LOW-NOX DOMESTIC WATER-HEATING APPLIANCES

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.