Energy conservation and CO emissions of fuel oil boilers used in domestic heating

According to the European standard, the CO emissions of new fuel oil boilers, whose output power is lower than 400 kW, are limited in steady-state regime to 110 mg/kWh of energy consumption. In order to assess the performance of actual fuel oil boilers, an experimental work has been performed on a boiler available in the laboratory and whose output nominal power is about 370 kW.
The two main parameters investigated are the nozzle capacity and the corresponding fuel injection pressure to keep the same burner consumption. Four nozzles whose capacity ranges from 30 to 45 kg/h at 20 bar were fitted on the burner equipping the fuel oil boiler. All the nozzles used are spill return and 45° type.
To assess the CO emissions of the fuel oil boiler, tests have been performed in transient and steady-state regimes. The tests were performed so that the output power was kept constant in steady-state regime (same water flow rate and inlet and outlet temperatures). That is why for each nozzle tested, the fuel injection pressure has been adjusted in order to maintain the fuel mass flow rate at around 37.7 kg/h. On the other hand, the oxygen content of the flue gas in the chimney was kept constant (around 3%) by adjusting only the position of the burner air damper (the position of the flame holder was kept constant) and thus the air combustion mass flow rate. Classical gas analysers were used to measure the CO2, O2, CO and NOx concentrations in the flue gas.
The tests performed showed that in steady-state regime, the CO emissions are very low compared to the European standard. On the other hand, in the transient regime (start-up regime for a cycling boiler), the CO emissions are not negligible and increase significantly if a high capacity nozzle is fitted on the fuel oil burner.
In short, the paper demonstrates that in order to increase the boiler thermal efficiency and thus to reduce CO2 emissions (for the same output power), one has to avoid the CO emissions in the transient regime. When performing a replacement of the burner nozzle, a low capacity nozzle associated with an adjustment of a high fuel injection pressure is recommended, since a reduction of CO emissions in the starting-up regime is achieved and, on the other hand, the thermal efficiency and the pollutants emissions of the fuel oil boiler in steady state regime are not affected.