Emissions from a gas-burning pulse combustor

• Main Author: David, Jiri
• Published: Middlesex University 1993
• Subjects: 621
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.574391

The pulse combustor has a number of attractive features for heating applications. These include simplicity of construction, compactness for a given heat input rate, enhanced exhaust gas heat transfer and most importantly low CO and NOx emissions. With more stringent air quality and emissions standards soon to be in force in many countries, the latter has received much attention among the producers of gas appliances. This study investigates the performance of and emissions from a Helrnholtz-type pulse combustor with varying parameters such as gas input rate, tailpipe length, combustion chamber volume and composition I)f the natural gas being used as a fuel. The experimental work was primarily focused on the measurements of O2, CO2, CO and NOx concentrations in the pulse combustor exhaust gas; such data are difficult to find in the general literature. Measurements of the aspirated combustion air flow rate and operating frequency were also made. Initially some modifications to the natural-gas-fired pulse burner of 5kW output constructed by A. Suthenthiran were made and new gas sampling ports and probes were designed and constructed. An orifice plate flow meter and air-box were also designed and constructed in an attempt to measure the pulsating air flow of the combustor. It was found, however, that the combustor would not operate with the air-box fitted. The air/fuel ratios of the combustor were therefore calculated from the measured exhaust gas analysis by means of a specially written computer program. Much attention was also given to the selection of reliable gas analysis equipment and as a result five different gas analyzers were tested. It was found that the investigated pulse combustor was capable of operation only under fuel-lean conditions since the stoichiometric air/fuel ratio was never reached. Percentage excess air levels when using the mains gas (NGA) as a fuel were typically found in the range of 5 - 40 % The variation of carbon monoxide exhaust levels with gas flow rate displayed, in most cases, a characteristic 'U' shaped curve gently sloping down and reaching minimum CO concentration (typically below 300 ppm), before rising steeply as stoichiometry was approached. It was also found that in the lower and middle operating range CO levels were notably reduced with increase in combustion chamber volume and slightly reduced with increase in tailpipe length. Furthermore, the CO/C02 ratio did not exceed the maximum permissible limit of 0.02 set by appliance safety standards for any of the conducted trials. Test results for all investigated variables indicated that production of nitrogen oxides was strongly temperature dependent; a finding which is consistent with the literature. In addition, NOx emissions were observed to rise with increase in gas flow rate, tailpipe length and combustion chamber volume respectively. With measured NOx levels below 60 ppm our results agree with the values reported elsewhere. Furthermore, oxides of nitrogen monitored in this study consisted mainly of thermal NO formed by the Zeldovich mechanism. The pulse combustor was operated on family 2H test gases ie. NGA (mains), NGB, NGC (high in Hz) and NGD (high in N2 ) in order to evaluate the effect of changes in fuel composition on its performance. A conventional method based on the modified Harris & Lovelace diagram was used to define an acceptable area within which all family 2H gases would be interchangeable. Propane was found not to be interchangeable with the reference gas NGA.