Combustion – Timer – programmer – retarder or condition responsive control – By combustion or combustion zone sensor
Reexamination Certificate
1999-02-18
2001-06-12
Lazarus, Ira S. (Department: 3743)
Combustion
Timer, programmer, retarder or condition responsive control
By combustion or combustion zone sensor
C431S021000, C431S168000, C431S088000, C431S326000, C126S908000
Reexamination Certificate
active
06244856
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to burner assemblies and in particular, but not exclusively, to burner assemblies for delivering heat to a rotary heat pump or other rotary heat receiver. It is emphasized however that the invention extends to burner assemblies for general use; and particularly to radian plaque burners with low NOx emissions.
DESCRIPTION OF THE RELATED ART
A rotary heat pump is described in U.S. Pat. No. 5,009,085, and further developments are described in our published International Patent Application No. WO 97/14924, the contents of which are incorporated herein by reference. In U.S. Pat. No. 5,009,085 a putative burner geometry is proposed in which a burner receives gas and air and is fitted with radiant plaques which emit the energy of combustion in approximately equal amounts of radiant heat and heat contained in the combustion products. The heat energy omitted from the stationary plaques impinges on the rotary dished plate of the heat pump generator. Hot flue gas from the burner flows over the outer surface of the generator and then is expelled via an annular slot. In the region of the slot, heat transfer to the generator plate is primarily by forced convection.
SUMMARY OF THE INVENTION
We have built and tested burner assemblies similar to those described in U.S. Pat. No. 5,009,085 but have found that the burner efficiency is relatively low, around 60%, to such an extent that jeopardises the prospects of a commercially viable heat pump.
Two further problems were encountered in attempting to follow the teachings of U.S. Pat. No. 5,009,085; firstly the apparent need to provide a gap of precisely defined thickness between the rotating generator surface and the burner housing, and secondly the need to provide a satisfactory support for the burner housing which is cantilevered to reach over the convexly dished generator surface.
A further design objective in this invention is to provide a burner assembly which provides sufficient heat but which does not have unacceptably high levels of NOx emissions. The formation of NOx in combustion is a complex process involving the reaction of oxygen, nitrogen and other species within the flame. In general the amount of NOx formed depends upon the temperature conditions in the flame and the residence time of the reacting species at the high temperatures.
Although some fuels contain significant quantities of nitrogen-bearing compounds, which can lead to NOx production, the levels in natural gas are very low and so the predominant source of nitrogen for NOx formation is the combustion air.
There are generally recognised to be two mechanisms for NOx formation. These are known as Fenimore NOx (F-NOx) or prompt NOx and Zeldovich NOx (Z-NOx) or thermal NOx. F-NOx is formed very rapidly in a flame, but in a fully pre-mixed flame will only be significant at sub-stoichiometric conditions. Z-NOx is strongly temperature dependent and is formed later in the flame. In a fully pre-mixed lean combustion Z-NOx is the predominant mechanism and is the reason why most NOx reduction strategies concentrate on the time and temperature dependence of this mechanism.
However, the Z-NOx temperature dependency is only strong at flame temperature conditions above about 1600° C. and most blue gas flames contain temperature conditions above 2000° C. In lean combustion, the oxygen in the air will continue to form NOx beyond the flame (i.e. when the combustion reactions are complete) if the temperatures are high enough.
Fully pre-mixed radiant plaque burners are increasingly being used in gas appliances because of their good turndown and low NOx characteristics. In a ceramic radiant plaque burner, the gas is fed into a plenum chamber closed by a ceramic plaque which may be porous or provided with an array of apertures. In general these burners operate in two distinct modes (although the transition is not a sharp one). At lower heat inputs the burner surface radiates strongly and the flame is very short, close to the plaque surface. At higher heat inputs the flame temperature and flame length increase and the plaque surface temperature may be lower the burner is said to be in blue flame mode.
When not in blue flame mode, ceramic radiant plaque burners have inherently low NOx characteristics because the flames are very short and temperatures are low as heat is dissipated from the flame by the radiating surface In most applications radiant plaques only operate with very low NOx levels at relatively modest thermal loadings (<300 kW/m
2
). Increasing throughput results in longer flames, higher local flame temperatures and increasing NOx emissions as the burner goes towards the blue flame condition. Nevertheless, even in blue flame mode the NOx emissions from ceramic radiant plaques can be significantly lower than conventional metal burners, because the plaque still has a flame temperature reduction and flame shortening effect.
We have found that, surprisingly, by designing a burner assembly in which a ceramic plaque burner is generally enclosed, with a significant amount of heat being radiated back from the enclosure and the heated surface, the NOx emissions for a given heat output can be significantly reduced thus allowing either reduced NOx emissions or a higher heat output for a given threshold of NOx.
Accordingly, in one aspect of this invention, there is provided a burner assembly comprising a radiant plaque burner means disposed within a generally enclosed chamber or a generally confined volume and directing radiant heat to a heat receiving means.
By this arrangement, we have found that it is possible to raise the limiting heat throughput of a given burner before the NOx reaches a set limit. We have found that enclosing the burner tends to raise the temperature of the radiant plaque thereby increasing reaction rates and tending to shorten the flame length. We also found that this feature can reduce flame temperature, thereby again reducing NOx. A further benefit of shortening flame length is that it allows the radiant plaque burner means to be located closer to the heat-receiving means without causing flame impingement. In a conventional arrangement, without the substantial amount of back-radiation, the flame length is greater and so the distance at which flame impingement occurs is greater. Flame impingement causes the combustion reactions to be quenched, resulting in unacceptably high CO emission levels.
In a preferred embodiment of this invention, the NOx forming reactions are quenched, reducing NOx emission levels, but the combustion reactions are already completed at a shorter distance from the burner means.
In a preferred embodiment, said heat receiving means comprises a continuous surface extending beyond the periphery of the burner means.
Preferably, the walls of the chamber and/or said heat-receiving surface return a significant component of the radiant heat developed by said burner means, thereby further to increase the temperature of the plaque. The temperature of the plaque is preferably over 700° C. and more preferably around 1000° C. By raising the temperature of the plaque surface above conventional levels, e.g. by return of the back-radiation, combustion reaction rates are increased, thereby tending to reduce the flame length and delaying the onset of flame lift.
Another advantage of increasing the temperature of the plaque is that it may be increased to the levels to activate one or more catalysts in the plaque to stabilise the flame and reduce NOx levels. Thus in a preferred embodiment, the plaque may incorporate one or more catalysts such as platinum, palladium, alumina.
Certain available plaques already contain a substantial quantity of alumina, though not for its possible catalytic properties as it is not active in the normal operating temperatures.
In such an arrangement, the burner is converted so that the surface pores are hot enough to initiate catalytic action while the flame length is short enough and close enough to the plaque to be stabilised by the catalytic action. In addition, the forward velocity of
Somervell Thomas
Winnington Terence Leslie
Cocks Josiah C
Interotex Limited
Lazarus Ira S.
Young & Thompson
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