Plasma Gasification Process

Plasma Process

At the most rudimentary level, plasma is just very high temperature thermal energy. In nature, plasma is produced by lightning when it superheats the air around the lightning bolt converting the air to plasma with a temperature of about 20,000 °C. Because plasma behaves differently than the three common states of matter; solid, liquid and gas, plasma is sometimes referred to as the fourth state of matter.

Cahill intends to create plasma with its plasma torch systems from Westinghouse Plasma Corp. It creates electric arcs, similar to lightning, inside its torches and pushes air through the arcs to create plasma. The plasma, with temperatures close to 5000 °C, is controlled and directed into the gasifier.

Plasma torches are sophisticated devices but their purpose is simple – they are high temperature heating devices.

A plasma gasifier is an oxygen starved vessel that is operated at the very high temperatures achievable with plasma. Because the environment inside the vessel is deprived of oxygen, feedstock that is processed in the gasifier is not combusted. Rather, the heat breaks the feedstock down into elements like hydrogen and simple compounds like carbon monoxide and water. The gas that is created is call synthesis gas or “syngas”.

The syngas created in the gasifier, which contains dust (particulates) and other undesirable elements like mercury, undergoes a clean-up process to make it suitable for conversion into other forms of energy including power, heat and liquid fuels. The syngas clean-up process is tailored to meet the requirements for each project. But in most cases, especially where MSW is the feedstock, the syngas clean-up will include particulate removal, sulphur removal and mercury/heavy metals removal.

Plasma gasification differs from non-plasma gasification in one key area – temperature. Non plasma gasifiers typically operate between 800 and 900 °C. The temperatures inside Westinghouse Plasma’s gasifier reach over 3000 °C. The syngas exits the gasifier at 950 °C. The slag flows out of the gasifier at 1650 °C. The higher temperatures inside the plasma gasifier result in the complete destruction of tars, something that is not achievable with non-plasma technologies. It is not feasible to remove tars downstream of the gasifier and therefore the utility of the syngas produced by non-plasma gasifiers is very limited. It can be burned immediately but it cannot be conditioned for use in gas turbines, reciprocating engines or for conversion into liquid fuels.

Feedstocks

The feedstock for plasma waste treatment is most often municipal solid waste, green waste, commercial waste or a combination of the three. Feedstocks may also include biomedical waste and hazardous materials. Waste tires and auto shredder residue (the material that remains when a vehicle is recycled and the metals are removed) are also good feedstocks for plasma gasification.

Most feedstocks, including municipal solid waste, contain both organic and inorganic components. The organic components are converted into syngas which is subsequently converted into products like electricity. The inorganic components, like glass, metal and concrete, are melted inside the plasma reactor and flow out of the bottom as a non-toxic vitrified molten slag which can be used safely as aggregate.

The heat from the plasma torch systems and the relatively long residence time in the gasifier ensures complete destruction of the feedstock and allows for the processing of high moisture feedstock or feedstock containing high levels of inert materials like glass and metals.

Products and Yields

The plasma gasifier within a plasma gasification plant produces syngas which can subsequently be converted into numerous types of energy including electricity and liquids fuels. The amount of end product, such as electricity, depends on the type of feedstock used and the capacity of the plant. Higher energy feedstocks such as tires will produce more units of energy products than lower energy value feedstocks such as municipal solid waste.

As an example, a plasma gasification plant that processes a municipal solid waste that has undergone a rough sorting to remove inorganics like metal and glass could send to the electricity grid about 900-1000 kWh per tonne of feedstock.

Inorganics, like metals, glass and concrete, within the feedstock are melted and converted into a glassy aggregate. The amount of aggregrate depends on the amount of inorganic material in the feedstock. A plasma gasification plant that processes municipal solid waste that has undergone a rough sorting to remove inorganics like metal and glass, could produce about 200-250 kg of slag per tonne of feedstock.