1% 2%
Refineries 4%
Tertiary 5%
Residential 12%
3% <1%
Industrial 16%
Road transport 24%
Other 9%
Electricity
generation 30%
The latent potential gain in efficiency There is a mature UK market in high efficiency dedicated direct gas-fired water heaters, having been applied in the UK for more than 40 years. The gas market in the UK and Europe is principally natural gas (that is, approximately 90% methane (CH4
) – a hydrocarbon); natural gas has
Water heaters (incl. combi) 6%
Figure 1: EU-15 energy-related greenhouse gas (GHG) emissions (more than 80% of total GHG emissions) by sector, according to EEA 2007
consultation document6 for the 2013
revision to AD Part L has a proposed recommended minimum gross efficiency for direct gas fired hot water systems of 90% for new construction over 30kW, compared with the previously recommended minimum of 73%. A comparable mandate in 2005 that practically demanded all new domestic boilers be condensing – despite scepticism at the time about their practicality7 more than nine million installations8
– led to .
a high latent heat content (water vapour) in the products of combustion. The actual composition of natural gas will vary, depending on its source. The combustion of hydrocarbons in a water heater is a reaction of oxygen (O2
the air (air principally being comprised of 79% nitrogen and 21% oxygen), with the carbon (C) and hydrogen (H) compounds in the fuel creating CO2 water vapour (H2
and superheated O), plus the nitrogen
mainly passing straight through. A simplified combustion equation for the most effective combustion of methane (given in ‘moles’) is
methane + air =>
water + carbon dioxide + nitrogen CH4
+ 2 (O2 2H2 + 3.76N2 O + CO2 ) => + 7.52N2
with the flue gas (on the right hand sides of the expressions above) containing neither fuel nor oxygen, both having been completely utilised in the chemical reaction. This perfect ratio is called the stoichiometric ratio, and provides a theoretical best mixture for combustion of the methane gas to produce heat without wasting fuel or causing excessive harmful gases. The relationship established by
‘Avogadro’s principle’ means that each mole of gas will take up the same volume, and so whilst the water is in a vapour form (and the flue gas being known as ‘wet’):
1 volume CH4 2 volumes H2 + 7.52 volumes N2
+ 2 volumes O2 =>
O + 1 volume CO2 volumes N2
+ 7.52
So the stoichiometric air-to-fuel volume ratio for methane combustion is 9.52 (air) : 1 (methane). The percentages in this stoichiometric mixture’s 10.52 volumes of flue gas are 9.5% CO2
, 71.5% N2,
Figure 2: Proposed conventional water heater EU energy label. Practically, to achieve an ‘A’ rating, anything but small domestic hot water heaters will have to be condensing water heaters
46 CIBSE Journal August 2012 and 19% H2 O that
will be in vapour form and will include significant amounts of latent heat. (The instrumentation used to control the air/gas mixture is likely to monitor the
However, the increased air supply will
reduce the temperature of the flue gases (so reducing efficiencies) and potentially increase the amounts of environmentally significant oxides of nitrogen (NOx
).
By carefully monitoring the flue gas composition, the excess air levels can be controlled at the lowest possible level, so that the oxygen concentrations in the high temperature zone of the combustion process can be minimized to reduce the NOx
formation and maintain efficiencies.
Recovering latent heat to improve hot water heating efficiencies The application of dedicated water heaters has become common in small commercial and industrial applications. The development over the last 15 years of non-storage, high output ‘instantaneous’ continuous flow heaters allows the provision of large quantities of potable (domestic) hot water without the need to store hot water. If well designed, controlled and operated, such a (non-condensing) heater can provide gross efficiencies of more than 80%. Such systems can also provide operational and space advantages by not requiring storage; storing hot water will add to heat losses, and the additional pipework and controls add to the complexity of systems, both in design and maintenance, resulting in lower gross system efficiencies than for non-storage, continuous flow systems. However, they
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‘dry’ flue gas that has a total of 8.52 volumes and a resulting 11.74% CO2
by
volume, under stoichiometric conditions.) All combustion systems use slightly
) in
more air than theoretically needed to ensure complete combustion of the fuel (known as ‘excess air’) – this is represented by a lambda value, λ, of greater than 1, where λ = (actual air ratio)/(stoichiometric ratio). If systems were operated with a lambda of 1, it is unlikely that all the gas would react – due to imperfect mixing, as well as variations due to pressures and air moisture content – so wasting fuel. Incomplete combustion will also create poisonous carbon monoxide (CO) and particulate matter (soot) in the flue gas. A simplified example process with 20%
excess air would be: CH4
2H2 O + CO2 + 2.4 (O2 + 3.76N2 + 0.4O2 ) => + 9.02N2
and λ = (actual air ratio)/(stoichiometric ratio) = 10.02/9.52 = 1.05
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