Methane DRAX methane burnersThere seems to be little problem in scaling the DRAX technology to larger scale from the current 10-20kW output, using either electric or gas pre-heat. High powered gas pre-heat burners might be most attractive for high temperature processes, however, offering economic advantages over the electrical methods most often used at higher temperatures.
We have already demonstrated that the DRAX burner, equipped with a hot compressed air blowing nozzle, can cut heavy gauge sheet steel. For applications requiring a neat cut, or through thick material further development will be necessary.
Thermo-photovoltaic electric power generationDRAX technology has been tried using methane as a fuel gas. Although there is loss in performance, attributable to the lower combustion energy of the methane gas, DRAX burners running on methane still offer a huge increase in temperature over conventional methane/air burners, although designs would probably need to differ from those used for propane. (DRAX technology can be used on other fuel gases, such as butane, propylene, or mixtures of LPG, with no change in basic design configuration).
Thermo-photovoltaic (TPV) electric power generators : these comprise a gas burner pre-heated using exhaust heat, with the main flame heating a SiC emitter, the emitter being surrounded by Gallium Antimonide (GaSb) or Silicon photovoltaic cells ('solar' cells). TPV electric generators are used in military and outdoor recreational contexts, for example recreational vehicles (RVs), and have been proposed as a quiet low emission power source for electric vehicles. The advantage of the DRAX burner in the TPV is that it will heat the emitter to a much higher temperature, emitting more of the near infra-red and visible radiation that the photovoltaic cells require.
DRAX burners currently require access to pressurised air, with at least a few tens of millibar pressure. Further development could reduce the pressure requirement, allowing a low power fan to be used, which could then be powered by the TPV itself without losing significant output power. Working on the assumption of an achievable radiator temperature of 1300 C from a conventional burner, 1500 C from an air pre-heated burner, and 1700 C from a DRAX burner, a simple calculation gives some idea of the advantage which the DRAX burner would have. With the simplifying assumption of black body radiation, these are the figures for both Gallium Antimonide (GaSb) and Silicon photovoltaic cells:
Gallium Antimonide (band-gap 0.72 eV, 1.75 microns)
Fraction of energy available for conversion
Ordinary Bunsen burner 21% Air pre-heat burner 27% DRAX burner 32% (20% improved over Air pre-heat, 50% over Bunsen) The DRAX burner might offer further advantages in compactness, in that its intensity of emission would be increased 50% over Air pre-heat and
Silicon (band-gap c. 1.2eV, 1.05 microns - the indirect band-gap giving a less clear cut-off)
Ordinary burner 2.9% Air pre-heat burner 5.4% DRAX burner 8.4% (60% improved over Air pre-heat, 190% over Bunsen)
Clearly the DRAX burner would give spectacular increases in efficiency using silicon cells. Silicon has the advantage of very low cost, robustness and lower surface reflection losses which further boost its performance relative to the idealised GaSb result above.
