Considerable interest is being shown in the low-NO(x) combustion of na
tural gas, and this study concerns two approaches to the production of
low-NO(x) combustion units. In the first part the combustion of partl
y premixed natural gas/air mixtures in two domestic water heaters was
experimentally investigated. Measurements of gas temperature and stabl
e species concentrations were taken in and around the combustion zone,
and from the flue. The first appliance was a bladed burner, commonly
used in domestic water heaters, of maximum thermal input of 11 kW in a
combustion chamber 18 x 18 x 16 cm; the primary mixture fuel: air equ
ivalence ratio was 1.92 inside the burner body. The second appliance w
as fan-assisted and included a flue-gas valve allowing 0-15% of the co
mbustion products to be recirculated; its burner was of the horizontal
bar type with two lines of primary mixture ports each firing toward p
lates forming a small angle with the burner body. Transverse slots in
the plates allowed for secondary aeration, and the flames developed al
ong the slots. From an understanding of the temperature/species profil
es within the combustion zone, the main routes of NO formation could b
e determined. Thermal and prompt-NO formation routes were considered i
n both appliances, as combustion temperatures exceeded 1800 K and beca
use of the fuel-rich nature of the primary flame zone. It can be assum
ed that an important role is played by the prompt-NO mechanism in the
bar burner appliance, since the temperature did not reach 2000 K. In o
rder to analyse the experimental results, the appliances were modelled
by means of a commercial computational fluid dynamic package, togethe
r with a NO(x) post-processing package. In the second part, the use of
natural-gas catalytic combustors as a future means of ultra-low NO(x)
combustion was investigated. Particular detail was placed in the desi
gn of combustors and in the choice of catalyst, because of poisoning a
nd high-temperature de-activation. Experimental measurements were coup
led with theoretical modelling by means of the Sandia National Laborat
ories code PREMIX.