis is also a thermo-chemical process, however here the waste is heated in the complete absence of oxygen. The products are olefin liquid, syngas-type product, and char. The liquid fuel can be used as an input to produce gasoline, while the char can be recovered or passed along to a gasification process.

Other waste to energy alternatives include landfill gas recovery that convert methane gas from decomposing trash in to power. Methane is collected through a series of pipes and compressed for electricity generation. There are also waste to biofuels and waste to chemical processes. These are similar to aforementioned waste to energy technologies as they may utilize gasification or other thermo-chemical processes. Appropriate waste feedstocks include soybean straw, wood waste and used oils/fats.

Finally, there is anaerobic digestion (AD). This differs greatly from the other waste to energy technologies mentioned here in that AD uses microorganisms to convert organic waste into a biogas (primarily methane) and digestate. This biogas can be used to generate electricity, while the digestate has applications as a fertilizer. Anaerobic digestion is limited to wet organic wastes (including manure and sewage) or food waste. As such it requires that these materials to be separated from regular waste.

Many contest that the primary goals for waste management; to reduce, reuse and recycle, and increasing waste conversion (i.e. waste-to-energy) rates are not compatible. However, in the United States, the states making the most use of waste-to-energy facilities are also those that recycle the most.

In addition, according to a recent study conducted by the EPA, increasing recycling wastes actually improves the efficiency of waste conversion (i.e. waste-to-energy).

Consumers are increasingly recycling more biogenic waste (paper and food) and throwing away more non-biogenic waste (rubber and plastics).

The higher energy content of non-biogenic waste makes it a more productive feedstock for generating electricity through a waste-to-energy technology such as the TVRC. Conversely to previously held views then, recycling is not just compatible with waste conversion, it actually improves the energy content of the leftover waste, boosting the potential of key waste-to-energy technologies, including plasma-arc plasma gasification.