HTCW and Other Gasifiers
Gasifiers have an intrinsic process advantage over incineration, in that they use distinct stages to:
1). Process the feed to make a syngas
2). To clean the gas before its use
3). To burn it in an efficient engine to generate electricity or steam
4). Alternatively, synthesize the gas into diesel, gasoline or methanol
The quality of a gasification process can be measured by its syngas. A good syngas has a high CV and is effectively free of tars. Tars are eliminated by high temperature operation and by ensuring sufficient air or oxygen are supplied to complete the gasification reactions.
A high CV syngas is created by not adding excessive air
or oxygen. Good control is needed for optimal performance.
The computer controlled multiple injection of oxygen in the
HTCW furnace allows this optimisation.
Gasifiers come in many forms, they can be divided into those such as HTCW that use oxygen or enriched air and those using air. Air is cheaper than oxygen, so an air gasifier should be cheaper, but is it better?
- The nitrogen in the air used in an air gasifier ensures a greater volume of syngas is formed per tonne of feed. Consequently an air blown gasifier has to be physically bigger than an equivalent oxygen blown gasifier.
- The nitrogen in the air gasification process' syngas of typically 45% creates problems with maintaining engine efficiency and avoiding a substantial de-rating of the engines.
- The excessive nitrogen has to be heated to gasification temperature. The bulk or all of this energy is normally lost as the gas is cooled on leaving the gasifier.
Gasification Efficiency Graph
comparing an air-blown to an oxygen-blown gasifier (HTCW)
The illustration below shows that the efficiency of a gasification process decreases as the temperature is raised. According to an independent study, at 1400C (2550F) the HTCW operates with the same efficiency as an air blown gasifier operating at 850C (1550F), which invariably produces a tarry gas. Part of the air blown gasifier industry has responded to this problem by the introduction of plasma assisted gasifiers. These can remove tars etc with the same effectiveness as the HTCW, but the inefficiency associated with heating the inert nitrogen in the air obviously remains.