Enhancing condensation in boilers by oxygen enrichment

Abstract

A condensing boiler combustion process has enrichment of the non-combustible intake gas, for example, the intake air may be enriched with oxygen. An oxygen-enriched air supply as a consequence has a l

Description

TECHNICAL FIELD This invention relates to domestic condensing boilers where part of the useful heat delivered is due to water vapour condensing. BACKGROUND OF INVENTION Over a million condensing boilers are installed in the UK every year. In a condensing boiler the flue gas is cooled and some of the water vapour produced by burning the fuel is condensed from vapour to liquid. As it does so it gives up its latent heat of evaporation and provides additional useful heat to the condenser. What is less well known is that only a portion of the water vapour condenses, much of it is emitted still as vapour and the latent heat of this vapour is lost, thus limiting the efficiency of the boiler. The proportion of lost water vapour is determined by the dew point of the flue gas: if the condenser is warmer than the dew point, all the vapour is lost. If the condenser is cooler than the dew point, then some of the vapour is captured and condensed. The cooler the condenser, the larger the proportion of the vapour that is condensed. The dew point of any gas is related to the volume fraction of water vapour in it. The higher the proportion of water vapour, the higher the dew point. In domestic practice the temperature of the condenser is controlled by the return temperature of the water in the household central heating circuit. The proportion of water vapour in the flue gas is determined by the chemical composition of the fuel and the chemical composition of the inlet oxidiser, which domestically is invariably air. There remains a need to condense more of the water vapour without having to reduce the return temperature of the central heating. We can do this by manipulating the relevant chemical compositions and the pressure in the condenser. SUMMARY OF INVENTION The present invention provides a method whereby more of the water produced by burning the fuel is condensed. It does this by reducing the volume of inert gas in the inlet gases, such that the proportion of water vapour is higher in the flue (exit) gases. According to the present invention, the inlet air is enriched in oxygen before combustion of the fuel. This increases the proportion of water vapour in the flue gas and, for the same amount of inlet fuel and the same condenser temperature, more of the water vapour produced is condensed thus increasing the efficiency of the boiler. A condensing boiler is understood to be a boiler which bums a fuel with an oxidiser where some of the flue gases resulting from the combustion is condensed, either in a single condenser or in an external heat-recovery condenser. An additional refinement is to increase the pressure in a condenser, with or without electrical energy recovery of the higher pressure at the flue exit, which also increases the proportion of water which is condensed. BRIEF DESCRIPTION OF THE DRAWINGS FIG 1 shows the theoretical maximum efficiency of a condensing boiler as a function of condenser temperature for two fuels: hydrogen and natural gas. FIG. 2 plots the gradients of the curves in FIG.l showing that the greatest advantage in reducing the condensation temperature (or equivalently, reducing the dew point by changing the chemical composition of the flue gas) occurs just at the point where condensation starts to occur. FIG. 3 is the same as FIG.l in that it shows the theoretical maximum efficiency of a condensing boiler as a function of condenser temperature, but it is for a single fuel: natural gas ('NG' in the figure) but with different amounts of oxygen in the air. At 50°C flue gas exit temperature, natural gas and air have a maximum efficiency of about 91% whereas air enriched to have 30% oxygen in it has a maximum efficiency of about 94%, an efficiency improvement of an additional 3%. An equivalent plot can be produced for hydrogen fuel. F1G.4 is derived from FIG.3. It shows the difference between each curve and the case where NG is burned in ordinary air. Pure oxygen increases the maximum possible efficiency by an additional 9% at the dew point of the flue gas of natural gas in air, about 55°C. The additional nearly 3% improvement at 50°C described in the caption to FIG.3 is seen more clearly in FIG.4. An equivalent plot can be produced for hydrogen fuel which demonstrates even greater proportional efficiency improvements. DETAIL OF INVENTION Boilers always take in excess air, above what is needed to exactly bum the fuel. This is typically an additional 15% of air (the oxidiser). Reducing the excess air would increase efficiency and increase the dew point of the flue gas if the fuel could all be perfectly burned, but some excess is required to ensure that it is all burned. However reducing the proportion of inert gases in the inlet oxidiser does not negatively affect the combustion of the fuel as there is still an excess of oxygen. An advantage of the invention is that the effect of oxygen enrichment is greater the higher the proportion of hydrogen to carbon (or other combustible element) in the fuel. So the invention has a greater effect in the case of a hydrogen-fuelled boiler compared to a natural gas fuelled boiler. Additionally or alternatively, the fan injecting the fuel-oxidiser mixture, or an additional compressor, compresses the combustor and condenser such that more water is condensed than occurs under ambient air pressure. In the preferred practice of the invention, the boiler recirculates a proportion of the flue gas into the inlet gases before combustion in order to reduce the combustion temperature, to reduce the production of oxides of nitrogen and to reduce wear and erosion in the burner. This is a well-known design feature in large industrial oxycombustion systems. A more sophisticated control system than is currently used can modify the excess air such that the excess is reduced to the minimum required for complete combustion. This requires additional chemical and / or thermal sensors in the flue gas. In the preferred practice a weather-compensating control system reduces or disables the oxygen enrichment under conditions when the efficiency benefit of oxygen enrichment is low. The invention is timely as it is only in recent years that domestic medical oxygen enrichment devices have been produced in volume, thus demonstrating that the enrichment technology exists at scale. These devices are capable of running continuously for periods of weeks or months. EXAMPLE OF A SPECIFIC EMBODIMENT A specific example of the process of the invention is a modest oxygen enrichment to the inlet air in an existing boiler and burner design, such that the higher temperature of the flame is not so high as to produce nitrogen oxide pollution or to require flue gas recirculation. Such a design would be quicker to safety test and to commercialise as the combustion chamber and condenser would not need to be redesigned and an existing design of medical oxygen enrichment device could be bought as a unit. Enhancing condensation in boilers by oxygen enrichment

