Could someone explain Lower Heating Value and Higher Heating Value in relation to hydrogen?
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When you burn hydrogen (or any fuels containing hydrogen, such as oil/gas hydrocarbons), one of the combustion products is water – H2O.
As the combustion typically occurs at high temperature, this is usually in the gaseous form of steam.
It turns out that a decent chunk of energy is locked up in keeping the steam a gas, which can be released by condensing it back into liquid water. Therefore, there are two quoted figures for calorific (heating) value: the Lower figure that assumes the steam is not condensed, and the Higher figure that assumes it is.
For maximum efficiency, the flue gases should be condensed to achieve the higher heating value, but there are a number of circumstances when that is not desirable or possible. Perhaps you want to heat something above the boiling point of water – in which case it’s difficult to cool the gases enough to condense.
Another reason is that for many hydrocarbon fuels, impurities in the fuels mean that the condensed liquid is quite acidic which can cause issues if the exhaust materials are not designed to cope with it.
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