The Science
The Gasplasma Process comprises a conventional fluid bed gasification plant to convert the organic waste material into a synthetic fuel gas that is a mix of hydrogen circa 40%, carbon monoxide 40% and carbon dioxide 19%. The balance comprises mostly nitrogen.
The gasifier is a vertical refractory lined cylinder with a bed made up of sand that is fluidised by passing through it, a mix of oxygen and steam causing it to ‘boil’. The use of oxygen and steam allows the conditions within the gasifier to be carefully controlled, maintaining a starved air atmosphere. At the high temperatures of operation, material fed into the gasifier thermally degrades into a syngas with considerable energy still to be released.
Conventionally syngas would then be oxidised (i.e.: mixed with air) and combusted in an adjoining or nearby combustion chamber. The heat produced would be used to make steam that in turn would drive a steam turbine generating power. However, small-scale steam cycle power plants are relatively inefficient in that they convert only around 20% of the energy in the feedstock into power.
- We will be able utilising our syngas in a gas engine or turbine, which can achieve much higher electrical generating efficiencies of 35-40%,and exporting substantially more than half the total electrical output.
To date, there have been a number of attempts to clean syngas of the tars and other contaminants that will rapidly foul gas engines/turbines or block their filter systems. These have generally not been successful. Fouling occurs because most syngas contains tars in vapour form that condense at the low temperatures required for use in gas engines.
- Trials of the plasma treatment have already shown its effectiveness in removing these tars and producing a syngas of high quality and consistency.
The Fluid bed gasifier incorporated into the Gasplasma Process also creates a significant amount of char and ash (the percentage depends on ash material within the feedstock- usually 10-15% from MSW).
- The Gasplasma Process will recover this material as a form of recycled aggregate and so provides a solution that results in a minimal amount of residues having to be sent to landfill.
The plasma converter is a refractory lined closed vessel into which a graphite electrode is inserted. Electrical power is passed between the electrode and the melt, which is in contact with electrically conductive elements built into the refractory hearth of the converter, and these provide the return electric path to the power supply.
An inert gas such as nitrogen or argon is passed down the centre of the electrode. Above 3,000 degrees Celsius, this gas ionises and becomes electrically conductive. The gas changes its properties and becomes more viscous and forms a plasma; often referred to as the fourth state of matter. The plasma, which is usually around 30-40cm in diameter, typically operates at between 5,000-10,000 degrees Celsius and provides an intense source of heat and of Ultra Violet light that rapidly breaks down the feedstock materials. The heat from the plasma radiates out inside the vessel resulting in an average temperature of around 1,200 - 1,500 degrees Celsius.
As it leaves the plasma convertor the syngas, is at high temperature and requires cooling which is undertaken by conventional heat exchangers. It is then further cleaned to remove acid gases e.g. sulphur and chlorine containing vapours before being transferred into a buffer storage tank where it is stored as a fuel, prior to being fed into gas engines.