Claims

1. The enrichment of the inlet oxidiser by the reduction of inert gases in a condensing boiler where some of the useful heat comes from condensing water.

2. A boiler of claim 1 where the oxidiser is air and the principle inert gas being reduced is nitrogen.

3. A boiler of claim 1 or 2 where the fuel contains components which are found in natural gas.

4. A boiler of claim 1 or 2 where the fuel is natural gas.

5. A boiler of claim 1 or 2 where the fuel is a liquefied petroleum gas.

6. A boiler of claim 1, 2, 3,4 or 5 where the fuel is enriched in hydrogen by more than 1% by volume.

7. A boiler of claim 1 or 2 where the fuel is more than 80% hydrogen by volume.

8. A boiler of claims 1, 2, 3,4, 5, 6 or 7 where the boiler is installed in a domestic premises for the purpose of central heating and or the production of domestic hot water.

9. A boiler of claim 1 or 2 where the fuel is enriched with a nitrogen containing fuel such as ammonia.

10. A boiler of claim 1 or 2 where the fuel is more than 80% by volume a nitrogen containing fuel.

11. A boiler of claim 9 or 10 where the boiler is installed in a domestic premises for the purpose of central heating and or the production of domestic hot water.08 04 25Enhancing condensation in boilers by inerts reductionAMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS:Claims1. The reduction of inert gases in the condenser of a condensing boiler where some of the useful heat comes from condensing water.

2. A boiler of claim 1 where the oxidiser is air and the principle inert gas being reduced is nitrogen.

3. A boiler of claim 1 or 2 where the fuel contains components which are found in natural gas.

4. A boiler of claim 1 or 2 where the fuel is natural gas.

5. A boiler of claim 1 or 2 where the fuel is a liquefied petroleum gas.

6. A boiler of claim 1, 2, 3,4 or 5 where the fuel is enriched in hydrogen by more than 1% by volume.

7. A boiler of claim 1 or 2 where the fuel is more than 80% but less than 97% hydrogen by volume.

8. A boiler of claims 1, 2, 3,4, 5, 6 or 7 where the boiler is installed in a domestic premises for the purpose of central heating and or the production of domestic hot water.

9. A boiler of claim 1 or 2 where the fuel is enriched with a nitrogen containing fuel such as ammonia.

10. A boiler of claim 1 or 2 where the fuel is more than 80% by volume a nitrogen containing fuel.

11. A boiler of claim 9 or 10 where the boiler is installed in a domestic premises for the purpose of central heating and or the production of domestic hot water.

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